Active chromospheric fibril singularity: Coordinated observations from Solar Orbiter, SST, and IRIS

TL;DR

This paper investigates a puzzling chromospheric fibril singularity observed near a blow-out jet and an adjacent flaring loop, using coordinated multi-instrument observations from Solar Orbiter, SST, IRIS, and SDO. It employs reprojection to Earth-view geometry and FFT-based potential field extrapolation of $B_ ext{LOS}$ data to reconstruct the 3D magnetic topology, revealing a weak-field corridor between like-signed flux patches that hosts inverted Y structures and a chromospheric saddle point. The key finding is that a flaring loop and a jet are linked through a separator fibril singularity formed by reconnection at the interface between the fan separatrix and the saddle, with a coronal null at the jet base enabling the dynamic release. The work demonstrates how photospheric moat flows can drive convergence and magnetic reconnection in chromospheric fine structures, offering a new perspective on jet initiation and the coupling between lower and upper solar atmosphere dynamics. It also underscores the importance of high-resolution, multi-instrument campaigns for identifying and interpreting magnetic topological features like fibril singularities and quasi-separatrix layers.

Abstract

The fine structures of the solar chromosphere, driven by photospheric motions, play a crucial role in the dynamics of solar magnetic fields. Many have been already identified such as fibrils, filament feet, and arch filament systems. Still, high resolution observations show a wealth of structures that remain elusive. We have observed a puzzling, unprecedented chromospheric fibril singularity in close vicinity of a blow-out solar jet and a flaring loop. We aim to understand the magnetic nature of this singularity and the cause of its activity using coordinated high-resolution multi-wavelengths observations. We aligned datasets from Solar Orbiter, SST, IRIS, and SDO. We re-projected the Solar Orbiter datasets to match the perspective of the Earth-based instruments. We performed potential field extrapolations from Solar Orbiter/PHI data. We analysed the spatial and temporal evolution of the plasma structures and their link with the surface magnetic field. This leads us to derive a model and scenario for the observed structures which we explain in a general schematic representation. We have discovered a new feature, a singularity in the chromospheric fibril pattern. It is formed in a weak magnetic field corridor between two flux concentrations of equal sign, at the base of a vertically inverted-Y shape field line pattern. In this specific case some activity develops along the structure. Firstly a flaring loop at one end, secondly a blow-out jet at the other end, where a coronal null-point was present and associated with a chromospheric saddle point being located onto the fibril singularity. The observations sugge

Figures (9)

• Figure 1: Outline of the areas covered by the different instruments drawn on a full disk AIA 171 Å image. The diagram in the top right indicates the different viewing angle (44) and heliocentric distance (0.41 AU) of Solar Orbiter as compared to SST, IRIS, and SDO which all observe along the Earth-Sun viewing line.
• Figure 2: Observations from Solar Orbiter (reprojected to Earth's view), SST, and IRIS showing a fibril singularity and a blow-out jet around 09:40 UT. The Solar Orbiter/PHI magnetogram (left panel) shows the sunspot's moat flow, containing small-scale magnetic patches (see the attached animation). The black arrow in the SST panel points to the chromospheric material ejected from the peeling of the arch filament system. The white box shows the FOV of Fig. \ref{['fig:zoom']}. This figure is associated with an online animation (https://drive.google.com/file/d/1O4EYS0pFC0b3BJj3wQm26zneTUkW6_Ev/view?usp=sharing).
• Figure 3: A zoomed-in view on the area outlined by the white box in of Fig. \ref{['fig:context']}. A strong moat flow is observed near the jet base area and is marked with the black arrow in panel (a). Four main positive and negative polarity patches are labelled as P1, P2 and N1, N2 respectively. The blow-out jet in the coronal EUV 174 Å image, associated with a chromospheric surge in H $\alpha$, is indicated with the gray arrow in panels (b) and (c). Fibrils and the 'X'-shaped saddle point structure are marked in the H $\alpha$ image. A flaring loop associated with the chromospheric fibril singularity is shown in EUI and H $\alpha$. Potential field extrapolation on the PHI magnetogram is shown in panel (d-e). In panel (d), the yellow arrow points the jet base which consists of a fan-spine configuration with a null point and associated outer spine towards the west (black arrow). A zoomed-in version is shown in panel (e), where the several inverted 'Y' structures have been shown from the weak magnetic corridor between N1 and N2.
• Figure 4: Intensity light curves at the flaring loop location in different AIA channels. The curves display a flare-like evolution, with a rapid heating phase that is most pronounced in the hotter channels and a cooling phase after the peak around 09:46 UT that is slower in the cooler channels. The position of the box used to compute the intensity is indicated in Fig. \ref{['fig:aia+eui_jet']}.
• Figure 5: Schematic representation of the observations. The two colliding regions of negative polarity are overlaid with chromospheric fibrils, represented in brown. The flaring loop along the fibril singularity is marked by a curved orange arrow going to the left. The fibril singularity line is shown in magenta with a saddle-point at one end shown in green. A saddle-point is shown at the boundary of fan surfaces and the fibril singularity. Multiple inverted 'Y' structures along the fibril singularity are illustrated with parallel black colour behind the main null-point structure at the jet base.