Annals 47, 1, 2004, 01/07def 237 ANNALS OF GEOPHYSICS, VOL. 47, N. 1, February 2004 Mailing address: Dr. Yuri M. Mikhailov, Institute of Ter- restrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN), Russian Academy of Science, 142190 Troitsk (Moscow Region), Russia; e-mail: yumikh@izmiran.rssi.ru Key words atmospheric quasi-static electric field – atmospheric precipitation – atmospheric waves 1. Introduction Anomalous variations in the quasi-static electrical field Ez in the near-Earth atmosphere before the M ≥ 4 earthquakes have been record- ed in many seismo-active regions of the Earth. They usually occur at an interval of a few hours to a few days before the main shock and prove to be either a bay-like field intensity decrease (even to a sign reversal) or an oscillation train lasting a few hours. These observations were summarized and analysed by Rulenko (2000) and made it possible to establish that both types of Ez anomalies are caused by the deformation processes in the subsurface layers of the Earth’s crust during the earthquake preparation phase. The former type of anomalies were observed on Kamchatka for a few years and were referred to the radon (Rn) emission from subsurface gases into the atmosphere, resulting in variations of atmospheric conductivity and, therefore in Ez magnitude (Rulenko et al., 1992, 1996; Buze- vich et al., 1998; Smirnov, 2001). These results, as well as the observations in other seismo-ac- tive regions, have made it possible to assume that the quasi-static electric field in near-Earth atmosphere can be responsible for ionospheric disturbances occurring before seismic shocks (Morgunov, 1988; Pulinets et al., 1998; Mikhai- lov et al., 1999). Research of electrical field in- tensity in temporary series has shown that the field value is affected by meteorological char- acteristics (precipitation, winds) (Smirnov, 2001; Mikhailov et al., 2002). This fact makes it difficult to detect some anomalies in Ez be- haviour before the earthquake. The object of this paper is the investigation of the power spectrum in the near-Earth atmos- Power spectrum features of the near-Earth atmospheric electric field in Kamchatka Yuri M. Mikhailov (1), Galina A. Mikhailova (1), Olga V. Kapustina (1), Alexandr V. Buzevich (2) and Sergei E. Smirnov (2) (1) Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN), Russian Academy of Science, Troitsk (Moscow Region), Russia (2) Institute of Space Physics Researches and Radiowave Propagation, Far-East Branch of Russian Academy of Science, Petropavlovsk-Kamchatski (Kamchatka Region), Russia Abstract Power spectrum of the diurnal variations of the quasi-electrostatic field Ez in the near-Earth atmosphere have been presented for the first time. The Ez power spectrum variations in the period of fine weather have been shown to exhibit two bands of the periods of natural atmospheric oscillations with T = 1-5 and 6-24 h. These oscilla- tions are the modes of the internal gravity and tidal waves in the lower atmosphere. On the days under atmos- pheric precipitation, the spectral power of Ez increases by an order of magnitude. During the pre-earthquake pe- riod, when the diurnal Ez variation had an anomaly, the intensity of harmonics with T = 1.8, 2.2, and 3.8 h in- creased by an order of magnitude or more in comparison with the Ez spectra in fine weather. Two additional spec- tral bands with T = 0.6 and 1 h have appeared simultaneously. 238 Yuri M. Mikhailov, Galina A. Mikhailova, Olga V. Kapustina, Alexandr V. Buzevich and Sergei E. Smirnov -2000 -1000 0 1000 2000 0 5 10 15 20 UT, hours 0 200 400 600 800 -400 -200 0 200 400 0 100 200 300 400 Ez, V/m 16.09.1999 19.09.1999 18.09.1999 17.09.1999 1 2 3 4 Fig. 1. Typical diurnal variations in a quasi-static electric field Ez in the near-Earth atmosphere: 1-3 – days without atmospheric precipitation; 2 – a day before the earthquake; 3 – for the earthquake day; 4 – day with heavy atmospheric precipitation. 