F ‐layer irregularities as determined by backscatter studies at 19 MHz over half of a solar cycle

A continuous series of backscatter observations at 19 MHz made at Plum Island, Massachusetts, 56°N invariant latitude, over nearly half a solar cycle is utilized to derive occurrence characteristics of ground backscatter propagated by means of F ‐layer reflection, 1 F , as well as those of aspect‐sensitive field‐aligned echoes from the F layer, FAE( F ). Neither field‐aligned E ‐layer echoes nor ground backscatter propagated by means of sporadic E were found to produce any noticeable blanketing of the radar signal for this frequency and radar location. The 1 F propagation showed marked sunspot cycle and azimuthal dependence and was strongest in winter and weakest in summer. Under quiet magnetic conditions FAE( F ) was primarily a sunset phenomenon, occurring usually at delays of 7 to 10 msec, with a marked sunspot cycle dependence. Some daytime activity was observed at longer delays of 20 to 22 msec which could be explained in terms of F ‐layer field alignment after one intermediate ground reflection. The occurrence of FAE( F ) during magnetic storms showed a seasonal variation in agreement with the well‐known storm behavior of f 0 F 2 at these latitudes. In general, however, the occurrence of FAE( F ) increased monotonically with increasing magnetic activity until the threshold of K Fr = 4 was reached, beyond which the depletion in the F ‐layer background ionization caused it to decrease, with complete cut‐off occurring irrespective of season during very disturbed conditions, K Fr ≥ 7. With the aid of ray traces, the occurrence statistics of both 1 F and FAE( F ) are interpreted and discussed in terms of the geometry of the probing radar, the various factors controlling F ‐layer propagation, and the extent of the F ‐layer irregularity region.