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 syste...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) on17 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.展开更多
The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China （geomagnetic lati- tude 21.81°N, longitude 174.44°E, L=1.16） to detect low latitude extremely-low-frequen...The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China （geomagnetic lati- tude 21.81°N, longitude 174.44°E, L=1.16） to detect low latitude extremely-low-frequency （ELF： 0.3-3 kHz） and very-low-frequency （VLF： 3-30 kHz） emissions originating from either natural or artificial sources since February 2016. Dur- ing the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude sta- tion with the majority （97.0%） occurring on 28 February and 1 March 2016. Observed whistlers manifest various types includ- ing single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04435 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below -1.6 kHz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between -15 s1/2 and 23 s1/2 for observed one-hop whistlers and is greater than 50 sm for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low lati- tude whistlers.展开更多
基金supported by the National Natural Science Foundation of China (Grants Nos. 41674163, 41474141, 41204120, 41304127, 41304130, and 41574160)the Projects funded by China Postdoctoral Science Foundation (Grants Nos. 2013M542051, 2014T70732)+2 种基金the Hubei Province Natural Science Excellent Youth Foundation (2016CFA044)the Project Supported by the Specialized Research Fund for State Key Laboratoriesthe 985 funded project of School of Electronic information, Wuhan University
文摘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) on17 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.
基金supported by the National Natural Science Foundation of China(Grants Nos.41204120,41474141,41304127,41304130,and 41574160)the Projects funded by China Postdoctoral Science Foundation(Grants Nos.2013M542051,2014T70732)the 985 funded project of School of Electronic information,Wuhan University
文摘The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China （geomagnetic lati- tude 21.81°N, longitude 174.44°E, L=1.16） to detect low latitude extremely-low-frequency （ELF： 0.3-3 kHz） and very-low-frequency （VLF： 3-30 kHz） emissions originating from either natural or artificial sources since February 2016. Dur- ing the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude sta- tion with the majority （97.0%） occurring on 28 February and 1 March 2016. Observed whistlers manifest various types includ- ing single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04435 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below -1.6 kHz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between -15 s1/2 and 23 s1/2 for observed one-hop whistlers and is greater than 50 sm for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low lati- tude whistlers.
