Ari Viljanen

Calculation of geomagnetically induced currents in the 400 kV power grid in southern Sweden

Sweden has experienced many geomagnetically induced current (GIC) events in the past, which is obviously due to the high-latitude location of the country. The largest GIC, almost 300 A, was measured in southern Sweden in the earthing lead of a 400 kV transformer neutral during the magnetic storm on 6 April 2000. On 30 October 2003, the city of Malmo at the southern coast suffered from a power blackout caused by GIC, leaving 50,000 customers without electricity for about 20-50 min. We have developed a model that enables calculation of GIC in the southern Swedish 400 kV power grid. This work constitutes the first modeling effort of GIC in Sweden. The model is divided into two parts. The electric field is first derived using a ground conductivity model and geomagnetic recordings from nearby stations. The conductivity model is determined from a least squares fit between measured and calculated GIC. GIC are calculated using a power grid model consisting of the topology of the system and of the transformer, transmission line, and station earthing resistances as well as of the coordinates of the stations. To validate the model, we have compared measured and calculated GIC from one site. In total, 24 events in 1998 to 2000 were used. In general the agreement is satisfactory as the correct GIC order of magnitude is obtained by the model, which is usually enough for engineering applications.

At substorm onset, 40% of AL comes from underground

Magnetic variations observed at the Earths surface are caused by external and internal sources. External variations arise from currents in the ionosphere and magnetosphere, and internal variations arise from currents induced in the solid Earth. In this paper we examine how large the internal contribution is to magnetic variations measured at the Earths surface. We use IMAGE magnetometer measurements to analyze 77 substorms during 1997. For each event we evaluate the internal and external parts of a locally derived auroral electrojet index (IL index). The magnetic field separation is performed using the Siebert-Kertz equations. A superposed epoch analysis of all events clearly shows that the internal contribution peaks strongly at substorm onset, when the internal contribution is ∼ 40% of the total field. After the substorm peak intensity, the internal contribution decreases almost linearly to the quiet time value of 10-20%. The induction effects are largest during the times of rapid changes and at stations located over the Arctic Ocean.

Loading‐unloading processes in the nightside ionosphere

The loading-unloading processes during magnetospheric substorms are examined using the ’IL index’ from the IMAGE magnetometer chain together with solar wind and IMF data from the WIND spacecraft. The IL index is a local, midnight sector, AL index. Energy input throughout the substorm is evaluated by integrating the epsilon parameter from the start of the growth phase until the end of the substorm. Energy dissipated in the ionosphere is estimated by integrating the IL index from the substorm onset to the end of the substorm. We show that the best correlation with the energy dissipated in the ionosphere is given by the energy input to the system after the substorm onset. Hence, we conclude that the energy loaded during the growth phase is necessary for the magnetospheric reconfiguration before the substorm onset, but that the size of the substorm as measured by the IL index is mostly governed by the direct energy input during the expansion phase.

Modeling geomagnetically induced currents during different ionospheric situations

We use several realistic three-dimensional models of ionospheric currents to calculate geomagnetically induced currents (GIC) in the Finnish high-voltage power system. Of special interest are events during which the magnetic field changes rapidly and GIC are large. The geoelectric field driving GIC is determined with the complex image method, which is a fast and accurate tool for taking into account induction effects in the Earth. A detailed investigation is made applying a model of a westward traveling surge (WTS). It is capable of producing magnetic field variations and GIC which are of the same magnitude as the observed values. However, the WTS model yields too large time derivatives of the magnetic field. A much simpler line current model produces very realistic magnitudes of both the magnetic field and GIC. However, in contrast to WTS, it lacks the realistic spatial structure of the ground magnetic field. The requirement of accurate models of the Earths conductivity is demonstrated by comparing a resistive and conductive structure in connection with a very rapid change of the magnetic field. Consideration of some other typical ionospheric events (Harangs discontinuity, omega band, pulsation) indicates that these phenomena probably cannot produce extremely large GIC.

The relation between geomagnetic variations and their time derivatives and implications for estimation of induction risks

By using the IMAGE magnetometer network data, we compare the geomagnetic variation field B and its time derivative dB/dt in and near the auroral region. We show that although the auroral electrojet is the main reason for large Bs, it cannot alone produce large dB/dts, so smaller-scale current systems are important. This is evidenced by the horizontal field: the geographic north component (X) is clearly larger than the eastward one (Y) in the auroral region, but dX/dt and dY/dt are nearly equal. This result is important when geomagnetic induction risks on man-made conductors are estimated. The key quantity is the horizontal geoelectric field, and contrary to what is often assumed, it can have large values in any direction, not just parallel to the electrojet.

One-dimensional upward continuation of the ground magnetic field disturbance using spherical elementary current systems

Ionospheric equivalent currents are defined as spherical sheet currents, which reproduce the observed magnetic disturbances below the ionosphere. One way of determining these currents is to place several so called spherical elementary current systems (SECS) in the ionospheric height and to solve an inversion problem for the amplitudes of these systems. In previous studies this method has been applied to two-dimensional data sets, having both latitudinal and longitudinal spatial coverage (2D SECS method). In this paper a one-dimensional variant of this method (1D SECS) is developed. The 1D SECS method can be used even in those situations where the data set is one dimensional, e.g. with one meridionally aligned magnetometer chain. The applicability of the 1D SECS method is tested using both synthetic and real data. It is found that in real situations the errors in the 1D SECS results are 5—10% in current density profiles and ~5% in integrated currents, when compared to the results of the more accurate 2D SECS method.

