Eccentric Dipole Evolution during the Last Reversal, Last Excursions, and Holocene Anomalies. Interpretation Using a 360-Dipole Ring Model

Alicia González-López,Saioa A Campuzano,Alberto Molina-Cardín,Francisco Javier Pavón-Carrasco,María Luisa Osete,Pablo Rivera

doi:10.3390/geosciences11110438

Alicia González-López, Saioa A Campuzano + Show 4 more

Open Access

https://doi.org/10.3390/geosciences11110438

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Abstract

The eccentric dipole (ED) is the next approach of the geomagnetic field after the generally used geocentric dipole. Here, we analyzed the evolution of the ED during extreme events, such as the Matuyama-Brunhes polarity transition (~780 ka), the Laschamp (~41 ka) and Mono Lake (~34 ka) excursions, and during the time of two anomalous features of the geomagnetic field observed during the Holocene: the Levantine Iron Age Anomaly (LIAA, ~1000 BC) and the South Atlantic Anomaly (SAA, analyzed from ~700 AD to present day). The analysis was carried out using the paleoreconstructions that cover the time of the mentioned events (IMMAB4, IMOLEe, LSMOD.2, SHAWQ-Iron Age, and SHAWQ2k). We found that the ED moves around the meridian plane of 0–180° during the reversal and the excursions; it moves towards the region of the LIAA; and it moves away from the SAA. To investigate what information can be extracted from its evolution, we designed a simple model based on 360-point dipoles evenly distributed in a ring close to the inner core boundary that can be reversed and their magnitude changed. We tried to reproduce with our simple model the observed evolution of the ED, and the total field energy at the Earth’s surface. We observed that the modeled ED moves away from the region where we set the dipoles to reverse. If we consider that the ring dipoles could be related to convective columns in the outer core of the Earth, our simple model would indicate the potential of the displacement of the ED to give information about the regions in the outer core where changes start for polarity transitions and for the generation of important anomalies of the geomagnetic field. According to our simple model, the regions in which the most important events of the Holocene occur, or in which the last polarity reversal or excursion begin, are related to the regions of the Core Mantle Boundary (CMB), where the heat flux is low.

Highlights

Two types of anomalous events were studied in this work: extreme events related to polarity transitions and Holocene geomagnetic field anomaTwo types of anomalous events were studied in this work: extreme events related lies
The evolution of the eccentric dipole was analyzed during different extreme events of the geomagnetic field using paleoreconstructions: the last reversal (~780 ka), last excursions (~41 ka and ~34 ka), and two important Holocene anomalies: the Levantine Iron
During the Matuyama-Brunhes transition, the evolution can be separated into two parts: before the change of polarity the eccentric dipole (ED) moves towards the Atlantic sector between 0◦ E

Summary

Introduction

The Earth’s magnetic field is generated and sustained in the outer core by the convection that drives the geodynamo. The most remarkable feature of the geomagnetic field is the occurrence of polarity reversals, which have taken place many times in the past at irregular intervals (e.g., [1]). Reversals have been observed in geodynamo simulations (e.g., [2]), but their cause has yet eluded a convincing explanation (e.g., [3]). There are anomalous events, named excursions, where the geomagnetic direction departs greatly from the Geocentric Axial Dipole (GAD), sometimes achieving the reversed direction for a short time. Excursions have been interpreted as large secular variation events, a pair of full reversals, or as aborted reversals (e.g., [4,5,6,7,8])

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