Observational Analysis of Multi-thermal Counter-streaming Flows in a Forming Filament and Their Relationship with Local Heating at Filament Footpoints

TL;DR

This study uses multi-instrument solar observations to analyze counter-streaming flows in a forming filament, revealing their multi-thermal nature, origin from footpoint brightenings, and association with localized chromospheric heating and magnetic activities.

Contribution

It provides the first detailed analysis of multi-thermal counter-streaming flows in a forming filament, linking them to footpoint heating and magnetic flux emergence.

Findings

Counter-streaming flows are multi-thermal, with cool and warm components moving in opposite directions.

Flows originate from footpoint brightenings associated with localized heating.

Magnetic flux emergence and cancellation at the footpoints drive persistent upflows.

Abstract

Utilizing high-resolution imaging and spectroscopic observations from the New Vacuum Solar Telescope (NVST), the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO), we investigated the nature and origin of counter-streaming flows within a forming active region filament. The ever-present counter-streaming flows observed within the filament are identified as interleaved unidirectional mass flows in opposing directions occurring in neighboring threads. Multi-wavelength observations corroborate the multi-thermal nature of these counter-streaming flows: the cool H$\alpha$ 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~{\sc iv} 1400~{\AA} line reveals significant micro-turbulence in the filament's counter-streaming flows, with a nonthermal velocity width of 40 km s$^{-1}$. These multi-thermal 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 counter-streaming flows. At their base, the Si~{\sc iv} and C~{\sc 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 magnitude of $10^{17}$~Mx, are detected near their base. We suggest such weak magnetic-field activities, possibly associated with unresolved magnetic reconnection events, drive these persistent upflows and localized footpoint heating. This work elucidates the multi-thermal origin of counter-streaming flows within a forming filament and provides evidence of localized chromospheric heating at the filament footpoints.