Counter-streaming flows in a giant quiet-Sun filament observed in the extreme ultraviolet

TL;DR

This study observes and analyzes counter-streaming flows in a giant quiet-Sun filament using multiwavelength EUV data, revealing persistent flows with specific velocities and spatial coverage, confirming their omnipresence in such filaments.

Contribution

It introduces a novel application of noise adaptive fuzzy equalization combined with local correlation tracking to quantify counter-streaming flows in a large quiet-Sun filament.

Findings

Counter-streaming flows are visible in all examined EUV wavelengths.

Flows last for at least two hours with speeds up to 2.5 km/s.

Flows cover about 15-20% of the filament area.

Abstract

A giant solar filament was visible on the solar surface from 2011 Nov. 8-23. The filament stretched over more than half a solar diameter. Multiwavelength data from the SDO instrument AIA (171, 193, 304, and 211\AA) were used to examine counter-streaming flows within the spine of the filament. H$\alpha$ images from the Kanzelh\"ohe Solar Observatory provided context information. We apply local correlation tracking (LCT) to a two-hour time series on 2011 Nov. 16 of the AIA images to derive horizontal flow velocities of the filament. To enhance the contrast of the AIA images, noise adaptive fuzzy equalization is employed, which allows us to identify and quantify counter-streaming flows in the filament. We detect counter-streaming flows in the filament, which are visible in the time-lapse movies in all examined AIA wavelength bands. In the time-lapse movies we see that these persistent flows lasted for at least two hours. Furthermore, by applying LCT to the images we clearly determine counter-streaming flows in time series of 171 and 193\AA\ images. In the 304\AA\ wavelength band, we only see minor indications for counter-streaming flows with LCT, while in the 211\AA\ wavelength band the counter-streaming flows are not detectable. The average horizontal flows reach mean flow speeds of 0.5 km/s. The highest horizontal flow speeds are identified in the 171\AA\ band with flow speeds of up to 2.5 km/s. The results are averaged over a time series of 90 min. Because the LCT sampling window has finite width, a spatial degradation cannot be avoided leading to lower estimates of the flow velocities as compared to feature tracking or Doppler measurements. The counter-streaming flows cover about 15-20% of the whole area of the EUV filament channel and are located in the central part of the spine. In conclusion, we confirm the omnipresence of counter-streaming flows also in giant quiet-Sun filaments.