Response of energetic neutral atom flux in the heliosheath to fast variations in the solar wind and its dependence on pickup ion distribution models

Context. Interstellar Boundary Explorer images have revealed a globally distributed flux of energetic neutral atoms (ENAs) at ∼0.2–6 keV while Cassini observed an ENA belt at 5–55 keV likely originating from the inner heliosheath (IHS) protons via charge exchange with penetrating interstellar neutrals. Such ENAs are considered to reflect solar wind variations to some extent. Aims. We explore ENA flux sensitivity in the IHS to solar wind changes at Carrington rotation (CR) resolution and quantify its dependence on IHS ion distributions. Methods. We utilized three models for ion distributions designed to respond to solar wind changes upstream of the termination shock (TS), from which the corresponding variations in ENA fluxes were computed. All three ion models employ a regularized kappa distribution for solar wind protons. The models differ in the treatment of transmitted pickup ions (PUIs) and reflected PUIs with different combinations of regularized kappa and filled-shell distributions. Results. Our ENA estimates reveal the potential for substantial flux change rates between adjacent CR times, often exceeding several tens of percent, a feature not recognized in previous studies. Such rapid variations in ENA flux levels exhibit a correlation with concurrent fluctuations in solar wind speed and density upstream of the TS. However, the specific characteristics of these ENA changes are contingent on the ion distribution model and the energy considered. Most notably, employing the filled-shell distribution for transmitted PUIs induces noticeable alterations in ENA flux near their cutoff energy (∼0.5–1.5 keV), responding promptly to rapid variations in solar wind bulk speed. Furthermore, the inclusion of reflected PUIs is critical in the high-energy regime (> ∼10 keV), which is typically associated with coronal hole fast streams, where ENA fluxes exhibit strong correlations with changes in the solar wind bulk speed and dynamic pressure. Conclusions. The results underscore the importance of precise PUI information in the IHS for accurate ENA estimation during swift solar wind changes.