Particle acceleration and the evolution of nonthermal line broadening in solar flares

Stochastic particle acceleration by wave turbulence is one of the proposed mechanisms by which charged particles can be accelerated to high energies during solar flares, with the turbulence assumed to be generated during the primary energy release process. The role of Alfvén wave turbulence in the acceleration of protons and the broadening of soft X-ray lines was explored by Alexander & MacKinnon (1992) and applied to gamma-ray flares by Alexander & Matthews (1994). In this scenario, the peak of the line broadening corresponds to the peak wave flux, which precedes the peak in the hard X-ray (or gamma-ray) emission. This is observed to be the case in many flares (e.g. Alexander et al., 1998; Harra, Matthews & Culhane, 2001). More recently, Kontar et al. (2017) compared the turbulent kinetic energy density derived from Hinode EIS observations with the nonthermal electron energy inferred from RHESSI observations, concluding that in the flare studied there was a substantial reservoir of turbulence, the decay of which correlated well with the electron acceleration. Here, we expand on this previous work by exploiting EIS sit-and-stare observations to examine the evolution of the wave energy density at high cadence and at multiple temperatures to further quantify the link between turbulence and electron acceleration, as well as through unique spectral observations of the region around the flare current sheet.