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  • 51. Cresswell-Moorcock, Kathy
    et al.
    Rodger, Craig J.
    Kero, Antti
    Collier, Andrew B.
    Clilverd, Mark A.
    Häggström, Ingemar
    Pitkänen, Timo
    A reexamination of latitudinal limits of substorm-produced energetic electron precipitation2013In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, p. 6694-6705Article in journal (Refereed)
  • 52.
    De Spiegeleer, Alexandre
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Volwerk, M.
    Andersson, L.
    Karlsson, T.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mouikis, C. G.
    Nilsson, H.
    Kistler, L. M.
    Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function2019In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 4, p. 2736-2754Article in journal (Refereed)
    Abstract [en]

    Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2-D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0 degrees to 180 degrees in the Northern Hemisphere and from 180 degrees to 0 degrees in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time-of-flight effect from which we estimate that the source of particle is located similar to 5-25R(E) and similar to 40-107R(E) tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain similar to 21-46 R-E for the Southern Hemisphere event and tailward of X similar to - 65R(E) (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.

  • 53. Demir, Sinan
    et al.
    Akkerman, V'yacheslav
    Rangwala, Ali S.
    Bychkov, Vitaliy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    ANALYSIS OF "FINGER" FLAME ACCELERATION AS A STAGE OF A METHANE AIR-DUST FIRE IN A COAL MINE2016In: PROCEEDINGS OF THE ASME POWER CONFERENCE, 2015, The american society of mechanical engineers , 2016, article id V001T03A007Conference paper (Refereed)
    Abstract [en]

    To reveal the inner mechanism of gas explosion, the entire scenario of premixed flame front evolution within an accidental fire is prescribed. Specifically, "finger" flame shape, which is one of the key stages of flame evolution, is scrutinized with the situation of a methane-air explosion. A transition from a globally -spherical front to a finger-shaped one occurs when a flame starts approaching the passage walls. While this acceleration is extremely strong, it stops as soon as the flame touches the passage wall. This mechanism is Reynolds-independent; being equally relevant to micro channels and giant tunnels. The flame speed increases by an order of magnitude during this stage. To implement dusty environments, Seshadri formulation for the planar flame [Combustion and Flame 89 (7992) 333] is employed with a non-uniform distribution of inert dust gradients, specifically, linear, parabolic and hyperbolic spatial dust distribution gradients are incorporated into the "finger" flame shape. This study systematically investigates how the noncombustible dust distributions affect fire evolution, the flame shape, and propagation velocity.

  • 54. Dieckmann, ME
    et al.
    Drury, LO
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institute of Theoretical Physics IV, Ruhr-University Bochum, D-44780 Bochum, Germany.
    On the ultrarelativistic two-stream instability, electrostatic turbulence and Brownian motion2006In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 8, article id 40Article in journal (Refereed)
    Abstract [en]

    Experimental evidence indicates that bulk plasma flow at ultrarelativistic speeds is common in astrophysical settings, e. g. the collimated jets of active galactic nuclei and gamma ray bursts. The low-plasma density of such flows implies their collisionless relaxation by means of wave-particle interactions. Such processes are not well understood in the ultrarelativistic regime. The thermalization of two interpenetrating equally dense electron-proton (e(-)p) beams in the absence of a magnetic field is examined here by means of 1.5D particle-in-cell simulations. The relative beam speeds correspond to Lorentz factors in the range 200-1000. The constraint to one spatial simulation dimension, which is aligned with the beam velocity vectors, implies that only the two-stream (TS) instability and the Weibel-type instability can grow, while filamentation instabilities are excluded. With this constraint and for our plasma parameters, the TS instability dominates. The electrostatic waves grow, saturate by the trapping of electrons, and collapse. The interaction of the electrons with the electric fields after the wave collapse represents a relativistic Wiener process. In response, the electrons are rapidly thermalized. The final electron distribution can be interpreted as a relativistic Maxwellian distribution with a high-energy tail arising from ultrarelativistic phase space holes.

  • 55. Dieckmann, ME
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institute of Theoretical Physics IV, Ruhr-University Bochum, D-44780 Bochum, Germany; SUPA Department of Physics, University of Strathclyde, Glasgow, Scotland.
    Stenflo, L
    Simulation study of the filamentation of counter-streaming beams of the electrons and positrons in plasmas2009In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 51, no 6, article id 065015Article in journal (Refereed)
    Abstract [en]

    The filamentation instability (FI) driven by two spatially uniform and counter-streaming beams of charged particles in plasmas is modelled by a particle-in-cell simulation. Each beam consists of electrons and positrons. The four species are equally dense and have the same temperature. The one-dimensional simulation direction is orthogonal to the beam velocity vector. The magnetic field grows spontaneously and rearranges the particles in space, such that the distributions of the electrons of one beam and the positrons of the second beam match. The simulation demonstrates that as a result no electrostatic field is generated by the magnetic field through its magnetic pressure gradient prior to its saturation. This electrostatic field would be repulsive at the centres of the filaments and limit the maximum charge and current density. The filaments of electrons and positrons in this simulation reach higher charge and current densities than in one with no positrons. The oscillations of the magnetic field strength induced by the magnetically trapped particles result in an oscillatory magnetic pressure gradient force. The latter interplays with the statistical fluctuations in the particle density and it probably enforces a charge separation, by which electrostatic waves grow after the FI has saturated.

  • 56.
    Ekeberg, Jonas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Solitary waves and enhanced incoherent scatter ion lines2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis addresses solitary waves and their significance for auroral particle acceleration, coronal heating and incoherent scatter radar spectra. Solitary waves are formed due to a balance of nonlinear and dispersive effects. There are several nonlinearities present in ideal magnetohydrodynamics (MHD) and dispersion can be introduced by including theHall termin the generalised Ohm’s law. The resulting system of equations comprise the classical ideal MHD waves, whistlers, drift waves and solitarywave solutions. The latter reside in distinct regions of the phase space spanned by the speed and the angle (to the magnetic field) of the propagating wave. Within each region, qualitatively similar solitary structures are found. In the limit of neglected electron intertia, the solitary wave solutions are confined to two regions of slow and fast waves, respectively. The slow (fast) structures are associated with density compressions (rarefactions) and positive (negative) electric potentials. Such negative potentials are shown to accelerate electrons in the auroral region (solar corona) to tens (hundreds) of keV. The positive electric potentials could accelerate solar wind ions to velocities of 300–800 km/s. The structure widths perpendicular to themagnetic field are in the Earth’s magnetosphere (solar corona) of the order of 1–100 km (m). This thesis also addresses a type of incoherent scatter radar spectra, where the ion line exhibits a spectrally uniform power enhancement with the up- and downshifted shoulder and the spectral region in between enhanced simultaneously and equally. The power enhancements are one order of magnitude above the thermal level and are often localised to an altitude range of less than 20 km at or close to the ionospheric F region peak. The observations are well-described by a model of ion-acoustic solitary waves propagating transversely across the radar beam. Two cases of localised ion line enhancements are shown to occur in conjunction with auroral arcs drifting through the radar beam. The arc passages are associated with large gradients in ion temperature, which are shown to generate sufficiently high velocity shears to give rise to growing Kelvin-Helmholtz (K-H) instabilities. The observed ion line enhancements are interpreted in the light of the low-frequency turbulence associated with these instabilities.

