Optimization of a Radio-frequency Atomic Magnetometer Toward Very Low Frequency Signal Reception

Many patients with severe obstructive sleep apnea (OSA) do not complain of excessive daytime sleepiness (EDS), possibly due to increased sympathetic nervous activity (SNA) and accompanying heightened alertness. We hypothesized that in patients with OSA, those without subjective EDS (Epworth Sleepiness Scale, ESS score < 11) would have higher very low frequency (VLF) heart rate variability (HRV) during sleep, reflecting greater sympathetic heart rate modulation than patients with an ESS score ≥ 11. Patients with severe OSA (AHI ≥ 30: 26 with and 65 without heart failure) were divided into those with and without EDS. Heart rate (HR) signals were acquired in stage 2 sleep during periods of recurrent apneas and hypopneas and submitted to coarse graining spectral analysis, which extracts harmonic, neurally mediated contributions to HRV from total spectral power. Because the apnea-hyperpnea cycle entrains muscle SNA at VLF (0 to 0.04 Hz), VLF power was our principal between-group comparison. Subjects without EDS had higher harmonic VLF power (944 ± 839 vs 447 ± 461 msec(2), p = 0.003) than those with EDS, irrespective of the presence or absence of heart failure (1218 ± 944 vs 426 ± 299 msec(2), p = 0.043, and 1029 ± 873 vs 503 ± 533 msec(2), p = 0.003, respectively). ESS scores correlated inversely with VLF power in all (r = -0.294, p = 0.005) and in heart failure subjects (r = -0.468, p = 0.016). Patients with severe OSA but without EDS have higher VLF-HRV than those with EDS. This finding suggests that patients with severe OSA but without EDS have greater sympathetic modulation of HRV than those with EDS that may reflect elevated adrenergically mediated alertness. Taranto Montemurro L; Floras JS; Picton P; Kasai T; Alshaer H; Gabriel JM; Bradley TD. Relationship of heart rate variability to sleepiness in patients with obstructive sleep apnea with and without heart failure.

