AURORAL ACOUSTICS PROJECT

Short History of Phase I: 2000–2005 and Phase II: 2011–

 

(Modified: 27.2.2020/ Page opened: 9.10.2001 / ukl)

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Introduction

 

When the Auroral Acoustics project began in the spring of 2000, there was not a single sound recording in the world that could be attributed to the sounds accompanying Northern Lights. Speculations of the auroral sounds and mechanisms of their birth were solely related to the observations made by the human ears. In Finland, too, there was a claim that these sounds are just folklore and physical impossibility, i.e., they cannot even occur in theory. This attitude led to a situation where those who had observed these sounds did not want to talk openly about their experiences. The aurora related sounds had become a kind of taboo.

 

However, similar sound observations were made at different times and around the world under active aurorae. We assumed that this must be due to the fact that the sounds have an objective basis, they exist in physical reality and are not just imagination or fiction.

 

The Auroral Acoustics project started as a free co-operation between the Laboratory of Acoustics and Audio signal Processing at Helsinki University of Technology (HUT) and Sodankylä Geophysical Observatory (SGO).  Since 2010 Helsinki University of Technology has been a part of Aalto University.  Researchers from Finnish Meteorological Institute (FMI) have also co-operated with the project. 

 

Main steps during 2000-2005 (Phase I): 

·    Spring 2000 the design and test of portable measuring system.

·    Spring 2000 collection of Finnish observations of aurora related sounds started (Oct. 2003 about 300 reports collected).

·    April 6.-7. 2000 the first recording of aurora related sounds with a non-professional system.

·    Summer 2000 the first measurements by using low noise and highly sensitive measuring microphone mounted in the focus of a parabolic reflector.

·    Fall 2000 the first testing with vertical VLF antenna that was later included in the measuring system.

·    Winter 2000-2001 many measurements during aurora nights.

·    April 11.-12. 2001 successful measurements at Koli (eastern part of Finland, see our old news dated Nov 12, 2001).

·    August 2002 first international publication of measurements of aurora related sounds in Finland (URSI2002: publications).

·    Winter 2002-2003 collection and analysis of new measurement data.

·    Fall 2003 analysis of data collected during the April 6-7, 2000 geomagnetic storm.

·    Spring 2004 some more auroral nights measurements - preparing a new international publication (BNAM2004: publications).

·    Summer 2004 detailed analysis of the aurora night data continues.

·    July 2005 MSc thesis by Janne Hautsalo - statistically significant correlation found between the sound energies and the geomagnetic activity. This important work confirmed for the first time the fact that the measured sounds cannot be any random ambient sounds, because they are statistically strongly related to geomagnetic fluctuations and Northern Lights. This MSc thesis was selected as the best MSc thesis of the department in 2005. The work was supported by Helsinki University of Technology Foundation.

 

During the Phase I of the Auroral Acoustics project hundreds of GB data (audio and VLF) were collected. However, the methodology was very limited, e.g. it didn’t allow the localization of the sound source. Further, all our applications were rejected. The project was left without support. The lack of resources eventually led to the freezing of the entire project over five years.

 

Main steps during 2011-2020 (Phase II, still active): 

·    Fall/Summer 2011 Zoom H4n based four channel recording system was tested with two new microphones.

·    Summer 2011 New VLF loop antenna was designed and constructed.

·    September 2011 a test recording revealed the first time that the sound source must be on the open, clear sky about 70 m above the ground (ICSV19, 2012: publications). This was a surprising result, because at that time, no physical mechanism was known to explain this. How the clapping sounds can be created on a clear sky at that altitude?

·    December 2012 Zoom H4n was found to be of poor quality for this study and it was replaced with Roland R44 recorder.

·    March 17–18, 2013 GM-storm produced hundreds of high-quality aurora related sounds and associated magnetic field pulses preceding them.

·    February 2015 the project received a grant from the Finnish Cultural Foundation.

·    March 17–18, 2015 GM-storm produced again hundreds of high-quality aurora related sounds with associated magnetic field pulses preceding them.

·    July 2016 the new Temperature Inversion Layer Hypothesis was published in Stockholm (BNAM2016: publications). High rank correlation values were found between the RMS values of the VLF pulses and the sound events.

·    July 2017 new results related to the estimation of the average sound source altitude (inversion layer), that were based on the rank correlation method, were published in London (ICSV24: publications).

·    May 2018 New observations e.g. related to spectral similarities between single clap sounds and cracklings were published in AES 144th Convention in Milan, Italy (AES-144: publications).

·    During 2017–2018 it was discovered that the spectra of the temporal envelopes of the auroral crackling sounds contain Schumann resonance frequencies and their intermodulation distortion (IMD) components. These observations were published July 2019 in Montreal, Canada (ICSV26: publications).

·    During 2019-2020 new software was developed for faster and more accurate sound source localization.

 Go to: The MAIN PROJECT PAGE.