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  • 1
    Electronic Resource
    Electronic Resource
    Observational support of reconnection in solar flares (1994)
    Springer
    Space science reviews 68 (1994), S. 129-130 
    Details
    ISSN: 1572-9672
    Keywords: Flares ; Reconnection ; Magnetic Fields
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a detailed analysis of the magnetic topology of flaring active region. TheH α kernels are found to be located at the intersection of the separatrices with the chromosphere when the shear, deduced from the fibrils or/and transverse magnetic field direction, is taken into account. We show that the kernels are magnetically connected by field lines passing close to the separator. We confirm, for other flares, previous studies which show that photospheric current concentrations are located at the borders of flare ribbons. Moreover we found two photospheric current concentrations of opposite sign, linked in the corona by field lines which follow separatrices. These give evidence that magnetic energy is released by reconnection processes in solar flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00749130
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  • 2
    Electronic Resource
    Electronic Resource
    Helical structures around quiescent solar prominences computed from observable magnetic fields (1992)
    Springer
    Solar physics 142 (1992), S. 291-311 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We analyse the magnetic support of solar prominences in two-dimensional linear force-free fields. A line current is added to model a helical configuration, well suited to trap dense plasma in its bottom part. The prominence is modeled as a vertical mass-loaded current sheet in equilibrium between gravity and magnetic forces. We use a finite difference numerical technique which incorporates both vertical photospheric and horizontal prominence magnetic field measurements. The solution of this mixed boundary problem generally presents singularities at both the bottom and top of the model prominence. The removal of the singularities is achieved by superposition of solutions. Together with the line current equilibrium, these three conditions determine the amplitude of the magnetic field in the prominence, the flux below the prominence and the current intensity, for a given height of the line current. A numerical check of accuracy in the removal of singularities, is done by using known analytical solutions in the potential limit. We have investigated both bipolar and quadrupolar photospheric regions. In this mixed boundary problem the polarity of the field component orthogonal to the prominence is mainly fixed by the imposed height of the line current. For bipolar regions above (respectively below) a critical height the configuration is inverse (respectively normal). For quadrupolar regions the polarity is reversed if we refer the prominence polarity to the closest photospheric polarities. We introduce the polarity of the component parallel to the prominence axis with reference to a sheared arcade. Increasing the shear with fixed boundary conditions can increase or decrease the mass supported depending on the configuration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151455
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  • 3
    Electronic Resource
    Electronic Resource
    Common evolution of adjacent sunspot groups (1993)
    Springer
    Solar physics 145 (1993), S. 77-94 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The evolution of two adjacent bipolar sunspot groups is studied using Debrecen full-disc, white-light photoheliograms and Hα filtergrams as well as Meudon magnetograms. The proper motions of the principal preceding spots of both groups show quite similar patterns; the spots move along almost parallel tracks and change the direction of their motion on the same day at almost the same heliographic longitude. Also, three simultaneous emergences of magnetic flux were observed in both groups. These observations support the idea that these adjacent sunspot groups were magnetically linked below the photosphere. Matching the extrapolated magnetic field lines with the chromospheric fibril structure appears to be different in the two groups since they indicate quite different model solutions for each group, i.e., a near-potential magnetic field configuration in the older group (1) and a twisted force-free field configuration in the younger group (2). The latter configuration could be created by a considerable twist of the main bunch of flux tubes in Group 2, which is reflected in the relative sunspot motions. It is also showed how this twist contributed to the formation of a filament between the two groups.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00627984
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  • 4
    Electronic Resource
    Electronic Resource
    Structural characteristics of eruptive prominences (1992)
    Springer
    Solar physics 141 (1992), S. 289-301 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Nowadays the primordial importance of the magnetic field for coronal plasma physics is well known. However, its determination is only made in cool regions, mainly the photosphere and prominences. The extrapolation to the corona gives some indications of the magnetic structure but is not presently sufficiently reliable. So it is important to consider all the other observable physical effects of the magnetic field. In this puzzle, eruptive prominences may play a key role because the cool plasma is forced to move along field lines, which can then be visualized. In the strongest field regions, flares also give such information, while coronal mass ejections (CME) play such a role at larger scales. The magnetic field, which is at the base of the physical processes, is a common link between these different events. Observed properties of solar prominence eruptions are reviewed, then their relationships with CMEs and flares are discussed, with the help of present models. We emphasize the importance of magnetic measurements in future coordinated observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155181
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  • 5
    Electronic Resource
    Electronic Resource
    Relationship between electric currents, photospheric motions, chromospheric activity, and magnetic field topology (1994)
    Springer
