ISSN 00167932, Geomagnetism and Aeronomy, 2012, Vol. 52, No. 3, pp. 390–397. © Pleiades Publishing, Ltd., 2012. Original Russian Text © O.V. Zolotov, A.A. Namgaladze, I.E. Zakharenkova, O.V. Martynenko, I.I. Shagimuratov, 2012, published in Geomagnetizm i Aeronomiya, 2012, Vol. 52, No. 3, pp. 413–420.
Physical Interpretation and Mathematical Simulation of Ionospheric Precursors of Earthquakes at Midlatitudes O. V. Zolotova, A. A. Namgaladzea, I. E. Zakharenkovab, O. V. Martynenkoa, and I. I. Shagimuratovb a Murmansk
State Technical University, Murmansk, Russia email:
[email protected] b Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Russian Academy of Sciences, Kaliningrad, Russia Received May 15, 2010; in final form, September 29, 2010
Abstract—The paper presents the results of studying anomalous variations in the total electron content (TEC) of the ionosphere as probable precursors of strong seismic events. The vertical drift of the F2 layer’s ionospheric plasma under the effect of seismically generated zonal electric field is considered as a likely rea son for the observed variations in the TEC. An estimation of this drift effects is made by mathematical simu lation utilizing the global numerical model of the Earth’s upper atmosphere (UAM). Midlatitude ionospheric effects were simulated. Two types of seismogenerated electric fields (dipole and monopole) were used with various magnitudes and spatial configurations. The derived results were compared with the TEC data of GPS observations from the IGS for the Kitira earthquake in southern Greece (January 8, 2006; M 6.8). It was shown that variations generated by additional sources of the dipole type are consistent with the observed data; monopoletype sources did not reproduce some typical peculiarities of these observations and systematically underestimated the deviation value. DOI: 10.1134/S0016793212030152
INTRODUCTION The issue of early earthquake prediction has been the subject of research for several decades. The hopes to solve it were initially related to the growth in the number of seismic networks and based on analyzing seismic records, including analyzing the Earth’s sur face deformations, geochemical parameters, on varia tions in magnetic and electric fields, etc. Precursors were found among parameters of the ionosphere, which responds to the effects of penetration from below after strong earthquakes through the neutral atmosphere. Anomalous variations in ionospheric parameters (electron concentrations in the E, F1, and F2 layers, total electron content (TEC), electron and ion temperatures, critical frequencies of the F2 layer, variations in ELF/VLF signals, sporadic layer forma tion, large and mediumscale migrating ionospheric disturbances and electric fields, etc), attributed to the preparatory processes of strong seismic events, were reported in (Afraimovich et al., 2004; Depueva and Rotanova, 2001; Depueva et al., 2007; Gokhberg et al., 1988; Hayakawa and Molchanov, 2002; Krankowski, Zakharenkova, and Shagimuratov, 2006; Molchanov et al., 1993, 2006; Ouzounov et al., 2007; Pulinets, 1998; Pulinets et al., 2005; Ruzhin and Depueva, 1996; Zakharenkova et al., 2007).
