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  • 1985-1989  (2)
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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 123 (1989), S. 161-176 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Numerical simulations of flaring coronal loops, based on the assumption of an electron beam energy input, show that material is ablated from the chromosphere at approximately the local sound speed, thus giving rise to an overall blueshift in the spectrum of the soft X-ray Ca xix resonance line of up to 5 mÅ. In this paper we critically examine the observational evidence for the existence of this blueshift. From an exhaustive analysis of seven limb flares and nine disk flares, we find no indication of Ca xix blueshifts in excess of about 1 mÅ. This conclusion may present serious problems for the electron beam hypothesis: possible reasons for the apparent discrepancy between theory and observations are discussed.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 109 (1987), S. 355-363 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The conditions required for the stability of a steady-state electron beam propagating in the solar corona are determined using the quasi-linear theory. The growth rate for electron plasma waves in a magnetized plasma is evaluated, with the electron distribution function being given by an analytic solution of the linearized Fokker-Planck equation. It is shown that, when the gyrofrequency is less than the plasma frequency, the instability has a narrow angular range, with the maximum growth rate occuring along the magnetic field. A stability boundary in parameter space is determined, indicating that electron beams must be highly collimated at injection to be Langmuir unstable at any point in space. The implications of the results for alternative models of hard X-ray bursts are discussed and it is argued that Langmuir instability will not occur on either the trap model or the thermal model. Such models would, therefore, be refuted by the detection of a large flux of plasma microwave radiation associated with hard X-ray emission.
    Type of Medium: Electronic Resource
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