Ultra-low Frequency Research Articles

Online Algorithm for Deriving Heart Rate Variability Components and Their Time–Frequency Analysis

Heart rate variability (HRV) containing four components of high (HF), low (LF), very low (VLF), and ultra-low (ULF) frequencies provides insight into the cardiovascular and autonomic nervous system functions. Classical spectral analysis is most often used in research on HRV and its components. The aim of this work was to develop and validate an online HRV decomposition algorithm for monitoring the associated physiological processes. The online algorithm was developed based on variational mode decomposition (VMD), validated on synthetic HRV with known properties and compared with its offline adaptive version AVMD, standard VMD, continuous wavelet transform (CWT), and wavelet package decomposition (WPD). Finally, it was used to decompose 36 real all-night HRVs from two datasets to analyze the properties of the four extracted components using the Hilbert transform. The statistical tests confirmed that the online VMD (VMDon) algorithm returned results of comparable quality to AVMD and CWT, and outperformed standard VMD and WPD. VMDon, AVMD, and CWT extracted four components from the real HRV with frequency content slightly exceeding the previously recognized ranges, suggesting the possibility of their modes mixing. Their ranges of variability were assessed as follows: HF: 0.11–0.40 Hz; LF: 0.029–0.14 Hz; VLF: 4.7–31 mHz; and ULF: 0.002–3.0 mHz.

Narrow-linewidth microcavity Brillouin laser based on pump-locked high-Q silica microsphere resonator

Microcavity-based Brillouin lasers are promising high-performance light sources for integrating photonics and optoelectronics. One method to lock the pump light frequency is to utilize a complex system with optoelectronic feedback, which requires a high-cost narrow-linewidth pump laser and limits the application of microlasers in integrated optoelectronic systems. Another method reported recently is all-optical feedback to achieve the locking of microcavity laser. We propose to utilize Rayleigh scattering of microcavities to lock the frequency of the pump laser to the resonant frequency of the Brillouin laser microcavity with the all-optical method. While compressing the linewidth of the pump laser, it can greatly improve the long-term stability of the optically pumped microcavity Brillouin laser. In the experiment, the linewidth of the semiconductor pump laser is compressed from the MHz level to the kHz level. The microcavity Brillouin laser achieves an ultra-narrow intrinsic linewidth of 100 Hz, with an ultra-low frequency noise of 35 Hz2/Hz. The constructed microlaser obtains a locking time up to 1 h, which does not require any temperature control or vibration isolation of the laser system. This work demonstrated an optically pump-locked microcavity Brillouin laser, which provides a stable and reliable low-cost experimental platform for ultra-narrow-linewidth lasers, precision laser sensors, microwave-photonic signal synthesizer, and optomechanical systems.

Foreshock Ultralow Frequency Waves at Mars: Consequence on the Particle Acceleration Mechanisms at the Martian Bow Shock

Foreshock Ultralow Frequency Waves at Mars: Consequence on the Particle Acceleration Mechanisms at the Martian Bow Shock

Excitation of ULF, ELF, and VLF Resonator and Waveguide Oscillations in the Earth–Atmosphere–Ionosphere System by Lightning Current Sources Connected with Hunga Tonga Volcano Eruption

The simulations presented here are based on the observational data of lightning electric currents associated with the eruption of the Hunga Tonga volcano in January 2022. The response of the lithosphere (Earth)–atmosphere–ionosphere–magnetosphere system to unprecedented lightning currents is theoretically investigated at low frequencies, including ultra low frequency (ULF), extremely low frequency (ELF), and very low frequency (VLF) ranges. The electric current source due to lightning near the location of the Hunga Tonga volcano eruption has a wide-band frequency spectrum determined in this paper based on a data-driven approach. The spectrum is monotonous in the VLF range but has many significant details at the lower frequencies (ULF, ELF). The decreasing amplitude tendency is maintained at frequencies exceeding 0.1 Hz. The density of effective lightning current in the ULF range reaches the value of the order of 10−7 A/m2. A combined dynamic/quasi-stationary method has been developed to simulate ULF penetration through the lithosphere (Earth)–atmosphere–ionosphere–magnetosphere system. This method is suitable for the ULF range down to 10−4 Hz. The electromagnetic field is determined from the dynamics in the ionosphere and from a quasi-stationary approach in the atmosphere, considering not only the electric component but also the magnetic one. An analytical/numerical method has been developed to investigate the excitation of the global Schumann resonator and the eigenmodes of the coupled Schumann and ionospheric Alfvén resonators in the ELF range and the eigenmodes of the Earth–ionosphere waveguide in the VLF range. A complex dispersion equation for the corresponding disturbances is derived. It is shown that oscillations at the first resonance frequency in the Schumann resonator can simultaneously cause noticeable excitation of the local ionospheric Alfvén resonator, whose parameters depend on the angle between the geomagnetic field and the vertical direction. VLF propagation is possible over distances of 3000–10,000 km in the waveguide Earth–ionosphere. The results of simulations are compared with the published experimental data.

