Sudden commencements observed by MAGSAT above the ionosphere.

Abstract

Among 21 geomagnetic sudden commencements (SC), each of which was reported by more than 2 ground observatories, 16 were detected by MAGSAT above the ionosphere (260-560km altitude) in the dawn-dusk meridian. The amplitude of the remaining 5SCs was small on the ground (less than 10nT). The SCs at MAGSAT appeared mainly in the H-component in low latitudes and in both H- and D-components in high latitudes. The Z-component did not show significant variations. The amplitude of the SCs was usually larger at the satellite altitude than on the ground, and the averaged ratio was 1.3 for the low latitude SCs. The larger amplitude at the satellite altitude was interpreted in terms of a westward zonal shielding current flowing in the ionosphere. A detailed analysis was made for one SC which occurred when the satellite was flying very closely above a ground station (Magadan in east Siberia, geomag. lat.=51.1°) in a duskside (16h-17h LT) meridian. The H-component at the satellite showed a positive impulse of 52nT and the D-component a negative impulse of 43nT in the beginning part of the SC. The duration of the impulse was 2-3min. The SC observed by normal-run magnetographs at ground stations near the satellite orbit was preceded by a typical preliminary impulse (PI) of opposite deflection to that of the pulse observed by MAGSAT. By the use of high time resolution data from the IMS magnetometer network Alska chain, the pulse detected by MAGSAT was identified with the PI observed at the ground stations, and it was concluded that the main source current for the PI flowed between the satellite and the ground, that is, in the ionosphere; but effects of other source currents including field aligned currents should be taken into account.