Rui Wang^1,2, Ying D. Liu^1,3, J. Todd Hoeksema², I.V. Zimovets^1,4,5, & Yang Liu²

1. State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, China
2. W.W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA, USA
3. University of Chinese Academy of Sciences, Beijing 100049, China
4. International Space Science Institute — Beijing (ISSI-BJ), Beijing 100190, China
5. Space Research Institute (IKI) of the Russian Academy of Sciences, Moscow 117997, Russia

Active region (AR) 12673 produced the largest solar flare (X9.3) in the past decade. One of the most prominent features is the vigorous flux emergence process that even continued after the major eruption (Figure 1a). An extraordinary flux emergence rate of Mx hr^-1 on September 3 was measured^[1], which builds up nonpotentiality (Figures 1b and 1c) and accumulated free energy rapidly for the later eruptions. Meanwhile, our results show that the photospheric magnetic field experienced fast horizontal motions. What is the relationship between the vigorous flux emergence and the fast photospheric motions in the AR, and which one is the dominant contributor to the major eruption, are both what is concerned in this nugget.

Figure 1| Time evolution of magnetic parameters in AR 12673. These parameters, available on a 12-minute cadence, are provided by the keywords in the SHARP data series^[2]. (a) Total unsigned flux and magnetic free energy by magnetic field extrapolations. (b) Total unsigned vertical current. (c) Mean twist parameter α. The vertical red lines represent the onset times of the X-class flares. Uncertainties are overplotted for each of the parameters.

Using HMI/SHARP vector magnetic field data^[2], we study the helicity in the corona. The emergence helicity accumulated in the corona is Mx² before the major eruption, while the shear helicity accumulated in the corona is Mx², which contributes about 79% of the total helicity (Figure 2). The shear-helicity flux is dominant from the beginning of the flux emergence to the major eruption, and it continued after the eruption. Our results imply that the emerged field initially contains relatively low helicity. Much more helicity is built up by shearing and converging flows acting on the pre-existing or emerging flux. We tend to believe that parts of the shearing flows come from horizontal divergent flow due to the rising flux in the subphotosphere^[3], since the profile of the shear-helicity flux correlates well with the total unsigned flux, and the distributions of horizontal flows of the flux appear to connect with the vertical flows.

Figure 2| Temporal profiles of magnetic helicity of AR 12673. (a) Red and blue curves represent helicity fluxes across the photosphere from shear and emergence terms, respectively. (b) Red and blue curves refer to accumulated helicities in the corona from shear and emergence terms, respectively. Vertical green lines correspond to the onset times of the X-class flares.

The evolution of the vertical currents also shows that most of the intense currents do not appear initially with the emergence of the flux (Figure 3), which implies that the emerging flux is probably not strongly current carrying, i.e., field lines are not twisted strongly in the early emergence phase. This is consistent with our conclusion from the study of the magnetic helicity. On the other hand, we find that the intense vertical currents appear on both sides of the polarity inversion line (PIL) as two opposite magnetic polarities are pressed against each other (Figure 3). Shearing motions are getting stronger with the flux emergence, especially on both sides of the PIL of the core field region formed by the coalescence of magnetic polarities after September 3, which is reflected in the shear-helicity flux. Therefore, the vertical currents on both sides of the PIL in the core field region are mainly contributed by the photospheric motions. The presence of intense vertical current quantifies the twist and shear of the magnetic field. Accordingly, we think that the flux rope in the core field region, exhibited by the extrapolations, is formed by long-term photospheric motions. The converging motions that press the opposite polarities against each other enhance the extent of magnetic shears and the gradient of magnetic fields.

Figure 3| Top row: SHARP vector magnetic field in the core region of AR 12673 at specific times. The background image is the normal field, with positive field in white and negative field in black. It is scaled to ±500G (white/black contours). Arrows represent tangential component of magnetic field. The blue/red arrows indicate that the normal fields at those pixels are positive/negative. Bottom row: vertical current (J_z) distributions overplotted on the corresponding magnetograms. Yellow (blue) contours represent positive (negative) electric current density J_z of 150 (-150) mA m^-2. The red curve is the PIL.

AR 12673 is representative, as the photospheric motions contribute most of the nonpotentiality in the AR with vigorous flux emergence. The knowledge of the role of photospheric motions and flux emergence is important for predicting disastrous space-weather events. For more details, please see Ref. [4].

We would like to thank Dr. Keiji Hayashi for fruitful discussions and valuable suggestions on this work.

References

[1] Sun, X., & Norton, A. A. 2017, RNASS, 1, 24
[2] Bobra, M. G., Sun, X., Hoeksema, J. T., et al. 2014, Solar Phys., 289, 3549
[3] Toriumi, S., Hayashi, K., & Yokoyama, T. 2012, ApJ, 751, 154
[4] Wang, R., Liu, Y. D., Hoeksema, J. T., Zimovets, I. V., & Liu, Y. 2018, ApJ, 869, 90