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, 1F, 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 1F 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 f0F2 at these latitudes. In general, however, the occurrence of FAE(F) increased monotonically with increasing magnetic activity until the threshold of KFr = 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, KFr ≥ 7.
With the aid of ray traces, the occurrence statistics of both 1F 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.