Spherical Elementary Current Systems Research Articles

Analysis of the Solar Flare Effects of 6 September 2017 in the Ionosphere and in the Earth's Magnetic Field Using Spherical Elementary Current Systems

AbstractThe interval 4–10 September 2017 was one of the most flare‐productive periods of the solar cycle 24, producing strong‐to‐severe space weather episodes. Thus, on 6 September, the solar active region AR 30023 produced an X‐9 level flare. The arrival of the associated coronal mass ejection produced severe geomagnetic storming on 7 and 8 September, preceded by two significant solar flare effects (Sfe) that could be seen in the lit hemisphere. In this article, we analyze the impact of these flares on the ionosphere as registered on the vertical incidence ionospheric sounder located at Ebre Observatory. We put the emphasis on the use of this instrument to detect solar flares by means of the absorption observed in its ionograms. We also analyze the impacts of these flares on the Earth's magnetic field and the temporal evolution of the second Sfe event using the technique of spherical elementary current systems, which allows tracing in detail the current system evolution during the Sfe lifetime. As expected, a sharp increase in current intensities occurred at the beginning. The peak of the disturbance was reached after a few minutes, when hard X‐rays were dominant, while a slow decay followed the advent of soft X‐rays and EUV rays. The modeled current systems appear abnormally displaced in longitude with respect to the subsolar point. The northern vortex shows up about 2 hr in advance of the subsolar point meridian, while the southern vortex is about 3 hr behind it. Both remain static, showing no significant shift over the whole episode. Although the event occurred during the equinox, a clear prevalence of the Northern Hemisphere was observed.

An application of principal component analysis to the interpretation of ionospheric current systems

AbstractIonospheric currents are driven by several different physical processes and exhibit complex spatial and temporal structure. Magnetic field measurements of ionospheric sources are often spatially sparse, causing significant challenges in visualizing current flow at a specific time. Standard methods of fitting equivalent current models to magnetic observations, such as line currents, spherical harmonic analysis, spherical cap harmonic analysis, and spherical elementary current systems (SECS), are often unable to capture the full spatial complexity of the currents or require a large number of parameters which cannot be fully determined by the available data coverage. These methods rely on a set of generic basis functions which contain limited information about the geometries of the various ionospheric sources. In this study, we develop new basis functions for fitting ground and satellite measurements, which are derived from physics‐based ionospheric modeling combined with principal component analysis (PCA). The physics‐based modeling provides realistic current flow patterns for all of the primary ionospheric sources, including their daily and seasonal variability. The PCA technique extracts the most relevant spatial geometries of the currents from the model run into a small set of equivalent current modes. We fit these modes to magnetic measurements of the Swarm satellite mission at low and middle latitudes and compare the resulting model with independent measurements and with the SECS approach. We find that our PCA method accurately reproduces features of the equatorial electrojet and Sq current systems with only 10 modes and can predict ionospheric fields far from the data region.

Use of spherical elementary currents to map the polar current systems associated with the geomagnetic sudden commencements on 2013 and 2015 St. Patrick's Day storms

AbstractAraki's model of geomagnetic sudden commencements (SCs) establishes that the ground magnetic signatures globally observed after the onset produced by an increased solar wind dynamic pressure impacting on the Earth's magnetosphere are caused by the setting up of a system of electric currents in the coupled magnetosphere‐ionosphere. This current system consists of a particular evolving set of magnetopause currents closing in the ionosphere through geomagnetic field‐aligned currents (FACs) and their induced counterpart. The present paper confirms the starting assumptions of the referred model by use of spherical elementary current systems (SECS), namely, the existence of FACs reversing polarity during the first couple of minutes of the SC. It is the first time that SECS have been applied to the study of SCs. The method has been fed with data from more than 100 stations of the global network of geomagnetic observatories and variometer sites in the northern hemisphere so as to provide a reliable pattern of the equivalent current system flowing at ionospheric heights on the occasion of the SCs associated with the 2013 and 2015 St. Patrick's Day storms. The combined analysis of solar wind data and the synoptic view of the SC current patterns provided by SECS allows it to explain some of the differences observed between both events.

