Schumann resonance phenomena – its different

Schumann resonance phenomena – its different aspects S. S. De S. K. Mitra Centre for Research in Space Environment Institute of Radio Physics and Electronics University of Calcutta, Kolkata 700 009, India 1. Introduction A number of electromagnetic emissions originated from lightning discharges in the Earthionosphere waveguide are occurring predominantly in the lower atmosphere in ULF, ELF and VLF frequency ranges. These include Schumann resonances (SR), ELF-VLF sferics, sprites etc. In the presence of fair-weather electric field between the ionosphere and Earth’s surface, electric discharges take place through the atmosphere. Thunderstorm, dynamo interaction between solar wind and magnetosphere, and dynamo effect of the atmospheric tides in the thermosphere introduce electromotive force in the ionosphere. Thunderstorms are considered to be the more powerful of these sources. Figure 1 shows the diagram of the Earth-ionosphere cavity. The field is directed from the ionosphere to the ground in the eastern hemisphere and from the ground to the ionosphere in the western hemisphere at the given instant, and their direction will change to the opposite in a halfcycle of resonant oscillation of 8 Hz frequency. The lower part of Fig. 1 separately depicts a crosssection of the cavity with two types of feasible resonances. A wave traveling along the ground surface (shown in the left part) circles the globe and returns to the starting point. The first Schumann resonance occurs when the phase delay of the round the world wave is equal to 2π. When the wave bounces between the ground and ionosphere (as in the right part) it may be trapped between two surfaces, and the transverse resonance takes place.

Fig. 1 Diagram of Earth-ionosphere cavity. The lower scheme demonstrates two kinds of electromagnetic resonance (Nickolaenko and Hayakawa, 2002)

Fig. 2 Schumann resonance spectra 8, 14, 20, 26 etc. Hz (Nickolaenko and Hayakawa, 2002)

The longitudinal dimension of the cavity 2πa is equal to 40 Mm, and the corresponding resonance frequency may be evaluated from the condition that the Earth’s circumference is equal to the wavelength. By neglecting the sphericity, one gets c fn  n  7.5n Hz, Where c  3  10 8 m/s is the velocity of light and n =1, 2, etc. is the 2a resonance number. The transverse size (height) of the Earth-ionosphere cavity is much smaller h/a ≈ 10-2, and the relevant frequencies are found from c Fp  p  2  10 3 p Hz, the ionospheric height is equal to an integer of half the wavelength. 2h We observe that two series of resonance frequencies exist in the Earth-ionosphere cavity, they correspond to different directions of wave propagation. Resonance phenomena in a cavity are connected with the interaction of direct and reflected (or round the world) waves arriving at an observer. Depending on the phase shift, an amplitude increase or reduction may occur as a consequence, provided that amplitudes of arriving waves are of comparable level. This is so in the SR band. Reflections from the ionosphere in the kHz band become noticeable in the night, and the transverse resonance is observed at night as a rule. The main distinction between the two phenomena is that a SR wave multiply circles the globe, and the phenomenon is a global one. The transverse resonance has a local nature because its wave is trapped in a close vicinity of the causative stroke. Schumann resonance was first detected by Balser and Wagner (1960). They observed the resonance phenomenon as a succession of peaks in the power spectrum of the radio noise below 50 Hz frequency. Figure 2 shows the SR spectra measured by Ogawa et al. (1966) that demonstrate the amplitude spectrum of SR with four peaks (modes). The structure of the Earth-ionosphere cavity, is displayed in Fig. 2, formed by the dielectric layer of air bounded by spherical conducting surfaces of the Earth and the lower ionosphere having the radii r=a and r=b. A SR signal arises owing to the excitation of electromagnetic radiation from the global thunderstorm activity that is concentrated in the Earth-ionosphere cavity. From the spectrum of SR, the peak frequencies of individual modes n equal approximately 8, 14, 20 and 26 Hz. These values were first measured by Balser and Wagner in 1960 and were afterwards confirmed by many independent measurements. The peak frequencies are remarkably lower than those theoretically first evaluated by Winfried Otto Schumann in 1952 through his formula: c fn  n(n  1) . He obtained the eigen frequencies in the spherical Earth-ionosphere cavity as 2a 10.6, 18.4, …Hz. Schumann spectra is generally detected by ball antenna for electric field (Fig. 3) and two induction coil type antenna (magnetometers) for two horizontal magnetic field components (Fig. 4) set at remote locations far from artificial and electrical interferences.

