Global storm time auroral X-ray morphology and timing and comparison with UV measurements

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on. NASA's Polar spa cecraft provides the first global images of the amoral oval in X-rays and a llows very accurate measurements of the timing of geomagnetic disturbances to a degree of temporal resolution not available from previous imagers due to its photon counting characteristics. On October 19, 1998, a magnetic clo ud associated with a CME encountered the Earth's magnetopause near 0500 UT, generating a magnetic storm that reached a minimum value in Dst of -139 nT . The z component of the interplanetary magnetic field (IMF) (B-z) remained remarkably steady for the first 10 hours of the storm as did the solar win d particle pressure. The PIXIE and UVI instruments on the Polar spacecraft were both imaging the auroral oval from 0800 to 1800 UT; six distinct impul sive auroral enhancements were observed by the imagers during this time per iod. Global imaging combined with geosynchronous particle observations allo wed classification of the geomagnetic disturbances associated with the even ts. Only two of the events were classified as substorms;one was classified as a poleward boundary intensification, one was a convection bay, and one w as a pseudobreakup. A sixth event occurred after a dramatic northward turni ng of the IMF at the end of the 10-hour B-z south period but was very weak and transient. The effects of the northward turning were counteracted by a simultaneous increase in the B-y component of the IMF. The first sign of si gnificant substorm activity occurred over 8 hours after the cloud encounter ed the Earth and was not associated with any change in the solar wind magne tic field or particle pressure. The cross polar cap potential remained larg e (> 100 kV), and most of the X-ray emissions observed were associated with enhanced earthward convection caused by large cross-tail electric fields; 50% were collected from the 0000 - 0600 magnetic local time (MLT) sector.