The frequencies which give rise to whistlers lie in the VLF frequency range 50 Hz to 20 kHz and can easily be observed in the opposite hemisphere with a simple antenna and audio amplifier. A whistler on its path may travel out to tens of thousands of kilometres from the earth. The energy starts as a sharp pulse lasting a few milliseconds which is stretched out in the magnetosphere because higher frequencies travel faster than lower frequencies. A spectrogram of such a whistler is shown in Figure 1. It is also found that part of the energy can be reflected from the lower edge of the ionosphere, giving rise to whistler echoes which are heard in both hemispheres.
Whistlers follow field-aligned ducts of enhanced ionisation through the magnetosphere.
This figure shows 3 of many possible paths. The ionosphere is indicated by the dashed circle. Whistlers have many interesting properties, one of which is that they can resonate and exchange energy with high energy charged particles in the magnetosphere. In the process the particle pitch angles are modified and some of them may precipitate into the atmosphere. These particles are normally trapped in the Van Allen belts around the earth and increase in number during magnetic storms.