Cassini UVIS observations of Jupiter's auroral variability
ABSTRACT During the December 2000 Cassini flyby of Jupiter, the Cassini Ultraviolet Imaging Spectrograph observed episodic brightenings of the UV aurora caused by solar coronal mass ejections (CMEs). The integrated auroral output in the H2 band emissions increased by a factor of 3-4 in two major events, on days 280 and 325-326 of 2000. The CME events cause measurable changes in the solar wind that we are examining in search of triggers of the observed auroral brightening events. We have detrended the auroral data to remove the usual longitudinal variations in auroral output, making it easier to study the large events. This is done by using the auroral arc area observed at each central meridian longitude in HST images to create an expected lightcurve. Jupiter's auroral response to CME shocks is not as prompt as the terrestrial response, but both involve nearly simultaneous radio bursts and auroral emissions.
- SourceAvailable from: Xiaoyan Zhou[show abstract] [hide abstract]
ABSTRACT: We present two cases of abrupt dayside auroral brightenings and very fast auroral propagation using the POLAR UV imaging data. The brightenings occur first at noon and then propagate along the auroral oval towards dawn and dusk. Ionospheric speeds of 6 to 11 k d s are determined. The auroral brightenings and motion are associated with the arrival and propagation of interplanetary shocks/pressure waves. The brightening at noon occurs within minutes of the shock com-pression of the noon-time magnetopause. The speed of the auroral propagation in the ionosphere towards dawn and dusk corresponds extremely well to the solar wind downstream flow. Our model assumes that shocks/pressure waves compress the outer dayside magnetosphere, and plasma contained therein. This plasma compression leads to the loss cone instability, wave-particle interactions, and concomitant particle loss into the ionosphere.01/1999; 26:1097-1.
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ABSTRACT: We use Wind solar wind data and Polar UV imaging data to study the nightside magnetospheric/magnetotail responses to interplanetary shocks/pressure pulses. Of 53 interplanetary shock/pressure pulse events that occurred in 1997 and 1998 at Wind, there are 18 cases where Polar near-midnight UV images are available. All of these 18 events are used in this study. The nightside auroral responses can be classified into three types: substorm expansion phase (SS) (or substorm further intensification) events, pseudobreakup (PB) events, and quiescent (QE) events. It is found that the solar wind preconditions determine the causes of the different auroral responses. A ~1.5-hour interplanetary magnetic field (IMF) Bs ``precondition'' (upstream of the interplanetary shock) gives good empirical results. The upstream IMF is strongly southward prior to substorm expansion phase triggerings (44% of all events), the IMF Bz is ~0 nT for PB triggerings (39% of all events), and the IMF is purely northward for quiescent events (17%). The evidence for IMF Bs preconditioning is interpreted in terms of a plasma sheet loading mechanism. The interplanetary shock compression effects on the near-Earth tail are discussed in light of existing substorm/PB triggering models.Journal of Geophysical Research 01/2001; 106:18957-18968. · 3.17 Impact Factor
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ABSTRACT: Spatially resolved spectra in four 50-Å FUV spectral windows were obtained across the jovian aurora with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope. Nearly simultaneous ultraviolet imaging makes it possible to correlate the intensity variations along the STIS slit with those observed in the images and to characterize the global auroral context prevailing at the time of the observations. Spectra at ∼1-Å resolution taken in pairs included an unabsorbed window and a spectral region affected by hydrocarbon absorption. Both sets of spectra correspond to an aurora with a main oval brightness of about 130 kilorayleighs of H2 emission. The far ultraviolet color ratios I(1550–1620 Å)/I(1230–1300 Å) are 2.3 and 5.9 for the noon and morning sectors of the main oval, respectively. We use an interactive model coupling the energy degradation of incoming energetic electrons, auroral temperature and composition, and synthetic H2 spectra to fit the intensity distribution of the H2 lines. It is found that the model best fitting globally the spectra has a soft energy component in addition to a 10 erg cm−2 s−1 flux of 80 keV electrons. It provides an effective H2 temperature of 540 K. The relative intensity of temperature-sensitive H2 lines indicates differences between the auroral main oval and polar cap emissions. The amount of methane absorption across the polar region is shown to vary in a way consistent with temperature. For the second spectral pair, the polar cap shows a higher attenuation by CH4, indicating a harder precipitation along high-latitude magnetic field lines.Icarus 01/2002; 157(1):91-103. · 3.16 Impact Factor