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Abstract

In the paper, long-term data sets of the Schumann resonance (SR) signals observed at the Ukrainian Antarctic Station (UAS) are analyzed.

It has been found that the long-time trends of the peak frequencies and intensity of the fundamental SR mode correlate with 11-year solar cycle.

No trend of yearly averaged numbers of lightning flashes is observed in LIS satellite data for the same period.

It is suggested that the long-term variations of the intensity and frequency of SR peaks were caused by changes in the ionosphere controlled by solar activity.

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1. Introduction

As is known, Schumann resonances (SR) are stimulated by global lightning activity.

Lightning discharges excite resonance signals in the cavity formed by the Earth surface and the lower boundary of the ionosphere [1, 2].

As was shown in the paper [3], long-term records of the intensity of the fundamental harmonic of SR signals can be used to monitor trends in the tropical surface air temperature. However, the mechanism of the lightning activity-global temperature coupling has not been well-described theoretically. Long-term high accuracy observations of Schumann

signals can be helpful for experimental study of this effect.

To the best author’s knowledge, there are only few papers [4, 5] reporting on records of the kind.

An important issue while processing SR data is to isolate signals produced by the

global lightning activity from interference radiation produced by local lightning discharges and technogenic sources, which can significantly deteriorate quality of the data.

As is suggested in paper [6], the best location for a Schumann observatory is the polar region owing to the absence of local lightning and technogenic activities.

This is a reason why monitoring of Schumann fields has been initiated at the Ukrainian Antarctic Station (UAS).

It should be noted that an additional advantage of the UAS is its location on the same arc of a great circle with American and Asian centers of the global lightning activity.

Such geometry makes it possible to separate contributions of different centers of the global

lightning activity [7, 8] from the total SR signal. In the present paper, the Schumann data recorded at the UAS from 2002 through 2012 are analyzed.

It has been find that long-term trends of intensity and peak frequencies of the fundamental Schumann mode correlate with the 11-year cycle of the solar activity.

The data are compared with the seasonally averaged numbers of flashes obtained from the Lightning Imagine Sensor (LIS) satellite records.

As a result it has been found that the numbers of lightning flashes demonstrate practically no variability from one year to another.

This suggests that the long-time variations of Schumann parameters detected at UAS were caused changes in the lower ionosphere controlled by the solar activity rather than by variability of the parameters of the world lightning centers.

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2. Measuring and data processing techniques

Two orthogonal horizontal components of the magnetic field oriented in south-north and east-west directions of the geographic coordinate system have been monitored at UAS (coordinates: 65º15' S, 64º16' W) since March 2002.

To that end an induction-coil magnetometer Lemi-112 was used till 2011, which recorded SR signals in the frequency band 0.1-300 Hz.

In April 2011 the hardware was changed to a Lemi-419ant with larger dynamic range and wider operation frequency band 0.001 Hz to 80 Hz.

The devices have been both manufactured by the Lviv Centre of the Institute of

Space Research (Ukraine, http://www.isr.lviv.ua ).

The instruments are capable of recording the absolute values of the magnetic field in a digital form and are equipped with GPS system for synchronizing with the Universal Time (UT).

The spectral and polarization techniques [8] were used for data processing.

At the first stage, the power and cross-spectra of the signals averaged for each 600 seconds interval are calculated.

These data are used to determine the Stokes parameters and compute the intensities, degree of polarization, ellipticity and position angle for the first three modes of

the Schumann resonances.

The values of peak frequencies for both polarization channels are calculated as well.

Then, using the asymptotic theory developed by the author of paper [7] the activities of the world lightning centers are estimated.

The activity of the African (A1) and the sum activity of the Asian and American (A2+A3) centers are calculated using the technique described in paper [8].

In the present study, we analyze monthly averaged parameters of the fundamental SR mode calculated for the period from March 2002 to December 2012.

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3. Data analysis and discussion

The dashed curves in Figure 1 show the times of the maximum activity of the world lightning centers as observed in the monthly averaged data calculated from the UAS SR records for 2002-2012.

Figure 1. Times of maxima observed in the monthly averaged activities of the world lightning centers calculated from the SR data recorded at UAS (dashed lines) and estimated from seasonal distributions of lightning flashes obtained by LIS satellite (solid lines).

The green, blue and red lines correspond to the Asian, African and American centers, respectively.

The time of the maximum activity changes with annual periodicity for all the lightning centers.

