Magnetosphere of Jupiter

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 (the internal field), 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. The dipole is tilted roughly 10° from Jupiter's axis of rotation; the tilt is similar to that of the Earth (11.3°). 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. In front of the magnetopause (at a distance from 80 to 130 RJ from the planet's center) 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. The shape of Jupiter's magnetosphere described above is sustained by the neutral sheet current (also known as the magnetotail 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 (or Chapman–Ferraro currents), which flow against rotation along the dayside magnetopause. 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 RJ 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 (between 10 and 40 RJ) and outer (further than 40 RJ) magnetospheres, the magnetic field is not a dipole, and is seriously disturbed by its interaction with the plasma sheet (see magnetodisk below). 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. 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. In the middle magnetosphere, at distances greater than 10 RJ from Jupiter, co-rotation gradually breaks down and the plasma begins to rotate more slowly than the planet. As cold, dense plasma moves outward, it is replaced by hot, low-density plasma, with temperatures of up to 20 keV (200 million K) or higher) moving in from the outer magnetosphere. may form the radiation belts in Jupiter's inner magnetosphere. The magnetodisk has a thin current sheet at the middle plane, 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. == Dynamics ==

Co-rotation and radial currents The main driver of Jupiter's magnetosphere is the planet's rotation. In this respect Jupiter is similar to a device called a Unipolar generator. When Jupiter rotates, its ionosphere moves relatively to the dipole magnetic field of the planet. Because the dipole magnetic moment points in the direction of the rotation, As a result, the poles become negatively charged and the regions closer to the equator become positively charged. Since the magnetosphere of Jupiter is filled with highly conductive plasma, the electrical circuit is closed through it. The strong direct current flowing into the magnetodisk originates in a very limited latitudinal range of about ° from the Jovian magnetic poles. These narrow circular regions correspond to Jupiter's main auroral ovals. (See below.) The return current flowing from the outer magnetosphere beyond 50 RJ enters the Jovian ionosphere near the poles, closing the electrical circuit. The total radial current in the Jovian magnetosphere is estimated at 60 million–140 million amperes. process, which separates the magnetic field from the plasma. The plasma flowing down the tail along the open field lines is called the planetary wind. The reconnection events are analogues to the magnetic substorms in the Earth's magnetosphere. Conversely, in Jupiter's magnetosphere the rotational energy is stored in the magnetodisk and released when a plasmoid separates from it. particularly as a source of high-energy protons. however, it could be especially strong at times of elevated solar activity. The auroral radio, optical and X-ray emissions, as well as synchrotron emissions from the radiation belts all show correlations with solar wind pressure, indicating that the solar wind may drive plasma circulation or modulate internal processes in the magnetosphere. == Emissions ==

