Cause of the localized maximum of X-ray emission in the morning sector: A comparison with electron measurements

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung. While other remote sensing techniques like ultraviolet (UV) and visible imaging sense emissions that are dominantly produced by low-energy electrons (<10 keV), the PIXIE X-ray images used in this study respond to electrons of energy above ∼3 keV. From a statistical study of global X-ray emission a localized maximum in the morning sector, delayed with respect to substorm onset, is found to be a common feature during substorms. The time delay of this morning precipitation relative to substorm onset strongly indicates that this localized maximum is caused by electrons injected in the midnight sector drifting into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone. In this study we have examined two isolated substorms that occurred on July 31, 1997, and September 4, 1997, to investigate these features in more detail. PIXIE images are used to examine the global structures of the two events. Particle measurements from several low-altitude satellites, NOAA and DMSP, have provided fine structure information of the spatial and spectral development of the morning precipitation. We find good spatial correlation between X-ray emission and electron measurements for both these events. Comparison of measured electron spectra and electron spectra derived from X-ray measurements by PIXIE are also presented and correlates fairly well. The technique for deriving electron characteristics from the X-ray measurements are described and discussed. We find that both the electron spectra and the X-ray spectra can be represented by double or single exponentials. The electron spectra measured in the early stage of the localized morning maximum of X-ray emission strongly indicate that the scattering of >2-10 keV electrons by wave-particle interaction into the loss cone is the main mechanism for this precipitation. Copyright 2000 by the American Geophysical Union.

Duke Scholars

Author Steven A. Cummer Electrical and Computer Engineering