Soft X-rays from the sunlit earth's atmosphere

The HEAO-1 A-2 experiment low energy proportional counters have been used to measure the X-ray spectrum of the sunlit earth in the energy range 0.2–0.8 keV. The X-rays arise by coherent scattering of, or fluorescence of atmospheric constituents by, solar coronal X-rays incident on the atmosphere. Although the relative spectral contributions of the two processes depend upon the sun-earth-satellite geometry, fluorescent oxygen and nitrogen K X-ray emission is always important. The observed spectra were compared with calculations in order to derive the coronal temperature and emission measure, parameters that characterize the incident solar spectrum. These derived parameters agree well with the expected values for the nonflaring sun, and good agreement was obtained between measurements closely spaced in time but having a wide range of geometries and counting rates. Thus X-ray observations of the sunlit earth's atmosphere can be a useful monitor of solar activity for satellite-borne instrumentation unable to view the sun directly. The total measured fluorescent line flux agreed well with calculations, but the N : O line ratio did not. This disagreement is attributed to several causes, including the relative weakness of N emission at high altitudes where fluorescence is particularly important, the presence of line emission in the solar spectrum, and possible small calibration errors. Since present detectors cannot resolve the oxygen and nitrogen K X-rays, observation of X-ray fluorescence from the sunlit atmosphere will be useful in monitoring atmospheric constituents only to the extent that total line counting rates depend upon composition. X-rays from the sunlit earth are briefly examined as a source of background in auroral X-ray observations. During nonflare periods this background should be unimportant above about 2 keV.