Aurorae Jupiter demonstrates bright, persistent aurorae around both poles. Unlike Earth's aurorae, which are transient and only occur at times of heightened solar activity, Jupiter's aurorae are permanent, though their intensity varies from day to day. They consist of three main components: the main ovals, which are bright, narrow (less than 1000 km in width) circular features located at approximately 16° from the magnetic poles; Auroral emissions have been detected in almost all parts of the electromagnetic spectrum from radio waves to X-rays (up to 3 keV); they are most frequently observed in the mid-infrared (wavelength 3–4 μm and 7–14 μm) and far ultraviolet spectral regions (wavelength 120–180 nm). The main ovals are the dominant part of the Jovian aurorae. They have roughly stable shapes and locations, As mentioned above, the main ovals are maintained by the strong influx of electrons accelerated by the electric potential drops between the magnetodisk plasma and the Jovian ionosphere. These electrons carry field aligned currents, which maintain the plasma's co-rotation in the magnetodisk. The total energy input into the ionosphere is 10–100 TW. In addition, the currents flowing in the ionosphere heat it by the process known as Joule heating. This heating, which produces up to 300 TW of power, is responsible for the strong infrared radiation from the Jovian aurorae and partially for the heating of the thermosphere of Jupiter. Spots were found to correspond to the Galilean moons Io, Europa and Ganymede and Callisto. They develop because the co-rotation of the plasma interacts with the moons and is slowed in their vicinity. The brightest spot belongs to Io, which is the main source of the plasma in the magnetosphere (see above). The Ionian auroral spot is thought to be related to Alfvén currents flowing from the Jovian to Ionian ionosphere. Europa's is similar but much dimmer, because it has a more tenuous atmosphere and is a weaker plasma source. Europa's atmosphere is produced by sublimation of water ice from its surfaces, rather than the volcanic activity which produces Io's atmosphere. Ganymede has an internal magnetic field and a magnetosphere of its own. The interaction between this magnetosphere and that of Jupiter produces currents due to magnetic reconnection. The auroral spot associated with Callisto is probably similar to that of Europa. Normally, magnetic field lines connected to Callisto touch Jupiter's atmosphere very close to or along the main auroral oval, making it difficult to detect Callisto's auroral spot. In September 2025, the scientists running the Juno spacecraft finally confirmed the existence Callisto's footprint on Jupiter's auroras after intensively analyzing data that Juno obtained back in September 2019. The reading happened during a time when a massive solar stream buffeted the Jovian magnetosphere which temporarily revealed Callisto's obscure auroral footprint. Bright arcs and spots sporadically appear within the main ovals. These transient phenomena are thought to be related to interaction with either the solar wind or the dynamics of the outer magnetosphere. The polar auroral emissions could be similar to those observed around Earth's poles: appearing when electrons are accelerated towards the planet by potential drops, during reconnection of solar magnetic field with that of the planet. Jupiter at radio wavelengths Jupiter is a powerful source of radio waves in the spectral regions stretching from several kilohertz to tens of megahertz. Radio waves with frequencies of less than about 0.3 MHz (and thus wavelengths longer than 1 km) are called the Jovian kilometric radiation or KOM. Those with frequencies in the interval of 0.3–3 MHz (with wavelengths of 100–1000 m) are called the hectometric radiation or HOM, while emissions in the range 3–40 MHz (with wavelengths of 10–100 m) are referred to as the decametric radiation or DAM. The latter radiation was the first to be observed from Earth, and its approximately 10-hour periodicity helped to identify it as originating from Jupiter. The strongest part of decametric emission, which is related to Io and to the Io–Jupiter current system, is called Io-DAM. The majority of these emissions are thought to be produced by a mechanism called "cyclotron maser instability", which develops close to the auroral regions. Electrons moving parallel to the magnetic field precipitate into the atmosphere while those with a sufficient perpendicular velocity are reflected by the converging magnetic field. This results in an unstable velocity distribution. This velocity distribution spontaneously generates radio waves at the local electron cyclotron frequency. The electrons involved in the generation of radio waves are probably those carrying currents from the poles of the planet to the magnetodisk. The intensity of Jovian radio emissions usually varies smoothly with time. However, there are short and powerful bursts (S bursts) of emission superimposed on the more gradual variations and which can outshine all other components. The total emitted power of the DAM component is about 100 GW, while the power of all other HOM/KOM components is about 10 GW. In comparison, the total power of Earth's radio emissions is about 0.1 GW. This periodical modulation is probably related to asymmetries in the Jovian magnetosphere, which are caused by the tilt of the magnetic moment with respect to the rotational axis as well as by high-latitude magnetic anomalies. The physics governing Jupiter's radio emissions is similar to that of radio pulsars. They differ only in the scale, and Jupiter can be considered a very small radio pulsar too. while the leading contribution comes from the electrons with energy in the range 1–20 MeV. This radiation is well understood and was used since the beginning of the 1960s to study the structure of the planet's magnetic field and radiation belts. The particles in the radiation belts originate in the outer magnetosphere and are adiabatically accelerated, when they are transported to the inner magnetosphere. These streams are highly collimated and vary with the rotational period of the planet like the radio emissions. In this respect as well, Jupiter shows similarity to a pulsar. == Interaction with rings and moons ==

