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  • 1.
    Andersson, Nils
    et al.
    School of Mathematics, University of Southampton.
    Haskell, Brynmore
    Astronomical Institute "Anton Pannekoek, University of Amsterdam.
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Lagrangian perturbation theory for a superfluid immersed in an elastic neutron star crust2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 416, nr 1, s. 118-132Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The inner crust of mature neutron stars, where an elastic lattice of neutron-rich nuclei coexists with a neutron superfluid, impacts on a range of astrophysical phenomena. The presence of the superfluid is key to our understanding of pulsar glitches, and is expected to affect the thermal conductivity and hence the evolution of the surface temperature. The coupling between crust and superfluid must also be accounted for in studies of neutron star dynamics, discussions of global oscillations and associated instabilities. In this paper we develop Lagrangian perturbation theory for this problem, paying attention to key issues like superfluid entrainment, potential vortex pinning, dissipative mutual friction and the star's magnetic field. We also discuss the nature of the core-crust interface. The results provide a theoretical foundation for a range of interesting astrophysical applications.

  • 2.
    Bamford, R. A.
    et al.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Kellett, B.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Bradford, W. J.
    RAL Space, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Norberg, Carol
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Box 812, SE-981 28 Kiruna, Sweden.
    Thornton, A.
    York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom.
    Gibson, K. J.
    York Plasma Institute, Department of Physics, University of York, Heslington, York, YO10 5DD, United Kingdom.
    Crawford, I. A.
    Department of Earth and Planetary Sciences, Birkbeck College, London, United Kingdom.
    Silva, L.
    Instituto Superior Técnico, 1049-00, Lisboa, Portugal.
    Gargate, L.
    Instituto Superior Técnico, 1049-00, Lisboa, Portugal.
    Bingham, Ruth
    University of Strathclyde, Glasgow, Scotland, United Kingdom and Central Laser Facility, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, United Kingdom.
    Minimagnetospheres above the lunar surface and the formation of lunar swirls2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 8, s. 081101-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper we will present the in-situ satellite data, theory and laboratory validation that show how small scale collisionless shocks and mini-magnetospheres can form on the electron inertial scale length. The resulting retardation and deflection of the solar wind ions could be responsible for the unusual "lunar swirl" patterns seen on the surface of the Moon.

  • 3. Behar, E.
    et al.
    Tabone, B.
    Saillenfest, M.
    Henri, P.
    Deca, J.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holmstrom, M.
    Nilsson, H.
    Solar wind dynamics around a comet: A 2D semi-analytical kinetic model2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikel-id A35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. We aim at analytically modelling the solar wind proton trajectories during their interaction with a partially ionised cometary atmosphere, not in terms of bulk properties of the flow but in terms of single particle dynamics.

    Methods. We first derive a generalised gyromotion, in which the electric field is reduced to its motional component. Steady-state is assumed, and simplified models of the cometary density and of the electron fluid are used to express the force experienced by individual solar wind protons during the interaction.

    Results. A three-dimensional (3D) analytical expression of the gyration of two interacting plasma beams is obtained. Applying it to a comet case, the force on protons is always perpendicular to their velocity and has an amplitude proportional to 1/r2. The solar wind deflection is obtained at any point in space. The resulting picture presents a caustic of intersecting trajectories, and a circular region is found that is completely free of particles. The particles do not lose any kinetic energy and this absence of deceleration, together with the solar wind deflection pattern and the presence of a solar wind ion cavity, is in good agreement with the general results of the Rosetta mission.

    Conclusions. The qualitative match between the model and the in situ data highlights how dominant the motional electric field is throughout most of the interaction region for the solar wind proton dynamics. The model provides a simple general kinetic description of how momentum is transferred between these two collisionless plasmas. It also shows the potential of this semi-analytical model for a systematic quantitative comparison to the data.

  • 4.
    Bello Arufe, Aaron
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gravitational Waves in General Relativity2017Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [en]

    In this paper, we write a summary about general relativity and, in particular,gravitational waves. We start by discussing the mathematics that generalrelativity uses, as well as the geometry in general relativity's spacetime. Afterwards,we explain linearized general relativity and derive the linearizedversions of Einstein's equations. From here, we construct wave solutionsand explain the polarization of gravitational waves. The quadrupole formulais derived, and generation and detection of gravitational waves is brie ydiscussed. Finally, LIGO and its latest discovery of gravitational waves isreviewed.

  • 5. Berthomier, M.
    et al.
    Fazakerley, A. N.
    Forsyth, C.
    Pottelette, R.
    Alexandrova, O.
    Anastasiadis, A.
    Aruliah, A.
    Blelly, P. -L
    Briand, C.
    Bruno, R.
    Canu, P.
    Cecconi, B.
    Chust, T.
    Daglis, I.
    Davies, J.
    Dunlop, M.
    Fontaine, D.
    Genot, V.
    Gustavsson, B.
    Haerendel, G.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hapgood, M.
    Hess, S.
    Kataria, D.
    Kauristie, K.
    Kemble, S.
    Khotyaintsev, Y.
    Koskinen, H.
    Lamy, L.
    Lanchester, B.
    Louarn, P.
    Lucek, E.
    Lundin, R.
    Maksimovic, M.
    Manninen, J.
    Marchaudon, A.
    Marghitu, O.
    Marklund, G.
    Milan, S.
    Moen, J.
    Mottez, F.
    Nilsson, H.
    Ostgaard, N.
    Owen, C. J.
    Parrot, M.
    Pedersen, A.
    Perry, C.
    Pincon, J. -L
    Pitout, F.
    Pulkkinen, T.
    Rae, I. J.
    Rezeau, L.
    Roux, A.
    Sandahl, I.
    Sandberg, I.
    Turunen, E.
    Vogt, J.
    Walsh, A.
    Watt, C. E. J.
    Wild, J. A.
    Yamauchi, M.
    Zarka, P.
    Zouganelis, I.
    Alfven: magnetosphere-ionosphere connection explorers2012Ingår i: Experimental astronomy (Print), ISSN 0922-6435, E-ISSN 1572-9508, Vol. 33, nr 2-3, s. 445-489Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The aurorae are dynamic, luminous displays that grace the night skies of Earth's high latitude regions. The solar wind emanating from the Sun is their ultimate energy source, but the chain of plasma physical processes leading to auroral displays is complex. The special conditions at the interface between the solar wind-driven magnetosphere and the ionospheric environment at the top of Earth's atmosphere play a central role. In this Auroral Acceleration Region (AAR) persistent electric fields directed along the magnetic field accelerate magnetospheric electrons to the high energies needed to excite luminosity when they hit the atmosphere. The "ideal magnetohydrodynamics" description of space plasmas which is useful in much of the magnetosphere cannot be used to understand the AAR. The AAR has been studied by a small number of single spacecraft missions which revealed an environment rich in wave-particle interactions, plasma turbulence, and nonlinear acceleration processes, acting on a variety of spatio-temporal scales. The pioneering 4-spacecraft Cluster magnetospheric research mission is now fortuitously visiting the AAR, but its particle instruments are too slow to allow resolve many of the key plasma physics phenomena. The Alfv,n concept is designed specifically to take the next step in studying the aurora, by making the crucial high-time resolution, multi-scale measurements in the AAR, needed to address the key science questions of auroral plasma physics. The new knowledge that the mission will produce will find application in studies of the Sun, the processes that accelerate the solar wind and that produce aurora on other planets.

  • 6.
    Bradley, Michael
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Forsberg, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Keresztes, Zoltán
    Gravitational Waves in Locally Rotationally Symmetric (LRS) Class II Cosmologies2017Ingår i: Universe, ISSN 2218-1997, Vol. 3, nr 4, artikel-id 69Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work we consider perturbations of homogeneous and hypersurface orthogonal cosmological backgrounds with local rotational symmetry (LRS), using a method based on the 1 + 1 + 2 covariant split of spacetime. The backgrounds, of LRS class II, are characterised by that the vorticity, the twist of the 2-sheets, and the magnetic part of the Weyl tensor all vanish. They include the flat Friedmann universe as a special case. The matter contents of the perturbed spacetimes are given by vorticity-free perfect fluids, but otherwise the perturbations are arbitrary and describe gravitational, shear, and density waves. All the perturbation variables can be given in terms of the time evolution of a set of six harmonic coefficients. This set decouples into one set of four coefficients with the density perturbations acting as source terms, and another set of two coefficients describing damped source-free gravitational waves with odd parity. We also consider the flat Friedmann universe, which has been considered by several others using the 1 + 3 covariant split, as a check of the isotropic limit. In agreement with earlier results we find a second-order wavelike equation for the magnetic part of the Weyl tensor which decouples from the density gradient for the flat Friedmann universes. Assuming vanishing vector perturbations, including the density gradient, we find a similar equation for the electric part of the Weyl tensor, which was previously unnoticed.                

