A detailed investigation of low latitude tweek atmospherics observed by the WHU ELF/VLF receiver: 2. Occurrence features and associated ionospheric parameters

As a companion paper to Zhou RX et al. (2020), this study describes application of the automatic detection and analysis module to identify all the tweek atmospherics detectible in the WHU ELF/VLF receiver data collected at Suizhou station during the period of 3 February through 29 February 2016. Detailed analysis of the identified low-latitude tweek events reveals that the occurrence rate varies considerably — from 800 to 6000 tweeks per day, and exhibits a strong diurnal and local time dependence, the peak occurring before local midnight. The diurnal variation of identified tweeks was similar to that of the lightning data obtained by the World-Wide Lightning Location Network (WWLLN).. Estimates of the propagation distance and ionospheric reflection height of tweek atmospherics suggest that the majority (~92%) of the low latitude tweeks originate from the lightning activity within a radius of 4000 km and that they are very likely to reflect from the lower ionospheric D-region at the height range of 75–85 km. At these lower ionospheric reflection altitudes, ~74% of the corresponding electron densities from the tweek spectral measurements are within 24.5–27.5 cm^-3. The daily variation of estimated D-region electron densities in the considered period (February 2016) also exhibits a small overall increasing trend from early to later in the month.