    Solar physics 149 (1994), S. 309-330 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Through coordinated observations made during the Max'91 campaign in June 1989 in Potsdam (magnetograms), Debrecen (white light and Hα), and Meudon (MSDP), we follow the evolution of the sunspot group in active region NOAA 5555 for 6 days. The topology of the coronal magnetic field is investigated by using a method based on the concept of separatrices - applied previously (Mandriniet al., 1991) to a magnetic region slightly distorted by field-aligned currents. The present active region differs by having significant magnetic shear. We find that the Hα flare kernels and the main photospheric electric current cells are located close to the intersection of the separatrices with the chromosphere, in a linear force-free field configuration adapted to the observed shear. Sunspot motions, strong currents, isolated polarities, or intersecting separatrices are not in themselves sufficient to produce a flare. A combination of them all is required. This supports the idea that flares are due to magnetic reconnection, when flux tubes with field-aligned currents move towards the separatrix locations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00690619
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  • 6
    Electronic Resource
    Electronic Resource
    Relationship between magnetic field evolution and flaring sites in AR 6659 in June 1991 (1994)
    Springer
    Solar physics 150 (1994), S. 199-219 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract During the international campaign of June 1991, the active region AR 6659 produced six very large, long-duration flares (X10/12) during its passage across the solar disk. We present the characteristics of four of them (June 4, 6, 9, 15). Precise measurements of the spot motions from Debrecen and Tokyo white-light pictures are used to understand the fragmentation of the main sunspot group with time. This fragmentation leads to a continuous restructuring of the magnetic field pattern while rapid changes are evidenced due to fast new flux emergence (magnetograms of MFSC, Huairou). The first process leads to a shearing of the field lines along which there is energy storage; the second one is the trigger which causes the release of energy by creating a complex topology. We conjecture that these two processes with different time scales are relevant to the production of flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712886
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  • 7
    Electronic Resource
    Electronic Resource
    A model for an inverse-polarity prominence supported in a dip of a quadrupolar region (1993)
    Springer
    Solar physics 144 (1993), S. 283-305 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We investigate the formation and support of solar prominences in a quadrupolar magnetic configuration. The prominence is modeled as a current sheet with mass in equilibrium in a two-dimensional field. The model possesses an important property which is now thought to be necessary, namely that the prominence forms within the dip, rather than the dip being created by the prominence. The approach of two bipolar regions of the same sign gives a natural way to form a dip in the magnetic field in a horizontal band above the photospheric polarity inversion line. As the approach proceeds, the height of the dip region decreases but, in agreement with observations, a corridor, free of significant magnetic field, is needed in order to obtain a dip at low heights. Support is achieved locally just as for normal-polarity configurations, so the model avoids the strong self-pinching effect of several inverse-polarity configurations (such as the Kuperus and Raadu model). The role of the strong field component along the prominence axis, which is here modelled by a uniform field in that direction, may well be to provide the necessary thermal properties for prominence formation. The model thus has several attractive features which make it credible for inverse polarity prominences: (i) both the dip and the inverse orientation are naturally present; (ii) prominence formation is by converging rather than shearing motions, in agreement with observations; converging photospheric motions induce a horizontal upward motion in the filament; (iii) the orientation of the axial field, opposite to what is expected from differential rotation, is naturally accounted for; (iv) the observed relation between chromospheric and prominence magnetic field strengths is naturally reproduced; (v) the field configuration is more complex than a simple bipole, in agreement with observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00627594
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  • 8
    Electronic Resource
    Electronic Resource
    Development of a topological model for solar flares (1992)
    Springer
    Solar physics 139 (1992), S. 105-123 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The main theoretical studies of the process involved in solar flares have been made in the two-dimensional approximation. However, the preliminary studies made with three field components suggest that reconnection could take place in the separatrices, the separator (intersection of separatrices) being a privileged location for this process. As a consequence the sites of flare kernels must be located on the intersections of the separatrices with the photosphere. Therefore, in order to understand the role of interacting large-scale structures in solar flares, we have analysed the topology of three-dimensional potential and linear force-free fields. The magnetic field has been modelled by a distribution of charges or dipoles located below the photosphere. This modelling permits us to define the field connectivity by the charges or the dipoles at both ends of every field line. We found that the appearance of a separator above the photosphere is more likely when a parasitic bipole emerges outside the axis that joins the main polarities and when the field lines are characteristic of a field created by dipoles. The separatrices derived in the potential and force-free hypothesis have different shapes. However, in the strong field regions where flares usually occur, the separatrices of the potential and force-free field models become closer. This property makes possible the use of the potential field, as a first estimate, for computing the location in the photosphere of the separatrices and for comparing this location with the position of observed Hα kernels. Displacements of the separatrices of a force-free field result from modifications of the free energy of the field. Then force-free fields have the further capability of predicting the kernel displacement. In all cases a configuration suitable for prominence support is found above the separator.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147884
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  • 9
    Electronic Resource
    Electronic Resource
    Interpretation of multiwavelength observations of November 5, 1980 solar flares by the magnetic topology of AR 2766 (1994)
    Springer
    Solar physics 150 (1994), S. 221-243 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712887
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  • 10
    Electronic Resource
    Electronic Resource
    Book reviews (1992)
    Springer
    Space science reviews 61 (1992), S. 421-433 
    Details
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00222314
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