Formation of current helicity and emerging magnetic flux in solar active regions

We present the results of a photometric study of X-ray-active weak-lined T Tauri (WTT) stars in the η Chamaeleontis star cluster. Multi-epoch V -band photometric monitoring during 1999 and 2000 of the 10 X-ray-active WTT stars found that all were variable in one or both years, with periods ascribed to rotational modulation of starspots. Comparison between the rotational and X-ray properties of these objects indicates the saturation level,     observed in other studies of X-ray-active pre-main-sequence stars, persists in the η Cha stars from the slow- to the fast-rotator regimes. Cousins VRI photometry of the WTT stars has enabled us to investigate further the photometric properties of these stars. The stars appear sufficiently coeval to distinguish near-equal-mass binaries within the sample. A new Hertzsprung–Russell diagram for these objects suggests ages of 4–9 Myr for M-type RECX primaries using the tracks of D'Antona & Mazzitelli.     

Fractal Brownian surface and distribution of longitudinal magnetic fields in two solar active regions

The distribution of photospheric longitudinal magnetic fields in a solar active region can be considered to be a natural surface in a mathematical sense. Just as with landscapes on the Earth, the surface may be a fractal Brownian surface (FBS). A method suggested in the paper can manifest whether the surface is an FBS or not. The method has been applied to the longitudinal magnetic fields in AR 5988 on March 24, 1990 and AR 6233 on August 30, 1990, in which the observational characteristics are quite different. The testing results indicate that the distributions of longitudinal magnetic field in both regions are not in agreement with the FBS model     

Photographic maps of magnetic fields of solar active regions

This paper describes our daily photography of maps of the strong magnetic fields in solar active regions. In one case, the isogauss line at 2000 G in a complex magnetic region is seen to coincide with the optical outline of a sunspot umbra.     

Evolution of network magnetic fields of solar quiet regions

There are 4 types of evolution patterns of network magnetic fields: (1) flux cancellation, the mutual disappearance of encountering fluxes of opposite polarity, (2) flux increase by emergence of ephemeral regions, (3) flux decrease of one polarity and (4) flux increase of one polarity, without emergence of ephemeral regions.From a time sequence of magnetograms of a quiet region of 1983 October 14, the evolution of 300 network features was measured. The magnetograms have a spatial resolution of 2 to 3 arcsec and a time resolution of about 2 hr. The statistics show that the contribution to flux decrease by Type 3 is 1.28 times that by Type 1, and the contribution to flux increase by Type 4 is 7 times that by Type 2.     

On heating of solar active regions by magnetic energy dissipation

The Tucker's proposal of heating of active regions by magnetic energy dissipation has been found to be unsatisfactory in its present form because it predicts unacceptably high values of the twisting velocity for the magnetic field.     

Magnetic flux transport of decaying active regions and enhanced magnetic network

Several series of coordinated observations on decaying active regions and enhanced magnetic network regions have been carried out jointly at Big Bear Solar Observatory (BBSO) and Huairou Solar Observing Station of the Bejing Astronomical Observatory in China. The evolution of magnetic fields in several regions was followed closely for 3 to 7 days. The transport of magnetic flux from the remnants of decayed active regions was studied. Three related topics are included in this paper. (1) We studied the evolution and lifetime of the magnetic network which defines the boundaries of supergranules. The results are consistent with our earlier studies: network cells have an average lifetime of about 70 hours; 68% of new cells appeared by growing from a single network magnetic element; 50% of decaying cells disappeared by contracting to a network element. (2) We studied the magnetic flux transport in an enhanced network region in detail, and found the diffusion rate to be negative, i.e., there was more flux moving towards the decayed active region than away from it. We found several other cases where the magnetic diffusion rate does not agree with Leighton's model. The slow diffusion rate is likely due to the fact that the average velocity of larger magnetic elements, which carry most of the magnetic flux, is less than 0.1 km s^–1; their average lifetime is longer than 100 hours. (3) We briefly described some properties of Moving Magnetic Features (MMFs) around a sunspot (detailed discussion on MMFs will be presented in a separate paper). In this particular case, the MMFs did not carry net flux away from the central spot. Instead, the polarities of MMFs were essentially mixed so that outflowing positive and negative fluxes were roughly balanced. During the 3-day period, there was almost no net flux accumulation to form a moat. The cancellation of MMFs of opposite polarities at the boundary of the super-penumbra caused quite a few surges and H brightenings.     

