Suman Ray

Correlelations between the Anomalous Behaviour of the Ionosphere and the Seismic Events for VTX-MALDA VLF Propagation

One of the most important application of the VLF signals is that it contains possible information about the lithosphere‐ionosphere coupling. In other words, in near future, it may be possible to predict seismic events by judging signatures of VLF signals. In this paper, we present the result of the monitoring of the VLF signals collected in the Malda branch of ICSP, located in Malda, West Bengal, for four years (2005, 2007–09) and we try to find out the co‐relations, if any, between the ionospheric activities and the earthquakes. Here we use that VLF signals which are transmitted from the VTX station (18.2 KHz), located near Vijayanarayanam in Tamilnadu, about 2290 km away from the receiver. To find out the co‐relation of the ionospheric activities with the seismic events such as earthquake, first we have to study the average signal throughout the year. For this, we plot the so‐called standardized calibration curve using the four years data. Here we use a total of 481 no. of data. To establish the co‐rela...

Inverse problem in ionospheric science: prediction of solar soft-X-ray spectrum from very low frequency radiosonde results

X-rays and gamma-rays from astronomical sources such as solar flares are mostly absorbed by the Earth’s atmosphere. Resulting electron-ion production rate as a function of height depends on the intensity and wavelength of the injected spectrum and therefore the effects vary from one source to another. In other words, the ion density vs. altitude profile has the imprint of the incident photon spectrum. In this paper, we investigate whether we can invert the problem uniquely by deconvolution of the VLF amplitude signal to obtain the details of the injected spectrum. We find that it is possible to do this up to a certain accuracy. This leads us to the possibility of uninterrupted observation of X-ray photon spectra of solar flares that are often hindered by the restricted observation window of space satellites to avoid charge particle damages. Such continuous means of observation are essential in deriving information on time evolution of physical processes related to electron acceleration and interaction with plasma in solar atmosphere. Our method is useful to carry out a similar exercise to infer the spectra of more energetic events such as the Gamma Ray Bursts (GRBs), Soft Gamma-ray Repeaters (SGRs) etc., by probing even the lower part of the Earth’s atmosphere. We thus show that to certain extent, the Earth’s atmosphere could be used as a gigantic detector of relatively strong astronomical events.

VLF observational results of total eclipse of 22nd July, 2009 by ICSP team

Solar eclipses provide us with exciting opportunity to study the VLF propagation effects under a controlled experiment in a cosmic scale. During the total eclipse of July 22nd, 2009, we conducted a campaign to obtain the data from more than a dozen places. We observe that in several places the signal amplitude is amplified, while in other places the amplitude is reduced. In yet other places, there are ups and downs in the signal during the obscuration period. In this paper, we present the results of our campaign during the total solar eclipse.

Anomalous behaviors of the VLF signals before earthquakes for VTX-Malda propagation path

We present the results of monitoring of four years (2005, 2007–09) of VLF signals collected in the Malda branch of ICSP, located in Malda, West Bengal and find the correlations, between the ionospheric activities and the earthquakes. Here we use that VLF signals which are transmitted from the VTX station (18.2 KHz). We first study the average signal throughout the year. We plot the so-called standardized calibration curve using the four years data. To establish the correlation between the ionospheric activities and the seismic events, we use the data of the year 2008 and we find that the deviations of the VLF day length (defined as the time difference between sunrise and sunset terminators) are correlated with the seismic event. We find that the highest deviation takes place one day prior to the seismic events. We also calculate the ‘D-layer preparation time’ (DLPT) and the ‘D-layer disappearance time’ (DLDT) for the data of 2008 and established the co-relation between the anomalous DLPT and DLDT with the seismic events. We find that the anomalous behaviour of the DLPT and DLDT are also correlated with the seismic events.

Correlation between seismic events and anomalous VLF day-length for west-east and east-west propagation paths

