Alphonse Sterling¹, Ronald Moore^{2, 1}, & Navdeep Panesar¹

1. NASA Marshall Space Flight Center, Huntsville, Alabama 35812, USA
2. University of Alabama in Huntsville, AL 35899, USA

What spawns CME-producing eruptions on the Sun?  A hint might come from an unexpected source: Coronal jets! Jet-producing regions are much smaller (size scale ~20,000 km)^[1] than CME-producing active regions (ARs; size ~100,000 km).  But recent studies show that jets result from the eruption of miniature  filaments (“minifilaments”), of size ~10,000 km, closely analogous to the larger-scale filament eruptions that produce CMEs^[2]. Minifilaments evolve from birth to  eruption over a few hours to a couple of days^[3], much shorter than the lifetime of large-scale filaments (weeks or months). This suggests that the buildup to jets is like the buildup to CMEs, but in “fast motion.”

On-disk studies of jets indicate that they are frequently spawned by magnetic-flux cancellation^[1,4].  Here (with details in Ref. [5]) we report on similar cancellation leading to two CME eruptions.

Figure 1| Time-distance map showing evolution of the first of two small ARs studied by Sterling et al.^[5], showing first  separation and then retraction of the region’s bipole poles over time. The vertical axis is the north-south direction on the solar disk, and the horizontal axis is time. (The displayed flux was summed in the east-west direction over the region of interest^[5].) Blue dashed lines show 00:00 UT for each day, while the orange solid line indicates the time of the CME-producing eruption. Reproduced from Figure 5 of Ref. [5].

We selected two ARs that were small (maximum flux ~10²¹ Mx) so that we could follow each within a single disk passage, from its emergence through its core-eruption time. We were also careful to select regions that were well isolated from surrounding regions, so that we could study their behavior as isolated magnetic systems. Our analysis used SDO/HMI magntograms, SDO/AIA EUV images, and SoHO/LASCO and STEREO/COR coronagraph images. 

Figure 1 shows a time-distance plot of the HMI magnetic evolution of the first region over a three-day period from early in its emergence, through the time of the CME-producing eruption on 2013 Oct 20 (orange line).  At first, the main positive-negative poles of the region separated from each other with time. Later however, the poles reversed direction, and started to converge upon each other. By mid-day on Oct 20 the opposing poles of the central portion of the region are undergoing cancellation. It is during this period that the eruption occurred, on Oct 20.

Figure 2| Variation of the positive flux with time in the region of Figure 1. The positive flux was selected because it was the minority polarity of the overall region, and hence could be more easily tracked within a limited area over the time period of interest (see Ref. [5] for precise region over which flux was integrated). The green line is a fit to the decreasing flux, indicating a cancelation rate of 1.1 x 10¹⁹ Mx/hr. Reproduced from Figure 6 of Ref. [5].

Figure 2 shows the positive-polarity flux evolution quantitatively. After increasing during emergence, from Oct 19 this flux rapidly reduces with time.  We attribute this to cancelation occurring along the central polarity inversion line (PIL) of the region, along with cancellation on the external PIL formed between the AR’s positive polarity and pre-existing negative field (that negative flux shows faintly in Fig. 1 to the south of the positive polarity, and canceling with the positive polarity from early on Oct 19). Filaments formed at both cancellation sites,  and both of these filaments erupted in close succession at the time of the orange line in  Figure 1. Together, this dual filament eruption was accompanied by a GOES C-class flare, and a CME of moderate angular extent observed by LASCO (Fig. 3). 

Figure 3| Coronagraph images from SOHO/LASCO C2, showing a CME erupting from the small AR of Figure 1. Radial lines in (b) indicated that the CME had an angular width of ~60 degrees. Reproduced from Figure 3 of Ref. [5].

A second isolated small AR examined in Ref. [5] showed similar behavior to the first case, with a filament (only one in that case) erupting from the region’s central PIL and producing a CME observed by STEREO on 2010 July 16.  

Because the regions were both nicely isolated, we could measure the percentage of each region’s peak total minority-polarity flux that disappeared — presumably through cancelation — before the eruption occurred. For the first case this was ~30% (Fig. 2), and for the second case it was ~50%. Similar measurements of magnetic flux canceling at the base of coronal jets outside of ARs give percentages of ~35%—45%^[1,5]. Based on this, we suggest that there may be some critical threshold, in the range of 50%,  of the flux of a twisted-field bipole that cancels to build a non-potential core region (often carrying a minifilament or filament) and trigger it to erupt explosively to make a jet or CME.

Please see Ref. [5] for full details of this study.

References

[1] Panesar, N. K., Sterling, A. C., Moore, R. L., & Chakrapani, P. 2016, ApJL, 832,L7
[2] Sterling, A. C., Moore, R. L., Falconer, D. A., & Adams, M. 2015, Nature, 523, 437
[3] Panesar, N. K., Sterling, A. C., & Moore, R. L. 2017, ApJ, 844, 131
[4] Sterling, A. C., Moore, R. L., Falconer, D. A., Panesar, N. K., & Martinez, F. 2017, ApJ, 844, 28 
[5] Sterling, A. C., Moore, R. L., & Panesar, N. K. 2018, ApJ, 864, 68