Blood pressure variability: a complex phenomenon The pathophysiological and clinical relevance of blood pressure variability (BPV) is still a matter of lively debate. Although BPV is increasingly recognized as an independent risk factor for target organ damage and cardiovascular events, including stroke in hypertensive patients [1–8], the mechanisms responsible for it are not yet fully elucidated. This is because BPV is a complex phenomenon that includes different components characterized by different frequencies of BP fluctuations [9–12], that is, the so-called high-frequency components (between 0.15 and 0.5 Hz in humans), mainly related to the mechanics of respiration; the low-frequency components (between 0.04 and 0.15 Hz), considered an expression of both sympathetic modulation of peripheral resistance and resonance in the baroreflex loop (see below); and the very low frequency (VLF) components (VLF <0.04 Hz) which include the slowest fluctuations with periods ranging from minutes to hours, as a function of the recording period length. The genesis of VLF components seems to depend on multifold mechanisms, including myogenic tone, thermoregulation, physical activity and sympathetic efferent modulation [9–12]. The BPV complexity in terms of frequency components is accompanied by the concomitant complexity in the responsible mechanisms, including neural and nonneural factors [11,13]. Neural influences may contribute to an increase or a reduction in total BPV by central modulation of cardiac and vascular targets through efferent sympathetic nerves, often triggered by behavioural factors such as exercise, emotion and sleep [11,14–16]. Neural influences originating in the arterial baroreflex loop are also involved and play a complex role in modulating BPV. On the one hand, the arterial baroreflex is actively involved in buffering total BPV, such that baroreflex sensitivity shows an inverse relationship with total BPV. This means that whenever the baroreflex is more effective, BPV tends to be reduced and vice versa [17–19]. On the other hand, arterial baroreflexes may paradoxically exert a promoting role for low-frequency BP oscillations through a resonance phenomenon in the baroreflex loop [20–24]. The role of arterial baroreflexes in modulating very low frequency components has been specifically and deeply investigated by assessing broad band spectral powers of BP and pulse interval [the reciprocal of heart rate (HR)] recorded in controlled conditions and after surgical baroreceptor deafferentation by sinoaortic denervation (SAD) in various animal models [18,25]. SAD was responsible for changes in all the spectral components of systolic BP and pulse interval, thus suggesting the involvement of baroreflex modulation in the genesis of not only the fast but also the slower components of BP and pulse interval variability. In particular, although the baroreflex seems to play a pro-oscillatory role on virtually all the components of pulse interval, the influence of the baroreflex on the spectral components of systolic BP seems to be a much more complex one. In fact, SAD was associated with a significant reduction in the power of spectral components in the low-frequency region, an increase in the power of components in the VLF region and almost no change in the power of high-frequency BP spectral components. These findings indicate that the baroreflex plays a different role in modulating BP fluctuations according to the frequency. It exerts a negligible effect on respiratory components, an expected buffering role in the VLF region and a paradoxical pro-oscillatory role in the low frequency region. In particular, VLF BP fluctuations seem to be the result of opposing interactions between haemodynamic perturbations and the corrective feedback provided by the sympathetic vascular component of the arterial baroreflex. This is demonstrated by the similarity of the effects of SAD [25], chronic chemical sympathectomy [26] and acute neurohumoral blockade combined with noradrenaline infusion on BPV [27]. Worth noting is the fact that these studies have also provided indirect evidence that one major source of slow haemodynamic perturbations is an autoregulatory-like (probably myogenic) response in several regional circulations. It is, thus, likely that in the conscious rat, the relative importance of myogenic and baroreflex responses largely determines the net changes in vascular resistance of regional vascular beds and hence the changes in total peripheral resistances (TPR). Finally, it should be considered that among nonneural mechanisms, apart from myogenic tone, endothelial factors and angiotensin II may also directly affect vasomotor activity and thus BPV [28,29], further increasing the overall complexity of the integrated modulation of this phenomenon. The role of myogenic responsiveness A recent contribution to the assessment of the mechanisms contributing to BPV and its clinical relevance is provided by a work by Stauss et al.[30] published in this issue. Stauss et al. put forward the provocative hypothesis that genetic predisposition to hemorrhagic stroke might be associated with a decreased rather than an increased BPV, and more specifically, with a decrease in its slow (or VLF) component. According to Stauss et al.[30], such a decrease is mostly mediated by a decreased myogenic responsiveness observed in the cerebral vasculature as well as in the whole arterial system of stroke-prone spontaneously hypertensive rats (SHRs). The suggestion is intriguing, but a few critical considerations need to be made in relation to this work. First of all, it is important to note that the bulk of BPV is concentrated at very low frequencies in both humans (VLF band starts at <0.04–0.05 Hz) [31] and rats (the corresponding frequencies being <0.15 Hz) [23]. Thus, overall BPV (when estimated from global indices such as the standard deviation of beat-to-beat BP values) is roughly equivalent to its VLF component, especially in the case of long-lasting recordings. The question is then which mechanism might link stroke with a decrease in the amplitude of VLF fluctuations in BP. In the brain, myogenic constriction of proximal arteries prevents BP surges from being transmitted to the downstream vasculature and is, thereby, an important physiological mechanism of protection against hemorrhagic stroke. Stroke-prone SHRs exhibit an impaired cerebrovascular myogenic responsiveness prior to stroke, which might facilitate the initiation of haemorrhage [32]. The hypothesis tested by Stauss et al.[30] is that the decreased myogenic responsiveness observed in the cerebral vasculature of stroke-prone SHRs is part of a generalized impairment of myogenic responses in the whole circulation, which, in turn, is reflected in a decreased VLF component of BPV. The myogenic response is the primary mechanism of pressure autoregulation of blood flow in regional circulations. The myogenic response and BP tend to form a vicious circle because any change in BP will be amplified by a parallel change in regional vascular resistances. Following this reasoning, Cowley et al.[33] stated that autoregulation ‘…would have devastating effects on arterial pressure control if (it) predominated in all regions of the systemic circulation…’. In other words, the myogenic response, if unopposed, would dramatically increase BPV. Reciprocally, a generalized impairment of myogenic responsiveness would result in a reduced BPV. Considering the kinetics of the myogenic response [34], this effect would be restricted to VLF fluctuations in BP. In their study, Stauss et al.[30] report that stroke-prone SHRs have a decreased VLF component of BPV compared with stroke-resistant SHRs. They demonstrate that nifedipine, which blocks the myogenic response, decreases the VLF fluctuations of BP in both stroke-prone and stroke-resistant SHRs, even after partial restoration of BP with an arginine vasopressin (AVP) infusion. This effect is not observed with sodium nitroprusside combined with AVP infusion. Finally, it is shown that in conscious dogs, nifedipine decreases the gain of the transfer function relating TPR to BP, which is also taken to indicate that the myogenic response contributes to VLF BPV. The approach followed by Stauss et al.[30], however, has some limitations. One of them is that dihydropyridine L-type calcium channel blockers not only block the myogenic response but also strongly interfere with the action of most endogenous vasoconstrictors. Therefore, any direct neurohumoral contribution to BPV, including that of sympathetic influences, was probably attenuated after nifedipine administration. In this respect, it is remarkable that the reduction of BPV after nifedipine administration was not restricted to the VLF band but was also observed in the low frequency band, where it is established that myogenic responses are not effective, and sympathetic effects predominate [24]. The experiment with sodium nitroprusside was important to control the effects of lowering the BP level, but giving nitric oxide is not equivalent to blocking L-type calcium channels in terms of vascular reactivity to endogenous vasoconstrictors. Another limitation of the study by Stauss et al.[30] relates to the use they make of the transfer function relating BP (input signal) and TPR (output signal) in conscious dogs. The properties of this transfer function (gain and phase functions) were supposedly supporting the statement that the myogenic response directly has an impact on BPV in the VLF range. However, the use of the cross-spectral transfer function between the mean arterial pressure (MAP) and TPR signals is simply invalid here. First, the cross-spectral transfer function is a noncausal correlation technique that does not have directionality. When this technique is used in an open loop condition, the results are clear, but in a closed loop condition, as in the present study, the results are open to interpretation. In this case, we absolutely know that TPR affects MAP, as the authors acknowledge, but the whole point of this article is to develop a technique to somehow demonstrate that MAP may affect TPR through a myogenic positive feedback mechanism. The phase of 0° with no change with frequency in the VLF range is highly consistent with TPR affecting MAP (and not vice versa), which can be expected to happen almost instantly as there is a direct link between TPR and MAP. There are autoregressive techniques that can be used to measure transfer relations during closed-loop operation [35] by building causality into the formulation, but the cross-spectral method cannot be used for the same purpose. The other option would have been to perform open-loop experiments. Indeed, to properly interpret the BP–TPR transfer function, the baroreflex system would need to be opened, either mechanically (by using the Moissejeff's procedure) or pharmacologically (by blocking the autonomic nervous system). In the open-loop configuration, myogenic responses are left unopposed and the BP–TPR phase decreases linearly with a slope proportional to the fixed time delay inherent in the response. As previously mentioned, in conscious rats, an effect common to SAD and chronic sympathectomy with guanethidine is a marked enhancement of VLF fluctuations in BP [36,37]. Taken together, these simple observations strongly suggest that the sympathetic component of the baroreceptor reflex plays an important role in limiting VLF fluctuations of BP. Surprisingly, the renal sympathetic nerve activity (RSNA) of normal rats is, however, only weakly related to BP in the VLF band as indicated by the low coherence values computed between the two variables [23,36]. This lack of coupling points either to nonlinearities in the relation or to the interference of noise sources affecting either variable. Obviously, applying coherence analysis to BP time series containing both neurally mediated and nonneurally mediated BP variations would inevitably result in low BP–RSNA coherence values. Similarly, coherence values would remain low if RSNA time series were containing both baroreflex-mediated and nonbaroreflex-mediated RSNA variations. Interestingly, the SAD procedure did not noticeably alter the amplitude of VLF fluctuations of RSNA, which demonstrates that central nervous structures can generate slow fluctuations of RSNA in the absence of baroreflex influences. On the other hand, coherence between BP and RSNA in the VLF band was not increased after SAD, thus indicating that BP tends to vary independent of RSNA after baroreceptor denervation [23,36]. In an attempt to identify the nonneural mechanisms contributing to VLF fluctuations in BP, haemodynamic studies have been performed on conscious rats after acute ganglionic blockade [27]. To allow for any BP fall due to vasodilator influences, the basal mean level of vascular tone was restored with a continuous infusion of noradrenaline. In this areflexic preparation, BP was highly unstable, and this lability was secondary to slow fluctuations (mainly increases) of systemic vascular conductance (the reciprocal of TPR). Interestingly, the BP changes were usually initiated by sharp decreases in stroke volume and cardiac output so that the conductance changes lagged behind the BP changes by about 1 s, and then amplified and prolonged the BP variations. The latter sequence of events was demonstrated by computing the phase function between BP (input signal) and systemic vascular conductance (output signal). Phase decreased linearly as a function of frequency in the 0–0.2 Hz frequency range, indicating the presence of a 1-s fixed-time delay between BP and conductance [27]. This haemodynamic pattern is strongly suggestive of predominant autoregulatory behaviour, possibly involving a myogenic response in regional vascular beds. The measurement of muscular and mesenteric blood flows in areflexic rats confirmed that autoregulatory-like (presumably myogenic) responses are a major source of BPV in the absence of a neural control of the circulation [27]. In sympathectomized rats, another source of haemodynamic perturbations has been identified in the skeletal muscle circulation where abrupt vasodilatations invariably occur at the onset of body movements and initiate a fall in BP [26]. As mentioned above, these experimental data led some investigators to formulate the hypothesis that VLF fluctuations in BP largely result from the continuous interplay between haemodynamic perturbations (especially the myogenic response) and the corrective action provided by the sympathetic limb of the baroreceptor reflex (Fig. 1) [38]. By using BP and RSNA time series collected in conscious SAD rats and parameters of the transfer function relating RSNA to BP, it was examined whether BP and RSNA variabilities actually observed in baroreceptor-intact rats could be predicted [23]. By progressively increasing the baroreflex gain, it was possible to compute virtual BP and RSNA power spectra that increasingly deviated from their progenitor spectra. In particular, BP spectral power decreased in the VLF range (as a result of baroreflex buffering of haemodynamic perturbations) and increased in the low-frequency band (as a result of increasing instability at the resonance frequency of the loop, i.e. at the frequency of the so-called Mayer waves).Fig. 1Therefore, the finding of a decreased VLF power in BP spectra can be interpreted as resulting either from a decreased influence of haemodynamic perturbations (including the myogenic response) or from an increased effectiveness of the sympathetic baroreflex (or, of course, from any combination of both). Luft et al.[39] have indeed reported that splanchnic sympathetic nerve activity is more reactive to BP changes in conscious stroke-prone SHRs than in WKY rats. In conclusion, whereas the study by Stauss et al.[30] indicates that the myogenic response may contribute to BP variability in addition to neural factors, it does not seem to provide a final demonstration that this mechanism is the main determinant of VLF BP fluctuations and, thus, that the latter might provide an indicator of the risk of hemorrhagic stroke, at least in the experimental animal. Such a stimulating issue would, thus, need to be further addressed through a more adequate methodological approach.

