Numerical solutions to the transport equation of galactic
cosmic rays in the interplanetary medium have been used to
investigate the applicability of commonly used approximate
analytic solutions for electrons (10 MeV to 10 GeV). We
find that for a given cosmic-ray diffusion coefficient:
(a) the "force-field" approximation is in reasonable agreement
with the numerical solution at energies ≳ 200 MeV, but
deviates significantly at lower energies, depending on the
shape of the interstellar electron spectrum, (b) the
"diffusion-convection" approximation agrees generally with
the numerical solution within a factor of ~ 2 over the
entire energy range
