Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS)

1. Gamma-Ray Imager/Polarimeter for Solar Flares (GRIPS) • Fundamental Goal: • Understand energy release at the Sun and its effect on Earth and the solar system by studying particle acceleration associated with solar flares • Science Objectives: • Determine the spatial distribution of flare-accelerated ions and electrons • Determine the angular distribution of relativistic electrons • Study the trapping of relativistic electrons in flare loops • Determine the accelerated-ion spectrum and associated elemental abundances • Associated RFAs: • F2. Understand the plasma processes that accelerate and transport particles. • H1. Understand the causes and subsequent evolution of solar activity that affects Earth’s space climate and environment. • J2. Develop the capability to predict the origin and onset of solar activity and disturbances associated with potentially hazardous space weather events. Ions accelerated in solar flares produce gamma-ray lines through nuclear interactions that reflect the nature and location of their acceleration. Relativistic electrons accelerated in solar flares produce polarized bremsstrahlung emission depending on their angular distribution. Images from RHESSI (e.g. Hurford et al. 2006) indicate that accelerated ions are interacting in localized regions that are spatially shifted from the locations of accelerated-electron interactions. • Mission Implementation Description: • One spacecraft in an equatorial or near-equatorial, ~600-km low-Earth orbit • Three-axis stabilized, mass: 190 kg, power: 340 W, data: 4 GB/day • Imaging: angular resolution ranging from 5 arcseconds to hundreds of arcseconds • Polarization: ~1% minimum detectable polarization at 150–650 keV energies • Spectroscopy: 3 keV to 17 MeV, spectral resolution of 1.8 keV FWHM at 662 keV • 3D spatially resolving germanium detectors (3D-GeDs): excellent spectral resolution (see above) and spatially locates energy depositions to within <0.1 mm3 • Single-grid tungsten Multi-Pitch Rotating Modulator (MPRM): exploits detector spatial resolution to provide a point-response function virtually free from sidelobes • 20-meter extendable boom separates the MRPM and the spectrometer to provide the angular resolution (see above), with loose requirements on twist and stability • Compton scatter reconstruction techniques used to determine the polarization of incident photons and to provide background rejection • Measurement Strategy: • Resolve gamma-ray line sources and X-ray sources • Measure the polarization of electron >150 keV bremsstrahlung emission • Determine source size and shape using high-quality images • Measure the spectral parameters of the various gamma-ray lines • Perform imaging spectral polarimetry • Enabling and Enhancing Technology Development: • 3D-GeDs with 0.5-mm strip pitch — detectors with coarser pitch and the associated electronics have been built and flown on NCT balloon payloads • Multi-Pitch Rotating Modulator grid — straightforward to fabricate (minimum pitch of 1 mm), and tungsten grids have been used on RHESSI and HEIDI • 20-meter extendable boom — already been developed for other space missions