P. Schmoldt, PhD - MTNet - DIAS

2. Sources for magnetotelluric recording
Fig. 2.4.: Layers of the ionosphere with their electron density and predominant ion populations; from Andersen and Fuller-Rowell
[1999].
Solar cycle variation
Regular solar variations are connected with wave radiation of the Sun with the fundamental
period for the solar cycle variations of 11 years [Schmucker, 1985], most easily
observed by the annual number of sunspots (Fig. 2.5). Even though the cause of the cycle
is not yet fully understood, changes in activity can be used to predict the observable
signal strength during a fieldwork campaign as the increased magnetic activity intensifies
pre-existing current systems used as MT sources [Mareschal, 1986]. Presently we are
situated in an elongated minimum of the twenty third 11 year cycle (Fig. 2.6) causing the
signal, and hence the signal-to-noise ratio (SNR), to be noticeably lower at mid-latitudes.
Moreover, the decreasing maximum field strength of annual sunspots, as observed by
Livingston and Penn [2009] using measured infrared intensity in the darkest position of
the sunspot umbrae, is raising the question of whether we are actually experiencing a
new period of severely reduced activity, similar to the Maunder Minimum from 1675 to
1715 AD [Luterbacher et al., 2001]. The present time frame, limited by the emergence of
the required instruments, is too short to verify such hypothesis.
(Semi-)Annual variation
Annual and semi-annual variations penetrate into the Earth down to a depth of approximately
1000 km and deeper but are commonly not used for MT investigation due to
the required extensive recording time. Furthermore, semi-annual variations exhibit very
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