Abstract The strong correlation between solar wind entropy and the frozen‐in ion charge state ratio indicates that entropy variability on timescales of hours or longer is preserved from its solar source. This prompts the question: where in the solar atmosphere does this entropy variability come from, and at what height does it become preserved? By cross‐correlating solar wind proton specific entropy and properties of heavy ion distributions measured by the Advanced Composition Explorer from 1998 to 2011, we have found that entropy is not strongly correlated with all frozen‐in properties of the solar wind plasma. In particular, we find a weaker correlation on average between entropy and Fe/O, relative to the strong negative correlation between entropy and , which indicates that entropy variability continues to evolve above the chromosphere on open magnetic fields. Similarly, we find a weaker correlation between entropy and , which freezes‐in at lower heliocentric distances than , , and , indicating that entropy variability must evolve above the freeze‐in distance. Together, these results suggest that the mechanisms generating entropy variability are active low in the solar atmosphere and cease between , corresponding to the transition from a collisional to collisionless plasma regime along open magnetic fields.
Nakhleh et al. (2026) studied this question.