  • 7.
    Bradley, Michael
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Forsberg, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Keresztes, Zoltán
    University of Szeged.
    Gergely, László A´
    University of Szeged.
    Dunsby, Peter K S
    University of Cape Town.
    Perturbations of Kantowski-Sachs models2015Ingår i: Proceedings of the Thirteenth Marcel Grossman Meeting on General Relativity / [ed] Robert T Jantzen, Kjell Rosquist, Remo Ruffini, Singapore: World Scientific, 2015, s. 2547-2549Konferensbidrag (Refereegranskat)
    Abstract [en]

    Perturbations of Kantowski-Sachs models with a positive cosmological constant are considered in a harmonic decomposition, in the framework of gauge invariant 1+3 and 1+1+2covariant splits of spacetime. Scalar, vector and tensor modes are allowed, however they remain vorticity-free and of perfect fluid type. The dynamics is encompassed in six evolution equations for six harmonic coefficients.

  • 8. Brosch, Noah
    et al.
    Häggström, Ingemar
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    EISCAT observations of meteors from the sporadic complex2013Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 434, nr 4, s. 2907-2921Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report meteor observation with the European Incoherent Scatter Scientific Association (EISCAT) radars obtained during a continuous 24-h period in 2009 December. The period, just after the Geminid meteor shower, was selected to have no strong meteor shower activity to allow a comparison with our previous observations collected during the 2008 Geminid shower. During the 2009 run, we used the very high frequency (VHF) and ultrahigh frequency systems, but most of the results presented here were derived from the VHF data. We discuss the statistical properties of the radar echoes, their Doppler velocity and altitude distributions, their radar cross-section, etc. We concentrate, as in our previous paper, on the population of high-altitude echoes, which we clearly detect, and discuss these specifically. We recognize a few echoes with positive Doppler velocities as produced by meteoroids that presumably entered the atmosphere at similar to grazing incidence angles and were leaving it when detected by radar. We detect meteor echoes with essentially zero Doppler velocity, reported here for the first time, which we interpret as meteoroids moving almost perpendicular to the beam and producing specular reflections off the meteor trail. We discuss meteors detected with tristatic measurements for which we find bunching in azimuth and depression angle that depends on the time of the day. Finally, we report again of the lack of extreme velocity meteors, a fact that weakens significantly the claim of the existence and abundance of interstellar meteors.

  • 9.
    Cetoli, Alberto
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pethick, Christopher
    The Niels Bohr International Academy, The Niels Bohr Institute, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark .
    Interaction of gravitational waves with normal and superconducting matter2012Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 85, nr 6, s. 064036-Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    We develop a unified formalism for describing the interaction of gravitational waves with matterthat clearly separates the effects of general relativity from those due to interactions in the matter.This allows one to take into account the microscopic character of the matter, and we derive a generalexpression for the dispersion of gravitational waves in matter in terms of correlation functions forthe matter in flat spacetime. The formalism enables one to derive simply previous results for thedispersion of gravitational waves in astrophysical plasmas. We also consider metals and show that,while simple estimates indicate that contributions of electrons to the stress tensor could be large,screening of the Coulomb interaction reduces these effects considerably. We consider both normaland superconducting metals, and show that in both cases, electrons and ions are locked togetherunder the influence of a gravitational wave with a frequency much less than the plasma frequencyand, consequently, charge separation has little effect on gravitational waves.

  • 10. de Vera, Jean-Pierre
    et al.
    Alawi, Mashal
    Backhaus, Theresa
    Baque, Mickael
    Billi, Daniela
    Boettger, Ute
    Berger, Thomas
    Bohmeier, Maria
    Cockell, Charles
    Demets, Rene
    de la Torre Noetzel, Rosa
    Edwards, Howell
    Elsaesser, Andreas
    Fagliarone, Claudia
    Fiedler, Annelie
    Foing, Bernard
    Foucher, Frederic
    Fritz, Joerg
    Hanke, Franziska
    Herzog, Thomas
    Horneck, Gerda
    Huebers, Heinz-Wilhelm
    Huwe, Bjoern
    Joshi, Jasmin
    Kozyrovska, Natalia
    Kruchten, Martha
    Lasch, Peter
    Lee, Natuschka
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Leuko, Stefan
    Leya, Thomas
    Lorek, Andreas
    Martinez-Frias, Jesus
    Meessen, Joachim
    Moritz, Sophie
    Moeller, Ralf
    Olsson-Francis, Karen
    Onofri, Silvano
    Ott, Sieglinde
    Pacelli, Claudia
    Podolich, Olga
    Rabbow, Elke
    Reitz, Guenther
    Rettberg, Petra
    Reva, Oleg
    Rothschild, Lynn
    Garcia Sancho, Leo
    Schulze-Makuch, Dirk
    Selbmann, Laura
    Serrano, Paloma
    Szewzyk, Ulrich
    Verseux, Cyprien
    Wadsworth, Jennifer
    Wagner, Dirk
    Westall, Frances
    Wolter, David
    Zucconi, Laura
    Limits of Life and the Habitability of Mars: The ESA Space Experiment BIOMEX on the ISS2019Ingår i: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 19, nr 2, s. 145-157Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    BIOMEX (BIOlogy and Mars EXperiment) is an ESA/Roscosmos space exposure experiment housed within the exposure facility EXPOSE-R2 outside the Zvezda module on the International Space Station (ISS). The design of the multiuser facility supports-among others-the BIOMEX investigations into the stability and level of degradation of space-exposed biosignatures such as pigments, secondary metabolites, and cell surfaces in contact with a terrestrial and Mars analog mineral environment. In parallel, analysis on the viability of the investigated organisms has provided relevant data for evaluation of the habitability of Mars, for the limits of life, and for the likelihood of an interplanetary transfer of life (theory of lithopanspermia). In this project, lichens, archaea, bacteria, cyanobacteria, snow/permafrost algae, meristematic black fungi, and bryophytes from alpine and polar habitats were embedded, grown, and cultured on a mixture of martian and lunar regolith analogs or other terrestrial minerals. The organisms and regolith analogs and terrestrial mineral mixtures were then exposed to space and to simulated Mars-like conditions by way of the EXPOSE-R2 facility. In this special issue, we present the first set of data obtained in reference to our investigation into the habitability of Mars and limits of life. This project was initiated and implemented by the BIOMEX group, an international and interdisciplinary consortium of 30 institutes in 12 countries on 3 continents. Preflight tests for sample selection, results from ground-based simulation experiments, and the space experiments themselves are presented and include a complete overview of the scientific processes required for this space experiment and postflight analysis. The presented BIOMEX concept could be scaled up to future exposure experiments on the Moon and will serve as a pretest in low Earth orbit.

  • 11. Dinu, Victor
    et al.
    Heinzl, Thomas
    Ilderton, Anton
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Infrared divergences in plane wave backgrounds2012Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 86, nr 8, s. 085037-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that the emission of soft photons via nonlinear Compton scattering in a pulsed plane wave (laser field) is in general infrared divergent. We give examples of both soft and soft-collinear divergences, and we pay particular attention to the case of crossed fields in both classical and quantum theories.

