Seismic signals illuminate the northern lights

Earth’s magnetosphere protects life from damaging high-energy radiation carried by the solar wind. As the stream of charged particles approaches Earth, Lorentz forces from the planet’s magnetic field deflect it and prevent the particles from bombarding the surface. For seismologists studying ground motion in high-latitude regions, variations in Earth’s magnetic field can manifest as false ground motions because of the ferromagnetic materials used in seismometers. Researchers began filtering out those fluctuations in 2020 using independent data from nearby magnetometers, but Carl Tape and Don Hampton from the University of Alaska Fairbanks and Adam Ringler of the US Geological Survey’s Albuquerque Seismological Laboratory have now found a signal in the noise. By combining optical and magnetic observations with more spatially dense seismic data, the researchers documented a magnetic ground signal associated with aurora borealis events.

Six all-sky cameras and 13 magnetometer stations operate in Alaska, and the number of seismic stations dwarfs those figures. The Incorporated Research Institutions for Seismology deployed 193 new seismometers and upgraded 35 existing stations from 2014 to 2017. To study how the optical, magnetic, and seismic observations capture aurora borealis activity, Tape and his colleagues used complementary data from those sources and focused on a few events on 2 March 2017. The time series for three magnetometer stations in the top portion of the figure below show the horizontal (red line), vertical (green line), and total (black line) magnetic-field intensity; the bottom panel shows the aurora image in the night sky as a time-versus-latitude plot. Corresponding data (not shown) from seismometers at those locations show magnetic anomalies measured on the ground and at the same time as the optical and magnetometer observations.

Tape and his colleagues suggest that seismometers could act as magnetometers for other research, provided that the local magnetic variations are well understood, the ground motion from earthquakes is separated from the seismic signal, and the metal materials encasing the sensor are documented. Alternatively, installing magnetometers at seismic stations would better capture the magnetic field variations and allow the unwanted signal to be removed from seismometer recordings. Multi-instrument sites could thus provide enhanced spatial information of the ground-based magnetic variability caused by ionospheric activity. (C. Tape, A. T. Ringler, D. L. Hampton, Seismol. Res. Lett., 2020, doi:10.1785/0220200161 .)