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arXiv · 2610.19718

Evidence for Suppressed Non-thermal Broadening in a Flare Ribbon from Stereoscopic Observations

Ziwen Huang·L. P. Chitta·L. Teriaca
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Abstract

Flare ribbons mark the footpoints of newly reconnected magnetic loops and are commonly associated with strong plasma flows and enhanced non-thermal broadening of transition-region lines due to impulsive energy deposition. However, the mechanisms governing energy transport into flare ribbons remain uncertain, particularly under different magnetic field configurations. We investigate the energy deposition process at a flare ribbon near a sunspot by determining the physical nature of its small-scale downflows. We study small-scale downflows in a remote ribbon during the M7.7-class event on 30 September 2024, using coordinated observations from SDO, IRIS, and Solar Orbiter. We use stereoscopic imaging from Solar Orbiter and IRIS to detect plane-of-sky motions on small scales (200 km). We conduct spectroscopic analysis of IRIS Si IV profiles, complemented by C II and Mg II diagnostics. Apparent downflow speeds measured from Solar Orbiter imaging (limb view) are 85-130 km/s, while IRIS images (disk view) reveal plane-of-sky speeds of 70-85 km/s. In contrast, IRIS Si IV profiles at the brightest ribbon pixels are well approximated by a single Gaussian, with minimal Doppler shifts and unexpectedly small non-thermal widths (10 km/s). Peak Si IV emission also coincides with a deeper Mg II central reversal, while electron densities are one to two orders of magnitude higher than typical quiet-Sun values. The absence of significant line-of-sight velocities and non-thermal broadening differs from typical flare-ribbon behavior. Our results are consistent with a scenario in which plasma motions are guided by highly inclined (nearly horizontal) magnetic fields in chromospheric canopy and are not accompanied by strong turbulence or wave motions. This study suggests that flare ribbons can exhibit a wide diversity of lower-atmospheric responses and emphasizes the critical role of magnetic field geometry.

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Ziwen Huang, L. P. Chitta, L. Teriaca. 2026-10-14. Evidence for Suppressed Non-thermal Broadening in a Flare Ribbon from Stereoscopic Observations. https://arxiv.org/abs/2610.19718

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