P. Schmoldt, PhD - MTNet - DIAS

2.2. Electric currents in the magnetosphere
Fig. 2.2.: Annual rate of lightning discharges, indicated by colours; from Global Hydrology Resource Center [2010].
field of thunderstorms in order to assure that the plane wave assumption is satisfied (cf.
Sec. 2.3).
2.2. Electric currents in the magnetosphere
The Earth’s magnetosphere is the region in which behaviour of charged particles is dominated
by the Earth’s internal magnetic field, with the resulting shape determined by interaction
with the electromagnetic components of the solar wind compressing it on the
dayside and causing an elongated tail on the night side (Fig. 2.3). The lowermost part
of the magnetosphere, extending from around 50 km to 1000 km above Earth’s surface,
is the relatively conductive ionosphere. The ionosphere consists of plasma and contains
mostly oxygen and nitrogen gases, ionised by solar radiation [Kelley, 1989; Vozoff , 1987;
Andersen and Fuller-Rowell, 1999] (Fig. 2.4).
Through the interaction of the solar wind, containing charged particles and plasma,
with elements of the Earth’s magnetosphere, various EM fields are generated, forming
the lower end of the MT source spectrum. The strength of generated EM fields are in
the range < 0.1 nT, for frequencies in the region of 1 s, and increases with period until it
reaches a quasi-steady level of around 3 nT, for periods greater than 10 5 s. Superimposed
on this is a multitude of spectral peaks with varying amplitude and sharpness (Fig. 2.1).
A list of the contributing signals together with their characteristics and sources is given in
Table 2.1 and a detailed description is given in Sections 2.2.1 to 2.2.5.
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