Ultra-low Frequency Research Articles

A Case Study of Pc1 Waves Observed at the Polar Cap Associated with Proton Precipitation at Subauroral Latitudes

The importance of ElectroMagnetic Ion Cyclotron (EMIC) ultra-low-frequency (ULF) waves (and their Pc1 counterparts) is connected to their critical role in triggering energetic particle precipitation from the magnetosphere to the conjugated ionosphere via pitch angle scattering. In addition, as a prominent element of the ULF zoo, EMIC/Pc1 waves can be considered a perfect tool for the remote diagnosis of the topologies and dynamic properties of near-Earth plasmas. Based on the availability of a comprehensive set of instruments, operating on the ground and in the top-side ionosphere, the present case study provides an interesting example of the evolution of EMIC propagation to both ionospheric hemispheres up to the polar cap. Specifically, we report observations of Pc1 waves detected on 30 March 2021 under low Kp, low Sym-H, and moderate AE conditions. The proposed investigation shows that high-latitude ground magnetometers in both hemispheres and the first China Seismo-Electromagnetic Satellite (CSES-01) at a Low Earth Orbit (LEO) detected in-synch Pc1 waves. In strict correspondence to this, energetic proton precipitation was observed at LEO with a simultaneous appearance of an isolated proton aurora at subauroral latitudes. This supports the idea of EMIC wave-induced proton precipitation contributing to energy transfer from the magnetosphere to the ionosphere.

Probing Polymorphic Stacking Domains in Mechanically Exfoliated Two-Dimensional Nanosheets Using Atomic Force Microscopy and Ultralow-Frequency Raman Spectroscopy.

As one of the key features of two-dimensional (2D) layered materials, stacking order has been found to play an important role in modulating the interlayer interactions of 2D materials, potentially affecting their electronic and other properties as a consequence. In this work, ultralow-frequency (ULF) Raman spectroscopy, electrostatic force microscopy (EFM), and high-resolution atomic force microscopy (HR-AFM) were used to systematically study the effect of stacking order on the interlayer interactions as well as electrostatic screening of few-layer polymorphic molybdenum disulfide (MoS2) and molybdenum diselenide (MoSe2) nanosheets. The stacking order difference was first confirmed by measuring the ULF Raman spectrum of the nanosheets with polymorphic stacking domains. The atomic lattice arrangement revealed using HR-AFM also clearly showed a stacking order difference. In addition, EFM phase imaging clearly presented the distribution of the stacking domains in the mechanically exfoliated nanosheets, which could have arisen from electrostatic screening. The results indicate that EFM in combination with ULF Raman spectroscopy could be a simple, fast, and high-resolution method for probing the distribution of polymorphic stacking domains in 2D transition metal dichalcogenide materials. Our work might be promising for correlating the interlayer interactions of TMDC nanosheets with stacking order, a topic of great interest with regard to modulating their optoelectronic properties.

Dayside Pc2 Waves Associated With Flux Transfer Events in a 3D Hybrid‐Vlasov Simulation

AbstractFlux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation.

Comparative oncology chemosensitivity assay for personalized medicine using low-coherence digital holography of dynamic light scattering from cancer biopsies

Nearly half of cancer patients who receive standard-of-care treatments fail to respond to their first-line chemotherapy, demonstrating the pressing need for improved methods to select personalized cancer therapies. Low-coherence digital holography has the potential to fill this need by performing dynamic contrast OCT on living cancer biopsies treated ex vivo with anti-cancer therapeutics. Fluctuation spectroscopy of dynamic light scattering under conditions of holographic phase stability captures ultra-low Doppler frequency shifts down to 10 mHz caused by light scattering from intracellular motions. In the comparative preclinical/clinical trials presented here, a two-species (human and canine) and two-cancer (esophageal carcinoma and B-cell lymphoma) analysis of spectral phenotypes identifies a set of drug response characteristics that span species and cancer type. Spatial heterogeneity across a centimeter-scale patient biopsy sample is assessed by measuring multiple millimeter-scale sub-samples. Improved predictive performance is achieved for chemoresistance profiling by identifying red-shifted sub-samples that may indicate impaired metabolism and removing them from the prediction analysis. These results show potential for using biodynamic imaging for personalized selection of cancer therapy.

