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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.

[1] During the flyby of the Earth by Cassini in 1999, the Radio and Plasma Wave Science (RPWS) instrument recorded a month of quasi-continuous observations of auroral kilometric radiation (AKR). Analyzing the full polarization of incoming radio waves, we found AKR to be 100% circular left-handed (LH) or right-handed (RH). As AKR is emitted predominantly on the extraordinary mode, we analyzed separately the northern (RH) emission, from the southern (LH) one with respect to the magnetic equator. AKR power variations reveal (1) a log-normal distribution at time scales of minutes, (2) bursts of emission at time scales of a few hours, and (3) a clear overall modulation at 24 h, together with a weaker modulation at 12 h (especially visible for LH emissions). The prominent 24 h modulation is found to modulate LH and RH AKR in phase opposition. This is interpreted as being due to visibility effects related to the precession of the terrestrial magnetic dipole, making Cassini oscillate relative to the average AKR beaming pattern from each hemisphere. We accordingly quantified the AKR beaming as a function of explored latitudes. On time scales shorter than a few hours, LH and RH emissions are found to be correlated. This is attributed to the actual conjugacy of the corresponding sources, simultaneously turned on by substorm occurrence. The geometrical anticorrelation (at 24 h) dominates close to Earth, while the short-term correlation (substorms) dominates far from Earth, where the detection threshold makes the visibility less important than the occurrence of substorms. Finally, the 12 h modulation is detected when it is not masked by strong visibility effects, namely, for the LH emission, which is observed mostly near the magnetic equator along the path of Cassini. As a 12 h modulation of some geomagnetic indices was also observed, we suggest that the AKR semidiurnal modulation may have a physical cause, for which we favor the variable geometry and efficiency of the reconnection between interplanetary and geomagnetic fields, or magnetotail oscillations.

[1] Similarly to other magnetized planets, Saturn displays auroral emissions generated by accelerated electrons gyrating around high-latitude magnetic field lines. They mainly divide in ultraviolet (UV) and infrared (IR) aurorae, excited by electron collisions with the upper atmosphere, and Saturn's kilometric radiation (SKR), radiated from higher altitudes by electron-wave resonance. Whereas spatially resolved UV and IR images of atmospheric aurorae reveal a continuous auroral oval around each pole, the SKR source locus was only indirectly constrained by the Voyager radio experiment to a limited local time (LT) range on the morningside, leading to interpretation of the SKR modulation as a fixed flashing light. Here, we present resolved SKR maps derived from the Cassini Radio and Plasma Wave Science (RPWS) experiment using goniopolarimetric techniques. We observe radio sources all around the planet, organized along a high-latitude continuous auroral oval. Observations of the Hubble Space Telescope obtained in January 2004 and January 2007 have been compared to simultaneous and averaged Cassini-RPWS measurements, revealing that SKR and UV auroral ovals are very similar, both significantly enhanced on the dawnside. These results imply that the SKR and atmospheric aurorae are triggered by the same populations of energetic electron beams, requiring a unified model of particle acceleration and precipitation on Saturn.

[1] We present goniopolarimetric (also known as direction finding) results of the Saturn kilometric radiation (SKR), using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver data. Tools to retrieve the characteristics of the SKR sources have been developed that allow us to measure their 3-D location and beaming angle relative to the magnetic field in the source and, thus, to deduce the location of the footprints of the active magnetic field lines. We present results from these analyses on SKR observed during the revolution 29 perikrone (25–26 September 2006) with a relatively high orbital inclination. These results provide for the first time the observed beaming angle, the invariant latitude, and the local time of the SKR sources. We provide evidence that the SKR is mainly emitted in the right-hand extraordinary (R-X) mode and marginally in the left-hand ordinary (L-O) mode. We observe the footprint of the active magnetic field lines in the ∼70° to ∼80° northern and southern latitudinal range and in the 0400 to 1600 local time range. The northern sources are observed at slightly higher latitude than southern sources. The location matches that of the UV and IR aurorae. Duskside and nightside sources are also detected.

[1] Accelerated electrons gyrating around Saturn's auroral (high latitude) magnetic field lines generate the intense Saturnian kilometric radiation (SKR). This radio emission is thought to be generated via the cyclotron maser instability (CMI) that predicts a strong anisotropy of the beaming pattern of the emission. Resulting visibility effects were suspected to be at the origin of characteristic features observed in Cassini's dynamic spectra as arc-shaped structures and shadow zones. By using the Planetary Radio Emissions Simulator (PRES) code, we model these visibility effects and their consequence on Cassini's observations of SKR: we compute the dynamic spectra resulting from the beaming pattern of CMI-generated SKR and its intersection with Cassini's trajectory. The SKR beaming pattern and its dependence on the frequency is computed for two typical electron distributions observed in auroral regions: a loss cone (favoring oblique emission) or a shell (favoring perpendicular emission). We successfully fit observed arc-shaped structures and shadow zones. Although oblique and perpendicular emissions both allow to produce radio arcs, the fit is better in the oblique case. Similarities and differences between observations and model results are discussed and perspectives are outlined.

[1] Since Cassini entered Saturn's magnetosphere in July 2004, the auroral Saturnian kilometric radiation (SKR), which dominates the kronian radio spectrum, is observed quasi-continuously. Consecutive orbits of the spacecraft covered distances to Saturn down to 1.3 Saturn radii, all local times and, since December 2006, latitudes as high as 60°. On the basis of carefully calibrated and cleaned long-term time series and dynamic spectra, we analyze the average properties, and characteristics of the SKR over 2.75 years starting at Cassini's Saturn orbit insertion. This study confirms and expands previous results from Voyager 1 and 2 studies in the 1980s: the SKR spectrum is found to extend from a few kHz to 1200 kHz; extraordinary mode emission dominates, i.e., left-handed (LH) from the southern kronian hemisphere and right-handed (RH) from the northern one, for which we measure directly a degree of circular polarization up to 100%; the variable visibility of SKR along Cassini's orbit is consistent with sources at or close to the local electron cyclotron frequency f ce , in the Local Time (LT) sector 09 h-12 h, and at latitudes >70°, with emission beamed along hollow cones centered on the local magnetic field vector; this anisotropic beaming results in the existence of an equatorial radio shadow zone, whose extent is quantified as a function of frequency; it also causes the systematic disappearance of emission at high latitudes above 200 kHz and below 30 kHz. In addition, we obtain new results on SKR: LH and RH intensity variations are found to match together at all timescales >30 min; moreover their spectra are found to be conjugated as a function of the latitude of the observer; we use this conjugacy to merge LH and RH spectra and derive pronounced systematic dependences of the SKR spectrum as a function of the spacecraft latitude and LT (that will be the input of a subsequent modeling study); we identify for the first time ordinary mode SKR emission; finally, in addition to the SKR and n-SMR components, we discuss the narrowband kilometric component (named here n-SKR) which extends mainly between 10 and 40 kHz, preferentially observed from high latitudes.

A new efficient radiative transfer algorithm for nonhomogeneous model atmospheres has been applied to the Uranian atmosphere. The contribution of the scatter solar Lyman-alpha to the Uranain emission is of the order of 300 R, and the Rayleigh contribution may reach 450 R for small values of the eddy diffusion coefficient (EDC). The total solar contribution may then reach about 750 R for a solar flux of 2.5 x 10 to the 11th photons/sq cm/s/A. A level of up to 400 R is confirmed in some directions for the interstellar wind contribution. The values of the atmospheric EDC necessary to mimic the observations are 50-100 sq cm/s. A small additional source located on the dayside Uranian atmosphere seems necessary correctly to fit the shape of the limb to limb intensity variation, especially near the limbs. Its contribution to the emergent intensity would range from 100 to 500 R.