Photospheric electric current and transition region brightness within an active region

Distributions of vertical electric current density (J _z) calculated from vector measurements of the photospheric magnetic field are compared with ultraviolet spectroheliograms to investigate whether resistive heating is an important source of enhanced emission in the transition region. The photospheric magnetic fields in Active Region 2372 were measured on 6 and 7 April, 1980 with the MSFC vector magnetograph; ultraviolet wavelength spectroheliograms (L and Nv 1239 Å) were obtained with the UVSP experiment aboard the Solar Maximum Mission satellite. Spatial registration of the J _z (5 arc sec resolution) and UV (3 arc sec resolution) maps indicates that the maximum current density is cospatial with a minor but persistent UV enhancement, but there is little detected current associated with other nearby bright areas. We conclude that although resistive heating may be important in the transition region, the currents responsible for the heating are largely unresolved in our measurements and have no simple correlation with the residual current measured on 5 arc sec scales.National Research Council Resident Research Associate.National Oceanic and Atmospheric Administration Space Environment Laboratory; currently at MSFC/SSL.     

1986年2月初太阳活动区的形态研究

根据Marshall空间飞行中心(MSFC)太阳天文台的矢量磁场测量和云南天文台的黑子细节照相资料,作者们详细研究了1986年2月初太阳大活动区(AR4711)的形态和演化。主要结论如下: i)几乎在活动区中每处地方,相距五小时观测到横向磁场排列方向和黑子半影纤维形态之间存在良好的相似性。 ii)利用文[4]的方法,推断了本活动区强的垂直电流源和强的水平电流渠道。 iii)与1972年8月初著名的太阳活动区(McMath 11976)相类似,沿老活动区的中性线的新浮磁通管的两足点(偶极黑子)的分离运动导致了一个密集四极磁结构的形成。 iv)新浮磁通管似乎是本活动区最强的电流系统。 上述结论将为进一步研究本区电流/磁场环系的演化及其与耀斑活动的关系提供一个基础数据。     

Subphotospheric current systems and flares

Subphotospheric current systems inferred from recent vector magnetograph observations (e.g. Gary et al., 1987) imply the existence of electric currents penetrating the photosphere and thus flowing deep in the solar convection zone. These currents presumably originate in an internal dynamo that supplies the observed photospheric magnetic fields through the buoyant motions of the initially deeply-buried flux tubes. The coronal fields resulting from this process therefore must carry slowly-varying currents driven by emf's remote from the surface. These currents may then drive solar-flare energy release. This paper discusses the consequences of such a deep origin of the coronal parallel currents. Simple estimates for a large active region suggest a mean current-closure depth 10,000 km, with a subphotospheric inductance 100 H and a subphotospheric stored energy 10³³ ergs.     

Vector magnetic field evolution,energy storage,and associated photospheric velocity shear within a flare-productive active region

The evolution of vector photospheric magnetic fields has been studied in concert with photospheric spot motions for a flare-productive active region. Over a three-day period (5–7 April, 1980), sheared photospheric velocity fields inferred from spot motions are compared both with changes in the orientation of transverse magnetic fields and with the flare history of the region. Rapid spot motions and high inferred velocity shear coincide with increased field alignment along the B _L= 0 line and with increased flare activity; a later decrease in velocity shear precedes a more relaxed magnetic configuration and decrease in flare activity. Crude energy estimates show that magnetic reconfiguration produced by the relative velocities of the spots could cause storage of 10³² erg day^–1, while the flares occurring during this time expended 10³¹ erg day^–1.Maps of vertical current density suggest that parallel (as contrasted with antiparallel) currents flow along the stressed magnetic loops. For the active region, a constant-, force-free magnetic field (J = B) at the photosphere is ruled out by the observations.Presently located at NASA/MSFC, Huntsville, Ala. 35812, U.S.A.     

