Effect of oblique interplanetary magnetic field on shape and behavior of the magnetosphere

The oblique angle made by the spiral interplanetary magnetic field with the radially expanding solar wind is shown to result in an easterly deflection of the solar wind as it traverses the standing hydromagnetic shock wave a few earth radii upstream of the magnetosphere. A quantitative estimate of the deflection angle can be obtained from the plasma shock relations, which are solved generally to yield the particle density, temperature, magnetic field, and plasma flow field immediately behind the shock front in terms of the corresponding interplanetary (preshock) parameters. For typical values of the interplanetary plasma and magnetic field parameters, the calculated deflection angle lies between 5° and 20°. The earth's orbital velocity relative to the radial solar wind direction contributes approximately an additional 5° deflection. The two effects combined cause the axis of symmetry of the magnetosphere, on the average, to be tilted away from the earth-sun line as if the solar wind came from an apparent direction between 10° and 25° to the west of the sun. These conclusions are consistent with a considerable body of experimental data and also suggest an alternate explanation for Explorer 10's numerous encounters with the magnetospheric boundary.