Are Solar Active Regions Born with Neutralized Currents?

Solar active regions(ARs)are formed by the 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 ARs 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 ARs.Such a large-sample statistical study suggests that most of the ARs 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 AR deviates from current neutrality,which supports previous event studies that eruption-productive ARs often have non-neutralized currents.

From fundamental theory to realistic modeling of the birth of solar eruptions

Solar eruptions,primarily manifested as solar flares,filament eruptions and coronal mass ejections,represent explosive releases of magnetic energy stored in the solar corona,with the potential to drive severe space weather.The initiation of solar eruptions remains an open question,leading to various theoretical models that are inferred from observations.However,these models are subjects of debate due to the absence of direct measurements of the three-dimensional(3D)magnetic fields in the corona.Numerical simulations,based on solving magnetohydrodynamics(MHD)equations that govern the macroscopic dynamics of solar corona,serve as a touchstone for testing these theoretical models.One early proposed model suggested that eruptions could be triggered by reconnection within a single sheared magnetic arcade,which is known as the tether-cutting reconnection model,but it was never confirmed through 3D MHD simulations until very recently.Consequently,two models have gained more popularity:one involving the eruption of a twisted magnetic flux rope(MFR)due to ideal instability(or loss of equilibrium),and the other known as the breakout eruption,which requires a quadrupolar configuration with a delicately located magnetic null point.Other mixed mechanisms,involving both ideal instability and reconnection,are also proposed in association with localized magnetic flux emergence.Now with the validation of the tether-cutting model,the fundamental mechanisms are boiled down to two types of models,one primarily based on the ideal instability of a pre-existing MFR,and the other based on the reconnection of sheared field lines with or without an MFR.Recently,the modelling of the birth of solar eruption using observed data-based MHD simulations has advanced rapidly,becoming a crucial research tool in the study of the initiation mechanisms.These realistic modellings reveal a higher level of complexity compared to all currently available theories and idealized models.

Data-driven modeling of solar coronal magnetic field evolution and eruptions

Magnetic fields play a fundamental role in the structure and dynamics of the solar corona.As they are driven by their footpoint motions on the solar surface,which transport energy from the interior of the Sun into its atmosphere,the coronal magnetic fields are stressed continuously with buildup of magnetic nonpotentiality in the form of topology complexity(magnetic helicity)and local electric currents(magnetic free energy).The accumulated nonpotentiality is often released explosively by solar eruptions,manifested as solar flares and coronal mass ejections.