Zhenghua Huang, Lidong Xia, Bo Li, Maria S. Madjarska

Cool transition region loops observed by the Interface Region Imaging Spectrograph

Figure 3. Radiance image in Si iv 1402.8 Å (top-left, in reversed color table), Doppler velocity image (bottom-left), nonthermal velocity image (bottom-right) and HMI magnetogram (top-right) of region "A". The dotted lines mark 15 loops that were visually identified. The blue contours over-plotted on the magnetogram denote the emission of the IRIS 2832 Å continuum raster image. The yellow box on the Doppler image outlines the region where anti-parallel flows are seen. The box (dashed lines) on the magnetogram marks the area from which the magnetic cancellation rate is obtained.


We report on the first Interface Region Imaging Spectrograph (IRIS) study of cool transition region loops. This class of loops has received little attention in the literature, mainly due to instrumental limitations. A cluster of such loops was observed on the solar disk in active region NOAA11934, in the Si iv 1402.8 Å spectral raster and 1400 Å slit-jaw (SJ) images. We divide the loops into three groups and study their dynamics and interaction. The first group comprises relatively stable loops, with 382–626 km cross-sections. Observed Doppler velocities are suggestive of siphon flows, gradually changing from −10 km s-1; at one end to 20 km s-1 at the other end of the loops. Nonthermal velocities from 15 km s-1 to 25 km s-1 were determined. These physical properties suggest that these loops are impulsively heated by magnetic reconnection occurring at the blue-shifted footpoints where magnetic cancellation with a rate of 1015 Mx s-1 is found. The released magnetic energy is redistributed by the siphon flows. The second group corresponds to two footpoints rooted in mixed-magnetic-polarity regions, where magnetic cancellation occurred at a rate of 1015 Mx s-1 and line profiles with enhanced wings of up to 200 km s-1 were observed. These are suggestive of explosive-like events. The Doppler velocities combined with the SJ images suggest possible anti-parallel flows in finer loop strands. In the third group, interaction between two cool loop systems is observed. Evidence for magnetic reconnection between the two loop systems is reflected in the line profiles of explosive events, and a magnetic cancellation rate of 3 × 1015 Mx s-1 observed in the corresponding area. The IRIS observations have thus opened a new window of opportunity for in-depth investigations of cool transition region loops. Further numerical experiments are crucial for understanding their physics and their role in the coronal heating processes.

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Last Revised: 2015 August 3rd