Jupiter's extensive magnetosphere envelops its ring system and the orbits of all four Galilean satellites. The plasma's co-rotation with the planet means that the plasma preferably interacts with the moons' trailing hemispheres, causing noticeable hemispheric asymmetries. Close to Jupiter, the planet's rings and small moons absorb high-energy particles (energy above 10 keV) from the radiation belts. The planetary magnetic field strongly influences the motion of sub-micrometer ring particles as well, which acquire an electrical charge under the influence of solar ultraviolet radiation. Their behavior is similar to that of co-rotating ions. Resonant interactions between the co-rotation and the particles' orbital motion has been used to explain the creation of Jupiter's innermost halo ring (located between 1.4 and 1.71 RJ). This ring consists of sub-micrometer particles on highly inclined and eccentric orbits. The particles originate in the main ring; however, when they drift toward Jupiter, their orbits are modified by the strong 3:2 Lorentz resonance located at 1.71 RJ, which increases their inclinations and eccentricities. Another 2:1 Lorentz resonance at 1.4 Rj defines the inner boundary of the halo ring. All Galilean moons have thin atmospheres with surface pressures in the range 0.01–1 nbar, which in turn support substantial ionospheres with electron densities in the range of 1,000–10,000 cm−3. The co-rotational flow of cold magnetospheric plasma is partially diverted around them by the currents induced in their ionospheres, creating wedge-shaped structures known as Alfvén wings. The interaction of the large moons with the co-rotational flow is similar to the interaction of the solar wind with the non-magnetized planets like Venus, although the co-rotational speed is usually subsonic (the speeds vary from 74 to 328 km/s), which prevents the formation of a bow shock. The pressure from the co-rotating plasma continuously strips gases from the moons' atmospheres (especially from that of Io), and some of these atoms are ionized and brought into co-rotation. This process creates gas and plasma tori in the vicinity of moons' orbits with the Ionian torus being the most prominent. The icy Galilean moons, Europa, Ganymede and Callisto, all generate induced magnetic moments in response to changes in Jupiter's magnetic field. These varying magnetic moments create dipole magnetic fields around them, which act to compensate for changes in the ambient field. The interaction of the Jovian magnetosphere with Ganymede, which has an intrinsic magnetic moment, differs from its interaction with the non-magnetized moons. Some of the energetic particles are trapped near the equator of Ganymede, creating mini-radiation belts. Energetic electrons entering its thin atmosphere are responsible for the observed Ganymedian polar aurorae. where they are implanted preferentially on the trailing hemispheres of Europa and Ganymede. On Callisto however, for unknown reasons, sulfur is concentrated on the leading hemisphere. Plasma may also be responsible for darkening the moons' trailing hemispheres (again, except Callisto's). Oxidants produced by radiolysis, like oxygen and ozone, may be trapped inside the ice and carried downward to the oceans over geologic time intervals, thus serving as a possible energy source for life. == Discovery ==

provided the first in situ and definitive discovery of the Jovian magnetosphere The first evidence for the existence of Jupiter's magnetic field came in 1955, with the discovery of the decametric radio emission or DAM. As the DAM's spectrum extended up to 40 MHz, astronomers concluded that Jupiter must possess a magnetic field with a maximum strength of above 1 milliteslas (10 gauss). In 1959, observations in the microwave part of the electromagnetic (EM) spectrum (0.1–10 GHz) led to the discovery of the Jovian decimetric radiation (DIM) and the realization that it was synchrotron radiation emitted by relativistic electrons trapped in the planet's radiation belts. These synchrotron emissions were used to estimate the number and energy of the electrons around Jupiter and led to improved estimates of the magnetic moment and its tilt. The definitive discovery of the Jovian magnetic field occurred in December 1973, when the Pioneer 10 spacecraft flew near the planet. == Exploration after 1970 ==

As of 2009 a total of eight spacecraft have flown around Jupiter and all have contributed to the present knowledge of the Jovian magnetosphere. The first space probe to reach Jupiter was Pioneer 10 in December 1973, which passed within 2.9 RJ The level of radiation at Jupiter was ten times more powerful than Pioneer's designers had predicted, leading to fears that the probe would not survive; however, with a few minor glitches, it managed to pass through the radiation belts, saved in large part by the fact that Jupiter's magnetosphere had "wobbled" slightly upward at that point, moving away from the spacecraft. However, Pioneer 11 did lose most images of Io, as the radiation had caused its imaging photo polarimeter to receive a number of spurious commands. The subsequent and far more technologically advanced Voyager spacecraft had to be redesigned to cope with the massive radiation levels. Voyagers 1 and 2 arrived at Jupiter in 1979 and traveled almost in its equatorial plane. Voyager 1, which passed within 5 RJ from the planet's center, Voyager 2 passed within 10 RJ When the Cassini spacecraft flew by Jupiter in 2000, it conducted coordinated measurements with Galileo. In July 2016 Juno was inserted into Jupiter orbit, its scientific objectives include exploration of Jupiter's polar magnetosphere. The coverage of Jupiter's magnetosphere remains much poorer than for Earth's magnetic field. Further study is important to further understand the Jovian magnetosphere's dynamics. ==Exploration after 2010==

The Juno New Frontiers mission to Jupiter was launched in 2011 and arrived at Jupiter in 2016. It includes a suite of instruments designed to better understand the magnetosphere, including a magnetometer as well as other devices such as a detector for plasma and radio waves called Waves. The Jovian Auroral Distributions Experiment (JADE) instrument should also help to understand the magnetosphere. Juno revealed a planetary magnetic field rich in spatial variation, possibly due to a relatively large dynamo radius. The most surprising observation until late 2017 was the absence of the expected magnetic signature of intense field aligned currents (Birkeland currents) associated with the main aurora. One of the goals of the European Space Agency's Jupiter Icy Moons Explorer (JUICE) mission, launched April, 2023, is to understand the magnetic field from Ganymede and how it impacts Jupiter. Tianwen-4 is a proposed Chinese mission that will either explore the moon Callisto or gather more information on Io. == Notes ==

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