Effect of the orientation of interplanetary shock on the geomagnetic sudden commencement

Abstract

The rise time of the geomagnetic sudden commencement (SC) in low latitudes generally lies within a range from 2 to 10 min. An SC observed on 15 December 1995 was remarkable because it had an extremely long rise time of ∼30 min, although as seen by Wind, the interplanetary shock responsible for the SC was sharp as usual. The amplitude of the SC was consistent with the estimated value experimentally deduced from the increase in the solar wind dynamic pressure, implying that a complete compression of the magnetosphere occurred. However, the in situ measurement of the magnetotail by Geotail exhibited this event as an unusually gradual compression compared with a typical SC. Even this gradual compression of the magnetosphere produced a preliminary impulse on the ground that was lengthy, small, but detectable at auroral latitudes. The multisatellite observations of the interplanetary shock and the SC signals indicate that the shock normal was highly inclined duskward. Thus the orientation of the interplanetary shock or discontinuity is important in determining the SC field observed on the ground. Even if the local measurement of the solar wind dynamic pressure at a spacecraft appears to be rapid, a tilted pressure front could require many minutes to compress the forward part of the magnetosphere, producing a long rise time. Thus we can not predict the speed of the magnetospheric responses on the basis of the real‐time solar wind observations without knowing the orientation of the causative shock/discontinuity.