Showing papers on "Very low frequency published in 2017"

VLF waves from ground‐based transmitters observed by the Van Allen Probes: Statistical model and effects on plasmaspheric electrons

Abstract: Whistler-mode Very Low Frequency (VLF) waves from powerful ground-based transmitters can resonantly scatter energetic plasmaspheric electrons and precipitate them into the atmosphere. A comprehensive 4-year statistics of Van Allen Probes measurements is carried out to assess their consequences on the dynamics of the inner radiation belt and slot region. Statistical models of the measured wave electric field power and of the inferred full wave magnetic amplitude are provided as a function of L, magnetic local time, season, and Kp over L=1-3, revealing the localization of VLF wave intensity and its variation with geomagnetic activity over 2012-2016. Since this VLF wave model can be directly used together with existing hiss and lightning-generated wave models in radiation belt simulation codes, we perform numerical calculations of the corresponding quasilinear pitch angle diffusion rates, allowing us to demonstrate the crucial role played by VLF waves from transmitters in energetic electron loss at L<2.5.

The very low-frequency band of heart rate variability represents the slow recovery component after a mental stress task.

Abstract: The very low-frequency (VLF) band of heart rate variability (HRV) has different characteristics compared with other HRV components. Here we investigated differences in HRV changes after a mental stress task. After the task, the high-frequency (HF) band and ratio of high- to low-frequency bands (LF/HF) immediately returned to baseline. We evaluated the characteristics of VLF band changes after a mental stress task. We hypothesized that the VLF band decreases during the Stroop color word task and there would be a delayed recovery for 2 h after the task (i.e., the VLF change would exhibit a "slow recovery"). Nineteen healthy, young subjects were instructed to rest for 10 min, followed by a Stroop color word task for 20 min. After the task, the subjects were instructed to rest for 120 min. For all subjects, R-R interval data were collected; analysis was performed for VLF, HF, and LF/HF ratio. HRV during the rest time and each 15-min interval of the recovery time were compared. An analysis of the covariance was performed to adjust for the HF band and LF/HF ratio as confounding variables of the VLF component. HF and VLF bands significantly decreased and the LF/HF ratio significantly increased during the task compared with those during rest time. During recovery, the VLF band was significantly decreased compared with the rest time. After the task, the HF band and LF/HF ratio immediately returned to baseline and were not significantly different from the resting values. After adjusting for HF and LF/HF ratio, the VLF band had significantly decreased compared with that during rest. The VLF band is the "slow recovery" component and the HF band and LF/HF ratio are the "quick recovery" components of HRV. This VLF characteristic may clarify the unexplained association of the VLF band in cardiovascular disease prevention.

On the Use of VLF Narrowband Measurements to Study the Lower Ionosphere and the Mesosphere–Lower Thermosphere

Abstract: The ionospheric D-region (~60 km up to ~95 km) and the corresponding neutral atmosphere, often referred to as the mesosphere–lower thermosphere (MLT), are challenging and costly to probe in situ. Therefore, remote sensing techniques have been developed over the years. One of these is based on very low frequency (VLF, 3–30 kHz) electromagnetic waves generated by various natural and man-made sources. VLF waves propagate within the Earth–ionosphere waveguide and are extremely sensitive to perturbations occurring in the D-region along their propagation path. Hence, measurements of these signals serve as an inexpensive remote sensing technique for probing the lower ionosphere and the MLT region. This paper reviews the use of VLF narrowband (NB) signals (generated by man-made transmitters) in the study of the D-region and the MLT for over 90 years. The fields of research span time scales from microseconds to decadal variability and incorporate lightning-induced short-term perturbations; extraterrestrial radiation bursts; energetic particle precipitation events; solar eclipses; lower atmospheric waves penetrating into the D-region; sudden stratospheric warming events; the annual oscillation; the solar cycle; and, finally, the potential use of VLF NB measurements as an anthropogenic climate change monitoring technique.

