Effects of the weather, underground

Subatomic particles detected in a deep mine yield stratospheric insights

If you think the effects of weather can’t be perceived far below Earth’s surface, think again. Sudden changes in air temperature in the stratosphere that can ultimately steer major storms can also influence the number of subatomic particles slamming into detectors located hundreds of meters below ground, a new study reveals.

FAR DOWN EARTH AND SKY. Particles that bombard this underground detector (one end seen in the background) provide hints about meteorological conditions high overhead in Earth’s stratosphere. IMAGE: Fermilab

As strong weather systems sweep across irregularities on Earth’s surface — a mountain range, say, or the sharp edge where a continent rises from the sea — airflow can be deflected upward, sending waves of energy into the stratosphere, at altitudes above where jets normally cruise. Once every couple of years, such perturbations are large enough to cause what scientists call a “sudden stratospheric warming,” in which the energy pumped into the stratosphere can heat a continent-sized region of high-altitude air by as much as 50 degrees Celsius in just a few days, says Scott Osprey, an atmospheric scientist at the University of Oxford in England.

These abrupt changes in high-altitude temperature, which normally occur in the Northern Hemisphere during winter months, can take weeks to dissipate, Osprey notes. In the meantime, the changes in stratospheric winds can influence weather at ground level, typically shunting storm systems farther south.

Turns out that these sudden stratospheric warmings, readily seen by ground-based meteorologists and satellite-based sensors, can also be discerned by particle physicists collecting data deep underground, Osprey and his colleagues report online March 7 in Geophysical Research Letters. In their study, the researchers analyzed the numbers of muons passing through a 5,400-metric-ton particle detector in an abandoned iron mine in northeastern Minnesota during autumn and winter months between October 2003 and March 2007.

Muons, negatively charged particles about 200 times the mass of an electron, can be created in a variety of ways, but most of the ones detected in this study originated in the upper atmosphere, says Giles Barr, a particle physicist at Oxford and a coauthor of the new study. “Ironically, the detector was built deep underground to get away from the muons” that bathe Earth’s surface, thereby making muons generated within the Earth easier to detect, he notes.

On average, the subterranean detector is struck by a muon about once every two seconds, Barr says. In summer, however, the frequency is about 4 percent higher than it is in winter, because muons — a by-product of reactions triggered when cosmic rays strike the upper atmosphere — are produced more often in warm, relatively thin air typical of summer months than in the cold, dense air of winter. When the air is warmer, the atmosphere expands upward and gas molecules move farther apart. In this lower density air, molecules energized by cosmic rays are more likely to decay and produce muons than to lose their energy as heat by colliding with other molecules.

At the height of a sudden stratospheric warming, however, temperatures at the altitudes where most muons are created approach those common in summer — which, in turn, causes the air to expand, become less dense and voilà! Muon counts surge at the underground detector.

“It’s remarkable that you can go deep within the Earth and see what’s going on high in the stratosphere,” says Lorenzo Polvani, an atmospheric scientist at Columbia University. By reviewing data gathered at subterranean particle detectors elsewhere in the world in the past five decades, scientists could find evidence of previously undetected sudden stratospheric warmings — as well as insights into whether this infrequent phenomenon has become more or less common as climate has changed.

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