ABSTRACT

The return strokes of lightning discharges are powerful natural transmitters of the electromagnetic (EM) energy over a wide EM spectrum extending from few Hz to few tens of MHz with maximum spectral energy in the Extremely Low Frequency (ELF, 3-3000 Hz) and the Very Low Frequency (VLF, 3-30 kHz) bands. These EM waves at the ELF-VLF frequency propagate by multiple reflections in the atmospheric waveguide formed between the surface of the earth and the lower boundaries of ionosphere called Earth-Ionosphere Waveguide (EIWG) with very low attenuation rate (2-3 dB/1000 km) and hence are observed around the globe. On propagating large distances in the EIWG, EM waves undergo appreciable dispersions near the cutoff frequencies of different modes. Such dispersed waves are known as ‘tweeks’ as they sound like ‘tweet’ when heard with loudspeaker. The fact that tweeks are reflected from lower ionosphere makes them a useful probing tool to investigate the nighttime D-region ionosphere. Tweek covers most part of presentation, which includes study of tweek occurrence features during four part of local night and during three seasons. They have been utilized to estimate ionospheric reflection height and equivalent electron density at the reflection height, in order to understand nighttime D-region variability during selected quite days in three seasons. Our results show a day to day variability up to 8km, under pure nighttime propagation (21:00-03:00 LT) on magnetically quiet days. An equivalent electron density profile of night time D-region ionosphere is estimated using tweek up to 100 km altitudes, which shows comparable results with IRI-2001 and rocket data.