Seasonal variations of nighttime D-region ionosphere in 2013 solar maximum observed from a low-latitude station
© Tan et al. 2015
Received: 6 February 2015
Accepted: 19 September 2015
Published: 5 October 2015
We present the observation of tweek atmospherics with harmonics m = 1–8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65° N, 108.02° E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5–6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4–225 cm −3, which corresponds to the harmonics m = 1–8 at the mean reflection height of 81.5–87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12–58 %, 3–67 %, and 24–76 % than those obtained using the International Reference Ionosphere (IRI-2012) model.
Collisions between charged and neutral particles dominate the physical interaction of the D-region ionosphere (∼60–90 km). These activities play an important role in the propagation of the extremely low frequency (ELF; 3–3000 Hz) and the very low frequency (VLF; 3–30 kHz) waves through the Earth-ionosphere waveguide (EIWG) (Hargreaves 1992; Kumar and Kumar 2013). By monitoring the ELF-VLF waves, Friedrich and Rapp (2009) showed that the nighttime D-region properties varies with zenith angle, solar flux, season, and latitude. The D-region ionosphere is too high for balloons and too low for the satellite measurements (Ohya et al. 2003). The attachment and recombination processes are too fast and that makes the free electron density very low (< 103 cm −3), especially in the nighttime. This inhibits the observability of the ionosondes and incoherent scatter radars (Hargreaves 1992). Rockets are used to measure the ionospheric parameters (Maeda 1971; Nagano and Okada 2000), but they are restricted by the timing of flights. Therefore, observing ELF-VLF from the ground is an effective tool to study the D-region ionosphere.
Lightning discharges emit ELF-VLF signals, which propagate into the EIWG and expose as “hooks” on the frequency-time spectrogram. They appear as chirping sounds on a loudspeaker. This type of sferics is called “tweeks” (Helliwell 1965; Yamashita 1978). Using the tweek method, many researchers have investigated the properties of the lower ionosphere. In the previous works, the first modes of tweeks are used to study the seasonal variations of the nighttime D-region ionosphere during the low solar activity period (Maurya et al. 2012a,b). However, the seasonal variation of those parameters at higher harmonics is poorly understood, which limits our view of the overall morphology of the low-latitude D-region.
We observed tweeks at Tay Nguyen university, Vietnam (Geog. 12.65° N, 108.02° E) during winter, equinox, and summer seasons of 2013 to calculate the cut-off frequency, reflection height, and electron density of the D-region ionosphere. From these parameters, we evaluate the morphology of this region. Our survey site is at low latitudes around the equatorial region, which is near the Asia Oceanic Region; hence, the lightning density is very high. This creates favorable opportunities to study the turbulence of the D-region using the ELF-VLF waves radiated by lightning. Furthermore, the observation period is during the high solar activity year of the solar cycle 24, which we expect that the D-region perturbs much more violently. We present our experiment and data analysis method in section “Experimental data and analysis”. In section “Results and discussion”, we will discuss the results obtained over the year 2013 and compare them with the International Reference Ionosphere (IRI-2012) model. Finally, we draw out conclusion in section “Conclusions”.
Experimental data and analysis
Our ELF-VLF receiver consists of a magnetic antenna, a preamplifier, an ADC (Analog to Digital Converter), a SU (Service Unit), a Global Positioning System (GPS) receiver, a PC, and recording software. The details of this receiver are described in our previous work (Tan et al. 2014). It has been developed to record the atmospheric signals. The antenna includes two orthogonal loops which are right isosceles triangles with the base of 2.6 m. Each loop consists of 8 turns of 18 American Wire Gause copper wire. The surface of one loop orients in the north-south (N-S) direction, while the surface of other loop orients in the east-west (E-W) direction. If the magnetic component of electromagnetic waves changes and passes the loop of antennas, a voltage will be generated in the coils of the antenna (Ramo et al. 1994). In order to minimize signal interference, the loops are wrapped with the silver shields. The plane of the antenna which is perpendicular to E-W is chosen because it is very sensitive to sferics generated by lightning discharges (Dahlgren et al. 2011; Wood 2004). The signals from antenna are amplified and filtered by a preamplifier near the antenna and then transmitted by the coaxial cable to the ADC at a distance of 150 m. The preamplifier is powered by a particular DC power supply to remove the AC noise. The ELF-VLF signals from the E-W channel of the preamplifier go into the sound card. The SpectrumLab v2.77b22 records the sferics with audio files having extension “wav.” The GPS is utilized for time synchronization with an accuracy of 100 ns. To record the sferics, SpectrumLab is configured with the sample rate of 44.1 kHz, 16-bit sampling, and fast Fourier transform (FFT) input size of 512. The Sonic visualiser software developed by Cannam et al. (2010) is used to analyze the audio files.
where f H is the gyrofrequency and f p is the plasma frequency. Considering the low-latitude (30° N-30° S) location of our site, we assume f H = 1.3 ± 0.16 MHz by using the IGRF (International Geomagnetic Reference Field) model. This result is consistent with the results calculated by Saini and Gwal (2005) and Kumar et al. (2009).
where c is the speed of light in free space, m is the mode number and f cm is cut-off frequency.
where R is the radius of the Earth.
