Improving the modeling of geomagnetically induced currents in Spain

Vulnerability assessments of the risk posed by geomagnetically induced currents (GICs) to power transmission grids benefit from accurate knowledge of the geomagnetic field variations at each node of the grid, the Earth's geoelectrical structures beneath them, and the topology and relative resistances of the grid elements in the precise instant of a storm. The results of previous analyses on the threat posed by GICs to the Spanish 400 kV grid are improved in this study by resorting to different strategies to progress in the three aspects identified above. First, although at midlatitude regions the source fields are rather uniform, we have investigated the effect of their spatial changes by interpolating the field from the records of several close observatories with different techniques. Second, we have performed a magnetotelluric (MT) sounding in the vicinity of one of the transformers where GICs are measured to determine the geoelectrical structure of the Earth, and we have identified the importance of estimating the MT impedance tensor when predicting GIC, especially where the effect of lateral heterogeneities is important. Finally, a sensitivity analysis to network changes has allowed us to assess the reliability of both the information about the network topology and resistances, and the assumptions made when all the details or the network status are not available. In our case, the most essential issue to improve the coincidence between model predictions and actual observations came from the use of realistic geoelectric information involving local MT measurements

Validating GIC modeling in the Spanish power grid by differential magnetometry

series of experiences and recommendations are presented concerning the derivation of geomagnetically induced currents (GIC) by use of the differential magnetometry method (DMM) under power lines. This indirect technique, intended to obtain observations to validate GIC models, is an alternative to measuring the current flow in the transformer neutrals. It is a non-intrusive and autonomous technique, as the procedure does not depend on the grid operator. In contrast, the selection of suitable sites devoid of human interferences, the need for power to supply the magnetometer, the data acquisition and transmission system, along with the choice of the appropriate instrumentation are difficulties that make not just any site suitable for installation and often require costly solutions. We focus on the methodology followed to estimate the GIC flowing in several transmission lines of the Spanish power grid with the aim of validating our GIC models, and we share our experience on the installation of the measuring points. Uncertainty inherent in the DMM is assessed, showing that noise is the main handicap, although it can be minimized with appropriate filtering. According to such experience, on some occasions only total DC currents above a significant fraction of 1 A give magnetic signatures well above the noise level, so this figure can roughly be considered as the threshold limit for detection. The low solar activity, combined with the mid-latitude condition of Spain, limited the significance of available recorded data, but we can already report and analyze the results for several minor geomagnetic storms

On the observation of magnetic events on broad-band seismometers

The objective of this contribution is to get new insights into the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have investigated if the Sudden Storm Commencements (SSC) cataloged during the 24th solar cycle (2008-2019) can be systematically identified in broad-band seismic stations distributed worldwide. The results show that the 23 SSC events with a mean amplitude above 30 nT and most of those with lower energy but still clearly identified in the magnetometer detection network can be observed at broad-band stations' network using a simple low-pass filter. Although the preliminary impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we focus on the short period seismic signals recorded in urban environments which are correlated with the activity of the railway transportation system. We have analyzed collocated measurements of electric field and seismic signals within Barcelona, evidencing that significant changes in the electric field following the activity of the transportation systems can be attributed to leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed

Expected geomagnetically induced currents in the Spanish islands power transmission grids.

The aim of this study is to evaluate the geomagnetically induced current (GIC) hazard in the power networks of the Canary and Balearic archipelagos. This is done in order to strictly complete the detailed assessment at national level of the power transmission system of mainland Spain, including the 400 and 220 kV levels. We have constructed models for the grids in each of the individual systems and used resistivity models of the lithosphere for each group of islands, from which we have calculated the surface impedances. The respective models of electrical admittances of the grids have been combined with the geoelectric field derived from the convolution of the recorded (or expected in an extreme scenario) geomagnetic storms and the impedances calculated from the geoelectrical models to derive the expected GICs in the power lines, substations, and transformers. The low geomagnetic latitude of the Canary Islands combined with the small size of their power networks, makes them one of the least likely electrified locations to record significant GICs, with less than 3 A for the 100-year return period. Even the 13 A that could be reached for the upper limit of the 95% confidence interval at the 500-year return period does not seem likely to have a significant impact. The Balearic Islands, being at higher latitude and with a system length of approximately 300 km, including alternating current power lines connecting the Islands, shows GIC signals of moderate amplitude with up to 40 A for the 100-year return period.</p