Fig. 3 Schematic diagram of a Ball antenna

Fig. 4 Induction coil type magnetometer

The experimental evidences of this phenomenon came later, with subsequently developed more realistic analytical models taking into account the inhomogeneities due to asymmetry of the day and night time ionosphere, variations of conductivity of the ionosphere, the distribution of sources and other factors. These were not considered by Schumann in his theory. The parameters of the Schumann resonance phenomena contain informations on world thunderstorm activities since the thunderstorm emissions are the main source of the natural electromagnetic noise in the Schumann resonance frequency range (4 – 40 Hz). The distribution pattern of world thunderstorms is complicated and permanently varies both during a day and during a year (years), which includes variations in the location of thunderstorm sources, their intensity, width and shape of the regions occupied by them, etc. The conditions of wave propagation in the Earth-ionosphere cavity do not remain constant, as well. All these factors affect, to a greater or smaller degree, the Schumann resonance parameters, in particular, the intensities and peak frequencies of the electromagnetic field components. The variety of factors influencing the Schumann resonance impedes interpretation of the experimental data. The influences of these factors in the lower ionosphere cause the resonance frequencies to decrease to the experimentally observed value. Different aspects of Schumann resonance phenomenon as well as their observation and measurement techniques have been published by a plenty of workers. One such example is the continuous measurement of SR at the Lehta station (Lat: 65° N, Long: 34° E) at the St. Petersburg during August 1999 to March 2005 (Yatsevich et al., 2008). Some of their observations may be presented now. The data represent diurnal and seasonal variations in the peak frequencies and intensities of the first three modes of the electric- and magnetic-field components (Figs. 5a – 5d).

Fig. 5a Winter average diurnal variations in the intensities and peak frequencies of the first three modes of the electric and magnetic field components

The diurnal variation of peak frequencies and amplitudes of different modes were utilized to know (i) the characteristics of global thunderstorm activities, and (ii) the electron number density of the ionosphere. These two applications of SR continued for more than three decades from its year of prediction in 1952 without any additional important findings. Some frequency changes about the peak values and some amplitude changes were found to be present in the observed spectra which may be attributed to the uncertainties arising from spatial distribution of lightning sources exciting the SR modes. During 1990s and later, the scenario with these sub-ionospheric ELF SR waves changed and several new aspects of SR emerged. Some of those will be given here.

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Fig. 5b Spring average diurnal variations in the intensities and peak frequencies of the first three modes of the electric and magnetic field components

Fig. 5c Summer average diurnal variations in the intensities and peak frequencies of the first three modes of the electric and magnetic field components

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Fig. 5d Autumn average diurnal variations in the intensities and peak frequencies of the first three modes of the electric and magnetic field components

2. Different aspects of Schumann resonances 2.1 Global thunderstorm activity Schumann resonance intensity records are used to estimate the level of global thunderstorm activity. We developed a calibrated wide-band ELF measurement technique at the Institute of Radio Physics and Electronics. Observations of signals on Schumann resonance spectra have been made during the period of 1997 to 2002 continuously and after 2004, occasionally. The records are taken from different noise-free regions as far as were practicable. Figure 6 shows the variation of global thunderstorm activities over Kolkata for an observational period of 15 months (January 2000 – March 2001). At a particular time, the source intensity is calculated by taking the sum of the power of first three resonance modes. The monthly variation of the global thunderstorm activities deduced from Schumann resonance amplitude is shown by the bar graphs. 2.2 Tropical surface temperature of the Earth The specific link between SR peak power in the lower mode (8 Hz) with global and regional temperatures has been established (Williams, 1992; Balling and Hilderbrand, 2000). The relation between SR amplitude and variations in global tropical surface temperature strongly suggests that such measurements can serve as the diagnostic of temperature and deep convection in the tropical atmosphere. The intensity of SR signals is found to be linked through logical chain to the tropical surface temperature of the Earth (Nickolaenko et al., 1998). It may be shown as follows: Surface temperature  atmospheric convection  formation of the clouds  lightning strokes  EM radiation and the SR signal level

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Fig.6 Variation of global thunderstorm activities over Kolkata for a period of 15 months (January 2000 – March 2001). [From the measurements of the heights of Schumann resonance amplitudes, different bars are obtained].

Thus, SR behaves as a sensitive global tropical thermometer on diurnal, seasonal time scales. The SR amplitude follows the temperature variation quite closely (specially, for long-period variation). Warmer and cooler periods of the entire tropical belt are due to the enhanced and suppressed magnetic field amplitudes of SR. These temperature fluctuations generate El-Nino-Southern Oscillations. 2.3 Global Electric Circuit The atmospheric global electric circuit is a current system in which current flows upward through the troposphere into the ionosphere and magnetosphere along the magnetic field lines to the opposite hemisphere which in the course of time returns to the Earth’s surface as the fair-weather air-earth current, thus closing the circuit. Among the three quasi-DC sources of electromotive force driving the global circuit (e.g., thunderstorms, dynamo-interaction between the solar wind and the magnetosphere, and dynamo effect of atmospheric tides in the thermosphere), the first one (thunderstorms) is taken to be the most powerful. Cloud-to-ground lightning strokes return the charge to the thunderstorm and close the global circuit. The SRs are caused by electromagnetic emissions from lightning activities and can be considered as a state of excitations of AC global circuit. Here, lightning activity is the energy source, global electrical circuit is the receiver of energy and SR is the vibration mode. 2.4 Effects of frequency change Diurnal variation of SR mode frequencies is attributed to the movement of thunderstorms. As the effective size of storm decreases, the magnitude of the variations grows. Diurnal frequency variations of the first mode frequency allow the evaluation of effective size of area covered with the global thunderstorms.

