The FOXSI-4 Sounding Rocket

The Focusing Optics X-ray Solar Imager (FOXSI) has demonstrated the utility of direct-focusing optics in observations of the quiet sun, active regions, and microflares through three prior sounding rocket flights. High-energy X-ray instruments with direct-focusing optics are regularly used to observe astrophysical sources and are capable of measuring emission from the sun, even if they have not been optimized for solar observation. Direct-focusing optics provide greater sensitivity and improved imaging capabilities compared to indirect measurement techniques, allowing detailed measurements to be made of relatively weak and bright sources simultaneously. The FOXSI instrument leverages these strengths of direct-focusing X-ray optics for dedicated solar observation. FOXSI-4 will fly in the spring of 2024 in the first-ever solar flare sounding rocket campaign, in a launch coordinated with the Hi-C Flare and SNIFS instruments. The fourth flight of FOXSI will incorporate improved hardware: high-resolution Wolter-I optics from Marshall Space Flight Center and from Nagoya University, improved CMOS soft X-ray sensors, variable-pitch CdTe strip detectors, a Timepix detector, and microfabricated pixelated attenuators from Goddard Space Flight Center. This complement of sensors will be used in flight to probe flare particle acceleration and energy deposition throughout the solar atmosphere and into interplanetary space. The flare campaign will provide an unprecedented view of these phenomena and will be a significant step forward in the development of future high-energy X-ray solar instruments.