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

Abstract

The chromospheric Lyman-alpha line of neutral hydrogen (Lyα; 1216 Å) is the strongest emission line in the solar spectrum. Fluctuations in Lyα are known to drive changes in planetary atmospheres, although few instruments have had the ability to capture rapid Lyα 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 Extreme UltraViolet Sensor on board the 15th Geostationary Operational Environmental Satellite, which has been monitoring the full-disk solar Lyα irradiance on 10-s timescales over the course of Solar Cycle 24. The vast majority (95%) of these flares produced Lyα enhancements of 10% or less above background levels, with a maximum increase of ∼30%. The irradiance in Lyα was found to exceed that of the 1-8 Å 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 Lyα 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 Lyα was found to have a power-law slope of 2.8±0.27. We also show that increased Lyα 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.

Solar Irradiance Variability Due to Solar Flares Observed in Lyman-Alpha Emission

“…These values are in agreement with those reported by Raulin et al (2013) for several C-class flares observed by Project for On-Board Autonomy 2/Large Yield Radiometer (PROBA-2/LYRA). Most individual M-and X-class flares can be readily observed in Lyα, as shown by Milligan et al (2020), they are included here for completeness. M-class flares were measured to increase the solar irradiance by 1.5% on average, while X-class flares exhibited a 3.8% increase in Lyα above background.…”

“…During quiescent solar conditions, Lyα photons get absorbed in the D-layer of Earth's ionosphere, along with solar X-rays, although Raulin et al (2013) were unable to detect any appreciable D-layer effects during seven small-to-moderate flares observed in Lyα; these flares were found to produce Lyα enhancements < 1%. However, during an Xclass flare that exhibited a ≈30% Lyα enhancement, Milligan et al (2020) noted that the impulsive increase in Lyα emission correlated with induced currents in the E-layer due to the ionization of nitric oxide as determined from magnetometer data. The corresponding D-layer response occurred several minutes later in line with the more gradually varying soft X-rays (SXR).…”

“…Lyα is believed to be a significant radiator of flare energy, accounting for several percent of the total radiated energy in a single emission line (Rubio da Costa et al, 2009;Milligan et al, 2014). It has also recently been shown that flare-induced acoustic oscillations can be detected in disc-integrated Lyα observations, suggesting a potential diagnostic for the dissipation of mechanical energy as well (Milligan et al, 2017(Milligan et al, , 2020. The study of flares in Lyα is also crucial for the field of space weather as photons at this wavelength can induce chemical and dynamic changes in planetary atmospheres (Chubb et al, 1957;Lean, 1985;Woods et al, 1995).…”

“…The few reports of solar flare observations in Lyα that previously existed were mostly from broadband, disc-integrated irradiance measurements, and often focused on small numbers of events due to the limited duty cycles of instruments capable of observing flares at this wavelength (Kretzschmar, Dominique, and Dammasch, 2013;Raulin et al, 2013;Milligan et al, 2014;Kretzschmar, 2015;Milligan and Chamberlin, 2016;Dominique et al, 2018). However, Milligan et al (2020) recently carried out a statistical study of 477 M-and X-class flares in broadband, disc-integrated Lyα emission, and calculated the distribution of contrasts. It was found that most flares (95%) produced less than a 10% enhancement above background, with a maximum of ≈30%, although X-class flares that occurred closer to the solar limb were found to produce less of an enhancement.…”

“…The study of Milligan et al (2020) focused on larger flares as smaller GOES class events (B-and C-class) did not produce an appreciable response in Lyα above the intense background. This may be due to instrumental sensitivity, or perhaps Lyα emission from weaker events is not inherently significant.…”