Direction-finding studies of large-scale ionospheric irregularities

Abstract

The paper describes the results of directional measurements on pulse signals reflected from the ionosphere, on various frequencies in the range 2 to 15 Mc/s. The measurements have been made at vertical and oblique incidence (distance 700 km), and the observed directional variations have been interpreted in terms of ionospheric tilts. At any instant the direction of the tilt may be regarded as random; the magnitude is found to be similar, on the average, in each of two orthogonal directions. The way in which the tilts change with time and distance have been studied, the latter by means of observations using either spaced receivers or transmitters. The tilts at a given point in region F are found to be uncorrelated at a time interval of about 5 min during the day and about 10 min during the night. At a given time they are uncorrelated at points separated horizontally by about 50 km. There is appreciable negative correlation at greater time intervals at a single point, and there may be a similar effect in the space-correlation. Observations are described which indicate large-scale horizontal movements in the iono­sphere. In the sporadic E layer these appear to be in the nature of drifting clouds of ioniza­tion, while in the F layer the effects are such as would be produced by horizontally travelling ripples in the surfaces of constant ionization density. These ripples have wave-lengths of 50 to 400 km and speeds up to 350 m/s. The direction of motion tends to be more often towards the east or west than towards the north or south, and evidence of a diurnal change has been obtained.