  • 57.
    Ekman, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quantum Kinetic Theory for Plasmas: spin, exchange, and particle dispersive effects2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is about developing and studying quantum mechanical models of plasmas. Quantum effects can be important at high densities, at low temperatures, and in strong electromagnetic fields, in various laboratory and astrophysical systems. The focus is on the electron spin, the intrinsic magnetic moment; exchange interactions, a purely quantum mechanical effect arising from particles being indistinguishable; and particle dispersive effects, essentially the Heisenberg uncertainty principle. The focus is on using phase-space formulations of quantum mechanics, namely Wigner and -functions. These methods allow carrying over techniques from classical plasma physics and identifying quantum as opposed to classical behavior.

    Two new kinetic models including the spin are presented, one fully relativistic and to first order in ħ, and one semi-relativistic but to all orders in ħ. Among other example calculations, for the former, conservation laws for energy, momentum, and angular momentum are derived and related to “hidden momentum” and the Abraham-Minkowski dilemma. Both models are discussed in the context of the existing literature.

    A kinetic model of exchange interactions, formally similar to a collision operator, is compared to a widely used fluid description based on density functional theory, for the case of electrostatic waves. The models are found to disagree significantly.

    A new, non-linear, wave damping mechanism is shown to arise from particle dispersive effects. It can be interpreted as the simultaneous absorption or emission of multiple wave quanta. This multi-plasmon damping is of particular interest for highly degenerate electrons, where it can occur on time scales comparable to or shorter than that of linear Landau damping.

  • 58.
    Ekman, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Al-Naseri, Haidar
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zamanian, Jens
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Relativistic kinetic theory for spin-1/2 particles: Conservation laws, thermodynamics, and linear waves2019In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 100, no 2, article id 023201Article in journal (Refereed)
    Abstract [en]

    We study a recently derived fully relativistic kinetic model for spin-1/2 particles. First, the full set of conservation laws for energy, momentum, and angular momentum are given together with an expression for the (nonsymmetric) stress-energy tensor. Next, the thermodynamic equilibrium distribution is given in different limiting cases. Furthermore, we address the analytical complexity that arises when the spin and momentum eigenfunctions are coupled in linear theory by calculating the linear dispersion relation for such a case. Finally, we discuss the model and give some context by comparing with potentially relevant phenomena that are not included, such as radiation reaction and vacuum polarization.

  • 59.
    Ekman, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Al-Naseri, Haidar
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zamanian, Jens
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Short-scale quantum kinetic theory including spin-orbit interactionsManuscript (preprint) (Other academic)
    Abstract [en]

    We present a quantum kinetic theory for spin-1/2 particles, including the spin-orbit interaction, retaining particle dispersive effects to all orders in ℏ, based on a gauge-invariant Wigner transformation. Compared to previous works, the spin-orbit interaction leads to a new term in the kinetic equation, containing both the electric and magnetic fields. Like other models with spin-orbit interactions, our model features "hidden momentum". As an example application, we calculate the dispersion relation for linear electrostatic waves in a magnetized plasma, and electromagnetic waves in a unmagnetized plasma. In the former case, we compare the Landau damping due to spin-orbit interactions to that due to the free current. We also discuss our model in relation to previously published works. 

  • 60.
    Ekman, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Asenjo, F. A.
    Zamanian, Jens
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Relativistic kinetic equation for spin-1/2 particles in the long-scale-length approximation2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 2, article id 023207Article in journal (Refereed)
    Abstract [en]

    In this paper, we derive a fully relativistic kinetic theory for spin-1/2 particles and its coupling to Maxwell's equations, valid in the long-scale-length limit, where the fields vary on a scale much longer than the localization of the particles; we work to first order in (h) over bar. Our starting point is a Foldy-Wouthuysen (FW) transformation, applicable to this regime, of the Dirac Hamiltonian. We derive the corresponding evolution equation for the Wigner quasidistribution in an external electromagnetic field. Using a Lagrangian method we find expressions for the charge and current densities, expressed as free and bound parts. It is furthermore found that the velocity is nontrivially related to the momentum variable, with the difference depending on the spin and the external electromagnetic fields. This fact that has previously been discussed as "hidden momentum" and is due to that the FW transformation maps pointlike particles to particle clouds for which the prescription of minimal coupling is incorrect, as they have multipole moments. We express energy and momentum conservation for the system of particles and the electromagnetic field, and discuss our results in the context of the Abraham-Minkowski dilemma.

  • 61. Eliasson, B
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV and Centre for Plasma Science and Astrophysics.
    Instability and dynamics of two nonlinearly coupled laser beams in a two-temperature electron plasma2006In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 74, no 4, article id 046401Article in journal (Refereed)
    Abstract [en]

    We consider nonlinear interactions between two colliding laser beams in an electron plasma, accounting for the relativistic electron mass increase in the laser fields and radiation pressure driven electron-acoustic (EA) perturbations that are supported by hot and cold electrons. By using the hydrodynamic and Maxwell equations, we obtain the relevant equations for nonlinearly coupled laser beams and EA perturbations. The coupled equations are then Fourier analyzed to obtain a nonlinear dispersion relation. The latter is numerically solved to show the existence of new classes of the parametric instabilities in the presence of two colliding laser beams in a two-electron plasma. The dynamics of nonlinearly coupled laser beams in our electron plasma is also investigated. The results should be useful in understanding the nonlinear propagation characteristics of multiple electromagnetic beams in laser-produced plasmas as well as in space plasmas.

