No abstract

Abstract:This paper reports the first observation of extremely small dispersion (ESD) whistlers from thunderstorms recorded during magnetic storm period at a low latitude ground station Srinagar (geomag. lat., 24 0 10' N; L=1.28). ESD whistlers with dispersion varying from 4-5 sec ½ surprisingly in large numbers were recorded on 06 April, 2011 at Srinagar during local thunderstorm period in the daytime afternoon. Such type of daytime ESD whistlers from local thunderstorm recorded during magnetic storm period at Srinagar has never been reported from any of the low latitude ground stations and is the first observation from low latitudes. Results of the study of the characteristics of daytime ESD whistlers from local thunderstorms observed at Srinagar shows that these whistlers are ducted whistlers and are found to obey perfectly the Eckersley law. Since the dispersion of these daytime ESD whistlers are found in the extremely small range of the order of 5 sec ½ which clearly shows that these whistlers have propagated over an extremely small path along the ducted field lines. It is proposed that daytime ESD whistlers from local thunderstorms recorded at Srinagar are the VLF waves radiated from the return stroke of lightning discharge present in local thunderstorms penetrated the ionosphere in waveguide mode and then they propagated to the other hemisphere along the geomagnetic field line of latitude ~ 5 0 in whistler duct formed by the local thunderstorm electrostatic electric fields. After reaching the opposite hemisphere, these whistler waves again traverse in the waveguide mode to reach the observing station Srinagar. The generation and propagation mechanisms of these ESD whistlers observed during local thunderstorms are discussed.