Tweek Atmospherics Research Articles

Automatic recognition of tweek atmospherics and plasma diagnostics in the lower ionosphere with the machine learning method

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide over long distances. In this paper, we developed an automatic method to recognize tweek atmospherics and diagnose the lower ionosphere based on the machine learning method. The differences (automatic-manual) of each ionosphere parameter between the automatic method and the manual method are 0.07  2.73 km, 0.03  0.92 cm-3, and 91  1068 km for h, Ne, and d, respectively. Moreover, the automatic method is capable of recognizing higher harmonic tweek sferics as well. These evaluation results of the model suggest that the automatic method is a powerful tool to investigate the long-term variations of the lower ionosphere.

Automatic Detection of Atmospherics and Tweek Atmospherics in Radio Spectrograms Based on a Deep Learning Approach

AbstractLightning strikes can be routinely observed by the radio receivers that measure electromagnetic signals in the very low frequency range. The acquired pulses called atmospherics provide valuable details about source lightning discharges and also about the Earth‐ionosphere waveguide where they can propagate thousands of kilometers. The automatic acquisition of the events requires also automatic methods for extraction of atmospherics' details to confirm the observed trends with statistical significance. For this purpose, we have developed a method based on a deep learning approach to automatically obtain the type of atmospherics, their exact time, and the frequency range from the frequency‐time spectrograms. The method that employs convolutional neural networks is designed to be adaptable to scientific needs and provide reliable results according to the requirements on the sensitivity to events, computation performance, and precision of extracted details. The efficiency and specific steps of our method are demonstrated for data recorded by the ELMAVAN‐G instrument. The comprehensive description of the method allows its usage for regular characterization of the ionospheric D‐layer or for other similar applications in the future.

Estimation of nighttime ionospheric D-region parameters using tweek atmospherics observed for the first time in the North African region

Dispersive wave modes called tweek atmospherics produced by lightning strokes were observed using Stanford University designed AWESOME VLF receiver system installed at the University of Tunis El Manar, Tunisia (Geog. lat. 36.50° N; lon. 10.08° E). About 84 tweek atmospherics were analyzed during geomagnetically quiet days to provide important clues for estimating the effective ionospheric reflection height and electron density using the cut-off frequency of first modes. Variations of nighttime Wait ionospheric parameters (h′ and β) have been studied using tweeks observed for the first time at the low latitude station in the North African region. The distance of the lightning located in a range of 500–5000 km and the accuracy obtained by single-site technique had been estimated by a comparison with independent data from the World-Wide Lightning Location Network (WWLLN).

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.

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

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.

Electron density variability of nighttime D region ionosphere in Vietnamese and Japanese sectors

AbstractRecording tweek atmospherics on geomagnetically quiet days in 2014 at Tay Nguyen University (TNU) (12.65°N, 108.02°E), Vietnam, and at Kagoshima (KAG) (31.48°N, 130.72°E), Japan, we investigated the nighttime electron density variability of the D region ionosphere between the equatorial‐low‐latitude Vietnamese and the low‐ to middle‐latitude Japanese sectors. We estimated the reflection height and electron density of the D region ionosphere using the first‐order mode cutoff frequency of tweek atmospherics. The results observed at both stations show that the mean electron density at the reflection height during winter season was higher than that during summer and equinox seasons. The electron density observed at TNU gradually decreased from 20:15 to 4:15 LT from winter to equinox and to summer. The electron density observed at KAG increased from 20:20 to 4:20 LT during summer and winter seasons. The mean electron density during 2014 observed at TNU (25.0 cm−3) was higher by 2.1 cm−3 than that observed at KAG (22.9 cm−3). During 2014, the nighttime electron density variations show a moderate positive correlation with the sunspot number but show weak to no correlation with the galactic cosmic rays. We suggest that the seasonal variations in the nighttime electron density could be significantly caused by the enhancement of geocoronal hydrogen Lyman α intensity and seasonal variation of nitric oxide density in the lower ionosphere.

Development of ground-based ELF/VLF receiver system in Wuhan and its first results

A new digital low-frequency receiver system has been developed at Wuhan University for sensitive reception of low-latitude broadband Extremely Low Frequency (ELF) and Very Low Frequency (VLF) radio waves originating from either natural or artificial sources. These low-frequency radio waves are useful for ionospheric remote sensing, geospace environment monitoring, and submarine communications. This paper presents the principle and architecture of the system framework, including magnetic loop antenna design, low-noise analog front-end and digital receiver with data sampling and transmission. A new structure is adopted in the analog front end to provide high common-mode rejection and to reduce interference. On basis of field programmable gate array (FPGA) device and Universal Serial Bus (USB) architecture, the digital receiver is developed along with time keeping and synchronization module. The validity and feasibility of the self-developed ground-based ELF/VLF receiver system is evaluated by first results of experimental data that show the temporal variation of broadband ELF/VLF wave spectral intensity in Wuhan (30.54°N, 114.37°E). In addition to the acquisition of VLF transmitter signals at various frequencies, tweek atmospherics are also clearly captured to occur at multiple modes up to n=6.

