Transport and chemical loss rates in Saturn's inner plasma disk

Abstract

The Kronian moon Enceladus is constantly feeding its surrounding with new gas and dust, from cryovolcanoes located in its south polar region. Through photoionization and impact ionization of these neutrals, a plasma disk is created, which mainly contains hydrogen ions and water group ions. This paper investigates the importance of ion loss by outward radial transport and ion loss by dissociative recombination, which is the dominant chemical loss process in the inner plasma disk. We use plasma densities derived from several years of measurements by the Cassini Radio and Plasma Wave Science electric field power spectral density and Langmuir probe to calculate the total flux tube content NL2. Our calculation shows that NL2 agrees well with earlier estimates within dipole L shell 8. We also show that loss by transport dominates chemical loss between L shells 4 and 10. Using extrapolation of available measurements, we extend the study to include L shells 2.5 to 4. The results indicate that loss by transport dominates chemical loss also between L shells 2.5 and 4. The loss rate by transport is around five times larger at L shell 5, and the difference increases as L7.7 beyond L = 5, for the net ion population. Chemical loss may still be important for the structure of the plasma disk in the region closest to Enceladus (around ±0.5 RS) at 3.95 RS (1 RS = Saturn's equatorial radius = 60,268 km), since the transport and chemical loss rates only differ by a factor of ∼2 in this region. We also derive the total plasma content of the plasma disk between L shells 4 and 10 to be 1.9 × 1033 ions and the total ion source rate for the same region to be 5.8 × 1027 s−1. The estimated equatorial ion production rate P ranges from 2.6 × 10−5 cm−3 s−1 (at L = 10) to 1.1 × 10−4 cm−3 s−1 (at L = 4.8). The net mass loading rate is derived to be 123 kg/s for L shells 4 to 10.

Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk

“…As new ions are produced by hot electron impact ionization the largest ion production rate, in the region close to the equatorial plane, will be found in between the maxima of the neutral density and the hot electron density. It should be noted that the equatorial plasma density peak does not correspond to the peak of the total flux tube content, which instead peaks around magnetic L shell 6.3 (Sittler Jr et al, ) and 6.6 (Holmberg et al, ).…”

“…Assuming quasineutrality of the plasma disk, that is, n i ≈ n e ≈ n , for the steady state situation we get κ n n + k n eh n n = α e f f n 2 + n / t . Holmberg et al () showed that the dominant loss process in the plasma disk is transport loss, so for a rough estimate we use κ n n + k n eh n n = n / t .…”