Lightning has been declared as a new Essential Climate Variable by the World Meteorological Organization. Schumann resonance is a valuable parameter to monitor the global lightning activity, thus, the Atmospheric Observation Panel for Climate accepted Schumann resonance (SR) measurements as an emerging tool for studying lightning-related large-scale processes in the atmosphere. Previous studies showed a clear extraterrestrial influence on the SR parameters at different time scales (e.g., solar cycle). For all these reasons, a growing new interest arises in the scientific community to exploit the potential of SR better in gaining more information on electrodynamic coupling mechanisms taking place in the atmosphere. This has motivated the installation of new instruments worldwide to monitor SR measurements.We performed a multi-station spectral analysis of the SR parameters (frequency and intensity) by using wavelet transformation. SR records from different monitoring sites around the globe were analyzed simultaneously for the first time: Hornsund (~12 years of data) and Belsk (~7 y.) managed by Poland, Rovaniemi and Ivalo in Finland (~16 y.), Eskdalemuir in Scotland (~10 y.), Nagycenk in Hungary (~22 y.), Boulder Creek in USA (~4 y.) and Shillong in India (~9 y.). For all SR sites, the periodicities of 0.5, 1, ~180 and 365-day appeared both in the frequency and the intensity of SR modes. Evidence was also found for the ~27- and ~45-day periods at specific time intervals. Cross-wavelet transform and wavelet coherence analyses were made between SR frequencies and the Kp index, and between SR intensities and Madden-Julian Oscillation index. Time periods of highly coherent 27-day as well as 45-day periodicities were found in the time series of these parameters intermittently. These preliminary results suggest that these periodicities are likely related to the solar rotation and Madden-Julian Oscillation, respectively. A detailed analysis about our findings will be presented and discussed.

The importance of lightning has long been recognized from the point of view of climate-related phenomena. However, the detailed investigation of lightning on global scales is currently hindered by the incomplete and spatially uneven detection efficiency of ground-based global lightning detection networks and by the restricted spatio-temporal coverage of satellite observations. We are developing different methods for investigating global lightning activity based on Schumann resonance (SR) measurements. SRs are global electromagnetic resonances of the Earth-ionosphere cavity maintained by the vertical component of lightning. Since charge separation in thunderstorms is gravity-driven, charge is typically separated vertically in thunderclouds, so every lightning flash contributes to the measured SR field. This circumstance makes SR measurements very suitable for climate-related investigations. In this study, 19 days of global lightning activity in January 2019 are analyzed based on SR intensity records from 18 SR stations and the results are compared with independent lightning observations provided by ground-based (WWLLN, GLD360 and ENTLN) and satellite-based (GLM, LIS/OTD) global lightning detection. Daily average SR intensity records from different stations exhibit strong similarity in the investigated time interval. The inferred intensity of global lightning activity varies by a factor of 2-3 on the time scale of 3-5 days which we attribute to continental-scale temperature changes related to cold air outbreaks from polar regions. While our results demonstrate that the SR phenomenon is a powerful tool to investigate global lightning, it is also clear that currently available technology limits the detailed quantitative evaluation of lightning activity on continental scales.

<p>Multi-station observations of Schumann resonance (SR) intensity document common behavior in the evolution of continental-scale lightning activity in two super El Niño events, occurring in 1997/98 and 2015/16. The vertical electric field component of SR at Nagycenk, Hungary and the two horizontal magnetic field components in Rhode Island, USA in 1997, and in 2014-2015, the two horizontal magnetic field components at Hornsund, Svalbard and Eskdalemuir, United Kingdom as well as in Boulder Creek, California and Alberta, Canada exhibit considerable increases in SR intensity from some tens of percent up to a few hundred percent in the transition months preceding the two super El Niño events. The UT time distribution of anomalies in SR intensity indicates that in 1997 the lightning activity increased mainly in Southeast Asia, the Maritime Continent and India, i.e. the Asian chimney region. On the other hand, a global response in lightning is indicated by the anomalies in SR intensity in 2014 and 2015. SR-based results are strengthened by comparison to independent lightning observations from the Optical Transient Detector and the World Wide Lightning Location Network, which also exhibit increased lightning activity in the transition months. The increased lightning is attributable to increased instability due to thermodynamic disequilibrium between the surface and the mid-troposphere during the transition. Our main conclusion is that variations in SR intensity may act as a precursor for the occurrence and magnitude of these extreme climate events, and in keeping with earlier findings, as a precursor to maxima in global surface air temperature. As a continuation of our research we plan to set up a webpage dedicated to monitor the actual state of global lightning activity based on SR measurements which may contribute to the early identification of increased instability preceding the next super El Niño event. </p>

A fundamental tectonic boundary between the Precambrian East European Craton (EEC) and the younger Phanerozoic mountain belts of Europe runs through Poland. Whereas the transition zone between both tectonical units was intensely investigated by means of electromagnetic methods in the past, the image of electric conductivity of the EEC itself has not been detailed so far. In the present study the lithospheric structure beneath the northeast of Poland is investigated by means of three parallel magnetotelluric profiles. As a result the Polish part of the craton does not appear as homogeneously highresistive as repeatedly reported. Beneath a shallow (from 2 to 4 km) sedimentary layer a less resistive structure (several hundred Ωm) under all three profiles in mid-crustal to upper mantle depth becomes apparent in the high-resistive background. It spatially coincides with a magmatic body of the Upper Proterozoic age called Śniardwy lake gabbro intrusion.

