John Brown and the thick-target model

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Number: 44
1st Author: Lyndsay Fletcher
2nd Author: Hugh Hudson
Published: 8 January 2007
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This Nugget is to bring attention to John Brown's work in the RHESSI context, following his 60th birthday symposium on Jan 28th in Glasgow. John's actual 60th birthday on February 4th precedes by a day the 5th anniversary of the launch of RHESSI, and his body of work on hard X-rays has been fundamental in our interpretation of RHESSI X-ray data.

John (right) and his legion of students and postdocs basically defined the "thick target model" which is the current practical basis of our understanding of the impulsive phase of a solar flare. John's work thus set the main theoretical framework for understanding RHESSI's solar hard X-ray observations, just as Reuven Ramaty's extensive work did for the solar gamma-rays. He has always been skeptical of simplistic models, though, and nowadays he and others are seeking ways to go beyond the restrictions of the thick-target model (the slogan is "Death to the Thick Target!"), which we will discuss below.

People often judge our scientific achievements via their "impact" in terms of citations of papers. This is a pretty flawed measurement by any objective standard, but it is somehow sort of democratic. We illustrate this with a summary of the citation statistics of John's scientific papers. The plot below shows the citation rate, per year, of the Brown publications. The impact of the RHESSI launch (red arrow) is clear: the rate doubled!

The thick-target model - yes!

The basic premise of this model is that the observed hard X-rays result from the bremsstrahlung emitted by a beam of electrons accelerated in the corona, but then "precipitating" into the dense atmosphere where they stop collisionally (via Coulomb collisions). The term "thick target" describes just this property of collisional termination of a particle beam, as might happen in a dentist's X-ray machine. If the electrons stop completely in the target (anode, chromosphere), it is a thick target, and the resulting X-ray spectrum has a fixed dependence on the beam properties. That simplification is one of the model's virtues, permitting an easy use of inverse theory to learn about the (assumed) electron beam.

The other major strength of this model is that we understand the physics of Coulomb collisions (Coulomb developed this physics in the early 19th century). Only a small fraction of the beam energy goes into direct X-ray emission in the impulsive phase of a flare; most of the beam energy produces the powerful UV and white-light emission as well as other essentially hydrodynamical consequences. Better yet, a clever enough application of the laws of hydrodynamics and of radiative transfer theory can use this energy input to characterize the dynamic as well as the static characteristics of the target. During the impulsive phase, there is such an intense energy release locally that the solar atmosphere (the target) must change drastically, and so one cannot use a static model.

The thick-target model - no!

John never intended that the thick-target model explain all aspects of a solar flare. His style of research emphasizes carefully-abstracted mathematical representations of physical problems and his nature is to challenge all simplistic models. The thick-target model fits squarely in his mathematical mold: it ignores many interesting and even fundamental aspects of solar flares in order to deal rigorously with others. It separates the transport of the non-thermal particles from the physics of their acceleration and other broad aspects of a general theory, such as the magnetic field itself. Another very popular theoretical playground, MHD theory, has comparable limitations that we could tackle in a separate Nugget some time. The real problems of the thick-target model lie in the complicated plasma physics of the particle beam. There are complexities requiring deep thought, such as the mechanism of particle acceleration and the physics of the return current (see a previous nugget). Our solar plasma laboratory is not readily reproducible in the basement of a physics department on Earth!

One of the implications of the thick-target theory about which we - including John - are increasingly concerned, is the issue of energetics. As John and others pointed out decades ago, to make the amounts of hard X-radiation that we observe basically requires implausible efficiencies of energy conversion. As data get better and better, the footpoints of the coronal loops seem to get smaller and smaller. Thus the intensity of this acceleration, per square cm, gets greater and greater. Thus from the point of view of plasma physics, we are not talking about the acceleration of a small fraction of the coronal electrons, but the complete derangement of the plasma. At present this seems to present intractable theoretical problems. Observations to confirm the collimation of the electrons into a strong beam - for example the "dentist's mirror" work by John and Eduard Kontar - now suggest strongly that in fact there are no strong beams in the chromosphere. The thick target model may not survive the onslaught being led by its own inventor!

Biographical note: Lyndsay Fletcher and Hugh Hudson are RHESSI team members in Glasgow and Berkeley.

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