ISSN 00167932, Geomagnetism and Aeronomy, 2009, Vol. 49, No. 2, pp. 252–262. © Pleiades Publishing, Ltd., 2009. Original Russian Text © A.A. Namgaladze, M.V. Klimenko, V. V. Klimenko, I.E. Zakharenkova, 2009, published in Geomagnetizm i Aeronomiya, 2009, Vol. 49, No. 2, pp. 267–277.
Physical Mechanism and Mathematical Modeling of Earthquake Ionospheric Precursors Registered in Total Electron Content A. A. Namgaladzea, M. V. Klimenkob, V. V. Klimenkoc, and I. E. Zakharenkovac a
Murmansk State Technical University, ul. Sportivnaya 13, Murmansk, 183010 Russia b Kaliningrad State Technical University, Kaliningrad, Russia c Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Western Division, Russian Academy of Sciences, pr. Pobedy 41, Kaliningrad, 236017 Russia email:
[email protected] Received July 7, 2008; in final form, September 23, 2008
Abstract—The physical mechanism by which the regions with increased or decreased total electron content, registered by measuring delays of GPS satellite signals before strong earthquakes, originate in the ionosphere has been proposed. Vertical plasma transfer in the ionospheric F2 region under the action of the zonal electric field is the main disturbance formation factor. This field should be eastward, generating the upward compo nent of plasma electromagnetic drift, in the cases of increased total electron content at midlatitudes and deepened minimum of the F2 layer equatorial anomaly. Upward plasma drift increases electron density due to a decrease in the O+ ion loss rate at midlatitudes and decreases this density above the equator due to an enhancement of the fountain effect (plasma discharge into the equatorial anomaly crests). The pattern of the spatial distribution of the seismogenic electric field potential has been proposed. The eastward electric field can exist in the epicentral region only if positive and negative electric charges are located at the western and eastern boundaries of this region, respectively. The effectiveness of the proposed mechanism was studied by modeling the ionospheric response to the action of the electric field generated by such a charge configuration. The results of the numerical computations indicated that the total electron content before strong earthquakes at middle and low latitudes is in good agreement with the observations. PACS numbers: 91.30.Px, 94.20.dj DOI: 10.1134/S0016793209020169
1. INTRODUCTION
2. OBSERVATIONS
Observable variations in the state of the ionosphere before earthquakes are called ionospheric precursors of these events. Numerous groups of researchers (see, e.g. [Liperovskii et al., 1992] and references in [Zakharenkova, 2007]) have sought these precursors for the last two decades. This search has also been per formed in the scope of the specialpurpose projects of natural disaster space monitoring: COMPASS1, COMPASS2, Sich1M, QuakeSat, and DEMETER. The global system of navigation satellites (GPS) and the network of receivers of signals from these satellites have been widely used to study the ionospheric effects related to seismic activity [Liu et al., 2002, 2004; Plot kin, 2003; Afraimovich et al., 2004; Zakharenkova et al., 2006; Krankowski et al., 2006]. Measurements of time delays of these signals can be used to map the total electron content (TEC) in the unitsection col umn and to study the time evolution of this content, which reflects the variations in the maximal electron density (NmF2) in the ionospheric F2 layer (the main ionospheric maximum).
Extensive studies of the ionospheric precursors of earthquakes in TEC have been performed for the last time at the Institute of Terrestrial Magnetism, Iono sphere, and Radiowave Propagation, Russian Acad emy of Sciences (IZMIRAN) and its western division (Kaliningrad) together with the Kant Russian State University. In [Zakharenkova et al., 2005, 2006, 2006a, 2006b; Zakharenkova, 2007], it was found that the manifestation of strong midlatitude earthquakes in TEC has the form of a local increase in electron den sity, which is observed two–three days before an earth quake, and the disturbed region maximum is located in close proximity to the epicentral region. The spatial scale is several thousand kilometers along the parallel and about 1000 km along the meridian. As we approach an earthquake instant, a disturbance ampli tude increases and reaches 40–100% of the back ground level. It was revealed that electron density above an epicentral region tends to decrease 10–30 h before an earthquake. The value of this negative effect can reach –30% relative to an undisturbed state. Under quiet geomagnetic conditions, the sign reversal of a seismoionospheric disturbance can be interpreted
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Fig. 1. (a) Differential map of the TEC percentage deviations from the background level for 0200 UT (LT = UT + 9 h) on Sep tember 24, 2003, a day before the earthquake of September 25, 2003, in Japan (M = 8.3). (b) The same map for 2000 UT (LT = UT + 2 h) on January 7, 2006, a day before the earthquake of January 8, 2006, in Greece (M = 6.8). Geographic coordinates are plotted on the axes. A circle marks the earthquake epicenter.
