Jupiter’s Plasma Torus Throws a Tantrum—Blame It on Io?

Table of Links

Abstract

1. Introduction

   1.1. Io as the main source of mass for the magnetosphere

   1.2. Stability and variability of the Io torus system

   1.3. Hypothesized volcanic mass supply events

   1.4. Objective of this review

2. Review of the relevant components of the Io-Jupiter system

   2.1. Volcanic activity: hot spots and plumes

   2.2 Io’s bound atmosphere

   2.3 Exosphere and atmospheric escape

   2.4 Electrodynamic interaction, plasma-neutral collisions, and the related atmospheric loss processes

   2.5. Neutrals from Io in Jupiter’s magnetosphere

   2.6. Plasma torus and sheet, energetic particles

   2.7 Jupiter’s aurora and connections to the Io torus

   2.8 Dust from Io

3. Summary: What we know and what we do not know and 3.1 Current understanding for normal (stable) conditions

   3.2 Canonical number for mass supply

   3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora

   3.4 Gaps in understanding, contradictions, and inconsistencies

4. Future observations and methods and 4.1 Spacecraft measurements

   4.2 Remote Earth-based observations

   4.3 Modeling efforts

Appendix, Acknowledgements, and References

3.3 Transient events in the plasma torus, neutral clouds and nebula, and aurora

As reviewed in Section 2, there are several phenomena observed in the Jovian system which indicate significant transient changes in the magnetosphere and which are often explained by some change in volcanic activity. It is argued that this volcanic event enhances the mass output from Io over some short period of time. Primarily, these are observations of

(1) significant changes in plasma torus UV emissions, or

(2) an increase in the brightness of the sodium cloud or sodium nebula, or

(3) a particular morphology or periodic intensifications of the Jovian aurora.

A simultaneous change in the bulk (oxygen or sulfur) neutral cloud, which has been observed for O in the 2015 plasma torus event, would be a diagnostic as well but has never been detected independently.

Table 3 lists all events published in the literature of significant changes in the bulk torus (1) and one event in 2007 where an increase in the sodium nebula (2) was observed as well as a change in the aurora morphology (3). Events where only an increase in sodium nebula brightness was reported (e.g., Wilson et al., 2002; Mendillo et al., 2004; Morgenthaler et al., 2019) or only aurora signatures potentially indicative of enhanced mass output from Io were detected are not listed because of the following reasons. The brightening in the sodium nebula was observed relatively frequently (about 7 observed instances reported since 1990) and the abundances and pathways of the trace species sodium might not be representative for the bulk mass abundance and transfer in the system (Section 2.5). Thus Na changes might not always coincide with changes in the bulk torus and reconfigurations of the magnetosphere. Jupiter’s aurora is shaped and affected by various magnetospheric and external processes and we therefore consider it not a reliable diagnostic for changes triggered by Io. The caveats with sodium and aurora observations as diagnostics are discussed in more in the following Section 3.3.

3.3.1 Time scales of transient events

Out of the five listed events, two relate to measurements of spacecraft visiting Jupiter: Voyager 1 and 2 (1979) as well as Cassini on the inbound and outbound leg (2000/2001). In these cases the timeline of the variations in the torus is difficult to determine. The flybys of Voyager 1 and 2 happened ~4 months apart and the change (increase from Voyager 1 to 2) in the torus emissions as inferred by Delamere and Bagenal (2003) thus must have happened in between the flybys. The Cassini torus UV observations revealed a decrease in emissions from the start of the observations over a period of about 50 days. However, the timelines of the increase and the high emission phase were not observed and could only be projected in simulations (Figure 18). In both spacecraft cases modeling suggests a change in the net supply by approximately factor 3 (Delamere and Bagenal, 2003; Delamere et al., 2004). The event observed by Brown and Bouchez (1997) suggest a period of about 25 days of increasing torus sulfur ion emissions followed by a declining phase of roughly 50 days (Figure 2, left). The simultaneously monitored sodium cloud (banana) emissions seem to increase much more rapidly within <10 days. Due to the relatively large statistical spread of the observed brightnesses and gaps in temporal coverage, these inferred times have some uncertainty. For the 2015 event, the plasma torus, neutral oxygen cloud, and sodium nebula emissions were monitored at higher cadence. In this case, both the neutral oxygen cloud emissions and the sodium nebula (up to 50 RJ) followed a similar timeline with an increase phase (including possibly a high stable phase) of around 50 days, as well as a declining phase of ~40 days. Hence, the total transient event in the neutrals lasted for about 3 months. For the singly charged torus ions (S+), the onset is close to the onset for the neutrals due to the short lifetime in the neutral clouds and the declining phase is somewhat longer. The cadence of production of higher charged ions can be seen in the lag of their emissions.

The length of the declining phase of the transient events in torus and neutral gas is consistent with a period of around 1-2 months, somewhat longer than but similar to the timescale for the outward radial transport (Section 2.6). The length of the increase period is usually associated with the length of a putative change in supply from Io, but might also relate to the timescales of the atmosphere (lifetime of ~10 days, Section 2.2) or of a transient mechanism that increases the loss from Io until a new limit and equilibrium are reached.

The 2007 observations show a relatively short transient enhancement of the sodium nebula for only 10 days. The observed aurora changes are first seen during this 10 day period and continued thereafter for at least a few days (Bonfond et al., 2012). Given the uncertainties in the relation of the sodium and aurora features to the bulk neutral gases and plasma torus, it is not worth estimating or interpreting time scales for this event.

3.3.2 Inferred changes in the neutral source rate for the torus

Through modeling of the mass and energy flows in the torus, effective supply rates as well as transient changes in these rates were inferred for the three events where UV torus emission enhancements were monitored (Table 2 in Section 2.6). According to the modeling, the total mass supply rate under normal conditions is mostly around 0.7 tons/s, so somewhat lower than the canonical number of 1 tons/s. During the transient events an increase of factor 3-4 is derived for the three cases with highest rates around ~3 tons/s.

The enhancement in neutral oxygen emissions around Io’s orbit observed by Hisaki for the 2015 event is a key evidence that changes in the torus are preceded by a change in the bulk neutral clouds, at least in the one case for which monitoring of neutral oxygen emissions exists (Section 2.5). This supports the hypothesis that a change of the supply of neutrals from Io to the neutral clouds is preceding and possibly causing the transient changes in the plasma torus and magnetosphere.

We note again that these inferred changes relate to the supply rate of material to the bulk (sulfur and oxygen ion) plasma torus and the oxygen neutral cloud for the 2015 event. Thus, the total atmospheric loss rate must not change by the same factor. The atmospheric loss to space includes, e.g., losses to the inner magnetosphere and material that leaves the Jovian system.

However, for triggering a change in neutral cloud and torus supply rate of factor 3-4, a substantial change at Io would in any case be required. In the next section, we discuss what such a change might imply for the loss processes from Io, the lack of evidence for aperiodic changes in the atmosphere, and summarize caveats about often made assumptions on the narrative of transient changes triggered by Io.