A detailed investigation of low latitude tweek atmospherics observed by the WHU ELF/VLF receiver: I. Automatic detection and analysis method

Abstract

As a dispersive wave mode produced by lightning strokes, tweek atmospherics provide important hints of lower ionospheric (i.e., D‐region) electron density. Based on data accumulation from the WHU ELF/VLF receiver system, we develop an automatic detection module in terms of the maximum‐entropy‐spectral‐estimation (MESE) method to identify unambiguous instances of low latitude tweeks. We justify the feasibility of our procedure through a detailed analysis of the data observed at the Suizhou Station (31.57°N, 113.32°E) on 17 February 2016. A total of 3961 tweeks were registered by visual inspection; the automatic detection method captured 4342 tweeks, of which 3361 were correct ones, producing a correctness percentage of 77.4% (= 3361/4342) and a false alarm rate of 22.6% (= 981/4342). A Short‐Time Fourier Transformation (STFT) was also applied to trace the power spectral profiles of identified tweeks and to evaluate the tweek propagation distance. It is found that the fitting accuracy of the frequency–time curve and the relative difference of propagation distance between the two methods through the slope and through the intercept can be used to further improve the accuracy of automatic tweek identification. We suggest that our automatic tweek detection and analysis method therefore supplies a valuable means to investigate features of low latitude tweek atmospherics and associated ionospheric parameters comprehensively.

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

“…Agreement before midnight between our finding and theirs is good; after midnight, our results diverge to some degree. The possible reasons can be multifold: (1) our studies employeddifferent intensity thresholds in selecting unambiguous tweeks to be included in our analyses (refer toZhou RX et al, 2020 for details); (2) multiple factorsaffect the propagation and detection of tweeks, including the location of receivers.…”

“…We then evaluate the low latitude tweek propagation distance from the lightning source. As described above in Zhou RX et al (2020), there are two ways to calculate the propagation distance on the basis of fits to the dispersive power spectrum profiles. Theoretically and ideally, the results from the two methods should be identical, which however is unlikely in practice.…”

“…Adopting the automatic detection and analysis module developed by Zhou RX et al (2020), we have performed a comprehensive analysis of the occurrence features of low latitude tweek atmospherics and associated ionospheric parameters, using the high quality ELF/VLF signal spectrogram data acquired at 22–02 LT in February 2016 by the Suizhou station of the WHU ELF/VLF receiver system. To improve knowledge of the characteristic low latitude tweek atmospheric over central China, we have investigated in detail the temporal and local time dependence of a number of important parameters, including the tweek occurrence rate, tweek propagation distance, tweek reflection height, and associated D‐region electron density.…”

“…Due to the orthogonal configuration of the two magnetic loop antennas, we further choose the spectrogram data of the E‐W channel for our analysis. Based on the automatic detection and analysis module developed by Zhou RX et al (2020), only tweeks whose intensity is a minimum of 20 dBFS above averaged background noise are identified.…”

The Generation of ULF/ELF/VLF Waves in the Ionosphere by Modulated Heating

“…Investigate electrojet-independent modulation methods. It is important to carry out experiments using heaters at middle and low latitudes, such as SURA (56.13° N, 46.1° E) [11] and Arecibo (18° N, 67° W, no longer available, but expected for building and using other heaters at low latitudes like Arecibo in the future) [88], as well as receivers at middle and low latitudes, such as the WHU (Wuhan University) ELF/VLF receiver (30.54° N, 114.37° E) [89,90].…”