Magnetic mirroring as the basic cause of parallel electric fields

In order to explain thick spatial structures of precipitation, like 'inverted V' events, in terms of a static parallel electric field the mechanism responsible for a low parallel conductivity may have to be operative even at fairly low current densities. As a solution of this problem it is suggested that the prallel electric field is due to magnetic mirroring of downflowing hot electrons from regions of negative space charge in the outer magnetosphere or the solar wind (the ambient cold electrons being depleted at high altitudes). It is found that an extremely thin precipitation structure, like an auroral ray, is likely due to a local increase of the parallel conductivity rather than due to a current-induced local decrease of the conductivity. The magnetic mirroring may thus provide a solution also in this case either in terms of a spatially nonuniform source of electrons or in terms of a spatially nonuniform wave-particle interaction by which electron gyroenergy is transformed into wave energy. Furthermore, a parallel electric field supported by the magnetic mirror is compatible with a strongly variable pitch angle distribution of precipitating electrons including distributions peaked at 90degree as well as at 0degree. This model may also provide a simple explanation of the simultaneous precipitation of electrons and collimated positive ions