Extracting Spatial-Temporal Coherent Patterns in Geomagnetic Secular Variation Using Dynamic Mode Decomposition

Rapid growth of magnetic-field observations through SWARM and other satellite missions motivate new approaches to analyze it. Dynamic mode decomposition (DMD) is a method to recover spatially coherent motion with a periodic time dependence. We use this method to simultaneously analyze the geomagnetic radial field and its secular variation from CHAOS-7 at high latitudes. A total of five modes are permitted by noise levels in the observations. One mode represents a slowly evolving background state, whereas the other four modes describe a pair of waves; each wave is comprised of a complex DMD mode and its complex conjugate. The waves have periods of T-1 = 19.1 and T-2 = 58.4 years and quality factors of Q(1) = 11.0 and Q(2) = 4.6, respectively. A 60-year wave is consistent with previous predictions for zonal waves in a stratified fluid. The 20-year wave is also consistent with previous reports at high latitudes, although its nature is less clear.