The magnetic fields of active regions

The Mount Wilson coarse array magnetograph data set is analyzed to determine characteristics of magnetic regions as a function of distance from the average latitude, ₀, of regions in each hemisphere, a quantity which varies during the activity cycle. Regions with normal polarity axis orientations are distributed asymmetrically about ₀ with the median latitude about 1 deg equatorward of ₀. Reversed polarity orientation regions show a somewhat broader and more symmetric distribution. Average sizes for regions at = 0 ( ₀) are nearly twice as large as those located at 10 deg latitude in either direction. Regions poleward of ₀ tend to show a net magnetic field biased toward the following polarity, and regions equatorward of ₀ are biased toward the leading polarity, both by around 10%. Neither region growth rates nor decay rates are related to . The average polarity axis tilt angles of regions are lower for regions near the equator than for those nearer the poles. It is most likely that this is basically an effect of latitude rather than . Meridional motions of young regions are shown to be toward ₀. Older regions do not show this behavior. This may be a magnetic effect rather than being due to large-scale circulatory motion, as has been suggested in the past. East-west inclination angles of active region magnetic fields show a slight tendency to trail the rotation direction (eastward inclination) by a few deg for regions with ₀> 0 and lead the rotation (westward inclination) by a few deg for regions with ₀ > 0. This effect may be related to the torsional oscillations. These various results are discussed in terms of a hypothetical subsurface magnetic flux tube which gives rise to the surface activity.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

The magnetic fields of active regions

The Mount Wilson coarse array data set is used to define active regions in the interval 1967 to August, 1988. From the positions of these active regions on consecutive days, rotation rates are derived. The differential rotation of the active regions is calculated and compared with previous magnetic field and plage rates. The agreement is good except for the variation with time. The active region rates are slower by a few percent than the magnetic field or facular rates. The differential rotation rate of active regions with reversed magnetic polarity orientations is calculated. These regions show little or no evidence for differential rotation, although uncertainties in this determination are large. A correlation is found between rotation rate and region size in the sense that larger regions rotate more slowly. A correlation between rotation rate and cycle phase is suggested which is in agreement with earlier sunspot results. Leading and following portions of active regions, unlike leading and following spots, show little or no difference in their rotation rates. The regions with polarity orientations nearest the normal configuration tend to show rotation rates that are nearest the average values. Most of these results generally support the conclusion that old, weaker magnetic fields have evolved different subsurface connections from the time they were a part of sunspots or plages. It seems possible that they are connected at a shallower layer than are sunspot or plage fields.Operated by the Association of Universities for Research in Astronomy, Inc., under Contract with the National Science Foundation.     

The magnetic fields of active regions

The Mount Wilson daily magnetogram data set is used in its coarse format to determine various statistical properties of magnetic regions. The method of defining magnetic regions is described, and also the criteria for a return of a magnetic region from one day to the next are given. Region sizes, polarity separations, total and net magnetic fluxes, magnetic complexities, and polarity orientations are defined. A relationship is found between polarity orientation and region size in the sense that regions with less magnetic flux tend to show greater deviation on average from the usual polarity orientation.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The magnetic fields of active regions

Magnetogram data are analyzed to study east-west magnetic flux differences interpreted as the component of magnetic field line inclination at the photospheric level in a plane parallel to the solar equator. This component is determined by comparing average east-west pairs of flux values at equal distances from the central meridian. The average inclination of a whole region is such as to trail the rotation (incline toward the east) by about 1.9 deg. Leading and following polarities tilt toward each other by about 16 deg. Growing regions are strongly inclined to the west (to lead the rotation) with large differences between leading and following portions. Decaying regions are slightly inclined to the east with more normal differences between leading and following portions. These results concerning growing and decaying regions are seen with greater amplitude for reversed polarity regions. As the activity cycle progresses, the average inclination of the field lines of the following portions of regions varies from about 10 to about 3 deg (leading the rotation), and the average difference in inclination of the leading and following portions of regions decreases monotonically during the cycle from nearly 20 to about 11 deg. A slight difference is seen between the average east-west inclination angles of regions that are rotating faster than average and those that are rotating slower than average in the sense that slower regions are slightly inclined toward the east and faster regions toward the west. Some of these results may be related to the location or nature of the subsurface flux tubes to which the active regions fields are connected and also, perhaps, to the nature of this connection.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Vertical gradients of the magnetic fields in active regions

