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Uncovering the High Energy Particles in Solar Flares

Solar flares are the most powerful explosions in the solar system. They represent the release of magnetic energy stored in the million degree solar corona. However, the processes that lead to the triggering of a flare and the production of very energetic particles is poorly understood. This is due in large part because we have been limited by technology poor sensitivity and low spatial and spectral resolution in the X-ray wavelengths of light which are emitted as flare gets underway and the particles are accelerated. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was launched in 2002 and has yielded solar flare hard X-ray spectra and images with unprecedented resolution, both spectrally and spatially. These X-rays are produced as high energy electrons collide with the solar atmosphere. Therefore, in order to study the distribution of high energy electrons produced in a solar flare, you must use some technique to uncover the electron spectrum from the X-ray spectrum. This can be done by coming up with a model about what types of electron spectra should be produced, then calculating the X-ray spectrum that would result from it and comparing it to the observations. While perfectly valid, your results are fundamentally limited by the assumptions that go into the original model. Another method is to deconvolve, or invert the X-ray spectrum to find the electron spectrum. This turns out to be an ill-posed problem mathematically for a number of reasons. Nevertheless, several methods have been developed to do this over the past few years; however, they have not been applied much to solar flare data since before RHESSI, there was not sufficient data to use. Initial application of these methods to RHESSI data showed very unexpected features in the electron spectra, leading critics to challenge the validity of the inversion techniques. This paper is a blind study of many of these techniques. Fake data with various features (or not) was produced, random noise at a level typical of RHESSI data was added to the data, and then the proponents of the different inversion methods were given this data for analysis. In all cases, the inversion techiques accurately recovered the original assumed electron spectra, with some minor differences between the techniques. This suggests that solar flare modelers will be forced to explain the unusual features observed in the electron spectra of a number of solar flares seen by RHESSI.


An artist's impression of the RHESSI satellite pointing at the Sun from Earth orbit.