It is commonly known that the Earth's atmosphere is a spherical-shell atmosphere (SSA) rather than a plane-parallel atmosphere (PPA). Thus, the light scattered by the Earth's atmosphere is governed by physics of the radiative transfer equation (RTE) with proper boundary conditions in the SSA system. In satellite and aircraft remote sensing, however, the PPA model is usually assumed to compute the lookup tables and convert the sensor-measured signals to the desired physical and optical quantities.
The PPA assumption is a simple yet very good approximation for solar and sensor zenith angles < ∼80°. Note that the solar and sensor zenith angles here are defined as a measure at the local surface. In the PPA assumption, however, the solar zenith angles ≥90° is not defined. When the solar zenith angles ≥90° in the PPA system; i.e., when the Sun is below the horizon, there will be no light scattered out to the top of the atmosphere (TOA) or at the bottom of the surface. We would experience complete darkness in this situation.