as an impending earthquake signal. The negative effects in NmF2 one–two days before the earthquake were also considered in [Gokhberg et al., 1988; Puli nets et al., 1998; Pulinets, 1998; Depueva et al., 2007]. Figures 1a and 1b show the characteristic TEC dis turbances, which were observed before the midlatitude earthquakes of September 25, 2003 (Japan) [Zakharenkova, 2007] and January 8, 2006 (Greece) [Zakharenkova et al., 2006]. The precursors of strong lowlatitude (equatorial) earthquakes are changes in the shape of the equatorial anomaly in the F2 layer: daytime deepening and wid ening of the electron density minimum above the mag netic equator or, on the contrary, filing of this mini mum; displacement of the anomaly crests [Depueva and Ruzhin, 1993; Depueva and Rotanova, 2000; Pulinets and Legen’ka, 2002; Depueva et al., 2007; Zakharenkova et al., 2006] (see Fig. 2, where large black circles show the seismogenic equatorial anomaly in the ionosphere a day before the earthquake of April 12, 1963, in Peru according to the ALOUETTE1 sat ellite data. Two other lines show the foF2 variation dur ing undisturbed periods (before and after the earth quake). The epicenter position is marked by EQ [Depueva and Ruzhin, 1995; Ruzhin and Depueva, 1996]).
results in an increase in the ion production rate and electric conductivity of the lower atmosphere and in the generation of an extraneous electric field. A joint action of these processes results in an enhancement of the ionospheric electric field to several–several tens of m V m–1 [Chmyrev et al., 1989]. The following forc ible arguments for the hypothesis of a seismogenic electric field are available: (a) geomagnetic conjugacy of ionospheric precursors [Pulinets et al., 2003] and (b) the above effects in the equatorial anomaly, the development of which is controlled by the zonal elec tric field [Depueva and Ruzhin, 1993; Pulinets and Legen’ka, 2002; Zakharenkova et al., 2006]. In [Namgaladze, 2007; Namgaladze et al., 2007], the possible physical processes of formation of TEC disturbances with spatial and time scales typical of foF2, MHz 10
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3. ORIGINATION OF EARTHQUAKE IONOSPHERIC PRECURSORS Numerous works [ShortTerm …, 1999; Pulinets et al., 1999; Sorokin et al., 1999, 2005, 2006; Sorokin and Chmyrev, 2002; Pulinets and Boyarchuk, 2004] proposed to physically interpret the formation of largescale ionospheric earthquake precursors based mainly on the hypothesis of a seismogenic electric field related to vertical turbulent transfer of charged aerosols and radioactive substances (radon isotopes) injected into the atmosphere. An increase in the level of atmospheric radioactivity before an earthquake GEOMAGNETISM AND AERONOMY
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Fig. 2. Seismogenic equatorial anomaly (large filled cir cles) in the ionosphere a day before the earthquake of April 12, 1963, in Peru according to the ALOUETTE1 satellite data. Two other lines correspond to the foF2 variations dur ing the undisturbed period (before and after the earth quake). The epicenter position is marked by EQ. 2009
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western and eastern boundaries of this region, respec tively. To verify the above hypothesis and to elucidate the effectiveness of the proposed mechanism, we per formed the model calculations of the ionospheric response to the action of the electric field generated by such a configuration of charges in the vicinity of the earthquake epicenter (in the epicentral region) for midlatitude and equatorial earthquakes.
Geomagnetic latitude 90 75 60 45 30 15 0 −15 −30 −45 −60 −75 −90 0 30 60 90 120 150 180 210 240 270 300 330 360 Geomagnetic longitude Fig. 3. The geomagnetic coordinate difference grid used in the model calculations. Circles mark grid nodes where additional potentials were specified (filled and open circles correspond to positive and negative potentials, respec tively); crosses show the positions of the earthquake epi centers.