  • 62. Eliasson, Bengt
    et al.
    Liu, Chuan S
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK.
    Kumar, Naveen
    Dynamics of relativistic laser pulses in plasmas2006In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 73, no 6, p. 632-638Article in journal (Refereed)
    Abstract [en]

    The dynamics of intense laser pulses in plasmas are investigated both theoretically and numerically. The linear growth and nonlinear saturation of relativistic stimulated Raman scattering of plasmons are investigated by means of a nonlinear dispersion relation and via direct Vlasov simulations. We observe acceleration of electrons up to ultra-relativistic energies by a positive electrostatic potential that is created by intense short laser pulses.

  • 63.
    Eliasson, Bengt
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Theoretische Physik IV, Ruhr – Universität Bochum, Bochum, Germany.
    Stenflo, Lennart
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, Linköping University, Linköping, Sweden.
    Full-scale simulation study of the initial stage of ionospheric turbulence2008In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, no A2, article id A02305Article in journal (Refereed)
    Abstract [en]

    We present a full-scale simulation of the nonlinear interaction between an intense electromagnetic wave and the Earth's ionosphere, by means of a generalized Zakharov model. The radio wave propagates from the neutral atmosphere into the ionospheric plasma layer and reaches the turning points of the ordinary and extraordinary wave modes. At the turning point of the ordinary mode, a parametric instability takes place in which the electromagnetic wave decays into an electron plasma wave and an ion acoustic wave with a typical wavelength of one meter. This is followed by collapse and caviton formation and trapping of the intense electron plasma wave. The cavitons lead to an efficient excitation of slow X (or Z) waves that propagate further into the denser ionospheric layer at higher altitudes. We use a realistic ion (oxygen) mass, length scales, and other plasma parameters. This numerical study should be useful for understanding the nonlinear interaction between intense radio waves and the ionosphere.

  • 64.
    Eliasson, Bengt
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Theoretische Physik IV, Ruhr – Universita¨t Bochum, Bochum, Germany.
    Thidé, Bo
    Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma2008In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 113, no A2, article id A02313Article in journal (Refereed)
    Abstract [en]

    We have performed a simulation study of Langmuir turbulence in the Earth's ionosphere by means of a Zakharov model with parameters relevant for the F layer. The model includes dissipative terms to model collisions and Landau damping of the electrons and ions, and a linear density profile, which models the ionospheric plasma inhomogeneity whose length scale is of the order 10–100 km. The injection of energy into the system is modeled by a constant source term in the Zakharov equation. Langmuir turbulence is excited “on-demand” in controlled ionospheric modification experiments where the energy is provided by an HF radio beam injected into the overhead ionospheric plasma. The ensuing turbulence can be studied with radars and in the form of secondary radiation recorded by ground-based receivers. We have analyzed spectral signatures of the turbulence for different sets of parameters and different altitudes relative to the turning point of the linear Langmuir mode where the Langmuir frequency equals the local plasma frequency. By a parametric analysis, we have derived a simple scaling law, which links the spectral width of the turbulent frequency spectrum to the physical parameters in the ionosphere. The scaling law provides a quantitative relation between the physical parameters (temperatures, electron number density, ionospheric length scale, etc.) and the observed frequency spectrum. This law may be useful for interpreting experimental results.

  • 65.
    Eliasson, Lars
    Umeå University, Faculty of Science and Technology, Rymdfysik.
    Satellite observations of auroral acceleration processes1994Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Measurements with satellite and sounding rocket borne instruments contain important information on remote and local processes in regions containing matter in the plasma state. The characteristic features of the particle distributions can be used to explain the morphology and dynamics of the different plasma populations. Charged particles are lost from a region due to precipitation into the atmosphere, charge exchange processes, or convection to open magnetic field lines. The sources of the Earth’s magnetospheric plasma are mainly ionization and extraction of upper atmosphere constituents, and entry of solar wind plasma. The intensity and distribution of auroral precipitation is controlled in part by the conditions of the interplanetary magnetic field causing different levels of auroral activity. Acceleration of electrons and positive ions along auroral field lines play an important role in magnetospheric physics. Electric fields that are quasi-steady during particle transit times, as well as fluctuating fields, are important for our understanding of the behaviour of the plasma in the auroral region. High-resolution data from the Swedish Viking and the Swedish/German Freja satellites have increased our knowledge considerably about the interaction processes between different particle populations and between particles and wave fields. This thesis describes acceleration processes influencing both ions and electrons and is based on in-situ measurements in the auroral acceleration/heating region, with special emphasis on; processes at very high latitudes, the role of fluctuating electric fields in producing so called electron conics, and positive ion heating transverse to the geomagnetic field lines.

  • 66. El-Taibany, WF
    et al.
    Moslem, WM
    Wadati, Miki
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, D-44780 Bochum, Germany; School of Physics, University of KwaZulu-Natal, Durban 4000, South Africa.
    On the instability of electrostatic waves in a nonuniform electron-positron magnetoplasma2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 22, p. 4067-4075Article in journal (Refereed)
    Abstract [en]

    The dispersion properties of three-dimensional electrostatic waves in a nonuniform electron-positron (EP) magnetoplasma are analyzed. A new dispersion relation is derived by use of the electron and positron density responses arising from the electron and positron continuity and Poisson equations. In the local approximation, the dispersion relation admits two wave modes with different velocities. The growth rates of various modes are illustrated both analytically and numerically. Considering the temperature gradients produces a linearly stable transverse mode. The growth rate of the slow mode instability due to the density inhomogeneity only is the highest one, though it appears at higher thermal energy. The angle of the wave propagation affects drastically on the instability features in each case. The applications of the present analysis are briefly discussed.

  • 67. Fatemi, S
    et al.
    Holmström, M
    Futaana, Y
    Barabash, S
    Lue, Charles
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    The lunar wake current systems2013In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 1, p. 17-21Article in journal (Refereed)
    Abstract [en]

    We present the lunar wake current systems when the Moon is assumed to be a non-conductive body, absorbing the solar wind plasma. We show that in the transition regions between the plasma void, the expanding rarefaction region, and the interplanetary plasma, there are three main currents flowing around these regions in the lunar wake. The generated currents induce magnetic fields within these regions and perturb the field lines there. We use a three-dimensional, self-consistent hybrid model of plasma (particle ions and fluid electrons) to show the flow of these three currents. First, we identify the different plasma regions, separated by the currents, and then we show how the currents depend on the interplanetary magnetic field direction. Finally, we discuss the current closures in the lunar wake.

