mentioned yesterday, low frequency radio waves travel through plasma more slowly than high frequency waves. While this has consequences in astrophysics, it also has consequences here on Earth. The upper layer of our atmosphere also contains a low density plasma, known as the ionosphere. When radio waves travel through the ionosphere, lower frequency waves travel more slowly than higher frequency ones, just like in interstellar space. When lightning strikes, it generates an electromagnetic pulse of radio waves, which travel along the Earth’s magnetic field lines through the ionosphere. As a result, the higher frequency waves from the pulse will reach you first, while lower frequencies reach you a little later. If you listen for these pulses on a special radio, you can pick them up as a short descending tone known as a whistler. You can hear several of them in the video above.
In 1979, Voyager I detected whistlers near Jupiter, and it was the first confirmation of extraterrestrial lightning. You can even hear a recording of this Jovian whistler. We have since discovered whistlers on several planets and moons.