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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. 2017. "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 = 2017,
month = 9
}
  • 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
  • We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations. We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Large-scale magnetic fluctuations-known to exist inmore » plasmas and known to have important effects on the nonthermal electron acceleration at shocks-are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (above post-flare loops) could accelerate electrons to a kinetic energy of a few MeV within 100 ion cyclotron periods, which is of the order of a millisecond. Given a sufficient turbulence amplitude level ({delta}B{sup 2}/B 2{sub 0} {approx} 0.3), about 10% of thermal test-particle electrons are accelerated to more than 15 keV. We find that protons are also accelerated, but not to as high energy in the available time and the energy spectra are considerably steeper than that of the electrons for the parameters used in our simulations. Our results are qualitatively consistent with the observed hard X-ray emissions in solar flares.« less
  • Studies made so far with one-dimensional hydrodynamic simulations have shown that it is difficult to reproduce the soft X-ray spectral line profile observed in the early phase of solar flares. Simulated line profiles predict a dominant emission from a large blueshifted component, while observations show persistently strong stationary components. We resolve these discrepancies by utilizing a multiple-loop system instead of just a single loop for conductively heated flare simulations. Under a fixed heat input rate, we examine how the heating duration {tau}{sub heat} affects the Ca xix resonance ({ital w}) line emission from single and multiple flare loops. In themore » multiple-loop model, the flare energy is released into individual loops with a specified time delay, which implicitly mimics the successive formation of flare loops due to continuous reconnection. We find that whether or not {tau}{sub heat} is longer than {tau}{sub {ital c}} affects the hydrodynamic response in an individual flare loop, where {tau}{sub {ital c}} corresponds to the time when the loop is filled with evaporated plasma. The Ca xix spectral line shape is characterized by an intensity ratio of emission from evaporated plasma to emission from accumulated plasma after evaporation. This ratio is mainly determined by the parameter {tau}{sub heat}/{tau}{sub {ital c}}. Our findings suggest that the following scenario can naturally explain the observed spectral line features. Flare energy is injected into a bundle of loops successively in two steps: in the preflare stage, {tau}{sub heat} {le} {tau}{sub {ital c}} for the inner loops, and then in the main flare stage, {tau}{sub heat} {gt} {tau}{sub {ital c}} for the outer loops. A large initial coronal density is not necessary in this scenario. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}« less