The E layer is a region of the ionosphere, extending from about 90-150 kilometres above the Earth, which influences long-distance communications by strongly reflecting radio waves in the 1-3 megahertz. It is also called E region, Heaviside layer, or Kennelly-Heaviside layer. This region reflects radio waves of medium wavelength and allows their reception around the surface of the Earth. The layer approaches the Earth by day and recedes from it at night. In medical terms, it is a cylinder of relativistic electrons gyrating in the magnetic field, which produces a self field strong enough to dominate the externally applied field and produces half reversal in the system. Since the mid '20s, another connection regarding the ionosphere has been hypothesized that lightning can interact with the lower ionosphere. According to this theory, thunderstorms could modulate the transient, localized patches of relatively high-electron density in the mid-ionosphere E layer, which significantly affects radiowave propogation.