Reduced Geomagnetic Shielding During the Laschamps Excursion and Its Impact on Cosmic-Ray-Induced Atmospheric Radiation

Abstract

The Laschamps geomagnetic excursion (≈41,000 years BP) was a period of significant weakening and incomplete reversal of the Earth's magnetic field. The weakening substantially reduced geomagnetic shielding against cosmic rays (CRs), which contribute to phenomena at Earth, such as cosmogenic isotope production, and atmospheric ionisation and radiation. In this work, we expand upon previous modeling of geomagnetic shielding during excursions and provide a robust methodology for assessing the CR impacts during such an event, focusing on CR-induced atmospheric radiation. This was achieved by updating the open-source OTSO CR trajectory tool to allow for paleomagnetic field models, namely LSMOD.2, to be used as inputs to compute global apparent geomagnetic cut-off rigidities at 100-year intervals throughout the excursion. The CRAC:DOMO model was used to assess the CR-induced atmospheric radiation, and the potential impact on the aviation industry was investigated by computing the effective dose rates for two representative flights, Helsinki to New York and Helsinki to Dubai, under various conditions. Results suggest low-latitude flights, normally well shielded under modern conditions, can experience significant increases in dose rates; in contrast, some high-latitude flight routes may observe decreases in radiation exposure due to the irregular geomagnetic structure during the excursion. These findings reveal that geomagnetic excursions can greatly enhance the levels of CR-induced atmospheric radiation, with wider implications that excursion events can likewise significantly affect other CR-induced processes, such as cosmogenic isotope production and atmospheric ionization. The methodology provided here outlines a framework under which CR impacts can be assessed under non-standard geomagnetic conditions.