Cosmic-ray induced ionization in the lower atmosphere (below ~ 15 km) is mainly caused by Galactic cosmic rays. However, some strong solar events can produce high energy ions with large intensity, which can penetrate to the lower atmosphere and produce significant ionization. We have modeled this effect for the solar storm of 20 January 2005, one of the strongest solar particle events ever recorded. This event produced energetic particles that can be detected by ground-based detectors, especially near the polar regions. We use the cosmic ray intensity vs. time as inferred from two neutron monitors, one located near the north pole (Inuvik) and the other located near the south pole (McMurdo). Then we performed Monte Carlo simulations of particle-air interactions using a realistic atmospheric model created from measured meteorological data to calculate atmospheric ionization at different altitudes and times for Inuvik and McMurdo. For the case of galactic cosmic rays, our simulation results of atmospheric ionization are consistent with balloon measurements. We have also studied the equivalent dose rate at airplane altitude. This research project is supported by Mahidol University and the Thailand Research Fund.