  The effect of precipitating high-energy particles on atmospheric physics and
chemistry is extensively studied over the last decade. In majority of the
existing models, the precipitating particles induced ionization plays an
essential role. For such effects, it is necessary to possess enhanced increase
in ion production, specifically during the winter period. In this study, we
focus on highly penetrating particles - cosmic rays. The galactic cosmic rays
are the main source of ionization in the Earth's stratosphere and troposphere.
On the other hand, the atmospheric ionization may be significantly enhanced
during strong solar energetic particle events, mainly over the polar caps. A
specific interest is paid to the most energetic solar proton events leading to
counting rate enhancement of ground-based detectors, namely the so-called
ground level enhancements (GLEs). During solar cycle 23, several strong ground
level enhancements were observed. A sequence of three GLEs was observed in
October-November 2003, the Halloween events. Here, on the basis of 3-D Monte
Carlo model, we computed the energetic particles induced atmospheric
ionization, explicitly considering the contribution of cosmic rays with
galactic and solar origin. The ion production rates were computed as a function
of the altitude above sea level using reconstructed solar energetic particles
spectra. The 24 hours and event averaged ionization effects relative to the
average due to galactic cosmic rays were also computed.
