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Linear vs nonlinear methods for detecting magnetospheric and ionospheric current systems patterns

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Papers from SWICo members

T. Alberti, F. Giannattasio, P. De Michelis and G. Consolini.

The geomagnetic field time series can be viewed as a superposition of signals which represent the different contributions to it at different scales coming from various sources both internal and external to the Earth. Here, we apply two different methods of analysis in order to recognize in the magnetic field of external origin the different contributions coming from external sources, making use of measurements of the vertical component of the geomagnetic field (directed away from the Earth’s center) recorded by the European Space Agency (ESA) Swarm A and B satellites at low and mid latitudes during a geomagnetically quiet period.

The main external contributions to the geomagnetic field as reconstructed through the Multivariate Empirical Mode Decomposition (MEMD). From top to bottom: 1) the residue (res) of the MEMD method can be attributed to the ring current contribution; 2) the c5 mode pattern can be related to the main Sq pattern; 3) the c4 mode pattern can be attributed to a sub-harmonic structure of the Sq current; 4) short-scale reconstructions expressed by C1−3 modes could be related to different source mechanisms (external drivers, magnetopause current).

The first is a linear method, that is, the Empirical Orthogonal Functions (EOF); the second is a nonlinear one, i.e. the Multivariate Empirical Mode Decomposition (MEMD). As the external contributions to the magnetic signal are intrinsically nonlinear, the MEMD seems to give better results with respect to EOF, and only five modes and a residue are necessary to reproduce the different contributions coming from the external sources against the 26 modes that are necessary in the case of the EOF. This study is an example of the potential of the MEMD for the analysis of the geomagnetic field of external origin, and of its capability to separate the ionospheric from the magnetospheric contributions in a simple and rapid way.

Publication: T. Alberti, F. Giannattasio, P. De Michelis and G. Consolini, Linear vs nonlinear methods for detecting magnetospheric and ionospheric current systems patterns, Earth and Space Science, 2020.
https://doi.org/10.1029/2019EA000559