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2.5 Shift in Solar Terminator Effect The EM effects associated with earthquake are thought to be responsible for ionospheric perturbation which could be detected by VLF / LF subionospheric transmitted signals. Variations in terminator times of the two different transmitted sub-ionospheric VLF signals at 40 kHz from Japan (Lat: 36.18°N, Long: 139.85°E) and 19.8 kHz from Australia (Lat: 21.82°N, Long: 114.16°E) during morning and evening periods about the days of two earthquakes at two different places are measured regularly at Kolkata (Lat: 22.56°N, Long: 88.5°E). The amplitude of signals recorded at Kolkata showed diurnal variations over which other geophysical or solar effect may be superimposed. The diurnal patterns of the signal variations are lost during the day of earthquake, prior to some days of earthquake and some days after the earthquake. In Fig. 7(a), we show the diurnal pattern of the amplitudes of two signals as received at Kolkata. These graphs are plotted considering the average of days excluding ± 5 days about the day of earthquake. The average terminator time has been defined with respect to the diurnal variation, which is the minimum value of amplitude during sunrise and sunset. Over large distances (about 5120 km between Japan and Kolkata and 5670km between Australia and Kolkata, respectively), only two modes are dominant. The sunrise minimum and the sunset minimum are the result of interference between the first and the second modes. During sunrise and sunset, due to discontinuity at the boundary of day and night, the second mode gains energy from the first mode. Two modes are then comparable, a fact called mode conversion. The destructive interference between the two modes provides the signal minimum. From the point of view of propagation, the time of the signal minima during sunrise and sunset are called terminator times. During the days of spiky variations, since the superposition of EM radiation emitted prior to the time and after the earthquake destroys the diurnal pattern of signal amplitude, it is not possible to find out the terminator time from raw data. But removal of abruptly large data points (data values of higher by 6 dB or more above the mean) produce a diurnal pattern of the signal as usual. Figure 7(b) is the sample of diurnal pattern recovered for the day of earthquake. In this way, we determined the terminator times both for morning and evening. The difference of terminator times from their average values are depicted in Fig. 8(a) for April, 2009. During this undisturbed month, the difference in terminator time from their mean value shows a gradual variation from negative to positive value. This is true for both signals and for both terminator times, morning and evening terminator time. In Fig. 8(b), the variations in terminator times around the day of earthquake have been shown. It is clear that the difference in terminator time from their mean value shows oscillatory variations between positive and negative values rather than systematic variation of increased type or decreased type. The deviations from the normal kind of variation in terminator time both in morning and evening values and for both the signals at 40 kHz and 19.8 kHz clearly indicate the influence of perturbation in the ionosphere due to earthquakes. The shift gradually changes while the date of occurrence would approach. 2.6 Anomalies of the 4th mode of Schumann resonance spectra Significant enhancement in amplitude of the fourth mode of Schumann resonance spectra and the increase in its peak frequency are obtained during the period of these two earthquakes (vide section: 2.5). Figure 9 depicts the results of analyses of the recorded data of August 11, 2009, the day of occurrence of earthquakes, along with the regular normal day data. Magnitude of Fourier transform results in arbitrary unit have been plotted against frequency. Thin continuous line represents the Schumann resonance spectra within the undisturbed Earth-ionosphere waveguide and the bold line indicates the variation when the disturbance over the earthquake zone sets in. -8-

Fig. 7(a) The diurnal pattern of the amplitudes of the LF and VLF signals at 40 kHz and 19.8 kHz as received at Kolkata. These graphs have been plotted considering the average of days excluding ± 5 days about the day of earthquake. The amplitude of VLF signals are in dB above 1 mV/m.

Fig. 7(b) The diurnal pattern of the amplitudes of the LF and VLF signals at 40 kHz and 19.8 kHz as received at Kolkata. These graphs have been plotted by removing abruptly large data points which are higher by 6 dB or more above the average on the day of earthquake (11.08.2009). The amplitude of VLF signals are in dB above 1 mV/m.

Fig. 8(a) Shift in terminator time in the days of April, 2009 about the monthly mean value. Upper two panels are for 40 kHz morning and evening terminator time while lower two panels represent the same parameter for 19.8 kHz.

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Fig. 8(b) Shift in terminator time surrounding the day of occurrence of the two earthquakes, Andaman Island, India and South Coast of Honsu, Japan. The results are presented in terms of their histogram. Upper two panels are for 40 kHz morning and evening terminator time while lower two panels represent the same parameter for 19.8 kHz.

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Amplitude as well as peak frequency around the fourth mode of Schumann resonance spectra (26 Hz) are found to increase. Shift in peak frequency about 1.25 Hz is obtained. Amplitude variation of the fourth mode of Schumann resonance spectra is plotted in Fig. 10. Continuous line curve is the plot of signals on the day of occurrence of the earthquakes. Large dashed and short dashed curves, respectively, are for the two days earlier and two days later from the earthquake date. Around the time of main shocks of both the earthquakes, amplitude sustains higher values about 2.4 in arbitrary unit. It is nearly 17% increase from the pre- and post- seismic values (De et al., 2011).

Fig. 10 variation of amplitude of the fourth mode of Schumann resonance spectra on the day of occurrences of the two earthquakes on August 11, 2009 (bold line curve) and variation of the same parameter two days earlier (large dashed line curve) and two days later (short dashed line curve) from the date of occurrence.

Fig. 9 Frequency spectrum of Schumann resonance on the day of occurrence of Andaman Island, India and Honsu, Japan earthquake on August 11, 2009 (bold line curve) along with the 15 days average plot adjacent to the day of occurrence (thin line curve).