Many authors (Plotkin, 2003; Pulinets and Legen’ka, 2006; Pulinets et al., 2003, 2005, 2010; Liu, Chuo, and Chen, 2001; Lui et al., 2002, 2004; Zakharenkova, 2007; Zakharenkova et al., 2007) investigated the characteristics of TEC modifications preceding strong earthquakes based on ionosondes, special satellites, and by other methods; in recent years, the most popular way is utilizing data of global navigation systems and their groundbased receiver networks. For example, according to (Zakharenkova, 2007; Zakharenkova, Krankowski, and Shagimuratov, 2006; Zakharenkova et al., 2006, 2007), TEC effects are manifested several (usually two to three) days prior to a strong earthquake and are seen as locally large longliving increase in the electron content, with the maximum of the disturbed region in the immediate vicinity of the epicenter. The spatial extent of such a disturbance is several thousand kilometers along the parallel and about 1000 km along the meridian. As the moment of the earthquake occurrence approaches, the amplitude of the disturbance increases and reaches a value 40–100% above the background level. A ten dency is revealed that the electron content decreases above the epicentral area 10–30 h prior to an earth quake. The value of this drop can be –30% relative to the undisturbed level. Under quiet geomagnetic con ditions, the sign alternation of a seismoionospheric disturbance may be interpreted as a signal of an
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impending earthquake. Largescale disturbances in the F2 layer’s electron content are observed under cer tain conditions in geomagnetically coupled areas (Ruzhin, Larkina, and Depueva, 1998; Pulinets and Legen’ka, 2006; Pulinets et al., 2003). In the present work, we study the effects of the ion ospheric TEC. The TEC is an integrated characteristic that is largely determined by the maximal electron concentrations of the F2 layer and is calculated as the electron number in a single column above the Earth’s surface. In recent years, TEC monitoring has been implemented utilizing global navigation systems (such as GPS in the United States and GLONASS in Rus sia) and their groundbased receivers. This allows researchers to perform continuous observations of the ionospheric TEC with a good spatial and temporal res olution, i.e., TEC observations allow the spatial extent and time dynamics of seismoionospheric effects in seismoactive regions to be assessed. TEC values are initially calculated at the raypath based on the phase and group delays of signals; then, the “sloped” TEC is transformed into a vertical one with the help of, for example, the onelayered model of the ionosphere. The observation results (and the treated TEC maps as well) are often freely available, for example, the IGS network data (Dow, Neilan, and Gendt, 2005) used in the present paper. Many authors (Sorokin and Chmyrev, 1999, 2002; Pulinets, 1998; Pulinets and Boyarchuk, 2004; Sorokin, Chmyrev, and Yaschenko, 2005; Sorokin, Yaschenko, and Hayakawa, 2006) attribute the physics of precursor formation in the TEC to the hypothetical seismogenic electric field. The ionospheric effects of such a field were mainly studied in terms of the one dimensional model, largely with respect to the lower ionosphere (Kim, Hegai, and IllichSvitych, 1993; Hegai, Kim, and Nikiforova, 1997; Kim, Pulinets, and Hegai, 2002). In the present study, we proceed from the hypothe sis that the principal reason why anomalous variations in the TEC appear is the vertical ionospheric plasma drift from the F2 layer under the effect of a seismically generated zonal electric field (Namalgadze et al., 2008). At midlatitudes, the vertical component of the electromagnetic drift, created by a field directed to the east and being directed upwards, leads to an increase in the electron content within the F2 layer’s maximum (NmF2) owing to plasma transfer to areas possessing a smaller concentration of neutral molecules О2 and N2 and, hence, with a lower rate of loss for О+ ions that dominate in the F2 layer (Bryunelli and Namalgadze, 1988). The oppositely directed field (directed to the west with the vertical drift component directed down wards) causes the opposite (negative) effect on the TEC. To determine the field’s spatial distribution that generates the observed disturbances in the TEC and to verify the suggested way of how plasma is also affected
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by the ionospheric F region, we performed a series of numerical experiments utilizing the global nonstation ary selfconsistent upper atmosphere model (UAM) (Namalgadze et al., 1988, 1991; Namalgadze A.A., Martynenko, and Namalgadze A.N., 1998). OBSERVATIONS Let us consider an earthquake that occurred on January 8, 2006, at 1135 UT (1335 LT) in Kitira, southern Greece. The earthquake’s epicenter was located on the seafloor east of Kitira Island (36°20′N, 23°20′E), the hypocenter was at a depth of 66 km (USGS data), and the magnitude was M = 6.8. After the main shock, aftershocks of up to M = 5.5 were recorded during the three following days. The geomagnetic activity was low and changed lit tle during the earthquake preparation (see Fig. 1); therefore, we can assume that there were no distur bances capable of making the studied seismoiono spheric effects indistinguishable. To determine the spatial characteristics of the ion ospheric response to the discussed seismic event, we applied differential mapping of the TEC deviation. The TEC values for the set time moment were com pared with the variations under quiet (calculated as the median of a certain range) background conditions. The respective maps of the TEC deviation on January 7, 2006, are given in Fig. 2. 2012
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According to Fig. 2, the region of positive varia tions in the TEC started to form at 1000 UT in the epi central area of the impending earthquake. The ampli tude of the anomalous variations in the TEC reached 38–55% relative to the background level and was observed during the following 10–12 h, reaching peak values (55%) in the period 1800–2000 UT. The region has a typical dome shape with a clearly expressed local character and peak position in the immediate vicinity of the earthquake’s epicenter. The characteristic linear dimension of the anomaly, which exceeded the back ground TEC level by more than 35%, was about 4000 km in longitude and 1500 km in latitude.
