Title:

Observations of EUV and X-ray Emission from Comets



Casey M. Lisse, 
University of Maryland


Abstract:

The unexpected discovery of x-ray emission from Comet Hyakutake in
March 1996 (Lisse et al. 1996) produced a number of questions about
the physical mechanism producing the radiation. The original detection
and subsequent observations have shown that the very soft (best fit
thermal bremsstrahlung model kT ~ 0.2 keV) emission is due to an
interaction between the solar wind and the comet's atmosphere. Using
the results from the more than 18 comets detected to date in x-rays,
we report here on the latest results on cometary x-ray emission,
including new results from Chandra, and show that charge exchange
between highly ionized minor ions in the solar wind and neutral gases
in the cometary coma is the most likely operative mechanism. We then
use this result to study a number of problems of astrophysical
interest: the nature of the cometary coma, other possible sources of
x-ray emission in the solar system, the structure of the solar wind in
the heliosphere, and the source of the local x-ray background.