Suman K. Dhakal^[1], Jie Zhang^[1], Panditi Vemareddy^[2], and Nishu Karna^[3]
1. Department of Physics and Astronomy, George Mason University, Fairfax, VA 22030, USA
2. Indian Institute of Astrophysics, II Block, Koramangala, Bengaluru-560 034, India
3. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA

Homologous solar eruptions, while originating from the same location in the same active region in a time sequential manner, are found to be similar in a variety of properties^[1,2]. Such eruptions are possible only if there is a continuous accumulation of magnetic energy in the same local region. Therefore, their studies are important to understand the continuous magnetic energy build-up and impulsive energy release in the corona.

Figure 1| Three homologous eruptions from NOAA AR 11429. (a-c) Time profiles of GOES soft X-ray intensity flux during the three eruptions. The maximum flux in these profiles corresponds to X1.3, M6.3, M8.4 flares, respectively. Red dotted line in (a) shows the contamination in the X-ray profile due to an earlier eruption. (d-f) Overlay of the flare ribbons (in blue) observed in AIA 1600 Å on the HMI LOS-magnetogram. (g-i) Difference images of AIA 193 Å showing the EUV dimming associated with each eruption. White box represents the FOV of the magnetograms shown in (d-f).

NOAA AR 11429 had a very complex magnetic configuration and was very flare productive. Episodic new flux emergence was observed in this AR until 2012 March 9. Dhakal et al.^[2] presented the analysis of three homologous eruptions from AR 11429. The first eruption happened on 2012 March 7 at ~01:05 UT, the second eruption happened on March 9 at ~03:22 UT, while the third eruption on March 10 at 16:50 UT. The first eruption was associated with an X-class flare and the other two eruptions were associated with M-class flares. The same photospheric location within the AR, similar evolution and location of flare ribbons, EUV dimmings, and nearly identical coronographic morphology of the CME invoked them as truly homologous eruptions (see Figure 1). This large and complex AR divided into two relatively simple sub-regions: northeast (NE) and southwest (SW) (see Figure 2). Homologous eruptions occurred from the SW sub-region over the period of four days.

Figure 2| Left panel: HMI LOS-magnetogram on March 9. Teal box shows the SW sub-region. The green asterisks show the location of strong gradient PIL and the pink line shows the distance between the centroids of positive and negative magnetic poles. Right panel shows the convergence of opposite magnetic polarities and magnetic flux cancellation in the southwest sub-region of the AR 11429. Convergence rate is examined by monitoring the distance between the centroids of positive and negative magnetic polarities and is shown in black asterisks. The time profile of negative flux in the SW subregion is in red. Vertical dashed lines refer to the time of three solar eruptions from the AR.

During the evolution of the AR, opposite magnetic poles in the SW sub-region were continuously moving towards each other (see the plot in black asterisks in Figure 2). The rate of convergence was higher after the flux emergence stopped in the AR. Further, shearing motion between the opposite magnetic fluxes, along the SW-PIL, was observed throughout the evolution. Though it was impossible to isolate the SW sub-region completely, the isolation and analysis of negative polarity of the SW sub-region showed that there was continuous flux cancellation in the SW sub-region. Convergence of opposite fluxes along the PIL, shearing motion, and flux cancellation is the classical scenario that can form sheared and twisted magnetic structure^[3].

Figure 3| Observation of pre eruptive coronal structure and magnetic structure modeled by NLFFF extrapolation. Left panels: the HCS before each eruption in AIA 131 Å. The brighter part of the HCS is outlined by orange asterisk. Middle (Right) panels: the top (side) view of magnetic field lines rendered on HMI LOS-magnetograms before each eruption. The sheared and twisted field lines, along the SW-PIL, are shown in blue-red lines. The green lines show the overlying magnetic field in the corona.

A filament was lying along the SW-PIL throughout the observational period and was observed soon after the eruption. A surviving filament suggested a partial eruption of the magnetic system^[4]. Also, a coherent hot channel structure (HCS) was observed to lie along the SW-PIL before each eruption (see Figure 3). The shape and location of reformed HCS, before the second and third eruptions, were identical to the first observed HCS. HCS are considered as the observational proxy of MFR^[5]. Non-linear-force-free-field (NLFFF) extrapolation results showed that long, sheared and twisted magnetic field lines were lying along the SWPIL. Thus, both observational and extrapolation results suggest the formation and existence of an MFR in the SW sub-region before each homologous eruption.

Our results suggest that the shearing motion and magnetic flux cancellation of opposite fluxes were: (1) the dominant factor, irrespective of the evolutionary phase, that contributed the recurrent homologous eruption, and (2) the key processes of forming the erupting structure, likely a magnetic flux rope, and its long-lasting continuation results in reformation of identical erupting structure.

References

[1] Zhang, J., & Wang, J. 2002, ApJ Lett., 566, L117
[2] Dhakal, S. K., Zhang, J., Vemareddy, P., & Karna, N. 2020, ApJ, 901, 1
[3] van Ballegooijen, A. A., & Martens, P. C. H. 1989, ApJ, 343, 971
[4] Gilbert, H. R., Holzer, T. E., Burkepile, J. T., & Hundhausen, A. J. 2000, ApJ, 537, 503
[5] Zhang, J., Cheng, X., & Ding, M.-D. 2012, Nature Comm., 3, 747