  • 12.
    Edgar, S Brian
    et al.
    Linköpings Universitet.
    Bradley, Michael
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Machado Ramos, M Piedade
    Universidade do Minho, Guimarães.
    Symmetry analysis of radiative spacetimes with a null isotropy using GHP formalism2014Ingår i: General Relativity and Gravitation, ISSN 0001-7701, E-ISSN 1572-9532, Vol. 46, nr 10, s. 1797-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A complete and simple invariant classification of the conformally flat pure radiation metrics with a negative cosmological constant that were obtained by integration using the generalised invariant formalism is presented. We show equivalence between these metrics and the corresponding type O subclass of the more general spacetime studied by Siklos. The classification procedure indicates that the metrics possess a one degree of null isotropy freedom which has very interesting repercussions in the symmetry analysis. The Killing and homothetic vector analysis in GHP formalism is then generalised to this case were there is only one null direction defined geometrically. We determine the existing Killing vectors for the different subclasses that arise in the classification and compare these results to those obtained in the symmetry analysis performed by Siklos for a larger class of metrics with Ricci tensor representing a pure radiation field and a negative cosmological constant. It is also shown that there are no homothetic Killing vectors present.

  • 13.
    Ekenbäck, Andreas
    et al.
    Swedish Institute of Space Physics, P.O. Box 812, SE-98128 Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, P.O. Box 812, SE-98128 Kiruna, Sweden.
    Wurz, Peter
    Griessmeier, Jean-Mathias
    Lammer, Helmut
    Selsis, Franck
    Penz, Thomas
    Energetic neutral atoms around HD 209458b: estimations of magnetospheric properties2010Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 709, nr 2, s. 670-679Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    HD 209458b is an exoplanet found to transit the disk of its parent star. Observations have shown a broad absorption signature about the Ly alpha stellar line during transit, suggesting the presence of a thick cloud of atomic hydrogen around the "hot Jupiter" HD 209458b. This work expands on an earlier work studying the production of energetic neutral atoms (ENAs) as a result of the interaction between the stellar wind and the exosphere. We present an improved flow model of HD 209458b and use stellar wind values similar to those in our solar system. We find that the ENA production is high enough to explain the observations, and we show that-using expected values for the stellar wind and exosphere-the spatial and velocity distributions of ENAs would give absorption in good agreement with the observations. We also study how the production of ENAs depends on the exospheric parameters and establish an upper limit for the obstacle standoff distance at approximately 4-10 planetary radii. Finally, we compare the results obtained for the obstacle standoff distance with existing exomagnetospheric models and show how the magnetic moment of HD 209458b can be estimated from ENA observations.

  • 14.
    Fatemi, Shahab
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden ; Department of Computer Science, Electrical and Space Engineering, Division of Space Technology, Luleå University of Technology, Luleå, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Lue, Charles
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Collier, Michael R.
    NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Effects of protons reflected by lunar crustal magnetic fields on the global lunar plasma environment2014Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 8, s. 6095-6105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Solar wind plasma interaction with lunar crustal magnetic fields is different than that of magnetized bodies like the Earth. Lunar crustal fields are, for typical solar wind conditions, not strong enough to form a (bow) shock upstream but rather deflect and perturb plasma and fields. Here we study the global effects of protons reflected from lunar crustal magnetic fields on the lunar plasma environment when the Moon is in the unperturbed solar wind. We employ a three-dimensional hybrid model of plasma and an observed map of reflected protons from lunar magnetic anomalies over the lunar farside. We observe that magnetic fields and plasma upstream over the lunar crustal fields compress to nearly 120% and 160% of the solar wind, respectively. We find that these disturbances convect downstream in the vicinity of the lunar wake, while their relative magnitudes decrease. In addition, solar wind protons are disturbed and heated at compression regions and their velocity distribution changes from Maxwellian to a non-Maxwellian. Finally, we show that these features persists, independent of the details of the ion reflection by the magnetic fields.

  • 15. Fatemi, Shahab
    et al.
    Lue, Charles
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holmstrom, Mats
    Poppe, Andrew R.
    Wieser, Martin
    Barabash, Stas
    Delory, Gregory T.
    Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 6, s. 4719-4735Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the results of the first local hybrid simulations (particle ions and fluid electrons) for the solar wind plasma interaction with realistic lunar crustal fields. We use a three-dimensional hybrid model of plasma and an empirical model of the Gerasimovich magnetic anomaly based on Lunar Prospector observations. We examine the effects of low and high solar wind dynamic pressures on this interaction when the Gerasimovich magnetic anomaly is located at nearly 20 degrees solar zenith angle. We find that for low solar wind dynamic pressure, the crustal fields mostly deflect the solar wind plasma, form a plasma void at very close distances to the Moon (below 20km above the surface), and reflect nearly 5% of the solar wind in charged form. In contrast, during high solar wind dynamic pressure, the crustal fields are more compressed, the solar wind is less deflected, and the lunar surface is less shielded from impinging solar wind flux, but the solar wind ion reflection is more locally intensified (up to 25%) compared to low dynamic pressures. The difference is associated with an electrostatic potential that forms over the Gerasimovich magnetic anomaly as well as the effects of solar wind plasma on the crustal fields during low and high dynamic pressures. Finally, we show that an antimoonward Hall electric field is the dominant electric field for similar to 3km altitude and higher, and an ambipolar electric field has a noticeable contribution to the electric field at close distances (<3km) to the Moon.

  • 16.
    Forsberg, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gravitational perturbations in plasmas and cosmology2010Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Gravitational perturbations can be in the form of scalars, vectors or tensors. This thesis focuses on the evolution of scalar perturbations in cosmology, and interactions between tensor perturbations, in the form of gravitational waves, and plasma waves.

    The gravitational waves studied in this thesis are assumed to have small amplitudes and wavelengths much shorter than the background length scale, allowing for the assumption of a flat background metric. Interactions between gravitational waves and plasmas are described by the Einstein-Maxwell-Vlasov, or the Einstein-Maxwell-fluid equations, depending on the level of detail required. Using such models, linear wave excitation of various waves by gravitational waves in astrophysical plasmas are studied, with a focus on resonance effects. Furthermore, the influence of strong magnetic field quantum electrodynamics, leading to detuning of the gravitational wave-electromagnetic wave resonances, is considered. Various nonlinear phenomena, including parametric excitation and wave steepening are also studied in different astrophysical settings.

    In cosmology the evolution of gravitational perturbations are of interest in processes such as structure formation and generation of large scale magnetic fields. Here, the growth of density perturbations in Kantowski-Sachs cosmologies with positive cosmological constant is studied.

  • 17.
    Forsberg, Mats
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Brodin, Gert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Linear theory of gravitational wave propagation in a magnetized, relativistic Vlasov plasma2010Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 82, nr 12, artikel-id 124029Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider propagation of gravitational waves in a magnetized plasma, using the linearized Maxwell-Vlasov equations coupled to Einstein's equations. A set of coupled electromagnetic-gravitational wave equations are derived that can be straightforwardly reduced to a single dispersion relation. We demonstrate that there is a number of different resonance effects that can enhance the influence of the plasma on the gravitational waves.

  • 18. Futaana, Y
    et al.
    Barabash, S
    Wieser, M
    Lue, C
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Wurz, P
    Vorburger, A
    Bhardwaj, A
    Asamura, K
    Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly2013Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, nr 2, s. 262-266Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human exploration on the Moon. A large positive electric potential was predicted and detected from single point measurements. Here, we demonstrate a remote imaging technique of electric potential mapping at the lunar surface, making use of a new concept involving hydrogen neutral atoms derived from solar wind. We apply the technique to a lunar magnetized region using an existing dataset of the neutral atom energy spectrometer SARA/CENA on Chandrayaan-1. Electrostatic potential larger than +135 V inside the Gerasimovic anomaly is confirmed. This structure is found spreading all over the magnetized region. The widely spread electric potential can influence the local plasma and dust environment near the magnetic anomaly. Citation: Futaana, Y., S. Barabash, M. Wieser, C. Lue, P. Wurz, A. Vorburger, A. Bhardwaj, and K. Asamura (2013), Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly, Geophys. Res. Lett., 40, 262-266, doi:10.1002/grl.50135.

  • 19. Glampedakis, K.
    et al.
    Jones, D. I.
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gravitational Waves from Color-Magnetic "Mountains" in Neutron Stars2012Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, nr 8, s. 081103-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Neutron stars may harbor the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color and flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter, we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of nonaxisymmetry in systems such as radio pulsars. We show that gravitational radiation from such color-magnetic "mountains" in young pulsars, such as the Crab and Vela, could be observable by the future Einstein Telescope, thus, becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.

