Ultraviolet emissions from the upper atmospheres of the planets

A large number of atoms and molecules have strong emission lines in the vacuum ultraviolet. As a result, this spectral region is particularly suited to studying the upper atmospheres of the planets. The observed emissions not only identify the constituents, but also provide information on the solar and magnetospheric excitation processes. Long term monitoring of these emissions, particularly with modest spatial resolution, can elucidate the effects of variations in the solar input as well as changes in magnetospheric conditions. Also, earth orbiting telescopes generally provide better spectral resolution than is available on flyby vehicles. A modest beginning in planetary upper atmospheric studies from earth orbit has been made using orbiting observatories designed primarily for stellar astronomy. As examples of the power of this technique, some recent results will be reviewed with an emphasis on Jupiter and the Io torus. The unusual scheduling requirements and the effects of scattered intense long wavelength radiation put demands on orbiting planetary observatories which are somewhat different from those of stellar astronomy. The implications of these demands for continued advances in this area are discussed.