| Abstrakt: |
Although a large number of magnetic declination and inclination measurements are available for the historical period from 1400 to 1900 CE, even the evolution of the axial dipole moment, the most prominent quantity of the geomagnetic field, is highly disputed for this time interval. Here, a new iterative Bayesian spherical harmonic model is constructed that combines historical (direct) and archeomagnetic or volcanic (indirect) records to better constrain the geomagnetic field evolution over this historical epoch. The resulting geomagnetic field reconstruction (BIGMUDIh.1) benefits from recently published and revised data for critical periods and regions. The highly variable data uncertainties and qualities are tackled with a weighting and selection scheme. Model uncertainties are mainly controlled by selection criteria applied to indirect intensity records and are estimated using a bootstrapping approach. A major improvement with respect to the established historical model gufm1 consists in the incorporation of archeointensities, which enables a direct estimation of the axial dipole evolution prior to 1840 CE. BIGMUDIh.1 yields a ∼2 µT (6%) decrease of the axial dipole component between 1500 and 1600 CE, followed by a relatively stable period until 1900 CE. Modeled declinations agree well with historical records and the gufm1 model derived from such records. Furthermore, the evolution of the South Atlantic Anomaly, driven by reverse flux patches (RFPs) at the core‐mantle boundary, is outlined. Periods of accelerated movement of the northern magnetic dip pole appear to be connected to the formation and changes of RFPs in the Northern Hemisphere. Plain Language Summary: The geomagnetic field, generated in the Earth's outer core, is our primary shield against energetic cosmic particles, thus protecting living organisms and human infrastructures. The Earth's magnetic field has been systematically monitored during the last ∼180 years, by a growing network of ground‐based observatories, which has been complemented by satellite recordings during the last decades. These measurements revealed significant changes in the strength and directions of the geomagnetic field. A decrease of the global field (axial dipole) strength of ∼7% has been determined since 1900 CE. This decline can be compared to more ancient geomagnetic variations derived from additional data sources. Man‐made observations of the Earth's magnetic field date back to the 15th century and mainly provide information on the angle between the geographic and magnetic North direction (declination) derived from compass measurements for navigation purposes. Knowledge about the field strength prior to first man‐made absolute intensity measurements in the 1830s can be gained from the laboratory analysis of rocks and archeological artifacts, which preserve the geomagnetic field over geological timescales. A combination of these different record types yields a decrease in the axial dipole strength of 6% during the 16th century followed by a nearly constant plateau until 1900 CE. Key Points: Development of a new geomagnetic field model for the historical period from 1400 to 1900 CEThe model evaluates and incorporates instrumental measurements and high‐quality archeointensity data and bootstraps lower quality dataAxial dipole intensity decreases between 1500 and 1600 CE and remains nearly constant between 1600 and 1900 CE [ABSTRACT FROM AUTHOR] |
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