2. Anomalous Signals While Radiosounding From Altitudes Below the F-Region Maximum (h_mF)

[4]  Ionograms with AS were registered under various geophysical conditions and in different regions of the globe. Table 1 provides information on AS ionograms registered on board Mir.

Figure 1

[5]  To describe the event thoroughly, Table 1 contains several groups of AS on various days when ionograms of this type have been registered. On 31 March 1999 a diurnal series of observations was conducted from 0949 UT to 0930 UT on the following day. This group is shown to demonstrate the quantitative side of the phenomena. During the period mentioned, six groups of ionograms of the type described were observed (see Table 1). They were predominantly observed in the latitude range from 20^o S to 20^o N. However, Mir sometimes was below the F2 -layer maximum even orbiting at high latitudes. Case 8 in Table 1 provides information on such a case. Cases 9 and 10 are shown to illustrate especially long periods of observations of the ionograms considered. These cases show that the region forming the extra trace is extended in space and is a structure of the global scale. Figure 1 shows a typical ionogram of this series of AS.

[6]  One can see that Mir is located below the ionospheric maximum. The trace of reflection from the ground is fairly typical for the topside sounding signals [Pulinets and Benson, 1999]. However, its lowest frequency is equal not to the ionospheric critical frequency (as it always takes place at topside sounding) but to the plasma frequency in the vicinity of Mir, the latter fact being the strongest confirmation of the Mir location below the layer maximum height. It should be noted that the equality of the frequency of the inflection point of the reflection from the ground to the plasma frequency in the vicinity of Mir is also confirmed by the plasma resonance denoted in Figure 1 by the number 1. Very short traces of the reflection from the ionosphere above Mir are also seen: z, o, and x are the reflection traces of the z, ordinary, and extraordinary components, respectively. The group delay of AS is slightly larger than that of the signals reflected from the ground. The frequency range of AS registration is higher than the F2 -layer critical frequency in the place of Mir location (range B in Figure 1). In such situations the hypothesis on the presence of large-scale irregularities of the electron concentration (with horizontal dimensions l > 10³ km) in the region of the main maximum makes it possible to consider the typical features of AS in the scope of the "triangle" (or "returning" from the Danilkin [1994] terminology) trajectories concept. AS are formed because of the refraction of signals of different frequencies at the sharp lateral electron concentration gradient of the ionospheric irregularity. This refraction precedes or follows the oblique reflection of the signals from the ground and returning of the signals to the point of the Mir location. The calculations in the scope of the solution of the problem of fitting parameters of such irregularities of the simplest form to provide coincidence of the calculated and observed group delays of AS have shown a presence of sharp positive gradients of the electron density. On the whole, the entire spectrum of ionograms with AS indicates to very large dimensions of the irregularities. All the above-said makes reasonable a comparison of the macroirregularities detected in the experiment in question to the corresponding macroirregularities described by Kalinin and Sergeenko [2002] and Kalinin et al. [2001].