Planetary magnetospheres

Six planets in the solar system possess magnetic fields apparently generated through internal processes. These fields extend some distance into surrounding space and interact with charged particles streaming from the sun. Electromagnetic interactions occur in the upper atmospheres of these planets, causing phenomena including auroral displays.

Overview

Planetary magnetospheres are usually created by magnetic force fields generated from the deep interiors of the planets. Six planets of the solar system—Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune—exhibit magnetospheres of differing magnitudes, apparently produced essentially in this way. The mechanics of the generating processes are not well understood. They differ somewhat among the planets, but the effect is as if strong bar magnets lay inside each of the affected planets. Lines of magnetic force radiating from the poles of each of these magnetic fields arc outward into surrounding space. The stronger ones link with corresponding force lines from the opposite pole to create a roughly spherical magnetic field extending several planetary radii into space around the planet. These three-dimensional magnetic structures are complex and dynamic regions. They contain plasmas of varying temperature and density, through which high-energy particles move. Venus and Mars also have magnetospheres, but they are the result of induced magnetism created by the action of the solar wind impacting directly on these planets’ atmospheres rather than by internal processes near their cores.

The importance of planetary magnetospheres lies in the ways they control the behavior of electrically charged particles, such as electrons, protons, and atomic and molecular ions. In the inner solar system, the heliosphere contains several charged particles per cubic centimeter on average. Although this is incredibly tenuous by Earth’s standards, it amounts to an important factor in the overall environment in which the planets exist.

Most of these charged particles are solar wind particles streaming outward from the sun at speeds averaging four hundred kilometers per second. They derive from the solar corona, a region outside the visible disk of the Sun, where the solar plasma is extremely rarefied and incredibly hot—about one million kelvins. Here, protons have such kinetic energy that they escape easily from the sun’s gravity, with the result that they create a steady “plasma wind” that blows out through the solar system. In addition, the complicated and energetic dynamics of the sun’s photosphere and chromosphere create a variety of circumstances in which bursts of charged particles from within are hurled out into the solar system, similar to water from a lawn sprinkler spewing out in a circular pattern.

Were it not for the effects of the solar wind, planetary magnetospheres would be completely spherical in shape. The pressure exerted by the onrushing plasma distorts the magnetic field lines, and therefore the magnetospheres, into teardrop shapes. In the sunward direction, the boundary of a magnetosphere occurs where the internal pressure of the planet’s magnetic field balances the pressure of the solar wind. In the direction away from the sun, the magnetosphere stretches out into a very elongated region called the magnetotail.

The actual size of a magnetosphere varies somewhat from day to day, even from hour to hour, depending on the intensity of the solar wind. Bursts of intense solar activity result in “gusts” in the solar wind that, when they encounter the planetary magnetic fields, compress the magnetospheres. Magnetospheres expand again when the solar wind diminishes. The farther out in the solar system a planet lies, the less is the ambient pressure exerted by the solar wind. This fact explains why the magnetospheres of Jupiter and Saturn are so large that they dwarf the sun.

Magnetic field lines rotate with the planet’s interior, from which they originate. As a result, the entire magnetosphere rotates with what is called “rigid body motion,” behaving as if it were a rigid extension of the planet’s core rather than a tenuous fluid on the fringes of the planet’s sphere of influence. The energy expended to force the magnetosphere to rotate with the interior of the planet is small but significant in the long run and gradually drains a planet of rotational momentum.

At the outer boundary of a planet’s magnetosphere, the solar wind breaks like sea waves on a reef. Much of the oncoming material is deflected and flows around the planet in the teardrop-shaped magnetosheath, but not without much turbulence and the formation of a shock wave identical to the wave that forms around aircraft traveling at supersonic speeds. This is known as the bow shock. A small percentage of the particles penetrate the magnetopause (the outer boundary of the magnetosphere) and enter the planet’s magnetic field.

