摘要：Context. The new Interface Region Imaging Spectrograph (IRIS) mission provides high-resolution observations of UV spectra and slit-jaw images (SJIs). These data have become available for investigating the dynamic features in the transition region (TR) below the on-disk filaments.
Aims. The driver of “counter-streaming” flows along the filament spine is still unknown yet. The magnetic structures and the upflows at the footpoints of the filaments and their relations with the filament mainbody have not been well understood. We study the dynamic evolution at the footpoints of filaments in order to find some clues for solving these questions.
Methods. Using UV spectra and SJIs from the IRIS, along with coronal images and magnetograms from the Solar Dynamics Observatory (SDO), we present the new features in a quiescent filament channel: subarcsecond bright points (BPs) and quasi-periodic upflows.
Results. The BPs in the TR have a spatial scale of about 350−580 km and lifetimes of more than several tens of minutes. They are located at stronger magnetic structures in the filament channel with a magnetic flux of about 1017−1018 Mx. Quasi-periodic brightenings and upflows are observed in the BPs, and the period is about 4−5 min. The BP and the associated jet-like upflow comprise a “tadpole-shaped” structure. The upflows move along bright filament threads, and their directions are almost parallel to the spine of the filament. The upflows initiated from the BPs with opposite polarity magnetic fields have opposite directions. The velocity of the upflows in the plane of sky is about 5−50 km s-1. The emission line of Si IV 1402.77 Å at the locations of upflows exhibits obvious blueshifts of about 5−30 km s-1, and the line profile is broadened with the width of more than 20 km s-1.
Conclusions. The BPs seem to be the bases of filament threads, and the upflows are able to convey mass for the dynamic balance of the filament. The “counter-streaming” flows in previous observations may be caused by the propagation of bi-directional upflows initiated from opposite polarity magnetic fields. We suggest that quasi-periodic brightenings of BPs and quasi-periodic upflows result from small-scale oscillatory magnetic reconnections, which are modulated by solar p-mode waves.
