**2. Research of GIC**

28 Space Science

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-to-

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.,

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

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

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.,

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

their equipment and the discussion of mitigation means against GIC problems.

soil voltages implying larger harmful currents at possible defects in the coating.

2008; Wik et al., 2009; Eroshenko et al., 2010).

the same storm (Kappenman & Albertson, 1990).

2009a, 2009b).

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

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 Wik et al. (2008).