Partial discharge (PD) diagnostic tests plays an important role on condition monitoring and fault diagnosis of power equipment insulation. With the very low frequency (VLF) diagnostic test widely used in the power system, the power capacity can be remarkably reduced by lowering the applied frequency. However, simulating the corona discharge process in insulating oil from power frequency to very low frequency is difficult because the corona discharge process in insulating oil at different frequencies takes much longer than a single corona discharge in insulating oil. Therefore, it is necessary to establish the simulation and experiment of corona discharge in insulating oil from power frequency to very low frequency for this problem. A simulation model of corona discharge in insulating oil from power frequency to very low frequency based on the finite element method is proposed in this paper, in which needle-plate electrode model based on CIGRE Method II was selected. Then making the dielectric constant of materials at frequencies from 50 Hz to 0.1 Hz was measured from experiments as parameter variable. Finally, with simulation model based on the Poisson equation and hydrodynamic continuity equations performed in COMSOL Multiphysics, the current intensity, the distribution of electric field intensity, and the development of streamer in corona discharge in insulating oil were analyzed. According to the simulation results, with the decrease of frequency, the electric field in insulating oil increased slightly at first and then decreased sharply. Simulation results were in good agreement with experiments, in which the maximum discharge, average discharge, and pulse repetition rate decreased significantly at low frequencies. This paper shows the characteristics of corona discharge in insulating oil from power frequency to very low frequency by simulation and experiment, providing the reference for insulation diagnosis of power equipment with PD test at very low frequency and studying the mechanism of frequency effect on corona discharge.

