Dynamics of large-scale isolated irregularities in the F2  ionospheric region during strong earthquakes

Yu. K. Kalinin

Institute of Applied Geophysics, Moscow, Russia

N. P. Sergeenko

Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Troitsk, Moscow Region, Russia

Contents

Abstract

Introduction

Figure 1

The hourly values of the F2 -region critical frequencies ( f_cF2 obtained at 12 vertical sounding (VS) ionospheric stations serve as an initial information for this study. The stations located near the segments of the great circle arc were chosen (see Figure 1a). We use part of the world map in the Merkator's projection. The ionospheric stations Moscow, Sverdlovsk, Tomsk, Irkutsk, and also the Japanese stations Akita, Kokubunji, and Wakkanai (the latter stations are not considered as independent measurers) are located along segment 1. The Slough, Moscow, Sverdlovsk, Alma-Ata, Novokazalinsk, Tashkent (three stations lie within a distance of 1 Mm), Delhi, and Canberra stations are located along segment 2. First letters of the name of each station show their position in the fragment of the map. For all f_cF2 sets the transition to sets of relative variations (df_cF2) was performed [Kalinin et al., 1998, 1999]:

(1)

In equation (1) f_cF2 _med is a running median of the f_cF2 multitude for each hour of the day. We considered the periods of two strong earthquakes in Japan (August, 1985) and Tashkent (December, 1980).

Not attempting to review all studies of the ionospheric precursors of earthquakes we note here two methods which use df_cF2(t) variations. Pulinesc [1998] suggested to use the quasi-periodic components df_cF2(t) as some precursor of an earthquake and Kalinin et al. [1999] used as a precursor the third and fourth sampling statistical invariants under various lengths and shifts of the sample. In our opinion, both approaches provide no significant progress in creation of a decision-making rule on whether there is a danger of an earthquake occurrence, for example, on the next day. We do not mean properties which are not related to the ionosphere in general or df_cF2(t) variations in particular. Currently the authors are developing a concept of large-scale instability according to which the F2 region (and probably the lower-lying regions) has a capacity to form (spontaneously or as a result of a "pulsed" impact) large-scale irregularities of the electron concentration with the size up to 5 Mm and the life-time longer than 10⁴ s. In particular, Kalinin and Romanchuk [1991] found a relation between the launches of the satellites with almost circular orbits at a height of 1-2 Mm and sampled invariants of changes of the critical frequency df_cF2(t). The large values of the invariants are related to large-scale irregularities and the arbitrary satellite localization to VS stations tends to suppose that these objects exist long and can move to large distances. At the same time, Kalinin and Romanchuk [1991] failed to establish similar relation between the df_cF2 distribution invariants and the temporal intervals preceding an earthquake by several hours. Therefore the search of such relation was carried out within the framework of the concept of large-scale ionospheric instability in terms of concentrated pulsed structures (the Duhamels approach) but not in terms of periodic variations (the Fourier approach). Figure 1b shows the scheme of pulse processing in relative variations of the F2 -layer critical frequencies.

The goal of this paper is to study peculiarities of the concentrated formation dynamics in the df_cF2(t) variations and also to try to answer the question whether there is a ground for the hypothesis that strong earthquakes are by several hours preceded by a particular type of compositions of the df_cF2(t) pulses.

Trajectories, Motion Velocity, and Dimensions of the Objects

Figure 2

We consider the data shown in Figure 2. Here the df_cF2(t) variations and Kp indices are shown for the period August 10, 1985, 0000 UT-August 12, 1985, 0800 UT for the Japan stations (complementing each other because of the information deficiency) and the Irkutsk, Tomsk, Sverdlovsk, and Moscow stations which are located near the great circle arc 1 (see Figure 1a). Two groups of pulses near the dashed lines 1 and 2 are distinguished. The Moscow-Japan distance is ~7.5 Mm, the temporary shifts are 8 and 7 hours, the velocity is about 1 Mm h ^-1, the duration is 3-5 hours, and the aligned (along the motion direction) dimensions are l 3-5 Mm. The arc 1 passes at a distance of ~2 Mm from the Slough station and both dashed lines 1 and 2 in Figure 2 show no corresponding positive bursts in the df_cF2(t) dependence at the Slough station. The object "misses" this station. Neither has the group of bursts at the Tomsk and Irkutsk stations at the end of August 11, 1985 any corresponding manifestation in the df_cF2(t) dependence at the Alma-Ata station. The object "misses" this station also, the fact leading to a direct estimate of the transversal dimensions: l < 4 Mm.

Figure 3

Figure 3 shows the data for December 10-11, 1980 (left) and December 1, 1980 (right) for Alma-Ata, Tashkent, Novokazalinsk, Sverdlovsk, and Slough, that is, for the stations located along great circle arc 2 of Figure 1a. Based on the data for December 11-12, 1980, the group of positive bursts or pulses is revealed which allows us to draw the trajectory as dashed line 1. There is some deficiency in the information: the Novokazalinsk station data have a gap in the "necessary" time. The positive pulse is almost absent in the Sverdlovsk station data. At the same time the Alma-Ata, Tashkent, Moscow, and Slough stations demonstrate such pulses. The Irkutsk station does not, which means the object was not stretched up to it. The Irkutsk station data were combined with the Moscow station data. The object velocity may be estimated from the slope of line 1 as 2 Mm h ^-1. The burst duration and dimensions are 2-3 hours and l 4-6 Mm, respectively. l does not exceed l. It should be noted that our consideration here is based on two hypotheses which are in general confirmed: first, the objects move along great circle arcs and, second, the objects themselves have rather simple structure similar to those of the figures rotating around their own peak.

