Nighttime D region electron density measurements from ELF-VLF tweek radio atmospherics recorded at low latitudes

Dispersive atmospherics (tweeks) observed during 2010 simultaneously at two low-latitude stations, Allahabad (geomagnetic latitude, 16.05 degrees N) and Nainital (geomagnetic latitude, 20.48 degrees N), have been used to estimate the nighttime D region electron density at the ionospheric reflection height under the local nighttime propagation (21:00-02:00 LT or 15:30-20:30 UT). The analysis of simultaneously recorded tweeks at both the stations on five international quiet days during one month each from summer (June), winter (January), and equinox (March) seasons shows that the D region electron density varies 21.5-24.5 cm(-3) over the ionospheric reflection height of 85-95 km. The average values of Wait lower ionospheric parameters: ionospheric reference height h' and sharpness factor beta are almost same during winter (85.9-86.1 km, 0.51-0.52 km(-1)) and equinox (85.6-85.7 km, 0.54 km(-1)) seasons. The values of h' and beta during summer season are about 83.5 km and 0.60 km(-1) at both stations. Overall, equivalent electron density profile obtained using tweek method shows lower values of electron density by about 5-60% than those obtained using the International Reference Ionosphere (IRI-2007) model and lower/higher by 2-68% than those obtained using rocket technique. The electron density estimated using all three techniques (tweek, IRI-2007, and rocket) is consistent in the altitude range of 82-98 km. The estimated geographic locations of causative lightnings of tweeks were matched with the locations and times of lightnings detected by the World-Wide Lightning Location Network (WWLLN). The WWLLN detected about 27.5% of causative lightnings of tweeks simultaneously observed at both the stations.