Evolution of the Solar Lyman-Alpha line profile during the solar cycle

TL;DR

This paper presents a new model of the solar Lyman-alpha line profile evolution over the solar cycle, improving upon previous models and impacting the understanding of radiation pressure on interstellar hydrogen in the heliosphere.

Contribution

The authors developed a comprehensive, observation-based model of the solar Lyman-alpha profile that accounts for solar cycle variations, addressing limitations of earlier models.

Findings

The new model differs significantly from previous models like TB09.

It incorporates full-disk solar Lyman-alpha observations from SUMER/SOHO.

The model suggests notable impacts on interstellar hydrogen distribution predictions.

Abstract

Recent studies of interstellar neutral (ISN) hydrogen observed by the Interstellar Boundary Explorer (IBEX) suggested that the present understanding of the radiation pressure acting on hydrogen atoms in the heliosphere should be revised. There is a significant discrepancy between theoretical predictions of the ISN H signal using the currently used model of the solar Lyman-alpha profile by Tarnopolski et al. 2009 (TB09) and the signal due to ISN H observed by IBEX-Lo. We developed a new model of evolution of the solar Lyman-alpha profile that takes into account all available observations of the full-disk solar Lyman-alpha profiles from SUMER/SOHO, provided by Lemaire et al. 2015 (L15), covering practically the entire 23rd solar cycle. The model has three components that reproduce different features of the profile. The main shape of the emission line that is produced in the chromosphere is modeled by the kappa function; the central reversal due to absorption in the transition region is modeled by the Gauss function; the spectral background is represented by the linear function. The coefficients of all those components are linear functions of the line-integrated full-disk Lyman-alpha irradiance, which is the only free parameter of the model. The new model features potentially important differences in comparison with the model by TB09, which was based on a limited set of observations. This change in the understanding of radiation pressure, especially during low solar activity, may significantly affect the interstellar H and D distributions in the inner heliosphere and their derivative populations.