A voltage mode grounded capacitance multiplier with widely tunable gain for ultra-low cutoff frequency filter.

A voltage mode capacitance multiplier for ultra-low frequency physiological signal processing is designed with a circuit model. With the proposed multiplier, a filter can achieve a cutoff frequency of 12 mHz with a 1 pF basic capacitance and a 10 kΩ resistor. The corresponding multiplication factor will be 1.35 × 109. By changing the controlling terminal, the multiplication factor can be widely tuned from 1950 to 1.35 × 109 and the corresponding filter cutoff frequency will be from 12 mHz to 8.15 kHz. According to the circuit model, to further increase the multiplication factor to decrease the chip area, more multiplication stages can be added to the feedback loop.

Ultra-Low Frequency Vibration Isolation of Cockroach-Inspired Structures With Electromagnetic Shunt Damping Enhanced by Geometric Nonlinearity

Ultra-Low Frequency Vibration Isolation of Cockroach-Inspired Structures With Electromagnetic Shunt Damping Enhanced by Geometric Nonlinearity

Low-Power Capacitively Coupled AC Amplifiers With Tunable Ultra Low-Frequency Operation

Low-Power Capacitively Coupled AC Amplifiers With Tunable Ultra Low-Frequency Operation

Investigating ultra-low frequency emissions and total electron content anomalies as earthquake precursors in Sumatra (2019–2020)

Ultra-low frequency (ULF) emissions and total electron content (TEC) anomalies are potential earthquake precursors for short-term earthquake prediction. This study comprehensively examines ULF emissions and TEC anomalies associated with the Sumatra earthquakes spanning 2019–2020. Precursor ULF emissions are identified through the polarization power ratio calculations involving Z-component and H-component geomagnetic data, meticulously recorded by the Magnetik Acquisition Data System (MAGDAS). Employing the Single Station Function Transfer technique, ULF anomalies directed toward the epicenter of earthquakes are recognized as reliable precursors. Among the 29 earthquakes scrutinized, 25 were heralded by ULF emissions acting as precursors. Among these, 15 instances solely exhibited ULF emissions, unaccompanied by TEC anomalies, while the remaining ten were linked to TEC anomalies. The investigation of TEC anomalies employs advanced autocorrelation techniques rooted in the analysis of correlation coefficient deviations. TEC anomalies are meticulously acquired from the observations of the Sumatran GPS Array (SuGAr) stations. Furthermore, this study illuminates a positive correlation (R = 0.65) between the lead time of ULF emissions and TEC anomalies. Moreover, the frequency of ULF emissions and TEC anomalies also showcases a direct relationship with earthquake magnitude. Heightened frequencies of ULF emissions and TEC anomalies as earthquake precursors correspond to more significant magnitudes in ensuing earthquakes. The outcomes of this research significantly augment the understanding of employing ULF emissions and TEC anomalies as effective earthquake precursors in the context of Sumatra.

Theoretical and experimental research on a Quasi-Zero-Stiffness-Enabled nonlinear piezoelectric energy harvester

Piezoelectric material provides a convenient and efficient avenue to convert ambient vibration energy into electrical energy to power wireless sensor network nodes of Internet of Things. However, the design of effective ultralow-frequency and low-amplitude harvesters using piezoelectric materials remains a challenge. To address this challenging problem, this paper devises a quasi-zero-stiffness-enabled (QZSE) piezoelectric energy harvester based on a geometrical nonlinear structure. The basic configuration and operation principles of the QZSE piezoelectric energy harvester ware elaborated firstly, then the nonlinear electromechanical coupling equation is derived, whose solution informs on both the dynamic and electrical responses of the harvester. The electromechanical-coupled equations are solved with the aid of the fourth-order Runge-Kutta algorithm. The energy harvesting performance is then investigated with respect to the parameter effects of excitation, damping and proof mass. For validating numerical results and demonstrating the advantages of the proposed design, a prototype is fabricated and experimentally tested. The results confirm the ability of the QZSE piezoelectric energy harvester in producing effective vibration-to-electricity energy conversion in ultralow frequency range. This study provides new impetus to energy harvesting from ultralow frequency ambient vibration using piezoelectric materials.

Dynamic change of vestibular function and the long-term prognosis of vestibular neuritis.

