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  • 1
    Electronic Resource
    Electronic Resource
    Explorer 34 magnetic field measurements near the tail current sheet and auroral activity (1981)
    Springer
    Astrophysics and space science 77 (1981), S. 409-442 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Explorer 34 (Imp 4) 2.56 s magnetic data during 131 traversals of the tail current sheet are presented along with simultaneous 2.5 min auroral electrojet indices AE and AL. The normal magnetic field,B ⊥, satellite crossing times and positions are tabulated for these 131 crossings.B ⊥ is defined in the center of the sheet: it is the vector magnetic field at the time of field minimum during the crossing (B x component changes sign). It is remarkable that the only normal components too large in magnitude to be classified as ‘fine structure’ occur near the time of onset of an ‘AE event’. Cases are discussed where the normal component, defined near the plasma sheet edges, has the opposite sign compared to the normal component defined at the sheet center. For quiet times, the current sheet may be only about 1000 km thick within a 3R e (Earth-radii) plasma sheet, and may carry some 10–15% of the total tail current.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00649469
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  • 2
    Electronic Resource
    Electronic Resource
    Shocks produced by impulsively driven reconnection (1988)
    Springer
    Solar physics 117 (1988), S. 97-121 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Shock waves produced by impulsively driven reconnection may be important during flares or during the emergence of magnetic flux from the photosphere into the corona. Here we investigate such shock waves by carrying out numerical experiments using two-dimensional magneto-hydrodynamics. The results of the numerical experiments imply that there are three different categories of shocks associated with impulsively driven reconnection: (1) fast-mode, blast waves which rapidly propagate away from the reconnection site; (2) slow-mode, Petschek shocks which are attached to the reconnection site; and (3) fast-mode, termination shocks which terminate the plasma jets flowing out from the reconnection site. Fast-mode blast waves are a common feature of many flare models, but the Petschek shocks and jet termination shocks are specific to reconnection models. These two different types of reconnection shocks might contribute to chromospheric ablation and energetic particle acceleration in flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148576
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  • 3
    Electronic Resource
    Electronic Resource
    A strong limitation on the rapidity of flux-pile-up reconnection (1996)
    Springer
    Solar physics 167 (1996), S. 445-448 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The reconnection rate which can be achieved in the steady-state flux-pile-up regime is severely limited by the gas pressure of the plasma. Using the family of solutions obtained previously by Priest and Forbes, we show that the Alfvén Mach number of the plasma flowing towards the reconnection site cannot exceed πβ e/(8 ln R me), where β e and R me are the plasma β and magnetic Reynolds numbers at large distance. This limit corresponds to a very weak flux-pile-up, and it is a factor of β e slower than the maximum Petschek rate. Thus the maximum flux-pile-up reconnection rate in the corona is at least two orders of magnitude smaller than the rate implied by flare observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146353
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  • 4
    Electronic Resource
    Electronic Resource
    Numerical study of line-tied magnetic reconnection (1982)
    Springer
    Solar physics 81 (1982), S. 303-324 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A two-dimensional configuration, analogous to that at the start of the main phase in two-ribbon flares, is modelled numerically by self-consistently solving the time-dependent MHD equations. The initial state consists of a vertical current sheet with an external plasma beta value of 0.1 and a magnetic Reynolds number of 10−3. Although the model does not yet include gravity or a full energy equation, many of the principal dynamical features of the main phase in a flare are present. In particular, the numerical results confirm the earlier prediction of the kinematic Kopp-Pneuman (1976) model that a neutral line forms at the base of the corona and rises upwards as open, extended field lines close back down to form loops (i.e., ‘post’-flare loops). By the end of the computation a state of nonlinear reconnection containing slow shocks has developed, and the velocity of the plasma flowing into the neutral line region is approximately 0.06 times the corresponding inflow Alfvén velocity - a value consistent with the steady-state nonlinear reconnection theory of Soward and Priest (1977). The speed at which the neutral line rises in the numerical simulation varies from an initial value of ≲ 0.02 to a final value of ∼- 0.12 times the inflow Alfvén speed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151304
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  • 5
    Electronic Resource
    Electronic Resource
    A numerical experiment relevant to line-tied reconnection in two-ribbon flares (1983)
    Springer
    Solar physics 84 (1983), S. 169-188 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The nonlinear evolution of a reconnecting magnetic field configuration similar to that occurring just before the onset of ‘post’-flare loops in two-ribbon flares is determined. The evolution, which is obtained by numerically solving the resistive MHD equations, shows two new features that have not yet been incorporated into contemporary models of ‘post’-flare loops. The first of these new features is the formation of a nearly stationary fast-mode shock above the region corresponding to the top of the loops. This fast-mode shock occurs just below the magnetic neutral line and between the slow-mode shocks associated with fast magnetic reconnection at the neutral line. The second new feature is the creation and annihilation of large-scale magnetic islands in the current sheet above the loops. The annihilation of the islands occurs very rapidly and appears to be a manifestation of the coalescence instability. The creation and annihilation of magnetic islands could be important in understanding the energetics of ‘post’-flare loops since the coalescence instability can produce an intermittent energy release more than an order of magnitude faster than that predicted by steady-state reconnection theories.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00157455
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  • 6
    Electronic Resource
    Electronic Resource
    Mass upflows in ‘post’-flare loops (1983)
    Springer