Актуальность исследования обусловлена экономическим интересом к использованию простых, эффективных и высокопроизводительных геофизических технологий для поиска и разведки рудных месторождений. Цель: оценить информативность геоэлектрических разрезов, построенных по импедансным данным метода радиокомпарации и пеленгации (радиокип), при изучении коры выветривания и кровли коренных пород в различных геологических условиях. Объекты исследования находятся в Дегтярско-Полевском рудном районе Среднего Урала, в приконтактовой зоне Серовско-Маукского разлома, разделяющего комплексы пород Тагильского прогиба и Восточно-Уральского поднятия. Геофизические профили располагались на территории Крылатовского золотокварцевого месторождения, Чусовского медно-колчеданного месторождения, Кунгурском и Волчихинском участках. Методы. Полевые измерения методом радиокип проводились широкополосным приемником «ОМАР-2м» со стелющейся линией и индукционным датчиком переменного магнитного поля. Осуществлялись наблюдения ортогональных компонент поля сигналов сверхдлинноволновых радиостанций. В качестве сравнения использовались результаты аудиомагнитотеллурических и вертикальных электрических зондирований, а также информация по геологическому изучению территории. Количественная обработка электроразведочных данных осуществлялась с применением программного обеспечения, разработанного в различных научных учреждениях. Результаты. Проведенные исследования показали хорошую сопоставимость результатов инверсии радиокип сверхдлинноволновых радиостанций с геоэлектрическими разрезами, полученными другими методами электромагнитных зондирований. Радиокип не может отобразить всех особенностей строения верхней части разреза, но позволяет выделить характерные черты коренных пород и определить основную границу между рыхлыми отложениями и основанием разреза. Полученные результаты указывают на принципиальную возможность обнаружения глубокозалегающих рудных объектов с помощью метода радиокип сверхдлинноволновых радиостанций. Метод отличается хорошей производительностью, поэтому можно добиться высокой детальности наблюдений при небольших затратах. Геоэлектрические разрезы радиокип могут использоваться для оперативной оценки общей мощности коры выветривания и выбора точек зондирований другими электромагнитными методами для уточнения геологической обстановки.