Tracking changes in autonomic function by coupled analysis of wavelet-based dispersion of heart rate variability and gastrointestinal symptom severity in individuals with hypermobile Ehlers–Danlos syndrome

IntroductionPeople with hypermobile Ehlers–Danlos syndrome (hEDS) experience multisystemic dysfunction with varying severity and unpredictability of flare occurrence. Cohort studies suggest that individuals with hEDS have a higher risk for autonomic dysfunction. The gold standard for assessing autonomic function, clinically, is the heart rate variability (HRV) assessment from 24-h Holter monitor electrocardiogram data, but this is expensive and can only be performed in short durations. Since their advent, biometric devices have been a non-invasive method for tracking HRV to assess autonomic function. This study aimed to understand the intra- and inter-individual variability in autonomic function and to associate this variability with gastrointestinal symptoms in individuals with hEDS using wearable devices.MethodsWe studied 122 days of biometric device data from 26 individuals, including 35 days highlighted as high gastrointestinal (GI) dysfunction and 48 days as low GI dysfunction. Utilizing wavelet analysis to assess the frequency domains of heart rate signals, we compared participants’ HRV data for high, low, very low (VLF), and ultralow (ULF) frequency domains associated with physiological differences.ResultsWe found a significant difference between the VLF and ULF signals on high-GI symptom days compared with low-symptoms days for 92 and 76% of the signals sampled, respectively.DiscussionOur pilot data show a change in HRV for individuals with hEDS experiencing a flare day for a single-body system. Future research will focus on evaluating the relationship between longitudinal multisystemic symptom severity fluctuations and HRV.

Numerical simulation and parametric optimization of harmonic pulse water flooding technology

Pulse water injection technology is an emerging enhanced oil recovery method that improves oil-water interface and flow characteristics by periodically varying the injection rate. This study, using the TOUGH2 simulator, examines the impact of different pulse injection waveforms (sine, triangle, and square) on the recovery factor in medium to low-permeability carbonate reservoirs. It highlights that pulse water injection outperforms constant rate injection, with the triangle waveform yielding the best recovery rates. Through controlled simulations, findings indicate that increasing pulse injection time and amplitude enhances recovery and water flow. However, frequency variations in ultra-low frequency pulses have minimal effects on recovery. Additionally, the efficiency of pulse water injection is inversely related to reservoir porosity and permeability, while higher oil viscosity significantly boosts recovery efficiency. Conversely, increasing water temperature reduces dynamic viscosity, affecting the oil-water interface lag and decreasing overall displacement efficiency. The study also notes significant differences between heterogeneous and homogeneous reservoir models, emphasizing the need for future large-scale numerical modeling to consider reservoir heterogeneity. These findings provide valuable insights for optimizing pulse water injection in medium to low-permeability reservoirs, supporting practical oil field development strategies.

Study of a 2DOF bistable energy harvester with variable potential well for ultra-low frequency vibration energy harvesting.

A two-degree-of-freedom bistable energy harvester with a spring-magnet oscillator designed for ultra-low frequency vibration energy harvesting is presented in this paper. It combines magnetic plucking frequency upconversion and a variable potential function to achieve a high-efficiency response while also being suitably installed for applications with spatial limitations. A lumped parameter model of the piezoelectric energy harvester and the magnetic dipoles is applied to develop the theoretical model for the system. The prototype is fabricated, and experimental parameter investigations are undertaken to examine the effects of various factors, including magnetic distance, stiffness, and tip mass. The results indicate that at a frequency of 1.2Hz and an acceleration of 0.23g, the maximum output power reaches 264.6 µW at an optimal resistance of 250 kΩ.