In this paper, we describe the results of an investigation of magnetic field structures in two active regions. The photospheric magnetic fields were measured simultaneously in all three components with the Crimean vector magnetograph in the Fe i 5250 line. In our analysis, we compare the observed magnetic field with the potential field. The potential field vector was calculated according to the potential-field theory, and the H _z component was taken as a boundary condition. From these data vertical gradients are calculated from the condition div H = 0. Averaged gradients of both fields increase with the H _z field intensity and within the error limits they do not differ from one another for field strengths up to 1200 G. For larger H _z the potential field gradients become higher than those of the observed field. In large spots, observed field gradients are about two times less than those of the potential field. It is shown that this difference is connected with the observed field twisting.     

On the development of magnetic fields in active regions

Transverse and longitudinal magnetic field scans together with K₂₃₂ spectroheliograms that cover the early phases of active region formation reveal the following:

1. The new active region forms near the periphery of an old magnetic region. There is evidence that the new region forms an interrelated system with the old magnetic structures on the sun.
2. Noticeable changes in the background magnetic field are seen nearly 3 days prior to the appearance of the sunspot. Magnetic hills of the longitudinal component appear along with bright localized K₂₃₂ emission. Subsequently the K₂₃₂ emission spreads along the boundary of one or two adjacent supergranules and at the time of sunspot formation occupies the whole supergranular cell.
3. Transverse fields with strengths of 100–150 gauss form closed regions in the area of the longitudinal component hills, in the very early phases of the region. These fields stretch and link up the two areas later, at which time the peak transverse fields with values near 250 gauss coincide with the zero line of the longitudinal field. When subsequently the spots appear in the new region, the transverse fields are located about the hills of the longitudinal field. The total field vectors just prior to sunspot formation are pressed to the surface. These are inclined about 45° to the surface after the spot appears. The findings indicate that the magnetic field of a new region emerges from the sub-photospheric layers. It is highly likely that the dynamics of a supergranule influences only the emergence of the magnetic field into the upper layers of the solar atmosphere.

     

Observations of vector magnetic fields in flaring active regions

We present vector magnetograph data of 6 active regions, all of which produced major flares. Of the 20 M-class (or above) flares, 7 satisfy the flare conditions prescribed by Hagyard (high shear and strong transverse fields). Strong photospheric shear, however, is not necessarily a condition for a flare. We find an increase in the shear for two flares, a 6-deg shear increase along the neutral line after a X-2 flare and a 13-deg increase after a M-1.9 flare. For other flares, we did not detect substantial shear changes.Visiting Associate from Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, China.     

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.     

The three-dimensional structure of atmospheric magnetic fields in two active regions

The magnetic field above two unrelated active regions on 11 and 12 September, 1974 has been studied using magnetograms obtained in C I 9111, Fe I 8688, Ca II 8542, and H. In C I 9111, originating low in the photosphere, the fields are strong and sharply defined. In Ca II 8542 and H they are very diffuse, with significant diffuseness also in Fe I 8688, due to the spreading of the field with height to form almost horizontal magnetic canopies over regions free of field at lower levels.Within a region between two small sunspots some 140 Mm apart, the canopy height found is typically 300–400 km. Within a small superpenumbra, the canopy height is 150–250 km. In extensive areas surrounding the active regions, over one-half the canopy bases are less than 400–500 km above the _c = 1 level, and over 80% less than 700 km.Arguments are given that the chromospheric fibrils (e.g., in H), taken to delineate the field configuration, are not due primarily to lateral variations in field but rather to differences in density or excitation of gas across the lines of force.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.     

Characteristics of proton flare active regions and force-free magnetic fields

Proton flare active regions have the common characteristic of an S-shaped neutral line caused by relative movements of sunspots. This observational feature suggested to us a method of estimating the force-free parameter. For three typical active regions we have estimated the variation in the force-free parameter, and assuming a constant force-free parameter, we calculated the structure of the force-free field and the magnetic energy of the potential field. Our calculations show that before the occurence of proton flares, the force-free parameter increases and the magnetic energy of the potential field decreases, this decrease may well be the source for the development of the force-free field, and its magnitude is sufficient for the requirement of the proton flare.     

Properties of the magnetic fields of coronal holes with active regions

We determine the structure of the magnetic fields of coronal holes (CHs) and investigate its change in connection with the emergence of active regions (ARs) in CHs. Based on our observations in the HeI 1083 nm line performed with the CrAO TST-2 telescope, we have selected CHs of two types: without (15 CHs) and with (28 CHs) ARs. Magnetograms obtained at the Kitt Peak National Solar Observatory have been used to calculate the magnetic fields of the same objects.     

