The Astrophysics Spectator
The Astrophysics Spectator
Of the planets in the solar system, Jupiter has the strongest magnetic field. The magnetic field interacts with the solar wind to form a bubble that is called a magnetosphere, and within this bubble an energetic plasma emits radio waves to make Jupiter one of the brightest radio sources in the sky.
The magnetic field of Jupiter is nearly a dipole field that is tilted 10 to Jupiter's rotation axis. The magnetic field rotates with the planet. The strength of the magnetic field is estimated to be 4.2 Gauss at the equator and 10 to 14 Gauss at the poles; by way of comparison, Earth's magnetic field is 0.3 Gauss at the equator.
The basic theory for all magnetic field generation in astrophysics is the dynamo theory. In this theory, magnetic fields are created by the convection of a conducting fluid. In Jupiter, the conducting fluid is the metallic hydrogen of the inner mantle. The dynamo converts gravitational potential energy into magnetic field energy. As Jupiter shrinks, gravitational potential energy is converted into heat at the core of the planet. The hot fluid of the inner mantle is buoyant, so it rises, transferring some of its thermal energy into the kinetic energy of convective motion. Some of the energy in convective motion is extracted in the process of creating the magnetic field. The greater the kinetic energy of the convective motion, the greater the energy that is put into the magnetic field.
The cartoon of how the dynamo mechanism works is somewhat reminiscent of a taffy pull. In a perfect conductor, magnetic field lines are frozen to the fluid they pass through, so when an element of the fluid moves, it carries a piece of magnetic field with it. If there is a shear in the fluid perpendicular to the magnetic field lines, the magnetic field lines are stretched, and the magnetic field strength is amplified. It is through this mechanism that energy is transferred from a convective fluid to the magnetic field. A hard limit to the strength of the magnetic field is set by the amount of kinetic energy carried by the fluid. Other factors that limit the strength of the magnetic field are the electrical resistance within the fluid, the increased buoyancy of magnetized fluids, and the tendency of conductors to expel magnetic fields.
As with Earth, Jupiter's magnetic field creates a teardrop-shaped bubble in the solar wind around Jupiter. The boundary of this bubble is called the magnetopause. The magnetopause is the boundary between the plasma that is static within Jupiter's magnetic field and the solar wind. In the direction of the Sun, the magnetopause ranges from 45 to 110 Jupiter radii (3 to 7.7 million kilometers) from the planet. From 10 to 30 Jupiter radii (0.7 to 2 million kilometers) ahead of the magnetopause in the direction of the Sun is a shock caused by the supersonic solar wind striking the subsonic cushion of wind ahead of the magnetopause. The magnetosphere has a radius of from 150 to 200 Jupiter radii (10.5 to 15 million kilometers), and it can trail the planet for half a billion kilometers, although this length can vary dramatically over time.
The plasma within the magnetosphere is very energetic, with nonthermal electrons of energies above 30MeV and nonthermal ions of energies above 7MeV.1 The composition of the ions is of hydrogen, helium, oxygen, and sulfur. The first two elements are no surprise, given that both Jupiter and the solar wind are predominately hydrogen and helium. In the magnetosphere, the hydrogen is thought to come from Jupiter's atmosphere, and the helium comes from the solar wind. The sulfur, however, is quite surprising unless you pay attention to what the moons are doing. The oxygen and sulfur in the magnetosphere are from the moon Io, which has active volcanos powered by the tidal heating of Io in its orbit of Jupiter. When a volcano erupts on Io, it sends a fountain of sulfur and oxygen high above the surface, and some of this material is injected into the magnetosphere.
Much of the heating of the magnetospheric plasma is from the compression of the magnetic field as Jupiter rotates. As the magnetic field is drawn from the more spacious dark side of Jupiter to the sunlit side, where the solar wind pushed the boundary of the magnetosphere closer to Jupiter, the magnetic field is compressed. A fundamental property of magnetic fields is that when they change with time, an electric field is generated. This electric field accelerates plasma particles, heating the magnetospheric plasma. The energy that goes into the plasma is extracted from Jupiter's rotation.
Storms of radio bursts are emitted from Jupiter's magnetosphere in the 0.6 to 30 MHz range. Individual bursts last from seconds to minutes, and the storms last from one to two hours. This decameter radio emission is cyclotron emission, which is the emission produced by an electron spiraling in a magnetic field. These storms are synchronized to the orbit of Io around Jupiter; as Io moves through Jupiter's magnetic field, it behaves as a conductor, and it generates an electric field. This field drives a current that flows along the magnetic field lines to Jupiter's atmosphere in much the same way that a changing electric field drives a current along a coaxial cable. The radios emission is associated with this current.
1 The electron volt (eV) is a unit of energy. Formally, it is the amount of energy an electron acquires when it passes through a 1 volt potential. Optical photons carry an energy of roughly 1eV. The unit of measure MeV is 106eV, and it is the unit of energy encountered when characterizing gamma-rays. The rest mass energy of the electron is 0.511MeV.
