Annals 47, 1, 2004, 01/07def 229 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Key words electromagnetic emission – temperature – Earth surface – impedance – earthquake precursor – 1 Hz frequency range New electromagnetic emission termed «thermal geomagnetic emission» has been de- scribed elsewhere (Gokhberg, 1998). This emis- sion occurs during the daytime and looks like broad band noise in the frequency range 1-5 Hz. The noise is supposed to be related to a thermal regime of the near Earth surface atmospheric layer. The example of the so-called type 1 emis- sion is shown in fig. 1a. During the local evening hours and night time the emission occurs with rising frequency noise bands, which look like a «fan». This type of emission (that we called as type 2) has been described in several papers (Larsen and Ege- land, 1968; Beamish and Tzanis, 1986; Bely- aev et al., 1987) and explained using the theory of the ionospheric Alfven resonator (see fig. 1b). Diurnal and seasonal variations of intensity for both types are represented in fig. 2. One can see that the diurnal maximum of «thermal emis- sion» (type 1, dark area) take place in between 12-14 UT or 15-16 LT for Borok Observatory (58.03°N, 39.97°E) and for «fan» (type 2, light area) between 16-18 UT or 19-21 LT. The seasonal distribution for type 1 emis- sion has one maximum during the summer time while for type 2 emission two maxima take place during the spring and autumn. Shown in fig. 3a are the thermal emission in- tensity (black vertical line) for each day dur- ing July 1998 and daily temperature variations measured at Borok Observatory. One can see a good correlation between the sharp gradient of the temperature decrease and deep decrease of the emission intensity starting from July 12 to 13. Similar features are seen in fig. 3b with in- tensity and temperature variations measured at Mondy Observatory (52.2°N, 104.5°E) during August 1997. For both observatories the gen- eral features of such an emission are similar if we take into account the shift of about 5 h in UT relative to their longitudinal position. The analysis of the impedance Z = E /H was produced for close to simultaneous records of Pc-1 and «thermal emission». The Pc-1 imped- Background electromagnetic noise in the vicinity 1 Hz as a possible indicator of earthquake-related anomalies Mikhail B. Gokhberg Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia Abstract Measurements of space-time distributions of the background emissions in the frequency range 0.5-5 Hz and their seasonal and diurnal variations are presented. The analysis of impedance ratio determines the presence of high space harmonics in the emission spectral distribution. The absence of this emission during the winter time cor- relates with the absence of any precursors before local earthquakes with M ∼ 6. It is shown that the knowledge of the background emission behavior and its origin are very important for precursor phenomena monitoring. Mailing address: Dr. Mikhail B. Gokhberg, Schmidt United Institute of Physics of the Earth, Russian Academy of Sciences, ul. Bolshaya Gruzinskaya 10, D-242, GSP-5, 123995 Moscow, Russia; e-mail: gmb@ifz.ru 230 Mikhail B. Gokhberg ance can be described by the plane wave imped- ance Z = − iωµ / k, where ω = 2π f, µ = 4π10−7 G/m, i- ~nvk = , where σ is the conductivi- ty of the Earth crust upper layer. In general cases, the «thermal emission» im- pedance can be described as Z * = − iωµ / k* where k2* = k2 + n2 and n is the space harmonic number. If we know σ and ω we can determine n which is related to the source dimension and the distance from it. The 3D fig. 4 shows the dependence of n−1 number on frequency ω and ratio Z /Z* for σ = = const. One can see that for the 20% ratio Z/Z*increase the value 1/n (meter) is about sever- al hundred meters if σ ≈ 1 S/m (Rokitianski, 1962) which is roughly related to Borok Obser- vatory conditions. The experimental statistical value for Borok Observatory is Z/Z* (1.2-1.4). It means that due to global temperature modulation of the emission intensity, the space distribution is less than one kilometer and is related to the local observation condition and local sources. The same results were obtained by gradient measurement on the distance less than one hun- dred meters for the local geomagnetic variation network at Borok. Using this experimental data one can conclude that type 1 emission correlates with the temperature regime and probably relates to the parametric instability of the near Earth