Shudong Bao

Patterns of current helicity for solar cycle 22

Using a 1988-1997 data set of original photospheric vector magnetograms from the Solar Magnetic Field Telescope (SMFT) of the Huairou Solar Observing Station of Beijing Astronomical Observatory, we computed the local current helicity Bz(∇×B)z for 422 active regions. We found that any given active region contained mixed signs of current helicity, but in most cases current helicity with a particular sign was dominant over a whole region area. In our data set, 84% of the active regions in the northern hemisphere have negative helicity, and 81% in the southern hemisphere have positive helicity. It is estimated that the noise and error in our calculation are at the 2 σ level. In addition, we have studied the evolution of the large-scale surface current helicity during the cycle 22, which is a running mean of absolute current helicity of the active regions observed over a Carrington rotation period. By comparing with monthly mean sunspot number, we found that the average current helicity has a good correlation with solar activity.

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.

The hemispheric sign rule of current helicity during the rising phase of cycle 23

We compute the signs of two different current helicity parameters (i.e., αbest andHc) for 87 active regions during the rise of cycle 23 The results indicate that 59% of the active regions in the northern hemisphere have negative αbest and 65% in the southern hemisphere have positive. This is consistent with that of the cycle 22. However, the helicity parameterHcshows a weaker opposite hemispheric preference in the new solar cycle. Possible reasons are discussed.

The distribution of current helicity at the solar surface at the beginning of the solar cycle

A fraction of solar active regions are observed to have current helicity of a sign that contradicts the polarity law for magnetic helicity; this law corresponds to the well-known polarity law for sunspots. A significant excess of active regions with the “wrong” sign of helicity is seen to occur just at the beginning of the cycle. We compare these observations with predictions from a dynamo model based on principles of helicity conservation, discussed by Zhang et al. (2006). This model seems capable of explaining only a fraction of the regions with the wrong sign of the helicity. We attribute the remaining excess to additional current helicity production from the twisting of rising magnetic flux tubes, as suggested by Choudhuri et al. (2004a). We estimate the relative contributions of this effect and that connected with the model based on magnetic helicity conservation. c

Three Super Active Regions in the Descending Phase of Solar Cycle 23

We analyze the magnetic configurations of three super active regions, NOAA 10484, 10486 and 10488, observed by the Huairou Multi-Channel Solar Tele- scope (MCST) from 2003 October 18 to November 4. Many energetic phenomena, such as flares (including a X-28 flare) and coronal mass ejections (CMEs), occurred during this period. We think that strong shear and fast emergence of magnetic flux are the main causes of these events. The question is also of great interest why these dramatic eruptions occurred so close together in the descending phase of the solar cycle.

Probing signatures of the alpha-effect in the solar convection zone

An attempt to extract maximum information on signatures of the alpha-effect from current helicity and twist density calculations in the solar photosphere is carried out. A possible interpretation of the results for developing the dynamo theory is discussed. The analysis shows that the surface magnetic current helicity is mainly negative/positive in the northern/southern hemispheres of the Sun. This indicates the actual alpha-effect at the photospheric level to be positive/negative, respectively. However, at the bottom of the convection zone, we may assume this effect to change the sign to negative/positive. We reveal some quantities related to the alpha-effect and discuss its spatial and temporal distribution. It is also found that there are a small number of active regions where the sign of the alpha-effect is opposite to that in most active regions. Such exceptional active regions seem to localize at certain active longitudes. We compare the determined regularities with theoretical predictions of the alpha-effect distribution in the solar convection zone.

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.

Distribution of Helical Properties of Solar Magnetic Fields

We summarize studies of helical properties of solar magnetic fields such as current helicity and twist of magnetic fields in solar active regions (ARs), that are observational tracers of the alpha-eect in the solar convective zone (SCZ). Information on their spatial distribution is obtained by analysis of systematic mag- netographic observations of active regions taken at Huairou Solar Observing Station of National Astronomical Observatories of Chinese Academy of Sciences. The main property is that the tracers of the alpha-eect are antisymmetric about the solar equator. Identifying longitudinal migration of active regions with their individual rotation rates and taking into account the internal dierential rotation law within the SCZ known from helioseismology, we deduce the distribution of the eect over depth. We have found evidence that the alpha-eect changes its value and sign near the bottom of the SCZ, and this is in accord with the theoretical studies and numerical simulations. We discuss other regularities which can be revealed by fur- ther analysis such as possible dependence on longitude, time, and magnetic field strength, etc.

Twist of magnetic fields in solar active regions

We study the twist properties of photospheric magnetic fields in solar active regions using magnetographic data on 422 active regions obtained at the Huairou Solar Observing Station in 1988– 1997. We calculate the mean twist (force-free field αf ) of the active regions and compare it with the mean current-helicity density of these same active regions, hc = B‖ · ( ×B)‖. The latitude and longitude distributions and time dependence of these quantities is analyzed. These parameters represent two different tracers of the α effect in dynamo theory, so we might expect them to possess similar properties. However, apart from differences in their definitions, they also display differences associated with the technique used to recalculate the magnetographic data and with their different physical meanings. The distributions of the mean αf and hc both show hemispherical asymmetry—negative (positive) values in the northern (southern) hemisphere—although this tendency is stronger for hc. One reason for these differences may be the averaging procedure, when twists of opposite sign in regions with weak fields make a small contribution to the mean current-helicity density. Such transequatorial regularity is in agreement with the expectations of dynamo theory. In some active regions, the average αf and hc do not obey this transequatorial rule. As a whole, the mean twist of the magnetic fields αf of active regions does not vary significantly with the solar cycle. Active regions that do not follow the general behavior for αf do not show any appreciable tendency to cluster at certain longitudes, in contrast to results for hc noted in previous studies. We analyze similarities and differences in the distributions of these two quantities. We conclude that using only one of these tracers, such as αf , to search for signatures of the α effect can have disadvantages, which should be taken into account in future studies. c

The most important origin of twist and tilt of magnetic fields in solar active regions

In this paper, we show observed properties of tilt and twist of bipolar active regions magnetic fields, using magnetograph data at Huairou, Beijing Astronomical Observatory. We deduce that origin of the twist of magnetic fields is same as that of tilt of bipolar magnetic fields. In another word, they are primarily produced by Coriolis force acting on a rising flux tube through convection zone.