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‐backscattered echoes which are propagated via sporadic E ( E 8 ) 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 E 8 echo. A weak secondary maximum is observed in the winter with no detectable field‐aligned structures being evident in the daytime E 8 . During disturbed magnetic conditions, the E 8 ground‐backscattered 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, E 8 ‐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.