239 Power spectrum features of the near-Earth atmospheric electric field in Kamchatka pheric quasi-static electric field Ez in a period of the fine weather, under atmospheric precipi- tation and during the pre-earthquake period of anomalous diurnal variations Ez. 2. Experimental data and processing procedure The Pole-2M electrostatic flux-meter, whose output data are transmitted into the information processing centre in the analogue form, is used at Paratunka (ϕ = 53°N, λ = 158.3°E) to meas- ure the vertical component of the atmospheric electric field (Ez). The Ez values are digitized with sampling rate ∆t = 1 min. The results of the investigations of the power spectrums of the near-Earth atmospheric electric field in Sep- tember-October 1999 are presented below. Fig- ure 1 shows the typical diurnal Ez variations in the fine weather (curve 1), on the day before the earthquake (curve 2, September 17), on the day of the earthquake (curve 3, September 18, 21.28:33.17 UT, ϕ = 51.21°N, λ = 157.56°E, h = = 60 km, Mp = 6.0) and on the day with intense precipitation (curve 4, September 19). The character of the diurnal Ez variations in the fine weather is described qualitatively by physical processes, occurring in the near-Earth atmos- phere. The increase in Ez magnitude at 19-24 UT (06.00-11.00 LT) is related to the convec- tive processes in the atmosphere due to the air temperature variations in the morning. In the fine weather in the daytime and at night the electrical fields are ∼ 100-120 V/m, varying in the range ∼ ± 20 V/m. At high and durable precipitation, the elec- tric field value decreased to − 1 kV/m. On the rainy day (September 19), which carries to the Earth’s surface volume changes, the atmospher- ic conductivity increases and Ez decreases, even to a sign reversal. Other examples of the diurnal Ez variations in fine weather and at high pre- cipitation were shown in the paper (Mikhailov et al., 2002). On the day before the earthquake (curve 2), the bay of decreased Ez magnitude was ob- served at 15.00-24.00 UT against a background of quiet meteorological conditions. Similar ef- fects were observed in the Paratunka during pe- riod 1997-2000 in 1-36 h before earthquakes (Smirnov, 2001). By example of 29 September 1999, fig. 2 il- lustrates the spectral processing of the diurnal Ez data. The upper curve 1 represents the diur- nal variations Ez. The next curve below 2 is the same data obtained by subtracting the daily av- erage Ez value from curve 1. Curve 2 shows the well-marked semidiurnal wave, whose daytime amplitude is less than the nighttime one. These values have been added by zeros to have 2048 points, needed for using the fast Fourier trans- form algorithm (FFT). The power spectrum of the augmented Ez time series was computed by the method of periodograms with rectangular time «window» at frequencies of fk = k ⋅ ∆f with a corresponding frequency step of ∆f = 1/2048 ⋅ ⋅∆t = 8.14 ⋅ 10−6 Hz and with periods of Tk = = 1/fk, where k = 1, 2, …, 1024. This spectrum is shown in the panel 4 (solid line, left ordinate axis). The period of the predominant harmonic is T = 12 h and less intensive harmonics with periods of 1-5 h are also present. To distinguish them we excluded periods longer than 5 h from the spectrum (the dash-dotted line, right ordi- nate axis). The time form of these filtered series is shown by curve 3. The spectra labelled 5 are shown in the bottom of fig. 2 as a function of the periods varying to 24 h (on the left) and T < < 5 h (on the right). 