부산대학교 도서관

Since the Voyager mission flybys in 1979, we have known the moon Io to be extremely volcanically active as well as to be the main source of plasma in the vast magnetosphere of Jupiter. Material lost from Io forms neutral clouds, the Io plasma torus and ultimately the extended plasma sheet. This material is supplied from the upper atmosphere and atmospheric loss is likely driven by plasma-interaction effects with possible contributions from thermal escape and photochemistry-driven escape. Direct volcanic escape is negligible. The supply of material to maintain the plasma torus was estimated from various methods at roughly one ton per second. Most of the time the magnetospheric plasma environment of Io is stable on timescales from days to months. Similarly, Io's atmosphere was found to have a stable average density on the dayside, although it exhibits lateral, diurnal and seasonal variations. There is a potential positive feedback in the Io torus supply: collisions of torus plasma with atmospheric neutrals likely are a significant loss process, which increases with torus density. The stability of the torus environment might be maintained by limiting mechanisms of either torus supply from Io or the loss from the torus by centrifugal interchange in the middle magnetosphere. Various observations suggest that occasionally the plasma torus undergoes major transient changes over a period of several weeks, apparently overcoming possible stabilizing mechanisms. Such events (and more frequent minor changes) are commonly explained by some kind of change in volcanic activity that triggers a chain of reactions which modify the plasma torus state via a net increase in supply of new mass. However, it remains unknown what kind of volcanic event can trigger torus events, whether Io's atmosphere undergoes a change before or during such magnetospheric events, and what processes could enable such a change.