Field‐aligned currents observed in the inner magnetosphere and their magnetic signature on the ground

Abstract

ISEE 2 magnetometer data and ISEE 1/2 particle data from the first half of 1978 were examined to search for signatures of field‐aligned currents in the Earth's inner magnetosphere (2.6–9.5 RE, L = 4.8–13.1). Eighteen field‐aligned current sheet “events” were found. Most of them occur during magnetically disturbed periods (AE = 226–724 nT). The surface current densities range from 12 to 113 mA/m and are compatible with magnitudes measured at low altitudes by TRIAD. In nearly every “event” gradients in the proton flux (32–50 keV and 80–126 keV are found in the vicinity of the field‐aligned currents indicating that the currents are located on or adjacent to the outer boundary of the plasma sheet. Using the Tsyganenko‐Usmanov magnetospheric field model to trace the field line from where the field‐aligned current signature was observed down to the ground, we found that the pattern is consistent with Iijima and Potemra's (1978) observation of large‐scale Birkeland currents in the polar region. Geomagnetic field data on the nightside, measured by about 10 observatories for each “event”, were used to study the magnetic field behavior on the ground. Even though we found both region 1 and region 2 currents from the ISEE data in the inner magnetosphere during magnetically disturbed periods, the ground magnetic signatures favor only the region 1 currents and also indicate the transient nature of the current flows.