Cosmic ray modulation of infra-red radiation in the atmosphere

TL;DR

This study detects and quantifies the absorption of infra-red radiation by molecular cluster ions produced by cosmic rays in the atmosphere, revealing a small but measurable effect on Earth's radiative balance.

Contribution

First direct measurement of atmospheric IR absorption by cosmic ray-induced molecular cluster ions, linking cosmic ray activity to radiative effects.

Findings

Infra-red absorption by molecular cluster ions is approximately 7 mW/m^2.

Each cosmic ray event causes an atmospheric energy change of about 2 J/m^2.

The effect has a negligible impact on climate due to its small magnitude.

Abstract

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.