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Particle acceleration in solar flares and its contribution to coronal heating are among the main unsolved problems in heliophysics. Accelerated electrons in a plasma radiate hard X-ray (HXR) emission through the well-known process of bremsstrahlung. HXR observations therefore are a powerful diagnostic tool, providing quantitative measurements of flare-accelerated electrons. Since bremsstrahlung emission depends on the density of the ambient medium, solar HXR emission is usually brightest from below the transition region, where the density increases rapidly towards the photosphere. Electron beams entering the chromosphere lose energy quickly through collisions and produce relatively intense HXR emission at the footpoints of magnetic field lines. Electron beams moving in the relatively tenuous corona suffer very few collisions, losing little energy and producing only faint HXR emission. Present-day HXR instrumentation does not have the sensitivity to see faint HXR emission from electrons traveling in the corona, nor the dynamic range to see such faint emission in the presence of bright HXR footpoint emission in the chromosphere. Existing observations therefore show us only where energetic electrons are stopped, but not where they are accelerated, nor along what path they escape from the acceleration site. The most sensitive solar HXR observations so far are provided by the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) (Lin et al. 2002). These measurements are obtained with a non-focusing rotation modulation collimator (RMC) imaging technique (Hurford et al. 2002). RMCs and other types of non-focusing imaging, however, have intrinsically limited dynamic range and sensitivity. HXR focusing optics can overcome both of these limitations (Section 1.2.2). </p>
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The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost Access to Space (LCAS) program to test HXR focusing optics combined with silicon strip detectors for solar observations (Krucker et al. 2009). The FOXSI program is being led by the Space Sciences Laboratory at UC Berkeley in collaboration with the Marshall Space Flight Center (MSFC) and the Japan Aerospace Exploration Agency (JAXA). FOXSI is on schedule and on budget for a launch in October 2010. FOXSI will offer imaging spectroscopy and unprecedented HXR sensitivity and dynamic range. FOXSI will be !100 times more sensitive than RHESSI at 10 keV, and, for the first time, detect the non-thermal counterparts of quiet sun network flares (Section 1.2.4). </p>
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Here we propose a continuation of the FOXSI program which includes data analysis and a second flight with an upgraded version of FOXSI. At moderate cost, we propose to enhance the effective area, in particular at higher energies (by a factor of !4 at 15 keV), by adding 3 more shells to the existing 7-shell optics (see Figure 9). Furthermore, our Japanese collaborators will provide, at no cost, newly available double-sided cadmium telluride (CdTe) detectors as a replacement for the Si detectors to allow us to take full advantage of the effective area at higher energies. A second flight will therefore not only allow us to continue testing HXR focusing optics for solar observations and also test newly developed CdTe strip detectors in flight but is also expected to provide a significant increase in scientific return. In this two year proposal, the first year (2011) will be used to upgrade the FOXSI payload and to analyze data from the first flight, while the second flight is planned for the midd