Cosmic ray modulation of infra-red radiation in the atmosphere

Cosmic rays produce molecular cluster ions as they pass through the lower atmosphere. Neutral molecular clusters such as dimers and complexes are expected to make a small contribution to the radiative balance, but atmospheric absorption by charged clusters has not hitherto been observed. In an atmospheric experiment, a thermopile filter radiometer tuned to a 9.15{\mu}m absorption band, associated with infra-red absorption of molecular cluster ions, was used to monitor changes following events identified by a cosmic ray telescope sensitive to high energy (>400MeV) particles, principally muons. The change in longwave radiation in this absorption band due to molecular cluster ions is 7 mWm^-2. The integrated atmospheric energy change for each event is 2Jm^-2, representing an amplification factor of 10^12 compared to the estimated energy density of a typical air shower. This absorption is expected to occur continuously and globally, but calculations suggest that it has only a small effect on climate.