Very Low Frequency Research Articles

Assessment of Cardiac Autonomic Function by Short-Term Sensor-Based and Long-Term Heart Rate Variability Analyses in Individuals with Spinal Cord Injury After Long-Term Table Tennis Training

This study aimed to examine the acute and chronic effects of an exercising table tennis program on cardiac Autonomic Nervous System (ANS) and functional capacity in people with tetraplegia. Twenty males with tetraplegia (C6–C7), with a mean age of 38.50 ± 4.04 years old, were randomly assigned into two equal groups: A, who followed a 6-month exercise training program with table tennis 3 times per week, and B, who remained untrained. Additionally, 11 healthy sedentary men (group C) with a mean age of 39.71 ± 5.87 years old participated in the study as healthy controls. At baseline, all participants underwent a short-term (5 min) and a long-term (24 h ambulatory) ECG monitoring to evaluate the heart rate variability (HRV) indices and a maximal arm ergometric and dynamometric testing of the upper limbs. Moreover, the acute cardiac autonomic responses to maximal arm cycle exercise test were evaluated by Polar S810i sensor chest strap. At the end of the 6-month study, all parameters were revaluated only in groups A and B. At baseline, there was no statistically significant difference between the two patient groups. However, intra-group changes at the end of the 6-month study regarding the 24-h HRV monitoring indicated that group A statistically increased the standard deviation of R-R intervals (SDNN) by 13.9% (p = 0.007), the standard deviation of R-R intervals calculated every 5 min (SDANN) by 8.4% (p = 0.007), the very low frequency (VLF) by 7.1% (p = 0.042), and the low frequency [LF (ms2)] by 10.5% (p = 0.009), which almost reached the levels of group C. Favorable improvements were also noticed at the end of the study for group A in maximal exercise time of the upper limbs by 80.4% (p < 0.001) and maximal strength of the right hand by 27.8% (p < 0.001). Linear regression analysis after training showed that maximal exercise time was positively correlated with SDNN (r = 0.663, p = 0.036) and with LF (ms2) (r = 0.623, p = 0.045). Our results indicate that a 6-month table tennis training program is efficient and can improve cardiac ANS activity mainly by increasing sympathovagal balance.

Implication of heart rhythm complexity in predicting long-term outcomes in pulmonary hypertension

Pulmonary hypertension (PH) is a serious disease, however simple tools to predict outcomes are lacking. In our previous investigation, we found that heart rate variability (HRV) and heart rhythm complexity (HRC) were associated with the detection and severity of PH, however their association with PH mortality remains unclear.The aim of this study was to investigate these metrics as a tool for determining long-term outcomes in PH patients. We enrolled 74 Asian PH patients with WHO PH group 1 or 4 at a single hospital in Taiwan between March 2012 and June 2018. After a median follow-up duration of 58 months (to January 2023), 22 patients had died. The patients who died had a significantly lower lean body mass index (BMI), impaired renal function, higher N-terminal pro B-type natriuretic peptide (NT-proBNP) level, lower very low-frequency (VLF), lower short-term detrended fluctuation analysis α1 (DFAα1), and lower multiscale entropy scale 5 value. In multivariable analysis, BMI, VLF and multiscale entropy scale 5 were significantly associated with survival. The best cut-off VLF and scale 5 values were 115.13 and 0.738, respectively. We then categorized the study population into three groups: both elevated VLF/scale 5 (group 1), either depressed VLF or depressed scale 5 (group 2), and both depressed VLF/scale 5 (group 3). The results showed that group 1 had the best outcomes, whereas group 3 had the worst survival (P < 0.001).Combining HRV and HRC metrics appears to be a good non-invasive tool to predict the long-term outcomes of patients with PH.

