Are Solar Active Regions Born with Neutralized Currents?

Abstract

Solar active regions are formed by emergence of current-carrying magnetic flux tubes from below the photosphere. Although for an isolated flux tube the direct and return currents flowing along the tube should balance with each other, it remains controversial whether such a neutralization of currents is also maintained during the emergence process. Here we present a systematic survey of the degrees of the current neutralization in a large sample of flux-emerging active regions which appeared on the solar disk around the central meridian from 2010 to 2022. The vector magnetograms taken by Helioseismic and Magnetic Imager onboard Solar Dynamic Observatory are employed to calculate the distributions of the vertical current density at the photosphere. Focusing on the main phase of flux emergence, i.e., the phase in which the total unsigned magnetic flux is continuously increased, we statistically examined the ratios of direct to return currents in all the active regions. Such a large-sample statistical study suggests that most of the active regions were born with currents close to neutralization. The degree of current neutralization seems to be not affected by the active-region size, the active-region growing rate, and the total unsigned current. The only correlation of significance as found is that the stronger the magnetic field nonpotentiality is, the further the active region deviates from current neutrality, which supports previous event studies that eruption-productive active regions often have non-neutralized currents.