On the minimum of line width of the backscattered solar Ly-α profile measured by SOHO/SWAN

Advances in Space Research. 2026. Vol. 78. No. 2. P. 1702–1710.

Anastasia Titova, Izmodenov V., Sergey Korolkov

Solar Ly-alpha photons are scattered by interstellar hydrogen (H) atoms that penetrate into the heliosphere through the region of the solar wind interaction with the local interstellar medium. The spectra of scattered Ly-alpha reflect the properties of the velocity distribution of the interstellar H atoms. The spectra of backscattered Ly-alpha have been reconstructed in anti-sunward directions by Quemerais et al. (Quémerais et al., 1999, Quémerais et al., 2006a). One of the spectacular results of this reconstruction was the discovery of the minimum of the width of the backscattered spectra in the direction (nearly) perpendicular to the interstellar flow direction. Originally, the minimum was interpreted as a proof of the secondary interstellar atom presence inside the heliosphere. However, later Katushkina and Izmodenov (2011), using a two-population kinetic model of the interstellar H atoms in the heliosphere, demonstrated that the minimum cannot be produced in their quite complete model.

In this paper, we performed numerical calculations of solar backscattered Ly-alpha spectra with the same model as in Katushkina and Izmodenov (2011). A key novel aspect of the presented analyses is the procedure of the obtaining the width values. Specifically, we obtain the widths following the same approach as in the analysis of SOHO/SWAN data: the numerically computed spectra are truncated at velocities of +-30 km/s and subsequently fitted with symmetric Gaussian functions. Our results show that the minimum arises as a direct consequence of applying this procedure to spectra generated by a two-population model. When the same analysis is performed on a single-population model, no such minimum appears. Thus, the longstanding issue of providing a theoretical explanation for the observed minimum is resolved.

Keywords: heliosphere, гелиосфера, Лайман-альфа, межзвездные атомы, Interstellar hydrogen, Solar Ly-alpha radiation