ON REGIONAL AND GLOBAL ASPECTS OF ARCHEOMAGNETIC INTENSITY SPIKES

Variations of the geomagnetic field prior to direct observations can be inferred from archeo- and paleomagnetic experiments. Investigations of past secular variation are relevant towards understanding the geodynamo process in Earth's core. Apparent unusual variations not seen in the present day and historical field are of particular interest to constrain the full range core dynamics. Recently, archeomagnetic “intensity spikes'', characterised by very high field values associated with rapid secular variation rates, have been widely discussed. They have first been noted in data from the Levant at around 900~BCE and have subsequently been confirmed by data from Turkey and Georgia. The most recent assessment of the Levantine data, involving a rigorous quality assessment, interprets the observations as an extreme local geomagnetic high with at least two intensity spikes between the 11th and 8th centuries BCE (Shaar et al., 2016, Earth and Planetary Science Letters). However, studies invoking intense magnetic flux spikes at the core-mantle boundary, or core flow models under the frozen flux approximation, cannot reconcile a strongly localized surface spike with presently accepted understanding of the geodynamo process in Earth’s core. Moreover, recently reported high intensity values from other parts of the world, e.g., China and Texas, at similar times raise the question of whether such features might be common occurrences, or whether they might even be part of a global magnetic field feature.

We use spherical harmonic modelling to test two hypotheses: firstly, whether the Levantine and other potential spikes might be related to higher than previously thought dipole field intensity around 1000~BCE, and secondly, whether the observations from different parts of the world are compatible with a single drifting intense flux patch. Preliminary results indicate that a connection of the spikes to high dipole moment seems conceivable if assuming rather high dipole variation rates, but the observed spikes in China and the Levant do not seem to be caused by westward drift of an intense magnetic flux patch.