Rapid Electron Acceleration in Low‐Density Regions of Saturn's Radiation Belt by Whistler Mode Chorus Waves. Issue 13 (8th July 2019)

• Record Type: Journal Article
• Authors: Woodfield, E. E.; Glauert, S. A.; Menietti, J. D.; Averkamp, T. F.; Horne, R. B.; Shprits, Y. Y.
• Is Part Of: Geophysical research letters. Volume 46:Issue 13(2019)
• Page Start: 7191
• Page End: 7198
• Publication Date: 2019-07-08
• Subjects: Saturn -- radiation belt -- electron -- wave‐particle interaction -- whistler mode chorus waves; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL083071
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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 10 4 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 R S and 5.5 R S 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. Plain Language Summary: Radiation belts are hazardous regions found around several of the planets in our Solar System. They consist of very hot, electrically charged particles trapped in the magnetic field of the planet. At Saturn the most important way to heat these particles has for many years been thought to involve the particles drifting closer toward the planet. This paper adds to the emerging idea at Saturn that a different way to heat the particles is also possible where the heating is done by waves, in a similar way to what we find at the Earth. We use recent information from the Cassini spacecraft on the number and location of particles and also of the waves strength and location combined with computer simulations to show that a particular wave called chorus is excellent at heating the particles where the surrounding number of cold particles is low. Key Points: Very low plasma densities found at Saturn significantly enhance the electron acceleration due to whistler mode chorus waves Extra diffusion where the plasma frequency/gyrofrequency <1 leads to rapid formation of butterfly pitch angle distributions Chorus acceleration of electrons at Saturn is an important mechanism for radiation belt dynamics between 2.5 R S and 5.5 R S