Lyman-alpha Variability During Solar Flares Over Solar Cycle 24 Using GOES-15/EUVS-E

TL;DR

This study analyzes Lyman-alpha emission variability during 477 solar flares in Solar Cycle 24, revealing that most flares cause modest enhancements, with significant implications for planetary atmospheres and ionospheric currents.

Contribution

It provides the first comprehensive statistical analysis of Lyman-alpha flare emissions using GOES-15 data over an entire solar cycle, highlighting center-to-limb effects and correlations with ionospheric responses.

Findings

95% of flares increased Lyman-alpha by 10% or less

Maximum Lyman-alpha enhancement observed was about 30%

Lyman-alpha flux can exceed X-ray irradiance by two orders of magnitude

Abstract

The chromospheric Lyman-alpha line of neutral hydrogen (\lya; 1216\AA) is the strongest emission line in the solar spectrum. Fluctuations in \lya\ are known to drive changes in planetary atmospheres, although few instruments have had the ability to capture rapid \lya\ enhancements during solar flares. In this paper we describe flare-associated emissions via a statistical study of 477 M- and X-class flares as observed by the EUV Sensor on board the 15th Geostationary Operational Environmental Satellite, which has been monitoring the full-disk solar \lya\ irradiance on 10~s timescales over the course of Solar Cycle 24. The vast majority (95\%) of these flares produced \lya\ enhancements of 10\% or less above background levels, with a maximum increase of $\sim$30\%. The irradiance in \lya\ was found to exceed that of the 1-8 \AA\ X-ray irradiance by as much as two orders of magnitude in some cases, although flares that occurred closer to the solar limb were found to exhibit less of a \lya\ enhancement. This center-to-limb variation was verified through a joint, stereoscopic observation of an X-class flare that appeared near the limb as viewed from Earth, but close to disk center as viewed by the MAVEN spacecraft in orbit around Mars. The frequency distribution of peak \lya\ was found to have a power-law slope of $2.8\pm0.27$. We also show that increased \lya\ flux is closely correlated with induced currents in the ionospheric E-layer through the detection of the solar flare effect as observed by the Kakioka magnetometer.