Sushanta Kumar Mondal

Bio: Sushanta Kumar Mondal is an academic researcher from Sidho Kanho Birsha University. The author has contributed to research in topics: Ionosphere & Solar eclipse. The author has an hindex of 9, co-authored 29 publications receiving 277 citations. Previous affiliations of Sushanta Kumar Mondal include Indian Centre for Space Physics.

Modeling of very low frequency (VLF) radio wave signal profile due to solar flares using the GEANT4 Monte Carlo simulation coupled with ionospheric chemistry

Abstract: . X-ray photons emitted during solar flares cause ionization in the lower ionosphere (~60 to 100 km) in excess of what is expected to occur due to a quiet sun. Very low frequency (VLF) radio wave signals reflected from the D-region of the ionosphere are affected by this excess ionization. In this paper, we reproduce the deviation in VLF signal strength during solar flares by numerical modeling. We use GEANT4 Monte Carlo simulation code to compute the rate of ionization due to a M-class flare and a X-class flare. The output of the simulation is then used in a simplified ionospheric chemistry model to calculate the time variation of electron density at different altitudes in the D-region of the ionosphere. The resulting electron density variation profile is then self-consistently used in the LWPC code to obtain the time variation of the change in VLF signal. We did the modeling of the VLF signal along the NWC (Australia) to IERC/ICSP (India) propagation path and compared the results with observations. The agreement is found to be very satisfactory.

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.

Ionospheric anomaly due to seismic activities-III: correlation between night time VLF amplitude fluctuations and effective magnitudes of earthquakes in Indian sub-continent

Abstract: We present the results of an analysis of year-long (2007) monitoring of night time data of the VLF signal amplitude from the Indian Navy station VTX at 182 kHz, received by the Indian Centre for Space Physics, Kolkata We analyzed this data to find out the correlation, if any, between night time amplitude fluctuation and seismic events We found, analyzing individual cases (with magnitudes >5) as well as statistical analysis (of all the events with effective magnitudes greater than 35), that night time fluctuation of the signal amplitude has the highest probability to be beyond the 2σ level about three days prior to seismic events Thus, the night time fluctuation could be considered as a precursor to enhanced seismic activities

Modeling of very low frequency (VLF) radio wave signal profile due to solar flares using the GEANT4 Monte Carlo simulation coupled with ionospheric chemistry

Abstract: . X-ray photons emitted during solar flares cause ionization in the lower ionosphere (~60 to 100 km) in excess of what is expected to occur due to a quiet sun. Very low frequency (VLF) radio wave signals reflected from the D-region of the ionosphere are affected by this excess ionization. In this paper, we reproduce the deviation in VLF signal strength during solar flares by numerical modeling. We use GEANT4 Monte Carlo simulation code to compute the rate of ionization due to a M-class flare and a X-class flare. The output of the simulation is then used in a simplified ionospheric chemistry model to calculate the time variation of electron density at different altitudes in the D-region of the ionosphere. The resulting electron density variation profile is then self-consistently used in the LWPC code to obtain the time variation of the change in VLF signal. We did the modeling of the VLF signal along the NWC (Australia) to IERC/ICSP (India) propagation path and compared the results with observations. The agreement is found to be very satisfactory.

Precursory effects in the nighttime VLF signal amplitude for the 18th January, 2011 Pakistan earthquake

Abstract: We have presented the result of the analysis of the nighttime VLF signals transmitted from the Indian Navy station VTX (latitude 8.43°N, longitude 77.73°E) at 19.2 kHz and received at Kolkata (latitude 22.57°N, longitude 88.24°E). On 18th January, 2011 an earthquake of magnitude 7.4 occurred at Southwestern Pakistan (latitude 28.9°N, longitude 64°E). We have analyzed the nighttime VLF signals for 2 weeks around 18th of January, 2011 to see if there have been any precursory effects of this earthquake. We have found that the amplitude of the nighttime VLF signals anomalously fluctuated 4 days before the earthquake. This agrees well with our previous findings based on the analysis of 1 year of earthquake data.

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Abstract: Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. VLF signal amplitude perturbation (ΔA) and amplitude time delay (Δt) (vis-a-vis corresponding X-ray light curve as measured by GOES-15) of NWC/19.8 kHz signal have been computed for solar flares which is detected by us during Jan–Sep 2011. The signal is recorded by SoftPAL facility of IERC/ICSP, Sitapur (22∘ 27′N, 87∘ 45′E), West Bengal, India. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (αeff) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay (Δt). For the C-class flares we find that there is a direct correspondence between Δt of a solar flare and the average solar zenith angle Z over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the Δt for different local diurnal time slots DT. We find that while the time delay is anti-correlated with the flare peak energy flux ϕmax independent of these time slots, the goodness of fit, as measured by reduced-χ2, actually worsens as the day progresses. The variation of the Z dependence of reduced-χ2 seems to follow the variation of standard deviation of Z along the Tx-Rx propagation path. In other words, for the flares having almost constant Z over the path a tighter anti-correlation between Δt and ϕmax was observed.

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.

A statistical study on the correlation between lower ionospheric perturbations as seen by subionospheric VLF/LF propagation and earthquakes

Abstract: The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over seven years from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion and nighttime fluctuation) and earthquakes with magnitude greater than 6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40km). Finally some comments on the mechanism of seismo-ionospheric perturbations are discussed.