PARTICLE-ACCELERATION AT COMETS

The presence of energetic particles in the cometary coma has been pred icted and has been observed at comets P/Halley, Giacobini-Zinner and G rigg-Skjellerup. A cometary ion is subject successively to different a cceleration mechanisms: the pick up process, adiabatic compression and finally first and second Fermi acceleration processes. From 1 km s-1 outflow speed, a water ion can, in this way, reach hundreds of keV. A several millions of km wide region around the cometary nucleus is ther efore pervaded by energetic cometary ions.