Renée Prangé

More about the structure of the high latitude Jovian aurorae

Abstract This study is based on the determination of a ‘reference’ main oval for Jupiters aurora from a series of high-resolution images taken with the Faint Object Camera on board the Hubble Space Telescope in the H2 Lyman bands centered near 1550 A . We have taken advantage of the visibility of the northern auroral oval over a large range of longitudes on June 24, 1994, especially for longitudes smaller than 170° where it is generally very faint and thus undetectable. In the south, where there is no such problem, we combined images taken from various points of view between June 1994 and September 1996. We find that the northern main oval is consistent in size and in general aspect with the footprint locus, in the Connerney (1998)s VIP4 model, of magnetic field lines crossing the equator near 20RJ. However, the precise shape of this oval differs from the model (and from previous ‘reference’ main ovals) in that it exhibits a ‘bean-like’ aspect with excursions toward lower latitudes in the SIII longitude range 190–240° (an already reported feature), and toward higher latitudes in the poorly documented 120–150° range. In the south, our reference oval covers an area about that of the 30RJ VIP4 model. As in the north, it is shifted from a VIP4 model oval, toward lower latitudes from 110 to 200° and toward higher latitudes from 310 to 100°. The accurate definition of these (magnetically conjugate) oval loci puts additional strong constraints on magnetic field models at high latitude. Based on our reference main oval, we have then extrapolated still higher latitude ovals. Very interestingly, we find that we can fit (i) the highest latitude arc of oval detected well inside the main oval at longitudes greater than 170°, and (ii) the high latitude edge of what we had previously named the ‘transpolar emission’ at longitudes less than 170° (both also detected on images taken at other dates), by a single empirical oval. We suggest that this oval indicates the location of the northern polar cap boundary, within the uncertainty related to the temporal variability of the actual emission features. We can also fit another secondary arc of oval and a second branch of our ‘transpolar emission’ by a slightly larger empirical oval, presumably connected to the outer magnetosphere. This implies that the region of permanent high latitude diffuse emission seen inside the main oval in the dusk sector must be at the footprint of closed field lines connected all the way out from the middle magnetosphere to the magnetopause. Finally a bright spot is sometimes observed just equatorward of the northern polar cap boundary with an average brightness of 0.5-1 MR (comparing with a bright main oval). We establish that this spot does not rotate with the planet, but rather remains close to magnetic noon. We thus tentitatively identify it, by reference to the Earth aurora, with the footprint of the northern Jovian polar cusp, or with a transient dayside aurora. We also highlight the differences observed in the southern high latitude structure.

Jovian ultraviolet auroral activity, 1981-1991

Abstract The distribution of auroral brightness on Jupiters surface as a function of magnetic longitude (System III, 1965) is studied using International Ultraviolet Explorer (IUE) observations of H 2 ultraviolet emissions over the years 1981 to 1991. The brightness is diagnostic of energy input to the atmosphere and of magnetospheric processes. The brightness distribution is determined by assuming model distributions, simulating the observations, and then comparing the predicted vs empirical brightnesses. The model brightness distributions consist of a single peak on top of a baseline emission, constrained to a set of candidate auroral ovals. The north auroral data are best fit by brightness distributions on the oval mapped by the Voyager UVS experiment, compared with footprints of magnetic field lines at L = 5.9, 8, 10, 12, and the last closed field lines. The southern auroral data weakly favor distributions using the L = 8 oval, but all the candidates for the southern ovals fit the observations adequately and produce similar brightness distributions, a consequence of the observational geometry and the limited angular resolution of the IUE instrument. The north and south aurorae appear to be correlated in brightness and in variations of the longitude of peak brightness. The north and south auroral brightness peaks are at ∼194° and ∼24° respectively on average with significant variability about these positions. The average auroral input power to both poles together is (8.2 ± 3.2) × 10 13 W (heavy ion primaries) or (4.3 ± 1.5) × 10 13 W (electron primaries). No long-term trend in the auroral brightness or morphology is detected between solar minimum and solar maximum, but there are strong fluctuations in all the parameters of the brightness distribution on much shorter time scales.

