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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 65 (1980), S. 39-52 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The HXIS, a joint instrument of the Space Research Laboratory at Utrecht, The Netherlands, and the Department of Space Research of the University of Birmingham, U.K., images the Sun in hard X-rays: Six energy bands in energy range 3.5–30 keV, spatial resolution 8″ over Ø 2′40″ and 32″ over Ø 6′24″ field of view, and time resolution of 0.5–7 s depending on the mode of operation. By means of a ‘flare flag’ it alerts all the other SMM instruments when a flare sets in and informs them about the location of the X-ray emission. The experiment should yield information about the position, extension and spectrum of the hard X-ray bursts in flares, their relation to the magnetic field structure and to the quasi-thermal soft X-rays, and about the characteristics and development of ‘type IV’ electron clouds above flare regions.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 67 (1980), S. 379-384 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 48 (1976), S. 197-254 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The Utrecht solar hard X-ray spectrometer S-100 on board the ESRO TD-1A satellite covers the energy range above 25 keV with 12 logarithmically spaced channels. Continuous sun-pointing is combined with high time resolution: 1.2 s for the four low energy channels (25–90 keV) and 4.8 s for the others. It is emphasized that the instrument design and calibration yield data virtually free of pile-up and other instrumental defects. A complete set of observations is presented for all well-observed flares during the period March 12, 1972 to October 1, 1973, including four from the highly active period August 1–8, 1972. Photon spectra are computed every 1.2 s for each event by deconvolution through the instrument response, rather than by fitting techniques. Using these actual photon spectra, the index γ for the best fitting single power law and the minimum (thick target) injection rate of electrons above 25 keV, F 25, are calculated. Results for γ and F 25 at 1.2 s intervals are presented for each event. Examination of all these results tentatively suggests a real distinction between events of a purely impulsive nature and prolonged events. Techniques of time series analysis are applied to the burst time profiles. Specifically: (1) The fluctuations present in the series are shown to be compatible with Poisson noise in the count rate. (2) It is emphasized that, without spatial resolution, the X-ray source must be characterized by the e-folding time scale τ of the total count rate; examination of individual τ's through the event shows very few statistically real τ's as short as 1.2 s, confirming (1). (3) For all events, the series are Fourier analysed; no small events showed statistically significant periodicities, but the large event of August 4, 1972 exhibited real periods of 30, 60 and 120 s in both the flux and the spectral index. (4) Statistically real, small timing differences (∼0.2 s) are shown to exist between spike peaks at different photon energies. A search is made for correlations between instantaneous values of inferred parameters (e.g. F 25, γ and the time scales). Most results are negative, but in the August 4 and 7, 1972 events a very well defined path was followed through the (F 25, γ)-plane, giving insight into the electron acceleration process. Finally some general conclusions are drawn concerning the implications of our analysis for the physics of particle acceleration, including the possibility of two classes of event. Specifically, the severe problems posed by the large electron fluxes (equivalent current ∼1017 A) demanded by the data are discussed in relation to flare theories. Some possibilities for getting around these problems, such as by reacceleration in a confinement region, are briefly considered.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 81 (1982), S. 137-157 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The impulsive phases of three flares that occurred on April 10, May 21, and November 5, 1980 are discussed. Observations were obtained with the Hard X-ray Imaging Spectrometer (HXIS) and other instruments aboard SMM, and have been supplemented with Hα data and magnetograms. The flares show hard X-ray brightenings (16–30 keV) at widely separated locations that spatially coincide with bright Hα patches. The bulk of the soft X-ray emission (3.5–5.5 keV) originates from in between the hard X-ray brightenings. The latter are located at different sides of the neutral line and start to brighten simultaneously to within the time resolution of HXIS. Concluded is that: (1) The bright hard X-ray patches coincide with the footpoints of loops. (2) The hard X-ray emission from the footpoints is most likely thick target emission from fast electrons moving downward into the dense chromosphere. (3) The density of the loops along which the beam electrons propagate to the footpoints is restricted to a narrow range (109 〈 n 〈 2 × 1010 cm-3), determined by the instability threshold of the return current and the condition that the mean free path of the fast electrons should be larger than the length of the loop. (4) For the November 5 flare it seems likely that the acceleration source is located at the merging point of two loops near one of the footpoints. It is found that the total flare energy is always larger than the total energy residing in the beam electrons. However, it is also estimated that at the time of the peak of the impulsive hard X-ray emission a large fraction (at least 20%) of the dissipated flare power has to go into electron acceleration. The explanation of such a high acceleration efficiency remains a major theoretical problem.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Solar physics 58 (1978), S. 139-148 
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The dynamics of hard X-ray producing electron beams in solar flares can be strongly affected by the occurrence of a reverse current. The parameter diagram for a beam can be divided into three regimes, one of which is the usual thick target case, the two others being due to two different possible consequences of the reverse current. The use of this parameter diagram as a possible diagnostic tool for solar flare hard X-ray sources is discussed, together with the necessary observations and their interpretation. The forthcoming Solar Maximum Mission, complemented with concurrent ground-based efforts provide the next possibility to obtain these observations, given a good coordination of observing programs. We stress the importance of microwave (GHz) ratio observations with good temporal (≲few sec) and spatial resolution (≲1″) in one dimension, and of reliable spectroscopic methods to determine the density in solar flare hard X-ray sources.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1572-9672
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Type of Medium: Electronic Resource
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