  We study the structure and reveal the physical nature of the reversed
granulation pattern in the solar photosphere by means of 3-dimensional
radiative hydrodynamics simulations. We used the MURaM code to obtain a
realistic model of the near-surface layers of the convection zone and the
photosphere. The pattern of horizontal temperature fluctuations at the base of
the photosphere consists of relatively hot granular cells bounded by the cooler
intergranular downflow network. With increasing height in the photosphere, the
amplitude of the temperature fluctuations diminishes. At a height of z=130-140
km in the photosphere, the pattern of horizontal temperature fluctuations
reverses so that granular regions become relatively cool compared to the
intergranular network. Detailed analysis of the trajectories of fluid elements
through the photosphere reveal that the motion of the fluid is non-adiabatic,
owing to strong radiative cooling when approaching the surface of optical depth
unity followed by reheating by the radiation field from below. The temperature
structure of the photosphere results from the competition between expansion of
rising fluid elements and radiative heating. The former acts to lower the
temperature of the fluid whereas the latter acts to increase it towards the
radiative equilibrium temperature with a net entropy gain. After the fluid
overturns and descends towards the convection zone, radiative energy loss again
decreases the entropy of the fluid. Radiative heating and cooling of fluid
elements that penetrate into the photosphere and overturn do not occur in equal
amounts. The imbalance in the cumulative heating and cooling of these fluid
elements is responsible for the reversal of temperature fluctuations with
respect to height in the photosphere.
