Investigation of the D-Region Ionosphere Characteristics using Tweek Atmospherics at Low

Latitudes

Khairul Khaizi Mohd Shariff1, Mohammad Mohammadpour Salut1, Mardina Abdullah1,2 , Kevin Lloyd Graf 3 1Department of Electrical, Electronics, and Systems Engineering, Faculty of Engineering and Build Environment, Universiti

Kebangsaan Malaysia (UKM), 43600, Selangor, Malaysia

2Institute of Space Science (ANGKASA), Faculty of Engineering and Build Environment, Universiti Kebangsaan Malaysia (UKM), 43600, Selangor, Malaysia

3Space, Telecommunications and Radioscience Laboratory, Stanford University, Stanford California, USA

khairulkhaizi@yahoo.com, ashkanpoursalut@yahoo.com, mardina@eng.ukm.my, graf@stanford.edu

Abstract—Tweek atmospherics emitted from lightning discharges can be utilized to monitor the night-time D-region ionosphere. This paper presents the observation of tweek atmospherics received at Universiti Kebangsaan Malaysia (UKM) (2.55°N, 101.46°E) during August 2009 and October 2010. Analysis of 1383 tweeks demonstrates that these ELF/VLF signals travel considerable distances, up to 7200 km from the causative lightning discharges. The occurrence rates of tweeks in higher modes drop significantly from 962 for the first mode to 2 for the sixth mode due to the higher attenuation for the higher harmonic tweeks. The fundamental cut-off frequency of tweeks is used to measure the night-time variations of the lower ionosphere’s height and equivalent electron density near the equatorial regions. the estimated ionosphere reflection height varies from 73 to 87 km and the estimated equivalent electron density at the reflection height varies from 24 to 28 el/cm3.

Keywords-Tweek atmospherics, Very Low Frequency (VLF), Lightning Discharge, D-Region Ionosphere

I. INTRODUCTION Lightning return strokes emit extremely powerful bursts of

electromagnetic energy ranging from a few hertz to gigahertz. The vast majority of this enormous energy is in the extremely low frequency (ELF; 3-3000 Hz) and very low frequency (VLF; 3-30 kHz) bands. The low propagation loss for ELF/VLF radio waves (a few decibels per megameter) allows them to travel over considerable distances. The ELF/VLF radio atmospheric radiated from the lightning electromagnetic pulse can undergo multiple reflections between the Earth’s surface and the ionosphere as it propagates sub-ionospherically. Many waveguide modes are established as propagation progresses, and dispersion can be seen in the atmospheric near the cutoff of each of these modes. An atmospheric that experiences many reflections and possesses many dispersive mode cutoffs is called a tweek atmospheric.

Tweek atmospherics have been used to examine the night-time D-region ionospheric reflection height (h), equivalent electron density (ne) at reflection height and propagation distance (d) from its source [1, 2, 3]. Saini and Gwal in 2010 estimated the lower ionospheric reflection height at Antarctica

using the cut-off frequency of the fundamental mode of tweek atmospherics. Their observation from January to March of 2003 and 2005 indicated that the lower ionospheric reflection height varies from 64 to 79 km [1]. Maurya et al. (2010) observed tweeks at a low-latitude station of Allahabad during the night of 23 March 2007. They estimated the equivalent electron density of the D-region to be in the range of 20 to 25 el/cm³ at 80 to 95 km altitude [2].

This paper aims to estimate, the night-time ionospheric reflection height, the D-region equivalent electron density and tweek propagation distance from the causative lightning discharge at a low-latitude station in UKM, Malaysia.

II. INSTRUMENTATION An AWESOME VLF receiver was installed at the Institute

of Space Science in UKM (ANGKASA; 2.55°N, 101.46°E) in 2009. The receiver consists of three major parts: a 1.7 m2 triangle shaped orthogonal crossed loop antenna aligned in the N/S and E/W directions, a preamplifier placed close to the antennas to minimize the resistive loss then passes the signals in the range of 0.8 to 47 kHz to the line receiver that is connected to a PC. The received continuous data is sampled with 100 kHz signal and converted to digital format using a 16-bit ADC card installed into the PC. Data acquisition software produced by Stanford University saves the data in MAT-file format [4].

