Masuda (1994) proposed two different classes of loop-top HXR emission sources: impulsive and gradual. The impulsive sources occur during the impulsive phase simultaneously with the footpoint sources that are evidently stronger. The similar temporal and spectral characteristics suggest the intimate relation of loop-top impulsive sources with the footpoint sources. The gradual loop-top sources are detected in principle after the impulsive phase. They show gradual variation and are much softer than the impulsive sources.
Such a division, however, can be questionable to some extent. First, almost the same area around the SXR loop-top is called either the impulsive HXR emission source or the gradual one, depending on the flare evolution. Moreover, times of accumulation around 10-s or more, that were needed to obtain one HXR image, limited the ability to study short-time fluctuations of loop-top emission sources. Owing to a lack of definitive evidences for impulsiveness of the impulsive sources some authors (e.g. Kosugi 1994) have called them the loop-top HXR emission sources in the impulsive phase. Finally, as was mentioned in Sect. 1, the presence of stronger footpoint sources could distinctly deform spectral characteristics of impulsive loop-top HXR emission sources.
In this Paper, I selected flares in which footpoint HXR emission sources were occulted by the solar limb. In the absence of footpoint sources it was very difficult to classify the resolved loop-top HXR emission sources as impulsive or gradual, the more so as all obtained sources have showed impulsive and gradual (smooth component) behavior simultaneously. Impulses were the strongest during the maximum of the HXR light curve but they were detectable also far from the maximum. The relative contribution of impulses to the total HXR flux was different for different loop-top sources (Table 2).
All ten flares investigated by Masuda (1994) had gradual HXR emission sources, and seven of them had impulsive loop-top HXR sources. In the present paper, each of fourteen investigated events shows the loop-top HXR emission sources. All sources found by Masuda were single, whereas half of the events in this paper show more than one (2 or 3) different sources. The lower number of loop-top HXR emission sources in Masuda (1994) was probably caused by the dynamical range limit (the footpoint sources were too strong to allow detection of the weaker sources) or/and by the spatial resolution limit of the HXT telescope (the flaring structures were too small to separate individual sources).
If we consider size, shape and connection with the bright loop-top SXR kernels, the loop-top HXR emission sources obtained by Masuda are similar to the type A sources in this paper. The sources of type B do not have any evident equivalent among the sources of Masuda. On the other hand, some of their characteristics suggest a similarity between type B sources and a class of HXR sources called "remote-site impulsive sources'' (Kosugi 1994).
Masuda found a relationship between SXR/HXR shift and energy spectrum of the impulsive loop-top HXR emission sources. Three sources showing larger shift (5-10 arcsec), among them the Masuda flare, were more energetic than sources which were almost co-spatial with the bright loop-top SXR kernels. This relationship has not been confirmed for the loop-top HXR emission sources investigated in this paper, e.g., for the two most energetic events, Nos. 7 and 12, their SXR/HXR shift was similar to the coalignment accuracy (Table 4).
If we compare energy spectra of loop-top HXR emission sources presented by Masuda and in the present paper, a fairly good similarity is seen with the exception of three impulsive sources showing the large SXR/HXR shifts, mentioned in the previous paragraph. The maximal temperatures estimated for them from the hardness ratio M1/L were within the interval 108 - >250 MK, that is far above the values obtained in this paper including the most energetic impulses (Table 3).
Two possible explanation of the reported difference should be mentioned. First, the impulsive loop-top sources showing such high-energy spectra were formed in a special magnetic configuration that occurs only exceptionally in the corona. Second, the strong footpoint HXR sources that were observed simultaneously have deformed the real temperature values of the impulsive loop-top HXR source which actually were similar to those obtained in the present paper. Further progress in the HXR image reconstruction as well as more comprehensive survey of behind-the-limb flares with occulted footpoints is needed to decide which explanation is correct.
Copyright ESO 2001