High-resolution spectroscopic imaging of counter-streaming motions in solar active region magnetic loops

We carried out high-resolution spectroscopic imaging in He i 10830 Å and H_α for a set of active region (NOAA 12569) magnetic loops of different sizes (classified into short and long loops) with the Goode Solar Telescope at the Big Bear Solar Observatory on 2016 July 18. The long loops take the form of an chromospheric arch filament system, yet their extreme ultraviolet (EUV) counterparts are observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Animations of blue- and red-wing images give counter-streaming motions; i.e., chromospheric absorption features in blue- and red-wing images move in opposite directions at different strands. The moving pattern is detected with the local correlation tracking method and confirmed by Doppler shifts. We speculate that, combined with the results of wavelet analysis that gives obvious 4 minute oscillation along trailing polarity, counter-streaming motions for short loops could be powered by p-mode leakage. However, for counter-streaming motions in long loops, we show that unidirectional mass flows in two opposite directions are accompanied with simultaneous weak EUV brightenings. Heating processes, probably by magnetic reconnection at footpoints, may have occurred. In addition, plasma flows along the magnetic loops, tracked with absorption features in He i 10830 Å, are found to be ejected from and drained out into inter-granule lane areas at different ends of the loop system.