Heating solar coronal holes

Abstract

It has been shown that the coronal hole, and the associated high-speed stream in the solar wind, are pow-ered by a heat input of the order of 5 x 10 5 ergs cm -2 s -1 , with most of the heat injected in the first 1-2 R Q , and perhaps 1 x 10 5 ergs cm -2 s -1 introduced at distances of several solar radii to provide the high speed of the issuing solar wind. The traditional view has been that this energy is obtained from Alfvén waves generated in the subphotospheric convection, which dissipate as they propagate outward, converting the wave energy into heat. This paper reviews the generation of waves and the known wave dissipation mechanisms, to show that the necessary Alfvén waves are not produced under the conditions presently understood to exist in the Sun, nor would such waves dissipate significantly in the first 1-2 R ö if they existed. Wave dissipation occurs only over distances of the order of 5 R Q or more. The alternative to wave heating is the activity of the small-scale magnetic fields—the network and intranet-work fields. Existing estimates indicate an adequate energy input at the base of the coronal hole and a suffi-cient generation of Alfvén waves to account for the distant heat input. It appears, then, that the network activity is the principal energy source for the coronal hole and hence is the basis for creating the heliosphere.