Detection of acoustic-gravity waves in lower ionosphere by VLF radio waves

Abstract: We present a new method to study harmonic waves in the low ionosphere (60 - 90 km) by detecting their effects on reflection of very low frequency (VLF) radio waves. Our procedure is based on amplitude analysis of reflected VLF radio waves recorded in real time, which yields an insight into the dynamics of the ionosphere at heights where VLF radio waves are being reflected. The method was applied to perturbations induced by the solar terminator motions at sunrises and sunsets. The obtained results show that typical perturbation frequencies found to exist in higher regions of the atmosphere are also present in the lower ionosphere, which indicates a global nature of the considered oscillations. In our model atmosphere, they turn out to be the acoustic and gravity waves with comparatively short and long periods, respectively.

Perturbations to the lower ionosphere by tropical cyclone Evan in the South Pacific region

Abstract: Very Low Frequency (VLF) electromagnetic signals from navigational transmitters propagate worldwide in the earth-ionosphere waveguide formed by the earth and the electrically conducting lower ionosphere. Changes in the signal properties are signatures of variations in the conductivity of the reflecting boundary of the lower ionosphere which is located in the mesosphere and lower thermosphere, and their analysis is, therefore, a way to study processes in these remote regions. Here we present a study on amplitude perturbations of local origin on the VLF transmitter signals (NPM, NLK, NAA and JJI) observed during tropical cyclone (TC) Evan, 9-16 December 2012 when TC was in the proximity of the transmitter-receiver links. We observed a maximum amplitude perturbation of 5.7dB on JJI transmitter during 16 December event. From Long Wave Propagation Capability model applied to three selected events we estimate a maximum decrease in the nighttime D-region reference height (H') by ~5.2 km (13 December, NPM) and maximum increase in the daytime D-region H' by 6.1 km and 7.5 km (14 &16 December, JJI).The results suggest that the TC caused the neutral densities of the mesosphere and lower thermosphere to lift and sink (bringing the lower ionosphere with it), an effect that may be mediated by gravity waves generated by the TC. The perturbations were observed before the storm was classified as a TC, at a time when it was a tropical depression, suggesting the broader conclusion that severe convective storms, in general, perturb the mesosphere and the stratosphere through which the perturbations propagate.

Finite-Difference Modeling of Very-Low-Frequency Propagation in the Earth-Ionosphere Waveguide

Abstract: Simulations of Very-low-frequency (VLF) transmitter signals are conducted using three models: the long-wave propagation capability, a finite-difference (FD) time-domain model, and an FD frequency-domain model. The FD models are corrected using Richardson extrapolation to minimize the numerical dispersion inherent in these models. Using identical ionosphere and ground parameters, the three models are shown to agree very well in their simulated VLF signal amplitude and phase, to within 1 dB of amplitude and a few degrees of phase, for a number of different simulation paths and transmitter frequencies. Furthermore, the three models are shown to produce comparable phase changes for the same ionosphere perturbations, again to within a few degrees. Finally, we show that the models reproduce the phase data of existing VLF transmitter–receiver pairs reasonably well, although the nighttime variation in the measured phase data is not captured by the simplified characterization of the ionosphere.

The effects of solar activity: Electrons in the terrestrial lower ionosphere

Abstract: Solar flare X-ray energy can cause strong enhancements of the electron density in the Earth’s atmosphere. This intense solar radiation and activity can cause sudden ionospheric disturbances (SIDs) and further create ground telecommunication interferences, blackouts as well as some natural disasters and caused considerable material damage. The focus of this contribution is on the study of these changes induced by solar X-ray flares using narrowband Very Low Frequency (VLF, 3–30 kHz) and Low Frequency (LF, 30–300 kHz) radio signal analysis. The model computation and simulation were applied to acquire the electron density enhancement induced by intense solar radiation. The obtained results confirmed the successful use of applied technique for detecting space weather phenomena such as solar explosive events as well for describing and modeling the ionospheric electron density which are important as the part of electric terrestrial-conductor environment through which external-solar wind (SW) electrons can pass and cause natural disasters on the ground like fires.

