Volcanic Activity, Atmospheric Loss, and Plasma Interactions in Jupiter’s Magnetosphere

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

Appendix

List of terms relevant to the mass supply from Io

A) Surface and Volcanism

Active volcano

On Earth an active volcano is a structure that is either erupting or is likely to erupt in the future. A terrestrial active volcano which is not currently erupting, is known as a dormant volcano (while extinct volcanoes are not expected to become active again). On Io, every volcano is probably either active or dormant.

We note also that from the perspective of hot spot detections or other measurements of volcanic activity at Io, it has not been possible to identify different states or levels of the global volcanic activity. This means there are no indications of general “volcanically active” or “volcanic quiet” periods (yet).

Hot spot

This term has different definitions depending on context. It is best to specify mantle hot spot, volcanic hot spot, or IR hot spot. A mantle hot spot may manifest itself as seismic anomalies, elevated topography, gravity anomaly, concentration of volcanoes, etc. A volcanic hot spot is any evidence for current or recent volcanic activity, such as eruptions observed and recorded by humans, gas venting, or volcanic deposits with very young radiometric dates. Io astronomers call a remotely-sensed IR emission enhancement that is clearly above the expected background emission a hot spot, an observational definition, maybe best called an IR hot spot.

Volcanic eruption

Generally, a volcanic eruption is an event when lava and/or gases are expelled from a volcanic site through a vent or fissure. At Io, the term eruption is primarily used for detection of a hot spot, i.e., thermal emission from hot lava. This is the only type of observations taken with sufficient cadence and coverage to actually infer temporal activity changes and thus observe and define an eruption event. The cadence of detections of (large) dust (or gas) plumes determined by the availability and timelines for plume activities are hardly constrained yet.

Plume (gas vs dust)

A plume consists of gas and particulates rising from a volcanic source often creating a large umbrella-shaped structure capped by a gasdynamic shockwave. Plume constituents may move differently and form umbrellas or jets within umbrellas, for example with large grains forming a more compact shape within a much larger gas canopy. Ionian plumes tend to be dense enough to be collisional (intermolecular collisions matter to the dynamics and molecules/grains do not simply move ballistically).

Stealth plume

A predominantly gaseous plume that has an insufficient dust component to make it detectable in visible light. The most and most well known images of Io’s plumes show them through Mie scattering of solar light in the visible, often at high phase angles (forward scattering). If there is very little dust in plumes and only gas, they are not seen in visible images.

Plume type (Pele vs Prometheus)

The size of a plume depends on the energy given to the rising gas/particle flow at the source. Giant Pele-class plumes appear to be sourced directly from hot lavas, perhaps a bubbling lava lake, and rise 200-400 km and have a high gas-to-dust mass ratio. Smaller (50-70 km high) Prometheus-class plumes contain more dust and appear to arise from a region where lava is encroaching upon pre-existing sulfur dioxide ice.

B) Mass supply from Io to the magnetosphere

Source for the bound atmosphere

The most recent observational studies suggest that both sublimation of surface SO2 frost and direct outgassing at volcanic sites sustain Io’s bulk dayside atmosphere. There are mutual effects between the two sources and atmospheric parts. Sublimation equilibrium is maintained above frost patches on the dayside but most of the SO2 should condense at night. A source rate can not be defined in this dynamic atmosphere.

Atmospheric mass loss

Neutral gas lost from Io’s gravitationally bound atmosphere and corona to space (not to the surface). Material is lost as neutrals through acceleration (above the exobase) to velocities higher than the escape speed (through various processes including collisions with plasma, heating of the neutrals or recombination), or as ions through local ionization and pick-up by the magnetic field. The fraction lost as neutrals at velocities below Jupiter’s escape velocity at Io’s distance (25 km/s in the reference frame of Jupiter) feeds into the neutral clouds. Neutrals with effective velocity exceeding the Jupiter escape speed populate the extended nebulae and leave the system. Locally ionized material is supplied to the plasma torus directly.

Mass loss from Io

The total mass loss from Io is the atmospheric mass loss plus direct mass loss (thermal or sputtered) from the solid surface or direct escape from volcanic plumes. Both loss from surface and volcanic plume escape are likely much smaller than the atmospheric mass loss and thus the mass loss from Io is nearly identical to the atmospheric mass loss. Loss from the surface is likely small because the shielding atmosphere is even sustained in absence of sunlight by volcanoes as recently shown. The velocity of ejected plume gas (or dust particulates) is well below Io’s escape (less than half for the largest plumes) velocity and the plumes interact with the sublimated atmosphere.

