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Title: Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration

Abstract

In a solar flare, a large fraction of the magnetic energy released is converted rapidly to the kinetic energy of non-thermal particles and bulk plasma motion. This will likely result in non-equilibrium particle distributions and turbulent plasma conditions. We investigate this by analyzing the profiles of high temperature extreme ultraviolet emission lines from a major flare (SOL2014-03-29T17:44) observed by the EUV Imaging Spectrometer (EIS) on Hinode . We find that in many locations the line profiles are non-Gaussian, consistent with a kappa distribution of emitting ions with properties that vary in space and time. At the flare footpoints, close to sites of hard X-ray emission from non-thermal electrons, the κ index for the Fe xvi 262.976 Å line at 3 MK takes values of 3–5. In the corona, close to a low-energy HXR source, the Fe xxiii 263.760 Å line at 15 MK shows κ values of typically 4–7. The observed trends in the κ parameter show that we are most likely detecting the properties of the ion population rather than any instrumental effects. We calculate that a non-thermal ion population could exist if locally accelerated on timescales ≤0.1 s. However, observations of net redshifts in the lines also implymore » the presence of plasma downflows, which could lead to bulk turbulence, with increased non-Gaussianity in cooler regions. Both interpretations have important implications for theories of solar flare particle acceleration.« less

Authors:
; ;  [1]
  1. School of Physics and Astronomy, University of Glasgow, G12 8QQ, Glasgow (United Kingdom)
Publication Date:
OSTI Identifier:
22663714
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; ACCELERATION; DISTRIBUTION; EMISSION; EQUILIBRIUM; EXTREME ULTRAVIOLET RADIATION; GAMMA RADIATION; HARD X RADIATION; HEAT EXCHANGERS; IONS; KINETIC ENERGY; PLASMA; RED SHIFT; SOLAR FLARES; SPACE; SPECTROMETERS; SUN; TURBULENCE; VELOCITY

