Formation of the thermal infrared continuum in solar flares

Simões, Paulo J. A.; Kerr, Graham S.; Fletcher, L.; Hudson, H. S.; Castro, C. G. Giménez de; Penn, M. J.

doi:10.1051/0004-6361/201730856

Cited by 47 publications

(41 citation statements)

References 60 publications

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“…Two mid‐IR sources were observed to be cospatial with WL and hard X‐ray foot points. The flux at both wavelengths is of the same order, lending support to a thermal optically thin origin, a conclusion further reinforced by results of radiative hydrodynamic simulations by Simões et al (). Two other events were registered at 10 μm: SOL2014‐08‐01T14:47 (Miteva et al, ) and SOL2014‐10‐27T14:22 (Kaufmann et al, ), the last one classified as X2.…”

Section: Introductionsupporting

confidence: 68%

“…The mid‐IR flux can be explained, as in Trottet et al (), as optically thin thermal bremsstrahlung emission of a heated chromospheric plasma, since fluxes of the two events are similar. Moreover, Simões et al (), using radiative hydrodynamic simulations, have demonstrated that the mid‐IR emission from solar flares may be accounted by optically thin thermal bremsstrahlung due to the increase of the electron density in the chromosphere, as a consequence of the ionization of hydrogen under non local thermodynamical equilibrium (non‐LTE) conditions.…”

Section: Discussionmentioning

confidence: 99%

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The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

et al. 2018

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Active Region 12673 is the most productive active region of solar cycle 24: in a few days of early September 2017, four X‐class and 27 M‐class flares occurred. SOL2017‐09‐06T12:00, an X9.3 flare also produced a two‐ribbon white light emission across the sunspot detected by Solar Dynamics Orbiter/Helioseismic and Magnetic Imager. The flare was observed at 212 and 405 GHz with the arcminute‐sized beams of the Solar Submillimeter Telescope focal array while making a solar map and at 10 μm, with a 17 arcsec diffraction‐limited infrared camera. Images at 10 μm revealed that the sunspot gradually increased in brightness while the event proceeded, reaching a temperature similar to quiet Sun values. From the images we derive a lower bound limit of 180‐K flare peak excess brightness temperature or 7,000 sfu if we consider a similar size as the white light source. The rising phase of mid‐IR and white light is similar, although the latter decays faster, and the maximum of the mid‐IR and white light emission is ∼200 s delayed from the 15.4‐GHz peak occurrence. The submillimeter spectrum has a different origin than that of microwaves from 1 to 15 GHz, although it is not possible to draw a definitive conclusion about its emitting mechanism.

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Section: Introductionsupporting

confidence: 68%

Section: Discussionmentioning

confidence: 99%

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

et al. 2018

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“…Allred et al (2005) used more sophisticated chromospheric heating models and found that back-warming contributes only 10% to the heating. In a recent study, Simões et al (2017) used RADYN simulations to determine the formation of the infrared continuum in flares and found no enhancement in the photospheric blackbody emission.…”

Section: Introductionmentioning

confidence: 99%

Reproducing Type II White-light Solar Flare Observations with Electron and Proton Beam Simulations

Prochazka

Reid

Milligan

et al. 2018

ApJ

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We investigate the cause of the suppressed Balmer series and the origin of the white-light continuum emission in the X1.0 class solar flare on 2014 June 11. We use radiative hydrodynamic simulations to model the response of the flaring atmosphere to both electron and proton beams, which are energetically constrained using Ramaty High Energy Solar Spectroscopic Imager and Fermi observations. A comparison of synthetic spectra with the observations allows us to narrow the range of beam fluxes and low energy cutoff that may be applicable to this event. We conclude that the electron and proton beams that can reproduce the observed spectral features are those that have relatively low fluxes and high values for the low energy cutoff. While electron beams shift the upper chromosphere and transition region to greater geometrical heights, proton beams with a similar flux leave these areas of the atmosphere relatively undisturbed. It is easier for proton beams to penetrate to the deeper layers and not deposit their energy in the upper chromosphere where the Balmer lines are formed. The relatively weak particle beams that are applicable to this flare do not cause a significant shift of the τ=1 surface and the observed excess WL emission is optically thin.

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“…Many of these models are currently widely used (e.g., Heinzel & Avrett 2012;Trottet et al 2015;Kleint et al 2016;Simões et al 2017). When velocity or the position of the flare transition region is modified in phenomenological models, the gas density must also change and is not correctly given by hydrostatic equilibrium.…”

Section: Discussion and Applicationmentioning

confidence: 99%

Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres

Kowalski

Allred

2018

ApJ

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The origin of the near-ultraviolet and optical continuum radiation in flares is critical for understanding particle acceleration and impulsive heating in stellar atmospheres. Radiative-hydrodynamic simulations in 1D have shown that high energy deposition rates from electron beams produce two flaring layers at T ∼ 10 4 K that develop in the chromosphere: a cooling condensation (downflowing compression) and heated non-moving (stationary) flare layers just below the condensation. These atmospheres reproduce several observed phenomena in flare spectra, such as the red wing asymmetry of the emission lines in solar flares and a small Balmer jump ratio in M dwarf flares. The high beam flux simulations are computationally expensive in 1D, and the (human) timescales for completing NLTE models with adaptive grids in 3D will likely be unwieldy for a time to come. We have developed a prescription for predicting the approximate evolved states, continuum optical depth, and the emergent continuum flux spectra of radiative-hydrodynamic model flare atmospheres. These approximate prescriptions are based on an important atmospheric parameter: the column mass (m ref ) at which hydrogen becomes nearly completely ionized at the depths that are approximately in steady state with the electron beam heating. Using this new modeling approach, we find that high energy flux density (>F11) electron beams are needed to reproduce the brightest observed continuum intensity in IRIS data of the 2014-Mar-29 X1 solar flare and that variation in m ref from 0.001 to 0.02 g cm −2 reproduces most of the observed range of the optical continuum flux ratios at the peaks of M dwarf flares.

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Formation of the thermal infrared continuum in solar flares

Cited by 47 publications

References 60 publications

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

Reproducing Type II White-light Solar Flare Observations with Electron and Proton Beam Simulations

Parameterizations of Chromospheric Condensations in dG and dMe Model Flare Atmospheres

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