To comprehensively evaluate the dynamic change of vestibular function during long-term follow-up of vestibular neuritis, as well as the co-relationship with the outcomes of vestibular neuritis (VN), which provides the recommendations for vestibular function tests during the course of VN. A prospective cohort study was conducted on 16 patients with acute VN. Caloric test, vHIT, rotatory chair tests, VEMP, dizziness handicap inventory (DHI) score, and dynamic dizzy scales (VAS-DD) was first performed within 7 days of neuritis onset, which were further re-evaluated during the 6-12 months of follow-up. The dynamic changes on multiple objective vestibular examinations were analyzed during the acute and recovery stage of VN. We further evaluated the co-relationship between the vestibular dysfunction scales and the prognosis of VN. In more than 6 months of follow-up, 44% of the ultralow frequency, 94% of the low-to-mid frequency, and 44% of the high-frequency function of the horizontal semicircular canal returned to normal (p < 0.05). The change degree in symmetry of the rotatory chair test was correlated with the gain of the horizontal semicircular canal on the vHIT and the unilateral weakness (UW) value on the caloric test (p < 0.05). The change in DHI score was correlated with the phase; change in VAS-DD level correlated with the symmetry and TC of the rotatory chair test at VN recovery stage (p < 0.05). There was no significant correlation between the change in DHI score or change in VAS-DD and the degree of vestibular function recovery (p > 0.05). In general, vestibular function improved during the course of VN. The rotatory chair test can be used to evaluate the overall function of the vestibular system and the compensatory state in patients with VN.

Spectroscopic and imaging considerations of THz-TDS and ULF-Raman techniques towards practical security applications.

Nitrogen-containing high-energy organic compounds represent a class of materials with critical implications in various fields, including military, aerospace, and chemical industries. The precise characterization and analysis of these compounds are essential for both safety and performance considerations. Spectroscopic characterization in the far-infrared region has great potential for non-destructive investigation of high energetic and related compounds. This research article presents a comprehensive study of common organic energetic materials in the far-infrared region (5-200 cm-1), aiming to enhance security measures through the utilization of cutting-edge spectroscopic techniques. Broadband terahertz time-domain spectroscopy and ultra-low frequency Raman spectroscopy are employed as powerful tools to probe the vibrational and rotational modes of various explosive materials. One of the key objectives of this present work is unveiling the characteristic spectral features and optical parameters of five common nitrogen based high energy organic compounds towards rapid and accurate identification. Further, we have explored the potential of terahertz reflection imaging for non-contact through barrier sensing, a critical requirement in security applications. Based on the spectral features obtained from the spectroscopic studies and using advanced imaging algorithms we have been able to detect these compounds under various barriers including paper, cloth, backpack, etc. Subsequently, this study highlights the capabilities of the two techniques offering a pathway to enhance their utility over a wide range of practical security applications.

Customized quasi-zero-stiffness metamaterials for ultra-low frequency broadband vibration isolation

Quasi-zero-stiffness (QZS) metamaterials play an important role in ultra-low frequency vibration mitigation. In this study, a customized QZS metamaterial is proposed to isolate ultra-low frequency broadband vibration and its isolation performance is verified. A representative unit cell (RUC) that satisfies both optimal QZS performance and minimum size is designed for the metamaterial and optimized by matching the parameters of positive stiffness heart-shaped beam with the properties of negative stiffness cosine beam. Then, a new composite structure with multiple RUCs is designed by matching the optimal mass block for each RUC to meet the vibration isolation requirements of ultra-low frequency and broadband. The transmissibility test is performed, and the isolator made of the metamaterial is then integrated into an underwater glider to test the vibration isolation performance. The results show that the proposed QZS metamaterial can isolate broadband vibrations of 3.2 to 60 Hz and the isolator can attenuate the vibration signals of gliders to the background noise in air and water. Therefore, the customized metamaterial can be used in the engineering case of ultra-low frequency broadband vibration isolation and the design method of the metamaterial can be used for reference by other equipment.

