Interaction of Solar Plasma Streams with the Outer Geomagnetic Field
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概要
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The general nature of the magnetic field surrounding the earth is discussed using various measurements made by space probes since 1958. It is shown that the outer geomagnetic field consists of two regions : the standing shock layer and the geomagnetic cavity. The pattern of these regions in the equatorial plane is shown in the frame fixed to the sun. The shock layer, in which turbulent fluctuations of magnetic field are characteristic, appears beyond 10 earth's radii in the sunward side even in a geomagnetically quiet period. The shape of the shock front (the boundary to a quiescent interplanetary plasma) is approximated as a hyperbola, with a stand-off distance of the order of 5 earth's radii. This standing shock layer is terminated by the geomagnetic field cavity inside, being a spherical shape in the day-side and an elongated tail in the night-side. It is also noted that the axis of symmetry of this pattern seems to be tilted somewhat towards west of the sun. The ordinary shock theory of a continuous flow is applied, assuming that the interacting plasma is magnetized so that the mean free path may be replaced by the Larmor radius of the medium. The theoretical estimations of the shock position and the size of the geomagnetic cavity are then compared with the observed ones. It is found that the observed shock wave is consistent for a supersonic flow of Mach number 2-5. The estimated values of a typical quiescent solar plasma stream are U=400-500km/s, n=5/cm^3 and B=10 gamma.
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