4. Analysis of the SXT data
The SXT data and filter ratio method (Hara et al. 1992) were used to derive the time evolution of mean temperature and emission measure during the decay of the loop system. The intensity, in each filter (Al1, AlMg), was integrated from an area of 189 pixels which lay inside an intensity contour 39% above the background containing the whole loop (see Fig. 5). However, during the analysis we found out that the general behaviour of the mean plasma temperature and emission measure is almost independent of the chosen area of the loop system where the intensity was integrated from. A similar result was obtained by Schmieder et al. (1996).
The time evolution of the mean plasma temperature in the loop (upper graph in Fig. 5) looks rather complicated. At the beginning of the observational sequence, from 13:07:28 UT to 13:17:04 UT, a manifestation of slow plasma cooling from the initial temperature 2.8 MK down to 2.5 MK in approximately 600 s can be seen. Then the temperature behaves rather chaotic. We believe that this could be accounted for by the influence of hot rare coronal plasma surrounding the PFL system, the existence of which we mentioned in the previous section. When the emission measure of plasma inside the examined loop was much greater than the emission measure of other plasma along the line of sight, the manifestation of plasma properties in the examined loop prevailed over the manifestation of properties of surrounding plasma. But as the emission measure of plasma inside the loop was decreasing with time (see lower plot in Fig. 5), the measured quantities were more and more influenced by the properties and behaviour of other plasma lying along the line of sight. So we believe that plasma inside the PFL could continue cooling even after 13:17:04 UT because the values of the temperature later on are probably very strongly influenced by other hot coronal plasma along the line of sight (see Fig. 2).
The behaviour of the total emission measure along the line of sight averaged over the area of the whole loop , presented in the lower graph of Fig. 5, looks much simpler. It decreases very quickly (in approximately 960 s) from its original value cm-5 at 13:07:28 UT to its final value cm-5 at 13:23:28 UT. Because such a rapid decrease of can be explained only by plasma depletion from the PFL (see also Fig. 2), we can expect a strong down-flow of hot plasma from the loop to its footpoints along the magnetic field lines. Later on, the behaviour of becomes slightly chaotic again, which we believe can be accounted for by rare hot coronal plasma along the line of sight, with a mean emission measure , which does not belong to the PFL system. Because the emission measure is an additive quantity, the mean emission measure of plasma inside the loop system itself is
can be used to estimate the mean value of the second power of the electron density in the loop system. If we suppose that all the emitting plasma is deposited inside some filaments, which are not resolved by SXT and which have the same electron density, we can calculate the mean electron density in the volume occupied by the loop system: . is the area over which the intensity is integrated.
From the SXT images and CDS rasters (Figs. 2, 3) we determined the apparent diameter of the loop system in the plane perpendicular to the line of sight cm. If we assume that this apparent diameter obtained above equals D and , so that all the emitting plasma along the line of sight is concentrated in the loop system, the maximum mean electron density is cm-3 and the minimum density is cm-3. On the other hand, if we suppose that equals one half of the almost constant value of at times from 13:23:28 UT to 13:39:28 UT, which is approximately cm-5 then the maximum mean electron density in the loop is cm-3 and the minimum density is cm-3, for the same value of D as in the previous case.
From the emission measure analysis we found that there has to be a plasma outflow from the examined loop system. Using the quantities obtained above and with the help of the continuity equation it is possible to estimate, though very roughly, the velocity of plasma outflow along the magnetic field at the base of the loop. If we suppose that the cross-sectional area along the loop is constant, we get for a velocity of a symmetrical plasma outflow to both footpoints km s-1. If the plasma outflow is directed only to one footpoint of the loop system then the velocity would be approximately twice larger. This rather low velocity contrasts with much stronger flows ( km s-1) observed in cool H loops (Wiik et al. 1996). This could be explained by quite different pressure scale-heights in these two cases.
© European Southern Observatory (ESO) 2000
Online publication: March 9, 2000