On the large-scale diffuse magnetic field of the Sun – II. The Contribution of Active Regions

Dikpati and Choudhuri (1994, 1995) developed a model for the poleward migration of the weak diffuse magnetic field on the Sun's surface. This field was identified with the poloidal component produced by the solar dynamo operating at the base of the convection zone, and its evolution was studied by considering the effects of meridional circulation and turbulent diffusion. The earlier model is extended in this paper by incorporating the flux from, the decay of tilted active regions near the solar surface as an additional source of the poloidal field. This extended model can now explain various low-latitude features in the time-latitude diagram of the weak diffuse fields. These low-latitude features could not be accounted for in the earlier model, which was very successful in modeling the behavior at high latitudes. The time-latitude diagrams show that regions of a particular polarity often have tongues of opposite polarity. Such tongues can be produced in the theoretical model by incorporating fluctuations in the source term arising out of the decaying active regions.     

EVIDENCE OF MAGNETIC RECONNECTION FROM Hα, SOFT X-RAY AND PHOTOSPHERIC MAGNETIC FIELD OBSERVATIONS

A conventional view of magnetic reconnection is mainly based on the 2-D picture of an X-type neutral point, or on the extension of it to 3-D, and it is thought to be accompanied by flux transport across separatrices (places where the field-line mapping is discontinuous). This view is too restrictive when we realize the variety of configurations that are seen flaring. We designed an algorithm, called Source Method (SM), to determine the magnetic topology of active regions (ARs). The observed photospheric field was extrapolated to the corona using subphotospheric sources and the topology was defined by the link between these sources. H flare brightenings were found to be located at the intersection with the chromosphere of the separatrices so defined. These results and the knowledge we gained on the properties of magnetic field-line linkage, led us to generalize the concept of separatrices to quasi-separatrix layers (QSLs) and to design a new method (quasi-separatrix layers method, QSLM) to determine the magnetic topology of ARs. QSLs are regions where the magnetic field-line linkage changes drastically (discontinuously when they behave like separatrices) and the QSLM can be applied to ARs where the photospheric field has been extrapolated using any kind of technique. In this paper we apply the QSLM to observed flaring regions presenting very different configurations and also to a decaying AR where a minor phenomenon, like an X-ray bright point (XBP), is observed. We find that the locations of flare and XBP brightenings are related to the properties of the field-line linkage of the underlying magnetic region, as expected from recent developments of 3-D magnetic reconnection. The extrapolated coronal field lines representing the structures involved in the analyzed events have their photospheric footpoints located at both sides of QSLs. Our results strongly support the hypothesis that magnetic reconnection is at work in various coronal phenomena, ranging from the less energetic ones to large-scale eruptions.     

Polarization of solar active regions at 9.5 mm wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 9.5 mm wavelength, using the 85 ft reflector and polarimeter at the Naval Research Laboratory Maryland Point Observatory. The angular resolution of the telescope at this wavelength is 1.6. All important active regions observed at 9.5 mm are bipolar in nature, the degree of polarization is about the same for both right and left circular components and it ranges up to about 4%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates clearly the neutral line between the regions of opposite polarity on magnetograms. Unipolar regions in magnetograms also show up as unipolar regions at 9.5 mm. Magnetic fields as low as 5–10 G on magnetograms manifest as distinctly polarized regions on 9.5 mm maps. A line of zero polarization seems to delineate the extent of absorption features observed at 9.5 mm in coincidence with H dark filaments.     

Resonance Scattering of 30.4 nm Chromospheric Radiation by Coronal Singly Ionized Helium Observed with EIT

A diffuse emission is observed above the solar limb in the 304 Å channel of the Extreme-Ultraviolet Imaging Telescope (EIT) onboard the SOHO spacecraft. Part of this emission is attributed to the presence of residual singly-ionized helium in the solar corona, which resonantly scatters the intense helium Lyman alpha radiation of the chromosphere. This emission can be distinguished from other coronal emissions in the EIT bandpass. Maps of the helium ion density integrated along the line of sight are derived. These agree well with models in the low latitude, closed magnetic field regions of the solar corona. However, the helium ions' abundance seems to be enhanced in the polar, open field regions above coronal holes. This may be related to acceleration processes of the fast solar wind close to the Sun.     

Noise storms and particular photospheric magnetic structures

A comparison of flux and polarization of solar radio noise storms with photospheric source position and magnetic field configuration for six year observations is reported. Three independent results pointing to a predominance of plus magnetic structures as regards noise-storm generation are outlined. A rather strong proof towards a cause-effect connection of photospheric magnetic structure development and noise-storm evolution is stressed.     

