Trapping of ion conics by downward parallel electric fields

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Abstract
Several examples of upflowing field‐aligned electrons at altitudes between 1000 and 15000 km at high latitudes have been reported recently. The velocity‐space distributions of these electrons suggest that they were accelerated out of the ionosphere by downward parallel electric fields, and parallel potential drops of a few tens to a few hundred volts over the altitude range 1000–6000 km are implied. The electron beams are associated with ion conics. Data from electrostatic analyzers onboard the S3‐3 satellite show that ion conics can be trapped at low altitudes by the downward electric field. Evidence for the parallel electric field is obtained not only from observations of upflowing electron beams but also from observations of retardation of downflowing electrons and local acceleration of downflowing ions. The inferred parallel electric fields have magnitudes large enough to locally balance the magnetic mirror force on perpendicular ion conics in the few hundred electron volt energy range. This electrostatic trapping has important consequences for wave heating of ion conics since it extends the residence time of ions in the heating region. It is demonstrated that this multipass heating mechanism leads to perpendicular ion heating far in excess of that predicted by present theories of ion conic heating.