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  • 1
    Electronic Resource
    Electronic Resource
    Chromospheric heating by electron and proton bombardment in the solar flare of June 7, 1980 (1983)
    Springer
    Solar physics 84 (1983), S. 263-269 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Using observations of both hard X-rays and γ-rays in the large solar flare on June 7, 1980, we infer the amount of chromospheric heating due to bombardment both by non-thermal electrons and by protons, respectively. If a thick-target model for the X-ray bremsstrahlung is adopted, then proton heating is shown to be important only in the lower chromosphere; however, if the hard X-rays are substantially thermal in origin, then proton heating may play an important or indeed dominant role in determining the structure of the entire flaring chromosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00157459
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  • 2
    Electronic Resource
    Electronic Resource
    The interpretation of hard X-ray polarization measurements in solar flares (1985)
    Springer
    Solar physics 96 (1985), S. 331-337 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We review recent observations of polarization of moderately hard X-rays in solar flares and compare them with the predictions of recent detailed modeling of hard X-ray bremsstrahlung production by non-thermal electrons. We find that the recent advances in the complexity of the modeling lead to substantially lower predicted polarizations than in earlier models and more fully highlight how various parameters play a role in determining the polarization of the radiation field. The new predicted polarizations are comparable to those predicted by thermal modeling of solar flare hard X-ray production, and both are in agreement with the observations. In the light of these results, we propose new polarization observations with current generation instruments which could be used to discriminate between non-thermal and thermal models of hard X-ray production in solar flares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00149688
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  • 3
    Electronic Resource
    Electronic Resource
    The structure of high-temperature solar flare plasma in non-thermal flare models (1985)
    Springer
    Solar physics 98 (1985), S. 281-291 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We solve the energy equation for the high-temperature (coronal) component of flare plasma for two models of energy input: (i) direct collisional heating by a beam of suprathermal electrons, and (ii) ohmic heating by the beam-neutralizing reverse current. We discuss the regimes where each case is applicable, and solve for the differential emission measure distribution of the coronal plasma in each case. ‘Scaling laws’ between loop temperatures and injected electron fluxes are derived for both models; these are testable observationally through coordinated soft X-ray and hard X-ray observations, thus providing a method of discriminating between the two cases. We also readdress the question of the energetic importance of a return current which is below the instability threshold for generation of ion-acoustic plasma turbulence. We find that unless the ambient coronal density is very low (≲ 109 cm −3), collisional heating will always dominate there, in agreement with the findings of previous authors. However, in the chromosphere/corona transition region, the relatively low temperature and correspondingly high plasma resistivity imply that reverse current ohmic heating can predominate the flare energetics, by up to an order of magnitude.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152461
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  • 4
    Electronic Resource
    Electronic Resource
    Can the Einstein-de Sitter model adequately describe the Universe at any epoch? (1978)
    Springer
    Astrophysics and space science 58 (1978), S. 181-188 
    Details
    ISSN: 1572-946X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The characteristics of Friedmann model universes containing decoupled matter and radiation are investigated to establish the range in redshift over which the frequently assumed Einstein-de Sitter approximation is valid. This approximation is found to have decidedly doubtful value for small, yet entirely plausible, values of the material density parameter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00645385
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  • 5
    Electronic Resource
    Electronic Resource
    Application of the trap-plus-precipitation hard X-ray burst model to the flare of August 4, 1972 (1979)
    Springer
    Solar physics 63 (1979), S. 175-185 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We apply the theory of Melrose and Brown (1976), concerning the X-ray spectrum resulting from the combined precipitation and within-trap acceleration of non-thermal electrons, to the large solar flare of August 4, 1972, using high time resolution hard X-ray data from the ESRO TD-1A scintillation counter for the calculations. By so doing, we infer the time behavior of the electron source function Q(E, t), as a function of energy E and of the mean ambient trap densityn (cm−3). We then interpret these results in the context of a variety of models: (i) a purely thick-target model; (ii) a static trap with continuous injection of fresh electrons; and (iii) a dynamic trap, with continuous redistribution of-electron energies due to, for example, modulations in the trapping magnetic field. It is concluded that there exists a minimum trap density n crit below which the static trap description cannot be valid, but that n 〈 n crit is possible if a dynamic trap is invoked. Further, in such low density dynamic traps, the electron source function at a given instant may bear little or no resemblance to a power-law spectrum despite its production, via non-thermal bremsstrahlung, of a power-law X-ray spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00155708
