Plasma near Io: Estimates of some physical parameters

Abstract

Models of Io's interaction with the Jovian plasma are examined to assess whether conditions hazardous to a spacecraft flying by Io at about 1000 km above its surface should be anticipated. Only two models are regarded as consistent with most of the presently available data. The ‘ionospheric’ model allows field‐aligned currents to close through Io and its ionosphere. The ‘magnetospheric’ model allows the currents to close on an Ionian magnetopause and across a tail neutral sheet. Although only the latter model provides an explanation for the pitch angle distribution of energetic electrons measured by the LECP detector on Voyager 1, the implications of both models for near‐Io plasma are explored. The plasma density is found to be roughly the same near Io and elsewhere in the torus. In Io's wake the plasma is hotter than elsewhere in the torus but in the extreme case the ion temperature is merely of order the temperature in the terrestrial ring current. No special problems are anticipated for a spacecraft subject to these plasma conditions.