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Zusammenfassung:
We study the dynamics of low-lying cool loop systems for three data sets as observed by the Interface Region Imaging Spectrograph. Radiances, Doppler shifts, and line widths are investigated in and around observed cool loop systems using various spectral lines formed between the photosphere and transition region (TR). Footpoints of the loop threads are either dominated by blueshifts or redshifts. The cospatial variation of velocity above the blueshifted footpoints of various loop threads shows a transition from very small upflow velocities ranging from (−1 to +1) $\mathrm{km}$ ${\rm{s}}$ −1 in the Mg ii k line (2796.20 Å; formation temperature: $\mathrm{log}(T/{\rm{K}})=4.0$) to the high upflow velocities from (−10 to −20) $\mathrm{km}$ ${\rm{s}}$ −1 in Si iv. Thus, the transition of the plasma flows from redshift (downflows) to blueshift (upflows) is observed above the footpoints of these loop systems in the spectral line C ii (1334.53 Å; log(T/${\rm{K}}$) = 4.3) lying between Mg ii k and Si iv (1402.77 Å; $\mathrm{log}(T/{\rm{K}})=4.8$). This flow inversion is consistently observed in all three sets of the observational data. The other footpoint of the loop system always remains redshifted, indicating downflowing plasma. The multispectral line analysis in the present paper provides a detailed scenario of the plasma flow's inversions in cool loop systems leading to the mass transport and their formation. The impulsive energy release due to small-scale reconnection above the loop footpoint seems to be the most likely cause for sudden initiation of the plasma flows evident at TR temperatures.
