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
...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. linklink Basics of the Magnetotelluric method, S. Thiel, U. Adelaide linklink Some on-line ebooks linklink