| 1. |
- Beth, Arnaud, et al.
(författare)
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First investigation of the diamagnetic cavity boundary layer with a 1D3V PIC simulation
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 667
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Tidskriftsartikel (refereegranskat)abstract
- Context: Amongst the different features and boundaries encountered around comets, one remains of particular interest to the plasma community: the diamagnetic cavity. Crossed for the first time at 1P/Halley during the Giotto flyby in 1986 and later met more than 700 times by the ESA Rosetta spacecraft around Comet 67P/Churyumov-Gerasimenko, this region, almost free of any magnetic field, surrounds nuclei of active comets. However, previous observations and modelling of this part of the coma have not yet provided a definitive answer as to the origin of such a cavity and on its border, the diamagnetic cavity boundary layer.Aims: We investigate which forces and equilibrium might be at play and balance the magnetic pressure at this boundary down to the spatial and temporal scales of the electrons in the 1D collisionless case. In addition, we scrutinise assumptions made in magneto-hydrodynamic and hybrid simulations of this environment and check for their validity.Methods: We simulated this region at the electron scale by means of 1D3V particle-in-cell simulations and SMILEI code.Results: Across this layer, depending on the magnetic field strength, the electric field is governed by different equilibria, with a thin double-layer forming ahead. In addition, we show that the electron distribution function departs from Maxwellian and/or gyrotropic distributions and that electrons do not behave adiabatically. We demonstrate the need to investigate this region at the electron scale in depth with fully kinetic simulations.
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| 2. |
- Chong, Ghai Siung, et al.
(författare)
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Dawn-Dusk Ion Flow Asymmetry in the Plasma Sheet : Interplanetary Magnetic Field B-y Versus Distance With Respect to the Neutral Sheet
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:4
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies have shown that the average dawn-dusk component of the perpendicular plasma flow in the plasma sheet (V-perpendicular to) can vary depending on the distance relative to the neutral sheet and the dawn-dusk component of the interplanetary magnetic field (IMF B-y). In this study, we combined 33 years of data from the Geotail, Time History of Events and Macroscale Interactions during Substorms, Cluster, and magnetospheric multiscale missions to study the slow (<200 km/s) ion flows perpendicular to the magnetic field. We find that IMF B-y has a hemispheric dependent influence on both the tail B-y and tail V-perpendicular to. Particularly, the influence is more prominent in the midnight sector (compared to both the pre- and post-midnight sectors) and at distances far from the neutral sheet (compared to the distances close to the neutral sheet). However, at distances close to the neutral sheet, there is an increased dominance of duskward flows which dominates over the systematic influence of IMF B-y on tail V-perpendicular to. Our results indicate that IMF B-y has a major influence on the magnetic flux transport in the magnetotail, mainly at distances far from the neutral sheet. The influence is weaker at distances close to the neutral sheet.
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| 3. |
- Chong, Ghai Siung, et al.
(författare)
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Ion Convection as a Function of Distance to the Neutral Sheet in Earth's Magnetotail
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 126:12
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Tidskriftsartikel (refereegranskat)abstract
- We utilized 33 years of data obtained by the Geotail, THEMIS, Cluster and MMS missions to investigate the slow (<200 km/s) ion flows perpendicular to the magnetic field in Earth's magnetotail plasma sheet. By using plasma β as a proxy of distance to the neutral sheet, we find that the ion flow patterns vary systematically within the plasma sheet. Particularly, in regions farther from the neutral sheet, earthward (tailward) flows exhibit a strong tendency to diverge (converge) quasi-symmetrically, with respect to the midnight meridional plane. As the distance becomes closer toward the neutral sheet, this tendency to diverge and converge gradually weakens. Moreover, duskward flows become the dominant components in both the earthward and tailward flows. These variations in ion flow patterns with distance to neutral sheet are hemispherically independent. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.
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| 4. |
- Chong, Ghai Siung, et al.
(författare)
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Tailward Flows in the Vicinity of Fast Earthward Flows
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:4
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Tidskriftsartikel (refereegranskat)abstract
- The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs – Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ∼75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical-like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the “freshly” rebounded BBFs from the near-Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear-induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs.
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| 5. |
- De Spiegeleer, Alexandre, et al.
