Post on 01-Apr-2015
transcript
Natural and artificial
geophysical magneto-tellurics
... diagram from NOAA
Earth's geomagnetic field:Solar interactions
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
...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