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Title: 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

Abstract

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.

Authors:
 [1]; ;  [2];  [3];  [4]
  1. Department of Astrophysical and Planetary Sciences, University of Colorado Boulder, 2000 Colorado Ave, Boulder, CO 80305 (United States)
  2. NASA/Goddard Space Flight Center, Code 671, Greenbelt, MD 20771 (United States)
  3. INAF-Osservatorio Astrofisico di Arcetri, I-50125 Firenze (Italy)
  4. Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, NO-0315 Oslo (Norway)
Publication Date:
OSTI Identifier:
22663876
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 836; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASYMMETRY; ATMOSPHERES; COMPARATIVE EVALUATIONS; COSMIC ELECTRONS; ELECTRON BEAMS; EMISSION; FLUX DENSITY; FORECASTING; HARD X RADIATION; HEATING; HYDRODYNAMICS; HYDROGEN; LAYERS; LIMITING VALUES; RADIANT HEAT TRANSFER; RECOMBINATION; SIMULATION; SOLAR FLARES; SUN; ULTRAVIOLET RADIATION

Citation Formats

Kowalski, Adam F., Allred, Joel C., Daw, Adrian, Cauzzi, Gianna, and Carlsson, Mats, E-mail: Adam.Kowalski@lasp.colorado.edu. 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. United States: N. p., 2017. Web. doi:10.3847/1538-4357/836/1/12.
Kowalski, Adam F., Allred, Joel C., Daw, Adrian, Cauzzi, Gianna, & Carlsson, Mats, E-mail: Adam.Kowalski@lasp.colorado.edu. 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. United States. doi:10.3847/1538-4357/836/1/12.
Kowalski, Adam F., Allred, Joel C., Daw, Adrian, Cauzzi, Gianna, and Carlsson, Mats, E-mail: Adam.Kowalski@lasp.colorado.edu. Fri . "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". United States. doi:10.3847/1538-4357/836/1/12.
@article{osti_22663876,
title = {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},
author = {Kowalski, Adam F. and Allred, Joel C. and Daw, Adrian and Cauzzi, Gianna and Carlsson, Mats, E-mail: Adam.Kowalski@lasp.colorado.edu},
abstractNote = {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.},
doi = {10.3847/1538-4357/836/1/12},
journal = {Astrophysical Journal},
number = 1,
volume = 836,
place = {United States},
year = {Fri Feb 10 00:00:00 EST 2017},
month = {Fri Feb 10 00:00:00 EST 2017}
}
  • Helioseismic data from the Helioseismic Magnetic Imager instrument have revealed a sunquake associated with the X1 flare SOL2014-03-29T17:48 in active region NOAA 12017. We try to discover if acoustic-like impulses or actions of the Lorentz force caused the sunquake. We analyze spectropolarimetric data obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Fortunately, the FIRS slit crossed the flare kernel close to the acoustic source during the impulsive phase. The infrared FIRS data remain unsaturated throughout the flare. Stokes profiles of lines of Si I 1082.7 nm and He I 1083.0 nm are analyzed. At themore » flare footpoint, the Si I 1082.7 nm core intensity increases by a factor of several, and the IR continuum increases by 4% ± 1%. Remarkably, the Si I core resembles the classical Ca II K line's self-reversed profile. With nLTE radiative models of H, C, Si, and Fe, these properties set the penetration depth of flare heating to 100 ± 100 km (i.e., photospheric layers). Estimates of the non-magnetic energy flux are at least a factor of two less than the sunquake energy flux. Milne-Eddington inversions of the Si I line show that the local magnetic energy changes are also too small to drive the acoustic pulse. Our work raises several questions. Have we missed the signature of downward energy propagation? Is it intermittent in time and/or non-local? Does the 1-2 s photospheric radiative damping time discount compressive modes?.« less
  • Enhanced continuum brightness is observed in many flares (“white light flares”), yet it is still unclear which processes contribute to the emission. To understand the transport of energy needed to account for this emission, we must first identify both the emission processes and the emission source regions. Possibilities include heating in the chromosphere causing optically thin or thick emission from free-bound transitions of Hydrogen, and heating of the photosphere causing enhanced H{sup −} continuum brightness. To investigate these possibilities, we combine observations from Interface Region Imaging Spectrograph (IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based Facility Infrared Spectrometer instrument, coveringmore » wavelengths in the far-UV, near-UV (NUV), visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures ∼6000–6300 K. The NUV emission, formed in the upper photosphere under undisturbed conditions, exceeds these simple fits during the flare, requiring extra emission from the Balmer