Spherical Elementary Current Systems Method Applied to Geomagnetic Field Modeling for the Adriatic

The aim of this work was to derive an accurate regional model of geomagnetic components on the Adriatic. Data of north, east and vertical geomagnetic components at repeat stations and ground survey sites enclosing the Adriatic Sea were used to obtain a geomagnetic model at 2010.5 epoch. The core field was estimated by use of the global Enhanced Magnetic Model, while the crustal field by a mathematical technique for expanding vector systems on a sphere into basis functions, known as spherical elementary current systems method. The results of this method were presented and compared to the crustal field estimations by the Enhanced Magnetic Model. The maps of isolines of the regional model are also presented.

Effects of space weather on GOCE electrostatic gravity gradiometer measurements

We examine the presence of residual nongravitational signatures in gravitational gradients measured by GOCE electrostatic gravity gradiometer. These signatures are observed over the magnetic poles during geomagnetically active days and can contaminate the trace of the gravitational gradient tensor by up to three to five times the expected noise level of the instrument (\(\sim \)11 mE). We investigate these anomalies in the gradiometer measurements along many satellite tracks and examine possible causes using external datasets, such as interplanetary electric field measurements from the ACE (advanced composition explorer) and WIND spacecraft, and Poynting vector (flux) estimated from equivalent ionospheric currents derived from spherical elementary current systems over North America and Greenland. We show that the variations in the east-west and vertical electrical currents and Poynting vector components at the satellite position are highly correlated with the disturbances observed in the gradiometer measurements. The results presented in this paper reveal that the disturbances are due to intense ionospheric current variations that are enhanced by increased solar activity that causes a very dynamic drag environment. Moreover, successful modelling and removal of a high percentage of these disturbances are possible using external geomagnetic field observations.

On Data Interpolation at Three Croatian Repeat Stations by Using the Spherical Elementary Currents Systems Method

A general mathematical tool for expanding vector systems on a sphere into basis functions, spherical elementary current system (SECS) method, was applied for separation of the geomagnetic field variations into external and internal parts, over a limited region of central and southeastern Europe. The registered variations at three Croatian repeat stations were compared to the variations estimated by the SECS method using the variations from the different sets of observatories. The results of the SECS method were also compared to a simple assumption that the variations at repeat station are equal to those at particular observatory. The relevance of this comparison was to get an insight about the possibility of using the SECS method for estimating the geomagnetic field variations over Croatia. The guidelines for the application of the SECS method for the purpose of reducing repeat station data were also given.

Comparison of DMSP and SECS region-1 and region-2 ionospheric current boundary

The region-1 and region-2 boundary has traditionally been identified using data from a single spacecraft crossing the auroral region and measuring the large scale changes in the cross track magnetic field. With data from the AUTUMN, CANMOS, CARISMA, GIMA, DTU MGS, MACCS, McMAC, STEP, THEMIS, and USGS ground magnetometer arrays we applied a state-of-art technique based on spherical elementary current system (SECS) method developed by Amm and Viljanen (1999) in order to calculate maps of region-1 and region-2 current system over the North American and Greenland auroral region. Spherical elementary current (SEC) amplitude (proxy for vertical currents) maps can be inferred at 10s temporal resolution, ~1.5° geographic latitude (Glat), and 3.5° geographic longitude (Glon) spatial resolution. We compare the location of the region-1 and region-2 boundary obtained by the DMSP spacecraft with the region-1 and region-2 boundary observed in the SEC current amplitudes. We find that the boundaries typically agree within 0.2°±1.3°. These results indicate that the location of the region-1 and region-2 boundary can reasonably be determined from ground magnetometer data. The SECS maps represent a value-added product from the magnetometer database and can be used for contextual interpretation in conjunction with other missions as well as help with our understanding of magnetosphere-ionosphere coupling mechanisms using the ground arrays and the magnetospheric spacecraft data.

