We study the effect of atmospheric electric fields on the radio pulse emitted
by cosmic ray air showers. Under fair weather conditions the dominant part of
the radio emission is driven by the geomagnetic field. When the shower charges
are accelerated and deflected in an electric field additional radiation is
emitted. We simulate this effect with the Monte Carlo code REAS2, using
CORSIKA-simulated showers as input. In both codes a routine has been
implemented that treats the effect of the electric field on the shower
particles. We find that the radio pulse is significantly altered in background
fields of the order of ~100 V/cm and higher. Practically, this means that air
showers passing through thunderstorms emit radio pulses that are not a reliable
measure for the shower energy. Under other weather circumstances significant
electric field effects are expected to occur rarely, but nimbostratus clouds
can harbor fields that are large enough. In general, the contribution of the
electric field to the radio pulse has polarization properties that are
different from the geomagnetic pulse. In order to filter out radio pulses that
have been affected by electric field effects, radio air shower experiments
should keep weather information and perform full polarization measurements of
the radio signal.Comment: 26 pages, 12 figures, accepted for publication in Astroparticle
Physic