Jupiter's volcanic moon Io spews out volcanic gas, which reaches its atmosphere and becomes ionized, forming what is known as the Io plasma torus. This plasma torus can interact with Jupiter's magnetosphere, possibly affecting auroral activity there.
To help determine whether Io's volcanic activity affects Jupiter's magnetosphere, Yoneda et al. analyzed ground-based observations of Jupiter's sodium nebula, which provides an indication of Io's volcanic activity and plasma content in the Io plasma torus, along with satellite-based measurements of radio emission called HOM emission, which is a sign of Jupiter's auroral activity.
They observe that Jupiter's sodium nebula was enhanced in late May through early June 2007, indicating that Io's volcanic activity increased during that period. The researchers observe that shortly after this enhancement began, Jupiter's HOM emission intensity decreased.
As a result, the authors conclude that increased volcanic activity on Io lessens auroral activity in Jupiter's magnetosphere.
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More information: Io's volcanism controls Jupiter's radio emissions, Geophysical Research Letters, doi: 10.1002/grl.50095, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50095/abstract