An empirical orthogonal function reanalysis of the northern polar external and induced magnetic field during solar cycle 23

Shore, R.M. ORCID:; Freeman, M.P. ORCID:; Gjerloev, J.W.. 2018 An empirical orthogonal function reanalysis of the northern polar external and induced magnetic field during solar cycle 23. Journal of Geophysical Research, 123 (1). 781-795.

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We apply the method of data-interpolating Empirical Orthogonal Functions (EOFs) to ground-based magnetic vector data from the SuperMAG archive to produce a series of month-length reanalyses of the surface external and induced magnetic field (SEIMF) in 110,000 km2 equal-area bins over the entire northern polar region at 5-minute cadence over solar cycle 23, from 1997.0 to 2009.0. Each EOF re-analysis also decomposes the measured SEIMF variation into a hierarchy of spatio-temporal patterns which are ordered by their contribution to the monthly magnetic field variance. We find that the leading EOF patterns can each be (subjectively) interpreted as well-known SEIMF systems or their equivalent current systems. The relationship of the equivalent currents to the true current flow is not investigated. We track the leading SEIMF or equivalent current systems of similar type by inter-monthly spatial correlation, and apply graph theory to (objectively) group their appearance and relative importance throughout a solar cycle, revealing seasonal and solar cycle variation. In this way, we identify the spatiotemporal patterns which maximally contribute to SEIMF variability over a solar cycle. We propose this combination of EOF and graph theory as a powerful method for objectively defining and investigating the structure and variability of the SEIMF or their equivalent ionospheric currents for use in both geomagnetism and space weather applications. It is demonstrated here on solar cycle 23, but is extendable to any epoch with sufficient data coverage.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BAS Programmes > BAS Programmes 2015 > Space Weather and Atmosphere
ISSN: 0148-0227
Additional Keywords: empirical orthogonal functions, external magnetic field, polar ionosphere, decomposition of variability, disturbance polar equivalent current systems, graph theory
Date made live: 13 Dec 2017 13:39 +0 (UTC)

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