Modeling of temporal variation of very low frequency radio waves over long paths as observed from Indian Antarctic stations

Abstract: Characteristics Very Low Frequency (VLF) signal depends on solar illumination across the propagation path. For a long path, solar zenith angle varies widely over the path and this has a significant influence on the propagation characteristics. To study the effect, Indian Center for Space Physics participated in the 27th and 35th Scientific Expedition to Antarctica. VLF signals transmitted from the transmitters, namely, VTX (18.2 kHz), Vijayanarayanam, India and NWC (19.8 kHz), North-west Cape, Australia were recorded simultaneously at Indian permanent stations Maitri and Bharati having respective geographic coordinates 70.75∘S, 11.67∘E and 69.4∘S, 76.17∘E. A very stable diurnal variation of the signal has been obtained from both the stations. We reproduced the signal variations of VLF signal using solar zenith angle model coupled with Long Wavelength Propagation Capability (LWPC) code. We divided the whole path into several segments and computed the solar zenith angle (χ) profile. We assumed a linear relationship between the Wait's exponential model parameters effective reflection height (h′), steepness parameter (β) and solar zenith angle. The h′ and β values were later used in the LWPC code to obtain the VLF signal amplitude at a particular time. The same procedure was repeated to obtain the whole day signal. Nature of the whole day signal variation from the theoretical modeling is also found to match with our observation to some extent.

Modeling long recovery early events (LOREs) produced by lightning-induced ionization of the nighttime upper mesosphere

Abstract: We present results of a cylindrically symmetric, coupled electrodynamic and photo-chemical model which simulates diffuse ionization of the middle atmosphere induced by strong lightning discharges (peak currents >150 kA). Scattering of sub-ionospherically propagating, very low frequency radio waves is then evaluated using the Long-Wave Propagation Capability code. Some modeled sprite halos exhibit continued electron density growth up to timescales of seconds due to O− detachment, though it is not yet clear how this might relate to the slower onset durations (>20 msec) of some early VLF events. Modeled electron density enhancements in sprite halos, capable of strong VLF scattering, can persist for long periods of time (greater than hundreds of seconds) even at lower altitudes where their recovery is initially controlled by fast attachment processes. Consequently, our modeling results indicate that both typical recovery (20 to 240 seconds) and long recovery (LOREs, >300 second) VLF scattering events can be explained by scattering from conductivity changes associated with sprite halos. In contrast, modeled scattered fields resulting from elve-associated conductivity changes, though exhibiting long recovery times, are too weak to sufficiently explain typical LORE observations. Theoretical scattering from structured ionization events (e.g., sprites columns and gigantic jets) is not considered in this work.

Lower Ionosphere Sensitivity to Solar X-ray Flares Over a Complete Solar Cycle Evaluated From VLF Signal Measurements

Abstract: The daytime lower ionosphere behaves as a solar X-ray flare detector, which can be monitored using Very Low Frequency (VLF) radio waves that propagate inside the Earth-ionosphere waveguide. In this paper, we infer the lower ionosphere sensitivity variation over a complete solar cycle by using the minimum X-ray fluence (FXmin) necessary to produce a disturbance of the quiescent ionospheric conductivity. FXmin is the photon energy flux integrated over the time interval from the start of a solar X-ray flare up to the beginning of the ionospheric disturbance recorded as amplitude deviation of the VLF signal. FXmin is computed for ionospheric disturbances that occurred in the time interval December-January from 2007 to 2016 (Solar Cycle 24). The computation of FXmin uses the X-ray flux in the wavelength band below 0.2 nm and the amplitude of VLF signals transmitted from France (HWU), Turkey (TBB) and USA (NAA); which were recorded in Brazil, Finland and Peru. The main result of this study is that the long-term variation of FXmin is correlated with the level of solar activity, having FXmin values in the range (1 − 12) × 10−7J/m2. Our result suggests that FXmin is anti-correlated with the lower ionosphere sensitivity, confirming that the long-term variation of the ionospheric sensitivity is anti-correlated with the level of solar activity. This result is important to identify the minimum X-ray fluence that an external source of ionization must overcome in order to produce a measurable ionospheric disturbance during daytime.

