Abstract
AbstractGeomagnetically trapped protons forming Earth's proton radiation belt pose a hazard to orbiting spacecraft. In particular, solar cell degradation is caused by non‐ionising collisions with protons at energies of several megaelectron volts (MeV), which can shorten mission lifespan. Dynamic enhancements in trapped proton flux following solar energetic particle events have been observed to last several months, and there is a strong need for physics‐based modeling to predict the impact on spacecraft. However, modeling proton belt variability at this energy is challenging because radial diffusion coefficients are not well constrained. We address this by using the British Antarctic Survey proton belt model BAS‐PRO to perform 3D simulations of the proton belt in the region 1.15 ≤ L ≤ 2 from 2014 to 2018. The model is driven by measurements from the Radiation Belt Storm Probes Ion Composition Experiment and Magnetic Electron Ion Spectrometer instruments carried by the Van Allen Probe satellites. To investigate sensitivity, simulations are repeated for three different sets of proton radial diffusion coefficients DLL taken from previous literature. Comparing the time evolution of each result, we find that solar cycle variability can drive up to a ∼75% increase in 7.5 MeV flux at L = 1.3 over four years due to the increased importance of collisional loss at low energies. We also show how the anisotropy of proton pitch angle distributions varies with L and energy, depending on DLL. However we find that phase space density can vary by three orders of magnitude at L = 1.4 and μ = 20 MeV/G due to uncertainty in DLL, highlighting the need to better constrain proton DLL at low energy.
Highlights
The proton radiation belt is formed by protons with energies from several hundred kiloelectron volts up to hundreds of megaelectron volts (MeV) that orbit Earth along closed drift shells under the influence of the geomagnetic field
We introduce the physics-based British Antarctic Survey proton belt model BAS-PRO, and use it to investigate the variability in ∼MeV proton phase space density at 1.15 ≤ L ≤ 2 as a function of the three adiabatic invariants μ, K and L
The differences in DLL between runs affects the region of time variability, with phase space density at L ∼ 1.5 staying relatively constant over the four years during the SA19 run, but increasing by a factor of ∼2 at 20 MeV/G in the S16 run
Summary
The proton radiation belt is formed by protons with energies from several hundred kiloelectron volts (keV) up to hundreds of megaelectron volts (MeV) that orbit Earth along closed drift shells under the influence of the geomagnetic field. The three adiabatic invariants associated with gyration, bounce and drift motion are stable for a trapped particle, but may be altered by a variety of processes leading to a finite lifetime. The proton belt at L ≲ 1.5 tends to be shielded from time-variation of the geomagnetic field and so exhibits variability over long timescales of years to decades (Selesnick & Albert, 2019). At L ∼ 2, variation in MeV flux nominally occurs on timescales of a year or so (Albert & Ginet, 1998), and at L ≳ 2 the proton belt may exhibit rapid variability due to geomagnetic disturbances. The proton belt is a challenging environment to model due to complex processes with a wide range of timescales applying over different coordinates of the trapped population
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