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  • 11
    Electronic Resource
    Electronic Resource
    The Importance of Photospheric Intense Flux Tubes for Coronal Heating (1997)
    Springer
    Solar physics 175 (1997), S. 123-155 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract 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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1004986503291
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  • 12
    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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  • 13
    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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  • 14
    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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  • 15
    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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  • 16
    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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  • 17
    Electronic Resource
    Electronic Resource
    A topological approach to understand a multiple-loop solar flare (1995)
    Springer
    Solar physics 161 (1995), S. 103-121 
    Details
    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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  • 18
    Electronic Resource
    Electronic Resource
    Magnetic Activity Associated With Radio Noise Storms (2000)
    Springer
    Solar physics 193 (2000), S. 227-245 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract As it crossed the solar disk in May and June 1998, AR 8227 was tracked by TRACE, Yohkoh, SOHO, and many ground-based observatories. We have studied how the evolution of the magnetic field resulted in changes in activity in the corona. In particular, we examine how the evolving field may have led to the acceleration of electrons which emit noise storms observed by the Nançay Radio Heliograph between 30 May and 1 June 1998, in the absence of any flare. The magnetic changes were related to moving magnetic features (MMFs) in the vicinity of the leading spot and are related to the decay of this spot. Within the limits of the instrumental capabilities, the location in time and space of the radio emissions followed the changes observed in the photospheric magnetograms. We have extrapolated the photospheric magnetic field with a linear force-free approximation and find that the active region magnetic field was very close to being potential. These computations show a complex magnetic topology associated to the MMFs. The observed photospheric evolution is expected to drive magnetic reconnection in such complex magnetic topology. We therefore propose that the MMFs are at the origin of the observed metric noise-storms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005218007132
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  • 19
    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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  • 20
    Electronic Resource
    Electronic Resource
    3D magnetic reconnection at an X-ray bright point (1996)
    Springer
    Solar physics 168 (1996), S. 115-133 
    Details
    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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