Geomagnetic depth sounding by induction arrow representation: A review

Considerable important research in upper atmosphere geophysics is carried out through the use of arrays of ground-based magnetometers. In order to better delineate the ionospheric and magnetospheric currents and waves as measured by these arrays, it is important to understand the conductivity of the earth's structure under the individual stations. Geomagnetic depth sounding studies are used to deduce the earth's conductivity profiles. In most studies, ‘induction arrows,’ or ‘induction vectors,’ are plotted on maps for graphical representations of lateral inhomogeneities in underground conductivity structures. Different methodologies and different arrow conventions have been used by a number of authors for deriving these vectors, often without relating their techniques to other work in the field. We review herein the various methodologies (except transfer functions) and present a unifying picture to the representations that should prove useful to researchers in both space physics and solid earth physics.