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  • 1. Bader, A.
    et al.
    Wieser, G. Stenberg
    Andre, M.
    Wieser, M.
    Futaana, Y.
    Persson, M.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Nilsson, H.
    Zhang, T. L.
    Proton Temperature Anisotropies in the Plasma Environment of Venus2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 5, s. 3312-3330Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Velocity distribution functions (VDFs) are a key to understanding the interplay between particles and waves in a plasma. Any deviation from an isotropic Maxwellian distribution may be unstable and result in wave generation. Using data from the ion mass spectrometer IMA (Ion Mass Analyzer) and the magnetometer (MAG) onboard Venus Express, we study proton distributions in the plasma environment of Venus. We focus on the temperature anisotropy, that is, the ratio between the proton temperature perpendicular (T-perpendicular to) and parallel (T-parallel to) to the background magnetic field. We calculate average values of T-perpendicular to and T-parallel to for different spatial areas around Venus. In addition we present spatial maps of the average of the two temperatures and of their average ratio. Our results show that the proton distributions in the solar wind are quite isotropic, while at the bow shock stronger perpendicular than parallel heating makes the downstream VDFs slightly anisotropic (T-perpendicular to/T-parallel to > 1) and possibly unstable to generation of proton cyclotron waves or mirror mode waves. Both wave modes have previously been observed in Venus's magnetosheath. The perpendicular heating is strongest in the near-subsolar magnetosheath (T-perpendicular to/ T-parallel to approximate to 3/2), which is also where mirror mode waves are most frequently observed. We believe that the mirror mode waves observed here are indeed generated by the anisotropy. In the magnetotail we observe planetary protons with largely isotropic VDFs, originating from Venus's ionosphere.

  • 2.
    Bergman, Sofia
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Wieser, Martin
    Johansson, Fredrik
    Eriksson, Anders
    The Influence of Spacecraft Charging on Low‐Energy Ion Measurements Made by RPC‐ICA on Rosetta2020Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 125, nr 1, artikkel-id e2019JA027478Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Spacecraft charging is problematic for low‐energy plasma measurements. The charged particles are attracted to or repelled from the charged spacecraft, affecting both the energy and direction of travel of the particles. The Ion Composition Analyzer (RPC‐ICA) on board the Rosetta spacecraft is suffering from this effect. RPC‐ICA was measuring positive ions in the vicinity of comet 67P/Churyumov‐Gerasimenko, covering an energy range of a few eV/q to 40 keV/q. The low‐energy part of the data is, however, heavily distorted by the negatively charged spacecraft. In this study we use the Spacecraft Plasma Interaction Software to model the influence of the spacecraft potential on the ion trajectories and the corresponding distortion of the field of view (FOV) of the instrument. The results show that the measurements are not significantly distorted when the ion energy corresponds to at least twice the spacecraft potential. Below this energy the FOV is often heavily distorted, but the distortion differs between different viewing directions. Generally, ions entering the instrument close to the aperture plane are less affected than those entering with extreme elevation angles.

  • 3.
    Bergman, Sofia
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg Wieser, Gabriella
    Wieser, Martin
    Johansson, Fredrik
    Eriksson, Anders
    The Influence of Varying Spacecraft Potentials and Debye Lengths on In Situ Low-Energy Ion Measurements2020Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402Artikkel i tidsskrift (Fagfellevurdert)
  • 4. 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 precipitation2013Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 6694-6705Artikkel i tidsskrift (Fagfellevurdert)
  • 5.
    De Spiegeleer, Alexandre
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Volwerk, M.
    Andersson, L.
    Karlsson, T.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Mouikis, C. G.
    Nilsson, H.
    Kistler, L. M.
    Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 4, s. 2736-2754Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 6.
    De Spiegeleer, Alexandre
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Volwerk, M.
    Mann, Ingrid
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Department of Physics and Technology, The Arctic University of Norway, Tromsø, Norway.
    Nilsson, H.
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Andersson, L.
    Vaverka, Jakub
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Low-frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 7, s. 7042-7056Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Using plasma sheet data from Cluster 1 spacecraft from 2001 till 2011, we statistically investigate oscillatory signatures in the plasma bulk flow. These periodic oscillations are compared to high-speed and quiet flows. Periodic oscillations are observed approximately 8% of the time, while high-speed flows and quiet flows are observed around 0.5% and 12% of the time, respectively. We remark that periodic oscillations can roughly occur everywhere for x(gsm) < -10 R-E and |y(gsm)| < 10 RE, while quiet flows mainly occur toward the flanks of this region and toward x = -10 R-E. The relation between the geomagnetic and solar activity and the occurrence of periodic oscillations is investigated and reveal that periodic oscillations occur for most Kp values and solar activity, while quiet flows are more common during low magnetospheric and solar activity. We find that the median oscillation frequency of periodic oscillations is 1.7 mHz and the median duration of the oscillation events is 41 min. We also observe that their associated Poynting vectors show a tendency to be earthward (S-x >= 0). Finally, the distribution of high-speed flows and periodic oscillations as a function of the velocity is investigated and reveals that thresholds lower than 200 km/s should not be used to identify high-speed flows as it could result in misinterpreting a periodic oscillations for a high-speed flow.

