| 1. |
- Agapitov, Oleksiy, et al.
(författare)
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Statistics of whistler mode waves in the outer radiation belt : Cluster STAFF-SA measurements
- 2013
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Ingår i: Journal of Geophysical Research-Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 118:6, s. 3407-3420
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Tidskriftsartikel (refereegranskat)abstract
- ELF/VLF waves play a crucial role in the dynamics of the radiation belts and are partly responsible for the main losses and the acceleration of energetic electrons. Modeling wave-particle interactions requires detailed information of wave amplitudes and wave normal distribution over L-shells and over magnetic latitudes for different geomagnetic activity conditions. We performed a statistical study of ELF/VLF emissions using wave measurements in the whistler frequency range for 10years (2001-2010) aboard Cluster spacecraft. We utilized data from the STAFF-SA experiment, which spans the frequency range from 8Hz to 4kHz. We present distributions of wave magnetic and electric field amplitudes and wave normal directions as functions of magnetic latitude, magnetic local time, L-shell, and geomagnetic activity. We show that wave normals are directed approximately along the background magnetic field (with the mean value of the angle between the wave normal and the background magnetic field, about 10 degrees-15 degrees) in the vicinity of the geomagnetic equator. The distribution changes with magnetic latitude: Plasmaspheric hiss normal angles increase with latitude to quasi-perpendicular direction at approximate to 35 degrees-40 degrees where hiss can be reflected; lower band chorus are observed as two wave populations: One population of wave normals tends toward the resonance cone and at latitudes of around 35 degrees-45 degrees wave normals become nearly perpendicular to the magnetic field; the other part remains quasi-parallel at latitudes up to 30 degrees. The observed angular distribution is significantly different from Gaussian, and the width of the distribution increases with latitude. Due to the rapid increase of , the wave mode becomes quasi-electrostatic, and the corresponding electric field increases with latitude and has a maximum near 30 degrees. The magnetic field amplitude of the chorus in the day sector has a minimum at the magnetic equator but increases rapidly with latitude with a local maximum near 12 degrees-15 degrees. The wave magnetic field maximum is observed in the night sector at L>7 during low geomagnetic activity (at L approximate to 5 for K-p>3). Our results confirm the strong dependence of wave amplitude on geomagnetic activity found in earlier studies.
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| 2. |
- Kamaletdinov, Sergey R., et al.
(författare)
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Spacecraft Observations and Theoretical Understanding of Slow Electron Holes
- 2021
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 127:16
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Tidskriftsartikel (refereegranskat)abstract
- We present Magnetospheric Multiscale observations showing large numbers of slow electron holes with speeds clustered near the local minimum of double-humped velocity distribution functions of background ions. Theoretical computations show that slow electron holes can avoid the acceleration that otherwise prevents their remaining slow only under these same circumstances. Although the origin of the slow electron holes is still elusive, the agreement between observation and theory about the conditions for their existence is remarkable.
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| 3. |
- Nakamura, Rumi, et al.
(författare)
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Multiscale Currents Observed by MMS in the Flow Braking Region
- 2018
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Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 123:2, s. 1260-1278
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Tidskriftsartikel (refereegranskat)abstract
- We present characteristics of current layers in the off-equatorial near-Earth plasma sheet boundary observed with high time-resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn-dusk direction. Field-aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field-aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field-aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold ExB drifting ions, and magnetized electrons. Our observations show that both the near-Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field-aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field-aligned currents in the off-equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field-aligned current system.
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| 4. |
- Nakamura, Rumi, et al.
