Waving goodbye to a standard model

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Number: 68
1st Author: Lyndsay Fletcher
2nd Author: Hugh Hudson
Published: 4 January 2008
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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. Figure 1 below shows a clear example by way of definition.

Figure 1: A protypical impulsive phase. The spiky curve shows the hard X-ray time profile, and the dash-dot curve the soft X-rays. The hard X-rays show the timing of the energy release, and the soft X-rays reflect the absorption of some of this energy by plasma that is heated and driven up into coronal loops.

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 burst. This led to the explanation described previously in these nuggets, for instance in [44]. Figure 2 shows its essence, and one might at this point wonder how such a simple cartoon could have monopolized 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. Ultraviolet (UV) and optical (WL) continuum radiation results; these radiations are known to dominate the luminosity of a flare energy-wise.

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 Alfven waves, 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. The new maximum (see Figure 3 of the previous Nugget) brings new observations with new spacecraft (plus RHESSI) and we are eager to see observations progress in the light of this new framework.

Biographical note: Lyndsay Fletcher are RHESSI researchers at Glasgow and Berkeley, respectively.

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