Second flight of the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS)

We propose to refly the NASA Gamma-Ray Imager/Polarimeter for Solar flares(GRIPS) high altitude balloon payload, the next-generation instrument for high-energy solar observations. 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. The key new technology, developed at our institution, was that of a 3D position-sensitive germanium detector (3D-GeD) that allows the position and energy deposition of every photon interaction to be recorded individually, even within the same detector. With the 3D-GeDs, GRIPS uses a new single-grid imaging system, called the Multi-Pitch Rotating Modulator (MPRM), which provides a near-ideal point response function with twice the throughput per detector area compared to the rotating modulation collimator (RMC) imaging of RHESSI. In addition, since Compton scattering (the dominant interaction in germanium at >150 keV) of the incident photons can be tracked by the 3D-GeDs, much of the background can be rejected for spectroscopy and imaging, and the polarization of photons can be determined.

GRIPS consists of a Spectrometer/Polarimeter which houses the 3D-GeDs, together with a single rotating MPRM grid placed on a boom 8 meters away, providing a full-Sun field-of-view and angular resolution (12.5-arcsec FWHM) sufficient to separate the footpoints of 2.2 MeV sources for almost all flares. The spacecraft-borne version of GRIPS is envisioned to have sixty-four 3D-GeDs, with a single grid ~20 meters away, in a spinning spacecraft. Such a space GRIPS would provide a tremendous leap forward for high-energy solar physics, with order of magnitude increases in effective area for gamma-ray line imaging, and much better angular resolution and image quality. A space GRIPS would allow the 0.511 MeV positron-annihilation line and the weaker prompt nuclear de-excitation lines to be imaged for the first time. Together with line shifts and shapes from the simultaneous high-resolution spectroscopy, such an instrument will provide powerful new diagnostics of the flare energy release and particle acceleration process. Furthermore, precise polarization measurements would be obtained for the first time to constrain models and determine beaming of energetic electrons.

"GRIPS-1" successfully flew in January 2016 from Antarctica. Unfortunately, no flare with emission above the atmospheric cutoff occurred during that interval. With the next solar maximum expected to peak around 2025, the time is ripe to propose GRIPS for a second long-duration balloon flight (LDBF), providing a high likelihood of catching a flare large enough for gamma-ray line imaging and precise polarization measurement, and a quasi-certainty of imaging a flare in hard X-rays. We propose to rebuild the gondola/boom that had to be left in Antarctica, replace and add GeDs and associated electronics, add a noise-dampening active balancer to the cryocooler, and add coarse sun sensors for easier initial solar acquisition.

GRIPS directly addresses key elements in NASA’s Science Goals for Heliophysics: "Explore the physical processes at work in the space environment from the sun to Earth and throughout the solar system". In particular, flare ions are difficult to observe but are likely an essential component to a complete theory of flare particle energization.

The Space Sciences Laboratory (SSL) at UC Berkeley (UCB) has a long tradition of training world-class graduate students in experimental solar and space physics and astrophysics. The development and flight of balloon missions is a key to this training, allowing students to get hands-on experience with every development stage on every hardware system that goes into flying a space mission, from concept, instrument development and calibration, through power, aspect, pointing, telemetry,commanding, ground-support system, and finally scientific analysis and dissemination.

Enabling lower cost innovative remote sensing instrument development from concept through breadboard and demonstration

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