  • 7.
    De Spiegeleer, Alexandre
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Volwerk, M.
    Karlsson, T.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Chong, Ghai Siung
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China.
    Nilsson, H.
    Oxygen Ion Flow Reversals in Earth's Magnetotail: A Cluster Statistical Study2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 11, s. 8928-8942Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present a statistical study of magnetotail flows that change direction from earthward to tailward using Cluster spacecraft. More precisely, we study 318 events of particle flux enhancements in the O+ data for which the pitch angle continuously changes with time, either from 0 degrees to 180 degrees or from 180 degrees to 0 degrees. These structures are called "Pitch Angle Slope Structures" (PASSes). PASSes for which the pitch angle changes from 0 degrees to 180 degrees are observed in the Northern Hemisphere while those for which the pitch angle changes from 180 degrees to 0 degrees are observed in the Southern Hemisphere. These flux enhancements result in a reversal of the flow direction from earthward to tailward regardless of the hemisphere where they are observed. Sometimes, several PASSes can be observed consecutively which can therefore result in oscillatory velocity signatures in the earth-tail direction. The PASS occurrence rate increases from 1.8% to 3.7% as the AE index increases from similar to 0 to similar to 600 nT. Also, simultaneously to PASSes, there is typically a decrease in the magnetic field magnitude due to a decrease (increase) of the sunward component of the magnetic field in the Northern (Southern) Hemisphere. Finally, based on the 115 (out of 318) PASSes that show energy-dispersed structures, the distance to the source from the spacecraft is estimated to be typically R-E along the magnetic field line. This study is important as it sheds light on one of the causes of tailward velocities in Earth's magnetotail.

  • 8. Echim, M. M.
    et al.
    Lamy, H.
    De Keyser, J.
    Maggiolo, R.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Royal Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Wedlund, C. L. Simon
    A Method to Estimate the Physical Properties of Magnetospheric Generators From Observations of Quiet Discrete Auroral Arcs2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 12, s. 10283-10293Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We discuss a method to estimate the properties of a magnetospheric generator using a quasi-electrostatic magnetosphere-ionosphere coupling model and in situ or remote sensing observations of discrete quiet arcs. We first construct an ensemble of Vlasov equilibrium solutions for generator structures formed at magnetospheric plasma interfaces. For each generator solution, we compute the ionospheric electric potential from the current continuity equation. Thus, we estimate the field-aligned potential drop that allows us to assess several properties of the discrete auroral arc, such as the field-aligned potential difference, the field-aligned current density, the flux of precipitating energy, and the height-integrated Pedersen conductance. A minimization procedure based on comparing the numerical results with observations is defined and applied to find which solution of the current continuity equation and which generator model give auroral arc properties that best fit the observations. The procedure is validated in a case study with observations by DMSP and Cluster and can be generalized to other types of data.