In addition to the charged particles acquired from the solar wind, the magnetospheres contain ions and electrons derived from the planet to which they belong and any satellites or rings that may orbit the planet within its magnetosphere. These particles may have drifted from the main body of the planet’s atmosphere through natural kinetic activity, or may have been knocked free by collisions with high-energy particles impacting the upper atmosphere. Once within the magnetosphere, charged particles migrate to particular regions, depending on their kinetic energy, their electrical charge, and other physical factors. Planetary magnetospheres differ with respect to some of these regions. One that appears to be similar in all is the plasma sheet that lies at the center of the magnetotail. The tail consists of two lobes of opposing polarity, between which is a narrow zone where the magnetic field strength is nil. This narrow but elongated region becomes a deep “pocket” in which charged particles can collect. Periodic disturbances such as intense solar activity produce temporary changes in the size and shape of the magnetotail. These can result in sudden injections of large numbers of these stored particles back into the polarized regions of the magnetosphere. Such injections become the source of the particles that create the aurora borealis (northern lights) and aurora australis (southern lights) long known to be associated with solar activity.

Methods of Study

Information about Mercury’s interior structure and magnetosphere depended upon observations and measurements acquired by instruments aboard the American spacecraftMariner 10, which reached the planet on March 29, 1974. Mariner10 obtained measurements that enabled researchers to establish with considerable accuracy the size and shape of the magnetosphere and thereby infer the strength of the magnetic field. The field is inclined twelve degrees to Mercury’s rotational axis; however, it is much smaller and has only about 1 percent the strength of Earth’s field. In two other important respects, Mercury’s magnetosphere differs from the magnetospheres of Earth and the Jovian planets. It does not possess any radiation belts, and because Mercury lacks any appreciable atmosphere, none of the interactions between a magnetosphere and an ionosphere are present. The Mercury Surface Space Environment, Geochemistry, and Ranging (MESSENGER), which entered Mercury's orbit in March 2011, has revealed, among other things, that bursts of energetic particles in the planet’s magnetosphere result from the interplay between the solar wind and Mercury’s magnetic field.

Venus lacks an internal magnetic field on a planetary scale; there may be residual magnetism in its rocks. This is believed to be the result of the fact that the planet spins so slowly on its axis that no dynamo effect occurs at the planet’s core. Nevertheless, Venus exhibits a weak ionospheric magnetic field created by an induced magnetism that results from the interaction of the solar wind with the ionosphere. Lacing through the ionosphere are a number of unique magnetic structures called flux ropes, which are long, twisted lines of magnetic force. Within the flux ropes, the plasma temperature and particle densities are high and a wide variety of ions are found.

The Earth’s magnetosphere extends about sixty thousand kilometers in a sunward direction. The magnetotail stretches out one thousand radii in the opposite direction. Clear evidence that the magnetic axis is inclined 11 degrees to Earth’s rotational axis is seen in the locations of the magnetic poles; the magnetic north pole lies in northernmost Canada and the magnetic south pole is found on the edge of Antarctica. The center of the Earth’s magnetic field also lies about five hundred kilometers away from the geometric center of Earth.

Many of the charged particles trapped within the Earth’s magnetosphere are concentrated in regions called the plasmasphere and the Van Allen radiation belts. The plasmasphere begins just beyond the ionosphere and grades outward into the Van Allen radiation belts. There are two belts: The inner belt extends radially from about one thousand to ten thousand kilometers, and the outer belt from twenty-five thousand to sixty thousand kilometers. The Van Allen radiation belts surround the planet like two concentric doughnuts, encasing the equatorial and temperate latitudes but leaving the poles relatively exposed. Electrons, the least massive of the charged particles and most easily deflected by magnetic fields, spiral around the lines of force in these radiation belts as they travel back and forth between the magnetic poles at speeds of thousands of meters per second.

Poleward of the Van Allen radiation belts is a region of the magnetosphere that does not trap charged particles but funnels them down toward the atmosphere. The impact of the charged particles upon atoms and molecules in the polar ionosphere raises the temperature of this region to more than 1,650 kelvins, causing atmospheric gases to fluoresce and create what is known as the northern lights and southern lights. Seen from a perspective high above the planet, auroral activity appears as rings of glowing light girdling the polar ionosphere, like halos over the Arctic and Antarctic regions.

