  We investigate the influence of the geomagnetic field (GF) on the Imaging Air
Cherenkov Telescope technique for two northern (Tenerife and San Pedro Martir)
and three southern (Salta, Leoncito and Namibia (the H.E.S.S.-site)) site
candidates for Cherenkov Telescope Array (CTA) observatories. We use the
CORSIKA and sim_telarray programs for Monte Carlo simulations of gamma ray
showers, hadronic background and the telescope response. We focus here on gamma
ray measurements in the low energy, sub-100 GeV, range. Therefore, we only
consider the performance of arrays of several large telescopes. Neglecting the
GF effect, we find (in agreement with previous studies) that such arrays have
lower energy thresholds, and larger collection areas below 30 GeV, when located
at higher altitudes. We point out, however, that in the considered ranges of
altitudes and magnetic field intensities, 1800-3600 m a.s.l. and 0-40 uT,
respectively, the GF effect has a similar magnitude to this altitude effect. We
provide the trigger-level performance parameters of the observatory affected by
the GF effect, in particular the collection areas, detection rates and the
energy thresholds for all five locations, which information may be useful in
the selection of sites for CTA. We also find simple scaling of these parameters
with the magnetic field strength, which can be used to assess the magnitude of
the GF effect for other sites; in this work we use them to estimate the
performance parameters for five sites: South Africa-Beaufort West, USA-Yavapai
Ranch, Namibia-Lalapanzi, Chile-La Silla and India-Hanle. We roughly
investigate the impact of the geophysical conditions on gamma/hadron separation
procedures involving image shape and direction cuts.
