Joachim Köppen Kiel 2018

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Whistlers are the most spectacular of the natural radio emissions that are produced by lightning. Under certain conditions in the ionosphere the radio pulse from the lightning can partially penetrate and cross the ionosphere to enter the Earth magnetosphere. In this low density plasma the radio waves can only propagate by following the field lines of the Earth magnetic field. Because these emerge in one hemisphere and curve out up to distances of 3 Earth radii, the waves traverse a long path through space and come down only in the other hemisphere. Here they may cross the ionosphere and can be received again. As they had to travel over a long path through the plasma, they are heavily subject to dispersion: the lowest frequencies arrive a few seconds after the highest frequencies. The initial 'click' is drawn out into a majestically sweeping down in frequency, which sounds like 'piouuu' or a more hissy 'shpiouu': a whistler.

Note that because the radio waves cannot but follow the magnetic field lines, whistler waves always travel north/south. Hence thunderstorms in Asia never make whistlers in Europe.

When the whistler wave comes down to the ionosphere, only part of its energy can cross the ionosphere and become audible as a one hop whistler. The other part is reflected back to the magnetosphere and travels back to the other hemisphere. There again, one part penetrates the ionosphere and is audible as a two hop whistler in the hemisphere from whence it came, but the other part is reflected ... and so on ... Under certain conditions a whistler wave can go back and forth several or many times between the hemispheres. This is called an Echo Train.

To hear whistlers depends on several factors: first of all at the other end of the magnetic field line that goes into space from the receiver's site there should be a thunderstorm to furnish the lightnings. At both ends of the magnetic field line the ionosphere must be in a condition to let the whistler wave pass through. Finally the conditions along the magnetic field line must be favourable for the wave to propagate.

All these conditions are not met all the time. Thus, one needs a bit of patience to hear whistlers. The best way is to listen as much as possible, and in a regular way. When I am on the beach, I try to listen in once per hour, for about a minute. If I hear nothing but spherics, I switch off again, but if I hear a swishing sound, I start recording. You never know what might happen next!

Three Whistlers from Two Hemispheres

On 24 aug 2016 at CEST 20:13 the recording shows three whistlers: The first is fairly strong and occurs 2.5 sec after a strong spheric (marked by an arrow) which must be the source lightning. This means that it is a two-hop whistler starting somewhere in Europe. After another 2.5 sec a fainter and longer whistler follows, obviously the 4-hop echo. Then, at 14 sec, a short whistler arrives. It is composed of two pure-tone whistlers.

listen to the sound file

For the analysis it is more convenient to use a logarithmic frequency scale, as the whistlers will follow nearly straight lines:

The numbers at the marks are the times in seconds after the start of the recording. They show:

There are several national and international networks of receiving stations that listen for spherics. Using the time delays measured by several stations, one can triangulate the location of the lightning that caused a spheric. Fortunately, the World Wide Lightning Location Network was so kind to provide me with the data of the strong lightnings they had picked up during the 60 seconds of my observations. On a map of these lightning locations one sees strong activity in Europe, but also two lightning strikes in southern Africa. This accounts well for the recorded three whistlers!

The "Attack of the Martians" Echo Train

It seems that August 24th is my lucky day: in 2006 many whistlers were heard and recorded on that day. The most spectacular series was at CEST 18:21, it sounded as if the Earth was being invaded by the Martians:

listen to the sound file
Here is a zoom of the groups with the loudest whistler:

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last update: Apr 2018 J.Köppen