Summary

Description

<p> 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> <p> 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> <p> 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 middle of the second  year (Spring 2012). </p> <p> FOXSI will be a pathfinder for the future generation of HXR solar spectroscopic  imagers. The NASA roadmap for Heliophysics 2009 promotes two future missions: the Solar  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  sensitive than RHESSI at 10 keV (about 200 times at 50 keV), will have a dynamic range of several  hundred, spatial resolution around 7 arcsec, and an excellent spectral resolution of <1 keV (Sec- tion 1.4). An instrument with this kind of sensitivity and dynamic range will be able to  image where electrons are accelerated, along which field line they travel away from the  acceleration site, where they are stopped, and how some electrons escape into interplanetary  space. Simultaneously, spectroscopy will provide quantitative measurements such as  the energy spectrum, density, and energy content of the accelerated electrons. Such an instrument  will revolutionize our understand of electron acceleration in solar flares. </p>