Observational Analysis of Multithermal Counterstreaming Flows in a Forming Filament and Their Relationship with Local Heating at Filament Footpoints

Abstract Utilizing high-resolution imaging and spectroscopic observations from the New Vacuum Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory, we investigated the nature and origin of counterstreaming flows within a forming active region filament. The ever-present counterstreaming flows observed within the filament are identified as interleaved unidirectional mass flows in opposing directions occurring in neighboring threads. Multiwavelength observations corroborate the multithermal nature of these counterstreaming flows: the cool H α component flows at about 10−20 km s −1 , while the warm ultraviolet and extreme-ultraviolet components reach 40–70 km s −1 . The Si iv 1400 Å line reveals significant microturbulence in the filament’s counterstreaming flows, with a nonthermal velocity width of ∼40 km s −1 . These multithermal flows emanate from compact brightenings at the filament’s ends, manifesting as small-scale, collimated upflows at their nascent phase. They continuously inject both chromospheric and transition region plasma into the filament channel, thereby feeding the counterstreaming flows. At their base, the Si iv and C ii spectral lines show pronounced line broadening and intensity enhancements, indicating significant localized chromospheric heating. Additionally, numerous small-scale photospheric flux emergence and cancellation events, with a magnitude of ∼10 17 Mx, are detected near their base. We suggest that such weak magnetic-field activities, possibly associated with unresolved magnetic reconnection events, drive these persistent upflows and localized footpoint heating. This work elucidates the multithermal origin of counterstreaming flows within a forming filament and provides evidence of localized chromospheric heating at the filament footpoints.