We present results of year-long analysis of VLF signals, both for VTX-Malda (west-east propagation path) and VTX-Pune (east-west propagation path). We analyzed whole year data of 2008 and 2009 for VTX-Malda and VTX-Pune propagation path, respectively. In both the cases we found that VLF day-length (defined as time difference between sunset terminator time and sunrise terminator time) became anomalously high 1-2 days before an earthquake. Besides this year-long study we have also done some case by case analysis. On 9th January, 2009 an earthquake of magnitude 5.5 occurred at Carlsberg Ridge (latitude 10.3° N, longitude 57.1° E). In a separate incident, on 3rd November, 2009, another earthquake of magnitude 5.4 occurred at Andaman Islands (latitude 14.1° N, longitude 93.1° E). We analyze VLF signals for VTX-Pune (east-west propagation path) propagation path around these two earthquake days and found that for both of these cases, the VLF-day-length became anomalously high two days before the event. This agrees well with statistical analysis based on year-long data for VTX-Pune path. Furthermore, during time period of June, 2010, two major earthquakes of low-depth (10Km) and high magnitude (M > 5) occurred near NWC-Salt Lake (east-west propagation path) propagation paths. First one occurred on 13th June, 2010 at Nicobar Islands, India (latitude 7.8° N, longitude 92.0° E). Its magnitude was 5.1 (in Richter scale). The second one of magnitude 6.0 (in Richter Scale) occurred on 19th June, 2010 at Andaman Islands, India (latitude 13.4° N, longitude 93° E). For both of these two earthquakes, we found that VLF-day-length, became anomalously high one day before earthquakes. We claim that these were pre-cursors of the earthquakes which occurred in Andaman and Nicober Islands.

Studies of VLF signal anomalies due to earthquake

We present evidences of anomalous VLF signals which we interpret to be precursors of seismic activities. These anomalies could be in VLF day lengths, or in D-layer preparation times, D-layer disappearance times, or, night time fluctuations. We made many case by case studies of major earthquakes and found that the anomalies occur 1-5 days before seismic events. We also made statistical analysis of several earthquakes together and came to same conclusions. Furthermore, we present results from several VLF receivers for a single earthquake and show that the result depends on the proximity of the propagation path to the epicenter.

Unusual shifts in terminator times of the VLF signals before the Pakistan earthquake (M=7.4), occurred on 18th Jan., 2011

On 18th Jan., 2011, at 20:23 UT, an earthquake of magnitude 7.4 occurred in Southwestern Pakistan (latitude 28° 54 N, longitude 64° E). We present the results of the analysis of Very Low Frequency (VLF) radio signals, received at three receiving stations located in India. We analyze the VLF signal around this earthquake day for four different VLF propagation paths namely DHO-IERC(Sitapur), VTX-Pune, VTX-ICSP(Kolkata) and NWC-IERC. We observed significant shifts of the `sunrise terminator time (SRT) for DHO-IERC and VTX-Pune paths. We also observed the SRT-shift for VTX-Kolkata path, but here the shift is not so strong. We found no significant shifts of SRT for NWC-IERC propagation path. Higher deviation in the VTX-Pune path as compared to VTX-ICSP path could be due to the formers proximity to the epicentre. Similarly, DHO-IERC path is over the epicentre while that of NWC-IERC path is away from the epicenter. This could be the reason why the effect in DHO-IERC path is stronger than that in NWC-IERC path.

Studies of precursors of earthquakes using anomalies in very low frequency signal

It is long conjectured that the ionosphere is affected by seismic activities and these effects can be detected in VLF signals. We present long term statistical studies for a quiet solar condition of three types of anomalies in the VLF signal amplitude and observed significant precursory effects in 1-5 days before major earthquakes. These are anomalies of (a) terminator shifts, (b) D-Layer preparation and disappearance times and (c) night time amplitude fluctuations. We compute correlations between earthquake magnitude and present feasible precursory effects in the signal before major earthquakes. We also present case by case studies for different earthquakes.

Results of VLF campaigns in Summer and Winter in Indian subcontinent

VLF propagation effects can be understood using the earth-ionosphere waveguide. However due to emergence of the new fields such as VLF Astronomy where the ionosphere is treated as a giant detector for extraterrestrial energetic phenomena and the seismo-electromagnetism where the disturbances of this giant detector is influenced by terrestrial events, especially earthquakes and other seismic activities. In order to interpret observed data correctly, we need to have the basic propagation characteristics throughout India. In this paper, we concentrate on the results of the VLF campaigns we conducted using over a dozen receiving stations in Summer and in Winter. We observe the well-known asymmetry in East to West and West to East propagation. From the signal amplitude characteristics, we divide the sub-continent in four zones as far as the VTX transmitter is concerned.

Precursor of earthquake using night time VLF amplitude

The results of the analysis of the year-long (2007) monitoring of the night time data of the VLF signal amplitude from the Indian Navy station VTX at 18.2kHz, received at the Indian Centre for Space Physics, Kolkata, are presented. We analyzed this data to find out the correlation, if any, between the night time fluctuation and the seismic events. We found, using individual cases (with magnitudes > 5) as well as statistical analysis (of all the events with effective magnitudes greater then 3.5) that the night time fluctuation of the signal amplitude has the highest probability to be beyond 2<r level about three days prior to the events. Thus the night time fluctuation could be considered as a precursor to the enhanced seismic activities.