Daniel Rego

Detailed study of FUV Jovian auroral features with the post-COSTAR HST faint object camera

A set of Hubble Space Telescope faint object camera images taken in the H 2 bands near 1550 A is used to infer the morphological properties of the steady state Jovian FUV aurorae. We focus on issues best addressed using the excellent spatial resolution available after correction of the spherical aberration, i.e., those related to high latitude or small auroral features. A thorough comparison of the emission loci with model ovals highlights the improvement of the VIP4 magnetic field model over previous ones at all latitudes. The northsouth conjugacy of the main oval is now correctly accounted for, and second-order discrepancies suggest non axially symmetric contributions of external origin. This oval is usually amazingly narrow (down to 80 ± 50 km) and very bright, although quite variable with time (100 kR to 1-2 MR, i.e., peak input flux of∼ 10-200 ergs cm -2 s -1 ). We discuss its structure, in latitude and longitude, and show that it is consistent with precipitation by pitch angle scattering. Fainter concentric narrow ovals are also present on the north polar cap, presumably at the footprint of open field lines. Both polar caps are partly covered by a faint diffuse emission, confined to the afternoon sector in magnetic local time. A bright extended feature, previously identified across the north polar cap along the 160° meridian, might be not a specific auroral feature but rather a region where inner arcs and diffuse polar cap emissions are intensified, maybe by a solar wind driven ionospheric process. Equatorward of the main oval, we identify a belt of moderate emission, attributed to a precipitation process distributed between the In torus and the distant magnetosphere. Longitudinally confined bright areas lie in the same latitude range and consist of series of short segments of concentric arcs. We also present the discovery of a narrow faint oval at the footprint of Ios orbit. Finally, we confirm that the FUV footprints of the Io flux tube are very small (a few hundreds of kilometers or less), implying an interaction close to Io. The input energy flux in this spots is huge and variable (0.8-5 x 10 11 W).

The role of H3+in planetary atmospheres

Spectroscopic studies of the upper atmospheres of the giant planets using infrared wavelengths sensitive to the H3+ molecular ion show that this species plays a critical role in determining the physical conditions there. For Jupiter, we propose that the recently detected H3+ electrojet holds the key to the mechanism by which the equatorial plasma sheet is kept in (partial) co–rotation with the planet, and that this mechanism also provides a previously unconsidered source of energy that helps explain why the jovian thermosphere is considerably hotter than expected. For Saturn, we show that the H3+ auroral emission is ca. 1% of that of Jupiter because of the lower ionospheric/thermospheric temperature and the lower flux of ionizing particles precipitated there; it is probably unnecessary to invoke additional chemistry in the auroral/polar regions. For Uranus, we report further evidence that its emission intensity is controlled by the cycle of solar activity. And we propose that H3+ emission may just be detectable using current technology from some of the giant extra–solar planets that have been detected orbiting nearby stars, such as Tau Bootes.

Supersonic winds in Jupiter's aurorae

Jupiter has a giant magnetosphere that is coupled to the planets upper atmosphere; as the planet rotates, its magnetic field drags a dense ionized equatorial sheet of plasma, which must interact with the upper atmosphere. Jupiters aurorae are much more powerful, than the Earths, and cause significant local heating of the upper atmosphere. Auroral electrojets—ion winds that race around Jupiters auroral ovals—play a key role in theoretical models of how Jupiters rotational energy is transferred to the plasma sheet, and how winds may transport energy from auroral heating to lower latitudes. But there has hitherto been no direct observational evidence for the existence of such electrojets. Here we report observations of electrojets that have winds approaching or in excess of the local speed of sound. The energy produced by these electrojets could heat the whole upper atmosphere, if the auroral regions couple efficiently with the rest of the planet.

HST far-ultraviolet imaging of Jupiter during the impacts of comet Shoemaker-Levy 9

Hubble Space Telescope far-ultraviolet images of Jupiter during the Shoemaker-Levy 9 impacts show the impact regions darkening over the 2 to 3 hours after the impact, becoming darker and more extended than at longer wavelengths, which indicates that ultraviolet-absorbing gases or aerosols are more extended, more absorbing, and at higher altitudes than the absorbers of visible light. Transient auroral emissions were observed near the magnetic conjugate point of the K impact site just after that impact. The global auroral activity was fainter than average during the impacts, and a variable auroral emission feature was observed inside the southern auroral oval preceding the impacts of fragments Q1 and Q2.

