Publication Name: Geophysical Journal International; First HAO Author's Name: Astrid Maute; Authors: G.D. Egbert, P. Alken, A. Maute, H. Zhang, A.D. Richmond

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.

Magnetic Bx component[nT] at five representative sites for 9/20/2002 - 10/10/2002

Magnetic Bx component[nT] at five representative sites for 9/20/2002 - 10/10/2002, the first half of this interval geomagnetic conditions were quiet (mean Kp 4) in the second half, with a significant storm (Kp = 7) on 10/01/2002. For each site we show the original observatory Bx time series (blue lines), the time domain PCA approximation using 20 modes for all bands (black lines), and global time domain model (red lines).

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.