Jupiter’s main auroral emission; local time and temporal variability

Jupiter's main auroral oval is associated with the ionosphere-magnetosphere coupling current system
which is related to the breakdown of corotation in the middle magnetosphere. Its auroral footpath is usually
represented as a smooth line closing around the pole. However, this simplistic view is misleading in many
regards. We have constructed a new reference contour in the northern hemisphere (Figure 1), based on more
than 1000 HST/UV images, which does not look like an oval and does not close around the pole. We use
this reference contour to quantify the effects of temporal and local time variability of the
magnetospheric plasma characteristics on the location of the main auroral emission.
Beyond the orbit of Ganymede (15RJ), two key ingredients are expected to have a measurable
influence on the instantaneous shape of the main emission contour: the azimuthal current flowing in the
current sheet [1,2] and the corotation breakdown distance. The former affects the radial extent of the
magnetic field lines, and the latter determines the radial location of the field aligned currents
transmitting momentum from the planet to the lagging plasma. So far, models used to magnetically map the
auroral main emission between the ionosphere and the equatorial plane assumed that these two parameters
are constant and axisymmetric. However, in situ observations, mainly by Galileo, have revealed large
local time asymmetries and temporal variations in the plasma flows and distribution. These variations have
an impact on the azimuthal current and the distance at which the plasma angular velocity becomes
significantly smaller than planetary rotation. We use a new magnetic field model [3], inherited
from VIP4 and including a magnetic anomaly in the northern hemisphere, to simulate the effects of these
asymmetries on the location of the main auroral emission, and interpret the large scattering of the
corresponding HST data point.