Co-estimation of geomagnetic field and in-orbit fluxgate magnetometer calibration parameters

Patrick Alken,Nils Olsen,Christopher C Finlay

doi:10.1186/s40623-020-01163-9

Abstract

For the past 20 years, state of the art geomagnetic core field models have relied heavily on magnetic measurements made from space-based instrumentation. These models have revealed rapid global magnetic field variations on sub-decadal timescales originating in Earth’s core. With the end of the CHAMP mission in 2010 and the launch of Swarm in late 2013, there has been a 3-year gap in high-quality satellite measurements of the geomagnetic field. Geomagnetic field models have therefore relied on ground observatory data to fill in this gap period. However, ground observatories are unable to provide a truly global picture of the core field and its temporal changes. Many satellites in operation carry vector fluxgate “platform” magnetometers for attitude control, which can offer an alternative to relying on ground observatory measurements during the gap period. However, these instruments need to be carefully calibrated in order to provide meaningful information on Earth’s core field. Some previous studies attempted to calibrate such instruments with a priori geomagnetic field models. This approach has several disadvantages: (1) errors in the model will introduce errors in the calibration parameters, and (2) relying on an a priori model may not be feasible in the post-Swarm era. In this paper, we develop a novel approach to build a time-dependent geomagnetic field model from platform magnetometer data, by co-estimating their calibration parameters with the internal field parameters. This method does not require an a priori geomagnetic field model, but does require a dataset of previously calibrated data. We use CHAMP, Swarm, and ground observatory measurements to supply this dataset, and incorporate platform magnetic measurements from DMSP and Cryosat-2 during the gap years. We find that the calibration parameters of DMSP and Cryosat-2 can be reliably estimated, and these missions provide meaningful information on rapid core field variations during the gap period.

Highlights

Earth’s internal magnetic field is constantly changing due to what is believed to be a geodynamo inside the liquid outer core
The CHAllenging Minisatellite Payload (CHAMP), Swarm, and observatory datasets have been calibrated previously against absolute magnetic field measurements, and act as truth data to enable a robust estimation of the Cryosat/Defense Meteorological Satellite Program (DMSP) calibration parameters
Model A is regularized by minimizing the third time derivative of the radial field component averaged over the core mantle boundary (CMB) ( 3 ), smoothing the alignment splines for CHAMP and Swarm ( α1,2,3 ), and smoothing the calibration splines for Cryosat and DMSP ( m, β )

Summary

Introduction

Earth’s internal magnetic field is constantly changing due to what is believed to be a geodynamo inside the liquid outer core. According to this theory, cooling of the core when combined with gravitational forces drives convective fluid motion. An alternative approach for studying geomagnetic core field variations is to analyze measurements recorded by ground observatories as well as satellite missions carrying magnetic instrumentation in Low-Earth Orbit (LEO). While many satellite missions carry fluxgate magnetometers, these instruments are subject to drift due to mechanical effects (such as vibrations), temperature variations, or aging of sensor material, and extreme care must be taken before using these data in geomagnetic field modeling

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Conclusion