Current Helicity In Active Regions Research Articles

Current helicity constraints in solar dynamo models

AbstractWe investigate to what extent the current helicity distribution observed in solar active regions is compatible with solar dynamo models. We use an advanced 2D mean-field dynamo model with dynamo action largely concentrated near the bottom of the convective zone, and dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic 2D mean-field dynamo model with simple algebraic alpha quenching only. Using these numerical models we obtain butterfly diagrams for both the small-scale current helicity and the large-scale magnetic helicity, and compare them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by −A·B evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation. Here B and A are respectively the dynamo generated mean magnetic field and its vector potential.

CURRENT HELICITY OF ACTIVE REGIONS AS A TRACER OF LARGE-SCALE SOLAR MAGNETIC HELICITY

We demonstrate that the current helicity observed in solar active regions traces the magnetic helicity of the large-scale dynamo generated field. We use an advanced 2D mean-field dynamo model with dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic 2D mean-field dynamo model with simple algebraic alpha quenching only. Using these numerical models we obtained butterfly diagrams both for the small-scale current helicity and also for the large-scale magnetic helicity, and compared them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by $-{\bf A \cdot B}$ evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation. Here ${\bf B}$ and ${\bf A}$ are respectively the dynamo generated mean magnetic field and its vector potential. A theoretical interpretation of these results is given.

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.

Studies of solar intranetwork fields and current helicity in active regions

In this paper we begin with a brief review of the most recent observational studies of intranetwork elements. Next by analyzing well observed quantities of other magnetic elements, we suggest that the magnetic flux of all elements is proportional to its lifetime, mainly depending on each size. Then we present an empirical way to extrapolate the size and magnetic field of intranetwork elements, and find that their average size is about 210 km and the corresponding field strength is 350 G. Finally, our studies of current helicity in active regions are summarized as follows: (1) most of the active regions in solar cycle 22 follow the hemispheric helicity sign rule, and the evolution of surface large-scale current helicity has a good correlation with solar activity; (2) the active regions with reversed helicity sign tend to emerge at certain longitudes; (3) the rapid and substantial changes of current helicity in an area or in its vicinity are most likely to trigger flares.

A survey of flares and current helicity in active regions

We examine the spatial and temporal rela- tionship between chromospheric H flares and photo- spheric current helicities in active regions. All of the data were obtained by the vector magnetograph system at Huairou Solar Observing Station of Beijing Astronomical Observatory. We focus our analysis on NOAA Active Region 6233, which was observed on 30 August 1990. The result shows that rapid and substantial changes of dis- tribution of current helicity in an area or in its vicinity are most likely to trigger flares, but no compelling cor- relation between peaks of current helicity and flare sites. Furthermore, we study the influence of flares on helicity for several other active regions. We nd that the time vari- ations of current helicity in the active regions with highly productive flares are more signicant than those of the poorly flare-productive active regions, and that the mag- nitude of current helicity does not always decrease after flares. Therefore we conclude that the rate of variation of current helicity may be considered as an indicator of flare activity. In this paper we describe the specication of the in- strumentation and data used, and we describe the meth- ods for computing helicity. The noises and errors in our data are estimated to be at the 2 3 level.

Distribution of Photospheric Electric Current Helicity and Solar Activities

In this paper, we describe the distribution of photospheric current helicity of active regions in the solar surface. The sign of helicity is mainly negative in the northern hemisphere of the Sun and positive in the southern hemisphere. We also discuss the relationship between the current helicity parameter B∥ (∇ × B)∥ and the α-factor of the force-free field. Our observations show that active regions with current helicity signs opposite to most others in the same hemispheres occur normally at some heliographic longitudes and persist over long periods.In addition, we analyze a possible mechanism of the sign rule of current helicity that shows opposite signs in both hemispheres.

Analysis of electric current helicity in active regions on the basis of vector magnetograms

The problem of (dc) magnetic field energy build up in the solar atmosphere is addressed. Although large-scale current generation may be due to large-scale shearing motions in the photosphere, recently a new approach was proposed: under the assumption that the magnetic field evolves through a sequence of force-free states, Seehafer (1994) found that the energy of small-scale fluctuations may be transferred into energy of large-scale currents in an AR (the α-effect). The necessary condition for the α-effect is revealed by the presence of a predominant sign of current helicity over the volume under consideration. We studied how frequently such a condition may occur in ARs. On the basis of vector magnetic field measurements we calculated the current helicity B z · (▽ × B) z in the photosphere over the whole AR area for 40 active regions and obtained the following results: The method proposed can be applied to those ARs where the determination of the predominant sign of current helicity by traditional visual inspection of Hα-patterns is not reliable.