Constructing the Coronal Magnetic Field By Correlating Parameterized Magnetic Field Lines With Observed Coronal Plasma Structures

A method is presented for constructing the coronal magnetic field from photospheric magnetograms and observed coronal loops. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal flux tubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures that (i) the normal component of the photospheric field remains unchanged, (ii) the field is given in the entire corona over an active region, (iii) the field remains divergence-free, and (iv) electric currents are introduced into the field. It is demonstrated that a parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 November 26. The result is a non-force-free magnetic field with the Lorentz force being of the order of 10^–5.5 g cm s^–2 resulting from an electric current density of 0.079 A m^–2. Calculations show that the plasma beta becomes larger than unity at a relatively low height of 0.25 r supporting the non-force-free conclusion. The presence of such strong non-radial currents requires large transverse pressure gradients to maintain a magnetostatic atmosphere, required by the relatively persistent nature of the coronal structures observed in AR 7999. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field.     

The evolution of the photospheric network

A time-lapse sequence of spectroheliograms in the bandhead of CN at λ3883 reveals the following behavior of the photospheric network with time:

1. There is a steady flow of bright ‘points’ (? 1000 km in diameter) laterally outward from sunspots at speeds on the order of 1 km·sec^?1. After traveling about 10 000 km from a sunspot they either conglomerate to form fragments of the photospheric network or disappear.
2. Spatial changes in the network pattern seem to take place by means of the shifting of network fragments laterally on the solar surface. Although most small-scale details are recognizable after 5–10 minutes, within 30 minutes nearly all the details have changed completely. In contrast to this, the large-scale network pattern seems relatively unchanged after 2 1/2 hours.
3. Occasionally ‘new’ network, not resulting from the lateral motion of bright features from either previously existing network or sunspots, appears on the solar surface. This process consists of the formation in approximately 10 minutes of bright points and a darker-than-average feature between them. The dark feature disappears in another 5–10 minutes and the bright points separate at a relative speed of a few km·sec^?1. If the event is of a sufficiently large magnitude, a sunspot will appear.

These observed changes of the photospheric network with time are interpreted as formation and motions of photospheric magnetic fields. It is suggested that these motions reflect the presence of both short-lived small-scale and long-lived large-scale photospheric currents such as one might expect from the granulation and the supergranulation.     

Pre-flare association of magnetic fields and millimeter-wave radio emission

Observations of radio emission at 3.3 mm wavelength associated with magnetic fields in active regions are reported. Results of more than 200 regions during the years 1967–1968 show a strong correlation between peak enhanced millimeter emission, total flux of the longitudinal component of photospheric magnetic fields and the number of flares produced during transit of active regions. For magnetic flux greater than 10²¹ maxwells flares will occur and for flux of 10²³ maxwells the sum of the H flare importance numbers is about 40. The peak millimeter enhancement increases with magnetic flux for regions which subsequently flared. Estimates of the magnetic energy available and the correlation with flare production indicate that the photospheric fields and probably chromospheric currents are responsible for the observed pre-flare heating and provide the energy of flares.This work was supported in part by NASA Contract No. NAS2-7868 and in part by Company funds of The Aerospace Corporation.     

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.     

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.     

Coronal force-free magnetic field: Source-surface model

Models of the magnetic field in the solar chromosphere and corona are still mainly based on theoretical extrapolations of photospheric measurements. For the practical calculation of the global field, the so-called source-surface model has been introduced, in which the influence of the solar wind is described by the requirement that the field be radial at some exterior (source) surface. Then the assumption that the field is current-free in the volume between the photosphere and this surface allows for its determination from the photospheric measurement. In the present paper a generalization of the source-surface model to force-free fields is proposed. In the generalized model the parameter( = ×B·B/B ²)must be non-constant (or vanish identically) and currents are restricted to regions with closed field lines. A mathematical algorithm for computing the field from boundary data is devised.     

Magnetic diffusion and flare energy buildup

Photospheric motion shears or twists solar magnetic fields to increase magnetic energy in the corona, because this process may change a current-free state of a coronal field to force-free states which carry electric current. This paper analyzes both linear and nonlinear two-dimensional force-free magnetic field models and derives relations of magnetic energy buildup with photospheric velocity field. When realistic data of solar magnetic field (B ₀ 10³ G) and photospheric velocity field (v _max 1 km s^–1) are used, it is found that 3–4 hours are needed to create an amount of free magnetic energy which is of the order of the current-free field energy. Furthermore, the paper studies situations in which finite magnetic diffusivities in photospheric plasma are introduced. The shearing motion increases coronal magnetic energy, while the photospheric diffusion reduces the energy. The variation of magnetic energy in the coronal region, then, depends on which process dominates.     

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.     

