1. A circularly polarized wave found in a plasma that runs parallel to the magnetic field produced by electric currents outside the plasma.
2. A wave in a plasma that moves parallel to the magnetic field produced by currents outside the plasma at frequencies less than that of the electron cyclotron resonance, and which is circularly polarized, rotating in the same sense as electrons in the plasma; responsible for whistlers.

A whistler is defined as an effect that occurs when a plasma disturbance, caused by a lightning discharge, travels out along lines of magnetic force of the earth's field and is reflected back to its origin from a magnetic point on the earth's surface.