Data for December 1, 1980 (see the right-hand part of Figure 3) relate to the situation when neither Sverdlovsk nor Slough show bursts of the corresponding sign at the "necessary" time. Thereby the observation trajectory length is reduced down to the Moscow-Alma-Ata distance, that is, becomes equal approximately to l and l. Therefore the ground to draw dashed line 2 as trajectory of the object motion is minimum.

Discussion

The data shown in Figures 2 and 3 are interested because they cover the daily periods preceding earthquakes. Their moments are marked by circles at the abscissas. In both cases a few hours before the earthquake (7 hours in Japan and 14 hours in Tashkent) a positive disturbance of the 15-20% amplitude moving with a velocity up to 1 Mm h ^-1 to a distance of not less than 7-8 Mm arises. We note that in both cases these disturbances were observed on the quiet geomagnetic background (see Kp indices in Figures 2 and 3). These cases correspond evidently to the rare situation when the motion of an object formed before the earthquake coincides with chains of VS stations situated along a great circle arc. The objects so formed were although large but concentrated nevertheless and moved to distances of at least twice their sizes. By this they significantly differ from a non-directed cylindrical wave diverging from a center. More exactly they may be Legendre conic waves. Such pulsed waves with the front laying on a cone diverging in a coarse of time evidently exist at once after an earthquake. For example, the data on the Alaska earthquake (0336 UT, March 1964) published by Leonard and Barnes [1965] contain the information about negative pulses with an amplitude of ~ -15% registered at the distances of 2, 3, and 4.5 Mm at the Adak, Stenford, and Maui VS station, respectively. The azimuths of this stations lie in a sector of ~120^o. Therefore only the wave with axial symmetry is a hypothetical object to which all the registered negative splashes can belong. Their duration is 1-2 hours and evidently increases with the distance. The motion velocity is 1.5 Mm h ^-1. There are reasons to believe that during and after an earthquake a wave motion occurs from which the large-scale concentrated formations in the F2 ionospheric region considered in this paper differ considerably. At the same time, a positive pulse with an amplitude of ~20% leading by approximately 2 hours the earthquake moment is seen among the data of the Maui station [Leonard and Barnes, 1965]. If we accept that we have there a wave motion propagating with a velocity of ~1 Mm h ^-1 then one could suppose (bearing in mind that the distance from Alaska to Maui is ~4.5 Mm) that the positive pulse have been formed in the vicinity of the earthquake center approximately 7 hours before the earthquake moment. This agrees to both the data on the Japanese earthquake and the absence of the positive pulses in the appropriate time at the Canberra station. The arc of the Alaska-Maui great circle is away from Canberra by more than 4 Mm.

Thus the main hypothesis follows from the consideration of the ionospheric data on three earthquakes in the Northern hemisphere. The earthquake moments are preceded by 7-15 hours by the formation of a positive disturbance with a contrast of 15-20%. The disturbance is of a local character with aligned and lateral dimensions of ~4 Mm and moves as a whole along the great circle arc to the distances not less than 6-7 Mm. The detection of such object with corresponding motion may be considered as a precursor of an earthquake 2-3 hours in advance.

Naturally this hypothesis should be, first of all, checked against large experimental data. Also a series of alternatives should be solved, some of which need formulating already at this stage of the study. Below we enumerate they.

The length of the predictor trajectory do or does not exceed 7 Mm and the life-time of the objects is or is not, respectively, longer than 7 hours. For example, a recognition or non-recognition of the hypothesis that the parts of trajectories 1 and 2 fit each other forming a round-the-globe signal depends on the solution of this alternative. However the data on the Tashkent earthquake does not confirm the possibility of existence of round-the-globe signals. The second aspect of this alternative is a recognition or non-recognition of the fact that the trajectory length < 7 Mm is or is not a sign of the precursor. According to this the data shown in Figure 3 either are not (because of an excessive trajectory length) or are false configurations not related to earthquakes.

Figure 4

There is also an azimuthal alternative: the large-scale formations considered can move only along one trajectory outgoing from the earthquake site of origin, or a few objects moving along different trajectories can arise. The question on existence or absence of trajectories with increased probability of object appearance is the adjacent to the previous one. Some indications to such a hypothesis can be seen in the data obtained from the IRI 90 [Bilitza, 1990] (see Figure 4). The positions of the line of equal critical frequency f_cF2 = 6 MHz for various moments of Moscow time are show there. It is essential that only this line in the whole world map in the Merkator's projection (part of which is shown in Figure 4) contains the elements of increased curvature corresponding to presence of moving objects with contrast up to 10-15% and dimensions of ~3-4 Mm moving with a velocity of ~1 Mm h ^-1. It should be emphasized that the IRI 90 data [Bilitza, 1990] manifest stable formations in the ionosphere on the temporal basis of 11 years.

Conclusion

Analysis of the data on the F2 region vertical sounding at two crossing chains of stations demonstrated an informativity of the approach, in which isolate formations which form successions shifted in time at station chains are sought for in time dependence of the relative variations of the critical frequency df_cF2(t). This approach made it possible to determine their motion velocities ~1 Mm h ^-1 and longitudinal and lateral dimensions ~4 Mm. The objects can move to a distance of up to 10 Mm. The analysis of three earthquakes shows that formation of such isolate objects occurs 7-15 hours before the earthquake. It is proposed to consider registration of the motion of such objects to distances up to 7 Mm as one of the predictors of earthquakes 2-3 hours in advance.

Acknowledgments

References

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Kalinin, Yu. K., A. A. Romanchuk, N. P. Sergeenko, and A. A. Tolmacheva, Variabilities of statistic sample invariants for F2 critical frequency of the midlatitude ionosphere over a prolonged period, Geomagn. Aeron. (in Russian), 38, 610, 1998.

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