Tweeks result when sferics are ducted in the earth-ionosphere
waveguide distances much greater than a cuple of thousand kilometers. The
distance can be as great as halfway around the earth (20,000 kilometers).
When ducted over large distances, the VLF radio waves undergo a process
called dispersion - that is the higher frequencies travel slightly faster
than the lower frequencies. This is especially true of the frequencies between
3 and 2 kilohertz. Tweeks sound much different than sferics. Instead of
the sharp crackling sound, tweeks have a quick mujsical sound somewhat like
the ricochet sound bullets make (at least in the movies). The spectrogram
of a tweek shows a vertical line at the higher frequencies with a curved
section (called the "hook") appearing at a frequency of about
2 kilohertz.
