Remote sensing of the energy of auroral electrons in Saturn’s atmosphere: Hubble and Cassini spectral observations

in Icarus (2013), 223

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ... [more ▼]

Saturn’s north ultraviolet aurora has been successfully observed twice between March and May 2011 with the STIS long-slit spectrograph on board the Hubble Space Telescope. Spatially resolved spectra at ∼12 Å spectral resolution have been collected at different local times from dawn to dusk to determine the amount of hydrocarbon absorption. For this purpose, the HST telescope slewed across the auroral oval from mid-latitudes up to beyond the limb while collecting spectral data in the timetag mode. Spectral images of the north ultraviolet aurora were obtained within minutes and hours with the UVIS spectrograph on board Cassini. Several daytime sectors and one nightside location were observed and showed signatures of weak absorption by methane present in (or above) the layer of the auroral emission. No absorption from other hydrocarbons (e.g. C2H2) has been detected. For the absorbed spectra, the overlying slant CH4 column varies from 3x1015 to 2x1016 cm-2, but no clear dependence on local time is identified. A Monte Carlo electron transport model is used to calculate the vertical distribution of the H2 emission and to relate the observed spectra to the energy of the primary auroral electrons. Assuming electron precipitation with a Maxwellian energy distribution into a standard model atmosphere, we find that the mean energy ranges from less than 3 to ∼10 keV. These results are compared with previous determinations of the energy of Saturn’s aurora based on ultraviolet spectra and limb images. We conclude that the energies derived from spectral methods indicate a wide range of electron energies while the nightside limb images suggest that the auroral precipitation is consistently soft. We emphasize the need for more realistic model atmospheres with temperature and hydrocarbon distributions appropriate to high-latitude conditions. [less ▲]

Anticipating Juno Observations of the Magnetosphere of Jupiter

Poster (2012, December 07)

The Juno spacecraft will arrive at Jupiter in 2016 and will go into polar orbit. Juno will make the first exploration of the polar regions of Jupiter's vast magnetosphere, combining in situ particles and ... [more ▼]

The Juno spacecraft will arrive at Jupiter in 2016 and will go into polar orbit. Juno will make the first exploration of the polar regions of Jupiter's vast magnetosphere, combining in situ particles and fields measurements with remote sensing of auroral emissions in the UV, IR and radio. The primary science period comprises ~30 orbits with 11-day periods with a~1.06Rj perijove, allowing Juno to duck under the hazardous synchrotron radiation belts. Apojove is at ~38Rj. The oblateness of the planet causes the orbit to precess with the major axis moving progressively south at about 1 degree per orbit, eventually bringing the spacecraft into the radiation belts. This orbit allows unprecedented views of the aurora and exploration of the auroral acceleration regions. We present an overview of anticipated Juno observations based on models of the Jovian magnetosphere. On approach to Jupiter and over a capture orbit that extends to ~180Rj on the dawn flank, Juno will traverse the magnetosheath, magnetopause and boundary layer regions of the magnetosphere. Due to the high plasma pressures in the magnetospheric plasmasheet the magnetosphere of Jupiter is known to vary substantially with the changes in the solar wind dynamic pressure. We use Ulysses solar wind data obtained around 5 AU to predict the conditions that Juno will observe over the several months it will spend in these boundary regions. [less ▲]

Likely Detection of UV Auroral Emission from the Magnetic Footprint of Callisto

Poster (2012, December 06)

A large number of UV images of Jupiter's aurora were obtained in 2007/2008 with the Hubble Space Telescope (HST) ACS/SBC camera. The initial results on variations with the solar wind conditions have been ... [more ▼]

A large number of UV images of Jupiter's aurora were obtained in 2007/2008 with the Hubble Space Telescope (HST) ACS/SBC camera. The initial results on variations with the solar wind conditions have been published elsewhere, but the large database permits other studies to be performed. In particular, while auroral emissions have been detected from the magnetic footprints of Io, Europa, and Ganymede in Jupiter's atmosphere, the footprint of Callisto has been located too close to the main auroral oval to be detected. We have thus analyzed images of the ultraviolet auroral emissions of Jupiter taken using the F115LP filter on the HST/ACS instrument. Using a unique co-addition method, we have identified a strong candidate for the footprint of Callisto on May 24, 2007. We tested this finding by applying the same co-addition method to a nearly identical auroral configuration on May 30, 2007 when Callisto was well removed in its orbit. Comparing the two co-added images, we can clearly see the presence of Callisto’s footprint on the 24th and its absence on the 30th. The method relies as well on the motion of Callisto's footprint remaining under the satellite, while most of the auroral emissions rotate with the planet. The images and analysis method will be presented in this presentation. [less ▲]

Dynamics of the main/rotation aurora

Conference (2012, November 27)

Satellite-induced electron acceleration and related auroras

Conference (2012, September 26)

Satellite-induced auroral emissions are known since decades, in particular those associated with the interaction of Io with the Jovian ionosphere. These emissions range from low frequency radio to UV ... [more ▼]