sur- face local atmospheric layer. Fig. 1a,b. Frequency versus time presentation of Hy, Hx magnetic components during (a) daytime (thermal or type 1 emission) and (b) nighttime (fan or type 2 emission) conditions. a b 231 Background electromagnetic noise in the vicinity 1 Hz as a possible indicator of earthquake-related anomalies Fig. 2. Seasonal and diurnal intensity variations for type1 (thermal) and type 2 (fan) emissions. Vertical scale shows the product of events number (N) by intensity of thermal emission (I), horizontal scale shows UT (in hours). For type 2 emission the absence of such a structure in electric field component demon- strates still higher differences Z * from plane wave. Moreover the presence of the structure in vertical Z magnetic component (as in the fig. 5) also shows the presence of a source nearby and strongly complicates the explanation from the far zone ionospheric source. 232 Mikhail B. Gokhberg Fig. 3a,b. Intensity (I) and temperature (T) variations (a) at Borok measured during July, 1998 and (b) at Mondy during August, 1997. a b Fig. 4. Dependence of wave number (n) on impedance ratio and frequency (Z is the impedance of plane wave for Pc-1 emission and Z * is the impedance of thermal emission). 233 Background electromagnetic noise in the vicinity 1 Hz as a possible indicator of earthquake-related anomalies Fig. 5. Evidence of occurrence of the essential z component in fan emission that indicates the near source presence. Fig. 6. Evidence of absence of the thermal emission during winter time (24-25 February, 1999) observed at Mondy Observatory before the moment of a local earthquake (shown by the vertical solid line). Note that the wavelength for Borok Earth crust conductivity condition is about several kilometers. An increase in the noise electromagnetic emission intensity around 1 Hz before local earthquakes have been reported by Wolfgang Boerner (Chicago University) and Jack Dea (San Diego, Navy) (private communications). Space and time distributions of the natural background noise level in the frequency range 1-5 Hz together with the impedance ratio algo- rithm which are represented here probably help to discriminate the anomaly related to the local earthquake preparation processes. For example, in fig. 6 the absence of this emission during winter time for Siberia does not determine such a precursor anomaly before earthquake with M∼ ∼ 6 (52°N, 105°E) on 25.02.1999 18:58 UT. One can see only the pulsations of the magne- tospheric origin. Another example of the thermal emission during the summer time is presented in fig. 7. The emission cannot be considered a precursor before the local earthquake with M ∼ 5.4 (the time of the earthquake is shown by black solid line) that occurs in the vicinity of the observa- tion point, because it is the usual background level. These two examples show that we have to be very careful before making any conclusions about precursor occurrence. In addition, it is important to investigate the behavior of the background level. We still hope to use type 1 emission for an earthquake preparation moni- toring. An increase in the noise electromagnet- ic emission intensity around 1 Hz before local earthquakes detected by W. Boerner (Chicago University) and J. Dea (San Diego, Navy) in Southern California can be related to the local near surface atmospheric layer heating (see Gorny et al., 1988). Thus the question is whether it is easier to measure anomalies in the temperature gradient directly or to detect the emission. 234 Mikhail B. Gokhberg Fig. 7. Typical diurnal distribution of the thermal emission before the moment of a local earthquake (shown by the vertical solid line) possibly not related with earthquake preparation. 235 Background electromagnetic noise in the vicinity 1 Hz as a possible indicator of earthquake-related anomalies REFERENCES BEAMISH, D. and A. TZANIS (1986): High resolution spectral characteristics of the Earth-Ionosphere cavity reso- nances, J. Atmos. Terr. Phys., 48 (2), 187-205. BELYAEV, P.P., S.V. POLYAKOV, V.O. RAPOPORT and V.I. TRA- CHTENGERZ (1987): Estimation of the atmosphere elec- tromagnetic noice in the short frequency range of geo- magnetic variation, Dokl. Akad. Nauk SSSR, 297 (4), 840-843 (in Russian). GOKHBERG, M.B. (1998): New type of electromagnetic emission in the range of short-period geomagnetic pul- sations, Dokl. Acad. Nauk SSSR, 359 (4), 543-544 (in Russian). GORNY, V.I., A.G. SALMAN, A.A. TRONIN and B.B. 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