Abstract
AbstractSaturn'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.
Highlights
Saturn's icy moon Enceladus ejects between tens to hundreds of kilograms of water‐group molecules each second (e.g., Burger et al, 2007; Jurac et al, 2002; Jurac & Richardson, 2005)
In this paper we present a case study, using partially coincident auroral and energetic neutral atom (ENA) images captured during a southern high‐latitude Cassini orbit on 9 April 2014, in which three injection signatures are observed at various stages of development
We highlight a case study using partially coincident auroral and ENA images captured during a southern high‐latitude Cassini orbit on 9 April 2014, in which three injection signatures are observed at various stages of development
Summary
Saturn's icy moon Enceladus ejects between tens to hundreds of kilograms of water‐group molecules each second (e.g., Burger et al, 2007; Jurac et al, 2002; Jurac & Richardson, 2005). Most pertinent to this study, auroral signatures have been associated with large‐scale plasma injection events, appearing to broadly follow the local time position of ENA emission signatures as they rotate around the planet (e.g., Lamy et al, 2013; Mitchell et al, 2009; Nichols et al, 2014). These auroral “spots” or “patches” first appear poleward of the main emission on the nightside of the planet in response to dipolarization of the stretched magnetotail field following reconnection (Jackman et al, 2013). We discuss the interpretation of these rotating signatures as indicators of possible injection activity, and surmise that their occurrence is likely modulated by periodic reconnection processes in the magnetotail
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
