A spectroscopic study of Epsilon Aurigae

Epsilon Aurigae has been observed during ingress and totality between 1982 and 1983 at Okayama. Analyses of profiles of H line and of radial velocities of neutral hydrogen and metals show that the secondary component consists of at least three parts in structure.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983.     

Magnetic Model of HD 2453

A model is constructed for the magnetic field of the star HD 2453, which has a very long rotation period (P=521^d). It is found that the structure of the field corresponds to the model of a dipole shifted by r=0.09R from the center. The angle of inclination of the axis of the dipole to the axis of rotation, =5°; that is, the star is viewed almost from its equator of rotation and magnetic equator. This explains the low amplitude of the phase dependence of the magnetic field, Be(P), and the low amplitude of the photometric variability. The field at the magnetic poles is equal to Bp=+4400 and -7660 G. The magnetic field parameters turn out to be close to those obtained by Landstreet and Mathys assuming a dipole-quadrupole-octupole model. A Mercator map of the magnetic field distribution of HD 2453 is produced.     

Measuring Magnetic Oscillations in the Solar Photosphere: Coordinated Observations with MDI,ASP and MWO

A comprehensive observing effort was undertaken to simultaneously obtain full Stokes profiles as well as longitudinal magnetogram maps of a positive plage region on 8 December, 1998 with the Michelson Doppler Imager, the Advanced Stokes Polarimeter and Mt. Wilson Observatory magnetograph. We compare 1.2 spatially-averaged signals of velocities as well as filter magnetograph longitudinal flux signals with Stokes determined fluctuations in filling factor, field inclination, magnetic flux and field strength. The velocity signals are in excellent agreement. Michelson Doppler Imager magnetic flux correlates best with fluctuations in the Advanced Stokes Polarimeter filling factor, not inclination angle or field strength. A correlated flux and filling factor change in the absence of a field strength fluctuation can be understood in terms of internally unperturbed flux tubes being buffeted by external pressure fluctuations. The 12.5 square aperture spatially averaged Mt. Wilson magnetograph signals are compared with Michelson Doppler Imager signals from the corresponding observing area. Velocity signals are in superb agreement. Magnetic signals exhibit similar oscillatory behavior. Lack of Advanced Stokes Polarimeter data for this time excludes interpretation of magnetic fluctuations as due to filling factor or field inclination angle. Mt. Wilson Observatory simultaneous sampling of the nickel and sodium spectral line profiles with several wing pairs allowed inter-comparison of signals from different heights of formation. Slight phase shifts and large propagation speeds for the velocity signals are indicative of modified standing waves. Phase speeds associated with magnetic signals are characteristic of photospheric Alfvén speeds for plage fields. The phase speed increase with height agrees with the altitude dependence of the Alfvén speed. The observed fluctuations and phases are interpreted as a superposition of signatures from the horizontal component of the driving mechanism sweeping the field lines in/out of the resolution area and the magnetic response of the flux tube to this buffeting.     

The flux ejection dynamo with small diffusivity

A number of examples are worked out to illustrate the consequences of reverse flux ejection from the surface of a convective layer of conducting fluid. Generally the reverse flux ejection has the opposite effect of magnetic buoyancy, tending to bury the fields rather than bringing them through the surface. Even a weak flux ejection effect prevents the excape of magnetic field through the surface. Reverse flux ejection at the surface of an -dynamo profoundly alters the character of the solutions of the dynamo equations. Altogether, flux ejection serves to obscure the interpretation of magnetic observations. The outstanding problem now is to determine under what circumstances there exists cyclonic convection with rotations in excess of ±1/2 in the rising columns of fluid. Negative turbulent diffusion is expected to be a close companion of the flux ejection effect.This work was supported by the National Aeronautics and Space Administration under grant NGL 14-001-001.     

A model of interplanetary and coronal magnetic fields

A model of the large-scale magnetic field structure above the photosphere uses a Green's function solution to Maxwell's equations. Sources for the magnetic field are related to the observed photospheric field and to the field computed at a source surface about 0.6 R above the photosphere. The large-scale interplanetary magnetic field sector pattern is related to the field pattern at this source surface. The model generates magnetic field patterns on the source surface that compare well with interplanetary observations. Comparisons are shown with observations of the interplanetary magnetic field obtained by the IMP-3 satellite.     