  • 68.
    Fatemi, Shahab
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Poirier, Nicolas
    École nationale supérieure de mécanique et d’aérotechnique, Chasseneuil-du-Poitou, France .
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wieser, Martin
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Barabash, Stas
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna, Sweden.
    A modelling approach to infer the solar wind dynamic pressure from magnetic field observations inside Mercury's magnetosphere2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id A132Article in journal (Refereed)
    Abstract [en]

    Aims: The lack of an upstream solar wind plasma monitor when a spacecraft is inside the highly dynamic magnetosphere of Mercury limits interpretations of observed magnetospheric phenomena and their correlations with upstream solar wind variations.

    Methods: We used AMITIS, a three-dimensional GPU-based hybrid model of plasma (particle ions and fluid electrons) to infer the solar wind dynamic pressure and Alfvén Mach number upstream of Mercury by comparing our simulation results with MESSENGER magnetic field observations inside the magnetosphere of Mercury. We selected a few orbits of MESSENGER that have been analysed and compared with hybrid simulations before. Then we ran a number of simulations for each orbit (~30–50 runs) and examined the effects of the upstream solar wind plasma variations on the magnetic fields observed along the trajectory of MESSENGER to find the best agreement between our simulations and observations.

    Results: We show that, on average, the solar wind dynamic pressure for the selected orbits is slightly lower than the typical estimated dynamic pressure near the orbit of Mercury. However, we show that there is a good agreement between our hybrid simulation results and MESSENGER observations for our estimated solar wind parameters. We also compare the solar wind dynamic pressure inferred from our model with those predicted previously by the WSA-ENLIL model upstream of Mercury, and discuss the agreements and disagreements between the two model predictions. We show that the magnetosphere of Mercury is highly dynamic and controlled by the solar wind plasma and interplanetary magnetic field. In addition, in agreement with previous observations, our simulations show that there are quasi-trapped particles and a partial ring current-like structure in the nightside magnetosphere of Mercury, more evident during a northward interplanetary magnetic field (IMF). We also use our simulations to examine the correlation between the solar wind dynamic pressure and stand-off distance of the magnetopause and compare it with MESSENGER observations. We show that our model results are in good agreement with the response of the magnetopause to the solar wind dynamic pressure, even during extreme solar events. We also show that our model can be used as a virtual solar wind monitor near the orbit of Mercury and this has important implications for interpretation of observations by MESSENGER and the future ESA/JAXA mission to Mercury, BepiColombo.

  • 69.
    Forsberg, Mats
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gravitational perturbations in plasmas and cosmology2010Doctoral thesis, comprehensive summary (Other academic)
    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.

  • 70.
    Giang, Tony
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yamauchi, Masatoshi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundin, Rickard
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nilsson, Hans
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ebihara, Yusuke
    Institute for Advanced Research, Nagoya University, Japan.
    Rème, Henri
    Centre d'Etude Spatiale des Rayonnements, Toulouse, France.
    Dandouras, Iannis
    Centre d'Etude Spatiale des Rayonnements, Toulouse, France.
    Vallat, C.
    VEGA contracted to Solar System Science Operations Division, ESA/ESAC, Madrid, Spain.
    Bavassano-Cattaneo, M. B.
    L'Istituto di Fisica dello Spazio Interplanetario, Roma, Italy.
    Klecker, B.
    Max Planck Institute for Extraterrestrial Physics, Garching, Germany.
    Korth, A.
    Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany.
    Kistler, L. M.
    University of New Hampshire, Durham, New Hampshire, USA.
    McCarthy, M.
    University of Washington, Seattle, USA.
    Outflowing protons and heavy ions as a source for the sub-keV ringcurrent2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 2, p. 839-849Article in journal (Refereed)
    Abstract [en]

    Data from the Cluster CIS instrument have been used for studying proton and heavy ion (O+ and He+ ) char- acteristics of the sub-keV ring current. Thirteen events with dispersed heavy ions (O+ and He+ ) were identified out of two years (2001 and 2002) of Cluster data. Allevents took place during rather geomagnetically quiet periods. Three of those events have been investigated in detail: 21 August 2001, 26 November 2001 and 20 February 2002. These events were chosen from varying magnetic local times (MLT), and they showed different characteristics. In this article, we discuss the potential source for sub-keV ring current ions. We show that: (1) outflows of terrestrialsub-keV ions are supplied to the ring current also during quiet geomagnetic conditions; (2) the composition of the out-flow implies an origin that covers an altitude interval from the low-altitude ionosphere to the plasmasphere, and (3) terrestrial ions are moving upward along magnetic field lines, at times forming narrow collimated beams, but  frequently also as broad beams. Over time, the ion beams are expected to gradually become isotropised as a result of wave-particleinteraction, eventually taking the form of isotropic drifting sub-keV ion signatures. We argue that the sub-keV energy-time dispersed signatures originate from field-aligned terrestrial ion energising and outflow, which may occur at all local times and persist also during quiet times.

  • 71.
    Gräns Samuelsson, Linnéa
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Conservation laws in kinetic theory for spin-1/2 particles2015Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    In this thesis a kinetic theory for spin-1/2 particles is given a brief overview, focusing on the derivation of an evolution equation for the quasiprobability distribution function used in the theory to describe certain types of quantum plasma. The current theory is expanded upon by exploring conservation laws. A local conservation law for momentum is derived using two different expressions for electromagnetic momentum, given by Abraham and Minkowski respectively. There has been some controversy over which of these expressions should be used; in the case considered here the expression given by Minkowski seems to be more suitable. Based on the conservation law for momentum, a conservation law for angular momentum is also derived.

