Energetic Electron Pitch Angle Distributions During the Cassini Final Orbits. Issue 7 (13th April 2018)

• Record Type: Journal Article
• Authors: Carbary, J. F.; Mitchell, D. G.; Kollmann, P.; Krupp, N.; Roussos, E.; Dougherty, M. K.
• Is Part Of: Geophysical research letters. Volume 45:Issue 7(2018)
• Page Start: 2911
• Page End: 2917
• Publication Date: 2018-04-13
• Subjects: Saturn magnetic field -- Saturn energetic electrons; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1002/2018GL077656
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Pitch angle distributions (PADs) of very energetic electrons (110–365 keV) are examined during the ring‐grazing and proximal orbits of the Cassini spacecraft, from day 320 2016 (15 November) to day 257 2017 (14 September). These repeating orbits allowed a statistical evaluation of the PADs within the magnetopause on the nightside of Saturn. Along L ‐shells (i.e., equatorial crossing distances of magnetic field lines) near and outside that of Titan and north of the equator, the electron fluxes were unidirectionally field‐aligned going away from Saturn. Along L ‐shells inside Titan's and south of the equator, the electrons had bidirectional or pancake (trapping) PADs. This behavior suggests that the field lines within Titan's L ‐shell are generally closed, while those outside of that L ‐shell are generally open. This result strictly applies only to the nightside local times sampled during the final Cassini orbits, but one may infer a similar behavior at other times. Plain Language Summary: The angle between the magnetic field vector and the velocity of an electron is known as the pitch angle. Sufficiently energetic electrons move at essentially the speed of light, so they can effectively sample the entire length of a magnetic field line. Therefore, the pitch angle distributions of energetic electron fluxes can serve as an indicator of whether the field line is open (i.e., electrons at pitch angles of 0° or 180°) or closed (i.e., electrons at pitch angles of 0° and 180° or at pitch angles of 90°). The pitch angle distributions of electrons during the final orbits of Cassini indicate the field lines are closed inside the magnetic shell of the moon Titan and open outside this shell. This finding has important implications for the global topology of Saturn's magnetic field. Key Points: Energetic electrons (110–365 keV) move away from Saturn along field lines outside Titan's L ‐shell, indicating open field lines Bidirectional and pancake distributions exist inside the L ‐shell of Titan, indicating closed field lines Thus, Titan's L ‐shell is an approximate boundary for open/closed field lines