MAGNETIC DRIFTS AT Io: DEPLETION OF 10-Mev ELECTRONS AT VOYAGER 1 ENCOUNTER DUE TO A FORBIDDEN ZONE.

The possibility that the partial dropout of the 10-Mev electrons observed at Voyager 1 encounter with Io might be explained by a forbidden zone is investigated. Because the convection velocity in the vicinity of Io decreases, grad B drifts of energetic electrons opposing the convection result in a forbidden region around Io. It is found that the location and magnitude of the 10-Mev electron depletion calculated on this basis agrees well with the location found by mapping along draped field lines from the Voyager trajectory to the location of Io. If the forbidden zone explanations for the 10-Mev dropout observed by Voyager is correct, then no electrons in this energy range would be lost to the surface of Io and there should not be a wake extending behind Io in which electrons of energies 10 Mev are depleted. Such a depletion of electrons near 9 Mev was observed by Pioneer, an apparent discrepancy.