Chaotic echoes on ionograms: Underdense scattering or total reflections?

Apart from spread F and some forms of sporadic E, little attention is usually paid to the variable occurrence of “chaotic” or disordered “scatter” in ionosonde measurements. In contrast to the echoes of broad radio bandwidth and high amplitude from the plane-stratified ionosphere, for which total internal reflection is unmistakably applicable, the processes responsible for occasional scattering are not obvious. With the multiparameter information of a digital ionosonde it should be possible to develop criteria by which the echoing processes can be decided, if not on an echo-by-echo basis, at least for groups of echoes that bear similar properties. The distinction is important since these chaotic echoes may be identified with significant physical processes, whereupon the ionosonde becomes one of their more powerful diagnostics. Three examples illustrate these points; they arise in consequence of (1) the (magnetic) equatorial electrojet; (2) some suggested neutral-atmosphere interactions near sporadic E at mid latitudes; and (3) the auroral electrojet, respectively.