DYNAMIC SPECTRAL ANALYSIS OF HUMAN BREATHING AND HEARTBEAT BY RESCUER RADARS

Specialized rescuer radars, employed for detecting living individuals during rescue operations amid natural or man-made disasters, operate by estimating the nonstationary spectral characteristics of reflected signals induced by breathing and heartbeats. The nonstationary spectra resulting from Doppler phase shifts in reflected radar signals are attributed to the breathing and heartbeat processes of living humans. These phase fluctuations occur at ultra-low frequencies and can be effectively detected using extended pseudo-noise probing signals, such as those based on Mersenne codes. The method for tracking the power spectrum of a process, where the rates of change in the correlation function at various lags differ over time, is presented. The method is discussed in terms of its application with finite-length correlation processes. It allocates a short memory for rapidly varying terms in the correlation while concurrently employing longer-term averaging for the more stationary terms. Time-varying spectral estimates of rescuer radar signals are derived through the processing of resulting correlation estimates of signals reflected from a living human body. Examples of real spectrum estimates are provided to illustrate that improved tracking can be achieved compared to employing a constant lag-invariant averaging method.

Tree Dielectric Response as an Earthquake Forecasting

Seismic precursor identification is vital in the Himalayan range and near active fault lines to offer early earthquake warnings, as India and nearby areas face a high risk of seismic activity. This laboratory-based research conducted simulations aimed at replicating electromagnetic signals generated by artificial seismic events spanning frequencies from 1 Hz to 59 Hz. High-frequency seismic signal attenuation upon interaction with the Earth's surface was observed, focusing on emulating electromagnetic waves within the ultra-low frequency (ULF) to very high frequency (VHF) range and investigating energy transfer dynamics within conductive materials. Findings included a reduction in green stem ring diameter, an inversely correlated penetration depth with frequency, and fluctuations in the dielectric constant of tree tissues, linked to water molecule polarization with frequency changes. By applying an electric field, lower dielectric constants were observed at lower frequencies, stabilizing at around 40 eV as frequency increased from 1 Hz to 59 Hz. As frequency levels escalated, especially transitioning into the radio or microwave spectrum, alterations in the dielectric constant of tree tissues were noted, often marked by an increase in dielectric loss, signifying heightened absorption of electromagnetic energy and its conversion into heat. Banyan tree roots exhibited responses well in advance of any discernible alterations in the subsurface environment, offering valuable early indicators of impending seismic activity. This discovery encourages further exploration and holds the potential to advance earthquake prediction using deep-rooted trees as biosensors, ultimately contributing significantly to the preservation of human lives and overall safety.

Ultra-Low Phase Noise Frequency Division With Array of Direct Digital Synthesizers.

In this article, we present a four-channel direct digital synthesis (DDS) design that operates with a common clock ranging from 500 MHz to 24 GHz and generates output frequencies up to 1.75 GHz. A key feature of this board is its custom field-programmable gate array (FPGA)-based synchronization method, which ensures alignment accuracy of 170 ps between the channels, enabling precise frequency and phase relationship settings. In addition, the DDS board incorporates a user-friendly web interface that allows for continuous control and monitoring capabilities over TCP/IP. Multiple synchronized channels can be power-combined to produce a low-phase noise output due to coherent addition of the common carriers and the noncoherent addition of the residual DDS noise. By exploiting these principles and combining eight parallel channels of two DDS boards, we achieve exceptional residual phase noise performance, with and for a 9.765625 MHz output signal. These noise levels surpass the previously reported results achieved with regenerative frequency dividers. We also present a method for obtaining accurate residual noise measurements using an absolute phase modulation (PM) noise and amplitude modulation (AM) noise nalyse. Furthermore, we nalyse the phase alignment tolerances required to minimize the AM-to-PM and PM-to-AM conversion that commonly occurs in power-combined signals. Finally, we demonstrate the synthesis of a highly stable 9.765625 MHz signal obtained from a cavity-stabilized optical frequency comb (OFC), with an absolute white phase noise of -180 dBc/Hz.

A 5.4-7.4 GHz Ultra-Low Jitter Injection-Locked Frequency Tripler With 3rd Harmonic Current Boosting Input Buffer

A 5.4-7.4 GHz Ultra-Low Jitter Injection-Locked Frequency Tripler With 3rd Harmonic Current Boosting Input Buffer

On the importance of low-frequency modes in predicting pressure-induced phase transitions.