Satellite-induced auroral emissions are known since decades, in particular those associated with the interaction of Io with the Jovian ionosphere. These emissions range from low frequency radio to UV. Flyby of Io allowed to better understand the power generation close to the satellite, and showed the existence of electron beams accelerated at high latitude. We will present a study of the power transfer between the local interaction at Io and the electron accelerated close to Jupiter. It shows that Alfvén acceleration can explain the morphology and brightness of the Io-related auroraeand the observed accelerated electrons in Io’s wake. The study is extended to the Europa, Ganymede and Enceladus for which auroral emissions have been observed, as well as to Callisto and to the principal inner satellites of Saturn. [less ▲]

Conversion from HST ACS and STIS auroral counts into brightness, precipitated power and radiated power for H2 giant planets

Conference (2012, September)

Conversion from HST ACS and STIS auroral counts into brightness, precipitated power, and radiated power for H2 giant planets

in Journal of Geophysical Research. Space Physics (2012), 117

The STIS and ACS instruments onboard HST are widely used to study the giant planet's aurora. Several assumptions have to be made to convert the instrumental counts into meaningful physical values (type ... [more ▼]

The STIS and ACS instruments onboard HST are widely used to study the giant planet's aurora. Several assumptions have to be made to convert the instrumental counts into meaningful physical values (type and bandwidth of the filters, definition of the physical units, etc…), but these may significantly differ from one author to another, which makes it difficult to compare the auroral characteristics published in different studies. We present a method to convert the counts obtained in representative ACS and STIS imaging modes/filters used by the auroral scientific community to brightness, precipitated power and radiated power in the ultraviolet (700-1800 Å). Since hydrocarbon absorption may considerably affect the observed auroral emission, the conversion factors are determined for several attenuation levels. Several properties of the auroral emission have been determined: the fraction of the H[SUB]2[/SUB] emission shortward and longward of the HLy-α line is 50.3% and 49.7% respectively, the contribution of HLy-α to the total unabsorbed auroral signal has been set to 9.1% and an input of 1 mW m[SUP]-2[/SUP] produces 10 kR of H[SUB]2[/SUB] in the Lyman and Werner bands. A first application sets the order of magnitude of Saturn's auroral characteristics in the total UV bandwidth to a brightness of 10 kR and an emitted power of ˜2.8 GW. A second application uses published brightnesses of Europa's footprint to determine the current density associated with the Europa auroral spot: 0.21 and 0.045 μA m[SUP]-2[/SUP] assuming no hydrocarbon absorption and a color ratio of 2, respectively. Factors to extend the brightnesses observed with Cassini-UVIS to total H[SUB]2[/SUB] UV brightnesses are also provided. [less ▲]

Main oval expansion and spectacular outer emissions at Jupiter: Is Io responsible?

Conference (2012, May 25)

Results from the combined HST-Cassini campaign 2011: hydrocarbon absorption and auroral electron energy

Conference (2012, May 24)

Expansion of the main auroral oval at Jupiter : evidence for Io’s control over the Jovian magnetosphere

Poster (2012, April)

In spring 2007, New Horizons' Jupiter fly-by provided a unique opportunity for the largest observation campaign dedicated to the Jovian aurora ever carried out by the Hubble Space Telescope. UV images of ... [more ▼]

In spring 2007, New Horizons' Jupiter fly-by provided a unique opportunity for the largest observation campaign dedicated to the Jovian aurora ever carried out by the Hubble Space Telescope. UV images of the aurora have been acquired on a quasi-daily basis from mid-February to mid-June 2007. Polar projection of the auroral emissions clearly show a continuous long-term expansion of main oval additionally to day by day variations. The main oval moved so much that the Ganymede footprint, which is usually located equatorward of the main emissions, has even been observed inside of it. Simultaneously, the occurrence rate of large equatorward isolated auroral features increased over the season. These emission patches are generally attributed to injections of depleted flux tubes. On 6th June, one of these features exceptionally moved down to the Io footpath. The Io footprint seemed to disappear while the footprint moved through this patch of emission. This disappearance is a unique case among all the UV images of the aurora acquired during the last 12 years. We suggest that all these changes seen in the Jovian aurora are evidence for a major reconfiguration of the magnetosphere induced by increased volcanic activity on Io. Indeed, New Horizons observed particularly intense activity from the Tvashtar volcano in late February 2007. Moreover, sodium cloud brightening caused by volcanic outbursts have also been seen in late May 2007. According to our interpretation, repeated volcanic outbursts beefed up the plasma torus density and its mass outflow rate. This caused the corotation breakdown boundary to migrate closer to Jupiter. Consequently, the main auroral oval moved equatorward. As heavy flux tubes move outward, sparsely filled ones should be injected into the inner magnetosphere in order to conserve the magnetic flux in this region. This phenomenon could explain the large number of injection signatures observed in May-June 2007. Such a cloud of depleted flux tubes probably disrupted the Io-magnetosphere interaction, leading to an abnormally faint Io footprint. [less ▲]