  • 72. Guillaume, E.
    et al.
    Döpp, A.
    Thaury, C.
    Ta Phuoc, K.
    Lifschitz, A.
    Grittani, G.
    Goddet, J.-P.
    Tafzi, A.
    Chou, S.-W.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Malka, V.
    Electron Rephasing in a Laser-Wakefield Accelerator2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 15, article id 155002Article in journal (Refereed)
    Abstract [en]

    An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

  • 73. Gunell, H.
    et al.
    Andersson, L.
    De Keyser, J.
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Can the downward current region of the aurora be simulated in the laboratory?2016In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 58, no 5, article id 054003Article in journal (Refereed)
    Abstract [en]

    A laboratory plasma device is proposed to simulate the downward current region of the aurora. In this device, a discharge in neon is used as a plasma source to represent the hot plasma of the magnetosphere, and a sodium Q-machine source represents the ionospheric plasma. An electrostatic Vlasov model is used to simulate both the downward current region itself and the proposed laboratory analogue. All important phenomena that appear in the simulation of space are found in the laboratory simulation too: a double layer carries most of the potential difference; double layers are in constant motion in the direction of decreasing magnetic field; electron phase space holes appear on the high potential side of double layers and these holes carry a part of the potential difference during double layer disruptions. The ability to simulate auroral physics in the laboratory is better for the downward than the upward current region, because of the lower levels of ion acoustic-like waves in the laboratory model of the former region. Better laboratory-space agreement is found when the discharge and Q-machine ions are of similar masses. If the masses differ significantly, as they do when using helium together with sodium ions, waves on the ion time scale dominate the plasma on the low potential side of the double layer, and there is a tendency toward multiple double layers appearing simultaneously at different locations. The experiment is suitable for the study of heating processes that occur in the downward current region and to address the behaviour of the current-voltage relationship for low voltages.

  • 74. Gunell, H.
    et al.
    De Keyser, J.
    Gamby, E.
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vlasov simulations of parallel potential drops2013In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 31, no 7, p. 1227-1240Article in journal (Refereed)
    Abstract [en]

    An auroral flux tube is modelled from the magnetospheric equator to the ionosphere using Vlasov simulations. Starting from an initial state, the evolution of the plasma on the flux tube is followed in time. It is found that when applying a voltage between the ends of the flux tube, about two thirds of the potential drop is concentrated in a thin double layer at approximately one Earth radius altitude. The remaining part is situated in an extended region 1-2 Earth radii above the double layer. Waves on the ion timescale develop above the double layer, and they move toward higher altitude at approximately the ion acoustic speed. These waves are seen both in the electric field and as perturbations of the ion and electron distributions, indicative of an instability. Electrons of magnetospheric origin become trapped between the magnetic mirror and the double layer during its formation. At low altitude, waves on electron timescales appear and are seen to be non-uniformly distributed in space. The temporal evolution of the potential profile and the total voltage affect the double layer altitude, which decreases with an increasing field aligned potential drop. A current-voltage relationship is found by running several simulations with different voltages over the system, and it agrees with the Knight relation reasonably well.

  • 75. Gunell, Herbert
    et al.
    Andersson, Laila
    De Keyser, Johan
    Mann, Ingrid
    Umeå University, Faculty of Science and Technology, Department of Physics. EISCAT Scientific Association, Kiruna, Sweden.
    Vlasov simulations of trapping and loss of auroral electrons2015In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 33, p. 279-293Article in journal (Refereed)
    Abstract [en]

    The plasma on an auroral field line is simulated using a Vlasov model. In the initial state, the acceleration region extends from one to three Earth radii in altitude with about half of the acceleration voltage concentrated in a stationary double layer at the bottom of this region. A population of electrons is trapped between the double layer and their magnetic mirror points at lower altitudes. A simulation study is carried out to examine the effects of fluctuations in the total accelerating voltage, which may be due to changes in the generator or the load of the auroral current circuit. The electron distribution function on the high potential side of the double layer changes significantly depending on whether the perturbation is toward higher or lower voltages, and therefore measurements of electron distribution functions provide information about the recent history of the voltage. Electron phase space holes are seen as a result of the induced fluctuations. Most of the voltage perturbation is assumed by the double layer. Hysteresis effects in the position of the double layer are observed when the voltage first is lowered and then brought back to its initial value.

  • 76.
    Gunell, Herbert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Goetz, C.
    Eriksson, A.
    Nilsson, H.
    Wedlund, C. Simon
    Henri, P.
    Maggiolo, R.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    De Keyser, J.
    Rubin, M.
    Wieser, G. Stenberg
    Cessateur, G.
    Dhooghe, F.
    Gibbons, A.
    Plasma waves confined to the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 469, p. S84-S92Article in journal (Refereed)
    Abstract [en]

    Ion acoustic waves were observed in the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft on 2015 August 3, when the comet was 1.25 au from the Sun. Wave spectra recorded by the Langmuir probe (RPC-LAP), peak near 200 Hz, decrease for higher frequencies and reach the noise floor at approximately 1.5 kHz. These waves were observed only when the spacecraft was in the diamagnetic cavity or at its boundary, which is identified as a sharp drop in magnetic field magnitude, measured by RPC-MAG. The plasma, on both sides of the boundary, is dominated by a cold (a few hundred K) water group ion population, one cold (k(B)T(e) similar to 0.1 eV) and one warm (k(B)T(e) similar to 10 eV) electron population. The observations are interpreted in terms of current-driven ion acoustic waves, generated by currents that flow through bulges on the boundary of the diamagnetic cavity.

  • 77.
    Gunell, Herbert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium.
    Goetz, Charlotte
    Wedlund, Cyril Simon
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Nilsson, Hans
    LLera, Kristie
    Eriksson, Anders
    Holmström, Mats
    The infant bow shock: a new frontier at a weak activity comet2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id L2Article in journal (Refereed)
    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.

  • 78.
    Gunell, Herbert
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Maggiolo, Romain
    Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Nilsson, Hans
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Slapak, Rikard
    EISCAT Scientific Association, Kiruna, Sweden.
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hamrin, Maria
    Umeå University, Faculty of Science and Technology, Department of Physics.
    De Keyser, Johan
    Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Belgium.
    Why an intrinsic magnetic field does not protect a planet against atmospheric escape2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, article id L3Article in journal (Refereed)
    Abstract [en]

    The presence or absence of a magnetic field determines the nature of how a planet interacts with the solar wind and what paths are available for atmospheric escape. Magnetospheres form both around magnetised planets, such as Earth, and unmagnetised planets, like Mars and Venus, but it has been suggested that magnetised planets are better protected against atmospheric loss. However, the observed mass escape rates from these three planets are similar (in the approximate (0.5–2) kg s−1 range), putting this latter hypothesis into question. Modelling the effects of a planetary magnetic field on the major atmospheric escape processes, we show that the escape rate can be higher for magnetised planets over a wide range of magnetisations due to escape of ions through the polar caps and cusps. Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss. Estimates of the atmospheric escape rates from exoplanets must therefore address all escape processes and their dependence on the planet’s magnetisation.

