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

摘要

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.