VLF Band Research Articles

Unusually high frequency natural VLF radio emissions observed during daytime in Northern Finland

Geomagnetic field variations and electromagnetic waves of different frequencies are ever present in the Earth’s environment in which the Earth’s fauna and flora have evolved and live. These waves are a very useful tool for studying and exploring the physics of plasma processes occurring in the magnetosphere and ionosphere. Here we present ground-based observations of natural electromagnetic emissions of magnetospheric origin at very low frequency (VLF, 3–30 kHz), which are neither heard nor seen in their spectrograms because they are hidden by strong impulsive signals (sferics) originating in lightning discharges. After filtering out the sferics, peculiar emissions are revealed in these digital recordings, made in Northern Finland, at unusually high frequencies in the VLF band. These recently revealed emissions, which are observed for several hours almost every day in winter, contain short (∼1–3 min) burst-like structures at frequencies above 4–6 kHz, even up to 15 kHz; fine structure on the 1 s time scale is also prevalent. It seems that these whistler mode emissions are generated deep inside the magnetosphere, but the detailed nature, generation region and propagation behaviour of these newly discovered high latitude VLF emissions remain unknown; however, further research on them may shed new light on wave-particle interactions occurring in the Earth’s radiation belts.

Frequency Management for Electromagnetic Continuous Wave Conductivity Meters.

Ground conductivity meters use electromagnetic fields for the mapping of geological variations, like the determination of water amount, depending on ground layers, which is important for the state analysis of embankments. The VLF band is contaminated by numerous natural and artificial electromagnetic interference signals. Prior to the determination of ground conductivity, the meter’s working frequency is not possible, due to the variable frequency of the interferences. Frequency management based on the analysis of the selected band using track-before-detect (TBD) algorithms, which allows dynamical frequency changes of the conductivity of the meter transmitting part, is proposed in the paper. Naive maximum value search, spatio-temporal TBD (ST-TBD), Viterbi TBD and a new algorithm that uses combined ST-TBD and Viterbi TBD are compared. Monte Carlo tests are provided for the numerical analysis of the properties for a single interference signal in the considered band, and a new approach based on combined ST-TBD and Viterbi algorithms shows the best performance. The considered algorithms process spectrogram data for the selected band, so DFT (Discrete Fourier Transform) could be applied for the computation of the spectrogram. Real–time properties, related to the latency, are discussed also, and it is shown that TBD algorithms are feasible for real applications.

Temporal fine structure frequency bands criticality in perception of the speech in the presence of noise

Context: In the competing background, noise speech cues are compromised. In quiet and noisy conditions, the distribution and weighting of temporal cues for the speech perception vary across different frequency spectrum. Temporal fine structure (TFS) cues help in perceiving speech in noise. Knowledge about the perceptual weighing of fine structure cues is essential for the design of assistive listening devices Aims: In the present study, the perceptual weighing of the fine structure cues across frequency bands in different listening conditions is measured. Settings and Design: Data were collected in a double room set up and adapted Plomp (1986) sentence speech recognition threshold (sSRT) method was used. Subjects and Methods: Forty normal hearing individuals presented without and with (very low frequency [VLF], low frequency, mid frequency [MF], and high frequency) filtered fine structure bands of hearing in noise test sentences. sSRTs were measured for uninterrupted and interrupted stimulus in different signal to noise conditions (quiet, 0 dB, +10 dB, and −10 dB). ANOVA, post hoc , and bivariate Karl Pearson correlation statistical analysis were used. Results: The sentence recognition for filtered stimulus in quiet and in noise resulted in significantly higher relative perceptual weight placed upon the TFS in the VLF band and MF at 0 dB signal to noise ratio (SNR) ( P < 0.001). The relative weighting of fine structure cues demonstrated the importance VLF band in quiet and noisy conditions and of the MF band that contains dynamic formant movement at 0 dB SNR condition.

