Earth's magnetic field is probably not reversing

Thèmes de recherche:
Année:
2018
Auteurs:
  • Maxwell Brown
  • Monika Korte
  • Richard Holme
  • Ingo Wardinski
  • Sydney Gunnarson
Journal:
Proceedings of the National Academy of Sciences
Volume:
115
Numéro:
20
Pages:
5111-5116
ISSN:
0027-8424
BibTex:
Résumé:
Earth's magnetic field is generated in Earth's convecting liquid iron outer core and protects Earth's surface from harmful solar radiation. The field has varied on different timescales throughout geological history, and these variations reflect changes deep within the Earth. Two of the field's most extreme variations are reversals and excursions. During such events, the strength of the field decreases and the magnetic poles rapidly flip polarity, with reversals characterized by the pole retaining an opposite polarity, while excursions are marked by a return to the original polarity. Field strength over the past centuries has also been decreasing strongly; however, through analyzing previous excursions, we infer that Earth's magnetic field is not in an early stage of a reversal or excursion.The geomagnetic field has been decaying at a rate of \~{}5\% per century from at least 1840, with indirect observations suggesting a decay since 1600 or even earlier. This has led to the assertion that the geomagnetic field may be undergoing a reversal or an excursion. We have derived a model of the geomagnetic field spanning 30{\textendash}50 ka, constructed to study the behavior of the two most recent excursions: the Laschamp and Mono Lake, centered at 41 and 34 ka, respectively. Here, we show that neither excursion demonstrates field evolution similar to current changes in the geomagnetic field. At earlier times, centered at 49 and 46 ka, the field is comparable to today's field, with an intensity structure similar to today's South Atlantic Anomaly (SAA); however, neither of these SAA-like fields develop into an excursion or reversal. This suggests that the current weakened field will also recover without an extreme event such as an excursion or reversal. The SAA-like field structure at 46 ka appears to be coeval with published increases in geomagnetically modulated beryllium and chlorine nuclide production, despite the global dipole field not weakening significantly in our model during this time. This agreement suggests a greater complexity in the relationship between cosmogenic nuclide production and the geomagnetic field than is commonly assumed.