3. Main results The method described above was used to make the spectral analysis of the diurnal varia- tions in Ez for September-October 1999. Natu- rally, the spectrum forms varied from day to day. To obtain statistically reliable spectra and to distinguish stable maximums the data have been averaged over days with specific Ez be- haviour. We used days without atmospheric precipitation and earthquakes, when the spec- tral variations may be caused by global effects in the Earth’s crust in the given region and, con- sequently, in the near-Earth atmosphere. Figure 3 shows the control atmospheric precipitation data for the observation period. The resulting averaged spectra for the days without (22 days, curve 1) and with (11 days, 240 Yuri M. Mikhailov, Galina A. Mikhailova, Olga V. Kapustina, Alexandr V. Buzevich and Sergei E. Smirnov 0 200 400 600 0 2 4 6 8 10 12 14 16 18 20 22 24 UT, hours -200 0 200 Ez- Ez, V/m 0 5 10 15 20 25 30 35 t, hours 0 5 10 15 20 25 30 35 t, hours 0 4E+10 8E+10 S 2, V 2 m-2 Hz-1 0.0000 0.0002 0.0004 0.0006 f, Hz 0 4E+10 8E+10 0 2 4 6 T = 1/f, hours -100 0 100 Ez (T<5 h), V/m 0 4E+9 8E+9 0 4E+9 8E+9 0 5 10 15 20 29.09.1999 1 2 3 4 5 _ Ez, V/m S 2, V 2 m-2 Hz-1 Fig. 2. The method for spectral processing of the diurnal Ez variations. 241 Power spectrum features of the near-Earth atmospheric electric field in Kamchatka curve 2) atmospheric precipitation are shown in fig. 4 (T < 5 h) together with the individual spec- trum for September 17 (curve 3). The vertical bars in curves 1 and 2 are the mean-square devi- ations. Curve 1 indicates the spectrum weaken- ing from ∼ 2.5 ⋅ 109 V2/m2 Hz at T ∼ 4 h to ∼ 2 ⋅ ⋅ 108 V2/m2 Hz at T ∼ 1 h without pronounced peaks at a level of 0.5 relative to the maximum value. The character of the spectrum shown by curve 2 has changed insignificantly, namely, the energy enhancements at T ∼ 4 h and ∼ 2 h, but the power level have increased twofold com- pared to curve 1, with very large mean-square deviations indicating that individual spectra are widely variable. Finally, curve 3 represents the individual Ez power spectrum for 17 September 1999 without atmospheric precipitation, but with anomalous diurnal variations (see fig. 1). 0 10 20 30 0 5 10 15 20 25 30 October 1999 0 10 20 30 40 0 5 10 15 20 25 30 September 1999 A tm o sp h e ri c p re ci p ita tio n , m m A tm o sp h e ri c p re ci p ita tio n , m m Fig. 3. Variations in the atmospheric precipitation level measured twice a day during September-October 1999. 242 Yuri M. Mikhailov, Galina A. Mikhailova, Olga V. Kapustina, Alexandr V. Buzevich and Sergei E. Smirnov 0 2E+9 4E+9 0 2E+10 0 1 2 3 4 5 6 7 T, hours 0 2E+10 4E+10 1 2 3 S 2, V 2 m-2 Hz-1 Fig. 4. The averaged Ez power spectrum in the band of periods T < 5 h on the days without precipitation (curve 1) and with precipitation (curve 2) and 17 September with the anomalous diurnal variations Ez (curve 3). 243 Power spectrum features of the near-Earth atmospheric electric field in Kamchatka The following features are clearly defined in this curve: i) There are the spectral bands (at 0.5 level of the maximum) at T = 0.6, 1, 1.8, 2.2 and 3.8 h. ii) The intensity at maxima of these bands exceeds the corresponding values for the days without atmospheric precipitation by an order of magnitude or more. The T > 5 h harmonics were distinguished by calculating the spectra of longer initial date series with a time step of ∆t = 1 h only for days of the fine weather, i.e. September 1-4, Septem- ber 20-24, October 20-24 and October 25-29. Figure 5 represents the resulting spectrum, av- eraged over these days, which shows the clear maximums with T ∼ 8, 10, 12 and 24 h. During September-October 1999 (except September 18), two more Kamchatka earth- quakes occurred, whose preparation zone in- cluded the Ez measurement point. The earth- quakes occurred on October 5 (05.01:35.94 UT, ϕ = 51.21°N, λ = 157.61°E, h = 76 km, M = 5.6) and on October 18 (20.49:47.92 UT, ϕ = = 51.30°N, λ = 157.12°E, h = 138 km, M = 4.8). From fig. 3 it is seen, however, that rainfalls were recorded during these days, which prob- ably screened the Ez effects caused by other sources. 