Optimization of the periodogram average for the estimation of the power spectral density (PSD) of weak signals in the ELF band

The frequencies between the ELF, ULF and VLF bands are widely used as indicators of the electrical activity in the atmosphere, as precursors of seismic activity and are of interest in the study of geophysical phenomena. These signal methods of analysis are varied, and among them, we find a method of averaged periodograms, which is based on the Fast Fourier Transform (FFT). In this type of analysis, the optimal number of segments to be averaged must be determined; however, there seems to be no satisfactory method for choosing them. In this paper, we provide a method that will allow us to select the optimal number of segments to be averaged.

Anomalously strong vertical magnetic fields from distant ELF/VLF sources

AbstractThere are many sources of very low frequency (VLF—3–30 kHz) and extremely low frequency (ELF—3–3000 Hz) radiation in the Earth‐ionosphere waveguide (e.g., lightning and ELF/VLF communication transmitters). At distances of thousands of kilometers from these sources, the vertical component of the ELF/VLF AC magnetic fields is expected to be very weak and several orders of magnitude lower than the horizontal magnetic components. However, measurements in Israel show a relatively strong vertical magnetic component in both the ELF and VLF bands, at the same order of magnitude as the horizontal components. Our measurements suggest that the real Earth‐ionosphere waveguide might often be very different from the theoretical waveguide used in model calculations. In addition, our results imply that using only the horizontal components for direction finding or the absolute magnetic field strength may result in errors, since often a significant fraction of the magnetic field energy hides in the vertical component.

Precursory Anomaly in VLF/LF Recordings Prior to the Çaglayan (Erzincan-Turkey) Earthquake on July 30th, 2009

An international project network consisting of six receivers for sampling LF and VLF radio signals has been going on to record the data in Europe from different transmission stations around the World. One of them was established in Resadiye, Turkey, located just on the North Anatolian Fault Zone. The receiver works in VLF and LF bands monitoring ten frequencies (16.4, 21.75, 37.5, 45.9, 153, 180, 183, 216 and 270 kHz) with one minute sampling interval. An earthquake of Mw = 4.9 took place 225 km away from the VLF/LF station at the eastern tip of the Erzincan basin at 4 km depth on July 30, 2009. We observed some anomalies on the radio signals (37.5 and 153 kHz) that initiated about 7 days before the earthquake and disappeared soon after the earthquake. We attribute this anomaly to the Mw=4.9 earthquake as a seismo-electromagnetic precursor. The radio anomaly that appeared 7 days before the occurrence of the 2009 Caglayan (Erzincan) earthquake is in good agreement with other results indicating precursory anomalies in the project network mostly observed in seismically active countries such as Italy and Greece.

Design of Log Periodic T-Antenna for Operation in S Band

In this paper, the design and simulation of a Log Periodic T Antenna is presented. T antennas are generally small sized antennas used in VLF, LF and shortwave bands. These antennas can be used in a wide variety of applications due to their ease of designing and simplicity but are restricted to small bandwidth, low efficiency and low frequency band applications. To overcome the deficiencies’ of this antenna, Log Periodic T antenna (LPTA) is proposed. The proposed antenna can be used in higher frequency band applications (S band) and has a broad bandwidth. The proposed T-antenna is implemented by means of a patch. The substrate is chosen to be RT Duroid. The antenna dimensions are calculated using Transmission Line model. The antenna layout is simulated in Agilent’s Advanced Design System (ADS) simulator.Keywords: S band, Shortwave band, T-Antenna

Short-term Heart Rate Variability And Its Correlation With Acute Maximal Graded Exercise In Healthy Adults

