Solar Flare Effects on Propagation of Sferics and Transmitted Signal
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Bulg. J. Phys. 35 (2008) 151–160
Solar Flare Effects on Propagation of Sferics
and Transmitted Signal
S.S. De1 , B.K. De2 , M. Pal2 , B. Bandyopadhyay1 , A. Guha1 ,
S. Paul1 , D.K. Haldar1 , S. Barui1 , R. Roy2
1
Centre of Advanced Study in Radio Physics and Electronics,
University of Calcutta, Kolkata 700 009, India
2
Department of Physics, Tripura University, Tripura 799 130, India
Received 19 July 2008
Abstract. The solar flare effects on the characteristic features of Integrated
Field Intensity of Sferics (IFIS) have been presented in this paper. Recordings
of atmospherics at frequencies 1, 3, 6, 9 and 12 kHz from Agartala (Lat. 23◦
N) showed sudden enhancement in the IFIS throughout the time of solar flares
occurred during July, 2004. The influence of solar flares on the transmitted
signals (VTX1) at 16.3 kHz from one navy station in India, recorded in Kolkata,
on November 23, 2004 has also been reported here.
PACS number: 94.20.Ee, 94.20.-y, 96.60.Rd
1 Introduction
Solar flares cause significant perturbations in the received VLF signals propagat-
ing in the Earth-ionosphere wave-guide [1]. During solar flare, a sudden, rapid
and intense variation in brightness of sun takes place. Flares release energy in
the form of electromagnetic radiation (from radio waves at the long wavelength
end, through optical emission of X-rays and gamma rays at the short wavelength
end), energetic particles (electrons and protons) and mass. Flares are frequent
around the peaks of sunspot cycles. Depending upon the brightness in X-ray
fluxes, the flares are classified as A, B, C, M and X classes with A indicating the
weakest and X being the extreme. A factor of ten in X-ray intensity separates
the latter classes. All these flares are high emissions in the wavelength range of
1–8 Å. These wave-lengths ionize the D-region, which results in radio absorp-
tion [2,3]. The understanding and prediction of solar flare lies on the structure of
magnetic field around sunspots which are the dark spots on photosphere of the
sun. The magnetic fields in sunspots are so intense that it suppresses the flow of
hot gases surrounding them. The gases in the spots become slightly cooler and
1310–0157
c 2008 Heron Press Ltd. 151S.S. De et al. appear dark in contrast with their surroundings. If the structure of magnetic field around the sunspots becomes twisted and sheared, then the magnetic field lines can cross and reconnect with the explosive release of energy, i.e., solar flare. The energy released in a flare is derived from the coronal electromagnetic field. The intense radiations from solar flares when travel towards the earth, there will be enhancement of D-region ionization [3-5]. For this, the characteristic fea- tures of Integrated Field Intensity of Sferics (IFIS) are greatly affected. The Fourier spectrum of IFIS extends from extra low frequency to high frequency. The contribution is mostly from VLF band. Stratospheric electric fields get modified due to conductivity enhancements caused by the energetic particles during solar flares [6]. Several works were reported earlier about the statistical relationship between the occurrences of so- lar flares and the variations of lower atmospheric electricity parameters [7,8]. There is always a correlation among atmospheric electricity, aurora, sunspots, geomagnetic activity, and solar X-ray flares. Very often, the observed responses on Sferics are confusing and are difficult to understand in terms of solar terres- trial interactions. The present paper deals with the observations of solar flare effects on the In- tegrated Field Intensity of Sferics (IFIS) recorded at Agartala (Lat. 23◦ N) at frequencies 1, 3, 6, 9 and 12 kHz. Observations exhibited substantial enhance- ment in the IFIS during the occurrences of solar flares in July, 2004. The effects of solar flares on sub-ionospheric signals at 16.3 kHz will also be reported here. It has been recorded on November 23, 2004 from Kolkata (Lat. 22◦ 34 N). The occurrences of flares are justified by GOES satellite data both in long and short X-ray range. 2 Instrumentation The receiver system mainly consists of (a) antenna (b) AC amplifier (c) selective circuit (d) detective circuit (e) logarithmic amplifier, and (f) recording device. The receiver system is presented by block diagram in Figure 1. The effective height of antenna is fixed to 8.63 meters and the terminal capacitance of the antenna wire is kept at 694 pF. The experimental setup installed at Tripura University consists of an inverted L- type antenna to receive vertically polarized atmospherics in the ELF-VLF bands from near and far sources. By selecting the bands, unwanted noise has been reduced. The cut-off 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 152
Solar Flare Effects on Propagation of Sferics and Transmitted Signal
Figure 1. Diagram of the ELF-VLF receiving system at Tripura (Lat. 23◦ N).
