P. Schmoldt, PhD - MTNet - DIAS
P. Schmoldt, PhD - MTNet - DIAS 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 20
2.3. Deviation from plane wave assumption Fig. 2.15.: Geomagnetic field recorded at mid-latitudes showing the effect of a solar flare effect (sfe) shortly after 14h with overhead Sq currents toward the equator indicated by a local eastward deflection of the Earth magnetic field (positive deflection of the declination measurements, D); from Schmucker [1985]. they are initially generated as magnetohydrodynamic (MHD) waves by processes induced in a plasma under influence of the magnetic field; the plasma is herein part of either solar wind, foreshock, Earth’s bow shock, magnetopause, or magnetosphere. The portion of MHD waves from sources external to the Earth’s magnetosphere that reach the Earth’s surface interact with each of the regions between the initial source location and the point of detection. Induced effects are known as field line resonance, current induction in the ionosphere, and cavity resonance, determining the actually observed pulsation characteristics. Internal sources of ULF wave include earthward directed plasma flow as well as gyro, drift and bounce resonances, responsible for Pi1 and Pi2 signals as well as Pc1, Pc2, and Pc3. An overview about the pulsations and their process of generation as they are understood by today is given in Table 2.3. 2.3. Deviation from plane wave assumption To simplify mathematic principles of EM induction processes forming the base of the MT method (cf. Sec. 6.2), it is commonly assumed that primary magnetic waves meet the characteristics of a plane wave for the frequency range and study area, i.e. it is assumed that the wave can be considered uniform. A plane wave requires either a uniform source of infinite length or a source at infinite distance, both of which are obviously not physically realisable. Hence, it needs to be examined under which circumstances the deviation of uniformity for a wave can be considered small enough such that the effect of the deviation is negligible for a given resolution. 2.3.1. Mathematical description Traditionally, the plane wave assumption was considered valid when the recording is made in the far-field, i.e. when the distance between source and recording location r is much greater than the wavelength λ (i.e. r ≫ λ). In the review paper by Mareschal [1986] on natural MT sources, it is suggested to instead compare the magnitude of the 21
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2.3. Deviation from plane wave assumption<br />
Fig. 2.15.: Geomagnetic field recorded at mid-latitudes showing the effect of a solar flare effect (sfe) shortly after 14h with overhead Sq<br />
currents toward the equator indicated by a local eastward deflection of the Earth magnetic field (positive deflection of the declination<br />
measurements, D); from Schmucker [1985].<br />
they are initially generated as magnetohydrodynamic (MHD) waves by processes induced<br />
in a plasma under influence of the magnetic field; the plasma is herein part of either solar<br />
wind, foreshock, Earth’s bow shock, magnetopause, or magnetosphere.<br />
The portion of MHD waves from sources external to the Earth’s magnetosphere that<br />
reach the Earth’s surface interact with each of the regions between the initial source location<br />
and the point of detection. Induced effects are known as field line resonance, current<br />
induction in the ionosphere, and cavity resonance, determining the actually observed pulsation<br />
characteristics. Internal sources of ULF wave include earthward directed plasma<br />
flow as well as gyro, drift and bounce resonances, responsible for Pi1 and Pi2 signals<br />
as well as Pc1, Pc2, and Pc3. An overview about the pulsations and their process of<br />
generation as they are understood by today is given in Table 2.3.<br />
2.3. Deviation from plane wave assumption<br />
To simplify mathematic principles of EM induction processes forming the base of the MT<br />
method (cf. Sec. 6.2), it is commonly assumed that primary magnetic waves meet the<br />
characteristics of a plane wave for the frequency range and study area, i.e. it is assumed<br />
that the wave can be considered uniform. A plane wave requires either a uniform source of<br />
infinite length or a source at infinite distance, both of which are obviously not physically<br />
realisable. Hence, it needs to be examined under which circumstances the deviation of<br />
uniformity for a wave can be considered small enough such that the effect of the deviation<br />
is negligible for a given resolution.<br />
2.3.1. Mathematical description<br />
Traditionally, the plane wave assumption was considered valid when the recording is<br />
made in the far-field, i.e. when the distance between source and recording location r<br />
is much greater than the wavelength λ (i.e. r ≫ λ). In the review paper by Mareschal<br />
[1986] on natural MT sources, it is suggested to instead compare the magnitude of the<br />
21
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