Twist In Active Regions Research Articles

NON-NEUTRALIZED ELECTRIC CURRENT PATTERNS IN SOLAR ACTIVE REGIONS: ORIGIN OF THE SHEAR-GENERATING LORENTZ FORCE

Using solar vector magnetograms of the highest available spatial resolution and signal-to-noise ratio we perform a detailed study of electric current patterns in two solar active regions: a flaring/eruptive, and a flare-quiet one. We aim to determine whether active regions inject non-neutralized (net) electric currents in the solar atmosphere, responding to a debate initiated nearly two decades ago that remains inconclusive. We find that well-formed, intense magnetic polarity inversion lines (PILs) within active regions are the only photospheric magnetic structures that support significant net current. More intense PILs seem to imply stronger non-neutralized current patterns per polarity. This finding revises previous works that claim frequent injections of intense non-neutralized currents by most active regions appearing in the solar disk but also works that altogether rule out injection of non-neutralized currents. In agreement with previous studies, we also find that magnetically isolated active regions remain globally current-balanced. In addition, we confirm and quantify the preference of a given magnetic polarity to follow a given sense of electric currents, indicating a dominant sense of twist in active regions. This coherence effect is more pronounced in more compact active regions with stronger PILs and must be of sub-photospheric origin. Our results yield a natural explanation of the Lorentz force, invariably generating velocity and magnetic shear along strong PILs, thus setting a physical context for the observed pre-eruption evolution in solar active regions.

Twisted solar active region magnetic fields as drivers of space weather: Observational and theoretical investigations

The properties and dynamics of magnetic fields on the Sun's photosphere and outer layers—notably those within solar active regions—govern the eruptive activity of the Sun. These photospheric magnetic fields also act as the evolving lower boundary of the Sun–Earth coupled system. Quantifying the physical attributes of these magnetic fields and exploring the mechanisms underlying their influence on the near-Earth space environment are of vital importance for forecasting and mitigating adverse space weather effects. In this context, we discuss here a novel technique for measuring twist in the magnetic field lines of solar active regions that does not invoke the force-free field assumption. Twist in solar active regions can play an important role in flaring activity and the initiation of CMEs via the kink instability mechanism; we outline a procedure for determining this solar active region eruptive potential. We also discuss how twist in active region magnetic fields can be used as inputs in simulations of the coronal and heliospheric fields; specifically, we explore through simulations, the formation, evolution and ejection of magnetic flux ropes that originate in twisted magnetic structures. The results and ideas presented here are relevant for exploring the role of twisted solar active region magnetic fields and flux ropes as drivers of space weather in the Sun–Earth system.

On the Availability of Sufficient Twist in Solar Active Regions to Trigger the Kink Instability

The question of whether there is sufficient magnetic twist in solar active regions for the onset of the kink instability is examined using a blind test of analysis methods commonly used to interpret observational data. Photospheric magnetograms are constructed from a recently developed numerical simulation of a kink-unstable emerging flux rope with nearly constant (negative) wind. The calculation of the best-fit linear force-free parameter αbest is applied, with the goal of recovering the model input helicity. It is shown that for this simple magnetic structure, three effects combine to produce an underestimation of the known helicity: (1) the influence of horizontal fields with lower local α values within the flux rope; (2) an assumed simple relation between αbest and the winding rate q does not apply to nonaxis fields in a flux rope that is not thin; and (3) the difficulty in interpreting the force-free twist parameter measured for a field that is forced. A different method to evaluate the magnetic twist in active region flux ropes is presented, which is based on evaluating the peak α value at the flux rope axis. When applied to data from the numerical simulation, the twist component of the magnetic helicity is essentially recovered. Both the αbest and the new αpeak methods are then applied to observational photospheric vector magnetic field data of NOAA AR 7201. The αbest approach is then confounded further in NOAA AR 7201 by a distribution of α that contains both signs, as is generally observed in active regions. The result from the proposed αpeak approach suggests that a larger magnetic twist is present in this active region's δ-spot than would have been inferred from αbest, by at least a factor of 3. It is argued that the magnetic fields in localized active region flux ropes may indeed carry greater than 2π winds, and thus the kink instability is a possible trigger mechanism for solar flares and coronal mass ejections.

The Sources of Magnetic Field Twist in Solar Active Regions

Observations have revealed that a hemispheric preference of magnetic chirality (handedness) exists throughout the solar atmosphere. For example, the current helicity of active regions is predominantly negative (left-handed twist) in the northern hemisphere and positive (right-handed twist) in the southern. The explanation of this hemispheric tendency is still open to question. In this paper we first review several possible mechanisms and clarify some misunderstandings. In our views, in the photosphere, the differential rotation acting on already emerged sunspot magnetic fields will lead to negative current helicity in the northern hemisphere and positive in the southern, but the same effect caused by the Coriolis force is opposite in sign. In the turbulent convection zone, the Coriolis force acting on the rising magnetic flux tubes will result in negative/positive helicity in the northern/southern hemisphere, but the corresponding action by the differential rotation will give rise to a reversed result. Moreover, in this region the α-effect will produce the wrong sign to explain the observed sense of magnetic twist. It should be noteworthy that the two current helicities generated by the α-effect, that in the mean field and that in the fluctuations, have opposite signs, and the former is positive/negative in the northern/southern hemisphere while the latter is negative/positive in the northern/southern hemisphere. In the overshoot region at the base of the convection zone, the current helicity created by the α-effect has the sign needed. Finally, we suggest that some surface flows (e.g., converging flows that can lead to cancellation of opposite-polarity flux in the photosphere) and magnetic reconnection are also important to the redistribution (or regeneration) of magnetic twist in active regions.