When you think of regions that are vulnerable to solar storms, the first places that probably come to mind are at high latitudes, like Canada, the northern United States, and Scandinavia—regions that regularly experience strong geomagnetic activity and spectacular displays of aurora.
A country like Spain, with its balmy Mediterranean climate and geomagnetic latitude equivalent to Florida, probably doesn’t leap to mind. However, history shows that lower-latitude regions need to be on guard as well. During the famous solar storm of 1859, the strongest on record, auroras were reported as far south as Cuba, and telegraphs failed across the United States and Europe.
Now a new study by Torta et al. demonstrates how to improve vulnerability assessments of Spain’s power grid by measuring the conductivity of the bedrock below critical substations. The method could be used by other countries seeking to quickly assess their vulnerability to space weather.
The threat to power grids during such storms stems from the storms’ ability to induce strong geomagnetic activity, which, in turn, can induce strong currents in power lines. Complicating matters is the fact that the strength of these currents also depends on the conductivity of the ground underneath. This varies with the type of rock and can change significantly across geological boundaries or where large bodies of water are present. Although some nations have dedicated resources to comprehensively map these differences over thousands of locations, like the EarthScope project in the United States, other nations may be able to survey only at a few places, near important substations in the power grid.
To evaluate surveys’ usefulness, the authors conducted one such survey near a substation in Spain’s power grid located in Vandellòs, in Catalonia on the Mediterranean Sea, near a nuclear power plant.
These surveys evaluate the natural electromagnetic signal present at these sites, which is a function of the region’s induction response to the natural magnetic field variations. Fluctuations in this signal behave like the firing of an electromagnetic pulse into the ground, sending current racing through the rock below. Electromagnetic disturbances then propagate back to the surface, where instruments can measure them and infer the conductivity of the underlying rock.
The team then used these measurements, called magnetotelluric (MT) soundings, in a model to predict how strong the currents would be at the Vandellòs substation during a solar storm and compared them to actual data from several storms in 2011 and 2012. The team found that MT readings significantly improved predictions, in particular, because they revealed that the nearby Mediterranean Sea strongly affects Earth’s electromagnetic response in the region. By one metric of model performance, MT readings improved the accuracy of the predictions by a factor of 8.
Furthermore, the team found that another key input in the model, the strength of Earth’s magnetic field overhead, could be interpolated from existing data without significantly hurting the model’s performance. That’s partly because at lower latitudes, Earth’s magnetic field is more uniform than at the poles. The team concluded that conducting MT surveys at critical power stations would be an effective way for regions at lower latitudes to assess their risk and be prepared when the next solar storm strikes. (Space Weather, https://doi.org/10.1002/2017SW001628, 2017)
—Mark Zastrow, Freelance Writer