296 Spectral analysis of cardiovascular control after spinal cord injury in rats: Effect of time post-injury

Abstract. The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures, but data are inevitably contaminated by noise from various sources. It is common practice to gate signals from TEM systems to reduce the amount of data and improve the signal-to-noise ratio (SNR). Gating acts as a filter, and optimum gating will pass the TEM signal un-attenuated while suppressing noise. In systems based on analog boxcar integrators, the gating corresponds to filtering with a square window. The frequency response of this window shape has large side lobes, which are often insufficient in attenuating noise, e.g., from radio signals in the very low frequency (VLF) 3–30 kHz band. Tapered gates have better side lobe suppression and attenuate noise better, but tapering with analog boxcar integrators is difficult. We propose using many short boxcar gates, denoted sub-gates, and combine the sub-gates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR. The semi-tapering approach is analyzed and tested experimentally on data from a roving TEM system. We quantify the effect of semi-tapered gates by computing an improvement factor as the ratio between the standard error of data measured with boxcar gates and the standard error of data measured with semi-tapered gates. Data from a test survey in Gedved, Denmark, with 1825 measurements gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centered between 78.7 and 978.1 µs. After inversion of the data, we find that semi-tapering increases the depth of investigation by about 20 % for this specific survey. We conclude that the semi-tapered approach is a viable path towards increasing SNR in TEM systems based on analog boxcar integrators.