  • 79. Haas, F
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; GOLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal; SUPA, Department of Physics, University of Strathclyde, Glasgow, G40NG, UK; School of Physics, University of Kwazulu-Natal, Durban 4000, South Africa.
    Nonlinear stationary solutions of the Wigner and Wigner-Poisson equations2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 11, article id 112302Article in journal (Refereed)
    Abstract [en]

    Exact nonlinear stationary solutions of the one-dimensional Wigner and Wigner-Poisson equations in the terms of the Wigner functions that depend not only on the energy but also on position are presented. In this way, the Bernstein-Greene-Kruskal modes of the classical plasma are adapted for the quantum formalism in the phase space. The solutions are constructed for the case of a quartic oscillator potential, as well as for the self-consistent Wigner-Poisson case. Conditions for well-behaved physically meaningful equilibrium Wigner functions are discussed.

  • 80. Haas, F
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; GOLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal; SUPA, Department of Physics, University of Strathclyde, Glasgow, G40NG, United Kingdom; School of Physics, University of Kwazulu-Natal, Durban 4000, South Africa.
    Nonlinear structure in a current-carrying collisional dusty plasma2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 9, article id 093702Article in journal (Refereed)
    Abstract [en]

    A perpendicular ion drift is proposed as a possible mechanism for the generation of magnetic field structures in a highly collisional dusty plasma. The basic dissipation mechanism is assumed to be the dust-neutral momentum exchange, so that plasmas with a small ionization fraction are natural candidates for experiments. The model reduces to a nonlinear partial differential equation for the vector potential. The conditions for linear instability are presented. Possible stationary states are periodic arrangements for the magnetic field, described by a Lienard equation. The fully depleted (ion-dust) case is also considered in detail. Applications of the present work to magnetic field structures in planetary rings, comets, and low-temperature dusty plasma experiments are discussed. A necessary condition for the validity of the model is a sufficiently slow time scale of the generated magnetic fields in dusty plasmas.

  • 81. Haas, F
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Ruhr-Universität Bochum, D-44780 Bochum, Germany; GOLP/Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal; SUPA, Department of Physics, University of Strathclyde, Glasgow, G40NG, UK; School of Physics, University of Kwazulu-Natal, Durban 4000, South Africa.
    Quantum and classical dynamics of Langmuir wave packets2009In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 79, no 6, article id 066402Article in journal (Refereed)
    Abstract [en]

    The quantum Zakharov system in three spatial dimensions and an associated Lagrangian description, as well as its basic conservation laws, are derived. In the adiabatic and semiclassical cases, the quantum Zakharov system reduces to a quantum modified vector nonlinear Schroumldinger (NLS) equation for the envelope electric field. The Lagrangian structure for the resulting vector NLS equation is used to investigate the time dependence of the Gaussian-shaped localized solutions, via the Rayleigh-Ritz variational method. The formal classical limit is considered in detail. The quantum corrections are shown to prevent the collapse of localized Langmuir envelope fields, in both two and three spatial dimensions. Moreover, the quantum terms can produce an oscillatory behavior of the width of the approximate Gaussian solutions. The variational method is shown to preserve the essential conservation laws of the quantum modified vector NLS equation. The possibility of laboratory tests in the next generation intense laser-solid plasma compression experiment is discussed.

  • 82.
    Haas, Fernando
    Umeå University, Faculty of Science and Technology, Department of Physics.
    On quantum plasma kinetic equations with a Bohmian force2010In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 76, no 3-4, p. 389-393Article in journal (Refereed)
    Abstract [en]

    The dispersion relation arising from a Vlasov-Poisson system with a Bohmian force term is examined and compared to the more fundamental Bohm and Pines dispersion relation for quantum plasmas. Discrepancies are found already when considering the leading order thermal effects. The time-averaged energy densities for longitudinal modes are also shown to be noticeably different.

  • 83. Hajra, Rajkumar
    et al.
    Henri, Pierre
    Vallières, Xavier
    More, Jeromé
    Gilet, Nicolas
    Wattieaux, Gaetan
    Goetz, Charlotte
    Richter, Ingo
    Tsurutani, Bruce T.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, B-1180 Brussels, Belgium.
    Nilsson, Hans
    Eriksson, Anders I.
    Nemeth, Zoltan
    Burchdegrees, James L.
    Rubin, Martin
    Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 475, no 3, p. 4140-4147Article in journal (Refereed)
    Abstract [en]

    The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov-Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of similar to 2.8 (+/- 1.9), skewness similar to 0.43 (+/- 0.36), mean duration of similar to 2.7 (+/- 0.9) min and relative density variation Delta N/N of similar to 0.5 (+/- 0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region.

  • 84.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Ganguli, G
    Gavrishchaka, VV
    Koepke, ME
    Zintl, MW
    Ivchenko, N
    Karlsson, T
    Clemmons, JH
    Inhomogeneous transverse electric fields and wave generation in the auroral region: a statistical study2001In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 106, no A6, p. 10803-10816Article in journal (Refereed)
    Abstract [en]

    We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current-driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.

  • 85.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    The importance of a dark ionosphere for ion heating and auroral arc formation2000In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 27, no 11, p. 1635-1638Article in journal (Refereed)
    Abstract [en]

    We present observations from the Freja Satellite to show that density reductions and ion heating at Freja heights are anticorrelated with solar illumination of the ionosphere. When the ionospheric foot-point of a flux-tube is in shadow, the ambient density is lower, transverse ion energization is more common and more intense, and the associated density cavities are deeper. In combination with the suggestion that the electrons must be accelerated to keV energies to carry an imposed current in a low density auroral cavity, these observations may explain the recent observation that auroras are more common when the ionosphere below is in darkness.