Long-term climate change in the D-region.

Abstract: Controversy exists over the potential effects of long-term increases in greenhouse gas concentrations on the ionospheric D-region at 60–90 km altitudes. Techniques involving in-situ rocket measurements, remote optical observations, and radio wave reflection experiments have produced conflicting results. This study reports a novel technique that analyses long-distance subionospheric very low frequency radiowave observations of the NAA 24.0 kHz transmitter, Cutler, Maine, made from Halley Station, Antarctica, over the period 1971–2016. The analysis is insensitive to any changes in the output power of the transmitter, compensates for the use of different data logging equipment, and can confirm the accuracy of the timing systems operated over the 45 year long record. A ~10% reduction in the scale size of the transmitter nighttime interference fringe pattern has been determined, taking into account the quasi-11 year solar cycle. Subionospheric radiowave propagation modeling suggests that the contraction of the interference fringe pattern about the mid-latitude NAA transmitter is due to a 3 km reduction in the effective height of the nighttime ionospheric D-region over the last 45 years. This is consistent with the effect of enhanced infra-red cooling by increasing greenhouse gases.

Line radiation events induced by very low frequency transmitters observed by the DEMETER spacecraft

Abstract: Electromagnetic wave data measured by the low-altitude DEMETER spacecraft sometimes exhibit intense emissions at discrete, harmonically spaced frequencies. We present a comprehensive analysis of this type of phenomenon, and we demonstrate that it is caused by very low frequency (VLF) transmitters in Europe. All available data were investigated for a possible presence of the events. Altogether, 87 events were identified, occurring exclusively during the nighttime. The occurrence of the events does not show any clear relation to the AE and Dst geomagnetic activity indices. The events are clearly localized, occurring either close to Great Britain or in the vicinity of its geomagnetically conjugated region. The frequency spectra of the events typically consist of up to 4 spectral peaks at multiples of about 1.3 kHz. Additional weaker spectral peaks at frequencies of about 1.9 kHz and 3.2 kHz are occasionally present. The events observed in the northern hemisphere tend to have larger frequency bandwidths than the events observed in the southern hemisphere. The occurrence of these events is correlated with the simultaneous detection of signals from VLF transmitters. A bicoherence analysis is employed to demonstrate that wave-wave coupling takes place. Finally, it is shown that the occurrence of the events is associated with a significantly increased precipitation of energetic electrons in a wide range of energies.

A comparative study of dielectric dissipation factor measurement under very low and power frequencies

Abstract: Very low frequency (VLF) high voltage testing had been used as a simple withstand test (go/no-go). Recently, VLF diagnostic testing has emerged as a promising tool for insulation assessment of power cables. The term VLF implies testing insulation with an excitation voltage at frequency of 0.1 Hz or lower. In this paper, a comparative analysis of the dielectric dissipation factor (DDF) or tanδ measurement is presented for 11-kV cross-linked polyethylene (XLPE) cable at power frequency (50 Hz) and at VLF. Here, the DDF response is measured from frequency 50 Hz to 0.1 Hz for four short cable sections where each sample has a capacitance lower than 300 pF. The experimental results show that the DDF value at 0.1 Hz is much higher than that at 50 Hz. Furthermore, differential tangent delta (DTD) and correlation of DDF values are calculated to incorporate with experimental results.

