Rousan Ali

Bio: Rousan Ali is an academic researcher from Tripura University. The author has contributed to research in topics: Amplitude & Ionosphere. The author has an hindex of 3, co-authored 4 publications receiving 27 citations.

Effects of a Solar Eclipse on the Propagation of VLF-LF Signals: Observations and Results

Abstract: The results from the measurements of some of the fundamental parameters (amplitude of sferics and transmitted signal, conductivity of lower ionosphere) of the ionospheric responses to the 22 July 2009 solar eclipse (partial: 91.7%) are shown. This study summarizes our results from sferics signals at 81 kHz and subionospheric transmitted signals at 19.8 and 40 kHz recorded at Agartala, Tripura (latitude: 23°N, longitude: 91.4°E). We observed significant absorption in amplitude of these signals during the eclipse period compared to their ambient values for the same period during the adjacent 7 days. The signal strength along their propagation paths was controlled by the eclipse associated decrease in ionization in the D-region of the ionosphere. Waveguide mode theory calculations show that the elevation of the height of lower ionosphere boundary of the Earth-ionosphere waveguide to a value where the conductivity parameter was 10 6 unit. The absorption in 81 kHz sferics amplitude is high compared to the absorption in the amplitude of 40 kHz signal transmitted from Japan. The simultaneous changes in the amplitudes of sferics and in the amplitude of transmitted signals assert some sort of coupling between the upper atmosphere and the Earth’s near-surface atmosphere prevailing clouds during solar eclipse.

Effects on atmospherics at 6 kHz and 9 kHz recorded at Tripura during the India-Pakistan Border earthquake

Abstract: . The outcome of the results of some analyses of electromagnetic emissions recorded by VLF receivers at 6 kHz and 9 kHz over Agartala, Tripura, the North-Eastern state of India (Lat. 23° N, Long. 91.4° E) during the large earthquake at Muzaffarabad (Lat. 34.53° N, Long. 73.58° E) at Kashmir under Pakistan have been presented here. Spiky variations in integrated field intensity of atmospherics (IFIA) at 6 and 9 kHz have been observed 10 days prior (from midnight of 28 September 2005) to the day of occurrence of the earthquake on 8 October 2005 and the effect continued, decayed gradually and eventually ceased on 16 October 2005. The spikes distinctly superimposed on the ambient level with mutual separation of 2–5 min. Occurrence number of spikes per hour and total duration of their occurrence have been found remarkably high on the day of occurrence of the earthquake. The spike heights are higher at 6 kHz than at 9 kHz. The results have been explained on the basis of generation of electromagnetic radiation associated with fracture of rocks, their subsequent penetration into the Earth's atmosphere and finally their propagation between Earth-ionosphere waveguide. The present observation shows that VLF anomaly is well-confined between 6 and 9 kHz.

A correlation study of solar activity index and amplitude of VLF trans-equatorial propagation

Abstract: Diurnal and seasonal behavior of amplitude of NWC 22.3 kHz signal transmitted from Australia (22049/S, 114023/E), and received at Kolkata (22034/N, 88024/E), India have been investigated The signal amplitude is remarkably low in October. The variations of daily maximum and daily minimum of signal amplitude have been analyzed. The signal on the present propagation path showed non-correlation with A p index, but showed a moderate but negative correlation with 10.7 cm solar flux.

