Recurrent energization of plasma in the midnight-to-dawn quadrant of Saturn's magnetosphere, and its relationship to auroral UV and radio emissions

in Planetary and Space Science (2009), 57

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although ... [more ▼]

We demonstrate that under some magnetospheric conditions protons and oxygen ions are accelerated once per Saturn magnetosphere rotation, at a preferred local time between midnight and dawn. Although enhancements in energetic neutral atom (ENA) emission may in general occur at any local time and at any time in a Saturn rotation, those enhancements that exhibit a recurrence at a period very close to Saturn's rotation period usually recur in the same magnetospheric location. We suggest that these events result from current sheet acceleration in the 15-20 Rs range, probably associated with reconnection and plasmoid formation in Saturn's magnetotail. Simultaneous auroral observations by the Hubble Space Telescope (HST) and the Cassini Ultraviolet Imaging Spectrometer (UVIS) suggest a close correlation between these dynamical magnetospheric events and dawn-side transient auroral brightenings. Likewise, many of the recurrent ENA enhancements coincide closely with bursts of Saturn kilometric radiation, again pointing to possible linkage with high latitude auroral processes. We argue that the rotating azimuthal asymmetry of the ring current pressure revealed in the ENA images creates an associated rotating field aligned current system linking to the ionosphere and driving the correlated auroral processes. [less ▲]

Comparisons of Saturn Kilometric Radiation and Saturn's UV Aurora

Conference (2004, May 17)

During the period 8 to 30 January 2004, a campaign to study the correlation between the solar wind and the response of Saturn's aurora was carried out using Cassini and the Hubble Space Telescope. In ... [more ▼]

During the period 8 to 30 January 2004, a campaign to study the correlation between the solar wind and the response of Saturn's aurora was carried out using Cassini and the Hubble Space Telescope. In particular, fields and particles instruments on Cassini were used to monitor the solar wind near Saturn and Saturn kilometric radio emissions nearly continuously. STIS images from Hubble were obtained approximately every other day to record Saturn's UV auroral morphology and intensity. In this paper we focus particularly on the relationship between the Saturn kilometric emissions and the auroral brightness and morphology. The radio emissions are generally believed to be generated by the cyclotron maser instability on auroral field lines similar to the situation for auroral kilometric radiation at Earth. A number of studies have shown a direct relationship between the radio emissions and discrete auroral arcs at Earth. Hence, one expects a relationship between the radio emissions and the aurora at Saturn. During the campaign, two corotating interaction regions (CIRs) swept past Cassini and Saturn (which were of order 0.5 AU apart at the time). Accompanying the CIRs were high solar wind densities. As expected from Voyager studies, the higher solar wind density resulted in generally more intense radio emissions. The UV images show that Saturn's UV aurora brighten considerably in response to the CIRs, as well. Furthermore, the brightest aurora usually appear in the local morning, consistent with the Voyager-determined SKR source region on field lines connecting to the magnetopause and the Kelvin-Helmholtz hypothesis for the origin of accelerated electrons. A more detailed examination of the auroral phenomena show much more complex variations, however. The radio emission frequency extent and peak frequency vary remarkably from one Saturn rotation to the next. Similarly, the auroral morphology changes dramatically. For example, it appears the evolution of the auroral oval to higher latitudes (higher L-shells) is correlated with a shift in the frequency of peak radio emissions to lower frequencies. This can be explained through an analysis of the cyclotron maser beaming geometry. We examine this and other aspects of the correlations between the radio emissions and the aurora. [less ▲]

Cassini UVIS Observations of Jupiter's Auroral Variability

Poster (2001, October 27)

In the December 2000 Cassini flyby of the Jupiter system, the Cassini Ultraviolet Imaging Spectrograph (UVIS) monitored Jupiter's auroral emissions from day 275 of 2000 to day 81 of 2001. Much of the ... [more ▼]

In the December 2000 Cassini flyby of the Jupiter system, the Cassini Ultraviolet Imaging Spectrograph (UVIS) monitored Jupiter's auroral emissions from day 275 of 2000 to day 81 of 2001. Much of the auroral variability can be explained simply in terms of the rotation of Jupiter's auroral arcs (measured by Hubble Space Telescope) with the planet. However, several brightening events were seen in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and are tied to large solar coronal mass ejection events. The auroral UV emissions from these bursts also correspond to hectometric radio emission increases reported by the Galileo and Cassini Radio and Plasma Wave experiments. The 2 largest events were on 2000 day 280 and on 2000 day 325-326. We will look at these events in some detail, and compare them with corresponding information on the interplanetary magnetic field, solar wind conditions, and energetic particle environment to try to understand the cause of these auroral brightness increases. [less ▲]