Authors: M. Neugebauer, T. E. Cravens, C. M. Lisse, F. M. Ipavich, D. Christian, R. von Steiger, P. Bochsler, P. D. Shah and T. P. Armstrong
Reference: J. Geophys. Res. 105 (A9), 20949-20995, 2000.
Both the Röntgen X-ray satellite (ROSAT) and the Extreme Ultraviolet Explorer (EUVE) have detected soft X-ray emission from comet C/Hyakutake 1996 B2. This emission varied by a factor of about 2 over a few hours and by a factor of 4 from day to day. One explanation for the excitation of cometary X-rays is the charge-transfer mechanism suggested by Cravens. This process involves charge-exchange collisions between highly charged heavy ions in the solar wind and neutral gas in the cometary coma. Oxygen ion fluxes observed by the CELIAS/MTOF instrument on the SOHO spacecraft and proton fluxes measured by near-Earth spacecraft are mapped to the location of the comet to demonstrate that the comet X-ray variability can be explained on the basis of variability in the solar wind. There is a good correlation between cometary X-ray emission and oxygen ion fluxes and a poorer correlation with proton flux. The correlation between the solar wind oxygen flux and cometary X-rays degrades with increasing latitudinal separation of SOHO from the comet. Cometary X-ray emission is not sensitive to variations in solar X-ray fluxes and is unlikely to be caused by crossing of the heliospheric current sheet. The charge-transfer mechanism appears to be supported by all the data examined to date. Cometary X-rays have some shortcomings as remote sensors of the solar wind, however, because of variations in cometary gas production rates and in the charge states and abundances of heavy solar wind ions.