As mentioned in the last chapter, when the mass of the star is greater than about three times the mass of the Sun, even the neutron Fermi pressure and other outward forces exerted by neutrons are not sufficient to withstand the force of gravity. There are no known forces in nature that can balance the force of gravity under these circumstances, and the star collapses and collapses until it reaches a very small volume with very high density. The precise nature of the final form that matter takes in this case is not known. What is indicated by Einstein's theory of gravitation is that when all the matter in the star goes within a certain small volume, no further communication is possible with the matter inside this volume, since any rays of light (which are the fastest possible signals) leaving it are pulled back towards the central region by the strength of gravity. The star then becomes a ‘black hole’. It is called ‘black’ because no radiation of any kind comes from within it. If the star has no rotation initially, the black hole quickly settles down to a spherical shape, the radius of the hole (from within which nothing can come out) being dependent on the total mass. For a mass M, the radius of the black hole, known as the Schwarzschild radius (after the German astronomer Karl Schwarzschild (1873–1916), who first found the solution of Einstein's equations in 1916 corresponding to a black hole) is 2GM/c2, where G is the Newtonian gravitational constant and c is the velocity of light.

Plasma waves observed near Jupiter exhibit characteristics similar to waves in the terrestrial magnetosphere and a quantitative analysis suggests a universality of the generation mechanisms. Electromagnetic whistler-mode waves are enhanced within the high-density plasma torus surrounding the orbit of Io and evidence for electron stable trapping is found in the inner torus. The observed intensity of low-frequency hiss is sufficient to scatter resonant (≳ 100 keV) electrons and cause continuous precipitation loss to the atmosphere at a rate comparable to 5% of the limit imposed by strong pitch-angle diffusion. The energy flux into the Jovian atmosphere is estimated to be approximately 3 mW/m2 over a broad invariant latitude range 65° ≲ Λ ≳ 70° mapping from the Io torus. Total power dissipation may therefore approach 1013 W. But because the energetic electron deposition occurs deep in the Jovian atmosphere, little of the concomitant auroral emission should be detectable by Voyager. The intermittent bursts of electromagnetic chorus and the equatorially confined electrostatic (n + 1/2)fc,e waves can at times scatter lower energy (keV) electrons on strong diffusion. The net energy deposition, however, is typically less than 0.5 mW/m2 and the total power dissipation over the entire auroral zone should be ≲ 1012 W. Energetic ions exhibit evidence for rapid precipitation loss throughout the plasma torus. Although the waves responsible for such scattering have not yet been identified, ion precipitation loss near the strong diffusion limit apparently offers the only viable mechanism to excite the observed auroral emissions.

The highly extended magnetic field-line configuration of the Jovian magnetosphere with a near-equatorial current sheet and associated plasma sheet arises from mechanical stresses in the rotating plasma balanced by magnetic stresses. The relative geometrical thinness of the current sheet permits the use of several approximations in the description of the stress balance, each with a specified regime of validity in terms of taillike vs. dipolar field and hot vs. cold plasma; these include pressure balance, a simplified tangential stress balance, and an estimate of current-sheet thickness. A number of simple but quantitative models of the magnetic field are now available, including both theoretical models based on various assumptions about the distribution and degree of corotation of the plasma and empirical models intended to represent the observations. From the empirical models, values of plasma parameters required to maintain stress balance can be estimated. To obtain agreement between the estimated and the observed mass density values, it is necessary to assume that the azimuthal velocity of the plasma decreases significantly below rigid corotation in the outer magnetosphere. The uncertainties in the magnetic field component normal to the current sheet lead to sizable discrepancies among various estimates of the density or of the current sheet thickness. Azimuthal magnetic fields over the midnight-to-dawn quadrant are nearly independent of local time, in contrast to the situation in the terrestrial magnetosphere; they imply radial currents whose closure through the ionosphere is related to partial corotation. Generalization of Parker-spiral arguments to include a finite ionospheric conductivity provides a quantitative model for the azimuthal field.

