Ground-based observations of Saturn’s auroral ionosphere over three days: trends in H3+ temperature, density and emission with Saturn local time and planetary period oscillation

O'Donoghue, J. ORCID: https://orcid.org/0000-0002-4218-1191, Melin, H., Stallard, T. S., Provan, G., Moore, L., Badman, S. V., Cowley, S. W. H., Baines, K. H., Miller, S. and Blake, J. S. D. (2016) Ground-based observations of Saturn’s auroral ionosphere over three days: trends in H3+ temperature, density and emission with Saturn local time and planetary period oscillation. Icarus, 263. pp. 44-55. ISSN 0019-1035 doi: 10.1016/j.icarus.2015.04.018

Abstract/Summary

On 19–21 April 2013, the ground-based 10-m W.M. Keck II telescope was used to simultaneously measure H3+ emissions from four regions of Saturn’s auroral ionosphere: (1) the northern noon region of the main auroral oval; (2) the northern midnight main oval; (3) the northern polar cap and (4) the southern noon main oval. The H3+ emission from these regions was captured in the form of high resolution spectral images as the planet rotated. The results herein contain twenty-three Hþ3 temperatures, column densities and total emissions located in the aforementioned regions – ninety-two data points in total, spread over timescales of both hours and days. Thermospheric temperatures in the spring-time northern main oval are found to be cooler than their autumn-time southern counterparts by tens of K, consistent with the hypothesis that the total thermospheric heating rate is inversely proportional to magnetic field strength. The main oval H3+ density and emission is lower at northern midnight than it is at noon, in agreement with a nearby peak in the electron influx in the post-dawn sector and a minimum flux at midnight. Finally, when arranging the northern main oval H3+ parameters as a function of the oscillation period seen in Saturn’s magnetic field – the planetary period oscillation (PPO) phase – we see a large peak in H3+ density and emission at ~115° northern phase, with a full-width at half-maximum (FWHM) of ~44° This seems to indicate that the influx of electrons associated with the PPO phase at 90° is responsible at least in part for the behavior of all H3+ parameters. A combination of the H3+ production and loss timescales and the ±10° uncertainty in the location of a given PPO phase are likely, at least in part, to be responsible for the observed peaks in H3+ density and emission occurring at a later time than the peak precipitation expected at 90° PPO phase.