Equation (5) shows that as f reaches near f cm , v gm approaches zero, while as f becomes much larger than f cm , v gm approaches the speed of light. In the case of f is less than cut-off frequency, the radio wave is quickly attenuated along the propagation distance (Wood 2004).
where t 2 - t 1 is the difference in arrival times of the two frequencies, f 2 and f 1, close to the tweeks of any mode, and v g f1 and v g f2 are the corresponding group velocities of the radio waves centered at frequencies f 1 and f 2.
The cut-off frequency captured from tweeks during three seasons is used to estimate the ionospheric refection height and D-region electron density at the reflection height. The arrival times t 1 and t 2 of two frequencies f 1 and f 2 closed to cut-off frequency from tweek spectrograms were measured to estimate the propagation distance of the tweeks. The frequency and time resolutions of measurement are 35 Hz and 1 ms, respectively. The estimated error in the reflection height is about 1.5 km for first-order mode, and it reduces with the increase in the modes. The errors in the electron density and propagation distance for all modes are found to be ∼ 0.6 cm −3 and ∼ 470 km, respectively.
Results and discussion
Tweek occurrence observed at low latitudes
Tweek occurrence observed during winter, equinox, and summer seasons
Tweek occurrence in the pre-midnight and post-midnight periods
The variations of reflection height and fundamental frequency with mode number
The mode number (m), the mean fundamental frequency (f cm /m), reflection height (h), propagation from lightning discharges to the receiver (d), and electron density (n e ) obtained from tweeks on the spectrograms (a–c) of Fig. 1
Figure 3 b shows the variation of f cm /m with the harmonics m. It reveals that f cm /m during winter, equinox and summer seasons slightly reduces from 1.81–1.72, 1.81–1.71, and 1.79–1.74 kHz, respectively. The trend of reduction of f cm /m with mode number for winter is f winter=−11.60m+1816 with R 2=0.85, for equinox is f equinox=−13.08m+1823 with R 2=0.96, and for summer is f summer=−5.17m+1782 with R 2=0.58. The variation of f cm /m is less steep during summer season as compared to that during winter and equinox seasons.
Morphology of nighttime D-region ionosphere
To present the seasonal variation of h for different modes, the data was smoothed by using the Adjacent-Averaging method with 5 points of window (Fig. 5). In the pre-midnight, the h for m = 1–6 is higher during summer season as compared to that during winter and equinox seasons. In the post-midnight, the seasonal variation of h for m = 2–3 is not significant, and the h for m = 4–6 is lower during summer season as compared to that during winter and equinox seasons. The trend of variation of h for the first harmonic is not clear because lower harmonics may interfere with AC noise. The increase of h at nighttime corresponds to the decrease of electron density due to its loss by the attachment and recombination processes. The scattered Lyman- α is an important source of the D-region ionization. Ohya et al. (2011) found that about 67 % of the nighttime D-region ionization is caused by Lyman- α and Lyman- β which ionize NO and O2 at altitude of 95 km. The lower h in the pre-midnight period during winter as compared to equinox and summer seasons is explained by the fact that the lower n e of daytime during winter giving rise to slower the loss processes of electrons in the pre-midnight period (Maurya et al. 2012b). Those phenomena clearly show at lower altitudes of the D-region. The h for m = 4–6 in the post-midnight period is higher during winter and equinox seasons as compared to that during summer season. It can be explained that tweek with higher modes could reflect at the less different altitudes because the D-region during summer season becomes less inhomogeneous. Around 19:00 LT, the tweek reflection height during winter season is higher than that during summer for m = 3–4. The higher h of winter after sunset transition could be caused by lower daytime n e during winter season.
Recording tweeks with a maximum up to the eighth harmonics from January–April, 1991 (R z =140), Shvets and Hayakawa (1998) calculated h which varies from 81–83 km. Capturing tweeks with m = 1–6 during the period of September, 2003–July, 2004 (R z =45) at Suva (Geog. 18.2° S; 178.3° E), Kumar et al. (2008) found that h varied from 83–92 km. At a low-latitude station, Allahabad (India), Maurya et al. (2012a) observed tweeks with m = 1–6 from April, 2007 (R z =7.5) to March 2008 (R z = 2.9), the mean h increased from 80–95 km. In our work, the mean h varies from 81.5–87.7 km, which is higher than that reported by Shvets and Hayakawa (1998), but lower than that shown in the results of Kumar et al. (2008) and Maurya et al. (2012a). Shvets and Hayakawa (1998) indicated that when the Sun ,s activity decreases hence the reflection height increases. Bremer and Singer (1977) and Danilov (1998) also illustrated that the Sun ′s activities affect the electron density of the nighttime D-region. Surveying the seasonal variation of tweek reflection height over three solar cycles, Ohya et al. (2011) found that the reflection height was relatively low in March–April and high in July–October.
