Solar radiation enters the Earth's atmosphere with a portion being scattered by clouds and aerosols.

Processing, archiving and distributing Earth science data
at the NASA Langley Research Center

What is TOA?

Top-of-the-Atmosphere (TOA) is a surface approximately 20 km above the Earth's surface.
The TOA is an ellipsoid x²/a² + y²/a² + z²/b² = 1
where a = 6408.1370 km and b = 6386.6517 km.