Speaker
Description
The study of solar flares offers a unique laboratory for investigating high-energy plasma physics and magnetic reconnection processes. A central open question in solar physics concerns the mechanisms responsible for particle acceleration during the impulsive phase of flares. While magnetic reconnection is identified as the primary energy release trigger, the specific processes accelerating electrons to non-thermal energies remain debated.
Standard diagnostics based on spectral and imaging data often face degeneracies, making it difficult to distinguish between thermal and non-thermal components or to constrain the anisotropy of the electron distribution. Hard X-ray polarimetry represents the "missing observable" capable of breaking these degeneracies. Theoretical models predict that collimated electron beams should produce a high degree of linear polarization, whereas the presence of strong magnetohydrodynamic (MHD) turbulence would tend to isotropize the electron pitch-angle distribution, significantly reducing the polarization signature of the non-thermal bremsstrahlung emission.
To address this observational gap a new generation of Hard X-ray polarimeters is under development. A Compton scattering polarimeter sensitive in the 25-100 keV energy range, currently in Phase B, is foreseen aboard the CUSP (CUbesat Solar Plarimeter) space mission funded by ASI and led by INAF-IAPS. Moreover, a new gas polarimeter effective in the 10-35 keV enegy range based on the photoelectric effect (as part of the HypeX - High Yield Polarimetry experiment in X-rays - project) is under development by INAF-IAPS in collaboration with Bonn University with the support of the Ministry of University and Research (MUR), the Ministry of Foreign Affairs and International Cooperation (MAECI) and ASI.
New missions concepts with a glance beyond the first pathfinder missions will be introduced.
References:
- Fabiani, S., et al. (2025). The CUbesat Solar Polarimeter (CUSP): mission overview II. Proceedings of SPIE.
- De Angelis, N., et al. (2025). Solar Flare Hard X-ray Polarimetry with the CUbesat Solar Polarimeter (CUSP) mission. PoS SISSA, https://pos.sissa.it/501/622/
- Fabiani, S. et al. (2025). Towards imaging-spectro-polarimetry of solar flares in the X-rays. Proceedings of SPIE.
- Jeffrey, N. L. S., et al. (2020). Probing solar flare accelerated electron distributions with prospective X-ray polarimetry missions. Astronomy & Astrophysics.
- Zharkova, V. et al. (2010), Diagnostics of energetic electrons with anisotropic distributions in solar flares I. Hard X-rays bremsstrahlung emission. A&A 512, A8
- Petrosian, V. (2012). Stochastic Acceleration by Turbulence. Space Science Reviews.
- Tsuneta, S., & Naito, T. (1998). Fermi Acceleration at the Fast Shock in a Solar Flare and the Impulsive Loop-Top Hard X-Ray Source. The Astrophysical Journal Letters.
