rmost large moon of Jupiter, orbits within a plasma torus that forms when neutral atoms in Ios atmosphere are ionized and swept into Jupiters magnetosphere. This torus is tilted with respect to Jupiter's equator, so at times Io is below or above the toruss core. Ios orbit also crosses Jupiters magnetic field lines, coupling the moon and the planets upper atmosphere. This generates an electric current that ultimately produces ultraviolet light emissions in Jupiters northern and southern polar regions facing Io. Scientists term these aurorae the Io footprints. In each hemisphere, the footprint consists of a bright spot sometimes followed by secondary spots. To study the Io footprint, Bonfond et al. analyze measurements from the Hubble Space Telescope and discover the systematic appearance of a new faint spot, this time preceding the main bright spot. If Io is orbiting in the southern part of the plasma torus, this leading spot is seen in the northern polar regions, and vice versa. The authors hypothesize that current loops in Jupiters magnetosphere might accelerate electrons toward the other hemisphere, generating the footprints leading and secondary signatures.
See press release: http://www.agu.org/sci_soc/prrl/2008-09.html
UV Io footprint leading spot: A key feature for understanding the UV Io footprint multiplicity"
B. Bonfond, D. Grodent, J.-C. Grard, and A. Radioti: Laboratoire de Physique Atmosphrique et Plantaire, Universit de Lige, Lige, Belgium;
J. Saur and S. Jacobsen: Institute fr Geophysik und Meteorologie, Universitt zu Kln, Cologne, Germany.
Geophysical Research Letters (GRL) paper 10.1029/2007GL032418, 2008; http://dx.doi.org/10.1029/2007GL032418
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