This study is an attempt to close a gap between recent research on geomagnetic pulsations and their usage as source signals in electromagnetic induction soundings (i.e., magnetotellurics, geomagnetic depth sounding, and magnetovariational sounding). The plane-wave assumption as a precondition for the proper performance of these methods is partly violated by the local nature of field line resonances which cause a considerable portion of pulsations at mid latitudes. It is demonstrated that and explained why in spite of this, the application of remote reference stations in quasi-global distances for the suppression of local correlated-noise effects in induction arrows is possible in the geomagnetic pulsation range. The important role of upstream waves and of the magnetic equatorial region for such applications is emphasized. Furthermore, the principal difference between application of reference stations for local transfer functions (which result in sounding curves and induction arrows) and for inter-station transfer functions is considered. The preconditions for the latter are much stricter than for the former. Hence a failure to estimate an inter-station transfer function to be interpreted in terms of electromagnetic induction, e.g., because of field line resonances, does not necessarily prohibit use of the station pair for a remote reference estimation of the impedance tensor.

The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500Ωm) has three arms of approximately 100km width and 300km to 400km length at 120° angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

AbstractWe present a deep electrical resistivity image from the passive continental margin in Namibia. The approximately 700 km long magnetotelluric profile follows the Walvis Ridge offshore, continues onshore across the Kaoko Mobile Belt and reaches onto the Congo Craton. Two‐dimensional inversion reveals moderately resistive material offshore, atypically low for oceanic lithosphere, reaching depths of 15–20 km. Such moderate resistivities are consistent with seismic P wave velocity models, which suggest up to 35 km thick crust. The Neoproterozoic rocks of the Kaoko Mobile Belt are resistive, but NNW‐striking major shear‐zones are imaged as subvertical, conductive structures in the upper and middle crust. Since the geophysical imprint of the shear zones is intact, opening of the South Atlantic in the Cretaceous did not alter the middle crust. The transition into the cratonic region coincides with a deepening of the high‐resistive material to depths of more than 60 km.

Geomar, Helmholtz Centre for Ocean Research, Wischhofstr. 1-3, 24148 Kiel, Germany 6 now at University of Leicester, University Road, Leicester LE1 7RH, United Kingdom 7 now at GEUS, Øster Voldgade 1, 1350 Copenhagen, Denmark 8 Institute of Geophysics, Polish Academy of Science, ul. Księcia Janusza 64, 01452 Warsaw, Poland 9 University of Barcelona, C. Marti I Franques, 08028 Barcelona, Spain 10 now at Forschungsanstalt für Wasserschall und Geophysik, WTD71, Klausdorfer Weg 2, D-24148 11 Kiel, Germany 12 Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, 13 Japan 14 GFZ German Research Centre for Geosciences, Telegrafenberg 14473, Potsdam, Germany 15

This paper is a review of studies which, by applying the magnetotelluric, geomagnetic deep sounding, and magnetovariational sounding methods (the latter refers to usage of the horizontal magnetic tensor), investigate Central Europe for zones of enhanced electrical conductivity. The study areas comprise the region of the Trans-European Suture Zone (i.e. the south Baltic region and Poland), the North German Basin, the German and Czech Variscides, the Pannonian Basin (Hungary), and the Polish, Slovakian, Ukrainian, and Romanian Carpathians. This part of the world is well investigated in terms of data coverage and of the density of published studies, whereas the certainty that the results lead to comprehensive interpretations varies within the reviewed literature. A comparison of spatially coincident or adjacent studies reveals the important role that the data coverage of a distinct conductivity anomaly plays for the consistency of results. The encountered conductivity anomalies are understood as linked to basin sediments, asthenospheric upwelling, large differences in lithospheric age, and—this concerns most of them, which all concentrate in the middle crust—tectonic boundaries that developed during all mountain building phases that have taken place on the continent.

The analysis of marine magnetotelluric data is often complicated by disturbing signals that are caused by small-scale periodic movements of the instrument. The motion-induced noise leads to a bias and/or severe scattering in the derived magnetotelluric transfer functions. Both the motion itself and its effects on the magnetic and telluric time-series are investigated in this study using an 80 d magnetotelluric data set that includes dynamic tilt records measured in the Pacific Ocean off Costa Rica. We apply a standard motion removal technique as well as a newly developed method to correct for motion-induced noise. The resulting magnetotelluric transfer functions are of significantly better quality than the uncorrected ones. Furthermore, the study of the properties of motion noise leads to conclusions about the optimal processing approach even in case of data sets where an explicit correction for that noise is not possible.

