Review of secular variation studies in the South Atlantic Anomaly

The South Atlantic Anomaly (SAA) is one of the most remarkable structural features of the Earth's magnetic field. The region is characterized by weak magnetic field intensity. Because of this weakening, particles within the inner radiation belt can mirror at lower altitudes, consequently increasing the local particle flux. Over centuries, the SAA has displayed notable secular variation (SV) characteristics, with a consistent decline in strength, expansion in area, and an overall westward drift. Both paleomagnetic and modern magnetic data indicate that SAA is currently undergoing rapid changes, with a rapid expansion in area and a rapid decreasing in intensity. These changes are coinciding with an increasing field asymmetry. Recent observations suggest that the evolution of SAA has become increasingly anisotropic, showing two magnetic intensity minima. This may indicate that the evolution of SAA has entered a new stage, which has aroused people's attention and discussion on the long-term evolution and future of SAA. The manifestation of the SAA on the Earth’s surface corresponds to the response of an inverse flux path at the core-mantle boundary (CMB) to the radial component of the geomagnetic field, positioned approximately beneath the South Atlantic Ocean. Consequently, studying the SV of SAA can understand the details of the Earth's internal dynamics. Moreover, these SV scenarios provide some speculative predictions for the SAA. The evolutionary details of SAA not only serve as a reference for future trends but also imposes additional constraints on Earth's core dynamic model, rendering it a compelling subject for scholarly inquiry. In this paper, we review the results of paleomagnetic and modern geomagnetic models concerning the past, present, and future evolution of the SAA, while also summarizing advancements in understanding the origin of this anomaly.