It is known that the galactic cosmic rays are the main source of ionization in the Earth’s strato-
sphere and troposphere. Occasionally, with greater probability during solar maximum periods
eruptive solar processes, namely solar flares and coronal mass ejections (CMEs) lead to a pro-
duction of high energy solar energetic particles (SEPs). Special class of events, called ground
level enhancements (GLEs) can drastically enhance the atmospheric ionization. The induced by
primary cosmic ray particles ionization is important in various processes related to atmospheric
physics and chemistry, specifically the minor constituents. During the Solar cycle 23 (1996 May
– 2008 January) we observed several strong GLEs, namely the Bastille day event on 14 July 2000
(GLE 59), the Easter event on 15 April 2001 (GLE 60), October–November 2003 Halloween
events (GLEs 65, 66 and 67), mid–January 2005 (GLE 68 and 69) and the last event from the
cycle on 13 December 2006 (GLE 70) (gle.oulu.fi). Here we study the long term ionization
effect of two of the mentioned events (specifically GLE 59 and GLE 70) and perform a compar-
ative analysis, studying the ionization in the tropospheric region. We use a full Monte Carlo 3-D
model and compute the cosmic ray induced ionization as a function of the altitude above the sea
level. Ionization maps of several altitudes in the troposphere and stratosphere are presented. The
investigated ionization effects are much more essential at the altitudes above 12 km a.s.l., and
especially in the Regener-Pfotzer maximum (15–20 km) and marginal at altitudes below 8 km.