Emma Thomas
When Voyager 2 flew by Uranus in 1986 we first detected and successfully mapped the the ultraviolet aurora across the planet’s surface. Since then we have only observed the planet’s aurora in the ultraviolet using the Hubble Space Telescope in 2012 and 2017 but never in the infrared using ground-based facilities.
Our aim is to detect and produce the initial mapping of the infrared aurora across Uranus through H3+ emissions. To do this we have used data from an observation run in September 2006 using the NIRSPEC instrument (Near InfraRed SPECtrograph) on the W. M. Keck II Telescope, where approximately 6 hours of Uranus observations were taken with a 0.43x24” slit, whilst Uranus measured 3.7” in the sky. We present two spatial mappings across Uranus using five emission lines: known as the Q branch of H3+ fundamental emission lines. From these we can measure intensity values, which can then be used to calculate and successfully map H3+ ro-vibrational temperatures and column density across a 121 degree longitudinal period.
Our results show significant intensity variations across Uranus which peak above estimated errors. In contrast we see no peaks in ro-vibrational temperature across these areas but instead find increased column density of H3+ (which occur close to or at these peaks in intensity). The implications of these results suggest that there is an increase in ionisation rates in the upper atmosphere, which in confirms our results to be the first discovery of the infrared aurora at Uranus!
Our future work will require line of sight adjustments, three-dimensional modelling and comparing our own models to those from the results of Voyager II. Furthermore, we hope to use the same method to carry out analysis on later observations to produce a complete aurora mapping across Uranus.
These past observations have either shown increases in H3+ density from average emissions across Uranus on a single observation night, or have revealed bright spots across Uranus but have been unable to calculate the temperature and density of these spots, and thus unable to confirm if these patches of emissions are auroral in nature.
Using the Near InfraRed SPECtrometer (NIRSPEC) on the Keck II telescope (which has a resolution of over 20,000), we have investigated a night of observations to measure variations in H3+ emissions across Uranus’s ionosphere and to measure both the ro-vibrational temperatures and column densities, to determine the cause of these emissions, if auroral.
Observations were taken between 07:26 and 13:23 UTC on 5th September 2006. Using a 0.288x24” slit aligned to the planet’s North-South (N-S) rotational axis, a total 224 spectra images were used in our dataset. To subtract the effects of sky emission in our data and to increase the signal to noise ratio, we present a total 13 emission spectra sets. These contain five H3+ emission lines of the Q branch, known as Q(1,0-) to Q(5,0-) between 3.94 and 4.00µm.
By measuring the brightness (intensity) of all five emissions lines across all 13 datasets, intensity variations across Uranus’s upper atmosphere can be measured and then used to calculate temperature and column density. Due to a break (calibration star observations) and slit nonalignment (between 10:52 and 11:31 UTC), there is a gap in our data.
In figure 4 and 5, our observations show an area between 0º and 70ºS latitude starting from 0º to 34º longitude with peak intensities between 0.78 to 0.49 μW m-2 sr-1 (15% above average). A second, much larger area of peak intensities was also observed from 70ºN and 30ºS latitude, at 85º to 112º longitude with values between 0.77 to 0.50 μW m-2 sr-1 (14% above average).
To determine if these intensity peaks were due to thermal processes within the ionosphere (upper atmosphere of Uranus containing free electrons and ions), we investigated the temperatures for these regions. Average temperatures were found to be 581K and 563K, which are within the error margin for the average temperature for these datasets (576K). Hence these peaks in brightness could not be due to increases in temperature.
We then investigated the column density, to see if an increase in the number of ions could explain the increase in brightness. From our results we found significant peaks of column density with values between 65% to 59% greater than the total dataset average. Additionally, these peaks of column density align closely with or exactly with the positions of peak intensities, suggesting that column density is the cause for these emissions.
We have observed two areas of H3+ intensity peaks, one in the southern hemisphere and a larger area in the northern hemisphere. These peaks are not due to thermal processes and instead occur due to peaks in column density at these regions, proving that we have discovered the infrared aurora at Uranus for the first time!