Power spectrum features of the near-Earth atmospheric electric field in Kamchatka

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 oscillations are the modes of the internal gravity and tidal waves in the lower atmosphere. On the days under atmospheric precipitation, the spectral power of Ez increases by an order of magnitude. During the pre-earthquake period, when the diurnal Ez variation had an anomaly, the intensity of harmonics with T = 1.8, 2.2, and 3.8 h increased by an order of magnitude or more in comparison with the Ez spectra in fine weather. Two additional spectral bands with T = 0.6 and 1 h have appeared simultaneously.


Introduction
Anomalous variations in the quasi-static electrical field E z in the near-Earth atmosphere before the M ≥ 4 earthquakes have been recorded 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(Rulenko et al., , 1996;;Buzevich et al., 1998;Smirnov, 2001).These results, as well as the observations in other seismo-active 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;Mikhailov et al., 1999).Research of electrical field intensity in temporary series has shown that the field value is affected by meteorological characteristics (precipitation, winds) (Smirnov, 2001;Mikhailov et al., 2002).This fact makes it difficult to detect some anomalies in Ez behaviour before the earthquake.
The object of this paper is the investigation of the power spectrum in the near-Earth atmos- pheric quasi-static electric field Ez in a period of the fine weather, under atmospheric precipitation and during the pre-earthquake period of anomalous diurnal variations E z.

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 measure 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 September-October 1999 are presented below.Figure 1 shows the typical diurnal E z 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, 21.28:33.17UT, 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 atmosphere.The increase in E z magnitude at 19-24 UT (06.00-11.00LT) is related to the convective 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 electric field value decreased to − 1 kV/m.On the rainy day (September 19), which carries to the Earth's surface volume changes, the atmospheric conductivity increases and Ez decreases, even to a sign reversal.Other examples of the diurnal E z variations in fine weather and at high precipitation were shown in the paper (Mikhailov et al., 2002).
On the day before the earthquake (curve 2), the bay of decreased Ez magnitude was observed at 15.00-24.00UT against a background of quiet meteorological conditions.Similar effects 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 illustrates the spectral processing of the diurnal E z data.The upper curve 1 represents the diurnal variations Ez.The next curve below 2 is the same data obtained by subtracting the daily average 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 transform algorithm (FFT).The power spectrum of the augmented E z 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/f k, 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 ordinate 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).

Main results
The method described above was used to make the spectral analysis of the diurnal variations in Ez for September-October 1999.Naturally, 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 behaviour.We used days without atmospheric precipitation and earthquakes, when the spectral variations may be caused by global effects in the Earth's crust in the given region and, consequently, 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,  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, September 20-24, October 20-24 and October 25-29.Figure 5 represents the resulting spectrum, averaged over these days, which shows the clear maximums with T ∼ 8, 10, 12 and 24 h.

Discussion
The continuous measurements of quasi-static 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 E z diurnal variations for September-October 1999.Under fine weather, clearly defined maximums (at a level of 0.5) were absent, but the intensity trended to weaken when the periods decreased from 5 to 1 h.These periods are known to be the periods of internal gravity waves, which are clearly seen in the seismo-gravitational oscillations of the Fig. 5.The averaged Ez power spectrum with periods of 2 h < T < 30 h for the fine weather days from 1 September to 29 October 1999.
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 seismogravitational oscillations, allows us to state that the Ez variations are of seismic nature.If we accept 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 concentration 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 September 1999, the E z power spectrum showed three distinct maximums (relative to the halfmaximum 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 E z 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 observed by Lin'kov et al. (1990) and their enhancement in the variations in the Earth's seismo-gravitational oscillations and in the simultaneous pressure variations in the near-Earth atmosphere before a strong earthquake.The agreement between the Ez spectra and the results of those papers indicate that not only the quiet diurnal variations in Ez, but also anomalies before earthquakes are of the same seismo-gravitational nature.

Conclusions
The fine spectral analysis of diurnal variations 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 atmospheric precipitation the natural frequencies of atmospheric oscillations have been distinguished 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 period, 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 spectral bands with T = 0.6 and 1.0 h were distinguished.
iii) The nature of the E z spectral anomalies from the Earth's seismo-gravitational oscillations has been confirmed experimentally.However, the question has still to be answered as to whether the pre-earthquake bay-like weakening anomalies of E z 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.

Fig. 1 .
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.

Fig. 2 .
Fig. 2. The method for spectral processing of the diurnal Ez variations.