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Title: Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop

Abstract

We explore the temporal relationship between microwave/hard X-ray (HXR) emission and Doppler velocity during the impulsive phase of a solar flare on 2014 October 27 (SOL2014-10-27) that displays a pulse on the light curves in the microwave (34 GHz) and HXR (25–50 keV) bands before the flare maximum. Imaging observation shows that this pulse mainly comes from one footpoint of a solar flare loop. The slit of the Interface Region Imaging Spectrograph ( IRIS ) stays at this footpoint during this solar flare. The Doppler velocities of Fe xxi 1354.09 Å and Si iv 1402.77 Å are extracted from the Gaussian fitting method. We find that the hot line of Fe xxi 1354.09 Å (log T ∼ 7.05) in the corona exhibits blueshift, while the cool line of Si iv 1402.77 Å (log T ∼ 4.8) in the transition region exhibits redshift, indicating explosive chromospheric evaporation. Evaporative upflows along the flare loop are also observed in the AIA 131 Å image. To our knowledge, this is the first report of chromospheric evaporation evidence from both spectral and imaging observations in the same flare. Both microwave and HXR pulses are well correlated with the Doppler velocities, suggesting that the chromospheric evaporationmore » is driven by nonthermal electrons around this footpoint of a solar flare loop.« less

Authors:
; ; ;  [1]
  1. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS, Nanjing 210008 (China)
Publication Date:
OSTI Identifier:
22654473
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 841; 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; EVAPORATION; EXPLOSIVES; GAMMA RADIATION; HARD X RADIATION; INTERFACES; KEV RANGE; MICROWAVE RADIATION; RED SHIFT; SOLAR FLARES; SUN; ULTRAVIOLET RADIATION; VISIBLE RADIATION