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In order to verify whether the observed variations were local rather than global, we applied the technique for global electron content (GEC) calculation, sug gested in (Afraimovich et al., 2006). A GEC analysis was made for several sectors centered relative to the epicenter zone: four square domains of 10° × 10° (a), 20° × 20° (b), 30° × 30° (c), and 40° × 40° (d) in size and two rectangular ones (extended along the latitude) 24 а b 22 20 18 16 14 12 10 8 6 03 04 05 06 07 08 09 10 11 12 13 Days (January 3–13, 2006)
Fig. 3. GEC variations in a sector of 10° × 10° in size (cen tered to the epicenter) (a) and in a latitudinally extended zone of 45° × 150° in size for the period January 3–13, 2006. The upwarddirected arrow indicates the moment of the earthquake.
of 40° × 90° (e) and 45° × 150° (f) in size. The GEC calculation results for domains (a) and (f) are given in Fig. 3. It follows from the data in Fig. 3 that as the covered domain becomes larger, the anomaly is rendered less expressive. In the case shown in Fig. 3b, any visible deviation from the background TEC level on January 7, 2006, cannot be distinguished; in contrast to this, the anomaly is clearly seen in the case shown in Fig. 3a. These points allow us to believe that the studied seis moionospheric effects are regional ones and localized in the epicentral area. Thus, the ionospheric TEC anomaly associated with the mentioned earthquake in Greece (midlatitudes) on January 8, 2006, was a local one and was found on January 7, i.e., the day before this strong earthquake. PHYSICAL INTERPRETATION OF THE OBSERVED DATA Since TEC variations reflect those in NmF2, it is natural to find the causes of the observed positive dis turbances in the TEC among the known ways of how positive disturbances in NmF2 are formed; such ways are discussed in detail, for example, in (Bryunelli and Namalgadze, 1988). These causes may be (a) plasma fluxes from the plasmasphere along the geomagnetic field lines, (b) changes in the neutral composition (an increase in the ratio between the concentrations of atomic and molecular components of the thermo sphere), (c) thermospheric winds directed towards the equator, (d) upward plasma transport by the zonal (directed towards the east) electric field, and (e) zonal plasma transport by the meridional electric field. It seems that flows in the plasmasphere should be excluded from the possible causes, because they only induce a nighttime increase in NmF2 but do not change the TEC, only leading to plasma redistribution within the geomagnetic field tube. Changes in the neutral composition must not be excluded, because they may be caused by changes in the turbulent diffusion regime in the lower and middle
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atmosphere and, hence, those of the turbopause alti tude in the epicentral area. The respective effects can be easily studied utilizing modern global ionospheric– thermospheric models through numerical experi ments with different altitude profiles of the turbulent diffusion coefficient. However, this way does not explain the observed magnetic coupling of the iono spheric effects preceding a seismic event. Generating thermospheric winds directed towards the equator requires certain gradients of neutral gas pressure at an altitude of the F2 layer proper, but this seems doubtful. Electric fields can easily be transferred from the lower ionosphere (if they emerge there) into the upper one along geomagnetic field lines and further to the opposite hemisphere, so they are readily considered to be the main cause of earthquake precursor formation in the TEC because they provide geomagnetic cou pling of ionospheric effects. Ionospheric plasma in the F2 layer is magnetized and travels perpendicularly to the geomagnetic field at the velocity of an electromag netic drift under the effect of the electric field. Depending on the field orientation, plasma transport by this drift can both increase and decrease the elec tron content in the F2 layer. At low latitudes, vertical plasma transport upwards by an electromagnetic drift forms the socalled equatorial anomaly (trough of electron concentration above the geomagnetic equa tor) in the daytime. At midlatitudes, the upward verti cal component of the drift leads to an increase in the electron content owing to the plasma influx into domains with lower concentrations of neutral mole cules О2 and N2 and, hence, with a lower rate of loss for О+ ions that dominate in the F2 layer. Zonal plasma drifts can increase the electron con tent in convergence regions (where oppositely directed currents are met), i.e., when the direction of move ment is changed. Due to the embedding of the geo magnetic field into the plasma, such a transverse downward plasma flow into the tube will mean plasma compression with growth in both gas and magnetic pressures, but it is unlikely that this compression can be longterm because an opposite movement must be under the effect of the gradients of these pressures (analogous to how it occurs in magnetoacoustic waves). Finally, an electromagnetic drift causes Joule heat ing of ion and neutral gases; then, the respective increases in the temperatures of gases change the ambipolar diffusion rates and accelerate the process of ion–molecular reactions. The resulting effect on the electron concentration will be mainly negative (decrease) or just lead to plasma redistribution in the geomagnetic field tube that will not have any influence on the TEC. GEOMAGNETISM AND AERONOMY
