Abstract
The solar quiet (Sq) source morphology changes on a daily basis and becomes disturbed during periods of increased magnetic activity. Therefore, it may be preferable to use single-day magnetic field recordings for the analysis of Sq variations. However, in short recordings, Sq and ocean tidal magnetic signals are often indistinguishable because of the close periods. As a result, the tidal magnetic signals can be erroneously attributed to signals of Sq origin, which can potentially lead to wrong interpretations, especially when small signals, such as those induced by the 3-D heterogeneities in the mantle, are sought. In this work, we quantitatively estimate the effect of ocean tidal signals in daily variations by performing rigorous 3-D modeling and comparing the results with real measurements from ground and sea floor observatories. We found that the vertical magnetic field component, Z, is affected the most such that at some locations the tidal signals explain the majority of the observed daily variation. Further, horizontal tidal magnetic fields at the sea floor are larger in amplitude and exhibit different spatial structures compared to signals estimated at the sea level. We propose a scheme aimed at correcting data for the ocean tidal signals and show that such correction suppresses the tidal signals in the observed field variations.
Highlights
Electromagnetic (EM) induction sounding with natural sources contributes to our knowledge of the composition, temperature, and presence of fluids in the Earth’s interior.Among other methods, daily ionospheric solar quiet (Sq) variations can be used to probe the electrical conductivity of the upper mantle (e.g., Schmucker 1999b; Olsen1998; Koch and Kuvshinov 2013, 2015)
Daily ionospheric solar quiet (Sq) variations can be used to probe the electrical conductivity of the upper mantle (e.g., Schmucker 1999b; Olsen 1998; Koch and Kuvshinov 2013, 2015)
In order to transform the tidal magnetic fields from the frequency to time domain, the conventional Fourier transform is complemented with amplitude and phase correction factors resulting in fk BTides bTides (t, r) = Re
Summary
Electromagnetic (EM) induction sounding with natural sources contributes to our knowledge of the composition, temperature, and presence of fluids in the Earth’s interior. N by using a volume integral equation solver given by Kuvshinov (2008), hereinafter called X3DG This solver computes an EM field induced by (arbitrary) sources for a specified 3-D conductivity model of the Earth. By virtue of linearity of Maxwell’s equations with respect to the source, magnetic fields at any location rj = (rj , θj , φj ) can be expressed by a sum of the unit magnetic fields scaled by the unknown source coefficients (Koch and Kuvshinov 2013). We can calculate the synthetic Sq variations in the frequency domain by multiplying the source coefficients with the unit fields. With ra = (a, θ, φ), k represents the kth tidal constituent, ωk is the corresponding angular frequency, σs is the depth-averaged seawater conductivity, u are the depthintegrated seawater transport, and Bmain is Earth’s main magnetic field.
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