Indian Journal of Radio & Space Physics Vol. 34, December 2005, pp. 408-412

Studies on the integrated field intensity of ELF-VLF sferics at Tripura *, India

B K De & M Pal

Tripura University, Suryamaninagar 799 130, Tripura West, India

and

S S De, R Bera, S K Adhikari & A Guha

Centre of Advanced Study in Radio Physics and Electronics, University of Calcutta, Kolkata 700 009, India

and

S K Sarkar

Department of Physics, University of Calcutta, Kolkata 700 009, India

Received 14 June 2004; revised 13 December 2004; accepted 29 July 2005

The results of observation of the Integrated Field Intensity of Sferics (IFIS) at 0.9, 3, 6, 9 and 12 kHz at Tripura (lati­tude: 23 °N) are reported here. Some specific characteristics of ELF-VLF atmospherics found from thi s hilly place are pre­sented, along with a brief description of the experimental set-up for their detection.

Key word: Atmospheric noise, ELF-VLF sferics, Sferics

PACS No: 92.60.Ta; 92.60.Pw

1 Introduction Severe thunderstorms make a remarkable contribu­

tion to the ELF band spectra. An important character­istic of the electrical behaviour of thunderclouds is the consistent polarity of their cloud-to-ground lightning flashes. Most flashes bring negative charge to the sur­face of the Earth. There are various mechanisms, which explain the negative charge on the lower part of thunderclouds 1•

The atmospherics are very significant with regard to electric phenomena in different types of cloud during meteorologicaiiy active periods. Atmospheric radio noise field strength (ARNFS) measurements are expected to provide different features for the study2

·3

of ionospheric propagation. The geographical position of Tripura (north-eastern part of India, latitude: 23 °N) favours ARNFS investigations from local cloud dis­charges, as weii as from the distant sources of Aus­tralia, Japan and Africa.

The experimental study of ELF in the north-eastern part of India is scanty. The ELF-VLF spectrum be- · tween 50 Hz and 10 kHz is unusual , because there is no broadcasting transmitter in this range. As long as

*A shorter version of thi s paper was presented at the National Conference of Radio Science in Indi a (I.N.C.U.R.S.I.-2003) held at National Physical Laboratory. New Delhi 110 012, during No­vember 27-29, 2003.

rhe receiver is placed well away from unintentional short range electromagnetic interferences, such as electrical power lines and electrical machinery, the spectrum is dominated by natural noise from cloud discharges.

During locally clear days and nights ARNFS ex­hibit regular behaviour at this latitude. During the lo­cal monsoon and local thunderstorms, their charac­teristic is modulated. The nature of the modulation varies from one place to another and with latitude4

• It is well known that at Tripura, maximum rainfall oc­curs in July-August, whereas in Australia maximum rainfall occurs in December-January.

The results of observations on the integrated field intensity of sferics (IFIS) at frequencies 0.9, 3, 6, 9 and 12 kHz are now regularly recorded at Tripura. Observations at 0.9 and 9 kHz have been made since March 2003 . Some of the features observed since May 2003 are reported in this paper.

The goal and objectives of the present work are to study the nature of variation of ARNFS with short period showers (duration 5-60 min), which are pecu­liar event~ in the north-eastern part of India. ARNFS exhibit a sudden rise and gradual fail in relation to short period showers. Also, the variation of ARNFS with cyclonic thunderstorms, the characteristics of ARNFS in relation to sea thunderstorms, the geomag­netic influence on the sunrise effect and the variation

DE eta/.: INTEGRATED FIELD INTENSITY OF ELF-VLF SFERICS AT TRIPURA 409

of ARNFS with solar X-ray flares may also be inves­tigated.

The uniqueness of the position of Tripura is that the location is on the slope from the Himalayas to the Bay of Bengal. The region is very prone to rainy weather, even in winter. The features of ARNFS are therefore expected to be somewhat different from the worldwide average.

