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Abstract

Context. The details of the spectral profiles of extreme UV emission lines from solar active regions contain key information for investigating the structure, dynamics, and energetics of the solar upper atmosphere.

Aims. We characterize the line profiles not only through the Doppler shift and intensity of the bulk part of the profile. More importantly, we investigate the excess emission and asymmetries in the line wings to study twisting motions and helicity.

Methods. We used a raster scan of the Interface Region Imaging Spectrograph (IRIS) in an active region. We concentrated on the Si IV line at 1394 Å, which forms just below 0.1 MK, and followed the plasma that moves in a cool loop from one footpoint to the other. We applied single-Gaussian fits to the line core, determined the excess emission in the red and blue wings, and derived the asymmetry of the red and blue wings.

Results. The blue wing excess at one footpoint shows injection of plasma into the loop that then flows to the other side. At the same footpoint, redshifts in the line core indicate that energy is deposited at around 0.1 MK. The enhanced pressure would then push the cool plasma down and inject some plasma into the loop. In the middle part of the loop, the spectral tilts of the line profiles indicate that the magnetic field has a helical structure, and the line wings are symmetrically enhanced. This is an indication that the loop is driven through the injection of helicity at the loop feet.

Conclusions. If the loop is driven to be helical, then the magnetic field can be expected to be in a turbulent state, as has been shown by existing magnetohydrodynamics models. The turbulent motions might explain the (symmetric) line wing enhancements that have also been seen in loops at coronal temperatures, but are not understood so far.