The mass loss from Io is larger than the neutral source rate for the torus (assuming Io is the only viable source). This is because some processes eject neutrals at a velocity larger than Jupiter’s escape velocity at the orbit of Io (25 km/s), which are then lost to the Io/Jupiter system and do not contribute to the supply.

Canonical number / Neutral source rate

Production of fresh torus plasma (ions) through ionization of neutrals (kg/s or particles/s) from the neutral clouds or Io’s neutral atmosphere. Ionization and ion-neutral collisions supply slow ions (and electrons) to the torus, which are then accelerated to corotate contributing to the supply of energy powering the torus UV emissions (the other significant energy contribution is hot electrons ~40-400 eV). To sustain the balance, the supply of new slow ions equals the rate at which neutrals must be resupplied, and therefore it is often called the neutral source rate in Neutral Cloud Theory modeling. The rate of this generation of fresh ions (and corresponding destruction of (atomic) neutral) was estimated based on the emitted energy at ~ 1 ton/s or (1-3)x1028 particles/s of S and O neutrals. This value is the canonical number of mass transfer frequently cited in the literature, yet often inaccurately for related but not identical processes like the mass loss from Io’s atmosphere (which is larger) or the net torus mass-loading rate (which is lower and only from ionization and not ion-neutral collisions).

Local torus ion supply at Io

The number of ions coming directly from Io has been estimated from the J0 pass of Galileo through the Io wake (Bagenal et al., 1997) at 18-58% of the canonical 1 tons/s. This ion loss rate was further refined to ~300 kg/s (Saur et al., 2003; Dols et al., 2008), which amounts to roughly 20% of the rate of neutrals being supplied to the neutral clouds from Io (see canonical number). As the magnetic field is slowed down in the vicinity of Io, the energization from pick-up of the ions generated in this region is significantly lower and likely negligible compared to the ionization in the neutral clouds at full corotation.

Torus mass-loading

Net production of plasma (ions) in the torus due to ionization of the extended neutral clouds and Io’s atmosphere. This is smaller than the neutral source rate, because charge exchange and momentum transfer collisions do not change the net number of ions in the plasma torus. Instead, a co-rotating ion is lost in the same process as a new slow ion is generated as well as a fast neutral, and the latter is lost from the system. This net source of torus plasma is balanced primarily by losses from effective radially outward transport.

Mass loss from the Jupiter system

Combined loss of ions and neutrals from Jupiter’s magnetosphere and gravity field. Is expected to be similar to the mass loss from Io, since no other substantial loss pathways are known.

Momentum transfer

Newly generated torus plasma has little to zero momentum. The plasma is then accelerated by the corotating local magnetic field effectively transferring momentum to the plasma flow from the angular momentum of Jupiter’s ionosphere, which is collisionally coupled to the planet’s upper atmosphere.

C) Atmospheric and plasma-atmosphere processes

Atmospheric sputtering

A process in which high-energy particles, either ions or atoms, collide with the atmosphere neutrals, causing the ejection or removal of atoms or molecules from the atmosphere. The process itself is similar to the surface (knock-out) sputtering, but the cascade/recoil processes are localized in few scale-heights instead of few nm. The yield (Y) for this process is defined as the average number of target neutrals released per incident projectile. For each sputtered particle, a cascade of multiple collisions is usually necessary. Atmospheric loss by sputtering has been assumed in a series of publications to model the formation of the Na extended cloud (Banana cloud) and O and S extended clouds. Surface sputtering rate at Io is negligible on the dayside hemisphere but may contribute to some neutral losses at night or in eclipse.

Ion/neutral elastic collisions

Collisions that conserve both the momentum and kinetic energy of the incoming particles.

