Io’s Mass Supply to Jupiter’s Magnetosphere: What We Know and What We Don’t | HackerNoon

Altogether, there is a qualitatively consistent understanding of how material from Io feeds into and is distributed over the Jovian system for stable conditions, However, we find that it is currently not understood how the mass loss from Io to supply the torus can change significantly and explain observed changes of the plasma torus and neutral clouds . The hypothesis of a significant transient increase of mass loss from Io is in fact difficult to reconcile with the current understanding of the atmosphere and escape from it. In Section 3.1 and 3.2, we summarize our understanding of the stable conditions, based on the current knowledge on the different parts presented in Section 2. In 3.3 we present an overview on transient events that were reported for the magnetosphere and that are commonly interpreted to be triggered by Io. After that we present caveats about the connections in the system in Section 3.4.

There is a general consensus on how the bulk mass is transferred in the Io-Jupiter system under normal conditions. “Normal conditions” refers here to a stable torus as observed by near constant emissions and in-situ measurements over several weeks to months (a Jupiter observing season is ~6-8 months per year for Earth bound observations) and that otherwise no unusual conditions are observed in the magnetosphere, like an increase in neutral clouds or nebulae.

Material is ejected from volcanic sites from the subsurface, delivering volatiles to the atmosphere and surface. Sublimation of surface frost deposits (50% – 80% atmosphere source) and the direct outgassing at volcanic sites (20% – 50% atmosphere source) sustain Io’s atmosphere. The atmosphere reveals strong lateral and diurnal density variations, but appears to have a stable averaged SO2 abundance on the dayside (Section 2.2). Despite potentially varying volcanic outgassing, the atmospheric stability is likely maintained by the effects of the sublimated fraction (which maintains vapor pressure equilibrium) and possible mutual effects between outgassed and sublimated gases. The bulk SO2 atmosphere is then eroded primarily from the interaction with the surrounding plasma. This creates new torus ions locally at Io (roughly 200-300 kg/s) and ejects atomic and molecular neutrals into the neutral clouds in and near Io’s orbit (ionized later in the torus leading to supply of fresh torus ions) and into the extended neutral nebulae (never added to plasma torus), see Section 2.4. All other processes that allow volatiles to escape Io and be added to the neutral clouds or plasma torus are at least an order of magnitude lower and are thus expected to be only secondary contributions to the supply of new ions into the torus (Section 2.3, Table 1).

Electron-impact ionization of the bulk neutral cloud gases constitutes the main production of plasma sourced into the plasma torus (Section 2.5). Finally, there is a net radial outward transport of plasma (on a time scale of 10-60 days, Section 2.6 and Table 2), which feeds the Io-genic material into the outer torus and then the plasma sheet, which extends far out into the magnetosphere. At the radial distance where essential momentum input is required to maintain corotation of the plasma, field-aligned currents lead to energy transport processes along the magnetic field lines causing the main emission in Jupiter’s aurora (Section 2.7).

The potential positive feedback on the mass supply from Io that would be expected because the loss depends on the torus plasma density (via collisions of plasma with the atmosphere and neutral clouds) is likely mitigated by one or several limiting mechanisms. The outward transport was shown to be faster during times of enhanced torus density which suggests that a loss-limited mechanism is effective (Section 2.6). The diversion of the incoming plasma due to the plasma-atmosphere interaction could potentially work as an additional balancing factor by limiting the supply, although simulations suggest only minor effects (Section 2.4).

Although it is still not fully understood which processes drive the mass transport through the magnetosphere, there is a relatively consistent picture of the mass fluxes, pathways and time scales of mass transfer in the Io-Jupiter system for the stable conditions. The limiting or stabilizing mechanism(s) maintain(s) the stability of the torus density and should make it rather insensitive to at least minor changes at Io.

The mass rate of ~1 tons/s was first derived by Broadfoot et al. 1979 based on the assumption that the power radiated away in the extreme-UV (EUV) and far-UV (FUV) is balanced by energy input from the pickup of freshly produced ion, which are entrained in the local bulk plasma flow and into a cyclo-motion at the local flow velocity. Hence, it is the rate of neutrals (kg/s or particles/s) removed by the interaction with the Io plasma torus (primarily electron-impact ionization and charge exchange) from the neutral clouds and Io’s corona. In modeling papers it is called the torus neutral source rate or the neutral source strength (e.g., Delamere et al. 2004).

The electron-impact ionizations of the neutrals supply additional new plasma to the torus without plasma losses in the same processes. This is thus net mass-loading of the torus. Charge exchange results in a new slow ion and converts an “old” torus ion into a fast neutral that leaves the system. Both ionizations and charge exchanges contribute to the supply of energy to power the torus UV emissions and sustain the torus ion and electron temperatures. (Hot electrons also have a significant contribution to the energy input to the torus, see Section 2.6.)

In equilibrium, the neutrals removed from the neutral clouds have to be resupplied from Io. To contribute to the neutral clouds, neutrals from Io must reach a sufficient velocity to overcome Io’s gravitation and at least reach the Hill sphere, where particles could continue on orbits bound to Jupiter. At the surface this velocity is 2.33 km/s. The escape to infinity, the escape velocity is 2.56 km/s. Neutrals ejected at lower speeds supply a corona that remains bound to Io. Neutrals ejected at speeds faster than the Jovian escape velocity at Io’s orbit (25 km/s in Jupiter’s reference frame, while Io’s orbital velocity is ~ 17 km/s) escape the Io system on hyperbolic trajectories and do not provide neutrals to the neutral clouds or plasma torus. Instead, they contribute to the formation of the nebulae. In addition, some of the material likely migrates radially inwards (also forming the cold torus).

Thus, the canonical rate (or neutral source rate) to the plasma torus does not equal the mass loss from Io but instead represents a lower limit for Io’s total neutral loss and the energy needed to support the UV power radiated by the plasma torus.