Data reduction of Croatian geomagnetic repeat stations surveys by using the spherical elementary current systems method

The geomagnetic declination, inclination and total field data surveyed at the Croatian Geomagnetic Repeat Stations Network were reduced to 2008.5, 2009.5 and 2010.5 epochs by a well-known observatory-based and a proposed method, in instances when an influence of the differences of secular variations at a repeat station and at an observatory were not taken into account. The motivation came from distances and distribution of the nearby geomagnetic observatories. A technique for expanding vector systems on a sphere into basis functions, known as the Spherical Elementary Current System (SECS) method, was applied to obtain separation of the geomagnetic field variations into external and internal parts, over Croatia and surrounding countries. The aim of this work is to propose the SECS method for determination of the annual mean values of north, east and vertical components of Earth’s magnetic field at the Croatian repeat stations. The results of the SECS method were compared to an observatory-based method which assumes that the variations of components about their annual mean values at a repeat station are equal to those at a particular nearby reference observatory. For reducing a repeat station data by this simple method the Furstenfeldbruck, Grocka and Tihany observatories were used, while for the SECS method additional three observatories were employed. The reduction methods were found comparable, while in the majority of instances the SECS method demonstrated smaller standard deviations of the components’ annual means. Besides, the annual mean values of the vertical component had the highest accuracies for both methods. The results indicate that the SECS method can be an acceptable method for data reduction to annual mean values.

A method to derive maps of ionospheric conductances, currents, and convection from the Swarm multisatellite mission

AbstractThe European Space Agency (ESA) Swarm spacecraft mission is the first multisatellite ionospheric mission with two low‐orbiting spacecraft that are flying in parallel at a distance of ~100–140 km, thus allowing derivation of spatial gradients of ionospheric parameters not only along the orbits but also in the direction perpendicular to them. A third satellite with a higher orbit regularly crosses the paths of the lower spacecraft. Using the Swarm magnetic and electric field instruments, we present a novel technique that allows derivation of two‐dimensional (2‐D) maps of ionospheric conductances, currents, and electric field in the area between the trajectories of the two lower spacecraft, and even to some extent outside of it. This technique is based on Spherical Elementary Current Systems. We present test cases of modeled situations from which we calculate virtual Swarm data and show that the technique is able to reconstruct the model electric field, horizontal currents, and conductances with a very good accuracy. Larger errors arise for the reconstruction of the 2‐D field‐aligned currents (FAC), especially in the area outside of the spacecraft orbits. However, even in this case the general pattern of FAC is recovered, and the magnitudes are valid in an integrated sense. Finally, using an MHD model run, we show how our technique allows estimation of the ionosphere‐magnetosphere coupling parameter K, if conjugate observations of the magnetospheric magnetic and electric field are available. In the case of a magnetospheric multisatellite mission (e.g., the ESA Cluster mission) several K estimates at nearby points can be generated.

Surface electric fields for North America during historical geomagnetic storms

To better understand the impact of geomagnetic disturbances on the electric grid, we recreate surface electric fields from two historical geomagnetic storms—the 1989 “Quebec” storm and the 2003 “Halloween” storms. Using the Spherical Elementary Current Systems method, we interpolate sparsely distributed magnetometer data across North America. We find good agreement between the measured and interpolated data, with larger RMS deviations at higher latitudes corresponding to larger magnetic field variations. The interpolated magnetic field data are combined with surface impedances for 25 unique physiographic regions from the United States Geological Survey and literature to estimate the horizontal, orthogonal surface electric fields in 1 min time steps. The induced horizontal electric field strongly depends on the local surface impedance, resulting in surprisingly strong electric field amplitudes along the Atlantic and Gulf Coast. The relative peak electric field amplitude of each physiographic region, normalized to the value in the Interior Plains region, varies by a factor of 2 for different input magnetic field time series. The order of peak electric field amplitudes (largest to smallest), however, does not depend much on the input. These results suggest that regions at lower magnetic latitudes with high ground resistivities are also at risk from the effect of geomagnetically induced currents. The historical electric field time series are useful for estimating the flow of the induced currents through long transmission lines to study power flow and grid stability during geomagnetic disturbances.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti-parallel to one another. Our results show that in general the equivalent ionospheric currents are anti-parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti-parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value-added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere-ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti‐parallel to one another. Our results show that in general the equivalent ionospheric currents are anti‐parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti‐parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value‐added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere‐ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Interpolation of externally-caused magnetic fields over large sparse arrays using Spherical Elementary Current Systems

