SM34A-01 INVITED
Multispectral Observations of Jupiter's Aurora
The auroral emissions of Jupiter have been studied from ground-based observatories (e.g., IRTF, CFHT, Keck, Nançay), Earth-orbiting satellites (e.g., IUE, EUVE, HST, ROSAT, Chandra, XMM), flyby spacecraft (e.g., Voyager, Cassini, New Horizons), and orbiting spacecraft platforms (e.g., Galileo) at x-ray, ultraviolet, visible, infrared, and radio wavelengths. While jovian auroral processes are highly variable, the total emitted auroral power is generally a few tens of TW. The main emissions are from various compounds of hydrogen (e.g., H and H2 at UV, visible, and near-IR (2-4 μm) wavelengths; H3+ in the near-IR) excited either directly by a beam of energetic precipitating particles or by secondary electrons produced as the beam slows down in the atmosphere. Very energetic primary particles (or the heat they produce) can penetrate to beneath the homopause, resulting in hydrocarbon auroras---particularly emissions from CH4, C2H2, C2H6, and C2H4 at thermal-IR (7-16 μm) wavelengths (however, these emissions may also be excited by Joule heating). The x-ray and radio emissions are produced by the precipitating particles themselves. The emissions at different wavelengths provide unique and complimentary information about the physical processes operating in the atmospheric and magnetospheric regions where the auroras originate. Here we review our current understanding of auroral emissions from Jupiter, as revealed through multispectral observations.
SM34A-02 INVITED
Recent results from HST and ground-based observations of Saturn's aurora
Current observations of Saturn's aurora performed from Earth-orbit with HST and ground based instruments more than complement the in situ measurements obtained by the Cassini spacecraft. These remote observations focus on two spectral windows revealing different facets of the same auroral phenomenon. The auroral photons captured in the ultraviolet bandwidth result from direct impact excitation of H and H2 by charged particles accelerated along magnetic field lines, while the thermal infrared emission involves additional steps in order to produce hot H3+ from the auroral energy. Each spectral window presents its own advantages. The high spatial and temporal resolution of the recent UV images obtained with HST make it possible to discriminate auroral sub-structures, such as short lived arcs and spots, and to map them into the magnetosphere where they can be associated with in situ observations. Infrared high resolution spectroscopy and emission-line imaging from ground observatories (IRTF, UKIRT) have more modest spatial resolution; however they recently pinned down emissions barely observed in the UV. Furthermore, they offer a direct measurement of the ion wind velocities in the auroral ionosphere. These ion flow patterns might then be used to untangle the origin of the auroral particles. The complementarity of observations obtained in the UV and IR bandwidths provides a powerful tool to study the auroral mechanisms in the Kronian magnetosphere and the atmospheric response to the auroral input.
SM34A-03 INVITED
Coordinated measurements of auroral processes at Saturn from the Cassini spacecraft and HST
One of the primary Cassini mission objectives at Saturn is to characterize Saturn's aurora—its spatial morphology, associated particle energization, radio wave generation, and magnetospheric currents, relationship with solar wind pressure and magnetic field, and its large scale mapping to the magnetosphere. By design, the Cassini orbital tour included high inclination and low periapsis orbits late in the prime mission specifically to address many of these topics. In this presentation, we will provide a snapshot of the current state of our investigation into the relationship between magnetospheric measurements of particles and fields, and the aurora. For in situ data, we will show measurements of upward traveling light ion conics (~30 keV to 200 keV), often accompanied by electron beams (<20 keV to ~1 MeV) and enhanced broadband noise (10 Hz to a few kHz), throughout the outer magnetosphere on field lines that nominally map from well into the polar cap (dipole L > 50) to well into the closed field region (dipole L < 10). Sometimes the particle phenomena and the broadband noise occur in pulses of roughly five-minute duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. Correlative observations of solar wind (Cassini) and aurora (HST) have established a strong relationship between solar wind pressure and auroral activity (brightness) (Crary et al., Nature, 2005; Clarke et al., JGR, 2008). A similar correspondence between bright auroral arcs and ring current ion acceleration will be shown here. So while some auroral forms seem to be associated with the open/closed field boundary (i.e. in the cusp—Bunce et al., JGR, 2008), we also demonstrate that under some magnetospheric conditions for which protons and oxygen ions are accelerated once per Saturn magnetosphere rotation at a preferred local time between midnight and dawn, simultaneous auroral observations by the HST reveal a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent energetic neutral atom enhancements coincide closely with bursts of Saturn kilometric radiation, again suggesting a linkage with high latitude auroral processes. Finally, we will show some intriguing results of auroral movie sequences from the Cassini UVIS instrument with corresponding ring current movies from the Magnetospheric Imaging Instrument Ion and Neutral Camera (MIMI/INCA).