The influence of short-term heart rate variability (HRV) on acute maximal graded exercise performance is not clearly understood and there are considerable discrepancies in the literature regarding the same. PURPOSE: To investigate the correlation if any, between the short-term HRV parameters and the acute maximal graded treadmill exercise parameters in healthy adult volunteers. METHODS: Designed as a cross-sectional study, consecutive, consenting healthy adult volunteers of mean age 20.56 ± 2.49 years (n=30, M/F 22/8) underwent recording of supine resting electrocardiogram under standard test conditions for HRV analysis. The volunteers then underwent pre-exercise assessment of resting heart rate (HR), blood pressure (BP) and respiratory rate (RR) in supine position and after five minutes of assuming standing posture followed by an acute maximal graded treadmill exercise test (Ball State University Protocol) until volitional exhaustion providing total test time (TT), cumulative work output (WO) achieved, cumulative metabolic equivalent of task (METs) achieved, derived VO2max achieved and drop in recovery HR data. Spectral analysis of short-term heart rate variability was performed using Kubios HRV software version 2.1 yielding time-domain (mean of RR intervals, root mean square of successive differences - rMSSD, standard deviation of normal to normal intervals - SDNN, the number of pairs of successive NNs that differ by more than 50 ms - NN50, the proportion of NN50 divided by total number of NNs - pNN50) and frequency-domain (VLF, LF, and HF band powers and peak frequencies) measures. Data were analysed using Pearson’s Correlation Coefficient with Bootstrap confidence interval statistics. RESULTS: The spectral analysis of heart rate variability parameters do not correlate significantly with the acute maximal graded treadmill exercise parameters. However, resting supine heart rate and resting supine systolic BP show significant negative correlation with drop in recovery heart rate at 5th minute (r = -0.501 and -0.496 at 95% confidence interval of -0.686 to -0.304 and -0.710 to -0.242 respectively, at p value < 0.05). CONCLUSIONS: Short-term heart rate variability parameters do not have any influence on the acute maximal exercise performance parameters of an individual. Resting heart rate and systolic blood pressure correlate with heart rate recovery which is a predominantly parasympathetic function.

Frequency-specific network topologies in the resting human brain.

A community is a set of nodes with dense inter-connections, while there are sparse connections between different communities. A hub is a highly connected node with high centrality. It has been shown that both “communities” and “hubs” exist simultaneously in the brain's functional connectivity network (FCN), as estimated by correlations among low-frequency spontaneous fluctuations in functional magnetic resonance imaging (fMRI) signal changes (0.01–0.10 Hz). This indicates that the brain has a spatial organization that promotes both segregation and integration of information. Here, we demonstrate that frequency-specific network topologies that characterize segregation and integration also exist within this frequency range. In investigating the coherence spectrum among 87 brain regions, we found that two frequency bands, 0.01–0.03 Hz (very low frequency [VLF] band) and 0.07–0.09 Hz (low frequency [LF] band), mainly contributed to functional connectivity. Comparing graph theoretical indices for the VLF and LF bands revealed that the network in the former had a higher capacity for information segregation between identified communities than the latter. Hubs in the VLF band were mainly located within the anterior cingulate cortices, whereas those in the LF band were located in the posterior cingulate cortices and thalamus. Thus, depending on the timescale of brain activity, at least two distinct network topologies contributed to information segregation and integration. This suggests that the brain intrinsically has timescale-dependent functional organizations.

First results of MWC SAS3 electromagnetic wave experiment on board of the Chibis-M satellite

The main goals of the Chibis-M mission are the testing of a new micro-satellite technology, the study of new physical processes related to lightning activity and the verification of possible monitoring techniques of Space Weather phenomena. In frames of the Chibis-M mission an electromagnetic wave complex MWC is installed on board of the satellite composed of electromagnetic sensors and SAS3 measuring unit. The obtained data show that the scientific instrumentation operates properly and produces interesting information. Here we present the first results of the first year of operation of the MWC in the ELF–VLF bands in different operation modes. An important conclusion is that basing on the experience of the first year it is possible to realize an effective and reliable Space Weather monitoring system using micro-satellites and simultaneously operating ground support equipments.

Heart rate variability during the MSLT and MWT in patients with and without excessive daytime sleepiness

