Energy Deposition by Galactic Cosmic Rays and Implications for Ozone Chemistry

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, $α$, 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.

Figures (19)

• Figure 1: Monthly mean total column ozone (in Dobson Units, DU) as a function of latitude ($90^{\circ}$S--$90^{\circ}$N) and time (1979--2025), derived from the combined TOMS, OMI, and OMPS satellite records. The color scale indicates the ozone column density, with warmer colors corresponding to higher values. White cells denote missing data.
• Figure 2: Two-dimensional wireframe representation of the atmospheric geometry implemented in the Geant4 simulations. The model consists of four concentric spherical layers corresponding to the altitude ranges 15--20 km, 20--25 km, 25--30 km, and 30--35 km, surrounding the Earth's surface.
• Figure 3: Comparison of sharp and smooth geomagnetic transmission functions for Polar ($R_{\rm c} = 0.1$ GV, blue) and Equatorial ($R_{\rm c} = 15$ GV, red) scenarios. Shaded regions indicate the penumbra zones, where transmission varies continuously from 0 to 1. The smooth function suppresses particle acceptance near threshold, with the effect being most pronounced for the Polar case where the penumbra width is large relative to $R_c$.
• Figure 4: Primary CR injection characteristics comparing Polar and Equatorial geomagnetic scenarios. Left panel: Differential energy spectrum of incident CR at the injection surface ($R_{\mathrm{inj}} = 36$ km). The Polar scenario ($R_{\rm c} = 0.1$ GV, blue solid line) follows the solar-modulated power law ($\gamma = 2.7$, $\phi = 550$ MV) with all four nuclear components contributing: protons (91%), helium (8%), CNO-group (0.7%), and Si-group (0.3%). The Equatorial scenario ($R_{\rm c} = 15$ GV, red dashed line) exhibits strong geomagnetic suppression below $\sim$14 GeV. A smooth cutoff with penumbra width $\Delta R = 2.0$ GV is implemented, but the transition appears abrupt on the logarithmic scale due to the narrow energy range spanned by the penumbra ($\sim$0.11 decades). Above the cutoff, the Equatorial flux is dominated by protons ($>$99%), as the required minimum energies for heavier nuclei (He: 26.5 GeV; CNO: 92.8 GeV; Si: 185.5 GeV) exceed or approach the spectrum maximum at 28 GeV. Right panel: Normalized angular distribution of incident primaries, demonstrating uniform isotropic sampling over the inward hemisphere ($\theta \in [0^\circ, 90^\circ]$) relative to the local radial direction.
• Figure 5: Geant4 event display showing the interaction of a 20 GeV primary cosmic-ray proton with the atmospheric layers implemented in the model. The concentric white spherical shells represent the different atmospheric layers. The trajectory of the primary proton is shown as a cyan line entering from the left. From the first interaction point, numerous green tracks are visible, corresponding to photons ($\gamma$) produced in the cascade, mainly from neutral-meson decays and bremsstrahlung. Yellow tracks indicate secondary neutrons generated in hadronic interactions between the primary (and secondary) particles and atmospheric nuclei. Red tracks correspond to electrons produced by electromagnetic processes within the cascade. Grey tracks correspond to neutrinos produced in hadronic interactions and decays (for example from pion and muon decays), which traverse the geometry essentially without interacting with the medium.