  • 20. Glampedakis, Kostas
    et al.
    Andersson, Nils
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Nordita, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden.
    Magnetohydrodynamics of superfluid and superconducting neutron star cores2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 410, nr 2, s. 805-829Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Mature neutron stars are cold enough to contain a number of superfluid and superconducting components. These systems are distinguished by the presence of additional dynamical degrees of freedom associated with superfluidity. In order to consider models with mixtures of condensates, we need to develop a multifluid description that accounts for the presence of rotational neutron vortices and magnetic proton fluxtubes. We also need to model the forces that impede the motion of vortices and fluxtubes, and understand how these forces act on the condensates. This paper concerns the development of such a model for the outer core of a neutron star, where superfluid neutrons co-exist with a type II proton superconductor and an electron gas. We discuss the hydrodynamics of this system, focusing on the role of the entrainment effect, the magnetic field, the vortex/fluxtube tension and the dissipative mutual friction forces. Our final results can be directly applied to a number of interesting astrophysical scenarios, e.g. associated with neutron star oscillations or the evolution of the large-scale magnetic field.

  • 21.
    Glampedakis, Kostas
    et al.
    Theoretical Astrophysics, University of Tübingen.
    Jones, D. Ian
    School of Mathematics, University of Southampton.
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Nordita, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden.
    Ambipolar diffusion in superfluid neutron stars2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, nr 3, s. 2021-2030Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    In this paper we reconsider the problem of magnetic field diffusion in neutron star cores. We model the star as consisting of a mixture of neutrons, protons and electrons, and allow for particle reactions and binary collisions between species. Our analysis is in much the same spirit as that of Goldreich & Reisenegger (1992), and we content ourselves with rough estimates of magnetic diffusion timescales, rather than solving accurately for some particular field geometry. However, our work improves upon previous treatments in one crucial respect: we allow for superfluidity in the neutron star matter. We find that the consequent mutual friction force, coupling the neutrons and charged particles, together with the suppression of particles collisions and reactions, drastically affect the ambipolar magnetic field diffusion timescale. In particular, the addition of superfluidity means that it is unlikely that there is ambipolar diffusion in magnetar cores on the timescale of the lifetimes of these objects, contradicting an assumption often made in the modelling of the flaring activity commonly observed in magnetars. Our work suggests that if a decaying magnetic field is indeed the cause of magnetar activity, the field evolution is likely to take place outside of the core, and might represent Hall/Ohmic diffusion in the stellar crust, or else that a mechanism other than standard ambipolar diffusion is active, e.g. flux expulsion due to the interaction between neutron vortices and magnetic fluxtubes.

  • 22. Gunell, H
    et al.
    Andersson, L
    De Keyser, J
    Mann, Ingrid
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. EISCAT Scientific Association, Kiruna, Sweden.
    Self-consistent electrostatic simulations of reforming double layers in the downward current region of the aurora2015Ingår i: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 33, nr 10, s. 1331-1342Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The plasma on a magnetic field line in the downward current region of the aurora is simulated using a Vlasov model. It is found that an electric field parallel to the magnetic fields is supported by a double layer moving toward higher altitude. The double layer accelerates electrons upward, and these electrons give rise to plasma waves and electron phase-space holes through beam-plasma interaction. The double layer is disrupted when reaching altitudes of 12 Earth radii where the Langmuir condition no longer can be satisfied due to the diminishing density of electrons coming up from the ionosphere. During the disruption the potential drop is in part carried by the electron holes. The disruption creates favourable conditions for double layer formation near the ionosphere and double layers form anew in that region. The process repeats itself with a period of approximately 1 min. This period is determined by how far the double layer can reach before being disrupted: a higher disruption altitude corresponds to a longer repetition period. The disruption altitude is, in turn, found to increase with ionospheric density and to decrease with total voltage. The current displays oscillations around a mean value. The period of the oscillations is the same as the recurrence period of the double layer formations. The oscillation amplitude increases with increasing voltage, whereas the mean value of the current is independent of voltage in the 100 to 800 V range covered by our simulations. Instead, the mean value of the current is determined by the electron density at the ionospheric boundary.

  • 23. Gunell, H
    et al.
    Mann, Ingrid
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Eiscat Scientific Association, Kiruna, Sweden.
    Wedlund, C Simon
    Kallio, E
    Alho, M
    Nilsson, H
    De Keyser, J
    Dhooghe, F
    Maggiolo, R
    Acceleration of ions and nano dust at a comet in the solar wind2015Ingår i: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 119, s. 13-23Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A quasi-neutral hybrid simulation of the interaction of the solar wind with the atmosphere of a comet is used together with a test particle simulation of cometary ions and dust to compute trajectories and velocity distribution functions of charged particles, starting outside the diamagnetic cavity at 150 km cometocentric distance. The simulations are run with parameters suited to make predictions for comet 67P/Churyumov-Gerasimenko when it is at a heliocentric distance of 1.45 AU. It is found that the shape of the ion trajectories depends on the location of the source, and that a velocity distribution that is observed at a given point in space is influenced by the spatial structure of the source. Charged dust grains with radii in the 1-10 nm range are accelerated from the nucleus to a distance of 2.9 x 104 km in between 15 min and 2 h approximately. Dust particles smaller than 10 nm in radius are accelerated to speeds over 10 km/s.

  • 24. Gunell, H.
    et al.
    Nilsson, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Eriksson, A.
    Odelstad, E.
    Maggiolo, R.
    Henri, P.
    Vallieres, X.
    Altwegg, K.
    Tzou, C. -Y
    Rubin, M.
    Glassmeier, K. -H
    Wieser, G. Stenberg
    Wedlund, C. Simon
    De Keyser, J.
    Dhooghe, F.
    Cessateur, G.
    Gibbons, A.
    Ion acoustic waves at comet 67P/Churyumov-Gerasimenko: Observations and computations2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, artikel-id A3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. On 20 January 2015 the Rosetta spacecraft was at a heliocentric distance of 2.5 AU, accompanying comet 67P/Churyumov-Gerasimenko on its journey toward the Sun. The Ion Composition Analyser (RPC-ICA), other instruments of the Rosetta Plasma Consortium, and the ROSINA instrument made observations relevant to the generation of plasma waves in the cometary environment. Aims. Observations of plasma waves by the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) can be explained by dispersion relations calculated based on measurements of ions by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA), and this gives insight into the relationship between plasma phenomena and the neutral coma, which is observed by the Comet Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA-COPS). Methods. We use the simple pole expansion technique to compute dispersion relations for waves on ion timescales based on the observed ion distribution functions. These dispersion relations are then compared to the waves that are observed. Data from the instruments RPC-LAP, RPC-ICA and the mutual impedance probe (RPC-MIP) are compared to find the best estimate of the plasma density. Results. We find that ion acoustic waves are present in the plasma at comet 67P/Churyumov-Gerasimenko, where the major ion species is H2O+. The bulk of the ion distribution is cold, k(B)T(i) = 0.01 eV when the ion acoustic waves are observed. At times when the neutral density is high, ions are heated through acceleration by the solar wind electric field and scattered in collisions with the neutrals. This process heats the ions to about 1 eV, which leads to significant damping of the ion acoustic waves. Conclusions. In conclusion, we show that ion acoustic waves appear in the H2O+ plasmas at comet 67P/Churyumov-Gerasimenko and how the interaction between the neutral and ion populations affects the wave properties.

  • 25.
    Gunell, Herbert
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium.
    Goetz, Charlotte
    Wedlund, Cyril Simon
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Nilsson, Hans
    LLera, Kristie
    Eriksson, Anders
    Holmström, Mats
    The infant bow shock: a new frontier at a weak activity comet2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, artikel-id L2Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The bow shock is the first boundary the solar wind encounters as it approaches planets or comets. The Rosetta spacecraft was able to observe the formation of a bow shock by following comet 67P/Churyumov-Gerasimenko toward the Sun, through perihelion, and back outward again. The spacecraft crossed the newly formed bow shock several times during two periods a few months before and after perihelion; it observed an increase in magnetic field magnitude and oscillation amplitude, electron and proton heating at the shock, and the diminution of the solar wind further downstream. Rosetta observed a cometary bow shock in its infancy, a stage in its development not previously accessible to in situ measurements at comets and planets.

