Energetic particles over Io's polar caps

Abstract

We present results obtained from the Galileo satellite's Energetic Particles Detector during its final two encounters in 2001 with Jupiter's moon Io. These encounters returned the first data from just above Io's polar caps. They complement previous low‐latitude data and provide a new perspective of Io's interaction with Jupiter's magnetosphere and ionosphere. The evolution of electron and ion distributions was measured from the upstream region throughout the polar cap traversals. From the time of initial field contact with Io and continuing throughout the encounter these distributions evolve in a manner consistent with adiabatic motion along the Io‐Jupiter field line. At encounter all particles develop narrow trapped‐like distributions indicative of the creation of a near‐Io magnetic bottle caused by an enhancement of field at Io's upstream surface. The measured pitch angle distributions indicate a field enhancement of up to 10%–15% higher than the field observed at Galileo's position. Distribution evolution times agree roughly with particle bounce times on the Io‐Jupiter field line. The ion distribution evolution times provide an estimate of ∼3–7 km/s for the field line convection speed across Io's polar caps, a value small (∼10%) compared with the upstream convection speed. Along with these trapped distributions, beams of ions and electrons are observed streaming into Io's polar caps throughout the encounters. The continued observation of ion beams across the polar cap is consistent with their half‐bounce times. The data further indicate that the convection speed may vary as the polar cap is traversed. The one exception to the adiabatic particle behavior discussed above is the observation of intense electron beams streaming into Io's polar caps. The polar cap electron beams are similar to those previously measured in Io's wake [Williams et al., 1996] and apparently originate from the same source. The source has been located at low (∼0.5 RJ) altitudes on the Io‐Jupiter field line [Williams et al., 1999]. The intensities of the electron beams indicate strong acceleration in the source region. Following the suggestion of Mauk et al. [2001], we conclude that the electron beams most likely form the downward (toward Jupiter) current portion of the overall current system established by Io's interaction with Jupiter's magnetosphere and ionosphere. A map of the observed beam locations for all Io encounters provides a rough measure of the spatial extent of this downward current.