Saturn's auroral/polar H +3 infrared emission : I. General morphology and ion velocity structure

Abstract

We present an analysis of an extensive dataset of observations of the auroral/polar regions of Saturn, carried out in 2004 and 2005 using the high-resolution facility spectrometer CSHELL on the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The paper outlines the morphology of emission in the H + 3 ν 2 Q ( 1 , 0 − ) line at 3.953 μm across the planet's southern auroral/polar region, and gives a description of the main classes of morphology observed. Similarly, it presents the ion velocity as a function of location across the auroral/polar region, with the main classes of structure observed. In presenting these, it sets out the baseline phenomenological parameters, both averaged across our entire dataset and for specific nights, for use in future studies. Averaged across our dataset, the (planetary) west–east infrared H + 3 emission profile of Saturn's auroral/polar regions indicates the presence of a clear auroral oval, roughly coincident with that delineated by EUV images, although there is clearly relatively more H + 3 emission across the polar cap than would be expected from a straight correlation with the EUV images. The integrated H + 3 emission can vary by a factor of ∼50 between observing runs, and a factor of two on a night-to-night basis. Again averaged across our dataset, the lag to co-rotation of ions in the auroral/polar regions is considerable. We also find that the lag to co-rotation shows large variations, and that it is possible to distinguish sub-regions within the auroral/polar region and characterise them by the rate at which the velocity changes as a function of distance from the centre of the planet.