Ca II 854.2 nm spectropolarimetry compared with ALMA and with scattering polarization theory

Ca II 854.2 nm spectropolarimetric observations of the Sun are compared with nearly simultaneous ALMA observations. These two types of chromospheric observations show rough agreement but also several notable differences. High-sensitivity (≃ 0.01%) observations reveal ubiquitous linear polarization structures across the solar disk in the core of the 854.2 nm line that are consistent with previous theoretical studies.