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

Extremely low frequency electromagnetic investigation on Mars

Natural electromagnetic (EM) signals of extremely low frequencies (ELF, 3 Hz-3 kHz) can be used to study many of the electromagnetic processes and properties occurring in the Martian environment. Sources of these signals, related to electrical activity in the atmosphere, are very significant since they can influence radio wave propagation on the planet, the atmospheric composition, and the ionospheric structure. In addition, such EM signals can be employed in many purposes such as: surveying the subsurface of Mars or studying the impact of the space weather on the Martian ionosphere. As ELF waves propagate on very long distances, it is possible to explore properties of the entire planet using single-station recordings. In this study, we propose an experiment that allows measuring ELF signals from the Martian surface. Such measurements can be used for detection of electric discharges in the atmosphere and water reservoirs in the planetary subsurface

Analysis of ELF electromagnetic field pulses recorded by the Hylaty station coinciding with terrestrial gamma-ray flashes

Terrestrial gamma-ray flashes (TGFs) were registered the first time by the NASA's Compton Gamma Ray Observatory. The physical mechanism of TGF generation is not fully known, but there is a consensus among researchers that the radiation is produced by bremsstrahlung of relativistic electrons in the thunderstorm regions of the atmosphere. Therefore, TGFs have been linked to positive-polarity intracloud lightning discharges, strong positive cloud-to-ground discharges or upward discharges from a thundercloud top. The currently operating Fermi Gamma-ray Space Telescope is equipped with a Gamma-ray Burst Monitor that can detect terrestrial gamma-ray flashes. It opens up a new possibility to search for lightning discharges responsible for TGFs. Ground-based lightning monitoring systems in the ELF, LF and VLF ranges can be used for that purpose. The ELF systems are especially useful, since they provide a large monitoring range of several thousand kilometers for strong atmospheric discharges (charge moments above several tens of C km). In this paper we have described the data analysis method for ELF electromagnetic field pulses and applied it to study our first examples of TGFs registered by Fermi GBM coinciding with ELF pulses recorded by the Hylaty ELF station located in the Carpathian Mountains in Poland. Using our ELF electromagnetic wave propagation model we have evaluated charge moments for the two registered events to be 320 and 110 C km and provided upper limits for the remaining events

Solar variations in extremely low frequency propagation parameters : 2. Observations of Schumann resonances and computation of the ELF attenuation parameter

Observations of resonant electromagnetic fields caused by global lightning activity are employed in determining the averaged parameters of the lower ionosphere. Using the twodimensional telegraph equation (TDTE) transmission line model described by Kul/ ak et al. [2003], we have computed the attenuation rate of the Earth-ionosphere waveguide from diurnal observations of the N-S magnetic component of the ELF field performed irregularly for 6 years in the East Carpathian mountains. As the measurements were carried out during both the minimum and the maximum of the solar cycle 23 we present how solar activity influence the first Schumann resonance frequency and the attenuation rate. The analysis of all the data indicates that the first Schumann resonance frequency increases from 7.75 Hz at solar minimum to about 7.95 Hz at solar maximum while the global mean attenuation rate a at 8 Hz varies from 0.31 dB/Mm at minimum to about 0.26 dB/Mm at maximu

An unusual sequence of sprites followed by a secondary TLE : An analysis of ELF radio measurements and optical observations

We present an extraordinary case of sprites in rapid succession–four sprite clusters in only 400 ms–followed by a secondary jet. Simultaneous ELF and optical observations, as well as lightning data, enabled us to thoroughly document this unique event. Locations of the transient luminous events (TLEs) were triangulated using video recordings from Nydek (Czech Republic) and Sopron (Hungary). We found that sprites were displaced up to 70 km from their parent lightning. The current moment waveform and charge moment changes associated with the event were reconstructed from the ELF electromagnetic signature recorded at the Hylaty station (Poland) by a new method. The results suggest that both a short-delayed and a long-delayed sprite were generated by a single positive cloud-to-ground discharge that had an intense continuing current. It had an unusual progression and lasted 200 ms. A large increase in the current moment during the development of a massive carrot sprite provides evidence in favor of sprite current. Our results also support the formation of an electric environment hypothesized to be necessary for the generation of the secondary TLEs

Application of the Schumann resonance spectral decomposition in characterizing the main African thunderstorm center

In this paper we present a new method for quantifying the main tropical thunderstorm regions based on extremely low frequency (ELF) electromagnetic wave measurements from a single station – the Hylaty ELF station in Central Europe. Our approach is basedon Schumann resonance (SR) measurements, which we apply as an example to thunderstorms in Africa. By solving the inverse problem, using the SR power spectrum templates derived analitically, we calculate distances to the most powerful thunderstorm centers and present simplified 1-D thunderstorm lightning activity ”maps” in absolute units C2m2/sec. We briefly describe our method of SR power spectrum analysis and present how this method is used with real observational data. We obtained the monthly lightning activity maps of the African storm centers with a spatial resolution of 1 degree and temporal resolution of 10 minutes for January and August 2011. This allowed us to study the varying location and intensities of the African storm centers in different seasons of the year. A crosscheck of the obtained lightning activity maps with TRMM satellite data recorded by the Lightning Imaging Sensor (LIS), as well as the derived correlation coefficients between SR and optical data were used to validate the proposed method. We note that modeling a maximum possible number of resonance modes in the SR power spectra (in our case 7 resonances) is essential in application of the proposed approach