2 Instrumentation The experimental set-up installed at Tripura Uni­

versity consists of an inverted L-type antenna to re­ceive vertically polarized atmospherics in the ELF­VLF bands from near and far sources. By selecting the bands, unwanted noise has been reduced. The cut­off frequencies of the low-pass filter and tuning fre­quencies are different. As for example, to receive at­mospherics at 3 kHz, the antenna-induced voltage is passed through a low-pass filter with a cut-off fre­quency of 5 kHz as shown in Table 1. The filter out­put is amplified with an AC amplifier using OP AMP IC531 in a non-inverting mode. The gain has been limited within the value to check transients that may trigger sustained oscillations in the amplifier. The amplifier is followed by a series resonant circuit tuned to the desired frequency and another buffer. The se­lective circuit is a series combination of an inductance and a capacitance. The set-up is depicted in a block diagram (Fig. 1).

To ensure high selectivity , the inductive coil is mounted inside a pot-core of ferrite material. The se­lected sinusoidal Fourier components of atmospherics are then passed to the input of a detector circuit through a unit gain buffer using OP AMP IC531 . In the detector circuit, the diode OA 79 is used in the negative rectifying mode. The output of the diode is applied across a parallel combination of resistance and capacitauce, so that the detecting time constant is 0.22 s. The level of the detected envelope is propor­tional to the amplitude of the Fourier component.

Tablel - Cut-off frequency of low-pass filter and corresponding tuned frequ encies

Cut-off frequency (Low-Pass Filter)

kHz

3 5 8 12 15

Tuned Frequency kHz

0.900 (in lieu of I) 3 6 9 12

AC AMPLIFIER VARIABLE GAIN

Fig. !-Diagram of the ELF-VLF receiving system at Tripura (latitude: 23 °N)

The detected output is amplified by a quasi­logarithmic de amplifier using OP AMP 741 in the inverting mode operation. The calibration of the recording system has been done using a standard signal generator with an accuracy of ±0.86 dB . During calibration, the antenna was disconnected from the filter circuit and replaced by the signal generator through a capacitance having a value equal to the terminal capacitance. First, the outputs are calibrated in terms of induced voltages at the antenna. To get very low signals from the function generator, a dB-attenuator is used. The output is calibrated in terms of values of dB above 1 J..I.V . Then it is converted to an absolute value in units of J..I.V. The absolute value of induced voltage has been divided by the effective height of the antenna to calculate the field strength in ll V /m.

The data are recorded in two ways - analog re­cording using chart recorder and digital recording us­ing a data acquisition system. The analog recorder records data on chart, moving at the rate of 2 crnlh or 4 em/h. The digital data acquisition system uses a PCI 1050, 16 channel 12 bit DAS card (Dynalog) . It has a 12 bit ND converter, 16 digital input and 16 digital output. The input multiplexer has a built-in over­voltage protection arrangement. All the 110 parts are accessed by 32 bit 110 instructor, thereby increasing the data input rate. It is supported by a powerful 32-bit API, which functions for 110 processing under the Win 98/2000 operating system.

410 INDIAN J RADIO & SPACE PHYS, DECEMBER 2005

3 Results of observation In the absence of local thundercloud activity, at­

mospherics from distant thunderclouds are character­ized by sunrise (SR) <:nd sunset (SS) effects. Figure 2 has been obtained by taking the average for the days which were locally clear in the month of August, 2003. The figure depicts standard deviations as ± val­ues about the mean points. It is seen that the diurnal pattern reveals higher levels at night and in the local afternoon hours. In the sunrise period, the level of atmospherics decreases gradually to a very low value. During sunset, the level decays somewhat to show the sunset minimum. This diurnal pattern is repeated

Augus~ 2003

a ...... 36.6 > ::1.

