Article

Cassini UVIS observations of Jupiter's auroral variability

Boston University, Boston, MA, USA
Icarus (Impact Factor: 2.84). 11/2005; DOI: 10.1016/j.icarus.2005.05.021

ABSTRACT The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H2 band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2–4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5–16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325–326 (November 20–21). The global brightening events on November 20–21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45–19:35 UT UVIS H2 band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6–5 kHz Galileo PWS data.

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    • "A single solar wind event traced from 1 AU has been proposed to correspond to auroral brightenings at both Jupiter and Saturn, within a large uncertainty in the arrival time at the planets and limited coverage of the auroral activity [Prangé et al., 2004]. Evidence exists from two events recorded at the time of the Cassini spacecraft flyby (late 2000 to early 2001) that Jupiter's auroral emissions may brighten at times of solar wind disturbances [Gurnett et al., 2002; Pryor et al., 2005; Nichols et al., 2007], but the data were insufficient to establish the repeatability or the physical nature of the correlation. The observed brightening of the aurora at a time of solar wind pressure increase is opposite to the initial prediction based on the corotation enforcement current system. "
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    ABSTRACT: {We have analyzed the Cassini Ultraviolet Imaging Spectrometer (UVIS) observations of the Jupiter aurora with an auroral atmosphere two-stream electron transport code. The observations of Jupiter by UVIS took place during the Cassini Campaign. The Cassini Campaign included support spectral and imaging observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). A major result for the UVIS observations was the identification of a large color variation between the far ultraviolet (FUV: 1100 1700 Å) and extreme ultraviolet (EUV: 800 1100 Å) spectral regions. This change probably occurs because of a large variation in the ratio of the soft electron flux (10 3000 eV) responsible for the EUV aurora to the hard electron flux (tilde15 22 keV) responsible for the FUV aurora. On the basis of this result a new color ratio for integrated intensities for EUV and FUV was defined (4$pi$I$_$/4$pi$I$_$) which varied by approximately a factor of 6. The FUV color ratio (4$pi$I$_$/4$pi$I$_$) was more stable with a variation of less than 50% for the observations studied. The medium resolution (0.9 Å FWHM, G140M grating) FUV observations (1295 1345 Å and 1495 1540 Å) by STIS on 13 January 2001, on the other hand, were analyzed by a spectral modeling technique using a recently developed high-spectral resolution model for the electron-excited H$_2$ rotational lines. The STIS FUV data were analyzed with a model that considered the Lyman band spectrum (B $Sigma$u+1rarrX$Sigma$g+1) as composed of an allowed direct excitation component (X $Sigma$g+1rarrB$Sigma$u+1) and an optically forbidden component (X $Sigma$g+1rarrEF,GK,HHmacr,ldots$Sigma$}g+1 followed by the cascade transition $Sigma$g+1rarrB$Sigma$u+1). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen to emphasize the cascade component. The ratio of the two components is a direct measurement of the mean secondary electron energy of the aurora. The mean secondary electron energy of the aurora varies between 50 and 200 eV for the polar cap, limb and auroral oval observations. We examine a long time base of Galileo Ultraviolet Spectrometer color ratios from the standard mission (1996 1998) and compare them to Cassini UVIS, HST, and International Ultraviolet Explorer (IUE) observations.
    Icarus 11/2005; 178:327-345. DOI:10.1016/j.icarus.2005.01.023 · 2.84 Impact Factor
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