  The possible effect of solar variability, accordingly cosmic rays variation
on atmospheric physics and chemistry is highly debated over the last years.
According to several recent models the induced by cosmic rays atmospheric
ionization plays a key role in several different processes. At recent, an
apparent effect on minor constituents and aerosols over polar regions during
major solar proton events was observed. The ion production rate during ground
level enhancements is a superposition of the contribution of cosmic rays with
galactic and solar origin. The solar cycle 23 provided several strong ground
level enhancements. The period of end October - beginning of November 2003 was
characterized by a strong cosmic ray variability, namely a sequence of three
GLEs was observed. In addition, there were several Forbush decreases, which led
to a suppression of galactic cosmic ray flux. As a consequence the cosmic ray
induced ion production in the atmosphere and the corresponding ionization
effect were subject of dynamical changes. Here we compute the complex ion
production due to cosmic rays during the Halloween sequence of ground level
enhancements on October-November 2003 and we estimate the ionization effect.
The spectral and angular characteristics of the solar protons are explicitly
considered throughout the events as well their time evolution. The ionization
effect during the period is computed at several altitudes above the sea level
in a region with $R_{c}$ $\le$ 1 GV and $R_{c}$ $\le$ 2 GV.
