Field-aligned ionospheric E-region irregularities and sporadic E

A continuous series of oblique backscatter observations at 19 MHz made at Plum Island, Massachusetts (56°N invariant), over nearly half a solar cycle is utilized to derive the occurrence characteristics of aspect-sensitive field-aligned irregularities in the E-layer and their association with ground-barkscattered echoes which are propagated via sporadic E (E8) reflection. Under quiet magnetic conditions, it is found that the field-aligned echo from E-layer heights, FAE(E), displays a summer evening maximum in conjunction with the E8 echo. A weak secondary maximum is observed in the winter with no detectable field-aligned structures being evident in the daytime E8. During disturbed magnetic conditions, the E8 ground-back-scattered echo is greatly suppressed together with a simultaneous increase of the FAE(E) which also appear in the daytime; no seasonal control is evident. One year's continuous observations at 49 MHz with a narrowbeam antenna showed the quiet time, E8-associated FAE(E) to be of the discrete type, whereas the FAE(E) obtained during storms were found to be diffuse in nature. These observations are discussed from the point of view of current-plasma instability theories.