One day prediction of nighttime VLF amplitudes using nonlinear autoregression and neural network modeling

Abstract: The electric field amplitude of very low frequency (VLF) transmitter from Hawaii (NPM) has been continuously recorded at Chofu (CHF), Tokyo, Japan. The VLF amplitude variability indicates lower ionospheric perturbation in the D region (60–90 km altitude range) around the NPM-CHF propagation path. We carried out the prediction of daily nighttime mean VLF amplitude by using Nonlinear Autoregressive with Exogenous Input Neural Network (NARX NN). The NARX NN model, which was built based on the daily input variables of various physical parameters such as stratospheric temperature, total column ozone, cosmic rays, Dst, and Kp indices possess good accuracy during the model building. The fitted model was constructed within the training period from 1 January 2011 to 4 February 2013 by using three algorithms, namely, Bayesian Neural Network (BRANN), Levenberg Marquardt Neural Network (LMANN), and Scaled Conjugate Gradient (SCG). The LMANN has the largest Pearson correlation coefficient (r) of 0.94 and smallest root-mean-square error (RMSE) of 1.19 dB. The constructed models by using LMANN were applied to predict the VLF amplitude from 5 February 2013 to 31 December 2013. As a result the one step (1 day) ahead predicted nighttime VLF amplitude has the r of 0.93 and RMSE of 2.25 dB. We conclude that the model built according to the proposed methodology provides good predictions of the electric field amplitude of VLF waves for NPM-CHF (midlatitude) propagation path.

VLF modal interference distance and nighttime D region VLF reflection height for west-east and east-west propagation paths to Fiji

Abstract: Very low frequency (VLF) signals from navigational transmitters propagate through the Earth-ionosphere waveguide formed by the Earth and the lower conducting ionosphere and show the pronounced minima during solar terminator transition between transmitter and receiver. Pronounced amplitude minima observed on 19.8 kHz (NWC transmitter) and 24.8 kHz (NLK transmitter) signals recorded at Suva (18.149°S, 178.446°E), Fiji, during 2013–2014, have been used to estimate the VLF modal interference distance (DMS) and nighttime D region VLF reflection height (hN). The NWC transmitter signal propagates mostly in west-east direction, and the NLK transmitter follows a transequatorial path propagating significantly in the east-west direction. The values of DMS calculated using midpath terminator speed are 2103 ± 172 km and 2507 ± 373 km for these paths having west-east and east-west components of VLF subionospheric propagation, respectively, which agree with previously published results and within 10% with theoretical values. We have also compared the DMS estimated using a terminator time method with that calculated using terminator speed for a particular day and found both the values to be consistent. The hN values were found to be maximum during winter of Southern Hemisphere for NWC signal and winter of Northern Hemisphere for NLK signal VLF propagation paths to Suva. The hN also shows significant day-to-day and seasonal variabilities with a maximum of about 10 km and 23 km for NWC and NLK signal propagation paths, respectively, which could be due to the atmospheric gravity waves associated with solar terminator transition, as well as meteorological factors such as strong lightnings.

Exploiting LF/MF signals of opportunity for lower ionospheric remote sensing

Abstract: We introduce a method to diagnose and track the D–region ionosphere (60−100 km). This region is important for long distance terrestrial communication, and is impacted by a variety of geophysical phenomena, but it is traditionally very difficult to detect. Modern remote sensing methods used to study the D-region are predominately near the Very Low Frequency (VLF, 3−30 kHz) band, with some work also done in the High Frequency and Very High Frequency bands (HF/VHF, 3−300 MHz). However, the frequency band between VLF and HF has been largely ignored as a diagnostic tool for the ionosphere. In this paper, we evaluate the use of 300 kHz radio reflections as a diagnostic tool for characterizing the D-region of the ionosphere. We present radio receiver data, analyze diurnal trends in the signal from these transmitters, and identify ionospheric disturbances impacting LF/MF propagation. We find that 300 kHz remote sensing may allow a unique method for D-region diagnostics compared to both the VLF and HF/VHF frequency bands, due to a more direct ionospheric reflection coefficient calculation method with high temporal resolution without the use of forward modeling.