Convolutional CFS-PML for the 2-D Crank-Nicolson FDTD Scheme and Its Application in Simulation of Ultra-Low Frequency Electromagnetic Problems

Convolutional CFS-PML for the 2-D Crank-Nicolson FDTD Scheme and Its Application in Simulation of Ultra-Low Frequency Electromagnetic Problems

An ultra low frequency spike timing dependent plasticity based approach for reducing alcohol drinking

Alcohol use disorder (AUD) is a chronic relapsing brain disorder characterized by an impaired ability to stop or control alcohol consumption despite adverse social, occupational, or health consequences. AUD affects nearly one-third of adults at some point during their lives, with an associated cost of approximately $249 billion annually in the U.S. alone. The effects of alcohol consumption are expected to increase significantly during the COVID-19 pandemic, with alcohol sales increasing by approximately 54%, potentially exacerbating health concerns and risk-taking behaviors. Unfortunately, existing pharmacological and behavioral therapies for AUD are associated with poor success rates, with approximately 40% of individuals relapsing within three years of treatment.Pre-clinical studies have shown that chronic alcohol consumption leads to significant changes in synaptic function within the dorsal medial striatum (DMS), one of the brain regions associated with AUD and responsible for mediating goal-directed behavior. Specifically, chronic alcohol consumption has been associated with hyperactivity of dopamine receptor 1 (D1) medium spiny neurons (MSN) and hypoactivity of dopamine receptor 2 (D1) MSNs within the DMS. Optogenetic, chemogenetic, and transgenic approaches have demonstrated that reducing the D1/D2 MSN signaling imbalance decreases alcohol self-administration in rodent models of AUD.Here, we present an electrical stimulation approach that uses ultra-low (≤ 1 Hz) frequency (ULF) spike-timing-dependent plasticity (STDP) in mouse models of AUD to reduce DMS D1/D2 MSN signaling imbalances by stimulating D1-MSN afferents into the GPi and ACC glutamatergic projections to the DMS in a time-locked stimulation sequence. Our data suggest that GPi/ACC ULF-STDP selectively decreases DMS D1-MSN hyperactivity leading to reduced alcohol consumption without evoking undesired affective behaviors using electrical stimulation rather than approaches requiring genetic modification. This work represents a step towards fulfilling the unmet need for a reliable method of treating severe AUD through cell-type-specific control with clinically available neuromodulation tools.

Alterations in Gut Microbiota Composition Are Associated with Changes in Emotional Distress in Children with Obstructive Sleep Apnea.

Emerging evidence underscores the pivotal role of the gut microbiota in regulating emotional and behavioral responses via the microbiota-gut-brain axis. This study explores associations between pediatric obstructive sleep apnea (OSA), emotional distress (ED), and gut microbiome alterations before and after OSA treatment. Sixty-six children diagnosed with OSA via polysomnography participated, undergoing adenotonsillectomy alongside routine educational sessions. ED was assessed using the OSA-18 questionnaire, categorizing participants into high ED (scores ≥ 11, 52%) and low ED (scores < 11, 48%) groups. Gut microbiome analysis revealed significant diversity differences, with high ED linked to a reduced Shannon index (p = 0.03) and increased beta diversity (p = 0.01). Three months post-treatment, significant improvements were observed in OSA symptoms, ED scores, and gut microbiome alpha diversity metrics among 55 participants (all p < 0.04). Moreover, changes in the relative abundances of Veillonella, Bifidobacterium, Flavonifractor, and Agathobacter, as well as ultra-low frequency power and low frequency power of sleep heart rate variability, were independently associated with ED score alterations. These findings underscore the gut microbiome's critical role in the emotional and behavioral symptoms associated with pediatric OSA, suggesting that microbiome-targeted interventions could complement traditional treatments for ED reduction and emphasizing the need for further research.