Pulsar radio emission

A new version of the theory of pulsar radio emission is developed for the case of a coaxial rotator. It is based on the electric field that we established [G. S. Sahakian, Astrofizika, 37, 97 (1994)] for the radiation channel (the channel of open magnetic field lines) and on convenient approximations for the electron energy obtained in [G. S. Sahakian and É. S. Chubarian, Astrofizika, 37, 255 (1994)]. It is shown that, owing to the emission of photons of curvature radiation by particles, e e+_c', and photon annihilation, _c e⁺e^– in the lower part of the radiation channel, a special region (the magnetic funnel) is formed in which vigorous cascade multiplication of particles occurs. The height of the magnetic funnel is h 6R^0.2, where R is the radius of the neutron star and is its angular rotation rate. As a result of supersaturation of the plasma density in the magnetic funnel, a discharge occurs after each time intervalt5·10^–7–0.8B ₁₂ ^–1.4 R ₆ ^–0.2 , i.e., the longitudinal electric field disappears (B is the magnetic induction in the star). During the active radiative processes in the magnetic funnel, two main fluxes of particles with high ultrarelativistic energies are formed: an upward flux of electrons and a positron flux falling onto the star's magnetic cap. These fluxes are accompanied by narrow strips of positron and electron fluxes, respectively, of considerably lower energy, which are fairly powerful, coherent radio sources. The pulsar's radio luminosity is calculated to be L7.4·10^223.8 ₃₀ ³ R ₆ ^–2 erg/sec, where =BR 3/2 is the star's magnetic moment. Comparing this result with observations, we conclude that the magnetic moment and hence the mass of the neutron star evidently must be considerably smaller, on the average, for fast pulsars than for slow ones. It is shown that the magnetic moment of the neutron star can be determined from the intervals between micropulses in the pulse profiles. The problem of the origin of the macrostructure of the radio pulse is discussed.Translated from Astrofizika, Vol. 38, No. 1, pp. 141–185, January – March, 1995.     

Infrared array measurements of sunspot magnetic fields

We have used a 128 × 128 format HgCdTl infrared array with the Sacramento Peak Observatory Vacuum Telescope (VTT) and Echelle spectrograph to obtain two-dimensional observations of the true magnetic field strength in a sunspot. The system we describe retains all of the spectral information contained in the unpolarized IR Fraunhofer line profile with time resolution of about a minute (depending on the scan area and spatial resolution). Unlike previous optical observations (cf. Adam, 1990), infrared observations readily allow direct field strength measurements out to the outer edge of the penumbra. Our data suggest that the magnetic flux density in the outer penumbra is not well described by an extrapolation of the quadratic polynomial, in normalized central distance, that describes the umbral field. We measure a relatively high field strength of 800 G at the penumbra-quiet-Sun boundary, which is consistent with the return-flux model of Osherovich and Garcia (1989).     

The formation of current sheets during the emergence of new magnetic flux from below the photosphere

Solar flares are frequently observed to occur where new magnetic flux is emerging and pressing up against strong active region magnetic fields. Since the solar plasma is highly conducting, current sheets develop at the boundary between the emergent and ambient flux, provided the two magnetic fields are inclined at a non-zero angle to one another.The present paper gives a simple two-dimensional model for the development of such sheets under the assumptions that no reconnection occurs and that the surrounding field remains a potential one. By using complex variable techniques, the position, orientation and shape of a current sheet may be determined, as well as the excess magnetic energy associated with it. Two examples are considered. The first, in which the ambient field is bipolar, may model new flux emergence near the edge of an active region, while the second example assumes a constant ambient field and may approximate the so-called fibril crossings which occur prior to some flares. In each case, the current sheets are curved, and the magnetic energy which is stored in excess of potential is sufficient to supply a solar flare when the sheets are long enough.     

Structure and stability of prominences with helical structure

Observations of internal structure and development of four helical prominences are presented. We assume that the helically twisted fine structure threads are outlining magnetic field lines and we found that it is possible to describe the magnetic fields by the uniform twist configuration, with the twists ranging between 2 and 7. The estimated lower limits for the magnetic fields were about 20 G which give lower limits for the currents flowing along the prominences in the range between 2 × 10¹⁰ A and 2 × 10¹¹ A and current densities at the axis of the prominences about 10^-4 A m^-2. The upper limit of electron drift velocity could be estimated as 1 m s^-1, which is far below the critical velocities for the onset of plasma microinstabilities.The stability of the studied prominences is discussed and the criteria for the onset of eruptive instability are established for a prominence modelled as a twisted and elliptically curved magnetic flux tube which is anchored in the photosphere and affected by its mirror-current. The eruption starts when the prominence attains a critical height which must be larger than half of the footpoint separation and depends on the values of twist, radius, and footpoint distance of the magnetic flux tube. The observed examples of eruptive prominences agree very well with the predictions. Possible applications to the two-ribbon flare process are outlined.Properties of stable cylindrical prominences in equilibrium are analyzed and a criterion for the distinction between the Kuperus-Raadu and Kippenhahn-Schlüter types of prominences is proposed. According to established criteria, two of the studied prominences were of the Kuperus-Raadu type, while the other two were of the Kippenhahn-Schlüter type.     

