4. Ionospheric Macroirregularities Observed Jointly by Satellite and Ionospheric Ionosondes

Figure 3

[10]  Figure 3 shows the information on this experiment in the part of the global map in Mercator projection. (left, bottom) There are three fragments (three ionograms) illustrating by bright marks at the frequency-distance ( f,D ) planes the signals detected by the ionosondes on board Mir on 29 April 1999 in a complete analogy to Figure 1. (left, top) The Mir coordinates of fragment I. The date and time are shown in the left, bottom corner. The time is so called "winter Moscow" and goes ahead of UT by 3 hours. The track caused by the reflection from the ground is shown in the ( f,D ) plane by red points in the first ionograms and by white line without points in the second and third ionograms. The AS tracks are designated by green points. The position of the irregularity is shown by green oval. The part where Mir registered anomalous signals is marked by thick vertical line. The GIS locations at Darwin, Ahmedabad, and Roma are shown by circles with the corresponding letters in Figure 3 (fragment II). The dashed curve DD' (the arc of a great circle passing through the epicenter and Roma) is the approximate trajectory of the irregularity motion. The GIS, which in proper time detected no positive "pulses" in d f_c(t) are shown by the circles with minus sign. This is assumed to be an indication of the absence of the irregularity related to the earthquake. Those are GISs (in the westward order) located in Petropavlovsk, Tomsk, Novosibirsk, Ashkhabad, and Moscow. No positive pulses has been registered by GIS in England and Holland. This shows that soon after passing over Roma the irregularity ceased its existence. Comparing the SS' and DD' trajectories, one can see that the anomalous signals were observed by the onboard ionosonde exactly when the Mir trajectory passed along the irregularity edge. In Figure 3 this fact is illustrated by the green semioval with green dashes within.

[11]  The dashed SS' curve in fragment II shows the part of the great circle which is a projection of the Mir orbit on the globe. The position of the Mir station during the registration of the ionogram presented in fragment I is marked at the SS' projection by a circle with a cross. The circle is located nearly in the middle of the solid stretch aligned from Philippines to New Guinea Island. This very stretch corresponds to the period of AS observation. Danilkin [1994] was the first to publish ionograms of an onboard ionosonde with AS. He suggested that the AS were due to the interaction of the signals transmitted by the onboard ionosonde to positive macroscale irregularities of the electron concentration. Sufficiency of this assumption was illustrated by the calculations of "return" AS trajectories under corresponding choice of the irregularity parameters. The irregularity parameters and arrival angles to the ground (generally speaking, they can be different for different frequencies) were chosen in such a way that the calculated virtual distances "fell" on the AS track. There appear at least two questions concerning the AS registered by the ionosonde on board Mir. First, why AS are not registered at every ionogram? Actually, the stretch containing the circle with a cross and corresponding to the periods of anomalous signal registration presents only a small part of the Mir trajectory shown in Figure 3 (fragment II) by dashed curve SS'.

[12]  Second, how in general irregularities with the horizontal dimension above 10 Mm can be formed in the ionospheric F2 region? The answers to both these questions are in the hypothesis proposed for the first time and substantiated by Kalinin and Sergeenko [2002]. According to this hypothesis, in the ionospheric F2 region 10-15 hours prior to a strong earthquake, there are formed in the vicinity of the epicenters macroscale irregularities which then travel horizontally to distances up to 20 Mm.

[13]  One should check whether the synchronism condition was fulfilled for any of the earthquakes on 20-21 April 1999. The place and time of the earthquake should fulfill the following conditions. Counting time from the moment 10-15 hours prior to the earthquake and accepting the irregularity velocity of about V_r 10³ km h^-1 out of the epicenter, the irregularity should come to the place where the Mir ionosonde has registered AS to the time of the registration. For the date in question (21 April 1999) and the preceding date, the number of potentially "suitable" earthquakes (with M >5 ) is not large. There are five of them, and all of them have occurred in the vicinity of Australia. However, the first four earthquakes have occurred too "early." Even if we assume that from each of the earthquakes the irregularity moved with a velocity of V_r directly to the point (j = 3.22; l = 134.16), these earthquakes should have reached the point earlier than at 0715 UT. The synchronism condition is fulfilled only for the earthquake (with M = 5.6 ) which took place at 1108 UT on 21 April 1999 northeastward from New Zealand. All the following earthquakes occurred too "late."

[14]  The epicenter if this earthquake is marked by a circle with a letter E in Figure 3 (fragment II). It is of a principal importance that the motion of the irregularity formed before this earthquake has been registered by the ground-based ionosondes in Darwin (at the north of Australia, the circle with a letter "D" in Figure 3 (fragment II), Ahmedabad (India, the circle with a letter "A"), and Roma (the circle with a letter "R"). All these ground-based ionosondes were registering the critical frequency of the ionospheric F2 layer ( f_oF2 ). The dependence of f_oF2 on time was recalculated into the relative variations of f_oF2 using the well-known method [Gaivoronskaya et al., 1971].

Figure 4

[15]  It is worth repeating that (see Table 1) well-pronounced AS were registered on board Mir fairly often. Figure 4 shows one more case of joint observation of AS (on board Mir) and the motion of the same irregularity induced by the earthquake and registered at the ground-based ionospheric stations. In the same way as in Figure 3, here the ground-based stations which registered the passage of the irregularity are marked by red. The station not registering this passage are marked yellow. Contrary to Figure 3, in this case some sort of an opposite motion of Mir and the irregularity is observed. Nevertheless, the picture is nearly the same as in Figure 3. The latter may be explained by the fact that Mir moves considerably faster than the irregularity.

[16]  Here we succeeded to choose among many earthquakes those what generated irregularities which "in time" came to the region of anomalous signal registration.