Solar flares are huge explosions on the Sun that release a tremendous amount of energy from the coronal magnetic field, up to 10³³ ergs, in a short time (100--1000 seconds), with much of the energy going into accelerated electrons and ions. An efficient acceleration mechanism is needed, but the details of this mechanism remain relatively unknown. A fraction of this explosive energy reaches the Earth in the form of energetic particles, producing geomagnetic storms and posing dangers to spaceborne instruments, astronauts, and Earthbound power grids. There are thus practical reasons, as well as intellectual ones, for wishing to understand this extraordinary form of energy release.

Through imaging spectroscopy of the hard X-ray (HXR) emission from solar flares, the behavior of flare-accelerated electrons can be studied. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) spacecraft launched in 2002 with the goal of better understanding flare particle acceleration. Using rotation modulation collimators, RHESSI is able to cover a wide energy range (3 keV--17 MeV) with fine angular and energy resolutions. RHESSI's success in the last 10 years in investigating the relationship between energetic electrons and ions, the nature of faint sources in the corona, the energy distribution of flares, and several other topics have significantly advanced the understanding of flares.

But along with the wealth of information revealed by RHESSI come some clear observational challenges. Very few, if any, RHESSI observations have come close to imaging the electron acceleration region itself. This is undoubtedly due to a lack of both sensitivity (HXRs from electron beams in the tenuous corona are faint) and dynamic range (HXR sources at chromospheric flare footpoints are much brighter and tend to obscure faint coronal sources). Greater sensitivity is also required to investigate the role that small flares in the quiet Sun could play in heating the corona.

The Focusing Optics X-ray Solar Imager (FOXSI) is a developing project to address these observational difficulties. FOXSI is a sounding rocket payload developed under NASA's Low Cost Access to Space program. The project spearheads a shift to using direct imaging via focusing grazing-incidence HXR optics rather than the indirect Fourier techniques used by RHESSI and its predecessors. Such optics can attain higher sensitivity since photons are focused onto a small detector volume and have significantly better dynamic range than Fourier methods do. On November 2, 2012 the FOXSI rocket payload was flown for a 6-minute observation and successfully imaged a solar flare, providing the first focused HXR spectroscopic images of the Sun above 5 keV.

The motivation, construction, testing, and flight of FOXSI will be described in this text, along with case studies on the use of FOXSI to analyze unique coronal HXR sources from two solar flares.

We investigate the injection at the Sun for three scatter-free impulsive electron events (7 August 1999, 28 June and 22 August 2000) observed from ~0.4 to 300 keV by the WIND 3DP instrument. Taking into account the interplanetary scatter-free propagation and instrumental effects, we find that the observed time profiles of electron fluxes at all energies fit well to triangular injections at the Sun with equal rise and fall times. We find two distinct injections: that of ~0.4 to 6–9 keV electrons begins 9.1 ± 4.7 min before the start of the type III radio burst and lasts for 50–300 min, while that of ~3 to 300 keV electrons starts 7.6 ± 1.3 min after the start of the type III burst and lasts for a factor of 5–10 times shorter. Electrons of the low-energy injection are likely to be the source of the type III radio bursts, and the delayed high-energy injection occurs when the associated CME passes altitudes of ~1–6 Rs. The observed electron energy spectra fit to double power-law with a downward break at ~40–50 keV, but exhibit a smooth power-law across the transition (~6–13 keV) between the two injections, suggesting the low-energy injection may provide the seed electrons for the high-energy injection.