A climatology of infrasound observations at the ARCI array in Norway

Infrasound was first discovered after the violent eruption of the Krakatoa, Indonesia, in 1883. Low frequency pressure waves were observed at traditional barographs. These appeared to have traveled with the sound speed and up to four passages where noticed at some instruments (Symons, 1888). The first microbarometer recordings date from 1908 when a meteoroid, or asteroid, exploded over Siberia in Russia, the so-called Tunguska event. The societal and scientific interest in infrasound increased during World War I, (e.g., Whipple, 1939), and later in the nuclear testing era (Posey & Pierce, 1971). With the signature of the Limited Test Ban Treaty in 1963, most interest in infrasound promptly came to a stop, since nuclear tests were confined to the underground. Only a few studies could be maintained (Balachandran et al., 1977; Liszka, 1978). In recent years, the study of infrasound gained renewed interest with the signature of the CTBT in 1996, where it is used a verification technique for atmospheric tests (Dahlman et al., 2009).

Sources of infrasound are in general large, since an enormous amount of air has to be displaced to generate such low frequencies (Gossard & Hooke, 1975). Natural sources are: avalanches, lightning, meteors, oceanic waves, severe weather, volcanoes, sprites and earthquakes. Among anthropogenic sources are: explosions, supersonic flights, military activity, rocket launches and nuclear tests. Identifying the sources of infrasound out of this zoo of coherent waves in the atmosphere, is one of the major challenges in infrasound research.

The propagation of infrasound through the highly dynamic atmosphere plays an important role in source identification. Infrasound travels up to thermospheric altitudes of 120 km and experiences refractions due to an increase in wind and/or temperature as a function of altitude. If the gradients in the propagation velocity are strong enough, infrasound will be bended back to the earth\'s surface (Drob et al., 2003). There are three regions in the atmosphere where such gradients might exist. These are of importance in long range sound propagation, i.e., over distances larger than 150 km. The regions are marked by (1) a strong jet stream at 10 km altitude, near the tropopause, (2) the combined effect of wind and temperature at the stratopause, around 50 km altitude and (3) the temperature increase in the thermosphere from 100 km and upwards.

The aim of this study is to identify the sources around the ARCES infrasound and to build up a climatology of station specific detections. Each infrasound array has its own detection capabilities as the atmospheric conditions and source characteristics are highly variable as function of geographical location and time.