Jupiter’s Glowing Secrets

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

2.7 Jupiter’s aurora and connections to the Io torus

Various auroral features have been identified and characterized by observations as diagnostics for the state of the magnetosphere. The main auroral structures—as seen moving from the mid-latitude towards the poles of Jupiter—are the footprint aurora (Io, Europa and Ganymede) and the low-latitude emission, the main auroral emission, and finally the polar emission. The main aurora is additionally subdivided in three different zones (Mauk et al., 2020). In this section, we mainly focus on auroral observation and modeling studies related to the magnetospheric mass balance and changes in the torus environment. For general details of the auroral process and dynamics, see review papers by Badman et al. (2016), Grodent (2015), or Mauk et al. (2020). The Io auroral footprint is discussed in Section 2.4, as it relates to the local interaction of Io’s atmosphere with the surrounding magnetosphere.

2.7.1. Main emission and mass balance

The rotational motion of out-flowing Io plasma in Jupiter’s magnetosphere is considered to be maintained by the transfer of angular momentum from the gas giant to the plasma itself (e.g., Hill, 1979). In the standard picture before the Juno mission, the main aurora was suggested to be related to the quasi steady-state field-aligned current system produced in such angular momentum transfer (e.g., Cowley and Bunce, 2001; Hill, 2001). Theoretical and numerical models indicate that the location of the main aurora would be shifted toward lower latitude in the case of increased plasma mass loading to the torus, because the momentum transfer is supposed to occur efficiently over a more limited radial distance of the equatorial magnetosphere (e.g., Nichols, 2011; Nichols and Cowley, 2005; Tao et al., 2010; Ray et al., 2012). Nichols (2011) found that the correlation or anti-correlation of the field-aligned current with the mass-loading rate would depend on the assumption in the model, i.e., whether the cold plasma density depends on the mass-loading rate or not.

However, recent Juno observations complicate this paradigm, and suggest that the main aurora is predominantly caused by broad-band bi-directional electron beams, which can deposit up to 3000 mW/m2 (Mauk et al., 2017; 2020; Salveter et al., 2022). These electron distributions might be generated by highly time-variable, turbulent electric currents and fields caused by the radial transport constantly perturbing the magnetosphere (Saur et al., 2018). The ionospheric Alfvén resonator is proposed to produce additional high frequency waves (Lysak et al., 2021). A simulation study considering dispersive scale Alfvén waves shows that a large ratio between the torus and high-latitude densities can act to enhance the broadband aurora (Damiano et al., 2019).

2.7.2. Aurora signatures connected to transient torus events

Particular aurora observations were proposed to be connected to events in the torus and at Io. Bonfond et al. (2012) found that the main aurora was expanded to lower latitudes - up to equatorward of the Ganymede footprint location - and that the occurrence rate of large equatorward isolated auroral features increased during a period in 2007, close to a brightening in the sodium nebula (Yoneda et al. 2009). The presence of equator isolated features suggests the increase of injection activities, replacing a large amount of outward-moving heavy flux tubes and flux tubes sparsely filled with hot plasma. The Io footprint aurora disappeared (power <1 GW) compared to observations of the footprint main spot at similar Jovian System-III longitude at Io, where the power is usually around 3-6.5 GW (Bonfond et al., 2012). For the same event, the activity of aurora-related hectometric radio emission (HOM), which is an indicator of Jupiter’s auroral acceleration, is decreased (Yoneda et al., 2013).

Bonfond et al. (2012) interpreted these aurora characteristics observed in 2007 to be caused by an increase in mass loading triggered by Io. We note, however, that a larger survey of Jupiter’s aurora by Grodent et al. (2018) found that the aurora revealed similar features in 18,5% of all observed cases in a period between November 2016 and July 2017, where both the sodium nebula and torus ion emissions were constantly at a low and stable level (Roth et al., 2020).

During the transient changes in Io torus in the winter of 2015 (Section 2.6), simultaneous monitoring of plasma torus emission and polar-integrated auroral spectra showed interesting responses indicating magnetospheric dynamics (see Figure 18, bottom panel with aurora intensity). Auroral sporadic enhancements lasting less than ~10 h are sometimes observed, followed ~7-20 h (average 11 h) later by sporadic enhancements of the ion brightness in the plasma torus (e.g., Yoshikawa et al., 2016). The auroral sporadic enhancements would represent transient energy and were linked with auroral signatures of injections between the main oval and the Io footprint (Kimura et al., 2015), which may have been driven by reconfigurations in the outer magnetosphere, as shown in Figure 20.

The changes in the torus emissions indicate an enhancement of the hot electron population in the inner magnetosphere. Pairs of these intensifications were frequently identified from ~20 days after the start of torus S+ emission increase and until the decrease in emissions to the common lower level. After that only the auroral intensification continued (Tsuchiya et al., 2018). The ~11 h time delay of a torus brightening from a corresponding aurora intensification did not change compared with the lower standard torus state (Yoshikawa et al., 2017). Auroral sporadic intensifications are much larger and more frequent during the enhanced torus emission interval (Kimura et al., 2018; Tao et al., 2021). A change in the auroral spectrum during the enhanced torus emission interval indicates a decrease in auroral electron energy and higher density magnetospheric source plasma in the middle magnetosphere (Tao et al., 2018).