  • 9.
    Eliasson, Bengt
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. 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 plasma2008Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 113, nr A2, artikkel-id A02313Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 10.
    Fatemi, Shahab
    et al.
    Swedish Institute of Space Physics, Kiruna, Sweden ; Department of Computer Science, Electrical and Space Engineering, Division of Space Technology, Luleå University of Technology, Luleå, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Lue, Charles
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Collier, Michael R.
    NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Stenberg, Gabriella
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Effects of protons reflected by lunar crustal magnetic fields on the global lunar plasma environment2014Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 8, s. 6095-6105Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 11. Fatemi, Shahab
    et al.
    Lue, Charles
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Holmstrom, Mats
    Poppe, Andrew R.
    Wieser, Martin
    Barabash, Stas
    Delory, Gregory T.
    Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 6, s. 4719-4735Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 12.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Andersson, L.
    Vaivads, A.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, H.
    The use of the power density for identifying reconnection regions2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 10, s. 8644-8662Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 13.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Brussels,Belgium.
    Goncharov, Oleksandr
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Fuselier, S.
    Mukherjee, J.
    Vaivads, A.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Space Sciences,Shandong University, Weihai, China.
    Torbert, R. B.
    Giles, B.
    Can Reconnection be Triggered as a Solar Wind Directional Discontinuity Crosses the Bow Shock?: A Case of Asymmetric Reconnection2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 11, s. 8507-8523Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Here we present some unique observations of reconnection at a quasi-perpendicular bow shock as an interplanetary directional discontinuity (DD) is crossing it simultaneously with the Magnetospheric Multiscale (MMS) mission. There are no burst data, but available data show indications of ongoing reconnection at the shock southward of MMS: a bifurcated current sheet with signatures of Hall magnetic and electric fields, normal magnetic fields indicating a magnetic connection between the two reconnecting regions, field-aligned currents and electric fields, E . J > 0 indicating a conversion of magnetic to kinetic energy, and subspin resolution ion energy-time spectrograms indicating ions being accelerated away from the X-line. The DD is also observed by four upstream spacecraft (ACE, WIND, Geotail, and ARTEMIS P1) and one downstream in the magnetosheath (Cluster 4), but none of them resolve signatures of ongoing reconnection. We therefore suggest that reconnection was temporarily triggered as the DD was compressed by the shock. Reconnection at the bow shock is inevitably asymmetric with both the density and magnetic field strength being higher on one side of the X-line (magnetosheath side) than on the other side where the plasma flow also is supersonic (solar wind side). This is different from the asymmetry exhibited at the more commonly studied case of asymmetric reconnection at the magnetopause. Asymmetric reconnection of the bow shock type has never been studied before, and the data discussed here present some first indications of the properties of the reconnection region for this type of reconnection.

  • 14.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Gunell, Herbert
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Belgian Institute for Space Aeronomy, Brussels, Belgium.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    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 closure2018Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, s. 242-258Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 15.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, T.
    Nilsson, H.
    Fu, H. S.
    Buchert, S.
    Andre, M.
    Marghitu, O.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Klecker, B.
    Kistler, L. M.
    Dandouras, I.
    The evolution of flux pileup regions in the plasma sheet: Cluster observations2013Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, nr 10, s. 6279-6290Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 16.
    Hamrin, Maria
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norqvist, Patrik
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    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 Earth2014Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 11, s. 9004-9018Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 17. Juusola, Liisa
    et al.
    Kubyshkina, Marina
    Nakamura, Rumi
    Pitkänen, Timo
    Amm, Olaf
    Kauristie, Kirsti
    Partamies, Noora
    Rème, Henry
    Snekvik, Kristian
    Whiter, Daniel
    Ionospheric signatures of a plasma sheet rebound flow during a substorm onset2013Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 350-363Artikkel i tidsskrift (Fagfellevurdert)
  • 18. Karlsson, T.
    et al.
    Kullen, A.
    Liljeblad, E.
    Brenning, N.
    Nilsson, H.
    Gunell, H.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    On the origin of magnetosheath plasmoids and their relation to magnetosheath jets2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 9, s. 7390-7403Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 19.
    Khurana, Krishan K.
    et al.
    Institute of Geophysics and Planetary Physics and Dept. of Earth, Planetary and Space Sciences, University of California at Los Angeles, CA, 90095, USA.
    Fatemi, Shahab
    Space Sciences Laboratory, University of California, Berkeley, California, USA.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Roussos, Elias
    Max Planck Institute, Göttingen, Germany.
    Krupp, Norbert
    Max Planck Institute, Göttingen, Germany.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Russell, Christopher T.
    Institute of Geophysics and Planetary Physics and Dept. of Earth, Planetary and Space Sciences, University of California at Los Angeles, CA, 90095, USA.
    Dougherty, Michele K.
    Imperial College, London, U.K.
    The role of plasma slowdown in the generation of Rhea's Alfvén wings2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 2, s. 1778-1788Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Alfvén wings are known to form when a conducting or mass-loading object slows down a flowing plasma in its vicinity. Alfvén wings are not expected to be generated when an inert moon such as Rhea interacts with Saturn's magnetosphere, where the plasma impacting the moon is absorbed and the magnetic flux passes unimpeded through the moon. However, in two close polar passes of Rhea, Cassini clearly observed magnetic field signatures consistent with Alfvén wings. In addition, observations from a high-inclination flyby (Distance > 100 R Rh ) of Rhea on 3 June 2010 showed that the Alfvén wings continue to propagate away from Rhea even at this large distance. We have performed three-dimensional hybrid simulations of Rhea's interaction with Saturn's magnetosphere which show that the wake refilling process generates a plasma density gradient directed in the direction of corotating plasma. The resulting plasma pressure gradient exerts a force directed toward Rhea and slows down the plasma streaming into the wake along field lines. As on the same field lines, outside of the wake, the plasma continues to move close to its full speed, this differential motion of plasma bends the magnetic flux tubes, generating Alfvén wings in the wake. The current system excited by the Alfvén wings transfers momentum to the wake plasma extracting it from plasma outside the wake. Our work demonstrates that Alfvén wings can be excited even when a moon does not possess a conducting exosphere.

