The region surrounding Solar System bodies having magnetic fields, in which the field is confined under the influence of the streaming solar wind. It is a teardrop-shaped region whose size and shape are constantly readjusting to the variations of the solar wind. Charged particles from both solar wind and Earth's atmosphere are stored in the terrestrial magnetosphere, which has been extensively explored since Van Allen radiation belts were discovered by Explorer 1 in 1958. Stored particles are periodically ejected into N and S regions of the atmosphere along the magnetic field and accelerated to high speeds by mechanisms which are poorly understood. Collisions with atmospheric atoms cause emissions of light seen as aurora. Other planets known to have magnetospheres include Jupiter, Saturn, Uranus, Neptune, and Mercury.
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A magnetosphere is the region around an astronomical object in which phenomena are dominated or organized by its magnetic field. The term "magnetosphere" has also been used to describe regions dominated by the magnetic fields of celestial objects, e.g. Before this, scientists knew that electric currents flowed in space, because solar eruptions sometimes led to "magnetic storm" disturbances.
In 1959 Thomas Gold proposed the name magnetosphere, when he wrote:
"The region above the ionosphere in which the magnetic field of the earth has a dominant control over the motions of gas and fast charged particles is known to extend out to a distance of the order of 10 earth radii; 1219/1Earth'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). The
motion of particles trapped in the magnetosphere (MOT), physics of magnetic storms and plasma flows (MSPF), and the history of magnetospheric research (HIST)will be covered separately.
General properties
Two factors determine the structure and behavior of the magnetosphere: (1) The internal field of the Earth, and (2) The solar wind.
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, i.e. at a distance of R Earth radii it only amounts to 1/R3 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. 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 section on magnetic storms and plasma flows, referred to in what follows as simply MSPF.Physical reasons (MSPF) 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.
To understand the magnetosphere, one needs to visualize its magnetic field lines (or "lines of force"), lines that everywhere point in the direction of the magnetic force--e.g., diverging out near the southern magnetic pole, and converging again around the north magnetic pole, where they enter the Earth.
Radiation belts
When the first scientific satellites were launched in the first half of 1958--Explorers 1 and 3 by the US, Sputnik 3 by the Soviet Union--they observed an intense (and unexpected) radiation belt around Earth, held by its magnetic field. It is centered on field lines crossing the equator about 1.5 RE from the Earth's center.
Later a population of trapped ions and electrons was observed on field lines crossing the equator at 2.5-8 RE.
The trapping of charged particles in a magnetic field can be quite stable. If plasma is pushed hard enough, it generates electric fields which allow it to move in response to the push, often (not always) deforming the magnetic field in the process.
Magnetic Tails
A magnetic tail is formed by solar winds blowing electrified gases, plasma, trapped in a planet's magnetosphere away from the sun. Earth's magnetic tail extends beyond the orbit of the Moon, while Jupiter's magenetic tail is believed to extend beyond the orbit of Saturn.
Electric currents in space
Most people first encounter magnetism as a strange property of permanent magnets made of iron, or of a small range of ferromagnetic materials. In space, however, magnetic fields owe their existence solely to electric currents, with no role for ferromagnetism.
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. 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 MOT, also "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, as described in the MSPF.
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. (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. 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.
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.
The internal field of the Earth ("main field") arising from electric currents in the core. 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). The field confining the Earth's plasma and magnetic field inside the magnetospheric cavity. 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. 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. 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 Birkeland current field (and its branches in the ionosphere and ring current), a circuit is associated with the polar aurora. 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).Magnetic substorms and storms
Earlier it was stated that "if plasma is pushed hard enough, it generates electric fields which allow it to move in response to the push, often (not always) deforming the magnetic field in the process."
The more common one occurs when the north-south component Bz of the interplanetary magnetic field (IMF) is appreciable and points southward. In this state field lines of the magnetosphere are relatively strongly linked to the IMF, allowing energy and plasma to enter it at relatively high rates. In the end, this squeezing breaks apart field lines in the plasma sheet ("magnetic reconnection"), and the distant part of the sheet, no longer attached to the Earth, is swept away as an independent magnetic structure ("plasmoid"). That happens in magnetic storms, when following an eruption on the sun (a "coronal mass ejection" or a "solar flare"--details are still being debated, see MSPF) a fast-moving plasma cloud hits the Earth. If the IMF has a southward component, this not only pushes the magnetopause boundary closer to Earth (at times to about half its usual distance), but it also produces an injection of plasma from the tail, much more vigorous than the one associated with substorms.
The plasma population of the ring current may now grow substantially, and a notable part of the addition consists of O+ oxygen ions extracted from the ionosphere as a by-product of the polar aurora. In addition, the ring current is driven earthward (which energizes its particles further), temporarily modifying the field around the Earth and thus shifting the aurora (and its current system) closer to the equator. The magnetic disturbance may decay within 1-3 days as many ions are removed by charge exchange, but the higher energies of the ring current can persist much longer.
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