The physical processes responsible for adiabatic electron heating in Saturn's radiation belts

Abstract:

It has been established that energetic electrons in the outer regions of Saturn's radiation belts (L-shell, L>5) are primarily generated through adiabatic transport from electron sources in the planet's middle and outer magnetosphere. In this presentation we show which physical processes mediate adiabatic transport and we define their boundaries in L-shell and energy. In particular, high energy resolution energetic electron spectra obtained by Cassini (20 keV - 1.6 MeV) reveal that electron fluxes are regularly distributed along discrete energy bands, the topology and structure of which appears to change drastically across ~100 keV, revealing the radial transport processes responsible. Below that energy threshold and down to L~5, transport is effected through a magnetospheric interchange instability, as evidenced also by simultaneous observations in magnetic field and plasma. Above ~100 keV we observe that the bands are always oriented such that with decreasing L-shell their characteristic energy increases. We find that this behaviour derives from slow radial plasma flows associated to a persistent convection pattern in Saturn's magnetosphere (noon to midnight electric field). Various forms of evidence indicate that noon to midnight electric field dynamics are key also for lower L-shells, where competing, local acceleration processes act, and may be ultimately responsible for the bulk of quasi and fully relativistic electrons trapped throughout Saturn's radiation belts. Superimposed on these two processes, stochastic, radial diffusion of electrons across small spatial scales has also been observed to take place but its driving mechanism remains elusive. We discuss whether Ultra Low Frequency (ULF) variations of Saturn's noon to midnight electric field may be responsible or other source(s) of ULF waves should be sought.