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External source field characterization and electromagnetic induction response of the deep earth using CHAMP satellite magnetic data

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Kunagu,  Venkata Praveen Kumar
2.3 Earth's Magnetic Field, 2.0 Physics of the Earth, Departments, GFZ Publication Database, Deutsches GeoForschungsZentrum;

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Zitation

Kunagu, V. P. K. (2013): External source field characterization and electromagnetic induction response of the deep earth using CHAMP satellite magnetic data, PhD Thesis.


Zitierlink: https://gfzpublic.gfz-potsdam.de/pubman/item/item_247142
Zusammenfassung
The steady progresses in the precise measurement of geomagnetic field using land-based observatories and satellites and adapting sophisticated mathematical tools, viz., spherical harmonics, splines, wavelets etc., for data analysis and interpretation, have improved our knowledge about the presence of several contributing sources (both external and internal) in the measured magnetic field. However, for determining a precise global geomagnetic field model, a uniform and full global coverage data is required. Ground based magnetic observatories, because of their sparse distribution over the globe, fail to provide the requisite adequate global coverage and thus hinder the correct understanding of the contributing sources in the observed magnetic field variations. This problem can be overcome, by using the data from Low Earth Orbiting (LEO) satellite missions, owing to their continuous, uniform, and adequate global coverage, which aptly suit to understand and characterize the Earth’s magnetic field variations in a broader perspective. Knowledge of the external source field contributions in the observed magnetic field variations is important, not only for understanding their spatio-temporal nature, but also for their use in geomagnetic induction studies. Thus, the present work concentrates on the characterization of magnetospheric source current system in view of induction studies. For the present study, CHAMP (CHAllenging Minisatellite Payload) satellite vector magnetic data are used. The data covers almost a complete solar cycle from July, 2001 to September, 2010, spanning 55,417 orbits. A better characterization of the external source field can be achieved by decomposing it into outer- and inner-magnetospheric contributions, which are best represented in Geocentric Solar Magnetospheric (GSM) and Solar Magnetic (SM) reference frames, respectively. Thus, a spherical harmonic (SH) model has been proposed to estimate the outer magnetospheric contribution using orbit-averaged CHAMP data following the Iterative Reweighted Least Squares (IRLS) technique. It was established that, there exists a stable outer magnetospheric field of about 7.39 nT. This stable field transforms to a periodic signal having diurnal and annual variations, when rotated to the SM frame. The residual time series obtained after removing the stable outer magnetospheric field from the CHAMP orbit-averaged data, contains contribution mainly from the inner magnetosphere and its induced counterpart. It is the field that is originated in the inner magnetosphere is generally responsible for induction studies. Accordingly, by determining the ratio of vertical (induced) to the horizontal (inducing) components of the residual field, as a function of frequency (period) the depth to the perfect substitute conductor can be estimated. The obtained residual field of inner magnetosphere was subjected to Continuous Wavelet Transform (CWT) to identify the temporal characteristics of the long period geomagnetic variations. The CWT was performed using Morlet wavelet as mother wavelet. Results of CWT showed the dominant 27-day periodicity, in all the vector components irrespective of the type of the day (disturbed or quiet) along with the well known diurnal, 90-day, annual and semi-annual variations. The CWT power spectra also revealed another interesting long period signal with 210-day periodicity, which has been identified in geomagnetic data for the first time. Central to these observations is the local time dependency of 27-day variations, which has also been identified for the first time in CHAMP magnetic data. The LT dependency of the 27-day variations thus supports the recent observations of non-axisymmetric nature of the source field of magnetospheric origin. Such a precise characterization of the inducing source field led to the determination of robust EM induction response of the Earth to the external inducing signal of 27-day periodicity through the complex C-response C(ω) function. Thus, the depth to the perfect substitute conductor is determined to be around 1132 km with a conductivity of about 1.05 S/m. The C(ω) responses determined with such high precision external field characterization also matches well with the earlier obtained global estimates.