Detection of Auroral Emissions from Callisto’s Magnetic Footprint at Jupiter

Poster (2011, October)

HST observations of Jupiter’s aurora in a large campaign reveal several cases where the main oval emission appeared at unusually low latitudes, making it possible to search for the first time for auroral ... [more ▼]

HST observations of Jupiter’s aurora in a large campaign reveal several cases where the main oval emission appeared at unusually low latitudes, making it possible to search for the first time for auroral emissions from the magnetic footprint of Callisto without the overlapping bright emissions from the main oval. Several cases have been found where point-source emissions have now been detected from locations consistent with Callisto’s magnetic footprint on Jupiter at a brightness of ten’s of kilo- Rayleighs. These observations confirm that there is an electrodynamic interaction between Callisto and Jupiter’s magnetospheric environment that is similar to those at Io, Europa, and Ganymede, which all have auroral footprints. The properties of the emissions and a comparison with other observations and theoretical expectations will be presented in this paper. [less ▲]

Response of Jupiter's and Saturn's auroral activity to the solar wind

in Journal of Geophysical Research. Space Physics (2009), 114

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and ... [more ▼]

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and Saturn are driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter's aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn. [less ▲]

The Degree of Correlation of Jovian and Saturnian Auroral Emissions With Solar Wind Conditions

Conference (2008, December 01)

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven ... [more ▼]

While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth's magnetosphere with the solar wind, auroral emissions on Jupiter and Saturn are thought to be driven primarily by internal processes, with the main energy source being the planets' rapid rotation. Limited evidence has suggested there might be some influence of the solar wind on Jupiter's aurorae, and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from one AU and measured near each planet. The data indicate a consistent brightening of both the auroral emissions and Saturn Kilometric Radiation (SKR) at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. This correlation has been strengthened by the final campaign observations in Feb. 2008. At Jupiter the situation is less clear, with increases in total auroral power seen near the arrival of solar wind forward shocks, while little increase has been observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn. [less ▲]

Oscillation of Saturn's southern auroral oval

in Journal of Geophysical Research (2008), 113(A11),

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently ... [more ▼]

Near-planetary-period oscillations in the Cassini plasma and magnetic field data have been observed throughout Saturn's magnetosphere despite the fact that Saturn's internal magnetic field is apparently highly axisymmetric. In addition, the period of the Saturn kilometric radiation has been shown to vary over time. In this paper we present results from the recent Hubble Space Telescope observations of Saturn's southern ultraviolet auroral emission. We show that the center of the auroral oval oscillates with period 10.76 h +/- 0.15 h for both January 2007 and February 2008, i.e., close to the periods determined for oscillations in other magnetospheric phenomena. The motion of the oval center is described for 2007 by an ellipse with semimajor axis similar to 1.4 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.93, and center offset from the spin axis by similar to 1.8 degrees toward similar to 04 h LT. For 2008 the oscillation is consistent with an ellipse with semimajor axis similar to 2.2 degrees +/- 0.3 degrees oriented toward similar to 09-21 h LT, eccentricity similar to 0.99, and a center offset from the spin axis by similar to 2.2 degrees toward similar to 03 h LT. The motion of the auroral oval is thus highly elliptical in both cases, and the major oscillation axis is oriented toward prenoon/premidnight. This result places an independent constraint on the magnitude of the planet's dipole tilt and may also indicate the presence of an external current system that imposes an asymmetry in the ionospheric field modulated close to the planetary period. [less ▲]

The HST UV Auroral Imaging Campaign of Jupiter and Saturn during the International Heliophysical Year

Conference (2006, December 01)

An extended campaign of observations of the UV auroral emissions from Jupiter and Saturn is scheduled for three periods beginning in Jan. 2007 and ending in late June 2008. This will be by far the most ... [more ▼]

An extended campaign of observations of the UV auroral emissions from Jupiter and Saturn is scheduled for three periods beginning in Jan. 2007 and ending in late June 2008. This will be by far the most extensive series of remote high resolution imaging of planetary aurora to date, and should provide new physical insight into the cause and effect relationships governing the controlling processes for the giant planet auroral emissions. These observations will overlap with in situ measurements of local solar wind and magnetospheric plasma conditions by Cassini at Saturn in Jan. 2007 and by the New Horizons mission approaching Jupiter in Feb. 2007. The UV auroral emission brightness and distributions will also be compared with extrapolated estimates of the solar wind conditions near each planet from periods just before planetary opposition in Jan. 2007 (Saturn) and June 2007 (Jupiter). The HST observations will also be coordinated with ground-based observations of near-IR auroral and nonthermal radio emissions. This paper will give an overview of the program, including the schedule of HST observations and the schedule of known coordinated observations. While a preliminary schedule has been submitted for the HST observations, this schedule will be finalized only when the HST orbit is sufficiently well known for the periods of observation for detailed pointing to be specified. By the time of Fall AGU, it should be possible to show the detailed schedule and pointing for the Jan-Feb 2007 observations. The paper will include a presentation of the plans for the rapid reduction and distribution of the HST auroral images to the community. [less ▲]

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

in Nature (2005), 433(7027), 717-719

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven(1), and those of Jupiter, where processes are ... [more ▼]

It has often been stated that Saturn's magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven(1), and those of Jupiter, where processes are driven by a large source of internal plasma(2-4). But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind(5) and Saturn kilometric radio emission(6), demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturn's auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure(5) Saturn's aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturn's auroral emissions behave fundamentally differently from those at the other planets. [less ▲]