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VOLCANIC BULLS-EYE--GRAVITY WAVES ASSOCIATED WITH CALBUCO When Chile's Calbuco volcano erupted on April 22nd, plumes of ash and volcanic gas shot more than 50,000 ft above Earth's surface. Orbiting overhead in the darkness of space, the NOAA/NASA Suomi NPP satellite observed the ripple effect of the blast. Night had fallen over the volcano during the early hours of April 23rd when a low-light camera on the satellite photographed a "bulls-eye" pattern of waves centered on the rising plume.  Ripples like these have been observed before, high above powerful thunderstorms. They are called "gravity waves", essentially, waves of pressure and temperature excited by the upward motion of air. (Gravity does not vary inside the waves; the waves get their name from the fact that gravity acts as a vertical restoring force that tries to restore equilibrium to up-and-down moving air.) The waves are visible because they glow. The phenomenon is called "airglow." Airglow is caused by an assortment of chemical reactions in the upper atmosphere driven mainly by solar ultraviolet radiation. Gravity waves rippling away from the central axis of a thunderstorm or, in this case, a volcano, cause temperature and density perturbations in the upper atmosphere. Those perturbations alter the chemical reaction rates of airglow, leading to more-bright or less-bright bands depending on whether the rates are boosted or diminished, respectively. Airglow occurs about 100 km above Earth's surface alongside meteors, noctilucent clouds and even some auroras. This makes airglow, and the bulls eye above Calbuco, a true space weather phenomenon. ] |
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