  • 20.
    Lindkvist, Jesper
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Khurana, Krishan K.
    Department of Earth and Space Sciences, University of California, Los Angeles.
    Fatemi, Shahab
    Space Sciences Laboratory, University of California, Berkeley.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Callisto plasma interactions: Hybrid modeling including induction by a subsurface ocean2015Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 6, s. 4877-4889Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    By using a hybrid plasma solver (ions as particles and electrons as a fluid), we have modeled the interaction between Callisto and Jupiter's magnetosphere for variable ambient plasma parameters. We compared the results with the magnetometer data from flybys (C3, C9, and C10) by the Galileo spacecraft. Modeling the interaction between Callisto and Jupiter's magnetosphere is important to establish the origin of the magnetic field perturbations observed by Galileo and thought to be related to a subsurface ocean. Using typical upstream magnetospheric plasma parameters and a magnetic dipole corresponding to the inductive response inside the moon, we show that the model results agree well with observations for the C3 and C9 flybys, but agrees poorly with the C10 flyby close to Callisto. The study does support the existence of a subsurface ocean at Callisto.

  • 21.
    Lue, Charles
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Futaana, Yoshifumi
    Barabash, Stas
    Saito, Yoshifumi
    Nishino, Masaki
    Wieser, Martin
    Asamura, Kazushi
    Bhardwaj, Anil
    Wurz, Peter
    Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath2016Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 1, s. 432-445Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study hydrogen energetic neutral atom (ENA) emissions from the lunar surface, when the Moon is inside the terrestrial magnetosheath. The ENAs are generated by neutralization and backscattering of incident protons of solar wind origin. First, we model the effect of the increased ion temperature in the magnetosheath (>10 times larger than that in the undisturbed solar wind) on the ENA scattering characteristics. Then, we apply these models to ENA measurements by Chandrayaan-1 and simultaneous ion measurements by Kaguya at the Moon, in the magnetosheath. We produce maps of the ENA scattering fraction, covering a region at the lunar near-side that includes mare and highland surfaces and several lunar magnetic anomalies. We see clear signatures of plasma shielding by the magnetic anomalies. The maps are made at different lunar local times, and the results indicate an extended influence and altered morphology of the magnetic anomalies at shallower incidence angles of the magnetosheath protons. The scattering fraction from the unmagnetized regions remains consistent with that in the undisturbed solar wind (10%-20%). Moreover, the observed ENA energy spectra are well reproduced by our temperature-dependent model. We conclude that the ENA scattering process is unchanged in the magnetosheath. Similarly to the undisturbed solar wind case, it is only magnetic anomalies that provide contrast in the ENA maps, not any selenomorphological features such as mare and highland regions.

  • 22. Maggiolo, R.
    et al.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Keyser, J.
    Pitkanen, T.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Cessateur, G.
    Gunell, H.
    Maes, L.
    The Delayed Time Response of Geomagnetic Activity to the Solar Wind2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 11, s. 109-127Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 23. Nilsson, Hans
    et al.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Karlsson, Tomas
    Slapak, Rikard
    Andersson, Laila
    Gunell, Herbert
    Schillings, Audrey
    Vaivads, Andris
    Oxygen ion response to proton bursty bulk flows2016Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 8, s. 7535-7546Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 24.
    Pellinen-Wannberg, Asta K.
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Haggstrom, Ingemar
    Sanchez, Juan Diego Carrillo
    Plane, John M. C.
    Westman, Assar
    Strong E region ionization caused by the 1767 trail during the 2002 Leonids2014Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 119, nr 9, s. 7880-7888Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Intensive E region ionization extending up to 140 km altitude and lasting for several hours was observed with the European Incoherent Scatter (EISCAT) UHF radar during the 2002 Leonids meteor shower maximum. The level of global geomagnetic disturbance as well as the local geomagnetic and auroral activity in northern Scandinavia were low during the event. Thus, the ionization cannot be explained by intensive precipitation. The layer was 30-40 km thick, so it cannot be classified as a sporadic E layer which are typically just a few kilometers wide. Incoherent scatter radars have not to date reported any notable meteor shower-related increases in the average background ionization. The 2002 Leonids storm flux, however, was so high that it might have been able to induce such an event. The Chemical Ablation Model is used to estimate deposition rates of individual meteors. The resulting electron production, arising from hyperthermal collisions of ablated atoms with atmospheric molecules, is related to the predicted Leonid flux values and observed ionization on 19 November 2002. The EISCAT Svalbard Radar (ESR) located at some 1000 km north of the UHF site did not observe any excess ionization during the same period. The high-latitude electrodynamic conditions recorded by the SuperDARN radar network show that the ESR was within a strongly drifting convection cell continuously fed by fresh plasma while the UHF radar was outside the polar convection region maintaining the ionization.

