Morgan Stores

MHD plasma turbulence is believed to play a vital role in the production of energetic electrons during solar flares and the non-thermal broadening of spectral lines is a key sign of this turbulence. Here, we determine how flare turbulence evolves in time and in space using spectral profiles of Fe xxiv, Fe xxiii and Fe xvi, observed by Hinode/EIS. To determine the non-thermal velocity, Gaussian fits were applied to every pixel in the EIS data field of view for each ion studied. To demonstrate the resulting fits I show two example spectral lines with Gaussian fits applied. The first example is Fe xvi and the second is Fe xxvi. The spectral line Fe xvi (254.8853 Å) sits in the blue wing of Fe xvii (255.1136 Å). A simple two Gaussian fit is successful in eliminating the influence of Fe xvii in the wing.
From these Gaussian fits the Full Width at Half Maximum of the lines is determined which allows the non-thermal velocity to be calculated. Maps of non-thermal velocity are then created for five observation times which cover the X-ray rise, peak, and decay of the flare. The maps are overlaid with AIA contours (94 Å and 304 Å channels) to show the coronal loops and the ribbon. The maps show non-thermal velocity is not localised in the loop apex but distributed throughout the entire flare; decreasing as the flare evolves and during and after the X-ray peak clear spatial patterns appear from the the loop apex towards the ribbon features.
The space-averaged non-thermal velocity was then determined for each ion as the flare evolves and shows that for the two hotter ions (Fe xxiii and Fe xxiv) the non-thermal velocity decreases as the flare evolves. But for the cooler ion (Fe xvi) it remains almost constant, slightly increasing as the flare evolves. The standard deviation (the spread) of the non-thermal velocity values is also shown and is fairly constant for each ion as the flare progresses. However, at the last observation time, all ions have similar standard deviation values
Next, we studied how the non-thermal velocity evolves in time in different regions across the flare. For the hotter ions, three coronal loop tops and a loop leg were studied where it was shown that the coronal loop tops at larger radial distance have a higher non-thermal velocity than the loops at a smaller radial distance. For the cooler ion an area corresponding to a southern ribbon feature was extracted where the non-thermal velocity against time steeply increases, peaking just prior to the peak in hard X-rays, before declining.
Then, we studied how the non-thermal velocity changes in space. We calculated the spatial gradients along different lines in the map connecting different flare features. For times close to and after the peak in hard X-rays where spatial patterns appeared from the coronal loop apex along the loop leg towards the ribbon feature. This showed a clear spatial variation decreasing linearly from loop apex towards the ribbon feature. The results suggest that turbulence has a more complex temporal and spatial structure than previously assumed.
Finally, the kinetic energy at every spatial pixel was calculated, using the non-thermal velocity maps and creating density maps by employing spectral line density diagnostics for Fe xvi. The creation of kinetic energy density maps for each ion reveal where energy is available for electron energisation, suggesting that similar levels of energy may be available to heat and/or accelerate electrons in large regions of the flare.