IGY Bulletin, Number 6, December 1957 - Whistlers and related phenomena

According to the start of this article in the IGY Bulletin, "whistlers are naturally occurring audio frequency radio waves which, received on an antenna, amplified, and fed to a transducer for conversion into sound, can be heard as long descending whistles."

What do these sound like? Check out this YouTube video of a recording by amateur Mark Dennison in 2010:

This image of the spectrum of a whistler

shows the descending frequency (High pitch to low) of a 3-second whistler signal from over 2.5 kilohertz to about 500 hertz. This falls within the audio spectrum of human hearing from 20 hertz to 20 kilohertz. (Other animals can hear much higher frequencies than humans). 

A figure in the article shows a frequency-vs.-time graph for a whistler that looks the same as the above.

Figure 3 from the Bulletin article, for a whistler bouncing between Alaska and New Zealand. Dispersion D increases with time.

I had my first audiogram two years ago, and although I have the typical moderate falloff for older folks at higher frequencies, I fit into the range above:

My left ear

The Bulletin article points out that the significance of whistlers is that they can provide information about the very high atmosphere, above the altitudes accessible by rocket soundings.

As mentioned in an earlier post, whistlers are generated by the propagation of sferics which then exhibit the property of dispersion whereby signal velocity depends on the frequency. Sferics are transient radio waves arising from naturally occurring electric discharges in the atmosphere. The most intense sferics are produced by lightning.

The figure below from the article indicates how whistler will penetrate the ionosphere, then bounce back and forth between places like Annapolis and Cape Horn that are at the same geomagnetic latitudes in the northern and southern hemispheres, following flux lines (like the dashed one) of the geomagnetic dipole field. Data collected earlier in the year verified such whistler paths that had previously been theoretically calculated.

The article further lists stations set up for whistle observations for the US-IGY program, and gives some of the preliminary findings as of this December article.

I guess if you were an ionospheric scientist working on whistlers, this would be your theme song:

Interesting to read about Adriana Caselotti, the uncredited voice of Snow White.