Linear force-free fields in the lower corona

We have computed the surface Green's function for linear force-free magnetic fields, where × B = B and is a constant, for application to low coronal levels of the solar atmosphere. Boundary conditions are imposed on the normal component of B on two parallel planes which delineate the force-free volume. This procedure ensures that the magnetic field energy remains bounded, and that the field lines have a smooth behavior. A simple bipolar source distribution is treated and representative field line tracings are shown.     

Structure and evolution of magnetic network features

Magnetogram series, obtained with the 512-diode-array magnetograph at KPNO, are used to investigate several properties of magnetic flux tubes in the solar atmosphere. Average size, lifetime and inclination of the flux elements are determined. Further we discuss the question, how magnetic flux appears and disappears on the solar surface. At last it is investigated, if our observational results are consistent with Piddington's flux-rope-fibre theory of solar magnetism.Visiting Astronomer at Kitt Peak National Observatory.     

Radiative heating and the buoyant rise of magnetic flux tubes in the solar interior

We study the effect of radiative heating on the evolution of thin magnetic flux tubes in the solar interior and on the eruption of magnetic flux loops to the surface. Magnetic flux tubes experience radiative heating because (1) the mean temperature gradient in the lower convection zone and the overshoot region deviates substantially from that of radiative equilibrium, and hence there is a non-zero divergence of radiative heat flux; and (2) the magnetic pressure of the flux tube causes a small change of the thermodynamic properties within the tube relative to the surrounding field-free fluid, resulting in an additional divergence of radiative heat flux. Our calculations show that the former constitutes the dominant source of radiative heating experienced by the flux tube.In the overshoot region, the radiative heating is found to cause a quasi-static rising of the toroidal flux tubes with an upward drift velocity 10^-3|| cm s^-1, where _e – _ad < 0 describes the subadiabaticity in the overshoot layer. The upward drift velocity does not depend sensitively on the field strength of the flux tubes. Thus in order to store toroidal flux tubes in the overshoot region for a period comparable to the length of the solar cycle, the magnitude of the subadiabaticity (< 0) in the overshoot region must be as large as 3 × 10^–4. We discuss the possibilities for increasing the magnitude of and for reducing the rate of radiative heating of the flux tubes in the overshoot region.Using numerical simulations we study the formation of -shaped emerging loops from toroidal flux tubes in the overshoot region as a result of radiative heating. The initial toroidal tube is assumed to be non-uniform in its thermodynamic properties along the tube and lies at varying depths beneath the base of the convection zone. The tube is initially in a state of neutral buoyancy with the internal density of the tube plasma equal to the local external density. We find from our numerical simulations that such a toroidal tube rises quasi-statically due to radiative heating. The top portion of the nonuniform tube first enters the convection zone and may be brought to an unstable configuration which eventually leads to the eruption of an anchored flux loop to the surface. Assuming reasonable initial parameters, our numerical calculations yield fairly short rise times (2–4 months) for the development of the emerging flux loops. This suggests that radiative heating is an effective way of causing the eruption of magnetic flux loops, leading to the formation of active regions at the surface.The National Solar Observatory is one of the National Optical Astronomy Observatories by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.     

On the ratio between the mechanical fluxes in- and outside the solar chromospheric mottles

Available information about the relative areas on the quiet and active parts of the sun covered with magnetic elements, together with theoretical results on the relation between the photospheric mechanical flux and the consequent coronal electron density, allows one to conclude that the mechanical flux generated in the photospheric magnetic elements is about seven times as large as the flux generated in non-magnetic regions.     

The dynamo dilemma

The recent determination that the angular velocity of the Sun declines downward through the convective zone raises serious questions about the nature of the solar dynamo. The principal qualitative features of the Sun are the azimuthal fields that migrate toward the equator in association with an oscillating poloidal field which reverses at about the time of maximum appearance of bipolar magnetic regions. If decreases downward, or is negligible, the horizontal gradient in produces a dynamo with some of these essential characteristics. There is reason to think that the dynamo is confined to the lower half of the convective zone where has the opposite sign from the usual ( > 0 in the northern hemisphere) producing equatorward migration but reversing the sign of the associated poloidal field. Meridional circulation may play an essential role in shaping the dynamo. At the present time it is essential to measure accurately and determine the nature of the meridional circulation.Solar Cycle Workshop Paper.     

