P. Schmoldt, PhD - MTNet - DIAS

2.2. Electric currents in the magnetosphere
Fig. 2.8.: Projection of the dip equator (dashed) and the geomagnetic equator (dip-pointed) onto the Earth’s surface; from Mareschal
[1986].
Fig. 2.9.: Equivalent current system for lunar daily variations L exhibiting two separate current loops on the day-lit side of either
hemisphere (U.T.: Universal time); from Malin [1973]
three degrees around the dip equator [e.g. Vozoff , 1987; Padilha et al., 1997; Padilha,
1999; Brasse, 2003], i.e. the region of horizontal geomagnetic field lines that are usually
in close proximity to the Earth’s equator (Fig. 2.8). Amplitudes of regular solar daily
variations can be enhanced through such irregular variations by up to 20 nT on disturbed
days S D [Schmucker, 1985], having serious effects on local MT observations [e.g. Hesse,
1982; Padilha et al., 1997; Padilha, 1999].
Lunar daily variation
Lunar daily variations are due to tidal effects in the oceans and atmosphere of the Earth
originating from the Moon orbiting the Earth, thereby forcing tidal currents to move
across field lines inducing effects on the magnetosphere. Such effects can be represented
by an equivalent current system exhibiting two characteristic separate current loops on the
day-lit side of either hemisphere [Schmucker, 1985] (Fig. 2.9). The effect of lunar daily
variations is commonly neglected given its minor amplitude in comparison with S q and
the effect of thermal instabilities in the recording of the MT electric components which
both occur in a similar frequency range.
15