Cassini VIMS observations of H3+ emission on the nightside of Jupiter

We present the first detailed analysis of H3+ nightside emission from Jupiter, using data from the Cassini flyby in 2000-2001, producing the first jovian maps of nightside H3+ emission, temperature and column density. Using these, we identify and characterise regions of H3+ nightside emission, compared against past observations of H3+ emission on the dayside. We focus our investigation on the region previously described as ‘mid-to-low latitude emission’, the source for which has been controversial. We find that the brightest of this emission is generated at jovigraphic latitudes similar to the most equatorward extent of the main auroral emission, but concentrated at longitudes eastward of this emission. The emission is produced by enhanced H3+ density, with temperatures dropping away in this region. This emission has a loose association with the predicted location of diffuse aurora produced by pitch angle scattering in the north, but not in the south. This emission also lays in the path of sub-rotating winds flowing from the aurora, suggesting a transport origin. Some differences are seen between dayside and nightside sub-auroral emission, with dayside emission extending more equatorward, perhaps caused by the lack of sunlight ionisation on the nightside, and unmeasured changes in temperature. Ionospheric temperatures are hotter in the polar region (~1100-1500K), dropping away towards the equator (as low as 750K), broadly similar to values on the dayside, highlighting the dominance of auroral effects in the polar region. No equatorial emission is observed, suggesting that very little particle precipitation occurs away from the polar regions.