III. RESULTS AND DISCUSSIONS In this study, we used the electromagnetic propagation

model proposed by IEEE Hertz Medal winners J. R. Wait and K.G. Budden. ELF/VLF electromagnetic pulses (EMPs) radiated by lightning discharges are trapped between the Earth and lower ionosphere due to the earth’s surface and the lower ionosphere’s high conductivity for these frequency bands. ELF/VLF waves propagate in different modes (m) with diverse group velocities. The modal equation for EMPs propagation is [5]

)2()cos2()().( mihkiig eeRR πθθθ −− = )1(

This research is partly funded by the Malaysian Government through Universiti Kebangsaan Malaysia under the UKM-LL-02-FRGS0033-20/2007 and UKM-OUP-NBT-30-153/2011.

Proceeding of the 2011 IEEE International Conference on Space Science and Communication (IconSpace) 12-13 July 2011, Penang, Malaysia

978-1-4577-0564-9/11/$26.00 ©2011 IEEE 130

where Rg(θ) and Ri(θ) are the reflection coefficients for the Earth and ionosphere respectively. θ is the ray incident angle and h is the ionospheric reflection height. Fig. 1 illustrates the propagation characteristics of ELF/VLF electromagnetic waves in the Earth-ionosphere waveguide (EIWG).

During the period of two months, 1383 tweeks were observed and classified according to their mode numbers. It has been observed that tweeks with third and higher harmonics occur sporadically and typically after midnight. The rare occurrences of the higher harmonic tweeks imply the increasing attenuation rate for higher mode numbers to propagate in EIWG [3]. Fig. 2 displays four selected spectrograms of tweeks with second, third and sixth harmonics. The time scale is set to 1 second to clearly show the dispersion of the tweek signal.

Figure 1. ELF/VLF electromagnetic waves propagation in EIWG.

Figure 2. Four examples of multimode tweek atmospherics observed in

August 2009 and November 2010. (LT= UT+ 8)

Equation (2) is used to calculate the ionospheric reflection height [6]

cmf

mch2

= (2)

where c is the speed of light, and fcm is the cut off frequency for each mode obtained from the spectrograms.

The complex refractive index of wave propagation in collisional magnetized plasma is given by the Appleton-Hartree equation [7]

−= 12rn

4sin)1(cos

2sin)1)(1(

)1(44

22222 θθθ YjZXYYjZXjZ

jZXX

+−−±−−−−

−−(3)

where the terms X, Y and Z are

ω

ωω

ωω

vZ

Y

X

H

H

p

=

=

⎟⎟⎠

⎞⎜⎜⎝

⎛=

2

where nr is the refractive index, v the is angular collision frequency with neutral, ω is the angular frequency of the wave’s and θ is the angle between the propagation direction of the wave to the external magnetic vector and ωP is the angular plasma frequency.

Electron density is estimated using (4) as proposed by [8]

cmHe ffn 810241.1 −×= (4)

where fH = 1.1±0.2 MHz for low-latitude and equatorial regions based on the model proposed by the International Geomagnetic Reference Field (IGRF) [8].

ELF/VLF signal travels near the speed of light in EIWG. The group velocity is given by Prasad [9]

2

1 ⎟⎟⎠

⎞⎜⎜⎝

⎛−==

ffc

kV cm

g δδω (5)

where ω is the wave’s angular velocity and k is the wave number. The horizontal propagation distance, d travelled by the tweeks in the mth mode is given as [9]

( )

21

21

gfgf

gfgf

VVVVt

d−×Δ

= (6)

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where Δ t is the time interval between f1 and f2 which are calculated from the spectrogram for each mode.

Table 1 shows the measurements of cut-off frequency, the D-region reflection height and electron density of the tweek atmospherics shown in Fig. 2 using (2, 4). The table shows that tweek atmospherics of higher harmonics are reflected at lower ionospheric heights as compared to the one at the fundamental mode (m=1). Furthermore, ionospheric electron density variation is estimated at 22.3-24.68 el/cm3.