Magnetic field configuration of active region NOAA 6555 at the time of a long-duration flare on 23 March 1991 – An Exception to Standard Flare Reconnection Model

The high-resolution H images observed during the decay phase of a long-duration flare on 23 March 1991 are used to study the three-dimensional magnetic field configuration of the active region NOAA 6555. Whereas all the large flares in NOAA 6555 occurred at the location of high magnetic shear and flux emergence, this long-duration flare was observed in the region of low magnetic shear at the photosphere. The H loops activity started soon after the maximum phase of the flare. There were a few long loop at the initial phase of the activity. Some of these were sheared in the chromosphere at an angle of about 45° to the east-west axis. Gradually, an increasing number of shorter loops, oriented along the east-west axis, started appearing. The chromospheric Dopplergrams show blue shifts at the end points of the loops. By using different magnetic field models, we have extrapolated the photospheric magnetograms to chromospheric heights. The magnetic field lines computed by using the potential field model correspond to most of the observed H loops. The height of the H loops were derived by comparing them with the computed field lines. From the temporal evolution of the H loop activity, we derive the negative rate of appearance of H features as a function of height. It is found that the field lines oriented along one of the neutral lines were sheared and low lying. The higher field lines were mostly potential. The paper also outlines a possible scenario for describing the post-flare stage of the observed long-duration flare.     

Multi-channel magnetograph observations

Simultaneous observations of photospheric magnetic fields, Caii K emission, the photospheric network and continuum faculae show that these four quantities are correlated in a complicated manner. The photospheric and calcium networks show increasing contrast with increasing magnetic field strength up to field strengths of about 500 G. Higher values of the magnetic field are found only in pores and sunspots. Continuum faculae also show increasing contrast with increasing magnetic field strength (even at the disk center), but this contrast reaches a maximum at field strengths of about 200 G. At higher field strengths, continuum faculae become monotonically darker until pore or spunspot conditions are reached.Measurements of the photospheric network and the continuum faculae over a wide range of result in families of limb contrast curves. These curves indicate that the dependence on H is as important as the dependence on . They also indicate that the magnetic field has a preferred inclination of about 50°. The facular contrast shows little dependence on resolution. This is interpreted in terms of a geometric model in which faculae are clumps of many individual flux tubes. These tubes are closely packed and unresolvable in the photosphere, but are more widely spaced, and therefore resolvable, in the low chromosphere.Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Photoelectric polarimetry of a dark unipolar sunspot

Photoelectric polarisation measurements in a stable sunspot (type H) with a particularly dark umbra carried out with the Capri magnetograph have been evaluated in terms of Unno's (1956) theory to give the value and direction of the magnetic field vector. A linear increase of the inclination angle with distance from the spot centre up tor 1.2R _s results, as originally found by Hale and Nicholson (1938) with a different procedure. The field strength decreases from the maximum value (about 3300) to about 15% at the penumbral border, still continuously decreasing outside the spot. The projected field direction deviates considerably from the radial symmetry in several parts of the spot region, but it is in good agreement with that of the overlaying chromospheric structures.Mitteilungen aus dem Fraunhofer Institut Nr. 97.     

Shear angle of magnetic fields

In this paper we introduce a new parameter, the shear angle of vector magnetic fields, , to describe the non-potentiality of magnetic fields in active regions, which is defined as the angle between the observed vector magnetic field and its corresponding current-free field. In the case of highly inclined field configurations, this angle is approximately equal to the angular shear, , defined by Hagyardet al. (1984). The angular shear, , can be considered as the projection of the shear angle, , on the photosphere. For the active region studied, the shear angle, , seems to have a better and neater correspondence with flare activity than does . The shear angle, , gives a clearer explanation of the non-potentiality of magnetic fields. It is a better measure of the deviation of the observed magnetic field from a potential field, and is directly related to the magnetic free energy stored in non-potential fields.     

A possible diagnostic method for magnetic field and velocity gradients in sunspots

We present a diagnostic method for detecting magnetic field gradients and velocity gradients in sunspots through the analysis of Stokes parameters profiles in magnetoactive lines. On one hand, the method is based on the concept of response functions for the Stokes profiles introduced by Landi Degl'Innocenti and Landi Degl'Innocenti (1977); on the other hand, it takes advantage of the diagnostic content of the residuals between observed Stokes profiles and best-fit Unno-profiles. The analysis of synthesis profiles obtained for the line 6302.502 FeI forming in the Sunspot Sunspot Model (Avrett, 1981) suggests that the method could actually be promising, at least if the dependence of the magnetic field vector on optical depth is sufficiently simple.     

