Atmospheric turbulence generated by mountain ranges that interrupt the tropospheric wind flow can produce aerodynamic infrasound that propagates thousands of kilometers from the source regions. These mountain associated infrasonic waves (MAW) have been observed for many years by infrasonic arrays operated by the University of Alaska in both Antarctica and interior Alaska. Now that the CTBTO/IMS infrasonic arrays I53US at Fairbanks and I55US at Windless Bight, Antarctica have been installed, we have accumulated a very large data set of MAW events at both stations from 2002 through 2005.

The detection algorithm that we use in searching for coherent MAW infrasonic waves that propagate across the sensor array is based upon the mean of each of the maxima of all the inter-microphone cross-correlations. That is, the normalized cross-correlation function is computed for each microphone pair and its maximum is identified. Next the mean of all of these maxima (mean (maximum (Cij)) is then defined to be the output value of the detection algorithm (MCCM) for that data window. For search analysis the data are segmented into small windows a few minutes long and the detection algorithm is applied to each window resulting in a series of detection values of the following parameters: (1) The F-Stat which is a measure of the signal to noise ratio for coherent signals in the analysis window (2) MCCM = the mean(maximum(Cij)); (3)  V  =   the trace-velocity in Km/sec; and (4) A =  the azimuth of arrival in degrees  for each data window in the time series of data. This set of four values of: F-Stat, MCCM, Vel and Az is then the final output of the MCCM algorithm.

For the MAW signal search we scan an entire 24 hour record with a sliding window that is 10000 points or 500 seconds in length.  We have found that the choice of the length of the data window is not of highest importance as long as it wide enough to contain a few cycles of the frequency of interest. MAW are long period waves therefore all the I53US and I55US data were first passband filtered from 0.015 to 0.10 Hz before application of the detection algorithm for signal search.

Thus far we have found many examples of MAW events at I53US that are characterized by (1) long period waves  in the range from 70 to 20 second periods, (2) long durations of quasi-sinusoidal wavetrains, of a few tenth of a Pascal amplitude,  lasting up to several days, (3)  a total lack of diurnal variation of frequency of occurrence, (4) a strong tendency to be observed only during winter months, and (5)  a  fixed azimuth of arrival from definite direction bands. The three principal direction bands for MAW at Fairbanks are: Band(1)  1100  to 1500 azimuth (the Saint Elias Range); Band(2)  1700   to 2300  (Alaska and Aleutian Ranges); and  Band(3)  2750   to 3000 (Seward and Chukotsk peninsulas ). Although there are mountain ranges located at virtually every direction, as seen from Fairbanks, it is principally within the three bands listed above that most of the MAWs are observed at I53US. Three examples for I53US have been chosen as typical of the MAW events that are seen in each of the three principal directions bands. One example of a typical MAW event for each of the three MAW azimuth bands that are observed at I53US in Fairbanks is shown below.

Examples of MAW events at I53US in Fairbanks and I55US in Antarctica

On January 2 , 2004, at about 11:00 UT , an MAW event began from an  azimuth of   200 degrees from  the Alaska Range that persisted in the I53US infrasound record for three days ending on Jan. 4th around 11:00 UT. The beginning of this MAW event can be seen clearly in Figure 1 in the azimuth plot in bottom panel where at 11 UT the detector locks on to the persistent coherent signal from the 200 degree direction of the Alaska Range as seen from Fairbanks.

At this same time the MCCM value rises to a higher average value of 0.88 while the trace-velocity becomes steady at an average value of 0.435 km/sec for the rest of the day. For the 45.5 hour period of this MAW event the detector output data were concatenated in order to obtain the average values over the entire event as follows: Azimuth = 173.8. +/ - 29.3 deg, Trace-Velocity = 0.531 +/ - 0.199 km/sec, and MCCM = 0.873 +/ - 0.058. The rather large STD value of 29.3 deg for the azimuth can be seen in Figure 2 to be associated with the eastward drift in the MAW source as a function of time from 200 deg at the beginning to 150 deg at the end of the event.

The location of the atmospheric turbulence region, due to the tropospheric winds and the extensive Alaska Range mountains, is itself drifting eastward as the storm progresses. The high waveform coherence across the I53US microphone array for this January MAW event is shown in Figure 3 as a histogram of the MCCM values over a 45.5 hour period beginning at 11:00 UT on Jan 2nd.

The mean of MCCM is 0.8733 with an STD of 0.058. A sample of the MAW waveform from Jan 3rd from 03:05 to 03:15 UT is shown from the Datascan plot window in Figure 4. For this particular segment of the continuous MAW wavetrain the azimuth = 181.4 deg, the trace-velocity = 416.6 m/sec, the MCCM = 0.933, and sigma tau = 0.16 sec. Sigma Tau is a measure of the planarity of the wave fronts. The 10 minute intervals of the waveforms, shown for each of the 8 sensors I53H1 to I53H8, are practically identical even though the waveform is quite irregular throughout its length. This is an important diagnostic characteristic for identifying MAW signals. In the bottom panel of Figure 4 all 8 traces are shown in a phase-aligned overlay. The high coherence from sensor to sensor of the MAW is evident from the plot in this bottom panel.

Two additional examples are shown in Figure 5 of MAW events at I53US from year 2004. In the top panel of this figure, for the event of Jan 21, the azimuth plot shows that the MAW signals are detected, from an azimuth of 300 degrees, and continue from the beginning of the data file at 00:00 UT until about 15 hours when the scatter in azimuth indicates that coherent MAW are no longer observed at I53US on that day. The end of the MAW signal train on Jan 21st may result from either a change in propagation conditions or a weakening in the source. The MAW signals on Jan. 21st are from the Seward and Chukotsk peninsulas band of azimuths from 275 to 300 deg.

In the bottom panel of figure 5 the azimuth plot for I53US on Jan 18th shows MAW from two different direction bands. The first band from 00:00 to 15:00 UT is from an azimuth around 150 degrees and is associated with the Saint Elias Range along the southwest coast of Alaska. Then from 17:00 to 24:00 UT there is an interval of coherent MAW signals is again detected at I53US from around 300 deg. The MCCM detector used at both I53US and I55US only displays the one signal with the highest coherence value in each successive analysis window. Thus on Jan. 18th there may have been Maw from both the 300 and the 150 degree bands occurring simultaneously. PMCC analysis on the other hand would allow the simultaneous detection of two MAW signal trains from different directions that may have been occurring on Jan 18th at I53US.

An example is shown in Figure 6 of the azimuth versus time plot of an MAW event in Antarctica at I55US for Feb. 6, 2004. The azimuth of arrival of the MAW signals at Windless Bight is about 350 degrees from a source in the Victoria Land mountains that is probably Mt. Supernal, a 11,991 foot high peak that stands alone above the Evans Neve glacier.