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Title: Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line

Abstract

Solar flares are among the most energetic phenomena that occur in the solar system. In the standard solar flare model, a fast mode shock, often referred to as the flare termination shock (TS), can exist above the loop-top source of hard X-ray emissions. The existence of the TS has been recently related to spectral hardening of a flare’s hard X-ray spectra at energies >300 keV. Observations of the Fe xxi 1354.08 Å line during solar flares by the Interface Region Imaging Spectrograph ( IRIS ) spacecraft have found significant redshifts with >100 km s{sup −1}, which is consistent with a reconnection downflow. The ability to detect such a redshift with IRIS suggests that one may be able to use IRIS observations to identify flare TSs. Using a magnetohydrodynamic simulation to model magnetic reconnection of a solar flare and assuming the existence of a TS in the downflow of the reconnection plasma, we model the synthetic emission of the Fe xxi 1354.08 line in this work. We show that the existence of the TS in the solar flare may manifest itself in the Fe xxi 1354.08 Å line.

Authors:
 [1];  [2]; ;  [3]
  1. Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA (United States)
  2. Department of Space Science and CSPAR, University of Alabama in Huntsville, Huntsville, AL (United States)
  3. Harvard-Smithsonian Center for Astrophysics, Boston, MA (United States)
Publication Date:
OSTI Identifier:
22654395
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 846; 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; EMISSION; HARD X RADIATION; INTERFACES; KEV RANGE; MAGNETIC RECONNECTION; MAGNETOHYDRODYNAMICS; MASS; PLASMA; RED SHIFT; SIMULATION; SOLAR FLARES; SOLAR SYSTEM; SPACE VEHICLES; SPECTRAL HARDENING; SUN; X-RAY SPECTRA

Citation Formats

Guo, Lijia, Li, Gang, Reeves, Kathy, and Raymond, John, E-mail: gang.li@uah.edu. Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA866A.
Guo, Lijia, Li, Gang, Reeves, Kathy, & Raymond, John, E-mail: gang.li@uah.edu. Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line. United States. doi:10.3847/2041-8213/AA866A.
Guo, Lijia, Li, Gang, Reeves, Kathy, and Raymond, John, E-mail: gang.li@uah.edu. Fri . "Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line". United States. doi:10.3847/2041-8213/AA866A.
@article{osti_22654395,
title = {Solar Flare Termination Shock and Synthetic Emission Line Profiles of the Fe xxi 1354.08 Å Line},
author = {Guo, Lijia and Li, Gang and Reeves, Kathy and Raymond, John, E-mail: gang.li@uah.edu},
abstractNote = {Solar flares are among the most energetic phenomena that occur in the solar system. In the standard solar flare model, a fast mode shock, often referred to as the flare termination shock (TS), can exist above the loop-top source of hard X-ray emissions. The existence of the TS has been recently related to spectral hardening of a flare’s hard X-ray spectra at energies >300 keV. Observations of the Fe xxi 1354.08 Å line during solar flares by the Interface Region Imaging Spectrograph ( IRIS ) spacecraft have found significant redshifts with >100 km s{sup −1}, which is consistent with a reconnection downflow. The ability to detect such a redshift with IRIS suggests that one may be able to use IRIS observations to identify flare TSs. Using a magnetohydrodynamic simulation to model magnetic reconnection of a solar flare and assuming the existence of a TS in the downflow of the reconnection plasma, we model the synthetic emission of the Fe xxi 1354.08 line in this work. We show that the existence of the TS in the solar flare may manifest itself in the Fe xxi 1354.08 Å line.},
doi = {10.3847/2041-8213/AA866A},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 846,
place = {United States},
year = {Fri Sep 01 00:00:00 EDT 2017},
month = {Fri Sep 01 00:00:00 EDT 2017}
}
  • Using radiative magnetohydrodynamic simulations of the magnetized solar photosphere and detailed spectro-polarimetric diagnostics with the Fe I 6301.5 Å and 6302.5 Å photospheric lines in the local thermodynamic equilibrium approximation, we model active solar granulation as if it was observed at the solar limb. We analyze general properties of the radiation across the solar limb, such as the continuum and the line core limb darkening and the granulation contrast. We demonstrate the presence of profiles with both emission and absorption features at the simulated solar limb, and pure emission profiles above the limb. These profiles are associated with the regionsmore » of strong linear polarization of the emergent radiation, indicating the influence of the intergranular magnetic fields on the line formation. We analyze physical origins of the emission wings in the Stokes profiles at the limb, and demonstrate that these features are produced by localized heating and torsional motions in the intergranular magnetic flux concentrations.« less
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  • We have measured the line width of the forbidden line Fe XXI lambda1354 in flare spectra observed from Skylab. The measured line widths are examined using three sets of ionization equilibrium calculations by Jordan, Summers, and Jacobs et al. The results indicate that the ionization equilibrium temperature of Fe XXI is more likely to be approx.1.0 x 10/sup 7/ K, a value given by Jacobs t al. and Jordan, rather than the higher value of approx.2.0 x 10/sup 7/ K given by Summers.
  • The Interface Region Imaging Spectrometer (IRIS) is the first solar instrument to observe ∼10 MK plasma at subarcsecond spatial resolution through imaging spectroscopy of the Fe XXI λ1354.1 forbidden line. IRIS observations of the X1 class flare that occurred on 2014 March 29 at 17:48 UT reveal Fe XXI emission from both the flare ribbons and the post-flare loop arcade. Fe XXI appears at all of the chromospheric ribbon sites, although typically with a delay of one raster (75 s) and sometimes offset by up to 1''. 100-200 km s{sup –1} blue-shifts are found at the brightest ribbons, suggesting hot plasmamore » upflow into the corona. The Fe XXI ribbon emission is compact with a spatial extent of <2'', and can extend beyond the chromospheric ribbon locations. Examples are found of both decreasing and increasing blue-shift in the direction away from the ribbon locations, and blue-shifts were present for at least six minutes after the flare peak. The post-flare loop arcade, seen in Atmospheric Imaging Assembly 131 Å filtergram images that are dominated by Fe XXI, exhibited bright loop-tops with an asymmetric intensity distribution. The sizes of the loop-tops are resolved by IRIS at ≥1'', and line widths in the loop-tops are not broader than in the loop-legs suggesting the loop-tops are not sites of enhanced turbulence. Line-of-sight speeds in the loop arcade are typically <10 km s{sup –1}, and mean non-thermal motions fall from 43 km s{sup –1} at the flare peak to 26 km s{sup –1} six minutes later. If the average velocity in the loop arcade is assumed to be at rest, then it implies a new reference wavelength for the Fe XXI line of 1354.106 ± 0.023 Å.« less
  • H-alpha observations of the impulsive phase of a 1B/M1 solar flare on May 24, 1987, were carried out with high temporal resolution. A line center imaging system and an imaging spectrograph for line profile acquisition have been operated simultaneously with 0.3 s and 2.3 s temporal resolution, respectively. The temporal evolution at line center and in the blue and red wing have been correlated with hard X-ray data from HXRBS. The observed line profiles have been analyzed in terms of dynamic H-alpha line profile calculations by Canfield and Gayley. The combined H-alpha and hard X-ray signatures of two flare kernelsmore » are compatible with the theoretical predictions for strong nonthermal electron heating and the formation of a downward moving chromospheric condensation. A third kernel also shows the predicted downward moving chromospheric material, but its temporal evolution is not compatible with electron beam heating. 15 refs.« less