40 KHz used for the detection of Solar Flares

Current VLF Solar Chart

Firstly may I say the Sun is Very dangerous NEVER, NEVER LOOK AT THE SUN you will not see flares and blindness and permanent eye damage can result.

Solar flares are sudden and intense variations in brightness in the solar atmosphere. Flares occur when the magnetic energy that has built up in the solar atmosphere suddenly releases. Radiation across most of the electromagnetic spectrum is thus emitted into space. This radiation covers from radio waves to optical, X-ray and Gamma waves. Flares can extent out to the Suns outer layer called the Corona which is the outer most level of the Suns atmosphere.
The numbers of flares have a relationship to the solar cycle which comes in 11 year cycles the next maximum is in or about 2011.
So why study solar flares?
Well they are the biggest explosions in the solar system.
Solar flares effect space and land based electrical equipment, ranging from satellites to national power grids.
Flares also affect the Earth’s atmosphere directly.
We may even discover how to control and produce power from thermonuclear reactions like on the Sun.
So how does a VLF radio at 40 KHz detect solar flares? Well it doesn’t directly, Very Low Frequency signals tend to be absorbed by the various layers of the Ionosphere. So what are we looking at, it’s the indirect effect of the solar flare on the Earth’s ionosphere.
When a large solar flare erupts a constituent of the emitted radiation is high energy X-ray particles. These particles travel at the speed of light much as light waves travel in the form of photons. It takes the particles about 8-9 minutes to reach the Earth’s ionosphere. Now the X-ray energy becomes ionised to a high degree and changes the existing ionospheres layers ability to reflect radio signals below 100 KHz. This means terrestrial radio signals that would have been absorbed by the D layer are now reflected back towards Earth. The effect is known as a SID or Sudden Ionospheric Disturbance. So by charting the reception of a Very Low Frequency Earth based transmitter with at least one sky hop we can infer/detect major solar flare eruptions. This means we are not receiving the VLF signals from the Sun but seeing the enhancement of the signal as a result of the flare. Most flares can be recognised by the sharp increase in signal strength and the gradual drop off. This is known as a shark fin trace.