Charge exchange

Process that occurs when a molecule or atom (neutral or not) collides with a charged ion and one or more electrons are transferred from one to the other. In the case for Io, the most common reactions happen between atoms of O and S and molecules of SO2 , charged or not. The charge exchange rates/cross sections depend on the relative velocity of the colliding particles, which is ~60 km/s for the corotation plasma at Io's radial distance. If there is a (slow gravitational bound) neutral atom is produced, it will have roughly the corotation speed of th ion, which in this case is enough to escape the Jupiter system. This is the main mechanism of generation of ENAs. Charge exchange processes are considered to dominate the production of oxygen and sulfur ions inside Io's orbit, although it does not change significantly the net ionization level of the plasma, and are, along with electron-impact ionization, the main contributor to the torus mass-loading.

Joule heating

j.E with j the electric current density and E the electric field. For the choice of the rest-frame of the electric field see Vasyliūnas and Song, 2005.

Pick-Up process

Process of entrainment of freshly ionized neutrals initially at rest in Io’s reference frame (in Io’s atmosphere or in Io’s extended neutral clouds) in the torus plasma flow. Fresh ions resulting from electron-impact ionization, photo-ionization or charge exchange in Io’s atmosphere (or in neutral clouds) experience an ExB drift in the frame of Io. They are entrained in the local bulk flow of the plasma (corresponding to the velocity of their gyrocenter) and also start a gyromotion at the local flow velocity. In the frame of Io, the trajectories of pick-up ions (and electrons) are cycloids perpendicular to the local magnetic field. An O or S ion picked-up in the neutral cloud at the corotation velocity ~ 60 km/s results in a supply of energy to the torus of 270 eV and 540 eV respectively, larger than the average ion energy of the torus ~100 eV. Ions picked-up in the atmosphere of Io result either in a local heating or cooling of the plasma depending on the velocity of the local plasma flow around Io where the pickup process takes place (slower than corotation close to Io, and accelerated on the flanks of Io)

Io-genic material

Neutral gas, plasma or dust in the Jovian system that ultimately originates from the interior or surface of Io. The majority of all sulfur (S) and sodium (Na) material is likely from Io, while there are possible other viable sources for H (Jupiter) and O (icy moons).

D) Specific regions and components in the systems

Exobase

The general definition is given in Section 2.3. As Io’s atmospheric density and temperature vertical distributions are still unknown, the exobase altitude is still undetermined. A wide range of estimates of the exobase altitudes have been given in the literature, ranging from several thousand kilometers to a few tens of kilometers on the dayside atmosphere, down to Io’s surface in eclipse. Our ISSI group consensus places the exobase at a few hundred kilometers, based on numerical simulations of the plasma properties along the Galileo flybys of Io, atmospheric modeling that includes the plasma/atmosphere interaction, and OI (630 nm) emissions, which can be collisionally quenched within the 110s radiative lifetime yet glow closely to Io’s limb (Geissler et al., 1999; 2004), precluding higher exobase altitudes.

Io corona (exosphere)

Neutral gas bound to Io beyond the exobase of Io. In this region, the gases are non-collisional but still bound gravitationally to Io. It extends in the Hill sphere (~5.8 RIo with RIo ~ 1821 km) and smoothly merges with the neutral clouds. Atomic O and S neutral coronae have been detected by HST in the UV, extending radially to ~10 RIo in all directions (Wolven et al., 2001). Energetic ion absorption features detected by Galileo far from Io are also consistent with an extended corona but the neutral species cannot be determined (Huybrighs et al., 2024). Electro-Magnetic Ion Cyclotron waves (EMIC) at the SO+ and SO2 + gyro frequencies have been observed by Galileo extending as far as ~7-20 Rio from Io in the downstream direction (Russell et al., 2003), which suggests a pickup process in extended SO2 and SO coronae.

Neutral cloud(s)

Structures of neutral gas extending along Io’s orbit, subjected to the gravitational field of Jupiter. These neutral clouds are fed by the plasma-atmosphere interaction at Io and are shaped by magnetospheric loss processes. Depending mostly on the ionization energy of the neutrals, these clouds have a limited extension along Io’s orbit (Na banana cloud, S cloud etc.). The use of plural in clouds originates from the different species and different shapes of the neutral structures (Figure 15, Section 2.5). They can also encompass the whole Io orbit as for atomic O (Koga et al., 2018a; Smith et al., 2022; Section 2.5) and can in that sense also be named neutral torus or tori. However, in stark contrast to the azimuthally rather homogeneous plasma torus, the neutral density is significantly larger close to Io for all species and we propose neutral clouds as the general term. The S and O neutral clouds are (likely) the main source of plasma for the torus (~80%).