The temporal variations of the ionospheric reflection height, equivalent electron density and the tweek propagation distance from its source (using (5, 6)) are shown throughout the night of 6 October 2010 in Fig. 3 separately. A considerable number of tweeks and tweeks exhibiting multiple modal cutoffs were observed during this period. About 125 tweeks were selected and the parameters were plotted only for the fundamental mode. Fig. 3a and b show that the D-region reflection height and equivalent electron density dramatically change during sunset (18:30-20:00 LT) and sunrise (6:00-7:00 LT). However these parameters are quite stable during the rest of the night. Tweeks atmospherics were reflected at the altitude of 73.4 km before sunset and 85.2 km at night due to the absence of extra ionization created by the sun. The ionospheric reflection height decreases from 85.2 km to 77.8 km during sunrise. We found that the electron density at the reflection height of 73-87 km changes from 24 to 28 el/cm3. Fig. 3c shows that the tweeks travelled distances of between 800 and 7200 km.

TABLE I. CUT-OFF FREQUENCY, D-REGION REFLECTION HEIGHT, PROPAGATION DISTANCE AND ELECTRON DENSITY MEASURED FROM FIG. 2.

Spectrogram Mode No. m

Cut-off Freq.

fcm (kHz)

Reflection Height h (km)

Equiv. Electron Density

ne (el/cm3)

a 1 2

1.64 3.49

91.40 85.90

22.39 23.82

b 1 2 3

1.64 3.39 5.16

91.40 88.43 87.15

22.39 23.14 23.48

c 1 2 3

1.64 3.47 5.24

91.40 86.40 85.82

22.39 23.68 23.84

d

1 2 3 4 5 6

1.73 3.49 5.35 7.14 8.93 10.78

86.65 85.90 84.05 83.98 83.93 83.43

23.62 23.82 24.34 24.37 24.38 24.53

Figure 3. Variations of D-region ionosphere in reflection heights, the

equivalent electron density at the reflection heights and the tweeks propagation distance.

IV. CONCLUSIONS The characteristics of the D-region ionosphere were

investigated by measuring tweek atmospherics at UKM station Malaysia. The result shows that tweek atmospherics are able to travel long distances up to 7200 km due to low attenuation for ELF/VLF electromagnetic waves. Analysis of the 1383 tweek atmospherics indicated that all tweeks occurred during the night. Moreover, the distribution of tweek mode numbers reveal that the occurrence of tweeks significantly drops as the mode number increases. It is estimated that the ionospheric electron density varies from 24 to 28 el/cm3 at the altitude of 73-87 km during the local night.

ACKNOWLEDGMENT This work was supported by the research grant UKM-LL-

02-FRGS0033-20 and UKM-OUP-NBT-30-153/2011. We are grateful to Stanford University VLF research group, and Prof. Umran Inan and Dr. Morris Cohen in particular for providing the AWESOME receiver and for their continuous encouragement and support. K. S, M. S and M. A wish to thank Alina Marie Hasbi for reading the paper.

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[4] M. B. Cohen, U. S. Inan, E. W. Paschal, "Sensitive Broadband ELF/VLF Radio Reception With the AWESOME Instrument," IEEE Trans. Geosci. Remote Sens., vol. 48, no.1, pp. 3-17, Jan. 2010.

[5] J. R. Wait, Electromagnetic Waves in Stratified Media, 2nd ed. New York: Pergamon, 1970, pp. 147-153.

[6] M. Yamashita, “Propagation of tweek atmospherics,” J. Atmos. Terr. Phys., vol. 40, pp. 151–156, 1978.

[7] K. G. Budden, Radio Waves in the Ionosphere, Cambridge, UK: Cambridge Univ. Press, 1961.

[8] H. Ohya, M. Nishino, Y. Murayama, and K. Igarashi, “Equivalent electron densities at reflection heights of tweek atmospherics in the low‐middle latitude D‐region ionosphere,” Earth Planets Space, vol. 55, pp. 627–635, 2003.

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