More about the structure of the high latitude Jovian aurorae

This study is based on the determination of a ‘reference’ main oval for Jupiterʹs aurora from a series of high-resolution images taken with the Faint Object Camera on board the Hubble Space Telescope in the H2 Lyman bands centered near 1550 Å. We have taken advantage of the visibility of the northern auroral oval over a large range of longitudes on June 24, 1994, especially for longitudes smaller than 170° where it is generally very faint and thus undetectable. In the south, where there is no such problem, we combined images taken from various points of view between June 1994 and September 1996. We find that the northern main oval is consistent in size and in general aspect with the footprint locus, in the Connerney (1998)ʹs VIP4 model, of magnetic field lines crossing the equator near 20RJ. However, the precise shape of this oval differs from the model (and from previous ‘reference’ main ovals) in that it exhibits a ‘bean-like’ aspect with excursions toward lower latitudes in the SIII longitude range 190–240° (an already reported feature), and toward higher latitudes in the poorly documented 120–150° range. In the south, our reference oval covers an area about that of the 30RJ VIP4 model. As in the north, it is shifted from a VIP4 model oval, toward lower latitudes from 110 to 200° and toward higher latitudes from 310 to 100°. The accurate definition of these (magnetically conjugate) oval loci puts additional strong constraints on magnetic field models at high latitude. Based on our reference main oval, we have then extrapolated still higher latitude ovals. Very interestingly, we find that we can fit (i) the highest latitude arc of oval detected well inside the main oval at longitudes greater than 170°, and (ii) the high latitude edge of what we had previously named the ‘transpolar emission’ at longitudes less than 170° (both also detected on images taken at other dates), by a single empirical oval. We suggest that this oval indicates the location of the northern polar cap boundary, within the uncertainty related to the temporal variability of the actual emission features. We can also fit another secondary arc of oval and a second branch of our ‘transpolar emission’ by a slightly larger empirical oval, presumably connected to the outer magnetosphere. This implies that the region of permanent high latitude diffuse emission seen inside the main oval in the dusk sector must be at the footprint of closed field lines connected all the way out from the middle magnetosphere to the magnetopause. Finally a bright spot is sometimes observed just equatorward of the northern polar cap boundary with an average brightness of 0.5-1 MR (comparing with a bright main oval). We establish that this spot does not rotate with the planet, but rather remains close to magnetic noon. We thus tentitatively identify it, by reference to the Earth aurora, with the footprint of the northern Jovian polar cusp, or with a transient dayside aurora. We also highlight the differences observed in the southern high latitude structure.