Sushanta K. Mondal

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.

Earth’s Ionosphere as a Gigantic Detector of Extra‐terrestrial Energetic Phenomena: A Review

The paper presents the ionospheric effects due to radiation from the transient extraterrestrial sources like Gamma Ray Bursts, Soft Gamma Ray Repeaters, Anomalous X‐ray Pulsars, X‐ray novae and X‐ray transient sources. Gamma rays could penetrate deep in the atmosphere due to their high energy in comparison with other types of radiation. If the transient radiation from the above sources is strong enough to enhance the ionospheric ionization, VLF radio propagation could be affected. In the paper, we discuss the evidences of detection of such phenomena in VLF data and explain some of the observations using theoretical considerations.

Partial Effects on VLF Data due to a Solar Flare During 2010 Annular Solar Eclipse

The VLF radio waves propagate through the Earth‐ionosphere waveguide. Irregularities caused by excesses or deficient soft X‐rays which sustain the ionosphere changes the waveguide properties and hence the signals are modified. We report the results of our monitoring of the NWC transmitter from Khukurdaha (∼80 km away from Kolkata) during the partial solar eclipse (75%) of 15th January, 2010. The receiving station and the transmitter were on two opposite sides of the annular eclipse belt. We got clear depression in the data during the period of partial eclipse. There was also a solar flare (spot no. 1040) on that day during the time the eclipse was near maximum. The flare started from B, reaching maximum to C1.3 (as observed by GOES 14 satellite). We saw the partial effect of this flare since a part of the active region was blocked by the moon. To our knowledge this is the first such incident where the solar flare was observed through lunar occultation.

Effect of solar flares on ionospheric VLF radio wave propagation, modeling with GEANT4 and LWPC and determination of effective reflection height

Solar flares are among the most prominent transient events affecting the radio wave propagation in the ionosphere. In this paper we model the ionizing effect on VLF wave, reflected from D-layer of the ionosphere during different classes of flares and compare with observed VLF modulation. A well known detector simulation software GEANT4 is used to calculate the ionization produced in different heights (~ 60-80 km) of the D layer. The chemical balancing between the ions and neutrals in the region is taken into account with a simplified chemical model, the GPI model. The VLF modulation is reproduced with LWPC. We have seen that the peaks of electron density arise at different times at different heights. Matching with the time delay of the VLF signals we have proposed an alternative effective reflection height during such events.

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.

VLF observation of a solar flare by lunar occultation during annular solar eclipse of January 15th, 2010

We report the results of our monitoring of the NWC transmitter from Khukurdaha (∼ 80 km away from Kolkata) during the partial solar eclipse (75%) of 15th January, 2010. The receiving station and the transmitter were on two opposite sides of the annular eclipse belt. We got clear depression in the data during the period of partial eclipse. However, most extraordinarily, there was a solar flare (spot no. 1040) on that day during the time when the eclipse was near maximum. The flare started from B type, reaching maximum to C1.3 (as observed by GOES 14 satellite). We saw the occultation of this flare by the moons limb. To our knowledge this is the first such incident where the solar flare was observed through lunar occultation.

First VLF detections of ionospheric disturbances due to Soft Gamma Ray Repeater SGR J1550–5418

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

Very Low Frequency detection of ionospheric disturbances due to GRB 090424

We report the detection of a Gamma Ray Burst GRB 090424 through the Very Low Frequency signal at three receiving stations. Very Low Frequency data recorded corresponding to 18.2 kHz at Salt Lake, ICSP and Malda situated on somewhat different propagation paths. The initial strong burst from GRB 090424 was double peaked followed by two much smaller peaks as observed by gamma ray space observatories. Double peaks are also seen in Very Low Frequency signals. However, because of slower response of the ionosphere the initial strong peaks were not resolved. The second Very Low Frequency peak is correlated to the smaller peaks. All the three VLF receivers got almost the same nature. We verified that there were no other disturbances (e.g. from solar flares) during this period of observation.

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.

Gamma‐Ray Bursts from RT‐2 payloads and VLF signals

RT‐2 payloads in Russian Satellite CORONAS‐PHOTON observed several Gamma Ray Bursts (GCN 9665, GCN 9833, GCN 10009 and GCN 10010). We show these results. We have compared simultaneous VLF data from many worldwide VLF receiving stations. However, we do not see sufficient signal disturbances. We discuss the implications of these observations.