C37

Presentation Title:	Evaluating the level of turbulence in the flaring transition region from IRIS spectral line observations
Author(s):	Gordon Emslie, Simon Shepard (WKU), Graham Kerr (CUA), Joel Allred (GSFC), Stephen Bradshaw (Rice)

Abstract:

It is well established that spectral line profiles in the EUV are significantly broadened compared to their thermal widths, and this has been taken as evidence for turbulent nonthermal motions of the emitting ions. The excess line width is typically evaluated by fitting a Gaussian profile and comparing its standard deviation $\sigma$ with the thermal width $\sqrt(kT/m_i)$. However, several studies have shown that turbulent motions can result in a line profile that is intrinsically non-Gaussian in shape. In particular, several authors have modeled the turbulent mean free path as a two-parameter function, with one parameter ($R$) measuring the ratio of the turbulent and collisional mean free paths at the thermal speed, and another parameter ($\alpha$) measuring the velocity dependence of the turbulent mean free path (compared to $\alpha = 4$ for collisions). Predicted line profiles for various values of $R$ and $\alpha$ show not only excess widths (i.e., standard deviations $\sigma$), but also significantly non-Gaussian wings, parametrized by the reduced kurtosis $K$, a quantity determined by the ratio of the fourth normalized moment and the fourth power of $\sigma$. The two-parameter physical parameter set ($R, \alpha$) may therefore be mapped onto a two–parameter observational set ($\sigma, K$). We report on the analysis of the profiles of several spectral lines observed by IRIS during flares, and on the behavior of $\sigma$ and $K$ with position along the slit and with time. In this way we shed light on the variation of $R$ and $\alpha$ throughout the observed area, and hence on the role of turbulence in the flare transition region.

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