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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 University, Faculty of Science and Technology, Department of Physics.
    Lagrangian perturbation theory for a superfluid immersed in an elastic neutron star crust2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 416, no 1, p. 118-132Article in journal (Refereed)
    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. Brosch, Noah
    et al.
    Häggström, Ingemar
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.
    EISCAT observations of meteors from the sporadic complex2013In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 434, no 4, p. 2907-2921Article in journal (Refereed)
    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.

  • 3.
    Brosch, Noah
    et al.
    The Wise Observatory and the Raymond and Beverly Sackler School of Physics and Astronomy, the Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel.
    Häggström, Ingemar
    EISCAT Scientific Association.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Westman, Assar
    EISCAT Scientific Association.
    Unusual features in high statistics radar meteor studies at EISCAT2010In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 401, no 2, p. 1069-1079Article in journal (Refereed)
    Abstract [en]

    We describe results of an experiment conducted with the European Incoherent Scatter (EISCAT) radars during three 8-h runs on consecutive nights in 2008 December aiming to detect and study the high-altitude meteor population along with the meteors detected at classical ~100-km altitudes. The experiment used coaxial ultra-high-frequency (UHF) and very high-frequency (VHF) radar beams pointed vertically to the zenith of Ramfjordmoen near Tromsø (Norway), and remote UHF receivers at Kiruna (Sweden) and Sodankyl¨a (Finland) for tristatic observations of a very limited volume at an altitude of 170 km above the transmitter site.

    The EISCAT VHF radar detected during the 24-h period 22 698 echoes identified as meteors. The number of UHF echoes in the same period was 2138, most detected also at VHF. Among the VHF meteors, 11 were detected at altitudes higher than 150 km. Of these, the record highest meteor was at 246.9 km. No high-altitude UHF echoes were detected, none was tristatic, and no echoes with a Doppler velocity above ~60 km s−1 were identified. Given the large number of echoes, which argues in favour of a highly significant characterization of the meteoroid population, we discuss the statistical properties of the detections and their possible physical nature.

    The average detection rate of VHF radar meteors was about 16 min-1. Comparing this high rate with that of the faintest optically detected meteors indicates that the radar detections originate from a meteoroid population that could be as optically faint as 13–14 mag. We did not observe a marked enhancement of the rates at the peak of the Geminid shower, confirming once again the proposal that most faint meteors, be these radar or optical, belong to the sporadic population and not to a specific shower.

    For a few meteors, our data show definite deceleration and possible fragmentation. A simple calculation indicates that one of the detected meteoroids was a submillimetre body that fragmented when the ram pressure reached about 0.5 pascal. This is much lower than the pressure that fragments brighter cometary meteors, which is at least two orders of magnitude higher.

  • 4. Glampedakis, Kostas
    et al.
    Andersson, Nils
    Samuelsson, Lars
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetohydrodynamics of superfluid and superconducting neutron star cores2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 410, p. 805-829Article in journal (Refereed)
    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.

  • 5.
    Glampedakis, Kostas
    et al.
    Theoretical Astrophysics, University of Tübingen.
    Jones, D. Ian
    School of Mathematics, University of Southampton.
    Samuelsson, Lars
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ambipolar diffusion in superfluid neutron stars2011In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 413, p. 2021-2030Article in journal (Other academic)
    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.

  • 6. 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.

  • 7.
    Kero, J.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fujiwara, Y.
    Abo, M.
    Szasz, C.
    Nakamura, T.
    MU radar head echo observations of the 2011 October Draconids2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 424, no 3, p. 1799-1806Article in journal (Refereed)
    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.

  • 8.
    Kero, J.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    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 rates2012In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 425, no 1, p. 135-146Article in journal (Refereed)
    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.

  • 9.
    Marklund, Mattias
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Clarkson, C.
    The general relativistic magnetohydrodynamic dynamo equation2005In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 358, p. 892-900Article in journal (Refereed)
  • 10.
    Szasz, Csilla
    et al.
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Kero, Johan
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Meisel, David D.
    Suny Geneseo, NY, USA.
    Pellinen-Wannberg, Asta
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Wannberg, Gudmund
    Umeå University, Faculty of Science and Technology, Physics. Institutet för rymdfysik.
    Westman, Assar
    EISCAT Scientific Association.
    Orbit characteristics of the tristatic EISCAT UHF meteors2008In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 388, no 1, p. 15-25Article in journal (Refereed)
    Abstract [en]

    The tristatic EISCAT 930-MHz UHF system is used to determine the absolute geocentric velocities of meteors detected with all three receivers simultaneously at 96 km, the height of the common radar volume. The data used in this study were taken between 2002 and 2005, during four 24-h runs at summer/winter solstice and vernal/autumnal equinox to observe the largest seasonal difference. The observed velocities of 410 tristatic meteors are integrated back through the Earth atmosphere to find their atmospheric entry velocities using an ablation model. Orbit calculations are performed by taking zenith attraction, Earth rotation as well as obliquity of the ecliptic into account. The results are presented in the form of different orbital characteristics. None of the observed meteors appears to be of extrasolar or asteroidal origin; comets, particularly short-period (<200 yr) ones, may be the dominant source for the particles observed. About 40 per cent of the radiants can be associated with the north apex sporadic meteor source and 58 per cent of the orbits are retrograde. There is evidence of resonance gaps at semimajor axis values corresponding to commensurabilities with Jupiter, which may be the first convincing evidence of Jupiter's gravitational influence on the population of small sporadic meteoroids surveyed by radar. The geocentric velocity distribution is bimodal with a prograde population centred around 38 km s−1 and a retrograde population peaking at 59 km s−1. The EISCAT radar system is located close to the Arctic Circle, which means that the North Ecliptic Pole (NEP) is near zenith once every 24 h, i.e. during each observational period. In this particular geometry, the local horizon coincides with the ecliptic plane. The meteoroid influx should therefore be directly comparable throughout the year.

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