In the past two decades, measurements of E-region drifts, using the conventional closely spaced receiver method with pulse signals incident normally on the ionosphere, have been made by many workers. The great majority of such measurements have been carried out at frequencies in the 2 to 3 MHz range and refer to drift movements in the lower part of the normal E region. Measurements of sporadic E drifts by the fading method have almost always been limited to night-time hours, when the normal E layer is absent, and the results have generally been used to extend daytime normal E measurements over the full 24 h. The objectives of the present work, in which the fading method has again been used, were to obtain some simultaneous measurements of drift in the normal E and the sporadic E regions, to obtain some qualitative data on height gradient of sporadic E-layer drifts, and to compare the horizontal drifts of small- and large-scale sporadic E irregularities. (A survey of current experimental and theoretical work on sporadic E, and of outstanding problems, has recently been published by Whitehead (1970).) The closely spaced receiver technique measures changes in a diffraction pattern at the ground and the interpretation of these changes in terms of ‘ionospheric drift motion’ is, of course, open to question. Certain basic ambiguities in the interpretation of experimental data obtained in the fading method cannot readily be resolved and are likely to remain until some direct comparisons can be made with actual movements at ionospheric levels of both the ionized and neutral constituents. In continuing the practice of referring to the processed data as ‘ionospheric drifts’ we recognize that the more appropriate term would be ‘apparent ionospheric drifts’, and that the ‘drifts’ refer only to possible motion of, or within the ionization.
HF Doppler radar observations of sporadic E at an Indian low latitude station, Visakhapatnam