  • 26.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Andersson, L.
    Vaivads, A.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, H.
    The use of the power density for identifying reconnection regions2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 10, s. 8644-8662Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the vicinity of magnetic reconnection, magnetic energy is transferred into kinetic energy. A reconnection region hence corresponds to a load, and it should manifest itself as large and positive values of the power density, E·J ≫ 0, where E is the electric field and J the current density. In this article we analyze Cluster plasma sheet data from 2001–2004 to investigate the use of the power density for identifying possible magnetic reconnection events from large sets of observed data. From theoretical arguments we show that an event with   pW/m3 in the Earth's magnetotail observed by the Cluster instruments (X <− 10RE and  ) is likely to be associated with reconnection. The power density can be used as a primary indicator of potential reconnection regions, but selected events must be reviewed separately to confirm any possible reconnection signatures by looking for other signatures such as Hall electric and magnetic fields and reconnection jets. The power density can be computed from multispacecraft data, and we argue that the power density can be used as a tool for identifying possible reconnection events from large sets of data, e.g., from the Cluster and the Magnetospheric Multiscale missions.

  • 27.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Marghitu, O
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Buchert, S
    André, M
    Klecker, B
    Kistler, LM
    Dandouras, I
    The role of the inner tail to midtail plasma sheet in channeling solar wind power to the ionosphere2012Ingår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, nr A6, s. A06310-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this article we use Cluster power density (E . J) data from 2001, 2002, and 2004 to investigate energy conversion and transfer in the plasma sheet. We show that a southward IMF B-z is favorable for plasma sheet energy conversion, and that there is an increased particle and Poynting flux toward the Earth at times when Cluster observes an enhanced energy conversion in the plasma sheet. Conversion from electromagnetic to kinetic energy is increasingly dominant farther down-tail, while the generation of electromagnetic power from kinetic energy becomes important toward the Earth with a maximum at roughly 10 R-E. By linking observations of the key quantity E . J to observations of the solar wind input and earthward energy flux, our results demonstrate the role of the inner tail to midtail plasma sheet as a mediator between the solar wind energy input into the magnetosphere and the auroral dissipation in the ionosphere.

  • 28.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, T.
    Nilsson, H.
    Fu, H. S.
    Buchert, S.
    Andre, M.
    Marghitu, O.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    The evolution of flux pileup regions in the plasma sheet: Cluster observations2013Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, nr 10, s. 6279-6290Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, E<bold></bold>J<0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, E<bold></bold>J>0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R(E). Our observations are compared to recent simulation results and previous observations.

  • 29. Horneck, Gerda
    et al.
    Walter, Nicolas
    Westall, Frances
    Grenfell, John Lee
    Martin, William F.
    Gomez, Felipe
    Leuko, Stefan
    Lee, Natuschka
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap. Department of Microbiology, Technical University München, München, Germany.
    Onofri, Silvano
    Tsiganis, Kleomenis
    Saladino, Raffaele
    Pilat-Lohinger, Elke
    Palomba, Ernesto
    Harrison, Jesse
    Rull, Fernando
    Muller, Christian
    Strazzulla, Giovanni
    Brucato, John R.
    Rettberg, Petra
    Capria, Maria Teresa
    AstRoMap European Astrobiology Roadmap2016Ingår i: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 16, nr 3, s. 201-243Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    The European AstRoMap project (supported by the European Commission Seventh Framework Programme) surveyed the state of the art of astrobiology in Europe and beyond and produced the first European roadmap for astrobiology research. In the context of this roadmap, astrobiology is understood as the study of the origin, evolution, and distribution of life in the context of cosmic evolution; this includes habitability in the Solar System and beyond. The AstRoMap Roadmap identifies five research topics, specifies several key scientific objectives for each topic, and suggests ways to achieve all the objectives. The five AstRoMap Research Topics are

    1. • Research Topic 1: Origin and Evolution of Planetary Systems

    2. • Research Topic 2: Origins of Organic Compounds in Space

    3. • Research Topic 3: Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life

    4. • Research Topic 4: Life and Habitability

    5. • Research Topic 5: Biosignatures as Facilitating Life Detection

    It is strongly recommended that steps be taken towards the definition and implementation of a European Astrobiology Platform (or Institute) to streamline and optimize the scientific return by using a coordinated infrastructure and funding system.

  • 30.
    Ilderton, Anton
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Torgrimsson, Greger
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Scattering in plane-wave backgrounds: infrared effects and pole structure2013Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 87, nr 8, artikel-id 085040Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We consider two aspects of scattering in strong plane-wave backgrounds. First, we show that the infrared divergences in elastic scattering depend on the structure of the background, but can be removed using the usual Bloch-Nordsieck approach. Second, we analyze the infinite series of shifted-mass-shell poles in the particle (Volkov) propagator using lightfront quantization. The complete series of poles is shown to describe a single, on-shell, propagating particle.

  • 31.
    Jörgenfelt, Erik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Invariant Construction of Cosmological Models2016Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Lösningar till Einsteins fältekvationer kan konstrueras från invarianta objekt. Om en lösning hittas är den lokalt ekvivalent till alla andra lösningar konstruerade från samma mängd. Här appliceras metoden på spatialt homogena lösningar med förbestämda algebror, under antagandet att den invarianta ramen med standard-vektorer är ortogonal på banorna. Lösningar med ortogonalt fluidflöde av Bianchi typ III undersöks och två klasser av dammlösningar (noll tryck) hittas, liksom en vakum-energilösning. Dammlösningar med lut och noll-skild vorticitet av Bianchi typ I--III undersöks också, och det visas att inga sådana lösningar existerar givet antagandena.

  • 32. Karlsson, T.
    et al.
    Kullen, A.
    Liljeblad, E.
    Brenning, N.
    Nilsson, H.
    Gunell, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    On the origin of magnetosheath plasmoids and their relation to magnetosheath jets2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 9, s. 7390-7403Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate localized magnetosheath and solar wind density enhancements, associated with clear magnetic field changes, and therefore referred to as magnetosheath/solar wind plasmoids, respectively. Using Cluster data, we show that there are two distinct populations of magnetosheath plasmoids, one associated with a decrease of magnetic field strength (diamagnetic plasmoids), and one with an increased magnetic field strength (paramagnetic plasmoids). The diamagnetic magnetosheath plasmoids have scale sizes of the order of 1-10 RE, while the paramagnetic ones are an order of magnitude smaller. The diamagnetic plasmoids are not associated with any change in the magnetosheath plasma flow velocity, and they are classified as embedded plasmoids in the terminology of Karlsson et al. (2012). The paramagnetic plasmoids may either be embedded or associated with increases in flow velocity (fast plasmoids). A search for plasmoids in the pristine solar wind resulted in identification of 62 diamagnetic plasmoids with very similar properties to the magnetosheath diamagnetic plasmoids, making it probable that the solar wind is the source of these structures. No paramagnetic plasmoids are found in the pristine solar wind, indicating that these are instead created at the bow shock or in the magnetosheath. We discuss the relation of the plasmoids to the phenomenon of magnetosheath jets, with which they have many properties in common, and suggest that the paramagnetic plasmoids can be regarded as a subset of these or a closely related phenomenon. We also discuss how the results from this study relate to theories addressing the formation of magnetosheath jets.

  • 33. Keresztes, Zoltán
    et al.
    Forsberg, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Bradley, Michael
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Dunsby, Peter K S
    Gergely, László Á
    Gravitational, shear and matter waves in Kantowski-Sachs cosmologies2015Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 11, artikel-id 042Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A general treatment of vorticity-free, perfect fluid perturbations of Kantowski-Sachs models with a positive cosmological constant are considered within the framework of the 1+1+2 covariant decomposition of spacetime. The dynamics is encompassed in six evolution equations for six harmonic coefficients, describing gravito-magnetic, kinematic and matter perturbations, while a set of algebraic expressions determine the rest of the variables. The six equations further decouple into a set of four equations sourced by the perfect fluid, representing forced oscillations and two uncoupled damped oscillator equations. The two gravitational degrees of freedom are represented by pairs of gravito-magnetic perturbations. In contrast with the Friedmann case one of them is coupled to the matter density perturbations, becoming decoupled only in the geometrical optics limit. In this approximation, the even and odd tensorial perturbations of the Weyl tensor evolve as gravitational waves on the anisotropic Kantowski-Sachs background, while the modes describing the shear and the matter density gradient are out of phase dephased by pi/2 and share the same speed of sound.