The velocity fields in active regions

From line-shift observations in two spectrum lines it is determined that the downward motions observed in plages may represent a real downward transport of material, not an apparent downward flow due to brightness or ionization differences in a multistream velocity model.     

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     

Magnetic and velocity fields of active regions

We have observed about 15 active regions on the Sun, with the Advanced Stokes Polarimeter and Dick Dunn Telescope at NSO/SP to map the Stokes parameters in the photospheric Fe 6302.5 Å and chromospheric Mg I 5173 Å lines, during 1999‐2002. The observations are corrected for dark current, gain, instrumental polarization and cross‐talk using ASP pipeline. The wavelength calibration is carried out using the O₂ telluric line 6302 Å which is also present in the observations. The photospheric and chromospheric longitudinal magnetograms are made from the Stokes V profiles, which were intercalibrated with the Kitt Peak magnetograms. The plasma motions are inferred from the line bisector measurements at different positions of the spectral line. In this paper we present the height dependence of Doppler velocity scatter plots of a sunspot in the photospheric Fe I 6302 Å line.     

Open magnetic fields and the solar cycle

Models of open magnetic structures on the Sun are presented for periods near solar minimum (CR 1626–1634) and near solar maximum (CR 1668–1678). Together with previous models of open magnetic structures during the declining phase (CR 1601–1611) these calculations provide clues to the relations between open structures, coronal holes, and active regions at different times of the solar cycle. Near solar minimum the close relation between active regions and open structures does not exist. It is suggested that near solar minimum the systematic emergence of new flux with the proper polarity imbalance to maintain open magnetic structures may occur primarily at very small spatial scales. Near solar maximum the role of active regions in maintaining open structures and coronal holes is strong, with large active regions emerging in the proper location and orientation to maintain open structures longer than typical active region lifetimes. Although the use of He I 10830 Å spectroheliograms as a coronal hole indicator is shown to be subject to significant ambiguity, the agreement between calculated open structures and coronal holes determined from He I 10830 Å spectroheliograms is very good. The rotation properties of calculated open structures near solar maximum strongly suggest two classes of features: one that rotates differentially similar to sunspots and active regions and a separate class that rotates more rigidly, as was the case for single large coronal holes during Skylab.     

Large-scale structure of background fields and active regions

An attempt is made to study the relations between emergence of active regions and the solar background large-scale structures on the basis of Solar Geophysical Data, including Kitt-Peak magnetograms, H filtergrams, and Ca images.The emergence of 217 active regions (a.r.s) that have appeared on the solar disk not farther than ± 60° from the central meridian is studied. The a.r.s are divided into two classes A and B according to their birth location. Class A contains a.r.s emerged far (8–10°) from the background field boundaries, and class B- those emerged near to (55°) or just at the boundaries.It was found that a.r.s of class A differ appreciably from those of class B; in particular, the dimensions and the intensity (S, I) of class B a.r.s are nearly twice as large as those of class A. For class A a.r.s some alterations of the solar large-scale structure boundaries were found in 15% of all the cases, whereas for those of class B in 60%.     

A comparison of the temperature and emission measure of X-ray active regions with coronal magnetic fields

The distribution of temperature and of emission measure in X-ray active regions relative to the coronal magnetic fields has been investigated. The position of maximum temperature and the position of maximum emission measure were found to lie along the magnetic neutral line, with the maximum temperature tending to lie above the position of an abrupt change in direction of the neutral line. Several simple structural models of these regions are compared to the emission measure. The total magnetic energy and the total emission measure appear to be related by a power law in the regions studied by us.     

A comparison of coronal X-ray structures of active regions with magnetic fields computed from photospheric observations

The appearances of several X-ray active regions observed on March 7, 1970 and June 15, 1973 are compared with the corresponding coronal magnetic field topology. Coronal fields have been computed from measurements of the longitudinal component of the underlying magnetic fields, under the current-free hypothesis. An overall correspondence between X-ray structures and calculated field lines is established, and the magnetic counterpart of different X-ray features is also examined. A correspondence between enhanced X-ray emission and the location of compact closed field lines is suggested by this study. Representative magnetic field values calculated under the assumption of current-free fields are given for heights up to 200″.