Cosmic ray induced ionization model CRAC:CRII: An extension to the upper atmosphere

A new version of the CRAC:CRII model computing ionization induced by cosmic rays in the atmosphere is presented, which is extended to the upper atmosphere and can be now applied to the entire atmosphere. The model is able to compute the ionization rate in the atmosphere at any given location and time provided the energy spectrum of incoming cosmic rays is known. It is discussed that the use of earlier models, either analytical or Monte Carlo, with the limited upper energy of 500 MeV, is well validated for the upper atmosphere (above a few g/cm(2) atmospheric depth, which corresponds to the altitude about 40 km) to study the effect of solar energetic particles but may lead to a significant underestimate of the background ionization due to galactic cosmic rays. The use of a full model accounting for the atmospheric cascade and full energy range of incoming cosmic rays, rather than earlier simplified models, is recommended to study the ionization effects of galactic cosmic rays in the upper atmosphere. On the other hand, transient strong effects of solar energetic particle events can be studied using truncated models.