The galactic cosmic rays are the main source of ionization in the troposphere of the Earth. Solar energetic particles of MeV energies cause an excess of ionization in the atmosphere, specifically over polar caps. The ionization effect during the major ground level enhancement 69 on January 20, 2005 is studied at various time scales. The estimation of ion rate is based on a recent numerical model for cosmic-ray-induced ionization. The ionization effect in the Earth atmosphere is obtained on the basis of solar proton energy spectra, reconstructed from GOES 11 measurements and subsequent full Monte Carlo simulation of cosmic-ray-induced atmospheric cascade. The evolution of atmospheric cascade is performed with CORSIKA 6.990 code using FLUKA 2011 and QGSJET II hadron interaction models. The atmospheric ion rate is explicitly obtained for various latitudes, namely, 40°N, 60°N and 80°N. The time evolution of obtained ion rates is presented. The short- and medium-term ionization effect is compared with the average effect due to galactic cosmic rays. It is demonstrated that ionization effect is significant only in subpolar and polar atmosphere during the major ground level enhancement of January 20, 2005. It is negative in troposphere at midlatitude, because of the accompanying Forbush effect. 1. Introduction Cosmic rays are high, ultrahigh, and extremely high energy particles of extraterrestrial origin, mostly protons. Cosmic rays (CRs) constantly impinge the Earth's atmosphere. While the low-energy particles are absorbed in the atmosphere, those with energies greater than 1？GeV/nucleon generate new particles through interactions with atomic nuclei. They are an important source of ionization in the Earth atmosphere [1]. The ionization in the stratosphere and troposphere is governed by galactic cosmic rays [2]. They initiate a complicated nuclear-electromagnetic-muon cascade resulting in an ionization of the ambient air. In such a cascade a small fraction of the initial primary particle energy reaches the ground as high energy secondary particles. Most of the primary energy is released in the atmosphere by ionization and excitation of the air molecules, resulting in an ionization of the ambient air. The maximum in secondary particle energy release is observed at altitudes of 15–26？km depending on latitude and solar activity level. This is the Pfotzer maximum. The galactic cosmic ray (GCR) is affected by solar activity. They follow the 11-year solar cycle and respond to long and short time scale solar-wind variations. They are modulated with the opposite phase,
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