We study the dynamics of low-lying cool loop systems for three datasets as observed by the Interface Region Imaging Spectrograph ( IRIS ) .
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 co-spatial variation of velocity above the blue-shifted footpoints of various loop threads shows a transition from very small upflow velocities ranging from ( -1 to +1 ) { \mathrm { km } } { \mathrm { s } } ^ { -1 } in the Mg ii k line ( 2796.20 Å ; formation temperature : \log ( T / { \mathrm { K } } ) = 4.0 ) to the high upflow velocities from ( -10 to -20 ) { \mathrm { km } } { \mathrm { s } } ^ { -1 } in Si iv .
Thus , the transition of the plasma flows from red-shift ( downflows ) to the blue-shift ( upflows ) is observed above the footpoints of these loop systems in the spectral line C ii ( 1334.53 Å ; \log ( T / { \mathrm { K } } ) = 4.3 ) lying between Mg ii k and Si iv ( 1402.77 Å ; \log ( T / K ) = 4.8 ) .
This flow inversion is consistently observed in all three sets of the observational data .
The other footpoint of loop system always remains red-shifted indicating downflowing plasma .
The multi-spectral line analysis in the present paper provides a detailed scenario of the plasma flows inversions in cool loop systems leading to the mass transport and their formation .
The impulsive energy release due to small-scale reconnection above loop footpoint seems to be the most likely cause for sudden initiation of the plasma flows evident at TR temperatures .