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)

摘要

We have statistically investigated the electron density ne,M and temperature T_(e,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)(∝ n_(e,M) T_(e,M)~(1/2) ), 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(∝ n_(e,M) T_(e,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 n_(e,M) and T_(e,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.