Solar magnetohydrodynamics

The sun is a magnetohydrodynamics (MHD) system that is not well understood. It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating:

The first is wave heating, in which sound, gravitational or magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events—nanoflares.Currently, it is unclear whether waves are an efficient heating mechanism.

The field of MHD was initiated by Hannes Alfvén, for which he received the Nobel Prize in Physics in 1970. He described the class of MHD waves now known as Alfvén waves. Observations show that all waves except Alfvén waves have been found to dissipate or refract before reaching the corona. Current research focus has therefore shifted towards flare heating mechanisms.

The magnetic filament above erupted on April 19, 2010. The black “hair-like object” is a speck of dust on the CCD camera.

Credit: SDO/AIA