Spacecraft have detected only a faint trace of a magnetic field around Mars. Its strength is less than one-tenth of 1 percent that of the Earth. This is not considered surprising, since the planet lacks a liquid core and cannot generate a field internally. Evidently, what has been detected is similar to the induced magnetism in the ionosphere of Venus, but the Martian atmosphere is much thinner, making even this effect weak. In situ examination of rocks on the surface by the Mars Exploration Rovers Spirit and Opportunity found residual magnetism in certain rocks.

Detection of sharp bursts of radio energy from Jupiter in 1955 was the first indication that this planet might possess a magnetosphere. Further study—particularly of data provided by Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, and Galileo—confirmed the existence of a huge magnetosphere and provided many insights into its features. Jupiter’s magnetic axis is inclined about 10 degrees to the axis of rotation, and its field is nineteen thousand times stronger than Earth’s. The magnetosphere extends in the sunward direction an average distance of three million kilometers, while the magnetotail stretches beyond the orbit of Saturn. The polarity of the field is inverted, as compared with Earth’s (that is, the magnetic north pole lies near the planet’s geographic south pole). In the huge sea of plasma surrounding Jupiter are protons, electrons, ions, and neutral atoms from three distinct sources: the solar wind, the planet’s atmosphere, and the surfaces of some of its satellites. The third source contributes the heavy ions that are present through the process of sputtering. There is no correspondence for this component in Earth’s Van Allen radiation belts.

Io, the innermost of the Galilean satellites, is the site of several active volcanoes. Each of these sites discharges as much as 100,000 tons of sulfur or sulfur dioxide per second. Most of this material rains back down on the surface, but a very small fraction of it (0.01 percent) escapes from Io’s gravity and enters the Jovian magnetosphere. There it is quickly broken down by sunlight and fast-moving solar particles whose impacts cause the formation of heavy ions of oxygen, sodium, and sulfur. A doughnut-shaped volume within the Jovian magnetosphere, known as the Io Torus, contains a concentration of these heavy ions. Io’s movement through this charged environment generates an intense electrical current that follows a huge arcing path between Io and Jupiter, known as the Io flux tube. As Io orbits above the portion of Jupiter facing toward Earth, the magnetic “footprint” of this flux tube faces Earth and unusual bursts of radio energy are detected as this concentration of particles interacts with the Jovian atmosphere.

The existence of Saturn’s magnetic field was not detected until Pioneer 11 approached the planet in 1979. The field is inclined exactly with the planet’s axis of rotation and is one thousand times stronger than Earth’s but nineteen times weaker than Jupiter’s. Like Jupiter, the magnetic polarity is inverted as compared with Earth’s. On the sunward side, Saturn’s magnetosphere extends an average of 1 million kilometers from the planet. Measurement of the field strength at the cloud tops shows the magnetic north pole to be stronger than the south pole, suggesting that the magnetic center of Saturn lies about twenty-four hundred kilometers north of its geometric center. Titan orbits Saturn at a mean distance of 1,221,400 kilometers, passing out of the magnetosphere as it comes around on the sunward side, only to plunge back into the magnetotail as it continues its journey. Titan’s significant atmosphere (1.6 times as dense as Earth’s) is the source of heavy ions of nitrogen and other molecules, along with a significant amount of neutral hydrogen. The innumerable particles in Saturn’s rings, however, have been shown to be excellent absorbers of the charged particles in the magnetosphere. Absorption of electrons by the rings is so complete that a region beginning at the outermost of the three main rings (the A ring) and extending all the way to the planet’s surface is the most radiation-free zone in the solar system.

Uranus’s magnetosphere is generally similar in size and strength to Saturn’s, but not in composition. It contains little other than protons and electrons derived from hydrogen escaping from the planet’s atmosphere. The inner magnetosphere includes an extensive corona of protons similar to Earth’s Van Allen radiation belts. The absence of significant numbers of heavy ions in the magnetosphere may be attributable to the fact that the energies of the protons are lower than is the case with Jupiter and Saturn, consequently limiting their ability to sputter heavy ions off the surfaces of the satellites and rings. The magnetic axis is inclined 60 degrees to the planet’s rotational axis, but since Uranus’s rotational axis is out of phase radically with the rotational axis of the other planets, this places the magnetosphere in a fairly normal orientation to the plane of the solar system. Its polarity matches that of Jupiter and Saturn. Not only is the magnetic axis steeply inclined to the rotational axis, but also it is offset a distance equal to about one-third of the planet’s radius from the geometric center of Uranus. The radical inclination of the two axes gives rise to some unusual dynamics in the magnetosphere. As the magnetosphere rotates with the planet’s interior, it creates a corkscrewing effect in the plasma of the magnetotail.

