Nighttime D-region equivalent electron density determined from tweek sferics observed in the South Pacific Region

Atmospherics or sferics that originate from lightning discharges on propagating large distances in the Earth-ionosphere waveguide, particularly at the night, form dispersed sferics called tweeks. Tweeks are novel diagnostic tool to monitor the nighttime D-region ionosphere. Mean equivalent electron density n(cm) at mean tweek reflection heights h(m) and electron density profile have been estimated using the higher harmonic tweeks recorded in the time between 21-03 hrs LT at Suva (18.2 degrees S, 178.3 degrees E), Fiji, during a period March-December 2006. The values of n(cm), vary from 29-170 cm(-3) in the altitude range of about 3.5 km at h(m) of about 83 km. In terms of usually used exponential electron density profile, the ionospheric reference height and the exponential sharpness factor are calculated to be 83.1 km and 0.64 km(-1), respectively. The scale height is calculated to be 1.9 km. Equivalent electron density profile of the nighttime lower ionosphere, using tweek method, shows lower values of electron density by about 20-45% than those obtained from the IRI-2001 model.