natural electric current flowing on and beneath the surface of the Earth and generally following a direction parallel to the Earth's surface. Telluric currents arise from charges moving to attain equilibrium between regions of differing electric potentials; these differences in potential are set up by several conditions, including very low-frequency electromagnetic waves from space, particularly from the magnetosphere incident upon the Earth's surface, and moving charged masses in the ionosphere and the atmosphere. Telluric currents are often used by geophysicists to map subsurface structures, such as sedimentary basins, layered rocks, and faults. An anomalous current density or gradient may be indicative of a subsurface structural feature.