The relation of temporal variations of soft X-ray emission from comet Hyakutake to variations of ion fluxes in the solar wind

Both the Rontgen X-Ray Satellite (ROSAT) and the Extreme Ultraviolet Explor er (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 fact or of 4 from day to day. One explanation for the excitation of cometary X r ays is the charge transfer mechanism suggested by Cravens. This process inv olves charge exchange collisions between highly charged heavy ions in the s olar wind and neutral gas in the cometary coma. Oxygen ion fluxes observed by the Charge, Element, and Isotope Analysis System (CELIAS) Mass Time-of-F light (MTOF) instrument on the SOHO spacecraft and proton fluxes measured b y near-Earth spacecraft are mapped to the location of the comet to demonstr ate that the comet X-ray variability can be explained on the basis of varia bility in the solar wind. There is a good correlation between cometary X-ra y emission and oxygen ion fluxes and a poorer correlation with proton flux. The correlation between the solar wind oxygen flux and cometary X rays deg rades with increasing latitudinal separation of SOHO from the comet. Cometa ry 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 s olar wind, however, because of variations in cometary gas production rates and in the charge states and abundances of heavy solar wind ions.