%0 Journal Article
%T Red and Blueshifts in Multi-stranded Coronal Loops: A New Temperature Diagnostic
%A S. Regnier
%A R. W. Walsh
%J Physics
%D 2014
%I arXiv
%X [...] redshifts are predominantly observed in the core of active regions, while blueshifts are observed at the edge of active regions. [...] Using a nanoflare heating model for multi-stranded coronal loops (Sarkar and Walsh, 2008, 2009), we reproduce the above Dopplershift observations using spectral lines covering a broad range of temperature (from 0.25 MK to 5.6 MK). We first show that red- and blueshifts are ubiquitous in all wavelength ranges; redshifts/downflows dominating cool spectral lines (from O v to Si vii) and blueshifts/upflows dominating the hot lines (from Fe xv to Ca xvii). By computing the average Dopplershift, we derive a new temperature diagnostic for coronal loops: the temperature at which the average Dopplershift vanishes is the mean temperature along the coronal loop. In addition, the temperature estimate at the footpoints of the loop when the average Dopplershift vanishes is a lower bound of the temperature along the loop. To compare closely with observations, we thus model typical Hinode/EIS rasters with a spatial resolution of 1", an exposure time of 50s and a step of 3". Even if the raster reproduce the global features of up and downflows along the loop, we show that this type of raster cannot provide information on the heating mechanism. We also discuss the fact that observing a single spectral line can lead to false interpretation of the physical processes at play. For instance, an observed increased in blueshift velocity in the Fe xii channel can indicate a cooling event (decrease of energy input). We also investigate the existence of coronal loops having Dopplershifts of opposite signs which could characterise a unidirectional flow along the loop: about 50% of loops have opposite Dopplershifts at the footpoints for the spectral line closest to the mean temperature of the loop.
%U http://arxiv.org/abs/1405.3450v1