Initial results from AWESOME VLF receivers: set up in low latitude Indian regions under IHY2007/UNBSSI program

(ULF; 1–30 Hz), extremely low frequency (ELF; 30– 3000 Hz), very low frequency (VLF; 3–30 kHz), low frequency (LF; 30–300 kHz), medium frequency (MF; 0.3– 3 MHz), high frequency (HF; 3–30 MHz) and ultra high frequency (UHF; >30 MHz). However, maximum energy radiation is contained in the ELF/VLF band 1 . Continuous monitoring of these ELF/VLF waves provides a powerful remote sensing tool for understanding the processes in ionosphere and magnetosphere. Since both the Earth and the ionosphere are good reflectors at these frequencies, the lightning radiated impulses, commonly known as radio atmospheric or sferics (short form of atmospherics), travel thousands of kilometres in the so-called earthionosphere waveguide (EIWG) with little attenuation (~2–3 dB/1000 km). When this radiated energy is received at VLF/ELF bands, the received signals do not exhibit any dispersion, except near the cut-off frequency of the waveguide and are known as sferics/tweeks. Part of the radiated energy also penetrates the lower boundary of the ionosphere and is guided through the magnetosphere by field-aligned irregularities called ducts with no appreciable attenuation, before re-entering the EIWG to be received as whistlers, emissions, etc. This mode of propagation in magnetized plasma is called the whistlermode