Abstract
AbstractThe 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.
Highlights
The Lyman‐alpha (Lyα; 1216 Å) line of neutral hydrogen, resulting from the 2p–1s transition, is the brightest emission line in the solar spectrum
This paper presents an overview of over 6 years of solar flare observations in Lyα emission (477 events) taken with the E‐channel of the EUVS instrument on Geostationary Operational Environmental Satellite (GOES)‐15
After removing flaring events that were affected by geocoronal absorption, this work shows that Lyα is significantly (1 − 100 × ) more energetic than the more commonly studied soft X‐ray (SXR) (1–8 Å), which are a driver of ionospheric disturbances, in agreement with an earlier study of ∼100 events by Kretzschmar (2015)
Summary
The Lyman‐alpha (Lyα; 1216 Å) line of neutral hydrogen, resulting from the 2p–1s transition, is the brightest emission line in the solar spectrum. Brekke et al (1996) reported a 6% increase in Lyα irradiance using Upper Atmosphere Research Satellite/Solar‐Stellar Irradiance Comparison Experiment (UARS/SOLSTICE), whereas Woods et al (2004) reported a 20% increase in the line core and a factor of two increase in the line wings during the famous 28 October 2003 X28 flare from a serendipitous Solar Radiation and Climate Experiment (SORCE) SOLSTICE (McClintock et al, 2005) observation Both SOLSTICE instruments capture disk‐integrated emission, their cadence is not sufficient for capturing rapid temporal variations during flares. As well as the space weather implications of changes in Lyα emission, Milligan et al (2014) showed that some 6%–8% of the energy deposited in the chromosphere by nonthermal electrons can be radiated away by the Lyα line alone (1030 erg; see Milligan et al, 2012) This single emission line becomes one of the most important observables for studies of flare energetics.
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