Bursty magnetic reconnection at Saturn's magnetosphere

in Geophysical Research Letters (in press)

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 ▲]

On the origin of Saturn's polar auroral arcs

Conference (2012, September 27)

Saturn’s main auroral emission similarly to Earth’s is suggested to be associated with the open-closed field line boundary. The polar auroral emissions at Saturn, emissions located poleward of the main ... [more ▼]

Saturn’s main auroral emission similarly to Earth’s is suggested to be associated with the open-closed field line boundary. The polar auroral emissions at Saturn, emissions located poleward of the main emission consist of several arc-like and spotty features. In this study we focus on the arc-like structures observed with the UVIS instrument onboard Cassini and we characterize them into three groups: ‘bending arcs’ arcs whose one end is connected to the main emision, ‘oval aligned arcs’ arcs oriented parallel to the main emission and ‘moving arcs’ arcs which move with time inside the main emission. We study their occurrence rate, location, size as well as their associated expansion or contraction of the main emission. Finally, we compare the auroral arcs at Saturn with those in the terrestrial aurora and we examine their relation to a combination of solar wind parameters such as northward IMF, strong IMF magnitude and high solar wind speed. [less ▲]

Auroral counterpart of magnet ic dipolarizations in Saturn’s tail

Poster (2012, September 27)

Following magnetic reconnection in a planetary magnetotail, newly closed field lines can be rapidly accelerated back towards the planet, becoming “dipolarized” in the process. At Saturn, dipolarizations ... [more ▼]

Following magnetic reconnection in a planetary magnetotail, newly closed field lines can be rapidly accelerated back towards the planet, becoming “dipolarized” in the process. At Saturn, dipolarizations are initially identified in magnetometer data by looking for a southward turning of the magnetic field, indicating the transition from a radially stretched configuration to a more dipolar field topology. The highly stretched geometry of the kronian magnetotail lobes gives rise to a tail current which flows eastward (dusk to dawn) in the near equatorial plane across the centre of the tail. During reconnection and associated dipolarization of the field, the inner edge of this tail current can be diverted through the ionosphere, in a situation analogous to the substorm current wedge picture at Earth. We present a picture of the current circuit arising from this tail reconfiguration, and outline the equations which govern the field- current relationship. We show the first in situ example of a dipolarization identified in the Cassini magnetometer data and use this formalism to estimate the ionospheric current density that would arise for this example and the implications for auroral electron acceleration in regions of upward directed field-aligned current. We then present a separate example of data from the Cassini UVIS instrument where we observe small ‘spots’ of auroral emission lying near the main oval; features suggested to be associated with dipolarizations in the tail. In the example shown, such auroral features are the precursor to more intense activity associated with recurrent energisation via particle injections from the tail following reconnection. [less ▲]

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

Conference (2012, September)

Observations of equatorward patchy auroral ultraviolet emissions

Conference (2012, May 25)

Saturn's temperature profiles at high, medium and low latitudes derived from UVIS occultations

Conference (2012, May 24)

Short term studies of the giant planets aurorae

Conference (2012, May 23)

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 ▲]

Review of M-I coupling and auroral emissions at the outer planets

Conference (2012, March 09)

The bulk morphology of Jupiter's and Saturn's UV aurorae is conveniently divided into three components: 1) the main emission (main oval), 2) the satellites auroral footprints (equatorward of the main ... [more ▼]

The bulk morphology of Jupiter's and Saturn's UV aurorae is conveniently divided into three components: 1) the main emission (main oval), 2) the satellites auroral footprints (equatorward of the main emission): Io, Europa and Ganymede for Jupiter and Enceladus for Saturn, 3) the polar emissions (poleward of the main emission).This schematic view is already providing useful information on the giant planets aurorae. However, a quick inspection of HST and Cassini UV images directly shows that this simplified classification does not really match the complex morphology of the auroral emissions. As an example, in the case of Jupiter's UV aurorae, it appears that the main emission is not forming an oval, not even a closed shape; it is far from uniform and its position and size vary with time and depend on the viewing geometry. A secondary emission appears equatorward of the main emission and many small scale features regularly appear, some of them periodically. Satellites auroral footprints themselves appear to be much more intricate than predicted by present models. These footprints are actually multiple, their location and number vary periodically with time and with the satellites orbital longitude. The polar emissions are also much more complicated than predicted. Each of the above effects is related to a specific physical phenomenon in the atmosphere, the magnetosphere, or even in the planet's interior. For instance, the spatial distribution of the satellites auroral footprints made it possible to demonstrate the existence of a magnetic field anomaly near the surface of Jupiter's northern hemisphere. Observations which might appear insignificant, like the multiplicity of the satellites footprints or their periodicity are actually extremely valuable because they reveal the complexity of the interaction, in this case between a moon and the magnetospheric plasma of the planet around which it is orbiting. The same applies to small scale auroral structures which depict crucial magnetospheric processes like hot plasma injection, flux-tube interchange or magnetic reconnection mechanisms. The growing HST and Cassini databases are shedding new light on the origin of Jupiter's and Saturn's aurorae. Mechanisms that we thought could be taken for granted may even be challenged. [less ▲]