White-light continuum in stellar flares

Kowalski, Adam F.

doi:10.1017/s1743921316000028

Cited by 5 publications

(4 citation statements)

References 43 publications

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“…Spontaneous hydrogen Balmer b-f emissivity from a narrow height range in the CC at z∼900 km dominates the emergent NUV continuum intensity at this time. 13 Figures 2, 4, and 6 of Kowalski et al (2015b) illustrate how the contribution function of Balmer continuum emission over large optical depth does not follow the electron density in a flare atmosphere with a much more dense CC than in the 5F11 model, and Figure 2 of Kowalski (2016) shows how the physical depth range of emergent continuum intensity is affected by large optical depth. See also Appendix C here.…”

Section: I2826mentioning

confidence: 97%

“…In the 5F11 electron beam model, the Balmer continuum emission originates over a large physical depth range (Δz=50-390 km; Table 3), which is not strongly wavelength dependent, due to the low optical depth in the CC. In the F13 simulation, the Balmer continuum emission originates over a much smaller physical depth range of ∼1 km (Kowalski 2016) 21 due to the larger density and optical depth in the CC. In the F13 model, the physical depth range is strongly wavelength dependent, and only the blue optical photons (λ∼4300 Å) have an optical depth that is low enough to escape from the stationary flare layers.…”

Section: Appendix C Comparison Of 5f11 and F13 Rhd Modelsmentioning

confidence: 99%

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The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

Kowalski

Allred

Daw

et al. 2017

ApJ

Self Cite

101

174

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The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright continuum emission in the far-and near-ultraviolet (NUV) and highly asymmetric chromospheric emission lines, providing long-sought constraints on the heating mechanisms of the lower atmosphere in solar flares. We analyze the continuum and emission line data from the Interface Region Imaging Spectrograph (IRIS) of the brightest flaring magnetic footpoints in this flare. We compare the NUV spectra of the brightest pixels to new radiative-hydrodynamic predictions calculated with the RADYN code using constraints on a nonthermal electron beam inferred from the collisional thick-target modeling of hard X-ray data from Reuven Ramaty High Energy Solar Spectroscopic Imager. We show that the atmospheric response to a high beam flux density satisfactorily achieves the observed continuum brightness in the NUV. The NUV continuum emission in this flare is consistent with hydrogen (Balmer) recombination radiation that originates from low optical depth in a dense chromospheric condensation and from the stationary beam-heated layers just below the condensation. A model producing two flaring regions (a condensation and stationary layers) in the lower atmosphere is also consistent with the asymmetric Fe II chromospheric emission line profiles observed in the impulsive phase.

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Section: I2826mentioning

confidence: 97%

Section: Appendix C Comparison Of 5f11 and F13 Rhd Modelsmentioning

confidence: 99%

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

Kowalski

Allred

Daw

et al. 2017

ApJ

Self Cite

101

174

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“…The Hβ/Hγ and Hα/Hγ line flux ratios are much smaller in the models than typical values from observations, but the Hδ/Hγ ratios are in general agreement. Such small values of Hα and Hβ to Hγ indicate a "reverse decrement" and are only observed to be as low as ∼0.8 (for Hα; Figure 4.22 of Kowalski 2012). The Hα/Hγ line flux ratios are very low in the F13 models because the optical depth necessary to produce the hot blackbody-like continuum shape results in a very large Hα optical depth and radiation thus escapes from a much smaller physical depth range of the atmosphere across this line compared to Hγ.…”

Section: Balmer Decrementsmentioning

confidence: 99%

“…The observed decrements in dMe flares are typically Hβ/H 1.0 1.2 g~-and for Hδ/H 0.75 1.05 g~- (Table 4.20 of Kowalski (2012), Table 1 of Allred et al 2006). Decrements are similarly used in solar flare studies as well, but the lack of broad wavelength coverage spectra make the measurements rare in the modern era (Johns-Krull et al 1997;Kowalski et al 2015a).…”

Section: Balmer Decrementsmentioning

confidence: 99%

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

Kowalski

Allred

Uitenbroek

et al. 2017

ApJ

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108

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The broadening of the hydrogen lines during flares is thought to result from increased charge (electron, proton) density in the flare chromosphere. However, disagreements between theory and modeling prescriptions have precluded an accurate diagnostic of the degree of ionization and compression resulting from flare heating in the chromosphere. To resolve this issue, we have incorporated the unified theory of electric pressure broadening of the hydrogen lines into the non-LTEradiative-transfer code RH. This broadening prescription produces a much more realistic spectrum of the quiescent, A0 star Vega compared to the analytic approximations used as a damping parameter in the Voigt profiles. We test recent radiative-hydrodynamic (RHD) simulations of the atmospheric response to high nonthermal electron beam fluxes with the new broadening prescription and find that the Balmer lines are overbroadened at the densest times in the simulations. Adding many simultaneously heated and cooling model loops as a "multithread" model improves the agreement with the observations. We revisit the threecomponent phenomenological flare model of the YZ CMi Megaflare using recent and new RHD models. The evolution of the broadening, line flux ratios, and continuum flux ratios are well-reproduced by a multithread model with high-flux nonthermal electron beam heating, an extended decay phase model, and a "hot spot" atmosphere heated by an ultrarelativistic electron beam with reasonable filling factors: ∼0.1%, 1%, and 0.1% of the visible stellar hemisphere, respectively. The new modeling motivates future work to understand the origin of the extended gradual phase emission.

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Stellar flares

Kowalski

2024

Living Rev Sol Phys

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Magnetic storms on stars manifest as remarkable, randomly occurring changes of the luminosity over durations that are tiny in comparison to the normal evolution of stars. These stellar flares are bursts of electromagnetic radiation from X-ray to radio wavelengths, and they occur on most stars with outer convection zones. They are analogous to the events on the Sun known as solar flares, which impact our everyday life and modern technological society. Stellar flares, however, can attain much greater energies than those on the Sun. Despite this, we think that these phenomena are rather similar in origin to solar flares, which result from a catastrophic conversion of latent magnetic field energy into atmospheric heating within a region that is relatively small in comparison to normal stellar sizes. We review the last several decades of stellar flare research. We summarize multi-wavelength observational results and the associated thermal and nonthermal processes in flaring stellar atmospheres. Static and hydrodynamic models are reviewed with an emphasis on recent progress in radiation-hydrodynamics and the physical diagnostics in flare spectra. Thanks to their effects on the space weather of exoplanetary systems (and thus in our search for life elsewhere in the universe) and their preponderance in Kepler mission data, white-light stellar flares have re-emerged in the last decade as a widely-impactful area of study within astrophysics. Yet, there is still much we do not understand, both empirically and theoretically, about the spectrum of flare radiation, its origin, and its time evolution. We conclude with several big-picture questions that are fundamental in our pursuit toward a greater understanding of these enigmatic stellar phenomena and, by extension, those on the Sun.

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White-light continuum in stellar flares

Cited by 5 publications

References 43 publications

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

The Atmospheric Response to High Nonthermal Electron Beam Fluxes in Solar Flares. I. Modeling the Brightest NUV Footpoints in the X1 Solar Flare of 2014 March 29

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

Stellar flares

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