Finite-difference time-domain analysis of ELF radio wave propagation in the spherical Earth–ionosphere waveguide and its validation based on analytical solutions

The finite-difference time-domain (FDTD) model of electromagnetic wave propagation in the Earth–ionosphere cavity was developed under assumption of an axisymmetric system, solving the reduced Maxwell equations in a 2D spherical coordinate system. The model was validated on different conductivity profiles for the electric and magnetic field components for various locations on Earth along the meridian. The characteristic electric and magnetic altitudes, phase velocity, and attenuation rate were calculated. We compared the results of numerical and analytical calculations and found good agreement between them. The undertaken FDTD modeling enables us to analyze the Schumann resonances and the propagation of individual lightning discharges occurring at various distances from the receiver. The developed model is particularly useful when analyzing ELF measurements.</p

Subtraction of correlated noise in global networks of gravitational-wave interferometers

The recent discovery of merging black holes suggests that a stochastic gravitational-wave background is within reach of the advanced detector network operating at design sensitivity. However, correlated magnetic noise from Schumann resonances threatens to contaminate observation of a stochastic background. In this paper, we report on the first effort to eliminate intercontinental correlated noise from Schumann resonances using Wiener filtering. Using magnetometers as proxies for gravitational-wave detectors, we demonstrate as much as a factor of two reduction in the coherence between magnetometers on different continents. While much work remains to be done, our results constitute a proof-of-principle and motivate follow-up studies with a dedicated array of magnetometers

Measurement and subtraction of Schumann resonances at gravitational-wave interferometers

Correlated magnetic noise from Schumann resonances threatens to contaminate the observation of a stochastic gravitational-wave background in interferometric detectors. In previous work, we reported on the first effort to eliminate global correlated noise from the Schumann resonances using Wiener filtering, demonstrating as much as a factor of two reduction in the coherence between magnetometers on different continents. In this work, we present results from dedicated magnetometer measurements at the Virgo and KAGRA sites, which are the first results for subtraction using data from gravitational-wave detector sites. We compare these measurements to a growing network of permanent magnetometer stations, including at the LIGO sites. We show the effect of mutual magnetometer attraction, arguing that magnetometers should be placed at least one meter from one another. In addition, for the first time, we show how dedicated measurements by magnetometers near to the interferometers can reduce coherence to a level consistent with uncorrelated noise, making a potential detection of a stochastic gravitational-wave background possible

The Accuracy of Radio Direction Finding in the Extremely Low Frequency Range

In this work, we study the accuracy of direction finding in the extremely low frequency (ELF) range using a newly installed broadband receiver equipped with two active magnetic antennas. The main natural source of ELF radio waves is lightning. In this work, we analyzed 1000 atmospheric discharges at distances of up to 5000 km from the receiver. We identified the most important factors influencing the accuracy of the angle of arrival: the deviation of the radio waves propagating through the day-night terminator zone and the signal-to-noise ratio resulting from local electromagnetic noise and Schumann resonance background. The obtained results clearly show that the accuracy of estimating the direction of arrival is very high (an average error of 0.1° with the standard deviation of 2.3°) when the signal-to-noise ratio is large (the amplitude of the magnetic field component above 100 pT), except for short periods in the local morning and evening, when the day-night terminator is present on the propagation path of the direct wave. For the day-night propagation paths, the refraction angle was larger than the incidence angle, and for the night-day propagation paths, the refraction angle was smaller than the incidence angle, which is consistent with theory. Using our analytical ELF radio propagation model allowed us to explain the obtained results.Fil: Mlynarczyk, Janusz. AGH University of Science and Technology; PoloniaFil: Kulak, Andrzej. AGH University of Science and Technology; PoloniaFil: Salvador, Jacobo Omar. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. UNIDEF - Observatorio Atmosférico de la Patagonia Austral | Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa. UNIDEF - Observatorio Atmosférico de la Patagonia Austral; Argentin

Extremely low frequency electromagnetic field measurements at the Hylaty station and methodology of signal analysis

We present the Hylaty geophysical station, a high-sensitivity and low-noise facility for extremely low frequency (ELF, 0.03–300 Hz) electromagnetic field measurements, which enables a variety of geophysical and climatological research related to atmospheric, ionospheric, magnetospheric, and space weather physics. The first systematic observations of ELF electromagnetic fields at the Jagiellonian University were undertaken in 1994. At the beginning the measurements were carried out sporadically, during expeditions to sparsely populated areas of the Bieszczady Mountains in the southeast of Poland. In 2004, an automatic Hylaty ELF station was built there, in a very low electromagnetic noise environment, which enabled continuous recording of the magnetic field components of the ELF electromagnetic field in the frequency range below 60 Hz. In 2013, after 8 years of successful operation, the station was upgraded by extending its frequency range up to 300 Hz. In this paper we show the station's technical setup, and how it has changed over the years. We discuss the design of ELF equipment, including antennas, receivers, the time control circuit, and power supply, as well as antenna and receiver calibration. We also discuss the methodology we developed for observations of the Schumann resonance and wideband observations of ELF field pulses. We provide examples of various kinds of signals recorded at the station