Effect of transcutaneous electrical acupoint stimulation at different frequencies on mild hypertension: A randomized controlled trial

BackgroundTranscutaneous electrical acupoint stimulation (TEAS) may contribute to blood pressure (BP) control, but the evidence remains insufficient. Our objectives were to evaluate the impact of TEAS on hypertension and determine the optimal frequency. MethodsA total of 120 hypertensive patients were randomly allocated to the TEAS-2Hz group, TEAS-10Hz group, or usual care control group in a 1:1:1 ratio. All patients were advised to continue their usual antihypertensive regimen. Additionally, patients in the TEAS groups received TEAS therapy 3 times per week for 4 weeks, with a 4-week follow-up. ResultsThe primary outcome was the change in systolic BP (SBP) from baseline to week 4. Secondary outcomes included changes in diastolic BP (DBP), mean arterial pressure (MAP), heart rate (HR), heart rate variability (HRV), and 12-item health survey (SF-12) at different time points. Both TEAS groups showed reductions in SBP relative to control (TEAS-2Hz group vs. control, −4.70 mmHg [95 % CI, −7.00 to −2.40 mmHg]; P < 0.001; TEAS-10Hz group vs. control, −8.66 mmHg [95 % CI, −10.97 to −6.36 mmHg]; P < 0.001). TEAS-10Hz provided a significant decrease in SBP than TEAS-2Hz (-3.96 mmHg [95 % CI, −1.66 to −6.26 mmHg]; P< 0.001). TEAS groups also exhibited reductions in DBP, MAP, HR, LF/HF ratio（LF/HF）, very low frequency (VLF), and normalized low frequency (LF norm), and an increase in normalized high frequency (HF norm) than control. No differences were observed among groups in low frequency (LF), high frequency (HF), total power (TP), very low frequency (VLF), and SF-12. ConclusionTEAS might be a promising adjunctive therapy for hypertension, and the recommended frequency is 10 Hz, which should be confirmed in larger trials.

High sensitivity measurement of ULF, VLF, and LF fields with a Rydberg-atom sensor.

Fields with frequencies below megahertz are challenging for Rydberg-atom-based measurements, due to the low-frequency electric field screening effect caused by the alkali-metal atoms adsorbed on the inner surface of the container. In this paper, we investigate electric field measurements in the ultralow frequency (ULF), very low frequency (VLF), and low frequency (LF) bands in a Cs vapor cell with built-in parallel electrodes. With optimization of the applied DC field, we achieve high-sensitive detection of the electric field at frequencies of 1 kHz, 10 kHz, and 100 kHz based on the Rydberg-atom sensor, with the minimum electric field strength down to 18.0 μV/cm, 6.9 μV/cm, and 3.0 μV/cm, respectively. The corresponding sensitivity is 5.7 μV/cm/Hz1/2, 2.2 μV/cm/Hz1/2, and 0.95 μV/cm/Hz1/2 for the ULF, VLF, and LF fields, which is better than a 1-cm dipole antenna. Besides, the linear dynamic range of the Rydberg-atom sensor is over 50 dB. This work presents the potential to enable more applications that utilize atomic sensing technology in the ULF, VLF, and LF fields.

Automatic Extraction of VLF Constant‐Frequency Electromagnetic Wave Frequency Based on an Improved Vgg16‐Unet

AbstractConstant Frequency Electromagnetic Waves (CFEWs) refer to electromagnetic waves with a constant frequency. Man‐made CFEWs are mainly used in wireless communication, scientific research, global navigation and positioning systems, and military radar. CFEWs exhibit horizontal line characteristics higher than the background on spectrograms. In this study, we focus on Very Low Frequency (VLF) waveform data and power spectral data collected by the China Seismo‐Electromagnetic Satellite (CSES) Electromagnetic Field Detector (EFD). We utilize deep learning techniques to construct an improved Vgg16‐Unet model for automatically detecting horizontal lines on time‐frequency spectrogram and extracting their frequencies. First, we transform waveform data into time‐frequency spectrogram with a duration of 2 s using Short‐Time Fourier Transform. Then, we manually label horizontal lines on the time‐frequency spectrogram using the Labelme tool to establish the dataset. Next, we establish and improve the Vgg16‐Unet deep learning model. Finally, we train and test the model using the dataset. Statistical experimental results show that the error rate of line detection is 0, indicating high reliability of the model, with fewer parameters and fast computation speed suitable for practical applications. Not only do we detect lines through the model, but we also obtain their frequencies. Additionally, in batch‐generated power spectrogram of CFEWs, we discover some unstable phenomena such as frequency shifts and fluctuations, which contribute to understanding the propagation mechanism of CFEWs in the ionosphere and improving the accuracy of related systems.