precursors were analyzed, and the assumption was made that the regions with increased (decreased) TEC in the ionosphere, observed by measuring GPS satel lite signal delays before strong earthquakes [Zakharen kova, 2007], are mainly generated as a result of the upward (downward) plasma transfer in the iono spheric F2 layer under the action of the eastward (west ward) zonal electric field. At midlatitudes, the electromagnetic drift vertical component, generated by the eastward field and directed upward, results in an increase in the electron density at a maximum of the F2 layer (NmF2) due to plasma transfer into the regions with a lower density of O2 and N2 neutral molecules and, correspondingly, with a lower loss rate in the ion–molecular reactions of O+ ions dominating in the F2 layer [Bryunelli and Namgaladze, 1988]. The oppositely directed field (westward) causes an opposite (negative) effect in NmF2 and TEC. At low latitudes (near the geomag netic equator), an enhancement of the eastward field deepens the minimum of the equatorial anomaly of the ionospheric F2 layer due to an increase in the foun tain effect (plasma discharge into the equatorial anomaly crests). The pattern of the potential spatial distribution for the eastward electric field was proposed: the eastward electric field can exist in the epicentral region only if positive and negative electric charges are located at the
4. MODEL CALCULATIONS The calculations were performed using the global numerical theoretical model of the Earth’s upper atmosphere [Namgaladze et al., 1988, 1990, 1991] with the modified solution of the equation for the elec tric potential [V.V. Klimenko et al., M.V. Klimenko et al., 2006]. The model is based on the numerical integration of the quasihydrodynamic equations of continuity, momentum, and heat balance for neutral and charged particles of cold nearEarth plasma together with the equation for the electric field poten tial in the altitude range from 80 km to the geocentric distance of 15 Earth’s radii, taking into account the noncoincidence of the Earth’s geographic and geo magnetic axes. The results of model calculations were analyzed by comparing the global maps of distribution of different ionospheric parameters obtained for undisturbed (without an additional electric field) and disturbed (with additional (supposedly seismogenic) electric field sources) conditions. We assumed that the undisturbed state corre sponded to the quiet day of the March equinox at low solar activity. The disturbed state, which was suppos edly observed before a strong earthquake, was mod eled including additional electric field sources. 5. ADDITIONAL (SEISMOGENIC) ELECTRIC FIELD SOURCES These sources were included and maintained dur ing 24 h as additional positive and negative potentials of 5 kV (in the case of a lowlatitude source) and 10 kV (a midlatitude source) at the western and eastern boundaries of the epicentral regions, respectively. Additional potentials were added as sources to the modeling equation in order to calculate the global potential distribution dependent on the quiet mag netospheric and thermospheric (the dynamo field) sources, which was subsequently integrated taking into account these additional sources. We considered two epicentral regions, which extend for 10° in latitude and 30° in longitude and have epicenter at the points with the following geo magnetic coordinates: (1) Φ = 45°, Λ = 90° (conditionally, Rome); (2) Φ = –15°, Λ = 210° (conditionally, Vanimo).
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Such dimensions approximately correspond to the horizontal extensions of the regions with the increased TEC values (see [Zakharenkova, 2007]). The first region is a typical midlatitude ionosphere, and the sec ond region is the equatorial ionosphere where the electric field effects are more substantial than at mid latitudes. Figure 3 shows the geomagnetic coordinate differ ence grid with nodes (marked by circles), where addi tional potentials were specified (filled and open circles correspond to the positive and negative potentials, respectively).
(without additional sources) are shown in the lower panels of Figs. 4 and 5 for comparison. Filled circles mark the positions of the earthquake epicenters; a cir cle with a dot, the position of the subsolar point.
6. ELECTRIC POTENTIAL AND FIELD CALCULATED IN THE ABSENCE AND IN THE PRESENCE OF ADDITIONAL SOURCES
The values of the appeared additional eastward fields are pronouncedly larger than those of the back ground undisturbed fields in the cases of the equatorial (an increase in the zonal field component in Vanimo from 0.2 to 2.0–3.5 mV m–1) and midlatitude (an increase in the zonal field component in Rome from ~1 to ~ 4–10 mV m–1) sources but, however, are sub stantially smaller than the values of highlatitude elec tric field of the magnetospheric origin under quiet conditions (~15–25 mV m–1, see Fig. 5).
The calculated global distributions of the potential and electric field vectors are shown in geomagnetic coordinates in Figs. 4 and 5, respectively, for the cases of the equatorial (lefthand upper panel) and midlati tude (right, upper) sources. The potential and electric field vector distributions under undisturbed conditions GEOMAGNETISM AND AERONOMY
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Figures 4 and 5 show the regions of the eastward electric field above the assumed epicenters of future earthquakes, which appear when additional sources are superimposed, and similar magnetically conjugate regions in the opposite hemispheres. The potential and electric field symmetry about the magnetic equa tor is caused by an ideal conductivity of the iono spheric plasma along the geomagnetic field lines and, correspondingly, by their electric equipotentiality.
