The Properties of the Solar Corona and Its Connection to the Solar Wind

The corona is a layer of hot plasma that surrounds the Sun, traces out its complex magnetic field, and ultimately expands into interplanetary space as the supersonic solar wind. Although much has been learned in recent decades from advances in observations, theory, and computer simulations, we still have not identified definitively the physical processes that heat the corona and accelerate the solar wind. In this review, we summarize these recent advances and speculate about what else is required to finally understand the fundamental physics of this complex system. Specifically: (1) We discuss recent sub-arcsecond observations of the corona, some of which appear to provide evidence for tangled and braided magnetic fields, and some of which do not. (2) We review results from three-dimensional numerical simulations that, despite limitations in dynamic range, reliably contain sufficient heating to produce and maintain the corona. (3) We provide a new tabulation of scaling relations for a number of proposed coronal heating theories that involve waves, turbulence, braiding, nanoflares, and helicity conservation. An understanding of these processes is important not only for improving our ability to forecast hazardous space-weather events, but also for establishing a baseline of knowledge about a well-resolved star that is relevant to other astrophysical systems.