Non-neutralized Electric Currents in Solar Active Regions and Flare Productivity

Abstract

We explore the association of non-neutralized currents with solar flare occurrence in a sizable sample of observations, aiming to show the potential of such currents in solar flare prediction. We used the high-quality vector magnetograms that are regularly produced by the Helioseismic Magnetic Imager, and more specifically, the Space weather HMI Active Region Patches (SHARP). Through a newly established method that incorporates detailed error analysis, we calculated the non-neutralized currents contained in active regions (AR). Two predictors were produced, namely the total and the maximum unsigned non-neutralized current. Both were tested in AR time-series and a representative sample of point-in-time observations during the interval 2012 – 2016. The average values of non-neutralized currents in flaring active regions are higher by more than an order of magnitude than in non-flaring regions and correlate very well with the corresponding flare index. The temporal evolution of these parameters appears to be connected to physical processes, such as flux emergence and/or magnetic polarity inversion line formation, that are associated with increased solar flare activity. Using Bayesian inference of flaring probabilities, we show that the total unsigned non-neutralized current significantly outperforms the total unsigned magnetic flux and other well-established current-related predictors. It therefore shows good prospects for inclusion in an operational flare-forecasting service. We plan to use the new predictor in the framework of the FLARECAST project along with other highly performing predictors.

Appendix: Differential Versus Integral Form of Ampère’s Law

In the original work of Georgoulis, Titov, and Mikić (2012), the integral form of Ampère’s law was chosen as being more accurate, although more time consuming. To examine the trade-off between speed and accuracy, we used both versions to calculate the total unsigned non-neutralized current for AR 11158. The results are shown in Figure 6, which shows that the two versions produce practically the same results. For the longer part of the time series, currents calculated by the two versions are the same within the uncertainties. For most of the points, the differences are within the error bars, and the two curves differ only at a few points. Overall, the differential form of Ampère’s law produces slightly smoother curves, since it uses a larger number of pixels, which smoothes out differences from image to image (which could be due to noise or errors in the azimuth disambiguation and deprojection). Because it is based on the definition of a “correct” contour around each partition, the integral form is more sensitive to these variations. In view of this result and because the integral form of Ampère’s law is significantly more time consuming, we chose to use the differential form to calculate the electric currents that correspond to each partition.

Figure 6

Total non-neutralized current, $I_{\mathrm{NN},\mathrm{tot}}$ of AR 11158 as a function of time, calculated using the integral (black) and differential (red) form of Ampère’s law.