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11.
Using a simplified form of the bremsstrahlung cross-section, we obtain an analytic expression for the intensity of electron-beam-produced hard X-ray emission with depth in solar flares. The results show that footpoint emission is more likely than previously thought, and we discuss these results in the light of recent observations.Presidential Young Investigator.NAS/NRC Research Associate, on leave from CNIE, San Miguel, Argentina. 相似文献
12.
J. H. Emslie 《Boundary-Layer Meteorology》1979,16(3):409-419
Engineering air pollution dispersion studies sometimes require a knowledge of the frequency of occurrence of ground-based inversions. Hostler (1961) and Munn et al. (1970) have prepared maps of frequencies of ground-based inversions for the United States and Canada, respectively, using radiosonde data. The method presented here employs long-term climatological maximum and minimum temperature data recorded at 36 pairs of valley/ridge climatological stations in British Columbia with at least several years of coincident data. Daily maximum and daily minimum temperatures at each pair of stations were compared. Monthly totals of positive temperature differences, i.e., inversion occurrences, in these comparisons were totalled, and converted to seasonal percentages, over the period of record. The resulting seasonal sets of inversion percentages at the times of maximum and minimum temperature are tabled. The effects of station separation, both horizontally and vertically are discussed, and the influence of adjacent water bodies, acting as heat sources in winter and heat sinks in summer are illustrated. Comparisons with the results obtained by other investigators are made. 相似文献
13.
Eduard?P.?KontarEmail author Michele?Piana Anna?Maria?Massone A.?Gordon?Emslie John?C.?Brown 《Solar physics》2004,225(2):293-309
Hard X-ray spectra in solar flares provide knowledge of the electron spectrum that results from acceleration and propagation in the solar atmosphere. However, the inference of the electron spectra from solar X-ray spectra is an ill-posed inverse problem. Here, we develop and apply an enhanced regularization algorithm for this process making use of physical constraints on the form of the electron spectrum. The algorithm incorporates various features not heretofore employed in the solar flare context: Generalized Singular Value Decomposition (GSVD) to deal with different orders of constraints; rectangular form of the cross-section matrix to extend the solution energy range; regularization with various forms of the smoothing operator; and preconditioning of the problem. We show by simulations that this technique yields electron spectra with considerably more information and higher quality than previous algorithms. 相似文献
14.
Brown John C. McArthur Guillian K. Barrett Richard K. McIntosh Scott W. Emslie A. Gordon 《Solar physics》1998,179(2):379-404
The effects of non-uniform plasma target ionisation on the spectrum of thick-target HXR bremsstrahlung from a non-thermal electron beam are analysed. In particular the effect of the target ionisation structure on beam collisional energy losses, and hence on inversion of an observed photon spectrum to yield the electron injection spectrum, is considered and results compared with those obtained under the usual assumption of a fully ionised target.The problem is formulated and solved in principle for a general target ionisation structure, then discussed in detail for the case of a step function distribution of ionisation with column depth as an approximation to the sharp coronal–chromospheric step structure in solar flare plasmas. It is found that such ionisation structure has very dramatic effects on derivation of the thick-target electron injection spectrum F0(E0) as compared with the result F*0(E
0) obtained under the usual assumption of a fully ionised target: (a) Inferred F*0 contain more electrons than F
0 and in some cases include electrons at energies where none are actually present. Although the total (energy-integrated) beam fluxes in the two cases do not differ by a factor of more than Aee/AeH, the spectral shapes can differ greatly over finite energy intervals resulting in the danger of misleading results for total fluxes obtained by extrapolation. (b) The unconstrained mathematical solution for F0 for any photon spectrum is never unique, while that for F*0 is unique. When the physical constraint F0 0 is added, for some photon spectra solutions for F0 may not exist or may not be unique. (This is not an effect of noise but of real analytic ambiguity.) (c) For data corresponding to F*0 with a low-energy cut-off, or a cut-off or rapid enough exponential decline at high energies, a unique solution F0 does exist and we obtain a recursive summation for its evaluation.Consequently, in future work on the inversion of HXR bremsstrahlung spectra it will be vital for algorithms to include the effects of target ionisation if spurious results on thick-target electron spectra are not to be inferred. Finally it is pointed out that the depth of the transition zone, and its evaporative evolution during flares may be derivable from its effect on the HXR spectrum. 相似文献
15.
