Magnetosphere of Jupiter

The magnetosphere of Jupiter is the cavity created in the solar wind by Jupiter's magnetic field. Extending up to seven million kilometers in the Sun's direction and almost to the orbit of Saturn in the opposite direction, Jupiter's magnetosphere is the largest and most powerful of any planetary magnetosphere in the Solar System, and by volume the largest known continuous structure in the Solar System after the heliosphere. Wider and flatter than the Earth's magnetosphere, Jupiter's is stronger by an order of magnitude, while its magnetic moment is roughly 18,000 times larger. The existence of Jupiter's magnetic field was first inferred from observations of radio emissions at the end of the 1950s and was directly observed by the Pioneer 10 spacecraft in 1973.
Jupiter's internal magnetic field is generated by electrical currents in the planet's outer core, which is theorized to be composed of liquid metallic hydrogen. Volcanic eruptions on Jupiter's moon Io eject large amounts of sulfur dioxide gas into space, forming a large torus around the planet. Jupiter's magnetic field forces the torus to rotate with the same angular velocity and direction as the planet. The torus in turn loads the magnetic field with plasma, in the process stretching it into a pancake-like structure called a magnetodisk. In effect, Jupiter's magnetosphere is internally driven, shaped primarily by Io's plasma and its own rotation, rather than by the solar wind as at Earth's magnetosphere. Strong currents in the magnetosphere generate permanent aurorae around the planet's poles and intense variable radio emissions, which means that Jupiter can be thought of as a very weak radio pulsar. Jupiter's aurorae have been observed in almost all parts of the electromagnetic spectrum, including infrared, visible, ultraviolet and soft X-rays.
The action of the magnetosphere traps and accelerates particles, producing intense belts of radiation. The interaction of energetic particles with the surfaces of Jupiter's largest moons markedly affects their chemical and physical properties. Those same particles also affect and are affected by the motions of the particles within Jupiter's tenuous planetary ring system. Radiation belts present a significant hazard for spacecraft and potentially to human space travellers.

Structure

Jupiter's magnetosphere is a complex structure comprising a bow shock, magnetosheath, magnetopause, magnetotail, magnetodisk, and other components. The magnetic field around Jupiter emanates from a number of different sources, including fluid circulation at the planet's core, electrical currents in the plasma surrounding Jupiter, and the currents flowing at the boundary of the planet's magnetosphere. The magnetosphere is embedded within the plasma of the solar wind, which carries the interplanetary magnetic field.

Internal magnetic field

The bulk of Jupiter's magnetic field, like Earth's, is generated by an internal dynamo supported by the circulation of a conducting fluid in its outer core. But whereas Earth's core is made of molten iron and nickel, Jupiter's is composed of metallic hydrogen. As with Earth's, Jupiter's magnetic field is mostly a dipole, with north and south magnetic poles at the ends of a single magnetic axis. On Jupiter the north pole of the dipole is located in the planet's northern hemisphere and the south pole of the dipole lies in its southern hemisphere. This is opposite from the Earth. Jupiter's field also has quadrupole, octupole and higher components, though they are less than one-tenth as strong as the dipole component.
The dipole is tilted roughly 10° from Jupiter's axis of rotation; the tilt is similar to that of the Earth. Its equatorial field strength is about 417.0 μT, which corresponds to a dipole magnetic moment of about 2.83 T·m³. This makes Jupiter's magnetic field about 20 times stronger than Earth's, and its magnetic moment ~20,000 times larger. Jupiter's magnetic field rotates at the same speed as the region below its atmosphere, with a period of 9 h 55 m. No changes in its strength or structure had been observed since the first measurements were taken by the Pioneer spacecraft in the mid-1970s, until 2019. Analysis of observations from the Juno spacecraft show a small but measurable change from the planet's magnetic field observed during the Pioneer era. In particular, Jupiter has a region of strongly non-dipolar field, known as the "Great Blue Spot", near the equator. This may be roughly analogous to the Earth's South Atlantic Anomaly. This region shows signs of large secular variations.