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It follows from qualitative considerations that upward plasma transport under the action of zonal electric field directed towards the east is the most likely electromagnetic way of how the observed increases in the TEC are formed at midlatitudes. The presence of such a field in the epicentral area requires positive ions on western side of this area and negative ions on its eastern side; it is analogous to how positive and negative ions are located on the morning and evening sides of a polar cap, respectively, and thus form the electric field of magnetospheric–ionospheric convection. In this case, the pattern of the electric potential distribution in the ionosphere above the epi central area will be qualitatively similar to the pattern of the field potential of magnetospheric–ionospheric convection. NUMERICAL EXPERIMENT AND RESULTS Studies of the electromagnetic way of how anoma lous variations in the TEC are formed were performed by mathematical simulation utilizing the global physi cal–mathematical model of the Earth’s upper atmo sphere (UAM) (Namalgadze et al., 1988, 1991; Nama lgadze A.A., Martynenko, and Namalgadze A.N., 1998). The UAM is a global threedimensional nonsta tionary numerical model that describes the thermo sphere, ionosphere, and plasmosphere as a whole; the model covers an altitude range from 80 km to 15RE of the geocentric distance and takes into account the noncoincidence of the Earth’s geomagnetic and geo graphic poles. With the help of numerical integration of the system of quasihydrodynamical equations that describe the preservation laws for particles, impulse, and energy (equations of continuity, motion, and ther mal balance), it allows us to calculate the desired parameters, for example, the concentrations of the principal neutral (O2, N2, and O) and charged (XY+, O+, and H+) components of the atmosphere; temper atures of neutral, ion, and electron gases; ion motion rates, etc. In addition to these, the model enables to solve the equation for potential ϕ of the magneto spheric, thermospheric (thermospheric dynamo), and seismic electric fields: ˆ (∇ϕ − V × B) − j m ] = 0, ∇ [σ
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ˆ is the tensor of ionospheric conductivity; V is where σ the massaveraged motion velocity of neutral gas; B is the vector of the geomagnetic field density; jm is the density of the magnetospheric current. After integrating the given equation over the thick ness of the current sheet (80 to 175 km) with the alti tude dependence of components of the electric field’s vector neglected, the problem of electric potential cal culation is reduced to a twodimensional one. It is supposed that there are no charged particles lower 2012
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than 80 km and the plasma is magnetized above 175 km; i.e., geomagnetic field lines are equipotential. The ionospheric conductivities that are incorporated into ˆ are calculated by standard the tensor of conductivity σ formulas. To calculate the field of magnetospheric convection in the UAM, Eq. (1) is solved with bound ary conditions in the form of potential distributions and/or fieldaligned currents in zones 1 and 2 on the boundary of a polar cap. Additional electric, supposedly seismogenic, fields were calculated by solving Eq. (1) with boundary con ditions on the boundary of the epicentral area in the form of hypothetical sources of electric potential (essentially, these are electric charges and the respec tive potentials) of different types and spatial configu rations at the nodes of the numerical grid (at an alti tude of 175 km above the Earth’s surface). These sources were activated at 0000 UT and operated con tinuously during the whole day simulated. In this numerical experiment, we considered two types of additional sources of seismic origin (a dipole one that consists of charges with different signs and a monopole one that consists of only positive charges) and nine versions of their spatial configurations (Fig. 4). Calcu lations were made for quiet solar and geomagnetic conditions and for different magnitudes of additional electric potentials: (1) 10 kV for dipole sources and (2) 10 and 20 kV for positively charged ones. Resulting from these experiments, we derived the spatial distri butions of electric fields and TEC disturbances gener ated by them. The calculated TEC disturbances are compared with observational data on the Kitira mid latitude earthquake of January 8, 2006 (Fig. 5). Earlier, a study of how additional electric field sources influence TEC variations in midlatitude and equatorial regions was carried out by Namgaladze et al. (2009), but there were no sources related to con crete earthquakes (only one configuration and only one (dipole) type of sources); the calculation was made in a rough numerical grid; and, finally, the task
