Nonneutralized Electric Currents as a Proxy for Eruptive Activity in Solar Active Regions

Abstract

It has been suggested that the ratio of photospheric direct to return current, ∣DC/RC∣, may be a better proxy for assessing the ability of solar active regions to produce a coronal mass ejection (CME) than others such as the amount of shear along the polarity inversion line (PIL). To test this conjecture, we measure both quantities prior to eruptive and confined flares of varying magnitude. We find that eruptive-flare source regions have ∣DC/RC∣ > 1.63 and PIL shear above 45° (average values of 3.°2 and 68°, respectively), tending to be larger for stronger events, while both quantities are on average smaller for confined-flare source regions (2.°2 and 68°, respectively), albeit with substantial overlap. Many source regions, especially those of eruptive X-class flares, exhibit elongated direct currents (EDCs) bracketing the eruptive PIL segment, which typically coincide with areas of continuous PIL shear above 45°. However, a small subset of confined-flare source regions have ∣DC/RC∣ close to unity, very low PIL shear (<38°), and no clear EDC signatures, rendering such regions less likely to produce a CME. A simple quantitative analysis reveals that ∣DC/RC∣ and PIL shear are almost equally good proxies for assessing CME-productivity, comparable to other proxies suggested in the literature. We also show that an inadequate selection of the current-integration area typically yields a substantial underestimation of ∣DC/RC∣, discuss specific cases that require careful consideration for ∣DC/RC∣ calculation and interpretation of the results, and suggest improving photospheric CME-productivity proxies by incorporating coronal measures such as the decay index.

The Temporal Evolution of Nonneutralized Electric Currents and the Complexity of Solar Active Regions

“…An additional benefit of our methodology is that the derivation of I NN,tot is not based on assumptions concerning the chirality of the active region (as, e.g., in Avallone & Sun 2020) and/or specific region selection (Liu et al 2017(Liu et al , 2024, which are required to determine the so-called degree of current neutralization (Török et al 2014). The presented results are in line with those derived from the study of the degree of current neutralization (Liu et al 2017(Liu et al , 2024Vemareddy 2019;Avallone & Sun 2020;Barczynski et al 2020;He et al 2020;Wang et al 2023).…”

“…An additional benefit of our methodology is that the derivation of I NN,tot is not based on assumptions concerning the chirality of the active region (as, e.g., in Avallone & Sun 2020) and/or specific region selection (Liu et al 2017(Liu et al , 2024, which are required to determine the so-called degree of current neutralization (Török et al 2014). The presented results are in line with those derived from the study of the degree of current neutralization (Liu et al 2017(Liu et al , 2024Vemareddy 2019;Avallone & Sun 2020;Barczynski et al 2020;He et al 2020;Wang et al 2023). Another future step would be a comparative analysis of the evolution of active regions using both approaches, namely the degree of current neutralization along with the ratio I NN,tot /I z (or its complement 1 − I NN,tot /I z ) and the neutrality factor proposed by Georgoulis et al (2012).…”

“…Overall, flaring active regions exhibit at least an order of magnitude higher nonneutralized electric currents than flare-quiet ones (Kontogiannis et al 2017(Kontogiannis et al , 2018 and the same quantity is highly correlated with the kinematic characteristics of CMEs (Kontogiannis et al 2019). Regions prone to eruptions are reported to exhibit a higher degree of non-neutralization (Liu et al 2017(Liu et al , 2024Vemareddy 2019;Avallone & Sun 2020). Localized, intense changes in the vertical electric current density have also been associated with pre-flaring activity and free energy build-up (Mitra et al 2020;Sahu et al 2023).…”

“…Although several studies have focused on the evolution of the magnetic flux during flux emergence (Otsuji et al 2011;Norton et al 2017;Kutsenko et al 2019), no study so far has been dedicated to the evolution of nonneutralized electric currents in a sizeable sample of emerging active regions. Smaller samples of regions/events have been scrutinized using the degree of current neutralization (e.g., Liu et al 2017;Avallone & Sun 2020;He et al 2020;Wang et al 2023;Liu et al 2024), whose calculation is based on assumptions regarding direct and return currents.…”