Lijuan Liu¹, Yuming Wang², Zhenjun Zhou¹, Karin Dissauer³, Manuela Temmer³, & Jun Cui^{1, 4}
1．School of Atmospheric Sciences, Sun Yat-sen University, Zhuhai, Guangdong, 519082, China
2．CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Sciences, University of Science and Technology of China, Hefei, Anhui, 230026, China
3．Institute of Physics, University of Graz, Universita¨tsplatz 5/II, 8010 Graz, Austria
4．CAS Key Laboratory of Lunar and Deep Space Exploration, National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, China

Flares and coronal mass ejections (CME) are closely related, but not with a one-to-one correspondence. Flares with a sign of failed ejecta, are known as “failed/confined eruptions”, while the ones with CMEs are referred to as “successful eruptions”. Eruption details can be determined by the topology of the source magnetic field, hence similar eruptions are expected to be produced from a same source region, e.g., the same polarity inversion line (PIL). Therefore, failed and successful eruptions from a same PIL are worth a study.

Here we perform a detailed comparative study between two eruptions that were initiated from the same PIL (yellow line in Figure 1) within NOAA AR 11387 on 2011 December 25: one failed eruption associated with a C8.4-class GOES soft X-ray flare, and one successful eruption associated with an M4.0-class flare, to discover the physical explanation for the events that are initiated from a similar magnetic environment but with different eruptiveness^[1].

The AIA observations indicate that the two eruptions both started from a reconnection between sheared arcades above the source PIL. The core structure of the failed eruption rose slowly with a peak velocity of 178 km/s, and halted/disappeared at around 0.24 R_sun; the successful eruption released a CME rapidly, with a peak velocity of 1041 km/s.

Figure 1| Evolution of the vector magnetic fields and the velocities of the AR before both eruptions. (a)-(c) vector magnetograms before the failed eruption. (d) six hours-averaged V_⊥ calculated by DAVE4VM before the failed eruption. The orange arrows display the horizontal component, and the cyan contours outline the vertical component at [0.05, 0.09] km/s. (e)-(h) same layouts as (a)-(d) but for the successful eruption.

Clear magnetic flux emergence and converging motion were observed before both eruptions (see Figure 1). Helicity injection and magnetic free energy accumulation also existed at the same time. They accumulated more remarkably before the successful eruption.

Figure 2| Coronal magnetic conditions before the two eruptions. (a) magnetic free energy map before the failed eruption. Each pixel represents the free energy integrated from the photosphere to 42 Mm, saturating at [0, 2.0 × 1028] erg. The dotted line outlines the FPIL. (b) Bh and decay index n distribution before the failed eruption. The position where n reaches 1.5 is outlined by the dashed line. (c),(d) have the same layout as (a),(b) but before the successful eruption.

A flaring PIL (FPIL) mask (dotted lines in Figure 2) involving the source PIL and the flaring area was identified^[2], in which the parameters measuring the non-potentiality of the core region and the confinement of the overlying fields were calculated. Before the eruption onset, the core region of the successful eruption displayed a larger non-potentiality than the failed one. Their decay index distributions had a similar variation trend like a “saddle”, in which a local torus-stable region was enclosed by two torus-unstable regions (see Figure 2). The torus-stable region may play a role in confining the failed eruption^[3].

Figure 3| Eruption-related photospheric B_h change. (a) change of B_h after the failed eruption, saturating at [-600,600] G. The black curve outlines the FPIL. (b) similar layout as (a), but for the successful eruption.

The topology changes deduced from the extrapolated non-linear force free fields confirmed the reconnection process in both eruptions. Besides, significant enhancements of B_h were found in the FPILs after both eruptions (see Figure 3), although the value of the Lorentz force change (calculated based on change of B_h) during the successful eruption was an order of magnitude larger than during the failed eruption, which may give the successful ejecta a larger initial momentum^[4].

In summary, the two eruptions both started from a reconnection between sheared arcades above the PIL, having distinct pre-eruption conditions and eruption details: before the failed one, the magnetic fields of the core region had a weaker non-potentiality; the external fields had a similar critical height for torus instability, a similar local torus-stable region, but a larger magnetic flux ratio (of low corona and near-surface region) as compared to the successful one. During the failed eruption, a smaller Lorentz force impulse was exerted on the outward ejecta; the ejecta had a much slower rising speed. Factors that might lead to the initiation of the failed eruption are identified: (1) a weaker non-potentiality of the core region, and a smaller Lorentz force impulse gave the ejecta a small momentum; (2) the large flux ratio, and the local torus-stable region in the corona provided strong confinements that made the erupting structure regain an equilibrium state.

For more details please refer to our recent publication:
Liu, L., Wang, Y., Zhou, Z., et al., 2018, ApJ, 858, 121

References

[1] Liu, L., Wang, Y., Zhou, Z., et al., 2018, ApJ, 858, 121
[2] Sun X., Bobra M. G., Hoeksema J. T., et al., 2015, ApJL, 804, L28
[3] Wang, D., Liu, R., Wang, Y., et al., 2017, ApJL, 843, L9
[4] Fisher G. H., Bercik D. J., Welsch B. T., et al., 2012, Solar Physics, 277, 59.