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 bremsstrahlung and line k-shell radiation produced by interaction of energetic electrons with the cometary atmosphere. Electron energization inside 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 Ε x Β fields of the host (solar wind) plasma moves in cycloidal orbits, forming an unstable ring distribution, which drives the modified two stream instability and excites lower hybrid waves. These waves have the phase velocity along the magnetic field much larger than across it. For this reason, they can be in simultaneous Landau resonance with both slow unmagnetized ions and fast, but magnetized (field aligned), electrons efficiently transferring ion energy to the electrons and leading to their energization. We have investigated this process in a quasilinear approximation. By solving the Fokker-Planck equation for the field aligned electron energy diffusion, we have been able to calculate fluxes and energy spectra of electrons accelerated by the lower hybrid waves and to determine spectrum and total luminosity of x-ray radiation. Results of theoretical analysis are compared with ROSAT and Beppo-Sax observations of x-ray emission from the different comets.