  • 25. Persson, M.
    et al.
    Futaana, Y.
    Nilsson, H.
    Wieser, G. Stenberg
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Fedorov, A.
    Zhang, T. L.
    Barabash, S.
    Heavy Ion Flows in the Upper Ionosphere of the Venusian North Pole2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 6, s. 4597-4607Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate the heavy ion density and velocity in the Venusian upper ionosphere near the North Pole, using the Ion Mass Analyzer, a part of the Analyzer of Space Plasmas and Energetic Atoms 4, together with the magnetic field instruments on Venus Express. The measurements were made during June-July 2014, covering the aerobraking campaign with lowered altitude measurements (similar to 130 km). The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400-km altitude. A clear trend of dusk-to-dawn heavy ion flow across the polar ionosphere was found, with speeds of similar to 2-10 km/s. In addition, the flow has a significant downward radial velocity component. The flow pattern does not depend on the interplanetary magnetic field directions nor the ionospheric magnetization states. Instead, we suggest a thermal pressure gradient between the equatorial and polar terminator regions, induced by the decrease in density between the regions, as the dominant mechanism driving the ion flow. Plain Language Summary We have calculated the ion density and velocities in the Venusian polar ionosphere using measurements from the Ion Mass Analyzer on board the Venus Express spacecraft. During June-July 2014 the periapsis was lowered to similar to 130 km, which allowed for measurements down to low altitudes of the ionosphere near the North Pole. The plasma scale heights are similar to 15 km below 150-km altitude and similar to 200 km at 150-400 km, which is similar to what was found near the equatorial region by the Pioneer Venus mission. In addition, there is a clear trend of dusk-to-dawn flow, along the terminator, for the heavy ions. This is surprising, as a general flow from day-to-night is expected for the Venusian ionosphere due to the long nights and significant heating of the dayside upper atmosphere. The interplanetary magnetic field direction does not appear to affect the ion flow pattern. Instead, we propose a thermal pressure gradient as the dominant accelerating mechanism, induced by the decrease in density from the equator toward the pole.

  • 26. Pitkänen, Timo
    et al.
    Aikio, Anita T.
    Juusola, Liisa
    Observations of polar cap flow channel and plasma sheet flow bursts during substorm expansion2013Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 118, s. 774-784Artikkel i tidsskrift (Fagfellevurdert)
  • 27.
    Pitkänen, Timo
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Institute of Space Sciences, Shandong University, Weihai, China.
    Kullen, A.
    Shi, Q. Q.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Nishimura, Y.
    Convection electric field and plasma convection in a twisted magnetotail: t THEMIS case study 1-2 January 20092018Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, nr 9, s. 7486-7497Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

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

  • 28.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Solar wind- and EUV-dependent models for the shapes of the Martian plasma boundaries based on Mars Express measurements2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 7, s. 7279-7290Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The long operational life (2003-present) of Mars Express (MEX) has allowed the spacecraft tomake plasma measurements in the Martian environment over a wide range of upstream conditions. Wehave analyzed ∼7000 MEX orbits, covering three orders of magnitude in solar wind dynamic pressure, withdata from the on board Analyzer of Space Plasmas and Energetic Particles (ASPERA-3) package, mappingthe locations where MEX crosses the main plasma boundaries, induced magnetosphere boundary (IMB), ionosphere boundary (IB), and bow shock (BS). A coincidence scheme was employed, where data fromthe Ion Mass Analyzer (IMA) and the Electron Spectrometer (ELS) had to agree for a positive boundaryidentification, which resulted in crossings from 1083 orbit segments that were used to create dynamictwo-parameter (solar wind density, nsw, and velocity vsw) dependent global dynamic models for the IMB, IB,and BS. The modeled response is found to be individual to each boundary. The IMB scales mainly dependenton solar wind dynamic pressure and EUV intensity. The BS location closely follows the location of the IMB atthe subsolar point, though under extremely low nsw and vsw the BS assumes a more oblique shape. The IBclosely follows the IMB on the dayside and changes its nightside morphology with different trends for nswand vsw. We also investigate the influence of extreme ultraviolet (EUV) radiation on the IMB and BS, findingthat increased EUV intensity expands both boundaries.

