Near‐Magnetic‐Field‐Aligned Energetic Electrons Above Saturn's Dark Polar Regions

Abstract

AbstractSaturn's main auroral emissions define two oval‐shaped regions, one encircling each magnetic pole. The regions at higher latitudes are generally “dark” (i.e., devoid of auroras), and are magnetically connected to the distant planetary magnetosphere where there is a much‐debated interaction with the solar wind. Electric currents flow into the atmosphere along the magnetic field within these polar regions. Establishing whether polar magnetic flux is “open” or “closed” is key for diagnosing how the solar wind interaction works. Because energetic electrons moving almost parallel or anti‐parallel to the magnetic field shed light on the field topology, we survey Cassini energetic particle data for rare instances when the spacecraft was able to measure these parts of the distribution in the polar field environment close to the planet. Over the entire mission we find 16 intervals when measurements at ∼0 and ∼180 pitch angles were made simultaneously without sunlight contamination. Across all the events, above‐background field‐aligned fluxes were measured intermittently by the >15 keV electron channels, extending up to ∼300 keV when present. Uni‐directional anti‐planetward fluxes were observed during 10 of the events, and bi‐directional fluxes were observed during 6 of the events. We suggest the uni‐directional anti‐planetward fluxes indicate the presence of field‐aligned beams, and that the bi‐directional fluxes indicate regions of locally closed magnetic field. These results either mean the solar wind interaction is predominantly via global magnetic reconnection but is more complex than initially proposed, or that the interaction is instead predominantly “viscous‐like” at Saturn.