On the magnetic field line topology in solar active regions

The field lines of closed magnetic structures above the photosphere define a mapping from the photosphere to itself. This mapping is discontinuous, and the field line connectivity to the boundary can change discontinuously in response to continuous changes of field strength and direction, if field lines either end in a singular point of the field or are tangential to the photosphere at one end. Whereas the general existence of singular points is questionable, the field has typically a cell structure due to the presence of segments of the zero line of the photospheric longitudinal field on which the transversal field is directed from negative (pointing into the Sun) to positive fields. The cell boundaries are made up of field lines which all touch the photosphere on one of these line segments. Within each of the cells the field line mapping is continuous. When during a slow evolution a substantial part of a coronal loop or of an arcade has passed from one cell into another a fast dynamic instability may set in which was previously prevented by the anchoring of field lines in the dense photosphere.     

Emerging flux in active regions

We have compared the rates at which flux emerges in active and quiet solar regions within the sunspot belts. The emerging flux regions (EFRs) were identified by the appearance of arch filament structures in H. All EFRs in high-resolution films of active regions made at Big Bear in 1978 were counted. The comparable rate of flux emergence in quiet regions was obtained from SGD data and independently from EFRs detected outside the active region perimeter on the same films. The rate of flux emergence is 10 times higher in active regions than in quiet regions. A sample of all active regions in 31 days of 1983 gave a ratio of 7.5. We discuss possible mechanisms which might funnel new magnetic flux to regions of strong magnetic field.     

10.7-cm Solar radio flux and the magnetic complexity of active regions

During sunspot cycles 20 and 21, the maximum in smoothed 10.7-cm solar radio flux occurred about 1.5 yr after the maximum smoothed sunspot number, whereas during cycles 18 and 19 no lag was observed. Thus, although 10.7-cm radio flux and Zürich suspot number are highly correlated, they are not interchangeable, especially near solar maximum. The 10.7-cm flux more closely follows the number of sunspots visible on the solar disk, while the Zürich sunspot number more closely follows the number of sunspot groups. The number of sunspots in an active region is one measure of the complexity of the magnetic structure of the region, and the coincidence in the maxima of radio flux and number of sunspots apparently reflects higher radio emission from active regions of greater magnetic complexity. The presence of a lag between sunspot-number maximum and radio-flux maximum in some cycles but not in others argues that some aspect of the average magnetic complexity near solar maximum must vary from cycle to cycle. A speculative possibility is that the radio-flux lag discriminates between long-period and short-period cycles, being another indicator that the solar cycle switches between long-period and short-period modes.Operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Statistical mechanics of velocity and magnetic fields in solar active regions

A statistical mechanics of the velocity and magnetic fields is formulated for an active region plasma. The plasma subjected to the conservation laws emerges in a most probable state which is described by an equilibrium distribution function containing a lagrange multiplier for every invariant of the system. The lagrange multipliers are determined by demanding that the measured expectation values of the invariants be reproduced. For a numerical exercise, we have assumed some probable values for these invariants. The total energy of a coronal loop is estimated from energy balance considerations. Doppler widths of the UV and EUV lines excited in the coronal loop plasma give a measure of the root-mean-square velocities. Measurements of magnetic helicity are not available for the solar corona.     

A logistic model for magnetic energy storage in solar active regions

Previous statistical analyses of a large number of SOHO/MDI full disk longitu-dinal magnetograms provided a result that demonstrated how responses of solar flares to photospheric magnetic properties can be fitted with sigmoid functions. A logistic model reveals that these fitted sigmoid functions might be related to the free energy storage process in solar active regions. Although this suggested model is rather simple, the free energy level of active regions can be estimated and the probability of a solar flare with importance over a threshold can be forecast within a given time window.     

A logistic model for magnetic energy storage in solar active regions

Previous statistical analyses of a large number of SOHO/MDI full disk longitudinal magnetograms provided a result that demonstrated how responses of solar flares to photospheric magnetic properties can be fitted with sigmoid functions. A logistic model reveals that these fitted sigmoid functions might be related to the free energy storage process in solar active regions. Although this suggested model is rather simple, the free energy level of active regions can be estimated and the probability of a solar flare with importance over a threshold can be forecast within a given time window.     

A new technique of observing the magnetic field of solar active regions

By fitting a new type of polarization detector after the entrance slit of a solar spectrograph, the two dimensional magnetic field of solar active regions can be obtained. Not only the field intensity, but also the longitudinal map is obtained quickly. The simultaneous observation of a few lines will also provide data on the structure of the magnetic tubes etc.     