(författare)
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In Which Magnetotail Hemisphere is a Satellite? Problems Using in Situ Magnetic Field Data
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:2
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Tidskriftsartikel (refereegranskat)abstract
- In Earth's magnetotail plasma sheet, the sunward-tailward Bx component of the magnetic field is often used to separate the region above and below the cross-tail current sheet. Using a three-dimensional magneto-hydrodynamic simulation, we show that high-speed flows do not only affect the north-south magnetic field component (causing dipolarization fronts), but also the sunward-tailward component via the formation of a magnetic dent. This dent is such that, in the Northern Hemisphere, the magnetic field is tailward while in the Southern Hemisphere, it is earthward. This is opposite to the expected signatures where Bx > 0 (Bx < 0) above (below) the neutral sheet. Therefore, the direction of the magnetic field cannot always be used to identify in which hemisphere an in situ spacecraft is located. In addition, the cross-tail currents associated with the dent is different from the currents in a tail without a dent. From the simulation, we suggest that the observation of a dawnward current and a tailward magnetic tension force, possibly together with an increase in the plasma beta, may indicate the presence of a magnetic dent. To exemplify, we also present data of a high-speed flow observed by the Cluster mission, and we show that the changing sign of Bx is likely due to such a dent, and not to the spacecraft moving across the neutral sheet.
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| 6. |
- De Spiegeleer, Alexandre, 1990-
(författare)
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There and back again... An Earth magneto-tale : understanding plasma flows in the magnetotail
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- On average, the Earth's magnetotail plasma sheet seems to be a calm region of the magnetosphere where the plasma moves slowly towards Earth. However, the plasma sheet actually hosts many phenomena, some of which can affect Earth. For example, high-speed flows of plasma with speed larger than 400 km/s are observed in the plasma sheet and they can lead to aurorae. Such dynamical phenomena and the impact they may have on Earth naturally makes the plasma sheet an important region of study. Even though these high-speed flows can affect Earth, they are observed less than 5% of the time, meaning that most of the time, other disturbances take place in the plasma sheet. Our aim is to investigate and better understand the plasma dynamics in the plasma sheet.The plasma above and below the cross-tail current sheet was previously thought to convect in the same direction. However, we find that under clearly non-zero Interplanetary Magnetic Field (IMF)By (dawn-dusk component), the plasma has a tendency to convect in opposite dawn-dusk direction across the current sheet near the midnight sector depending on the sign of IMF By.The high-speed plasma flows are known to be associated with an increase of the northward component of the magnetic field as they propagate toward Earth. Using simulations, we notice that the magnetic field lines are bent by the high-speed flows and dents can appear. The deformation of the magnetic field is such that it may be directed towards the tail above the cross-tail current sheet and towards the Earth below it. This is opposite to the expected orientation of the magnetic field thus making this feature important in order to properly identify the region in which a spacecraft is located.At times, the plasma can be seen to move back and forth in an oscillatory manner. We investigate statistically such oscillatory behaviour and compare them to high-speed flows and to time periods when the plasma is calm. These oscillatory flows are observed about 8% of the time in the plasma sheet. They typically have a frequency of about 1.7 mHz (~10 min period) and usually last about 41 min.Some oscillations of the plasma velocity are observed along the magnetic field. The particles measured by the satellite initially have a velocity parallel to the magnetic field and towards Earth. Gradually with time, the measured velocity of the particles turns around to first become more perpendicular to the magnetic field and then be along the magnetic field but away from Earth. These signatures are interpreted simply as being due to mirroring particles injected tailward of the satellite and move toward Earth. The particles are then reflected, and move away from Earth. We investigate the general features of such oscillations along the magnetic field and find that the source of the particles is typically located less than 25 RE (Earth's radii) tailward of the satellite.
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| 7. |
- Dredger, Pauline M., et al.
(författare)
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A case study in support of closure of bow shock current through the ionosphere utilizing multi-point observations and simulation
- 2023
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Ingår i: Frontiers in Astronomy and Space Sciences. - : Frontiers Media S.A.. - 2296-987X. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- On the bow shock in front of Earth's magnetosphere flows a current due to the curl of the interplanetary magnetic field across the shock. The closure of this current remains uncertain; it is unknown whether the bow shock current closes with the Chapman-Ferraro current system on the magnetopause, along magnetic field lines into the ionosphere, through the magnetosheath, or some combination thereof. We present simultaneous observations from Magnetosphere Multiscale (MMS), AMPERE, and Defense Meteorological Satellite Program (DMSP) during a period of strong By, weakly negative Bz, and very small Bx. This IMF orientation should lead to a bow shock current flowing mostly south to north on the shock. AMPERE shows a current poleward of the Region 1 and Region 2 Birkeland currents flowing into the northern polar cap and out of the south, the correct polarity for bow shock current to be closing along open field lines. A southern Defense Meteorological Satellite Program F18 flyover confirms that this current is poleward of the convection reversal boundary. Additionally, we investigate the bow shock current closure for the above-mentioned solar wind conditions using an MHD simulation of the event. We compare the magnitude of the modeled bow shock current due to the IMF By component to the magnitude of the modeled high-latitude current that corresponds to the real current observed in AMPERE and by Defense Meteorological Satellite Program. In the simulation, the current poleward of the Region 1 currents is about 37% as large as the bow shock Iz in the northern ionosphere and 60% in the south. We conclude that the evidence points to at least a partial closure of the bow shock current through the ionosphere.
