Partha Chowdhury1, Belur Ravindra2, & Sanjiv Kumar Tiwari3,4
1. University College of Science and Technology, Chemical Technology Dept., University of Calcutta, 92, Acharya Prafulla Chandra Road, 700 009, Kolkata, India
2. Indian Institute of Astrophysics, II Block, Koramangala, Bengaluru, 560 034, India
3. Lockheed Martin Solar & Astrophysics Laboratory, 3251 Hanover St. Bldg. 203, Palo Alto, CA, 94306, USA
4. Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA, 94035, USA
Sunspot region dynamics are heavily influenced by electric currents within the Sun’s plasma, driven by charged particles moving along twisted magnetic field lines. Understanding these currents is vital for comprehending solar activity, as their presence signifies deviations from potential magnetic field configurations and plays a crucial role in energetic events like solar flares and coronal mass ejections[1]. While early theories suggested a net zero current in isolated twisted sunspots, supported by subsequent high-resolution observations indicating balanced net current in isolated sunspots, the presence of strong net currents have been found in ARs with sunspot groups having strong polarity inversion lines[1, 2]. The magnetic free energy stored in these current-carrying fields is believed to power solar flares, with strong net currents observed in highly flare-productive regions, exhibiting distinct patterns compared to CME-productive regions.
Figure 1| The left-panel presents a detailed, high-resolution image of Active Region NOAA~12192, as observed in visible light. The right-panel displays a corresponding magnetogram of the same solar region. In this magnetogram, areas of opposing magnetic polarity are clearly distinguished, with one color representing positive polarity and the other representing negative polarity.
In October 2014, the exceptionally large active region NOAA 12192 (Figure 1) produced a record-breaking sequence of flares but surprisingly no significant CMEs[3]. This region, with its complex magnetic structure, unleashed an X1.6 class flare on October 22nd, which was studied using high-resolution magnetograms and EUV images. Analysis of the total vertical current within the active region revealed that it remained relatively stable for a couple of days before increasing around mid-October 21st (Figure 2 (left)). Interestingly, the current’s orientation reversed in the early hours of October 22nd, growing significantly in strength, and the powerful X1.6 flare occurred precisely when this total current peaked at 5 Terra Amps, subsequently decreasing after the event. This suggests a link between the accumulation of magnetic free energy, reflected in the electric current, and the release of energy during the flare.
Figure 2| Left: This figure shows the change in electric current over four days within the active region. Right: The change in electric current in positive and negative polarity regions over four days. The solid line represents the current in the north polarity region, while the data points (diamonds) represent the current in the south polarity region. Vertical lines indicate the start, peak, and end times of the X1.6 class solar flare, consistent with previous representations. The vertical bars on each data series denote the measurement uncertainty.
Examining the currents in the northern and southern magnetic regions over four days revealed that they often flowed in opposite directions (Figure 2(right)). Both regions experienced an increase and subsequent decrease in current strength starting from October 21st. Notably, the southern region’s current reversed direction around the beginning of October 22nd and remained so until mid-afternoon. The significant X1.6 flare occurred when the currents in both regions flowed in the same direction and reached their strongest points simultaneously. Following the flare, the northern region’s current reversed direction again. These findings indicate that a specific configuration – currents in opposite magnetic regions flowing in the same direction and peaking concurrently – might create favorable conditions for the magnetic reconnection process that powers solar flares.
The study found a sudden surge in the total current just before the X1.6 flare, reaching its peak during the event, suggesting an influx of fresh magnetic energy. In the two days preceding the flare, the overall current in both opposing magnetic areas had been weakening, likely due to the intensification of opposing currents within each area, indicating a highly stressed magnetic field. This behavior suggests that the X1.6 flare might have been triggered when the total currents in the opposing magnetic regions not only reached their maximum but also aligned in the same direction, facilitating explosive magnetic reconnection[4]. The unusual lack of significant CMEs from this highly flare-productive region highlights the need for further investigation into the current behaviors in various active regions to fully understand their role in solar eruptions.
References
[1] Ravindra, B., et al. 2011, ApJ, 740, 19
[2] Wheatland, M. S., 2000, ApJ, 532, 616
[3] Panesar, N. K., et al. 2016, ApJ Lett, 822, L23
[4] Chowdhury, P., Ravindra, B., Tiwari, S. K., 2025, Sol Phys, 300, 36