Kink instability of solar coronal loops as the cause of solar flares

Solar coronal loops are observed to be remarkably stable structures. A magnetohydrodynamic stability analysis of a model loop by the energy method suggests that the main reason for stability is the fact that the ends of the loop are anchored in the dense photosphere. In addition to such line-tying, the effect of a radial pressure gradient is incorporated in the analysis.Two-ribbon flares follow the eruption of an active region filament, which may lie along a magnetic flux tube. It is suggested that the eruption is caused by the kink instability, which sets in when the amount of magnetic twist in the flux tube exceeds a critical value. This value depends on the aspect ratio of the loop, the ratio of the plasma to magnetic pressure and the detailed transverse magnetic structure. For a force-free field of uniform twist the critical twist is 3.3, and for other fields it is typically between 2 and 6. Occasionally active region loops may become unstable and give rise to small loop flares, which may also be a result of the kink instability.     

On the Correlation Between the he ii λ304 Network Brightening and the Photospheric Magnetic Field

Near simultaneous coronal EUV images were used to show that the He ii 304 network brightening is independent of coronal EUV radiation. We studied the quantitative relation between the intensity of He ii 304 network brightening with the associated magnetic elements. An almost linear relationship was found between the He ii 304 network brightening and the magnetic field for a field strength higher than 10 G with exceptions at neutral lines and in the intra-network. We also calculated the most probable formation height of He ii 304 network elements using the potential extrapolation of the photospheric magnetic field. The results show that He ii 304 network elements form at or around 3000 km above the height of the layer at which the sources of magnetic flux are deployed.     

Polarized radiation diagnostics of magnetohydrodynamic models of the solar atmosphere

Solar magnetic elements and their dynamical interaction with the convective surface layers of the Sun are numerically simulated. Radiation transfer in the photosphere is taken into account. A simulation run over 18.5 minutes real time shows that the granular flow is capable of moving and bending a magnetic flux sheet (the magnetic element). At times it becomes inclined by up to 30° with respect to the vertical around the level ₅₀₀₀ = 1 and it moves horizontally with a maximal velocity of 4 km/s. Shock waves form outside and within the magnetic flux sheet. The latter cause a distinctive signature in a time series of synthetic Stokes V-profiles. Such shock events occur with a mean frequency of about 2.5 minutes. A time resolution of at least 10 seconds in Stokes V recordings is needed to reveal an individual shock event by observation.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

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.     

The Importance of Photospheric Intense Flux Tubes for Coronal Heating

Dissipation of magnetic energy in the corona requires the creation of very fine scale-lengths because of the high magnetic Reynolds number of the plasma. The formation of current sheets is a natural possible solution to this problem and it is now known that a magnetic field that is stressed by continous photospheric motions through a series of equilibria can easily form such sheets. Furthermore, in a large class of 3D magnetic fields without null points there are locations, called quasi-separatrix layers (QSLs), where the field-line linkage changes drastically. They are the relevant generalisation of normal separatrices to configurations without nulls: along them concentrated electric currents are formed by smooth boundary motions and 3D magnetic reconnection takes place when the layers are thin enough. With a homogenous normal magnetic field component at the boundaries, the existence of thin enough QSL to dissipate magnetic energy rapidly requires that the field is formed by flux tubes that are twisted by a few turns. However, the photospheric field is not homogeneous but is fragmented into a large number of thin flux tubes. We show that such thin tubes imply the presence of a large number of very thin QSLs in the corona. The main parameter on which their presence depends is the ratio between the magnetic flux located outside the flux tubes to the flux inside. The thickness of the QSLs is approximately given by the distance between neighbouring flux tubes multiplied by the ratio of fluxes to a power between two and three (depending on the density of flux tubes). Because most of the photospheric magnetic flux is confined in thin flux tubes, very thin QSLs are present in the corona with a thickness much smaller than the flux tube size. We suggest that a turbulent resistivity is triggered in a QSL, which then rapidly evolves into a dynamic current sheet that releases energy by fast reconnection at a rate that we estimate to be sufficient to heat the corona. We conclude that the fragmentation of the photospheric magnetic field stimulates the dissipation of magnetic energy in the corona.     

The stability parameters of three-body periodic orbits

Formulae containing the elements of the variational matrix are obtained which determine the linear iso-energetic stability parameters of periodic orbits of the general three-body problem. This requires the numerical integration of the variational equations but produces the stability parameters with the effective accuracy of the numerical integration. The procedure is applied for the determination of horizontally critical orbits among the members of sets of vertical-critical periodic orbits of the threebody problem. These critical-critical orbits have special importance as they delimit the regions in the space of initial conditions which correspond to possibly stable three-dimensional periodic motion of low inclination.