The Very Low Frequency (VLF) signal of 10 kHz has strong penetrability of ground objects. Because of the antenna size, its application is limited. Therefore, it is important to study the VLF signal generation method based on appropriately sized high frequency radar antennas. The concept of generating VLF signal with high frequency array antenna is proposed in this paper. The waveform of the emission signal, staggered array structure design, and array parameter selection methods are presented and discussed. The pulse width of the composite signal is increased by using periodic pulse signals as radiation element signals. The resting period of the pulse signals is filled with the pulse width expansion generated by the array and the VLF signal with continuous time is composed in the target area. The performance of the composite VLF signal and the energy utilization of the emission signal are evaluated using Peak SideLobe Ratio (PSLR), Integrated SideLobe Ratio (ISLR) and through the spectrum comparison between the emission signal and the composite signal. With the 10 kHz VLF signal composed by 100 MHz radiant element signals, the hundred meter array is simulated. When the staggered array is constituted by nine arrays and the pulse width of the radiation element is set to 0.115 μs, PSLR and ISLR of the composite signal spectrum are –13.34 dB and –9.44 dB respectively, and the energy proportion of 10 kHz low-frequency signal in the composite signal is 89.79%. The effects of radiation element spacing error, time error, phase error and amplitude error of the radiation element signal and the target’s deviation are analyzed. It is found that the proposed method is an effective one and the simulation results have illustrated the effectiveness.

Just like in the case of any electromagnetic (EM) method, the electrical properties of the ground affect the behaviour of radio waves as well. The first EM measurement using radio frequencies applied wave-tilt techniques and was made at relatively high frequencies with shallow penetration depth [1]. The earliest EM measurement with radio waves of 3–30 kHz was carried out in 1963 with the aim of ore prospecting [2]. In the late sixties commercially available ground very low frequency (VLF) instruments were introduced into near-surface exploration. These instruments can be used to observe either the magnetic field and/or to determine the terrain’s apparent resistivity. Over 1D half-space the magnetic field at the surface is linearly polarized. However in the presence of a lateral conductivity inhomogeneity – situated between the surface and skin depth – the total magnetic field at the surface will be elliptically polarized due to the induced magnetic field. Usually the induced vertical magnetic field component is small compared with the primary azimuthal magnetic field component. In this case the ellipticity of the magnetic polarization ellipse is approximately equal to the quadrature component of the ratio of the vertical and the azimuthal magnetic component, and the tilt angle of the ellipse approximately equals the real component of the same ratio [3]. If the radial electric field and the azimuthal magnetic field component are known the apparent resistivity at the VLF frequencies can be derived. Takacs was the first in Hungary to develop the radiokip method with instruments to apply the EM fields of distant LF transmitters for near-surface geological explorations [4]. The VLF method was introduced and intensively used in Transdanubian Central Range bauxite exploration by ELGI [5]. In the frame of this work Farkas developed the VLF invariant resistivity method based on the concept of the magnetotelluric impedance tensor [6]. The VLF method utilizes the frequency range of 10 kHz–30 kHz, providing poor depth resolution. To overcome this resolution problem this frequency range was extended and in addition to the VLF carrier waves the low frequency (LF) signals from civilian radiotransmitters are also utilized by the RMT (Radiomagnetotelluric) method. Takacs carried out and interpreted the first RMT soundings in Hungary in the range of 18.3 kHz and 630 kHz. He measured both the electric and magnetic field components and MT 1D inversion was applied [7]. The radiofrequency resistivity (RF-R) device measuring in the range of 12 kHz–240 kHz was successfully applied to delineate karst structures [8]. In 1973 Tilsley applied a portable VLF transmitter as a supplementary source to the regular VLF transmitters [9]. To cope with the interpretation problem arising from the mutual position between transmitter and structural strike direction or to overcome the poor coupling with the target the use of a portable transmitter can be recommended. For the determination of sufficient distance between the portable VLF transmitter and VLF profiles numerical modelling is also needed [10].