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Fig. 5. Global distributions of the electric field vectors (in mV m–1) calculated for the cases without (lower panels) and with (upper panels) additional seismogenic sources of the electric field. The calculations for the equatorial and midlatitude sources are illustrated in the left and righthand panels, respectively. Filled circles and circles with dots mark the positions of the earthquake epicenters and the subsolar point, respectively. Dashed lines show the geographic equator and the noon–midnight and sunrise– sunset meridians.
7. IONOSPHERIC EFFECTS OF ADDITIONAL ELECTRIC FIELD SOURCES We now consider the effects that are caused by the appeared eastward fields in the modeled ionospheric parameters. Figures 6a–6c and 7a–7c show the calcu lated global distributions of the foF2 critical frequen cies and TEC under the action of constant additional sources and in the absence of these sources at 0300 (Figs. 6a, 7a), 0800 (Figs. 6b, 7b), and 2300 (Figs. 6c, 7c) UT. These instants correspond to the situations when the earthquake epicenters and, correspondingly, additional sources of the seismogenic electric field are differently sunlit. At 0300 UT, the midlatitude and equatorial sources are on the nightside and near the subsolar point, respectively. At 0800 UT, the midlatitude source is already on the sunlit morning side, and the equatorial source is still on the sunlit evening side. At 2300 UT, the midlatitude source is again on the nightside, and
the equatorial source is in the early morning zone. At 2300 UT, the midlatitude source is again on the night side; the equatorial source, in the early morning zone. The effects of TEC are similar to those of foF2 at middle and low latitudes. The maximal absolute devi ations of foF2 from the background values under the action of the midlatitude and equatorial sources of the additional electric field are ~0.5 and ~1.5 MHz, respectively. In this case the maximal relative TEC deviations from the background values under the action of the midlatitude source reach ~60% and are larger than 100% for the equatorial source. The action of the equatorial source of the eastward electric field results in the displacement of the equatorial anomaly crests from ±(10°–15°) to ±(20°–25°) geomagnetic latitude. The main specific features of the obtained model pattern are as follows.
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Fig. 6. Global distributions of foF2 (MHz) at (a) 0300 UT, (b) 0800 UT, and (c) 2300 UT calculated for the cases without (lower panels) and with (upper and middle panels) additional seismogenic sources of the electric field. The calculations for the equatorial and midlatitude sources are illustrated in the upper and middle panels, respectively. Filled circles and circles with dots mark the positions of the earthquake epicenters and the subsolar point, respectively.
7.1. The Equatorial Source The effect of the equatorial source intensifies the known equatorial anomaly of the F2 layer in the epi central region, namely: deepens the foF2 minimum above the equator in the epicentral region and shifts the anomaly crests from the equator to midlatitudes. Such a behavior absolutely agrees with the ALOU GEOMAGNETISM AND AERONOMY
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ETTE1 data a day before the earthquake of April 12, 1963, in Peru [Depueva and Ruzhin, 1995; Ruzhin and Depueva, 1996] (see Fig. 2) and with the latitudi nal variations in foF2 observed (according to the ALOUETTE1 data) more than six days and less than five days before the strong earthquake of August 15, 1963, which had the epicenter near the magnetic 2009
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precursors found in the TEC data [Zakharenkova, 2007]. The source has no pronounced effects at night, and the effects appear after the sunrise and remain after the sunset, which is shown when the illustrated situations are compared at 0300 and 2300 UT. Note that the calculated electric field value in the case of the midlatitude source is twice as large as in the case of the lowlatitude source, whereas the effects of this field are smaller at midlatitudes than at low lati tudes. In addition to the effects in the electron density in the F2 layer and in TEC, the effect of both sources is observed as distinct and, sometimes, very strong effects in almost all modeled ionospheric parameters: electron and ion temperature and, especially, equato rial electrojet.
