Radio atmospheric Sferics received from about distance or greater have their frequencies slightly offset in time, producing "tweeks". 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 (for the conditions at that particular instant). A lightning channel with all its branches and its electric currents behaves like a huge antenna system from which electromagnetic waves of all frequencies are radiated. Beyond
History of geophysics the waves reflected off the internal layers of the Earth, several theories developed as to what would cause variances in wave speed or loss of certain frequencies. This led to scientists like Inge Lehmann discovering the presence of the Earth's core in 1936. Beno Gutenberg and Harold Jeffreys worked at explaining the difference in Earth's density due to compression and the shear velocity of waves. Since seismology is based on elastic waves, the speed of waves could help determine density and therefore the behavior of the layers within the Earth. Nomenclature for the behavior of seismic waves was produced based