The theme of this paper is to establish a reliable ionospheric very low frequency (VLF) transmitter, which is also broad band. Two approaches are studied that generate VLF waves in the ionosphere. The first, classic approach employs a ground-based HF heater to directly modulate the high latitude ionospheric, or auroral electrojet. In the classic approach, the intensity-modulated HF heater induces an alternating current in the electrojet, which serves as a virtual antenna to transmit VLF waves. The spatial and temporal variations of the electrojet impact the reliability of the classic approach. The second, beat-wave approach also employs a ground-based HF heater; however, in this approach, the heater operates in a continuous wave mode at two HF frequencies separated by the desired VLF frequency. Theories for both approaches are formulated, calculations performed with numerical model simulations, and the calculations are compared to experimental results. Theory for the classic approach shows that an HF heater wave, intensity-modulated at VLF, modulates the electron temperature dependent electrical conductivity of the ionospheric electrojet, which, in turn, induces an ac electrojet current. Thus, the electrojet becomes a virtual VLF antenna. The numerical results show that the radiation intensity of the modulated electrojet decreases with an increase in VLF radiation frequency. Theory for the beat wave approach shows that the VLF radiation intensity depends upon the HF heater intensity rather than the electrojet strength, and yet this approach can also modulate the electrojet when present. HF heater experiments were conducted for both the intensity modulated and beat wave approaches. VLF radiations were generated and the experimental results confirm the numerical simulations. Theory and experimental results both show that in the absence of the electrojet, VLF radiation from the F-region is generated via the beat wave approach. Additionally, the beat wave approach generates VLF radiations over a larger frequency band than by the modulated electrojet.

A large number of sferics were photographically recorded in the very low frequency (VLF) and extremely low frequency (ELF) bands at a UCLA field station in Hawaii. From the characteristic VLF waveforms it was clear that the VLF signals were generated from lightning discharges. It was found that an observable ELF component (slow tail) followed the VLF component in almost all cases. It was also found that about one third of the sferics observed were ELF signals, similar in appearance to slow tails but not preceded by observable VLF oscillations. Peak amplitudes were measured for both the VLF and ELF components of almost 3000 sferics. The results were tabulated in groups according to (1) whether the sferics were recorded during the day or during the night, (2) whether the polarity of the initial excursion of the ELF signal was positive or negative, and (3) whether the VLF and ELF components appeared together or separately. Amplitude distribution histograms were plotted for all cases. For those sferics possessing both VLF and ELF components, the VLF to ELF peak amplitude ratios were also tabulated separately as in (1) and (2) above, and ratio-distribution histograms were plotted. The more important results obtained from the histograms were as follows. 1. No significant differences were found between the amplitude distributions for the ELF waveforms that were preceded by VLF oscillations and those that were not. Hence, it is probable that both groups were generated by lightning discharges. 2. For both daytime and nighttime sferics the median value of the ELF amplitude was greater for ELF waveforms of positive polarity than for waveforms of negative polarity. 3. For both daytime and nighttime sferics the median value of the VLF/ELF peak-amplitude ratio was greater for ELF waveforms of negative polarity than for waveforms of positive polarity. 4. The polarity of the ELF waveform was predominantly negative at night and positive during the day (verified by a count of the polarities of almost 6000 additional ELF waveforms). An attempt is made to explain the experimental results in terms of known properties of lightning discharges, and some of the difficulties in making such an interpretation are indicated.

The objective of this research is to eliminate very low frequency (VLF) communication signals with frequencies in the range of 15–25 kHz from streamed time-domain electromagnetic data. The frequency-domain approach of notch filtering or bandwidth limitation is unsatisfactory when early delay-time data are required in a time-domain system. United States military VLF transmitters presently communicate [Formula: see text] using minimum shift keying modulation, and it is possible to derive the encrypted bitstream from the data sampled at typical geophysical streaming rates. The method involves convolving the data with waveforms of frequency different by one-quarter of the bit rate above and below the carrier frequency and using the difference between the convolutions to predict the transmitted bits. The transmitted signal is then exactly recreated from the decoded bits, and the predicted signal at the receiver is subtracted from the data stream. It is possible to predict bit rates and encoding methods from other military transmitters through data analysis and again subtract the predicted signals from the streamed data. This procedure reduces the variance of data, implying that unwanted VLF signals have been reduced by a factor of three to nine in stations thousands of kilometers from a VLF transmitter. Much larger signal/noise improvements are predicted for stations within a few hundred kilometers of the VLF source. Lower degrees of improvement are noted from Indian and Chinese transmitters, which appear to have different encoding and modulation methods.

