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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. HST / STIS UV 135.6 nm Roesler et al., 1999.
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Variations ofthe auroral UV emissionfrom Io’s atmosphere Lorenz Roth*J. Saur*, P.D. Feldman, D.F. Strobel, K.D. Retherford *Institute of GeophysicsUniversity of Cologne, Germany
ObservationsofIo´sauroralemission HST / STIS UV 135.6 nm Roesler et al., 1999 Galileo SSI visible – 3 filters 380 - 710 nm Geissler et al., 1999 Cassini ISS near UV – blue visiblered Geissler et al., 2004 New HorizonsLORRI visible- UV 350 - 850 nm Roth et al., 2011
Generation of Io’s aurora • ExcitationofIo´satmospherebyelectronimpact • Atmosphericproperties:- columndensityofN≈ (1-5)∙1016 cm-2 (sunlit) (Lellouch et al., 2007)- mainconstituent: SO2 - various minor components: O, O2, S, S2, SO, Na, K, Cl, NaCl • Electrons in theplasmatorus:- thermal electrons: kTe= 5 eVne = 1200-3600 cm-3- non-thermal electrons: kTe≈ keV • The auroraastoolto find propertiesof:- atmosphericdistribution, compositionanddensity- plasmaconditions- magneticfieldconfiguration
HST / STIS observationsofIo´s UV aurora 50 exposuresduring 26 orbitsbetween1997 and 2001 2 exposures in eclipse 40 exposurescontainingOI 1356 Åemission- the clearest of emissions imaged 19 combinedobservationimages(2 exposuresduringoneorbit) ➩ 2 x better signal-to-noiseratio (afterRetherford, 2002)
MorphologyoftheOI 1356 Åemission • Morphologyonlydependson the: • 1. observinggeometry 2. tiltofthebackgroundfield • 3. positionwithrespecttothetorus
Model ofthe 3D emissiondistribution • Goal: Find a simple functionthatisabletodescribethe 3D emissionprofilearoundIofor all STIS observationswithasfewparametersaspossible • Main aurorafeatures:- Equatorialspots- backgroundemission- limbglow (north- southdifferences) • Constructionof 2D modelimagesbyintegratingalongline-of-sight • Comparisonbetweenmodelimagesand 18 combinedSTIS OI 1356 Åobservations (Io in sunlight)
Aurora model • Background emission: • Emission on theflanks(equatorialspots): • Rotatethe 3D profileintothe B fieldcoordinatesystem➩ Tiltdeterminedby • Brightness proportional todistancetotoruscenter ( ) • 13 fit parameters (in red) • 6 areas: - off-disk emission / on-disk emission - anti-Jovian spot / sub-Jovian spot (9x9 pixels) - limbglow North / South + positionoftheequatorialspots • Find least squaresfor all 18 observations:
Results Aurora modelisabletoreproducethemainfeaturesofthemorphologyobservedover 5 years(1997 – 2001) Tiltofaurora ~ 80% oftiltofbackgroundfield Spots:- longitudinal extension: ∼22°- spotcentershifteddownstreamfrom sub-/ anti-jovianmeridian: ~12° / ~4°- Distanceofcentertosurface: ~ 50 km- brightnessesandextensionssimilar! Eclipseobservation:- faintestemissionof all STIS observations- but: also faintestmodeledemission- sub-Jovian spotappearssmallerandfurtherupstream
Summary • Weexamined 18 HST/STIS observationsofthe OI 1356 ÅemissionfromIo`satmospheretakenbetween 1997 and 2001 • Assumption: Morphologyofthe OI 1356 Åauroraonlydepends on the:1. observationgeometry2. tiltofthebackgroundfield3. positionofIowithrespecttothetorus • Weestablished a phenomenologicalmodelforthe3D emissiondistributionaroundIothatreproducestheobservedauroramorphology. • Onlyweakvariationsofthe3D emissionprofilebetween 1997 and 2001. ➩ Comparisonsofother (new) observationswiththeauroramodel (Roth et al., Simulation of Io’s auroralemission…, Icarus, available online)