Radio atmospheric signal
A Radio Atmospheric or Sferic (sometimes also spelled "Spheric") is a broadband electromagnetic impulse that occurs as a result of natural atmospheric lightning discharges. Sferics may propagate from their lightning source without major attenuation for thousands of kilometers in the Earth-Ionosphere waveguide, and can be received far away - on other hemispheres or further - from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a spectrogram, a sferic appears as a vertical stripe (reflecting its broadband and impulsive nature) that may extend from a few kHz to several tens of kHz, depending on atmospheric conditions.
When the electromagnetic energy from a sferic escapes the Earth-Ionosphere waveguide and enters the magnetosphere, it becomes dispersed by the near-earth plasma, forming a whistler signal. Because the source of the whistler is an impulse (i.e., the sferic), a whistler may be interpreted as the impulse response of the magnetosphere.