I - 20.6

1 'H~ 0 f-(9 z 13.0 UJ 0::: f-

08.2 (/) A 0 _J

UJ 04.6 LL

0 4 8 12 16 20 24

TIME, hrs 1ST A: Sunrise minimum

C: Afternoon maximum B: Recovery effect D: Sunset minimum

Fig. 2-Diurnal behaviour of ARNFS over Tripura [The time is shown in IST (lndian Standard Time) and the field strength in V/m on a non-linear scale.]

36.6

·§ 14.6

~

u) 07 .31 LL z ~ 046

02.6

01 .8

0200

: .. : j

0400 0600 0800 1000 1200

TIME, hrs 1ST

Fig. 3- The record of ELF-VLF sferic field strengths during sun­rise; the steep fall during sunrise is evident

during locally clear periods. The pattern is apprecia­bly perturbed due to severe rain, thunderstorms and solar flares as discussed in later sections of the paper.

The recorded ELF-VLF sferic strengths during sun­rise on meteorogically clear days show a fall charac­terized by two or three small steps having magnitudes lying in the range 8-15 dB (Fig. 3). It is also.seen from the record during geomagnetically active days (Ap > 50) that the sumise effect disappears. Sometimes the effect is observed without the steps. Figure 4 depicts the nighttime level followed by the sunrise minimum. Throughout the time in rainy days, ARNFS exhibit quasi-periodic variations. During rainy days, the records exhibit irregular variations as shown in Fig. 5. On this very day, the whole sky was covered with cloud from dawn. Rain started over the locality at 0815 hrs LT. Showers continued up to 1530 hrs LT.

C/) u. z 0::: <(

Fig. 4-Nighttime and sunrise observations at 9kHz

E 46 .1

> :J.. 32.7

::C 0 23 .1 z ~ 16.4 1-C/) 11 .6 0 ul 08 .2 u:::

0800 1000 1200 1400 1600 1800 2000

TIME. hrs 1ST

Fig. 5-0bservations of ARNFS at 9 kHz made during rain

DE eta/.: INTEGRATED FIELD INTENSITY OF ELF-VLF SFERICS AT TRIPURA 411

The total rainfall was so high that the whole Agartala town 12 km away from the experimental site was flooded. The receivers have been installed in a room near the field and the antenna was connected to the input through a waterproof coaxial cable. Hence, the irregular variations are not due to electrical shortening at the input of the receiver due to rains.

Figure 6 shows two successive gradual rises in sig­nal strength from A to B and from C to D followed by sudden falls from B to C and D to E, respectively. The gradual rises in ARNFS during rainy days of pre­monsoon and monsoon seasons are the evidence of gradual development of thunderclouds over the local­ity of Tripura. With the commencement of rain, the charged drops fall vertically, decreasing the potential difference across the bipolar structure of cloud very

E > 65.2 :::1.

I ~ 36.6 z w a::: ~ 20.3

0 _J

w u:: 11.6

0915 1015 1115 1215 1315

TIME, hrs 1ST

Fig. 6-Gradual rise and sudden fall associated with a local thun­derstorm in the monsoon season. [A to B shows gradual rise and B to C a sudden fall; C to D is another gradual rise and D to E a sudden fall.]

~ 46.1 (!) z w 0:::: 25.9 . l­en 0 _. w u:

ARN!7S Ill 9kHz 12.05.2003

1300 1400 1500

TIME, hrs 1ST

1600

Fig. 7--Gradual rise and gradual fall associated with a local thun­derstorm in the pre-monsoon season

rapidly down to the critical value required for cloud discharge. On the next day, the overhead sky was normal and the record exhibited no gradual rise and sudden fall.

During monsoon periods, most of the events are in the form of gradual rise and sudden fall. In pre­monsoon (Fig. 7) and post-monsoon (Fig. 8) seasons, gradual rise is followed by a gradual fall or step fall during local daytime.

