Radio Atmospheric Signal
A Radio Atmospheric signal (sometimes referred to as Sferic or Spheric), is a broadband electromagnetic impulse that occurs during atmospheric lightning discharges. Sferics spread out from the lightning source and can be received thousands of miles away. A sferic, depending on atmospheric conditions, may extend anywhere from a few kHz to several tens of kHz. Sferics from far reaching storms, over 1500 miles away, are generally offset in frequency range and may be picked up as tweeks. A whistler signal forms when the sferic escapes the Earth-Ionosphere waveguide and enters the magnetosphere and is dispersed by the near-earth plasma.