Lower Hybrid Drift Waves and Electromagnetic Electron Space‐Phase Holes Associated With Dipolarization Fronts and Field‐Aligned Currents Observed by the Magnetospheric Multiscale Mission During a Substorm

Abstract

We analyze two ion scale dipolarization fronts associated with field‐aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on 10 August 2016. The first event corresponds to a fast dawnward flow with an antiparallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing and with a smaller lower hybrid drift wave activity. Electromagnetic electron phase‐space holes are detected near these low‐frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet we cannot rule out the possibility that the drift waves are produced by the antiparallel current associated with the fast flows, leaving the source for the electron holes unexplained.

Simultaneous Multispacecraft Probing of Electron Phase Space Holes

“…We have presented observations of EHs observed simultaneously at four MMS spacecraft in a region with intense parallel currents and fast plasma flows in the plasma sheet boundary layer. Similar EHs associated with intense parallel currents in the plasma sheet boundary layer have been recently reported by Le Contel et al (), but the spacecraft separation of about 50 km did not allow simultaneous observations of the EHs at four MMS spacecraft. We have used multispacecraft interferometry to estimate the EH velocities and direction of propagation.…”

“…For several of the EHs we have demonstrated that their 3‐D configuration is adequately described by Gaussian distributions with elliptic cross section and two perpendicular scales that can differ by as much as a factor of 3. Thus, in contrast to the assumption often invoked in single‐spacecraft studies (e.g., Chen et al, ; Le Contel et al, ; Vasko et al, ), the EHs are generally not axially symmetric. EHs exhibit d max / d ∥ ∼2–3 and d min / d ∥ ∼1–2 that is in satisfactory agreement with the gyrokinetic scaling relation (Franz et al, ) predicting d ⊥ / d ∥ ∼1.4–2.2 for ω p ∼ 1–2 ω c .…”

Multiscale Currents Observed by MMS in the Flow Braking Region

“…In the presence of high‐frequency wave activity, our averaged data processed beyond ion‐gyro time scales may smear out the responsible electric fields and may cause differences between the E × B drift and the electron motion in addition to the possible contribution from electron‐scale processes. Such wave activity was, however, weaker for event (ii) (Le Contel et al, ). The overall agreement between the electric‐field drift and electron perpendicular velocity shown in Figure , and the agreement between the curlometer current and the particle current during the intense dawn‐dusk current events indicated in Figure , suggested that the overall processes of these intense current events can be well described with Hall physics once different ion components are taken into account.…”

“…While we have so far concentrated on processes with ion scales or larger, the above description may need some modification once shorter time variations are taken into account. For example, the first type of Hall current described above contains highly structured electron currents due to interactions with lower‐hybrid waves, which was obtained based on the high‐resolutions particle and field data (Le Contel et al, ). In the presence of high‐frequency wave activity, our averaged data processed beyond ion‐gyro time scales may smear out the responsible electric fields and may cause differences between the E × B drift and the electron motion in addition to the possible contribution from electron‐scale processes.…”

“…Note that the relative thickness to the ion gyro radius may be smaller for events (iii) and (iv) due to possible underestimation of ion thermal velocity during these times. Hence, at least for events (i) and (ii), the overall pattern of the intense dawnward flow and reversal to duskward flows and associated FACs are expected to be a large‐scale process, although some ion‐scale substructures are embedded in these events (see Le Contel et al, for detail). Based on these flow and field patterns at MMS, observations from ground and other spacecraft, and comparisons with an MHD simulation, NAK17 suggested that events (i) and (ii) detected at MMS are most likely due to the crossing of the high‐latitude side of the current wedge produced in the flow‐braking region.…”