Dynamics of the Jovian Magnetosphere
Total Page：16
File Type：pdf, Size：1020Kb
25 Dynamics of the Jovian Magnetosphere N. Krupp, V.M. Vasyli¯unas, J. Woch, A. Lagg Max-Planck-Institut f¨ur Aeronomie, Katlenburg-Lindau K.K. Khurana, M.G. Kivelson IGPP and Dept. Earth & Space Sciences, University of California, Los Angeles B.H. Mauk, E.C. Roelof, D.J. Williams, S.M. Krimigis The Johns Hopkins University Applied Physics Laboratory W.S. Kurth, L.A. Frank, W.R. Paterson Dept. of Physics & Astronomy, University of Iowa 25.1 INTRODUCTION bit around Jupiter for almost 8 years (1995-2003), Galileo has collected in-situ data of the jovian system over an ex- Understanding the dynamic processes in the largest magne- tended duration and has for the first time made possible tosphere of our solar system is one of the outstanding goals the study of Jupiter on time scales of weeks, months, and of magnetospheric physics. These processes are associated even years, using the same instrumentation. New regions of with temporal and/or spatial variations of the global config- the jovian magnetosphere have been explored, especially the uration. Their time scales range from several 10s of minutes jovian magnetotail which was not visited by any of the previ- to sizeable fractions of the planetary rotation period to long- ous spacecraft (except, to a very minor extent, by Voyager term variations which take several days to develop. They oc- 2). Additional inputs have become available from ground- cur on both local and global spatial scales, out to dimensions based and Earth-orbit-based measurements and from ad- representing a large fraction of the magnetosphere. vanced global magnetohydrodynamic (MHD) simulations of Early indications of a “living”, varying jovian magneto- the jovian magnetosphere. Ideally, of course, all such re- sphere came from ground-based observations of jovian radio sources should be combined together. The dual spacecraft emissions. It was found that some of the jovian decametric constellation of Galileo and Cassini in and near the jovian emissions are correlated with changes in solar wind param- system at the end of 2000 and the beginning of 2001, to- et al. eters (see e.g., Teresawa 1978), and in addition there gether with a simultaneous observational campaign from was evidence of solar wind influence on hectometric radio Earth, provided a partial opportunity for such a combined et al. emissions and on jovian aurora (Baron 1996, Gurnett study, from which transient processes on a wide range of et al. 2002). Most of the information we have today, however, temporal and spatial scales could be identified and their about dynamical processes at Jupiter has come from in-situ physical nature disentangled at least in part. spacecraft observations. Of the seven spacecraft that have visited the gas giant over the last 30 years, six only flew past A comparison of the dynamical plasma processes be- the planet and hence provided only snapshots, each lasting tween Jupiter and Earth reveals both similarities and dif- a few days or a week at most, of the state of the magne- ferences (Russell 2001, e.g.,). Within the magnetospheres of tosphere. These flyby missions —Pioneer 10 (1973) and 11 both planets, plasma and energetic particles undergo large- (1974), Voyager 1 and 2 (both in 1979), Ulysses (1992), and scale flows, there are identifiable sources that supply the Cassini (2000/2001) — offered very limited possibilities to plasma, and large amounts of energy are transported and observe and investigate transient processes, because of the dissipated. At Earth, the primary flow is a large-scale cir- short duration of the flyby and the inability to distinguish culation, called magnetospheric convection, driven primar- between temporal and spatial variations. The state of knowl- ily by the solar wind. Furthermore, the solar wind consti- edge derived from the early flyby missions has been summa- tutes the major source for the magnetospheric plasma con- rized in the books by Gehrels (1976) and Dessler (1983) for tent (although the contribution of Earth’s ionosphere is also Pioneer and Voyager and in a series of special journal is- significant) and provides almost all of the energy that is sues (Science Vol. 257, Planet. Space Science Vol. 41 and J. stored and dissipated in the magnetosphere-ionosphere sys- Geophys. Res., Vol. 98) for Ulysses. tem. The energy flow is strongly modulated by solar-wind A whole new dimension in studying Jupiter’s magne- parameters (especially the angle between the interplane- tosphere became possible with Galileo, the first orbiting tary magnetic field and Earth’s dipole) and is highly un- spacecraft in the magnetosphere of an outer planet. In or- steady, with prolonged periods of enhanced activity (mag- 2 Krupp et al. D D A: interchange events D B: flow bursts Dusk C C: electron bursts D: upstream events ? massMass x x outflow ? x ? ? x x ? x x x x x ? A x x Sun x x x