Satellites and interactions P. A. Delamere University of Colorado The interaction of a satellite and its plasma environment results in a cascade of physical processes that couple the satellite to the distant planetary atmosphere. A key aspect of this topic are the complex electrodynamic processes that result from the interaction of inflowing plasma with neutral gas distributed near the satellite. The neutral gas can be in the form of, for example, a gravitationally bound atmosphere (e.g. Io) or an escaping plume (e.g. Enceladus). A directly-observable consequence of the plasma/neutral interaction are auroral emissions both at the satellite and/or at the magnetic footprint of the satellite in the planetary atmosphere. In this review, we will focus on aspects of the local electrodynamic interaction (i.e. within several satellite radii) that ultimately generate the large-scale current systems that couple the satellite to the planetary atmosphere as well as implications for the circumplanetary distribution of neutral gas.

Types of interactions Magnetized (internally generated magnetic field) Mini-magnetosphere (Ganymede) Non-magnetized Inert, no induced magnetic field and no neutral source (e.g. moon). Induced magnetic field due to time-varying external magnetic field (e.g. Europa and Io) Magnetic perturbation due to interaction with neutral source (.e.g. Io, Enceladus) Conductivity due to ionization, charge exchange, collisions Sub-Alfvenic flow (except Titan when outside of Saturn’s magnetosphere)

Ganymede: A Magnetosphere within a Magnetosphere Torrence Johnson

Ganymede HST observations of oxygen emissions - McGrath Jia et al. Figure 19: (Top) HST /STIS images of Ganymede’s aurora due to electron impact excitation of oxygen at OI 1356 A (M. McGrath, private communication). Contours illustrate variations in brightness. (a) The leading (downstream) hemisphere taken on 23 Dec. 2000. (b) Jupiter-facing hemisphere taken on 30 Nov. 2003. (c) Trailing (upstream) hemisphere taken on 30 Oct. 1998. (Bottom) Numerical model of the magnetosphere of Ganymede, with the satellite and the location of the auroral emissions superimposed (based on Jia et al., 2008). (d) The view looking at the anti-Jupiter side of Ganymede. (e) The view looking in the direction of the plasma flow at the upstream side (orbital trailing side) of Ganymede, with Jupiter to the left. The shaded areas show the regions of currents parallel to the magnetic field.Computer simulations are helpful in visualizing the interaction process (Paty et al. 2006, 2008; Jia et al. 2008, 2009, 2010) but lack of information about the conductivities of Ganymede’s tenuous patchy atmosphere and icy surface limit our understanding of the circuit of electrical currents that couple the magnetosphere to the moon. It is clear that electrical currents reach Jupiter, however, because of the strong auroral emissions (Figure 14) at the Ganymede footprint (Clarke et al. 2002; Grodent et al. 2009). Short-term (few seconds) variability of aurora at Jupiter associated with the magnetic footprint of Ganymede (Grodent et al. 2009) is perhaps associated with bursty reconnection on the upstream side of Ganymede’s magnetosphere (Jia et al. (2010). The local interaction also bombards electrons into Ganymede’s atmosphere, exciting auroral emissions (reviewed by McGrath et al. 2004) as shown in Figure 19c. The locations of the aurora on Ganymede are consistent with the boundaries between regions where the magnetic flux tubes connect at both ends to Ganymede and regions where the fluxtubes connect to Ganymede on one end and Jupiter at the other. Jia et al. Paty et al.

Types of interactions Magnetized (internally generated magnetic field) Mini-magnetosphere (Ganymede) Non-magnetized Inert, no induced magnetic field and no neutral source (e.g. moon). Induced magnetic field due to time-varying external magnetic field (e.g. Europa and Io) Magnetic perturbation due to interaction with neutral source (.e.g. Io, Enceladus) Conductivity due to ionization, charge exchange, collisions Sub-Alfvenic flow (except Titan when outside of Saturn’s magnetosphere)

Induced magnetic fields (Europa and Io) Khurana et al., 2011 Schilling et al., 2008 Ion number density in cm􏰀3 in the wake for the E4 flyby conditions (a) when induction is neglected and (b) with induction. Shown is the yz plane at x = 2.75 RE. The trajectory of the Galileo spacecraft is indicated by the solid black line.

Types of interactions Magnetized (internally generated magnetic field) Mini-magnetosphere (Ganymede) Non-magnetized Inert, no induced magnetic field and no neutral source (e.g. moon). Induced magnetic field due to time-varying external magnetic field (e.g. Europa and Io) Magnetic perturbation due to interaction with neutral source (.e.g. Io, Enceladus) Conductivity due to ionization, charge exchange, collisions Sub-Alfvenic flow (except Titan when outside of Saturn’s magnetosphere)

Who’s the culprit for this internally driven magnetosphere. Io Who’s the culprit for this internally driven magnetosphere? Io. This dramatic image of Io was taken by the Long Range Reconnaissance Imager (LORRI) on New Horizons at 11:04 Universal Time on February 28, 2007, just about 5 hours after the spacecraft's closest approach to Jupiter.

