Plasma turbulence and the standard solar flare model
E. P. Kontar, J. E. Perez, L. K. Harra, A. A. Kuznetsov, A. G. Emslie, N. L. S. Jeffrey, N. H. Bian, and B. R. Dennis
A long-standing solar-flare problem relates to the extraordinarily efficient conversion of magnetic energy into accelerated nonthermal particles. It has been hypothesized that magnetohydrodynamic turbulence plays a key role. In the recently published paper (Kontar et al, 2017), we use coordinated observations of a solar flare, from radio to EUV and X-ray, with unprecedented spatial, spectral and temporal resolutions, to determine the location of, and energy in, MHD turbulence, and thus evaluate its role in the energy conversion process in solar flares.
The paper reports on the observations of a GOES X1.2 flare that occurred on 2013 May 15, in NOAA active region 11748 (Fig 1). This flare was observed by many instruments, helping to place the EIS/Hinode observations in context. They include data from (1) the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), which produces high spatial-resolution hard X-ray (HXR) images with 1 keV spectral resolution and few-second time resolution, (2) the Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager (HMI), which measures the magnetic field geometry and strength in the flaring active region, (3) the SDO Atmospheric Imaging Assembly (AIA), which provides high-resolution spatial images in key EUV wavebands, the Geostationary Operational Environmental Satellite (GOES), which provides synoptic spatially-integrated SXR data in the soft X-ray band, and (5) the Nobeyama Radioheliograph and Radiopolarimeters, which measure the radio-wave radiation produced by mildly relativistic accelerated electrons and can be used to diagnose magnetic field strength.
Figure 1: Left: Background: Red contour: 50% of maximum intensity at 6-9 keV, blue contour: 50% of maximum intensity at 25-50 keV, white contour: EIS Fe XXIV (225 A) intensity map at 30% and 75%, and green contour: 34GHz radio emission 30% and 75%. The insert shows the extrapolated magnetic field in the corona. Right: Cartoon showing different flare elements and post-flare loops.
Using the observations, the energy of plasma motions, non-thermal electrons, thermal plasma and magnetic field were determined. Two aspects of the turbulent energy content inferred herein are significant. First, the turbulent energy is observed to be spatially concentrated in the coronal part of the magnetic loop below the observed cusp-like structure, where the primary energy release is believed to occur (Fig 1). Second, its energy content coincides with the HXR intensity, i.e., before the maximum rate of electron acceleration. Although the instantaneous turbulent energy content is only a percent or so of the available magnetic energy (and of the thermal energy in the SXR-emitting plasma), the transfer of energy out of the turbulent energy reservoir could be sufficiently rapid for the power to rival that associated with dissipation of the turbulence and the acceleration of non-thermal particles. The ratio of kinetic energy (K) to non-thermal power (P) shows that for such a scenario to be viable, the turbulent energy must be dissipated (and replenished) on a time scale ∼1 - 10 s (Fig 2). It is interesting to note that the dissipation time scale required is approximately the same as the Alfven crossing time (the critical balance in MHD turbulence), a quantity that is readily deducible from observations.
Figure 2: Flare energetics. Left panels, top to bottom: power P (erg s-1) in non-thermal electrons, density n (cm-3) of the SXR-emitting plasma, thermal energy content Uth (erg) of the SXR-emitting plasma. Right panels, top to bottom: turbulent kinetic energy K (erg), ratio K/P (s), and ratio K/Uth (dimensionless).
In summary, the suite of observations presented herein demonstrates the presence, in the acceleration region, of a significant energy reservoir in turbulent plasma motions that correlates well in time with the acceleration of HXR-producing electrons. An instantaneous energy content, produced and dissipated on a time scale of a few seconds, transfers a steady-state power rivalling the power in accelerated non-thermal particles. These observations not only enable quantitative testing of turbulent acceleration models; they lend considerable credence to the idea that turbulence acts as a crucial intermediary in the transfer of energy from reconnecting magnetic fields to accelerated particles during solar flares.
References: E. P. Kontar, J. E. Perez, L. K. Harra, A. A. Kuznetsov, A. G. Emslie, N. L. S. Jeffrey, N. H. Bian, and B. R. Dennis, Phys. Rev. Lett. 118, 155101
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