Astronomy and Astrophysics – Astronomy
Scientific paper
May 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21620104m&link_type=abstract
American Astronomical Society, AAS Meeting #216, #201.04; Bulletin of the American Astronomical Society, Vol. 41, p.844
Astronomy and Astrophysics
Astronomy
Scientific paper
Geocoronal charge exchange events occasionally approach the intensity of the 1/4-keV diffuse background and have very similar low-resolution spectra. The best models go on to indicate that charge exchange in interplanetary space should provide a less sporadic flux of average intensity at least equal to the fairly uniform level observed at low Galactic latitudes. The local hot bubble lives on in some form, however, because large features at intermediate and high latitudes with up to three times the low-latitude intensity are persistently observed over long periods of time. Charge exchange models do not naturally predict these large anisotropies, and would be seriously challenged to provide them consistently in fixed celestial directions. Multiple sky surveys carried out under quite different solar conditions agree well on absolute intensities in all parts of the sky. This by no means rules out charge exchange as an important contributor, but does bring it into question. There is good evidence for solar wind charge exchange providing a fairly steady contribution in the O VII line at 570 eV, and observations of geocoronal and cometary charge exchange show that 1/4-keV emission is usually much stronger than at this higher energy. Yet we have no direct observational evidence of a steady state contribution to the 1/4-keV background, and considerable evidence suggesting that any such contribution is probably not dominant. Time and directional dependencies seem to be an obvious observational handle on solar wind contributions, but models show a conspiracy to make these weak and uncertain levers for determining the fractional importance. We discuss possibilities for determining this fraction with high-resolution spectral diagnostics.
McCammon Dan
Wisconsin X-ray Group
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