The Io Footprint Morphology

The innermost Galilean moon Io is embedded in a dense
plasma torus. It disturbs the flow of the corotating torus
plasma and generates MHD waves. Especially the Alfv ́ n
e
mode, which carries electric currents along the magnetic
field, is responsible for the famous Io footprint in the Jo-
vian aurora. The Alfv ́ n waves are partly reflected at plasma
e
density gradients e.g. at the torus edges and form a compli-
cated wave pattern. The footprint morphology in the Jovian
aurora displays this pattern. Besides the main spot feature,
a trailing wake extending over up to 100 degrees and occa-
sionally occurring multiple spots indicating reflections have
been identified. The intensity, the occurrence of multiple
spots and the inter-spot distances have been found to vary
strongly [1].
Io is moving up and down in the plasma torus confined to the
centrifugal equator. It interacts with a dense plasma when
located in the torus center and considerably thinner plasma
at the torus flanks. The incoming plasma density controls
the interaction strength and wave amplitude. This primar-
ily leads to a relationship between the footprint brightness
and Io’s centrifugal latitude [1]. However, it has also been
shown that the reflection angle is strongly depending on the
wave amplitude due to nonlinear effects [2]. This has an
impact on the inter-spot distances. On the other hand this
parameter is also directly affected when Io changes its posi-
tion in the plasma torus.
To deconvolve this system we present the results of our 3D
MHD simulations showing the influence of Io’s centrifugal
latitude and nonlinearities on the footprint morphology par-
ticularly on the number of spots and inter-spot distances.
We observe interference phenomena leading to locally en-
hanced or reduced footprint brightness. We also discuss the
recently observed leading spot feature [3].