1. Introduction

[2]  In spring 1999 the manned space complex Mir with an onboard automatic ionospheric station (AIS) was orbiting at heights of about 330-350 km. Abilities of radiosounding almost directly from the maximum of the F2 layer had been studied earlier in the experiments on board the Intercosmos 19 and Cosmos 1809 satellites. A possibility of determining various ionospheric parameters and derive N(h) vertical profiles has been demonstrated in these experiments [Danilkin, 1994; Danilkin and Vaisman, 1997]. Orbiting at southern latitudes, Mir often was moving below the main maximum of the electron concentration in the ionosphere.

[3]  Among the ionograms registered in this period [see Danilkin, 2001], there are a large number of ionograms containing anomalous signals (AS). Currently, there is only one noncontroversial explanation coordinated with the entire volume of data of these traces in ionograms of oblique satellite sounding [Danilkin and Kotonaeva, 2002]. This explanation claims that the traces appear as a result of the oblique reflection of radiowaves within a large range of frequencies and then (or before) of the return of all radiowaves into the transmission point due to sharp gradients of the electron concentration in the ionosphere. Later, it was found that these gradients are lateral walls of stable singular macroirregularities with very specific properties [Kalinin and Sergeenko, 2002].