30 Years of sfe in the Ebro Observatory

International audienceThe magnetic effect produced by chromospheric fiares, sfe, are not a mere amplitude increa.se of the diurnal variation, Sq. In some cases they are a decrease. In base of an statistic study over 30 years of magnetic records in Ebre, we have found; a) a dependence in phase lag of sfe and Sq vectors with local time. In fact, phase lag greater than 902, reversed sfe, eoncentratc between 10 and 12 hours; b) the reversed sfe have an equinoctial character; c) It exists a weaker correlation of the solar activity with the reversed sfe than with the regular ones

Solar flare effects at Ebre: Regular and reversed solar flare effects, statistical analysis (1953 to 1985), a global case study and a model of elliptical ionospheric currents

International audienceThis paperp resentsth e resultso f the analysiso f geomagnetice ffectso f solarf lares (sfe) recordeda t Ebre observatory(4 0.8ø l atitudeN , 0.5ø l ongitudeE ) during3 3 years( 1953- 1985). At Ebre, locatedn eart he focusl atitude,t wo typeso f sfe can be observedr:e gulara nd reverseds fe.R egulars fea ret hosew hichh avep hased ifferencelse sst han9 0ø w ith the regular diurnal magnetic variation of the day, Sn. Reversed sfe are those which have phase differences greatert han 90ø with Sn.F rom these3 3 years,1 40 sfe eventsw ere selecteda nd a statistical studyw as performed.W e founda local time dependencoef the phased ifferencesb etweent he sfe and S n vectors. Morning hours have slightly positive values and afternoon hours have slightly negative ones. Reversed sfe, with a phase difference exceeding 90 ø, concentrate between1 0 and 12 hours.R everseds fe showa dominante quinoctiacl haracterA. lso, a weaker correlationw as foundb etweens olara ctivityw ith reverseds fe (r=-0.47)t han with regulars fe (r=-0.68).U sing data from 67 observatoriesw, e performeda globals tudyo f a sfe case,s een at Ebre as reverseds fe. In this case,i n the northernh emispheret,h e sfe systemw as about 1 hour of local time eastward of the S n system and formed 4 ø higher in latitude. Finally, we presenta model of two elliptical ionospherice quivalentc urrent systemsw ith focus offset about1 hour in local time to explaint he phased ifferenceb etweent he sfe and Sq magnetic vectorso bserveda t Ebre. The parameterso f this model have been fitted from the resultso f a previouss tatisticaal nalysisf rom Ebre data. Spatiala nd temporald istributiono f the sfe and $q vector phasesa re calculatedw ith this model, and conditionsf or reverseds fe occurrence are predicted

Solar flare effects at Ebre: Unidimensional physical, integrated model

International audienceA great increase of the ionizing radiation during solar flares results in an immediatei ncreaseo f the ionizationp roductionr ate, electrond ensitiesa nd electricc urrents in the ionospheref,o llowed simultaneouslyb y disturbanceos f the magnetice lementsa t ground level (solar flare effects (sfe)). In this paper an attempt is made to model sfe phenomena combining several semiempirical models derived from satellite and radar data obtainedd uringt he last two decadesT. he model allows us to quantifym odel valueso f the phased ifferenceb etweent he sfe and Sq vectors,f or comparisonto the measurableq uantity. It explains the cause of the change in magnetic perturbation during a flare at Ebre Observatory (40.8 ø latitude N, 0.5 ø longitude E). Large phase shift of the magnetic vector observed before noon, result from a descent of the "center of gravity" of the conducting mass that, combined with a very different regime of neutral winds in the lower and in the middle parts of the dynamo region, produce a change in the direction of the integrated currents