The Solar-Cycle Temporal Variation of the Solar Wind Charge Exchange X-ray Lines

TL;DR

This study reveals a solar-cycle related variation in X-ray emission lines caused by solar wind charge exchange, affecting measurements of the Milky Way's hot gas and providing an empirical model to correct for this contamination.

Contribution

It introduces an empirical model linking solar activity to X-ray line variations, accounting for a time lag, and improves estimates of the Milky Way's hot gas properties by removing SWCX contamination.

Findings

Detected a ~0.91-year lag between solar activity and X-ray line fluxes.

Quantified the reduction in hot gas emission measurements after SWCX correction.

Established a correlation between solar wind flux and X-ray emission lines.

Abstract

Solar wind charge exchange (SWCX) is the primary contamination to soft X-ray emission lines from the Milky Way (MW) hot gas. We report a solar-cycle ($\approx 10$ yr) temporal variation of observed \ion{O}{7} and \ion{O}{8} emission line measurements in the {\it XMM-Newton} archive, which is tightly correlated with the solar cycle traced by the sunspot number (SSN). This temporal variation is expected to be associated with the heliospheric SWCX. Another observed correlation is that higher solar wind (SW) fluxes lead to higher O VII or O VIII fluxes, which is due to the magnetospheric SWCX. We construct an empirical model to reproduce the observed correlation between the line measurements and the solar activity (i.e., the SW flux and the SSN). With this model we discovered a lag of $0.91_{-0.22}^{+0.20}$ yr between the O VII flux and the SSN. This time lag is a combination of the SW transit time within the heliosphere, the lag of the neutral gas distribution responding to solar activity, and the intrinsic lag between the SSN and the launch of a high-energy SW (i.e., $\rm O^{7+}$ and $\rm O^{8+}$). MW O VII and O VIII fluxes have mean values of 5.4 L.U. and 1.7 L.U., which are reduced by $50\%$ and $30\%$, compared to studies where the SWCX contamination is not removed. This correction also changes the determination of the density distribution and the temperature profile of the MW hot gas.