  The need for quantitative characterization of the solar energetic particle
(SEP) dynamics goes beyond being an academic discipline only. It has numerous
practical implications related to human activity in space. The terrestrial
magnetic field shields the International Space Station (ISS) and most uncrewed
missions from exposure to SEP radiation. However, extreme SEP events with hard
energy spectra are particularly rich in hundreds of MeV to several GeV protons
that can reach the altitudes of the Low Earth Orbit (LEO). These protons have a
high penetrating capability, thus producing significant radiation hazards for
human spaceflight. SEPs also have a significant effect on the atmosphere.
Sudden ionization of the upper atmosphere at high latitudes that occurs during
polar cap absorption (PCA) events can block high frequency (HF) communication
for hours, affecting communication with aircraft on intercontinental
high-altitude flights. Another effect of SEPs in the atmosphere is creating NOx
molecules in the upper atmosphere that can deplete the atmospheric ozone
population. The paper also presents an analysis of (1) how various pitch angle
diffusion coefficient approximations affect the properties of the simulated
SEPs population and (2) discusses how pitch angle scattering when SEPs are
beyond 1 AU affects a SEP event decay phase at the Earth's orbit.