continuum in the chromosphere. Thus, the continuum originates from enhanced radiation from photosphere (visible-IR) and chromosphere (NUV). From the standard thick-target flare model, we calculate the energy of the nonthermal electrons observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and compare it to the energy radiated by the continuum emission. We find that the energy contained in most electrons >40 keV, or alternatively, of ∼10%–20% of electrons >20 keV is sufficient to explain the extra continuum emission of ∼(4–8) × 10{sup 10} erg s{sup −1} cm{sup −2}. Also, from the timing of the RHESSI HXR and the IRIS observations, we conclude that the NUV continuum is emitted nearly instantaneously when HXR emission is observed with a time difference of no more than 15 s.« less
  • Spectroscopic observations of solar flares provide critical diagnostics of the physical conditions in the flaring atmosphere. Some key features in observed spectra have not yet been accounted for in existing flare models. Here we report a data-driven simulation of the well-observed X1.0 flare on 2014 March 29 that can reconcile some well-known spectral discrepancies. We analyzed spectra of the flaring region from the Interface Region Imaging Spectrograph ( IRIS ) in Mg ii h and k, the Interferometric BIdimensional Spectropolarimeter at the Dunn Solar Telescope (DST/IBIS) in H α 6563 Å and Ca ii 8542 Å, and the Reuven Ramatymore » High Energy Solar Spectroscope Imager ( RHESSI ) in hard X-rays. We constructed a multithreaded flare loop model and used the electron flux inferred from RHESSI data as the input to the radiative hydrodynamic code RADYN to simulate the atmospheric response. We then synthesized various chromospheric emission lines and compared them with the IRIS and IBIS observations. In general, the synthetic intensities agree with the observed ones, especially near the northern footpoint of the flare. The simulated Mg ii line profile has narrower wings than the observed one. This discrepancy can be reduced by using a higher microturbulent velocity (27 km s{sup −1}) in a narrow chromospheric layer. In addition, we found that an increase of electron density in the upper chromosphere within a narrow height range of ≈800 km below the transition region can turn the simulated Mg ii line core into emission and thus reproduce the single peaked profile, which is a common feature in all IRIS flares.« less
  • SOLRAD and many other satellite systems have provided a large data base showing the time-dependent behavior of broad and band solar fluxes in the X-ray and EUV spectral regions. These bands are broad in the sense that one band may contain many ionospherically important spectral lines. We present results of tests performed to determine how this information can be best be used to predict the effects of a solar flare on the ionosphere. Our approach has been to first adopt a model of the spectral line and continuum enhancements based on a synthesis of many types of flare observations. Thismore » detailed spectral model is used in a time-dependent ionosphere model to calculate the response of the electron and ion density profiles. Then the spectral model is mathematically filtered to show how it would appear to the SOLRAD EUV detectors, and this degraded information is used in the ionosphere model. Comparison of the two ionosphere shows that the two spectra produces changes in the total electron content in the ionosphere that differ by only a few percent. More significant changes which occur in the individual species densities are described.« less
  • SOLRAD and many other satellite systems have provided a large data base showing the time-dependent behavior of broad and band solar fluxes in the X-ray and EUV spectral regions. These bands are broad in the sense that one band may contain many ionospherically important spectral lines. We present results of tests performed to determine how this information can be best be used to predict the effects of a solar flare on the ionosphere. Our approach has been to first adopt a model of the spectral line and continuum enhancements based on a synthesis of many types of flare observations. Thismore » detailed spectral model is used in a time-dependent ionosphere model to calculate the response of the electron and ion density profiles. Then the spectral model is mathematically filtered to show how it would appear to the SOLRAD EUV detectors, and this degraded information is used in the ionosphere model. Comparison of the two ionosphere shows that the two spectra produces changes in the total electron content in the ionosphere that differ by only a few percent. More significant changes which occur in the individual species densities are described.« less