A recent study shows that seismometers on the ground are behaving like magnetometers that record auroral activity in Alaska.
By Jeng Hann Chong, Cal State University Northridge
Citation: Chong, J.H., 2020, Seismic sensors on the ground record auroras in the sky, Temblor, http://doi.org/10.32858/temblor.114
Auroras like the northern lights have fascinated people for as long as we have observed these vivid streaks of light swirling across the night skies. But auroras are not just pretty lights: They are accompanied by disturbances in Earth’s magnetic environment that affect local radio communications. These disturbances can be detected by seismometers on the ground, say scientists.
Auroras are caused by the solar wind — charged particles from the sun — interacting with Earth’s magnetic field, and sending high-energy electrons into the upper atmosphere. During magnetic storms, when more solar wind than usual interacts with Earth’s magnetosphere, large amounts of these electrons collide with gases in Earth’s atmosphere to produce stunning light displays. Auroras are commonly seen at high latitudes, as Earth’s magnetic field lines direct the charged particles to the poles. In 1741, scientists in Uppsala, Sweden, discovered that auroras are associated with magnetic perturbations. In the early 1990s, scientists linked magnetic storms with anomalous signals recorded on seismometers. In a new study, published in Seismological Research Letters, Carl Tape, a seismologist at the University of Alaska, Fairbanks, and his colleagues associated the occurrence of auroras using seismometer records, optical images and magnetic field data.
Alaska has more than 200 temporary seismic stations distributed across the state. As part of the EarthScope Transportable Array (TA), these stations record data that scientists use to study earthquakes and map the interior structure beneath North America. “This [dense seismic station array] has provided unprecedented coverage in the high Arctic region with broadband seismometers that can record subtle signals,” Tape says.
Noise in seismometers
Broadband seismometers are sensitive instruments that are designed to record ground motions. However, they can also detect nonearthquake-related signals, such as the expansion and contraction of seismometer springs as they respond to changes in the atmospheric temperature and pressure. Normally, scientists want to decrease this kind of “noise” in the ground motion data. To isolate these undesired signals, ferromagnetic materials that don’t expand or shrink with changing temperatures are used to construct the seismometers, says Robert Anthony, a geophysicist at the U.S. Geological Survey – Albuquerque Seismological Laboratory, who was not involved in this study. However, a previous study by Ringer et al. (2020) showed that these ferromagnetic materials make the TA seismometers more susceptible to low-frequency magnetic signals produced by auroras, says Anthony, who was also part of the Ringer et al study.
Only a few seismic stations, mostly permanent stations in Alaska, have magnetic shielding that can reduce the magnetic field variations caused by the auroras, Tape says. It costs thousands of dollars to install magnetic shielding for one seismic station and these magnetometers have to be installed alongside the seismometers to reduce the magnetic field variation signals (Ringer et al., 2020). Additionally, strong magnetic storms can still affect the seismometers with magnetic shielding, Tape notes.
Inspired by the discovery from the Ringer study, Tape and his colleagues investigated whether these unwanted signals picked up by TA seismometers could be used as improvised magnetometers to study the auroras.
Observations of the Aurora Borealis
Tape and his colleagues looked at three separate aurora events, one during March 2017 and two in February 2019. The team used two all-sky cameras located at the Poker Flat Research Range near Fairbanks to capture time-lapse, fisheye photographs of auroras in the sky. The team also used a keogram plot to show the appearances of aurora across different latitudes at different times. This helps researchers study the timing and location of auroras in a single plot.
During analysis of a March 2, 2017, aurora, Tape and his colleagues found a greater magnetic field variation recorded by magnetometers at locations with high auroral activity as observed in photographs and the keogram. This behavior was recorded in seismometers across different latitudes. Curiously, the researchers found that the magnetic variation detected in seismometers was stronger with green auroras as opposed to blue and red auroras in one of the February 2019 events. The reason for this observation was not the objective of this study and is unknown.
Increasing multisensor stations in Alaska
There are currently 13 magnetometers in Alaska, Anthony notes. But this study, inadvertently, showed that “we have an array of at least 200 seismometers acting as magnetometers,” he says. Nevertheless, the magnetic field variation signals can still be masked by other signals, such as earthquakes, Tape and his colleagues wrote, so it would be nice to have more actual magnetometers.
Having all-sky cameras and magnetometers at seismic stations provides an opportunity to study space weather, Tape and his team noted, and can help scientists distinguish the source of low-frequency noise.
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Ringer, A. T., R. E. Anthony, D. C. Wilson, A. C. Claycomb, and J. Spritzer (2020). Magnetic Field Variations in Alaska: Recording Space Weather Events on Seismic Stations in Alaska, Bull. Seismol. Soc. Am. XX,1–11, doi: 10.1785/ 0120200019
Tape, C., A. T. Ringler, and D. L. Hampton (2020). Recording the Aurora at Seismometers across Alaska, Seismol. Res. Lett. XX, 1–15, doi: 10.1785/0220200161
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