Jupiter's magnetic field and radiation belts

An understanding of Jupiter’s magnetic field has proved vital to furthering comprehension of this enormous planet’s singular structure, and such knowledge also enriches Earth, planetary, and solar system science.

Overview

Since 1610, when Galileo focused his telescope on Jupiter and four of its satellites, this immense planet, orbiting 628 million kilometers from Earth at its closest approach, has received much attention from astronomers. Jupiter’s mean distance from the Sun is 5.2 times the mean distance between Earth and the Sun, the latter being known as one astronomical unit (AU). After the Sun, Jupiter is the largest object in the solar system, possessing a mass 318 times greater than Earth’s and a diameter eleven times longer. Its volume is thirteen hundred times that of Earth. Because of its prominence, many of Jupiter’s basic characteristics were ascertained centuries ago. Sir Isaac Newton accurately calculated its mass and density, for example. Its radius, diameter, rate of rotation, chemical composition, and singular surface features have similarly been under study for centuries. However, this body, which is neither a star like the Sun nor a terrestrial planet like Earth, has retained certain mysteries, many of which have to do with the composition of Jupiter’s interior and the origins and behavior of its magnetic field and radiation environment.

During the 1950s and 1960s, prior to investigations by uncrewed spacecraft, radio astronomers gathered approximate data on Jupiter’s magnetosphere, the zone of powerful magnetic influence that surrounds the planet. In 1955, Bernard Burke and Kenneth Franklin, both radio astronomers, found evidence that Jupiter’s magnetosphere was a source of nonthermal radio activity (in contrast to the thermal radiation emitted by all objects with temperatures above absolute zero) at a frequency of 22.2 megahertz (MHz). Other astronomers later noted that this radio activity occurred, if not continuously, at least in a patterned way, at the same point in the planet’s rotation. Such emissions distinguished Jupiter from the other planets and raised provocative questions about the validity of previous theories of radiation.

Several years later, additional radio bursts were picked up by Earth-based radio telescopes in a different portion of the radio spectrum (300 to 3,000 MHz). Unlike the emissions detected earlier, which originated at the planet’s surface, these decametric radiations emanated from within Jupiter’s toroidal region—a region encircling the planet, tilted about 10° from its equatorial belt and extending about 286,400 kilometers from it into space. Within this region and on both sides of the planet at about 140,000 kilometers from its surface, there are two hot spots, areas of intense radiation activity, which evoked considerable scientific curiosity.

In 1959, two astronomers, Frank Drake and S. Hvatum, identified the source of this toroidal radio activity as synchrotron radiation. Atomic particles within the region were being accelerated to very high speeds by a powerful magnetic field and by changes in the frequency of the electric field. There has been a growing consensus among astronomers that these high-energy particles, electrons that came from the Sun, have been trapped by Jupiter’s external magnetic field, which is 19,000 times stronger than Earth’s magnetic field. Forming radiation belts around the planet, these high-energy particles, moving at high velocities, may produce radio emissions when they strike the top of Jupiter’s atmosphere.

Growing scientific curiosity about these prodigious emissions led to Jupiter’s magnetosphere becoming a subject for investigation during the flybys of Pioneer 10 and Pioneer 11. Pioneer 10, launched in March 1972, flew to within 130,000 kilometers of Jupiter on December 3, 1973, securing remarkable photographs of Jovian cloud tops and measuring Jupiter’s radiation belts before sailing farther into space. Pioneer 11, also referred to as Pioneer-Saturn, was launched in April 1973. On December 2, 1974, it flew to within 42,000 kilometers of Jupiter, photographing the vast but previously unstudied Jovian polar regions. Among other measurements, Pioneer 11 obtained fresh data on Jupiter’s complex magnetic field. The massive flow of data from the Pioneer flights greatly contributed to more precise assessments of the configurations of Jovian radiation belts and the magnetic field, as well as of the extent of the magnetosphere and the distribution of energetic electrons and protons within the planet’s interior. This information, in turn, encouraged fresh ideas about Jupiter’s structure and rotation. In the mid-1970s, for example, John D. Anderson of the Jet Propulsion Laboratory in Pasadena, California, and William B. Hubbard of the University of Arizona made use of Pioneer data to devise a new model of Jupiter’s internal structure that was consistent with data regarding both its gravitational field and its magnetic field.

Anderson and Hubbard proposed that beneath the dense and apparently chaotic Jovian atmosphere lies a thick layer of liquid molecular hydrogen, beneath which an even thicker layer of liquid metallic hydrogen exists. The heart of the planet, they proposed, consists of a small, rocky core of iron and silicates heated to temperatures of nearly 30,000 kelvins. Although the presence of such a core could never be proved by gravitational studies, its existence was plausibly deduced from the assumption that if its composition were similar to the Sun’s, it should contain some measure of the same elements. The liquid metallic hydrogen layer presumably extends 46,000 kilometers out from the core, is heated to 11,000 kelvins, and is compressed under a pressure of three million Earth atmospheres. This layer cannot be experimentally modeled in bulk in a laboratory, yet the construct is plausible: metallic hydrogen has been created in small amounts in the laboratory. The first to do it were researchers at Lawrence Livermore Laboratory in 1996. In a liquid metallic state, hydrogen molecules break down into atoms and become electrical conductors.

