Computation of energy- and pitch-angle-dependent drift shells shapes using particle tracing in GT package

S.A. Proshin 1, V.V. Malakhov 1, A.G. Mayorov 1

1. National Research Nuclear University MEPhI, Moscow, Russia


Standard methods for calculating drift shells (IRBEM, shellg) use a simplified approach by computing the value of parameter L through tracing Earth's magnetic field lines. 
This method provides sufficient accuracy for solving many problems in the area of Earth’s radiation belts but is generally not suitable for precise reconstruction of shape 
and position of drift shells in fields with complex configurations. Furthermore, this approach does not account for dependence on particle energy and equatorial pitch angle 
in radiation belt particles. Such differences can be significant especially at the lower boundary of the inner radiation belt where flux values exhibit strong gradients across geomagnetic coordinates.

We present rigorous calculations of drift shells in an undisturbed magnetosphere computed via trajectory simulations of trapped protons within Earth's magnetic field. 
The form and location of each shell are reconstructed based on girocenter calculations from obtained trajectories according to their strict definition [1]. 
Calculations were performed using the GT environment [3] for protons with energies ranging from 10 MeV to 4 GeV under conditions of an undisturbed magnetosphere 
covering values of parameter L between 1.1 and 6. Our analysis revealed that even for minimum-energy particles, the shapes and positions of drift shells calculated 
by standard methods differ significantly compared to those derived through tracers. The effect is particularly pronounced over the South Atlantic Magnetic Anomaly 
region near the lower boundary of Earth's inner radiation belt. Values of 𝐿, as determined by these two approaches, may vary up to 0.02 Re, which is substantial 
given the sharp dependency of fluxes on this variable in this region. Additionally, we employ more accurate models such as CHAOS[2] for better representation of 
the main component of Earth's magnetic field. We also include lithospheric components modeled with LCS.

1. J. G. Roederer, «Coordinates for representing radiation
belt particle flux», Journal of Geophysical Research: Space
Physics, 123:1381, 2018.
2. Finlay et al., «The CHAOS-7 geomagnetic feld model and observed changes in the South Atlantic Anomaly», Earth, Planets and Space, 72:156, 2020.
3. https://github.com/agmayorov/GTsimulation.git