THE CME EVENT OF FEBRUARY 21, 1994 - RESPONSE OF THE MAGNETIC-FIELD AT THE EARTH SURFACE

On February 21, 1994, a large coronal mass ejection (CME) passed by th e Earth. In the solar wind, the CME front was noted by very large chan ges in the solar wind dynamic pressure, flow direction, and magnetic f ield, as measured by several spacecraft. At the Earth's surface, many ground stations also recorded the passage of this CME, as a storm sudd en commencement onset. We examine here several interesting features of the surface magnetic field response to this sudden commencement onset . In particular, we use the magnetometer records to examine the propag ation time of the initial impulse along the Earth's surface. We also s tudy in detail the initial amplitude change at several stations, and n ote the variation in the response as a function of magnetic latitude. In addition, we crudely estimate the relative contributions of the Cha pman-Ferraro and the ionospheric current system to the surface respons e. Also of interest is the polarization and period of Psc oscillations superposed on the main impulse at several stations, including polariz ation reversals as a function of magnetic latitude. Corresponding iono spheric currents at stations along the 210 degrees MM chain are also c omputed, and very large and sudden changes in the ionospheric current flow patterns are reported during the initial minutes of the sudden co mmencement onset. These and other results are interpreted in terms of previous models, paradigms, and statistical results.