ELECTRON ENERGIZATION BY LOWER-HYBRID WAVES AS A POSSIBLE SOURCE FOR X-RAY-EMISSION FROM COMETS

A goal of this presentation is to demonstrate that recently observed X -ray emission from comets can be interpreted as a combination of brems strahlung and line k-shell radiation produced by interaction of energe tic electrons with the cometary atmosphere. Electron energization insi de the cometary bow-shock is due to mass-loading of the shocked solar wind flow by newly born heavy cometary ions created by photoionization of the neutral gas outflow. The new ion component in the crossed E x B fields of the host (solar wind) plasma moves in cycloidal orbits, fo rming an unstable ring distribution, which drives the modified two str eam instability and excites lower hybrid waves. These waves have the p hase velocity along the magnetic held much larger than across it. For this reason, they can be in simultaneous Landau resonance with both sl ow unmagnetized ions and fast, but magnetized (field aligned), electro ns efficiently transferring ion energy to the electrons and leading to their energization. We have investigated this process in a quasilinea r approximation. By solving the Fokker-Planck equation for the held al igned electron energy diffusion, we have been able to calculate fluxes and energy spectra of electrons accelerated by the lower hybrid waves and to determine spectrun and total luminosity of X-ray radiation. Re sults of theoretical analysis are compared with ROSAT and Beppo-Sax ob servations of X-ray emission from the different comets.