Magnetosphere of Jupiter - Interaction With Rings and Moons

Interaction With Rings and Moons

See also: Rings of Jupiter, Ganymedian magnetosphere, and Space weathering

Jupiter's extensive magnetosphere envelops its ring system and the orbits of all four Galilean satellites. Orbiting near the magnetic equator, these bodies serve as sources and sinks of magnetospheric plasma, while energetic particles from the magnetosphere alter their surfaces. The particles sputter off material from the surfaces and create chemical changes via radiolysis. The plasma's co-rotation with the planet means that the plasma preferably interacts with the moons' trailing hemispheres, causing noticeable hemispheric asymmetries. In addition, the large internal magnetic fields of the moons contribute to the Jovian magnetic field.

Close to Jupiter, the planet's rings and small moons absorb high-energy particles (energy above 10 keV) from the radiation belts. This creates noticeable gaps in the belts' spatial distribution and affects the decimetric synchrotron radiation. In fact, the existence of Jupiter's rings was first hypothesized on the basis of data from the Pioneer 11 spacecraft, which detected a sharp drop in the number of high-energy ions close to the planet. 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. The resonant interaction between the co-rotation and the orbital motion is thought to be responsible for the creation of Jupiter's innermost halo ring (located between 1.4 and 1.71 R_j), which 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 R_j, 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 Alfven 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. In effect, the Galilean moons (mainly Io) serve as the principal plasma sources in Jupiter's inner and middle magnetosphere. Meanwhile the energetic particles are largely unaffected by the Alfven wings and have free access to the moons' surfaces (except Ganymede's).

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 induction is thought to take place in subsurface layers of salty water, which are likely to exist in all of Jupiter's large icy moons. These underground oceans can potentially harbor life, and evidence for their presence was one of the most important discoveries made in the 1990s by spacecraft.

The interaction of the Jovian magnetosphere with Ganymede, which has an intrinsic magnetic moment, differs from its interaction with the non-magnetized moons. Ganymede's internal magnetic field carves a cavity inside Jupiter's magnetosphere with a diameter of approximately two Ganymede diameters, creating a mini-magnetosphere within Jupiter's magnetosphere. Ganymede's magnetic field diverts the co-rotating plasma flow around its magnetosphere. It also protects the moon's equatorial regions, where the field lines are closed, from energetic particles. The latter can still freely strike Ganymede's poles, where the field lines are open. 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.

Charged particles have a considerable influence on the surface properties of Galilean moons. Plasma originating from Io carries sulfur and sodium ions farther from the planet, 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). Energetic electrons and ions, with the flux of the latter being more isotropic, bombard surface ice, sputtering atoms and molecules off and causing radiolysis of water and other chemical compounds. The energetic particles break water into oxygen and hydrogen, maintaining the thin oxygen atmospheres of the icy moons (since the hydrogen escapes more rapidly). The compounds produced radiolytically on the surfaces of Galilean moons also include ozone and hydrogen peroxide. If organics or carbonates are present, carbon dioxide, methanol and carbonic acid can be produced as well. In the presence of sulfur, likely products include sulfur dioxide, hydrogen disulfide and sulfuric acid. 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.