**1. Introduction**

"Space Weather" refers to electromagnetic and particle conditions in the near-Earth space. It is controlled by solar activity. The whole space weather chain extending from the Sun to the Earth's surface is very complicated and includes plasma physical processes, in which the interaction of the solar wind with the geomagnetic field plays an essential role. Space weather phenomena statistically follow the eleven-year sunspot cycle but large space weather storms can also occur during sunspot minima. Changes of currents in the Earth's magnetosphere and ionosphere during a space weather storm produce temporal variations of the geomagnetic field, i.e. geomagnetic disturbances and storms. Technological systems, even humans, in space and on the ground may experience adverse effects from space weather (e.g. Lanzerotti et al., 1999).

At the Earth's surface, space weather manifests itself as "Geomagnetically Induced Currents" (GIC) in technological conductor networks, such as electric power transmission grids, oil and gas pipelines, telecommunication cables and railway circuits. GIC observations have a much longer history than the time when the concept of space weather has been used as GIC effects were already found in the first telegraph equipment in the mid-1800's (Boteler et al., 1998; Lanzerotti et al., 1999; Lanzerotti, 2010). Telecommunication systems have suffered from GIC problems several times in the past. Optical fibre cables generally used today are not directly affected by space weather. However, metal wires lying in parallel with fibre cables are used to provide power to repeater stations, and they may be prone to GIC impacts. Trans-oceanic submarine communication cables are a special category regarding GIC since their lengths imply that the end-to-end voltages associated with GIC can be very large.

Buried pipelines may suffer from serious corrosion of the steel due to GIC (e.g. Gummow, 2002). Corrosion is an electrochemical process occurring at points where a current flows from the pipe to the soil. Roughly speaking, a continuous dc current of 1 A for one year causes a loss of about 10 kg of steel. To prevent or minimise corrosion, so-called cathodic protection (CP) systems are used for pipelines. They keep the pipeline in a negative voltage of typically slightly less than 1 V with respect to the soil. Pipe-to-soil voltages associated with GIC may well exceed the CP voltage, thus possibly cancelling and invalidating the protection. Furthermore, control surveys of pipe-to-soil voltages during space weather storms may lead to

Geomagnetically Induced Currents as Ground Effects of Space Weather 29

(Elovaara et al., 1992; Elovaara, 2007) whereas the neighbour country, Sweden, has experienced harmful GIC effects several times (Elovaara et al., 1992; Pirjola & Boteler, 2006; Wik et al., 2009). Such a dissimilarity between Sweden and Finland is somewhat surprising but, concerning power systems, it can be explained based on differences in transformer

In Section 2 of this paper, we consider research of GIC, which can be done by measurements or by theoretical modelling. Section 3 is devoted to a more detailed discussion of the calculation of GIC in power networks and pipelines, and Section 4 contains concluding

Research of GIC is highly multidisciplinary since the subjects involved cover items from solar physics to engineering details of the operation of power systems or other networks. We often speak about the "space weather chain" that begins at solar activity, extends via the solar wind and magnetospheric-ionospheric processes to GIC in ground-based systems and to the mitigation of adverse effects of GIC (e.g. Pirjola, 2000; Pirjola et al., 2003). Roughly speaking, GIC studies can be divided into two parts, the first of which refers to the space physical and geophysical investigation of GIC in a network, whereas the second part includes the engineering evaluation of effects of GIC on the system in question as well as the design of techniques for mitigating the harmful impacts. This paper deals with the first part. The flow of GIC in a network is easy to understand based on Faraday's and Ohm's laws. The geomagnetic field experiences temporal variations during a space weather event. According to Faraday's law, they are accompanied by a geoelectric field, which, based on Ohm's law, drives currents in conductors, i.e. GIC in networks. In theoretical discussions, the determination of GIC in a system is usually divided into two parts or steps. The "geophysical part" and the "engineering part" refer to the modelling of the horizontal geoelectric field at the Earth's surface and to the calculation of GIC in the particular

GIC can naturally be studied by making measurements or by theoretical modelling. In practice, the validity of the models always has to be verified by measured data. If necessary, the data may enable adjusting model parameter values. Concerning an appropriate model of the Earth's conductivity in southern Sweden, the adjustment is explicitly demonstrated by

The usual place for installing GIC recording equipment in a power network is the earthing lead of a transformer neutral. In the normal situation, this particular lead carries no 50/60 Hz current because the sum of the ac currents in the three phases is equal to zero. Furthermore, measuring GIC in the neutral lead directly gives information about the currents flowing through transformer windings, where they can result in harmful saturation. Recordings of GIC can be performed with a coil around the earthing lead. However, for example in the Finnish 400 kV network, a small shunt resistor is utilised in the lead (e.g. Elovaara et al., 1992). The largest GIC magnitudes measured in Finland and in

design and specifications in the two countries.

network, respectively (e.g. Pirjola, 2002).

