  The geomagnetic field (Bgeo) sets a lower cutoff rigidity (Rc) to the entry
of cosmic particles to Earth which depends on the geomagnetic activity. From
numerical simulations of the trajectory of a proton using different models for
Bgeo (performed with the MAGCOS code), we use backtracking to analyze particles
arriving at the location of two nodes of the net LAGO (Large Aperture Gamma ray
burst Observatory) that will be built in the near future: Buenos Aires and
Marambio (Antarctica), Argentina. We determine the asymptotic trajectories and
the values of Rc for different incidence directions, for each node. Simulations
were done using several models for Bgeo that emulate different geomagnetic
conditions. The presented results will help to make analysis of future
observations of the flux of cosmic rays done at these two LAGO nodes.
