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  • 1
    Electronic Resource
    Electronic Resource
    Fundamental and harmonic radiation in type III solar radio bursts (1994)
    Springer
    Solar physics 154 (1994), S. 335-360 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Type III solar radio bursts are investigated by modelling the propagation of the electron beam and the generation and subsequent propagation of waves to the observer. Predictions from this model are compared in detail with particle, Langmuir wave, and radio data from the ISEE-3 spacecraft and with other observations to clarify the roles of fundamental and harmonic emission in type III radio bursts. Langmuir waves are seen only after the arrival of the beam, in accord with the standard theory. These waves persist after a positive beam slope is last resolved, implying that sporadic positive slopes persist for some time, unresolved but in accord with the predictions of stochastic growth theory. Local electromagnetic emission sets in only after Langmuir waves are seen, in accord with the standard theory, which relies on nonlinear processes involving Langmuir waves. In the events investigated here, fundamental radiation appears to dominate early in the event, followed and/or accompanied by harmonic radiation after the peak, with a long-lived tail of multiply scattered fundamental or harmonic emission extending long afterwards. These results are largely independent of, but generally consistent with, the conclusions of earlier works.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00681103
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  • 2
    Electronic Resource
    Electronic Resource
    Propagation and absorption of cyclotron maser radiation in solar microwave spike bursts (1993)
    Springer
    Solar physics 145 (1993), S. 317-338 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract It has been argued that the loss-cone-driven electron cyclotron maser instability can account for the properties of millisecond microwave spike bursts observed during some solar flares. However, as it propagates outward from the corona, maser radiation undergoes gyroresonance absorption when its frequency is a harmonic of the local electron-cyclotron frequency. Existing analytical models using slab geometries predict that this absorption should be sufficiently strong to prevent the radiation from being seen at the observed levels, except under highly restrictive conditions or for unrealistic plasma parameters. A more comprehensive analysis is presented here to determine if and when maser radiation can escape to produce microwave spike bursts. This analysis employs numerical raytracing and incorporates propagation and absorption of fundamental maser emission in a realistic model of a coronal flux loop. It is found that ranges of physical conditions do exist under which maser radiation can escape to an observer and that these conditions are much more limiting for fundamental emission in the extraordinary (χ)-mode than in the ordinary (o)-mode. Detailed investigation implies that escaping radiation in the χ-mode is highly directional and chiefly observable toward the center of the solar disk, while escapingo-mode radiation is found to emerge from the corona over a much wider range of directions, with some cases corresponding to radiation observable near the solar limb.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00690659
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  • 3
    Electronic Resource
    Electronic Resource
    Electron-cyclotron maser emission in solar microwave spike bursts (1991)
    Springer
    Solar physics 134 (1991), S. 299-314 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A new model is developed for electron-cyclotron maser emission from flaring loops, which incorporates competition between driving of the instability and maser-induced relaxation, together with interactions between small neighboring regions of unstable plasma. This results in a picture in which radiation is emitted in bursts from regions whose length scale is determined self-consistently by previous bursts, while the unstable plasma fluctuates about the point, close to marginal stability, at which driving of the instability is balanced by relaxation due to maser-induced electron diffusion. Under the conditions applicable to flaring loops, time scales of fundamental x-mode (x1) driving and saturation are approximately equal at ∼ 1 ms, resolving a (104–106)-fold discrepancy in previous models and agreeing with the observed time scales of microwave spike bursts. Saturation effects are found to be especially effective in suppressing amplification of the most strongly growing modes. This suppression enables fundamental o-mode (o1) and second-harmonic x-mode (x2) emission to compete more effectively against x1 emission for the available free energy than has previously been estimated. Consideration of mode competition, burst time scales, suppression of growth due to overlap between amplification and absorption bands, and escape of radiation through absorption layers to the observer, implies that the observed radiation probably escapes from the corona principally in the o-mode, either emitted directly as o1 radiation or mode converted from x1 emission.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152650
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of turbulence on the electron cyclotron-maser mechanism for solar microwave spike bursts (1991)
    Springer
