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

Milligan, Ryan O.

doi:10.1007/s11207-021-01796-3

Cited by 7 publications

(15 citation statements)

References 39 publications

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“…A significant contribution can be noticed from Lyα when the chromosphere is heated above 10 kK, which is possible in strong flares. A recent study of the Lyα contrasts by Milligan (2021) found that C-class flares, with some notable exceptions, produced a far smaller contrast compared to Mor X-class flares. Note, however, that those observations lacked spatial resolution and therefore don't represent the response of a single footpoint but of the global flare.…”

Section: Summary and Discussionmentioning

confidence: 96%

On the radiative losses in the chromosphere during a C-class flare

Yadav,

Rodríguez,

Kerr

et al. 2022

Preprint

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Context. Solar flares release an enormous amount of energy (∼10 32 erg) in the corona. A substantial fraction of this energy is transported to the lower atmosphere, which results in chromospheric heating. The mechanisms that transport energy to the lower solar atmosphere during a flare are still not fully understood. Aims. We aim to estimate the temporal evolution of the radiative losses in the chromosphere at the footpoints of a C-class flare, in order to set observational constraints on the electron beam parameters of a RADYN flare simulation. Methods. We estimated the radiative losses from hydrogen, and singly ionized Ca and Mg using semi-empirical model atmospheres, which were inferred from multi-line inversion of observed Stokes profiles obtained with the CRISP and CHROMIS instruments at the Swedish 1-m Solar Telescope. The radiative losses were computed taking into account the effect of partial redistribution and nonlocal thermodynamic equilibrium. To estimate the integrated radiative losses in the chromosphere the net cooling rates were integrated between the temperature minimum and the height where the temperature reaches 10 kK. We have also compared our time series of radiative losses with those from RADYN flare simulations. Results. We have obtained a high spatial-resolution map of integrated radiative losses around the flare peak time. The stratification of the net cooling rate suggests that the Ca IR triplet lines are responsible for most of the radiative losses in the flaring atmosphere. During the flare peak time, the contribution from Ca ii H & K and Mg ii h & k lines are strong and comparable to the Ca IR triplet (∼32 kW m −2 ). Since our flare is a relatively weak event the chromosphere is not heated above 11 kK, which in turn yields a subdued Lyα contribution (∼7 kW m −2 ) in the selected limits of the chromosphere. The temporal evolution of total integrated radiative losses exhibits sharply-rising losses (0.4 kW m −2 s −1 ) and a relatively slow decay (0.23 kW m −2 s −1 ). The maximum value of total radiative losses is reached around the flare peak time, and can go up to 175 kW m −2 for a single pixel located at footpoint. After a small parameter study, we find the best model-data consistency in terms of the amplitude of radiative losses and the overall atmospheric structure with a RADYN flare simulation in the injected energy flux of 5 × 10 10 erg s −1 cm −2 .

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Section: Summary and Discussionmentioning

confidence: 96%

On the radiative losses in the chromosphere during a C-class flare

Yadav,

Rodríguez,

Kerr

et al. 2022

Preprint

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“…Woods et al (2004) reported that the Lyα line wing is enhanced by about two times while the line core is enhanced by 20% in an X17 flare. Using the Lyα fluxes of more than 400 X-and M-class flares measured by the Geostationary Operational Environmental Satellite (GOES), Milligan et al (2020) found that the Lyα emission has an enhancement of 10% or less in most of the flares, with an average of 1%-4%, while the enhancement is only about 0.1%-0.3% on average for thousands of C-and B-class flares (Milligan 2021). The physical origin of flaring Lyα has also been studied by comparing Lyα with the hard X-ray (HXR) and soft X-ray (SXR) emissions that are associated with nonthermal electrons and thermal plasmas, respectively.…”

Section: Introductionmentioning

confidence: 99%

The Lyα Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

De-chao

et al. 2022

ApJ

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The hydrogen Lyα (H i Lyα) emission during solar flares has rarely been studied in spatially resolved images, and its physical origin has not been fully understood. In this paper, we present novel Lyα images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-wave-band and multiperspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Lyα emission has a good temporal correlation with the thermal emissions at 1–8 Å and 5–7 keV, indicating that the flaring Lyα is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Lyα at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Lyα emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Lyα emission shows fairly similar features to the He ii λ304 emission in light curve and spatiotemporal variation, along with small differences. These observational results improve our understanding of the Lyα emission in solar flares and also provide some insights for investigating the Lyα emission in stellar flares.

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“…The peak enhancement in Lyα emission among the flare sample was found to range from 1.5% to 6.4%. Comparatively, Milligan (2021) reported an average enhancement in Lyα emission of 1.5% for M-class flares, placing two of the three flares in this study above that average. Interestingly, FISM2 estimated Lyα enhancements of <2.5% for each flare.…”

Section: Discussionmentioning

confidence: 42%

“…In a statistical study of 477 M-and X-class flares observed in the E-channel of the Extreme Ultraviolet Sensor on board the Geostationary Operational Environmental Satellite (GOES/ EUVS-E; Viereck et al 2007;Evans et al 2010), Milligan et al (2020) found that typical increases in Lyα were <10%, with a maximum of approximately 30%. A follow-up study of several thousand flares using a superposed epoch analysis revealed average increases of 0.18%-0.35% for B-and C-class flares, and of 1%-4% for M-and X-class ones (Milligan 2021).…”

Section: Introductionmentioning

confidence: 99%

Observational Analysis of Lyα Emission in Equivalent-magnitude Solar Flares

Greatorex,

Milligan,

Chamberlin

2023

ApJ

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The chromospheric Lyα line of neutral hydrogen (1216 Å) is the most intense emission line in the solar spectrum, yet until recently observations of flare-related Lyα emission have been scarce. Here, we examine the relationship between nonthermal electrons accelerated during the impulsive phase of three M3 flares that were co-observed by RHESSI, GOES, and the Solar Dynamics Observatory, and the corresponding response of the chromosphere in Lyα. Despite having identical X-ray magnitudes, these flares showed significantly different Lyα responses. The peak Lyα enhancements above the quiescent background for these flares were 1.5%, 3.3%, and 6.4%. However, the predicted Lyα enhancements from FISM2 were consistently <2.5%. By comparing the properties of the nonthermal electrons derived from spectral analysis of hard X-ray observations, flares with a “harder” spectral index were found to produce a greater Lyα enhancement. The percentage of nonthermal energy radiated by the Lyα line during the impulsive phase was found to range from 2.0% to 7.9%. Comparatively, the radiative losses in He ii (304 Å) were found to range from 0.6% to 1.4% of the nonthermal energy while displaying enhancements above the background of 7.3% to 10.8%. FISM2 was also found to underestimate the level of He ii emission in two of the three flares. These results may have implications for space weather studies and for modeling the response of the terrestrial atmosphere to changes in the solar irradiance, and will guide the interpretation of flare-related Lyα observations that will become available during Solar Cycle 25.

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

Cited by 7 publications

References 39 publications

On the radiative losses in the chromosphere during a C-class flare

On the radiative losses in the chromosphere during a C-class flare

The Lyα Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

Observational Analysis of Lyα Emission in Equivalent-magnitude Solar Flares

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