Modeling of the Variability of D‐Region Ionospheric Electron Density During Solar Cycle‐24

AbstractSolar cycle variation of earth's atmosphere, particularly the ionosphere is of particular interest in the field of space science and space weather studies. In this article, we present the outcome of our detailed quantitative study on solar cycle variation of lower ionospheric properties using numerical investigation. First, we seek to model and compare the collective D‐region ionization rates (’s) due to the individual contributions from the ionizing sources, namely, (a) solar extreme ultraviolet (EUV) radiation including the Lyman‐ irradiation and (b) solar X‐ray irradiation throughout the solar cycle (C24). Then, we compute the electron density profiles using the ionization rates. We report significant solar cycle variation in ionization rate and electron density profiles for the entire span of C24. We use the sunspot number (SPN) profile to substantiate the finer details of profile during C24. For the segments of the D‐region above two different geographic coordinates, we report that profiles show a consistent “dual peak” nature. It is very similar to the SPN profile during C24. As a next‐order validation, we compare our modeled profiles with their IRI‐2020 counterpart (s). Their overall trends are found to be in agreement. Finally, we discuss the response of the D‐region in terms of due to C24. The work lays the foundation for our upcoming studies on D‐region response to solar cycle variation with Very Low Frequency (VLF) observation.

Influence of autonomic nervous dysfunction on eating during hemodialysis sessions: An observational study.

Although some studies have indicated that eating during hemodialysis may induce hypotension and cardiovascular events, some patients still consume food during their treatment. This prospective study was conducted to determine whether the need to eat during hemodialysis treatment was related to abnormal glucose metabolism and autonomic nerve dysfunction. Seventy patients were enrolled in this study, and their demographic features and various laboratory parameters were analyzed. At each routine hemodialysis visit, predialysis, intradialysis, and postdialysis blood pressure measurements were systematically conducted. A 24-hour ambulatory electrocardiogram (ECG) was performed during the hemodialysis interval, and heart rate variability (HRV) values were calculated. Additionally, whether the patients ate during the hemodialysis treatments was recorded. Another 20 people who underwent physical examinations during the same period and were matched for sex and age were included in the control group. The HRV values of the hemodialysis patients were generally lower than those of the control group. Univariate analysis revealed significant differences in sex, age, calcium antagonist use, blood calcium levels, insulin levels, diastolic blood pressure (DBP) measurements, and HRV indices between hemodialysis patients who ate and those who did not eat during hemodialysis (P < .05), whereas there were no significant differences in diabetes status or in the hemoglobin, albumin, blood glucose and C-peptide levels (P > .05). Multivariate analysis revealed that low values for very low frequency (VLF) and postdialysis DBP were risk factors for fasting intolerance during hemodialysis treatments. Autonomic dysfunction may affect whether hemodialysis patients tolerate fasting during dialysis. VLF evaluation may provide information that can be used to develop a more reasonable intradialytic nutritional supplementation method.

Dysfunction of the autonomic nervous system in gastro-esophageal reflux disease: Consequences for the cardiovascular system

ObjectiveThe pathophysiology of gastro esophageal reflux disease (GERD) implicates autonomic dysregulation of the lower esophageal sphincter tone. Our goal is to investigate whether this dysregulation of the autonomic nervous system (ANS) function observed in isolated GERD cases can affect other systems, such as cardiovascular regulation.Methods: Twenty-five participants were included in the study, 11 patients with isolated GERD and 14 controls. All patients and 7 controls responded to a COMposite Autonomic Symptoms Score 31 (COMPASS 31) questionnaire and underwent functional explorations including EMLA test, sympathetic skin response (SSR), 24-hour heart rate recording and ambulatory blood pressure measurement (ABPM). Seven additional controls underwent a 24-hour heart rate recording only. ResultsGERD patients (Age: mean 36.81±7.82; SR= 0.22) showed high clinically dysautonomic scores (COMPASS 31) (p = 0.015), increased Heart rate variability (HRV) parameters (daytime, nighttime, 24-hour SDNN (standard deviation of the RR interval (NN)), respectively p = 0.003, p < 0.001, p = 0.001; daytime and nighttime very low frequencies (VLF) respectively p = 0.03 and p = 0.007), impaired nocturnal dipping of blood pressure (3/11 patients) and high positivity of EMLA test (7/11, p = 0.037). These outcomes were strongly correlated with clinical dysautonomic assessment. No difference was observed between patients and controls regarding SSR. ConclusionOur data suggests a high parasympathetic tone amongst patients with GERD and a dysregulation of parasympathetic and sympathetic balance in the cardiovascular system with an impairment of the peripheral sympathetic fibers of cutaneous microcirculation, assessed by the EMLA test. GERD may be an inaugural symptom of autonomic neuropathy. Further functional exploration of peripheral small fibers seems to be necessary.