A. Gordon Emslie 《Solar physics》1989,121(1-2):105-115
We review the somewhat questionable concept of an isolated flare loop and the various physical mechanisms believed to be responsible, to some degree, for energy transport within the loop structure. Observational evidence suggests a predominant role for high-energy electrons as an energy transport mechanism, and we explore the consequences of such a scenario in some detail, focusing on radiation signatures in the soft X-ray, hard X-ray, and EUV wavebands, as observed by recent satellite observatories. We find that the predictions of flare loop models are in fact in excellent agreement with these observations, reinforcing both the notion of the loop as a fundamental component of solar flares and the belief that electron acceleration is an integral part of the flare energy release process. 相似文献
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17.
McConnell Mark L. Ryan James M. Smith David M. Lin Robert P. Emslie A. Gordon 《Solar physics》2002,210(1-2):125-142
Although designed primarily as a hard X-ray imager and spectrometer, the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
is also capable of measuring the polarization of hard X-rays (20–100 keV) from solar flares. This capability arises from the
inclusion of a small unobstructed Be scattering element that is strategically located within the cryostat that houses the
array of nine germanium detectors. The Ge detectors are segmented, with both a front and rear active volume. Low-energy photons
(below about 100 keV) can reach a rear segment of a Ge detector only indirectly, by scattering. Low-energy photons from the
Sun have a direct path to the Be and have a high probability of Compton scattering into a rear segment of a Ge detector. The
azimuthal distribution of these scattered photons carries with it a signature of the linear polarization of the incident flux.
Sensitivity estimates, based on Monte Carlo simulations and in-flight background measurements, indicate that a 20–100 keV
polarization sensitivity of less than a few percent can be achieved for X-class flares. 相似文献
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19.
We examine empirical atmospheric structures that are consistent with enhanced white-light continuum emission in solar flares. This continuum can be produced either by hydrogen bound-free emission in an enhanced region in the upper chromosphere, or by H- emission in an enhanced region around the temperature minimum. In the former case, weak Paschen jumps in the spectrum will be present, with the spectrum being dominated by a strong Balmer continuum, while in the latter case the spectrum exhibits a weaker, flat enhancement over the entire visible spectrum.We find that when proper account is taken of radiative backwarming processes, the two enhanced atmospheric regions above are not independent, in that irradiation by Balmer continuum photons from the upper chromosphere creates sufficient heating around the temperature minimum to account for the temperature enhancements there. Thus the problem of main phase white-light flare production reduces to one of creating temperature enhancements of order 104 K in the upper chromosphere; radiative backwarming then naturally accounts for the enhancements of order 100 K around the temperature minimum.Heating by electron and proton bombardment, and by XUV irradiation from above, are then considered as candidates for creating the necessary enhancements in the upper chromosphere. We find that electron bombardment can be ruled out, whereas bombardment by protons in the few-MeV energy range is a viable candidate, but one without strong observational support. The XUV irradiation hypothesis is examined by incorporating it self-consistently into the PANDORA radiative transfer algorithm used to construct the empirical model atmospheres; we find that the introduction of XUV radiation, with flux and spectrum appropriate to white-light flare events, does indeed produce sufficient radiative heating in the upper chromosphere to balance the radiative losses associated with the required temperature enhancements.In summary, we find that the radiative coupling of (i) the upper chromosphere and temperature minimum regions (through Balmer continuum photons) and (ii) the transition region and upper chromosphere (through XUV photons) can account for white-light emission in solar flares.Presidential Young Investigator. 相似文献
20.
We have measured the motion of facular points and granules in the same region near a decaying sunspot. It is found that both features move away across the moat surrounding the sunspot. The mean speed of facular points is larger than that of granules: 0.65 km s–1 and 0.4 km s–1, respectively. These results are consistent with previous measurements of the speed of bright network features and moving magnetic fields, as well as of non-magnetic photospherical material. They support models in which a decaying sunspot is at the center of a supergranule, whose horizontal motions sweep out granules and magnetic flux tubes associated to the facular points. It is also found that granules are dragged by supergranular motions away of the moat.Contributions from the Kwasan and Hida Observatories, University of Kyoto.A part of this work was done while one of the authors (R.M.) was staying at the Kwasan and Hida Observatories, University of Kyoto, Japan, as a JSPS research fellow. 相似文献