was to verify the principal possibility of the studied phenomenon. Several studied configurations of the electric field distribution generated TEC disturbances, which were not significantly different from each other. To simplify the analysis, we only presented the most different con figurations: two configurations for dipole sources (Figs. 4a, 4b) and two configurations for additional seismogenic positivetype sources of electric potential (Figs. 4c, 4d). The calculated magnitudes of the electric field’s eastward component do not exceed values of 16 mV/m and reach a peak at some isolated points (correspond ing to the nodes of the numerical grid that are the clos est to the grid nodes with the introduced additional potential). Such values of the electric field are consis tent with the estimates of other authors (Chmyrev et al., 1989; Sorokin, Chmyrev, and Yaschenko, 2005), as well as with rocketbased measurements of highden sity electric fields in the Е layer of the ionosphere (Yokoyama et al., 2002) (such fields are also associated with seismic activity). Additional dipoletype sources (10 kV) generate disturbances of TEC variations in the ionosphere, which are more intensive than monopole (positive) sources of electric fields, for both +10 and +20 kV magnitudes of the set electric potential. Figure 5 presents the latitudinal sequence of distur bances in the TEC in comparison with quiet back ground conditions for morning and evening time moments in the simulated case and based on GPS observations of the IGS network for the January 8, 2006, Kitira earthquake in southern Greece. It is seen in Fig. 5 that TEC variations, generated by dipole sources, reproduce the shape and peculiarities of these observations, as well as the positions of peaks and troughs for both daytime and nighttime condi tions (calculation variants). The deviation values of the TEC variations generated by these sources are consistent with the GPS observation data and remain within the error limits or, at least, within the limits of
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ordinary (natural) variability of this parameter at mid latitudes under quiet conditions. Additional sources of the monopole (positive) type do not reproduce the magnitude and shape (some typ ical peculiarities of the initially observed data) for both nighttime and daytime conditions at midlatitudes. An increase in the magnitude for this type of additional electric potential from +10 to +20 kV does not improve the consistency with the observation data. CONCLUSIONS In the paper, the hypothesis about the vertical drift of the ionospheric plasma of the F2 layer under the effect of the zonal component of the seismogenic elec tric field (the drift was assumed to be the cause of appearing anomalous longliving positive or negative anomalies in the TEC) has been verified by mathemat ical simulation utilizing the UAM model. The electric fields generated by hypothetical seismogenic sources, the concentrations of the principal neutral and charged components of the atmosphere, and the TEC variations have been calculated by numerically solving GEOMAGNETISM AND AERONOMY
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a system of quasihydrodynamical equations of conti nuity, motion, and thermal balance with a supplemen tary modified equation for the electric field potential. Nine spatial configurations and two types (dipole and monopole) and two magnitudes of additional electric potential sources have been studied, as well as the respective spatial distributions of electric fields. The disturbances in the TEC generated by these fields have been compared with GPS data from the IGS net work for the Kitira earthquake (southern Greece) of January 8, 2006 (M 6.8). It has been found that additional dipoletype sources generate disturbances in the ionosphere above middle latitudes and are well consistent with the observed data (at least qualitatively or within the limits of this parameter’s natural variability). Monopole (positive) additional sources of electric potential do not reproduce some peculiarities that are inherent to the observed data, such as extremum posi tions and the magnitude of TEC variations at midlati tudes. An increase in the magnitude of the additional potential from +10 to +20 kV poorly influences the 2012
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pattern of the generated ionospheric disturbances and does not improve the consistency either.
Liu, J.Y., Chuo, Y.J., and Chen, Y.I., Ionospheric GPS TEC Perturbations Prior to the 20 September 1999, ChiChi Earthquake, Geophys. Res. Lett., 2001, vol. 28, pp. 1383–1386.
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