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  • 11
    Electronic Resource
    Electronic Resource
    Inversion of synchrotron spectra (1983)
    Springer
    Astrophysics and space science 92 (1983), S. 105-112 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The problem of synchrotron radiation spectra is treated from the viewpoint of deconvolving the spectrum of ultrarelativistic source electrons from the observed photon spectrum. It is shown that for homogeneous sources the problem amounts to inversion of a Meijer transform with a modified Bessel finction kernel. A precise analytic inversion is only possible in the complex plane but Meijer transform tables are available for a wide range of functions. More convenient inversion formulae prove possible by use of a Laplace transform approximation or by analysing the spectra in terms of their integral moments. The filtering property of the transform is also established showing that the contribution to the synchrotron spectrum of high frequency components in the electron spectrum declines exponentially with their frequency. Thus, as with other Laplace-like transforms, only a few terms in an electron spectrum expansion can be deconvolved for any plausible noise level in the synchrotron spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00653590
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  • 12
    Electronic Resource
    Electronic Resource
    The structure and stability of coronal magnetic fields (1982)
    Springer
    Solar physics 113 (1982), S. 131-136 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The high degree of symmetry often assumed in studies of the structure and stability of coronal magnetic field configurations is restrictive and can yield misleading results. We have therefore developed fully three-dimensional numerical methods for constructing force-free equilibria and for examining their stability properties, which make no assumptions about symmetry. A test of the stability analysis has been performed by applying it to the Gold-Hoyle twisted flux tube, which is known to be kink-unstable if the helical field makes more than about one turn between the line-tying end-plates. Our preliminary result is that the critical number of turns is about 1.1, in good agreement with the previous best estimate. However, we find that the growth rate, which has not been discussed previously, is orders of magnitude smaller than expected, even when the flux tube is twisted far beyond the stability limit.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147691
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  • 13
    Electronic Resource
    Electronic Resource
    Two-component temperature analysis of OSO-5 X-ray flare data (1973)
    Springer
    Solar physics 28 (1973), S. 169-174 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using data covering the 2.6–10 Å wavelength range from the OSO-5 satellite a four-parameter model of the emitting region in a flare process is derived. The thermal emission spectrum of Landini and Fossi is used to calculate the plasma parameters, electron temperature and emission measure. The X-ray flare data is explained by a model which treats the source volume as two time-varying temperature regions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152921
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  • 14
    Electronic Resource
    Electronic Resource
    Mass motions in a heated flare filament (1976)
    Springer
    Solar physics 50 (1976), S. 133-151 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Energy transport in a hot flare plasma is examined with particular reference to the influence of fluid motion. On the basis of dimensional considerations the dynamical timescale of the flare plasma is shown to be comparable to the timescale for energy loss by conduction and radiation. It is argued that mass motion is likely to have a profound influence on the evolution of the flare. The detailed response of a flare filament to a localized injection of energy is then analyzed. Radiative, conductive and all dynamical terms are included in the energy equation. Apart from greatly enhancing the rate of propagation of the thermal disturbance through space, mass motion is found to be significant in transferring energy through the moving fluid. Finally the predicted thermal structure is discussed and it is concluded that the presence of mass motions in the flare may be inferred from the form of the soft X-ray differential emission measure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00206198
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  • 15
    Electronic Resource
    Electronic Resource
    The dynamic formation of quasi-static active region loops (1981)
    Springer
    Solar physics 70 (1981), S. 97-113 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A hydrodynamic model is used to analyse the formation of an active region coronal loop. After an impulse of energy that ‘lifts' part of a cool uniform plasma into the domain of radiative instability (T〉 105 K) the atmosphere is allowed to relax dynamically to a new quasi-steady equilibrium. Although the radiative-hydrodynamic coupling leads to quite complex physical phenomena, the final state of the plasma shows excellent agreement with previous quasi-static loop calculation. The bearing of this analysis on the evolution of flare plasmas is then discussed. In particular, it is shown that the energy flux emanating from an excited coronal source leads to the ‘dynamic evaporation’ of cool transition-zone and chromospheric material. The analysis indicates, however, that an accurate description of the transition zone in solar flares requires considerably more sophistication than has hitherto been employed. Finally, a discussion is given of the uniqueness and stability of the quasi-static loop. It is concluded that, despite thermal instability, a quasi-static model should adequately describe the gross features of quiescent coronal loops.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00154394