    Solar physics 136 (1991), S. 343-360 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract It is shown that small magnetic perturbations can significantly alter the rates of cyclotron growth, absorption, mode conversion, and refraction because of the sensitive dependence of these processes on the field strength in narrow layers. In particular, growth lengths are increased, absorption depths decreased, mode conversion becomes more effective, and turbulent refraction leads to isotropization of the emission. The criteria for significant effects to occur are derived and it is shown that they can be met by the few-percent field perturbations observed in coronal loops. Relative to the theory of cyclotron-maser emission in smoothly varying plasmas, perturbations enable fundamental o-mode (o1) and second-harmonic x-mode (x2) radiation to saturate more effectively, increase the chance of x1, o1, and x2 radiation escaping to infinity through absorption and mode-coupling windows, and partially isotropize radiation emitted near the x-mode cutoff. It is concluded that o1 and x1 emission are both likely to be present, and that x2 emission is possible under some circumstances. However, x1 radiation can escape only at near-parallel propagation (θ ≈ 0) or via mode conversion to the o-mode at θ ≈ 90°, whereas o1 and x2 emission can escape for a wide range of angles around θ = 0 and, under many circumstances, near θ = 90°.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146541
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  • 5
    Electronic Resource
    Electronic Resource
    Radial variation of type-III source parameters (1992)
    Springer
    Solar physics 137 (1992), S. 307-315 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The parameters of type-III sources have been observed to vary as powers of the distance of the source from the Sun. Here, the values of the observed exponents are reviewed and theoretical relationships between them are discussed and extended. It is shown that 11 observed exponents can be derived from a four-element subset. A least-squares fit is carried out by varying these four exponents and it is shown that the results are consistent with observaton to within the observational uncertainties. Best-fit expressions are given for the plasma density and temperature, the solar wind speed, beam velocity and density, frequency drift rate, peak Langmuir fields, brightness temperature, volume emissivity, beam duration and burst decay time.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00161852
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  • 6
    Electronic Resource
    Electronic Resource
    Stochastic-growth theory of Langmuir growth-rate fluctuations in type III solar radio sources (1993)
    Springer
    Solar physics 146 (1993), S. 357-363 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Fluctuations in type III beams are produced by quasilinear interactions with clumpy Langmuir waves in type III radio sources. The properties of these fluctuations are estimated and shown to yield Langmuir growth rates and growth-rate fluctuations consistent with those required by the recent stochastic-growth theory of type III radio bursts, with observations, and with existing theoretical constraints. This strengthens the basis of stochastic-growth theory and provides an essential consistency test for this model.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00662019
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  • 7
    Electronic Resource
    Electronic Resource
    Clumpy Langmuir waves in type III radio sources (1992)
    Springer
    Solar physics 139 (1992), S. 147-163 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A model is developed for the clumpy Langmuir waves observed in type III source regions. In this model the waves are generated by instability of a beam which propagates outward from the Sun in a state close to marginal stability. Ambient density perturbations cause fluctuations about the marginally stable state, leading to nonuniformities in both beam and waves and, hence, to spatially inhomogeneous growth. High damping rates and high wave levels are strongly anti-correlated, leading to suppression of the net damping. Below saturation stochastic growth causes the waves to follow a random walk in the logarithm of their energy density and the resulting probability of observing a field of magnitude E is approximately proportional to E -1. Comparison with observations shows that this model can account for the levels and clumpiness of the Langmuir waves, the small net dissipation required for the beams to propagate to 1 AU, the characteristic decay time of type III electromagnetic emission, and the negative mean growth rate observed in situ in type III sources. At 1 AU only the very highest fields approach the threshold for nonlinear wave collapse, but this threshold may be more commonly exceeded closer to the Sun.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147886
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  • 8
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    Effects of turbulence on the electron cyclotron-maser mechanism for solar microwave spike bursts (1991)
    Springer
    Details
    Publication Date: 1991-12-01
    Print ISSN: 0038-0938
    Electronic ISSN: 1573-093X
    Topics: Physics
    Published by Springer
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  • 9
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    Fundamental and harmonic radiation in type III solar radio bursts (1994)
    Springer
    Details
    Publication Date: 1994-10-01
    Print ISSN: 0038-0938
    Electronic ISSN: 1573-093X
    Topics: Physics
    Published by Springer
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  • 10
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    Clumpy Langmuir waves in type III radio sources (1992)
    Springer
    Details
    Publication Date: 1992-05-01
    Print ISSN: 0038-0938
    Electronic ISSN: 1573-093X
    Topics: Physics
    Published by Springer
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