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  • 6
    Electronic Resource
    Electronic Resource
    The heating of the temperature minimum region in solar flares — A reassessment (1979)
    Springer
    Solar physics 64 (1979), S. 129-134 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract As a sequel to the work by Machado et al. (1978), we discuss and evaluate the suggestions made by these authors on how to possibly reconcile the observed temperature enhancements at temperature-minimum levels in solar flares with some form of theoretical heating mechanism. After establishing the H− LTE assumption used by Machado et al., we then consider EUV irradiation, and joule heating by steady currents, as heating mechanisms. We find that, unless there are strong inhomogeneities associated with either mechanism, neither can reasonably be reconciled with observations. It is concluded that detailed, high resolution (both spatial and temporal) measurements are necessary to further our understanding of the flare process at temperature-minimum levels.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00151121
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  • 7
    Electronic Resource
    Electronic Resource
    Electron-heated flare models - alive and well (1982)
    Springer
    Solar physics 113 (1982), S. 175-181 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The idea that deka-keV electrons, accelerated during the primary energy release, are responsible for the bulk of the impulsive phase energy transport in solar flare atmospheres has been around for quite some time now. With the advent of (i) detailed numerical calculations of the hydrodynamic response of the atmosphere to the energy input from such electrons, and (ii) the observations made during the last solar maximum of the spatial, spectral, and temporal behavior of flare-associated emissions in a variety of wavelength ranges, we have now reached a point where quantitative tests of the thick-target electron-heated model (Brown, 1973) are possible. Here I describe some recent results of such quantitative tests.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00147695
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  • 8
    Electronic Resource
    Electronic Resource
    Prospects for solar flare X-ray polarimetry (1988)
    Springer
    Solar physics 118 (1988), S. 309-319 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The history of solar flare X-ray polarimetry is reviewed and it is shown that as yet, there is no experimental evidence for such polarization. The present experimental limits are at the level of a few percent but these results may be biased by a large thermal component at low energies which may decrease the apparent polarization. To avoid this difficulty it will be necessary to make observations at higher energies where thermal emission is less important. The theoretical estimates of the polarization expected in the solar flare are also reviewed. The best present theoretical estimates are in the range of a few percent and are consistent with the present experimental limits. In this paper we discuss a new satellite instrument that has sufficient sensitivity at high energies to detect the polarization that is predicted by the present theories. The instrument sensitivity for a moderate (M class) event approaches polarization levels of 1% in each of 7 energy bins spanning the 10 to 100 keV range for integration times as short as 10 s. Comparable results can be obtained for an X class flare in 1 s.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00148598
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  • 9
    Electronic Resource
    Electronic Resource
    Temperature minimum heating in solar flares by resistive dissipation of Alfvén waves (1982)
    Springer
    Solar physics 80 (1982), S. 99-112 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We examine the possibility that the strong heating produced at temperature-minimum levels during solar flares is due to resistive dissipation of Alfvén waves generated by the primary energy release process in the corona. It is shown how, for suitable parameters, these waves can carry their energy essentially undamped into the temperature-minimum layers and can then produce a degree of heating consistent with observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00153426
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  • 10
    Electronic Resource
    Electronic Resource
    The effects of viewing angle on the inference of magnetic shear in preflare active regions (1989)
    Springer
    Solar physics 119 (1989), S. 77-86 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The magnetic shear at a point within an active region field configuration can be defined (Hagyard et al., 1984b) as the difference in angle between the observed photospheric transverse field and that of a reference potential field calculated using the observed line-of-sight field as a boundary condition. Using analytic models for non-potential (but force-free) fields representative of preflaring active regions, we calculate the degree of magnetic shear along the magnetic neutral line that such fields would exhibit, as a function of the location and orientation of the active region on the solar disk. We find that, except for regions close to disk center, the position of the inferred neutral line (zero line-of-sight field) is significantly different from the actual neutral line (zero radial field), and that the calculated reference potential field also varies significantly with the position of the region. Thus the inferred degree of shear can vary significantly with the position and orientation of the region, due to (a) straightforward geometric projection effects, (b) the shift of the inferred neutral line relative to its true position, and (c) variations in the reference potential field. The significance of these results for flare prediction is considered.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146214
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