Seasonal variations of nighttime D-region ionosphere in 2013 solar maximum observed from a low-latitude station

We present the observation of tweek atmospherics with harmonics m = 1–8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65° N, 108.02° E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5–6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4–225 cm −3, which corresponds to the harmonics m = 1–8 at the mean reflection height of 81.5–87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12–58 %, 3–67 %, and 24–76 % than those obtained using the International Reference Ionosphere (IRI-2012) model.

Daytime tweek atmospherics

AbstractWe report the first observation of daytime tweek atmospherics based on measurements at Moshiri (44.37°N, 142.27°E) and Kagoshima (31.48°N, 130.72°E), Japan, during nonsolar eclipse days for 5 months in 1980–1994. The daytime tweeks were observed on geomagnetically quiet and stormy days. The daytime tweeks had clear frequency dispersion with an average duration of 12 ms, which was shorter than that in the nighttime (~50 ms). The average occurrences of the daytime tweeks at Moshiri and Kagoshima were 0.6 and 0.1 tweeks per minute during 10:00–15:00 LT, respectively. Daytime tweeks up to the second‐order mode were visible. There was no difference in the occurrence of each visible mode between storm time and magnetically quiet time. The daytime reflection heights were similar to those at night (85–100 km) but with greater variation. We evaluated the attenuation rate (αn) of tweeks by strictly taking the ionospheric reflection coefficient into account. For each frequency, αn was evaluated as a function of the electron density, electron density gradient, and ionospheric height. We found that αn had an inverse relationship with the electron density (or conductivity), electron density gradient, and ionospheric height. We suggest that the best conditions for daytime tweek observations are when the bottomside of the ionosphere is sharply defined and the ionospheric height is high.

Characteristic of Tweek Atmospherics Observed in Mid-latitude using AWESOME VLF Receiver

This study presents the analysis of tweek atmospherics received by AWESOME VLF receiver at station of Gakona (62.71°N, 143.99°W) during four months observation from January to April 2011. Tweek which originates from lightning discharge are used to monitor the nighttime D-region ionosphere using the fundamental cut-off frequency to measure the variations of the lower ionosphere’s reflection height, the equivalent electron density at the reflection height and the propagation distance travel by tweeks. In this study, a total of 1316 tweeks are analyzed and from the analysis, it shows that equinox’s season has the highest tweek occurrence compared to winter season in March and April. The maximum harmonic (m) of t weeks is found to be up to fourth (m = 4) and tweeks with mode number one (m = 1) are more dominantly occurred. Our observations indicate that the equivalent electron densities for tweeks varies from 22-27 eL/cm3 in the altitude ranged of 75 to 91 km and demonstrate that these ELF/VLF signals travel considerable distances up to 6700 km from the causative lightning discharges. The ionospheric parameters for three locations (high, middle and low latitude respectively) were compared and the results show that they are almost consistent for all the locations.

Reflection height of daytime tweek atmospherics during the solar eclipse of 22 July 2009

We report multipoint observations of daytime tweek atmospherics during the solar eclipse of 22 July 2009. Sixteen and sixty‐three tweek atmospherics were observed at Moshiri and Kagoshima, Japan, where the magnitudes of the solar eclipse were 0.458 and 0.966, respectively. This was the first observation of tweek atmospherics during a low‐magnitude eclipse (0.458). The average and standard deviation of the reflection height were 94.9 ± 13.7 km at Moshiri and 87.2 ± 12.9 km at Kagoshima. The reflection height at Moshiri was almost the same as that for normal nighttime conditions in July (96.7 ± 12.6 km) in spite of the low magnitude of the eclipse. The reflection height at Kagoshima seems be divided into two parts: propagation across the total solar eclipse path and propagation in the partial solar eclipse path. During the eclipse, we also observed the phase variation in the LF transmitter signals. The average change in the phase delay of the LF signals was 109° for the paths that crossed the eclipse path and 27° for the paths that did not cross the eclipse path. Assuming a normal daytime height for LF waves of 65 km, a ray tracing analysis indicates that the variations in phase correspond to a height increase of 5–6 km for the paths across the eclipse and 1–2 km for partial eclipse paths. The wide range of estimated tweek reflection heights at Kagoshima also suggests a difference in electron density in the lower ionosphere between total and partial solar eclipses.

