Interchange Injections at Saturn: Statistical Survey of Energetic H⁺ Sudden Flux Intensifications

Abstract

We present a statistical study of interchange injections in Saturn's inner and middle magnetosphere focusing on the dependence of occurrence rate and properties on radial distance, partial pressure, and local time distribution. Events are evaluated from over the entirety of the Cassini mission's equatorial orbits between 2005 and 2016. We identified interchange events from CHarge Energy Mass Spectrometer (CHEMS) H + data using a trained and tested automated algorithm, which has been compared with manual event identification for optimization. We provide estimates of interchange based on intensity, which we use to investigate current inconsistencies in local time occurrence rates. This represents the first automated detection method of interchange, estimation of injection event intensity, and comparison between interchange injection survey results. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the most intense events as defined by H + partial particle pressure. We determine that nightside occurrence dominates as compared to the dayside injection rate, supporting the hypothesis of an inversely dependent instability growth rate on local Pedersen ionospheric conductivity. Additionally, we observe a slight preference for intense events on the dawnside, supporting a triggering mechanism related to large-scale injections from downtail reconnection. Our observed local time dependence paints a dynamic picture of interchange triggering due to both the large-scale injection-driven process and ionospheric conductivity.Plain Language Summary Studying high-energy particles around magnetized planets is essential to understanding processes behind mass transport in planetary systems. Saturn's magnetic environment, or magnetosphere, is sourced from a large amount of low-energy water particles from Enceladus, a moon of Saturn. Saturn's magnetosphere also undergoes large rotational forces from Saturn's short day and massive size. The rotational forces and dense internal mass source drive interchange injections, or the injection of high-energy particles closer to the planet as low-energy water particles from the inner magnetosphere are transported outward. There have been many strides toward understanding the occurrence rates of interchange injections, but it is still unknown how interchange events are triggered. We present a computational method to identify and rank interchange injections using high-energy particle fluxes from the Cassini mission to Saturn. These events have never been identified computationally, and the resulting database is now publically available. We find that the peak rates of interchange occur between 7 and 9 Saturn radii and that this range coincides with the highest intensity events. We also find that interchange occurrence rates peak on the nightside of Saturn. Through this study, we identify the potential mechanisms behind interchange events and advance our understanding of mass transport around planets. neutral mass source from Enceladus re...

Spectral Signatures of Adiabatic Electron Acceleration at Saturn Through Corotation Drift Cancelation

“…Roussos et al () found cases of high correlation between radiation belt and ring current variations, indicating the ring current is a key reservoir of the belt electrons through inward adiabatic heating (Kollmann et al, ; Paranicas et al, ). Adiabatic transport could be accomplished through stochastic magnetic field fluctuations (Roussos et al, ), centrifugal interchange instability injections (Azari et al, ; Paranicas et al, ; Rymer et al, ), or variable global scale convective flows (Roussos et al, ). Nonadiabatic mechanisms contributing to electron acceleration such as wave particle interactions (Horne et al, ) have been discussed by Woodfield Horne et al (, ) through numerical simulations and Cassini measurements.…”

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Are Saturn's Interchange Injections Organized by Rotational Longitude?

“…Associated with the interchange process are injections of hot plasma from the middle into inner magnetosphere, which are often termed interchange injections. Interchange injections are typically characterized by an enhancement of energetic (>100 eV) plasma and a depletion of low‐energy plasma along with enhanced wave activity and magnetic field pressure (e.g., André et al, , ; Azari et al, ; Burch et al, ; Chen & Hill, ; Hill et al, ; Kennelly et al, ; Lai et al, ; Rymer et al, ). Within this very same spatial region in which interchange is occurring, modulations of the field and plasma conditions have been observed associated with rotational periodicities (e.g., Ramer et al, ).…”

“…Subsequent to the work by Kennelly et al (), several studies have evaluated interchange distributions at Saturn, extending to close to the end of the Cassini mission in mid‐September of 2017 (Azari et al, ; Lai et al, ). Azari et al () identified interchange injection events from 2005 to 2016 using a statistics‐based algorithm applied to H + flux intensitifications from the Magnetosphere Imaging Instrument's CHarge Energy Mass Spectrometer (CHEMS) (Krimigis et al, ). In their work, interchange injections were found to be both more prevalent and more intense between 7 and 9 R s .…”

“…In addition to the broad nightside peak, Kennelly et al () also found a dayside, post noon peak, which does not seem to fit this explanation of ionospheric conductivity. Beyond examining the local time and radial dependence of their identified injection events, Azari et al () also compared their interchange injection events found in the CHEMS data to previous interchange surveys that used different detection methods. They found only modest overlap between their high‐energy ion event catalogue and prior lists, and similarily for the other lists to each other, with 26 events shared by all published surveys.…”