SM34A-04
Cassini/VIMS observations of Saturn's infrared aurora
Here we report on the lastest images of Saturn's aurorae taken by the Visual and Infrared Mapping Spectrometer. These represent the first-ever images of the IR dayside and nightside aurorae at both the north and south poles of the planet, the first detailed temperature maps of Saturn's auroral region, as well as the highest spatial resolution images taken of any IR aurora beyond the Earth. The images show the aurorae to be highly complex and very dynamic. The main oval appears to be variable, as previous UV images have suggested. However, active polar emissions, as bright as the main auroral oval, are repeatedly observed to occur at very high latitudes (>85°). There are also significant levels of emission at all times inside the main auroral oval, though this is highly variable. In addition, there appears to be significant levels of auroral emission equatorward of the main oval, extending round from the midnight side.
SM34A-05
Saturn Auroral Movies from Cassini UVIS
Cassini's Ultraviolet Imaging Spectrograph (UVIS) has completed four years of study of Saturn's atmosphere and auroras. Two long slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn. Auroral emissions are seen from electron-excited molecular and atomic hydrogen. In 2008 UVIS obtained data with the spacecraft well out of Saturn's ring plane, permitting UVIS to obtain a number of short movies of the rotating auroral structures. In some movies a cusp-like feature is present near noon inside the oval. One movie from 2008 day 201 shows parallel linear features on the day side almost at right angles to the main auroral oval that appear, then lengthen, separate in the middle, and then fade away. The same movie also shows one bright "polar flare" inside the oval. A few of the most recent images were obtained at sufficiently close range that 2 spacecraft slews were needed to completely cover the oval. These images provide almost 100 pixels of information across the oval and clearly show multiple arcs of emission on the main oval and scattered emissions inside the oval. We will discuss these features, their locations, and possible interpretations.
SM34A-06
A Multi-Instrument Study of Auroral Hiss Emissions at Saturn Observed With Cassini
Over the last two years, the Cassini spacecraft has undergone a series of higher inclination orbits, allowing investigation and measurements of the Saturnian auroral zone. The Radio and Plasma Wave Science (RPWS) Investigation has detected low frequency funnel-shaped whistler mode emissions along the auroral field lines, much like the auroral hiss observed at Earth. The poleward and equatorward flaring of the auroral hiss funnel on the frequency-time spectrograms is the result of whistler mode waves propagating upward into a region of diminishing plasma density. These detections are important in understanding the auroral processes occurring at Saturn. The emissions usually display two periodicities, one on a short time scale of roughly one hour and a second on a time scale on the order of the SKR period. The short term variability appears to align with the one-hour periodic bursts of upward propagating field-aligned electrons at energies of tens to hundreds of keV. These detections also correlate well to the field-aligned currents observed at Saturn. Ray tracing calculations are underway to explain the funnel shape and determine the source position.
SM34A-07
A Rotating Plasma Density Structure in Saturn's High-Latitude Auroral Regions
Whistler-mode auroral hiss emissions produced by upgoing electrons associated with the auroral zone current system are commonly observed in the high-latitude regions of Saturn's magnetosphere by the Cassini radio and plasma wave instrument. As the auroral hiss emissions propagate upward from a source near the planet they are driven into resonance as they reach the level where the wave frequency is equal to the local electron plasma frequency. This resonance produces a well-defined upper cutoff frequency at the local electron plasma frequency that can be used to determine the local electron density. The densities measured over the polar cap region are extremely low, typically only about 10-4 cm-3 at radial distances of about 15 RS (Saturn radii). At lower latitudes, where auroral field-aligned currents are observed, the densities are higher, about 10-3 to 10-2 cm-3. The transition between these two density regimes is often very sharp, and shows a rotational modulation that stays in step with the approximately 10.8-hour rotational modulation of the auroral current system and Saturn Kilometric Radiation (SKR). This relationship suggests that there is rotating plasma density structure in the high latitude magnetosphere that is closely associated with the rotating auroral current system. This rotating plasma density structure may play a key role in driving various rotational effects observed in Saturn's magnetosphere.
SM34A-08
Location and correlation of Saturnian Kilometric Radiation (SKR) sources and UV aurorae
The Saturnian Kilometric Radiation (SKR) is the most intense kronian radio emission. It is an auroral emission similar to the auroral kilometric radiation (AKR) at Earth, supposedly associated with Saturn's polar aurorae. Here we investigate observations relating the SKR sources observed by the Radio Plasma and Wave Science experiment of the Cassini spacecraft to Ultraviolet (UV) aurorae as observed either by the Hubble Space Telescope (HST) or the UVIS instrument onboard Cassini. We take advantage of Cassini-HST combined observations in 2004, 2007 and 2008, during which the SKR was observed quasi-continuously whereas the HST took hundreds of images of Saturn's aurorae spread over more than 30 days. We also use UVIS observations taken when Cassini is located close to the planet (typically below 10 kronian radii). On the one hand, polar projection of UV images allow us to directly derive the intensity, latitude and local time of aurorae. On the other hand, assuming that the SKR is emitted via the Cyclotron Maser Instability, i.e. close to the local electron gyrofrequency, a goniometric (direction-finding) analysis of the data allows to measure the coordinates of the magnetic footprint of field lines carrying radio sources. Comparison of images in these two spectral ranges reveals a close association between SKR and discrete UV aurorae, in spite of the strong visibility effects affecting SKR.