ELF-VLF ARNFS exhibits a sudden enhancement following intense solar X-flares5

. There is some slug­gishness in the event. The increase in ionisation of the lower boundary of the ionosphere after a flare time increases the conductivity of the Earth-ionosphere waveguide, which leads to higher values of ARNFS. In Fig. 9, there is sudden enhancement of atmospher-

73.1

E ~ 51 .8 :X: 1-

36.8 (.9 z w a::: 1- 25.9 en 0 _J

w 18.3 u::

1050 1150 1350 TIME, hrs 1ST

Fig. 8-Gradual rise and steep fall associated with a local thun­derstorm in the post-monsoon season

29.10.2003

~ ::::1.

u) 11.6 u.. z a::: <(

4.6

0826 1126 1426 1726 2026

TIME, hrs 1ST

Fig. 9- Sudden enhancement of ARNFS at 0.9 kHz during a solar X-ray flare

412 INDIAN 1 RADIO & SPACE PHYS, DECEMBER 2005

ics (SEA) at 0.9 kHz soon after 1000 hrs 1ST. This is related to a major solar flare recorded in the 0.5-4 A and 1-8 A bands on 29 Oct. 2003 by GOES satellite. The commencement time of the flare is around (www. sec. noaa. Gov I ftpmenu/ plot /2004_plots/ xray.html) 1000 hrs 1ST, whereas atmospherics field started to rise with a delay of 6-10 min. The observations of SEA were reported earlier at the VLF range.4

4 Conclusions

The Tripura ARNFS data have been monitored and recorded since March, 2003 only at 900Hz and 9kHz frequencies. The preliminary reports given here are based on analog recordings. It is believed that the three minimum values given in Fig. 4 between 0515 and 0730 hrs 1ST are not due to sticking of the re­corder, as the record returns later to a lower level. We are also using digital data acquisition system simulta­neously.

The observed diurnal variation of ARNFS at Tripura (Fig. 2) is somewhat different from the obser­vations of the sferic rate at Roorkee6

, where as ARNFS at Tripura is well in agreement with the sferic rate at Pretoria.6 ARNFS at Tripura and the sferic rate at Pretoria have a minimum around the morning hours and a maximum during the afternoon hours. At Roor­kee the nighttime value is as small as the morning value6

. The sferics consists of two components6; radio

waves coming directly from cloud discharges and ra­dio waves coming from distant sources via iono­spheric reflection . The relative magnitude of these two components determines the diurnal behaviour, which is expected to be dependent on geographical positions. Roorkee is at the base of the Himalayan range and far from the sea. Like Pretoria, Tripura is also not too far from the sea.

From the results presented in different figures, it may be concluded that the nighttime level is typically 20 dB greater than the daytime level, the sunrise ef­fect consists of a fall in a few steps. At night, when the D-region is absent, VLF radio waves are reflected from the lower E-region. After sunrise, the electron density beneath the E-region increases. Before the complete formation of D-region, the VLF waves pass through a region of low electron density before and after being reflected from the lower E-region, and are partially absorbed. With the passage of time, the D­region is gradually formed, causing a decrease of the reflection height. At some discrete heights, construe-

. ·;i/ .,

tive interference occurs among the different modes. This is believed to be one cause of the steps in sunrise effect7

. Also, any change of cloud electrification in the locality during sunrise can give rise to steps. The cumulative effect of growth and decay of a number of thunderstorm cells in the neighbourhood of the re­ceiving station gives rise to the zigzag variations ob­served during rainy days (Fig. 5). The atmospheric radio noise field exhibits a sudden enhancement dur­ing solar X-ray flare (Fig. 9). The enhancement of solar X-rays at A< 10 A is responsible8 for this during solar flare conditions. During an X-ray flare, the electron density in the D region increases by 7-35 times.9 This increases the reflection coefficient of the ionospheric boundary of the Earth-ionosphere waveguide. Solar X-ray flares affect the noise level as the consequence of extra-ionization9

'10 in the D-region

of the ionosphere.

Acknowledgement This work is funded by the Indian Space Research

Organisation (ISRO) through the S K Mitra Centre for Research in Space Environment, University of Cal­cutta, Kolkata, India.

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