Internal Versus External Sources of Plasma at Saturn: Overview From Magnetospheric Imaging Investigation/Charge‐Energy‐Mass Spectrometer Data

Abstract

Plasma composition observations provide a useful mechanism for investigating plasma sources and subsequent evolution within planetary magnetospheres. While He ++ is the second most abundant ion species in the solar wind, there are no known sources of He ++ ions within the magnetosphere of Saturn, allowing He ++ to serve as a tracer of solar wind ions within the Kronian magnetosphere. Meanwhile, water group ions (W + , consisting of O + , OH + , H 2 O + , and H 3 O + ) and H 2 + , known to originate within the magnetosphere of Saturn, serve as a tracer of internally ionized plasma. In this paper, we investigate the relative abundances and properties of energetic (32-220 keV) ion species originating from sources within the magnetosphere and from the solar wind. Solar wind-originating ions are observed to have significant relative abundance (up to~0.05) in the midnight, dawn, and noon local time quadrants at high radial distances (~40,~45, and~20 R S , respectively). Several possible entry processes, such as reconnection and Kelvin-Helmholtz instabilities, are outlined in this paper as well as a discussion of subsequent transport.Plain Language Summary While the sources of plasma within the magnetosphere of Saturn have long focused on internal generation, this study seeks to estimate the amount of plasma entering into the magnetosphere from the solar wind. Using a mission averaged vantage point of the global relative abundances of 32 to 220 keV ions, the internal to external plasma is investigated in order to estimate the relative amount of solar wind-originating ions within the magnetosphere of Saturn, as well as the regions of their importance. Solar wind-originating ions are predominantly seen within the midnight and dawn quadrants of Saturn, while the dayside and inner magnetosphere is dominated by internally generated plasma. The energetic plasma is also seen to peak in density near a radial distance of 10 R S as a consequence of loss processes closer to the planet and radial diffusion further from Saturn.the relative contributions of both internally generated and externally generated plasma within the magnetosphere of Saturn. Additionally, studying the density profiles of energetic plasma, in general, provides insight into various energization and loss processes occurring at Saturn.

Convection in the Magnetosphere of Saturn During the Cassini Mission Derived From MIMI INCA and CHEMS Measurements

“…The heavier ions will be more affected, since they are more confined to the current sheet than other lighter ions and thus subject to reconnection. The average hot ion O + pressure calculated in this study is lower in the dawnside region, where Allen et al () found a depletion of water group hot ions.…”

“…This plasma could also reconnect to the external solar wind stream (Dungey, ) that would accelerate the plasma antisunward and down the magnetotail. Ion densities in the magnetotail measured by the CHEMS instrument indicate lower water group ion density in the dawn sector relative to dusk, which could imply reconnective mass loss during the transit of water group plasma across the magnetotail (Allen et al, ) but not necessarily antisunward plasma release.…”

Cassini/MIMI Observations on the Dungey Cycle Reconnection and Kelvin‐Helmholtz Instability in Saturn's Magnetosphere

“…Mass loss has been shown to occur through periodic reconnection events (Cowley et al, ), in the form of plasmoids (Jackman et al, ) or constantly from the postmidnight sector that is not necessarily related to reconnection (A. W. Smith et al, ). The lower W + density toward the dawn (compared to dusk) in the Allen et al () study is indicative of a magnetotail reconnection that favors mass loss through tailward plasmoids (Mitchell et al, ). Compared to W + , the H + , He + , and He ++ particles exhibit different behavior in the Allen et al () data set: the densities are increased toward the dawn sector, indicating that their sources outweigh the losses, that is, either due to additional energization of thermal plasma up to 32 keV or solar wind plasma entering the magnetosphere.…”

“…The focus of the Allen et al () study is on the interplay between external and internal charged particle sources in Saturn's magnetosphere from the perspective of suprathermal ions, that is, integrated fluxes and densities of 32 to 220 keV energetic ion populations measured by the Charge Energy Mass Spectrometer, a unique detector that measures ion masses and their charged states at such high energies (Krimigis et al, ), over an extended time period of ∼13 years (2004–2017). The authors exploit a robust method toward determining the relative abundances and group abundance ratios of protons ( H + ), water group ions (W + ), and H , He + , and He ++ near the equatorial plane (±15° in Latitude) for various local times and radial distances covered by the Cassini orbits, <70 Rs toward the magnetotail, adopting a conservative magnetopause boundary by the use of the Kanani et al () model: out to ∼20 Rs in the noon sector and ∼30–45 Rs in the postdusk and predawn sectors (see black line in Figure b).…”

“…The spatial distributions of charged ions are key to establishing all different plasma processes, together with the coupling of the solar wind with Saturn's magnetosphere, and the Allen et al () study provides a preliminary, yet important, insight by essentially asking a few fundamental questions: Has Cassini obtained statistically significant evidence on the existence of solar wind tracers (e.g., He ++ ) inside the magnetosphere? How are these distributed and how do they compare with the internally sourced plasma?…”