Multiscale Currents Observed by MMS in the Flow Braking Region.

Area of Science:

• Space Physics
• Plasma Physics
• Magnetospheric Physics

Background:

• Substorms involve dynamic changes in Earth's magnetosphere, including current systems and plasma flows.
• The near-Earth plasma sheet boundary is a critical region for substorm energy dissipation and particle acceleration.

Purpose of the Study:

• To characterize current layers in the off-equatorial near-Earth plasma sheet boundary during an intense substorm.
• To investigate the relationship between plasma flows, field-aligned currents, and Hall currents.
• To understand the role of these structures in the substorm current wedge and energy transfer.

Main Methods:

• Utilizing high time-resolution measurements from the Magnetospheric Multiscale (MMS) spacecraft.
• Analyzing data from four MMS spacecraft separated by approximately 50 km.
• Observing fast flow disturbances, field-aligned currents, and Hall currents in the southern hemisphere dusk sector.

Main Results:

• Observed fast flow disturbances (up to 500 km/s) and intense thin field-aligned current layers associated with Earthward streaming hot ions.
• Detected intense Hall current layers adjacent to field-aligned currents, comprising mixed ion components (hot unmagnetized, cold E×B drifting, and magnetized electrons).
• Identified near-Earth plasma jet diversion and Hall current layers at the reconnection jet boundary as sites of perpendicular current diversion, contributing to the observed field-aligned current pattern.

Conclusions:

• The multiscale structure of flow braking is preserved in the field-aligned currents in the off-equatorial plasma sheet.
• These structures are translated to the ionosphere, forming a part of the substorm field-aligned current system.
• Observations support simulation predictions of reconnection jets and their contribution to substorm dynamics.