Ellis A. Avallone1 and Xudong Sun2
1. Institute for Astronomy, University of Hawai’i at Mānoa, 2680 Woodlawn Dr., Honolulu, HI 96822, USA
2. Institute for Astronomy, University of Hawai’i at Mānoa, Pukalani, HI 96768, USA
Solar eruptions are energetic events that send high-energy radiation and charged particles through interplanetary space. If an eruption is directed towards Earth, satellites, astronauts, and electrical grids can be negatively affected. While there has been a significant effort to predict solar eruptions, we still don’t fully understand the environments where these eruptions originate.
Active regions (ARs) are a primary source of solar eruptions. They are rooted in sunspots and host strong magnetic fields that can be described by a flux rope, a tube-like region of space containing a twisted magnetic field. Like all evolving magnetic fields, flux ropes are current-generating structures and contain direct currents, which connect AR polarity centers, and return currents, which surround and oppose the direct currents. Figure 1 (taken from Ref [1]) shows a pre-emergence flux rope and a post- emergence flux rope along with their associated currents.
Figure 1| AR magnetic fields and currents. Panel (a) shows a pre-emergence flux rope, its associated magnetic field (rainbow), and direct (orange) and return (blue) currents. Panel (b) highlights the distribution of direct (orange) and return (green) currents in a post-flux emergence AR.
It’s a well-known fact that free magnetic energy associated with AR currents drives solar eruptions, but it’s unclear how well these driving currents are neutralized (i.e. how close the ratio of direct to return currents in each magnetic polarity is to 1). The degree of current neutralization in ARs has been debated for years. While previous theoretical studies suggested that the average net current density of ARs is zero [2], other theoretical studies and numerical simulations have shown that non-neutralized currents emerge alongside the development of substantial shear along an active region’s polarity inversion line (PIL) [1,3].
In this work, we build off these previous theoretical studies and an observational study[4] by analyzing a sample of eruptive and non-eruptive ARs in the SHARP datasets from SDO/HMI. We start by calculating the current density of ARs from vector magnetograms using Ampère’s Law. From these current density maps, we determine the ratio between direct and return currents in each polarity and in the full AR (|DC/RC|tot). By doing this for our sample of 15 flare-active and 15 flare-quiet ARs, we can compare the degree of current neutralization to flare productivity.
We compare the degree of current neutralization between flare-active and flare-quiet ARs and find that the degree of current neutralization differs between these two populations at a 90% confidence level. We also compare the degree of PIL shear and find that flare-active ARs are more sheared than flare-quiet ARs at a 90% confidence level. Figure 2 shows the distributions of current neutralization and PIL shear for our full sample.
Figure 2| Histograms showing the distribution of |DC/RC|tot (left) and PIL shear (right) in flare-active (orange/red) and flare-quiet (green/blue) ARs.
We also focus on examples (eg. Figure 3) of flare-active and flare-quiet ARs to look for other trends within the two populations. For flare-active ARs, we observe coherent current ribbons around the PIL in their current density maps. This indicates the presence of a coherent flux rope, which is a necessary condition for eruption. We don’t observe this trend in flare-quiet ARs, where the currents are more evenly distributed.
Figure 3| A Flare-active (left) and flare-quiet (right) AR. Top panels show the magnetograms, middle panels show the current density maps, where the flare-active AR is more coherent and the flare-quiet AR is more scattered, and bottom panels show |DC/RC| in the positive (red) and negative (blue) polarities of each AR and the unsigned flux (gray) over the on-disk lifetime of each AR.
Finally, we analyze contributions to systematic uncertainty in our results. We compare the region selection method we use to compute the degree of current neutralization to the region selection method used in Ref [4], which computationally determined what regions of AR magnetic fields contributed to flaring events. Instead, we use the entire cutout from the SHARP dataset and isolate strong magnetic fields using a threshold of 200 Gauss. We find that our method decrease the degree of current neutralization by a factor of 2. However, we do not expect this difference to change the trends observed between flare-active and flare-quiet ARs.
By measuring the degree of current neutralization for 30 solar ARs, we have determined that this pa- rameter is a good proxy for assessing an AR’s ability to erupt. For more details, check out our full paper Ref [5].
References
[1] Török, T., Leake, J. E., Titov, V. S., et al. 2014, ApJL, 782, L10
[2] Parker, E. N. 1996, ApJ, 471, 485
[3] Melrose, D. B. 1991, ApJ, 381, 306
[4] Liu, Y., Sun, X., Török, T., Titov, V. S., & Leake, J. E. 2017, ApJ, 846, L6
[5] Avallone, E. A., & Sun, X. 2020, arxiv e-prints, arXiv:2003.02814