WHAT ASTRONOMY HAS LEARNED FROM OBSERVATIONS OF H-3(+)

We review the uses to which H-3(+) astronomical observations have been put, in the five years since the first detection of this molecular ion in the aurorae of Jupiter. Spectroscopy of Jupiter shows that the ionospheric temperature is high (similar to 1000 K) compared with the lower atmosphere and that auroral column densities are between 10(12) and 10(13) cm(-2). The molecule is also distributed widely across the planet in concentrations about 10% of peak auroral densities. Imaging in H-3(+)-sensitive wavelengths links the aurorae to open magnetic field lines, rather than those encompassing the Io plasma torus. Uranus shows H-3(+) emission intensities of a few percent olf the peak jovian auroral lines, with somewhat weaker emission from Saturn. On Uranus, H-3(+) is distributed fairly evenly across the planet, with some indication that auroral enhancement is not more than a factor of two. Saturnine H: appears to be concentrated more towards the magnetic poles. The identification of H; in the spectrum of the type II supernova SN1987a constrains models of the explosion to avoid microscopic mixing during much of the first year of the event. So far no reliable detection of H-3(+) in the interstellar medium has been reported.