Information from the Pioneer flybys of Jupiter also led to revisions of the hypothesis that the planet’s excessive radiation of heat results either from radioactivity or from heat generated by gravitational contraction of the largely gaseous mass of which Jupiter is composed. Since it now appeared that Jupiter was a liquid body (and liquids are all but incompressible), it seemed likely that its excess radiated heat was merely a residue of the heat generated when the planet coalesced from the solar nebula. The implication would be that the planet’s original thermal energy is continuously finding its way to the surface and, in the process, creating convection currents, the rising of hot gases or liquids, and the downward movement of colder liquids or gases in the planet’s interior. Such grand-scale convection currents, as described by John H. Wolfe, who served as chief scientist for the Pioneer missions to Jupiter, could constitute a mechanism for generating the Jovian magnetic field. As primordial heat rose through Jupiter’s liquid metallic hydrogen, stirring it, the Coriolis force affected the resulting convection currents. The Coriolis force arises from planetary rotation and deflects other forces in motion—depending on whether they are north or south of the body’s equator—either to the right or to the left, much as a person walking across a moving carousel appears thrown off course as seen in an inertial reference frame. Deflected convection currents in such circumstances would set up loops of electric current, which could and may create a magnetic field.

The magnetosphere of Jupiter was determined to expand and contract under pressure from the solar wind. Where an equilibrium existed between magnetic forces and the solar wind’s incident stream of charged particles, a planetary magnetopause developed. Pioneer data confirmed the magnetopause to be as far out from the Jovian atmosphere as 7,135,000 kilometers and as close to its atmospheric layer as 3,565,000 kilometers. Just as there is a bow shock wave, there is also a magnetotail, which, much like a ship’s wake, extends several hundred kilometers behind Jupiter in a direction away from the Sun.

Like Earth’s magnetic field, the Jovian field is dipolar, but its magnetic axis is tilted between 9.5° and 10.8° from the planet’s rotational axis, a displacement of about 7,000 kilometers from Jupiter’s center. The strength of the magnetic field, as measured at Jupiter’s cloud tops, varies from 3 to 14 gauss (a unit of magnetic field strength), extremely powerful by comparison with the 0.3- to 0.8-gauss strength of Earth’s magnetic field at the surface. Probably because of the still unknown circulation patterns of the liquid metallic hydrogen in the Jovian interior, Jupiter’s magnetic field is far more complex than Earth’s. In addition to its dipolar field, the Jovian field, according to Pioneer 11 data, also has quadrupole and octupole movements. That is, components of the main field have four and eight poles, respectively, although these are much weaker than the main dipolar field and have been detected only close to the planet.

Further complications arise from the motion of the Jovian satellites that orbit within its magnetosphere. In the course of their orbits, Io, Ganymede, Europa, Callisto, Amalthea, and potentially several other satellites absorb highly charged particles that otherwise might be trapped by Jupiter’s magnetosphere. Thus, these satellites clear channels through Jupiter’s radiation belts. Io sputters material into space that forms a torus of charged particles along its orbit about Jupiter. This torus consists of sulfur, sodium, oxygen, and a few lesser constituents ejected from volcanic activity. Ganymede complicates Jupiter’s magnetosphere because that satellite itself possesses a magnetic field and therefore has a magnetosphere within Jupiter’s own magnetosphere.

Perhaps because Io, in particular, possesses an ionosphere that provides it with a conductive fluid, not only has it trapped charged particles otherwise destined for Jupiter’s magnetosphere, but it also produces and accelerates charged particles. When Io is in a fixed position along an Earth-Jupiter line, radio emissions increase; thus, it is believed that its charged particles are an additional source of these emissions. Amalthea has also shown peculiarities as it orbits through the Jovian magnetosphere. Charged particles in its magnetic field unexpectedly do not increase in density toward Jupiter’s magnetic equator or to its “surface.” Instead, particle density varies widely at many different points.

One of the major reasons that the Galileospacecraft was outfitted with a sophisticated magnetometer was to attempt to determine how plasma is transported through Jupiter’s magnetosphere. Naturally, the magnetometer was also intended to provide precise determinations of spatial and temporal variations of Jupiter’s magnetic field and the extent and shape of the planet’s complex magnetosphere, as well as its interactions with the solar wind.

Knowledge Gained

Between the recording of Jovian radio emissions in the 1950s and the Pioneer and Voyager robotic space flights more than twenty years later, our understanding of the Jovian magnetic field has increased by quantum leaps. Data not only have confirmed or disproved older theories about the field’s origins, extent, and inner and outer complexities but also led to more consistent and plausible theories about the structure of Jupiter itself and, indeed, of other planets as well.

