How Jupiter’s Plasma Torus Strips Away Io’s Atmosphere | HackerNoon

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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

o is embedded in the Io plasma torus and orbits Jupiter (Figure 10) with a period of 42 hours and 28 minutes. The plasma in the Io torus is magnetically coupled to Jupiter and thus rotates with the same angular velocity as Jupiter’s ionosphere corresponding to a period of 9 hours and 55 minutes. The plasma is therefore rotating faster than Io and overtakes the moon with a relative velocity of 57 km/s. The fast-moving plasma interacts with Io’s atmosphere and surface, which causes a large variety of plasma and atmospheric effects that contribute to mass loss from Io. Reviews on this plasma interaction are presented in, e.g., Kivelson et al. (2004), Bagenal and Dols (2020, 2023), Saur et al. (2004, 2021). A pre-Galileo analysis on losses due to various ion collisions is presented in Sieveka and Johnson (1984).

Various types of collisions of the torus plasma ions and electrons with Io’s atmosphere lead to an exchange of matter, momentum and energy between the ionized and neutral gases as depicted in Figure 10. These collisions slow down the plasma in Io’s ionosphere and its vicinity. The modified and slowed plasma around Io generates plasma waves traveling away from Io, a slowed wake behind Io, and draped magnetic field lines around Io. The most important wave mode excited by the interaction is the Alfvén mode which travels along Jupiter’s field lines towards Jupiter in the northern and southern direction (see pink structures in Figure 12, left).

2.4.1 Plasma-neutral collisions as primary loss process

The aforementioned collisions are the engine of Io’s plasma interaction. These collisions are also likely the primary reason for the loss of Io’s atmosphere into the torus and will be reviewed in this subsection. The loss of SO2 from Io’s atmosphere occurs in various collisional pathways that also include dissociation and ionization into sulfur and oxygen neutrals and ions (e.g.,Thomas et al., 2004; Nerney and Bagenal, 2020).

Other important processes are electron impact dissociation, electron impact ionization and photodissociation (Figure 13). Photoionization plays a smaller role at Io. Photodissociation does not affect the plasma interaction and is discussed in Section 2.4.1.

The electron impact ionization rate does not grow linearly with increasing neutral density because the total amount of electron energy available for ionization is limited by the amount of electron energy available in the torus electrons upstream of Io. Although the simulations in Saur et al. (2003) include potential negative feedbacks caused by an increased atmospheric column (increased diversion of the incoming plasma flow and electron cooling), these results show that the total elastic collision rate scales approximately linearly with increasing neutral density. Neutrals which are ionized by electron impact turn into plasma and are subsequently accelerated by the local electromagnetic forces. These accelerated ions and electrons are subsequently advected out of Io’s atmosphere into the plasma torus.

The electron ionization process is energetically limited and is significantly smaller than the elastic collision and photodissociation rates. It is estimated, based on the Galileo flyby in Io’s wake, at ~ 300 kg/s (Saur et al., 2003; Dols et al., 2008, Bagenal 1997). Although this local mass loading directly populates the torus, this rate is significantly smaller than the torus neutral supply rate of ~ 1 ton/s. Consequently, most of the mass that leaves Io’ s atmosphere is in the form of neutrals. Most of these neutrals have a velocity larger than the velocity to reach the Hill sphere (>2.33 km/s) but smaller than the escape velocity from the Jupiter system at Io’s orbit (<25 km/s), and the escaping neutrals feed the extended neutral clouds.

Another important aspect of the collisions of the magnetized torus plasma with Io’s atmosphere is, in addition to the momentum exchange, the energy exchange, i.e., the heating of the neutral atmosphere. The heating occurs in form of plasma and Joule heating (Vasyliunas and Song, 2005; Saur et al., 1999), which can significantly increase the temperature and thus the scale height of the atmosphere and ionosphere (Strobel et al., 1994) possibly leading to increased thermal escape into the torus. The heating rates are model-dependent and currently no consensus on the true thermal escape rate exists.

The role of individual volcanoes within Io’s plasma interaction has been studied by Roth et al. (2011) and Blöcker et al. (2018). The latter found, taking Tvashtar and Pele as exemplary plumes, that both modify the total production and collision rates in Io’s atmosphere by <3% due to the relatively small size of the plumes with respect to the global atmosphere. This indicates that individual volcanoes might only weakly influence the loss rate from Io’s atmosphere to the torus.

2.4.2 Remote observations of the local plasma-atmosphere interaction

Electron-impact excited emission from Io’s atmosphere is a diagnostic means to investigate the structure of Io’s atmosphere and its ion loss into the torus. This emission is often referred to as “auroral emission” (see glossary). Such remote observations provide significant information about the state of the atmosphere and plasma interaction, although they do not directly monitor the rate and variations of Io’s neutral losses.

The cross-sections for electron impact ionization of SO2, S and O have very similar energy dependencies as the cross-sections for electron impact excited UV emission from these species (e.g., Saur et al., 2003). Thus, the UV emission from Io’s atmosphere is a direct monitor of electron-impact ionization in Io’s atmosphere. Io’s auroral emission observed in the UV and at visible wavelengths shows two bright spots near the limb of Io at Io’s magnetic equator, defined as the plane perpendicular to Jupiter’s magnetic field through Io’s center (see Figure 14 left, Roesler et al., 1999; Retherford et al., 2000; Geissler et al., 2004; Roth et al., 2014; 2017). The physical reason is that the convection pattern of plasma through Io’s atmosphere and electron heat flux along Jupiter’s field lines control the transport of electron energy into Io’s atmosphere (Saur et al., 2000; Roth et al., 2014; 2017). The heat flux also explains why the northern or southern hemisphere facing the center of the torus is brighter in UV than the opposite one (Retherford et al., 2003; Roth et al., 2014). Analysis of observations taken over four years (Roth et al., 2014) showed that the variations in the UV emissions can be solely explained by changes in the plasma environment and collapse of Io’s atmosphere during eclipse. Variations caused by a change of the global atmospheric density putatively caused by sporadic volcanic eruptions were not detectable, supporting the hypothesis of a stable atmosphere (Section 2.2).

We consider the variability of the brightness of Io’s footprint in Jupiter’s aurora an indirect tool to study Io’s atmosphere and its supply to the magnetospheric environment, in particular because it relies on complex acceleration processes along the Alfvén wings (Hess et al., 2013; Szalay et al., 2018; Saur et al., 2013). Based on Juno UVS measurements, Hue et al. (2019) found that the brightness of Io’s footprint does not significantly change when Io passes through eclipse. This might imply that Io’s interaction and the power transmission is more strongly saturated than expected, i.e., a change in the atmosphere density does not change the power transmission (Blöcker et al., 2020). An alternative explanation would be that the atmosphere collapses less than derived from other observations.