Autonomic heart regulation changes during the transition from wakefulness to sleep. Heart rate variability (HRV) can be used to assess the autonomic nervous system function (ANS). We aimed to evaluate ANS in patients with and without excessive daytime sleepiness (EDS) during the maintenance of wakefulness test (MWT) and multiple sleep latency test (MSLT). From a series of 98 consecutive patients with symptoms of sleep disordered breathing (SDB) studied with 5 trials of MWT followed by a research version MSLT two groups of patients (matched by age and gender) were selected consecutively based on mean sleep latencies: the sleepy group (SG) with 17 patients ( MWT < 20 min and MSLT < 8 min) and the alert group (AG) with 20 patients (AG: MWT > 20 and MSLT > 8 min). For each test, the first 120-s window of RR signals during wakefulness was analyzed in the following frequency bands: VLF, < 0.04 Hz; LF, 0.04–0.15 Hz; HF, 0.15–0.4 Hz; TB, total-frequency band. ANS activity was described by measures obtained from traditional power spectral analysis (PSA) and from time–frequency representation (TFR). Non-linear measures – Correntropy (CORR) and auto-mutual-information function (AMIF) – were used to describe the RR regularity. Mean values from all day were compared between groups. Cohen’s effect size was calculated to measure the strength of each difference. Finally, a conditional forward logistic regression analysis was performed to find the best discriminatory variables between groups. TFR (in HF and VLF band), AMIF (in all bands) and CORR (in Total band) analysis showed that the SG had higher values than the AG ( p range 0.000–0.02) during the MSLT. The strongest differences were found in HF band from TRF and AMIF and in Total band from AMIF and CORR. The best sensitivity (82.4%) and specificity (85%) to distinguish patients with and without sleepiness were achieved with the combination of AMIF and CORR in Total band, respectively. No differences were found in traditional PSA measures. Heart rate variability analysis during the first 2 min of the MSLT while the patient is awake show specific changes in TFR and non-linear measures that differentiate sleepy from non-sleepy patients. Study supported by FIS PI07/0318 to MS, cofinanced by FEDER.

840 – Heart rate variability in panic disorder patients before and after the therapy

We assessed HRV (heart rate variability) in 31 patients with panic disorder before and after 6-weeks treatment with antidepressants and CBT and 18 controls. Patients were regularly assessed weekly on the CGI, BAI and BDI. HRV was assessed during 3 positions. Thirty one pharmacoresistant panic disorder patients entered a 6-week open-label treatment study. A combination of CBT and pharmacotherapy proved to be the effective treatment. Patients significantly improved in all rating scales. There were highly statistical significant differences between panic patients and control group in all components of power spectral analysis in 2 nd (VLF, LF and H in supine) and in two component of 3 rd (LF and HF in standing) positions. There was also statistically significant difference between these two groups in LF/HF ratio in supine position (2 nd ). We have found statistically significant differences in HRV frequencies between before and after CBT program in VLF in standing position. We have also found statistically significant differences in HRV frequency ratios in the third position in case of decrease VLF + LF / HF ratio. We can also see decreasing activity in VLF and LF band in standing position in1st measurement and an increase in VLF and LF band in standing position in 2nd measurement. These findings demonstrate a lower autonomic power in panic disorder patients and tendency to increase it during the treatment. Supported by: Project IGA MZ ČR NS10301-3/2009

Subdivision of heart rate variability VLF band contains prognostic value of all-cause mortality after acute myocardial infarction

Results: A comparison of EMG-EDR breath locations to the reference using a 1.6-s match window (representing 20% of average breath-to-breath interval) yielded a sensitivity of 94% and a positive predictive value of 95%. Discussion: EMG-EDR performed well with minimal errors in representation of respiration activity. However, most errors were caused by one subject's occasional atrial premature beats (APBs) which inhibited MRI triggering, and thus resulted in an apparent drop in the baseline EMG levels; this caused a subsequent but temporary distortion of the EMG-EDR waveform. Data from more subjects are currently being obtained. Conclusion: On this limited data set, the ECG/EMG-derived respiration technique performs well even in the electrically noisy MRI environment. Additional algorithm logic is needed to reduce the effects of APBs. Use of this technique may allow elimination of the additional hardware of the respiratorygating belt, since ECG signals are already acquired during cardiacMRI imaging.

