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.

Thirty-five sprite-producing lightning flashes were recorded in nine nights in different seasons at the east coast of Spain with a 3D Lightning Mapping Array (LMA) since July 2011. A low-frequency time-of-arrival network provided data on emissions from return strokes and intracloud processes and a very-high-frequency interferometer network produced complementary lightning data. This study analyzes the bidirectional development of flashes in order to understand the positioning and timing of the positive cloud-to-ground stroke (+CG) and its consequences for charge neutralization by negative leaders, affecting sprite morphology. A summary of negative leader extents, altitudes, and speeds before and after the + CG stroke is provided, as well as positive leader origins and inferred speeds. Negative leader speeds exhibited modes at 10 5 and 5 × 10 5 m s À1. Five examples with different evolutions are discussed: (1) Slow bidirectional development with negative leader termination before the + CG stroke; (2) Fast bidirectional development with the negative leader continuing after the + CG stroke. (3) Slow-fast bidirectional development with a negative leader exhibiting a sudden lowering and speed increase; (4) Fast secondary bidirectional development from an in-cloud horizontal positive leader. Negative leaders propagated rapidly into the upper positive charge layer, continuing after the + CG stroke; (5) Slow bidirectional development with a long negative leader (50 km) subject to cutoff while the original positive leader remained trapped inside negative charge. A + CG stroke subsequently occurred under the old negative leader channel. Carrot sprites tended to be associated with fast extending leaders after the stroke, columniform/mixed sprites with slower side branches.

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. PACS numbers:Introduction. A stochastic gravitational-wave background (SGWB) is a potential signal source for groundbased, second-generation interferometric gravitationalwave detectors such as Advanced LIGO [1] and Advanced Virgo [2]. An astrophysical SGWB could be produced by objects such as compact binary coalescences, pulsars, magnetars, or core-collapse supernovae. A cosmological background could be generated by various physical processes in the early universe [3,4]. Previous analyses have achieved interesting constraints on these processes [3][4][5]. In particular, with the recent discovery of a binary blackhole merger [6], there is a chance of observing a SGWB from these systems [7].

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.

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