TY - JOUR
T1 - Non-Gaussian Velocity Distributions in Solar Flares from Extreme Ultraviolet Lines
T2 - A Possible Diagnostic of Ion Acceleration
AU - Jeffrey, Natasha L.S.
AU - Fletcher, Lyndsay
AU - Labrosse, Nicolas
PY - 2017/2/10
Y1 - 2017/2/10
N2 - 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 Šline at 3 MK takes values of 3-5. In the corona, close to a low-energy HXR source, the Fe xxiii 263.760 Šline 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.
AB - 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 Šline at 3 MK takes values of 3-5. In the corona, close to a low-energy HXR source, the Fe xxiii 263.760 Šline 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.
KW - atomic data
KW - line: profiles
KW - Sun: flares
KW - Sun: UV radiation
KW - Sun: X-rays, gamma rays
KW - techniques: spectroscopic
UR - http://www.scopus.com/inward/record.url?scp=85014348946&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/836/1/35
DO - 10.3847/1538-4357/836/1/35
M3 - Article
AN - SCOPUS:85014348946
VL - 836
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 1
M1 - 35
ER -