UV Observations of the Io Plasma Torus from New Horizons and Rosetta UV Observations of the Io Plasma Torus from New Horizons and Rosetta Andrew J. Steffl 1, M. F. A'Hearn 2, J. L. Bertaux 3, P. D. Feldman 4, G. R. Gladstone 1, J. W. Parker 1, K. D. Retherford 1, D. C. Slater 1, S. A. Stern 5, M. Versteeg 1, H. A. Weaver 6 1 SwRI, 2 Univ. of Maryland, 3 Service d'Aeronomie, 4 JHU, 5 NASA HQ, 6 JHU-APL Presentation 4.09

Alice UV Spectrometers 4 Alice series UV spectrometers Alice (R-Alice) on Rosettain flight Alice (P-Alice) on New Horizonsin flight LAMP on LROdevelopment UVS on Junodevelopment Light-weight: 3 kg (R-Alice); 4.4 kg (P-Alice) Low-power: 4 W (R-Alice); 4.4 W (P-Alice) Wavelength Range: Å (R-Alice); Å (P-Alice) Spectral Resolution: ~5Å FWHM (point source); ~10Å (Filled-slit) Dispersion: ~1.8 Å/pixel Detector: 1024x32-pixel Double Delay Line (DDL)

Jovian Aurora Reflected sunlight Torus emissions The Jovian Magnetosphere from Rosetta ESAs Rosetta flew past Mars on (DOY 056) On R-Alice began observations of the Jovian magnetosphere Observations continued intermittently until (DOY128). 378 hours of integration were acquired (1.36 Ms). 4.2 AU from Jupiter, so no spatial resolution

No obviously significant temporal variations in the IPT Torus emissions ~2x fainter than during the Cassini epoch Torus emissions possibly brighter DOY , though poor sampling Jovian aurora significantly brighter on DOY 63, 69, 82 The Jovian Magnetosphere from Rosetta Alice 10-hour Averaged Luminosity

New Horizons scan of aurora on S/C at 22:15 local time; III = 170º Scan approx. east from +3 to -6 R J Total scan time: 1800s Sky in Alice narrow FOV for ~30s. Data are time-tagged (pixel list mode) The Jovian Magnetosphere from New Horizons Alice

B Stars S III 680Å O II 834Å Io Plasma Torus Noon Ansa H Å Sunlit Crescent of Jupiter Jovian Aurora New Horizons Alice Spectral Images

Torus mixing ratios (N ion /N e) Iogenic neutrals (not modeled) Modeling the Io Plasma Torus Seen by P-Alice High ionization states (S IV & O III) more abundant T e ~ 3.5 eV vs. ~4.5 eV during Cassini epoch

Both Rosetta Alice and New Horizons Alice observed the Jovian Magnetosphere –Observations yielded rich data sets The Io plasma torus was 2x fainter in 2007 than the Cassini epoch of Torus electron temperatures ~ 4 eV –Significantly cooler than the Cassini epoch Suggests (tentatively!) a lower rate of pickup ions –i.e. a lower amount of gas lost from Ios atmosphere –Somewhat surprising given the amazing Tvashtar plume images New Horizons Jupiter encounter data (through July 2007) available from the PDS Small Bodies Node Conclusions