Ultraviolet imaging of the Jovian aurora with the Hubble Space Telescope

We present here for the first time a Lyman-α image of the north polar region of Jupiter obtained with the Faint Object Camera (FOC) on board the Hubble Space Telescope a few hours after the encounter of the ULYSSES spacecraft with Jupiter. The presence of high latitude regions of enhanced emission is clearly observed. A comparison with the location of the “UVS oval”, the Io (L = 6) and high-latitude field-line footprints shows that the best agreement is obtained with the L ≥ 15 footprint and the UVS oval which are close to each other for the particular longitudinal sector (30° < λIII < 210°) observed. These two L-shells correspond to two possible sources of precipitation: particles originating respectively from the region of the plasma torus of Io in a distorted magnetic field or particles from the distant magnetosphere by analogy with the terrestrial aurora. The first direct determination of the latitudinal extent of the oval and of its intensity is made and compared with previous estimates.

Temporal monitoring of Jupiter's auroral activity with IUE during the Galileo mission. Implications for magnetospheric processes

Abstract In this study, we analyze FUV auroral spectra of Jupiter acquired on a quasi-continuous basis between August 17 and September 25, 1996 as part of the last IUE Key Project. This campaign was coordinated with Galileo measurements. We show that it is possible to define an “auroral activity index” which quantifies the variability of the flux radiated in the H2 bands (1560–1620 A). The activity indices in both hemispheres are generally similar and indicate a strong interhemispheric conjugacy, suggestive of particle precipitation on closed field lines. We identify variability on three different scales: small variations of 10–20% are observed on short time scales (∼a few hours), variations by a factor of 2–4 occur on scales of 5–10 days, and a long-term trend is observed on a scale which exceeds the 6 weeks of observations. The energy output in this spectral range is a direct measurement of the energy losses of the magnetosphere along auroral magnetic field lines. We establish here that its intermediate time-scale variations, the best documented ones in this study, are amazingly well correlated with the state (quiet or disturbed) of the magnetic field measured by the magnetometer on board Galileo, although distances from ∼15 to ∼115RJ and all local times were sampled during that time. This indicates temporal, rather than spatial, variations of the magnetic field, and suggests that some dynamical process, yet to be identified, affects the magnetosphere as a whole and triggers recurrent energy releases along auroral field lines. The recurrence time derived from this study is larger than the 3-day periodicity previously assigned to variabilities in the magnetotail on the basis of Galileo measurements during other time periods. A crude preliminary comparison also indicates correlations with the intensity of the auroral kilometer radio waves and anticorrelations with the local plasma density, both derived from the Galileo PWS experiment. Comparison with other particle and field instruments is underway.

The magnetic-field-aligned polarization electric field and its effects on particle distribution in the magnetospheres of Jupiter and Saturn

Abstract In the magnetospheres of Saturn and Jupiter, it is well known that, even in the absence of electric currents, a polarization electric field appears along field lines to tie together ions and electrons. The present study focuses on this parallel electric field (ifE∥), the potential drop it implies across these magnetospheres, and its consequences on the mapping of the particle densities. We start from general expression of this field, in terms of centrifugal, gravitational and mirror forces. We conduct simple numerical simulations for n-component plasmae, where we analyze the influence of different parameters on the plasma equilibrium: the plasma anisotropy, the mass and temperatures of all species and the field line geometry. We apply the equilibrium model to existing plasma distributions at Saturn and Jupiter. Typical potential drops between the ionosphere and the equator are ∼ 30 Volts at Saturn and ∼ 300 Volts at Jupiter. Effects of E∥ on the mapping of particle densities along field lines of Saturns and Jupiters magnetospheres are discussed in detail.

Detection of Self-reversed Lyα Lines from the Jovian Aurorae with the Hubble Space Telescope

We present Lyα profiles taken at high spectral resolution (~70 mA) in the Jovian aurorae, using the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope. They exhibit a strong central reversal, reminiscent of solar and stellar spectra. This effect was predicted by models as a consequence of radiative transfer effects on photons excited deep in the atmosphere by energetic charged particles from the magnetosphere. However, it had not been detected until these observations, the first detection of a reversed planetary Lyα line. The peaks on both sides of the reversal are separated by about 0.1 to 0.15 A depending on the region, indicating faint and variable atomic hydrogen vertical column densities of about (1-5) × 1016 cm-2 above the auroral source. We find that the line profiles are asymmetric, with differences in intensity between the blue and red peaks. The reversals and the nearby H2 emission lines seem to be tied to the planets rest frame, whereas the auroral photons may be shifted by as much as a few kilometers per second in either direction. Finally, the auroral profiles display almost symmetric weak wings extending over more than 1.5 A, possibly because of emission from a superthermal hydrogen population.