Performance Analysis and Optimization of Switch Device for VLF Communication Synchronous Tuning System Based on Coupled Inductors

The very low frequency (VLF) communication system is characterized by a limited transmit bandwidth. Due to the low operating frequency, the dimensions of VLF antennas are significantly smaller than the corresponding wavelength. Therefore, VLF antennas are considered electrically small antennas (ESAs) with high Q values and narrow bandwidth. To achieve broadband VLF communication, synchronous tuning technology is commonly employed. In this study, we focus on analyzing the performance of the switching device in the synchronous tuning system using coupled inductors and IGBT as core components. By considering the equivalent circuit of the controlled source associated with coupled inductors, we propose an adaptive input impedance optimization algorithm based on the variable capacitor (hereinafter referred to as VC-ADIO) to address issues arising from coupling coefficient variations and internal parameters within IGBTs that affect primary loop input impedance. The radiated power of the VLF antenna is improved effectively.

Spectra of GRB 221009A at Low Energies Derived from Ground-based Very Low-frequency Measurements

The gamma-ray burst (GRB) event GRB 221009A was the brightest event that has ever been detected to date. Owing to its unexpected brightness, the temporal and/or spectral information of the prompt emission cannot be accurately measured by many satellites (with the only exception of GECAM-C), since they suffered from significant pulse pileup and data saturation effects. Similarly, the X45 solar flare event occurring on 2003 November 4 saturated space-borne X-ray detectors, and it was through ground-based measurements of very low-frequency (VLF) signals that the magnitude of this event was determined, since VLF signals are particularly sensitive to the disturbance on the D-region ionosphere caused by low-energy photons. Therefore, in this study, we first report measurements of VLF signals from the JJI and VTX transmitter as recorded in Shiyan, China, when GRB 221009A occurred. The amplitude change was ∼1.25 and ∼2.31 dB for the JJI and VTX transmitter, respectively. Using a suite of well-validated models, we have further simulated the influence on the D-region ionosphere induced by low-energy photons (<100 keV) of GRB 221009A. Compared with the pre-GRB condition, the electron density was enhanced by 39.75% and 626.61% at 60 and 70 km altitude for the VTX-SYS path and 39.73% and 621.11% at 60 and 70 km altitude for the JJI-SYS path, respectively, with the altitude of notable electron density change being as low as ∼30 km. Moreover, we have compared modeling results of VLF signal change with our measurements during GRB 221009A. The good agreements obtained in terms of amplitude change and overall trend validate the fluxes and spectra of GRB 221009A at low energies (<20 keV) as measured by GECAM-C.

First observations of the transient luminous event effect on ionospheric Schumann resonance, based on the China Seismo-Electromagnetic Satellite

Abstract. In this study, we focus on the interactions and interrelated effects among the transient luminous events (TLEs) recorded at the Luoding ground station (22.76° N, 111.57° E), the lightning activities observed by the World Wide Lightning Location Network, and the ionospheric electric field deduced from the China Seismo-Electromagnetic Satellite (CSES). The results show that on 25 September 2021, the signal-to-noise ratio of the Schumann resonance at the first mode of 6.5 Hz and the second mode of 13 Hz dropped below 2.5 during the TLEs. A significant enhancement in the energy in extremely low frequencies (ELFs) occurred, and the power spectral density increased substantially. Distinct lightning whistler waves were found in the very low frequency (VLF) band, further indicating that the energy could possibly have been excited by the lightning. Our results suggest that the observations of the electric field from the satellite could possibly be utilised to monitor the lower-atmospheric lightning and its impact on the space environment.

