[Jolly44 (OP)]

I'm trying to find a recording of "whistlers" [https://spam.com]- [/url]audio frequency electromagnetic waves resulting from lightning disturbances travelling along the earth's magnetic lines of force. Wikipedia has a nice spectrogram but I can't locate an audio clip. Can anyone help?

[Kryptid]

Might this count?

[paul cotter]

"Magnetics lines of force" are a conceptual construct and have no real world existence. They represent contours of equal magnetic field strength similar to height contours on a map or isobars on a weather chart. How a disturbance could travel along an imaginary line, I don't know. If you tune an old style long wave am radio between stations distant(or local) thunderstorms can be monitored but it's more crackles than whistles.

[alancalverd]

I recall hearing good recordings of whistlers as an undergraduate.

brings it all back!

Paul: you won't hear these on LW AM because the signal itself is at an audio frequency. You need to tune around 1 - 20 kHz in a very "quiet" area. Fascinating stuff!

[evan_au]

Here are some more:
https://space-audio.org/sounds/EarthWhistlers/ewhist.html

How a disturbance could travel along an imaginary line, I don't know.

Electrons spiral around magnetic lines of force, producing the visible lines of the aurora.
- Iron filings line up along the field lines from a permanent magnet - the traditional high-school demonstration to visualize invisible lines of magnetic force.

Lightning impulses can generate pulses of electromagnetic energy (and even X-Rays, positrons and gamma rays) in the upper atmosphere.
- These pulses of energy start with energy at all frequencies (this sounds like a click or crackle).
- But the plasma in near-Earth space transmits carries this impulse in such a way that different frequencies travel at different speeds, producing the whistler effect.
- In a sense, this is what happens to light from the Sun when we see a rainbow - different frequencies are separated because they travel at different velocities.

[paul cotter]

Alancalverd, you have me somewhat confused as to the nature of these signals. I know a lw am receiver won't go below 150khz but surely some harmonics would be detectable unless these signals are very pure sinewaves. You suggest 1-20khz: does it sweep through this range or is it a single nominal frequency modulated by a separate audio tone? If it is a sweep one would need a wideband(relatively, compared to a radio) amp to monitor and there would be a lot of extraneous noise.

[Bored chemist]

You could listen for the harmonics but it won't really work.
If the signal runs from 20 KHz down to 1KHz then, to get it onto long wave radio (with a low frequency limit of 150KHz you have to listen to about the 150th harmonic.
That means  a signal that sweeps from 3MHz down to 150KHz over the course of a second or two.
But only the bit below about 300 KHz is long wave, so that's the only bit you will be able to receive.
Say you have your radio tuned to about 200 KHz,  It will pick up signals within about 9KHz of that frequency.
And the 150th harmonic of your whistler will sweep though that in well under a second.
All you would hear would be a click.
You would, I guess , get a series of clicks as each harmonic swept through the frequency you were tuned to.
I will leave the question of what you would get as an exercise for "the interested reader".

[paul cotter]

Evan au, yes I know electrons spiral in a magnetic field, the domestic microwave oven is an everyday example. I just feel this "lines of force" concept is outdated-personal opinion and probably not the place to raise it. Also the op suggested em radiation being entrained by "lines of force" which doesn't happen.

[evan_au]

surely some harmonics would be detectable

The dispersive nature of the plasma means that these are pure sine-waves - all the harmonics are removed, and separated in time.
Before it is dispersed, you get a wideband click, which would be audible on LW AM radio.

[alancalverd]

"Lines of force" was translated by my old physics teacher as equipotential contours, which makes a lot more sense.

[evan_au]

"Lines of force" was translated by my old physics teacher as equipotential contours

In a "bar magnet" magnetic field, the equipotentials would be a a set of nested doughnut-shaped surfaces.
- The Earth's magnetic field can be roughly approximated as a bar magnet
- If electrons orbited the equipotential surface, they would need to circle the entire planet
- This does not align with the observed linear structures in the aurora
- These lines of force seem to follow the path of a small "test compass", from one pole to the other; a path rather than a surface

This is equivalent to the situation with an electric dipole, where the lines of force follow the path of a small test charge. I expect these would cross the equipotential surfaces at right angles.

[paul cotter]

We are spammed again!

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