Sources and at least some causes of Jupiter’s high levels of radio emissions have been identified. Based on hard, if still incomplete, evidence gathered by uncrewed spacecraft, the configurations of the magnetosphere, magnetopause, magnetosheath, and magnetotail have been delineated. The magnetic field’s axis and its location have been defined. Quadrupole and octupole fields within the overall dipolar field have been discovered. The strength of the field and fluctuations within it have also been measured. Investigations of the magnetosphere have led to theories concerning the makeup of the Jovian interior and some of the convective and conductive functions of the liquid metallic hydrogen that compose much of it.

Further, many of the special characteristics of the outer magnetosphere have been explained as the result of two principal mechanisms. First, as observed by James Van Allen (discoverer of Earth’s Van Allen radiation belts), a mass of low-energy plasma trapped in the Jovian magnetic field has created pressures that have inflated the field as if it were a balloon. Second, because of interactions with the planet’s rotating magnetic field, the plasma co-rotates (over a period of 9 hours, 55.5 minutes), creating a centrifugal force that contributes to the outward pressure. Plasma analyzers on the two Voyager spacecraft indicated that the plasma originated from gases—principally sulfur dioxide and hydrogen sulfide with sodium and oxygen in lesser amounts—vented by Io’s vigorous volcanic activities. This plasma is responsible for the Io torus, a phenomenon unique to Jupiter’s magnetosphere. That is, Io is surrounded by a doughnut-shaped band of excited charged particles, some captured from the solar wind as it orbits Jupiter and some produced and agitated by Io’s own environment.

It is also understood now that Io and the other inner satellites, in the course of their orbits, attract many particles that enter Jupiter’s inner magnetosphere and thereby limit that region’s population of trapped particles. Particles trapped in the Jovian magnetosphere also affect the chemistry of the planet’s environment. Andrew Ingersoll has shown that trapped particles rain down from the magnetosphere into the Jovian atmosphere. There, lightning and other charged particles break down dominant chemical species, thus keeping a balance between production and breakdown of hydrocarbons in the Jovian atmosphere.

The Galileo spacecraft provided long-term measurements of Jupiter’s magnetic field and the interaction of the larger satellites with that complex structure. Magnetic measurements combined with gravitational information refined the understanding of Jupiter’s interior structure. Scientists continued to study Jupiter’s magnetic field in the twenty-first century. For example, in 2023, NASA’s Juno probe recorded conditions within the magnetic field that suggested it was highly impacted by solar wind.

Context

Jupiter attracts scientific attention because of its massive size, stunningly dynamic visual wavelengths, and noise in the radio range. By far the largest body in the solar system after the Sun, it is a natural target of interest for the astronomer’s gaze and telescopic observation. Consequently, the planet’s general outward appearance—its cloud cover, its bands, and its fascinating Great Red Spot—have been closely observed over several centuries.

An emitter of abundant radio emissions, Jupiter was sufficiently noisy to arouse the curiosity of radio astronomers, who were then led to explore the spectrum of these emissions and theorize about the planet’s structure. By the 1950s, thanks to analysis in the visible, infrared, and radio frequency portions of the electromagnetic spectrum, much was known of Jupiter’s chemical composition. The light gases hydrogen and helium were dominant, and the sheer size of Jupiter meant that it would require billions of years to dissipate them. Some believed that beneath the planet’s dense cloud cover, there were mountains, while others believed the planet to be entirely gaseous. In light of the vast quantities of information gained by the early 1980s, much of the pre-1950 understanding of the planet seems rudimentary, even laughable.

Between the 1950s and the early 1980s, several broad intellectual currents inspired scientific interest in planetary science, in Jupiter, and, ultimately, in the Jovian magnetosphere. One such current was the general scientific acceptance of a theory of the solar system’s origin that postulates condensation from a disk-shaped nebula of gas and dust. This theory proposes that about 4.6 billion years ago, a cloud of interstellar gas or dust, overwhelmed—as Carl Sagan and others have suggested—by an exploding star, collapsed and condensed to form the solar system. The central mass in the interstellar formation, contracting under its own gravity, produced such prodigious heat that it generated a thermonuclear reaction from which the Sun evolved. Lesser masses—Earth, for example—experienced less heating, and so, as planets, moons, asteroids, and comets bathed in and reflected the Sun’s light. Substantiation of this theory’s accuracy, validity, and reliability required a fresh empirical examination of each major object in the solar system.

With the dawn of the space age, it became possible to make in situ observations or at least the nearest equivalent. Chronologically speaking, data collected by the flybys of Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2, by Galileo in orbit about Jupiter, and by Cassini and New Horizons passing through the Jupiter system en route to other destinations, vastly increased the storehouse of knowledge concerning Jupiter’s magnetosphere. In 2011, the US National Aeronautics and Space Administration (NASA) launched the Juno spacecraft which will reach Jupiter by 2016; the spacecraft is equipped with a fluxgate magnetometer, which includes two sensors designed to measure the strength and direction of the Jovian magnetic field. Contributions of these exploratory missions to astronomy and comparative planetology helped advance a greater understanding of Earth’s relationship to the rest of the solar system.

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