Magnetic Fields and Solar Activities

We discuss the study of solar magnetic fields based on the photospheric vector magnetograms of solar active regions which were obtained at Huairou Solar Observing Station near Beijing in the period of 22nd and 23th solar cycles. The measurements of the chromospheric magnetic field and the spatial configuration of the field at the lower solar atmosphere inferred by the distribution of the solar photospheric and chromospheric magnetic field. After the analysis on the formation process of delta configuration in some super active regions based on the photospheric vector magnetogram observations, some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, proposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and the relationship with magnetic shear in some delta active regions completely. (3) The proposition is that the large-scale delta active regions are formed from contribution by highly sheared non-potential magnetic flux bundles generated in the subatmosphere. We present some results of a study of the magnetic helicity. We also compare these results with other data sets obtained by magnetographs (or Stokes polarimeters) at different observatories, and analyze the basic chirality of the magnetic field in the solar atmosphere.     

Gradients of the line-of-sight magnetic fields in active region NOAA 6659

The gradients of line-of-sight magnetic fields in active region NOAA 6659 on 1991 June 8 have been calculated based on the photospheric and chromospheric magnetograms taken at Huairou Solar Observing Station. We found that high gradients coincided with high strengths of the transverse magnetic fields, implying a complicated configuration of the magnetic field in the lower atmosphere.For this extraordinarily flare-prolific region, a possible relationship between the gradients and the flares was inferred.     

Structures of chromospheric magnetic fields in the solar flare producing active region 5747

In this paper, the formation and the measurement of the H line in chromospheric magnetic fields are discussed. The evolution of the chromospheric magnetic structures and the relation with the photospheric vector magnetic fields and chromospheric velocity fields in the flare producing active region AR 5747 are also demonstrated.The chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric field are found in the flare-producing region. This probably reflects the complication of the magnetic force lines above the photosphere in the active region. The evolution of the chromospheric magnetic structures in the active region is caused by the emergence of magnetic flux from the sub-atmosphere or the shear motion of photospheric magnetic fields. The filaments separate the opposite polarities of the chromospheric magnetic field, but only roughly those of the photospheric field. The filaments also mark the inversion lines of the chromospheric Doppler velocity field which are caused by the relative motion of the main magnetic poles of opposite polarities in the active region under discussion.     

Reversed-polarity structures of chromospheric magnetic field

Reversed-polarity structures of chromospheric magnetic fields are magnetic gulfs and islands of opposite polarity relative to the underlying photospheric fields. In this paper data measured with the Solar Magnetic Field Telescope of the Huairou Solar Observing Station in Beijing were analyzed. From more than 300 pairs of photospheric magnetograms (in FeI λ5324.19 Å) and relevant chromospheric magnetograms (Hβ λ4861.34 Å), the reality of the reversed-polarity structures is demonstrated. According to an analysis of the fine structure of the magnetic field in the two layers of active regions, we found that there are probably four different types as follows: Type A: magnetic islands of opposite polarity corresponding to photospheric fields appear in the chromospheric magnetogram. Type B: magnetic gulfs of opposite polarity corresponding to photospheric fields appear in the chromospheric magnetogram. Type C is the reverse of type B. That is, a magnetic gulf of opposite polarity corresponding to the chromospheric field appears in the photospheric magnetogram. Type D is the reverse of type A.     

Fine structures of chromospheric magnetic field and material flow in a solar active region

In this paper, we analyze the relations between photospheric vector magnetic fields, chromospheric longitudinal magnetic fields and velocity fields in a solar active region. Agreements between the photospheric and chromospheric magnetograms can be found in large-scale structures or in the stronger magnetic structures, but differences also can be found in the fine structures or in other places, which reflect the variation of the magnetic force lines from the photosphere to the chromosphere. The chromospheric superpenumbral magnetic field, measured by the Hline, presents a spoke-like structure. It consists of thick magnetic fibrils which are different from photospheric penumbral magnetic fibrils. The outer superpenumbral magnetic field is almost horizontal. The direction of the chromospheric magnetic fibrils is generally parallel to the transverse components of the photospheric vector magnetic fields. The chromospheric material flow is coupled with the magnetic field structure. The structures of the H chromospheric magnetic fibrils in the network are similar to H dark fibrils, and the feet of the magnetic fibrils are located at the photospheric magnetic elements.     