Theoretical ideas concerning Jovian magnetospheric phenomena are at least as diverse as the phenomena themselves, and there presently exists no single comprehensive model that encompasses all known phenomena within a unified theoretical framework. We identify here a number of important theoretical concepts, some subset of which (together with perhaps others yet unidentified) will ultimately provide the elements of such a comprehensive model. A number of ideas have been advanced to account for the copious plasma source associated with Io, but none of these has yet accounted satisfactorily for both the magnitude and the morphology of the inferred source. Nevertheless, given the observed fact that Io supplies the bulk of the magnetospheric plasma mass, and the corollary that the net plasma transport is predominantly outward, it follows that the rotational energy of Jupiter is an important if not dominant source of energy for magnetospheric phenomena. This rotational energy is expended in a variety of phenomena, including the electrodynamic Io-Jupiter interaction and associated radio and auroral emissions, the acceleration of charged particles to MeV energies, and the generation of a wide variety of spin-periodic phenomena as observed both remotely and in situ. The spin periodicities observed within the magnetosphere can be explained for the most part as resulting from the diurnal wobble of the magnetospheric current sheet caused by the offset between Jupiter's magnetic dipole axis and its spin axis. However, remotely observed spin periodicities (the “pulsar” phenomena) apparently require the existence of an intrinsic longitudinal asymmetry in the Jovian magnetosphere that corotates with Jupiter.

In the Jovian magnetosphere, electrons, protons, and heavier ions are accelerated to energies well above 10 MeV. These energetic particles constitute a valuable diagnostic tool for studying magnetospheric processes and produce the Jovian radio emissions. In the inner magnetosphere, both the electron and proton fluxes with energies above 1 MeV build up to ~ 108 per cm2 s and constitute a major radiation hazard to spacecraft passing through this region. Surprisingly, high fluxes of energetic oxygen and sulfur (> 7 MeV/nuc) are also found in the inner magnetosphere. Of particular interest are the interactions of these particles with the inner Jovian moons and with the Io plasma torus. Throughout much of the middle magnetosphere and magnetospheric tail, highest fluxes are found in the plasma sheet, which coincides closely with the tilted dipole equator out to 45 Rj (Jupiter radii). This plasma sheet has not been identified beyond 45 Rj in the subsolar hemisphere; however, on the night side, it extends to 200 Rj. On the day side, fluxes near the equator are relatively independent of distance (15 to 45 RJ) and fall into the range 104 to 10 per cm2 s each for protons and electrons above ~ 1 MeV. In the predawn direction, proton and electron fluxes decrease by three orders of magnitude from 20 to 90 Rj (105 to 102 per cm2 s) and then remain relatively constant to the boundary layer near the magnetopause.

During the early 1960s the dominant emphasis of the space program of the United States was on manned space flight, looking toward landings on the moon and the detailed investigation thereof. Parallel with these activities, but at a much lower level of emphasis, was the development of a national program of planetary exploration. The nearby terrestrial planets Venus and Mars were the most readily accessible. Also, interest in search for extraterrestrial life on Mars provided a strong motivation for landing on its surface an elaborate device called an automated biological laboratory. The mission for accomplishing this was called Voyager, a name that was later changed to Viking. Still later, the name Voyager was adopted for an altogether different planetary mission.

The development of a national program of planetary exploration had many sources and many aspects. But to a very considerable extent, all of these aspects came into focus most clearly within the Space Science Board (SSB) of the National Academy of Sciences and more specifically within the National Aeronautics and Space Administration's Lunar and Planetary Missions Board (LPMB), created in early 1967 under the chairmanship of John W. Findlay of the National Radio Astronomy Observatory, with Homer E. Newell, John E. Naugle, Donald P. Hearth, Oran Nicks, and Robert Kraemer as the principal NASA participants. The minutes of the LPMB over the period 1967–70 reflect intensive and comprehensive consideration of every subsequently conducted lunar and planetary mission, as well as several that have not yet been conducted.

