  Cosmic ray atmospheric showers provide an effective environment for the
production of MeV-scale dark sector particles. We show that, when available,
the resonant annihilation of positrons from the shower on atmospheric electrons
is the dominant production mechanism by more than an order of magnitude. We
provide a quantitative example based on dark photon production and update
existing constraints on a corresponding light dark matter model from kilotons
neutrino experiments and xenon-based direct detection experiments.
