Velocity field of an Active Region filament from GRIS IR He I and IRIS UV observations

TL;DR

This work tackles the scarce problem of measuring plasma flows in solar active region filaments using a simultaneous infrared and ultraviolet, disk-centered dataset. By combining GRIS He I 10830 Å inversions with IRIS Mg II k, C II, and Si IV spectroscopy, and grounding these in high-resolution Hα imaging, the authors construct a velocity field spanning the chromosphere to the transition region. They report persistent blueshifts within the filament across all spectral windows and identify redshifted footpoints likely caused by overlap with an arch filament system, offering observational benchmarks for filament models and simulations. The results demonstrate the value of coordinated ground-based and space-borne spectroscopy for constraining filament dynamics, mass loading, and energy balance across multiple atmospheric layers, with potential for future multi-instrument campaigns and advanced modeling.

Abstract

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. 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. Spectra from the IRIS satellite, including the Mg II k, C II and Si IV 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 and Hα lines. 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 the Si IV showing the highest values. Red shifted emission in the 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. 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.

Figures (12)

• Figure 1: Overall view of the AR12871. Panel A: Full-disk magnetogram clipped between $\pm$500 G from HMI taken at 10:55 UT on November 10. The black box indicates the FoV shown in the following AIA 304 and 193 Å bands (panels B and C). Panel D: High-spatial resolution HiFI H$\alpha$ image taken at 10:48 UT on November 10. Panel E: IRIS slit-reconstructed image in the center of the Mg ii k$_{3}$ line acquired between 11:03 UT and 11:52 UT on November 10. Panels F-G: HMI magnetogram clipped between $\pm$500 G and continuum taken at 10:55 UT. The red box in the continuum image indicates the GRIS FoV.
• Figure 2: Restored H$\alpha$ filtergrams acquired by the HiFI instrument at the GREGOR telescope showing the fine structure of the filament at various times on November 9 (top panels) and November 10 (bottom panels) November. The red and blue contours in first panels indicate the positive and negative magnetic flux density obtained from SDO/HMI at level equal to $\pm$200 G, respectively. The red box in the second bottom panel represents the GRIS acquired scanned FoV acquired between 09:17:32 and 09:37:05 UT. Movies of the HiFI data showing the filament in the time interval between 08:44 UT and 11:02 UT (10:49 UT) on November 9 and 10 are available online.
• Figure 3: Top panel: Zoom of the H$\alpha$ filtergram for reference. Middle panel: GRIS slit-reconstructed He i line core intensity of the RoI acquired between 09:17:32 and 09:37:05 UT on November 10. Bottom panel: Stokes-I profiles, normalized to the continuum intensity, observed at the red, blue and orange (1, 2 and 3) positions shown in the He i line core map. Arrows point to thin darker elongated threads where the optical thickness of the He i line is higher than their own surroundings (Fig. \ref{['fig4bis']}). White contours represent plasma velocity of about $-1$ km s$^{-1}$. The shaded area indicates where the He i line core is calculated (see for more detail Sect \ref{['sec_gris']}). The vertical lines represent the rest wavelengths for the three He i components as reported in Table 1 of Kuckein2012b.
• Figure 4: He i Doppler velocity and optical thickness of the GRIS observation, as calculated using the HAZEL code. Black and red contours represent plasma velocity of $-1$ km s$^{-1}$. Green and yellow contours indicate plasma velocity greater than 10 and 15 km s$^{-1}$.
• Figure 5: From top to bottom: Peak intensity maps and spectral profiles in the locations marked with an asterisk in the respective intensity maps and their relative fits of Mg ii k, Si iv 1394 Å and C ii 1335 Å during the middle IRIS raster scan (11:03 - 11:52 UT). Original spectral line profiles (in black) and corresponding Gaussian fits in red color (for Si iv and C ii spectral lines).