Similar to how a mountain in sunlight casts a shadow, large obstacles can create dark areas for radio waves. Unlike visible light, however, scientists believe that radio waves could travel along the ground to the top of a mountain and into the “shadowed” zone, thanks to a natural phenomenon known as surface plasmon. . Since surface plasmons are related to subsurface stress, this could be used to help monitor seismic activity.
Surface plasmons, also called surface plasma waves, are products of electromagnetic waves traveling across the Earth’s surface. If mobile positive electrical charges (similar to electrons, but positively charged instead of negatively) could travel, they could interact with radio signals and move toward nearby mountain peaks, oscillating in time with the radio waves. This collective oscillation is the surface plasmon, moving like a breeze over a lake.
Because surface plasmons need energy to oscillate, they can only be induced when the plasma frequencies of the surface electric charges are higher than that of the passing radio wave: the greater the difference between the plasma frequency and the frequency of the radio wave is greater, the greater the surface plasmon is.
The positive charges necessary for a surface plasmon are released when underground rocks are stressed. This seismic stress forces positive charge carriers to the surface, creating a plasma layer in the highest parts of the landscape. Because strong seismic activity corresponds to more tectonic stress and therefore more charge carriers (and therefore higher plasma frequency), there cannot be a surface plasmon without an earthquake. If there are enough of these positive charges on the surface of the ground, they could absorb and re-radiate radio wave energy, causing them to scatter randomly from the rugged terrain of a mountain.
In a new study, Fujii shows that when a surface plasmon is made up of a particularly high density of positive charges, it can propagate over a mountaintop and down the other side, radiating electromagnetic waves back into the shaded region of the mountain. Using both an ideal conical structure and a model of Japan’s Tsurugidake peak, the team’s supercomputer shows that surface plasmons scattered over surface bumps and mountain tops that randomly retransmit radio waves incoming into a tightly converging beam-like wave, concentrated in a small region.
The retransmitted waves can reach areas that would have been inaccessible to the original signal. By monitoring these anomalous radio waves, the researchers expect scientists to be able to monitor seismic activity over larger areas. (Radiosciencedoi:10.1002/2016RS006068, 2016)
—Leah Crane, freelance writer
Crane, L. (2016), Earthquakes Could Channel Radio Waves To Dark Mountain Areas, Eos, 97, https://doi.org/10.1029/2016EO059987. Published September 29, 2016.
Text © 2016. The authors. CC BY-NC-ND 3.0
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