Io’s SO2 Atmosphere Lyman-a Images  NSO2 ~ 1016 cm -2 1998 1999 Turns out Io has an SO2 atmosphere Because SO2 gas absorbs strongly at 1215A, Lyman-a images provide a map of the SO2 atmosphere on Io. Dark=more SO2 gas. Note the pronounced variability in the inferred abundance and distribution of the gas between 1998 & 1999. It is unclear at present whether this variability is temporal or longitudinal. 1999

Neutral Sources Spencer et al., 2007

Enceladus Dougherty et al., 2006

Interaction processes Electron impact dissocation of SO2 is the fastest reaction [Smyth and Marconi, 1998] Pickup=source of energy (at 60 km/s) Tpu(O+)=270 eV Tpu(S+)=540 eV Tpu(SO2+)= 1080 eV

Momentum loading (pickup) Ionization Charge exchange New ions stationary in satellite rest frame “Picked-up" by local plasma flow Ionization adds mass Charge exchange does not add mass (usually) Both transfer momentum from ambient plasma to new

Ionization and Charge Exchange .CharCharge Charge exchange amplified by “seed” ionization. Results in an avalanche of reactions [Fleshman et al., 2011] Ionization limited by electron temperature [Saur et al., 1999; Dols et al., 2008]

Electrodynamic consequences Momentum loading generates currents Ionization + charge exchange In limit where v = v_cor

Momentum transfer Magnetic field perturbation due to “pick up” (e.g. ionization and charge exchange) Alfven characteristic determined plasma mass density and magnetic field. vA vcor

Momentum transfer Alfven wing magnetic field topology results in forces on charged particles via Maxwell stresses. Acceleration of iogenic plasma at the expense of torus plasma. Ultimately, Jupiter’s atmospheric is the source of momentum and energy.

Estimate of momentum loading Maxwell stress Plasma mass coupling rate Momentum balance requires (ignoring upstream input, chemical processes) kg/s

Momentum transfer

Galileo Io Flyby - 1995 Flow Magnetic field Y (RIo) Bagenal, [1997] Electron Beams Galileo Fresh hot ions Flow Magnetic field Bagenal, [1997] Flyby geometry Flow Magnetic field Electron beams Density? Flux NeVx (1010 cm-2 s-1) Is the local interaction ionosphere-like (elastic collision dominated), or comet-like (mass loading dominated)? Bagenal, [1997]: 200-500 (kg/s) Saur et al., [2003]: 50-200 (kg/s) Y (RIo)

Mass transfer rate (Alfven wing) Escaping Fast Neutrals (? kg/s) New plasma (50-500 kg/s)

Io’s (partial) neutral torus M. Burger

Energetics

Precipitating electrons (100-1000 eV): ~1010-1011 W Talk by Sebastien Hess IR+UV auroral spots: 109-1010 W Power (Alfven wave) per hemisphere: 5x1011 W Io-DAM: 109-1010 W Io Interaction: ~1012 W How well can the wave power be transfered from Io to Jupiter? How well can the power of the Alfven waves be transfered to the electrons? Can we explain the radiated power?

Pickup Energetics Pickup involves two parts: Acceleration to corotation speed Heating at local flow speed (on time scale of gyromotion) Much of the ≈1 TW of power is necessary for the pickup of roughly 200 kg/s into corotational flow.

Outstanding issues How does the thermal electron temperature and hot electron beams affect the interaction? Enceladus, Te = 2 eV (little interaction) Io, Te = 5 eV (strong interaction) What are the important processes that shape the extended coronae/neutral clouds? Electron impact dissociation vs. charge exchange What is the feedback between the neutral source and ambient plasma conditions (i.e. plasma torus)? Enceladus’ variable plume source Io’s volcanic activity Under what circumstances are energetic particles (keV-MeV) important (Europa, Ganymede, Callisto)? These systems are complicated. The basic concepts are each fairly straightforward. But they interact in non-linear ways which makes a complex whole. This means we usually need to break down the problem and understand each component first. Then putting it all together. Ultimately, keys to understanding comefrom exploring sensitivity of the interactions to varying (i) the moon's conductivity, (ii) the moon'satmosphere, (iii) the ambient plasma - and, for numerical models, (iv) the boundary conditions.