Cassini UVIS observations of the Io plasma torus

Abstract

On January 14, 2001, shortly after the Cassini spacecraft's closest approach to Jupiter, the Ultraviolet Imaging Spectrometer (UVIS) made a radial scan through the midnight sector of Io plasma torus. The Io torus has not been previously observed at this local time. The UVIS data consist of 2-D spectrally dispersed images of the Io plasma torus in the wavelength range of 561Å-1912Å. We developed a spectral emissions model that incorporates the latest atomic physics data contained in the CHIANTI database in order to derive the composition of the torus plasma as a function of radial distance. Electron temperatures derived from the UVIS torus spectra are generally less than those observed during the Voyager era. We find the torus ion composition derived from the UVIS spectra to be significantly different from the composition during the Voyager era. Notably, the torus contains substantially less oxygen, with a total oxygen-to-sulfur ion ratio of 0.9. The average ion charge state has increased to 1.7. We detect S V in the Io torus at the 3σ level. S V has a mixing ratio of 0.5%. The spectral emission model used in can approximate the effects of a non-thermal distribution of electrons. The ion composition derived using a kappa distribution of electrons is identical to that derived using a Maxwellian electron distribution; however, the kappa distribution model requires a higher electron column density to match the observed brightness of the spectra. The derived value of the kappa parameter decreases with radial distance and is consistent with the value of κ=2.4 at 8 R J derived by the Ulysses URAP instrument (Meyer-Vernet et al., 1995). The observed radial profile of electron column density is consistent with a flux tube content, NL 2 , that is proportional to r -2 . transport timescale (see review by Thomas et al. 2004, Delamere and Bagenal 2003, Lichtenberg 2001, Schreier et al. 1998). Thus, one aims to relate observations of spatial and temporal variations in torus emissions to the underlying sources, losses and transport processes. Towards this ultimate goal, we present an analysis of observations of the Io torus made by the Cassini spacecraft's Ultraviolet Imaging Spectrograph (UVIS) on January 14, 2001, with emphasis on determining the radial structure. In a companion paper, (Steffl et al.2004), hereafter referred to as paper I, we present examples of the EUV spectra of the torus and its temporal variability as observed during the full 6-month encounter period.Analysis of the temporal structure of the torus is presented in Delamere et al. 2004. II. UVIS DATAUVIS consists of two independent, but coaligned, spectrographs: one optimized for the extreme ultraviolet (EUV), which covers a wavelength range of 561Å to 1181Å and the other optimized for the far ultraviolet (FUV), which covers a wavelength range of 1140Å to 1913Å (McClintock et al. 1993, Esposito et al. 1998, Esposito et al. 2000. Each spectrograph is equipped with a 1024 x 64 pixel imaging microchannel plate detector. UVIS pixels are rectangular, and subte...

Temporal Variations of Io's Magnetic Footprint Brightness

“…From Table 1, the expected dense regions in the plasma torus varies in different HST observation epochs. The results confirm the azimuthal variation of plasma density in the plasma torus, which was directly measured by Steffl et al (2008).…”

Extreme Ultraviolet Radiation Measurement for Planetary Atmospheres/Magnetospheres from the Earth-Orbiting Spacecraft (Extreme Ultraviolet Spectroscope for Exospheric Dynamics: EXCEED)

“…The S III line at 680 Å is by far the most energetic feature in the EUV spectrum of the torus. The general shape of the profile observed by EXCEED is fairly similar to the UVIS profile by the Cassini spacecraft (Steffl et al 2004a(Steffl et al , 2004b; however, our spectral image seems thinly. The weak emissions, e.g.…”

“…More than 1000 kg of neutral gases from Io's atmosphere are ionized per second and produce a dense (approximately 2000 cm −3 ) torus of electrons, sulfur, and oxygen ions, trapped in Jupiter's strong magnetic field. While in situ measurements of the Io plasma torus by the Voyager and Galileo spacecraft (Frank and Paterson 2000;Bagemal 1985) and remote-sensing observations from the ground and from space-based UV telescope have characterized the density, temperature and composition of the plasma as well as the basic structure (Broadfoot et al 1981;Steffl et al 2004aSteffl et al , 2004bThomas et al 2003), the temporal variability of the torus remains poorly determined.…”

“…The spectrum is sensitive to electron temperature (>5 eV). With the EUV spectrum and the CHIANTI atomic physics database, we can extract the electron temperature and ion mixing ratios based on the spectral diagnosis method (Steffl et al 2004a(Steffl et al , 2004b. The distribution and temporal variation of hot electrons is a potential target for measurement to clarify this issue ).…”

Isolating auroral FUV emission lines using compact, broadband instrumentation

“…For example: the OI emission multiplets at 130.4 nm and 135.6 nm seen at both Earth and Ganymede can also be observed at Europa (Hall et al 1995;, Io (Ballester et al 1987;Clarke et al 1994;Roesler et al 1999) and Enceladus (Zastrow et al 2012); SI emissions at 129.9 nm, 138.9 nm, 142.9 nm and 147.9 nm are seen at Io (Ballester et al 1987;Roesler et al 1999); and N 2 LBH band emissions have been observed at Titan (Broadfoot et al 1981;Strobel and Shemansky 1982;Ajello et al 2008;Stevens et al 2011;Ajello et al 2012) and are predicted to exist at Triton (Strobel et al 1991). The Io torus also emits in the FUV region, with SII, SIII and SIV emissions dominating the spectrum (A Cassini UVIS spectrum is shown in Steffl et al 2004). As with Ganymede, several of these objects exhibit variable emission morphologies for various reasons.…”