Two-stage primary acceleration in filament initial eruption under a fan-spine magnetic configuration

TL;DR

This study examines the initial eruption of a solar filament embedded in a fan-spine magnetic configuration, revealing a two-stage acceleration driven by distinct reconnection processes. Using multi-instrument observations (SDO/AIA, CHASE, IRIS, HMI) and magnetic-field extrapolations, the authors trace the formation of a hot channel and the gradual growth of a pre-eruptive flux rope, followed by a rapid ascent associated with an external reconnection at a null-point in a QSL. The first acceleration is tied to internal filament reconnection forming the hot channel and a slow rise, while the second acceleration is triggered by reconnection above the filament, producing an annular circle ribbon and a fast eruption with an axial-flux surge to over $1.1 \times 10^{21}$ Mx. The findings emphasize the critical role of fan-spine topology and QSL-related breakout reconnection in enabling successful eruptions in complex multipolar fields, with implications for CME forecasting and space weather prediction.

Abstract

Understaning the filament rising process is crucial for unveiling the triggering mechanisms of the coronal mass ejections and forecasting the space weather. In this paper, we present a detailed study on the filament initial eruption under a fan-spine structure. It was found that the filament underwent two distinct acceleration stages corresponding to a calss M1.0 and M4.6 flare event, respectively. The first acceleration stage commenced with the filament splitting, after which the upper portion was subsequently heated being a hot channel and slow rose at an average speed of 22 km/s. A set of hot reverse C-shaped loops appeared repeatedly during the filament splitting and a hook structure was recognized at this phase, suggesting ongoing growth of the magnetic flux rope (MFR). When it reached a certain altitude, the hot channel appeared to get into a quasi-static phase with its upper edge seriously decelerated and lower edge expanding downward. Approximately 30 minutes later, as a distinct annular ribbon appeared outside the hook structure, the hot channel rose again at a velocity over 50 km/s accompanied with rapid footpoints drifting, and experienced the second acceleration stage with its axial flux increased to 1.1 X 10^{21} Mx. It is deduced that the filament initial eruption under a magnetic dome possess multi kinetic process. We suggest that the magnetic reconnection taken place within and beneath the filament continues to trigger the growth of pre-eruptive MFR and the first acceleration, when the magnetic reconnection above the filament plays a key role in the second acceleration.

Figures (9)

• Figure 1: An overview of the filament eruption. (a) Photospheric continuum image of ARs observed by SDO/HMI. The black box outlines the field of view (FOV) for panels (b)-(d). (b)-(c) H$\alpha$ images showing the filament before (b) and during (c) the eruption observed by CHASE. (d) Line-of-sight magnetogram with the blue contours at B = - 500 G. The red and black regions represent positive and negative magnetic field, respectively. (e) Temporal evolution of GOES 1-8 Å soft X-ray flux. The vertical dotted, dashed and dot-dashed lines indicate the start, peak and end time for each flare. The shadow region shows the operation period of CHASE and the solid red line indicates the end time of the IRIS observation.
• Figure 2: Images of the filament eruption. (a1)-(a4), (b1)-(b4) and (c1)-(c4) show the filament eruption in AIA 131 Å, 211 Å and 304 Å passband, respectively. The red dashed lines in panels (a1)-(a3) denote the erupted filament F1 and hot channel. The black arrows in panels (a1)-(a3) indicate the occurred flare loop (FL). The white dashed lines in panel (b1) and (c1) denote the remained filament F2. The blue arrows in panels (b1)-(c4) indicate the foot points (FP1 and FP2) of the hot channel. The yellow arrows in panels (c1)-(c4) indicate the appeared flare ribbons. (d1)-(d4) show the average temperature derived by DEM method. The black box indicates the region to get the mean temperature of the hot channel in Figure \ref{['fig4']}. A 17 s animation including AIA 131 Å, 211 Å and 304 Å is available online to present the filament splitting and eruption from 19:00 UT to 22:30 UT.
• Figure 3: Evolution of the flare ribbons. (a1)-(a4) and (b1)-(b4) show the evolution of hot channel and flare ribbons in a larger filed of view (FOV) in AIA 131 Å and 304 Å. (c1)-(c4) and (d1)-(d4) show the evolution of flare ribbons observed by CHASE and IRIS. The yellow arrows denote the flare ribbon R1-R4 and the yellow dashed line contours the large circle ribbon R3. The black arrows in panels (a1) and (d4) indicate the slices S1-S3 to obtain the time-distance diagrams in Figure \ref{['fig4']}. The white boxes denote the integral region to get the light curves in Figure \ref{['fig4']}. The black boxes outline the FOV of panels (c1)-(c4) and (d1)-(d4), respectively. The black arrows in panels (c1) and (c4) indicate the south footpoint (FP1) of the filament F1. The red arrows indicate the identified hook structures. The orange dashed line contours the recongrized footpoint to integrate the axial flux of the MFR. A 10 s animation including AIA 131 Å, 304 Å from 20:26 UT to 22:30 UT and IRIS 1400 Å from 19:52 UT to 21:28 UT is available online to present the evolution of the flare ribbon.
• Figure 4: Dynamical evolution during the filament eruption. (a)-(d) Time-distance diagrams along the slice S1, S2 and S3 in Figure \ref{['fig3']}, respectively. (e) Normalized light curves at AIA 94, 131 Å in the white box in Figure \ref{['fig3']}(a3), and IRIS 1400 Å in box 1 and 2 in Figure \ref{['fig3']}(c4). (f) Velocity derived from the white dashed lines in panels (a)-(c). (g) Temporal evolution of the mean temperature of the hot channel as indicated by the black square in Figure \ref{['fig2']}. (h) Temporal evolution of the magnetic flux of the MFR. The vertical dotted and dashed lines denote the start and peak time of two flares, respectively.
• Figure 5: Configuration of coronal loops. (a)-(c) AIA 171 Å images showing the changes of coronal loops. The white, black and pink arrows indicate different coronal loops L1, L2 and L3, respectively. The green circles indicates the footpoints of sheared arcade L2. The black box shows the FOV in panel (h). (d)-(f) Running difference images at AIA 171 Å. The pink dashed lines show the opened coronal loop L3. (g) Footpoint drifting and coronal dimming at 211 Å. The yellow arrow indicates the post flare loops. The red arrow indicates the dimming region. (h) The remained H$\alpha$ filament overlaid with magentic field. The white and black contours represent the positive and negative magnetic polarities of $\pm$ 500 G. The blue and red contours represent the brightening edges at the footpoint. (i) H$\alpha$ profiles at the filament as pointed by the purple square in panel (c) and (h).