Apparent and intrinsic evolution of active region loops

DÉMOULIN, P.; JANVIER, M.; BAKER, D.; MANDRINI, C.H.

Workshop; 7th Solar Orbiter Workshop: Exploring the solar environs; 2017

We analyze the evolution of Fe xii coronal plasma upflows from the edgesof ten active regions (ARs) as they cross the solar disk using theHinode Extreme Ultraviolet Imaging Spectrometer (EIS) to do this.Confirming the results of Démoulin et al. ( Sol. Phys. 283, 341,2013), we find that for each AR there is an observed long-term evolutionof the upflows. This evolution is largely due to the solar rotation thatprogressively changes the viewpoint of dominantly stationary upflows.From this projection effect, we estimate the unprojected upflow velocityand its inclination to the local vertical. AR upflows typically fan awayfrom the AR core by 40° to nearly vertical for the followingpolarity. The span of inclination angles is more spread out for theleading polarity, with flows angled from -29° (inclined toward theAR center) to 28° (directed away from the AR). In addition to thelimb-to-limb apparent evolution, we identify an intrinsic evolution ofthe upflows that is due to coronal activity, which is AR dependent.Furthermore, line widths are correlated with Doppler velocities only forthe few ARs with the highest velocities. We conclude that for the linewidths to be affected by the solar rotation, the spatial gradient of theupflow velocities must be large enough such that the line broadeningexceeds the thermal line width of Fe xii. Finally, we find that upflowsoccurring in pairs or multiple pairs are a common feature of ARsobserved by Hinode/EIS, with up to four pairs present in AR 11575. Thisis important for constraining the upflow-driving mechanism as it impliesthat the mechanism is not local and does not occur over a singlepolarity. AR upflows originating from reconnection alongquasi-separatrix layers between overpressure AR loops and neighboringunderpressure loops is consistent with upflows occurring in pairs,unlike other proposed mechanisms that act locally in one polarity.