  The atmospheric response for MeV gamma rays (~ 0.1 - 10 MeV) can be
characterized in terms of two observed components. The first component is due
to photons that reach the detector without scattering. The second component is
due to photons that reach the detector after scattering one or more times.
While the former can be determined in a straightforward manner, the latter is
much more complex to quantify, as it requires tracking the transport of all
source photons that are incident on Earth's atmosphere. The scattered component
can cause a significant energy-dependent distortion in the measured spectrum,
which is important to account for when making balloon-borne observations. In
this work we simulate the full response for gamma-ray transport in the
atmosphere. We find that the scattered component becomes increasingly more
significant towards lower energies, and at 0.1 MeV it may increase the measured
flux by as much as a factor of ~2-4, depending on the photon index and off-axis
angle of the source. This is particularly important for diffuse sources,
whereas the effect from scattering can be significantly reduced for point
sources observed with an imaging telescope.
