  The GRAPES-3 muon telescope located in Ooty, India records 4x10^9 muons
daily. These muons are produced by interaction of primary cosmic rays (PCRs) in
the atmosphere. The high statistics of muons enables GRAPES-3 to make precise
measurement of various sun-induced phenomenon including coronal mass ejections
(CME), Forbush decreases, geomagnetic storms (GMS) and atmosphere acceleration
during the overhead passage of thunderclouds. However, the understanding and
interpretation of observed data requires Monte Carlo (MC) simulation of PCRs
and subsequent development of showers in the atmosphere. CORSIKA is a standard
MC simulation code widely used for this purpose. However, these simulations are
time consuming as large number of interactions and decays need to be taken into
account at various stages of shower development from top of the atmosphere down
to ground level. Therefore, computing resources become an important
consideration particularly when billion of PCRs need to be simulated to match
the high statistical accuracy of the data. During the GRAPES-3 simulations, it
was observed that over 60% of simulated events don't really reach the Earth's
atmosphere. The geomagnetic field (GMF) creates a threshold to PCRs called
cutoff rigidity Rc, a direction dependent parameter below which PCRs can't
reach the Earth's atmosphere. However, in CORSIKA there is no provision to set
a direction dependent threshold. We have devised an efficient method that has
taken into account of this Rc dependence. A reduction by a factor ~3 in
simulation time and ~2 in output data size was achieved for GRAPES-3
simulations. This has been incorporated in CORSIKA version v75600 onwards.
Detailed implementation of this along the potential benefits are discussed in
this work.
