Interaction of solar plasma streams with the outer geomagnetic field

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: a standing shock layer and the geomagnetic cavity. The pattern of these regions in the equatorial plane is shown in a frame fixed relative to the sun. The shock layer, having characteristic turbulent fluctuations of magnetic field, appears beyond 10 earth radii on the sunward side even during geomagnetically quiet periods. The shape of the shock front (the boundary of a quiescent interplanetary plasma) is approximated by a hyperbola, with a stand-off distance of the order of 5 earth radii. This standing shock layer is terminated on its interior by the geomagnetic field cavity, having a spherical shape on the day side and an elongated tail on the night side. The ordinary shock theory of a continuous flow is applied, assuming that the interacting plasma is magnetized so that the mean free path can be replaced by the Larmor radius of the medium. The theoretical estimates of the shock position and the size of the geomagnetic cavity are then compared with those observed. It is found that the observed shock wave is consistent for a supersonic flow of Mach number 3–4. The estimated values of a typical quiescent solar plasma stream are U = 400 ∼ 600 km/s, n = 5/cm3, and B = 10 γ.