What is the layer of the Sun just above the core?

The layer of the Sun just above the core is the radiative zone. It extends from roughly 0.25 of the Sun's radius to about 0.7 solar radii, forming the thickest major layer of the Sun's interior.

Energy produced by nuclear fusion in the core moves through the radiative zone mainly as electromagnetic radiation. Photons are repeatedly absorbed and scattered by the extremely dense plasma, so their outward journey is slow and follows a zigzag path rather than a straight line. Estimates for crossing this region are commonly on the order of hundreds of thousands of years.

The radiative zone is followed by the convection zone. At that boundary, called the tachocline, the temperature gradient becomes steep enough for hot plasma to rise and cooler plasma to sink. This convection carries energy toward the visible surface, or photosphere.

The radiative zone should not be confused with the chromosphere or photosphere, which are atmospheric layers above the Sun's surface. The core itself produces most of the Sun's energy; the radiative zone transports that energy outward without significant bulk convection.