  • 34.
    Kero, J.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Fujiwara, Y.
    Abo, M.
    Szasz, C.
    Nakamura, T.
    MU radar head echo observations of the 2011 October Draconids2012Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 424, nr 3, s. 1799-1806Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    On 2011 October 8, the Earth passed through a stream of dust ejected by the comet 21P/GiacobiniZinner during its perihelion passage of the year 1900, causing an outburst of October Draconid meteors. 13 Draconids were observed among similar to 6300 meteor head echoes with precisely determined orbits during an observational campaign ranging from October 8 05:00 ut to October 9 13:00 ut with the Shigaraki middle and upper atmosphere (MU) radar in Japan (34 degrees.85?N and 136 degrees.10?E). The meteor outburst occurred while the Draconid radiant was descending below and 2?h later rising up above the horizon. Therefore, 11 of the detections were from very low (<15 degrees) elevation. The detection altitudes of the Draconids were high compared to sporadic meteors of the same velocity and radiant elevation. The weighted mean geocentric velocity of the 13 Draconids was 20.6 +/- 0.4?km?s-1, and the weighted mean radiant located at right ascension alpha = 263 degrees.3 +/- 0 degrees.6 and declination delta = 55 degrees.8 +/- 0 degrees.2.

  • 35.
    Kero, J.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Szasz, C.
    Nakamura, T.
    Meisel, D. D.
    Ueda, M.
    Fujiwara, Y.
    Terasawa, T.
    Nishimura, K.
    Watanabe, J.
    The 2009-2010 MU radar head echo observation programme for sporadic and shower meteors: radiant densities and diurnal rates2012Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 425, nr 1, s. 135-146Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The aim of this paper is to give an overview of the monthly meteor head echo observations (528.8?h) conducted between 2009 June and 2010 December using the Shigaraki Middle and Upper atmosphere radar in Japan (34 degrees.85 N, 136 degrees.10 E). We present diurnal detection rates and radiant density plots from 18 separate observational campaigns, each lasting for at least one diurnal cycle. Our data comprise more than 106?000 meteors. All six recognized apparent sporadic meteor sources are discernable and their average orbital distributions are presented in terms of geocentric velocity, semimajor axis, inclination and eccentricity. The north and south apex have radiant densities an order of magnitude higher than other apparent source regions. The diurnal detection rates show clear seasonal dependence. The main cause of the seasonal variation is the tilt of the Earth's axis, causing the elevation of the Earth's apex above the local horizon to change as the Earth revolves around the Sun. Yet, the meteor rate variation is not symmetric with respect to the equinoxes. When comparing the radiant density at different times of the year, and thus at different solar longitudes along the Earth's orbit, we have found that the north and south apex source regions fluctuate in strength.

  • 36.
    Koblik, V.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för tillämpad fysik och elektronik.
    Polyakhova, E.
    Sokolov, L.
    Solar sail near the Sun: Point-like and extended models of radiation source2011Ingår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 48, nr 11, s. 1717-1739Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Some modifications of solar sail radiation pressure forces on a plate and on a sphere for use in the numerical simulation of 'local-optimal' (or 'instantaneously optimal') trajectories of a spacecraft with a solar sail are suggested. The force model development is chronologically reviewed, including its connection with solar sail surface reflective and thermal properties. The sail surface is considered as partly absorbing, partly reflective (specular and diffuse), partly transparent. Thermal balance is specified because the spacecraft moves from circular Earth orbit to near-Sun regions and thermal limitations on the sail film are taken into account. A spherical sail-balloon can be used in near-Sun regions for scientific research beginning with the solar-synchronous orbit and moving outward from the Sun. The Sun is considered not only as a point-like source of radiation but also as an extended source of radiation which is assumed to be consequently as a point-like source of radiation, a uniformly bright flat solar disc and uniformly bright solar sphere. (C) 2011 COSPAR. Published by Elsevier Ltd. All rights reserved.

  • 37.
    Kobyakov, Dmitry
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars2014Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In the present thesis, “Hydrodynamics of Binary Bose-Einstein Condensates and Hydro-elasticity of the Inner Crust of Neutron Stars”, the hydrodynamic effects, instabilities and superfluid turbulence in binary immiscible ultracold gases, and hydro-elastic macroscopic coupled modes and microscopic structure of the inner layers of the crust of neutron stars, are studied. The ultracold gas dynamics can be realized in the laboratory. The excitation modes of the inner crust determine a number of observable properties such as elasticity, thermal properties and mass transport properties. Here we focus on expanding the details, rather than repeating the results presented in the published articles.

    In the part of the thesis related to atomic ultracold gases, we utilize the physical parameters in the experimentally realizable parameter region. We numerically simulate the coupled non-linear Schrödinger equations, and calculate observable quantities, such as phase and modulus of the order parameter, conditions needed for observation of the Rayleigh-Taylor instability and for turbulence generation. The numerical calculations are accompanied by analytical description of the processes. The dispersion relation for capillary-gravitational waves at the interface between two ultracold gases, is derived straightforwardly from the superfluid Lagrangian. The equations of motion for centre-of-mass of the superfluids are derived, and then used in our model of the quantum swapping of immiscible superfluids pressed by a strong external force. By numerical simulation, we find that the Kelvin-Helmholtz instability which occurs at the non-linear stage of the Rayleigh-Taylor instability, can generate quantum turbulence with peculiar properties. We find that two-dimensional superfluid systems with weak inter-component repulsion are different from previously studied strongly repulsive binary superfluids, because the quantum Kelvin-Helmholtz instability in weakly repulsive superfluids rolls up the whole interface forming a vortex bundle, similarly to dynamics of the shear fluid layers in the classical hydrodynamics. Production of vortex bundles favours the Kolmogorov spectrum of turbulence, and we find that the Kolmogorov scaling indeed is present in a freely decaying turbulence.

    In the part of the thesis related to neutron stars, we study the inner crust of neutron stars, where the fully ionized atomic nuclei coexist with a superfluid of neutrons. The interaction between superfluid neutrons and the crystallized Coulomb plasma is due to the interaction between density perturbations (interaction of the scalar type), and between the current - the non-dissipative entrainment effect (interaction of the vector type). We calculate velocities of the collective modes of the crystal coupled to superfluid neutrons. As an input we use the results of microscopic nuclear calculations in the framework of the compressible liquid drop model (the Lattimer and Swesty equation of state), and more recent effective Thomas-Fermi calculations with shell corrections (N. Chamel, and the Brussels theoretical nuclear physics group). Knowledge of velocities as functions of the matter density in the inner crust is important for calculation of a number of dynamic and transport properties. The heat transport properties of the inner crust are directly observable in accreting binary systems (low-mass x-ray binaries). The mass transport properties of the inner crust are directly linked to the rotational evolution, being a key physical ingredient of the pulsar glitch phenomenon. The elastic properties are related to the vibrational modes of the star, and to the breaking stress of the crust.

    In the second part of our work on neutron stars we investigate the microscopic structure of the inner crust treating the structure as an anisotropic crystal coupled to s-wave superfluid neutron liquid. As we analyse dynamics of the elementary excitations at higher wavenumbers (smaller scales), we reach the edge of the first Brillouin zone. The Lattimer-Swesty data is applicable for wavenumbers much smaller than the edge of the first Brillouin zone. We extrapolate the data through the whole first Brillouin zone to calculate the fastest growth rate of the unstable modes. The crucial step is to calculate the mode velocities in anisotropic crystal incorporating both the induced neutron-proton interactions, and the electron screening properties. We find that the combined influence of these two effects leads to softening of the longitudinal phonon of the lattice above about the Thomas-Fermi screening wavenumber of the electrons. The critical wavenumber when the frequency becomes purely imaginary is about  1/5 - 2/3  of the reciprocal lattice vector, thus validating our assumption. The imaginary mode frequency implies instability at finite wavenumbers. Our calculations suggest that the mode at the first Brillouin zone edge is the most unstable, and thus the structure experiences a displacive phase transition when the central ion of a unit cell of the body-cubic-centred lattice, is displaced to the cube face. Thus, the electronic structure of matter at densities above the neutron drip [1], is richer than previously appreciated, and new microscopic calculations of nuclear structure are necessary which take into account the high-wavenumber physics. Such calculations will provide crucial input to models interpreting the quasi-periodic oscillations in Soft Gamma Repeaters as magnetar x-ray flares, and to the theory of glitches of neutron stars.

