Ground Effects of Space Weather

摘要

Space storms produce geomagnetically induced currents (GIC) in technological systems at the Earth's surface, such as electric power transmission grids, pipelines, communication cables and railways. Thus GIC are the ground end of the space weather chain originating from the Sun. The first GIC observations were already made in early telegraph equipment about 150 years ago, and since then several different systems have experienced problems during large magnetic storms. Physically, GIC are driven by the geoelectric field induced by a geomagnetic variation. The electric and magnetic fields are primarily created by magnetospheric-ionospheric currents and secondarily influenced by currents induced in the Earth that are affected by the ground conductivity. The most violent magnetic variations occur in auroral regions, which indicates that GIC are a particular high-latitude problem but lower-latitude systems can also experience GIC problems. In power networks, GIC may cause saturation of transformers with harmful consequences extending from harmonics in the electricity to large reactive power consumption and even to a collapse of the system or to permanent damage of transformers. In pipelines, GIC and the associated pipe-to-soil voltages can enhance corrosion and disturb corrosion control measurements and protection. Modelling techniques of GIC are discussed in this paper. Having information about the Earth's conductivity and about space currents or the ground magnetic field, a GIC calculation contains two steps: the determination of the geoelectric field and the computation of GIC in the system considered. Generally, the latter step is easier but techniques applicable to discretely-earthed power systems essentially differ from those usable for continuously-earthed buried pipelines. Time-critical purposes, like forecasting of G1C, require a fast calculation of the geoelectric field. A straightforward derivation of the electric field from Maxwell's equations and boundary conditions seems to be too slow. The complex image method (CIM) is an alternative but the electric field can also be calculated by applying the simple plane wave formula if ground-based magnetic data are available. In this paper, special attention is paid to the relation between CIM and the plane wave method. A study about GIC in Scotland and Finland during the large geomagnetic storm in April 2000 and another statistical study about GIC in Finland during SSC events are also briefly discussed.