  • 29.
    Ramstad, Robin
    et al.
    Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna.
    Effects of the crustal magnetic fields on the Martian atmospheric ion escape rate2016Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 43, nr 20, s. 10574-10579Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Eight years (2007–2015) of ion flux measurements from Mars Express are used to statisticallyinvestigate the influence of the Martian magnetic crustal fields on the atmospheric ion escape rate.We combine all Analyzer of Space Plasmas and Energetic Atoms/Ion Mass Analyzer (ASPERA-3/IMA)measurements taken during nominal upstream solar wind and solar extreme ultraviolet conditions tocompute global average ion distribution functions, individually for the north/south hemispheres and forvarying solar zenith angles (SZAs) of the strongest crustal magnetic field. Escape rates are subsequentlycalculated from each of the average distribution functions. The maximum escape rate (4.2 ± 1.2) × 1024 s−1 is found for SZA = 60–80, while the minimum escape rate (1.7±0.6)×1024 s−1 is found for SZA = 28–60,showing that the dayside orientation of the crustal fields significantly affects the global escape rate (p=97%). However, averaged over time, independent of SZA, we find no statistically significant difference inthe escape rates from the two hemispheres (escape from southern hemisphere 46% ± 18% of global rate).

  • 30.
    Ramstad, Robin
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Barabash, Stas
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Futaana, Yoshifumi
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Nilsson, Hans
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Holmström, Mats
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna.
    Global Mars-solar wind coupling and ion escape2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 8, s. 8051-8062Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Loss of the early Martian atmosphere is often thought to have occurred due to an effective transfer of the solar wind energy through the Martian induced magnetic barrier to the ionosphere. We have quantified the coupling efficiency by comparing the power of the heavy ion outflow with the available power supplied by the upstream solar wind. Constraining upstream solar wind density nsw, velocity vsw, and EUV intensity IEUV/photoionizing flux FXUV in varying intervals reveals a decrease in coupling efficiency, k,with solar wind dynamic pressure as ∝ pdyn−0.74±0.13 and with FXUV as k ∝ FXUV−2.28±0.30. Despite the decreasein coupling efficiency, higher FXUV enhances the cold ion outflow, increasing the total ion escape rate as Q(FXUV) = 1010(0.82 ± 0.05)FXUV. The discrepancy between coupling and escape suggests that ion escapefrom Mars is primarily production limited in the modern era, though decreased coupling may lead to an energy-limited solar wind interaction under early Sun conditions.

  • 31. Sanchez-Cano, Beatriz
    et al.
    Blelly, Pierre-Louis
    Lester, Mark
    Witasse, Olivier
    Cartacci, Marco
    Orosei, Roberto
    Opgenoorth, Hermann
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Lillis, Robert
    Leblanc, Francois
    Milan, Stephen E.
    Conroy, Philip
    Floury, Nicolas
    Plane, John M. C.
    Cicchetti, Andrea
    Noschese, Raffaella
    Kopf, Andrew J.
    Origin of the Extended Mars Radar Blackout of September 20172019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 6, s. 4556-4568Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express, which operates between 0.1 and 5.5 MHz, suffered from a complete blackout for 10 days in September 2017 when observing on the nightside (a rare occurrence). Moreover, the Shallow Radar (SHARAD) onboard the Mars Reconnaissance Orbiter, which operates at 20 MHz, also suffered a blackout for three days when operating on both dayside and nightside. We propose that these blackouts are caused by solar energetic particles of few tens of keV and above associated with an extreme space weather event between 10 and 22 September 2017, as recorded by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. Numerical simulations of energetic electron precipitation predict that a lower O-2(+) nighttime ionospheric layer of magnitude similar to 10(10) m(-3) peaking at similar to 90-km altitude is produced. Consequently, such a layer would absorb radar signals at high frequencies and explain the blackouts. The peak absorption level is found to be at 70-km altitude. Plain Language Summary Several instrument operations, as well as communication systems with rovers at the surface, depend on radio signals that propagate throughout the atmosphere of Mars. This is the case also for two radars that are currently working in Mars' orbit, sounding the ionosphere, surface, and subsurface of the planet. In mid-September 2017, a powerful solar storm hit Mars, producing a large amount of energetic particle precipitation over a 10-day period. We have found that high-energy electrons ionized the atmosphere of Mars, creating a dense layer of ions and electrons at similar to 90 km on the Martian nightside. This layer attenuated radar signals continuously for 10 days, stopping the radars to receive any signal from the planetary surface. In this work, we assess the properties of this layer in order to understand the implications of this kind of phenomenon for radar performance and communications.

