On the effect of a weak interplanetary magnetic field on the interaction between the solar wind and the geomagnetic field

Recent developments have indicated the possibility that the presence of a weak interplanetary magnetic field may lead to the formation of a collision-free shock wave upstream from the boundary of the geomagnetic field and to a transition region characterized by an irregular magnetic field in the intervening space. Previous calculations of the coordinates of the shock wave are improved upon by application of recent advances in the theory of gasdynamic shock waves developed for re-entry aerodynamic studies. It is found that the standoff distance of the shock wave is increased significantly over previous estimates and that the new results appear to be at least consistent with observations. Theoretical results are also ahown, to illustrate the differences to be anticipated in the transition region when the interplanetary field is aligned parallel or antiparallel to the geomagnetic field in the equatorial plane and when the Alfvén Mach number is greater or less than 2. A number of observational results that display properties similar to those calculated are reviewed to indicate that such phenomena may actually occur in space.