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| 8. |
- Fatemi, Shahab, et al.
(författare)
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Ion Dynamics at the Magnetopause of Ganymede
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 127:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the dynamics of the thermal O+ and H+ ions at Ganymede's magnetopause when Ganymede is inside and outside of the Jovian plasma sheet using a three-dimensional hybrid model of plasma (kinetic ions, fluid electrons). We present the global structure of the electric fields and power density (E ⋅ J) in the magnetosphere of Ganymede and show that the power density at the magnetopause is mainly positive and on average is +0.95 and +0.75 nW/m3 when Ganymede is inside and outside the Jovian plasma sheet, respectively, but locally it reaches over +20 nW/m3. Our kinetic simulations show that ion velocity distributions at the vicinity of the upstream magnetopause of Ganymede are highly non-Maxwellian. We investigate the energization of the ions interacting with the magnetopause and find that the energy of those particles on average increases by a factor of 8 and 30 for the O+ and H+ ions, respectively. The energy of these ions is mostly within 1–100 keV for both species after interaction with the magnetopause, but a few percentages reach to 0.1–1 MeV. Our kinetic simulations show that a small fraction ((Formula presented.) 25%) of the corotating Jovian plasma reach the magnetopause, but among those >50% cross the high-power density regions at the magnetopause and gain energy. Finally, we compare our simulation results with Galileo observations of Ganymede's magnetopause crossings (i.e., G8 and G28 flybys). There is an excellent agreement between our simulations and observations, particularly our simulations fully capture the size and structure of the magnetosphere.
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| 9. |
- Goncharov, Oleksandr, et al.
(författare)
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Evolution of High-Speed Jets and Plasmoids Downstream of the Quasi-Perpendicular Bow Shock
- 2020
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 125:6
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Tidskriftsartikel (refereegranskat)abstract
- Plasma structures with enhanced dynamic pressure, density, or speed are often observed in Earth's magnetosheath. We present a statistical study of these structures, known as jets and fast plasmoids, in the magnetosheath, downstream of both the quasi-perpendicular and quasi-parallel bow shocks. Using measurements from the four Magnetospheric Multiscale (MMS) spacecraft and OMNI solar wind data from 2015-2017, we present observations of jets during different upstream conditions and in the wide range of distances from the bow shock. Jets observed downstream of the quasi-parallel bow shock are seen to propagate deeper and faster into the magnetosheath and on toward the magnetopause. We estimate the shape of the structures by treating the leading edge as a shock surface, and the result is that the jets are elongated in the direction of propagation but also that they expand more quickly in the perpendicular direction as they propagate through the magnetosheath.
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| 10. |
- Gunell, Herbert, et al.
(författare)
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Ion acoustic waves near a comet nucleus : Rosetta observations at comet 67P/Churyumov-Gerasimenko
- 2021
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 39:1, s. 53-68
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Tidskriftsartikel (refereegranskat)abstract
- Ion acoustic waves were observed between 15 and 30 km from the centre of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft during its close flyby on 28 March 2015. There are two electron populations: one cold at k(B)T(e) approximate to 0.2 eV and one warm at k(B)T(e) approximate to 2 eV. The ions are dominated by a cold (a few hundredths of electronvolt) distribution of water group ions with a bulk speed of (3-3.7) km s(-1). A warm k(B)T(e) approximate to 6 eV ion population, which also is present, has no influence on the ion acoustic waves due to its low density of only 0.25 % of the plasma density. Near closest approach the propagation direction was within 50 degrees from the direction of the bulk velocity. The waves, which in the plasma frame appear below the ion plasma frequency f(pi) approximate to 2 kHz, are Doppler-shifted to the spacecraft frame where they cover a frequency range up to approximately 4 kHz. The waves are detected in a region of space where the magnetic field is piled up and draped around the inner part of the ionised coma. Estimates of the current associated with the magnetic field gradient as observed by Rosetta are used as input to calculations of dispersion relations for current-driven ion acoustic waves, using kinetic theory. Agreement between theory and observations is obtained for electron and ion distributions with the properties described above. The wave power decreases over cometocentric distances from 24 to 30 km. The main difference between the plasma at closest approach and in the region where the waves are decaying is the absence of a significant current in the latter. Wave observations and theory combined supplement the particle measurements that are difficult at low energies and complicated by spacecraft charging.
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