Net field-aligned currents observed by Triad

Abstract

The magnetometer observations on the Triad satellite frequently indicated the existence of a step-like level shift in the field component approximately in the magnetic east-west direction near the expected position of the polar cap boundary. Such a field change implies the presence of a net current flowing into or away from the ionosphere in a current layer parallel to the local L-shell. The current direction is toward the ionosphere in the morning half and away from it in the afternoon half of the.current layer. These current directions are the same as those of the field-aligned currents observed in the polar cap boundary layer by Ogo 5. Hence a substantial part of the field-aligned currents discussed in this paper is believed to originate in, and return to, the magnetospheric tail flowing on the high-latitude boundary of the plasma sheet. The effect of the net rarrent is largest in the late afternoon to the evening, with a maximum in the 1500 -1800 MLT interval.