The Solar Polarization and Directivity X-Ray Experiment (PADRE) is a NASA CubeSat mission built at just under 20 x 20 x 36cm, that orbits Earth. On it live two instruments, working in tandem to provide important insights into the behavior of high-energy electrons accelerated during solar flare events, helping refine models for space weather.
SHARP (Solar Hard X-ray Polarimeter) directly measures the azimuthal scattering distribution of HXRs using a cylindrical beryllium scatterer. 8 ~1 mm-thick CdTe photon-counting detectors, instrumented with Timepix3 ASICs surround the scatterer. Photons coming in get scattered, and because the spacecraft rotates at 1RPM, each of the 8 detectors is likely to collect a full 360° of data around the scatterer. Photons are significantly more likely to scatter perpendicular to their plane of oscillation, so the incoming linear polarization of the HXRs can be determined by analyzing the photon flux over time for each detector, calibrated and displayed over each other. With enough data accumulated, if there consistently appears two bumps and two valleys (when a detector enters and exits the angles perpendicular to the plane of oscillation, labeled E) then the data would strongly suggest that flare electrons accelerate as collimated beams. If instead, no consistent and significant increase or decrease in flux exists, then an argument claiming that flare electrons accelerate in an almost-random cloud is supported.
MeDDEA (Measuring Directivity to Determine Electron Anisotropy) is composed of four flight-spare detectors from STIX (The Spectrometer/Telescope for Imaging X-rays) beneath metal filters that ensure only HXR’s reach the detectors. The data from MeDDEA and STIX are calibrated based on the thermal component of a flare, and then compared to measure the directivity of the flare.
