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1

Electronic Resource

A topological approach to understand a multiple-loop solar flare (1995)

Bagalá, L. G. ; Mandrini, C. H. ; Rovira, M. G. ; [et al.]

Springer

Solar physics 161 (1995), S. 103-121

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ISSN: 1573-093X

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract We analyze the UV and X-ray data obtained by the SMM satellite for the flare starting at 02:36 UT on November 12, 1980 in AR 2779. From a detailed revision of the Ov emission, we find that the observations are compatible with energy being released in a zone above the magnetic inversion line of the AR intermediate bipole. This energy is then transported mainly by conduction towards the two distant kernels located in the AR main bipole. One of these kernels is first identified in this paper. Accelerated particles contribute to the energy transport only during the impulsive phase. We model the observed longitudinal magnetic field by means of a discrete number of subphotospheric magnetic poles, and derive the magnetic field overall topology. As in previous studies of chromospheric flares, the Ov kernels are located along the intersection of the computed separatrices with the photosphere. Especially where the field-line linkage changes ‘discontinuously’, these kernels can be linked in pairs by lines that extend along separatrices. Our results agree with the hypothesis of magnetic energy released by magnetic reconnection occurring on separatrices.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00732087

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2

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Interpretation of multiwavelength observations of November 5, 1980 solar flares by the magnetic topology of AR 2766 (1994)

Démoulin, P. ; Mandrini, C. H. ; Rovira, M. G. ; [et al.]

Springer

Solar physics 150 (1994), S. 221-243

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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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3

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3D magnetic reconnection at an X-ray bright point (1996)

Mandrini, C. H. ; Démoulin, P. ; Driel-Gesztelyi, L. ; [et al.]

Springer

Solar physics 168 (1996), S. 115-133

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ISSN: 1573-093X

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00145829

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4

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EVIDENCE OF MAGNETIC RECONNECTION FROM Hα, SOFT X-RAY AND PHOTOSPHERIC MAGNETIC FIELD OBSERVATIONS (1997)

Mandrini, C. H. ; DÉmoulin, P. ; BagalÁ, L. G. ; [et al.]

Springer

Solar physics 174 (1997), S. 229-240

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ISSN: 1573-093X

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract 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. Hα 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1004950009970

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5

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CAN WE EXTRAPOLATE A MAGNETIC FIELD WHEN ITS TOPOLOGY IS COMPLEX? (1997)

DÉmoulin, P. ; HÉnoux, J. C. ; Mandrini, C. H. ; [et al.]

Springer

Solar physics 174 (1997), S. 73-89

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ISSN: 1573-093X

Source: Springer Online Journal Archives 1860-2000

Topics: Physics

Notes: Abstract In order to understand various solar phenomena controlled by the magnetic field, such as X-ray bright points, flares and prominence eruptions, the structure of the coronal magnetic field must be known. This requires a precise extrapolation of the photospheric magnetic field. Presently, only potential or linear force-free field approximations can be used easily. A more realistic modelling of the field is still an active research area because of well-known difficulties related to the nonlinear mixed elliptic-hyperbolic nature of the equations. An additional difficulty arises due to the complexity of the magnetic field structure which is caused by a discrete partition of the photospheric magnetic field. This complexity is not limited to magnetic regions having magnetic nulls (and so separatrices) but also occurs in those containing thin elongated volumes (called Quasi-Separatrix Layers) where the photospheric field-line linkage changes rapidly. There is a wide range for the thickness of such layers, which is determined by the character (bipolar or quadrupolar) of the magnetic region, by the sizes of the photospheric field concentrations and by the intensity of the electric currents. The aim of this paper is to analyse the recent nonlinear force-free field extrapolation techniques for complex coronal magnetic fields.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1023/A:1004926513260

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6

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Observational support of reconnection in solar flares (1994)

Démoulin, P. ; Hénoux, J. C. ; Schmieder, B. ; [et al.]

Springer

Space science reviews 68 (1994), S. 129-130

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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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7

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Development of a topological model for solar flares (1992)

Démoulin, P. ; Hénoux, J. C. ; Mandrini, C. H.

Springer

Solar physics 139 (1992), S. 105-123

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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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