2.7 Non-linear heating of the lower ionosphere during interaction between HF and ELF signals Interaction experiment between Schumann resonance and HF (15 MHz) round-the-world signals (RWS) in the lower region of the ionosphere (D-layer) shows peaks at the Schumann resonance frequency modes in the spectra of the HF signals (Yampolski et al., 1997). This is due to the effect of cross-modulation between ELF and HF radio waves. The intensity of SR field produced by thunderstorm activity is strong enough to give rise to nonlinear effects in the D-layer. As a result, there will be heating of electrons by the ELF oscillations of SR fields. The fair-weather electric field also initiates the process of heating in the D-layer. The influence of this field gives a linear dependence of electron temperature fluctuations on the field strength on SR. These lead to non-linear effects comprising the variations of electron temperature, effective collision frequency and conductivity of the medium. The fluctuation of temperature due to electron heating including the influence of small-scale fair-weather electric field has been studied. - 11 -

The expression for temperature rise may be derived as T [V 2 A12  2V 0 RE s A21   02 R 2 E s2 ] T  m 3 02 R 2 KT 2  ( 2   k2 ) e Where,

A1   k

 pk ( k  i )  ( k  i ) 2   ck2

 pk  ck

 (

k

 (

k

 i ) 2   ck2  pk ( k  i )

k

k

 i ) 2   ck2

and A2   k

 pk ( k  i )  ( k  i ) 2   ck2

 ( k

 pk  ck

 i ) 2  i  pk ( k  i )

 (

k

 i ) 2   ck2 Other symbols to be followed from De et al. (2005). k

k

From numerical analysis, T is in the range (1.802 – 2.605) K. The variation arises due to the variation in the value of fair-weather field and electron-neutral particle collision frequency at the upper D-region height range. The value of SR field at the surface of the Earth is taken as 3×10-4 Vm-1. It is assumed to remain constant up to the upper boundary of SR cavity. The value of fair-weather field is about 3×10-1 Vm-1 at the height of D-layer. The electric field exhibits dependence of local influences. The observed field at the surface of the Earth is low at the daytime and remains relatively higher at night. The value of fair-weather field between 60 km and 80 km has considerable fluctuations (0.1–1) Vm-1. In the numerical analysis, the values of the geomagnetic field at the height range 60 – 80 km are taken between 0.3792 Oe and 0.3711 Oe, where  0  9×10-12 Fm-1 2.8 Schumann resonance data to monitor climate change Global energy balance of the planet is very sensitive to the water vapour content in the upper troposphere. Small changes in upper tropospheric water vapour can have large implications for the Earth’s climate. Measurements of water vapour in the upper troposphere are very difficult and expensive over long periods of time. Continental convective storms that pump the water vapour into the upper troposphere are also the storms that produce the majority of Earth’s lightning discharges, and hence the major source of ELF electromagnetic waves in the Earth-ionosphere cavity. Global water vapour measurements reveal the apparent correlations in time and space between the lightning activity, the ELF SR measurements and the atmospheric water vapour content. The SR may thus be of use in monitoring changes in atmospheric water vapour content, an extremely important parameter related to global climate change. Thus, from statistical analysis of long time water vapour data, global climate change can be predicted.

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2.9 Transient luminous events Some SR transients (Q-bursts) are released as the transient luminous events: sprites, elves, bluejets etc. Sprites are produced by positive cloud-to-ground (CG) lightning occurring in the thunderstorm stratiform region. It is accompanied by Q-burst in the SR band. For this, SR data is useful for the estimation of global occurrence rate of sprites. The return stroke current in the lightning discharge from the ground, in some cases, does not end in the cloud, but continues to move upward and terminate in the lower ionosphere. This transient current/field is associated with optical emissions (sprites, elves, blue jets, blue starters), in the space between the top of the cloud and the lower ionosphere, known as Transient Luminous Event (TLE). These are shown in Fig. 11.

Fig. 11 Lightning related transient optical emissions in atmosphere (Neubert, 2003)

Sprites are cluster of short lived (~50 ms) red luminous columns, stretching from 35 km to 90 km altitude having width less than one Km (Sentman, 1995; Lyons, 1996; Neubert, 2003) with the maximum brightness at 66 km altitude (Wescott et al., 2001). The upper portion of the sprites is red, with faint blue tendrils extending to 40 km or lower. About 80% of sprites are associated with ELF transient events and +ve CG lightning return strokes having large peak current (>35 kA) (Neubert et al., 2008). Some sprites are associated with –ve CG lightning (Williams et al., 2007). Sprites have been observed in Africa, South, Central and North America, Australia, Europe and Japan. These may occur over any area so long energetic thunderstorms are present. Several different types of transient luminous events above thunderstorms are found which include relatively slow moving fountains of blue light, known as blue jets, which emanate from the top of thunderstorms up to an altitude of 40 km. Elves, abbreviated from Emissions of Light and VLF perturbation from Electromagnetic pulse Sources, are luminous disks formed by electromagnetic pulse of the cloud-to-ground lightning - 13 -