  • 86.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gunell, Herbert
    Umeå University, Faculty of Science and Technology, Department of Physics. Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Lindkvist, Jesper
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lindqvist, Per-Arne
    Royal Institute of Technology, Stockholm, Sweden.
    Ergun, Robert E.
    Laboratory of Atmospheric and Space Physics, Boulder, CO, USA.
    Giles, Barbara L.
    NASA Goddard Space Flight Center, Greenbelt, MD, USA.
    Bow shock generator current systems: MMS observations of possible current closure2018In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, p. 242-258Article in journal (Refereed)
    Abstract [en]

    We use data from the first two dayside seasons of the Magnetospheric Multiscale (MMS) mission to study current systems associated with quasi‐perpendicular bow shocks of generator type. We have analyzed 154 MMS bow shock crossings near the equatorial plane. We compute the current density during the crossings and conclude that the component perpendicular to the shock normal (J⊥) is consistent with a pileup of the interplanetary magnetic field (IMF) inside the magnetosheath. For predominantly southward IMF, we observe a component Jn parallel (antiparallel) to the normal for GSM Y> 0 (<0), and oppositely directed for northward IMF. This indicates current closure across the equatorial magnetosheath, and it is observed for IMF clock angles near 0∘ and 180∘. To our knowledge, these are the first observational evidence for bow shock current closure across the magnetosheath. Since we observe no clear signatures of |J⊥| decreasing toward large |Y| we suggest that the main region of current closure is further tailward, outside MMS probing region. For IMF clock angles near 90∘, we find indications of the current system being tilted toward the north‐south direction, obtaining a significant Jz component, and we suggest that the current closes off the equatorial plane at higher latitudes where the spacecraft are not probing. The observations are complicated for several reasons. For example, variations in the solar wind and the magnetospheric currents and loads affect the closure, and Jn is distributed over large regions, making it difficult to resolve inside the magnetosheath proper.

  • 87.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marghitu, Octav
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Klecker, Berndt
    André, Mats
    Buchert, S
    Kistler, L
    McFadden, J
    Rème, H
    Vaivads, Andris
    Observations of concentrated generator regions in the nightside magnetosphere by Cluster/FAST conjunctions2006In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 24, p. 637-49Article in journal (Refereed)
  • 88.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perpendicular ion energization by waves in the auroral region2002In: Recent Research Developments in Geophysics / [ed] S. G. Pandalai, Research Signpost, 2002, p. 403-437Chapter in book (Other academic)
  • 89.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andre, M
    Eriksson, AI
    A statistical study of wave properties and electron density at 1700 km in the auroral region2002In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 107, no A8, p. SIA 21-1-SIA 21-13Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive overview of the electron density and six different wave types in the range similar to1 Hz to similar to1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty-one months of Freja observations from the Northern Hemisphere obtained at similar to1700 km altitude and invariant latitudes 40degrees-75degrees are used. We find that waves around the lower hybrid frequency occur in one low-latitude dayside band and one high-latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high-frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high-latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.

  • 90.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hellström, Thomas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Andre, Mats
    Eriksson, AI
    A statistical study of ion energization at 1700 km in the auroral region2002In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 20, no 12, p. 1943-1958Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive overview of several potentially relevant causes for the oxygen energization in the auroral region. Data from the Freja satellite near 1700 km altitude are used for an unconditional statistical investigation. The data are obtained in the Northern Hemisphere during 21 months in the declining phase of the solar cycle. The importance of various wave types for the ion energization is statistically studied. We also investigate the correlation of ion heating with precipitating protons, accelerated auroral electrons, suprathermal electron bursts, the electron density variations, K-P index and solar illumination of the nearest conjugate ionosphere. We find that sufficiently strong broadband ELF waves, electromagnetic ion cyclotron waves, and waves around the lower hybrid frequency are foremost associated with the ion heating. However, magnetosonic waves, with a sharp, lower frequency cutoff just below the proton gyrofrequency, are not found to contribute to the ion heating. In the absence of the first three wave emissions, transversely energized ions are rare. These wave types are approximately equally efficient in heating the ions, but we find that the main source for the heating is broadband ELF waves, since they are most common in the auroral region. We have also observed that the conditions for ion heating are more favourable for smaller ratios of the spectral densities S-E/S-B of the broadband ELF waves at the oxygen gyrofrequency.

  • 91.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marghitu, Octav
    Buchert, Stephan
    Klecker, Berndt
    Kistler, Lynn M
    Dandouras, Iannis
    Geomagnetic activity effects on plasma sheet energy conversion2010In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 28, p. 1813-1825Article in journal (Refereed)
    Abstract [en]

    In this article we use three years (2001, 2002, and 2004) of Cluster plasma sheet data to investigate what happens to localized energy conversion regions (ECRs) in the plasma sheet during times of high magnetospheric activity. By examining variations in the power density, E·J, where E is the electric field and J is the current density obtained by Cluster, we have studied the influence on Concentrated Load Regions (CLRs) and Concentrated Generator Regions (CGRs) from variations in the geomagnetic disturbance level as expressed by the Kp, the AE, and the Dst indices. We find that the ECR occurrence frequency increases during higher magnetospheric activities, and that the ECRs become stronger. This is true both for CLRs and for CGRs, and the localized energy conversion therefore concerns energy conversion in both directions between the particles and the fields in the plasma sheet. A higher geomagnetic activity hence increases the general level of energy conversion in the plasma sheet. Moreover, we have shown that CLRs live longer during magnetically disturbed times, hence converting more electromagnetic energy. The CGR lifetime, on the other hand, seems to be unaffected by the geomagnetic activity level. The evidence for increased energy conversion during geomagnetically disturbed times is most clear for Kp and for AE, but there are also some indications that energy conversion increases during large negative Dst. This is consistent with the plasma sheet magnetically mapping to the auroral zone, and therefore being more tightly coupled to auroral activities and variations in the AE and Kp indices, than to variations in the ring current region as described by the Dst index.

  • 92.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marghitu, Octav
    Institute for Space Sciences, Bucharest, Romania.
    Vaivads, Andris
    Swedish Institute of Space Physics, Uppsala, Sweden.
    Klecker, Berndt
    Max-Planck-Institut für extraterrestrische Physik, Garching, Germany.
    Kistler, Lynn M
    Space Science Center, University of New Hampshire, Durham, USA.
    Dandouras, Iannis
    CESR-CNRS, Toulouse, France.
    Scale size and life time of energy conversion regions observed by Cluster in the plasma sheet2009In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 27, no 11, p. 4147-4155Article in journal (Refereed)
    Abstract [en]

    In this article, and in a companion paper by Hamrin et al. (2009) [Occurrence and location of concentrated load and generator regions observed by Cluster in the plasma sheet], we investigate localized energy conversion regions (ECRs) in Earth's plasma sheet. From more than 80 Cluster plasma sheet crossings (660 h data) at the altitude of about 15–20 RE in the summer and fall of 2001, we have identified 116 Concentrated Load Regions (CLRs) and 35 Concentrated Generator Regions (CGRs). By examining variations in the power density, E·J, where E is the electric field and J is the current density obtained by Cluster, we have estimated typical values of the scale size and life time of the CLRs and the CGRs. We find that a majority of the observed ECRs are rather stationary in space, but varying in time. Assuming that the ECRs are cylindrically shaped and equal in size, we conclude that the typical scale size of the ECRs is 2 RE≲ΔSECR≲5 RE. The ECRs hence occupy a significant portion of the mid altitude plasma sheet. Moreover, the CLRs appear to be somewhat larger than the CGRs. The life time of the ECRs are of the order of 1–10 min, consistent with the large scale magnetotail MHD simulations of Birn and Hesse (2005). The life time of the CGRs is somewhat shorter than for the CLRs. On time scales of 1–10 min, we believe that ECRs rise and vanish in significant regions of the plasma sheet, possibly oscillating between load and generator character. It is probable that at least some of the observed ECRs oscillate energy back and forth in the plasma sheet instead of channeling it to the ionosphere.

