Comparative radiation belt studies by Juno and Cassini
Comparative radiation belt studies by Juno and Cassini E. Roussos 1, R. Thorne 2 1: Max Planck Institute for Solar System Research, Göttingen, Germany 2: Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, USA
Introduction Jupiter (Quasi)-dipolar region: Energies/Fluxes: L<15 -20 Saturn L<7 -10 e-: few tens of Me. V e-: up to 50 -100 Me. V ions: hundreds of Me. V ions: ~Ge. V ~108 cm-2 s-1 (e- > 1 Me. V) <107 cm-2 s-1 (e- > 1 Me. V) Ion Composition: H+, He++, O? +, S? +etc. H+, He++, W+, Fen+ etc. Loss regions: Moons, Io/Europa torus, diffuse rings, waveparticle scattering Main rings, Moons, Enceladus torus, diffuse rings, wave-particle scattering Major Datasets: Galileo, Ulysses (high. latitude), Pioneer, Voyager, Synchrotron & aurora emissions Cassini (including ENA imaging), Voyager, Pioneer, aurora imaging
Cassini radiation belt crossings Current coverage Roussos et al. (2014) Proximal/F-ring orbits
Juno Radiation Belt crossings Bagenal et al. (2014)
Types of measurements In situ • CAPS/JADE (Cold plasma/energetic particles) • MIMI/JEDI (Energetic particles) • RPWS/Waves (Radio & Plasma waves) • MAG/MAG (Magnetic field) Remote • • UVIS/UVS/Hubble, Hisaki (UV aurora) MIMI-INCA (Energetic neutral atoms) RPWS/Waves (Radio/Plasma waves) MWR/LOFAR (Synchrotron emission)
Inner radiation belts (1) Flux mapping Roussos et al. (2014) Bagenal et al. (2014) Saturn: • Limited coverage inside L=2. 8 & high latitudes • Limited pitch angle coverage Jupiter: • Limited high latitude coverage • Limited local time coverage • Contaminated/saturated measurements • Access mostly through synchrotron maps
Inner radiation belts (2) Flux mapping results • Transport coefficients: – DLL ~ L 3 (Jupiter – e. g. Tsuchiya et al. , 2011) (thermospheric winds) – DLL ~ Ln, n>6 (Saturn) (magnetic/electric fluctuations) Roussos et al. (2007) • Plasma/energetic particle convection – Noon/midnight electric field (Sat. ) (Andriopoulou et al. 2012) – Dawn-dusk electric field (Jup. ) (Barbosa & Kivelson, 1983) – Properties? (intensity, radial profile) Wilson et al. (2012)
Inner radiation belts (3) Flux mapping results • CRAND process – Observed at Saturn (Kollmann et al. 2013) – Ambiguous at Jupiter (Fisher et al. 1996) – Source of CRAND? • • • Rings or atmosphere at Saturn? Only atmosphere at Jupiter Are hydrogenous atmospheres effective for CRAND? Kotova et al. (this MOP) • Other local sources – Electron CRAND – Energy diffusion (lightning generated whistlers) – Efficiency of multiple charge exchange Krimigis et al. (2005)
Inner radiation belts (5) Long/short term variations • Monitoring when in SW (remote observations) – Impact on middle magnetosphere (MIMI/INCA) – Impact on inner belts (LOFAR, Juno/MWR) – Response to UV input – Transients/ other periodicities or time scales Roussos et al. (2014) • In-situ monitoring – Short period orbits, sufficient for SW time scales Tsuchiya et al. (2011)
Inner radiation belts (6) Transient phenomena • Transient radiation belts – – Linked with CMEs at Saturn (protons) (Mc. Donald et al. 1980; Roussos et al. 2008) Unclear picture for electrons (Jupiter and Saturn) (Russell et al. 2001) Combination of in-situ & remote observations will be helpful Time scales of transient belt evolution: days to months Garrett et al. (2012) Kimura et al. (2015)
Seed population (middle/outer magnetosphere) Energetic particle injections • Injections/Flux tube interchange (remote observations) – – Frequency of occurrence Spatial organization Radial velocities Impact on inner belts (LOFAR, Juno/MWR) • Injections/Flux tube interchange (in-situ) (e. g. Mauk et al. 1999) – Pitch angle distributions – Radial velocities – ENA imaging Dumont et al. (2014); Radioti et al. (2012)
Seed population (middle/outer magnetosphere) Wave particle energization • Electrons and whistler mode chorus waves – Important for initial acceleration of seed electron population – Significant for Jupiter, role unclear for Saturn – Latitudinal distribution of chorus waves is a determining factor – Plasma frequency from electron density model is important) • Association to injections (Bolton et al. 1997) Shprits et al. (2012)
Seed population (middle/outer magnetosphere) Quasi-periodic electron bursts • Impulsive electron/ion acceleration source – Electrons/ions up to few Me. V – Possibly originating at high latitudes, links to aurora (Badman et al. 2012; Mitchell et al. 2015) – PAD easier to obtain with Juno – Possibly mapping to closed field lines (Roussos et al. 2015) – Signature in several instruments (Mitchell et al. 2009; Mc. Dowall et al. 1993) Palmaerts et al. (this MOP); Simpson et al. (1992)
Summary • Juno/Cassini observations excellent opportunity for radiation belt studies • Combination of remote/in-situ measurements unique • Measurements by Cassini/Juno complementary – many common elements in the two systems • Earth-based observations (e. g. LOFAR) will also be important • Radiation belts are not closed systems – studying also the seed population region is essential
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