Heart rate variability for assessing comatose patients with different Glasgow Coma Scale scores

ObjectiveTo assess the autonomic nervous system (ANS) in coma by heart rate variability (HRV). MethodsSixteen comatose patients and 22 normal subjects with comparable ages and genders were studied. Patients were classified in two subgroups according to the Glasgow Coma Scale (GCS). Time, frequency, and informational HRV domain indices were calculated. ResultsA notable reduction of HRV was found in patients. Regarding the time domain indices, the triangular index, and the Delta_RRs, were significantly reduced in the subgroup with GCS=3. Absolute power for the whole frequency spectrum decreased whenever GCS scores were lower. A significant decrement was found for absolute power of the VLF and LF bands in the subgroup of GCS=3, and although it was lower for the HF band in these patients, those changes were not statistically significantly different. The LF/HF ratio and the Shannon´s entropy indices were significantly reduced in the subgroup with GCS=3. Our results are discussed regarding the progressive dysfunction the ANS networks when coma deepens. ConclusionsThe HRV procedure is a powerful tool to assess the ANS in comatose patients. SignificanceHRV is a minimally invasive, low-cost methodology, suitable for assessing the ANS in coma.

Neuronal oscillations in the EEG under varying cognitive load: A comparative study between slow waves and faster oscillations

ObjectiveThis study has been specifically designed to investigate very low frequency neuronal oscillations (VLFO, <0.5Hz) during resting states and during goal-directed tasks of graded difficulty levels, quantify the changes that the slow waves undergo in these conditions and compare them with those for higher frequency bands (namely delta, theta and alpha). MethodsTo this end we developed a multistage signal processing methodology comprising blind source separation coupled with a neural network based feature extraction and classification method. ResultsChanges in the amplitude and phase of brain sources estimates in the VLF band between rest and task were enhanced with increased task difficulty, but remained lower than those experienced in higher frequency bands. The slow wave variations were also significantly correlated with task performance measures, and hence with the level of task-directed attention. ConclusionsThese findings suggest that besides their prominent sensitivity to external stimulation, VLFOs also contribute to the cortical ongoing background activity which may not be specifically related to task-specific attention and performance. SignificanceOur work provides important insight into the association between VLF brain activity and conventional EEG frequency bands, and presents a novel framework for assessing neural activity during various mental conditions and psychiatric states.

Characteristics of flux-time profiles, temporal evolution, and spatial distribution of radiation-belt electron precipitation bursts in the upper ionosphere before great and giant earthquakes

The analysis of energetic electron observations made by the DEMETER satellite reveals that radiation belt electron precipitation (RBEP) bursts are observed in general several (~1-6 days) before a large (M > 6.5) earthquake (EQ) in the presence of broad band (~1-20 kHz) VLF waves. The EBs show in general a relative peak-to-background flux increase usually < 100, they have a time duration of ~0.5 – 3 min, and their energy spectrum reach up to energies <~500 keV. The RBEP activity is observed as one, two or three EBs throughout a semi-orbit, depended on the magnetic field structure above the EQ epicenter. A statistical analysis has been made for earthquakes in Japan, which reveals a standard temporal variation of the number of EBs, which begins with an incremental rate several days before major earthquakes, and after a maximum, decreases so that the electron precipitation ceases above the epicenter. Some earthquake induced EBs were observed not only in the nightside ionosphere, but also in the dayside ionosphere.

Development of regulation of the cardiac chronotropic function in albino rats during the early postnatal ontogeny according to the results of spectral analysis of heart rhythm variability

Regulation of the cardiac chronotropic function was studied by spectral analysis of cardiac rhythm variability in HF, LF, and VLF bands in rats at various stages of the early postnatal ontogeny. The inadequacy of the regulatory mechanisms during the first days of life manifested by low power of all waves (particularly HF) in the cardiac rhythm variability spectrum. On day 14 of life, the cardiointerval variability was formed by HF waves, their low power together with increasing heart rate indicating more intense sympathetic effects. On day 21 of life, a potent elevation of the VLF power reflected a stronger centralization of regulation from higher autonomic centers. The age of 28 days was characterized by a sharp increase of HF activity and could be regarded as the turning point in the development of parasympathetic effects and activity of the autonomic regulation contour. From the age of 35 days, the wave power and the proportion of the spectral components of cardiac rhythm variability in albino rats corresponded to the adult pattern; a trend to the central regulation predominance and to greater rigidity of cardiac rhythm formed only with the onset of sexual maturation.

