Energy Deposition by Galactic Cosmic Rays and Implications for Ozone Chemistry

TL;DR

This study uses Monte Carlo simulations to quantify how galactic cosmic rays deposit energy in Earth's atmosphere and influence ozone chemistry through the production of reactive radicals, providing a detailed link between cosmic rays and ozone depletion.

Contribution

It introduces a comprehensive Monte Carlo framework for modeling cosmic-ray energy deposition and its impact on atmospheric ozone chemistry, bridging high-energy particle transport with photochemical processes.

Findings

96% of energy budget from secondary particles within 15-35 km altitude

Ozone decrease estimated at 0.1% to 1% due to cosmic-ray induced radicals

Production rates peak between 18 and 22 km altitude

Abstract

We present a Monte Carlo study of galactic cosmic-ray (GCR) energy deposition and its implications for stratospheric chemistry, performed with the Geant4 toolkit. Primary nuclei (protons, $\alpha$, CNO, and Si) were propagated through an atmosphere modeled from 0 to 120~g~cm$^{-2}$, considering both Polar ($R_{\mathrm{c}}=0.1$~GV) and Equatorial ($R_{\mathrm{c}}=15$~GV) geomagnetic cutoff conditions. The simulations resolve the variation of energy deposition with altitude for primary and secondary particles, revealing that $\sim$~96\% of the stratospheric energy budget arises from cascade secondaries within the 15--35~km domain. By converting layer-resolved energy deposition into ion pair production rates, we quantify the resulting formation of odd nitrogen (NO$_{\rm x}$) and odd hydrogen (HO$_{\rm x}$) radicals, which catalyze the destruction of ozone. The modeled production rates peak between 18 and 22~km altitude, leading to an estimated fractional ozone decrease of order $10^{-3}$--$10^{-2}$ under average GCR fluxes, consistent with observed background modulation over the solar cycle. These results establish a physically consistent link between cosmic-ray induced energy deposition and ozone chemistry, providing a benchmark framework for coupling high-energy particle transport to atmospheric photochemical models.