Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini

S A Taylor,A J Coates,R T Desai,P Schippers,G H Jones,J H Waite,A Wellbrock,M W Michiko,R Caro‐Carretero,A N Fazakerley

doi:10.1002/2017ja024536

Abstract

AbstractThe Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer has observed photoelectrons produced in the plume of Enceladus. These photoelectrons are observed during Enceladus encounters in the energetic particle shadow where the spacecraft is largely shielded from penetrating radiation by the moon. We present a complex electron spectrum at Enceladus including evidence of two previously unidentified electron populations at 6–10 eV and 10–16 eV. We estimate that the proportion of “hot” (>15 eV) to “cold” (<15 eV) electrons during the Enceladus flybys is ≈ 0.1–0.5%. We have constructed a model of photoelectron production in the plume and compared it with ELS Enceladus flyby data by scaling and energy shifting according to spacecraft potential. We suggest that the complex structure of the electron spectrum observed can be explained entirely by photoelectron production in the plume ionosphere.

Highlights

Photoionization is the physical process where a neutral atom or molecule becomes ionized by the interaction with a photon producing a photoelectron and a positive ion
We have constructed a model of photoelectron production in the plume and compared it with Electron Spectrometer (ELS) Enceladus flyby data by scaling and energy shifting according to spacecraft potential
The negatively charged particle environment near Enceladus consists of several populations: magnetospheric electrons, photoelectrons, cluster ions, and charged ice grains

Summary

Introduction

Photoionization is the physical process where a neutral atom or molecule becomes ionized by the interaction with a photon producing a photoelectron and a positive ion. The particular electron energy spectrum observed in an atmosphere depends on the neutrals present and the branching ratios for each photoionization reaction (Mantas & Hanson, 1979) Another feature is an observed dropoff of photoelectron production above ≈60 eV which is due to a reduction in solar photon intensity at wavelengths below 16 nm (Coates et al, 2011; Galand et al, 2006). Plumes of neutral particles (Waite et al, 2006) and micron-sized icy dust particles (Spahn et al, 2006) have been discovered emanating from “tiger stripe” features on the icy surface at high southern latitudes (Porco et al, 2006) Significant quantities of these neutral particles become charged, and Cassini has detected plasma (Tokar et al, 2006, 2009) (including water group ion clusters; Coates et al, 2010) and charged nanograins (Hill et al, 2012; Jones et al, 2009) in the plumes. In addition to the two previously identified photoelectron peaks, we identify further low-energy photoelectron energy structure and study ratios between the two main peaks (C and D)

Instrumentation and Flyby Orientations

Observations

Analysis and Discussion

Summary and Conclusions