Sujay Pal

Bio: Sujay Pal is an academic researcher from University of Calcutta. The author has contributed to research in topics: Very low frequency & Ionosphere. The author has an hindex of 9, co-authored 44 publications receiving 217 citations. Previous affiliations of Sujay Pal include Indian Centre for Space Physics & University of Electro-Communications.

Modeling of sub-ionospheric VLF signal perturbations associated with total solar eclipse, 2009 in Indian subcontinent

Abstract: During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2 kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4 km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by h ′ = + 3.0 km for the VTX-Malda path and h ′ = + 1.8 km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj.

First ever VLF monitoring of the lunar occultation of a solar flare during the 2010 annular solar eclipse and its effects on the D-region electron density profile

Abstract: A ground based Very Low Frequency (VLF) radio receiver of Indian Centre for Space Physics located at Khukurdaha ( 22 ° 27 ′ N , 87 ° 45 ′ E ) monitored the VLF signal at 19.8 kHz from the NWC station during a partial solar eclipse (maximum obscuration 75%) which took place on January 15, 2010. The receiver and the transmitter were on two opposite sides of the annular eclipse belt. During the same period, a solar flare also occurred and it was partly blocked by the lunar disk. Thus the resultant signal was perturbed both by the eclipse and by the flare. The deviation of the signal from the normal value was obtained by subtracting from the average diurnal signal on days bracketing the eclipse. The deviation was analysed. We compare the data from GOES-14, HINODE and RHESSI satellites during the event. We got a clear depression in the data during the period of the partial eclipse. Most interestingly, there was also a flaring activity in the sun which reached its peak (C-type) just after the time when the eclipse was near maximum. By superposing the lunar disk on the image obtained by HINODE mission, we pin-point the time frame of blocking hard and soft X-rays. We extract the time variation of the electron density profile in the D-region of the ionosphere due to occulted solar flare from the combined effect of the eclipse and the flare. We also compare the results with a normal solar flare.

Theoretical study of lower ionospheric response to solar flares: sluggishness of D-region and peak time delay

Abstract: The rates of ion production and loss processes in the lower ionosphere during solar and other astronomical ionizing events vary with height. This variations influence the time lags of the response in different ionospheric layers. Very Low Frequency (VLF) signals reflected from any of these layers follow this time lag or delay during a transient cosmic events. One of the easiest ways to study this property is to observe the shift in the peak of VLF signal amplitude with respect to the peak of solar flares. We numerically model to find ion densities and resulting VLF signal perturbations during some solar flares. We clearly find from the model that the delay in the peak of the electron densities (with respect to peak of the ionizing event) in the lower ionosphere varies from height to height. The result also depends on the properties of events, such as peak intensity and sharpness. We investigate analytically how the delay of electron density peak should depend on height varying chemical rate parameters as well as the nature of transient events. Our capability is demonstrated using three classes (namely, X, M and C) of solar flares. The work is a step forward in our goal to employ ionosphere as a natural detector for astronomical observations.

Theoretical models for Computing VLF wave amplitude and phase and their applications

Abstract: We present a review of the present theoretical models for computing the amplitude and phase of the VLF signal at any given point on earth. We present the basics of the wave hop theory and the Mode theory. We compute the signal amplitudes as a function of distance from a transmitter using both the theories and compare them. We also repeat a similar exercise for the diurnal signal. We note that the signal variation by wave hop theory gives more detailed information in the day time. As an example of using LWPC code, we compute the variation of the effective height h′ and steepness β parameters for a solar flare and obtain the time dependence of the electron number density along both VTX‐Kolkata and NWC‐Kolkata propagation paths.

Sub-ionospheric VLF signal anomaly due to geomagnetic storms: a statistical study

Abstract: . We investigate quantitatively the effect of geomagnetic storms on the sub-ionospheric VLF/LF (Very Low Frequency/Low Frequency) propagations for different latitudes based on 2-year nighttime data from Japanese VLF/LF observation network. Three statistical parameters such as average signal amplitude, variability of the signal amplitude, and nighttime fluctuation were calculated daily for 2 years for 16–21 independent VLF/LF transmitter–receiver propagation paths consisting of three transmitters and seven receiving stations. These propagation paths are suitable to simultaneously study high-latitude, low-mid-latitude and mid-latitude D/E-region ionospheric properties. We found that these three statistical parameters indicate significant anomalies exceeding at least 2 times of their standard deviation from the mean value during the geomagnetic storm time period in the high-latitude paths with an occurrence rate of anomaly between 40 and 50 % presumably due to the auroral energetic electron precipitation. The mid-latitude and low-mid-latitude paths have a smaller influence from the geomagnetic activity because of a lower occurrence rate of anomalies even during the geomagnetically active time period (from 20 to 30 %). The anomalies except geomagnetic storm periods may be caused by atmospheric and/or lithospheric origins. The statistical occurrence rates of ionospheric anomalies for different latitudinal paths during geomagnetic storm and non-storm time periods are basic and important information not only to identify the space weather effects toward the lower ionosphere depending on the latitudes but also to separate various external physical causes of lower ionospheric disturbances.

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.

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.

Modeling of sub-ionospheric VLF signal perturbations associated with total solar eclipse, 2009 in Indian subcontinent

Abstract: During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2 kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4 km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by h ′ = + 3.0 km for the VTX-Malda path and h ′ = + 1.8 km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj.

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

Abstract: The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.

Low‐mid latitude D region ionospheric perturbations associated with 22 July 2009 total solar eclipse: Wave‐like signatures inferred from VLF observations

Abstract: We present first report on the periodic wave-like signatures (WLS) in the D region ionosphere during 22 July 2009 total solar eclipse using JJI, Japan, very low frequency (VLF) navigational transmitter signal (22.2 kHz) observations at stations, Allahabad, Varanasi and Nainital in Indian Sector, Busan in Korea, and Suva in Fiji. The signal amplitude increased on 22 July by about 6 and 7 dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5 dB at Nainital, Busan, and Suva, respectively, as compared to 24 July 2009 (normal day). The increase/decrease in the amplitude can be understood in terms of modal interference at the sites of modes converted at the discontinuity created by the eclipse intercepting the different transmitter-receiver great circle paths. The wavelet analysis shows the presence of WLS of period ~16–40 min at stations under total eclipse and of period ~30–80 min at stations under partial eclipse (~85–54% totality) with delay times between ~50 and 100 min at different stations. The intensity of WLS was maximum for paths in the partially eclipsed region and minimum in the fully eclipsed region. The features of WLS on eclipse day seem almost similar to WLS observed in the nighttime of normal days (e.g., 24 July 2009). The WLS could be generated by sudden cutoff of the photo-ionization creating nighttime like conditions in the D region ionosphere and solar eclipse induced gravity waves coming to ionosphere from below and above. The present observations shed additional light on the current understanding of gravity waves induced D region ionospheric perturbations.