Uniformly spaced field‐aligned ionization ducts

Abstract

MF and HF radio wave signals transmitted from topside sounders can often become trapped in magnetic‐field‐aligned ionization ducts located approximately vertically below the satellite. The resulting traces observed on ionograms are called combination mode traces. A number of cases of combination mode traces observed with the ISIS topside sounders were studied for passes over mid‐ and low‐latitude regions which exhibited distinguishable, nearly uniformly spaced traces on the ionograms typical of this type of propagation in the frequency range above 1 MHz. Associated extraordinary vertical and ducted traces were also evident in all cases. The ducted trace is interpreted as due to propagation in a duct encompassing the satellite at the time of measurement, whereas the combination mode traces are interpreted as due to ducts located well below the satellite. The simultaneous presence of a ducted trace and combination mode traces suggests the presence of a parallel system of ducts that are field aligned. On interpreting the ionograms for events studied by ray tracing, it was found that the ducts are separated by as much as 70 km transverse to their field alignment. The data indicate that, at times, the ducts may exist down to the F peak and may extend into the conjugate region as well. A multiplicity of ducts with such large, relatively uniform separations was not previously appreciated, and the mechanism for creating them in this fashion is not clear. One possibility for the even spacing of these irregularities is that they result from acoustic gravity waves at lower altitudes. Possible relationships to other geophysical phenomena, particularly equatorial phenomena, are discussed.