THE RESPONSE OF THE MAGNETOSPHERE TO THE PASSAGE OF A CORONAL MASS EJECTION ON MARCH 20-21 1990

The geomagnetic response to the passage of a coronal mass ejection (CM E) is studied. The passage of the CME resulted in a storm sudden comme ncement (SSC) at 2243 UT on March 20 1990 with disturbed magnetic acti vity during the following 24 h. The auroral, sub-auroral and equatoria l magnetic response to the southward turning at 1314 (+/-5) UT on Marc h 21 and the equatorial response to the southward turning associated w ith the SSC on 20 March are discussed in terms of existing models. It is found that the auroral and sub-auroral response to the southward tu rning associated with the SSC is a factor 2 or more quicker than norma l due to the shock in the solar wind dynamic pressure. The low-latitud e response time to the southward turning, characterised by Dst and the magnetopause current corrected Dst, is unaffected by the shock. Dst and Dst, characteristic of the equatorial magnetic field, responded t o the 1314 (+/-5) UT southward turning prior to the first observed sub storm expansion phase onset, suggesting that a dayside loading process was responsible for the initial enhancement in the ring current rathe r than nightside particle injection. The response time of the auroral and sub-auroral magnetic field to the southward turning at 1314 (+/-5) UT on March 21 is measured at a variety of longitudes and latitudes. The azimuthal propagation velocity of the response to the southward tu rning varied considerably with latitude, ranging from similar to 8 km s(-1) at 67 degrees N to similar to 4 km s(-1) at 55 degrees N. The so uthward velocity of the equatorward boundary of the northern polar con vection pattern has been measured. This velocity was similar to 1.2 km s(-1) at 1600 MLT, although there was evidence that this may vary at different local times.