Abstract. A physically-based technique for interpolating external magnetic field disturbances across large spatial areas can be achieved with the Spherical Elementary Current System (SECS) method using data from ground-based magnetic observatories. The SECS method represents complex electrical current systems as a simple set of equivalent currents placed at a specific height in the ionosphere. The magnetic field recorded at observatories can be used to invert for the electrical currents, which can subsequently be employed to interpolate or extrapolate the magnetic field across a large area. We show that, in addition to the ionospheric currents, inverting for induced subsurface current systems can result in strong improvements to the estimate of the interpolated magnetic field. We investigate the application of the SECS method at mid- to high geomagnetic latitudes using a series of observatory networks to test the performance of the external field interpolation over large distances. We demonstrate that relatively few observatories are required to produce an estimate that is better than either assuming no external field change or interpolation using latitudinal weighting of data from two other observatories.

Characteristics of Magnetic Variations and Current Wedge in the Sawtooth Event on 30 September 2000

AbstractThe purpose of this paper is to study the response of auroral electrojets during the sawtooth event observed by LANL satellites on 30 September 2000. Firstly, we establish a method to determine large‐scale 2‐dimensional ionospheric equivalent current systems in the high‐latitude ionosphere using the spherical elementary current system method. Then we apply this method and magnetic data of high‐latitude magnetometers in the Northern Hemisphere to study the variations of auroral electrojets during the sawtooth event. By comparing characteristics of enhancements of nightside ionospheric westward electrojets with those of positive magnetic bays of H components in the middle/low‐latitude ground magnetic field after each sawtooth injection, it is demonstrated that there are current wedges formed during this sawtooth event. The local time width of current wedges suggested by the two observations are both around 11 hours (MLT). In addition, time durations of middle/low‐latitude magnetic bays to reach maximum variations were generally longer than those for high‐latitude electrojets, implying that source currents for middle/low‐latitude magnetic bays are multiple.

Local determination of ionospheric plasma convection from coherent scatter radar data using the SECS technique

A new technique for merging line‐of‐sight (LOS) data of the ionospheric plasma convection velocity, as obtained from coherent scatter radars, into a full velocity vector field on a sphere is presented. This technique is based on the expansion into Spherical Elementary Current Systems (SECS) which have been successfully applied to many other problems in ionosphere‐magnetosphere physics. Despite their name mentioning currents for historical reasons, SECS can be used as basis functions for any continuously differentiable vector field on a sphere. In contrast to the traditional modeling of the radar data with spherical harmonics over the whole auroral zone, the new technique does not require any “a priori” model input but relies solely on the measured data, nor does it need any explicit boundary conditions to be specified. The new technique is designed to be applied locally to areas where sufficient radar backscatter exists. The analysis area that satisfies this condition may have any shape and is not limited to, e.g., spherical caps. A test with synthetic data shows that the method performs excellently (less than 5% relative error) if 25% or more of the optimal coverage of input data are actually available as backscatter data, with respect to the scale on which the results for the velocity vector field are desired to be obtained. Still if only 10% of the optimal coverage of input data are available, the technique performs fairly well with a relative error of ∼12%. A second test with real LOS input data from the SuperDARN radars shows that on such a local area with sufficient backscatter, our new technique is able to reproduce mesoscale details of the LOS data significantly better than the current standard analysis based on the technique of Ruohoniemi and Baker (1998) which processes the radar data on the whole auroral zone. While the new technique is presented here for the application with LOS radar data, it can be applied for merging any kind of vector component data on a sphere to a vector field.

