Sudipta Sasmal

First VLF detections of ionospheric disturbances due to Soft Gamma Ray Repeater SGR J1550-5418 and Gamma Ray Burst GRB 090424

We present the first report of the detection of sudden ionospheric disturbances (SIDs) due to a Soft Gamma Ray Repeater (SGR) SGR J1550-5418 and a Gamma Ray Burst (GRB) GRB 090424. These detections were made with receiving stations of Indian Centre for Space Physics which were monitoring Very Low Frequency signals (VLFs) from the VTX transmitter located at the southern tip of Indian sub-continent. These positive detections add to the list of a handful of similar detections of other GRBs and SGRs throughout the world.

Results of VLF campaigns in Summer, Winter and during Solar Eclipse in Indian Subcontinent and Beyond

VLF propagation effects are generally understood in terms of the earth‐ionosphere waveguide. However, details of the theory are still incomplete. Particularly important are the newly emerging fields of VLF Astronomy where the ionosphere is treated as a giant detector for extraterrestrial energetic phenomena and the subject of lithosphere‐ionosphere coupling where the the disturbances of this giant detector is influenced by terrestrial events, especially earthquakes and other seismic activities. We review the activities of our group in these fields. In particular, we concentrate on the results of the VLF campaigns we conducted using over a dozen receiving stations in Summer, in Winter and during the Total Solar eclipse in July, 2009. We also discuss briefly the results we obtained in Antarctica and their implications.

Studies of the Correlation Between Ionospheric Anomalies and Seismic Activities in the Indian Subcontinent

The VLF (Very Low Frequency) signals are long thought to give away important information about the Lithosphere‐Ionosphere coupling. It is recently established that the ionosphere may be perturbed due to seismic activities. The effects of this perturbation can be detected through the VLF wave amplitude. There are several methods to find this correlations and these methods can be used for the prediction of these seismic events. In this paper, first we present a brief history of the use of VLF propagation method for the study of seismo‐ionospheric correlations. Then we present different methods proposed by us to find out the seismo‐ionospheric correlations. At the Indian Centre for Space Physics, Kolkata we have been monitoring the VTX station at Vijayanarayanam from 2002. In the initial stage, we received 17 kHz signal and latter we received 18.2 kHz signal. In this paper, first we present the results for the 17 kHz signal during Sumatra earthquake in 2004 obtained from the terminator time analysis method. ...

Modeling of temporal variation of Very Low Frequency (VLF) radio waves over long paths as observed from Indian Antarctic stations.

Characteristics Very Low Frequency (VLF) signal depends on solar illumination across the propagation path. For a long path, solar zenith angle varies widely over the path and this has a significant influence on the propagation characteristics. To study the effect, Indian Center for Space Physics participated in the 27th and 35th Scientific Expedition to Antarctica. VLF signals transmitted from the transmitters, namely, VTX (18.2 kHz), Vijayanarayanam, India and NWC (19.8 kHz), North-west Cape, Australia were recorded simultaneously at Indian permanent stations Maitri and Bharati having respective geographic coordinates 70.75∘S, 11.67∘E and 69.4∘S, 76.17∘E. A very stable diurnal variation of the signal has been obtained from both the stations. We reproduced the signal variations of VLF signal using solar zenith angle model coupled with Long Wavelength Propagation Capability (LWPC) code. We divided the whole path into several segments and computed the solar zenith angle (χ) profile. We assumed a linear relationship between the Waits exponential model parameters effective reflection height (h′), steepness parameter (β) and solar zenith angle. The h′ and β values were later used in the LWPC code to obtain the VLF signal amplitude at a particular time. The same procedure was repeated to obtain the whole day signal. Nature of the whole day signal variation from the theoretical modeling is also found to match with our observation to some extent.

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.

Modeling D-region ionospheric response of the Great American TSE of August 21, 2017 from VLF signal perturbation

Abstract Solar eclipse is a unique opportunity to study the lower ionospheric variabilities under a controlled perturbation when the solar ultraviolet and X-ray are temporally occulted by the lunar disk. Sub-ionospheric Very Low Frequency (VLF) radio signal displays the ionospheric response of solar eclipse by modulating its amplitude and phase. During the Total Solar Eclipse (TSE) on August 21, 2017 in North America, data was recorded by a number of receivers as presented in public archive. Out of these, two receiving stations YADA in McBaine and K5TD in Tulsa could procure a reasonable quality of noise free data where the signal amplitude was clearly modulated due to the eclipse. During the lunar occultation, a C3.0 solar flare occurred and the signal received from Tulsa manifested the effect of sudden ionization due to the flare. The VLF amplitude in Tulsa shows the effect which is generally understood by superimposing effects of both the solar eclipse and flare. However, the signal by YADA did not perturb by the solar flare, as the flaring region was totally behind the lunar disk for the entire period. We numerically reproduced the observed signal amplitude variation at both the receiving locations by using Wait’s two component D-region ionospheric model and the well-known Long Wavelength Propagation Capability (LWPC) code. The perturbed electron density for both the cases is computed which matches satisfactorily with the true ionospheric conditions.

A comparative study of VLF signals from several transmitters around the world as observed from Maitri station, Antarctica

To examine the behavior of the sunrise and sunset terminators, characteristics of the solar activities in the polar region and the possible seismic correlation with ionospheric anomalies a Stanford AWESOME VLF receiving system was installed at the Indian permanent Antarctic station Maitri during the 27th Indian scientific expedition to Antarctica. Almost five weeks of data was recorded successfully from several transmitters around the globe. The narrow-band signals show variation of the solar radiation and the signature of the prolonged day and night. We compare the signal of frequency 18.2 kHz with the same signal received at Kolkata and we present simultaneous observation of the variation of the sunrise and sunset terminators. We compare the diurnal signal amplitude variation with the LWPC model. We also present the broadband signal, contains the signature of sferics. The signals do not have a strong signature of whistlers because at the conjugate points, the lightning effects are minimum.

Simultaneous observation of Solar Events by Indian Payload (RT‐2) and ICSP‐VLF receiver

We present the results of analysis of simultaneous observation of several Solar Flares (SF) by RT‐2 Experiment onboard CORONAS‐PHOTON satellite and ICSP‐VLF receivers. The ionospheric response of the solar flare is clearly detected in the VLF signals. The hard X‐ray event shows several pulsations especially in the 5th July, 2009 event, whereas in the VLF signal, we did not detect any such kind of pulsations. Such results are very useful in modeling the ionosphere, especially to understand the atmospheric chemistry.

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.