Banana, jet, and stream

The banana, jet, and stream terminology in extended neutral cloud structures derives from sodium studies (e.g., Smyth and Combi, 1988b; Wilson et al., 2002), but has been broadly applied to other species in existing literature (see Section 2.5 for more details).

Neutral nebulae / sodium nebula

Neutral gas abundance that extends more than 100 (1000?) RJ around Jupiter and is not subject to the gravitational field of Jupiter. The only nebula observed so far is the Na nebula (also sometimes called Mendillo-disc after Mendillo et al., 1990) fed by neutral sodium ejection from Io’s atmosphere or Io’s torus at velocity larger than the escape velocity at the orbit of Io (25 km/s in the Jovian reference frame). The existence of nebulae in the major species O and S (and maybe SO and SO2) is suspected to exist but has not yet been detected. In the analysis of images, the emissions are analyzed in differently extended regions, like up to a radial distance of 25 RJ (with much contribution from the neutral cloud in the signal), or up to 100 RJ (with relatively more contributions from the nebulae).

Io plasma torus

The Io plasma torus is a structure that encompasses the orbit of Io and is mainly composed of S and O multiply-charged ions. It comprises three main regions: (1) the warm torus, (2) the ribbon, and (3) the cold torus.

Transient torus event / brightening / enhancement

Identified by an intensification of UV or optical ion emissions from the plasma torus for a limited period of usually 1-3 months w.r.t. a common stable background level. Such intensification is commonly explained by an enhanced mass loading of the torus, which leads to a higher density and possibly temperature in the torus causing the brighter emissions.

Plasma sheet

Structure of plasma in the magnetosphere, radially beyond the warm torus (beyond ~7-10 R_J) and out to roughly 30 RJ where the density drops below 1/cm3 . It is sourced from the effective radially outward transport of torus Io-genic material. The plasma in the plasma sheet is mostly corotating but corotation breaks down between 20-30 RJ. Ion and electron temperatures increase with radial distance, thus are higher in the plasma sheet than in the warm torus.

Io local aurora

Electron or ion impact excited emission from Io’s atmosphere. This choice of nomenclature is non-unique as no universal definition of aurora exists and many authors require in their definition active electron acceleration as part of the auroral processes. The latter is not the case at Io, i.e., electrons are not actively accelerated near Io in contrast to Ganymede.

Auroral footprint

Electron or ion impact excited emission from Jupiter’s atmosphere triggered by Io. Charged particle acceleration is powered by Io’s electrodynamic interaction with Jupiter’s magnetosphere. Particles are accelerated along the Alfvén wings connecting Io with Jupiter. Among the three auroral moon footprints that are clearly recognizable, Io’s one is considerably brighter, and always visible both in UV and IR. Especially in IR, the Io’s auroral footprint shows some structure, like secondary spots or a tail, at preceding and posterior longitudes (Section 2.7). Some of these structures are caused by the reflections of the Alfvén waves on speed gradients, and they depend on the latitudinal location of the moon within the plasma sheet, while part of the fine morphology of the footprint still seeks for a clear explanation.

Acknowledgements

This review is based on discussions within International Team project #515 (“Mass Loss from Io's Unique Atmosphere: Do Volcanoes Really Control Jupiter's Magnetosphere?”) funded by the International Space Science Institute (ISSI) in Bern. LR is supported by the Swedish National Space Agency through grant 2021-00153. Further travel support from the Swedish Foundation for International Cooperation in Research and Higher Education (STINT) is acknowledged. We thank Márton Galbács very much for preparing the sketch in Figure 10. HH is supported by a DIAS Research Fellowship in Astrophysics. HH gratefully acknowledges financial support from Khalifa University’s Space and Planetary Science Center (Abu Dhabi, UAE) under Grant no. KU-SPSC-8474000336. CS acknowledges the support of NASA through programs 80NSSC21K1138 and 80NSSC22K0954, and the NSF through program AST-2108416. KdK acknowledges funding from the National Science Foundation grant 2238344 through the Faculty Early Career Development Program. SVB was supported by STFC Fellowship and grant ST/M005534/1 and ST/V000748/1. CT, RK, MK, and FT are supported by JSPS KAKENHI under Grant Number 20KK0074. AB is supported by the Volkswagen Foundation Grant Az 97742.

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