Study of solar flare induced D-region ionosphere changes using VLF amplitude observations at a low-latitude site

Abstract: We obtained and analyzed 26 solar flare events from C2.56 to X3.2 classes at Tay Nguyen University, Vietnam (12.56 ° N, 108.02 ° E) during May – December, 2013 using Very Low Frequency remote sensing to understand the responses of low-latitude D-region ionosphere during solar flares. The observed VLF amplitude perturbations are used as the input parameters for the simulated LWPC program, using Wait’s model of lower ionosphere, to calculate two Wait’s parameters: the reflection height, H’ and the sharpness factor, b . Results reveal that when the X-ray irradiance increased, the b increased from 0.3 km -1 to 0.506 km -1 , while the H' decreased from 74 km to 60 km. The electron density increases at the height of 74 km with 1-3 orders of magnitude during solar flares . These phenomena can be explained that the ionization due to X-ray irradiance becomes greater than that due to cosmic rays and Lyman- a radiation, which increases the electron density profile. The changes rules of the Wait’s parameters and electron density of D-region ionosphere of our results are in agreement with the studied results shown by other authors. The 3D representation of the electron density changes with altitude and time supports to fully understand the shape of the electron density changes due to X-ray flares. The shape variation of electron density is roughly followed to the variation of the amplitude perturbation and keeps this rule for the different altitude. We also found that the electron density versus the height in lower latitude D-region ionosphere increases more rapidly during solar flares.

Studies of seismo-ionospheric correlations using anomalies in phase of very low frequency signal

Abstract: We present the results of the statistical analysis of very low frequency (VLF) signal transmitted from NWC transmitter (latitude 21.8° S, longitude 114.15° E) at 19.8 kHz received at Ionospheric and Earthquake Research Centre (IERC) (latitude 22.50° N, longitude 87.79° E). We analyse the phase of VLF signal for the whole year 2011 and compute the sunrise and sunset terminator time from it. We observe unusual shift of these sunrise and sunset terminator time during seismic events for which the earthquake epicentres are in India and its subcontinent region. We calculate the total energy accumulation by all those earthquakes for a single day and compute the effective magnitude of all the earthquakes for that day which behaves as a single quake. We compute a cross correlation between fluctuation in terminator time shifts from the normal value with effective magnitude of earthquake. We found that the unusual fluctuations in terminator time are well correlated with earthquake magnitude and the fluctuati...

The spatial distribution features of three ALPHA transmitter signals at the topside ionosphere

Abstract: The spatial distribution features of electric field over three Alpha transmitters in Russia were analyzed based on the Demeter satellite records at local nighttime during the solar minimum in December of 2008, where the three transmitters are with the same emitted power of 500 kW and the same radio waves at 11.9 kHz, 12.6 kHz, and 14.9 kHz. The results of observations showed that the maximal electric field reached −80 to −70 dB (hereafter referred as to V/m) at 660 km altitude, and the horizontal covered area even exceeded 80° in longitude with electric field above −100 dB at 14.9 kHz. The lowest electric field and the smallest longitude scale were detected over Krasnodar (KRA), which is demonstrated that the lower ionosphere plays an important role in attenuating the energy as suggested by the simulation results from the full-wave propagation model. Another feature over KRA was the significant decrease in electromagnetic field strength at 11.9 kHz and 12.6 kHz, being one order of magnitude lower than the other two transmitters, where the lower hybrid resonance waves affected severely the whistler mode wave mode propagation. Compared with the ground very low frequency observations at Tonghai and Ya'an in China, the most complex variations were observed from KRA, while the east transmitter Khabarovsk maintained high strength of electromagnetic power in a longer distance than the middle transmitter Novosibirsk in local nighttime, which is consistent with the large covering scale in the topside ionosphere due to the enhancement by wave-particle interaction from the other transmitter.