4. Discussion The continuous measurements of quasi-stat- ic electric field Ez with the sampling rate ∆t = 1 min have made it possible to study the fine structure of the power spectra with periods from 4 min and longer. Two spectral bands with periods T = 1-5 h and 6-24 h have been found by analysing the Ez diurnal variations for Sep- tember-October 1999. Under fine weather, clearly defined maximums (at a level of 0.5) were absent, but the intensity trended to weak- en when the periods decreased from 5 to 1 h. These periods are known to be the periods of in- ternal gravity waves, which are clearly seen in the seismo-gravitational oscillations of the 0 12 24 T, hours 0 5E+9 1E+10 1.5E+10 S 2, V 2 m-2 Hz-1 Fig. 5. The averaged Ez power spectrum with periods of 2 h < T < 30 h for the fine weather days from 1 Sep- tember to 29 October 1999. 244 Yuri M. Mikhailov, Galina A. Mikhailova, Olga V. Kapustina, Alexandr V. Buzevich and Sergei E. Smirnov Earth and related with pressure disturbances in the near-surface atmosphere (Garmash et al., 1989; Petrova, 1999). The coincidence of the Ez power spectra with those of the Earth’s seismo- gravitational oscillations, allows us to state that the Ez variations are of seismic nature. If we ac- cept the «piston» mechanism of generation of air pressure fluctuations by the Earth’s surface oscillations (Garmash et al., 1989), then we may expect similar variations in the radon con- centration spectra in subsurface gases. The T = 6-24 h spectral band shows the clear harmonics with T = 8, 12 and 24 h. These are the tidal waves in the lower atmosphere, related to the variations in the temperature of underlying surface. Their intensity is of the same order of magnitude as that of internal gravity waves. One day before the earthquake on 18 Sep- tember 1999, the Ez power spectrum showed three distinct maximums (relative to the half- maximum level) at T = 1.8, 2.2, and 3.8 h. Their intensity increased by an order of magnitude and more, especially at T < 3 h, as compared with the Ez spectra during the days of the fine weather. Moreover, spectral components at T = = 0.6 and 1 h appeared too. The similar spectral structures within the T = 1-5 h band were ob- served by Lin’kov et al. (1990) and their en- hancement in the variations in the Earth’s seis- mo-gravitational oscillations and in the simulta- neous pressure variations in the near-Earth at- mosphere before a strong earthquake. The agreement between the Ez spectra and the re- sults of those papers indicate that not only the quiet diurnal variations in Ez, but also anom- alies before earthquakes are of the same seis- mo-gravitational nature. 5. Conclusions The fine spectral analysis of diurnal varia- tions in the quasi-static electric field Ez in the near-Earth atmosphere has been carried out for the first time and has shown the following: i) In fine weather and on the days with at- mospheric precipitation the natural frequencies of atmospheric oscillations have been distin- guished in two bands with periods of T = 1-5 h and T = 6-24 h in the Ez power spectrum. The former band corresponds to internal gravity wave modes of the lower atmosphere, while the second band to the tidal waves. ii) During the earthquake preparation peri- od, the intensity of bands with T = 1.8, 2.2, and 3.8 h increased by an order of magnitude and more compared with spectra obtained in fine weather. During this period, two additional spec- tral bands with T = 0.6 and 1.0 h were distin- guished. iii) The nature of the Ez spectral anomalies from the Earth’s seismo-gravitational oscilla- tions has been confirmed experimentally. How- ever, the question has still to be answered as to whether the pre-earthquake bay-like weakening anomalies of Ez are caused by enhancement of conductivity in the lower atmosphere as a result of the additional ionization by a radioactive gas, or this weakening is a purely seismic electrical signal caused by the rock failure in a hypocenter. Acknowledgements The authors are deeply indebted to L.N. 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