discharge producing both light and ionization. It is seen at the altitude of 75 – 105 km. The horizontal width of elves: 100 – 300 km. Halos – have been observed in association with –CGs from the ground. Blue jets move with nearly 100 km s-1 and are characterized by a blue conical shape. An upward moving giganting jets (GJ) may have a direct path of electrical contact between the top of thundercloud and lower ionosphere. 2.10 Extra terrestrial lightning Within the planets: Venus, Mars, Jupiter, Saturn and moon Titan – Schumann resonances are detected. The existence of the exciting source of electromagnetic waves in ELF range and the existence of ionosphere with electrical conductivity increasing with height from the surface are needed to be present in the planets for SR. In the case of gaseous planets, high conducting layer would be necessary for SR. The lightning on Venus from the impulsive electromagnetic waves was detected through Venera 11 and 12 landers. Theoretical calculations of the Schumann resonances at Venus were reported (Nickolaenko and Rabinowicz, 1982; Pechony and Price, 2004). Its detection is not possible because of variation environment. Lightning activity has not been detected on Mars, but charge separation and lightning strokes may be possible in the Martian dust storms. Martian global resonances were modeled by Pechony and Price (2004); and Molina-Cuberos et al. (2006). Transmission line modeling method has been applied for predicting the Schumann frequencies on Mars (Molina-Cuberos et al., 2006). Their analyses reveal that the resonances have a fundamental mode with a peak frequency approximately 11 Hz, depending on the solar condition the resonance frequencies are very sensitive indicator of global conductivity profile. Therefore, experimental measurements of Schumann resonance by ground sensor are a valid tool for remote sensing of the lower ionosphere on Mars and could contribute to the detection of sporadic meteoroid in Mars. Schumann resonances on Titan have received more attention than on any other celestial body (Morente et al., 2003; Molina-Cuberos et al., 2006; Nickolaenko et al., 2003; Pechony and Price, 2004). It appears that only the first Schumann resonance mode might be detectable on Titan. Little is known about the electrical parameters of Jupiter and Saturn interior. There is no work in regard to Schumann resonances on Saturn. Jupiter is the only planet where lightning activity is well established. An attempt is there for detecting Schumann Resonance on Jupiter. But there is no possibility to place any sensor there. 3. Results of some observations from Kolkata The frequency response of different peaks of Schumann resonances in the received signals is shown in Fig. 12, which was recorded on April 2, 2000. Observations at different times show amplitude and frequency fluctuations which are due to the fluctuations of the causes producing such signal spectra from the middle ionosphere. Figure 13 is the recorded signal at Kolkata on April 12, 2000, which depicts amplitude and frequency changes in all the three modes.

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Fig. 12 Frequency response of received signal recorded on April 2, 2000.

Fig. 13 The recorded Schumann resonance spectra over Kolkata on April 12, 2000. Resonance frequencies experience variations.

The monthly intensity variations due to global thunderstorm activities obtained from Schumann resonance amplitude are shown by the bar graphs in Fig. 14. The intensity fluctuation of SR modes is directly related to the variation of thunderstorm activity. The black coloured bar graphs are drawn from the data recorded at Kolkata. The half-tone bar graphs of Fig. 14 are due to the data from the Geophysical Observatory, Modra, Slovakia (Lat: 48.61º N) for the same period. Intensity reaches maximum value during summer, which agrees with the results of earlier works (Nockolaenko et al., 1999; Nickolaenko and Rabinowicz, 1995). Because of different locations of these two recording centers, the influences of global thunderstorm activity zones for the three modes of SR are also different, for which the annual intensity variations at these two latitudes are different. Satellite observations show the existence of great variability in the longitudinal distribution of thunderstorm activity centers along with their temporal and seasonal variations. Observations of global lightning distribution during January 1998 to February 2005 taken from LIS (Lightning Imaging Sensor) NASA, confirm that the thunderstorm activity in Himalayan region is more prominent than Asia-Australia region. The Asia-Australia has long been thought of as one of the largest thunderstorm producing regions, but recent records confirm that the thunderstorm from the Himalayan regions is also strong enough to be one of the distinguished lightning centers all over the world. Kolkata is nearer to Himalayan and Asia-Australia lightning centers in comparison to Slovakia. This may be the reason for the observed variation of intensity of SR modes. The global variations of thunderstorm activity centers change the source-receiver distance which may be the reason for the seasonal and temporal variation in frequency shift.

Fig. 14 Variation of resultant intensity of SR modes. The black coloured bar graphs show the results recorded at Kolkata and the half-tone bar graphs depict the results from the data of Geophysical Observatory at Slovakia. Intensity fluctuations are directly connected to thunderstorm activities.

Figure 15 shows the resultant intensity fluctuations of SR modes recorded from four different latitudes during the whole year 2000. The black coloured and half-tone bar graphs indicate the results recorded from Kolkata and Slovakia, whereas the blank bar graphs and the bar graphs filled-up by slanting hatched lines are due to Moshiri (Lat: 44.29 N) and Nagycenk (Lat: 47.6 N), respectively. Because of the difference in the distance between the location of the source and observational sites, the recorded data exhibited the difference in SR intensity. Moshiri is very close to Asia-Australia violent thunderstorm center than the situation of Nagycenk. Because of geographical location of lightning centers, the magnitude of intensity of SR at Nagycenk may be more dependent on African and European sources than the American sources. The geographical location of Nagycenk is nearer to African sources of thunderstorm center. K o lk a t a S lo v a k ia M o s h ir i N agycenk

8

INTENSITY, pT/Hz

1/2

7 6 5 4 3 2 1 0 Jan

Feb

M ar

A pr

M ay

Jun

Jul

A ug

S ep

O ct

N ov

D ec

M O NTH S

Fig. 15 The resultant intensity fluctuations of SR modes recorded at four different places are shown by bar graphs. The black coloured and half-tone bars represent the results of Kolkata and Slovakia while the blank bars and bars filled-up by slanting hatched lines are due to the records at Moshiri (Lat. 44.29 N) and Nagycenk (Lat. 47.6 N), respectively.