  • 32.
    Vaverka, Jakub
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. National I nstitute of Polar Research, Tachikawa, Japan; Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic.
    Nakamura, Takuji
    Kero, Johan
    Mann, Ingrid
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norberg, Carol
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics,Kiruna, Sweden.
    Lindqvist, Per-Arne
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics,Kiruna, Sweden.
    Comparison of Dust Impact and Solitary Wave Signatures Detected by Multiple Electric Field Antennas Onboard the MMS Spacecraft2018Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, nr 8, s. 6119-6129Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dust impact detection by electric field instruments is a relatively new method. However, the influence of dust impacts on electric field measurements is not completely understood and explained. A better understanding is very important for reliable dust impact identification, especially in environments with low dust impact rate. Using data from Earth-orbiting Magnetospheric Multiscale mission (MMS) spacecraft, we present a study of various pulses detected simultaneously by multiple electric field antennas in the monopole (probe-to-spacecraft potential measurement) and dipole (probe-to-probe potential measurement) configurations. The study includes data obtained during an impact of a millimeter-sized object. We show that the identification of dust impacts by a single antenna is a very challenging issue in environments where solitary waves are commonly present and that some pulses can be easily misinterpreted as dust impacts. We used data from multiple antennas to distinguish between changes in the spacecraft potential (dust impact) and structures in the ambient plasma or electric field. Our results indicate that an impact cloud is in some cases able to influence the potential of the electric field antenna during its expansion.

  • 33.
    Vaverka, Jakub
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Pellinen-Wannberg, Asta
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Kero, Johan
    Mann, Ingrid
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Norberg, Carol
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Detection of meteoroid hypervelocity impacts on the Cluster spacecraft: First results2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 6, s. 6485-6494Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the first study of dust impact events on one of the Earth-orbiting Cluster satellites. The events were identified in the measurements of the wide band data (WBD) instrument on board the satellite operating in monopole configuration. Since 2009 the instrument is operating in this configuration due to the loss of three electric probes and is therefore measuring the potential between the only operating antenna and the spacecraft body. Our study shows that the WBD instrument on Cluster 1 is able to detect pulses generated by dust impacts and discusses four such events. The presence of instrumental effects, intensive natural waves, noncontinuous sampling modes, and the automatic gain control complicates this detection. Due to all these features, we conclude that the Cluster spacecraft are not ideal for dust impact studies. We show that the duration and amplitudes of the pulses recorded by Cluster are similar to pulses detected by STEREO, and the shape of the pulses can be described with the model of the recollection of impact cloud electrons by the positively charged spacecraft. We estimate that the detected impacts were generated by micron-sized grains with velocities in the order of tens of km/s.

  • 34.
    Vorburger, Audrey
    et al.
    Physikalisches Institut, University of Bern, Bern.
    Pfleger, Martin
    Institute for Chemical Engineering and Environmental Technology, Graz.
    Lindkvist, Jesper
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Swedish Institute of Space Physics, Kiruna, Sweden.
    Holmström, Mats
    Swedish Institute of Space Physics, Kiruna, Sweden.
    Lammer, Helmut
    Space Research Institute, Austrian Academy of Sciences, Graz.
    Lichtenegger, Herbert I. M.
    Space Research Institute, Austrian Academy of Sciences, Graz.
    Galli, André
    Rubin, Martin
    Wurz, Peter
    Physikalisches Institut, University of Bern, Bern.
    Three‐Dimensional Modeling of Callisto's Surface Sputtered Exosphere Environment2019Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 8, s. 7157-7169Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the release of various elements from Callisto's surface into its exosphere by plasma sputtering. The cold Jovian plasma is simulated with a 3‐D plasma‐planetary interaction hybrid model, which produces 2‐D surface precipitation maps for magnetospheric H+, O+, O++, and S++. For the hot Jovian plasma, we assume isotropic precipitation onto the complete spherical surface. Two scenarios are investigated: one where no ionospheric shielding takes place and accordingly full plasma penetration is implemented (no‐ionosphere scenario) and one where an ionosphere lets virtually none of the cold plasma but all of the hot plasma reach Callisto's surface (ionosphere scenario). In the 3‐D exosphere model, neutral particles are sputtered from the surface and followed on their individual trajectories. The 3‐D density profiles show that whereas in the no‐ionosphere scenario the ram direction is favored, the ionosphere scenario produces almost uniform density profiles. In addition, the density profiles in the ionosphere scenario are reduced by a factor of ∼2.5 with respect to the no‐ionosphere scenario. We find that the Neutral Gas and Ion Mass Spectrometer, which is part of the Particle Environment Package on board the JUpiter ICy moons Explorer mission, will be able to detect the different sputter populations from Callisto's icy surface and the major sputter populations from Callisto's nonicy surface. The chemical composition of Callisto's exosphere can be directly linked to the chemical composition of its surface and will offer us information not only on Callisto's formation scenario but also on the building blocks of the Jupiter system.

