Magnetotelluric fields of a line current

Magnetotelluric fields in the equatorial and auroral electrojet zones violate the condition of plane waves because of the concentrated nature of sources in the ionosphere. To test if Cagniard's relation for apparent resistivity is still valid in these areas, a line current is used to simulate an electrojet. The integral expressions for the electromagnetic fields at the surface of a layered earth are numerically integrated for two simple models, one representing an average continent, the other representing an active tectonic area. It is shown that Cagniard's relation is particularly applicable to active tectonic areas where conductivities in the earth may be enhanced as a result of thermal processes. Cagniard's relation is also applicable to average continental areas, provided that the electrojet may be represented by a line current at a height greater than 500 km. This suggests than an electrojet at a height less than 500 km would permit invoking Cagniard's relation if the electro jet is reasonably broad.