Speaker
Description
The existence of linear polarization due to the impact of flare-accelerated particles with the lower atmosphere, known as impact polarization, has been a long-debated question. Although theories predict this phenomenon, convincing observational evidence has remained elusive. In the past, the H-alpha line has been used to search for impact polarization, but no convincing signature has been found (Bianda et al., 2005). We plan to revive this discussion through a combined set of flare observations with STIX and IRSOL/ZIMPOL, in the wavelength range that includes the He I D3 line at 587.6 nm and the Na I D1 and D2 lines at 589.6 nm and 589.0 nm, respectively. The He I D3 line, in particular, is a promising candidate to being sensitive to flare-accelerated particles because it forms in the upper chromosphere and it goes into emission at particle precipitation sites, such as optical continuum enhancements (e.g., Battaglia & Krucker, 2025).