    Solar physics 88 (1983), S. 211-218 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A self-consistent numerical model of a reconnecting magnetic field configuration similar to that occurring during the main-phase of two-ribbon flares is used to estimate the upflow caused by the fast-mode expansion of the magnetic field moving into the reconnection region. Such an expansion creates a field-aligned pressure gradient which accelerates plasma upward from the chromospheric base of magnetic field lines in the region external to the loops. The numerical results imply that the amount of mass sucked up in this way is even smaller than was previously estimated by Kopp and Pneuman who used a kinematic model. Therefore, some indirect mechanism (such as evaporation), which would probably derive its motive power from the thermal energy generated by the reconnection, is required to explain the large mass upflows inferred from observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00196188
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  • 7
    Electronic Resource
    Electronic Resource
    On the maximum energy release in flux-rope models of Eruptive Flares (1994)
    Springer
    Solar physics 150 (1994), S. 245-266 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We determine the photospheric boundary conditions which maximize the magnetic energy released by a loss of ideal-MHD equilibrium in two-dimensional flux-rope models. In these models a loss of equilibrium causes a transition of the flux rope to a lower magnetic energy state at a higher altitude. During the transition a vertical current sheet forms below the flux rope, and reconnection in this current sheet releases additional energy. Here we compute how much energy is released by the loss of equilibrium relative to the total energy release. When the flux-rope radius is small compared to its height, it is possible to obtain general solutions of the Grad-Shafranov equation for a wide range of boundary conditions. Variational principles can then be used to find the particular boundary condition which maximizes the magnetic energy released for a given class of conditions. We apply this procedure to a class of models known as cusp-type catastrophes, and we find that the maximum energy released by the loss of equilibrium is 20.8% of the total energy release for any model in this class. If the additional restriction is imposed that the photospheric magnetic field forms a simple arcade in the absence of coronal currents, then the maximum energy release reduces to 8.6%.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712888
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  • 8
    Electronic Resource
    Electronic Resource
    Models for the motions of flare loops and ribbons (1995)
    Springer
    Solar physics 159 (1995), S. 275-299 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have found a conformal mapping which is valid for any magnetic boundary condition at the photosphere and which can be used to determine the evolution of an open, two-dimensional magnetic field configuration as it relaxes to a closed one. Solutions obtained with this mapping are in quasi-static equilibrium, and they contain a vertical current sheet and have line-tied boundary conditions. As a specific example, we determine the solution for a boundary condition corresponding to a submerged, two-dimensional dipole below the photosphere. We assume that the outer edges of the hottest X-ray loops correspond to field lines mapping from the outer edges of the Hα ribbon to the lower tip of the current sheet where field lines reconnect at aY-type neutral line which rises with time. The cooler Hα loops are assumed to lie along the field lines mapping to the inner edges of the flare ribbons. With this correspondence between the plasma structures and the magnetic field we determine the shrinkage that field lines are observed to undergo as they are disconnected from the neutral line. During the early phase of the flare, we predict that shrinkage inferred from the height of the Hα and X-ray loops is close to 100% of the loop height. However, the shrinkage should rapidly decrease with time to values on the order of 20% by the late phase. We also predict that the shrinkage in very large loops obeys a universal scaling law which is independent of the boundary condition, provided that the field becomes self-similar (i.e., all field lines have the same shape) at large distances. Specifically, for any self-similar field containing aY-type neutral line, the observed shrinkage at large distances should decrease as (ΔX/X R)−2/3, where ΔX is the ribbon width andX Ris the ribbon separation. Finally, we discuss the relation between the electric field at the neutral line and the motions of the flare loops and ribbons.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00686534
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  • 9
    Electronic Resource
    Electronic Resource
    Material ejection (1994)
    Springer
    Solar physics 153 (1994), S. 73-89 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract This paper reviews the major discussions and conclusions of the Flares 22 Workshop concerning the physical processes involved in mass ejecta events, with an emphasis on large-scale phenomena, especially Coronal Mass Ejections (CMEs). New insights have been gained from recent data obtained from the SMM andYohkoh spacecraft and from several new ground-based radio and optical instruments, as well as from theoretical advances concerning the origins, driving mechanisms and long-term evolution of CMEs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00712493
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  • 10
    Electronic Resource
    Electronic Resource
    Steady magnetic reconnection in three dimensions (1989)
    Springer
    Solar physics 119 (1989), S. 211-214 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The concepts of magnetic reconnection that have been developed in two dimensions need to be generalised to three-dimensional configurations. Reconnection may be defined to occur when there is an electric field (E∥) parallel to field lines (known as potential singular lines) which are potential reconnection locations and near which the field has an X-type topology in a plane normal to that field line. In general there is a continuum of neighbouring potential singular lines, and which one supports reconnection depends on the imposed flow or electric field. For steady reconnection the nearby flow and electric field are severely constrained in the ideal region by the condition that E∥ = 0 there. Potential singular lines may occur in twisted prominence fields or in the complex magnetic configuration above sources of mixed polarity of an active region or a supergranulation cell. When reconnection occurs there is dynamic MHD behaviour with current concentration and strong plasma jetting along the singular line and the singular surfaces which map onto them.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146222
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