Moon–Magnetosphere Interactions: a Tutorial

Advances in Space Research 33 (2004) 2061–2077 www.elsevier.com/locate/asr Moon–magnetosphere interactions: a tutorial M.G. Kivelson a,b,* a University of California, Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA 90095-1567, USA b Department of Earth and Space Sciences, UCLA, Los Angeles, CA 90095-1567, USA Received 8 May 2003; received in revised form 13 August 2003; accepted 18 August 2003 Abstract The interactions between the Galilean satellites and the plasma of the Jovian magnetosphere, acting on spatial scales from that of ion gyroradii to that of magnetohydrodynamics (MHD), change the plasma momentum, temperature, and phase space distribution functions and generate strong electrical currents. In the immediate vicinity of the moons, these currents are often highly structured, possibly because of varying ionospheric conductivity and possibly because of non-uniform pickup rates. Ion pickup changes the velocity space distribution of energetic particles, f (v), where v is velocity. Distributions become more anisotropic and can become unstable to wave generation. That there would be interesting plasma responses near Io was fully anticipated, but one of the surprises of Galileo’s mission was the range of effects observed at all of the Galilean satellites. Electron beams and an assortment of MHD and plasma waves develop in the regions around the moons, although each interaction region is different. Coupling of the plasma near the moons to the Jovian ionosphere creates auroral footprints and, in the case of Io, produces a leading trail in the ionosphere that extends almost half way around Jupiter. The energy source driving the auroral signatures is not fully understood but must require field aligned electric fields that accelerate elections at the feet of the flux tubes of Io and Europa, bodies that interact directly with the incident plasma, and at the foot of the flux tube of Ganymede, a body that is shielded from direct interaction with the background plasma by its magnetospheric cavity. Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Moon–magnetosphere interactions; Jupiter; Jupiter’s moons 1. Introduction 2. Properties of importance to the interaction The major moons of Jupiter and Saturn, solid bodies Various intrinsic properties of the moons critically comparable in scale with Earth’s moon (whose radius is affect their interactions with the plasma that flows onto 1734 km), are embedded in the flowing plasma of a them. Of particular importance are the internal mag- planetary magnetosphere. Some relevant properties are netic fields of the moons, which may be permanent and/ presented in Table 1. Interactions with the surroundings or induced. Neutrals liberated from some of the moons depend on details of the bodies and of the plasma that can upon ionization become the source of a plasma that flows onto them. This tutorial presentation first intro- is not only locally denser than the ambient plasma of the duces the physical processes that must be considered in magnetosphere but may contain ion species not present understanding how the moons interact with the system in it. The electrical conductivity of the surfaces and the and then presents and interprets selected measurements interiors of the moons, their ionospheres, and the plas- in the vicinity of the moons. ma clouds that surround them are critical elements of the interaction. There are interesting similarities and differences between the interaction regions (magnetospheres or equivalent) that form around the planets and the * Tel.: +1-310-825-3435; fax: +1-310-206-8042. interaction regions that surround the Galilean moons. E-mail address: [email protected] (M.G. Kivelson). Critical is the fact that planets are embedded in a 0273-1177/$30 Ó 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.08.042 2062 M.G. Kivelson / Advances in Space Research 33 (2004) 2061–2077 Table 1 Selected properties of the Galilean moons and of Titan Io Europa Ganymede Callisto Titan Radius (km) 1818 1560 2634 2400 2575 Density (kg mÀ3) 3530 2990 1940 1851 1881 Orbital period 42.46 (h) 85.22 (h) 171.71 (h) 400.54 (h) 15.9 (days) super-magnetosonic plasma, the solar wind, whereas the Galilean moons are embedded in a sub- or trans-magnetosonic plasma, in this case Jupiter’s magnetospheric plasma, that overtakes them from their (orbital) trailing side. Properties of the magnetospheric plasma at the orbit of a moon that affect the interaction include: the MHD Mach numbers (fast, intermediate, and slow), the plasma beta, the Alfven conductance (which char- acterizes the effectiveness of the plasma in carrying current across the magnetic field), and the relatively predictable orientation and temporal variation of the external magnetic field. We shall return to a discussion of these features but let us first consider how the large- scale perturbations relate to MHD wave modes pro- duced in the interaction region. 