AURORAL ACOUSTICS - AURORA RELATED SOUNDS -
NEWS
(modified Aug 23 2007/ page opened Oct 9 2001 /
ukl)
THE LATEST NEWS OF THE PROJECT WILL APPEAR ON THIS PAGE
*August 21 2007 *
This excellent Master's Thesis
by Janne Hautsalo describes
statistical analysis of sound recordings made at Koli, Finland April
11-12 2001 during a strong geomagnetic
storm.
The main results of the thesis
- The analysis shows statistically
significant correlation between the sound power fluctuation and the geomagnetic
activity.
- The cross-correlations were performed at one-third octave bands with
different delay values.
- The delays between magnetic field activity and sound power
fluctuation at the highest correlation values speak for a sound source
relatively close to the measuring place (ground level). Thus the
possible audio range sound source (infrasounds excluded) is not located at the aurora light source,
80-100 km above the ground level, but relatively close to the ground.
- The same audio data
revealed a clear correlation peak at the upper infrasound range
(<20Hz) with a delay corresponding to the sound wave traveling-delay
from the level of the aurora light source to the ground (this outcome
confirms a known result published earlier).
- The possible physical mechanisms behind these sound effects are not yet known.
- The simultaneously measured local electric field signals (a VLF
antenna 20m from the microphone) indicated very low correlation with the
measured sound signals (at zero delay) which means that the local electric field cannot be the
cause of the acoustic signals detected.
- The acoustic measurements were done with a highly sensitive, low
noise, measuring microphone (B&K) which is carefully shielded
against ambient electric and magnetic field disturbances.
The thesis was selected among the five best
Master's Theses of
the year 2005 at TKK (Helsinki University of Technology,
Finland).
So far we know, this is the
first research
result ever published showing statistical connection between aurora
related sounds and simultaneous geomagnetic activity.
This outcome is based on 100 aurora night recordings made during
2000-2005 (the most active part of the cycle 23) at different locations
in Finland. The collected data archive is close to 0.5 Tb.
Up to now only one night
(April 11-12 2001) has shown clear correlations between the acoustic
signals and the geomagnetic activity.
This may speak for an event, which occurs very seldom and therefore is
difficult to study.
Depending on the available resources
- the study continues.
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