Formation of Solar Delta Active Regions： Twist and Writhe of Magnetic Ropes

We analyze the process of formation of delta configuration in some well-known super active regions based on photospheric vector magnetogram observations. It is found that the magnetic field in the initial developing stage of some delta active regions shows a potential-like configuration in the solar atmosphere, the magnetic shear develops mainly near the magnetic neutral line with magnetic islands of opposite polarities, and the large-scale photospheric twisted field forming gradually later. Some results are obtained: (1) The analysis of magnetic writhe of whole active regions cannot be limited in the strong field of sunspots, because the contribution of the fraction of decayed magnetic field is non-negligible. (2) The magnetic model of kink magnetic ropes, supposed to be generated in the subatmosphere, is not consistent with the evolution of large-scale twisted photospheric transverse magnetic field and not entirely consistent with the relationship with magnetic shear in some delta active regions. (3) T     

Structures of chromospheric magnetic field in the vicinity of the filament of active region 5669

In this paper, the chromospheric magnetic structures and their relation to the photospheric vector magnetic field in the vicinity of a dark filament in active region 5669 have been demonstrated. Structural variations are shown in chromospheric magnetograms after a solar flare. Filament-like structures in the chromospheric magnetograms occurred after a solar flare. They correspond to the reformation of the chromospheric dark filament, but there is no obvious variation of the photospheric magnetic field. We conclude that (a) some of the obvious changes of the chromospheric magnetic fields occurred after the flare, and (b) a part of these changes is perhaps due to flare brightening in the chromospheric H line.During the reforming process of the dark filament, a part of its chromospheric velocity field shows downward flow, and it later shows upward flow.     

On the origin of the prolate solar chromosphere

A simple geometric model is proposed to explain the recently reported effect of the prolateness of the solar chromosphere. We assume that a specific dynamical part of the solar atmosphere above the 2 Mm level, being a mixture of moving up and down jets of chromospheric matter with the coronal plasma between them, is responsible for the solar prolateness. Due to the dynamic nature of this layer, the magnetic field is considered to play a very important role in the density distribution with the height, guiding the mass flows along the field lines. The difference of the magnetic field topology in the polar and the equatorial regions leads to different heights of the chromospheric limb. Calculations show a satisfactory coincidence with observations when the mean separation between opposite polarity concentrations is about 9 Mm. The possible observational signature of this network in low photospheric and chromospheric layers is discussed.     

Some results of a morphological study of the evolution of the active region of July, 1982

The photospheric, chromospheric, and magnetic field structure of large active region No. 18474 of July 1982 are studied. Various morphological features observed are described and their possible role in the evolution of sunspots group discussed. The abundance of different light bridges is a characteristic feature for this group. It is shown that the light bridges in the photosphere coincide with the location and direction of arch filaments or fibril streams in the chromosphere, and appear in the region of a developing rotating magnetic field.     

Interpretation of multiwavelength observations of November 5, 1980 solar flares by the magnetic topology of AR 2766

We present a detailed analysis of the magnetic topology of AR 2776 together with Hα UV, X-rays, and radio observations of the November 5, 1980 flares in order to understand the role of the active region large-scale topology on the flare process. As at present the coronal magnetic field is modeled by an ensemble of sub-photospheric sources whose positions and intensities are deduced from a least-square fit between the computed and observed longitudinal magnetic fields. Charges and dipole representations are shown to lead to similar modeling of the magnetic topology provided that the number of sources is great enough. However, for AR 2776, departure from a potential field has to be taken into account, therefore a linear force-free field extrapolation is used. The locations of the four bright off-band Hα kernels in quadrupolar active regions have been studied previously. In this new study the active region is bipolar and shows a two-ribbon structure. We show that these two ribbons are a consequence of the bipolar photospheric field (the four kernels of quadrupolar regions merge into two bipolar regions). The two ribbons are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibril direction, is taken into account. This study supports the hypothesis that magnetic energy is stored in field-aligned currents and released by magnetic reconnection at the location of the separator, before being transported along field lines to the chromospheric level. It is also possible that part of the magnetic energy could be stored and released on the separatrices. Our study shows that meeting just one of two conditions- the presence of intense coronal currents or of a separator in a magnetic field configuration - is not sufficient for flaring. In order to release the stored energy, the coronal currents need either to be formed along the separatrices or to be transported towards the separator or separatrices. The location of the observed photospheric current concentrations on the computed separatrices supports this view. Member of the Carrera del Investigador Científico, CONICET.     

AR6659光球色球的磁场速度场的演化特征及其与大耀斑的关系

ＡＲ６６５９是２２周以来最重要的一个活动区，它爆发了２２周最强大的高能事件。本文用云南天文台的光球、色球精细结构照片和北京天文台怀柔站的磁场速度场资料，分析了该活动区磁场速度场的二维位形和大耀斑期间的演化特征。本文分析的４个大耀斑均爆发在中性线附近的Ｎ极区磁场梯度大的地方及色球速度场的红移区。偏带观测也显示耀斑物质是向红端移动的。耀斑波沿横场传播在离本黑子群几万至十几万公里的地方激起感生耀斑，在原生耀斑与感生耀斑之间往往有耀斑环相连。此外，本文还从演化特征出发分析了耀斑爆发前活动区等离子体的宏观不稳定性。     

