S Miller

Detection of H3(+) from Uranus

The detection of H3(+) in Uranus is reported. Using the CGS4 spectrometer on the UKIRT telescope, we clearly detected 11 emission features of the H3(+) fundamental vibration-rotation band between 3.89 and 4.09 microns. These features are composed primarily of lines from the Q-branch; the strongest of them is the Q(3) blend at 3.985 microns. Analysis of these features indicates a rotational temperature of 740 +/- 25 K, an ortho-H3(+) fraction of 0.51 +/- 0.03, and a disk-averaged H3(+) column abundance of 6.5 x 10 exp 10 (+/- 10 percent) molecules/sq cm. Comparison is made with Jupiter, Saturn, and Neptune. A detection of the H2 1-0 S(1) line in Uranus and an upper limit to H3(+) emission from Neptune also are reported. The rate of energy deposition into Uranus appears to be significantly higher than the rate reported during the Voyager 2 flyby in January of 1986.

The influence of H2O line blanketing on the spectra of cool dwarf stars

We present our initial results of model atmosphere calculations for cool M dwarfs using an opacity sampling method and a new list of H2O lines. We obtain significantly improved fits to the infrared spectrum of the M dwarf VB10 when compared to earlier models. H2O is by far the dominant opacity source in cool stars. To illustrate this, we show the Rosseland mean of the total extinction under various assumptions. Our calculations demonstrate the importance of a good treatment of the water opacities in cool stars and the improvements possible by using up-to-date data for the water line absorption.

The domination of Saturn’s low-latitude ionosphere by ring ‘rain’

Saturn’s ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation. At low latitudes the solar radiation should result in a weak planet-wide glow in the infrared, corresponding to the planet’s uniform illumination by the Sun. The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models. A planet-to-ring magnetic connection has been previously suggested, in which an influx of water from the rings could explain the lower-than-expected electron densities in Saturn’s atmosphere. Here we report the detection of a pattern of features, extending across a broad latitude band from 25 to 60 degrees, that is superposed on the lower-latitude background glow, with peaks in emission that map along the planet’s magnetic field lines to gaps in Saturn’s rings. This pattern implies the transfer of charged species derived from water from the ring-plane to the ionosphere, an influx on a global scale, flooding between 30 to 43 per cent of the surface of Saturn’s upper atmosphere. This ring ‘rain’ is important in modulating ionospheric emissions and suppressing electron densities.

The effect of the impact of comet Shoemaker Levy-9 on Jupiter's aurorae

We present infrared spectra and images of the jovian aurorae taken at wavelengths sensitive to the H3+ molecular ion during the period around the impact of Comet Shoemaker Levy-9. The spectra were obtained using CGS4 on the United Kingdom Infrared Telescope and the images using NSFcam on NASAs Infrared Telescope Facility. Comparison with spectra obtained in May, 1993, shows that while the relative intensities of the northern and southern auroral zones prior to and during impact week (July 16–22, 1994) were broadly comparable with those of 1993, a few days after the last collision the northern aurora was considerably enhanced and its southern counterpart somewhat depressed. The north/south auroral ratio was returning to more normal values a week later. The effect of material drifting from the impact sites to the southern auroral zone is discussed in relation to these results.

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Context. Measurement of linear polarisation in Earths thermospheric oxygen red line can be a useful observable quantity for characterising conditions in the upper atmosphere; therefore, polarimetry measurements are extended to other planets. Since FUV emissions are not observable from the ground, the best candidates for Jupiter auroral emissions are H-3(+) infrared lines near 4 mu m. This ion is created after a chemical process in the Jovian upper atmosphere. Thus the anisotropy responsible of the polarisation cannot be the particle impact as in the Earth case.Aims. The goal of this study is to detect polarisation of H-3(+) emissions from Jupiters aurora.Methods. Measurements of the H-3(+) emissions from Jupiters southern auroral oval were performed at the UK Infrared Telescope using the UIST-IRPOL spectro-polarimeter, with the instrument slit positioned perpendicular to Jupiters rotation axis. Data were processed by dividing the slit into 24 bins. Stokes parameters (u, q and v), polarisation degree and direction were extracted for each bin and debiased.Results. More than 5 bins show polarisation with a confidence level above 3 sigma. Polarisation degrees up to 7% are detected. Assuming the auroral intensity is constant during the 8 waveplate positions exposure time, i.e. around 10 min, strong circular polarisation is present, with an absolute value of the Stokes v parameter up to 0.35. Conclusions. This study shows that polarisation is detectable in the Jovian infrared auroras, but new measurements are needed to be able to use it to characterise the ionospheric environment. At present, it is not possible to propose a mechanism to explain this polarisation owing to the lack of theoretical work and laboratory experiments concerning the polarisation of H-3(+).

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Previous observations of Saturns infrared aurorae have shown that a mid-latitude aurora exists significantly equatorward of the main auroral oval. Here, we present new results using data from four separate observing runs in 1998, 2003, 2008, and 2010. When combined, these provide a view of the mid-latitude aurora under a considerable range of viewing conditions, allowing the first calculation of the latitudinal position of this aurora to be made. This has shown that the mid-latitude aurora is located at the magnetic footprint of the region within the magnetosphere where the initial breakdown in corotation occurs, between 3 R S and the orbit of Enceladus (~3.95 R S). We also confirm that this aurora is a continuous stable feature over a period of more than a decade and that an oval morphology is likely. When combined, these results indicate that the mid-latitude auroral oval is formed by currents driven by the breakdown process within the magnetosphere, in turn caused by mass loading from the torus of Enceladus, analogous with the volcanic moon Ios dominant role in the formation of Jupiters main auroral oval.

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Previous observations of Saturns infrared aurorae have shown that a mid-latitude aurora exists significantly equatorward of the main auroral oval. Here, we present new results using data from four separate observing runs in 1998, 2003, 2008, and 2010. When combined, these provide a view of the mid-latitude aurora under a considerable range of viewing conditions, allowing the first calculation of the latitudinal position of this aurora to be made. This has shown that the mid-latitude aurora is located at the magnetic footprint of the region within the magnetosphere where the initial breakdown in corotation occurs, between 3 R S and the orbit of Enceladus (~3.95 R S). We also confirm that this aurora is a continuous stable feature over a period of more than a decade and that an oval morphology is likely. When combined, these results indicate that the mid-latitude auroral oval is formed by currents driven by the breakdown process within the magnetosphere, in turn caused by mass loading from the torus of Enceladus, analogous with the volcanic moon Ios dominant role in the formation of Jupiters main auroral oval.