Detection of an ionospheric current for the preliminary impulse of the geomagnetic sudden commencement

Abstract
MAGSAT observed a geomagnetic sudden commencement at an altitude of 550 km on its dawnside orbit above the South Atlantic Ocean. The H‐component of the magnetic field increased monotonically by 46 nT in about 4 minutes, while the D‐component decreased westward by 10 nT in the first 1‐2 minutes and then increased gradually. There was no significant deviation in the Z‐component. The comparison with ground‐based magnetograms showed that the negative impulse of the D‐component provided the first experimental evidence of the global ionospheric current system for the preliminary impulse of the geomagnetic sudden commencement.