The seasonal variation of Wait ′s parameters and electron density profiles
Using first three modes of tweeks, Kumar et al. (2009) and Maurya et al. (2012a) calculated the reference height (h ′) and sharpness factor (β) by using the Wait ′s formula. Maurya et al. (2012a) used two tweeks in 1 min at each 15 min interval during pure nights from 21:00–02:00 LT of 2010, meanwhile, Kumar et al. (2009) used tweeks in 5 min at each hour interval during nights from 21:00–03:00 LT. In present work, we use the previous method of Kumar et al. (2009) and Maurya et al. (2012a) to obtain h ′ and β. We calculated the mean h and n e of the first three harmonics of tweeks recorded in 2 min at every 15 min interval. We analyzed tweeks in August, September, and November of 2013, as representative of summer, equinox, and winter seasons. Tweeks having d less than 5000 km observed from 21:00–02:00 LT are chosen, so that the error contribution due to in the pre-midnight period, some tweeks coming from dayside part (from west side of the station) and in post-midnight period, some tweeks coming from dayside part (from east side of the station) is avoided.
The values of reference height and sharpness factor during winter, equinox and summer seasons
h ′ (km)
β (k m −1)
h ′ (km)
β (k m −1)
h ′ (km)
β (k m −1)
Observing tweeks at Allahabad, India (16.05° N) during the low solar activity period, Maurya et al. (2012a) found that the mean h ′ during winter, equinox, and summer seasons are 85.9, 85.7, and 83.5 km, respectively, and the mean β during winter, equinox, and summer seasons are 0.51, 0.54, and 0.61 km −1, respectively. In our work, the mean h ′ during three seasons is lower by 0.2–2.2 km than that reported by Maurya et al. Our mean β is higher by 0.06 and 0.01 km −1 during winter and equinox seasons but lower by 0.06 km −1 during summer season than those reported by Maurya et al. (2012a), respectively. Cummer et al. (1998) observed sferics at mid-latitude of 37.43° N in July 1996 (R z =10) and used the Long Wave Propagation Capability (LWPC) program to calculate h ′ and β with 83.3 and 0.49 km −1, respectively. Using LWPC modeling of amplitude and phase of VLF signals, Thomson et al. (2007) studied the nighttime mid-latitude D-region near solar minimum and estimated the Wait’s parameters of h ′=85.1 km and β=0.63 km −1. Cheng et al. (2006) recorded sferics during 16 nights of summer season at Duke (36° N) and found that h ′ and β varied from 83.6–85.6 km and 0.4–0.5 km −1, and the mean h ′ and β are 84.5 km and 0.45 km −1, respectively. Our results on h ′ during summer are lower than those reported by those authors.
Tweeks occur about 51 % during summer season, 22 % during winter season, and 27 % during equinox season. Tweeks with m = 4–8 occur more often during summer season as compared to winter and equinox seasons.The occurrence rate of the tweeks with the propagation distance in the range of 1000–5000 km during equinox, winter, and summer seasons are 92, 89, and 92 %, respectively. The boundary of the waveguide becomes more sharp for tweeks with m = 5–6.
The mean n e increases from 28.4–225.0 cm −3 which corresponds to h of 81.5–87.7 km. During winter season, the variation of h makes a dip around 22:00 LT and has a crest around 3:00 LT and then slightly decreases. During summer season, the temporal variation of h has the crest around 20:30 LT and reduces to the dip from 1:00–2:00 LT and then increases. The h decreases from 19:00–23:00 LT for March and increases from 19:00–21:00 LT for September. The trend of the variation of h between 2 months which are in different seasons is nearly symmetric.
The h corresponding to higher harmonics (m = 4 – 6) in the post-midnight is higher during winter and equinox seasons as compared to summer season because n e is lower during winter and equinox seasons than that during summer season. At the higher altitudes, the D-region becomes less inhomogeneous during summer season as compared to that during other seasons; hence, the tweeks with higher modes during summer season could reflect at the less different altitudes, whereas those tweeks during winter and equinox seasons could reflect at the higher altitudes.
The mean h ′ during winter, equinox, and summer seasons are 84.4, 83.7, and 83.3 km, respectively. The β during winter, equinox, and summer seasons are 0.57, 0.55, and 0.55 km −1, respectively. The h ′ during summer season is lower in our work as compared to other works due to the higher level of solar activity.
The values of n e calculated by tweek method during summer, equinox, and winter seasons are lower by 12–58 %, 3–67 %, and 24–76 % than those obtained using the IRI-2012 model, respectively, and show a good comparison with IRI-2012 in the altitude ranges of 82–89 km, 87–92 km, and 88–93 km for summer, equinox, and winter seasons, respectively. The seasonal variation of the electron density profile during solar maximum period shows less significant than that during solar minimum activity period.
The authors are grateful to Department of Physics, Faculty of Natural Science and Technology, Tay Nguyen University for their encouragement and support.
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