Birbal Singh

On the lithosphere‐atmosphere coupling of seismo‐electromagnetic signals

[1]xa0Employing borehole and terrestrial antennas, vertical electric field components of the naturally occurring very low frequency electromagnetic emissions at the frequency of 3 kHz have been monitored simultaneously at Agra (geographic latitude 27.8°N, longitude 78°E), India, from 15 March 1999 to 30 September 1999. This period of observation included a major seismic swarm activity in the months of March and April which occurred in the Chamoli hills of north India, the next two months of May and June being quiet with respect to local lightning and spheric activities, and the rest of the three months from July to September being highly disturbed due to local lightning and thunderstorm activities. The abnormal electric field changes occurred in the form of noise bursts of varying amplitude and duration and included three kinds of data: (1) noise bursts observed by borehole antenna only, (2) noise bursts observed by terrestrial antenna only, and (3) noise bursts observed by both the antennas. We find that the occurrence of the first kind of data is positively correlated with major seismic activities in the region over the period of observations. The third kind of data indicates coupling between the two antennas dominated overall by atmospheric emissions. The long distance propagation of the seismo-electromagnetic emissions through the middle layer crust working as waveguide or through seismic faults is found to be associated with large attenuation ≈13 dB/km. Hence, the observation of the emissions at Agra, about 400 km from Chamoli, is interpreted in terms of leakage to the atmosphere through “windows” of low conductivity in the skin layer near the epicenter, possibly produced by some geophysical formations as discussed by other workers and then propagation in the Earth-ionosphere waveguide.

VLF electromagnetic noise bursts observed in a borehole and their relation with low-latitude hiss

Abstract In an attempt to monitor subsurface VLF electric field changes associated with earthquakes, a borehole antenna has been installed at Bichpuri, Agra (Geograph. Lat. 27.2°N, Geograph. Long. 78°E, Geomag. Lat. 17.1° N , L=1.1 ) in India and observations have been taken since February, 1998 using an analog system. The electric field changes have been found to occur in the form of noise bursts of varying amplitudes and durations. A thorough analysis of the data of the year 1999 has shown occasional occurrence of large amplitudes, long duration (∼5– 6 h or more) noise bursts which show seasonal, stormtime, and diurnal variations similar to those of low latitude magnetospheric hiss observed on the ground. Similar noise bursts have also been recorded in the vertical terrestrial antenna operated in conjunction with the borehole antenna. This result indicates penetration of ionospheric/magnetospheric VLF signals to large depths in the crustal region and cautions for careful identification of potential seismogenic signals in VLF data.

High dispersion whistlers observed at Agra station (L = 1.15)

Abstract Propagation paths of unusually large dispersion whistlers (25–35 s 1 2 ) observed in the low-latitude ground station of Agra (geomagnetic latitude, 17°12N, L = 1.15), India, are determined by employing the curve fitting technique. The results show that these whistlers propagated in the mid-latitude ionosphere along the field lines corresponding to L = 2.1–2.4 and reached our ground station through the Earth-ionosphere waveguide mode of propagation. These results are then confirmed using ray tracing computations.

Whistler triplets, bands, and fine structures observed in a low latitude ground station

Whistler triplets, banded whistlers, and temporal fine structure in whistlers observed for the first time in a low latitude ground station at Agra (geomag. lat. 17° 1′ N , L = 1.15) are reported here. We show that the whistler triplets recorded by us are the one hop multipath whistlers which propagated to the ground under the influence of the equatorial anomaly in the dayside of the earth and reached our station in the earth-ionosphere waveguide mode of propagation. The similar time intervals between the successive triplets are due to similar time intervals between the causative lightning discharges. The banded structure (showing frequency- fine structure) and temporal fine structure obseved in groups of high dispersion whistlers are interpreted in terms of the effects of multistroke lightning.