Magnetic field and electric current structure in the chromosphere

Three dimensional vector magnetic field structure throughout the chromosphere above an active region is deduced by combining high resolution H filtergrams with a simultaneous digital magnetogram. An analog model of the field is made with 400 metal wires representing fieldlines which are assumed to outline the H structure. The height extent of the field is determined from vertical field gradient observations around sunspots, from observed fibril heights and from an assumption that the sources of the field should be largely local. After digitization the magnetic field H matrix is retrieved. Electric current densities j are computed from j=curl H. The currents (typically 10 mA m^–2) flow in patterns not similar to observed features and not parallel to magnetic fields. Lorentz forces are computed from {ie0323-01}. The force structures correspond to observed solar features and a series of observed dynamics may be expected: downward motion in bipolar areas in lower chromosphere, an outflow of the outer chromosphere into the corona with radially outward flow above bipolar plage regions (where coronal streamers are observed) and motions of arch filament systems. Observed current structure and magnitude agree well with previous vector magnetograph observations but disagree with theoretical current-free or force-free concepts. A dynamic chromosphere with electromagnetic forces in action is thus inferred from observations.     

Topology of Magnetic Field and Coronal Heating in Solar Active Regions – II. The Role of Quasi-Separatrix Layers

The photospheric vector magnetic fields, H and soft X-ray images of AR 7321 were simultaneously observed with the Solar Flare Telescope at Mitaka and the Soft X-ray Telescope of Yohkoh on October 26, 1992, when there was no important activity in this region. Taking the observed photospheric vector magnetic fields as the boundary condition, 3D magnetic fields above the photosphere were computed with a new numerical technique. Then quasi-separatrix layers (QSLs), i.e., regions where 3D magnetic reconnection takes place, were determined in the computed 3D magnetic fields. Since Yohkoh data and Mitaka data were obtained in well-arranged time sequences during the day, the evolution of 3D fields, H features and soft X-ray features in this region can be studied in detail. Through a comparison among the 3D magnetic fields, H features and soft X-ray features, the following results have been obtained: (a) H plages are associated with the portions of QSLs in the chromosphere; (b) diffuse coronal features (DCFs) and bright coronal features (BCFs) are morphologically confined by the coronal linkage of the field lines related to the QSLs; (c) BCFs are associated with a part of the magnetic field lines related to the QSLs. These results suggest that as the likely places where energy release may occur by 3D magnetic reconnection, QSLs play an important role in the chromospheric and coronal heating in this active region.     

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.     

Coronal Magnetic Field Structure and Evolution for Flaring AR 11117 and Its Surroundings

In this study, photospheric vector magnetograms obtained with the Synoptic Optical Long-term Investigations of the Sun (SOLIS) survey are used as boundary conditions to model three-dimensional nonlinear force-free (NLFF) coronal magnetic fields as a sequence of NLFF equilibria in spherical geometry. We study the coronal magnetic field structure inside an active region and its temporal evolution. We compare the magnetic field configuration obtained from NLFF extrapolation before and after the flaring event in active region (AR) 11117 and its surroundings observed on 27 October 2010, and we also compare the magnetic field topologies and the magnetic energy densities and study the connectivities between AR 11117 and its surroundings. During the investigated time period, we estimate the change in free magnetic energy from before to after the flare to be 1.74×10³²?erg, which represents about 13.5?% of the NLFF magnetic energy before the flare. In this study, we find that electric currents from AR 11117 to its surroundings were disrupted after the flare.     

