Background:
Solar flares are the most powerful explosions in the solar system, which occur when some of the Sun’s magnetic fields reconnect, releasing magnetic energy as radiation and kinetic energy. These explosions are each equivalent to more than the entire world’s nuclear arsenal going off at once. As the magnetic fields reconnect, a large fraction of the energy goes into accelerating electrons and ions, which will stream along the magnetic field to the chromosphere, and then deflect and decelerate. This deceleration emits hard X-rays (HXRs), the highest energy X-rays, around the “footprints” (where the loop meets the chromosphere) of a flare loop. Whether the electrons travel as a collimated beam (with parallel rays, spreading minimally) or in a quasi-isotropic cloud (spreading close to uniformly in all orientations) is currently unknown.

What is PADRE?
The Solar Polarization and Directivity X-Ray Experiment (PADRE) is a NASA CubeSat mission built to measure the degree of anisotropy of flare electron distributions – the amount the electron distribution varies – via two independent and complementary instruments on board. MeDDEA measures HXR directivity – the variation of X-ray intensity with viewing angle – via stereoscopic spectroscopy. Stereoscopic spectroscopy is observing a single flare simultaneously from two widely separated viewing angles and comparing cross-calibrated – calibrated based on the thermal component of the flare – spectrums of photons/second/area, which is why PADRE is working in tandem with The Spectrometer/Telescope for Imaging X-rays (STIX), orbiting the Sun. SHARP, the other instrument, measures the linear polarization of flare HXR emission by Compton scattering. That is, measuring the alignment of a plane – the electric field vector – the HXRs come in aligned to, by scattering them across 8 detectors, and watching where the scattered photons go; scattering peaks perpendicular to the incoming polarization vector.