Size and shape

Jupiter's internal magnetic field prevents the solar wind, a stream of ionized particles emitted by the Sun, from interacting directly with its atmosphere, and instead diverts it away from the planet, effectively creating a cavity in the solar wind flow, called a magnetosphere, composed of a plasma different from that of the solar wind. The Jovian magnetosphere is so large that the Sun and its visible corona would fit inside it with room to spare. If one could see it from Earth, it would appear five times larger than the full moon in the sky despite being nearly 1700 times farther away.
As with Earth's magnetosphere, the boundary separating the denser and colder solar wind's plasma from the hotter and less dense one within Jupiter's magnetosphere is called the magnetopause. The distance from the magnetopause to the center of the planet is from 45 to 100 R_J at the subsolar point—the unfixed point on the surface at which the Sun would appear directly overhead to an observer. The position of the magnetopause depends on the pressure exerted by the solar wind, which in turn depends on solar activity. In front of the magnetopause lies the bow shock, a wake-like disturbance in the solar wind caused by its collision with the magnetosphere. The region between the bow shock and magnetopause is called the magnetosheath.
At the opposite side of the planet, the solar wind stretches Jupiter's magnetic field lines into a long, trailing magnetotail, which sometimes extends well beyond the orbit of Saturn. The structure of Jupiter's magnetotail is similar to Earth's. It consists of two lobes, with the magnetic field in the southern lobe pointing toward Jupiter, and that in the northern lobe pointing away from it. The lobes are separated by a thin layer of plasma called the tail current sheet.
The shape of Jupiter's magnetosphere described above is sustained by the neutral sheet current, which flows with Jupiter's rotation through the tail plasma sheet, the tail currents, which flow against Jupiter's rotation at the outer boundary of the magnetotail, and the magnetopause currents, which flow against rotation along the dayside magnetopause. These currents create the magnetic field that cancels the internal field outside the magnetosphere. They also interact substantially with the solar wind.
Jupiter's magnetosphere is traditionally divided into three parts: the inner, middle and outer magnetosphere. The inner magnetosphere is located at distances closer than 10 R_J from the planet. The magnetic field within it remains approximately dipole, because contributions from the currents flowing in the magnetospheric equatorial plasma sheet are small. In the middle and outer magnetospheres, the magnetic field is not a dipole, and is seriously disturbed by its interaction with the plasma sheet.

Role of Io

Although overall the shape of Jupiter's magnetosphere resembles that of the Earth's, closer to the planet its structure is very different. Jupiter's volcanically active moon Io is a strong source of plasma in its own right, and loads Jupiter's magnetosphere with as much as 1,000 kg of new material every second. Strong volcanic eruptions on Io emit huge amounts of sulfur dioxide, a major part of which is dissociated into atoms and ionized by electron impacts and, to a lesser extent, solar ultraviolet radiation, producing ions of sulfur and oxygen. Further electron impacts produce higher charge state, resulting in a plasma of S⁺, O⁺, S²⁺, O²⁺ and S³⁺. They form the Io plasma torus: a thick and relatively cool ring of plasma encircling Jupiter, located near Io's orbit. The plasma temperature within the torus is 10–100 eV, which is much lower than that of the particles in the radiation belts—10 keV. The plasma in the torus is forced into co-rotation with Jupiter, meaning both share the same period of rotation. The Io torus fundamentally alters the dynamics of the Jovian magnetosphere.
As a result of several processes—diffusion and interchange instability being the main escape mechanisms—the plasma slowly leaks away from Jupiter. As the plasma moves further from the planet, the radial currents flowing within it gradually increase its velocity, maintaining co-rotation. These radial currents are also the source of the magnetic field's azimuthal component, which as a result bends back against the rotation. The particle number density of the plasma decreases from around 2,000 cm⁻³ in the Io torus to about 0.2 cm⁻³ at a distance of 35 R_J. In the middle magnetosphere, at distances greater than 10 R_J from Jupiter, co-rotation gradually breaks down and the plasma begins to rotate more slowly than the planet. Eventually at the distances greater than roughly 40 R_J this plasma is no longer confined by the magnetic field and leaves the magnetosphere through the magnetotail. As cold, dense plasma moves outward, it is replaced by hot, low-density plasma, with temperatures of up to 20 keV moving in from the outer magnetosphere. Some of this plasma, adiabatically heated as it approaches Jupiter, may form the radiation belts in Jupiter's inner magnetosphere.

Magnetodisk

While Earth's magnetic field is roughly teardrop-shaped, Jupiter's is flatter, more closely resembling a disk, and "wobbles" periodically about its axis. The main reasons for this disk-like configuration are the centrifugal force from the co-rotating plasma and thermal pressure of hot plasma, both of which act to stretch Jupiter's magnetic field lines, forming a flattened pancake-like structure, known as the magnetodisk, at the distances greater than 20 R_J from the planet. The magnetodisk has a thin current sheet at the middle plane, approximately near the magnetic equator. The magnetic field lines point away from Jupiter above the sheet and towards Jupiter below it. The load of plasma from Io greatly expands the size of the Jovian magnetosphere, because the magnetodisk creates an additional internal pressure which balances the pressure of the solar wind. In the absence of Io the distance from the planet to the magnetopause at the subsolar point would be no more than 42 R_J, whereas it is actually 75 R_J on average.
The configuration of the magnetodisk's field is maintained by the azimuthal ring current, which flows with rotation through the equatorial plasma sheet. The Lorentz force resulting from the interaction of this current with the planetary magnetic field creates a centripetal force, which keeps the co-rotating plasma from escaping the planet. The total ring current in the equatorial current sheet is estimated at 90–160 million amperes.