Theory for preliminary negative impulse in storm sudden commencement inH at equatorial stations

Abstract

It is shown that the storm sudden commencement (SSC) inH field at low latitude station consists of only a positive excursion when the interplanetary shock due to the solar plasma impinging on the magnetosphere is associated with a southward excursion of the interplanetary magnetic field (IMF). When the signature of SSC at low latitude station consists of a preliminary negative excursion preceding the main positive excursion of theH field, the solar plasma causing the compression is associated with a northward excursion of the IMF. It is suggested that the signature of SSC(H) at equatorial stations is the result of combined effect of the compression of magnetosphere by the solar plasma as well as due to the electric field effects associated with the velocity of the solar plasma (v) interacting with the northward component (Bz) of the interplanetary field (i.e.,E=−v _x Bz).