Sources, Sinks, and Transport of Energetic Electrons Near Saturn's Main Rings

Abstract

The inner boundary of Saturn's electron radiation belts, near the planet's A ring (∼2.27 Rs), is studied using Cassini's proximal orbit measurements. We find that variable convective flows transport energetic electrons to the A ring, which absorbs them instantaneously, forming the inner belt boundary. These flows are also responsible for a variable and longitudinally asymmetric boundary configuration. Prenoon, the boundary oscillates toward and away from the A ring with a 2‐week period. Postnoon, it maps persistently near the F ring (∼2.32 Rs) and coexists with localized MeV electron intensity enhancements (microbelts). We propose that the microbelts contain electrons in drift resonance with corotation, trapped in local time‐confined trajectories, which result from the aforementioned convective flows. The microbelts' collocation with the F ring implies either a local, secondary electron production due to galactic cosmic ray collisions with F ring dust or an enhanced resonant electron trapping due to an electrodynamic interaction between the F ring and Saturn's magnetosphere.

Sustaining Saturn's Electron Radiation Belts Through Episodic, Global‐Scale Relativistic Electron Flux Enhancements

“…Large‐scale variations in the noon‐midnight electric field are known to occur in the dawn‐dusk electric field of Jupiter, triggering fast transport of energetic electrons (Han et al, ; Murakami et al, ). Similar variations at Saturn are hinted by electron belt observations near Saturn's A ring (Roussos et al, ).…”

Spectral Signatures of Adiabatic Electron Acceleration at Saturn Through Corotation Drift Cancelation

“…All the observations prove the importance of CDR electron transport due to time‐variable local time fixed convective electric fields (Roussos et al, ). The global noon‐to‐midnight electric field remains the best candidate for driving this electron transport process (Andriopoulou et al, ; Wilson et al, ), it causes a quasi‐uniform, dawnward E × B convection flow.…”

“…The global noon‐to‐midnight electric field remains the best candidate for driving this electron transport process (Andriopoulou et al, ; Wilson et al, ), it causes a quasi‐uniform, dawnward E × B convection flow. It will shift electrons' circular orbits toward the direction of noon if E < E CDR , or midnight if E > E CDR (Roussos et al, ). As electrons drift on constant μ curves, we traced electrons with the method of Roussos et al (), a guiding center test particle tracing in a dipole field considering a noon‐midnight electric field and corotation.…”

“…Those electrons' guiding centers remain nearly stationary in local time such that their drift is sensitive to inflows/outflows fixed at their local time. These flows may transport electrons across a large L shell range over short periods, and they could be associated with the noon‐midnight electric field, a well‐established convective pattern at Saturn inward of L ∼10 and down to the main rings (Andriopoulou et al, ; Roussos et al, ; Thomsen et al, ). Electrons with much lower or higher energy than E CDR drift fast across the radial flow channels and experience a smaller change in L shell.…”

A radiation belt of energetic protons located between Saturn and its rings

“…Magnetic mapping of the in situ measurements was performed by calculating the L-shell, equatorial pitch angle, and the loss cone through an empirical third-order magnetic field model (24). We define the L-shell value (L) as the distance from the planet (in Saturn radii) that a field line crosses the magnetic equator.…”