The geomagnetic field over the past 5 million years

Abstract

S U M M A R YThe modern geomagnetic field is usually expressed as a spherical harmonic expansion. Although the palaeomagnetic record is very incomplete in both space and time, sufficient data are available from a span of ages to generate time-averaged spherical harmonic field models with many degrees of freedom. Here three data sets are considered: directional measurements from lavas, inclination measurements from ocean sediments, and intensity measurements from lavas. Individual data are analysed, as well as siteaverages, using the same methods that have been developed for the modern field, to give models for the past 5 Myr. The normal-polarity field model has an axial-dipole intensity similar to that of the modern-day field, whilst the equatorial-dipole component is very much smaller. The field is not axisymmetric, but shows flux concentrations at the core's surface under Canada and Siberia similar to those observed in the field over historical timescales. Tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples. The reverse-polarity field model does not show such obvious features, but this may be due to the sparsity of the data.The patterns observed in the normal-polarity field, with persistent features in the northern hemisphere and a smooth southern hemisphere, could be explained if the present pattern of secular variation is typical of the past several million years. This would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation, with relatively little change over quieter regions. However, we have been unable to find any evidence for a geographical pattern of secular variation in the data.

Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

“…The longitudes of the flux patches and the large seismic shear velocity structures seem to be correlated (Gubbins 2003), although some patches exhibit significant mobility (Amit et al 2011b;Korte and Holme 2010). The time-average radial field of the dynamo model of Aubert et al (2008) decently reproduces the longitudes of the flux patches in the paleomagnetic field (Kelly and Gubbins 1997). However, a shift between the locations of the large heat flux structures and the patches was observed, with the shift value depending on the dynamo internal parameters (Aubert et al 2007;Olson and Christensen 2002;Takahashi et al 2008).…”

“…Following Aubert et al (2008), Amit and Choblet (2009) compared three long-term time-average properties of their dynamo models with relevant observations: the radial magnetic field on the outer boundary with the timeaverage paleomagnetic field over the past 5 Myr (Kelly and Gubbins 1997), the flow at the top of the shell with the core flow model of Amit and Christensen (2008) obtained from inversions of the historical geomagnetic secular variation (Jackson et al 2000) and averaged over the period , and the inner boundary buoyancy flux with seismic anomalies of the upper inner core (Tanaka and Hamaguchi 1997). As an example, we highlight here the latter comparison (Fig.…”

“…This suggests that accounting for post-perovskite improves the agreement of the dynamo models with the order 1 dominance in seismic properties of the upper inner core (Tanaka and Hamaguchi 1997), although the phase may vary substantially from one dynamo model to another. Amit and Choblet (2009) argued that accounting for post-perovskite also improves the recovery of the Atlantic/Pacific hemispheric dichotomy in core flow activity (Amit and Christensen 2008;Gillet et al 2009) and the single intense paleomagnetic flux patch in the southern hemisphere (Kelly and Gubbins 1997). Next, narrow features that are not captured by global tomography models were considered.…”

Theory and Modeling of Planetary Dynamos

“…They show up in paleomagnetic field models covering up to 5 Myr (Gubbins and Kelly 1993;Kelly and Gubbins 1997;Johnson and Constable 1995;Carlut and Courtillot 1998;Johnson et al 2003) and can also be identified in archeomagnetic (Korte and Constable 2006) and historic (Jackson et al 2000;Bloxham 2002) data (see Fig. 7).…”

Expedition 306 summary

“…If this is correct, paleointensity records from cores with sedimentation rates less than ~10 cm/k.y. are unlikely to record anything but the axial dipole Gubbins, 1997;Johnson and Constable, 1997;Carlut and Courtillot, 1998). Refinement of time-averaged field models as the paleomagnetic database is augmented will lead to a better grasp of how the nonzonal terms in the time-averaged field may influence paleointensity records.…”