Unstable Flow Structures Present at Different Rotational Velocities of the Centrifugal Compressor

Unstable flow structures cause inevitable energy losses in all power energy systems, including turbomachines. In this study, a set of analyses was conducted with the use of spectral maps on the pressure signals obtained from an industrial centrifugal compressor. The spectral maps provide one a detailed visualization of the flow conditions present in the machine along the performance curve and to distinguish the flow phenomena present prior to the surge. The method accuracy is especially useful in detecting the inlet recirculation. The study was conducted at four impeller rotational speeds with varying loads imposed by a valve at the outlet. At each speed, the machine experienced different stages of unstable flow conditions prior to the surge. Five main frequency peaks that appeared in all cases were identified and discussed. The surge was observed at all impeller speeds. At lower ones, however, it appeared at higher valve closures. At higher speeds, the surge was much more intense. The study has also shown that the inlet recirculation appears also for the closed-type industrial impeller. The phenomenon was present in all conditions. The higher impeller speed, the faster onset of the inlet recirculation was. This structure has a strong potential for an early instability warning because it appears in various types of impellers, has a very particular spectral structure and its positioning is very predictable. This study gives another example of the inlet recirculation universality and potential for efficient anti-surge protection

Salt Stress Mitigation via the Foliar Application of Chitosan-Functionalized Selenium and Anatase Titanium Dioxide Nanoparticles in Stevia (Stevia rebaudiana Bertoni)

High salt levels are one of the significant and major limiting factors on crop yield and productivity. Out of the available attempts made against high salt levels, engineered nanoparticles (NPs) have been widely employed and considered as effective strategies in this regard. Of these NPs, titanium dioxide nanoparticles (TiO2 NPs) and selenium functionalized using chitosan nanoparticles (Cs–Se NPs) were applied for a quite number of plants, but their potential roles for alleviating the adverse effects of salinity on stevia remains unclear. Stevia (Stevia rebaudiana Bertoni) is one of the reputed medicinal plants due to their diterpenoid steviol glycosides (stevioside and rebaudioside A). For this reason, the current study was designed to investigate the potential of TiO2 NPs (0, 100 and 200 mg L−1) and Cs–Se NPs (0, 10 and 20 mg L−1) to alleviate salt stress (0, 50 and 100 mM NaCl) in stevia. The findings of the study revealed that salinity decreased the growth and photosynthetic traits but resulted in substantial cell damage through increasing H2O2 and MDA content, as well as electrolyte leakage (EL). However, the application of TiO2 NPs (100 mg L−1) and Cs–Se NPs (20 mg L−1) increased the growth, photosynthetic performance and activity of antioxidant enzymes, and decreased the contents of H2O2, MDA and EL under the saline conditions. In addition to the enhanced growth and physiological performance of the plant, the essential oil content was also increased with the treatments of TiO2 (100 mg L−1) and Cs–Se NPs (20 mg L−1). In addition, the tested NPs treatments increased the concentration of stevioside (in the non-saline condition and under salinity stress) and rebaudioside A (under the salinity conditions) in stevia plants. Overall, the current findings suggest that especially 100 mg L−1 TiO2 NPs and 20 mg L−1 Cs–Se could be considered as promising agents in combating high levels of salinity in the case of stevia

The effects of the strong thunderstorms seen in the ionosphere above Poland by DEMETER and Swarm satellites. Michel Parrot (6)

International audienceLightnings and particularly TLEs (sprites, jets, elves, halos) are associated with the electromagnetic connections and interactions between atmosphere, ionosphere and magnetosphere. DEMETER was a low-altitude microsatel-lite, it operated from June 2004 till December 2010 on a polar, circular orbit which altitude at the beginning was 710km but it was decreased to 660 km. DEMETER measured variations of the electric field in low frequency range from 0 to 20 kHz. The plasma analyzer instrument measured variations of the ion density 4 s time resolution. Langmuir probe gave the value of the electron temperature and density. A detector of energetic particle measured electrons and protons with energies from 70 keV to 2.34MeV every 4 s in survey mode and 1 s in burst mode. DEMETER has clearly shown, that thunderstorms and TLE can affected the ionosphere even at altitude of its orbit (680km). It registered many strong thunderstorms in Poland during its time of operation. The Swarm constellation comprises 3 identical satellites launched on 22 November 2013 into a near-polar orbit. This set of satellites is still operating. Two of them are operating on the circular, polar orbits with initial altitude 460. Third one has also circular orbit, but with altitude 530. The orbits of the first 2 satellites are in almost the same plane, but third one is close to be perpendicular to the first two. The payload containing Vector Field Magnetometer, Absolute Scalar Magnetometer and Electric Field Instrument among other allows to study the effects in the ionosphere generated by thunderstorms. The measurements performed during flights over thunderstorms areas in Poland will be discussed in our presentation. The discussion of the cross correlation between the ground based registrations (PERUN and VERA systems) and DEMETER and Swarm measurements of the ULF/ELF/VLF waves, electron density and temperature variations in the ionosphere related to the strong thunderstorms in Poland will be presented