“Evolution and disappearance of the paleo-West Pacific Anomaly: Implications to the future of the South Atlantic Anomaly” by Yaochen Yue (LPG)

Palaeomagnetic and modern geomagnetic measurements indicate that the South Atlantic Anomaly (SAA) has undergone rapid changes over the past few hundred years. Its minimum intensity decreased at an average rate of 26 nT/yr, accompanied by a continuous westward drift and spatial expansion. Recently, a secondary minimum of SAA emerged near southern Africa, leading to speculation that expansion of the SAA could indicate an impending geomagnetic reversal. Here, we focus on the evolution and disappearance of another weak intensity anomaly: paleo-West Pacific Anomaly (WPA), which may have implications for the future of SAA evolution. We regard the WPA as SAA-like structure due to its weak intensity and its association with a reversed flux patch on the core-mantle boundary.

Based on models gufm1 and HistKalmag, the WPA occurred between 1600 and 1820 CE and experienced phases of rapid expansion, drift, split and diminishment. Consequently, we suggest that the observed evolutionary pattern in the WPA can serve as a reference for other weak anomalies, such as the SAA. The initial two evolutionary phases of the WPA are similar to the evolution of the SAA over the past century, and current state of the SAA may correspond to an early stage of splitting. This study provides valuable insight into the evolution of the SAA and highlights the potential utility of the WPA as an evolutionary reference for such geomagnetic phenomena.

Studies of the SAA and WPA have highlighted the importance of local minima in anomaly evolution. Here, we propose a new topological algorithm to define extrema and saddle points on magnetic intensity maps. The identification of saddle points further enables the subdivision of the SAA area into sub-regions associated with different minima. In addition, by computing partial-derivative contours, this method reveals non-dipole characteristics and hemispheric asymmetry. This approach has strong potential for advancing the study of geomagnetic weak anomaly evolution.