Statistical dependence of auroral ionospheric currents on solar wind and geomagnetic parameters from 5 years of CHAMP satellite data

Abstract. The effects of the solar wind dynamic pressure (P), the z component of the solar wind magnetic field (Bz), the merging electric field (Em), season and the Kp index on R1 and R2 field-aligned currents are studied statistically using magnetic field data from the CHAMP satellite during 2001–2005. The ionospheric and field-aligned currents are determined from the magnetic field data by the recently developed 1-D Spherical Elementary Current System (SECS) method. During southward IMF, increasing |Bz| is observed to clearly increase the total field-aligned current, while during northward IMF, the amount of field-aligned current remains fairly constant regardless of |Bz|. The dependence of the field-aligned current on Bz is given by |Ir[MA]|=0.054·Bz[nT]2−0.34·Bz[nT]+2.4. With increasing P, the intensity of the field-aligned current is also found to increase according to |Ir[MA]|=0.62·P[nPa]+1.6, and the auroral oval is observed to move equatorward. Increasing Em produces similar behaviour, described by |Ir[MA]|=1.41·Em[mV/m]+1.4. While the absolute intensity of the ionospheric current is stronger during negative than during positive Bz, the relative change in the intensity of the currents produced by a more intense solar wind dynamic pressure is observed to be approximately the same regardless of the Bz direction. Increasing Kp from 0 to ≥5 widens the auroral oval and moves it equatorward from between 66°–74° AACGM latitude to 59°–71° latitude. The total field aligned current as a function of Kp is given by |Ir[MA]|=1.1·Kp+0.6. In agreement with previous studies, total field-aligned current in the summer is found to be 1.4 times stronger than in the winter.

A model for estimating the relation between the Hall to Pedersen conductance ratio and ground magnetic data derived from CHAMP satellite statistics

Abstract. The goal of this study is to find a way to statistically estimate the Hall to Pedersen conductance ratio α from ground magnetic data. We use vector magnetic data from the CHAMP satellite to derive this relation. α is attained from magnetic satellite data using the 1-D Spherical Elementary Current Systems (SECS). The ionospheric equivalent current density can either be computed from ground or satellite magnetic data. Under the required 1-D assumption, these two approaches are shown to be equal, which leads to the advantage that the statistics are not restricted to areas covered by ground data. Unlike other methods, using magnetic satellite measurements to determine α ensures reliable data over long time sequences. The statistical study, comprising over 6000 passes between 55° and 76.5° northern geomagnetic latitude during 2001 and 2002, is carried out employing data from the CHAMP satellite. The data are binned according to activity and season. In agreement with earlier studies, values between 1 and 3 are typically found for α. Good compatibility is found, when α attained from CHAMP data is compared with EISCAT radar measurements. The results make it possible to estimate α from the east-west equivalent current density Jφ; [A/km]: α=2.07/(36.54/|Jφ|+1) for Jφ<0 (westward) and α=1.73/(14.79/|Jφ+1) for Jφ0 (eastward). Using the same data, statistics of ionospheric and field-aligned current densities as a function of geomagnetic latitude and MLT are included. These are binned with respect to activity, season and IMF BZ and BY. For the first time, all three current density components are simultaneously studied this way on a comparable spatial scale. With increasing activity, the enhancement and the equatorward expansion of the electrojets and the R1 and R2 currents is observed, and in the nightside, possible indications of a Cowling channel appear. During southward IMF BZ, the electrojets and the R1 and R2 currents are stronger and clearer than during northward BZ. IMF BY affects the orientation of the pattern.

