When we observe solar flares we are observing the explosive interaction of plasma and magnetic fields. To understand flares we must understand phenomena on many scales, including the very large scale of coronal loops and the very small scale of atomic particles. To understand the rapid heating, particle acceleration, mass motions, and radiation that characterize flares, we must understand the interrelationships among these multi-scale phenomena.

In these web pages we have shown how a relatively simple model can explain the x-ray structure of a flare observed by instruments on the Yohkoh satellite. In this model we specify the large-scale structure of the flaring loop, and then use the microscopic physics of collisional energy losses and bremsstrahlung production by high-energy electrons to calculate the structure of the x-ray emission from the model loop. The results demonstrate that a compact x-ray source can be present in the corona, at the cusp or the top of the flaring loop, even when the plasma density at this location is not enhanced over the density in the rest of the loop. They also demonstrate that the x-ray structure observed with Yohkoh is obtained with a plasma density on the order of 1011 cm-3 in the loop. The model provides predictions of how the appearance of the loop should change when the x-ray energy or the plasma density in the loop is changed, and predicts detailed x-ray spectra from the loop.

This model, like all models, is neither complete nor unique. It says nothing about how the energetic electrons are accelerated, for example. It does, however, imply that most of the electrons are accelerated high in the corona, above the bright loop observed in soft x-rays and above the hard x-ray cusp. Other physically plausible models may also explain the Yohkoh observations. This is especially true in view of the fact that we do not know if the x-ray emission from the cusp is from thermal or nonthermal electrons. We have seen that this model may in fact result in enough heating of the cusp plasma to produce a significant thermal hard x-ray source there.

Models such as the one presented here arise from a combination of observational data, knowledge of the physical universe and the mathematical constructs that have been developed to describe it, and imagination. They provide a framework for making sense out of an often complex or ambiguous body of data. Their relationship to reality is only determined through a careful comparison of the models with observations, and a comparison of the success of competing models at describing the data. Any acceptable model must, of course, be self-consistent and physically plausible. Most important, these models provide direction for future observations. What should the next generation mission be for studying the tremendous release of energy in solar flares? This question is addressed on the next web page.


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