Magnetosphere - General Properties

General Properties

Two factors determine the structure and behavior of the magnetosphere: (1) The internal field of the Earth, and (2) The solar wind.

1. The internal field of the Earth (its "main field") appears to be generated in the Earth's core by a dynamo process, associated with the circulation of liquid metal in the core, driven by internal heat sources. Its major part resembles the field of a bar magnet ("dipole field") inclined by about 10° to the rotation axis of Earth, but more complex parts ("higher harmonics") also exist, as first shown by Carl Friedrich Gauss. The dipole field has an intensity of about 30,000-60,000 nanoteslas (nT) at the Earth's surface, and its intensity diminishes as the inverse of the cube of the distance, i.e. at a distance of 2 Earth radii it only amounts to 1/8 of the surface field in the same direction. Higher harmonics diminish faster, like higher powers of 1/R, making the dipole field the only important internal source in most of the magnetosphere.
2. The solar wind is a fast outflow of hot plasma from the sun in all directions. Above the sun's equator it typically attains 400 km/s; above the sun's poles, up to twice as much. The flow is powered by the million-degree temperature of the sun's corona, for which no generally accepted explanation exists yet. Its composition resembles that of the Sun—about 95% of the ions are protons, about 4% helium nuclei (alpha particles), with 1% of heavier matter (C, N, O, Ne, Si, Mg...up to Fe) and enough electrons to keep charge neutrality. At Earth's orbit its typical density is 6 ions/cm3 (variable, as is the velocity), and it contains a variable interplanetary magnetic field (IMF) of (typically) 2–5 nT. The IMF is produced by stretched-out magnetic field lines originating on the Sun, a process described in the article Geomagnetic storm.

Physical reasons make it difficult for solar wind plasma with its embedded IMF to mix with terrestrial plasma whose magnetic field has a different source. The two plasmas end up separated by a boundary, the magnetopause, and the Earth's plasma is confined to a cavity inside the flowing solar wind, the magnetosphere. The isolation is not complete, thanks to secondary processes such as magnetic reconnection—otherwise it would be hard for the solar wind to transmit much energy to the magnetosphere—but it still determines the overall configuration.

An additional feature is a collision-free bow shock which forms in the solar wind ahead of Earth, typically at 13.5 R_E on the sunward side. It forms because the solar velocity of the wind exceeds (typically 2–3 times) that of Alfvén waves, a family of characteristic waves with which disturbances propagate in a magnetized fluid. In the region behind the shock ("magnetosheath") the velocity drops briefly to the Alfvén velocity (and the temperature rises, absorbing lost kinetic energy), but the velocity soon rises back as plasma is dragged forward by the surrounding solar wind flow.

To understand the magnetosphere, one needs to visualize its magnetic field lines, that everywhere point in the direction of the magnetic field—e.g., diverging out near the magnetic north pole (or geographic southpole), and converging again around the magnetic south pole (or the geographic northpole), where they enter the Earth. They can be visualized like wires which tie the magnetosphere together—wires that also guide the motions of trapped particles, which slide along them like beads (though other motions may also occur).