Why Jupiter? Is a book devoted solely to the magnetosphere of Jupiter too narrow, too specialized? With the present emphasis on solar-terrestrial relationships, why should we be studying other magnetospheres, and why Jupiter's? The primary reason is that Jupiter's magnetosphere is so unlike the Earth's in its fundamental workings. We study the Jovian magnetosphere because it is different. The difference challenges our understanding of magnetospheric physics. It leads us to a broader and more basic insight regarding both magnetospheric physics and the behavior of matter on a cosmic scale.

Jupiter is not an ordinary planet, nor does it have an ordinary magnetosphere. Although Jupiter's magnetosphere does most of the things Earth's does, it does them differently. For example, the Earth's magnetosphere extracts essentially all of its energy and some significant fraction of its plasma from the solar wind. In contrast, Jupiter's magnetosphere is powered by the slowing of Jupiter's spin, and nearly all of the magnetospheric plasma comes from internal sources – the satellite Io and the Jovian ionosphere. Jupiter also exhibits weak but genuine pulsar behavior. If we did not have the Earth's magnetosphere as a model, most theoretical work on the Jovian magnetosphere would probably be directed toward pulsar-type models.

The brief encounters of the two Pioneer and the two Voyager spacecraft with Jupiter have opened new frontiers of research in magnetospheric physics. Jupiter offers more than just another magnetosphere; it functions in a different mode and allows us to stretch our conceptions and develop better theories of the Earth's magnetosphere.

Jovian coordinate systems are not complicated or cabalistic, but they are different. The following is a description of these systems, as relevant to this book. I will also try to explain why things are as they are. There is logic behind the present system, even if some of the results seem curious or unfortunate.

Jovian longitude conventions

Latitude and longitude coordinates are usually established relative to some solid surface. Because Jupiter does not have a solid surface (at least none that is visible through the clouds), arbitrary, but convenient, coordinate grids have been prescribed. A spin equator is rather easily made out from observations of cloud motion, so the direction of the planetary spin axis is determined with relatively good accuracy. However, the determination of longitude is an entirely different matter.

Longitudes on a planet are fixed relative to an arbitrary, but well defined, prime- or zero-longitude meridian. For example, the Earth's prime meridian is the one that passes through the central cross-hair of the transit telescope at the Greenwich Royal Observatory. Its location is unique, and it stays put. The selection of this meridian as the prime or zero-longitude meridian was initially arbitrary, but the selection, once made, fixes the longitude grid with precision. The problem immediately faced in establishing a Jupiter longitude system is that the mean rotation period of the clouds is a function of latitude. The equatorial region rotates faster than the temperate and polar regions, as is common in all planetary upper atmospheres.

Introduction

A toroidal volume near Io's orbit is made luminous by multiple optical and ultraviolet line emissions excited by resonant scattering of sunlight and by electron collisions. These emitting atoms and ions have been lost from Io. Table 6.1 summarizes the species and detected transitions as of early 1982. In this chapter we focus on spectrophotometric measurements of these emissions and their physical interpretation. The reader is referred to Pilcher and Strobel [1982] for a more general view of torus emission phenomenology.

The concept of circumplanetary atoms of satellite origin was first proposed by McDonough and Brice [1973] for Titan, a satellite with a dense atmosphere from which Jeans escape is probably important. The Io phenomenon was not anticipated owing to Io's low atmospheric pressure [Smith and Smith, 1972; Pearl et al., 1979]; nevertheless, the discovery by R. A. Brown [1974] of sodium optical emission from Io's vicinity established the first example of a satellite that is a rich, continuing source of material for a planetary environment. We know now that the flow of material from Io dominates the particle and energy budgets of the Jovian magnetosphere.

The primary spatial reference here is to a toroidal volume of ~ 4 × 1031 cm3 between about 5 and 7 Rj from Jupiter, which includes Io's orbit but lies near the magnetic equatorial plane or centrifugal symmetry surface. This torus contains the bulk of Io's neutral atom clouds and coincides roughly with the UV source region seen by Voyager.

