Natural EM Fields from a controlled source perspective

From a controlled source exploration perspective, natural EM fields are usually regarded quite simply as unwanted noise, and major efforts are made to get rid of this "noise". The converse is also true. In textbooks and most research papers, the natural or magnetotelluric (MT) signals are assumed to propagate vertically, with variable horizontal electric field polarizations, and most processing software generally assumes sources are at infinity. Without source information, the horizontal magnetic fields are used as a time (phase) and amplitud~ reference for horizontal electric field or vertical magnetic field components. At audio magnetotelluric (AMT) frequencies, the sources are generally "sferics", the fields of distant lightning strikes. We now have extensive lightning and solar wind detection networks in 24 hour operation that define exactly where and when each source "event" occurs, and in theory it is possible to extract the source dipole characteristics from public data. Source information, not conventionally used in MT and AMT, can in concept (a) reduce noise levels, (b) improve our ability to correct static shifts, (c) better map excellent conductors, and (d) identify and assist in the measurement and thus elimination of static shifts.