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The magnetotelluric (MT) and auditomagnetotelluric (MAT) methods are passive techniques within the family of electromagnetic methods that use electric and magnetic fields generated by solar activity -such as the auroras- as well as cosmic radiation as their signal source.  These global sources can be measured at frequencies typically ranging from 0.0003 to 10000 Hertz, and they can provide resistivity information from the surface down to tens of kilometers, if the measurements are sufficiently long.  For exploration purposes, just a few hours of measurement can yield results down to a few kilometers in depth.  A variation of these methods involves the use of a controlled-source transmitter (CSMAT) to enhance the received signal, which is particularly useful during periods of low solar activity.