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Publications (10 of 18) Show all publications
Echim, M. M., Lamy, H., De Keyser, J., Maggiolo, R., Gunell, H. & Wedlund, C. L. (2019). A Method to Estimate the Physical Properties of Magnetospheric Generators From Observations of Quiet Discrete Auroral Arcs. Journal of Geophysical Research - Space Physics, 124(12), 10283-10293
Open this publication in new window or tab >>A Method to Estimate the Physical Properties of Magnetospheric Generators From Observations of Quiet Discrete Auroral Arcs
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 12, p. 10283-10293Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2019
Keywords
quiet auroral arcs, magnetosphere-ionosphere coupling, magnetospheric plasma interfaces, field-aligned currents, Vlasov equilibrium
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-167089 (URN)10.1029/2019JA026969 (DOI)000504543100001 ()
Available from: 2020-01-09 Created: 2020-01-09 Last updated: 2020-03-18Bibliographically approved
Hamrin, M., Gunell, H., Goncharov, O., De Spiegeleer, A., Fuselier, S., Mukherjee, J., . . . Giles, B. (2019). Can Reconnection be Triggered as a Solar Wind Directional Discontinuity Crosses the Bow Shock?: A Case of Asymmetric Reconnection. Journal of Geophysical Research - Space Physics, 124(11), 8507-8523
Open this publication in new window or tab >>Can Reconnection be Triggered as a Solar Wind Directional Discontinuity Crosses the Bow Shock?: A Case of Asymmetric Reconnection
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 11, p. 8507-8523Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2019
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-168193 (URN)10.1029/2019JA027006 (DOI)000505404700019 ()
Available from: 2020-03-10 Created: 2020-03-10 Last updated: 2020-03-10Bibliographically approved
Maggiolo, R., Gibbons, A., Cessateur, G., De Keyser, J., Dhooghe, F., Gunell, H., . . . Vaeck, N. (2019). Effect of the Surface Roughness of Icy Grains on Molecular Oxygen Chemistry in Molecular Clouds. Astrophysical Journal, 882(2), Article ID 131.
Open this publication in new window or tab >>Effect of the Surface Roughness of Icy Grains on Molecular Oxygen Chemistry in Molecular Clouds
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2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 882, no 2, article id 131Article in journal (Refereed) Published
Abstract [en]

Molecular cloud and protosolar nebula chemistry involves a strong interaction between the gas phase and the surface of icy grains. The exchanges between the gas phase and the solid phase depend not only on the adsorption and desorption rates but also on the geometry of the surface of the grains. Indeed, for sufficient levels of surface roughness, atoms and molecules have a significant probability to collide with the grain icy mantle several times before being potentially captured. In consequence, their net sticking probability may differ from their sticking probability for a single collision with the grain surface. We estimate the effectiveness of the recapture on uneven surfaces for the various desorption processes at play in astrophysical environments. We show that surface roughness has a significant effect on the desorption rates. We focus in particular on the production of O-2 since unexpectedly large amounts of it, probably incorporated in the comet when it formed, have been detected in the coma of comet 67P by the Rosetta probe. Our results suggest that the higher escape probability of hydrogen compared to heavier species on rough surfaces can contribute to enhancing the production of O-2 in the icy mantles of grains while keeping its abundance low in the gas phase and may significantly decrease the desorption probability of molecules involved in the O-2 chemical network.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019
Keywords
astrochemistry, comets: general, comets: individual (67P/Churyumov-Gerasimenko), ISM: abundances, ISM: clouds, ISM: molecules
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-164410 (URN)10.3847/1538-4357/ab3400 (DOI)000485693400011 ()
Available from: 2019-10-22 Created: 2019-10-22 Last updated: 2019-10-22Bibliographically approved
De Spiegeleer, A., Hamrin, M., Gunell, H., Volwerk, M., Andersson, L., Karlsson, T., . . . Kistler, L. M. (2019). Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function. Journal of Geophysical Research - Space Physics, 124(4), 2736-2754
Open this publication in new window or tab >>Oscillatory Flows in the Magnetotail Plasma Sheet: Cluster Observations of the Distribution Function
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 4, p. 2736-2754Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2019
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-162349 (URN)10.1029/2018JA026116 (DOI)000477707800027 ()
Available from: 2019-08-27 Created: 2019-08-27 Last updated: 2020-05-04Bibliographically approved
De Spiegeleer, A., Hamrin, M., Volwerk, M., Karlsson, T., Gunell, H., Chong, G. S., . . . Nilsson, H. (2019). Oxygen Ion Flow Reversals in Earth's Magnetotail: A Cluster Statistical Study. Journal of Geophysical Research - Space Physics, 124(11), 8928-8942
Open this publication in new window or tab >>Oxygen Ion Flow Reversals in Earth's Magnetotail: A Cluster Statistical Study
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2019 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 11, p. 8928-8942Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2019
Keywords
Earth's magnetotail, oxygen ions, velocity reversal, pitch angle distribution, Cluster, field aligned
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-166575 (URN)10.1029/2019JA027054 (DOI)000497604100001 ()
Available from: 2019-12-19 Created: 2019-12-19 Last updated: 2020-05-04Bibliographically approved
Gunell, H., Lindkvist, J., Goetz, C., Nilsson, H. & Hamrin, M. (2019). Polarisation of a small-scale cometary plasma environment: Particle-in-cell modelling of comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics, 631, Article ID A174.
Open this publication in new window or tab >>Polarisation of a small-scale cometary plasma environment: Particle-in-cell modelling of comet 67P/Churyumov-Gerasimenko
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 631, article id A174Article in journal (Refereed) Published
Abstract [en]

