P. Schmoldt, PhD - MTNet - DIAS

2. Sources for magnetotelluric recording
Fig. 2.14.: Equivalent current model representing DP2 with two current loops and flow direction towards low latitudes on the morning
and high latitudes on the evening sector respectively. The vectors represent the external part of the horizontal disturbance field, rotated
90 degrees to indicate the direction of overhead currents; from Schmucker [1985].
conductivity and intensifying S q variations (Sec. 2.2.2) on the day-side [Schmucker,
1985]. The sfe occurs in geomagnetic observations with a steep onset followed by a slow
decay of approximately exponential form with the sign and amplitude of their contribution
to the magnetic field components dependent on the location of the recording station
relative to the centre of the generated current loop (Fig. 2.15).
2.2.5. Ultra low frequency waves
Ultra low frequency (ULF) waves, or (micro-)pulsations as they are referred to in earlier
literature, are classified by their waveform and wave period, divided into continuous pulsations
(Pc) and irregular pulsations (Pi) that are further subdivided into bands related to
specific types of pulsations. ULF waves are part of the period range below the MT dead
band of which waves relevant for MT observations have been detected between 0.2 s and
600 s comprising 5 bands for the continuous and 2 for the irregular pulsations (Tab. 2.1).
The limits of these bands are not precise, and effects of different pulsation types exhibit
overlapping period ranges [McPherron, 2005].
Specifics of the circumstances that lead to the observed pulsation characteristics remain
elusive, but certain aspects about ULF wave generation and their modification are well
known. A comprehensive overview about the sources of ULF waves, and their effect of
the Earth’s magnetosphere, are given in an excellent review paper by McPherron [2005],
which is recommended to the inquisitive reader. A complete repetition of this topic is not
the aim of this Section and such an in-depth description would go beyond the scope of this
Thesis. Summarising in brief, it can be stated that all ULF waves have in common that
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