The Solar Atmosphere

Abstract

Looking at the solar photosphere, we see the top of the convection zone in the form of granulation: Hot gas rising from the solar interior as part of the energy transport process reaches a position where the opacity is no longer sufficient to prevent the escape of radiation. The gas expands, radiates, and cools and in so doing loses its buoyancy and descends. These motions, ultimately driven by the requirement that the energy generated by nuclear fusion in the Sun’s core be transported in the most efficient manner, represent a vast reservoir of “mechanical” energy flux. Looking closer, we see that granulation is not the only phenomenon visible at the solar surface. The quiet and semi-quiet photosphere is also threaded by magnetic fields that appear as bright points, as well as darker micro-pores and pores. These small scale magnetic structures are, while able to modify photospheric emission, subject to granular flows and seem to be passively carried by the convective motions. Convective flows are also known to generate the perturbations that drive solar oscillations. Oscillations, sound waves, with frequencies mainly in the band centered roughly at 3 mHz or 5 minutes are omnipresent in the solar photosphere and are collectively known as p-modes (‘p’ for pressure). These p-modes are a subject in their own right and studies of their properties have given solar physicists a unique tool in gathering information on solar structure — the variation of the speed of sound cs, the rotation rate, and other important quantities — at depths far below those accessible through direct observations. In this chapter we will consider them only insofar as they interact and possibly channel energy into the layers above the photosphere. The shuffling, buffeting, and braiding of magnetic structures that presumably continue on up into the upper solar layers, the propagation of the higher frequency component 1 photospheric oscillations through the chromosphere and into the corona — all may contribute to heating and thus the production