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

Abstract

Abstract. The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging 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 energy above 10 keV. Previous studies by ground-based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) electron populations, correlated with ground-based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X-ray emission at 5-9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it isfound that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.

Imaging riometer observations on energetic electron precipitation at SANAE IV, Antarctica

“…This and other studies [ Berkey et al , 1974] on energetic precipitation, as observed by CRNA, suggested that the maximum around midnight is to be related to the injection of fresh substorm electrons and that the morning maximum most probably is due to drifting electrons from substorms around midnight. This is confirmed by Østgaard et al [1999, 2000], who found that the delay in electron precipitation in the morning sector relative to the substorm onset time in the midnight sector is consistent with drifting electrons in the energy range of 90–170 keV.…”

“…The injection region, which is the region where particle fluxes become simultaneously enhanced at different energies, expands rapidly eastward and slowly westward. The UV emissions, which relate to visible aurora, are most intense at the duskward side of the auroral bulge, while the X rays increase significantly eastward [ Østgaard et al , 1999]. This indicates differences in electron energies resulting from the injection region.…”

Energetic electron precipitation and the NO abundance in the upper atmosphere: A direct comparison during a geomagnetic storm

“…The areas of largest X‐ray intensity have rotated to the dayside of the SNOE track some hours after 1245 UT. On the basis of studies of substorm events, we can expect that most of the precipitation associated with possible substorm expansions after 1245 UT occurs on the nightside around geomagnetic midnight [e.g., Østgaard et al , 1999]. The NO produced by this new energy input is thus not expected to be transported to the areas beneath the next SNOE passes in the late morning sector.…”

Comparisons of thermospheric high‐latitude nitric oxide observations from SNOE and global auroral X‐ray bremsstrahlung observations from PIXIE

“… Frahm et al , 1997]. The PIXIE instrument is the first X‐ray imager to be able to image the entire auroral oval at once, providing excellent coverage for use in both statistical [ Petrinec et al , 1999, 2000; Anderson et al , 2001] and case [ Imhof et al , 2000, 2001; Anderson et al , 2000; Østgaard et al , 1999, 2000a, 2000b] studies. We examine here how the global electron precipitation from the magnetosphere affects thermospheric chemistry (in particular, NO) as a function of altitude and a variety of temporal scales (long‐term, seasonal, transport processes, and gain and loss rates).…”