(författare)
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Near-Earth plasma sheet boundary dynamics during substorm dipolarization
- 2017
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Ingår i: Earth Planets and Space. - : Springer Berlin/Heidelberg. - 1343-8832 .- 1880-5981. ; 69
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Tidskriftsartikel (refereegranskat)abstract
- We report on the large-scale evolution of dipolarization in the near-Earth plasma sheet during an intense (AL similar to -1000 nT) substorm on August 10, 2016, when multiple spacecraft at radial distances between 4 and 15 RE were present in the night-side magnetosphere. This global dipolarization consisted of multiple short-timescale (a couple of minutes) Bz disturbances detected by spacecraft distributed over 9 MLT, consistent with the large-scale substorm current wedge observed by ground-based magnetometers. The four spacecraft of the Magnetospheric Multiscale were located in the southern hemisphere plasma sheet and observed fast flow disturbances associated with this dipolarization. The high-time-resolution measurements from MMS enable us to detect the rapid motion of the field structures and flow disturbances separately. A distinct pattern of the flow and field disturbance near the plasma boundaries was found. We suggest that a vortex motion created around the localized flows resulted in another fieldaligned current system at the off-equatorial side of the BBF-associated R1/R2 systems, as was predicted by the MHD simulation of a localized reconnection jet. The observations by GOES and Geotail, which were located in the opposite hemisphere and local time, support this view. We demonstrate that the processes of both Earthward flow braking and of accumulated magnetic flux evolving tailward also control the dynamics in the boundary region of the near-Earth plasma sheet.
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| 5. |
- Takahashi, Kazue, et al.
(författare)
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Propagation of Ultralow-Frequency Waves from the Ion Foreshock into the Magnetosphere During the Passage of a Magnetic Cloud
- 2021
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 126:2
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Tidskriftsartikel (refereegranskat)abstract
- We have examined the properties of ultralow-frequency (ULF) waves in space (the ion foreshock, magnetosheath, and magnetosphere) and at dayside magnetometer stations (L = 1.6-6.5) during Earth's encounter with a magnetic cloud in the solar wind, which is characterized by magnetic fields with large magnitudes (similar to 14 nT) and small cone angles (similar to 30 degrees). In the foreshock, waves were excited at similar to 90 m Hz as expected from theory, but there were oscillations at other frequencies as well. Oscillations near 90 mHz were detected at the other locations in space, but they were not in general the most dominant oscillations. On the ground, pulsations in the approximate Pc2-Pc4 band (5 mHz-120 mHz) were continuously detected at all stations, with no outstanding spectral peaks near 90 mHz in the H component except at stations where the frequency of the third harmonic of standing Alfven waves had this frequency. The fundamental toroidal wave frequency was below 90 mHz at all stations. In the D component spectra, a minor spectral peak is found near 90 mHz at stations located at L < 3, and the power dropped abruptly above this frequency. Magnetospheric compressional wave power was much weaker on the nightside. A hybrid-Vlasov simulation indicates that foreshock ULF waves have short spatial scale lengths and waves transmitted into the magnetosphere are strongly attenuated away from noon.
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| 6. |
- Zhou, Xu-Zhi, et al.
(författare)
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On the Origin of Perpendicular Ion Anisotropy Inside Dipolarizing Flux Bundles
- 2019
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 124:6, s. 4009-4021
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Tidskriftsartikel (refereegranskat)abstract
- Perpendicular anisotropy of suprathermal ions, observed inside some of the dipolarizing flux bundles (DFBs) in the magnetotail plasma sheet, have been attributed to successive, betatron-type accelerations during the DFB entry of ambient ions. It has been unclear, however, where and how these ions enter the DFBs. The proposed locations include the DFB flanks where cross-tail drifting ions are picked up, and the DFB leading edge with sharp magnetic field gradient (the dipolarization front, DF). Here we examine the latter scenario, based on a simplistic, test particle approach, to predict the preferred conditions for the appearance of the DFB ion anisotropy. Our model predicts that the ion anisotropy would be stronger at locations closer to the neutral sheet and would appear preferentially in the DFB dawnside and central sectors rather than the duskside sector. We also predict that the ion anisotropy would more likely be observed in DFBs with higher propagation speeds. These properties can be understood in our model by the dawnward drift of ions during their DF penetration (attributed to the large magnetic gradient). To examine these predictions, we carry out a statistical survey based on observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission, to show a clear dependence of the ion anisotropy on spacecraft location and the DFB propagation speed. These findings, therefore, are consistent with the scenario that the perpendicular ion anisotropy originates from the ion acceleration and penetration across sharp DFs.
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