Recordings and occurrence of geomagnetically induced currents in the Finnish natural gas pipeline network

Abstract A project implemented to study the effects of space weather on the Finnish natural gas pipeline was started in August 1998. The aims of the project were (1) to derive a model for calculating geomagnetically induced currents (GIC) and pipe-to-soil (P/S) voltages in the Finnish natural gas pipeline, (2) to perform measurements of GIC and P/S voltages in the pipeline and (3) to derive statistical predictions for the occurrences of GIC and P/S voltages at different locations in the pipeline network. GIC and P/S voltage were recorded at a compressor station. The GIC measurement was made with two magnetometers, one right above the pipe, and another at the Nurmijarvi Geophysical Observatory about 30 km southwest. The largest GIC since November 1998 has been 30 A. The P/S voltage recording was stopped in May 1999, but GIC is still measured. GIC statistics were derived based on the recordings of the geomagnetic field at Nurmijarvi. The geoelectric field was calculated by using the plane wave model. This field was input to the general pipeline model resulting in the distribution of currents and P/S voltages at selected points in the pipeline. As could be expected, the largest P/S voltage variations occur at the ends of the pipeline network, while the largest GIC flow in the middle parts.

Plasma sheet ion injections into the auroral bulge: Correlative study of spacecraft and ground observations

Multiple and sporadic time-of-flight velocity dispersed ion structures (TDIS) are systematically observed above the ionosphere at ∼3 Re altitude by Interball/Auroral spacecraft near the poleward edge of the auroral bulge. These events represent direct snapshots of the impulsive ion acceleration process in the equatorial plasma sheet which allow us to study the details of the connection between ionospheric and plasma sheet manifestations of the magnetospheric substorm. Two events are analyzed during which the spacecraft footpoints passed over the Scandinavian ground network. We found that the TDIS correlate with the intensifications of westward current and auroral activations at the poleward edge of the bulge, which confirms the association of these dispersed ion beams with the temporal evolution of impulsive reconnection in the tail. Furthermore, we present direct evidence of an active neutral line in the magnetotail during one of the events using plasma sheet measurements made concurrently by the Interball/Tail and Geotail spacecraft. The 2–3 min repetition period of these ∼1 min long activations indicates a fundamental time constant of the substorm instability. On the other hand, the estimated injection distances of the energy-dispersed ions were inferred to be smaller than the estimated position of the reconnection region in the tail. We also found that the TDIS ion beams are released within the closed plasma flux tubes deep inside the plasma sheet, and yet they are synchronized with auroral activations at the poleward boundary. These facts imply that the ion beams are formed in a spatially extended region of the plasma sheet rather than in the close vicinity of the neutral line. We argue that braking of the reconnection-induced fast flow bursts when they interact with the closed plasma flux tubes and the earthward propagating fast wave electric field generated in the braking region may be important in forming the observed multiple, sporadic, energy-dispersed ion beams.

Determination of ground conductivity and system parameters for optimal modeling of geomagnetically induced current flow in technological systems

In this work, methods to determine technological system parameters and the ground conductivity structure from different sets of geomagnetically induced current (GIC), magnetic field and geoelectric field observations are explored. The goal of the work is to enable optimal modeling of induced currents in any technological system experiencing GIC. As an additional product, the introduced methods can also be applied to utilize GIC observations in the imaging of the subsurface geological structures. Here a robust processing scheme and Occam’s inversion technique familiar from magnetotelluric (MT) studies are applied to the determination of the ground conductivity structure. The application of the methods to GIC data from the Finnish pipeline for a storm period of October 24-November 1, 2003 demonstrate that optimal system parameters and ground conductivity structure can be obtained using time series comprising only 8 days worth of data. Importantly, the obtained ground model is in agreement with models obtained in earlier MT studies. Furthermore, it is shown that although in an ideal case the magnetic field data used should be obtained from the immediate vicinity of the GIC observation site, some spatial separation (200–300 km) between the sites can be tolerated.

Modelling of space weather effects on pipelines

Abstract The interaction between the solar wind and the Earths magnetic field produces time varying currents in the ionosphere and magnetosphere. The currents cause variations of the geomagnetic field at the surface of the earth and induce an electric field which drives currents in oil and gas pipelines and other long conductors. Geomagnetically induced currents (GIC) interfere with electrical surveys of pipelines and possibly contribute to pipeline corrosion. In this paper, we introduce a general method which can be used to determine voltage and current profiles for buried pipelines, when the external geoelectric field and the geometry and electromagnetic properties of the pipeline are known. The method is based on the analogy between pipelines and transmission lines, which makes it possible to use the distributed source transmission line (DSTL) theory. The general equations derived for the current and voltage profiles are applied in special cases. A particular attention is paid to the Finnish natural gas pipeline network. This paper, related to a project about GIC in the Finnish pipeline, thus provides a tool for understanding space weather effects on pipelines. Combined with methods of calculating the geoelectric field during magnetic storms, the results are applicable to forecasting of geomagnetically induced currents and voltages on pipelines in the future.