University of Kansas
X-Ray Emission in the Solar System
Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals
by Cravens et al.
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University of KansasX-Ray Emission in the Solar System |
Draft Temporal Variations of Geocoronal and Heliospheric X-Ray Emission Associated with the Solar Wind Interaction with Neutrals by Cravens et al. |
Image: Jovian soft X-rays from ROSAT; courtesy of J. H. Waite.
Snowden et al. [1994] identified several sources of "noncosmic background contamination" in the ROSAT/PSPC soft X-ray back-ground data, including an energetic particle-induced internal background, a solar X-ray scattering background from the Earth's atmosphere, short-term enhancements (STE) background associated mainly with auroral X-rays, and long-term enhancements (LTE) background for which no explanation was found. The 1/4 keV channel LTE are observed to vary on time-scales of days [Snowden et al.,1994].
The ROSAT LTE data are derived from observations conducted in two distinct modes of operation: scanning and pointed. During survey operations, ROSAT was scanned around great circles on the sky with the scan axis in the ecliptic plane within 15 deg of the Earth/Sun line. The satellite was rotated at a rate of ~4 deg. min-1. LTE data are obtained in 20 deg segments for this mode. More commonly, LTE were determined in the pointed mode, and we use this data mode in this paper. These observations had fixed pointing directions and total exposures which ranged from a few to more than a hundred kiloseconds but because of scheduling idiosyncrasies were broken into exposures of ~1-2 ks. Because of spacecraft operational constraints, the pointing directions were limited to a swath of the sky between 75 deg and 105 deg away from the Sun.
The LTE values for the scanning data were determined by comparing adjacent segments from multiple scans. Specifically, an array was formed with segment number in one direction and scan number in the other direction (the maximum offset of the ROSAT field of view was only ~4 arc min between scans and the ROSAT field of view was ~1.9 deg). The variation of the average count rate as a scan progressed was relatively small and smooth, allowing the identification of any temporal variations up to time periods of a few days. These time variations have the steady (or any very slowly varying) background removed and are identified as LTEs. The LTE count rates for the pointing data were determined by examination of the observation light curves, comparing intensities with overlapping exposures if any existed, and comparison with the scanning intensities. The identification of LTEs from the ROSAT data alone was most sensitive to components with temporal variations of a few days or less and greater than a few kiloseconds. Shorter-duration enhancements in the ROSAT data were lumped into the STE category in processing and excluded from subsequent analysis. Slowly varying LTEs with temporal scales of greater than a week or two would be difficult to identify.
Next: 7. Comparison of Solar Wind Flux Data and Model Results With ROSAT LTE Data
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Tizby Hunt-Ward tizby@ku.edu |