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equator in the American longitudinal sector [Depueva et al., 2007]. 7.2. The Midlatitude Source The effect of the midlatitude source results in an increase in foF2 and TEC in the epicentral regions, if these regions are sunlit. The values of increases and their spatial scales coincide with the characteristics of
8. DISCUSSION The obtained model effects in foF2 and TEC in the epicentral regions, caused by the seismogenic sources of the electric field, are very similar to the effects observed before strong earthquakes at low and middle latitudes and described in the Section 2 (see Figs. 1, 2). The spatial scales, signs, and relative values of distur bances coincide, which indicates that the model of additional potentials is generally correct. By varying the location and value of these additional potentials, we can evidently make the model effects even more similar to the observed ionospheric precursors of earthquakes. Thus, in our opinion the performed model calcula tions and their agreement with the observations are convincing arguments for the hypothesis that zonal (eastward or westward) electric fields with magnitudes of about 3–10 mV m–1 appear in the epicentral regions several days before earthquakes and cause local increases (or decreases) in TEC at midlatitudes and intensifications (or weakening) of the equatorial anomaly in the lowlatitude ionospheric F2 layer. Such values of the seismogenic electric field agree with the estimates obtained in [Sorokin et al., 1999, 2005; Sorokin and Chmyrev, 2002] and with the rocket observations of intense (>10 mV m–1) electric field in the ionospheric F2 layer, related to seismic activity [Yokoyama et al., 2002]. Note that we do not discuss here the mechanism by which the seismogenic field is generated and do not explain why positive and negative charges should be accumulated at the western and eastern boundaries of the epicentral region, respectively, or vice versa. We specify the type of the required electric field and the mechanism by which this field affects plasma in the ionospheric F2 layer. For the accepted hypothesis, it is substantial that the additional electric field is zonal and, therefore, the vertical component of plasma motion exists in the region of the electron density
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Fig. 7. Global distributions of logTEC [cm–2] at (a) 0300 UT, (b) 0800 UT, and (c) 2300 UT calculated for the cases without (lower panels) and with (upper and middle panels) additional seismogenic sources of the electric field. The calculations for the equatorial and midlatitude sources are illustrated in the upper and middle panels, respectively. Filled circles and circles with dots mark the positions of the earthquake epicenters and the subsolar point, respectively.
maximum in the ionospheric F2 layer. We assume that magnetic conjugacy of earthquake ionospheric pre cursors and the wellknown fact that precisely the zonal electric field is responsible for the shape of the equatorial anomaly of the ionospheric F2 layer [Bryunelli and Namgaladze, 1988] are the conclusive GEOMAGNETISM AND AERONOMY
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arguments for the zonal electric field of any sign. Under usual conditions, the electric field of the equa torial anomaly is generated by the dynamo action of winds of the neutral atmosphere. These winds cannot substantially change during about a day and subse quently return to their initial state due to inertia of the 2009
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9. CONCLUSIONS
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We proposed the physical explanation of increases or decreases in the maximal electron density in the ionospheric F2 layer and TEC in the unitsection col umn, which are observed during one–four days before strong earthquakes and are localized in the epicentral regions extending for 10° in latitude and 30° in longi tude, and verified the adequacy of the proposed expla nation by using the method of mathematical modeling with the help of the global numerical model of the ion osphere. Vertical drift of the ionospheric F2 layer plasma under the action of the zonal electric field of the seis mogenic origin is the main factor of formation of NmF2 and TEC disturbances. If TEC increases at midlatitudes and the equatorial minimum of the F2 layer anomaly deepens, this field is eastward and causes electromagnetic plasma drift across the geo magnetic field at a velocity directed strictly up above the magnetic equator and up and toward the pole at midlatitudes. Upward plasma transfer increases electron density in the ionospheric F2 layer due to a decrease in the loss rate of dominating O+ ions in the ion–molecular reac tions at midlatitudes and decreases NmF2 above the equator due to an enhancement of the fountain effect (plasma discharge into the equatorial anomaly crests) at low latitudes. We proposed the pattern of the spatial distribution of the seismogenic electric field potential. The east ward electric field can exist in the epicentral region only if positive and negative electric charges are located at the western and eastern boundaries of this region, respectively. The effectiveness of the proposed mechanism was studied by modeling the ionospheric response to the action of the zonal electric fields with magnitudes of 3–10 mV m–1, generated by such a configuration of charges caused by seismogenic sources located at mid dle and low latitudes. The results of the numerical computations are in good agreement with the observa tions of TEC before strong earthquakes at middle and low latitudes in the spatial scales and amplitude char acteristics. ACKNOWLEDGMENTS
Fig. 7. (Contd.)
atmosphere, and the magnetospheric electric field cannot penetrate to low latitudes during this period due to the shielding effect of the zone2 fieldaligned currents [Lyatsky and Maltsev, 1983]. Consequently, it seems most probable that the electric field, disturbing the equatorial anomaly before strong earthquakes, is of the seismic origin.
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