I read with great interest “Mount St. Helens: A 30-year legacy of volcanism,” by J. W. Vallance et al., the feature in the 11 May 2010 issue of Eos (91(19), 169–170). This well-written article is about the volcano itself and ash and aircraft incidents but not about far-field effects such as traveling ionospheric disturbances (disturbances in the ionosphere that act like traveling waves) that were caused by the explosion.
I have a personal interest in the 1980 Mount St. Helens volcanic eruption. At that time I was working as a physicist at the U.S. Air Force Geophysics Laboratory (AFGL) at Hanscom Air Force Base, in Massachusetts. AFGL had a magnetometer array located across the United States, from which I and other scientists at Hanscom observed perturbations in the Earth's magnetic field, which we related directly to the explosion. These perturbations were observed at stations as far away from Mount St. Helens as Mount Clemens, Mich., 3122 kilometers distant. We were able to measure the velocity of the resulting disturbance at about 316 meters per second, in reasonable agreement with velocities of the pressure wave (from 305 to 309 meters per second) that was observed on microbarometers by the Japan Meteorological Agency.