| 1. |
- 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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| 2. |
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| 3. |
- 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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| 4. |
- De Spiegeleer, Alexandre, et al.
(författare)
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Low-frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail
- 2017
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Ingår i: Journal of Geophysical Research - Space Physics. - Washington : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 122:7, s. 7042-7056
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- De Spiegeleer, Alexandre, et al.
(författare)
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Oscillatory Flows in the Magnetotail Plasma Sheet : Cluster Observations of the Distribution Function
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 124:4, s. 2736-2754
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- De Spiegeleer, Alexandre, et al.
(författare)
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Oxygen Ion Flow Reversals in Earth's Magnetotail : A Cluster Statistical Study
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - : Wiley-Blackwell. - 2169-9380 .- 2169-9402. ; 124:11, s. 8928-8942
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- 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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| 8. |
- Hamrin, Maria, 1972-, et al.
(författare)
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Can Reconnection be Triggered as a Solar Wind Directional Discontinuity Crosses the Bow Shock? : A Case of Asymmetric Reconnection
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 124:11, s. 8507-8523
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Pitkänen, Timo, 1979-, et al.
(författare)
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Asymmetry in the Earth's magnetotail neutral sheet rotation due to IMF B-y sign?
- 2021
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Ingår i: Geoscience Letters. - : Springer Nature. - 2196-4092. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Evidence suggests that a non-zero dawn-dusk interplanetary magnetic field (IMF B-y) can cause a rotation of the cross-tail current sheet/neutral sheet around its axis aligned with the Sun-Earth line in Earth's magnetotail. We use Geotail, THEMIS and Cluster data to statistically investigate how the rotation of the neutral sheet depends on the sign and magnitude of IMF B-y. In our dataset, we find that in the tail range of -30 < XGSM < -15 R-E, the degree of the neutral sheet rotation is clearly smaller, there appears no significant rotation or even, the rotation is clearly to an unexpected direction for negative IMF B-y, compared to positive IMF B-y. Comparison to a model by Tsyganenko et al. (2015, doi:10.5194/angeo-33-1-2015) suggests that this asymmetry in the neutral sheet rotation between positive and negative IMF B-y conditions is too large to be explained only by the currently known factors. The possible cause of the asymmetry remains unclear.
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| 10. |
- Pitkänen, Timo, 1979-, et al.
(författare)
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Convection electric field and plasma convection in a twisted magnetotail : t THEMIS case study 1-2 January 2009
- 2018
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 123:9, s. 7486-7497
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 11. |
- Pitkänen, Timo, 1979-, et al.
(författare)
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IMF By Influence on Magnetospheric Convection in Earth's Magnetotail Plasma Sheet
- 2019
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Ingår i: Geophysical Research Letters. - : Blackwell Publishing Ltd. - 0094-8276 .- 1944-8007. ; 46:21, s. 11698-11708
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Tidskriftsartikel (refereegranskat)abstract
- We use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy<200 km/s) in the magnetotail plasma sheet under the influence of a clearly nonzero dawn-dusk interplanetary magnetic field (IMF By). We find that IMF By causes an interhemispheric asymmetry in the flows, which depends on the direction of IMF By. On the average, one magnetic hemisphere is dominated by a dawn-dusk flow component, which is oppositely directed compared to that in the other hemisphere. This asymmetry is observed for both earthward and tailward flows. A comparison to tail By reveals that the region where the asymmetry in the average flows appears agrees with the appearance of the tail By direction collinear to IMF By. The results imply that IMF By has a major influence on the direction of the magnetic flux transport in the magnetotail.
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| 12. |
- Schillings, Audrey, et al.
(författare)
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The fate of O+ ions observed in the plasma mantle: particle tracing modelling and cluster observations
- 2020
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Ingår i: Annales Geophysicae. - : Copernicus Publications. - 0992-7689 .- 1432-0576. ; 38:3, s. 645-656
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Tidskriftsartikel (refereegranskat)abstract
- Ion escape is of particular interest for studying the evolution of the atmosphere on geological timescales. Previously, using Cluster-CODIF data, we investigated the oxygen ion outflow from the plasma mantle for different solar wind conditions and geomagnetic activity. We found significant correlations between solar wind parameters, geomagnetic activity (Kp index), and the O+ outflow. From these studies, we suggested that O+ ions observed in the plasma mantle and cusp have enough energy and velocity to escape the magnetosphere and be lost into the solar wind or in the distant magnetotail. Thus, this study aims to investigate where the ions observed in the plasma mantle end up. In order to answer this question, we numerically calculate the trajectories of O+ ions using a tracing code to further test this assumption and determine the fate of the observed ions. Our code consists of a magnetic field model (Tsyganenko T96) and an ionospheric potential model (Weimer 2001) in which particles initiated in the plasma mantle region are launched and traced forward in time. We analysed 131 observations of plasma mantle events in Cluster data between 2001 and 2007, and for each event 200 O+ particles were launched with an initial thermal and parallel bulk velocity corresponding to the velocities observed by Cluster. After the tracing, we found that 98 % of the particles are lost into the solar wind or in the distant tail. Out of these 98 %, 20 % escape via the dayside magnetosphere.
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| 13. |
- Vaverka, Jakub, et al.
(författare)
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Comparison of Dust Impact and Solitary Wave Signatures Detected by Multiple Electric Field Antennas Onboard the MMS Spacecraft
- 2018
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 123:8, s. 6119-6129
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Vaverka, Jakub, et al.
(författare)
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Detection of EMPs generated by meteoroid impacts on the MMS spacecraft and problems with signal interpretation
- 2017
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Ingår i: 2017 XXXIInd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS). - : IEEE. - 9789082598704
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Konferensbidrag (refereegranskat)abstract
- Signatures of hypervelocity dust impacts detected by electric field instruments are still not completely understood. We have used the electric field instrument onboard one of the MMS spacecraft orbiting the Earth since 2015 to study various pulses in the measured electric field detected simultaneously by multiple antennas. This unique instrument allows a detailed investigation of registered waveforms. The preliminary results shown that the solitary waves can generate similar pulses as dust impacts and detected pulses can easily by misinterpreted when only one antenna is used.
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| 15. |
- Vaverka, Jakub, et al.
(författare)
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Detection of meteoroid hypervelocity impacts on the Cluster spacecraft : First results
- 2017
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 122:6, s. 6485-6494
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 16. |
- Vaverka, Jakub, et al.
(författare)
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Potential of Earth Orbiting Spacecraft Influenced by Meteoroid Hypervelocity Impacts
- 2017
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Ingår i: IEEE Transactions on Plasma Science. - 0093-3813 .- 1939-9375. ; 45:8, s. 2048-2055
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Tidskriftsartikel (refereegranskat)abstract
- Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth's magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth's magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.
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| 17. |
- Vaverka, Jakub, et al.
(författare)
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Spacecraft potential influenced by meteoroid hypervelocity impacts
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth's magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth's magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.
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| 18. |
- Yao, Shutao, et al.
(författare)
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Observations of kinetic-size magnetic holes in the magnetosheath
- 2017
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 122:2, s. 1990-2000
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic holes (MHs), with a scale much greater than ρi (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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| 19. |
- Yao, S. T., et al.
(författare)
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Propagation of small size magnetic holes in the magnetospheric plasma sheet
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : Wiley-Blackwell. - 2169-9380 .- 2169-9402. ; 121:6, s. 5510-5519
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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