A magnetosphere is the region of space surrounding an astronomical object in which charged particles are controlled by that object's magnetic field. The magnetic field near the surface of many astronomical objects resembles that of a dipole. The field lines farther away from the surface can be significantly distorted by the flow of electrically conducting plasma emitted from a nearby star (e.g. the solar wind from the Sun).
Planets with a magnetosphere, like Earth, are capable of mitigating or blocking the effects of cosmic radiation.
Study of Earth's magnetosphere began in 1600, when William Gilbert discovered that the magnetic field on the surface of Earth resembled that on a terrella, a small, magnetized sphere. In the 1940s, Walter M. Elsasser proposed the model of dynamo theory, which attributes Earth's magnetic field to the motion of Earth's iron outer core. Through the use of magnetometers, scientists were able to study the variations in Earth's magnetic field as functions of both time and latitude and longitude. Beginning in the late 1940s, rockets were used to study cosmic rays. In 1958, Explorer 1, the first of the Explorer series of space missions, was launched to study the intensity of cosmic rays above the atmosphere and measure the fluctuations in this activity. This mission observed the existence of the Van Allen radiation belt (located in the inner region of Earth's magnetosphere), with the Explorer 3 mission later that year definitively proving its existence. Also in 1958, Eugene Parker proposed the idea of the solar wind. The term 'magnetosphere' was proposed by Thomas Gold in 1959. The Explorer 12 mission (1961) led to the observation by Cahill and Amazeen in 1963 of a sudden decrease in the strength of the magnetic field near the noon meridian, later named the magnetopause. In 1983, the International Cometary Explorer observed the magnetotail, or the distant magnetic field.
The structure and behavior of magnetospheres are dependent on several variables: the type of astronomical object, the nature of sources of plasma and momentum, the period of the object's spin, the nature of the axis about which the object spins, the axis of the magnetic dipole, and the magnitude and direction of the flow of solar wind.
The distance at which a planet can withstand the solar wind pressure is called the Chapman–Ferraro distance. This is modeled by a formula wherein represents the radius of the planet, represents the magnetic field on the surface of the planet at the equator, and represents the velocity of the solar wind:
A magnetosphere is classified as "intrinsic" when , or when the primary opposition to the flow of solar wind is the magnetic field of the object. Mercury, Earth, Jupiter, Ganymede, Saturn, Uranus, and Neptune exhibit intrinsic magnetospheres. A magnetosphere is classified as "induced" when , or when the solar wind is not opposed by the object's magnetic field. In this case, the solar wind interacts with the atmosphere or ionosphere of the planet (or surface of the planet, if the planet has no atmosphere). Venus has an induced magnetic field, which means that because Venus appears to have no internal dynamo effect, the only magnetic field present is that formed by the solar wind's wrapping around the physical obstacle of Venus (see also Venus' induced magnetosphere). When , the planet itself and its magnetic field both contribute. It is possible that Mars is of this type.
The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the ambient medium. For stars, this is usually the boundary between the stellar wind and interstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause.
The magnetosheath is the region of the magnetosphere between the bow shock and the magnetopause. It is formed mainly from shocked solar wind, though it contains a small amount of plasma from the magnetosphere. It is an area exhibiting high particle energy flux, where the direction and magnitude of the magnetic field varies erratically. This is caused by the collection of solar wind gas that has effectively undergone thermalization. It acts as a cushion that transmits the pressure from the flow of the solar wind and the barrier of the magnetic field from the object.
The magnetopause is the area of the magnetosphere wherein the pressure from the planetary magnetic field is balanced with the pressure from the solar wind. It is the convergence of the shocked solar wind from the magnetosheath with the magnetic field of the object and plasma from the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are complex. The structure of the magnetopause depends upon the Mach number and beta of the plasma, as well as the magnetic field. The magnetopause changes size and shape as the pressure from the solar wind fluctuates.
Opposite the compressed magnetic field is the magnetotail, where the magnetosphere extends far beyond the astronomical object. It contains two lobes, referred to as the northern and southern tail lobes. Magnetic field lines in the northern tail lobe point towards the object while those in the southern tail lobe point away. The tail lobes are almost empty, with few charged particles opposing the flow of the solar wind. The two lobes are separated by a plasma sheet, an area where the magnetic field is weaker and the density of charged particles is higher.
Over Earth's equator, the magnetic field lines become almost horizontal, then return to reconnect at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On the dayside of Earth, the magnetic field is significantly compressed by the solar wind to a distance of approximately 65,000 kilometers (40,000 mi). Earth's bow shock is about 17 kilometers (11 mi) thick and located about 90,000 kilometers (56,000 mi) from Earth. The magnetopause exists at a distance of several hundred kilometers above Earth's surface. Earth's magnetopause has been compared to a sieve because it allows solar wind particles to enter. Kelvin–Helmholtz instabilities occur when large swirls of plasma travel along the edge of the magnetosphere at a different velocity from the magnetosphere, causing the plasma to slip past. This results in magnetic reconnection, and as the magnetic field lines break and reconnect, solar wind particles are able to enter the magnetosphere. On Earth's nightside, the magnetic field extends in the magnetotail, which lengthwise exceeds 6,300,000 kilometers (3,900,000 mi). Earth's magnetotail is the primary source of the polar aurora. Also, NASA scientists have suggested that Earth's magnetotail might cause "dust storms" on the Moon by creating a potential difference between the day side and the night side.
The magnetosphere of Jupiter is the largest planetary magnetosphere in the Solar System, extending up to 7,000,000 kilometers (4,300,000 mi) on the dayside and almost to the orbit of Saturn on the nightside. Jupiter's magnetosphere is stronger than Earth's by an order of magnitude, and its magnetic moment is approximately 18,000 times larger. Pluto, on the other hand, has no magnetic field.
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