A high‐resolution model of the external and induced magnetic field at the Earth's surface in the Northern Hemisphere. Issue 2 (17th February 2017)

• Record Type: Journal Article
• Authors: Shore, R. M.; Freeman, M. P.; Wild, J. A.; Gjerloev, J. W.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 2(2017)
• Page Start: 2440
• Page End: 2454
• Publication Date: 2017-02-17
• Subjects: empirical orthogonal functions -- external magnetic field -- polar ionosphere -- decomposition of variability -- disturbance polar equivalent current systems; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2016JA023682
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We describe a method of producing high‐resolution models of the Earth's combined external and induced magnetic field using the method of empirical orthogonal functions (EOFs) applied to the SuperMAG archive of ground‐based magnetometer data. EOFs partition the variance of a system into independent modes, allowing us to extract the spatiotemporal patterns of greatest dynamical importance without applying the a priori assumptions of other methods (such as spherical harmonic analysis, parameterized averaging, or multivariate regression). We develop an approach based on that of Beckers and Rixen (2003) and use the EOF modes to infill missing data in a self‐consistent manner. Applying our method to a north polar case study spanning February 2001 (chosen for its proximity to solar maximum and good data coverage), we demonstrate that 41.7% and 9.4% of variance is explained by the leading two modes, respectively, describing the temporal variations of the disturbance polar types 2 and 1 ( DP 2 and DP 1) patterns. A further 14.1% of variance is explained by four modes that describe separate aspects of the motion of the DP 1 and DP 2 systems. Thus, collectively over 65% of variance is described by the leading six modes and is attributable to DP 1 and DP 2. This attribution is based on inspection of the spatial morphology of the modes and analysis of the temporal variation of the mode amplitudes with respect to solar wind measures and substorm occurrence. This study is primarily a demonstration of the technique and a prelude to a model spanning the full solar cycle. Key Points: Disturbance polar equivalent currents types 1 and 2 ( DP 1 and DP 2) resolved at 5 min cadence in all LT sectors and all polar latitudes via EOF DP 1 and DP 2 shown to collectively account for 65% of the total external and induced magnetic field variance in the polar region Data predictions based on the EOF reanalysis were used to derive time series of the polar cap radius in two local time sectors