On The Torsional Oscillations In Babcock–Leighton Solar Dynamo Models

The torsional oscillations at the solar surface have been interpreted by Schüssler and Yoshimura as being generated by the Lorentz force associated with the solar dynamo. It has been shown recently that they are also present in the upper half of the solar convection zone (SCZ). With the help of a solar dynamo model of the Babcock–Leighton type studied earlier, the longitudinal component of the Lorentz force, L , is calculated, and its sign or isocontours, are plotted vs. time, t, and polar angle, (the horizontal and vertical axis respectively). Two cases are considered, (1) differential rotation differs from zero only in the tachocline, (2) differential rotation as in (1) in the tachocline, and purely latitudinal and independent of depth in the bulk of the SCZ. In the first case the sign of L is roughly independent of latitude (corresponding to vertical bands in the t, plot), whereas in the second case the bands show a pole–equator slope of the correct sign. The pattern of the bands still differs, however, considerably from that of the helioseismic observations, and the values of the Lorentz force are too small at low latitudes. It is all but certain that the toroidal field that lies at the origin of the large bipolar magnetic regions observed at the surface, must be generated in the tachocline by differential rotation; the regeneration of the corresponding poloidal field, B _p has not yet been fully clarified. B _p could be regenerated, for example, at the surface (as in Babcock–Leighton models), or slightly above the tachocline, (as in interface dynamos). In the framework of the Babcock-Leighton models, the following scenario is suggested: the dynamo processes that give rise to the large bipolar magnetic regions are only part of the cyclic solar dynamo (to distinguish it from the turbulent dynamo). The toroidal field generated locally by differential rotation must contribute significantly to the torsional oscillations patterns. As this field becomes buoyant, it should give rise, at the surface, to the smaller bipolar magnetic regions as, e.g., to the ephemeral bipolar magnetic regions. These have a weak non-random orientation of magnetic axis, and must therefore also contribute to the source term for the poloidal field. Not only the ephemeral bipolar regions could be generated in the bulk of the SCZ, but many of the smaller bipolar regions as well (at depths that increase with their flux), all contributing to the source term for the poloidal field. In contrast to the butterfly diagram that provides only a very weak test of dynamo theories, the pattern of torsional oscillations has the potential of critically discriminating between different dynamo models.     

Solar magnetic fields and convection

The flux-rope-fibre model of solar magnetic fields is developed further to cover post-spot evolution of the fields, faculae, and the influence of magnetic fields on some convective motions. (i) Unipolar magnetic regions of a strongly dominant polarity are explained, as are some fields outside the network, and some tiny reversed polarity fields. (ii) The migration of magnetic regions is explained: the following regions to the poles where most of the flux just vanishes and the preceding towards the equator. (iii) The model explains the rotation of the gross pattern of background fields with a period of 27 days. It explains the puzzling features of active longitudes and of magnetic longitudes extending across the equator. (iv) The magnetic model provides a framework for the various chromospheric fine structures, the rosettes, bushes, double chains, mottles and spicules. It provides qualitative models of these features and points the way to a very complicated quantitative model of the network. (v) Several new convective patterns are described and explained in terms of magnetic stresses. The first is the moat around sunspots, which replaces the supergranule motions there. The second is the long-lived (4–7 days) supergranule cell enclosed by strong fields. The third is a small-scale () convective motion, and the fourth is aligned or long granules, both caused by small-scale magnetic fields. (vi) Photospheric line faculae and photospheric continuum faculae are different phenomena. The former, like the chromospheric faculae, are caused by Alfvén-wave heating. The latter are caused by a new small-scale convective motion. (vii) A model of the 3-min oscillation is described.     

Limb Prominence Eruption on 11 August 2000 as Seen From Ground- and Space-Based Observations

We report on a prominence eruption as seen in H with the Crimean Lyot coronagraph, the global H network, and coronal images from the LASCO C2 instrument on board SOHO. We observed an H eruption at the northwest solar limb between 07:38:50 UT and 07:58:29 UT on 11 August 2000. The eruption originated in a quiet-Sun region and was not associated with an H filament. No flare was associated with the eruption, which may indicate that, in this case, a flux rope was formed prior to the eruption of the magnetic field. The H images and an H Dopplergram show a helical structure present in the erupted magnetic field. We suggest that the driving mechanism of the eruption may be magnetic flux emergence or magnetic flux injection. The limb H observations provide missing data on CME speed and acceleration in the lower corona. Our data show that the prominence accelerated impulsively at 5.5 km s^–2 and reached a speed slightly greater than 800 km s^–1 in a narrow region (h<0.14 R ) above the solar surface. The observations presented here also imply that, based only on a CME's speed and acceleration, it cannot be determined whether a CME is the result of a flare or an eruptive prominence.     

