&lt;p&gt;
	Particle acceleration in solar flares and its contribution to coronal heating are among the main&amp;nbsp; unsolved problems in heliophysics. Accelerated electrons in a plasma radiate hard X-ray (HXR)&amp;nbsp; emission through the well-known process of bremsstrahlung. HXR observations therefore are a&amp;nbsp; powerful diagnostic tool, providing quantitative measurements of flare-accelerated electrons. Since&amp;nbsp; bremsstrahlung emission depends on the density of the ambient medium, solar HXR emission is&amp;nbsp; usually brightest from below the transition region, where the density increases rapidly towards the&amp;nbsp; photosphere. Electron beams entering the chromosphere lose energy quickly through collisions and&amp;nbsp; produce relatively intense HXR emission at the footpoints of magnetic field lines. Electron beams&amp;nbsp; moving in the relatively tenuous corona suffer very few collisions, losing little energy and producing&amp;nbsp; only faint HXR emission. Present-day HXR instrumentation does not have the sensitivity to see&amp;nbsp; faint HXR emission from electrons traveling in the corona, nor the dynamic range to see such&amp;nbsp; faint emission in the presence of bright HXR footpoint emission in the chromosphere. Existing&amp;nbsp; observations therefore show us only where energetic electrons are stopped, but not where they&amp;nbsp; are accelerated, nor along what path they escape from the acceleration site. The most sensitive&amp;nbsp; solar HXR observations so far are provided by the Reuven Ramaty High Energy Spectroscopic&amp;nbsp; Imager (RHESSI) (Lin et al. 2002). These measurements are obtained with a non-focusing rotation&amp;nbsp; modulation collimator (RMC) imaging technique (Hurford et al. 2002). RMCs and other types&amp;nbsp; of non-focusing imaging, however, have intrinsically limited dynamic range and sensitivity. HXR&amp;nbsp; focusing optics can overcome both of these limitations (Section 1.2.2).&amp;nbsp;&lt;/p&gt;
&lt;p&gt;
	The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the&amp;nbsp; NASA Low Cost Access to Space (LCAS) program to test HXR focusing optics combined with&amp;nbsp; silicon strip detectors for solar observations (Krucker et al. 2009). The FOXSI program is being led&amp;nbsp; by the Space Sciences Laboratory at UC Berkeley in collaboration with the Marshall Space Flight&amp;nbsp; Center (MSFC) and the Japan Aerospace Exploration Agency (JAXA). FOXSI is on schedule&amp;nbsp; and on budget for a launch in October 2010. FOXSI will offer imaging spectroscopy and&amp;nbsp; unprecedented HXR sensitivity and dynamic range. FOXSI will be !100 times more sensitive than&amp;nbsp; RHESSI at 10 keV, and, for the first time, detect the non-thermal counterparts of quiet sun network&amp;nbsp; flares (Section 1.2.4).&amp;nbsp;&lt;/p&gt;
&lt;p&gt;
	Here we propose a continuation of the FOXSI program which includes data analysis&amp;nbsp; and a second flight with an upgraded version of FOXSI. At moderate cost, we propose to&amp;nbsp; enhance the effective area, in particular at higher energies (by a factor of !4 at 15 keV), by adding&amp;nbsp; 3 more shells to the existing 7-shell optics (see Figure 9). Furthermore, our Japanese collaborators&amp;nbsp; will provide, at no cost, newly available double-sided cadmium telluride (CdTe) detectors as&amp;nbsp; a replacement for the Si detectors to allow us to take full advantage of the effective area at higher&amp;nbsp; energies. A second flight will therefore not only allow us to continue testing HXR focusing&amp;nbsp; optics for solar observations and also test newly developed CdTe strip detectors&amp;nbsp; in flight but is also expected to provide a significant increase in scientific return. In&amp;nbsp; this two year proposal, the first year (2011) will be used to upgrade the FOXSI payload and to&amp;nbsp; analyze data from the first flight, while the second flight is planned for the middle of the second &amp;nbsp;year (Spring 2012).&amp;nbsp;&lt;/p&gt;
&lt;p&gt;
	FOXSI will be a pathfinder for the future generation of HXR solar spectroscopic&amp;nbsp; imagers. The NASA roadmap for Heliophysics 2009 promotes two future missions: the Solar&amp;nbsp; Energetic Particle Acceleration and Transport (SEPAT) mission and the Heliospheric Magnetics (HMag) misson. These include imaging spectrometers to study coronal HXR sources. Using focusing optics as in FOXSI, such a future space-based instrument will be about !1000 times more&amp;nbsp; sensitive than RHESSI at 10 keV (about 200 times at 50 keV), will have a dynamic range of several&amp;nbsp; hundred, spatial resolution around 7 arcsec, and an excellent spectral resolution of &amp;lt;1 keV (Sec- tion 1.4). An instrument with this kind of sensitivity and dynamic range will be able to&amp;nbsp; image where electrons are accelerated, along which field line they travel away from the&amp;nbsp; acceleration site, where they are stopped, and how some electrons escape into interplanetary&amp;nbsp; space. Simultaneously, spectroscopy will provide quantitative measurements such as&amp;nbsp; the energy spectrum, density, and energy content of the accelerated electrons. Such an instrument&amp;nbsp; will revolutionize our understand of electron acceleration in solar flares.&amp;nbsp;&lt;/p&gt;