Magnetosphere
The magnetosphere' or magnetosphere is a layer around a planet in which the planet's magnetic field deflects most of the part of the solar wind forming a protective shield against high-energy charged particles from the Sun. Earth's magnetosphere is not unique to the solar system, and all planets with a magnetic field—Mercury, Jupiter, Saturn, Uranus, and Neptune—have a magnetosphere own. Jupiter's satellite Ganymede has a magnetic field, but it is too weak to trap the plasma of the solar wind. Mars has a very weak surface magnetization with no outer magnetosphere.
Particles from the solar wind that are stopped form the Van Allen belts. At the magnetic poles, the areas where the Earth's magnetic field lines penetrate its interior, part of the charged particles are driven into the upper atmosphere producing the northern or austral lights. Such auroral phenomena have also been observed on Jupiter and Saturn.
History
The Earth's magnetosphere was discovered in 1958 by the US Explorer I satellite. Before that, some magnetic effects in space were known, since solar flares sometimes produced magnetic storms on Earth that were detectable by means of radio waves. However, no one knew how or why these currents occurred. The solar wind was also unknown.
Before this, scientists knew that electrical current flowed in space due to solar flares. It was not known, however, when these currents flowed or why. In August and September 1958, the United States Army began Project Argus in order to test a theory about the formation of radiation belts that may have tactical use in warfare.
In 1959, Thomas Gold proposed the name for the magnetosphere, when he wrote: "The region above the ionosphere, in which the earth's magnetic field predominates over streams of gas and fast-moving charged particles, is known which extends over a distance of the order of 10 Earth radii, so it could be appropriately called the magnetosphere”.
Structure
In the outermost and widest part of a planet's atmosphere, the magnetosphere interacts with the solar wind in a region called the magnetopause that is smaller in the direction of the sun, and extremely extended in the opposite direction. In the case of Earth it is about 100 000 km and in the case of Jupiter it is more than 4 million km. Ahead of the magnetopause is the collision surface between the solar wind and the magnetic field. In this region the solar plasma slows down rapidly before being deflected by the rest of the magnetosphere. The charged particles of the solar wind are dragged by the magnetic field over the magnetic poles, giving rise to the formation of polar auroras.
Bow Shock
The bow shock forms the outermost layer of the magnetosphere; the boundary between the magnetosphere and the environment. For stars, this is usually the boundary between the stellar wind and the interstellar medium; for planets, the speed of the solar wind there decreases as it approaches the magnetopause.
Magnetic cover
The magnetoenvelope is the region of the magnetosphere between the arc shock and the magnetopause. It forms mainly from the impacted solar wind, although it contains a small amount of plasma from the magnetosphere. It is an area that exhibits a high energy flux of particles, 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 of the solar wind flow and the barrier of the magnetic field of the object.
Magnetopause
The magnetopause is the area of the magnetosphere where the pressure of the planetary magnetic field is balanced by the pressure of the solar wind. It is the convergence of the impacted solar wind of the magnetoheath with the magnetic field of the object and plasma of the magnetosphere. Because both sides of this convergence contain magnetized plasma, the interactions between them are complex. The structure of the magnetopause depends on the Mach number and the beta of the plasma, as well as the magnetic field. The magnetopause changes size and shape as the pressure of the solar wind fluctuates.
Magnetic tail
Opposite the compressed magnetic field is the magnetic tail, where the magnetosphere extends well beyond the astronomical object. It contains two lobes, called the northern and southern tail lobes. Magnetic field lines in the northern tail lobe point toward the object, while those in the southern tail lobe point the other way. The tail lobes are nearly empty, with few charged particles opposing the flow of the solar wind. The two lobes are separated by a sheet of plasma, in an area where the magnetic field is weakest and the density of charged particles is greatest.
Earth's Magnetosphere
Above Earth's equator, magnetic field lines become nearly horizontal, then reconnect at high latitudes. However, at high altitudes, the magnetic field is significantly distorted by the solar wind and its solar magnetic field. On Earth's dayside, the magnetic field is significantly compressed by the solar wind up to a distance of approximately 65,000 kilometers (40,389.2 mi). Earth's bow shock is about 17 kilometers (10.6 mi) thick and is about 90,000 kilometers (55,923.5 mi) from Earth. The magnetopause exists at a distance of several hundred kilometers above the Earth's surface. Earth's magnetopause has been likened to a sieve because it allows particles from the solar wind to enter. Kelvin-Helmholtz instabilities occur when large eddies of plasma move along the edge of the magnetosphere at a different speed than the magnetosphere, causing the plasma to slip. This results in magnetic reconnection, and as the magnetic field lines break and reconnect, particles from the solar wind are able to enter the magnetosphere. On the night side of Earth, the magnetic field extends in the magnetotail, which spans more than 6,300,000 kilometers (3,914,648.2 mi). Earth's magnettail is the primary source of polar aurora.In addition, NASA scientists have suggested that Earth's magnettail could cause "dust storms" on the Moon by creating a potential difference between the day and night sides..
Plasmoids
The solar wind and currents in the tail lobes cause strong distortions of the field lines in the plasma layer of the magnetic tail. If these distortions become too strong (the details of the processes are not yet understood), pinch-offs can occur as a result of magnetic reconnections: the parts of the field lines closest to the earth close to form more lines of fields similar to dipoles, while parts farthest from Earth form a plasmoid, a plasma-filled region of space with closed field lines. On the one hand, the plasmoid is accelerated outwards by the released magnetic energy, on the other hand, it leads to a heating of the upper atmospheric layers and thus to an intensified feedback with the electrical flow system.
The process of detachment of plasmoids is called a magnetic substorm because it was initially considered only a subcomponent of magnetic storms. Today, however, we know that the partial storm is a phenomenon that occurs not only in the "storm phases" but also in calm phases; the course is very similar in both cases: a partial storm lasts about 45 minutes and gives rise to a plasma heating of about 2 keV. However, during a stormy phase, the plasma is already hotter at the beginning (about 3–4 keV in calm phases and about 8 keV in stormy phases) and the rise is steeper.
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