ABSTRACT
It will have been apparent that, despite our best efforts, we are still some way from being able to resolve these questions with any degree of certainty. Perhaps the closest to being understood is auroral electron acceleration. We can be fairly confident in picturing the acceleration as taking place just an Earth radius or so out along auroral-zone magnetic lines of force lying in or near the boundary between the plasma sheet and the tail lobes of the magnetosphere. The unstable nature of the relatively sharp transition between these plasmas gives rise to waves-lower-hybrid waves seem best to fit the bill-which draw their energy from plasma sheet ions and which are rapidly consumed by electrons. The stop-press news is that the waves now actually appear to have been observed in the acceleration region. However, we still need to know whether wave amplitudes and velocities are really capable of delivering the observed degree of acceleration. Another missing item is a good description of the particles and fields of the tail lobes. This is crucial since it appears to be the difference between tail-lobe and plasma-sheet properties that triggers the chain of events leading to the discrete aurora. In view of the particle fluxes encountered in the tail lobes being much lower than in the surrounding regions it may be necessary to mount a dedicated mission into this region. The tail lobes are almost voids, but they have a key role to play.