The derivation of temperature gradient and electron density maps from EUV spectroheliograms

We discuss spatial variations in electron density at the base of the corona and in the temperature gradient in the chromospheric-coronal transition layer as determined from analysis of maps constructed from Mgx and OVI spectroheliograms. Both the mapping techniques and results of analyzing EUV spectra from OSO 6 observations are presented. Comparisons of these maps with photospheric magnetograms and spectroheliograms made in chromospheric EUV lines and continua indicate that the electron density and temperature gradient in the transition layer tend to be enhanced in areas where the photospheric magnetic field and chromospheric EUV emission are enhanced. Relationships among the coronal electron density, transition-layer temperature gradient, chromospheric emission, and photospheric magnetic field strength are derived.     

Solar chromospheric magnetic fields in active regions inferred by monochromatic images of the Stokes' parameterV of the Hβ line

A series of H chromospheric magnetograms was obtained at various wavelengths near the line center with the vector video magnetograph at Huairou Solar Observing Station as a diagnostic of chromospheric magnetic structures. The two-dimensional distribution of the circular polarization light of the H line with its blended lines at various wavelength in active regions was obtained, which consists of the analyses of Stokes' profileV of this line. Due to the disturbance of the photospheric blended line Fei 4860.98 for the measurement of the chromospheric magnetic field, a reversal in the chromospheric magnetograms relative to the photospheric ones occurs in the sunspot umbrae. But in the quiet, plage regions, even penumbrae, the influence of the photospheric blended Fei 4860.98 line is not obvious. As regards the observation of the H chromospheric magnetograms, we can select the working wavelength between -0.20 and -0.24 from the line core of H to avoid the wavelengths of the photospheric blended lines in the wing of H.After the spectral analysis of chromospheric magnetograms, we conclude that the distribution of the chromospheric magnetic field is similar to the photospheric field, especially in the umbrae of the sunspots. The chromospheric magnetic field is the result of the extension of the photospheric field.     

A mechanism for solar ultraviolet flux variability

Solar UV emission observed by a filter photometer on Nimbus IV from 1969 to 1973 is examined in an attempt to understand the short term (27 day) and secular variability. Two models are discussed to explain the variations - a calcium plage model and a chromospheric network (faculae and spicule) structure model. Both relate to the remnant magnetic fields of active regions. An association between UV brightenings and the large scale magnetic field has been found consistent with the network model. An increase in UV emittance can be achieved by raising the effective chromospheric temperature closer to a photospheric level. If the Sun's luminosity is constant on these time intervals, the enhanced UV radiation could be partiallly offset by an overall decrease in photospheric temperature as measured by Livingston in visible photospheric profiles. Total solar luminosity may then show less variability, however, the UV to visible luminosity variation may have significant planetary influences. Lockwood and Thompson (1979) report a relation between solar activity and planetary albedos, and Schatten (1979) discussed a long-suspected relationship between solar activity and the Great Red Spot appearance.     

Abnormal Stokes Profiles of the Photospheric Lines in the Region of Chromospheric Dual Flows in the Surroundings of a Solar Pore: 1. Observations

The results of the analysis of the full Stokes profiles of the photospheric lines Fe I λ 630.15 nm and Fe I λ 630.25 nm in a region of chromospheric dual flows appearance in the vicinity of a small pore are presented. The analysis is based on the spectropolarimetric observations of the active region NOAA 11024 with the THEMIS French–Italian telescope (Tenerife Island, Spain). The temporal variations in the high-resolution Stokes parameters I, Q, U, and V were considered for each pixel. It was found that the dual chromospheric flows appeared in the region of the abnormal Stokes profiles of the photospheric lines. Most of the Stokes profiles Q, U, and V have a complex shape and vary greatly from pixel to pixel, which indicates strong inhomogeneities in the structure of the magnetic field in that region. The amplitude and shape of the Stokes profiles were rapidly changing during the observations. A change in the polarity of the photospheric magnetic field took place during the observations in the region of a bright chromospheric point. The evidence of the emergence of a new small-scale magnetic flux of the opposite polarity is obtained; this could lead to magnetic reconnections, appearance of dual chromospheric flows, and occurrence of a microflare.     

