Global nature of field‐aligned currents and their relation to auroral phenomena

Abstract

Low‐ and high‐altitude spacecraft carrying scientific magnetometers have probed the magnetic fields of the terrestrial magnetosphere. These measurements reveal that field‐aligned currents, possibly powered by dynamo action, flow over large regions of the earth's neighborhood. The purpose of this paper is to report on highlights of the U.S. contributions to the IMS effort in the area of understanding the global nature of the field‐aligned currents and their relation to auroral and ionospheric phenomena. It has been established now that field‐aligned currents flow on the same field lines connecting distant regions to low ionospheric altitudes. Similarly, currents flow on L shells extending over several hours of local time. However, it is becoming increasingly apparent that the sources of the currents in the various latitudes and longitudes are not the same. For this reason we discuss separately the following current‐laden regions: (1) the noon sector, (2) the dawn and dusk sectors, (3) the midnight sector, and (4) the polar cap. Special attention is paid to polar cap ionospheric irregularities and the relation of auroral kilometric radiation to field‐aligned currents and optical auroral emissions.