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  • 16
    Electronic Resource
    Electronic Resource
    A relaxation law for evaporating and draining coronal plasmas (1983)
    Springer
    Solar physics 84 (1983), S. 105-107 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Simple analytic expressions linking the pressure, density and fluid velocity of a relaxing coronal plasma are presented. These appear in good agreement with more extensive theoretical calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00157448
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  • 17
    Electronic Resource
    Electronic Resource
    The stability and uniqueness of coronal loops (1982)
    Springer
    Solar physics 76 (1982), S. 331-355 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A hydrodynamic model of high resolution is used to examine the stability of coronal loops to finite amplitude perturbations. The loop is heated by means of a low-amplitude energy input and its subsequent dynamic relaxation is followed. Firstly, the initial atmosphere is generated by solving the time independent form of the hydrodynamic equations. It is shown that the loop structure depends critically on the balance between the radiative losses and the quiescent heating at the base of the transition zone, i.e. on the concavity of the temperature profile in this region. This result already anticipates the need for high spatial resolution across the model transition zone. The dynamic evolution of the loop is then investigated for two classes of lower boundary conditions. In one case the chromospheric temperature is fixed throughout the simulation; in the other the low chromosphere is represented by a rigid insulating barrier. In both cases the loop is found to be stable: The loop is also unique to the extent that it relaxes to a state which is physically indistinguishable from its initial configuration. It is pointed out however, that a loop whose chromosphere is only marginally stable can evolve dynamically away from the initial static configuration. Finally, the observational consequences of the analysis are discussed. The differential emission measure profile is found to change its form as the loop cools, firstly, through an evaporative phase in which the coronal density increases; secondly, through a quasi-steady relaxation in which the enhanced coronal density gradually drains away to the chromosphere. This behaviour represents a possible observational test of the model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00170990
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  • 18
    Electronic Resource
    Electronic Resource
    An explanation for fast downflows on the Sun (1986)
    [s.l.] : Nature Publishing Group
    Nature 324 (1986), S. 128-129 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Observed flows exhibit several features that must be explained by any convincing theory. First, the flows are observed to persist much longer than the transit time of a gas element through the loop10'11. This fact alone suggests a quasi-steady model in which plasma flows continually from one ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/324128a0
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  • 19
    Electronic Resource
    Electronic Resource
    The kink instability in infinite cylindrical flux tubes: Eigenvalues for power-law twist profiles (1990)
    Springer
    Astrophysics and space science 166 (1990), S. 289-300 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We develop simple accurate methods of calculating ideal MHD instability eigenvalues λ for infinitely-long cylindrical tubes, with twist functionT(r)=B θ/rB z . A complete theoretical treatment is presented for force-free magnetic equilibria with arbitraryT(r), and detailed semi-analytic results for the kink instability are given for the particular case of a power-law twistT(r)=r ν, where the index ν is non-negative. Our results show that the most rapidly growing and energetic instabilities occur in the Gold-Hoyle ν=0 field, with the instability progressively weakening with increasing ν. However, the maximum force eigenvalue is always small, so that even in the Gold-Hoyle case (where λ=O(10−2) in dimensionless units) only a small proportion of the available magnetic energy can be released in the linear phase. Our results also confirm that the linear pinch (ν=∞) is remarkably weak (λ=O(10−3)) yet relatively resistant to line-tying! It is shown that the weakness of the force eigenvalue implies that the influence of uniform gas pressure on stability is negligible. Implications for the energy-release mechanism in solar flares are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01094900
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  • 20
    Electronic Resource
    Electronic Resource
    Linear MHD stability of infinitely long coronal flux tubes (1989)
    Springer
    Astrophysics and space science 151 (1989), S. 265-283 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Twisted magnetic flux tubes are often used to model the filed in coronal loops, and much attention has been given to analysing their stability. Previous astrophysical studies have concentrated on establishing the existence of an instability or determining stability bounds, and little information seems available on the associated eigenvalues, which give crucial information on the energy released. This paper develops methods of determining eigenvalues for infinitely long flux tubes. The most striking feature of the results is that the eigenvalues are always small-of order 10−2 (in dimensionless units) even for the fastest helical kink modes (m=1). The more localized higher-m modes have even smaller eigenvalues. A family of flux tubes with field line twist proportional tor ν is investigated, and it appears that the most energetic instabilities occur in the Gold-Hoyle tube with uniform twist (ν=0). Implications of these results are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00648384
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