A study of sub-ionospheric early VLF perturbations observed at Agra (L=1.15), India

In this paper, we present the results of sub-ionospheric VLF perturbations observed on NWC (19.8 kHz) ransmitter signal propagating in the Earth-ionosphere waveguide, monitored at Agra (Geomag. Lat 27°E, long. 78°N) using SoftPAL receiver. During the period of observation (June, 2011 to December, 2011), we found 75 cases of abrupt amplitude/phase perturbations showing early character. Most of the early events observed show an amplitude change lying between 0.16-4.5 dB, only few cases having >4.5 dB and phase change lying between 01-17 degree respectively. The onset duration of these early perturbations is up to ~ 5 sec, showing early slow character. The World Wide Lightning Location Network (WWLLN) data and the broadband VLF data is analysed to find the location of causative lightning associated with these early VLF perturbations. During the period of our study majority of the events are observed at nighttimes and only few cases are observed at daytimes most likely due to the occurrence of red sprite or elves in the daytime. The lightning discharge and associated processes that leads to the changes in the waveguide characteristics and hence VLF transmission have been discussed.