Abstract
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 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.
Highlights
Solar flare extreme ultraviolet (EUV) spectral line observations with the Hinode (Kosugi et al 2007) EUV Imaging Spectrometer (EIS; Culhane et al 2007) provide information on ion line emissions, plasma temperatures, mass flows, ion abundances, and electron densities
We show that in many locations in a flare, the Fe XVI and Fe XXIII line profiles observed by Hinode EIS are inconsistent with Gaussian spectral line shapes and are better described by emission from a kappa distribution of ion velocities
We find that the line profile analysis of suitable unblended lines such as Fe XVI and Fe XXIII can provide a powerful diagnostic for microscopic or macroscopic ion velocities during a solar flare, which may help constrain fundamental processes related to localized particle acceleration and/or turbulent magnetic or plasma fluctuations or flows
Summary
Solar flare extreme ultraviolet (EUV) spectral line observations with the Hinode (Kosugi et al 2007) EUV Imaging Spectrometer (EIS; Culhane et al 2007) provide information on ion line emissions, plasma temperatures, mass flows, ion abundances, and electron densities (cf. Milligan 2015). Jeffrey et al (2016) observed non-Gaussian spectral lines in flare EUV emission, showing that many unblended Fe XVI lines were consistent with a line shape produced by a kappa rather than a Maxwellian velocity distribution, in different flare regions. Ion kappa velocity distributions (cf Livadiotis & McComas 2009; Pierrard & Lazar 2010) are routinely detected in space physics (see, e.g., Gloeckler & Geiss 1998), but the high density flare environment (ne > 109 cm−3) with thermalizing Coulomb collisions is very different from the collisionless solar wind If such distributions can exist in flare conditions, they could provide a novel diagnostic technique of solar flare ion acceleration unavailable using other methods. We weigh the evidence for the line shapes being due to nonMaxwellian flare-accelerated ions or to non-Gaussian turbulent velocity fluctuations, which would be an observational first
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