Role of coupled processes on the radial and angular distributions of > 1 keV electrons at Saturn
Authors: Santos-Costa Daniel , Woodfield Emma , Menietti Doug , Hospodarsky George , Clark George , Kollmann Peter , Paranicas Chris , Tseng Wei-Ling 
Published: March 2020

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&lt;div&gt;We&amp;#160;present results from a three-dimensional diffusion theory model, which&amp;#160;solves the time dependent Fokker-Planck equation with physical terms representing energizing, source and loss processes&amp;#160;to&amp;#160;interpret key features in the radial and angular distributions of &gt; 1 keV-energy electrons&amp;#160;at Saturn.&amp;#160;Cassini observations of eV-keV electron Pitch-Angle Distributions (PADs) at Saturn have revealed a spatial structuring, with little temporal and longitudinal dependence, that can be broken up into three distinct regions [1]: (1) a region dominated by field-aligned PADs from ~12-15 Rs, (2) a transition region&amp;#160;from ~8-12 Rs&amp;#160;in which butterfly distributions&amp;#160;are typically observed, and (3) a region inside ~8 Rs dominated by trapped PADs.&amp;#160;Past studies have explained field-aligned PADs&amp;#160;by the presence of field aligned currents and acceleration mechanisms in the auroral region [2], while pancake profiles would be the result of inward adiabatic transport [3]. It was argued that energetic electrons are&amp;#160;adiabatically&amp;#160;energized&amp;#160;during inward motion and their PADs&amp;#160;would&amp;#160;radially evolve from field-aligned (&gt; 15 Rs) to butterfly to pancake/isotropic inside ~8 Rs [4,5,6].&amp;#160;Although Cassini had unveiled Enceladus' dense and extended&amp;#160;neutral cloud, little had been done&amp;#160;regarding the role of neutrals on the distributions of electrons.&amp;#160;We have subsequently combined multi-instrument data analyses of Cassini observations (particle, field and waves) and a diffusion theory model of charged particle fluxes to test the scenarios of the origins and&amp;#160;radial evolution of electrons' PADs in the region ~2-15 Rs. In our&amp;#160;work,&amp;#160;Cassini CAPS/ELS, MIMI/LEMMS and MAG are used to&amp;#160;both&amp;#160;constrain the model at its boundary conditions and discuss our simulation results with in-situ data. Our radial transport is initially constrained by MIMI/LEMMS observations of micro-signatures [7]&amp;#160;and assumed to be adiabatic [8]. Our simulation results show&amp;#160;that the adiabatic transport cannot entirely explain the radial and angular features of energetic electrons within the ~2-15 Rs region.&amp;#160;The coupling of different&amp;#160;mechanisms&amp;#160;is required into our model to&amp;#160;obtain better&amp;#160;agreements with in-situ data.&amp;#160;The implementation of a supra-thermal electron&amp;#160;population&amp;#160;at high-latitudes appears to be a reasonable source of magnetospheric particles beyond ~9 Rs. While impact-ionization and&amp;#160;Bremsstrahlung are&amp;#160;insignificant&amp;#160;mechanisms&amp;#160;for &gt; 1 keV-energy electrons, coulomb collisions with neutrals efficiently alter the electron distributions inside ~9 Rs. The&amp;#160;drastic&amp;#160;depletion&amp;#160;observed&amp;#160;in the electron fluxes&amp;#160;inside ~9-10 Rs&amp;#160;is partially explained by the interaction of electrons with neutrals. To&amp;#160;pursue our&amp;#160;understanding of&amp;#160;radial and angular distributions of &gt; 1 keV electrons inside ~7-8 Rs, we are&amp;#160;currently investigating the role of dust, cold plasma&amp;#160;and waves.&amp;#160;Interactions with dust and plasma particles seem to have limited effects. Past studies showed that wave-particle interactions at Saturn are&amp;#160;inconclusive&amp;#160;[9,10]. Nonetheless, we&amp;#160;propose to revisit the role of waves at Saturn as only the interaction with whistler mode chorus waves was examined&amp;#160;and the role of coupled processes not&amp;#160;discussed. We will thus present our latest results of the interactions of neutrals, dust and&amp;#160;plasma environments, and&amp;#160;electromagnetic waves with Saturn&amp;#8217;s energetic electron population from&amp;#160;a physics-based modeling approach.&amp;#160;&lt;/div&gt;
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