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The difference between maximum and minimum frequency values of the first Schumann resonance frequency recorded during the span of each day has been determined. These diurnal frequency shifts of the first mode have been averaged over the corresponding month. In this way, the average values are determined during the period January 2000 to December 2000 from the recorded data at Kolkata. The results are shown in Fig. 16.

Fig. 16 Annual changes in the averaged values of the Schumann resonance frequency variations of the first mode (January 2000 to December 2000).

In Fig.17, the mean diurnal and seasonal variations of the peak frequency of the first SR mode for the spring season (March to May) recorded at Kolkata in the year 2000 are compared with the variations over the same season of the year 2006 at Moshiri, Japan.

Fig. 17 The diurnal and seasonal variations of the peak frequency of the first mode of SR: (a) for the spring season (March to May) recorded at Kolkata, (b) results of Moshiri, Japan (Sekiguchi et al., 2008), for the same season of 2006

The curve (a) is drawn with Kolkata data while the curve (b) is due to Japanese work. The difference between the two graphs may be interpreted in term of the variations of the influence of - 17 -

thunderstorm activity centers, namely, Asia and Australia; Africa; and Europe and America (Orville and Henderson, 1986) between Kolkata and Moshiri. The great variability in the longitudinal distribution of thunderstorm activity centers along with their seasonal variations (Williams and Heckman, 1993) is also to be taken into account in the context of the observed difference. The observed variations of both frequency and amplitude in Schumann resonance phenomena are related to changes in global thunderstorm activity as well as due to consequence of complex effects in the earth-ionosphere cavity. The inhomogeneities in the earth-ionosphere cavity and the anisotropy due to geomagnetic field are supposed to introduce conductivity perturbation in the medium which modify the attenuation depending on the location of the source. In the recorded data of Fig. 14, the influence of Asia lightning activity center is more in Kolkata and Slovakia than African and American centers. The intensity fluctuation of SR modes due to the main influence of Asian lightning center can be understood from this figure. Thunderstorm occurrences are high during July – August (rainy season) and low during November – December (winter season) for which SR intensity is high in rainy season than in winter season. The thunderstorm center of Asia in the rainy season remains at Lat: 15° N, Long: 105° E and it is nearer to both the centers. For this reason, SR intensity bears almost the same value during this season. But the Asian thunderstorm center during winter season remains at Lat: 15° S and Long: 130° E and for this, its influence on Slovakia center is less in comparison to Kolkata center. Hence, larger variations of SR intensities at these centers during winter season are found. Moreover, there is extra influence over Kolkata recording center from the Himalayan regions also during the winter season. This explains why the thunderstorm activities are similar in some part of the year and differ in other part. Schumann resonance frequencies may be considered as the indicator of the thunderstorm distribution in global lightning activity level. The growth of the global lightning activity is generated by the thunderstorm spreading. The earth-ionosphere cavity acts as the resonator of electromagnetic waves generated due to excitation energy from lightning. Different records from Kolkata exhibit sub-peaks surrounding the three modes. The occurrences of these unequal subpeaks are due to the uneven influences of the global thunderstorm activity centers towards the interaction within the earth-ionosphere cavity generating the modes. Within the cavity, disturbances like random fluctuations of irregularities are occurring regularly along with other disturbances, like Polar-Cap absorption, X-ray bursts, etc. Solar proton precipitation introduces an abrupt frequency variation. The modification of frequency is enhanced by the interaction between the polar ionosphere and precipitating particles. It causes the whole polar ionosphere to shift downward causing the frequency of the first SR mode globally to move to lower values (Schlegel and Fullekrug, 1999; Roldugin et al., 2003). Protons with energy up to 100 MeV are most often emitted from the sun in conjunction with solar flares which can penetrate deep into the D-region and cause additional ionization leading to conductivity changes thereby modifying SR parameters. The variation of frequency of the first mode of SR spectra during solar proton events on July 14, 2000 is shown in Fig. 18 from the observed data recorded over Kolkata. The result shows the - 18 -

decrease in frequency during the period of occurrence. The severe solar X-ray bursts occur just before the proton event shows enhancement of the first mode frequency. The influences of solar proton events and X-ray bursts upon the SR frequency fluctuation in the results from some other locations are also exhibited. The variation may be explained in terms of changes of ionization incurred by the events. High energy solar proton flux of July 14, 2000 was detected by GOES 8 satellite at 1035 UT which showed maxima at 1145 UT. It was preceded by the solar X-ray bursts. Its onset occurred at 1010 UT and the maximum took place at 1035 UT. The variation of proton flux at energy (E) 510 < E < 700 MeV and X-ray at wavelength () at 0.05 <  < 0.4 nm are shown in Fig. 18 by continuous line curve and dashed line curve, respectively.