Integration of Geophysical and Hydrochemical Methods to Determine the Groundwater Contamination in Al-Hosayniat Landfill South East Mafraq City, Jordan

An integration of Geophysical – Electromagnetic and hydro-chemical methods was used to investigate the potential of groundwater contamination due to Al-Hosayniat Municipal Landfill (HL) southeast of Mafraq city. Twenty-five Time Domain Electromagnetic (TDEM) soundings were carried out inside and outside the HL in addition to one WNW-ESE Very Low Frequency (VLF) profile. The TDEM's results help to define the subsurface geology and structures down to 240m depth. The results also depict the presence of a major fault striking NW-SE that crosses the whole landfill, this fault is documented in the surface geological map, and it was delineated and imaged by TDEM measurements and detected at larger depths. VLF results also detected the presence of a major fault beneath the landfill. The presence of the fault just beneath the HL may enhance the leachate movement which can imply groundwater contamination hazards for a longer period as it is a preferential pathway for contaminants. The results of water quality analyses of adjacent wells show higher concentrations of Na+, SO-24, TDS in some wells and this can be ascribed to agricultural activities and over-pumping in the area rather than landfill leachate. The heavy metal concentrations were found to be within the permissible limits and did't indicate direct contamination due to the HL. However, Fluorite (F-) shows a high concentration of (1.3mg/l) in WS4 and WS5 wells and approaches the permissible limit (1.5mg/l) according to the Jordan Water Standards (JWS). Bromide (Br) shows high concentration of (0.6mg/l) in well Al-2462.

Influence of shift work on sleep quality and circadian patterns of heart rate variability among nurses.

Shift work implementation is essential for providing continuous patient care in hospitals. However, working in shifts on a routine basis may disrupt the circadian pattern and alter the sleep-wakefulness cycle in nurses. Stress due to shift work can influence the adaptability of the cardiovascular system, produce psychophysiological strain and deteriorate work performance in female nurses. This study investigated the effect of morning and night shift work on sleep quality and circadian patterns governing heart rate variability (HRV) in female nurses working in a tertiary care hospital. Thirty-eight healthy female nurses were recruited. Frequency and time domain parameters of HRV were recorded as markers of cardiac autonomic function. A student t-test was used to investigate differences in HRV between morning and night shift workers. Mann-Whitney non-parametric test was applied for the difference between Pittsburgh Sleep Quality Index (PSQI) scores in the two groups. Standard deviation of the normal-to-normal interval (SDNN) (msec), total power (ms2) and high-frequency (HF) band power (ms2) were significantly reduced in night shift nurses than in morning shift nurses. The low-frequency (LF)/HF ratio was significantly increased in night shift nurses. The differences in standard deviation of the averages of NN intervals (SDaNN) (msec), root mean square of successive differences between adjacent NN intervals (RMSSD), mean NN, very low-frequency (VLF) band power (ms2) and LF band power (ms2) were not statistically significant. The global PSQI score was significantly higher among night shift workers than in morning shifts. Inadequate sleep can disrupt the body's ability to regulate heart rhythm and increase the risk of cardiovascular diseases and mortality. The research suggests a propensity for autonomic imbalance in night shift workers when compared to their counterparts on morning shifts.

Algorithms and Resources for the Monitoring of Very-Low-Frequency Signal Deviations Due to Solar Activity Using a Web-Based Software-Defined Radio-Distributed Network.

This work presents the development and testing of an experimental web-based SDR (software-defined radio) monitoring system for indirect solar activity detection, which has the ability to estimate and potentially predict various events in space and on earth, including solar flares, coronal mass ejections, and geomagnetic storms. The proposed system can be used to investigate the effect of solar activity on the propagation of very-low-frequency (VLF) signals. The advantages and benefits of the given approach are as follows: increasing measurement accuracy and eventual solar activity identification by combining measurements from multiple spatially distributed SDRs. The verification process involves carrying out several experiments comparing data from the GOES satellite system and the Dunksin SuperSID system with information received by the SDR monitoring system. Then, utilizing Pearson correlation coefficients, the measured data from the SDRs, along with those from the GOES satellite system and the Dunsing monitoring station, are investigated. At the time of a solar flare, the correlation value is above 90% for most of the stations used. Combining the signal-to-noise ratio via summation also shows an improvement in the results, with a correlation above 98%.

