MAGNETOSPHERE
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MAGNETOSPHERE |
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The Earth's MagnetosphereA fundamental property of space plasmas is that they are collisionless, and as such have extremely high conductivity. As a result, over large scale lengths, the magnetic field can be considered to be frozen-in to the plasma flow. So, for example, the interplanetary magnetic field is convected away from the Sun with the solar wind flow. By the same argument, when the plasma flow encounters a magnetic obstacle, such as the field generated internally at the magnetized planets, the solar wind flow is to a first approximation excluded from the cavity occupied by the main planetary field. The solar wind is thus slowed and deflected around this cavity at the bow shock. The cavity itself is called the magnetosphere, and the boundary separating it from the shocked solar wind (the magnetosheath is called the magnetopause. In the absence of the solar wind, the magnetic field structure in this region would be approximately dipolar. However, the action of the solar wind is to compress the structure on the dayside of the planet, and drag it out into and extended structure, the magnetotail, on the nightside. Typically the subsolar dayside magnetopause stands about 10 Earth radii upstream of the planet, while the tail may be many hundreds of Earth radii long.
Much of the current research performed in magnetospheric physics is related to the finer details of interaction between the solar wind and the magnetosphere. Observationally it is known that solar wind plasma does gain entry to the magnetosphere and that convection patterns are set up within the cavity. It is believed that this is achieved through the breakdown of the frozen-in flux condition at certain key points within the system. This breakdown allows a process called "magnetic reconnection" to take place at those locations, which are termed magnetic neutral points or neutral lines. The sites of reconnection are themselves very small, but they can have a significant effect on the global structure of the magnetosphere. For example, the presence of a neutral line on the dayside magnetopause creates direct links between the interplanetary magnetic field and the Earth's main field. This allows the solar wind and magnetospheric plasma populations to mix along the field lines, so solar wind plasma has access to the magnetospheric cavity. Moreover, this creates magnetic flux tubes which have one end firmly embedded in the terrestrial ionosphere, while the other remains frozen-into the continued solar wind flow around the magnetosphere. These "open" flux tubes are therefore dragged antisunward over the poles of the Earth and are stretched out and added to the magnetotail structure, together with the associated plasma . This process thus taps the solar wind kinetic energy and stores it in the form of magnetic energy in the magnetotail lobes. The return leg of this convection cycle is again thought to be driven by reconnection, this time at a neutral line located in the centre of the tail. The stretched fields in the north and south lobes are oppositely directed, and therefore may reconnect to form two hairpin-like magnetic field structures. One of these has both ends connecting to the ionosphere, the other has both ends mapping back out into the solar wind flow well downstream of the reconnection site. As these flux tubes convect away from the neutral line, the magnetic energy is released back into the plasma, such that jets of accelerated and heated plasma form both Earthward and tailward of the neutral line. The Earthward flowing plasma eventually returns to the dayside, thus completing the convection cycle. Note that in general magnetospheric convection is not a steady state phenomena, but tends to occur in bursts. These bursts of activity are known as magnetic substorms. |
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Created February, 1999 by David Burgess |