Time variability and structure of quiet Sun sources at 6 cm wavelength

Using the Westerbork Synthesis Radio Telescope (WSRT) we produced a synthesized map of a quiet Sun region on June 15, 1976, and studied the structure and time variability of the quiet emitting regions at 6 cm wavelength with a spatial resolution of 6 arc sec. Comparison of the 12hr synthesis map with Ca⁺ K filtergram shows that bright and dark features on the 6 cm quiet Sun synthesized map correspond to the chromospheric networks and cells observed in Ca⁺ K. All 6 cm bright features lie over bright Ca⁺ K network elements. The reverse correlation is not true, that is, not all bright Ca⁺ K network features have their 6 cm counterparts. Comparison with the photospheric magnetogram shows that about 72% of the photospheric magnetic field enhancements (¦B¦ 5 G) are coincident with 6 cm emissive regions. Only one 6 cm feature could be positively identified with a bipolar magnetic structure. This implies that no more than 20–25% of the 6 cm emitting features could be associated with X-ray bright points. Intercomparison of our 12hr two-dimensional synthesis map, a 4hr two-dimensional synthesis map (around meridian) and the one-dimensional fan beam scans of the quiet Sun region at 6 cm, along with the Ca⁺ K filtergram and photospheric magnetogram shows that: (1) All of the 15 time-varying elements at 6 cm were located on Ca⁺ K networks; (2) about 40% of the 15 time varying elements at 6 cm are coincident with enhancements of the photospheric magnetogram; (3) individual time-varying sources have minimum source size (FWHM) of 15 arc sec and maximum brightness temperature of 10⁵ K; (4) the life time of the time varying sources varies from a few minutes to several tens of minutes; (5) the intensity of the sources varies by factors of 2 to 7 over time periods of 1 min to tens of minutes; and (6) the sources tend to disappear for periods of up to tens of minutes and to reappear at the same locations.     

3D magnetic reconnection at an X-ray bright point

On May 1, 1993, a flaring X-ray bright point (XBP) was observed for about 16 hours in the old, disintegrating, bipolar active region (AR) NOAA 7493. During this period, a minor magnetic bipole (10²⁰ Mx) emerged in the region. We have found observational evidence showing that the XBP brightenings were due to magnetic reconnection between the new bipole and pre-existing plage fields. The aim of the present work is to substantiate with magnetic modelling what has been shown by the observations. For this purpose we extrapolate the observed photospheric magnetic fields in the linear force-free approximation and follow its evolution during the lifetime of the XBP. From the computed coronal field lines we determine the location of regions of drastic change in field-line linkage, called quasi-separatrix layers or QSLs. QSLs are open layers that behave physically like separatrices: the break down of ideal magnetohydrodynamics and the release of free magnetic energy may occur at these locations when their thickness is small enough. The extrapolated field lines, with photospheric footpoints on both sides of QSLs, match the observed chromospheric and coronal structures (arch filament system, XBP and faint X-ray loops (FXL)). We study also the evolution of the width of the QSL located over the new negative polarity pore: the calculated QSL is very thin (typically less than 100 m) during the lifetime of the XBP, but becomes much thicker ( 10⁴ m) after the XBP has faded. Furthermore we show that peaks in X-ray brightness propagate along the FXL with a velocity of 670 km s^-1, starting from the XBP location, implying that the energy is released where the emerging bipole impacts against pre-existing coronal loops. We discuss the possible mechanism of energy transport and conclude that the energy is conducted to the remote footpoints of the FXL by a thermal front. These results strongly support the supposition that the XBP brightness and flaring are due to the interaction of different flux systems, through 3D magnetic reconnection, at QSLs.Member of the Carrera del Investigador Cientifíco, CONICET.Also at Konkoly Observatory, Budapest, Pf. 67, H-1525 Hungary.     

An analysis of photospheric vector magnetograms, Hα images and soft X-ray images in a superactive region NOAA 7321

Based on the photospheric vector magnetograms taken at Huairou Solar Observing Station (HSOS), H images taken at Hiraiso Solar Terrestrial Research Center of Communication Research Laboratory, soft X-ray images taken by Yohkoh and an extrapolation method, the magnetic field structures and some active phenomena of the active region AR 7321 around 04:12 UT on 27 October 1992 are analyzed in this paper. A divergence of the transverse magnetic field, located at a newly formed spot, was found. At least four highly sheared magnetic field systems separately spread from this divergence to four other sites around this divergence. Vertical current concentrations are upward in this region and downward in the other four sites, and the corresponding H bright patches and soft X-ray bright loops coincide with these structures, confirming the existence of these four systems. The extrapolated magnetic force lines reconstructed by the Boundary Element Method (BEM), force-free field assumption, and boundary condition of observed photospheric vector magnetic field, coincide in space with the H bright patches and soft X-ray bright loops, showing that this extrapolation method is very effective and suggesting that the H bright patches and soft X-ray bright loops in this case represent the magnetic field structures in the upper atmosphere of the Sun.The bright structures in the H images and the soft X-ray images have a close correlation with the non-potential characteristics of the magnetic fields.