ELECTRON HEATING BY THE CROSS-SHOCK ELECTRIC-POTENTIAL

Electrons can experience superadiabatic heating at a collisionless sho ck due to the presence of a cross-shock electric potential. This heati ng can greatly exceed the temperature rise associated with the conserv ation of the magnetic moment of the electrons as they move into the st ronger magnetic field downstream of the shock. The criterion for the o nset of superadiabatic heating and the effects of the detailed structu re of the shock are investigated for the special case where the magnet ic Field is perpendicular to the shock normal. A comparison of the nat ural length scale of the electron orbits and the shock thickness is us ed to derive a new criterion for the onset of superadiabatic heating. This new heating criterion facilitates an improved understanding of th e role of various physical parameters in determining the degree of ele ctron heating. It has the same functional form as the condition for th e electron equations of motion to permit exponential divergence of ele ctron orbits (an important feature of dynamical chaos) which it has pr eviously been argued determines the onset of superadiabatic heating. H owever, we show that the orbits of individual electrons undergoing sup eradiabatic heating do not diverge significantly because of the very s hort time they spend within the shock, the only region in which the eq uations of motion are nonlinear. The techniques used to analyze dynami cally chaotic systems are therefore of little assistance in understand ing this potentially important process.