Interaction of Very Low Frequency (VLF) and Extremely Low Frequency (ELF) Waves in the Ionospheric Plasma and Parametric Antenna Concept

Abstract: Kim, Tony C. Ph.D., Environmental Science Ph.D. Program, Department of Physics, Wright State University, 2017. Interaction of Very Low Frequency (VLF) and Extremely Low Frequency (ELF) Waves in the Ionospheric Plasma and Parametric Antenna Concept. This research dramatically increase radiation efficiency of very low frequency (VLF) and extremely low frequency (ELF) antenna in the ionosphere by implementing a concept of a parametric antenna. The research addresses the interaction of the electromagnetic waves in the atmosphere; analyzes the radiation efficiency of different types of RF frequencies (ex: Very low Frequency (VLF) and Extremely Low Frequency (ELF)); and includes different types of antennas, such as dipole and loop antennas, in the ionosphere environment and simulating the differences to verify the parametric antenna concept. This VLF analysis can be performed many ways and this VLF frequency is widely used in space antennas by both military and civilian elements. The VLF waves in the ionosphere are used to create high levels of density irregularities in the radiation belt region and to deflect the energetic electrons and ions from the region to prevent their negative effects on satellite electronics (including the antenna). Therefore, this research addresses the problem of low radiation efficiency of satellite based antenna on conventional loop and dipole antennas used for excitation of electromagnetic VLF/ELF waves in the ionosphere. The research results will be used in the field of ionospheric plasma physics research with applications in satellite space experiments. In particular, the results will be influential in the area of active space experiments for the removal of highly energetic particles in the ionosphere which are harmful to satellite electronics, VLF/ELF communications, and for different commercial applications. This research first looks at a theoretical solution followed by modeling and simulation to prove the parametric antenna concept. Finally, experimentation was performed in the laboratory to validate and verify a theoretical solution and modeling and simulation of parametric antenna.

Stochastic resonance for underwater VLF weak signal detection under lévy noise background

Abstract: In this paper, the second-order stochastic resonance (SR) system with Levy noise is analyzed, and the improved numerical method is proposed to obtain the output signal sequence after the SR system. Then the matched stochastic resonance (MSR) system is investigated and derived to produce a stable SR phenomenon in Levy noise circumstance. The matched performance is then evaluated which reflected the efficiency for arbitrary frequencies. And it seems achieve a better enhancing performance in the very low frequency band (0.01–100Hz), which is considered as the ship seismic signal. Moreover, the simulation results of the frequency detection verify the effectiveness of the MSR system. It is extremely helpful to detect the underwater very low frequency (VLF) weak signal under the strong background noise of the complex marine environment.

On the VLF wave generation by beating of two HF heaters

Abstract: Nighttime very low frequency (VLF) wave generation high frequency (HF) heating experiments were performed under three distinctive ionospheric situations, employing two x-mode transmissions with a frequency difference of 3.5–9.5 kHz. The three situations were HF heater transmissions: (1) reflecting well below, (2) reflecting slightly below, and (3) penetrating through the F-peak (i.e., foF2 layer). The results reveal that (1) the N-S component of the VLF wave magnetic field dominated in all measurements; (2) the VLF radiation intensity is the strongest when the HF heaters are reflected slightly below the foF2 layer, i.e., in situation (2); (3) the VLF radiation intensity does not vary strongly with frequency; however, there is an anomaly in situation (2), the 5.5 kHz radiation intensity is approximately 7 dB above the intensity trend with frequency in situations (1) and (3). In situation (2), foF2 was decreasing with time, while a density cusp appeared slightly below the heater reflection height. Theoretic...

Behaviour of electron content in the ionospheric D-region during solar X-ray flares

Abstract: One of the most important parameters in ionospheric plasma research also having a wide practical application in wireless satellite telecommunications is the total electron content (TEC) representing the columnal electron number density. The F region with high electron density provides the biggest contribution to TEC while the relatively weakly ionized plasma of the D region (60 km - 90 km above Earths surface) is often considered as a negligible cause of satellite signal disturbances. However, sudden intensive ionization processes like those induced by solar X ray flares can cause relative increases of electron density that are significantly larger in the D-region than in regions at higher altitudes. Therefore, one cannot exclude a priori the D region from investigations of ionospheric influences on propagation of electromagnetic signals emitted by satellites. We discuss here this problem which has not been sufficiently treated in literature so far. The obtained results are based on data collected from the D region monitoring by very low frequency radio waves and on vertical TEC calculations from the Global Navigation Satellite System (GNSS) signal analyses, and they show noticeable variations in the D region electron content (TECD) during activity of a solar X ray flare (it rises by a factor of 136 in the considered case) when TECD contribution to TEC can reach several percent and which cannot be neglected in practical applications like global positioning procedures by satellites.

