The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS): proposed reflight during the next solar maximum

The Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) high altitude balloon payload was successfully flown in January 2016 from Antarctica. GRIPS provides a near-optimal combination of high-resolution imaging, spectroscopy, and polarimetry of solar-flare gamma ray/hard X-ray emissions from ~20 keV to >~10 MeV. GRIPS's goal is to address questions raised by recent solar flare observations regarding particle acceleration and energy release, such as: DO Do flare ions and electrons share a common acceleration process? What causes the spatial separation between energetic electrons producing hard X-rays and energetic ions producing gamma-ray lines? Is there significant high-energy particle acceleration at flare footpoints? How anisotropic are the relativistic electrons, and why can they dominate in the corona? Where do flare accelerated ions deposit their energy over the course of a flare? How do the compositions of accelerated and ambient material vary with space and time, and why? The spectrometer/polarimeter consists of 3D position-sensitive germanium detectors (3D-GeDs), where each energy deposition is individually recorded with an energy resolution of a few keV FWHM and a spatial resolution ~0.1 mm3. Imaging is accomplished by a multi-pitch rotating modulator (MPRM), a single 2.5-cm thick tungsten alloy slit/slat grid with pitches that range quasi-continuously from 1 to 13 mm. The MPRM is situated 8 meters from the spectrometer to provide excellent image quality and unparalleled angular resolution at gamma-ray energies (12.5 arcsec FWHM), sufficient to separate 2.2 MeV footpoint sources for almost all flares. Polarimetry is accomplished by analyzing the anisotropy of reconstructed Compton scattering in the 3D-GeDs, with an estimated minimum detectable polarization of a few percent at 150-650 keV in an X-class flare. We describe our proposed second flight with modifications to the instrument, targeting the anticipated next maximum of solar activity around 2025.