Solving the Sun’s Heating Problem

Overview

Instead of growing colder at higher regions of its atmosphere, the Sun becomes unexpectedly hotter, soaring to a temperature of millions of degrees Kelvin in the low corona just above the photosphere at 5,780 degrees Kelvin. Heat should not emanate from a cold object to a hotter one anymore than water should flow up hill. The first plausible explanations for this heating paradox were in terms of either sound waves or magnetic waves, generated by motions in the turbulent convective zone. Spacecraft measurements in 1978, however, showed that although the lower chromosphere is probably heated by the dissipation of sound waves, there is not enough energy left over to heat the overlying corona by any substantial amount. Magnetic waves were also detected long ago, moving in the solar wind, but they seemed to travel through the corona without depositing significant amounts of energy into it.

Coronal heating is usually greatest where the magnetic fields are strongest. The Skylab, Yohkoh and SOHO spacecraft have demonstrated that the hottest, densest material in the low corona, with the most intense X-ray and extreme ultraviolet emission, is concentrated within thin, long and strongly magnetized loops located in solar active regions. Studies of loop physical parameters suggest that they are energized by stressed magnetism.

Most of the material we can see in the transition region, between the chromosphere and corona, is falling down into the Sun instead of moving out into the overlying corona, further complicating the heating problem.

Continued dynamic activity and forced magnetic connections are nevertheless ubiquitous features throughout the low solar corona. Magnetic concentrations merge together and cancel all the time and all over the Sun, providing a plausible explanation for heating the low corona outside active regions. Evidence for coronal heating by ongoing magnetic reconnection is obtained from rapid jets, X-ray bright points, ultraviolet blinkers, numerous low-level flares, the continual replenishment of the photosphere’s small-scale magnetism every 40 hours, and the uniform heating of the diffuse corona.