Global X-ray emission during an isolated substorm - a case study

The polar ionospheric X-ray imaging experiment (PIXIE) and the UV imager (U VI) onboard the Polar satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of t he atmospheric X-ray bremsstrahlung and the auroral UV emissions. While the UV images in the Lyman-Birge-Hopfield-long band used in this study respond to the total electron energy flux which is usually dominated by low-energy electrons (<10 keV), the PIXIE images of X-ray bremsstrahlung above simila r to 2.7 keV respond to electrons of energy above similar to 3 keV. Compari son of precipitation features seen by UVI and PIXIE provides information on essentially complementary energy ranges of the precipitating electrons. In this study an isolated substorm is examined using data from PIXIE, UVI, gr ound-based measurements, and in situ measurements from high- and low-altitu de satellites to obtain information about the global characteristics during the event. Results from a statistical study of isolated substorms, which h as reported a significant difference in the patterns of energetic electron precipitation compared to the less energetic precipitation are confirmed. A localized maximum of electron precipitation in the morning sector delayed with respect to substorm onset is clearly seen in the X-ray aurora, and the time delay of this morning precipitation relative to substorm onset strong ly indicates that this intensification is caused by electrons injected in t he midnight sector drifting into a region in the dawnside magnetosphere whe re some mechanism effectively scatter the electrons into the loss cone. In this study, we also present the results from two low-altitude satellite pas ses through the region of the localized maximum of X-ray emission in the mo rning sector. Measured X rays are compared with X-ray fluxes calculated fro m the electron spectral measurements. By fitting the electron spectra by a sum of two exponentials we obtain fairly good agreement between calculated and directly measured X-ray flux profiles. (C) 2000 Elsevier Science Ltd. A ll rights reserved.