OBSERVATIONS OF ELECTRON DISTRIBUTIONS IN MAGNETOSHEATH MIRROR MODE WAVES

The behavior of the electron distribution function within mirror mode waves in the Earth's magnetosheath is studied using observations made by the electron experiment on Active Magnetospheric Particle Tracer Ex plorers United Kingdom Subsatellite (AMPTE UKS). The results of a deta iled pitch angle analysis show that in the mirror mode troughs the ele ctron distribution displays two distinct features related to particle trapping: (1) The ''deeply''' trapped electrons (those with pitch angl es close to 90 degrees in the magnetic field troughs) are significantl y cooled with respect to the rest of the electron population and (2) t he ''shallowly'' trapped electrons (those with pitch angles just great er than the critical pitch angle for trapping) are significantly heate d with respect to the rest of the electron population. This is explain ed as a result of the trapped electron population undergoing a combina tion of Fermi acceleration and deceleration and betatron deceleration. The untrapped electron population appears to behave adiabatically as it travels alternately through mirror mode peaks and troughs. This beh avior is confirmed by the use of Liouville mapping techniques, which a lso allow us to estimate the electric potential associated with the mi rror mode wave.