The occurrence of ultra-low frequency oscillation mode as observed by means of periodic DFT calculations at Γ point in a molecular crystal at ambient conditions can be a valuable predictor of imminent pressure-induced phase transition. We illustrate it with a series of polymorphs of diacetyl-pyrenes, for two of which such soft-modes indicated the trajectory of pressure-induced structural reorganization at ≈0.8 GPa. Notably, susceptibility to pressure-induced phase transformations could not be predicted based on material's thermodynamic characteristics, and there were no unequivocal clues in their crystal packing.

A Portable Cardiac Dynamic Monitoring System in the Framework of Electro-Mechano-Acoustic Mapping.

Abnormalities in cardiac function arise irregularly and typically involve multimodal electrical, mechanical vibrations, and acoustics alterations. This paper proposes an Electro-Mechano-Acoustic (EMA) activity model for mapping the complete macroscopic cardiac function to refine the systematic interpretation of cardiac multimodal assessment. We abstract this activity pattern and build the mapping system by analyzing the functional comparison of the heart pump and Electronic Fuel Injection (EFI) system from the multimodal characteristics of the heart. Electrocardiogram (ECG), seismocardiogram (SCG) & Ultra-Low Frequency seismocardiogram (ULF-SCG), and Phonocardiogram (PCG) are selected to implement the EMA mapping respectively. First, a novel low-frequency cardiograph compound sensor capable of extracting both SCG and ULF-SCG is proposed, which is integrated with ECG and PCG modules on a single hardware device for portable dynamic acquisition. Afterward, a multimodal signal processing chain further analyses the acquired synchronized signals, and the extracted ULF-SCG is shown to indicate changes in heart volume. In particular, the proposed method based on waveform curvature is used to extract 9 feature points of the SCG signal, and the overall recognition accuracy reaches over 90% in the data collected by EMA portable device. Ultimately, we integrate the portable device and signal processing chains to form the EMA cardiovascular mapping system (EMACMS). As a next-generation system solution for cardiac daily dynamic monitoring, which can map the mechanical coupling and electromechanical coupling process, extract multi-characteristic heart rate variability (HRV), and enable extraction of important time intervals of cardiac activity to assess cardiac function.

Preface Special Issue: Developments in Earthquake Precursors Studies

This Special Issue of Annals of Geophysics is dedicated to present recent developments in the field of earthquake (EQ) precursors. Due to their potentially catastrophic nature, EQs have early attracted the attention of humans. During the last decades, a considerable amount of scientific effort has been devoted to the study of EQ precursors, aiming at EQ forecast. Prediction of large EQs, especially in the short-term, is of outmost importance for our society, since it could prevent, or at least restrict, their disastrous consequences, both in terms of infrastructure / material damages and in terms of casualties. It is clear that EQ preparation processes are complex and multifaceted, which call for a multidisciplinary approach. Many possible precursors have been reported and studied during the last decades, which include (just to mention some): foreshock activity, pre-slip effect, surface deformation, seismic electric signals (SES), ultra-low frequency (ULF) magnetic field anomalies, fracture-induced electromagnetic emissions (FEME) (fracture-induced electromagnetic radiation, FEMR), ULF/ELF (extremely low frequency) atmospheric electromagnetic radiation, atmospheric anomalies (such as SLHF, OLR etc.) and ionospheric (lower and upper regions) anomalies, lithosphere-atmosphere-ionosphere (LAI) coupling, even abnormal animal behavior.

The influence of a weak ultra-low frequency electromagnetic field 2 and 8 Hz on the consolidation of daytime sleep

The article present the hypothesis that weak electromagnetic fields of ultra-low frequencies (0.5–26 Hz) could affect daytime sleep features and structure. Parameters of daytime sleep continuity were compared in the study with counterbalanced control/exposition (50 min exposure to electromagnetic field at 2 and 8 Hz / 0.004 μT) scheme in 14 healthy volunteers (8 men, 6 women, mean age 19.43 ± 1.22 y.o.). Nonlinear regression model was used to assess daytime sleep continuity. Exposure to a weak electromagnetic field of ultra-low frequency significantly improved the quality of sleep, assessed by the indicator of sleep continuity, namely, there were fewer transitions from the second and deeper stages of sleep to the first stage and to the state of wakefulness. The results can be used to develop non-pharmacological methods of sleep correction, as well as to improve the quality of short-term sleep and its positive effect on wellbeing, cognitive function and working capacity