Partial Null Point Reconnection of an Eruptive Filament

TL;DR

The paper analyzes a 13 July 2015 filament eruption associated with a GOES B8.9 flare and a CME, using SDO/AIA/HMI, GONG, and PFSS modeling. It documents a three-phase eruption in which magnetic flux cancellation builds a flux rope that destabilizes, triggering a standard flare beneath the rising structure and, above it, interchange reconnection at a magnetic null point that produces a large circular brightening and an inner brightening. The authors connect the observed circular ribbon, loop contraction/expansion, and CME morphology to a fan-spine topology with a null point, offering a unified framework where reconnection occurs both below and above the erupting filament. This work highlights the role of null-point reconnection in shaping flare ribbons, loop dynamics, and CME development, contributing to a more complete understanding of complex filament eruptions in structured coronal fields.

Abstract

Solar filaments are cool and dense plasma structures suspended in the solar corona against gravity. We present observations of a quiescent filament eruption that occurs on 13 July 2015. The eruption is associated with a two-ribbon GOES B8.9 class flare. Photospheric magnetic flux cancellation is present below the filament during days. This builds up a flux rope which progressively rises until it gets unstable, first leading to a confined eruption and pre-flare brightenings, then to an ejection which starts $\approx$ 20 min later with the flare onset. An interesting feature of this event is the presence of a large circular brightening formed around the erupting region. This brightening is produced due to interchange reconnection of the ejected magnetic configuration with the surrounding open magnetic field. This null-point topology is confirmed by a potential-field extrapolation. The EUV loops located on the southern side of the filament eruption first contract during the null-point reconnection, then expand as the flux rope is ejected. The associated CME has both a classical flux rope shape and plasma ejected along open field lines on the flux rope side (a trace of interchange reconnection). Finally, we set all this disparate observations within a coherent framework where magnetic reconnection occurs both below and above the erupting filament.

Figures (13)

• Figure 1: Top panel: Temporal evolution of GOES X-ray flux in the 1 -- 8 Å wavelength range. Bottom panel: Temporal variation of the intensity of AIA in different wavelengths accumulated in the black box shown in Figure \ref{['fig:evolution']}h. For better visualization, the intensity of AIA 193 Å is divided by 6. Plots with different colors correspond to different wavelengths as indicated on top left corner of the panel. The gray shaded region highlights the pre-flare brightening in the active region.
• Figure 2: Evolution of the filament eruption in AIA 131 Å (a -- c), 193 Å (d -- f), 304 Å (g -- i), and in GONG H$\alpha$ (j -- l) wavelengths. The red and blue contours in panel d are the contours of positive and negative magnetic polarities ($\pm$ 40 G) from HMI, respectively. The black box marks the region used to calculate the AIA light curves in Figure \ref{['fig:goes_aia']} (bottom panel). The erupting filament is shown with white arrows in panels e, g, and h at different projected heights. The flare loops and flare ribbons are pointed with arrows in panels f and i. An animation (MOV_Fig2.mp4), related to this figure, is included as online supplementary material.
• Figure 3: Images of AIA 304 Å in the top panels show the filament feet with green circles. Panel b is the same as panel a with magnetic field contours ($\pm$ 20 G) from SDO/HMI, red for the positive and blue for negative magnetic polarity. The middle and bottom panels are the base difference images (subtracted from the image at 08:30 UT) showing the large circular ribbon around the erupting filament. The initial circular ribbon brightening is shown in (c) and (e) with white arrows. The black arrows in (d) show the inner brightenings inside the circular one. Red arrows in (d) show two inverse J-shaped flare ribbons.
• Figure 4: Time-distance analysis of the filament eruption in different AIA wavelengths. (a): AIA image in 304 Å showing the selected direction with a white arrow. (b), (c), (d), and (e): Display of the time-distance diagram along the chosen direction in AIA 304, 211, 193, and 131 Å, respectively. The dotted and dash-dotted lines in the time-distance diagrams are the fittings of linear and a combination of linear and exponential functions, respectively.
• Figure 5: Images of AIA 171 Å, using the MGN method, showing the evolution of the EUV loops at the south of the erupting filament. The white vertical lines represent the top of one set of loops before the eruption, i.e. at 8:40 UT. In panel a, two white arrows point to : (left) a straight elongated shape suggesting the presence of open magnetic field lines and (right) very stretched loops. The same vertical line has been added to panels b and c. An animation (MOV_Fig5.mp4) of this figure is available.