UV Observations of the Io Plasma UV Observations of the Io Plasma Torus from Torus from New HorizonsNew Horizons and and RosettaRosetta

Andrew J. StefflAndrew J. Steffl11, M. F. A'Hearn, M. F. A'Hearn22, J. L. Bertaux, J. L. Bertaux33, P. D. , P. D. FeldmanFeldman44, G. R. Gladstone, G. R. Gladstone11, J. W. Parker, J. W. Parker11, K. D. Retherford, K. D. Retherford11, D. , D.

C. SlaterC. Slater11, S. A. Stern, S. A. Stern55, M. Versteeg, M. Versteeg11, H. A. Weaver, H. A. Weaver66

11SwRI, SwRI, 22Univ. of Maryland, Univ. of Maryland, 33Service d'Aeronomie, Service d'Aeronomie, 44JHU, JHU, 55NASA NASA HQ, HQ, 66JHU-APLJHU-APL

Presentation 4.09Presentation 4.09

Alice UV Spectrometers

•4 “Alice” series UV spectrometers

• Alice (R-Alice) on Rosetta in flight

• Alice (P-Alice) on New Horizons in 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:

700-2050Å (R-Alice); 520-1870Å (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 sunlightTorus emissions

The Jovian Magnetosphere from Rosetta

• ESA’s Rosetta flew past Mars on 2007-02-25 (DOY 056)

• On 2007-02-28 R-Alice began observations of the Jovian magnetosphere

• Observations continued intermittently until 2007-05-08 (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 120-130, though poor sampling

• Jovian aurora significantly brighter on DOY 63, 69, 82

The Jovian Magnetosphere from Rosetta Alice

10-hour Averaged Luminosity

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• New Horizons scan of aurora on 2007-03-03

• S/C at 22:15 local time;III= 170º

• Scan approx. east from +3 to -6 RJ

• 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 TorusNoon Ansa

H2 1600Å

Sunlit Crescent of Jupiter

Jovian Aurora

New Horizons Alice Spectral Images

Torus mixing ratios (Nion/Ne)

Iogenic neutrals(not modeled)

Modeling the Io Plasma Torus Seen by P-Alice

• High ionization states (S IV & O III) more abundant

• Te ~ 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 2000-2001

• 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 Io’s atmosphere– Somewhat surprising given the amazing Tvashtar plume images

• New Horizons Jupiter encounter data (through July 2007) available from the PDS Small Bodies Node 2007-11-01

Conclusions