  The 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 might impact atmospheric ionization rates. In this study a
model that yields atmospheric ionization for the entire globe based on an input
magnetic field is presented. Taking the time dependent paleomagnetic data as
input, a 3D time series of the atmospheric ionization rates during the reversal
is produced. We show, that 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. All atmospheric
ionization data is made available for download.
