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.