Formation of electron radiation belts at Saturn by Z-mode wave acceleration

The energetic electron spectra of Saturn's radiation belts are studied using Cassini's 13 years of measurements by the Magnetosphere Imaging Instrument/Low Energy Magnetospheric Measurement System detector. We find that between L shells (L) of 10 and 4.5 the differential flux spectrum of 0.3‐ to 1.6‐MeV electrons evolves from a single power law to two power law functions separated at an energy cutoff (Ec). We show that inside L∼8, Ec has an L shell dependence that tracks consistently the energy of corotation drift cancelation (or resonance) ECDR, that is, the energy at which magnetic electron drifts and azimuthal corotation cancel, and is not associated with the absorbing effects of Saturn's moons. Ec also deviates from what conservation of the first adiabatic invariant would dictate. Our results verify that electrons around ECDR can be transported radially very efficiently by variable convective flows, such as those related to the noon‐midnight electric field in Saturn's magnetosphere.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Sun

Roussos

Krupp

et al. 2019

“…Losses could be due to collisions with the neutral torus and ring particles (which are slow, continuous losses; Woodfield et al, ; Lorenzato et al, ) or as yet unquantified wave‐particle interactions, particularly scattering due to whistler mode hiss (Menietti et al, ) and ion cyclotron waves (Leisner et al, ; Meeks et al, ). Recent results outside of 4 R S (Kollmann et al, ) show that electron energy spectra are time variable on top of a trend that follows the effect of radial diffusion; some of this variation may be due to chorus wave acceleration and subsequent losses.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…The electron radiation belts at Saturn until recently were thought to be the result of the production of high‐energy electrons from Cosmic Ray Albedo Neutron Decay and radial diffusion of lower‐energy seed population electrons (Kollmann et al, ; Kollmann et al, ; Roussos et al, ). Recently, however, Woodfield et al () showed that wave‐particle interactions with Z‐mode waves have a very important role in accelerating electrons inside the orbit of Enceladus. Whistler mode chorus waves, which are a key source of electron acceleration at the Earth (Horne et al, ) and Jupiter (Horne et al, ; Woodfield et al, ), were dismissed as having negligible effect on the electrons at Saturn due to the combination of plasma and magnetic field conditions (Lorenzato et al, ; Shprits et al, ).…”

Section: Introductionmentioning

confidence: 99%

Rapid Electron Acceleration in Low‐Density Regions of Saturn's Radiation Belt by Whistler Mode Chorus Waves

Woodfield

Glauert

Menietti

et al. 2019

Self Cite

Electron acceleration at Saturn due to whistler mode chorus waves has previously been assumed to be ineffective; new data closer to the planet show it can be very rapid (factor of 104 flux increase at 1 MeV in 10 days compared to factor of 2). A full survey of chorus waves at Saturn is combined with an improved plasma density model to show that where the plasma frequency falls below the gyrofrequency additional strong resonances are observed favoring electron acceleration. This results in strong chorus acceleration between approximately 2.5 RS and 5.5 RS outside which adiabatic transport may dominate. Strong pitch angle dependence results in butterfly pitch angle distributions that flatten over a few days at 100s keV, tens of days at MeV energies which may explain observations of butterfly distributions of MeV electrons near L=3. Including cross terms in the simulations increases the tendency toward butterfly distributions.

“…Second, we adopt a dipole magnetic field to approximate the Saturn's background magnetic field (Gu et al, 2013;Woodfield et al, 2018).…”

Section: Plasma Density and Wave Modelsmentioning

confidence: 99%

Combined Effects of Equatorial Chorus Waves and High‐Latitude Z‐Mode Waves on Saturn's Radiation Belt Electrons

Cui

et al. 2019

In this paper, we examined the combined effects of equatorial chorus waves and high‐latitude Z‐mode waves on the energy and pitch angle distribution of Saturn's radiation belt electrons. Our simulation results show that these two types of waves at different latitudes jointly control the fluxes of Saturn's radiation belt electrons, which is very different from the individual effects of each kind of waves. The presence of Z‐mode waves can efficiently inhibit the reversed energy spectrum of electrons driven by chorus waves, whereas the presence of chorus waves can further accelerate and scatter the relativistic electrons accelerated by Z‐mode waves toward the larger and smaller pitch angles. Our findings provide a new insight on how different types of waves at different latitudes jointly affect the radiation belt electron dynamics, including, but not limited to, Saturn.