Elongation of a Solar Filament and its Three-Dimensional Numerical Reconstruction for Magnetic Structures

TL;DR

This study investigates how a quiescent solar filament elongates and reorganizes its feet in response to photospheric magnetic activity. It combines multi-wavelength observations from THEMIS, IRIS, AIA/SDO, GONG, Hinode/SOT, and HMI with a data-driven 3D NLFFF reconstruction that inserts a flux rope into a potential field and relaxes it to a force-free state. The results show that feet are linked to parasitic polarities and flux cancellation at the PIL, while the filament extends as a full flux rope develops, with the two-day evolution captured by a two-part FR separated from the surrounding field by current layers on quasi-separatrix layers (QSLs). Longitudinal oscillations with a period near $70$ minutes emerge as heating from reconnection energizes plasma along the dipped field lines, illustrating how photospheric flux evolution shapes the coronal magnetic topology and filament dynamics.

Abstract

Quiescent filaments are prominent features of the solar atmosphere, and their evolution reflects the coronal magnetic field's response to photospheric magnetic activity. Here, we report on a quiescent filament observed from 2023 September 28-29, aiming to understand how the magnetic configuration shapes its feet and drives its extension. For this purpose, high-resolution spectral data in H$α$ and Mg II k are used from the Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires (THEMIS) and the Interface Region Imaging Spectrograph (IRIS), respectively. To track changes in the filament, we utilise long-term data from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and from the Global Oscillation Network Group (GONG). We analyse the longitudinal magnetic field in the photosphere using the Solar Optical Telescope (SOT) onboard Hinode, as well as SDO/Helioseismic and Magnetic Imager (HMI) data. In addition to this, we use GONG H$α$ data to analyze the longitudinal oscillations in the filament. Observations show that parasitic polarities and canceling flux play a key role in forming and reorganizing the filament feet and in lengthening the filament. A 3D MHD reconstruction using vector magnetograms reveals that its magnetic configuration evolves into a full flux rope (FR), whose extension on the second day matches the observed filament growth. The FR is separated from the surrounding nearly potential field by quasi-separatrix layers, which in turn are separated by current layers. They get more organized around the FR as it is growing up. Moreover, the longitudinal oscillations in the extended filament are attributed to heating from flux cancellation in underlying bright points.

Figures (12)

• Figure 1: Panels (a) and (b) show the full disk H$\alpha$ image observed by Meudon spectroheliograph on 28 and 29 September 2023, respectively. The black box indicates the filament observed by THEMIS and IRIS during the 2023 campaign. We note the change in shape of the east end of the filament on September 29.
• Figure 2: Zoom on the filament observed with GONG H$\alpha$ (panels a and d), the longitudinal magnetic field (HMI in panels b and e) and a local zoom of Hinode/SOT with levels $\pm$ 100 G (in panels c and f) for September 28 and September 29. The two FOVs of the filament are aligned with the FOV of 29 Sept at 11:00 UT. We note that the global pattern of the network polarities of HMI is well co-aligned. The FOV of SOT is represented by the dashed black box in HMI magnetograms. F$_1$, F$_2$, and F$_3$ are feet of the filament. On September 28, two feet, F$_1$ and F$_2$, are observed, while on September 29, feet F$_1$ and the extension F$_3$ are present. In the SOT maps, ovals indicate the parasitic polarities (n$_1$, p$_2$) and cancelling flux regions (n$_3$ p$_3$, n$_4$ p$_4$). More details on the observed data sets, including the observed wavelengths, are provided in the Appendix \ref{['appendix']}. An animation of panels a and d is available, starting on 27 September 2023 at 23:59 UT and ending on 29 September 2023 at 23:58 UT. The real-time duration of the animation is 3 min 12 s. (An animation of this figure is available in the online article.)
• Figure 3: Region of the filament observed in AIA 304 and 193 Å on the 28$^{th}$ and 29$^{th}$ September 2023 (panels a, d and b, e, respectively). The FOVs are aligned on the FOV of September 29 at 11:00 UT. HMI magnetograms with levels $\pm$ 100 G of the filament region for September 28 and 29 are shown in panels (c) and (f), respectively. The blue curve represents the filament spine traced along the dark part of the filament from AIA 193 Å images for the 28$^{th}$ and 29$^{th}$ of September, respectively. The feet F$_1$, F$_2$, and the extension F$_3$ are indicated in panels a, b, d, and e. The polarities n$_1$, p$_2$ are indicated in panel c, and n$_3$, p$_3$, n$_4$, p$_4$ in panel f. The ellipses marked in the AIA filter images correspond to those in the HMI magnetograms, denoting the parasitic polarities (n$_1$, p$_2$) and the bipoles (n$_3$–p$_3$ and n$_4$–p$_4$). The black contours in panels d and e outline the observed EUV bright points. An animation of this figure is available, starting at 00:00 UT on 28 September and ending at 23:57 UT on 29 September. The real-time duration of the animation is 32 s.(An animation of this figure is available in the online article.)
• Figure 4: Zoomed view of the filament observed by GONG (panel a) in H$\alpha$, and the slit-reconstructed images from the spectra in H$\alpha$ line center observed by THEMIS (panel b), and in Mg II k line center (2796.4 Å) by IRIS (panel c), for September 28 and in d, e, f for September 29. The contours (blue) of the filament observed in GONG are overlaid on the THEMIS image, and the filament contours from THEMIS are overlaid in white over GONG and IRIS images to show the THEMIS FOV, which is focused on F$_2$ on September 28 and on F$_1$ on September 29. These images are aligned with the X coordinate at 11:00 UT on September 29. Fine structures in the filament are visible in THEMIS and IRIS maps.
• Figure 5: Filament contours and magnetic field evolution. The magnetograms are displayed with magnetic field levels of $\pm$50 G for the positive and negative polarities, respectively. The white contours in each panel are the filament contours observed in GONG corresponding to the HMI magnetogram time. Panel a correspond to the initial phase of the formation of foot F$_2$ located near the very small positive polarity p$_2$. Panel b corresponds to the $1^{st}$ maximum of polarity p$_2$ at around 08 UT, shown by the left red arrow in Figure \ref{['HMI_time']} b. Panel c shows a slight decrease in p$_2$. Panel d is when F$_2$ has disappeared. Panel e is when F$_2$ reappears around 21 UT. Panel f corresponds to the $2^{nd}$ maximum of p$_2$. We also observe a small negative polarity within p$_2$ and the disappearance of F$_2$. In panel g, the negative polarity within p$_2$ is stronger, and none of the filament feet are clearly identifiable. In panel h, a strong negative polarity has emerged at the location of p$_2$, which corresponds to the maximum of the blue curve in Figure \ref{['HMI_time']} b at around 06 UT on September 29. In panels h and i, we observe two bipoles n$_3$ p$_3$ and n$_4$ p$_4$ (ellipses in panel i) and the extension of the filament up to F$_3$. An animation of this figure is attached. The animation starts at 23:59 UT on 27 September and ends at 23:53 UT on 29 September. The animation's real-time duration is 1 min 20 s. (An animation of this figure is available in the online article.)