A generally accepted theory of the enigmatic phenomenon of planetary radio emission is not yet available. In this chapter, we direct our attention primarily to the question of how the Jovian decameter radiation might be generated via both direct and indirect mechanisms. Direct mechanisms transform the free energy contained in an electron distribution (typically a loss-cone) directly into electromagnetic waves. Indirect mechanisms transform the free energy contained in an electron beam distribution first into electrostatic waves that can then couple, in some manner, to produce electromagnetic waves. The growth rates for the unstable electromagnetic and electrostatic waves are derived. Nonlinear theories are briefly discussed as they apply to the case of Jupiter's decametric radiation. Because most of the Jovian radio emission seems to be controlled by Io, we describe how Io, through the emission of kinetic Alfven waves, can produce a “beamlike” electron distribution. It is more difficult to understand how Io can enhance or produce a “loss-cone” distribution. Thus we conclude that, at least for Jovian radio phenomena, indirect mechanisms are preferred. We also describe theories and models for the generation of the dynamic spectral arcs that characterize the radio spectrum from hectometric to decametric wavelengths.

Introduction

Jupiter is the most powerful planetary source of nonthermal electromagnetic radiation in the solar system, with a radio spectrum extending from a few kHz to over 100 MHz. The phenomenology of the decimeter component in the GHz range has been discussed in Chapter 7.

The recent Voyager encounters with Jupiter have now provided us with the first comprehensive investigation of plasma waves in the magnetosphere of Jupiter. The most striking feature of the Jovian plasma wave observations is the close similarity to the plasma-wave phenomena observed in the Earth's magnetosphere. Essentially, all major types of plasma waves detected in the Jovian magnetosphere have analogs in the Earth's magnetosphere. These include, for example, electrostatic waves near and upstream of the bow shock, electromagnetic continuum radiation, lightning- generated whistlers, whistlermode chorus and hiss, electrostatic electron cyclotron and upper hybrid emissions, and broadband electrostatic noise.

Introduction

Any waves that are influenced by the presence of a plasma are called plasma waves. Because wave-particle interactions in a collisionless plasma produce scattering and thermalization effects somewhat similar to collisions in an ordinary gas, plasma waves are now recognized as being of fundamental importance for understanding the equilibrium state of planetary magnetospheres. In the Earth's magnetosphere, wave-particle interactions are known to be responsible for heating the solar wind at the bow shock, for the diffusion that allows plasma to enter the magnetosphere, and for the pitch-angle scattering that causes the loss of energetic particles trapped in the magnetic field. A large number of different types of waves can occur in planetary magnetospheres. The properties of some of the more important plasma wave modes are summarized in Table 8.1. In general, plasma waves can be classified as either electromagnetic, which have both electric and magnetic fields, or electrostatic, which have no magnetic field.

Our understanding of Jupiter's ionosphere has been enhanced by the Voyager encounters. Vibrationally excited H2 (υ ≥ 4) probably played an important role in providing a rapid loss mechanism for protons (the major topside ion) during the Voyager encounter. A straightforward calculation of the Voyager 1 entry electron concentration profile with chemistry and physics adequate to understand the Pioneer radio occultation profiles yields significant differences from the Voyager radio science measurements and suggests that substantial improvements in models may be necessary if the preliminary results of the observations remain unaltered after improved reduction and analysis of the data. It is argued that, although solar EUV radiation probably controlled the ionosphere during the Pioneer observations, particle precipitation as evidenced by strong H2 airglow emissions on a planetwide basis appears to have played an essential role in both heating the thermosphere and as an ionization source during the Voyager encounters. The main contribution to the Pedersen conductivity in Jupiter's ionosphere occurs in the region where multilayer structure is dominant and accurate reductions of the Voyager radio occultation measurements are not yet available. Theoretical estimates of the integrated Pedersen conductivity are in the range of 0.02–10 mho; the former values are representative of an ionosphere produced by solar EUV radiation and the latter of the auroral ionosphere under intense particle precipitation.

Introduction

The original interest in an ionosphere on Jupiter was generated by the discovery of strong radio-frequency emissions at ~ 20 MHz that were thought to be plasma frequencies associated with Jupiter's ionosphere [Gardner and Shain, 1958].