The aurora on Jupiter and Saturn have been studied with increasing sensitivity and resolution in a series of HST GO programs of UV imaging and spectroscopy. These have revolutionized our understanding of the auroral phenomenona on both planets, and the auroral emissions provide the only remote method to study the magnetospheres of these giant planets.
Much has been learned about the auroral morphology on both planets, but much remains to be understood about the Sun-Magnetosphere-Ionosphere coupling at both Jupiter and Saturn. Our present understanding is clearly limited by the fact that the auroral emissions on both planets vary rapidly compared with the cadence of HST observations. Missions that study the Earth aurora from space observe the global auroral activity nearly continuously for years at a time, coupled with satellite measurements of particle precipitation, ground-based measurements of ionospheric currents, neutral winds, and local auroral emissions at high resolution. It is through this high time coverage that the dependence of the Earth aurora on the solar wind has been untangled. By comparison, on average there have been about 10 HST orbits per year of giant planet auroral observations, while the auroral activity varies on timescales from localized flares lasting tens of seconds to giant storms lasting hours to days.
We propose to take the next step toward understanding the physical processes driving the aurora on Jupiter and Saturn, including the effects of the solar wind, through a concentrated series of HST observations coordinated with planetary spacecraft and other earth-based observations in 2007.
A unique opportunity exists in HST cycle 15 to discover the physical principles underlying the auroral processes on Jupiter and Saturn. This is the International Heliophysical Year (IHY), an international program on the 50th anniversary of the 1957 International Geophysical Year to perform intensive coordinated measurements of space physics phenomena at the Sun, the Earth, the other planets, the solar wind, and the heliosphere. This event fortuitously overlaps with the Cassini spacecraft orbital mission about Saturn, and the New Horizons (NH) spacecraft flyby of Jupiter. In addition, intensive observations of both Jupiter and Saturn from ground-based observatories around the world are planned, and especially intensive measurements of the solar wind will be carried out. It will therefore be possible to correlate all the data taken by the different spacecrafts with actual images from HST to better understand the morphology of aurorae as well as the influence of each parameter (Solar Wind, IMF...) on Auroral events in the giant planets.