Wik et al. (2008).

**2.1 Measurements of GIC** 

remarks.

**2. Research of GIC** 

incorrect data. Pipelines are covered with a highly-resistive coating, whose materials have a much larger resistivity today than in earlier times. But a high resistance also increases pipe-tosoil voltages implying larger harmful currents at possible defects in the coating.

GIC impacts on railways have not been much investigated yet but evidence of anomalies in railway signalling systems due to GIC exists at least in Sweden and Russia (Ptitsyna et al., 2008; Wik et al., 2009; Eroshenko et al., 2010).

Nowadays electric power transmission networks are the most important regarding GIC effects, and the importance continuously increases with the extension of power grids including complex continent-wide interconnections and with the even larger dependence of the society on the availability and reliability of electricity. The frequencies associated with GIC are typically very much lower than the 50/60 Hz ac frequency used in power transmission. Thus, from the viewpoint of a power system, GIC are (quasi-)dc currents. Consequently their presence may lead to half-cycle saturation of transformers, which can result in non-linear behaviour of transformers (e.g. Molinski, 2002; Kappenman, 2007). This further implies large asymmetric exciting currents producing harmonics, unnecessary relay trippings, increased reactive power demands, voltage fluctuations, and possibly even a collapse of the whole power network. Transformers can also be overheated with possible damage. The best-known GIC disturbance is a province-wide blackout in Québec, Canada, for several hours during a large geomagnetic storm in March 1989 (e.g. Bolduc, 2002). A transformer was permanently damaged and had to be replaced in New Jersey, USA, during the same storm (Kappenman & Albertson, 1990).

All this means that research of GIC and space weather is not only relevant and significant regarding space science but important practical applications also exist. As indicated above, today's GIC research is particularly concentrated on power networks, which constitute the main focus in this paper as well. The discussion is limited to space physical and geophysical aspects associated with GIC including the calculation of GIC but neglecting the consideration of engineering details of possible adverse impacts of GIC on networks and their equipment and the discussion of mitigation means against GIC problems.

The shape of the geomagnetic field implies that geomagnetic storms are the most intense and most frequent at high latitudes. So GIC are a special concern in the same areas. However, during major space weather storms, large geomagnetic disturbances may also occur at much lower latitudes, which indicates the possibility of GIC problems there, too. Moreover, GIC magnitudes in a system depend significantly on the network topology, configuration and resistances. GIC values also vary much from site to site in a system being generally large at ends and corners of a network. In addition, the sensitivity of a system to GIC depends on many technical matters and varies from one network to another. Consequently, a GIC value that can be ignored in one system may be hazardous in another. All this shows that GIC issues have to be taken into account in mid- and low-latitude networks, too (e.g. Kappenman, 2003; Trivedi et al., 2007; Bernhardi et al., 2008; Liu et al., 2009a, 2009b).

Finland is located at high latitudes. Consequently, GIC would be a potential problem in the country, and in fact, research of GIC has been carried out as collaboration between Finnish power and pipeline industry and the Finnish Meteorological Institute since the latter part of the 1970's. However, fortunately, GIC have never caused significant problems in Finland (Elovaara et al., 1992; Elovaara, 2007) whereas the neighbour country, Sweden, has experienced harmful GIC effects several times (Elovaara et al., 1992; Pirjola & Boteler, 2006; Wik et al., 2009). Such a dissimilarity between Sweden and Finland is somewhat surprising but, concerning power systems, it can be explained based on differences in transformer design and specifications in the two countries.

In Section 2 of this paper, we consider research of GIC, which can be done by measurements or by theoretical modelling. Section 3 is devoted to a more detailed discussion of the calculation of GIC in power networks and pipelines, and Section 4 contains concluding remarks.