The correlation of solar flare production with magnetic energy in active regions

An investigation of 531 active regions was made to determine the correlation between energy released by flares and the available energy in magnetic fields of the regions. Regions with magnetic flux greater than 10²¹ maxwell during the years 1967–1969, which included sunspot maximum, were selected for the investigation. A linear regression analysis of flare production on magnetic flux showed that the flare energy is correlated with magnetic energy with a coeificient of correlation of 0.78. Magnetic classification and field configuration also significantly affect the production of flares.This work was supported by the Aerospace Sponsored Research Program.     

Statistical properties of bipolar magnetic regions

Using observations from the Michelson Doppler Imager(MDI) onboard Solar and Heliospheric Observatory(SOHO), we develop a computational algorithm to automatically identify bipolar magnetic regions(BMRs) in active regions(ARs), and then study their statistical properties.The individual magnetic(positive or negative) pole of a BMR is determined from the region with an absolute strength above 55 G and with an area larger than 250 pixel~2(~495 Mm~2), while a BMR is identified as a pair of positive and negative poles with the shortest area-weight distance between them.Based on this method, 2234 BMRs are identified from MDI synoptic magnetograms between Carrington Rotations 1909(1996 May 06) and 2104(2010 December 10). 1005 of them are located in the northern hemisphere, while the other 1229 are in the southern hemisphere. We find that the BMR parameters(e.g., latitude, separation, fragment number and strength) are similar to those of ARs. Moreover, based on the maximum likelihood estimation(MLE) method, the frequency distributions representing the occurrence of these BMRs as functions of area and magnetic flux exhibit a power-law behavior, i.e.,dN/dx ∝ x~(-αx), with indices of α_A = 1.98 ± 0.06 and α_F = 1.93 ± 0.05 respectively. We also find that their orientation angles(θ) follow "Hale's Polarity Law" and deviate slightly toward the direction of the solar equator. Consistent with previous findings, we obtain the dependence of orientation angles on latitudes for normal BMRs during the 23 rd solar cycle. The north-south asymmetry of these BMRs is also detected here.     

A quantity characterizing variation of observed magnetic twist in solar active regions

An alternative parameter R_J_z is introduced as the ratio of one of two kinds of opposite-sign current to the total current and is used to investigate the relationship between this quantity and the hemispheric helicity sign rule(HSR) that has been established by a series of previous statistical studies. The classification of current in each hemisphere obeys the following rule: if the product of the current and the corresponding longitudinal field component contributes a consistent sign with respect to the HSR,it is called "HSR-compliant" current,otherwise it is called "HSR-noncompliant" current. Firstly,consistency between the butterfly diagram of R_J_z and current helicity was obtained in a statistical study.Active regions with R_J_zsmaller than 0.5 tend to obey the HSR whereas those with R_J_z greater than 0.5 tend to disobey it. The "HSR-compliant" current systems have a 60% probability of realization compared to 40% for "HSR-noncompliant" current systems. Overall,the HSR is violated for active regions in which the "HSR-noncompliant" current is greater than the "HSR-compliant" current. Secondly,the parameter R_J_z was subsequently used to study the evolution of current systems in the case analyses of flare-productive active regions NOAA AR 11158 and AR 11283. It is found that there is a "R_J_z-quasistationary" phase that is relatively flare quiescent and "R_J_z-dynamic" phase that is characterized by the occurrence of large flares.     

Skinning process stability of the magnetic field in the solar active regions

Skinning process stability of the magnetic field in homogeneous plasma is studied. A set of magnetohydrodynamic equations is used. Dependence of electrical conductivity on the plasma parameters and radiation intensity in grey-body approximation are taken into account. The investigation is carried out on the model problems in linear approximation and by means of numerical solution of MHD equations. Threshold of stability and critical gradient of magnetic field in skin-layer are obtained. The model of the phenomenon proposed in the paper indicates on overheating instability of plasma with electric current in large gradient magnetic field zones as a possible trigger mechanism of solar flare origin.     

Comments on the heating of solar active regions by magnetic energy dissipation

The method of Kumar and Narain (1985) to calculate the twisting velocity of magnetic field appears to be erroneous. Hence, their conclusion about Tucker's proposal for the heating of active regions by magnetic energy dissipation may not be correct.     

Infrared photometry of solar active regions

Simultaneous time series of broad-band images of two active regions close to the disk center were acquired at the maximum (0.80Μm) and minimum (1.55Μm) continuum opacities. Dark faculae are detected in images obtained as weighted intensity differences between both wave-length bands. The elements of quiet regions can be clearly distinguished from those of faculae and pores in scatter plots of brightness temperatures. There is a smooth transition between faculae and pores in the scatter plots. These facts are interpreted in terms of the balance between the inhibition of convective energy transport and the lateral radiative heating.