Long-range auroral backscatter echoes observed at 12 mc from college, Alaska

Auroral echoes with ranges between 1100 and 2500 km have frequently been observed both during the daytime and the nighttime by means of a 12 Mc radar using a rotating antenna. The range of these echoes exceeds the radio-horizon for direct E-layer backscatter. In the majority of cases, ground-scatter is observed simultaneously at shorter range. Twelve Mc auroral echoes exhibit a characteristic rapid fading rate and are aspect-sensitive. It is postulated that the long-range auroral echoes are due to backscatter or reflection of the pulse signals from clouds of ionization at E-layer height which are elongated in the direction of the magnetic lines of force. An alternate hypothesis advanced by Stanford workers which involves deviative penetration of the F-layer and backscatter from aligned columns at upper F-layer heights is rejected for the College observations for reasons of unfavorable geometry. The proposed return mechanism allows simple propagation modes which depend only on ground reflections and the presence of an ionospheric layer capable of reflecting 12 Mc waves at oblique incidence. For the long-range daytime echoes, the ionospheric reflection will take place in the F-layer and for the nighttime echoes in the E-layer, as indicated by the groundscatter. Some observations of double or split long-range daytime echoes support the proposed daytime mode. The exceptional cases of long-range auroral echoes which cannot be explained in terms of simple propagation modes involving one F or E-layer reflection are discussed. One type of these echoes may be associated with sunlit aurora.