Citation Formats

Li, D., Ning, Z. J., Huang, Y., and Zhang, Q. M., E-mail: lidong@pmo.ac.cn. Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA71B0.
Li, D., Ning, Z. J., Huang, Y., & Zhang, Q. M., E-mail: lidong@pmo.ac.cn. Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop. United States. doi:10.3847/2041-8213/AA71B0.
Li, D., Ning, Z. J., Huang, Y., and Zhang, Q. M., E-mail: lidong@pmo.ac.cn. 2017. "Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop". United States. doi:10.3847/2041-8213/AA71B0.
@article{osti_22654473,
title = {Explosive Chromospheric Evaporation Driven by Nonthermal Electrons around One Footpoint of a Solar Flare Loop},
author = {Li, D. and Ning, Z. J. and Huang, Y. and Zhang, Q. M., E-mail: lidong@pmo.ac.cn},
abstractNote = {We explore the temporal relationship between microwave/hard X-ray (HXR) emission and Doppler velocity during the impulsive phase of a solar flare on 2014 October 27 (SOL2014-10-27) that displays a pulse on the light curves in the microwave (34 GHz) and HXR (25–50 keV) bands before the flare maximum. Imaging observation shows that this pulse mainly comes from one footpoint of a solar flare loop. The slit of the Interface Region Imaging Spectrograph ( IRIS ) stays at this footpoint during this solar flare. The Doppler velocities of Fe xxi 1354.09 Å and Si iv 1402.77 Å are extracted from the Gaussian fitting method. We find that the hot line of Fe xxi 1354.09 Å (log T ∼ 7.05) in the corona exhibits blueshift, while the cool line of Si iv 1402.77 Å (log T ∼ 4.8) in the transition region exhibits redshift, indicating explosive chromospheric evaporation. Evaporative upflows along the flare loop are also observed in the AIA 131 Å image. To our knowledge, this is the first report of chromospheric evaporation evidence from both spectral and imaging observations in the same flare. Both microwave and HXR pulses are well correlated with the Doppler velocities, suggesting that the chromospheric evaporation is driven by nonthermal electrons around this footpoint of a solar flare loop.},
doi = {10.3847/2041-8213/AA71B0},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 841,
place = {United States},
year = 2017,
month = 5
}
  • The response of the solar chromosphere to flare heating by nonthermalelectrons is examined. A number of interesting phenomena appear in our numericalsolutions of the equations of hydrodynamics and radiative transfer. Here wediscuss one aspect of these results: the phenomenon of chromosphericevaporation. We present results for a range of heating fluxes and show how thesemay be understood in simple terms. Our major conclusions are as follows: (1)There is an energy flux threshold for ''explosive'' evaporation. Explosiveevaporation occurs when the upper chromosphere is unable to radiate the flareenergy deposited there, and is therefore heated rapidly to coronal temperatures.Energy fluxes less than thismore » threshold produce ''gentle'' evaporation, in whichthe chromosphere is eaten away by conduction at a much slower rate. (2) Theexpansion velocity of explosively evaporated plasma cannot exceedapprox.2.35c/sub s/, where c/sub s/ is the sound speed in the evaporated material. (3) We derive a simple analytic model for the temporal variation of velocity in explosively evaporated plasma. This ''gasbag'' model, based on isothermal expansion of an impulsively heated mass of plasma, is used successfully to reproduce our own numerical results, as well as those of MacNeice et al. (1984). (4) The lower transition region, in both gentle and explosive evaporation, quickly reaches a quasi-steady balance between conduction and radiation, so that the conductive flux at 10/sup 5/ K is given by 3.42 x 10/sup 5/P ergs cm/sup -2/ s/sup -1/, where P(dyn cm/sup -2/) is the pressure in the flare transition region. In the case of explosive evaporation, a short powerful pulse of EUV radiation is emitted from plasma with temperatures near 10/sup 5/ K during the adjustment to this equilibrium.« less
  • A GOES M1.5 solar flare was observed in NOAA AR 10652 on 2004 July 27 around 20:00 UT with the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. Images obtained with SOHO's Extreme-ultraviolet Imaging Telescope and with the Transition Region And Coronal Explorer satellite show that the CDS slit was positioned within the flare, whose emission extended 1 arcmin along the slit. Rapid cadence (9.8 s) stare spectra obtained with CDS include emission from the upper chromosphere (He I at 584.3 A), transition region (O V at 629.7 A), corona (Si XII at 520.7 A), andmore » hot flare plasma (Fe XIX at 592.2 A), and reveal that (1) the flare brightened in its southern parts before it did so in the north; (2) chromospheric evaporation was 'explosive' during the first rapid intensity increase observed in Fe XIX, but converted to 'gentle' during the second; (3) chromospheric evaporation did not occur in the northern portion of the flare observed by CDS: the brightening observed there was due to flare material moving into that location from elsewhere. We speculate that the initial slow, steady increase of Fe XIX intensity that was observed to start several minutes before its rapid increase was due to direct coronal heating. The change from explosive to gentle evaporation was likely due to either an increased absorption of beam energy during the gentle event because the beam passed through an atmosphere modified by the earlier explosive event, or to a weakening of the coronal magnetic field's ability to accelerate nonthermal particle beams (via reconnection) as the flare progressed, or both.« less
  • The entire profile of the Fe XXIII line at 263.8 A, formed at temperature Almost-Equal-To 14 MK, was blueshifted by an upward velocity -122 {+-} 33 km s{sup -1} when it was first detected by the Extreme-ultraviolet Imaging Spectrometer operating in rapid cadence (11.18 s) stare mode during a C1 solar flare. The entire profile became even more blueshifted over the next two exposures, when the upward velocity reached its maximum of -208 {+-} 14 km s{sup -1} before decreasing to zero over the next 12 exposures. After that, a weak, secondary blueshifted component appeared for five exposures, reached amore » maximum upward velocity of -206 {+-} 33 km s{sup -1}, and disappeared after the maximum line intensity (stationary plus blueshifted) was achieved. Velocities were measured relative to the intense stationary profile observed near the flare's peak and early during its decline. The initial episode during which the entire profile was blueshifted lasted about 156 s, while the following episode during which a secondary blueshifted component was detected lasted about 56 s. The first episode likely corresponds to chromospheric evaporation in a single loop strand, while the second corresponds to evaporation in an additional strand, as described in multi-strand flare loop models proposed by Hori et al. and Warren and Doschek. Line emission from progressively cooler ions (Fe XVII, XVI, and XIV) brightened at successively later times, consistent with cooling of flare-heated plasma.« less
  • Observations of gentle chromospheric evaporation during the cooling phase of a solar flare are presented. Line profiles of the low-temperature (T of about 6 x 10 to the 6th K) coronal Mg XI line, observed with the X-Ray Polychromator on the Solar Maximum Mission, show a blueshift that persisted for several minutes after the impulsive heating phase. This result represents the first detection of an evaporation signature in a soft X-ray line formed at this low temperature. By combining the Mg XI blueshift velocity data with simultaneous measurements of the flare temperature derived from Ca XIX observations, it is demonstratedmore » that the upward flux of enthalpy transported by this gently evaporating plasma varies linearly with the downward flux of thermal energy conducted from the corona. This relationship is consistent with models of solar flares in which thermal conduction drives chromospheric evaporation during the early part of the cooling phase. 22 references.« less
  • Magnetic energy released in the corona by solar flares reaches the chromosphere where it drives characteristic upflows and downflows known as evaporation and condensation. These flows are studied here for the case where energy is transported to the chromosphere by thermal conduction. An analytic model is used to develop relations by which the density and velocity of each flow can be predicted from coronal parameters including the flare's energy flux F. These relations are explored and refined using a series of numerical investigations in which the transition region (TR) is represented by a simplified density jump. The maximum evaporation velocity,more » for example, is well approximated by v{sub e} ≅ 0.38(F/ρ{sub co,} {sub 0}){sup 1/3}, where ρ{sub co,} {sub 0} is the mass density of the pre-flare corona. This and the other relations are found to fit simulations using more realistic models of the TR both performed in this work, and taken from a variety of previously published investigations. These relations offer a novel and efficient means of simulating coronal reconnection without neglecting entirely the effects of evaporation.« less