Galactic cosmic rays are the main source of ionization in the Earth’s stratosphere and troposphere. However, in some cases, specifically during solar maximum periods, eruptive solar processes, such as solar flares and coronal mass ejections lead to an acceleration of high-energy solar energetic particles (SEPs). The energy of SEPs is usually tens of MeV/nucleon, occasionally exceeding 100 MeV/nucleon. Rarely, in some cases, it reaches about several GeV/nucleon. Whilethe MeV particles can be absorbed in the atmosphere, those with energy about GeV/n initiate an atmospheric cascade similarly to the galactic cosmic rays. This special class of events, called ground level enhancement can drastically enhance the atmospheric ionization. The induced by primary cosmic ray particles ionization is important for various processes related to the physics and chemistry of the atmosphere and minor constituents. During the solar cycle 23 were observed several strong ground level enhancements. Here, we study the ionization effect during the sequence of three ground level enhancements of October-November 2003, the so-called Halloween events. We apply a full Monte Carlo 3-D model for computation of the cosmic ray induced ionization as a function of the altitude above sea level and derive the corresponding ionization effect.