Long-term variations in tweek reflection height in the D and lowerEregions of the ionosphere

[1] We investigated, for the first time, long-term (1976–2010) variations in reflection heights of tweek atmospherics based on very low frequency (VLF) observations at Kagoshima, Japan. The results revealed the effects of the solar cycle on the nighttime lower ionosphere at low to middle latitudes. The tweek reflection heights on geomagnetically quiet days were analyzed every month over three solar cycles by using an automated spectral fitting procedure to estimate the cutoff frequency. The average and standard deviation of the reflection height were 95.9 km and ±3.1 km, respectively. Typical periods of time variation for the reflection height were 13.3, 3.2, 1.3, 1.0, 0.6, and 0.5 years. The variations in tweek reflection heights did not show simple anticorrelation with solar activity. The correlation coefficient between tweek reflection height and sunspot number was 0.03 throughout the three solar cycles. Hilbert-Huang transform analysis successfully indicated the presence of 0.5–1.5 year and ∼10 year variations as intrinsic mode functions (IMF). The decomposed IMF with the ∼10 year variation had a positive correlation with sunspot numbers and a negative correlation with galactic cosmic rays (GCRs). We hypothesize that these variations in tweek reflection heights could be caused by coupling of several ionization effects at the D and lower E regions, effects such as geocorona, GCRs, particle precipitation, and variations in neutral density in the lower thermosphere. Among these processes, the geocorona and particle precipitation could show negative correlation, while the GCRs and neutral density could show positive correlation with solar activities.

Study of variation in the lower ionospheric reflection height with polar day length at Antarctic station Maitri: Estimated with tweek atmospherics

In the present paper investigations are made on the lower ionosphere with the help of observed tweek atmospherics at the Indian Antarctic research station Maitri during the second half of austral summer (January–March of 2003 and 2005). The variations in the lower ionospheric reflection height are found to be in correspondence with the sunrise and sunset at Maitri. The results indicate that the tweek atmospherics are helpful in tracking the changes taking place in the lower ionospheric height through the day‐night boundary. The ionospheric reflection height estimated using the cutoff frequency of fundamental (first‐harmonic or first‐mode) tweek atmospherics ranges from ∼64 to ∼79 km with a maximum calculated error of ±2.7 km. The recordings were carried out from 1800 to 0600 UTC. The analysis shows that the average cutoff frequency for the fundamental tweek atmospherics and hence the ionospheric reflection height varies from the months of January to March in accordance with the sunrise and sunset.

Development of an automatic procedure to estimate the reflection height of tweek atmospherics

AbstractThis paper presents an automated procedure to estimate apparent reflection heighth(from the cutoff frequency for the first waveguide mode,fc), horizontal propagation distanced, and propagation timeTgof tweek atmospherics. Tweek data recorded at the Kagoshima Observatory (31.48°N, 130.72°E), Japan, were used to evaluate the procedure by comparing the results estimated by the automatic method to those read manually by an operator. The two types of results showed differences (automatic–manual) of +0.58 km, −9.9 Hz, and +3058.9 km for meanh,fc, andd, respectively. The difference inh(fc) was less than the resolution of the fast Fourier transform used to obtain the tweek spectra. These comparisons indicate that the automatic estimation procedure of tweek parameters developed in this paper performs well and is a useful tool for studying long-term height variations of the ionosphericDand lowerEregions using very low frequency (VLF) and extremely low frequency (ELF) records observed in Japan over the past 30 years.

Higher harmonic tweek sferics observed at low latitude: estimation of VLF reflection heights and tweek propagation distance

Abstract. Lightning generated signals recorded at a low-latitude station, Suva (18.2° S, 178.3° E) Fiji, in the South Pacific region, during September 2003–July 2004, are used to study the propagation features and the reflection heights of tweek atmospherics in the waveguide formed by the Earth's surface and the lower ionosphere. Tweeks are observed only during the local night and the maximum harmonic (n) recorded is six. The occurrence of tweeks with higher n progressively decreases as n increases. The dispersed part of tweeks decreases as n increases. The attenuation factor has been calculated for tweeks with n=1–3. The ionospheric reflection heights obtained assuming the transverse magnetic mode of propagation for tweek signals vary from 83–92 km. A higher harmonic of the same tweek is reflected from about 2.0 km higher than the lower harmonic. For 90% of tweeks, propagation distances are estimated to be between 1000–5000 km. Tweeks with lower n propagate longer distances than the tweeks with higher n.