As a type of strategic communication, the long wave communication, with the operating frequency in the low frequency band and wavelength greater than tens of kilometers, plays a significant role in the military defense and national security, and has been drawn much attention. To satisfy the strategic demand of VLF(Very Low Frequency) in the underwater electromagnetic communication, a receiving system for weak signals in VLF band is designed, which mainly includes magnetic antenna and signal conditioning circuit. In this work, the magnetic antenna was designed by using the high-permeability magnetic core connected in series multi-layer coil around a 3D printing skeleton, which contributes to realize the miniaturization of the magnetic antenna. In order to reduce the noise of THE signal conditioning circuit, and meet the requirements of weak signal reception, the MMIC cascade chip is utilized and a specific static operating point is set. The measured results show that the designed magnetic antenna has higher inductance value and Q value in the operating frequency range, which indicates that the design has the higher receiving sensitivity. The size of this design is reduced by 50% with the operation performance, which provides a feasible technical scheme for the miniaturization of VLF underwater strategic communication system.

Solar winds, Coronal Mass Ejections（CME）, interplanetary shock waves and high-energy particle events, induced by the solar radio bursts, are called as solar electromagnetic storms. The propagation, acceleration, and evolution of the CMEs, solar winds, interplanetary shock waves and high-energy particle phenomena mainly happen at the frequency range from tens of kHz to tens of MHz, which is generally considered as the frequency band of Very Low Frequency（VLF）. Though, there are already few space instruments that have investigated the VLF solar radio emissions, most of the observations are spectral measurements, and only occupied part of the whole VLF band, while the imaging observations of the VLF solar radio bursts are still blank. Based on the characteristics of the vector antenna, it can estimate the Direction Of Arrival（DOA）of the incoming waves, which can be used to locate and image the solar radio bursts（Type Ⅱ&Ⅲ）. So all the latest space VLF radio instruments adopt the tripole antenna can do the observations. In this paper, in order to investigate the detection of the solar radio bursts with the space VLF radio explorer, different algorithms are proposed to detect the solar radio bursts with a tripole vector antenna, and simulations have been done with some space VLF radio observations for these algorithms under different conditions. The simulation results show that the vector antenna can locate the radio bursts with a degree-level accuracy. The capability of detecting the solar radio bursts is also analyzed in further for the space VLF radio explorer.

2-D nonlinear joint inversion of VLF and VLF-R data using simulated annealing

Power spectral analysis of fetal heart rate variability has been proposed to provide a non-invasive estimate of autonomic balance. However, there are few systematic data before birth. We therefore examined developmental changes in the frequency power spectrum at very low (0-0.04 Hz), low (0.04-0.15 Hz) and high frequencies (0.15-0.4 Hz), as well as the ratio of low- to high-frequency power (LF/HF), in chronically catheterized, healthy fetal sheep at 0.6 (n = 8), 0.7 (n = 7) and 0.8 gestational age (ga; n = 11). In a second study, 0.8 ga fetuses received either atropine (4.8 mg bolus, then 4.8 mg h(-1) for 30 min, n = 6) or 6-hydroxydopamine (20 mg ml(-1) at 2.5 ml h(-1) for 3 h; n = 9). Data were analysed by sleep state, defined by low-voltage-high-frequency (LV) or high-voltage-low-frequency (HV) EEG. Total spectral power increased with gestational age (P < 0.05), while LF/HF decreased from 0.6 to 0.7 ga. At 0.8 ga, heart rate and LF/HF were significantly higher during HV than LV sleep (P < 0.05). Consistent with this, although total spectral power was not significantly greater during HV sleep, there was a significant interaction between sleep state and frequency band (P = 0.02). Both atropine (P = 0.05) and 6-hydroxydopamine (P < 0.05) were associated with an overall reduction in spectral power but no significant effect on the LF/HF ratio. This study does not support substantial, consistent differences between the frequencies of sympathetic and parasympathetic activity in late-gestation fetal sheep.