A dynamical model of Jupiter's auroral electrojet

A global simulation for the auroral electrojet on Jupiter is presented. The required sequence of models was computed using JIM (the Jovian Ionospheric Model), a time-dependent, three-dimensional model for the thermosphere and ionosphere of Jupiter, and an a priori model for the planets ionospheric electric field. We describe the plasma dynamics in the model by considering ion and electron motions at pressure levels less than 2 ?bar, lying above Jupiters dynamo region, and including the region of maximum energy deposition by auroral particles. By considering the motions of the neutral species being `dragged by the electrojet, we quantify the electrodynamic coupling between the neutral thermosphere and the auroral ionosphere. Two distinct altitude regions evolve in the model simulations, distinguished by different thermospheric flow patterns. Higher-altitude regions are subject to gas dynamic flow, while lower-altitude regions are strongly influenced by electrodynamic flow, associated with the transfer of momentum from the electrojet to the neutral gas. The electrojet models provide a basis for physical interpretation of current observational detections of ion motions in the Jovian auroral regions; as well as a means of optimizing future observations, in order to make similar detections.

Auroral Lyman α and H2 bands from the giant planets: 2. Effect of the anisotropy of the precipitating particles on the interpretation of the “color ratio”

Previous spectral analyses have given evidence of collisionally excited Jovian and (at times) Saturnian H2 Werner bands being absorbed by hydrocarbons at the shortest wavelengths along the auroral ovals, and of a longitudinal dependence of this absorption in the Jovian aurorae. This “color ratio” has been used to estimate the energy of the primary particles. In such estimates, particles are generally assumed to penetrate vertically into the atmosphere. However, the precipitating particle angular distribution is unknown, and a model developed for a diffuse aurora by Prange and Elkhamsi (1991), for instance, predicts quite different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distributions peaking far from vertical may increase the energy derived from a given color ratio by as much as a factor of 3. We discuss previous interpretations of the color ratio longitudinal modulation (variation of the auroral atmosphere structure, or of the incident particle energy) in view of the subsequent increase in energy input. We argue that an interpretation in terms of energy variations only is not consistent with the energy available in the magnetosphere if the aurorae are diffuse, and we discuss this finding in the context of recent Hubble Space Telescope images.

The impact of comet Shoemaker-Levy 9 on the Jovian ionosphere and aurorae

Abstract We outline the effects of a “typical” fragment of Comet Shoemaker-Levy 9 on the ionosphere and aurorae of Jupiter, viewed chronologically from approach, through impact to longer term effects. The ionosphere is found to have been a sensitive tracer of the impact events. The powerful jovian magnetic field transmitted the effects of the collisions across hemispheres. Electrodynamical processes were particularly important around the time of fragment impact, along with shock heating of the ionosphere. Chemical reactions not usual in the jovian upper atmosphere played a key role in diminishing ionospheric and auroral emission intensities.

The UV and IR Jovian aurorae

Abstract Although the Jovian aurorae have been widely considered as due to the interaction of the Io torus with the magnetosphere, rather little had been learnt until the last few years from the remote UV observations as far as generation mechanisms are concerned, due both to the complexity of the observation analysis and the deficiency of the theoretical framework relating them to the primary magnetospheric particles. Recently developed methods, accounting for spectral and/or spatial features of IUE and Voyager FUV/EUV observations are now providing the first insights on the primary particle species, energy and origin. Simultaneously, new modeling of the trapped particle dynamics and of the atmospheric properties are beginning to reveal what are the critical features to look at. This review will include the comparison between UV and IR data, and a brief overview of the capabilities to be expected from the future missions (HST, Galileo, Cassini).