THE EVOLUTION OF FIELD-ALIGNED CURRENTS AS A FUNCTION OF SUBSTROM PHASE

ISEE 1 and 2 measurements of magnetic changes across nightside field-aligned currents at mid altitudes in the inner magnetosphere are examined to determine the average ionospheric extrapolated characteristics and properties of region 1 and 2 currents as a function of substorm phase. The properties of these currents studied include current intensity, density, layer width, and velocity and are consistent with earlier studies at lower altitudes. In general, region 1 and 2 currents behave differently during substorm growth and expansion, but are comparable in the recovery phase. During a substorm, region 1 current intensity ranges from 0.4 to 0.6 A/m and peaks during the expansion phase. Region 2 intensity varies from 0.15 to 0.35 A/m and reaches a maximum during recovery. The density of region 2 currents remains essentially steady at 1.4-mu-A/m2, while region 1 decreases from 5 to 1.3-mu-A/m2. The width of these current sheets throughout a substorm remains in the range from 100 to 500 km. The velocity of these sheets is for the most part inwards with speeds from 50 to 200 m/s. Even though these current sheet crossings were not originally selected as substorm associated events, they all occurred during one of the phases of a substorm. This indicates that substorms play a major role in the generation of nightside region 1 and 2 currents detected by the ISEE spacecraft. These results are consistent with a model in which these currents flow to couple the outer magnetosphere and the ionosphere in response to the stresses applied by both dayside and nightside reconnection. Region 1 currents are responsive to variations in the stress caused by both reconnection regions, whereas region 2 currents are responsive to only nightside reconnection.