  • 35. Yao, S. T.
    et al.
    Shi, Q. Q.
    Li, Z. Y.
    Wang, X. G.
    Tian, A. M.
    Sun, W. J.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Wang, M. M.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Bai, S. C.
    Shen, X. C.
    Ji, X. F.
    Pokhotelov, D.
    Yao, Z. H.
    Xiao, T.
    Pu, Z. Y.
    Fu, S. Y.
    Zong, Q. G.
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Liu, W.
    Zhang, H.
    Reme, H.
    Propagation of small size magnetic holes in the magnetospheric plasma sheet2016Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, nr 6, s. 5510-5519Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magnetic holes (MHs), characteristic structures where the magnetic field magnitude decreases significantly, have been frequently observed in space plasmas. Particularly, small size magnetic holes (SSMHs) which the scale is less than or close to the proton gyroradius are recently detected in the magnetospheric plasma sheet. In this study of Cluster observations, by the timing method, the minimum directional difference (MDD) method, and the spatiotemporal difference (STD) method, we obtain the propagation velocity of SSMHs in the plasma flow frame. Furthermore, based on electron magnetohydrodynamics (EMHD) theory we calculate the velocity, width, and depth of the electron solitary wave and compare it to SSMH observations. The result shows a good accord between the theory and the observation.

  • 36.
    Yao, Shutao
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik. Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai, China.
    Wang, X. G.
    Shi, Q. Q.
    Pitkänen, Timo
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Hamrin, Maria
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Yao, Z. H.
    Li, Z. Y.
    Ji, X. F.
    De Spiegeleer, Alexandre
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Xiao, Y. C.
    Tian, A. M.
    Pu, Z. Y.
    Zong, Q. G.
    Xiao, C. J.
    Fu, S. Y.
    Zhang, H.
    Russell, C. T.
    Giles, B. L.
    Guo, R. L.
    Sun, W. J.
    Li, W. Y.
    Zhou, X. Z.
    Huang, S. Y.
    Vaverka, Jakub
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för fysik.
    Nowada, M.
    Bai, S. C.
    Wang, M. M.
    Liu, J.
    Observations of kinetic-size magnetic holes in the magnetosheath2017Inngår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, nr 2, s. 1990-2000Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magnetic holes (MHs), with a scale much greater than ρ(proton gyroradius), have been widely reported in various regions of space plasmas. On the other hand, kinetic-size magnetic holes (KSMHs), previously called small-size magnetic holes, with a scale of the order of magnitude of or less than ρi have only been reported in the Earth's magnetospheric plasma sheet. In this study, we report such KSMHs in the magnetosheath whereby we use measurements from the Magnetospheric Multiscale mission, which provides three-dimensional (3-D) particle distribution measurements with a resolution much higher than previous missions. The MHs have been observed in a scale of 10-20 ρe (electron gyroradii) and lasted 0.1-0.3 s. Distinctive electron dynamics features are observed, while no substantial deviations in ion data are seen. It is found that at the 90 degrees pitch angle, the flux of electrons with energy 34-66 eV decreased, while for electrons of energy 109-1024 eV increased inside the MHs. We also find the electron flow vortex perpendicular to the magnetic field, a feature self-consistent with the magnetic depression. Moreover, the calculated current density is mainly contributed by the electron diamagnetic drift, and the electron vortex flow is the diamagnetic drift flow. The electron magnetohydrodynamics soliton is considered as a possible generation mechanism for the KSMHs with the scale size of 10-20 ρe.

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