Global-scale electron precipitation features seen in UV and X rays during substorms

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (WI) onboard the Polar satellite have provided the first simultaneou s global-scale views of the patterns of electron precipitation through imag ing of the atmospheric X-ray bremsstrahlung and the auroral ultraviolet (UV ) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9 .9-19.7 keV X-ray images used in this study respond only to electrons of en ergy above 10 keV. Previous studies by ground-based, balloon, and space obs ervations have indicated that the patterns of energetic electron precipitat ion differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron prec ipitation, one has not been able to establish a complete picture. In this s tudy the development of the electron precipitation during the different pha ses of magnetospheric substorms is examined. Comparisons are made between t he precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) e lectron populations, correlated with ground-based observations and geosynch ronous satellite data. We focus on one specific common feature in the energ etic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV im ages. Delayed relative to substorm onsets, we observe a localized maximum o f X-ray emission at 5-9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found t hat this maximum most probably is caused by electrons injected in the midni ght sector drifting (i.e,, gradient and curvature drift) into a region in t he dawnside magnetosphere where some mechanism effectively scatters the ele ctrons into the loss cone.