example, Bingham et al. (1996) invoked the generation of keV electrons in the cometary coma via wave-particle interaction as the source mechanism Dust measurements on the Vega and Giotto probes to Comet of collisional excitation of the X-ray emission; Cravens (1996) proposed Halley have shown that dust particles of very small sizes (10that charge transfer interaction between the highly charged solar wind 100 A) might be very abundant in cometary comas. A scenario heavy ions (C 6ϩ , C 5ϩ , O 7ϩ , O 6ϩ ...), could be responsible for the intense of self-generation of such population of very small grains X-ray luminosity. Because of the large cross sections of the related charge (VSGs) via collisional interaction with the cometary dust coma exchange process ( Ȃ 3 ϫ 10 Ϫ15 cm 2 ) and the high efficiency of radiative excitation (D. E. Shemansky, private communication, 1997) Cravens' is considered here. If the VSGs can be charged to an electrostatic model appears to be particularly attractive. What remains to be tested potential of տ10 V, they would be effectively trapped by elecis whether the X-ray spectra so generated match the observations.
tromagnetic interaction with the cometary plasma and the
Even though no spectral information was available for the ROSAT collisional erosion of the cometary dust coma would be faciliobservations of Comet Hyakutake, previous measurements of several tated. In the case that a small fraction of the impact kinetic comets using the position sensitive proportional counter (PSPC) instruenergy can be converted to soft X-ray emission, the resultant ment have shown that the spectra of two comets, Comet Tsuchiya-Kiuchi X-ray luminosity might reach 5-10% of the value observed by 1990i and Comet Levy 1990, can be described as power laws and are ROSAT thus providing a second component of the source consistent with thermal electron bremsstrahlung emission with the elec- mechanism.