The phrase "downward flux" refers to the movement or flow of something from a higher position or level to a lower one. It suggests that something is moving or going down. Full definition
The increase in downward flux, F ↓, is matched by the decrease in upward flux, F ↑. (scienceofdoom.com)
Equation (2) just tells us that ΔF and α ΔT have opposite intrinsic directions, ie, while ΔF reffers to net downward flux, α ΔT refers to net upward flux (as emphasized above). (skepticalscience.com)
However, only (more or less) downward fluxes do anything to counteract any convective transport that may be occurring. (hubpages.com)