Spectro-imaging observations of Jupiter's 2-μm auroral emission. I. H 3+ distribution and temperature

Abstract

We report on spectro-imaging infrared observations of Jupiter's auroral zones, acquired in October 1999 and October 2000 with the FTS/BEAR instrument at the Canada–France–Hawaii Telescope. The use of narrow-band filters at 2.09 and 2.12 μm, combined with high spectral resolution (0.2 cm −1), allowed us to map emission from the H 2 S 1(1) quadrupole line and from several H 3 + lines. The H 2 and H 3 + emission appears to be morphologically different, especially in the north, where the latter notably exhibits a “hot spot” near 150°–170° System III longitude. This hot spot coincides in position with the region of increased and variable hydrocarbon, FUV and X-ray emission, but is not seen in the more uniform H 2 S 1(1) emission. We also present the first images of the H 2 emission in the southern polar region. The spectra include a total of 14 H 3 + lines, including two hot lines from the 3 ν 2– ν 2 band, detected on Jupiter for the first time. They can be used to determine H 3 + column densities, rotational ( T rot) and vibrational ( T vib) temperatures. We find the mean T vib of the v 2=3 state to be lower (960±50 K) than the mean T rot in v 2=2 (1170±75 K), indicating an underpopulation of the v 2=3 level with respect to local thermodynamical equilibrium. Rotational temperatures and associated column densities are generally higher and lower, respectively, than inferred previously from ν 2 observations. This is a likely consequence of a large positive temperature gradient in the sub-microbar auroral atmosphere. While the signal-to-noise is not sufficient to take full advantage of the 2-D capabilities of the observations, the search for correlations between line intensities, T vib and column densities, indicates that variations in line intensities are mostly due to correlated variations in the H 3 + column densities. The thermostatic role played by H 3 + at ionospheric levels may provide an explanation. The exception is the northern “hot spot,” which exhibits a T vib about 250 K higher than other regions. A partial explanation might invoke a homopause elevation in this region, but a fully consistent scenario is not yet available. The different distributions of the H 2 and H 3 + emission are equally difficult to explain.