Recurrent geomagnetic storms and the solar wind

The structure that would be expected to develop when a fast stream in the solar wind is embedded in a slower ambient wind is discussed on the basis of gas dynamics. This structure is compared with that appearing in recurrent geomagnetic storms. It is found that the development of M-region storms follows the expected structure of the solar-wind stream. The sudden commencements on some M storms correspond to the shock that is expected to occur in the solar wind if the relative velocities of the fast stream face and the ambient wind are high enough. It is also shown that the typical quiet initial phase of the sudden commencement M storm corresponds to the relatively low-velocity region in space between the shock and the stream proper. The main phase corresponds to the high-velocity stream itself. In particular the fast M streams detected by Mariner 2 and their associated geomagnetic storms are discussed.