Exploring Ultra-short Heart Rate Variability Metrics in Patients with Diabetes Mellitus: A Reliability Analysis.

Ultra-short heart rate variability (HRV) metrics represent autonomic tone parameters derived using small epochs of interbeat interval data. These measures have risen in popularity with the advent of wearable devices that can capture interbeat interval data using electrocardiography (ECG) or photoplethysmography. Autonomic neuropathy in diabetes mellitus (DM) is well established, wherein 5-min HRV is conventionally used. Ultra-short measures have the potential to serve as markers of reduced autonomic tone in this patient population. Data of patients with Type I and Type II DM who had presented to our laboratory for autonomic neuropathy assessment were chosen for analysis. One-minute and 2-min epochs were chosen from 5 min of ECG data using standard software. Time domain, frequency domain, and nonlinear measures were computed from 1 to 2 min epochs, and reliability was compared with measures derived from 5-min HRV using intraclass correlation coefficients (ICCs). Data of 131 subjects (79 males, 52 females; mean age = 53.3 ± 12.16 years) were analyzed. All ultra-short HRV measures derived from 1 min to 2 min data showed good to excellent reliability (median ICC values ranging from 0.83 to 0.94) when compared with 5-min metrics. The notable exception was very low frequency (VLF) power, which showed poor reliability (median ICC = 0.43). Ultra-short HRV metrics derived from 1 to 2 min epochs of ECG data can be reliably used as predictors of autonomic tone in patients with DM. VLF power is poorly reproducible in these small epochs, probably due to variability in respiratory rates. Our findings have implications for ultra-short HRV estimation using short epochs of ECG data.

Inter-comparison of World Wide Lightning Location Network (WWLLN) and Lightning Detection Sensor Network (LDSN) data over India

Atmospheric lightning is a dominant weather related disaster which requires continuous monitoring efforts. While the world wide lightning location network (WWLLN) measures the very low frequency (VLF) signals emanating from the lightning, other regional networks, such as Lightning Detection Sensor Network (LDSN) use the broad band radio frequency signals. Present investigation compares the cloud-to-ground (CG) lightning occurrences recorded by the WWLLN and LDSN during 02 October – October 09, 2022 over India. Results show that though the WWLLN monitored the CG lightning occurrences about 20% lesser than the LDSN, spatial distribution of the occurrences and trends were in very good agreement.

Resonance behaviour of magnetoelectric (PSLZT multilayer - NZFO) layered composites: A comprehensive theory – Experimental perspective

In recent years, compact antenna and miniaturized power transfer devices are among the interesting applications of magnetoelectric (ME) composites in low frequency (LF) / very low frequency (VLF) range. Here we elucidate, in general, analytical calculations used to understand, the vibrational modes and resonance frequencies of the layered ME composite and; Finite Element Analyses (FEM) with isotropic and anisotropic properties to accurately predict the resonance frequencies pertaining to the axisymmetric and asymmetric mode of vibrations with respect to significant ME response. Further, ME coefficient of the composite with Longitudinal - Transverse (L-T) configuration based on the interface coupling model were utilized to completely study the ME composites. The configuration optimized through the design was in particular corroborated experimentally with a representative PSLZT multilayer on NZFO based ME layered composite system. The PSLZT multilayer on NZFO based ME composite was fabricated by tape casting the constituent thick films and co-sintering process which are further analysed for their ME responses at their resonance frequencies. ME voltage spectrum showed a maximum of 7.74 V/cm.Oe at its axisymmetric resonance mode at 30.6 kHz. Various peaks at different resonance frequencies with respect to axisymmetric and asymmetric modes of vibrations agree well and validates the above modeling studies. Overall, the complete study provides a systematic design guideline for optimizing the ME composite configuration suitable for specific application with required resonance frequency.

