  The reconstruction of very inclined air showers is a new challenge for
next-generation radio experiments such as the AugerPrime radio upgrade, BEACON,
and GRAND, which focus on the detection of ultra-high-energy particles. To
tackle this, we study the electromagnetic particle content of very inclined air
showers, which has scarcely been studied so far. Using the simulation tools
CORSIKA and CoREAS, and analytical modeling, we explore the energy range of the
particles that contribute most to the radio emission, quantify their lateral
extent, and estimate the atmospheric depth at which the radio emission is
strongest. We find that the distribution of the electromagnetic component in
very inclined air showers has characteristic features that could lead to clear
signatures in the radio signal, and hence impact the reconstruction strategies
of next-generation radio-detection experiments.