Velocity measures in the sunspot Rome no. 7490 (July 5, 1979)

Material motions on the solar surface have been deduced from the wavelength shift of Fe i 6302.5 Å, measured over the umbra and inner penumbra of a spot for which the magnetic field configuration has already been established with some confidence. The two vector fields are considered together in detail and the results support the convective roll sunspot model (Spruit, Galloway). For the magnetic field regions, both material flow along the field lines and field line motions are derived. A small upward motion only is deduced for the field free regions.     

Similarities between chromospheric and photospheric quiet-Sun magnetograms

Simultaneous observations of chromospheric (H) and photospheric (Fei 5324.19 Å) magnetograms in quiet solar regions enable us to study the spatial configuration of the magnetic field in the solar atmosphere. With the typical spatial resolution of the Huairou magnetograph, the photospheric and chromospheric magnetic structures of the quiet Sun maintain a very similar pattern. Moreover, the vertical magnetic flux is almost the same from the photosphere to the chromosphere. As an intermediate step, we analyze the formation of the working lines used by the Huairou video magnetograph of the Beijing Astronomical Observatory. The Stokes V contribution function of H and Fei 5324.19 Å are calculated. It is found that our H magnetograms provide the distribution of the chromospheric magnetic field at a height some 1000–1500 km above the photosphere.     

Ephemeral region flares and the diffusion of the network

The flare-like events which are frequently seen in H in apparently quiet regions of the solar disk can in all cases be identified with bipolar features (ephemeral regions, ER) on magnetograms. These events represent the H counterpart of X-ray bright point flares.Statistically, this phenomenon is associated with the proximity of the bipolar features to the super-granulation network, in the sense that an ER is likely to flare during its lifetime if the distance to the nearest network element is less than or equal to its own pole separation. This conclusion is supported by direct study of time sequences of magnetograms and H pictures, which manifest the interaction of ER with the supergranulation network. The flare-like brightenings in some examples occurred in the region of interaction between network flux and one pole of the ER.The consequence of this interaction is that small quantities of network flux are transported over distances of the order of the ER pole separations. This may have an important effect on the long-term diffusion of magnetic flux.     

The topological association of Hα structures and magnetic fields

The topological associations between H structures and magnetic fields are examined for an active region observed on two different dates. The structures seen in the on and off band of H filtergrams are compared with the contour maps of magnetic fields at the level of magnetogram observations. Similar comparisons are made also with the configurations of force-free magnetic lines of force at various heights evaluated with the use of formulations developed previously by Nakagawa and Raadu (1972).Among the results of significance, we may note that (1) H plages could be identified with regions of magnetic field larger than ±80 G, (2) the network of bright dots seen in H -1 Å filtergrams follow closely ±80 G contours. (3) stable prominences lie along either neutral lines or valleys of magnetic fields, (4) the configuration of magnetic lines of force shows discrete domain structures suggesting bipolar nature of local magnetic fields, and (5) within a domain the configuration is governed apparently by evalutional consequences. Details of analyses are described with discussions on the limitations and possible future improvements.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Magnetograph response to canopy-type fields

The response of longitudinal-field magnetographs to magnetic fields which are semi-infinite or confined to a horizontal layer is discussed with respect to the interpretation of solar diffuse fields, observed towards the limb, in terms of magnetic canopy models. Numerical results are presented for several reference solar models and typical calibration curves are shown for the C I 9111 Å, Fe I 8688 Å, and Ca II 8542 Å lines in magnetostatic atmospheres derived from a mean model. A procedure is developed for determining the base heights of magnetic canopies from observations with an uncertainty not exceeding the order of a pressure scale height. Until definitive information regarding atmospheric structure inside flux tubes can be developed from theory or observation, reliable field strengths cannot be derived from the data.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

A comparison between photospheric and chromospheric quiet-Sun magnetograms

Photospheric (Fei 5324.19 Å line) and chromospheric (H line) magnetic fields in quiet-Sun regions have been observed in the solar disk center by using the vector video magnetograph at Huairou Solar Observing Station of Beijing Astronomical Observatory. Observational results show that the quiet-Sun magnetic elements in the solar photosphere and chromosphere present similar magnetic structures. Photospheric and chromospheric magnetograms show corresponding time variations. This suggests that the magnetic fields in quiet-Sun regions present different 3-D magnetic configurations compared to those in solar active regions.     

Transport equations for low-energy solar particles in evolving interplanetary magnetic fields

Two new forms of a simplified Fokker-Planck equation are derived for the transport of low-energy solar energetic particles in an evolving interplanetary magnetic field, carried by a variable radial solar wind. An idealised solution suggests that the invariant anisotropy direction reported by Allum et al. (1974) may be explained within the conventional theoretical framework. The equations may be used to relate studies of solar particle propagation to solar wind transients, and vice versa.