Fig. 18 Energetic solar proton flux and X-ray bursts as recorded onboard GOES 10 satellite during the solar proton event (SPE) of July 14, 2000. The continuous line curve is for the proton flux while the dashed line curve represents X-ray bursts.

Fig. 19 Variation of the first mode SR frequency over Kolkata on July 14, 2000 depicted by the continuous line curve joining the square blocks. It is superposed on the curves for solar X-ray bursts and SPE. SR frequencies averaged over three days adjacent to the day of occurrence are shown by the dot-dashed curve.

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The changes of the first SR frequency on July 14, 2000 over Kolkata are shown by the continuous line graph joining the square blocks of Fig. 19. It is superposed on the curves for solar X-ray bursts (continuous line curve) and SPE (dashed line curve). The values of the first SR frequency averaged over other three days adjacent to the day of occurrence of these two events are plotted by dotdashed curve. The SR frequency starts enhancing from 0900 UT and reaches the highest value ~ 8.15 Hz at 1035 UT during solar bursts. Immediately after, during solar proton flare, its value approaches towards lower value showing the minima at ~ 7.45 Hz because of the instantaneous occurrence of high energy SPE. The decrease in the first mode SR frequency continued up to around 1200 UT and returned to its normal value at 1600 UT. During solar activity, along with the exponential variation of conductivity in the vertical direction, there may be two kinds of changes: i) increase in ionization level at the normal D-region reflection heights, and ii) the variation of D-region lower edge. According to this model, the variation of the first SR frequency is given by f1 N h  0.36(  ) f1 2N h Further, the frequency can change due to both electron density variations in the ionospheric Dregion and the change of height of the lower boundary of the D-region. Growth of X-ray flux enhances electron density without a significant change in ionospheric altitude. So the first SR frequency increases. Solar proton penetrating deep into the atmosphere causes a decrease of altitude of D-region. This gives rise to decrease of the first SR frequency. But, SPE can also affect ionization. So the two terms compete with each other and their relative magnitudes are the decisive factors. If two terms are comparable, no change in the first SR frequency may be obtained. Apart from this, two terms are interlinked. So this model will not be efficient in estimating the magnitude of variation of the first SR frequency. Later on, two characteristic layers were identified within the lower ionosphere responsible for height variation of vertical electric field and horizontal magnetic field components. Another kind of inhomogeneity can arise during enhancement of solar X-ray or proton flux (Greifinger and Greifinger, 1978). In this method, the eigen value depends on the value of the conductivity of the atmosphere in two independent characteristic layers (typically 40-55 km for frequency < 50 Hz). Of these, one layer is defined as that at which    0 , i.e., where displacement and conduction currents are equal to each other. The other, relatively upper layer is that at which 4 0 2  1 where ζ is the conductivity scale height at the altitude (typically 60-70 km) responsible for reflection of ELF wave. The unequal ionization of these two layers by solar extra-radiation can give rise to vertical inhomogeneity affecting SR frequency. An approximate variation of ionospheric conductivity around 30-80 km is shown in Fig. 20. The SR frequencies can be expressed as:

fn 

n(n  1) c 2 re

h1 h2

n is the mode number (n = 1); c, the velocity of light; re , mean radius of the earth; h1 and h2 are the two characteristic heights in the D-region.

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For n = 1, f 1 

1 c  re

h1 h2

Fig. 20 Conductivity model according to using the exponential equation with perturbation parameter. The line  = 0 signifies the altitude in which ionosphere becomes conductive (Sentman, 1983).

During solar flare, the lower height h1 is almost unaffected because it causes extra-ionization mainly around 70 km, i.e., height of reflection h2 is slightly reduced and let it be 2 so that h1 1 c modified first SR frequency becomes  f 1 SR  . So the first SR frequency  re h2   2 increases. In our observation, the proton event was preceded by a flare during which we observed an increase in the first SR frequency. Also, during solar flare, the reflectivity of the upper height increases. This fact also contributes additionally to the increase in the first SR frequency. During solar proton event, energetic protons can penetrate deep into the ionosphere upto 50 km and can ionize regions lower than the normal D-region (Clilverd et al., 2006). In this case, h1 decreases and 1 c h1   1 let it be by 1. If the effect on h2 is now neglected, the first SR frequency  f 1 SR  .  re h2 As a result, first SR frequency decreases. The histogram of the changes of the first mode of SR frequency during these two events on the day of occurrence over Kolkata is presented in Fig. 21. The results are analogous with the results as interpreted in the previous paragraph.

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Fig. 21 Histogram of the first mode SR frequency over Kolkata during the same events.

For brevity, diurnal variations of the first mode of SR frequency over Lovozero (68° N, 35.1° E) for three different SPE and solar X-ray flux events occurred on November 6, 1997; July 14, 2000 and April 2, 2001 are shown in Fig. 22 by curves (a), (b) and (c), respectively. The variations of Xray intensity at 0.05 <  < 0.4 nm and proton flux at 0 < E < 4500 MeV are obtained from the event of November 6, 1997. X-ray bursts last for 10 min and attains maximum value at 1145 UT while proton flux lasts for 7 hr 30 min (1230 UT – 2000 UT) with peak value at 1700 UT. In these two events, the value of frequency increase was ~ 0.3 Hz and the subsequent decrease was ~ 0.15 Hz, respectively. On July 14, 2000, the duration of X-ray bursts was of 50 min, maximum flux took place at 1035 UT having the same intensity. Proton flux at 510 < E < 700 MeV with total duration of 4 hr attained peak at 1145 UT. The variations of SR frequencies were ~ 0.3 Hz for Xray burst (increased maxima) and ~ 0.4 Hz for SPE (decreased minima). The SR frequency increases by ~ 0.25 Hz (maxima) on April 2, 2001 for X-ray bursts with the same intensity. Interval was of 1 hr 30 min with peak at 2200 UT.

