Natural and artificial geophysical magnetotellurics Earths geomagnetic field

Natural and artificial geophysical magneto-tellurics

Earth's geomagnetic field: Solar interactions . . . diagram from NOAA

Auroral electrojet . . . from Meta. Tech Corp.

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 Borealis © i. Stockphoto / Roman Krochuk . . . photo: NASA, site geology. com

Aurora are due to N 2 and atomic O excitations 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.

Aurorae during the day

Equatorial electrojet n. T . . . from Wiki. Pedia

. . . 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 electromagnetic cavity resonance in the spherical shell between the surface and the base of the ionosphere.

E and H fields at ground level Schumann resonances – the cavity resonance

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