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The magnetosphere is a region around the Earth formed by the motion of the solar wind ramming into the magnetic field of the Earth. The shape of the magnetosphere is the direct result of being blasted by the solar wind. The solar wind compresses the sunward side of the magnetosphere to a distance of 6-15 times the radius of the Earth. A supersonic shock wave is created sunward of Earth somewhat like a sonic boom. This shock wave is called the bow shock. Most of the solar wind particles are heated and slowed at the bow shock and detour around the Earth. The magnetosphere can be thought of as a cavity in the solar wind caused by the presence of the magnetic field. At the boundary of the magnetosphere, called the magnetopause, the inside pressure of the magnetosphere itself and the outside pressure of the solar wind are equal when the boundary is steady. The solar wind drags out the night-side magnetosphere to possibly 1000 times Earth's radius; the elongated region extending away from the Sun is called the magnetotail. The magnetotail's axis is approximately aligned with the solar wind flow direction. The magnetic field of the magnetosphere is the result of the combination of the intrinsic magnetic field of the Earth plus those fields due to the flowing electrical currents in the magnetosphere and on its boundaries.

The magnetotail acts as a reservoir of plasma and energy. A current sheet lies in the center of the tail, embedded within a region of hot plasma, the plasma sheet, that separates two regions called the tail lobes. The magnetic field lines that reach down the magnetotail have opposite directions on the northern and southern lobes, respectively. The current sheet is the region in which the open field lines may reconnect to form closed field lines that convect back to the Earth. As the hot plasma sheet flows toward the Earth into an increasing magnetic field, the different motion of the ions and electrons produces a ring current in a radial distance of 3-5 Earth radii. The term, trapped radiation belts, is used to describe the trapped particles in the ring current region. Most of the ring current is carried by the trapped particles, and all of the trapped particles contribute to the ring current. The plasmasphere is a region of relatively dense, cold plasma that surrounds the Earth, approximately in the same space as the radiation belts. A sharp boundary of plasma density at an altitude of 3-5 Earth radii is called the plasmapause. The region of low cold-plasma densities just outside the plasmapause is referred to as the density trough.



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