One-dimensional spherical elementary current systems and their use for determining ionospheric currents from satellite measurements

Abstract The method of 1D spherical elementary current systems (SECS) is a new way for determining ionospheric and field-aligned currents in spherical geometry from magnetic field measurements made by a low-orbit satellite. In contrast to earlier methods, the full ionospheric current distribution, including both divergence-free and curl-free horizontal currents, as well as field-aligned currents, can be determined. Placing infinitely many 2D SECSs of identical amplitudes at a constant latitude results in two types of 1D SECSs, which are independent of longitude, and by superposition can reproduce any ionospheric and field-aligned current system with the same property. One type of the 1D SECSs is divergence-free and toroidal with a poloidal magnetic field, and the other type is curl-free and poloidal. Associated with the divergence of the curl-free type are radial currents. The magnetic field of the combined curl-free 1D SECS and field-aligned currents is toroidal and restricted to the region above the ionosphere. Ionospheric currents are determined by placing several 1D SECSs at different latitudes and choosing their amplitudes in such a way that their combined magnetic field as closely as possible fits the one measured by the satellite. The 1D SECS method has been tested using both modeled and real data from the CHAMP satellite, and found to work excellently in 1D cases.

Ionospheric currents estimated simultaneously from CHAMP satelliteand IMAGE ground-based magnetic field measurements: a statisticalstudy at auroral latitudes

Abstract. One important contribution to the magnetic field measured at satellite altitude and at ground level comes from the external currents. We used the total field data sampled by the Overhauser Magnetometer on CHAMP and the horizontal magnetic field measurements of the IMAGE ground-based magnetometer network to study the ionospheric Hall current system in the auroral regions. For the CHAMP data a current model consisting of a series of lines and placed at a height of 110km is fitted to the magnetic field signature sampled on the passage across the polar region. The derived current distributions depend, among others, on season and on the local time of the satellite track. At dawn/dusk the auroral electrojets can be detected most clearly in the auroral regions. Their intensity and location are evidently correlated with the A E activity index. For a period of almost two years the results obtained from space and the currents determined from ground-based observations are studied. For the full IMAGE station array a newly-developed method of spherical elementary current systems (SECS) is employed to compute the 2-D equivalent current distribution, which gives a detailed picture of an area covering latitudes 60° – 80° N and 10° – 30° E in the auroral region. Generally, the current estimates from satellite and ground are in good agreement. The results of this survey clearly show the average dependence of the auroral electrojet on season and local time. This is particularly true during periods of increased auroral activity. The correlation coefficient of the results is close to one in the region of sizeable ionospheric current densities. Also the ratio of the current densities, as determined from above and below the ionosphere, is close to unity. It is the first time that the method of Hall current estimate from a satellite has been validated quantitatively by ground-based observations. Among others, this result is of interest for magnetic main field modelling, since it demonstrates that ground-based observations can be used to predict electrojet signatures in satellite magnetic field scalar data. Key words. Ionosphere (auroral Ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)

Fast computation of the geoelectric field using the method of elementary current systems and planar Earth models

Abstract. The method of spherical elementary current systems provides an accurate modelling of the horizontal component of the geomagnetic variation field. The interpolated magnetic field is used as input to calculate the horizontal geoelectric field. We use planar layered (1-D) models of the Earth's conductivity, and assume that the electric field is related to the local magnetic field by the plane wave surface impedance. There are locations in which the conductivity structure can be approximated by a 1-D model, as demonstrated with the measurements of the Baltic Electromagnetic Array Research project. To calculate geomagnetically induced currents (GIC), we need the spatially integrated electric field typically in a length scale of 100km. We show that then the spatial variation of the electric field can be neglected if we use the measured or interpolated magnetic field at the site of interest. In other words, even the simple plane wave model is fairly accurate for GIC purposes. Investigating GIC in the Finnish high-voltage power system and in the natural gas pipeline, we find a good agreement between modelled and measured values, with relative errors less than 30% for large GIC values. Key words. Geomagnetism and paleomagnetism (geomagnetic induction; rapid time variations) – Ionosphere (electric field and currents)