Solar cycle observations of the Neon abundance in the Sun-as-a-star

TL;DR

This study investigates the variation of the Sun's coronal Neon to Oxygen ratio over the solar cycle using SDO/EVE data, finding only weak cycle dependence and values too low to resolve the solar modeling problem.

Contribution

It provides the first detailed analysis of the Ne/O ratio variation as a star over the solar cycle using EUV observations, challenging the idea that neon abundance variations can solve the solar modeling discrepancy.

Findings

Ne/O ratio shows weak anti-correlation with the solar cycle.

Ratios at solar minimum are too low to resolve the solar modeling problem.

Higher temperature measurements exhibit stronger cycle dependence.

Abstract

Properties of the Sun's interior can be determined accurately from helioseismological measurements of solar oscillations. These measurements, however, are in conflict with photospheric elemental abundances derived using 3-D hydrodynamic models of the solar atmosphere. This divergence of theory and helioseismology is known as the $"$solar modeling problem$"$. One possible solution is that the photospheric neon abundance, which is deduced indirectly by combining the coronal Ne/O ratio with the photospheric O abundance, is larger than generally accepted. There is some support for this idea from observations of cool stars. The Ne/O abundance ratio has also been found to vary with the solar cycle in the slowest solar wind streams and coronal streamers, and the variation from solar maximum to minimum in streamers ($\sim$0.1 to 0.25) is large enough to potentially bring some of the solar models into agreement with the seismic data. Here we use daily-sampled observations from the EUV Variability Experiment (EVE) on the Solar Dynamics Observatory taken in 2010--2014, to investigate whether the coronal Ne/O abundance ratio shows a variation with the solar cycle when the Sun is viewed as a star. We find only a weak dependence on, and moderate anti-correlation with, the solar cycle with the ratio measured around 0.2--0.3 MK falling from 0.17 at solar minimum to 0.11 at solar maximum. The effect is amplified at higher temperatures (0.3--0.6 MK) with a stronger anti-correlation and the ratio falling from 0.16 at solar minimum to 0.08 at solar maximum. The values we find at solar minimum are too low to solve the solar modeling problem.