WawHelioGlow: a model of the heliospheric backscatter glow. II. The helioglow buildup and the potential significance of the anisotropy in the solar EUV output

TL;DR

This paper models the helioglow caused by interstellar atoms excited by solar EUV, analyzing how solar cycle phases and EUV anisotropy influence the helioglow distribution and matching observations from SOHO/SWAN.

Contribution

The study introduces a detailed helioglow model incorporating realistic solar EUV anisotropy and compares it with observations across solar cycle phases.

Findings

Model reproduces sky distribution of helioglow

EUV anisotropy improves fit during certain solar phases

Solar wind anisotropy consistent with scintillation data

Abstract

The helioglow is a fluorescence of interstellar atoms inside the heliosphere, where they are excited by the solar EUV. Because the mean free path between collisions for the interstellar gas is comparable to the size of the heliosphere, the distribution function of this gas inside the heliosphere strongly varies in space and with time and is non-Maxwellian. Coupling between realistically modeled solar factors and the distribution function of interstellar neutral gas is accounted for in a helioglow model that we have developed. WawHelioGlow is presented in the accompanying Paper I. Here, we present the evolution of the gas density, solar illumination, helioglow source function, and other relevant parameters building up the helioglow signal for selected lines of sight observed at 1 au. We compare these elements for various phases of the solar cycle and we present the sensitivity of the results to heliolatitudinal anisotropy of the solar EUV output. We assume a realistic latitudinal anisotropy of the solar wind flux using results from analysis of interplanetary scintillations. We compare the simulated helioglow with with selected maps observed by the SOHO/SWAN instrument. We demonstrate that WawHelioGlow is able to reproduce fundamental features of the sky distribution of the helioglow. For some phases of the solar cycle, the model with an anisotropy of the solar EUV output better reproduces the observations, while for other phases no EUV anisotropy is needed. In all simulated cases, the solar wind anisotropy following insight from interplanetary scintillation measurements is present.