Radial x ? B ? ? ? inward ? diffusion Planetary D wind D Dawn ? ? ? ? ? Sun ? ? ? Distant Galileo X-line at r=60–80Corotation R breakdownMagnetopauseBow shock X-line measurements end at r=150 R J J Figure 25.1. Sketch of the jovian magnetosphere in the equatorial plane (top) and a noon-midnight meridional cut (bottom). Question marks indicate regions where no data are available as yet. Black arrows represent the particle flow directions, grey arrows the mass outflow and the inward diffusion. The x-symbols show the location of a magnetic x-line in the jovian magnetotail, and the letters A-D mark the positions where various dynamic features have been inferred from in-situ data. During the two spacecraft (Galileo and Cassini) measurements in 2000/2001 there was evidence for wavy structure of the dusk magnetopause as indicated. The spatial extent of these unstable boundaries, however, is not fully known and may be as prevalent (or more so) on the dawn side. Dotted lines are drawn to separate magnetospheric regions. The solid and dashed lines in the bottom sketch represent magnetic field lines; the dashed portions lie beyond the distances reached by Galileo. The fat grey bar indicates the equatorial current/plasma sheet. netic storms) and with sudden global re-configurations of mass source, which may not be negligible and may well drive the magnetosphere probably connected with internal insta- or at least trigger some internal processes or disturbances. bilities (magnetospheric substorms, originally thought to be However, there are internal sources of energy and mass that just sub-units of magnetic storms but now recognized as a are much more important than those from the solar wind, distinct phenomenon). At Jupiter the situation is more com- unlike the case at Earth. The average plasma flow is driven plex. The solar wind still constitutes an external energy and primarily by the rotation of the planet throughout most of 25 Dynamics of Magnetosphere 3 the jovian magnetosphere (see Chapter 24 and e.g. Hill and moving predominantly in the corotation direction but with Dessler 1991). The plasma in the jovian magnetosphere is velocities around 200-400 km/s, less than the rigid corota- supplied mainly by internal sources, predominantly Io (see tion speed. The flow is larger at dawn than at dusk (see Chapter 23). Planetary rotation is also the dominant source Chapter 24). Beyond a comparable distance the magnetic of energy for much of jovian magnetospheric dynamics. We field becomes too weak to exert a radial inward force suf- may therefore expect magnetospheric transient processes at ficient to balance the centripetal acceleration of a rigidly Jupiter to be significally different from those at Earth. There corotating plasma, although it does balance the acceleration is evidence that reconfigurations, possibly involving large- of the actual sub-corotating plasma flow. scale instabilities and resembling terrestrial substorms in Dynamic processes in the jovian magnetosphere are ev- some ways, occur at Jupiter. The mechanism, however, may idenced by deviations from the average configuration. The involve energy extracted by torques on the rotating planet letters A-D in Figure 25.1 indicate the typical locations, in- and stresses exerted by rotating mass-loaded flux tubes, in side and outside the magnetosphere, where such deviations place of stresses from solar wind flow. Similarities with Earth (to be discussed in more detail below) have been observed. may be related in part to the common role of magnetic field The magnetopause is often unstable to surface waves at the line reconnection. boundary. During the two spacecraft (Galileo and Cassini) Although Jupiter’s magnetosphere is the second best- measurements in 2000/2001 there was evidence for wavy mo- known of all explored magnetospheres, it still ranks far be- tion of the magnetopause on the dusk flank of the magneto- hind Earth’s in this respect, and only a tiny fraction of it sphere as indicated in Figure 25.1. The spatial extent of sur- has already been explored with in-situ measurements. If one face waves, however, is not fully known and the dawn mag- scales all positions by the normal stand-off distance of the netopause surface could well be less stable. The planetary or magnetopause (10 REat Earth and 60-90 RJ at Jupiter), magnetospheric wind as inferred from Voyager observations ≈ the most distant measurements taken onboard Galileo in (Krimigis et al. 1981) is shown at distances beyond 130RJ Jupiter’s magnetotail (150 RJ ) correspond to only about in the predawn section of the magnetosphere. 20-25 RE in the Earth’ case (Woch et al. 2003). In fact, In this chapter we will focus on the observational ev- there is evidence that the jovian magnetotail may extend as idence and possible explanations for these various devia- far out as 5 AU, to the orbit of Saturn, thereby influencing tions from the average state of the jovian magnetosphere.