Sounds of the Sun

The acoustic waves detected by BiSON are analogous to sound waves on the Earth.  Imagine the oscillating surface of the Sun behaving like a sound source.  For instance, a loudspeaker produces sound by generating compression waves in the air due to the movement of the cone.  These pressure waves are detected by the ear and transmitted as electrical impulses to the brain.  Similarly, the BiSON spectrometers are sensitive to spectral lines in sunlight.  By observing the Doppler shift of these spectral lines we are able to reconstruct the movement of the solar surface and calculate the frequencies present using discrete Fourier methods.  We can think of this as listening to the sound of the Sun.  As with many sound sources, these are not pure tones and we need to extract the notes which are of interest to us.

The dominant modes (or 'notes') observed have frequencies of around 3 mHz corresponding to a period of approximately 5 minutes.  If you consider a speaker that takes 5 minutes to move the cone just one in-out cycle then it is clear that this sound is inaudible.  What we have done here is take some of our data and 'speed them up' some 100,000 times, which converts a 3 mHz signal into a 300 Hz signal.  The following table gives the frequencies of notes on the musical scale, showing that a frequency of 300 Hz corresponds to a note slightly above D.

The modes chosen are the most prominent in our data.

• Raw noise.  This is a raw data file containing all frequencies observed by BiSON.

• l=0, n=20 note.  A band pass filter has been used to isolate the specific mode, with a frequency of 2.899 mHz in the BiSON data increased to 289.9 Hz in the audible range.

• l=1, n=21 note.  The frequency of this mode is 3.098 mHz in our data, converted to 309.8 Hz.

• l=0, n=20, 21 chord.  In this file a filter has been used to pick out two modes, at 2.899 mHz and 3.034 mHz, which can be seen as a double spike in the power spectrum.

• l=1, n=20, 21 chord.  Again two modes have been isolated, at 2.963 mHz and 3.098 mHz.

• Extended raw file.  This mp3 file contains all frequencies observed over a 4 year period, corresponding to 18 minutes of audio.

• Wide-band-pass filter.  This wav file is the first minute of the extended raw file after it has been band-pass filtered to remove frequencies outside the range of detection for solar oscillations. The filter has a much wider response than the filters used to single out individual modes, so all modes are present with less noise.