3. Pertinent MHD wave properties The group velocity of a wave constrains the regions of space within which perturbations can be imposed by its action. Fig. 1 shows the properties of the group velocity of MHD waves for two different assumed plasma con- ditions in a uniform magnetic field (Kivelson, 1995). In the plasma rest frame, the fast mode carries information in all directions relative to the background field, B,and the speed depends on the angle relative to B. The Alfven wave or intermediate mode carries information strictly along the background field. The slow mode carries information in directions close to the background field but, like the fast mode, it does not carry electrical current along the field direction. Only the Alfven wave carries field-aligned current! Different types of perturbations are imposed by different wave modes. The fast mode is compressional. Fig. 1. Group velocities of the MHD fast (F), intermediate (I), and Thermal and magnetic pressure increase and decrease in slow (S) mode waves plotted vs. direction of wave vector relative to the phase producing pressure gradients that exert forces on background magnetic field. Labels above the plots indicate that in the upper (lower) panel the Alfven speed, V , exceeds (is less than) the the plasma. The slow mode is also compressional but A sound speed, cs. thermal and magnetic pressure vary in antiphase and therefore the total pressure (thermal plus magnetic) changes little, although density changes develop. The 4. Flow in the interaction region: analogy to planetary intermediate mode or Alfven wave does not change the magnetospheres field magnitude but plays an important role because it carries field-aligned current that couples regions sepa- There are similarities and differences between the in- rated along the background field direction. Where the teractions that occur at the moons and the interactions field changes direction without changing its magnitude, between the solar wind and the planets. In both cases, this mode is present. Its action reaccelerates downstream the flow slows when the plasma incident from upstream flow to the speed of the incident plasma. first senses the presence of an obstacle to the flow. In the M.G. Kivelson / Advances in Space Research 33 (2004) 2061–2077 2063 super-magnetosonic solar wind, the slowing occurs only the upstream boundary is the locus defined by abrupt downstream of a standing fast magnetosonic bow shock, bends of the magnetic field. Downstream of this surface whether the interaction is with a magnetized planet lies the region coupled to the moon by field-aligned (Mercury, Earth, Jupiter, for example) or an unmagne- currents. Because field-aligned current is carried only by tized body with an atmosphere (Venus, for example). the Alfven wave, whose group velocity in the plasma rest Downstream of the bow shock and within planetary frame is along ÆB, the perturbations carried by this magnetospheres, flows are typically sub-magnetosonic. mode appear downstream of a front represented sche- Within Jupiter’s magnetosphere magnetospheric plasma matically in Fig. 3. The angle of bendback, measured À1 approximately corotates with Jupiter and overtakes the relative to )B is given by a ¼ tan ðu=VAÞ in terms of moons, whose Keplerian orbital speeds are smaller than the flow velocity u and the Alfven speed of the back- the speed of plasma flow along the orbits. The relative ground plasma VA. In spacecraft measurements, the flow is sub-magnetosonic, so perturbations that slow signature is a field rotation with significant jdBxj where and divert the flow as the plasma approaches a moon x^ ¼ u=u. The surface defined by the field rotation is re- develop gradually. The need to decelerate the flow ferred to as an Alfven wing (Drell et al., 1965). Con- across a standing shock wave is absent. Indeed, no up- finement to a region downstream of a tilted front is stream shocks have been observed in the vicinity of the sometimes discussed in terms of wave characteristics moons on Galileo’s multiple passes, nor was a shock (Neubauer, 1980). Fig. 4 shows schematically the inter- identified in the, vicinity of Titan on the Voyager 1 pass. action region viewed from downstream in the flow. Mach numbers >1 may occur near Callisto at times Current flows into moon on the side facing toward when it is crossing the magnetic equator, but none of the Galileo encounters occurred in this plasma regime. Fig. 2 represents the qualitative behavior of the plasma flow near a moon. Both, at the moons and the planets, the flow slows when the plasma incident from upstream first senses the presence of an obstacle to the flow which, in the super- magnetosonic solar wind, occurs only downstream of a standing fast magnetosonic bow shock. Many features of the diversion of the flow behind the bow shock can be understood as analogues of arguments that we will in- troduce in the context of the interactions at the moons.

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