Insights from geodynamo simulations into long-term geomagnetic field behaviour

Abstract

8Detailed knowledge of the long-term spatial configuration and temporal variability of the geomagnetic field is lacking because of insufficient data for times prior to 10 ka. We use realisations from suitable numerical simulations to investigate three important questions about stability of the geodynamo process: is the present field representative of the past field; does a time-averaged field actually exist; and, supposing it exists, how long is needed to define such a field. Numerical geodynamo simulations are initially selected to meet existing criteria for morphological similarity to the observed magnetic field. A further criterion is introduced to evaluate similarity of long-term temporal variations. Allowing for reasonable uncertainties in the observations, observed and synthetic axial dipole moment frequency spectra for time series of order a million years in length should be fit by the same power law model. This leads us to identify diffusion time as the appropriate time scaling for such comparisons. In almost all simulations, intervals considered to have good morphological agreement between synthetic and observed field are shorter than those of poor agreement. The time needed to obtain a converged estimate of the time-averaged field was found to be comparable to the length of the simulation, even in non-reversing Preprint submitted to Earth and Planetary Science LettersJuly 25, 2014 models, suggesting that periods of stable polarity spanning many magnetic diffusion times are needed to obtain robust estimates of the mean dipole field. Long term field variations are almost entirely attributable to the axial dipole; non-zonal components converge to long-term average values on relatively short timescales (15 − 20 kyr). In all simulations, the time-averaged spatial power spectrum is characterised by a zigzag pattern as a function of spherical harmonic degree, with relatively higher power in odd degrees than in even degrees. We suggest that long-term spatial characteristics of the observed field may emerge on averaging times that are within reach for the next generation of global time-varying paleomagnetic field models.

Inner core translation and the hemispheric balance of the geomagnetic field

“…4), and we find that if we consider the progressive running average of the hemispheric measures it takes approximately 25,000 model years to accurately determine the final time-averaged field morphology. This is similar to the timescale found by Davies and Constable (2014) for the non-zonal components of the field to converge on their long-term averages.…”

“…Use of the advective scaling would result in time series of approximately 40,000 model years and does not significantly alter the discussion below. Further discussion of the merits of the two scalings can be found in, for example, Davies and Constable (2014), . After dimensionalisation the simulation time series are divided into consecutive windows lasting 400 model years for comparison to gufm1 (Jackson et al, 2000).…”

GEOMAGIA50.v3: 1. general structure and modifications to the archeological and volcanic database

“…As global field models are constructed to recreate the field at the core-mantle boundary, they have the potential to be used to understand the geodynamo and have been used to investigate core flow (Dumberry and Finlay 2007;Wardinski and Korte 2008;Livermore et al 2014), with possible implications for length of day variations on millennial time scales (Dumberry and Bloxham 2006); the behavior of high latitude flux patches (Korte and Holme 2010;Amit et al 2011); hemispheric field asymmetries related to archeomagnetic jerks (Gallet et al 2009); discrete scale invariance across geodynamo time scales (Jonkers 2007); and similarities with the characteristics of dynamo simulations (Christensen et al 2011;Heimpel and Evans 2013;Davies and Constable 2014). Calculations of dipole eccentricity using CALS3k.4b and CALS10k.1b , coupled with observations of hemispherical variations in seismic velocity at the top of the Earth's inner core, motivated Olson and Deguen (2012) to investigate persistent eccentricity in numerical dynamo simulations and they suggested lopsided solidification within the inner core.…”

Testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka

“…We used a compilation of 86 sediment RPI records (Tauxe and Yamazaki, 2007), and absolute paleointensity data from the 2014.01 version of the PINT database (Biggin and Paterson, 2014) and the Geomagia50.v2 database (Donadini et al, 2009). The RPI compilation was previously used to construct the PADM2M model , which spans 2 million years.…”