Natural and artificial

geophysical magneto-tellurics

... diagram from NOAA

Earth's geomagnetic field:Solar interactions

...from MetaTech Corp.

Auroral electrojet

Aurorae form around the polar electrojets

Aurora Australis captured by NASA's IMAGE satellite and overlaid onto NASA's satellite-based Blue Marble image.

More Aurora images!

Aurora Australis

... photo: NASA, site geology.com

Aurora Borealis

© iStockphoto / Roman Krochuk

Auroral light is mostly from electronically excited oxygen atoms. The green radiation prevails at low altitudes and the red at higher.

Excited nitrogen molecules and nitrogen molecular ions produce pink and red at low altitudes.

Aurora are due to N2 and atomic O excitations

Aurorae during the day

... from WikiPedia

nT

Equatorial electrojet

...and natural source magneto-tellurics

The ionospheric electrojet currents are not constant and because they vary in time, they are composed of a continuous spectrum of frequencies.

As well, lightning strikes produce an electro-magnetic cavity resonance in the spherical shell between the surface and the base of the ionosphere.

Schumann resonances – the cavity resonance

E and H fields at ground level

Wavefields – not necessarily normal to the surface

Controlled source MT

In principle, rather than using natural sources, we can use any source of electromagnetic waves. When using non-natural sources, we often use the term artificial source or controlled source MT.

Theory and practice of MT geophysics

Some references:

The Magnetotelluric Method, G.R. Jiracek, San Diego State Univ. link

Basics of the Magnetotelluric method, S. Thiel, U. Adelaide link

Some on-line ebooks link