A statistical study of plasma sheet electrons carrying auroral upward field-aligned currents measured by Time History of Events and Macroscale Interactions during Substorms (THEMIS)

Abstract

[1] We have statistically investigated the electron density ne,M and temperature Te,M in the near-Earth plasma sheet in terms of the magnetosphere-ionosphere coupling process, as measured by the electrostatic analyzer (ESA) on board the Time History of Events and Macroscale Interactions during Substorms (THEMIS-D) satellite from November 2007 to January 2010. To find out when and where an aurora can occur, either with or without electron acceleration, the thermal current j∥th and the conductivity K along the magnetic field line were also estimated from observations of the magnetospheric electrons with pitch angle information inside 12 RE. The thermal current, j∥th(∝ ne,M Te,M1/2), represents the upper limit of the field-aligned current that can be carried by magnetospheric electrons without a field-aligned potential difference. The conductivity, K(∝ ne,M Te,M−1/2), relates the upward field-aligned current, j∥, to the field-aligned potential difference, V∥, assuming adiabatic electron transport. The thermal current is estimated by two methods: (1) from the relation by using ne,M and Te,M and (2) from the total downward electron number flux. We find that in the dawnside inner magnetosphere, the thermal currents estimated by both methods are sufficient to carry typical region 2 upward field-aligned current. On the other hand, in the duskside outer magnetosphere, a field-aligned potential difference is necessary on the region 1 current since the estimated thermal current is smaller than the typical region 1 current. By using the relationship, j∥ = KV∥, where K is the conductivity estimated from Knight's relation and j∥ is the typical auroral current, we conclude that a field-aligned potential difference of V∥ = 2–5 kV is necessary on the duskside region 1 upward field-aligned current.