Evolution of the solar Ly-$\alpha$ line profile during the solar cycle. II. How accurate is the present radiation pressure paradigm for interstellar neutral H in the heliosphere?

TL;DR

This study compares two models of solar Lyman-alpha line evolution to assess their impact on interstellar neutral hydrogen distribution in the heliosphere, revealing that current radiation pressure models may need further revision for consistency.

Contribution

It evaluates the effects of a new, more accurate solar Lyman-alpha line model on ISN H distribution, highlighting potential inaccuracies in existing radiation pressure paradigms.

Findings

IKL18 model predicts higher ISN H densities during most of the solar cycle.

The ISN H density at the termination shock remains consistent with previous estimates.

A significant absorption of solar Lyman-alpha flux inside the heliosphere suggests the radiation pressure model needs revision.

Abstract

Following the derivation of a more accurate model of the evolution of the solar Lyman-$\alpha$ line with the changing solar activity by Kowalska-leszczynska et al. 2018 (IKL18) than the formerly used model by Tarnopolski et al. 2009 (ST09), we investigate potential consequences that adoption of the resulting refined model of radiation pressure has for the model distribution of interstellar neutral (ISN) H in the inner heliosphere and on the interpretation of selected observations. We simulated the ISN H densities using the two alternative radiation pressure models and identical models of all other factors affecting the ISN H distribution. We found that during most of the solar cycle, the IKL18 model predicts larger densities of ISN H and PUIs than ST09 in the inner heliosphere, especially in the downwind hemisphere. However, the density of ISN H at the termination shock estimated by Bzowski et al. 2008 obtained using ST09 does not need revision, and the detection of ISN D by IBEX is supported. However, we point out the existence of a considerable absorption of a portion of the solar Lyman-$\alpha$ spectral flux inside the heliosphere. Therefore, the model of radiation pressure for ISN H is still likely to need revision, and hence the available models of ISN H are not self-consistent.