Context: The plasma near the nucleus of a comet is subjected to an electric field to which a few different sources contribute: the convective electric field of the solar wind, the ambipolar electric field due to higher electron than ion speeds, and a polarisation field arising from the vastly different ion and electron trajectories.

Aims: Our aim is to show how the ambipolar and polarisation electric fields arise and develop under the influence of space charge effects, and in doing so we paint a qualitative picture of the electric fields in the inner coma of a comet.

Methods. We use an electrostatic particle-in-cell model to simulate a scaled-down comet, representing comet 67P/Churyumov-Gerasimenko with parameters corresponding to a 3.0 AU heliocentric distance.

Results: We find that an ambipolar electric field develops early in the simulation and that this is soon followed by the emergence of a polarisation electric field, manifesting itself as an anti-sunward component prevalent in the region surrounding the centre of the comet. As plasma is removed from the inner coma in the direction of the convectional electric field of the solar wind, a density maximum develops on the opposite side of the centre of the comet.

Conclusions: The ambipolar and polarisation electric fields both have a significant influence on the motion of cometary ions. This demonstrates the importance of space charge effects in comet plasma physics.

Place, publisher, year, edition, pages
EDP Sciences, 2019
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-167865 (URN)10.1051/0004-6361/201936004 (DOI)000515103500003 ()
Funder
Swedish National Space Board, 201/15Swedish National Space Board, 108/18
Available from: 2020-02-05 Created: 2020-02-05 Last updated: 2020-03-19Bibliographically approved
Wedlund, C. S., Bodewits, D., Alho, M., Hoekstra, R., Behar, E., Gronoff, G., . . . Beth, A. (2019). Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O. Astronomy and Astrophysics, 630, Article ID A35.
Open this publication in new window or tab >>Solar wind charge exchange in cometary atmospheres I. Charge-changing and ionization cross sections for He and H particles in H2O
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A35Article in journal (Refereed) Published
Abstract [en]

Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. Aims. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Methods. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds (<20 keV u-1/2000 km s-1). Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 105 to 106 K (12-130 eV). Conclusions. Single- and double-electron captures by He2+ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H+ and single-electron loss by H- dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above 20 keV and are expected to play a moderate role in the total ion production. However, the effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
comets: general, comets: individual: 67P/Churyumov-Gerasimenko, instrumentation: detectors, solar wind, methods: data analysis, plasmas
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-166463 (URN)10.1051/0004-6361/201834848 (DOI)000486989400034 ()
Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2019-12-17Bibliographically approved
Wedlund, C. S., Behar, E., Kallio, E., Nilsson, H., Alho, M., Gunell, H., . . . Hoekstra, R. (2019). Solar wind charge exchange in cometary atmospheres II. Analytical model. Astronomy and Astrophysics, 630, Article ID A36.
Open this publication in new window or tab >>Solar wind charge exchange in cometary atmospheres II. Analytical model
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A36Article in journal (Refereed) Published
Abstract [en]

Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet because they mass-load the solar wind through an effective conversion of fast, light solar wind ions into slow, heavy cometary ions. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided a unique opportunity to study charge-changing processes in situ. Aims. To understand the role of charge-changing reactions in the evolution of the solar wind plasma and to interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary. Methods. An extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines is presented. It is based on a thorough book-keeping of available charge-changing cross sections of hydrogen and helium particles in a water gas. Results. After presenting a general 1D solution of charge exchange at comets, we study the theoretical dependence of charge-state distributions of (He2+, He+, He-0) and (H+, H-0, H-) on solar wind parameters at comet 67P. We show that double charge exchange for the He2+-H2O system plays an important role below a solar wind bulk speed of 200 km s-1, resulting in the production of He energetic neutral atoms, whereas stripping reactions can in general be neglected. Retrievals of outgassing rates and solar wind upstream fluxes from local Rosetta measurements deep in the coma are discussed. Solar wind ion temperature effects at 400 km s-1 solar wind speed are well contained during the Rosetta mission. Conclusions. As the comet approaches perihelion, the model predicts a sharp decrease of solar wind ion fluxes by almost one order of magnitude at the location of Rosetta, forming in effect a solar wind ion cavity. This study is the second part of a series of three on solar wind charge-exchange and ionization processes at comets, with a specific application to comet 67P and the Rosetta mission.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
comets: general, comets: individual: 67P/Churyumov-Gerasimenko, instrumentation: detectors, waves, solar wind, methods: analytical
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:umu:diva-166462 (URN)10.1051/0004-6361/201834874 (DOI)000486989400035 ()
Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2019-12-17Bibliographically approved
Wedlund, C. S., Behar, E., Nilsson, H., Alho, M., Kallio, E., Gunell, H., . . . Hoekstra, R. (2019). Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics, 630, Article ID A37.
Open this publication in new window or tab >>Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko
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2019 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 630, article id A37Article in journal (Refereed) Published
Abstract [en]

Context: Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ.

Aims: To understand the role of these reactions in the evolution of the solar wind plasma and interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary.

Methods: We used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyser (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas.

Results: A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms (ENAs), with fluxes expected to rival those of H+ and He2+ ions.

Conclusions: Neutral outgassing rates are retrieved from local RPC-ICA flux measurements and match ROSINA estimates very well throughout the mission. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. This work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.

Place, publisher, year, edition, pages
EDP Sciences, 2019
Keywords
comets: general, comets: individual: 67P/Churyumov-Gerasimenko, instrumentation: detectors, solar wind, methods: analytical, plasmas
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-167009 (URN)10.1051/0004-6361/201834881 (DOI)000486989400036 ()
Available from: 2020-01-08 Created: 2020-01-08 Last updated: 2020-01-08Bibliographically approved
Hamrin, M., Gunell, H., Lindkvist, J., Lindqvist, P.-A., Ergun, R. E. & Giles, B. L. (2018). Bow shock generator current systems: MMS observations of possible current closure. Journal of Geophysical Research - Space Physics, 123, 242-258
Open this publication in new window or tab >>Bow shock generator current systems: MMS observations of possible current closure
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, p. 242-258Article in journal (Refereed) Published
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.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:umu:diva-143055 (URN)10.1002/2017JA024826 (DOI)000425637600018 ()
Available from: 2017-12-14 Created: 2017-12-14 Last updated: 2018-06-09Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0001-5379-1158

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