Electron acceleration in the heliosphere

We review the evidence for electron acceleration in the heliosphere putting emphasis on the acceleration processes. There are essentially four classes of such processes: shock acceleration, reconnection, wave particle interaction, and direct acceleration by electric fields. We believe that only shock and electric field acceleration can in principle accelerate electrons to very high energies. The shocks known in the heliosphere are coronal shocks, traveling interplanetary shocks, CME shocks related to solar type II radio bursts, planetary bow shocks, and the termination shock of the heliosphere. Even in shocks the acceleration of electrons requires the action of wave particle resonances of which beam driven whistlers are the most probable. Other mechanisms of acceleration make use of current driven instabilities which lead to electron and ion hole formation. In reconnection acceleration is in the current sheet itself where the particles perform Speiser orbits. Otherwise, acceleration takes place in the slow shocks which are generated in the reconnection process and emanate from the diffusion region in the Petschek reconnection model and its variants. Electric field acceleration is found in the auroral zones of the planetary magnetospheres and may also exist on the sun and other stars including neutron stars. The electric potentials are caused by field aligned currents and are concentrated in narrow double layers which physically are phase space holes in the ion and electron distributions. Many of them add up to a large scale electric field in which the electrons may be impulsively accelerated to high energies and heated to large temperatures.