Using tweek atmospherics to measure the response of the low-middle latitude D-region ionosphere to a magnetic storm

Tweek atmospherics are pulse signals from ELF/VLF electromagnetic waves caused by lightning discharges; these signals have characteristic cut-off frequencies due to long-distance propagation by the Earth–ionosphere waveguide mode. We developed a method of estimating tweek reflection heights (equivalent electron densities) in the low-middle latitude D-region ionosphere by accurate measurement of their first-order mode cut-off frequency (∼1.8 kHz). This research is the first to show reflection-height measurements of the D-region ionosphere's response to the great geomagnetic storm in October 2000 using tweek atmospherics observations. We found transient lowering/rising in the small bay D st-variation and rising near the maximum depression of the major bay D st-variation. Comparing this response in tweek reflection heights with the intensities of 40 kHz radio-wave signals, electron density profiles from the IRI-2001 model and measured by MF radars, and the ionospheric F-region parameter ( h′ F) from ionosonde data, we determined two cases where the lowering/rising of the D-region was coupled with vertical motion of the F-region and one where it was not. Based on simultaneous total electron content (TEC) perturbation data, we suggest the following possible mechanisms for these two results: E × B vertical plasma drift due to ionospheric electric fields, and the propagation of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral to low-latitude ionosphere, respectively.

Equivalent electron densities at reflection heights of tweek atmospherics in the low-middle latitude D-region ionosphere

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20—28 el./cm3 at ionospheric reflection heights of 80—85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

ELF-VLF atmospheric waveforms under night-time ionospheric conditions

Abstract. Tweek atmospherics generated by lightning discharges and propagated in the night-time Earth-ionosphere waveguide, have often very pronounced dispersive features near the first few waveguide cut-off frequencies ( fcm~m\\dot{s} fc1, fc 1~1.6–1.9 kHz, m=1, 2, . . . , ), being very extended in time, and have rather large amplitudes of oscillations with periods corresponding to the narrow vicinity of the cut-off frequencies. In this paper an analytical approach is developed to describe the waveform of distant tweeks. It is based on the solving of the Maxwell equations in two qualitatively different regions, whose changes are related in the first instance to the changes in the relative magnitudes of the displacement current and components of the conduction currents, and the following asymptotic matching of the solutions in the transitional region. The analytical night-time waveguide model accounts for both anisotropy and vertical inhomogeneity of the low ionosphere. The model is valid for upper ELF – lower VLF range and is well suitable for the analysis of the QTEm modes in the cut-off frequency regions, which determine the most important part of the tweek spectra and tweek amplitudes. The influence of the different ionospheric heights on the tweek characteristics is determined. The efficiency of the tweek generation by cloud-to-cloud discharge is also evaluated.

Very low latitude whistlers and the effect of earth‐ionosphere waveguide propagation

AbstractWhen a sonogram of the nighttime whistlers observed in Wuchang, China, at a low latitude (at a geomagnetic latitude of 19.4°N) was studied in detail, whisker‐type spectral components were observed, which may be due to the waveguide propagation effect of the second‐ and third‐order modes, in the spectra of intense whistlers accompanied by echo trains. By applying the field analysis direction‐finding method to this part, the propagation characteristics are derived and the results exhibited a tendency identical to that of the characteristics of tweek atmospherics. It was found that the whistlers transmitted through the ionosphere at a very low latitude in Zhanjiang (geomagnetic latitude of 10.0°N) and exhibited an extremely rare waveguide mode propagation after subionospheric propagation of over approximately 1000 km.

Wave characteristics of tweek atmospherics deduced from the direction‐finding measurement and theoretical interpretation

Detailed wave characteristics (incident and azimuthal angles, wave polarization, and their frequency dependences) of tweek atmospherics have been elucidated on the basis of the application of our field‐analysis direction finding to the VLF data observed in southern China. It is then found that the wave polarization of the first‐order mode at the frequency above its cutoff frequency (ƒ1c = 1.7 ∼ 1.8 kHz) is always left handed and it becomes exactly left‐handed circular when the wave frequency decreases down to the ƒ1c, together with the fact the incident angle becomes zero (vertical) when the frequency approaches ƒ1c. Whereas the zeroth‐order mode at frequency below ƒ1c is found to be linearly polarized. These wave properties are first interpreted qualitatively, and then we try to explain them in terms of the full wave theory in the Earth‐ionosphere waveguide in which the realistic electron density profiles of the lower ionosphere are assumed. Finally, it is suggested that the measurement of wave polarization, as given in this paper, would be useful in studying the formation mechanism of tweek tails, the coupling into whistler mode waves of lightning discharges, the lower ionospheric density profile, etc.