(modified 2.7.2012/ page opened  9.10.2001 / ukl)


*August 21 2007 *

Study of Aurora Related Sound and Electric Field Effects [pdf]

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.

*June 8 2004 *


Read the latest publication [5]: BNAM2004, Mariehamn, land  (pdf-file, 1.2 Mb).

-> The paper describes the very first, preliminary attempt to record and analyze aurora related sounds related to the April 6-7 2000 event.

-> More details of this pilot material will be published later. The work with high quality acoustical measurements made during other active aurorae continues.



*Nov 12 2001 * (News published only on this page)


Finally, the mystery of auroral sounds is gradually opening.

Two audio recordings: 6.-7.4.2000 and 11.-12.4.2001, contain sound material which can be related to (correlated with) other physical measurements of those geomagnetic storms. There are also other recordings made during active aurorae (2000-2001) which contain similar exceptional sound material. The recorded sounds and effects, auroral sounds, as we believe,  are not present in the reference recordings made with the same instruments at the same places during nights with normal geomagnetic activity.

The recordings have been made with professional instruments of highest quality. There is no possibility that, e.g., the microphone has directly picked up effects of geomagnetic or electrical fields.  These fields have been measured simultaneously separately.  Many audio professionals in our unit have heard the recordings and agree that these are real sounds with many natural acoustic features, like echoes, sounds which are somehow created by the nature, not by any technical failure.

The recordings contain almost all auroral sound types described earlier by observers around the world.
Up to now, three persons who have earlier heard auroral crackling have listened to the recordings and recognized the sounds similar to those they heard.

More details will be published later!