Whistlers: Under the right conditions, the VLF signal travels out away from earth and returns by traveling along a magnetic field line. During this long path, dispersion is much greater than with tweeks. While tweeks might disperse a few hundred kilohertz over a few thousandths of a second, whistler show a dispersion of a second or more over several thousand kilohertz. The sound of a whistler is a musical descending tone that lasts for a second or more. On the spectrogram, whistlers appear as long sweeping arcs showing the sequential arrival of the frequencies. It is important to remember that all of the frequencies start out at the same time (a sferic), but the path taken by a whistler is so long that the dispersion of the frequ-encies is quite pronounced