    [1] The neutron drip density is ~3×1011 g cm-3.

  • 38.
    Kobyakov, Dmitry
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Radiophysics Department, Nizhny Novgorod State University, Novgorod, Russia and The Niels Bohr International Academy, The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
    Pethick, Christopher J.
    The Niels Bohr International Academy, The Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark and NORDITA, KTH Royal Institute of Technology and Stockholm University, Stockholm, Sweden.
    Towards a metallurgy of neutron star crusts2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 112, nr 11, s. 112504-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the standard picture of the crust of a neutron star, matter there is simple: a body-centered-cubic (bcc) lattice of nuclei immersed in an essentially uniform electron gas. We show that at densities above that for neutron drip (~4×1011 g cm-3 or roughly one thousandth of nuclear matter density), the interstitial neutrons give rise to an attractive interaction between nuclei that renders the lattice unstable. We argue that the likely equilibrium structure is similar to that in displacive ferroelectric materials such as BaTiO3. As a consequence, properties of matter in the inner crust are expected to be much richer than previously appreciated and we mention possible consequences for observable neutron star properties.

  • 39. Kun, E.
    et al.
    Keresztes, Z.
    Simkó, A.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Szucs, G.
    Gergely, L. A.
    Comparative testing of dark matter models with 15 HSB and 15 LSB galaxies2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 608, artikel-id A42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. We assemble a database of 15 high surface brightness (HSB) and 15 low surface brightness (LSB) galaxies, for which surface brightness density and spectroscopic rotation curve data are both available and representative for various morphologies. We use this dataset to test the Navarro-Frenk-White, the Einasto, and the pseudo-isothermal sphere dark matter models.

    Aims. We investigate the compatibility of the pure baryonic model and baryonic plus one of the three dark matter models with observations on the assembled galaxy database. When a dark matter component improves the fit with the spectroscopic rotational curve, we rank the models according to the goodness of fit to the datasets.

    Methods. We constructed the spatial luminosity density of the baryonic component based on the surface brightness profile of the galaxies. We estimated the mass-to-light (M/L) ratio of the stellar component through a previously proposed color-mass-to-light ratio relation (CMLR), which yields stellar masses independent of the photometric band. We assumed an axissymetric baryonic mass model with variable axis ratios together with one of the three dark matter models to provide the theoretical rotational velocity curves, and we compared them with the dataset. In a second attempt, we addressed the question whether the dark component could be replaced by a pure baryonic model with fitted M/L ratios, varied over ranges consistent with CMLR relations derived from the available stellar population models. We employed the Akaike information criterion to establish the performance of the best-fit models.

    Results. For 7 galaxies (2 HSB and 5 LSB), neither model fits the dataset within the 1 sigma confidence level. For the other 23 cases, one of the models with dark matter explains the rotation curve data best. According to the Akaike information criterion, the pseudoisothermal sphere emerges as most favored in 14 cases, followed by the Navarro-Frenk-White (6 cases) and the Einasto (3 cases) dark matter models. We find that the pure baryonic model with fitted M/L ratios falls within the 1 sigma confidence level for 10 HSB and 2 LSB galaxies, at the price of growing the M/Ls on average by a factor of two, but the fits are inferior compared to the best-fitting dark matter model.

  • 40.
    Lee, Natuschka N
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap. Umeå universitet, Medicinska fakulteten, Institutionen för medicinsk kemi och biofysik.
    Fritz, Jörg
    Fries, Marc D
    Gil, Jose F
    Beck, Andreas
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Schmitz, Birger
    Steele, Andrew
    Hofmann, Beda A
    The extreme biology of meteorites: their role in understanding the origin and distribution of life on earth and in the Universe2017Ingår i: Adaption of microbial life to environmental extremes: novel research results and application / [ed] Helga Stan-Lotter, Sergiu Fendrihan, Cham: Springer Publishing Company, 2017, 2, s. 283-325Kapitel i bok, del av antologi (Refereegranskat)
    Abstract [en]

    Meteorites have captured our fascination since our early history – they have evoked awe, fear, an irresistible curiosity, and numerous lively debates. Former historians have indicated that many of the ancient cultures and civilizations in Europe, Africa, Asia, the Inuit, and the native Indians in America regarded both the meteorite and the location of their fall as sacred. Thus, they used the meteorites as religious objects or for craft design like jewelry, weapons, or even practical things like tools and horse shoes. Today, meteorites continue to capture our fascination through popular cultural formats such as science fiction and also as a scientific window that reveals the secrets of the Solar System formation. Within academia, meteorites have always fomented keen scientific debate. It was not until the early nineteenth century that the cosmic origin of meteorites, i.e., being truly not tellurian, was approved by the scientific community after the late eighteenth-century work of Ernst F. Chladni (1794). This implied for the first time that there are other smaller bodies in the sky besides the Moon. After this, several other lively debates followed on controversial findings and hypotheses around the role of meteorites in the universe and for the evolutionary course of life on Earth, often in connection with the profound difficulties to approach this subject in an adequate scientific way. Principally the different types of meteorites (asteroids, meteors, etc.) can be viewed as a most extreme or exotic substrate, habitat, and transport mode of chemicals and possibly even of cell-based life forms for several reasons:

    1. (i)

      They have experienced a remarkable history since their origin as condensates from the Solar Nebula, more or less metamorphosed or molten fragments of asteroids, or rocks from Mars or our Moon.

    2. (ii)

      The meteorites have been exposed to multiple extreme conditions ranging from milliseconds to billions of years duration when traveling through the interplanetary space, until they fell down on an astronomical body like Earth.

    3. (iii)

      Once on Earth, the meteorites get exposed to different weathering conditions, which often makes it a challenge to retrieve their former history in an unambiguous way.

  • 41. Lehto, Kirsi
    et al.
    Kauko, Anni
    Nurmi, Pasi
    Lee, Natuschka
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för ekologi, miljö och geovetenskap.
    Chatzitheodoridis, Elias
    Lehto, Harry
    eTimeTrek: digital deep history of the Universe2019Ingår i: EPJ Web of Conferences: The International Symposium on Education in Astronomy and Astrobiology (ISE2A 2017) / [ed] Deustua, S Eastwood, K TenKate, IL, EDP Sciences, 2019, Vol. 200, artikel-id 01020Konferensbidrag (Refereegranskat)
  • 42. Maggiolo, R.
    et al.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Keyser, J.
    Pitkanen, T.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Cessateur, G.
    Gunell, H.
    Maes, L.
    The Delayed Time Response of Geomagnetic Activity to the Solar Wind2017Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 11, s. 109-127Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate the lagged correlation between a selection of geomagnetic indices and solar wind parameters for a complete solar cycle, from 2000 to 2011. We first discuss the mathematical assumptions required for such a correlation analysis. The solar wind parameters and geomagnetic indices have inherent timescales that smooth the variations of the correlation coefficients with time lag. Furthermore, the solar wind structure associated with corotating interaction regions and coronal mass ejections, and the compression regions ahead of them, strongly impacts the lagged correlation analysis results. This work shows that such bias must be taken into account in a correct interpretation of correlations. We then evidence that the magnetospheric response time to solar wind parameters involves multiple timescales. The simultaneous and quick response of the PC and AE indices to solar wind dynamic pressure with a delay of similar to 5 min suggests that magnetospheric compression by solar wind can trigger substorm activity. We find that the PC and AE indices respond to interplanetary magnetic field (IMF) B-Z with a response time of respectively similar to 20 and similar to 35 min. The response of the SYM-H index takes longer (similar to 80 min) and is less sharp, SYM-H being statistically significantly correlated to the IMF B-Z observed up to more than similar to 10 h before. Our results suggest that the solar wind velocity's dominant impact on geomagnetic activity is caused by the compression regions at the interface of fast/slow solar wind regimes, which are very geo-effective as they are associated with high solar wind pressure and strong interplanetary magnetic field.

