Mapping Electrostatic Solitary Waves in Earth's Magnetotail with MMS

Electrostatic solitary waves (ESWs), fast-moving bipolar electric field structures lacking a magnetic field component, are observed by NASA's Magnetospheric Multiscale Mission (MMS) in multiple regions of the magnetosphere. These include the bow shock, magnetosheath, and magnetotail. ESWs observed in the near-Earth magnetotail (6-12 Re) are thought to be spatially and energetically associated with the braking of bursty bulk flows (BBFs), high-speed flows thought to originate from reconnection jets at the near earth neutral line. Bulk mapping of the ExB drift velocity with MMS' FIELDS instruments (Electric Double Probe and Fluxgate Magnetometer) shows strongly variable high-intensity BBF activity in the near magnetotail region (9-20 Re) with velocities ranging from 200-600 km/s, as well as slower and more random flows in the BBF braking region closer to Earth. MMS' Solitary Wave Detector (SWD), derived onboard from the V1/V2 spin-plane double probes, quantifies the occurrence and amplitude of ESWs throughout each spacecraft's orbit; analysis of burst (65 kS/s) parallel electric field data indicates that SWD identifies 70% of all ESWs encountered in the tail region. Simultaneous bulk mapping of ESW activity with SWD indicates an increase in the average frequency of both low- and medium-amplitude (0.5-3 mV/m and 12-50 mV/m) ESWs at the regions of transition from high-velocity, directed bulk flows to slow, turbulent activity in the near magnetosphere, suggesting that BBF activity is the main driver of ESW generation in the near-Earth magnetotail. An observed dawn-biased asymmetry in the ExB drift velocity is also seen in spatial maps of ESW counts.