Tracking counterpart signatures in Saturn's auroras and ENA imagery during large-scale plasma injection events

Kinrade, Joe and Badman, Sarah and Paranicas, C. and Mitchell, Donald. G. and Arridge, Chris and Gray, Rebecca and Bader, Alexander and Provan, G. and Cowley, S. W. H. and Martin, Carley and Achilleos, Nicholas (2020) Tracking counterpart signatures in Saturn's auroras and ENA imagery during large-scale plasma injection events. Journal of Geophysical Research: Space Physics, 152 (2): e2019JA027. ISSN 2169-9402

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Abstract

Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning.