Modeling the Atmospheric Ionization Rate During a Geomagnetic Reversal

Abstract

AbstractThe Matuyama‐Brunhes reversal of Earth's magnetic dipole field took place 0.78 Ma ago, and detailed temporally resolved paleomagnetic data are available for this period. A geomagnetic reversal is expected to impact the cosmic ray flux, which in turn affects atmospheric ionization rates. To assess the magnitude of this effect, three preexisting models are used in tandem to yield 3D time series of the atmospheric ionization rates for the entire globe during the reversal. Specifically the time variable paleomagnetic field data is input into a particle trajectory calculator to produce geomagnetic cutoff rigidity maps for the entire globe. This is combined with an empirical model of the local interstellar cosmic ray spectrum and ionization yield functions based on Monte Carlo atmospheric particle simulations to produce novel time variable 3D atmospheric ionization rates during the reversal. As the dipole field weakens, the atmospheric ionization increases at low latitudes. The increase is ca. 25% at the surface and up to a factor of 5 in the upper atmosphere. Globally, ionization rates increase around 13% at the surface and up to a factor of 2 in the upper atmosphere, whereas polar regions are largely unaffected. Finally, the change in ionization due to the solar 11‐year cycle is greatly affected by the reversal. The relative change in atmospheric ionization between solar‐minimum and solar‐maximum varies between 2 and two orders of magnitude.