Fig. 22 Diurnal variation of the first mode SR frequency over Lovozero (68° N, 35.1° E) during three different SPE and X-ray bursts. The curves (a), (b) and (c) show the events of November 6, 1997; July 14, 2000 and April 2, 2001, respectively.

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The influence of SPE and X-ray burst events of July 14, 2000 on the first mode of SR frequency at different latitudes are presented in Fig. 23. All the curves show the increase at 1035 UT, i. e., at the peak of X-ray flux and decrease during SPE. It attains maximum value 8.25 Hz at 1035 UT over Mitzpe Ramon (30.5° N, 34.4° E) during X-ray bursts while minimum value of 7.35 Hz at 1300 UT over Lovozero (68° N, 35.1° E) during SPE. Locations of three global thunderstorm regions are at Asia (15° N, 105° E), Africa (10° N, 10° E) and America (30° N, 80° W) (Barr et al., 2000). Mitzpe Ramon nearer to both the Asia and African lightning centers, whereas Lovozero though nearer to Asian center but far away from African center. Due to this variation of relative positions, the joint influence of Asia-African lightning centers on SR frequency is much stronger for Mitzpe Ramon than that for Lovozero. Moreover, the influence of American lightning center is more towards Mitzpe Ramon than Lovozero due to their geographical positions. No data are available after 1600 UT on Lekhta (64.26° N, 33.58° E) observatory.

Fig. 23 Variation of the first mode SR frequency over different latitudes on July 14, 2000.

Figures 23 and 24 show the seasonal and monthly variation of the first SR frequency over different stations. The amplitude of SR frequency is determined by the temporal and spatial distribution of global lightning. Lightning activity moves from northern to southern hemisphere as the winter sets in northern hemisphere. SR characteristics are, therefore, expected to be dependent on location and seasonal conditions. SR properties are strongly affected by global lightning activity and the distance of the SR receiver to the source regions (Sentman, 1995). So these observed seasonal and monthly changes are primarily determined by the source-observer geometry. The seasonal and monthly variations of the SR frequency in the observatories considered are not same. Figure 25 asserts that during solar flare followed by SPE, the short-time variations of the SR frequency at the three stations, viz., Lovozero, Karymshimo and Kolkata are almost same. The variations observed over Lekhta and Mitzpe Ramon are small but the nature of variations are almost same as that of the other three stations. The seasonal and monthly variations of the first SR frequency over different observatories are different due to different source-observer geometry, also due to movement of lightning sources from northern to southern region and vice versa. The variation of SR frequency due to solar X-ray - 23 -

flare and SPE are observed to be same at all the observatories. This is because of the fact that this kind of variation is not dependent on source-ionosphere geometry. This kind of variation is to be explained by the two-characteristic-layer-model of two different heights. Mainly, the decrease in first SR frequency during SPE requires two-characteristic-layer-model. For the purpose of such modeling, to account for the variation of the first SR frequency associated with SPE and solar Xray bursts, preliminary data of electron density at the two characteristic heights are to be known during respective solar events. For this purpose, VLF phase and amplitude observations could be used. Very low frequency signals are known to be seriously affected during SPEs propagating through the Earth-ionosphere cavity at about 50 – 60 km altitude (Clilverd at al., 2006). The Dregion electron density and ionization change during high energy particle precipitation down to this altitude depend upon the particle energy at this altitude.

Fig. 24 Seasonal variation of the first mode SR frequency over different Observatories on July 14, 2000.

Fig. 25 Monthly variation of the first mode SR frequency at different latitudes for the same phenomenon.

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The correlation between solar activity and SR parameters might be used for many studies, like the study of the sun and extraterrestrial sources. High solar proton fluxes are very serious radiation hazards in the inner solar system and might have serious effects on interplanetary spacecrafts, particularly on the component materials which are exposed to the high energy of these events. An analysis of long term observational data may provide knowledge regarding the dependence of the first SR frequency on quiet sun radiation or slowly varying components of the solar radiation. Thus, it can be concluded that as the ELF propagation gets influenced by SPE, the SR amplitude and ELF signal amplitude observations may be used to predict changes in the ionospheric D-region during any particle precipitation effects. Enormous possibilities are there with Schumann resonance phenomena and their applications. 4. Some of the unsolved problems (i) (ii) (iii) (iv) (v) (vi)

Development of methods and tools that will enable long-term continuous monitoring of SR recording of upper Tropospheric Water Vapour influencing the climate change. The relative importance of the day-night asymmetry in the ionospheric conductivity profile. Determination of spatial lightning distribution from background records. Latitudinal changes in the Earth’s magnetic field. Sudden ionospheric disturbances Polar cap absorption etc.

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