Strong magnetic field fluctuations within filamentary auroral density cavities interpreted as VLF saucer sources

The Geoelectrodynamics and Electro‐Optical Detection of Electron and Suprathermal Ion Currents (GEODESIC) sounding rocket encountered more than 100 filamentary density cavities associated with enhanced plasma waves at ELF (&lt;3 kHz) and VLF (3–10 kHz) frequencies and at altitudes of 800–990 km during an auroral substorm. These cavities were similar in size (∼20 m diameter in most cases) to so‐called lower‐hybrid cavities (LHCs) observed by previous sounding rockets and satellites; however, in contrast, many of the GEODESIC cavities exhibited up to tenfold enhancements in magnetic wave power throughout the VLF band. GEODESIC also observed enhancements of ELF and VLF electric fields both parallel and perpendicular to the geomagnetic fieldB0within cavities, though the VLF E field increases were often not as large proportionally as seen in the magnetic fields. This behavior is opposite to that predicted by previously published theories of LHCs based on passive scattering of externally incident auroral hiss. We argue that the GEODESIC cavities are active wave generation sites capable of radiating VLF waves into the surrounding plasma and producing VLF saucers, with energy supplied by cold, upward flowing electron beams composing the auroral return current. This interpretation is supported by the observation that the most intense waves, both inside and outside cavities, occurred in regions where energetic electron precipitation was largely inhibited or absent altogether. We suggest that the wave‐enhanced cavities encountered by GEODESIC were qualitatively different from those observed by earlier spacecraft because of the fortuitous timing of the GEODESIC launch, which placed the payload at apogee within a substorm‐related return current during its most intense phase, lasting only a few minutes.

The European VLF/LF radio network to search for earthquake precursors: setting up and natural/man-made disturbances

Abstract. In the last years disturbances in VLF/LF radio signals related to seismic activity have been presented. The radio data were collected by receivers located on the ground or on satellites. The ground-based research implies systematic data collection by a network of receivers. Since 2000 the "Pacific VLF network", conducted by Japanese researchers, has been in operation. During 2008 a radio receiver was developed by the Italian factory Elettronika (Palo del Colle, Bari). The receiver is equipment working in VLF and LF bands. It can monitor 10 frequencies distributed in these bands and, for each of them, it saves the power level. At the beginning of 2009, five receivers were made for the realization of the "European VLF/LF Network"; two were planned for Italy and one for Greece, Turkey and Romania, respectively. In 2010 the network was enlarged to include a new receiver installed in Portugal. In this work, first the receiver and its setting up in the different places are described. Then, several disturbances in the radio signals related to the transmitters, receivers, meteorological/geomagnetic conditions are presented and described.

Thermal ion upflow in the cusp ionosphere and its dependence on soft electron energy flux

We investigate the origin of low‐energy (Ek &lt; 10 eV) ion upflows in Earth's low‐altitude dayside cusp region. The Cusp‐2002 sounding rocket flew from Ny Ålesund, Svalbard, on 14 December 2002, carrying plasma and field instrumentation to an altitude of 768 km. The Suprathermal Ion Imager, a two‐dimensional energy/arrival angle spectrograph, observed large (&gt;500 m s−1) ion upflows within the cusp at altitudes between 640 km and 768 km. We report a significant correlation between ion upflow and precipitating magnetosheath electron energy flux in this altitude range. There is only very weak correlation between upflow and wave power in the VLF band. We find a small negative correlation between upflow and the magnitude of the DC electric field for fields less than about 70 mV m−1. The apparent relation between upflow and electron energy flux suggests a mechanism whereby ions are accelerated by parallel electric fields that are established by the soft electrons. Significant ion upflows are not observed for electron energy fluxes less than about 1010 eV cm−2 s−1. The lack of correspondence between ∣∣ and upflow on the one hand, and wave power and upflow on the other, does not rule out these processes but implies that, if operating, they are not local to the measurement region. We also observe narrow regions of large ion downflow that imply either a rebalancing of the ionosphere toward a low‐Te equilibrium during which gravity dominates over the pressure gradients or a convection of the upflowing ions away from the precipitation region, outside of which the ions must fall back into equilibrium at lower altitudes.