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R. Prangé (IAS, France), L. Pallier (IAS, France)
In this study we determine of a 'reference main oval' for Jupiter's aurorae from a series of high resolution images taken with the FOC on board HST in the H2 Lyman bands near 1550 Å. We have focused on images taken when most of the north auroral oval was active, including at longitudes \le 170\circ where it is generally undetectable. Our reference main ovals are consistent in size and in general aspect with the footprints of 20-30 RJ VIP4 model ovals (Connerney et al. 1998). However, in the north it differs from the model (and from previous 'reference' main ovals) in that it exhibits a 'kidney-like' aspect with excursions toward low latitude in the SIII longitude range 190\circ-240\circ (an already reported feature), and toward high latitude in the poorly documented 120\circ-150\circ range. In the south, it is also shifted from a VIP4 model oval, toward low latitude and toward high latitude, in the longitudes ranges (110\circ-200\circ) and (310\circ-100\circ), respectively. The accurate definition of both main oval loci puts additional strong constraints on magnetic field models. By extrapolation of the north reference main oval, we define higher latitude 'reduced reference ovals', which we can fit both the polewardmost arc detected longward of longitude 170\circ and the high latitude edge of what we had previously named the 'transpolar emission' shortward of 170\circ. This indicates that these two features must in fact be part of a single, very high latitude auroral oval, connected to the outer magnetosphere. We suggest that, subject to solar wind condition variations, this oval indicates the approximate location of the northern polar cap boundary. Finally we study a bright spot located just equatorward of this oval, we establish that it remains fixed near magnetic noon, and we tentitatively identify it with the northern Jovian polar cusp.
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