Effects of cosmic rays on the Earth’s atmosphere

2023-2024
The Earth is drenched in a bath of radiation (highly energetic particles) that constantly interacts with its geomagnetic field and its atmosphere.

With the Atmospheric Radiation Interaction Simulator (AtRIS), we can simulate the ionisation and dose rates induced by cosmic rays in different conditions. Understanding the effect of space radiations in the atmosphere is very important due to the role they play on health and on atmospheric chemistry.

Earth’s harsh radiation environment

The space environment of the Earth appears as deceptively empty to human eyes. This region is however, populated by a plethora of particles of different kinds, origins and energy.

When they reach the top of the atmosphere, particles with high energy collide with the molecules of the atmosphere leading to ionisation processes. For very energetic particles, the collisions with atmospheric constituents trigger the development of showers of secondary particles that propagate down to the surface of the Earth.

It is important to study and accurately model the effects of space radiation on the atmosphere since they play an important role in health and in the climate system.

  • On one hand, changes in the ionisation rates can lead to a temporary change in atmospheric composition and dynamics.
  • On the other hand, increases of the radiation dose in the atmosphere caused by a strong solar event can pose significant threat to humans, especially for aircraft crew who are exposed to higher levels of radiation at commercial flight altitudes than at the surface level.

Simulation with AtRIS (Atmospheric Radiation Interaction Simulator)

To compute the ionisation and the radiation dose rate induced by energetic particles from space, we use a simulation toolkit named AtRIS or the Atmospheric Radiation Interaction Simulator (Banjac et al., 2019). This GEANT4 based tool injects particles at the top of a simulated atmosphere and computes the resulting ionisation, dose rates and the production of secondary particles. 

When we compute those quantities, the most important consideration is the shielding effect of Earth’s geomagnetic field. It prevents some of the energetic particles to reach the top of the atmosphere, especially at low latitudes. The maps shown in Figure 1 illustrate this effect very well, with low induced ionisation at low latitudes (high shielding) and high ionisation over the polar regions (no shielding at all).

Moreover, we showed that the state of the atmosphere also had an influence on the computed quantities. Indeed, changes in the atmospheric density profile lead to variations in the ionisation and dose rates, as shown in Figure 2.

At high latitudes, there is a strong seasonal variability of the ionisation, which is maximum during local winter. Although the geomagnetic field has the most impact, Figure 1 shows that the different density profiles in the two hemispheres lead to an asymmetry of the ionisation between the polar regions (Winant et al., 2023).

Future work aims to include solar protons in the simulations to study the effect of solar storms on the atmosphere as well as the pathways of the resulting secondary radiation.

 

References

  • Banjac, S., Herbst, K., & Heber, B. (2019). The Atmospheric Radiation Interaction Simulator (AtRIS): Description and Validation. Journal of Geophysical Research: Space Physics, 124(1), 50-67.
     
  • Winant, A., Pierrard, V., Botek, E., & Herbst, K. (2023). The Atmospheric Influence on Cosmic-Ray-Induced Ionization and Absorbed Dose Rates. Universe, 9(12), 502.
Cosmic rays hit molecules in the Earth’s atmosphere and splinter into showers of secondary particles
When cosmic rays hit molecules in the Earth’s atmosphere, they splinter into showers of secondary particles that lose most of their energy before they reach the ground. Credits L.Bret/Novapix.
Map of the ionisation rate
Figure 1a: Maps of the ionisation rate (in pair/cm³ s, where a pair is the combination of the resulting ion and the released electron) due to galactic cosmic rays at 12 km altitude considering the effect of the geomagnetic field and the seasonal variability of atmospheric density.
Map of the ionisation rate
Figure 1b: Maps of the ionisation rate (in pair/cm³ s, where a pair is the combination of the resulting ion and the released electron) due to galactic cosmic rays at 12 km altitude considering the effect of the geomagnetic field and the seasonal variability of atmospheric density.
Seasonal variation of the ionisation rate
Figure 2: Seasonal variation of the ionisation rate (in pair/cm³ s) due to galactic cosmic rays as a function of time (in Month) and altitude (in km). Each of the three panels correspond to a given latitude.