P. Schmoldt, PhD - MTNet - DIAS

2. Sources for magnetotelluric recording
the plane wave assumptions are down-weighted on a statistical basis; see Section 6.2 for
details on data processing schemes.
Mid-latitude zone
As previously noted (Sec. 2.2.2), S q variations generated at mid-latitudes are of a global
nature and do not exhibit strong non-uniformity; source regions of other MT signals are
usually sufficiently far away for recordings carried out between 3 and 50 degree geomagnetic
latitude on either hemisphere. For datasets collected in these areas, no strong
constraints have to be applied to correct for source effects. Processing of such data usually
involves weighting of impedance estimates with the vertical field partial coherence
[Beamish, 1979], and rejection of events that exhibit low coherence with horizontal magnetic
fields [Gough and de Beer, 1980].
Low-latitude zone
Source effects at low latitudes are mainly caused by the EEJ, initiated by S q variations,
appearing on the day-side of the Earth as an eastward flowing ribbon of electric current
most prominent in the region within three degrees on both sides of the dip equator
(Sec. 2.2.2). Much work has been done on defining the origin and morphology of the
EEJ [e.g. Untiedt, 1967; Hutton, 1972; Richmond, 1973a,b; Fambitakoye and Mayaud,
1976a,b,c; Mayaud, 1977; Marriott et al., 1979; Onwumechili and Agu, 1982] and its
effect on MT measurements [e.g. Forbes, 1981; Padilha et al., 1997; Carrasquilla and
Rijo, 1998; Padilha, 1999], leading to the conclusion that the effects of the EEJ source are
essential during the daytime and can be eliminated by separate investigation of daytime
and nighttime records or down-weighting estimates made during the occurrence of EEJ’s
(analogue to the methods described for mid-latitudes).
High-latitude zone
The regions around the Earth’s poles (above 50 degrees geomagnetic latitude) are considered
to be the most complicated areas in terms of source effects on MT recordings, given
the variety of signals mapping via field lines into these domains. See Table 2.1 for a list of
MT sources and their occurrence at different latitudes and Section 2.2.2 for a description
of the PEJ dominating the electric current flow in the ionosphere at high-latitudes. The
PEJ varies between the different sectors of the Earth in intensity, width, and lateral extent,
which, in particular, leads to a different degree of non-uniformity of the MT source signal
generated in either the daytime or the nighttime sectors [Garcia et al., 1997]. Source
effects are much more prominent during nighttime intervals, exhibiting strong negative
excursions of the northern magnetic field and increased activity in the electric fields (Fig.
2.12) which can cause severe distortion of MT recordings.
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