Citation Formats

Jeffrey, Natasha L. S., Fletcher, Lyndsay, and Labrosse, Nicolas. Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration. United States: N. p., 2017. Web. doi:10.3847/1538-4357/836/1/35.
Jeffrey, Natasha L. S., Fletcher, Lyndsay, & Labrosse, Nicolas. Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration. United States. doi:10.3847/1538-4357/836/1/35.
Jeffrey, Natasha L. S., Fletcher, Lyndsay, and Labrosse, Nicolas. Fri . "Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration". United States. doi:10.3847/1538-4357/836/1/35.
@article{osti_22663714,
title = {Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines: A Possible Diagnostic of Ion Acceleration},
author = {Jeffrey, Natasha L. S. and Fletcher, Lyndsay and Labrosse, Nicolas},
abstractNote = {In a solar flare, a large fraction of the magnetic energy released is converted rapidly to the kinetic energy of non-thermal particles and bulk plasma motion. This will likely result in non-equilibrium particle distributions and turbulent plasma conditions. We investigate this by analyzing the profiles of high temperature extreme ultraviolet emission lines from a major flare (SOL2014-03-29T17:44) observed by the EUV Imaging Spectrometer (EIS) on Hinode . We find that in many locations the line profiles are non-Gaussian, consistent with a kappa distribution of emitting ions with properties that vary in space and time. At the flare footpoints, close to sites of hard X-ray emission from non-thermal electrons, the κ index for the Fe xvi 262.976 Å line at 3 MK takes values of 3–5. In the corona, close to a low-energy HXR source, the Fe xxiii 263.760 Å line at 15 MK shows κ values of typically 4–7. The observed trends in the κ parameter show that we are most likely detecting the properties of the ion population rather than any instrumental effects. We calculate that a non-thermal ion population could exist if locally accelerated on timescales ≤0.1 s. However, observations of net redshifts in the lines also imply the presence of plasma downflows, which could lead to bulk turbulence, with increased non-Gaussianity in cooler regions. Both interpretations have important implications for theories of solar flare particle acceleration.},
doi = {10.3847/1538-4357/836/1/35},
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}
}
  • We present detailed extreme ultraviolet (EUV) spectra of four large solar flares: M5.6, X1.3, X3.4, and X17 classes in the spectral ranges 176-207 Å and 280-330 Å. These spectra were obtained by the slitless spectroheliograph SPIRIT onboard the CORONAS-F satellite. To our knowledge, these are the first detailed EUV spectra of large flares obtained with a spectral resolution of ∼0.1 Å. We performed a comprehensive analysis of the obtained spectra and provide identification of the observed spectral lines. The identification was performed based on the calculation of synthetic spectra (the CHIANTI database was used), with simultaneous calculations of the differentialmore » emission measure (DEM) and density of the emitting plasma. More than 50 intense lines are present in the spectra that correspond to a temperature range of T = 0.5-16 MK; most of the lines belong to Fe, Ni, Ca, Mg, and Si ions. In all the considered flares, intense hot lines from Ca XVII, Ca XVIII, Fe XX, Fe XXII, and Fe XXIV are observed. The calculated DEMs have a peak at T ∼ 10 MK. The densities were determined using Fe XI-Fe XIII lines and averaged 6.5 × 10{sup 9} cm{sup –3}. We also discuss the identification, accuracy, and major discrepancies of the spectral line intensity prediction.« less
  • We have studied the relationship between the location of EUV non-thermal broadening and high-energy particles during large flares using the EUV Imaging Spectrometer on board Hinode, the Nobeyama Radio Polarimeter, the Nobeyama Radioheliograph, and the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. We have analyzed five large flare events that contain thermal-rich, intermediate, and thermal-poor flares classified by the definition discussed in the paper. We found that, in the case of thermal-rich flares, the non-thermal broadening of Fe XXIV occurred at the top of the flaring loop at the beginning of the flares. The source of 17 GHzmore » microwaves is located at the footpoint of the flare loop. On the other hand, in the case of intermediate/thermal-poor flares, the non-thermal broadening of Fe XXIV occurred at the footpoint of the flare loop at the beginning of the flares. The source of 17 GHz microwaves is located at the top of the flaring loop. We discussed the difference between thermal-rich and intermediate/thermal-poor flares based on the spatial information of non-thermal broadening, which may provide clues that the presence of turbulence plays an important role in the pitch angle scattering of high-energy electrons.« less
  • Simultaneous solar flare observations with SDO and RHESSI provide spatially resolved information about hot plasma and energetic particles in flares. RHESSI allows the properties of both hot (≳8 MK) thermal plasma and non-thermal electron distributions to be inferred, while SDO/AIA is more sensitive to lower temperatures. We present and implement a new method to reconstruct electron distribution functions from SDO/AIA data. The combined analysis of RHESSI and AIA data allows the electron distribution function to be inferred over the broad energy range from 0.1 keV up to a few tens of keV. The analysis of two well-observed flares suggests thatmore » the distributions in general agree to within a factor of three when the RHESSI values are extrapolated into the intermediate range 1-3 keV, with AIA systematically predicting lower electron fluxes. Possible instrumental and numerical effects, as well as potential physical origins for this discrepancy, are discussed. The inferred electron distribution functions in general show one or two nearly Maxwellian components at energies below ∼15 keV and a non-thermal tail above.« less
  • Extreme ultraviolet (EUV) spectra of highly charged iron ions in a wavelength range of 100-300 A have been observed from two different plasma sources of the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) with a monoenergetic electron beam and a Large Helical Device (LHD) with Maxwellian electron energy. The excitation process of the spectral lines is compared between the two plasmas, and it is found that the excitation process for Fe XIX - Fe XXII ions is clearly different. Namely, the EUV emission lines from the EBIT plasma are only dominated by electron impact excitation connected to the ground state, butmore » the excitation mechanism is not so simple in the LHD plasma. The difference in the excitation process is studied by measuring the intensity ratio of EUV emission lines (114.412 A [1s{sup 2}2s2p{sup 2} {sup 2}P{sub 3/2}{yields}1s{sup 2}2s{sup 2}2p {sup 2}P{sub 3/2}]/117.144 A [1s{sup 2}2s2p{sup 2} {sup 2}P{sub 1/2}{yields}1s{sup 2}2s{sup 2}2p {sup 2}P{sub 1/2}]) arising from different ground levels in the Fe XXII ions. The line intensity ratio has an extremely small value of 0.2 in the EBIT plasma with a low beam current of 30 mA and a beam energy of 2 keV, while the ratio varies with the electron density n{sub e} in the LHD plasmas, i.e., 0.35 for n{sub e} = 1 x 10{sup 13} cm{sup -3} and 0.65 for n{sub e} = 4 x 10{sup 13} cm{sup -3}. Here, the electron density of the EBIT plasma is estimated to be smaller than 10{sup 12} cm{sup -3} and the electron temperature of the LHD plasmas is 2 keV. The dependence of the line intensity ratio on the observed electron density is analyzed for both the EBIT and the LHD plasmas using several collisional-radiative (CR) models. The present experimental data can easily be reproduced by the analysis when the thermal proton impact excitation is taken into account. The importance of the proton impact excitation is also experimentally verified by injecting an iron pellet into the LHD plasmas and changing the ratio of the proton density to the electron density.« less