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.
Table 1. Cut-off frequency of low-pass filter and corresponding tuned frequencies
Cut-off frequency (Low-Pass Filter) [kHz] Tuned frequency [kHz]
3 0.900 (in lieu of 1)
5 3
8 6
12 9
15 12
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 OA79 is used in the negative
rectifying mode. The output of the diode is applied across a parallel combination
of resistance and capacitance, 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.
153S.S. De et al. The detected output is amplified by a quasi-logarithmic dc 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 cir- cuit and replaced by the signal generator through a capacitance having a value equal to the terminal capacitance. At 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 μV. Then it is converted to an absolute value in units of μV. The absolute value of induced voltage has been divided by the effective height of the antenna to calculate the field strength in μV/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 cm/h or 4 cm/h. The digital data acquisition system uses a PCI 1050, 16 channel 12 bit DAS card (Dynalog). It has a 12 bit A/D converter, 16 digital input and 16 digital output. The input multiplexer has a built-in over-voltage protection arrangement. All the I/O parts are accessed by 32 bit I/O instructor, thereby increasing the data input rate. It is supported by a powerful 32-bit API, which functions for I/O processing under the Win 98/2000 operating system. Figure 2. The enhanced Sferics amplitudes of frequencies 1, 3, 6, 9, 12 kHz are presented side by side during the flares observed from Agartala on July 15, 2004. Positive responses in IFIS are depicted. 154
Solar Flare Effects on Propagation of Sferics and Transmitted Signal
3 Observations over Agartala
To investigate the effect of solar flare on ELF-VLF Sferics, we have taken con-
tinuous observations of 1, 3, 6, 9 and 12 kHz Sferics in ELF-VLF receivers. The
solar flare data have been collected from GOES10 satellite. During different
dates in the month of July, 2004, positive responses in the form of enhancement
in IFIS have been obtained in the case of only two X-class and only two M-class
flares in the sunlit period (Figures 2–5). In these figures, VLF signal ampli-
tudes are presented side by side with NOAA GOES10 satellite signal data for
observation and comparison.
ELF-VLF ARNFS exhibit a sudden enhancement following intense solar X-
flares [9]. There is some sluggishness in the event. The increase in ionization 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
Figure 6, there is sudden enhancement of atmospherics (SEA) at 0.9 kHz soon
after 1000 hrs IST. This is related to a major solar flare recorded in the 0.5–4 Å
and 1–8 Å bands on October 29, 2003 by GOES satellite. The commencement
time of the flare is around 1000 hrs IST [10], whereas atmospherics field starts
to rise with a delay of 6–10 min. The observations of SEA were reported earlier
at the VLF range [6,9].
Figure 3. It shows similar changes in IFIS during the solar flares on July 16, 2004.
155S.S. De et al. Figure 4. Positive enhancements in IFIS are seen during the solar flares occurred on July 17, 2004. Figure 5. Enhancements in IFIS are observed during the solar flares on July 18, 2004. 156
Solar Flare Effects on Propagation of Sferics and Transmitted Signal
Figure 6. Occurrences of ARNFS at 0.9 kHz during a solar X-ray flare on October 29,
2003.
To study the effects of solar flares on ELF-VLF Sferics, continuous recording of
900 Hz Sferics are conducted separately from Tripura. The Solar flare data have
been collected from GOES satellite. During the month of July, 2004, positive
Figure 7. Whole day records on July 16, 2004.
157S.S. De et al.
Figure 8. Enhancement of IFIS during flares on July 16, 2004.
response in the form of enhancement in IFIS is obtained. The whole day records
of IFIS signals for July 16, 2004 are presented in Figure 7 and the recorded
enhancements of IFIS on the same day during flares are given in Figure 8. The
corresponding flares on July 16, 2004 are depicted in Figure 9.
4 Observations over Kolkata
At Kolkata we are continuously recording a transmitted VLF signal VTX1 at
16.3 kHz transmitted from one of the Indian Navy stations at Vijayanarayanam
(Lat. 8o 25’59.88” N, Long. 77o 48’ E). In the absence of local cloud activities,
Figure 9. Solar flares observed through GOES on July 16, 2004.
