Portal:Jupiter/Theory/1

Radiative dynamos

Main articles: Stars/Dynamos/Radiatives and Radiative dynamos

This is a diagram of the dynamo within Jupiter producing its axisymmetric dipole magnetic field. Credit: Robert MacDowall, Planetary Magnetospheres Laboratory, Code 695, GSFC, NASA.

"The interior of Jupiter is the seat of a strong dynamo that produces a surface magnetic field in the equatorial region with an intensity of ~ 4 Gauss. This strong magnetic field and Jupiter’s fast rotation (rotation period ~ 9 h 55 min) create a unique magnetosphere in the solar system which is known for its immense size (average subsolar magnetopause distance 45-100 R_J where 1 R_J = 71492 km is the radius of Jupiter) and fast rotation [...]. Jupiter’s magnetosphere differs from most other magnetospheres in the fact that it derives much of its plasma internally from Jupiter’s moon Io. The heavy plasma, consisting principally of various charge states of S and O, inflates the magnetosphere from the combined actions of centrifugal force and thermal pressure."

In "the absence of an internal heavy plasma, the dipole field would balance the average dynamic pressure of the solar wind (0.08 nPa) at a distance of ~ 42 R_J in the subsolar region [...] the observed average magnetopause location of ~ 75 R_J [...] The heavy plasma is also responsible for generating an azimuthal current exceeding 160 MA in the equatorial region of Jupiter’s magnetosphere where it is confined to a thin current sheet (half thickness ~ 2 R_J in the dawn sector)."

"The energization of plasma by various electrical fields as it diffuses inwards is responsible for the creation of radiation belts in the inner magnetosphere of Jupiter. It is believed that the radial diffusion is driven by the ionospheric dynamo fields produced by winds in Jupiter’s atmosphere"

"In situ and remote observations of Io and its surroundings from Voyager showed that Io is the main source of plasma in Jupiter’s magnetosphere [...] "

"It is estimated that upward of 6 × 10²⁹ amu/s (~ 1 ton/s) of plasma mass is added to the magnetosphere by Io. The picked-up plasma consists mostly of various charged states of S and O and populates a torus region extending from a radial distance of ~ 5.2 R_J to ~ 10 R_J."

"The next most important source of plasma in Jupiter’s magnetosphere is the solar wind whose source strength can be estimated by a consideration of the solar wind mass flux incident on Jupiter’s magnetopause and the fractional amount that makes it into the magnetosphere (< 1%). Such a calculation suggests that the solar wind source strength is < 100 kg/s (Hill et al. 1983) considerably lower than the Io source. Nevertheless, the number density of protons (as opposed to the mass density) may be comparable to the iogenic plasma number density in the middle and outer magnetospheres where the solar wind may be able to gain access to the magnetosphere."

"The escape of ions (mainly H⁺ and H₂⁺ ) from the ionosphere of Jupiter provides the next significant source of plasma in Jupiter’s magnetosphere. The ionospheric plasma escapes along field lines when the gravity of Jupiter is not able to contain the hot plasma (~ 10 eV and above). The escape however is not uniform and depends on the local photoelectron density, the temperature variations of the ionosphere with the solar zenith angle, other factors such as the auroral precipitation of ions and electrons and the ionospheric heating from Pedersen currents. In situ measurements show that in Io’s torus, protons contribute to less than 20% of total ion number density and constitute < 1% of mass suggesting that the ionosphere is not a major source of plasma in Jupiter’s magnetosphere. [The] ionospheric source strength [is] in the range of ~ 20 kg/s."

The "surface sputtering of the three icy satellites by jovian plasma provides the last significant source of plasma in Jupiter’s magnetosphere. Because the icy moons lack extended atmospheres and the fluxes of the incident plasma are low at the locations of these moons, the total pickup of plasma from these satellites is estimated to be less than 20 kg/s based on the plasma sputtering rates provided".

"Other minor constituents found in the torus [...] were Na⁺ (with an abundance of < 5%) and molecular ions SO⁺ and SO₂⁺ (both with abundances of < 1% of the total). The average mass of a torus ion is ~ 20 and the average fractional charge on an ion is ~ 1.2 [...]. The bulk velocity of the plasma was found to be ~ 75 km/s, close to the corotational value."