Geomagnetically Induced Currents

The solar wind – magnetosphere energy transfer creates currents flowing in the coupled magnetosphere – inosphere system. The time varying magnetic field of these changing currents creates, according to Faraday’s law of induction, an electric field on the Earth’s surface, the ground manifestation of space weather. This electric field, in turn, may drive currents in conductive infrastructures of mankind, such as earthed power grids or long-distance subterran oil or gas pipelines. These currents are commonly called as geomagnetically induced currents, GICs.

Considering power lines, in order to reduce transmission losses low resistant high voltage lines are used that favor GIC flow in the system. The quasi-DC GIC affects the normal duty cycle of power transformer by causing half cycle saturation of the core. This can produce undesired hamonics in the AC waveform that manifests itself in magnetic flux leakage, transformer overheating, power fluctuation, protective relay tripping. Eventually, it may cause the collapse of entire power grid, with considerable economical loss, such what happened in March 13, 1989 in Canada, where six millon people were affected. Regarding oil/gas pipelines, the effect is cumulative. GIC changes the pipe-soil-potential, resulting in speeded up corrosion, thus premature aging.

As mentioned above, induction drives GIC and today we determined empirical parameters dependent on ground conductivity that connect the measured dB/dt (first time derivative of magnetic field) with the produced ground electric field. Using specific parameters of the actual power grid in question, GIC may be modelled based on this elecric field estimate. dB/dt thus may be used as GIC proxy. The Finnish Meteorological Institute measures the actual GIC at Mäntsälä natural gas pipeline (http://space.fmi.fi/gic/?page=home). Considering the famous ‘Halloween storm’ on October 30, 2003 causing power outage in southern Sweden, figure 1 depicts the magnetic field variation, dB/dt, and the measured GIC in the Finnish natural gas pipeline.

GIC at Mäntsälä (top) and dX/dt at nearby Nurmijärvi observatory (corresponding to eastward E-field, thus to GIC)
GIC at Mäntsälä (top) and dX/dt at nearby Nurmijärvi observatory (corresponding to eastward E-field, thus to GIC) Source: FMI GASUM

It is clear that the two curves correlate well in this particular case, and magnetic disturbance of about 10-20 nT drove up to about 50 A GIC (about 1/4 the size of the largest value ever measured in Finland).

This week, on June 8, 2015, the elevated solar wind speed and lasting negative IMF caused a moderate space weather event with K=5 at Tromsø (TRO), following preceeding quiet days.

The magnetic H, D, and Z component at TRO on Jun 8, 2015
Geomagnetic H, D, and Z Components at TRO on June 8, 2015.
dBdT
The rate of change of magnetic components at TRO in nT/sec. (Note the different timescales when comparing).

Note that, although the disturbance amplitude remained relatively moderate (compare it to 22-23 UT), the rate of change, dB/dt, reached considerable level, such that is comparable, though smaller, than that during the Helloween strom dicussed above.

The NOSWE leads an international science consortium within the framework of ESA Space Situational Awareness (SSA) Programme, whose aim, among others, is to provide reliable GIC service for potential power- and pipeline operators, together with 30-min dB/dt (GIC proxy) forecast based on L1 solar wind data (see a preceeding post on L1 data).