Velocity field of an active region filament from GRIS infrared He I and IRIS ultraviolet observations

¹ INAF – Osservatorio Astronomico di Roma Via Frascati 33 I-00078 Monteporzio Catone Rome, Italy
² Space Science Data Center (SSDC), Agenzia Spaziale Italiana Via del Politecnico s.n.c. I-00133 Roma, Italy
³ INAF – Osservatorio Astronomico di Capodimonte Salita Moiarello 16 I- 80131 Napoli, Italy
⁴ Institut d’Astrophysique Spatiale, Bâtiment 121, Rue Jean Dominique Cassini, Université Paris Saclay 91405 Orsay, France
⁵ Instituto de Astrofísica de Canarias (IAC), Vía Láctea s/n E-38205 La Laguna Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna Tenerife, Spain
⁷ Institut für Sonnenphysik (KIS) Georges-Köhler-Allee 401a 79110 Freiburg, Germany
⁸ INAF – Osservatorio Astrofisico di Catania Via Santa Sofia 78 I-95123 Catania, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 December 2024
Accepted: 27 October 2025

Abstract

Context. Plasma flow measurements in solar active region filaments are rare, particularly in the infrared and ultraviolet ranges that probe the chromosphere and transition region. In addition, previous studies generally focused on prominences and filaments near the solar limb.

Aims. This study presents a multi-wavelength, multi-instrument analysis of an active region filament observed on the solar disk on November 9 and 10, 2020. Our goal is to characterize the plasma flows in the filament using spectroscopic measurements in both the infrared and ultraviolet spectral ranges. This is important for understanding the mechanisms for filament support, mass loading, and energy balance. Furthermore, this also offers observational benchmarks for filament modeling and simulations.

Methods. Spectra from the IRIS satellite, including the Mg II k 2796 Å, C II 1335 Å, and Si IV 1393 Å lines were analyzed alongside ground-based observations from the GREGOR Infrared Spectrograph and High-resolution Fast Imager instruments, whose observed spectral ranges include the chromospheric He I 10830 Å and Hα 6563 Å lines.

Results. Persistent blueshifts were measured within the filament structure in both spectral ranges. These can be interpreted as upflow velocities ranging from 0.5 to 15 km s⁻¹, with Si IV 1393 Å showing the highest values. Redshifted emission in He I and Mg II k₃ at the footpoints of a newly formed dark bundle suggest chromospheric downflows, likely due to spatial overlap between an arch filament system close to the filament footpoints. The weak redshifted signal in the Si IV emission may suggest confinement to lower atmospheric layers.

Conclusions. The observed velocity patterns provide, for the first time, a comprehensive and coherent view of the plasma dynamics from the chromosphere to the transition region, illustrating that the filament emission is consistently blueshifted in all the spectral windows, and thus in different temperature regimes.