  • 93.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fellgård, D
    The importance of solar illumination for discrete and diffuse aurora2005In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 23, p. 3481-3486Article in journal (Refereed)
  • 94.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pitkänen, Timo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Norqvist, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Karlsson, T.
    Nilsson, H.
    Andre, M.
    Buchert, S.
    Vaivads, A.
    Marghitu, O.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    Evidence for the braking of flow bursts as they propagate toward the Earth2014In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, no 11, p. 9004-9018Article in journal (Refereed)
    Abstract [en]

    In this article we use energy conversion arguments to investigate the possible braking of flow bursts as they propagate toward the Earth. By using EJ data (E and J are the electric field and the current density) observed by Cluster in the magnetotail plasma sheet, we find indications of a plasma deceleration in the region -20 R-E < X < - 15 R-E. Our results suggest a braking mechanism where compressed magnetic flux tubes in so-called dipolarization fronts (DFs) can decelerate incoming flow bursts. Our results also show that energy conversion arguments can be used for studying flow braking and that the position of the flow velocity peak with respect to the DF can be used as a single-spacecraft proxy when determining energy conversion properties. Such a single-spacecraft proxy is invaluable whenever multispacecraft data are not available. In a superposed epoch study, we find that a flow burst with the velocity peak behind the DF is likely to decelerate and transfer energy from the particles to the fields. For flow bursts with the peak flow at or ahead of the DF we see no indications of braking, but instead we find an energy transfer from the fields to the particles. From our results we obtain an estimate of the magnitude of the deceleration of the flow bursts, and we find that it is consistent with previous investigations.

  • 95.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Börlin, Niclas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Vaivads, Andris
    GALS: gradient analysis by least squares2008In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 26, no 11, p. 3491-3499Article in journal (Refereed)
  • 96.
    Hamrin, Maria
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vedin, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rönnmark, Kjell
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Börlin, Niclas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Stenberg, Gabriella
    IRF, Kiruna.
    GALS: gradient analysis by least squares2006In: Proceedings of the 2006 AGU Fall Meeting, American Geophysical Union , 2006, p. SM53A-1442-SM53A-1442Conference paper (Other academic)
  • 97. Hasegawa, Akira
    et al.
    Shukla, Padma Kant
    Umeå University, Faculty of Science and Technology, Department of Physics. Institut für Theoretische Physik IV, Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, Bochum, D-44780, Germany.
    A note on the ion surface waves in a pair-ion plasma2005In: Physica scripta, Stockholm: Royal Swedish academy of sciences , 2005, Vol. T116, p. 105-106Conference paper (Refereed)
    Abstract [en]

    The propagation of ion surface waves in a pair-ion plasma is considered. The dispersion relation for the ion surface waves is derived. The ion surface wave frequency is found to be in good agreement with observations from a recent laboratory experiment.

  • 98.
    Hedman, Robert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    How the interplanetary magnetic field affects the magnetic field line geometry in Earth’s magnetotail: statistical Cluster observations2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The effect of the IMF y-component to the geomagnetic field geometry down- wind from the Earth in the magnetotail was examined. Nine years of Cluster satellite data (2001-2009) were used to construct statistical averages of the magnetic field line geometry in the magnetotail between -20 and -10 Earth radii (rE ) in 5 cross sectional ”slices”. For each cross-section, all the data (the background) were plotted as well as separately data belonging to only time periods during which the IMF By was either positive or negative. The latter were plotted both without and with the removal of the background. It was found that the IMF By induces a collinear By component into the magnetotail, leading to twisting of the closed field lined. The induced By is constrained approximately to a band around the neutral sheet. The induction is most prominent on the flanks but equal out as the distance to the Earth increases. The positions of the nodes that connect the closed field lines within the tail, however, were found to change position asymmetrically to the side depending on the sign of the IMF By. Also, the rotation of the neutral sheet due to the IMF By was examined but no statistically significant rotation could be found.

    The results clearly demonstrate that the IMF By has a significant effect on the magnetic field in the Earth’s magnetotail. The statistical approach introduced in this thesis of using several cross sectional slices will be exploited in the future studies of the IMF By effects to the night side geospace magnetotail. 

  • 99. Heinzl, Thomas
    et al.
    Harvey, Chris
    Ilderton, Anton
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bulanov, Stepan S.
    Rykovanov, Sergey
    Schroeder, Carl B.
    Esarey, Eric
    Leemans, Wim P.
    Detecting radiation reaction at moderate laser intensities2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 91, no 2, article id 023207Article in journal (Refereed)
    Abstract [en]

    We propose a new method of detecting radiation reaction effects in the motion of particles subjected to laser pulses of moderate intensity and long duration. The effect becomes sizable for particles that gain almost no energy through the interaction with the laser pulse. Hence, there are regions of parameter space in which radiation reaction is actually the dominant influence on charged particle motion.

  • 100. Heissler, P.
    et al.
    Barna, A.
    Mikhailova, J. M.
    Ma, Guangjin
    Khrennikov, K.
    Karsch, S.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Földes, I. B.
    Tsakiris, G. D.
    Multi‑μJ harmonic emission energy from laser‑driven plasma2015In: Applied physics. B, Lasers and optics (Print), ISSN 0946-2171, E-ISSN 1432-0649, Vol. 118, no 2, p. 195-201Article in journal (Refereed)
    Abstract [en]

    We report on simultaneous efficiency and divergence measurements for harmonics from solid targets generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of aL≃1.5aL≃1.5, we demonstrate the generation of 30 μJ high-harmonic radiation in a 17±317±3 mrad divergence cone. This corresponds to a conversion efficiency of ≳≳ 10−4 in the 17–80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities.

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