Auroral Lyman α and H2 bands from the giant planets: 1. Excitation by proton precipitation in the Jovian atmosphere

This paper is part of a work aimed at modeling the ratio of the observed Jovian auroral intensity at H Lyman α and in the H2 Lyman and Werner bands and interpreting them as diagnostic of the incident magnetospheric particle species and energy. The work is planned in three steps: (1) modeling of the volume excitation rate, (2) modeling of the radiative transfer at Lyman α, (3) application to existing observational data and new data obtained from the Hubble Space Telescope. The present paper deals with the first step. Models of the volume excitation rate have previously been developed for low energy electrons and oxygen ions. However, the energy range of the study has to be extended towards higher energy in view of recent results on the penetration depth of the primary particles. Protons have not been modeled so far. We have used an existing electron code of degradation of energy [Gerard and Singh, 1982] which has been improved, updated and adapted to the case of precipitating protons. The issues of nonequilibrium beam H/H+ fractions and of getting reliable cross sections over a wide energy range have been considered with particular care. The altitude distribution of the volume excitation rate is compared for electrons and protons, for various initial energies in the range 10–50 keV and 50 keV to 1 MeV, respectively.

The H3+ Latitudinal Profile of Saturn

We present an H latitudinal profile of Saturn, obtained in 1998 October using the CSHELL spectrometer on the NASA Infrared Telescope Facility. The profile, measured at 3.953 μm, shows that the majority of the emission is concentrated in the auroral ovals, making Saturn similar to Jupiter and different from Uranus. The spatial resolution is sufficient to resolve the southern auroral oval, currently fully displayed around the south pole, into two peaks separated by 12. At the time of the observations reported here, the emission flux in the H line is 8.3 (±1.7) × 10-18 W m-2 for the intensity integrated over a 10 swath along the southern aurora and 5.8 (±1.3) × 10-18 W m-2 for the northern aurora. There may also be some mid- to low-latitude emission, similar to that on Jupiter. We suggest that planetwide H emission from Saturn is between 1.2 and 3.6 × 1011 W.

Correspondence between field aligned currents observed by Ulysses and HST auroral emission

Abstract Auroral emission is direct evidence for the magnetophere/ionosphere coupling which exists for a magnetised planet such as Jupiter. This coupling takes place by way of momentum transfer which occurs via field aligned currents. The driving mechanism for the auroral emission is charged particle precipitation along the field lines. During the Ulysses flyby of Jupiter in February 1992 high latitude auroral events were recorded by the Faint Object Camera on the Hubble Space Telescope using direct imaging of UV emissions. Measurements of magnetospheric field aligned current signatures were recorded by the dual fluxgate/vector helium magnetometer onboard the Ulysses spacecraft on the same day as the auroral, emissions. A correspondence is found to occur between the locus of auroral emission on Jupiter and the oval traced onto the auroral region by projection of the observed interval of field aligned currents. The field line mapping used to establish this correspondence is performed using a magnetospheric field model tailored to the conditions observed during the Ulysses flyby.

Comparison of IUE and HST diagnostics of the Jovian Aurorae

Analysis of international ultraviolet explorer (IUE) and Voyager ultraviolet spectrometer (UVS) spectra of the Jovian auroral emission indicates that the Jovian auroral brightness is modulated in longitude (brighter near 180° in the north and 20° in the south) and that there is a color ratio asymmetry associated with this brightening. The purpose of this study is to investigate the origin of this apparent asymmetry. To that end, we use a series of six typical images of the north auroral region taken in the H 2 Lyman bands with the faint object camera (FOC) aboard the Hubble space telescope (HST) and which cover a full Jovian rotation. Although the images do not display any strong brightening near 180°, once we have simulated the signal IUE would see through its aperture, we find the characteristic longitudinal modulation. We attribute most of this modulation to a combination of viewing geometry effects near the east and west ansae of the auroral oval (already taken into account in previous studies) and of the spatial degradation of the source by the IUE instrument function (never considered so far), and we suggest qualitatively that these effects may also affect the color ratio asymmetry. Nevertheless, part of the asymmetry seems to be clue to an intrinsic modulation associated with a bright feature crossing the polar cap along the 160° meridian (transpolar emission) and present in most of the images. We then use a series of FOC images taken during an atypically strong auroral event, and we show that the same effects can again account for the anomalous brightness variations observed simultaneously with IUE.

Infrared spectroscopic studies of the jovian ionsophere and aurorae

Abstract We review recent spectroscopic studies of the ionosphere of Jupiter. These demonstrate the importance of the H3+ molecular ion in understanding not only the ion-molecule chemistry of the jovian upper atmosphere, but of its energetics and dynamics as well. Comparisons are made with a new three-dimensional, global circulation model of Jupiters ionosphere and thermosphere, JIM.