Neptune’s magnetosphere was first revealed to the Voyager 2 spacecraft and made its presence known through weak nonthermal radiation with a sixteen-hour repetitive pattern that scientists believe is linked to the planet’s period of rotation. The field shares the same polarity as the fields of Earth and Mercury. Two unusual features reminiscent of Uranus are the magnetosphere’s rakish tilt of 47 degrees to the rotational axis and the location of the center of the field at a point fourteen thousand kilometers away from the geometric center of the planet. Particle density within the magnetosphere is the lowest of any of the planets, with no more than 1.4 protons and heavy ions per cubic centimeter present. Particle densities at Uranus and Jupiter are three and three thousand times greater, respectively.

Context

At the midpoint of the twentieth century, scientists believed that the Earth’s upper atmosphere trailed away to an utter and complete vacuum only a few hundred kilometers above the surface. Today, it is known that the atmosphere grades gradually into the huge magnetosphere, within which are pockets of surprisingly concentrated plasma as well as vast regions where little exists. The Earth’s magnetosphere and those of other planets are, in turn, embedded in the much larger solar magnetosphere, or heliosphere. This complex mesh of electrical and magnetic fields, electric currents, and particle flows becomes a conduit by which electromagnetic disturbances occurring on the sun are conveyed directly to the planet’s ionosphere, and even to its surface.

Some consequences of this linkage are relatively well known, such as the fact that high levels of solar activity interfere temporarily with shortwave radio broadcasts by altering the upper atmosphere’s reflectivity to radio waves. Solar disturbances also interfere with transmission of electric power and communications over long-distance cable systems. It is necessary to engineer such systems with features that prevent the disruption of service when Earth’s magnetosphere is in turmoil because of solar activity.

There is growing physical evidence suggesting a connection between solar activity and terrestrial weather. Historical evidence links a prolonged period of solar inactivity from 1645 to 1715 (known as the Maunder minimum) to severe climate changes. The mechanism for these linkages is not yet clear, but there is no doubt that the ionosphere responds profoundly to changes in the magnetosphere. Some scientists believe that changes in the ionosphere are conveyed to the lower atmosphere, where weather occurs, through an electrical current that flows between the upper atmosphere and the ground. This current, called the global circuit, is involved intimately in the thousands of thunderstorms occurring over Earth every day. It has also been suggested that there may be a connection between solar activity and geophysical events such as earthquakes.

Records in Earth’s rocks, particularly the iron-rich rocks of the seafloor, prove conclusively that Earth’s magnetic field has reversed polarity on many occasions. There is no cyclical pattern evident in the intervals between reversals, and their cause is not yet known. It has been speculated that major meteor impacts may cause the poles to “flip-flop.” However, the phenomenon may also be a response to changes in the sun’s polarity, transmitted through the charged heliosphere. Whether there are any environmental consequences to these polarity changes and whether they can be predicted still need to be determined.

On a cosmological scale, radio astronomy and space-age probes have demonstrated that the universe contains many vast structures of plasma organized around magnetic force fields. They are detectable by the radio energy they give off and are found to surround other stars and even entire galaxies. In fact, such structures may occur as frequently as the much more familiar gravitationally organized ones. Earth’s magnetosphere is, therefore, an important laboratory in which the behavior of ionized matter can be studied. It is reasonable to believe that as scientists understand plasmas and magnetospheres better, new insights will be gained, many of which may have important scientific and practical implications.

In July 2023, researchers detected the presence of massive swirling waves at the outer portion of the magnetic field of Jupiter. This was in an area separating Jupiter's magnetic field and incoming solar wind. The data was made available by NASA's Juno mission, which is a satellite in orbit around Jupiter. These waves indicated the delivery of plasma and energy from the sun into the magnetosphere of the planet.

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