Management and quality assurance of observational data recorded by geomagnetic observatories in Egypt

The National Research Institute of Astronomy and Geophysics (NRIAG) is an Egyptian research institution that operates two Geomagnetic Observatories (GMOs), namely the Misallat (MLT), and Abu‑Simbel (ABS) observatories. Data availability, reliability and quality are the basis for any scientific research. So, it is essential to check and manage the quality of Geomagnetic Data (GMD) and prepare them in a proper Data Format (DF) before publishing them for research studies. This paper focuses on the management and quality assurance of the GMD recorded by the MLT and ABS observatories, and presents the current efforts deployed for managing, examining and processing them as well as converting their original DF into international DF. The raw data has been processed mathematically using MATLAB scripts. Results show that GMD from the MLT and ABS observatories suffer from various non‑natural disturbances. Electric power lines, industrial and human activities nearby the MLT observatory as well as some internal problems with the measuring devices, might have caused these disturbances. In addition, the study examined the impact of these disturbances on the quality of recorded data. The comparison of raw data from the two observatories and other INTERMAGNET data showed a good correlation between them before 2015. From 2015 until October 2022, the noise level mostly at the MLT became very high and the natural Ultra Low Frequency (ULF) emissions at the MLT data were completely hidden by the background noise. The study recommends that the Magnetically Clean Environment (MCE) must be resorted again as well as fixing the measuring instruments especially at the MLT observatory to provide high quality data that can be suitable for all aspects of scientific research and satisfy the INTERMAGNET requirements.

Recording of ultra-low frequency electromagnetic emission during loading of a rock sample

Recording of ultra-low frequency electromagnetic emission during loading of a rock sample

Optimal design of a novel nested metamaterial with hybrid auxetic-locally resonant band gap for suppressing robotic grinding vibration

To suppress vibration during robotic grinding of aero-engine blades caused by low stiffness and robotic point-to-point movement, a novel metamaterial comprising a double-nested auxetic structure and a local resonator is proposed. It simultaneously achieves enhanced-stiffness and an ultra-low frequency wide-range band gap generated by the hybrid auxetic-locally resonant characteristic. Young’s modulus is studied analytically and numerically. A co-simulation procedure is applied to obtain the optimized band gap (the bottom boundary is 18.03 Hz) based on the modal analysis results, verified through the experimental study. The novel design of the nested metamaterial provides an effective strategy for suppressing robotic grinding vibration.

Ultralow frequency waves in the South Atlantic anomaly: Application of FY-3E observations

Ultralow frequency waves in the South Atlantic anomaly: Application of FY-3E observations

Geomagnetic Storm and Space Weather Perturbation via Magnetometer Analysis

Abstract A geomagnetic storm is a significant disturbance of Earth’s magnetosphere that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth. Solar flares and Coronal Mass Ejection (CME) are the events that cause space weather perturbations, which can disrupt all types of electronic equipment. This study aims to determine the relation between the Ultra Low Frequency (ULF) detection via magnetometer with space weather perturbation. The potential of this study is promising, as it can help to understand more about space weather perturbation and develop safer precaution steps. SpaceWeatherLive.com website was used to find the dates of solar events, including solar flares and CME. Solar parameter readings which were solar wind speed (Vsw), dynamic pressure (kPa), symmetric (SYM) disturbance index for the H component, and the interplanetary magnetic field (Bz), were recorded from the OMNI website. Next, the SuperMAG website was used to extract the N and Z components of ULF data from six magnetometers in six stations near the Earth’s equator. Proper graphs were plotted using the data obtained from both Omniweb and SuperMag websites, which averaged for every minute over a period of three days. The graphs obtained were analysed statistically using Pearson method correlations. The results show values of more than +0.5 or less than -0.5 for the Pearson method, proving that the ULF detection method could be related to space weather. Results with a value less than +0.5 or more than -0.5 have a weak correlation between both ULF data and space weather perturbation. ULF data could be used to understand space weather conditions better and help organize safety measures or create safer equipment when dealing with space weather perturbations in the future.

A Novel Determination of the Foreshock ULF Boundary: Statistical Approach.

The location and spatial extent of the region populated by the foreshock waves depend on the IMF orientation. We performed a systematic statistical study of wave activity in the frequency range of Hz observed during an initial phase of the THEMIS mission. Wave activity is quantified by standard deviations of the IMF magnitude and its components over 10-min intervals. We apply the foreshock coordinate system defined as the angle between the bow shock normal and upstream magnetic field vectors and the distance from the spacecraft to bow shock along the magnetic field line. We have found that the Ultra-low Frequency (ULF) foreshock boundary (a) is well defined in these coordinates, (b) it tends to shift outward with an increasing solar wind bulk speed, and (c) with an increasing Mach number. However, the change of the fluctuation level in the foreshock is not uniform because the increasing solar wind bulk speed enhances the fluctuation level mainly in a close proximity of the bow shock whereas the increasing Mach number leads to an intensification of fluctuation levels at the foreshock boundary.

