First Vertical Ion Density Profile in Jupiter’s Auroral Atmosphere: Direct Observations Using the Keck II Telescope

We present the first vertical ion density profiles of Jupiter's upper atmosphere derived directly from ground-based observations. Observations of infrared H + 3 emissions in Jupiter's auroral/polar regions were collected by the high-resolution spectrometer NIRSPEC on the Keck II telescope. We have calculated vertical density profiles for a latitude in the southern auroral region using the measured column densities and a shell model of the Jovian ionospheric H + 3 emission. We compare our resultant profiles to those generated by a recent one-dimensional model in both local thermodynamic equilibrium (LTE) and non-LTE conditions. We find good agreement with the model profiles up to 1800 km. Above that, however, our measurements show that more H + 3 is produced than is predicted by the model. Our observational method is a new tool for probing Jupiter's upper atmosphere from Earth and can possibly be extended to the study of other gas giant planets.