Modeling Diurnal Variation Magnetic Fields for Mantle Induction Studies

Accurate models of the spatial structure of ionospheric magnetic fields in the daily variation (DV) band (periods of approximately 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. 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. Combining the two steps, we have a Frequency domain model of DV band global magnetic fields that is continuous in both space and time. The frequency domain model can easily be transformed back to the time domain (TD) to directly fit time-domain data. So far, the model uses only ground-based data, from 127 geomagnetic observatories. We show that the model accurately reproduces surface magnetic fields, including those at sites not used for model construction. Although fits are best during geomagnetically quiet times, and at mid-latitudes, the model tracks even complex magnetic field variations during storms, at all latitudes. Our preliminary model thus already represents an advance in empirical DV modeling.