Effects of the India-Pakistan border earthquake on the atmospherics at 6 kHz and 9 kHz recorded at Tripura

Abstract: Normal 0 MicrosoftInternetExplorer4 The unusual variations observed in the records of the integrated field intensity of the atmospherics (IFIA) at 6 kHz and 9 kHz at Agartala, Tripura, in the north-eastern state of India (latitude, 23˚ N; longitude, 91.4˚ E) during the large earthquake on October 8, 2005 at Muzaffarabad (latitude, 34.53˚ N; longitude, 73.58˚ E) in Kashmir in Pakistan are here analyzed. Spiky variations in the IFIA at 6 kHz and 9 kHz were observed several days previous to the day of the earthquake (from midnight, September 28, 2005). The effects persisted for some days, decayed gradually, and eventually ceased on October 31, 2005. The spikes are distinctly superimposed on the ambient level, with mutual separation of 2–5 mins. The number of spikes per day and the total duration of their occurrence were particularly high on the day of the earthquake. The spike heights are higher at 6 kHz than at 9 kHz. The results are discussed here. The generation of electromagnetic radiation associated with the fracture of rocks, the subsequent penetration of this radiation into the Earth atmosphere, and finally its propagation through the Earth–ionosphere waveguide may be responsible for these observed spikes. The present observations show that the very low frequency anomaly dominates between 6 kHz and 9 kHz. The nature of the spikes presented here is a characteristic feature of the IFIA during the period of the earthquake. This has been established on the basis of time-series analyses over a period of one year.

Statistical model for standard seismicity and detection of anomalies by residual analysis

Abstract: A statistical point-process model is derived to describe the standard activity of earthquake occurrences by assuming that general seismicity is given by the superposition of aftershock sequences. The parameters are estimated ty the maximum likelihood method. Using the estimated model, the “residual point process” of the data is defined and used to find the anomalies which are included in the data set but not captured in the considered model for the standard seismicity. For instance, seismic quiescences can be measured quantitatively by using the residual process. Some examples are provided to illustrate such analyses. Furthermore, a time series of the magnitudes on the residual point process is considered, to investigate its dependence either on the time or on the history of the seismicity. By assuming the exponential distribution at each time and modelling of the b- value , we can examine such dependences and estimate them. Two practical examples are shown.

VLF signals in summer and winter in the Indian sub-continent using multi-station campaigns

Abstract: We have carried out 2 week-long campaigns in Indian winter and summer to study VLF signals from the Indian navy transmitter (VTX) operating at 18.2 kHz. We have used more than a dozen of receivers scattered throughout the Indian sub-continent in each of these campaigns. To our knowledge, this is the largest campaign of its kind in this region. The propagation paths range from 500 km to almost 3,000 km covering an area of about 4 million sq km. We have presented the results of the amplitude variation of the diurnal signal at each of these receiving stations in winter and summer and compare them. We have clearly found the non-reciprocity of the east to west and west to east propagation. Our results generally agree with the signal shapes obtained using the long wave propagation capability code based on mode propagation through the Earth-ionosphere cavity.

Spectral character of VLF sferics propagating inside the Earth-ionosphere waveguide during two recent solar eclipses

Abstract: [1] Effects of solar eclipses on the propagation characteristics of worldwide VLF sferics from lightning activity require more investigation. An attempt was made on the occasion of two solar eclipses during 22nd July, 2009 and 15th January, 2010 to study the effects of the two eclipses on the propagation characteristics of VLF sferics in the Earth-ionosphere waveguide. Identical experimental setups were used to study the VLF sferics during the two eclipse events. The spectral character of VLF sferics propagating inside the waveguide is studied at a fixed receiver location (23.75°N, 91.25°E) at six discrete frequencies in between 3 and 20 kHz. During both the eclipse events, it is observed that VLF sferics at all the six discrete frequencies is increased from the mean normal average ambient level. The increment peaks around 10–12 kHz with an overall increment of 6.4 dB with respect to its ambient level. The VLF spectral character of enhancement of sferics show similar characters in two eclipses. The percentage decrease in electron density using standard modeling equations is found to be 90% at the height of 71 km for both the eclipses, supporting linear variation of electron density with solar radiation at the D-region of the ionosphere. The results are explained qualitatively on the basis of a decrease in electron density at the lower ionosphere modifying the reflection coefficient which affected the propagation of VLF sferics in Earth-ionosphere waveguide during eclipsed condition.