2015 St. Patrick's Day aurora
© NASA/Sebastian Saarloos
The 2015 St. Patrick's Day aurora seen in Donnelly Creek, Alaska.
On St. Patrick's Day in 2015, people living as far south as Tennessee spotted brilliant green and red auroras glowing in the night skies. The northern lights-which are typically visible only at high latitudes-were caused by a space storm so intense it disrupted electrical fields on Earth's surface. Now, a new study helps to explain how space storms produce powerful, ground-level electric currents that disrupt power grids, gas and oil pipelines, and communication systems.

Scientists have long known that these currents, called geomagnetically induced currents (GICs), result from interactions between the fluctuating solar wind and Earth's magnetosphere, a region around the upper atmosphere dominated by the magnetic field that buffers our planet from space radiation. The ionosphere, a pulsating layer of charged particles that produces auroras, also plays an important role. Precisely how the storms produce the on-the-ground electric currents has been difficult to pinpoint, however.

To get a more detailed understanding, Ngwira et al. focused on two big space storms: the St. Patrick's Day event and a second space storm that occurred in March 2012. The team examined data from NASA's Time History of Events and Macroscale Interactions During Substorms mission, a suite of satellites that can sense the rapid release of energy just before auroras brighten and expand across the night sky. They also looked at ground monitoring data from several sites across Alaska, where the biggest GICs were detected during the storms.

The team found that the fastest, most intense fluctuations in the amplitude of Earth's magnetic fields occurred just as the auroras were spreading and brightening. The rapidly shifting magnetic fields pushed and pulled electrons at ground level, generating a powerful electric current. The findings support the hypothesis that the brief period leading up to an aurora, called a substorm, is the trigger for intense GICs. The finding could help scientists protect communication and energy systems from destructive electrical currents in the future. (Space Weather, https://doi.org/10.1029/2018SW001911, 2018)
Citation: Underwood, E. (2018), How two massive space storms zapped Alaska, Eos, 99, https://doi.org/10.1029/2018EO102291. Published on 06 August 2018. Text © 2018. The authors. CC BY-NC-ND 3.0