Saturn’s Recurrent Auroral Flashes in the Equinoctial Campaign

Abstract

The past decades of Cassini’s investigation around Saturn have revealed the frequent occurrence of ∼1 hr quasiperiodic (QP) signatures in the in situ magnetic field and plasma measurements and plasma wave observations. Saturn’s aurora, as the response to the planet’s magnetospheric dynamics, is also observed to exhibit transient features that can reoccur with a strikingly similar periodicity. Previous investigations have associated these features with energetic plasma injections, standing Alfvén waves, and dayside magnetodisk reconnections, which all require a closed-field configuration. However, both previous and our analyses of the Hubble Space Telescope (HST) images, as Saturn approached the equinox (when both poles are in the field of view), reveal that transient auroral emissions are strictly nonconjugate between hemispheres—they occur in either the north or south alone. This picture is matchable with the open-configured, north-south asymmetric magnetopause reconnection. We also present a detailed case analysis of Cassini’s measurements for a prolonged period before and after a sequence of transient auroral brightenings observed in HST’s visits G1-G4. We show that the northern lobe featured hourly QP pulsations in the magnetic field, electron flux, and whistler-mode hiss. In an interval outside HST exposures, we observed tens to hundreds of keV electrons in the antiparallel direction associated with upward field-aligned currents, which can in turn produce aurora. This is potentially suggestive of reconnection processes at the magnetopause, but are limited in the spatial scale.