Monday, January 29, 2007

Earth's Magnetosphere


The magnetosphere of Earth is a region in space whose shape is primarily determined by the distortion of Earth's internal magnetic field and by the solar wind plasma and the interplanetary magnetic field (IMF). In the magnetosphere, a mix of free ions and electrons is held mainly by magnetic and electric forces that are much stronger than gravity and collisions.

On the side facing the Sun, the distance to its boundary (which can vary) is about 70,000 km or between 10 - 12 Earth radii (or RE, where 1 RE=6371 km). 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. The tail region stretches well past 200 RE, and the way it ends is not known.

The neutral gas envelope of Earth ("geocorona") continues to about 4-5 RE, with diminishing density and minimal interaction with the plasmas of the magnetosphere. So does the upwards extension of the ionosphere, known as the plasmasphere.

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 Gauss. The dipole field has an intensity of about 30,000-60,000 nanotesla (nT) at the Earth's surface, and its intensity diminishes like the inverse of the cube of the distance.

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 as yet. Its composition resembles that of the Sun - about 95% of the ions are protons, about 4% helium nuclei, with 1% of heavier matter - and enough electrons to keep charge neutrality.

A magnetic tail is formed by solar winds blowing electrified gases, plasma, trapped in a planet's magnetosphere away from the sun. The magnetic tail can extend great distances away from its originating planet. Earth's magnetic tail extends beyond the orbit of the Moon, while Jupiter's magnetic tail is believed to extend beyond the orbit of Saturn. The plasma in the tail is revolving, reaching the end of the tail and then folding back in on itself and returning to the planet it originated from.

There are also gaps in the magnetic tail, called troughs, where no stream of material exists. These troughs change in size and location, and can reconnect at later points in the tail. The night-side magnetic tail can sometimes whip violently back, throwing large amounts of superheated plasma and highly charged particles at the originating planet.

Magnetic fields from currents that circulate in the magnetospheric plasma extend the Earth's magnetism much further in space than would be predicted from the Earth's internal field alone. Such currents also determine the field's structure far from Earth, creating the regions described in the introduction above.

Similarly, in everyday applications, electric currents always require a "voltage" to drive them, a sort of electric pressure difference (a pressure known as "electric potential"), similar to the pressure difference that drives water along a pipe. Ohm's law is observed to hold fairly well in metallic conductors used by electric technology (e.g. wires) and it predicts a current proportional to voltage. Double the voltage and the current doubles, remove it and no current can flow.

Not so in the magnetosphere (and in many plasmas) where currents (with one important exception) need no voltage to drive them. Any electric current is the transport of electric charge, but in many cases, such transport is already implied by the structure of the field and the plasma. For instance, electrons and positive ions trapped in the dipole-like field near the Earth tend to circulate around the magnetic axis of the dipole (the line connecting the magnetic poles), without gaining or losing energy (see "Guiding center motion").

Viewed from above the northern magnetic pole, ions circulate clockwise, electrons counterclockwise, producing a net circulating clockwise current, known (from its shape) as the ring current. No voltage is needed--the current arises naturally from the motion of the ions and electrons in the magnetic field.

Any such current will modify the magnetic field. The ring current, for instance, strengthens the field on its outside, helping expand the size of the magnetosphere. At the same time, it weakens the magnetic field in its interior. In a magnetic storm, plasma is added to the ring current, making it temporarily stronger, and the field at Earth is observed to weaken by up to 1-2%.

The deformation of the magnetic field, and the flow of electric currents in it, are intimately linked, making it often hard to label one as cause and the other as effect. Frequently (as in the magnetopause and the magnetotail) it is intuitively more useful to regard the distribution and flow of plasma as the primary effect, producing the observed magnetic structure, with the associated electric currents just one feature of those structures, more of a consistency requirement of the magnetic structure.

As noted, one exception (at least) exists, a case where voltages do drive currents. That happens with Birkeland currents, which flow from distant space into the near-polar ionosphere, continue at least some distance in the ionosphere, and then return to space. (Part of the current then detours and leaves Earth again along field lines on the morning side, flows across midnight as part of the ring current, then comes back to the ionosphere along field lines on the evening side and rejoins the pattern.) The full circuit of those currents, under various conditions, is still under debate.

Because the ionosphere is an ohmic conductor of sorts, such flow will heat it up. It will also give rise to secondary Hall currents, and accelerate magnetospheric particles - electrons in the arcs of the polar aurora, and singly-ionized oxygen ions (O+) which contribute to the ring current.

Labels: , ,