<P> In theory of hearing physiology, the traveling wave (TW) of Von Bekesy, resulted from an acoustic surface wave of the basilar membrane into the cochlear duct . His theory pretended to explain every feature of the auditory sensation owing to these passive mechanical phenomena . Jozef Zwislocki, and later David Kemp, showed that that is unrealistic and that active feedback is necessary . </P> <P> Ground wave refers to the propagation of radio waves parallel to and adjacent to the surface of the Earth, following the curvature of the Earth . This radiative ground wave is known as the Norton surface wave . Other types of surface wave are the non-radiative Zenneck surface wave or Zenneck--Sommerfeld surface wave, the trapped surface wave and the gliding wave . See also Dyakonov surface waves (DSW) propagating at the interface of transparent materials with different symmetry . </P> <P> Lower frequency radio space waves, below 3 MHz, travel efficiently as ground waves . In ITU nomenclature, this includes (in order): medium frequency (MF), low frequency (LF), very low frequency (VLF), ultra low frequency (ULF), super low frequency (SLF), extremely low frequency (ELF) waves . </P> <P> Ground propagation works because lower - frequency waves are more strongly diffracted around obstacles due to their long wavelengths, allowing them to follow the Earth's curvature . The Earth has one refractive index and the atmosphere has another, thus constituting an interface that supports the guided wave's transmission . Ground waves propagate in vertical polarization, with their magnetic field horizontal and electric field (close to) vertical . With VLF waves, the Ionosphere and earth's surface act as a waveguide . </P>

Where are the short distance surface currents located