A Controllable T-Type Broadband Impedance Matching Network for Ultra-Low Frequency Electroacoustic Transducers

A Controllable T-Type Broadband Impedance Matching Network for Ultra-Low Frequency Electroacoustic Transducers

Practical ultra-low frequency noise laser system for quantum sensors

The laser’s frequency noise is crucial to the sensitivity of quantum sensors. Two commonly used methods to suppress the laser’s frequency noise are locking the laser to an atomic transition by the lock-in technique or to an ultra-low thermal expansion (ULE) glass cavity by the PDH technique. The former cannot suppress rapidly changing frequency noise and hardly meets the needs; the latter has powerful performance but a heightened cost. The lack of high-performance and low-cost laser noise suppression methods dramatically limits the practical application of quantum sensors. This work demonstrates that, in many quantum sensing applications such as the Rydberg atomic superheterodyne receiver, by cascade locking the laser to both the atomic transition and a low-cost (LC) cavity, the same performance as locking to the ULE cavity can be achieved. This work is significant in promoting the practical application of quantum sensors.

Magnetospheric Control of Ionospheric TEC Perturbations via Whistler-Mode and ULF Waves.

The weakly ionized plasma in the Earth's ionosphere is controlled by a complex interplay between solar and magnetospheric inputs from above, atmospheric processes from below, and plasma electrodynamics from within. This interaction results in ionosphere structuring and variability that pose major challenges for accurate ionosphere prediction for global navigation satellite system (GNSS) related applications and space weather research. The ionospheric structuring and variability are often probed using the total electron content (TEC) and its relative perturbations (dTEC). Among dTEC variations observed at high latitudes, a unique modulation pattern has been linked to magnetospheric ultra-low-frequency (ULF) waves, yet its underlying mechanisms remain unclear. Here using magnetically conjugate observations from the THEMIS spacecraft and a ground-based GPS receiver at Fairbanks, Alaska, we provide direct evidence that these dTEC modulations are driven by magnetospheric electron precipitation induced by ULF-modulated whistler-mode waves. We observed peak-to-peak dTEC amplitudes reaching 0.5 TECU (1 TECU is equal to electrons/ ) with modulations spanning scales of 5-100km. The cross-correlation between our modeled and observed dTEC reached 0.8 during the conjugacy period but decreased outside of it. The spectra of whistler-mode waves and dTEC also matched closely at ULF frequencies during the conjugacy period but diverged outside of it. Our findings elucidate the high-latitude dTEC generation from magnetospheric wave-induced precipitation, addressing a significant gap in current physics-based dTEC modeling. Theses results thus improve ionospheric dTEC prediction and enhance our understanding of magnetosphere-ionosphere coupling via ULF waves.

Ultra-low frequency active vibration isolation system with quasi-zero stiffness characteristic using self-tuning filter-based feedforward control

Purpose of studyThis study aims to resolve the compromise between low suspension stiffness and high load-bearing capability in vibration isolation and enhance the suppression performance of ultra-low frequency vibration through advanced active feedforward control methods, featuring high-performance precision isolation for large-scale ultra-precision instruments. Describe methodsAn air-magnetic hybrid parallel configuration of positive and negative stiffness is employed to achieve adjustable stiffness, endowing the vibration isolation system with quasi-zero stiffness characteristics. A novel self-tuning feedforward control strategy is proposed through a self-tuning filter to update the controller parameters online, minimizing the loss caused by model uncertainties. ResultsThe vertical and horizontal natural frequencies of the system exhibit a remarkably low resonance frequency of lower than 0.5 Hz. With the self-tuning feedforward strategy, the maximum vibration attenuation reaches 78 dB in the vertical direction and 70 dB in the horizontal direction, reducing the cumulative power at 100 Hz by 61.4 % and 47.8 %, respectively. The above results showcase the proposed approach with excellent performance in isolating low-frequency vibrations. Conclusions/DiscussionThe ultra-low frequency active vibration isolation system designed in this paper achieves exceptionally low suspension stiffness. The implemented self-tuning feedforward controller isolates large precision machinery from broadband floor vibrations and significantly enhances the vibration isolation performance of the system.