158Solar Flare Effects on Propagation of Sferics and Transmitted Signal
Figure 10. Indian Navy VTX1 16.3 kHz VLF signal amplitude along with NOAA
GOES12 satellite during solar X-ray flares occurred on November 23, 2004 are shown.
the diurnal variation of the signal exhibits the well-known sunrise and sunset
effects. On November 23, 2004, we observed a C-class solar flare through its
effect on the propagation of the transmitted VLF signal. Figure 10 summarizes
the results. The commencement of flare time recorded by our receiver matches
nearly with the time recorded by GOES12 satellites both in long and short X-ray
range. The total duration of the flare was around 45 minutes. Though the flare
was a C-class flare, the effect on the ionosphere was very strong, which modified
the reflection coefficient of the ionosphere in VLF range.
5 Results and Discussion
The reflection coefficient of ionosphere varies from day to night. In general, the
reflection coefficient is higher by night than day. The normal diurnal variation
of Sferics changes during X-ray flares [4,6,9]. The signal increases or decreases
depending on the wave frequency. The waveguide formed by the earth and the
lower surface of the ionosphere, which helps the propagation of radio waves, is
greatly affected by solar flare effects through the conductivity parameter of lower
ionosphere [1,6]. Some of the null responses of solar flares on ELF-VLF Sferics
are due to strong local cloud activity. The comparative study of our recorded
Sferics spectra during solar flares with those of GOES satellite data suggests
159S.S. De et al.
some sluggishness, which is the time lag observed in between the occurrence
of solar flares and sudden enhancement of atmospherics (SEA). This observed
sluggishness is due to relaxation effect in the ionospheric region.
During solar flares, the intensity of solar X-ray fluxes heading towards the earth’s
atmosphere increases the ionization and hence the reflection coefficient as well
as reflection height are altered for a short period. This is observed as a sudden
change in the amplitude of ARNFS in the ELF-VLF range [3-5].
Acknowledgement
This work is funded by Indian Space Research Organization (ISRO) through
S.K. Mitra Centre for Research in Space Environment, University of Calcutta,
Kolkata, India.
References
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160Bulg. J. Phys. 35 (2008) 151–160
Contents
11. GENERAL THEORY OF FIELDS AND PARTICLES
H. DIMOV, R.C. RASHKOV
The Neumann-Rosochatius Integrable System and Multi-Spin Single
Spike
String Solutions on R×S 5 . Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
REGULAR PAPERS
26. NUCLEAR ASTROPHYSICS
S. BANERJEE
Neutrino Pair Bremsstrahlung in Supernova Environments . . . . . . . . . . . . 95
44. HEAT TRANSFER
V. ANTONOV, I. IORDANOVA
Development and Application of Three-Dimensional Numerical Model
for
Characterization of Thermal Fields during Surface Laser Treatment of
Solid Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
61. STRUCTURE OF SOLIDS AND LIQUIDS; CRYSTALLOGRAPHY
B. GUEORGIEV, I. IORDANOVA, C.M. SPRECHER
Evaluation of Spherical Grains in Flame- Sprayed Coatings for Medical
Purposes by Stereological Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
73. ELECTRONIC STRUCTURE AND ELECTRICAL PROPERTIES OF
SURFACES,
INTERFACES, THIN FILMS, AND LOW-DIMENSIONAL STRUC-
TURES
G. BESHKOV, D. SPASOV, E. MANOLOV, N. NEDEV, S. GEORGIEV,
Ts. IVANOV,
K. GRIGOROV
Properties of AlNx Nanofilms Prepared by Rapid Thermal Annealing . . 129
75. MAGNETIC PROPERTIES AND MATERIALS
D. UTHRA
Characterization of Doped Rare Earth Manganites, La0.68−x Ax Ca0.32
MnO3
where A = Y, Gd (x = 0.00, 0.08) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
1310–0157
c 2008 Heron Press Ltd. 161CONTENTS
87. BIOLOGICAL AND MEDICAL PHYSICS
M.W.M. AL-BADRANI
The Influence of Chemical and Color Quenching on the Shape of Energy
Spectrum for Beta Particles of 90 Sr – 90 Y by Using Liquid Scintillation
Cocktail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
94. PHYSICS OF THE IONOSPHERE AND MAGNETOSPHERE
S.S. DE, B.K. DE, M. PAL, B. BANDYOPADHYAY, A. GUHA, S. PAUL,
D.K. HALDAR, S. BARUI, R. ROY
Solar Flare Effects on Propagation of Sferics and Transmitted Signal . . 151
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