Examining the Capability of the VLF Technique for Nowcasting Solar Flares Based on Ground Measurements in Antarctica

Measurements of Very-Low-Frequency (VLF) transmitter signals have been widely used to investigate the effects of various space weather events on the D-region ionosphere, including nowcasting solar flares. Previous studies have established a method to nowcast solar flares using VLF measurements, but only using measurements from dayside propagation paths, and there remains limited focus on day–night mixed paths, which are important for method applicability. Between March and May of 2022, the Sun erupted a total of 56 M-class and 6 X-class solar flares, all of which were well captured by our VLF receiver in Antarctica. Using these VLF measurements, we reexamine the capability of the VLF technique to nowcast solar flares by including day–night mixed propagation paths and expanding the path coverage in longitude compared to that in previous studies. The amplitude and phase maximum changes are generally positively correlated with X-ray fluxes, whereas the time delay is negatively correlated. The curve-fitting parameters that we obtain for the X-ray fluxes and VLF signal maximum changes are consistent with those in previous studies for dayside paths, even though different instruments are used, supporting the flare-nowcasting method. Moreover, the present results show that, for day–night mixed paths, the amplitude and phase maximum changes also scale linearly with the logarithm of the flare X-ray fluxes, but the level of change is notably different from that for dayside paths. The coefficients used in the flare-nowcasting method need to be updated for mixed propagation paths.

Crafting very low frequency magnetoelectric antenna via piezoelectric and electromechanical synergic optimization strategy

A boom in exploration for marine geology and ocean resources has resulted in a huge demand for radio navigation or special environment communications, in turn spurring the rapid development of portable underwater wireless communication technology. State of the art acoustic communication methods used today are plagued by substantial transmission delays, multipath effects, and doppler frequency shifts, among other challenges, thus impeding the advancement of underwater wireless communication technology. Low-frequency electromagnetic transmission has proven to be a prospective solution for underwater communication, but the conventional electrical antennas is too large for portable underwater wireless communication. Emergent magnetoelectric (ME) antennas driven by piezoelectric materials have become a promising solution for miniaturizing very low frequency (VLF) communication systems. Here, a theoretical model between the radiation performance and piezoelectric material properties of the ME antenna was conducted. Guide by the theory analysis, Pb(In1/2Nb1/2)O3-Pb(Mn1/3Sb2/3)O3-Pb(Zr0.49Ti0.51)O3 (PIN-PMS-PZT) piezoelectric ceramic simultaneous with high d33 and Qm (d33 ∼ 401, Qm ∼ 1510) has been designed to enhance the magnetoelectric radiation of the VLF ME antenna. The PIN-PMS-PZT based ME antenna achieves a large converse magnetoelectric response 1.78 Gs·cm/V in EMR, which is almost doubled to commercial PZT based ME antenna. More importantly, a VLF communication system was built based on the VLF antenna, which successfully transmitted digital signals using Amplitude-Shift-Keying (ASK) modulation. It is believed that the presented work could provide a theoretical basis and feasible technical path for the employment of ME antennas in the future.

Estimation of the Ionospheric D‐Region Ionization Caused by X‐Class Solar Flares Based on VLF Observations

AbstractIn this paper, we study the ionization‐recombination processes in the lower ionosphere during solar flares of various classes in June 2014 and September 2017. For the first time, ionization and recombination rates of the ionospheric D‐region were estimated using the experimental data on variations in the amplitude and phase characteristics of very low frequencies (VLF) signals and two‐channel data from the GOES satellite (0.05–0.4 nm and 0.1–0.8 nm). The empirical two‐parameter Wait‐Ferguson model was used to calculate temporal changes in the electron concentration Ne during solar flares of different classes. GOES satellite data and black body model were used to estimate the X‐ray fluxes in the wavelength spectral range λ ≤ 0.3 nm. Joint analysis of the temporal evolution of the Ne vertical profile in the lower ionosphere and X‐ray fluxes in different wavelength ranges was carried out. As a result, the values of the ionization and recombination rate coefficients were obtained, and the spectral ranges of radiation that have the greatest impact on Ne at given heights were determined. Calculated ionization and recombination rate coefficients and ranges were successfully verified using VLF data during different solar flares. The results obtained in this work can be used in future studies for a more accurate assessment of the response of lower ionosphere to solar flares of various classes.