3) The Earth's magnetosphere was discovered in 1958 by Explorer 1.
4) The magnetosphere of Earth is a region in space whose shape is determined by the Earth's internal magnetic field, the solar wind plasma, and the interplanetary magnetic field (IMF). In the magnetosphere, a mix of free ions and electrons from both the solar wind and the Earth's ionosphere is confined by electromagnetic forces that are much stronger than gravity and collisions.
5) Despite its name, the magnetosphere is distinctly non-spherical. All known planetary magnetospheres in the solar system possess more of an oval tear-drop shape because of the solar wind.
6) On the side facing the Sun, the distance to its boundary (which varies with solar wind intensity) is about 70,000 km (10-12 Earth radii or RE, where 1 RE = 6371 km; unless otherwise noted, all distances here are from the Earth's center).
7) The boundary of the magnetosphere ("magnetopause") is roughly bullet shaped, about 15 RE abreast of Earth and on the night side (in the "magnetotail" or "geotail") approaching a cylinder with a radius 20-25 RE.
8) The tail region stretches well past 200 RE, and the way it ends is not well-known.
9) The outer neutral gas envelope of Earth, or geocorona, consists mostly of the lightest atoms, hydrogen and helium, and continues beyond 4-5 RE, with diminishing density.
10) The hot plasma ions of the magnetosphere acquire electrons during collisions with these atoms and create an escaping "glow" of energetic neutral atoms (ENAs) that have been used to image the hot plasma clouds by the IMAGE and TWINS missions.
11) The upward extension of the ionosphere, known as the plasmasphere, also extends beyond 4-5 RE with diminishing density, beyond which it becomes a flow of light ions called the polar wind that escapes out of the magnetosphere into the solar wind.
12) Energy deposited in the ionosphere by auroras strongly heats the heavier atmospheric components such as oxygen and molecules of oxygen and nitrogen, which would not otherwise escape from Earth's gravity. Owing to this highly variable heating, however, a heavy atmospheric or ionospheric outflow of plasma flows during disturbed periods from the auroral zones into the magnetosphere, extending the region dominated by terrestrial material, known as the fourth or plasma geosphere, at times out to the magnetopause.
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 like the inverse of the cube of the distance, i.e. at a distance of R 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, 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 RE 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).
Classification of magnetic fields
Regardless of whether they are viewed as sources or consequences of the magnetospheric field structure, electric currents flow in closed circuits. That makes them useful for classifying different parts of the magnetic field of the magnetosphere, each associated with a distinct type of circuit. In this way the field of the magnetosphere is often resolved into 5 distinct parts, as follows.
1. The internal field of the Earth ("main field") arising from electric currents in the core. It is dipole-like, modified by higher harmonic contributions.
2. The ring current field, carried by plasma trapped in the dipole-like field around Earth, typically at distances 3–8 RE (less during large storms). Its current flows (approximately) around the magnetic equator, mainly clockwise when viewed from north. (A small counterclockwise ring current flows at the inner edge of the ring, caused by the fall-off in plasma density as Earth is approached.)
3. The field confining the Earth's plasma and magnetic field inside the magnetospheric cavity. The currents responsible for it flow on the magnetopause, the interface between the magnetosphere and the solar wind, described in the introduction. Their flow, again, may be viewed as arising from the geometry of the magnetic field (rather than from any driving voltage), a consequence of "Ampére's law" (embodied in Maxwell's equations) which in this case requires an electric current to flow along any interface between magnetic fields of different directions and/or intensities.
4. The system of tail currents. The magnetotail consists of twin bundles of oppositely directed magnetic field (the "tail lobes"), directed earthwards in the northern half of the tail and away from Earth in the southern half. In between the two exists a layer ("plasma sheet") of denser plasma (0.3-0.5 ions/cm3 versus 0.01-0.02 in the lobes), and because of the difference between the adjoining magnetic fields, by Ampére's law an electric current flows there too, directed from dawn to dusk. The flow closes (as it must) by following the tail magnetopause—part over the northern lobe, part over the southern one.
5. The Birkeland current field (and its branches in the ionosphere and ring current), a circuit is associated with the polar aurora. Unlike the 3 preceding current systems, it does require a constant input of energy, to provide the heating of its ionospheric path and the acceleration of auroral electrons and of positive ions. The energy probably comes from a dynamo process, meaning that part of the circuit threads a plasma moving relative to Earth, either in the solar wind and in "boundary layer" flows which it drives just inside the magnetopause, or by plasma moving earthward in the magnetotail, as observed during substorms (below).
Posted on: March 7, 2008 12:15 PM, by Chris Rowan
The Earth's magnetic field may approximate to a simple dipole, but explaining precisely how that dipole is generated and maintained is not simple at all. The field originates deep in the Earth, where temperatures are far too high for any material to maintain a permanent magnetisation; the dynamism that is apparent from the wandering of the magnetic poles with respect to the spin axis (secular variation), and the quasi-periodic flips in field polarity, also suggest that some process is actively generating and maintaining the geomagnetic field. Geophysicists therefore look to the most dynamic region in the planetary depths, the molten outer core, as the source of the force that directs our compass needles.