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.
Abstract:
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.