Evolution of magnetic fields and mass flow in a decaying active region

Five days of coordinated observation were carried out from 24–29 September, 1987 at Big Bear and Huairou Solar Observatories. Longitudinal magnetic fields of an p sunspot active region were observed almost continuously by the two observatories. In addition, vector magnetic fields, photospheric and chromospheric Doppler velocity fields of the active region were also observed at Huairou Solar Observatory. We studied the evolution of magnetic fields and mass motions of the active region and obtained the following results: (1) There are two kinds of Moving Magnetic Features (MMFs). (a) MMFs with the same magnetic polarity as the center sunspot. These MMFs carry net flux from the spot, move through the moat, and accumulate at the moat's outer boundary. (b) MMFs in pairs of mixed polarity. These MMFs are not responsible for the decay of the spot since they do not carry away the net flux. MMFs in category (b) move faster than those of (a). (2) The speed of the mixed polarity MMFs is larger than the outflow measured by photospheric Dopplergrams. The uni-polar MMFs are moving at about the same speed as the Doppler outflow. (3) The chromospheric velocity is in approximately the opposite direction from the photospheric velocity. The photospheric Doppler flow is outward; chromospheric flow is inward. We also found evidence that downward flow appears in the photospheric umbra; in the chromosphere there is an upflow.     

Magnetic Energy Storage and Current Density Distributions for Different Force-Free Models

In the last decades, force-free-field modelling has been used extensively to describe the coronal magnetic field and to better understand the physics of solar eruptions at different scales. Especially the evolution of active regions has been studied by successive equilibria in which each computed magnetic configuration is subject to an evolving photospheric distribution of magnetic field and/or electric-current density. This technique of successive equilibria has been successful in describing the rate of change of the energetics for observed active regions. Nevertheless the change in magnetic configuration due to the increase/decrease of electric current for different force-free models (potential, linear and nonlinear force-free fields) has never been studied in detail before. Here we focus especially on the evolution of the free magnetic energy, the location of the excess of energy, and the distribution of electric currents in the corona. For this purpose, we use an idealised active region characterised by four main polarities and a satellite polarity, allowing us to specify a complex topology and sheared arcades to the coronal magnetic field but no twisted flux bundles. We investigate the changes in the geometry and connectivity of field lines, the magnetic energy and current-density content as well as the evolution of null points. Increasing the photospheric current density in the magnetic configuration does not dramatically change the energy-storage processes within the active region even if the magnetic topology is slightly modified. We conclude that for reasonable values of the photospheric current density (the force-free parameter α<0.25 Mm⁻¹), the magnetic configurations studied do change but not dramatically: i) the original null point stays nearly at the same location, ii) the field-line geometry and connectivity are slightly modified, iii) even if the free magnetic energy is significantly increased, the energy storage happens at the same location. This extensive study of different force-free models for a simple magnetic configuration shows that some topological elements of an observed active region, such as null points, can be reproduced with confidence only by considering the potential-field approximation. This study is a preliminary work aiming at understanding the effects of electric currents generated by characteristic photospheric motions on the structure and evolution of the coronal magnetic field.     

The Hβ Chromospheric Magnetic Field in a Quiescent Filament

We observed the line-of-sight magnetic field in the chromosphereand photosphere of a large quiescent filament on the solar disk on September 6, 2001 using the Solar Magnetic Field Telescope in Huairou Solar Observing Station. The chromospheric and photospheric magnetograms together with Hβ filtergrams of the filament were examined. The filament was located on the neutral line of the large scale longitudinal magnetic field in the photosphere and the chromosphere. The lateral feet of the filament were found to be related to magnetic structures with opposite polarities. Two small lateral feet are linked to weak parasitic polarity. There is a negative magnetic structure in the photosphere under a break of the filament. At the location corresponding to the filament in the chromospheric magnetograms, the magnetic strength is found to be about 40-70 Gauss (measuring error about 39 Gauss). The magnetic signal indicates the amplitude and orientation of the internal magnetic field in the filament. We discuss several possible causes which may produce such a measured signal. A twisted magnetic configuration inside the filament is suggested .     

The magnetic field in the solar atmosphere

This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.     

Magnetic Helicity Injection in Solar Active Regions

We present the evolution of magnetic field and its relationship with mag- netic(current)helicity in solar active regions from a series of photospheric vector magnetograms obtained by Huairou Solar Observing Station,longitudinal magne- tograms by MDI of SOHO and white light images of TRACE.The photospheric current helicity density is a quantity reflecting the local twisted magnetic field and is related to the remaining magnetic helicity in the photosphere,even if the mean current helicity density brings the general chiral property in a layer of solar active regions.As new magnetic flux emerges in active regions,changes of photospheric cur- rent helicity density with the injection of magnetic helicity into the corona from the subatmosphere can be detected,including changes in sign caused by the injection of magnetic helicity of opposite sign.Because the injection rate of magnetic helicity and photospheric current helicity density have different means in the solar atmosphere, the injected magnetic helicity is probably not proportional to the current helicity den- sity remaining in the photosphere.The evidence is that rotation of sunspots does not synchronize exactly with the twist of photospheric transverse magnetic field in some active regions(such as,delta active regions).They represent different aspects of mag- netic chirality.A combined analysis of the observational magnetic helicity parameters actually provides a relative complete picture of magnetic helicity and its transfer in the solar atmosphere.