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L1 data distributed prior to April 2023 attempted to rotate the telescope’s polarization coordinate frame to the solar frame; however, it only included the parallactic angle (ignoring the P-angle) and applied the rotation in the wrong direction. This led to an incorrect and time-variable polarization reference frame. The revised data products now apply the correct rotation so that the polarization coordinate frame is stable in time and consistent with canonical reference coordinates used by SDO/HMI and Hinode-SP, as shown in the figure below (courtesy of Ichimoto et al. Sol Phys 249, 233–261 (2008). https://doi.org/10.1007/s11207-008-9169-9). The figure references the solar cardinal points where N/S is aligned with the solar rotation axis. The solar rotation axis is misaligned from the geocentric celestial frame by the P angle which varies from +/- the Earth’s obliquity. Additionally, the celestial frame is rotated from the Alt/Az local frame by the parallactic angle. The two combined are sometimes referred to as the solar orientation angle.
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