http://sprg.ssl.berkeley.edu/%7Etohban/ ... icle_id=68
It seems that theory is getting more and more uncomfortably "electrical" for the mainstream. Their "little black box" seems to be giving way under further scrutiny.Introduction
Everything that is really interesting or important in a solar flare arguably happens in the impulsive phase. This is when non-thermal effects appear, when the corona gets loaded with mass to produce soft X-rays, and when the accompanying CME is accelerated most rapidly. Its effects dominate the discussion of many of the RHESSI Science Nuggets, simply because RHESSI provides such a good look at the hard X-ray and gamma-ray parts of the powerful nonthermal emissions.
The standard model
The standard model in this context is a black-box in the corona that accelerates nonthermal electrons, which stream down into the chromosphere and deposit energy there. The particle beam implied by this was inspired by radio Type III bursts. This led to the explanation described previously in these nuggets, for instance in . Figure 2 shows the essence of this model; its conceptual simplicity, plus various pieces of (ambiguous) observational support, meant that this model has occupied our best theoretical minds for almost four decades.
Figure 2: The standard model for the impulsive phase. The black box shows the coronal acceleration site (true location and nature considered to be irrelevant), and the arrows show the beam of electrons plunging Sunward.
A new scenario
The Nugget authors recently proposed a replacement model, inpired by earlier works that had been largely ignored, plus new solar observations and the realization that theories of the aurora borealis involved similar ideas. The cartoon in Figure 3 shows the idea. Instead of a black box accelerating electrons, the new idea is that large-scale restructuring of the coronal field launches a pulse of [urlhttp://en.wikipedia.org/wiki/Alfv%C3%A9n_wave]Alfven waves[/url], which transport energy via the Poynting flux S = E x B just as electromagnetic waves do. There are strong technical reasons for preferring this mode of energy transport, one of them being that the electron beam is very unlikely to be sufficiently intense (see the earlier Nugget on white-light flares).
Figure 3: The alternative model. Note the absence of a black box! The energy now arrives via the Poynting flux, as shown. The particle acceleration takes place in or near the chromosphere, where the waves damp.
This new scenario is quite different from the old one, and poses theoretical problems that require (and we hope will soon get) attention. In the meanwhile we are eager to see the new observations, especially from Hinode and STEREO that may shed more light on the particular problems here. The old thick-target model served its purpose well for three decades and, we believe, reached the end of its utility with the RHESSI and TRACE observations of the past solar maximum.