E. Roussos 1 , N. Krupp 1, P. Kollmann 1 , M. Andriopoulou 1 ,

1. Energetic charged particle absorption signatures in Saturn's magnetosphere: observations and applications E. Roussos 1, N. Krupp 1, P. Kollmann 1, M. Andriopoulou1, C. Paranicas2, D.G Mitchell 2, S.M. Krimigis2, 3, M.F. Thomsen4 1: Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany 2: John Hopkins Applied Physics Laboratory, Laurel, Maryland, USA 3: Office of Space Research and Technology, Academy of Athens, Greece 4: Los Alamos National Laboratory, USA

2. Electron Microsignatures • Localizeddropouts in energeticelectronfluxes, causedbyparticleabsorption on moonsor rings • Depth of dropoutisdependent on the angular separationfromtheabsorbingbody (typicallydecreasing) Jones et al. (2006) 0 deg 30 deg Angular separation

3. Microsignature formation • Plasmaabsorbinginteraction (e.g. similartoEarth‘sMoon – S.W. interaction) • Plasma absorbingmoonsat Saturn (Mimas, Tethys, Dione, Rhea…) • Main features: • Formation of plasma cavity (wake) &interactionregiondownstream • Refillingprocesses of thewake

4. Wake refilling I • Alongthefield (mosteffective): • Potentialdrops, fieldalignedparticleacceleration, two-streaminstability… • Does not workforenergeticelectrons (abovefewkeV) atSaturn´smoons: Low energyelectrons Wake / Accelerationregion Particleflow High energyelectrons Khurana et al. (2008)

5. Wake refilling II • 2. Perpendiculartothefield (lesseffective): • Electricfields due topressuregradients, deviationfromchargeneutrality • Evenlesseffectiveforenergeticparticles Tethys wakecrossing Khurana et al. (2008) • Magneticdrifts of energeticparticlesoccur on lines of equalBm • Energeticparticleshavethetendencytobeexcludedfromthewake

6. Electron microsignatures Saturn Dione position during absorption 80o position during detection Cassini orbit Orbit of Dione expected detection position Day 122, 2005

7. Studies • Refilling of microsignatures • Displacement of microsignatures

8. Magnetosphericdiffusion (I) Day 2005/258 06:37 UT Tethys (28-37 keV) DLL ~ 1.5 10-9 Rs2/s (Van Allen et al., 1980, JGR)

9. Magnetospheric diffusion (II) Roussos et al. (2007)

10. Studies • Refilling of microsignatures • Displacement of microsignatures

11. Typesofdisplacements (A) ORGANIZED (B) COMPLEX • Displacement origin: • Dipole assumption insufficient • Magnetospheric electric fields

12. Magneticfieldmodels (I) Insufficientmapping of equatorial microsignature locationwhenusing a dipole? Model fieldlinesbased on: GiampieriandDougherty (2005) Te Di • Succesfulltracingcanhelpsetconstrains on field model inputs: • Currentsheetboundaries/dimensions • Plasma/energeticparticleparameters of ring current • Solar wind parameters, magnetopausedistance

13. Magneticfieldmodels (II) • Example: • DOY 2008-168/ 40 deglatitude • 2 degdownstream of Dione • Inwardsdisplacedassuming a dipole Succesfulltracingwith: Currentsheet model + magnetopausescalinglawsbyBunce et al. (2006), (MP at 21.3Rs) + currentsheetthickness of 2.4-2.5 Rs K. Khurana‘s model (AGU, 2006) for SW dynamicpressurethatcorrespondsto a MP distance of 21.5 Rs Results not alwaysconsistentfrom different magneticfieldmodels .

14. Magneticfieldmodels (III) • Onlypartofthesolution • Displacementsvisible also atequatoriallatitudes • Currentsheetperturbationexplainsonlyinwarddisplacements • Drift shellsplitingweak(10-15% differencewouldberequiredbetweenday/night |B| atconstant radial distance & latitude) • Energydependentdisplacements, complexdisplacementprofilescannotbeexplained • Magnetospheric electricfieldsnecessary

15. ElectricFields (I) Tethys / Dione • Displacement calculation corrected for current sheet perturbation • On average: • outward at noon • inward at midnight • smaller amplitudes at dawn-dusk • Consistent with a noon-midnight electric field

16. ElectricFields (II) ORGANIZED COMPLEX

17. ElectricFields (III)(noontomidnightelectricfield) • Variousmethods for electricfieldestimation, eg: • Range: 0.1 – 1.0 mV/m • Method not applicable for small displacements • Other methods being tested currently • Pointing of E-field can also be set as free parameter

18. Electric Fields (IV)(magnetospheric dynamics) Roussos et al., (2010) • Complex displacement profilesareindicative of localdynamics • Microsignatureageenergydispersion + energydispersion in displacement  radial velocities/azimuthalelectricfields in thesedynamicregions

19. Electric Fields (IV)(magnetospheric dynamics) Organized Complex Ratio: Complex/Organized Complex profiles relevant to injections ? (Chen and Hill 2008; Mueller et al. 2010)

20. Additional/futureapplications • Plasma composition/chargedstates(Selesnickand Cohen, 2009) • Bimodaldiffusion(Selesnickand Cohen, 1993) • Energeticparticlesources(Paranicas et al. 1997) • Backwardstracing of microsignatures withmodelsforelectric/magneticfields • Organizationas a function of SKR longitudeat Saturn • Combinedinjection/microsignaturestudies • Applicationsto Jovian magnetosphere • Multi-instrument studies • Interdisciplinaryscience (detection/characterizationof ring arcs etc.) • +++ • Moon interactionsignaturesgiveusthecapabilitytoindirectlyperform ´´multi-point´´ observations in themagnetospheres of outerplanets.

21. Instrumentation • Energeticparticledetector • Lowestenergy: Wheremagneticdriftsstarttobeimportant(time dispersioneffectsof microsignatures becomevisible) • Upperenergy: Gradient/curvaturedriftscancel corotation (displacementsattheseenergies sensitive toweakelectricfields) • Time resolution: Seconds(most microsignatures last 1-2 minutesat a givenenergyrange) • Energyresolution: dE/E~0.1, 10 energychannelsat least (time/energydispersioneffectsof microsignatures becomevisible) • Pitch angle coverage: Spinning sensorsprobably not sufficient(difficulttocover all pitchangles in 1-2 minutes)