Variations of the auroral UV emission from Io’s atmosphere Lorenz Roth * J. Saur *, P.D. Feldman, D.F. Strobel, K.D. Retherford * Institute of Geophysics University of Cologne, Germany

Observations of Io´s auroral emission Galileo SSI visible – 3 filters nm Geissler et al., 1999 Cassini ISS near UV – blue visible red Geissler et al., 2004 New Horizons LORRI visible - UV nm Roth et al., 2011 HST / STIS UV nm Roesler et al., 1999

Generation of Io’s aurora Excitation of Io´s atmosphere by electron impact Atmospheric properties: - column density of N ≈ (1-5)∙10 16 cm -2 (sunlit) (Lellouch et al., 2007) - main constituent: SO 2 - various minor components: O, O 2, S, S 2, SO, Na, K, Cl, NaCl Electrons in the plasma torus: - thermal electrons:kT e = 5 eV n e = cm -3 - non-thermal electrons: kT e ≈ keV The aurora as tool to find properties of: - atmospheric distribution, composition and density - plasma conditions - magnetic field configuration

HST / STIS observations of Io´s UV aurora 50 exposures during 26 orbits between 1997 and exposures in eclipse 40 exposures containing OI 1356 Å emission - the clearest of emissions imaged 19 combined observation images (2 exposures during one orbit) ➩ 2 x better signal-to-noise ratio (after Retherford, 2002)

Morphology of the OI 1356 Å emission  Morphology only depends on the: 1. observing geometry 2. tilt of the background field 3. position with respect to the torus

Model of the 3D emission distribution Goal: Find a simple function that is able to describe the 3D emission profile around Io for all STIS observations with as few parameters as possible Main aurora features: - Equatorial spots - background emission - limb glow (north - south differences) Construction of 2D model images by integrating along line-of-sight Comparison between model images and 18 combined STIS OI 1356 Å observations (Io in sunlight)

Background emission: Emission on the flanks (equatorial spots): Rotate the 3D profile into the B field coordinate system ➩ Tilt determined by Brightness proportional to distance to torus center ( ) 13 fit parameters (in red) 6 areas: - off-disk emission / on-disk emission - anti-Jovian spot / sub-Jovian spot (9x9 pixels) - limb glow North / South + position of the equatorial spots Find least squares for all 18 observations: Aurora model

Observations - Model

Results Aurora model is able to reproduce the main features of the morphology observed over 5 years (1997 – 2001) Tilt of aurora ~ 80% of tilt of background field Spots: - longitudinal extension: ∼ 22° - spot center shifted downstream from sub-/ anti-jovian meridian: ~12° / ~4° - Distance of center to surface: ~ 50 km - brightnesses and extensions similar! Eclipse observation: - faintest emission of all STIS observations - but: also faintest modeled emission - sub-Jovian spot appears smaller and further upstream

Summary We examined 18 HST/STIS observations of the OI 1356 Å emission from Io`s atmosphere taken between 1997 and 2001 Assumption: Morphology of the OI 1356 Å aurora only depends on the: 1. observation geometry 2. tilt of the background field 3. position of Io with respect to the torus We established a phenomenological model for the 3D emission distribution around Io that reproduces the observed aurora morphology. Only weak variations of the 3D emission profile between 1997 and ➩ Comparisons of other (new) observations with the aurora model (Roth et al., Simulation of Io’s auroral emission…, Icarus, available online)