Cosmogenic ion production rate in the high–middle–low altitude troposphere and its influence on the terrestrial cloud properties

Abstract

Cosmic-ray-induced ion production rate in the atmosphere is important to evaluate the influence of the galactic cosmic ray (GCR) modulations on cloud properties, which control the global climate on the Earth. An air shower Monte Carlo simulation was carried out to estimate the amount of cosmic-ray-produced low-energy electrons ( δ-ray) at the high–middle–low troposphere. At low altitudes, energetic knock-on electrons by gamma rays, muons, and electrons/positrons produced by a muon decay process contribute to the total amount of electrons, whereas cascade generations initiated by neutral pions play a big role at higher altitudes. The total ion production rate was estimated by considering δ-ray production, initiated by gamma rays, low-energy electrons, knock-on electrons, muons, and muon-decay positrons. Other charged components such as protons rapidly decrease with atmospheric depth, and thus they cannot produce many ion pairs at low altitudes. Calculated ion production rates within the atmospheric depth intervals d=50–100 g cm −2, 350–400 g cm −2, and 650–700 g cm −2 were n = 13.72 , 7.88, and 2.41 s −1 cm −3, respectively. Our simulation indicates that the high-energy component of the primaries controls the ion production rate at low altitudes. We found that very high-energy GCRs play an important role in the ion production rate at low altitudes. These results are consistent with the previously reported values (∼2 ion pairs s −1 cm −3 at ground level and up to ∼20–30 ion pairs s −1 cm −3 in the upper troposphere). Based on the simulation results, we estimated the influence of GCR modulation on the ion production rate during the last 11-year solar cycle. The estimated variation in the nucleation rate was ∼1.0% between the solar maximum and minimum. The simulation result may explain the observed variation of low clouds by ∼1.7%. We also discussed other possibilities to change the cosmic-ray intensity, including the galactic spiral arm crossings. We propose a possible method to find evidence of the correlation between cosmic-ray intensity and the climate change in the past by comparing cosmogenic radionuclides recorded in the seafloor to that in the glacial layer.