COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations

Loïc Huder,Christopher C Finlay,Hervé Tchoungui,Nicolas Gillet,Magnus D Hammer

doi:10.1186/s40623-020-01194-2

Abstract

We present the geomagnetic field model COV-OBS.x2 that covers the period 1840–2020. It is primarily constrained by observatory series, satellite data, plus older surveys. Over the past two decades, we consider annual differences of 4-monthly means at ground-based stations (since 1996), and virtual observatory series derived from magnetic data of the satellite missions CHAMP (over 2001–2010) and Swarm (since 2013). A priori information is needed to complement the constraints carried by geomagnetic records and solve the ill-posed geomagnetic inverse problem. We use for this purpose temporal cross-covariances associated with auto-regressive stochastic processes of order 2, whose parameters are chosen so as to mimic the temporal power spectral density observed in paleomagnetic and observatory series. We aim this way to obtain as far as possible realistic posterior model uncertainties. These can be used to infer for instance the core dynamics through data assimilation algorithms, or an envelope for short-term magnetic field forecasts. We show that because of the projection onto splines, one needs to inflate the formal model error variances at the most recent epochs, in order to account for unmodeled high frequency core field changes. As a by-product of the core field model, we co-estimate the external magnetospheric dipole evolution on periods longer than 2 years. It is efficiently summarized as the sum of a damped oscillator (of period 10.5 years and decay rate 55 years), plus a short-memory (6 years) damped random walk.

Highlights

Two crucial characteristic time-scales of the geodynamo are the Alfvén time (τ A ≈ 4 years based on the propagation of torsional waves, see Gillet et al 2010) and the turn-over time (τ U ≈ 200 years based on the amplitude of core flow motions, e.g., Finlay et al 2010)
The COV‐OBS.x2 field model Statistics on prediction errors We provide in Table 2 some statistics concerning the COV-OBS.x2 misfits and biases to the new (GO and virtual observatories (VO))
We produce the COV-OBS.x2 geomagnetic field model, which extends to 2020 previous generations of COV-OBS series of models

Summary

Introduction

Two crucial characteristic time-scales of the geodynamo are the Alfvén time (τ A ≈ 4 years based on the propagation of torsional waves, see Gillet et al 2010) and the turn-over time (τ U ≈ 200 years based on the amplitude of core flow motions, e.g., Finlay et al 2010). We present how we derive the stochastic a priori information (temporal cross-covariances) used for the model construction, and some distinctions compared with previous generations of COV-OBS models concerning the field induced in the outer core by magnetospheric field changes. We show how the COV-OBS.x2 model uncertainties can be used to estimate the probability density function (PDF) of magnetic forecasts within the employed stochastic framework. We propose PDFs for 5-year forecasts based on the employed stochastic properties It is from this method that we derived ISTerre’s candidates models to IGRF-13 (Alken et al 2020). We propose a stochastic analysis of external dipole field changes

Methods

Results and discussion

Conclusions