MAIRE: Atmospheric Model

A detailed model of the atmosphere is required in order to produce accurate secondary particle spectra as a function of altitude/depth in the atmosphere. It is worth pointing out that a full scale model of the atmosphere is required as particle decays during flight are an important process of the simulation. We have modelled the atmosphere using 310 shells, from sea-level to 100 km altitude. The parameters of the shells are summarised in the table below. The air density and composition of each shell is calculated using the MSISE90 model [1] available from NSSDC/NASA. There are small changes in the air composition as a function of altitude, so each shell is defined as a different material in the model. Also, each shell has its own temperature, which is important for low energy neutron transport, but at the moment only Geant4 can perform temperature-dependent neutron transport simulations "on-the-fly", so these data are kept in the model but have not been used in the production of the current response matrices. The ground composition is also important in determining the radiation at sea-level as well as at low altitudes. In the current model it is simply assumed to be SiO2 with a density of 1.8 g/cm2.

 

Altitudes (km)
Layer thickness (km)
50 - 100
2.5
15 - 50
0.5
5 -15
0.1
1 - 5
0.05
0 - 1
0.025

The layer thickness of the atmosphere model.

 

[1] Hedin, A. E., Extension of the MSIS Thermospheric Model into the Middle and Lower Atmosphere, J. Geophys. Res. 96, 1159, 1991. http://nssdc.gsfc.nasa.gov/space/model/atmos/msise.html