As is known [1,2], the periodicities are caused by the seasonal drift of the centers.

The lightning centers shift over the respective continents from the equator in the northwest direction in the summer and southeastward in the winter.

So, the longitude and latitude of the centers both change. It is also known [1, 2] that the maximum lightning activity occurs at about 16-17 LT.

Therefore, a result of the longitudinal drift of the centers will be a shift of the time (as represented in UT) of the maximum activity toward later hours in summer and earlier hours in winter.

We estimated this effect also by an alternative way using seasonal maps of the numbers of lightning flashes calculated from Lightning Imagine Sensor satellite data available from Internet (http://thunder.msfc.nasa.gov/data/query/distributions.html) and compare the

results with the UAS data.

The calculations suggested that the lightning center location corresponds to the mass center of the flash density computed from the LIS data.

It is evident that variations observed at UAS and calculated from the satellite data are correlated and have comparable values for the American and African centers.

The span of variation estimated from SR records for the Asian center is greater.

A possible reason can be that the LIS satellite observes the latitudinal belt ±35о only while some number of lightnings occurs in summer at higher latitudes as well.

Note that neither SR nor satellite data demonstrate any noticeable trend during the 11-year period which suggests invariability of the seasonal drift of the centers during the solar activity cycle.

Analysis of other parameters computed from the SR records at UAS allows determining long-term trends of the peak frequencies and the intensity of the first Schumann mode. These variations are presented in Figure 2.a and Figure 2.b, respectively.

Comparison of these results with the GOES data for X-ray flux 1-8 A and Wolf sunspot number (Figure 2.c, http://sidstation.loudet.org/solar-activity-en.xhtml) shows that the SR parameters change in correspondence with the solar activity variations within 11-year cycle. It should be noted that papers [5, 9] confirm this result by observations of the long-term trends of the fundamental frequency of the electric component of Schumann field

measured in Europe.

There could be three possible explanations for these trends, specifically, variation of the lightning intensity within the solar cycle, interannual variability of the seasonal drift of the world lightning centers and long-term changes of the characteristics of the ionospheric boundary of the cavity.

The second reason seems to be unlikely in view of the absence of interannual variations of the time of the activity maxima (Figure. 1).

To test the first hypothesis, we have estimated seasonally averaged numbers of lightning flashes from the LIS data and have not found any noticeable variations during the solar cycle (Figure 2.d).

So, the third reason seems to be the most plausible.

Figure 2. Variations of the peak frequencies for the 1-st SR mode in the south-north (blue lines) and west-east (green lines) channels obtained from monthly (thin line) and yearly (thick line) averaged (panel a); monthly (blue line) and yearly (green line) averaged intensities of the horizontal component of the 1-st SR mode recorded at UAS (panel b); X-ray flux (blue line , data of GOES satellite) and Wolf number, R (green line) (panel c); and seasonally (blue line) and yearly (green line) averaged normalized number of lightning flashes estimated from the LIS satellite data.

This assumption is supported by huge changes of the X-ray flux (1-8 A) responsible for ionization of the lower ionosphere by two orders of magnitude during the cycle from about 1.3x10-8 W/m2 in the solar minimum to 1.15x10-6 W/m2 in the maximum of the cycle (Figure. 2.c).

The enhancement of the ionization coefficient can provide

changes in both the height of the cavity and conductivity of the D-layer of the ionosphere. The two factors are responsible both for changes in the Shuman resonance peak frequencies, and for the attenuation in the waveguide and hence, the amplitude of the SR signals durig the solar cycle maximum.

Thus, the most verisimilar reason for the 11-year variation of the Schumann field parameters observed in Antarctica is changes in the lower ionosphere controlled by the solar activity.

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4. Conclusion

Analysis of the parameters of the ELF fields recorded at the Ukrainian Antarctic Station during 11 years and spatial distributions of lightning flashes calculated from satellite observations for the same period show the interannual stability of the seasonal drift of the world lightning centers.

The satellite data also demonstrate no significant changes of yearly averaged number of lightning flashes during the solar cycle.

Therefore, the most probable cause of long-term variations of intensity and peak-frequencies of the first Schumann maximum detected at UAS is changes in the lower

ionosphere controlled by the solar activity.

Additional experimental and theoretical studies are necessary to include this effect in the models of ELF signal propagation in the Earth-ionosphere cavity, which would improve the calculation accuracy of the activity of the world lightning centers and could be useful for estimation of the global temperature changes on the Earth