Distributed Electric Currents in Solar Active Regions

Abstract

Using magnetographic data provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we analyzed the structure of magnetic fields and vertical electric currents in six active regions (ARs) with different level of flare activity. We found that electric currents are well balanced over the entire AR: for all of them the current imbalance is below 0.1%, which means that any current system is closed within an AR. Decomposition of the transverse magnetic field vector into two components allowed us to reveal the existence of large-scale vortex structures of the azimuthal magnetic field component around main sunspots of ARs. In each AR, we found a large-scale electric current system occupying a vast area of an AR, which we call distributed electric current. For ARs obeying the Hale polarity law and the hemispheric helicity sign rule, the distributed current is directed upward in the leading part of an AR and it appears to be closing back to the photosphere in the following part of an AR through the corona and chromosphere. Our analysis of the time variations of the magnitude of the distributed electric currents showed that lowflaring ARs exhibit small variations of the distributed currents in the range of ±20×10 12 A, whereas the highly flaring ARs exhibited significant slow variations of the distributed currents in the range of 30 − 95 × 10 12 A. Intervals of the enhanced flaring appear to be co-temporal with smooth enhancements of the distributed electric current.

Are Solar Active Regions Born with Neutralized Currents?

“…In observations, only two studies found that the photospheric current from an isolated sunspot is close to neutrality (Wilkinson et al 1992;Venkatakrishnan & Tiwari 2009). Other studies have shown that the RC vanishes, namely, the net current is not zero in the photosphere (e.g., Leka et al 1996;Longcope & Welsch 2000;Wheatland 2000;Ravindra et al 2011;Georgoulis et al 2012;Vemareddy et al 2015;Liu et al 2017;Vemareddy 2019;Avallone & Sun 2020;Fursyak et al 2020). For example, Ravindra et al (2011) analyzed the net currents in both the positive and negative polarities of AR NOAA 10930, and found that the net current in each polarity increased simultaneously with the flux emergence.…”

On the Origin of the Photospheric Magnetic Field

“…One popular observational approach to attempt to discriminate between locally bare and locally balanced current systems is to measure the net integrated normal current in the polarities of ARs. Statistical (Wheatland 2000;Kontogiannis et al 2017;Fursyak et al 2020) and event (Georgoulis et al 2012;Gosain et al 2014;Liu et al 2017) studies have largely concluded that some AR polarities support a net current and others support counterbalancing currents. However, these studies have not changed the conventional picture of coronal currents, perhaps because of the second major challenge in this controversy, namely, that determining the net current over partitions of positive and negative magnetic polarities in the photosphere is fraught with observational challenges.…”