RESPONSES OF THE IONOSPHERIC D-REGION TO PERIODIC AND TRANSIENT VARIATIONS OF THE IONIZING SOLAR Lyα RADIATION

Abstract: Solar radiation has the most important role in periodical variation of terrestrial atmospheric properties. Under unperturbed ionospheric conditions, the solar Lyα line has a dominant influence on ionization processes in the lowest ionospheric layer, the so called D-region. In this paper, we present periodical and transient variations in influences of the Lyα radiation on this ionospheric layer. In the case of periodical lower ionospheric changes we consider diurnal, seasonal and solar cycle variations and show analysis of acoustic and gravity waves induced by solar terminator. Influences of solar flares and eclipses on this atmospheric layer are analyzed as examples of sudden ionospheric disturbances. For decades, Very Low Frequency radio signals (3 – 30 kHz) are successfully used as a tool for monitoring of changes in the lower ionosphere, based on radio wave propagation through Earth-ionosphere waveguide along given trajectories and registration of their physical parameters (amplitude and phase delay). For the analysis conducted in this paper, we used records of the VLF DHO signal, emitted on 23.4 kHz frequency from transmitter in Germany and received in Serbia.

Environmental Noise Cancellation for High-TC SQUID-Based Magnetocardiography Systems Using a Bistage Active Shield

Abstract: An active noise cancellation method is proposed for superconducting quantum interference devices (SQUID)-based magnetocardiography systems working out of magnetically shielded rooms. Using YBCO high-Tc rf-SQUID magnetometers as magnetic field sensors, an active shielding system was implemented based on this method. This method incorporates two different shielding frequency regimes of operation simultaneously. This is because the unwanted background magnetic field signals range from very low frequencies up to high frequencies with a wide range of amplitudes at the upper and lower frequency spectra. Therefore, the shielding system is designed in a bistage configuration, and each stage covers one part of the frequency spectrum. Each shielding stage is based on a coil designed and optimized by finite element method calculations. One of the coils is used for compensating high amplitude and very low frequency (0–0.1 Hz) far-field environmental magnetic field noise, and the second one is used for relatively low amplitude and higher frequency (0.1–100 Hz) magnetic field noise. Because of these frequency and amplitude characteristics, each coil uses independent control circuit. For designing the controllers, methods based on the dynamic model approximation of the first and second orders were examined, which led to optimized proportional-integral-derivative controllers. The shielding efficiency of the proposed bistage active shielding system shows a significant improved effectiveness compared to that of the conventional one stage active shielding systems.

High Sensitivity Very Low Frequency Receiver for Earthquake Data Acquisition

Abstract: A high sensitivity very low frequency (VLF) receiver is developed based on AD744 monolithic operational amplifier (Op-Amp) for earthquake data acquisition. In research related natural phenomena such as atmospheric noise, lightning and earthquake, a VLF receiver particularly with high sensitivity is utterly required due to the low power of VLF wave signals received by the antenna. The developed receiver is intended to have high sensitivity reception for the signals in frequency range of 10-30 kHz allocated for earthquake observation. The VLF receiver which is portably designed is also equipped with an output port connectable to the soundcard of personal computer for further data acquisition. After obtaining the optimum design, the hardware realization is implemented on a printed circuit board (PCB) for experimental characterization. It shows that the sensitivity of realized VLF receiver is almost linear in the predefined frequency range for the input signals lower than -12dBm and to be quadratic for the higher level input signals.