The density distribution and 58.4 nm radiation intensity of interstellar helium in the heliosphere: a model simulation

Research on the interstellar medium and its interaction with the solar system constitutes a significant topic in planetary and heliospheric physics. As the Sun traverses the local interstellar cloud, interstellar neutrals penetrate the heliosphere, forming the interstellar wind and scattering solar extreme ultraviolet (EUV) emission lines. The intensity of this scattered radiation serves as a key diagnostic for the characteristic parameters of the interstellar wind, which are crucial for characterizing the structure of the heliosphere and the properties of the very local interstellar medium (VLISM). Meanwhile, EUV emission is a powerful tool for studying stellar and heliospheric evolution. Due to strong absorption by the interstellar medium at EUV wavelengths, accurate modeling is essential for interpreting observations and understanding these interactions. In this study, we review classical modeling methods for the density distribution of interstellar helium atoms in the heliosphere and the corresponding intensity of the resonantly scattered 58.4 nm radiation. We establish distinct density and intensity models for different orbital positions of Earth. Our results show that when Earth enters the helium focusing cone in the downwind region, both the helium density and the 58.4 nm radiation intensity increase rapidly, with the temperature effect playing a particularly important role. The radiation intensity in the downwind direction can reach up to 170 times that in the upwind direction. For simplicity, some secondary factors such as solar line width and Doppler shift effects were omitted. This modeling work provides valuable insights into the heliosphere-VLISM interaction and large-scale heliospheric structure, and can aid in the analysis of current and future measurements of the outer heliosphere and interstellar boundary.