The influence of shield wires on Geomagnetically Induced Currents (GICs) in power systems is considered. For the most simple power network, with one single transmission line and one shield wire connecting two substations, we derive the expressions for the voltage source and the resistance of the Thévenin equivalent circuits that, in parallel with the substations grounding resistances, produce the same effects on GICs as the full circuit. Our model extends results from previous studies that considered the effect of shield wires resistances by also including the induced geoelectric field.

We analyzed correlations between solar, interplanetary-medium parameters, and geomagnetic-activity proxies in 27-day averages (a Bartels rotation) for the 2009 – 2016 time interval. We considered two new proxies: I) Bzs GSM (Geocentric Solar Magnetic), calculated as the daily percentage of the IMF southward component along the GSM z-axis and then averaged every 27 days; ii) four magnetospheric indices (T-indices), calculated from the local north-south (X) contributions of the magnetosphere’s cross-tail (TAIL), the symmetric ring current (SRC), the partial ring current (PRC), and the Birkeland current (FAC), derived from the Tsyganenko and Sitnov (J. Geophys. Res. 110, A03208, 2005: TS05) semi-empirical magnetospheric model. Our results suggest, among the parameters tested here, solar facular areas, interplanetary-magnetic field intensity and new proxies derived here could be taken into account in an empirical model, with a 27-day resolution, to explain geomagnetic activity felt on the Earth’s surface in terms of solar surface features and the IMF condition. We further retrieve a clear annual oscillation in series of 27-day-mean values of toward/away asymmetries of geomagnetic-activity indices, which can be interpreted in the light of the Russell–McPherron hypothesis for the semiannual variation of geomagnetic activity (Russell, C.T., and R. L. McPherron (1973), J. Geophys. Res., 78, 92 – 108).

The solar quiet variation (Sq) observed at observatories near the latitude of the Sq vortex focus is difficult to assess because it is affected by both the geomagnetic activity level and the dynamics of the ionosphere and the upper atmosphere resulting in changes of the Sq ionospheric vortex shape and position. The use of only geomagnetically quiet days (QD) to calculate Sq for a given month can, to a certain extent, remove the effect of geomagnetic disturbances; however, the effect of the atmospheric dynamics still needs to be taken into account. The Sq vortex shape and position can be acquired from the horizontal vector of the geomagnetic field measured at geomagnetic observatories located in the European-African sector between 10ºN and 60ºN using vector rotation or calculating equivalent currents. Here we present results of a comparative analysis of two methods to extract Sq variations from the observations of the earth geomagnetic field. The analyzed data are measurements of the geomagnetic field done between 2007 and 2017 at the Coimbra Geomagnetic Observatory (COI, Portugal) located near 40ºN.The principal component analysis (PCA) based Sq curves are compared with the standard ones obtained using 5 international QD per month. For most of the analyzed years for the X component, the second PCA mode was identified as Sq variation whereas for the Y and Z components for all analyzed data sets the first PCA mode was identified as Sq variation. We studied differences and similarity of the PCA and IQD based Sq in relation to (1) the average geomagnetic activity level and (2) Sq vortex shape and position relatively to COI.

The irregular variation of geomagnetic activity, caused by the solar wind interaction with the magnetosphere/ionosphere, occurs in a wide time interval (from seconds to annual and even larger periods) and amplitude ranges (from few to hundreds of nT). Major variation events (geomagnetic storms) can cause damaging effects and important perturbations to different human activities such as satellite communications, long distance radio broadcasting, navigation, surveying, etc. Furthermore, the induction of electric currents (geomagnetically induced currents – GICs) may affect all grounded conducting networks, in particular electrical power transmission systems. Useful proxies of GIC effects are still under debate; here, we identify the pros and cons of some different candidates. The intensity of geomagnetic activity is usually characterized by geomagnetic indices, among which the 3-hourly indices Kp and local K. In this study we compare 3-hour K and 1-hour K-derived local range indices, using geomagnetic time series from the mid-latitude Coimbra observatory (COI), in Portugal. We also compute smaller time-resolution geomagnetic and GIC indices such as the geomagnetic horizontal field components and their time derivatives, horizontal field magnitude and its time derivative and the LDi/LCi indices [Cid19]. We compare the computed indices with GIC simulations in the Portuguese transmission power grid, to evaluate which of them may be used to nowcast the induced currents. We suggest as a better GIC proxy, an index obtained from geomagnetic field components filtered by convolution with the uniform conductivity Earth filter (new EGIC index). Previous studies have considered the local ground conductivity to be an important factor to determine GIC amplitudes (e.g. [Tri07] and [Rib21]). We then obtain GIC estimations for power grid substations lying at different geological regions. Acknowledgements: This study is funded by national funds through FCT (Portuguese Foundation for Science and Technology, I.P.), under the project MAG-GIC (PTDC/CTA-GEO/31744/2017). FCT is also acknowledged for support through projects UIDB/50019/2020-IDL, PTDC/CTA-GEF/1666/2020 (MN) and PTDC/CTA-GEO/031885/2017 (MN). CITEUC is funded by FCT (UIDB/00611/2020 and UIDP/00611/2020). We acknowledge the collaboration with REN (Redes Energéticas Nacionais).

The separation of contributions from different sources in the magnetic field signal measured at satellite altitude is an open challenge. An approach to this problem, using Principal Component Analysis, is here applied to geomagnetic external field series at Virtual Observatories (VO). These series are computed from an enlarged dataset of Swarm data covering all local times and all geomagnetic activity levels between January 2014 and December 2019. For each 30-days time window, the Equivalent Source Dipole technique is used to reduce all measurements inside a cylinder to one single ‘observation’ at its axis and 500 km altitude. Our results reveal a first principal mode with dipolar geometry and time variation following very closely the RC-index of geomagnetic activity. They display a resolved second principal mode with annual periodicity and of approximately zonal quadrupolar radial pattern, reminiscent of results in a previous study using VO series from a filtered satellite dataset and with lower time resolution. We resort to the recent comprehensive model CM6 to identify a possible source for this second mode. We propose that the dipolar mode is the expression of the magnetospheric ring current dynamics, at 30-day time resolution, and the quadrupolar mode is the expression of the annual asymmetry between local summer and winter Sq current vortices. Two fainter modes could be related to the equinoctial amplification of Sq vortices and the ionospheric dynamo modulation by nonmigrating tides. We show that a more uniform local time sampling could contribute to better resolve ionospheric structures.