Horizontal Movements in the Ionosphere

The object of the paper is to survey the existing knowledge of horizontal movements in the ionosphere as obtained by radio methods. After a brief introduction the various methods and associated results are described under the following main headings: Reflected radio waves observed at closely spaced points; Observations of movements of large scale irregularities in the F region; Observations of waves from radio stars; Observations of waves reflected from meteor trails; Observations of waves reflected from the sporadic-E layer. The most detailed discussion is accorded to the first of these methods, with which the authors have been personally associated. The results obtained at Cambridge, England, by this method are compared with the results obtained by other workers using this and the other methods in different parts of the world. The following conclusions are reached: At all levels in the ionosphere systematic as well as irregular horizontal movements are present. Typical velocities are of the order of 80 m/sec; there is a tendency for the velocity to increase with height. The systematic movements have the form of regular diurnal and seasonal variations and show considerable consistency between observations made in the Northern and Southern hemispheres. At heights around 100 km the semi-diurnal components of the velocity, as determined by Fourier analysis, are found to represent a uniformly rotating velocity vector; the direction of the rotation is clockwise in the Northern hemisphere and anti-clockwise in the Southern hemisphere. This is the type of movement predicted by the theory of atmospheric oscillations, though the phase of the rotation is not that expected theoretically. At times of magnetic disturbance the velocities of the horizontal movements are increased, and this phenomenon becomes more marked with increasing height in the atmosphere. At 300 km velocities as high as 1000 m/sec have been recorded during magnetic storms. The report ends with a brief summary of observations by non-radio methods.