  • 43. Nilsson, H.
    et al.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium.
    Karlsson, T.
    Brenning, N.
    Henri, P.
    Goetz, C.
    Eriksson, A. I.
    Behar, E.
    Wieser, G. Stenberg
    Vallieres, X.
    Size of a plasma cloud matters: The polarisation electric field of a small-scale comet ionosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A50Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.

    Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth's ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.

    Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.

    Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.

    Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.

  • 44. Nilsson, Hans
    et al.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, Tomas
    Slapak, Rikard
    Andersson, Laila
    Gunell, Herbert
    Schillings, Audrey
    Vaivads, Andris
    Oxygen ion response to proton bursty bulk flows2016Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 8, s. 7535-7546Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have used Cluster spacecraft data from the years 2001 to 2005 to study how oxygen ions respond to bursty bulk flows (BBFs) as identified from proton data. We here define bursty bulk flows as periods of proton perpendicular velocities more than 100 km/s and a peak perpendicular velocity in the structure of more than 200 km/s, observed in a region with plasma beta above 1 in the near-Earth central tail region. We find that during proton BBFs only a minor increase in the O+ velocity is seen. The different behavior of the two ion species is further shown by statistics of H+ and O+ flow also outside BBFs: For perpendicular earthward velocities of H+ above about 100 km/s, the O+ perpendicular velocity is consistently lower, most commonly being a few tens of kilometers per second earthward. In summary, O+ ions in the plasma sheet experience less acceleration than H+ ions and are not fully frozen in to the magnetic field. Therefore, H+ and O+ motion is decoupled, and O+ ions have a slower earthward motion. This is particularly clear during BBFs. This may add further to the increased relative abundance of O+ ions in the plasma sheet during magnetic storms. The data indicate that O+ is typically less accelerated in association with plasma sheet X lines as compared to H+.

  • 45. Nilsson, Hans
    et al.
    Wieser, Gabriella Stenberg
    Behar, Etienne
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Wieser, Martin
    Galand, Marina
    Wedlund, Cyril Simon
    Alho, Markku
    Goetz, Charlotte
    Yamauchi, Masatoshi
    Henri, Pierre
    Odelstad, Elias
    Vigren, Erik
    Evolution of the ion environment of comet 67P during the Rosetta mission as seen by RPC-ICA2017Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, s. S252-S261Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Rosetta has followed comet 67P from low activity at more than 3.6 au heliocentric distance to high activity at perihelion (1.24 au) and then out again. We provide a general overview of the evolution of the dynamic ion environment using data from the RPC-ICA ion spectrometer. We discuss where Rosetta was located within the evolving comet magnetosphere. For the initial observations, the solar wind permeated all of the coma. In 2015 mid-April, the solar wind started to disappear from the observation region, to re-appear again in 2015 December. Low-energy cometary ions were seen at first when Rosetta was about 100 km from the nucleus at 3.6 au, and soon after consistently throughout the mission except during the excursions to farther distances from the comet. The observed flux of low-energy ions was relatively constant due to Rosetta's orbit changing with comet activity. Accelerated cometary ions, moving mainly in the antisunward direction gradually became more common as comet activity increased. These accelerated cometary ions kept being observed also after the solar wind disappeared from the location of Rosetta, with somewhat higher fluxes further away from the nucleus. Around perihelion, when Rosetta was relatively deep within the comet magnetosphere, the fluxes of accelerated cometary ions decreased, as did their maximum energy. The disappearance of more energetic cometary ions at close distance during high activity is suggested to be due to a flow pattern where these ions flow around the obstacle of the denser coma or due to charge exchange losses.

  • 46.
    Pellinen-Wannberg, Asta
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Bjurström, Lukas
    Meteoritjakt i Ritsem2010Ingår i: Populär Astronomi, ISSN 1650-7177, nr 4, s. 50-52Artikel i tidskrift (Övrig (populärvetenskap, debatt, mm))
  • 47. Penner, A. J.
    et al.
    Andersson, N.
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hawke, I.
    Jones, D. I.
    Tidal deformations of neutron stars: The role of stratification and elasticity2011Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 84, nr 10, s. 103006-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We discuss the response of neutron stars to the tidal interaction in a compact binary system, as encoded in the Love number associated with the induced deformation. This problem is of interest for gravitational-wave astronomy as there may be a detectable imprint on the signal from the late stages of binary coalescence. Previous work has focused on simple barotropic neutron star models, providing an understanding of the role of the stellar compactness and overall density profile. We add realism to the discussion by developing the framework required to model stars with varying composition and an elastic crust. These effects are not expected to be significant for the next generation of detectors, but it is nevertheless useful to be able to quantify them. Our results show that (perhaps surprisingly) internal stratification has no impact whatsoever on the Love number. We also show that crust elasticity provides a (predictably) small correction to existing models.

  • 48. Penner, AJ
    et al.
    Andersson, N
    Jones, DI
    Samuelsson, Lars
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hawke, I
    Crustal failure during binary inspiral2012Ingår i: Astrophysical journal letters, ISSN 2041-8205, Vol. 749, nr 2, s. L36-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the first fully relativistic calculations of the crustal strain induced in a neutron star by a binary companion at the late stages of inspiral, employing realistic equations of state for the fluid core and the solid crust. We show that while the deep crust is likely to fail only shortly before coalescence, there is a large variation in elastic strain, with the outermost layers failing relatively early on in the inspiral. We discuss the significance of the results for both electromagnetic and gravitational-wave astronomy.

  • 49.
    Persson, Erik
    Lunds universitet.
    The Moral Status of Extraterrestrial Life2012Ingår i: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 12, nr 10, s. 976-984Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    If we eventually discover extraterrestrial life, do we have any moral obligations for how to treat the life-forms we find; does it matter whether they are intelligent, sentient, or just microbial—and does it matter that they are extraterrestrial? In this paper, I examine these questions by looking at two of the basic questions in moral philosophy: What does it take to be a moral object? and What has value of what kind? I will start with the first of these questions by looking at the most important attempts to answer this question on our own planet and by asking whether and how they could be applied to extraterrestrial life. The results range from a very strong protection of all extraterrestrial life and all extraterrestrial environments, whether inhabited or not, to total exclusion of extraterrestrial life. Subsequently, I also examine whether extraterrestrial life that lacks moral status can have value to human or alien life with moral status, and if that could generate any obligations for how to treat extraterrestrial life. Based on this analysis, I conclude that extraterrestrial life-forms can have both instrumental value and end value to moral objects, which has strong implications for how to treat them.

  • 50.
    Pitkänen, Timo
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Space Sciences, Shandong University, Weihai, China.
    Kullen, A.
    Shi, Q. Q.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Nishimura, Y.
    Convection electric field and plasma convection in a twisted magnetotail: t THEMIS case study 1-2 January 20092018Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, nr 9, s. 7486-7497Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate THEMIS satellite measurements made in a tail-aligned constellation during a time interval on 1-2 January 2009, which has previously been attributed to an interval of an interplanetary magnetic fieldB(y)-driven magnetotail twisting. We find evidence for that the orientation of the convection electric field in the tail is twist-mode dependent. For earthward flow and a negative twist (induced tail B-y < 0), the electric field is found to have northward E-z and tailward E-x components. During a positive twist (induced tail B-y > 0), the directions of E-z and E-x are reversed. The E-y component shows the expected dawn-to-dusk direction for earthward flow. The electric field components preserve their orientation across the neutral sheet, and a quasi-collinear field is observed irrespective to the tail distance. The electric field associated with the tailward flow has an opposite direction compared to the earthward flow for the negative twist. For the positive twist, the results are less clear. The corresponding plasma convection and thus the magnetic flux transport have an opposite dawn-dusk direction above and below the neutral sheet. The directions depend on the tail twist mode. The hemispherically asymmetric earthward plasma flows are suggested to be a manifestation of an asymmetric Dungey cycle in a twisted magnetotail. The role of tailward flows deserve further investigation.

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