Modeling Diurnal Variation Magnetic Fields for Mantle Induction Studies

Abstract:

Accurate models of the spatial structure of ionospheric magnetic fields in the diurnal variation (DV) band (periods of a few hours to a day) would enable use of magneto-variational methods for three-dimensional imaging of upper mantle and transition zone electrical conductivity. Constraints on conductivity at these depths, below what is typically possible with magnetotellurics, would in turn provide valuable constraints on mantle hydration and Earth’s deep water cycle. As a step towards this objective, we present here a novel approach to empirical modeling of global DV magnetic fields. First, we apply frequency domain (FD) principal components analysis (PCA) to ground-based geomagnetic data, to define the dominant spatial and temporal modes of source variability. Spatial modes are restricted to the available data sites, but corresponding temporal modes are effectively continuous in time. Second, we apply FD PCA to gridded surface magnetic fields derived from outputs of the physics-based Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM), to determine the dominant modes of spatial variability. The TIEGCM spatial modes are then used as basis functions, to fit (or interpolate) the sparsely sampled data spatial modes. Combining the two steps, we have a FD model of DV band global magnetic fields that is continuous in both space and time. We show that the FD model can easily be transformed back to the time domain (TD) to directly fit time-series data, allowing use of satellite, as well as ground-based, data in the empirical modeling scheme. We have constructed global FD and TD models of DV band source fields for 1997-2018, using data from 127 geomagnetic observatories. We show that this model accurately reproduces surface magnetic fields in both active and quiet times, including those at sites not used for model construction. After a brief summary of the source modeling methodology, we discuss ongoing work to incorporate satellite date from Swarm and CHAMP, and to use the new source models for mantle induction studies.