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 } \leq 1 GV and R _ { c } \leq 2 GV .