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unpredictable arrivals, high speeds, and sometimes- unknown compositions, meteors are difficult to model. Substantial improvements have been recently made (e.g., References 13 and 14) by using a combination of multiple monitoring technologies, more realistic atmospheric speci- fications, and hydrodynamic source models.
Volcanoes
Eruptions are driven by the excess pressure of volcanic
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fluids .Thesefluidsmayconsistofacombinationofmagma,
water, gas, rock particles, and in some cases, mud. When the fluids breach the surface of a volcano, the pressure release
azimuth of arrival and trace velocity of the acoustic waves associated with the earthquake indicated that the source of the waves moved eastward along the Denali Fault. These observations were in agreement with the description by local seismologists of a propagating rupture along the fault. As the rupture moved along the Denali Fault it produced large, local ground motions and these motions produced infrasound waves that were detected at the Fairbanks, Alaska array I53US. Despite the large displacements, the infrasonic sources were not attributed to the horizontal motions of the ground, but rather to the motion of the mountains in the Alaska Range associated with the Denali Fault. Submarine earthquakes may also pro-
duce infrasound through the displacement of the ocean surface. This subject will be dis- cussed in more detail in the Tsunami section.
Auroral infrasound
Researchers at the Geophysical Institute, University of Alaska have identified two forms of infrasound associated with visible auroras. Early work, dating to the 1960s, found large “bow waves” associated with the passage of
10 auroral curtains in the overhead ionosphere .
The ground footprint of these waves produces an impulse in the infrasound record as they sweep across the array. Recently, a new form of auroral infrasound has been discovered at the Geophysical Institute. These infrasound sig- nals are associated with pulsating auroral forms that are common after magnetospheric substorms. The pulsating auroral forms appear stationary in the sky with intensity fluctua- tions that are quasi-periodic in the period range from 10 – 30 seconds. When adjusted for propagation delays from the ionosphere to the ground, the infrasound signals show good correlation with the fluctuations in auroral intensity. Presumably, the precipitating auroral electrons that produce the optical auroras also deposit enough heat to produce detectable pressure variations. However, the details of the energy transfer have yet to be delineated.
Meteors
Sensitive infrasound sensors11 can rou- tinely detect the rumble of hypersonic objects tearing through the atmosphere. A meteor entry may generate sound either as a shock wave radiating from a near-cylindrical Mach cone, or from the explosion of the meteor. Approximately ten thousand small meteors hit the Earth’s atmosphere each year with suf- ficient energy to generate an acoustic signal, while several bolides per year have explosive energies that are comparable to a thousand tons of dynamite. Such kiloton bolide bursts occur approximately once per year and can
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cause significant damage . Because of their

Infrasound 11