Geomagnetic variation data from the observatories in Belsk (BEL, Poland) and Lviv (LVV, Ukraine) significantly suffer from disturbances caused by direct current (DC) electric railways. The aim of this study is to quantify the impact of these disturbances on quantities derived from such data, as the K index of magnetic activity and the induction arrow used in the geomagnetic deep sounding method to indicate lateral contrasts of electric conductivity in the solid earth. Therefore, undisturbed data have been reconstructed by means of a frequency-domain transfer function that relates the horizontal magnetic field components of the observatory to the ones synchronously recorded at a noise-free reference station. The comparison of the K index derived from original and reconstructed data shows an increase of quiet time segments by 29% for LVV and by 14% for BEL due to our noise removal procedure. Furthermore, the distribution of the corrected K indices agrees well with the one from the Niemegk observatory in Germany.

In the presented study, an attempt is made to model the propagation behavior of signals emitted by a Polish DC railway line with a number of grounded horizontal electric dipoles on the surface of a homogeneous half-space. The signals were measured on a magnetotelluric profile perpendicular to the railway line mainly in its near-field and transition zone, and they were separated from the natural electromagnetic variations by means of a reference site. Fitting the DC signals to the dipole model yields the conductivity of the homogeneous halfspace. Its value for the vertical and for the perpendicular horizontal magnetic component is confirmed by the usual 2D MT model obtained from the profile.

The magnetic field observations made at the geomagnetic observatory Brorfelde (BFE), Denmark, are subject to disturbances of several nT caused by two single wire high-voltage DC power lines connecting Scandinavia with Central Europe. We have analysed how the magnetic disturbances relate to the currents in the power lines and found a linear relationship. Using linear regression on carefully selected parts of the available data series we have determined the factors of proportionality. Based on the simple relationship between the currents in the power lines and the observed magnetic disturbances, we have derived a method to correct the observatory data for a significant part of the disturbances. In addition we have investigated the influence of the disturbances on the determination of the K-index of the observatory. We found that the K-index based on the original disturbed data compared to the K-indices calculated from corrected data was different in 4% of the 3-hour intervals for the investigated period.

The Trans-European Suture Zone runs through the North Sea, South Scandinavia, the Baltic Sea, and Poland into the Black Sea. It divides old Precambrian lithosphere in the Northeast from younger, Caledonian and Variscan one in the Southwest. 2D models of four magnetotelluric profiles crossing this prominent tectonic border are presented and an attempt to integrate them into quasi 3D view on the regional conductivity structure is made in this paper.

A large‐scale international electromagnetic experiment has been carried out in northwest Poland and northeast Germany. The main goal was to study the deep conductivity structure across the Trans‐European Suture Zone, which is the most prominent tectonic structure of Phanerozoic age in Europe. Electromagnetic measurements were carried out mainly along seismic profiles P2, LT‐7, and LT‐2 crossing the suture zone and running in the northeastern direction. Strike and dimensionality analyses indicate that a geo‐electrical strike of N60°W common to both profiles LT‐7 and P2 can be estimated. This strike direction was used to project and rotate all transfer functions and both profiles were subjected to 2D inversion using three different approaches. The results show the presence of highly conductive Cenozoic‐Mesozoic sedimentary cover reaching depths up to 3 km. A significant conductivity anomaly beneath the central part of the TESZ, called the Central Polish Anticlinorium, has been well resolved at mid‐crustal depths. The upper mantle of the Precambrian East European Craton is more resistive than, adjacent to the West, the younger Paleozoic Platform.

Title and contents 1 Introduction: On comparableness of magnetotelluric processing methods 5 1\. Fundamentals: The single site method 9 1.1 The impedance equation and its solution 9 1.2 On cascade decimation, Fourier coefficients, spectra, and instruments' responses 14 1.3 Noise 21 1.3.1 Statistic noise in output channels 21 1.3.2 Statistic noise in input channels 26 1.3.3 Correlated noise 27 2\. Evading noise: The Remote Reference technique 31 2.1 New transfer functions due to a second site 31 2.2 Application to data in need 34 2.3 Requirements to a remote site 40 3\. Inspecting the evil: Larsen's Signal-Noise Separation 50 3.1 Description of the method 51 3.1.1 Discerning signal and noise 51 3.1.2 A transfer function for correlated noise 54 3.1.3 Some general consequences 57 3.2 Comparison with Remote Reference 59 3.2.1 The debate and open questions 60 3.2.2 Answers from a consistent least-square point of view 61 4\. Rescuing the Separation approach: Oettinger's extension 71 4.1 A remote reference approach stabilizes the Separation tensor 71 4.2 Supporting independency of input variables 78 4.3 Information about correlated-noise sources 90 Conclusions 97 Acknowledgements 99 Bibliography 101