What do the results in Sect. 4 tell us about the transition region? Firstly we will do a comparison of the s3477r01 and 165749 datasets using just the QUB results. As discussed above, both these datasets observe a region some 16 arc sec apart in X. In dataset s3477r01 the region 14 to 55 arc sec was measured using the Fe XIII 359/348 ratio and found to have an average electron density over the observation time of Log Ne=9.7 cm-3. This implies a coronal electron pressure of 15.9 cm-3 K. The corresponding region 8 to 49 arc sec measured in the transition region by dataset 165749 was found to have an electron density of Log Ne=10.8 cm-3, corresponding to a transition region electron pressure of Log 16.0 cm-3 K. These results clearly offer little evidence of a constant electron density from the transition region to the corona, despite the fact that we are not even observing the same spatial region. Instead, they offer strong evidence for a constant electron pressure, i.e. the measured electron pressures from both diagnostics differ by only 0.1 dex.
Similar conclusions are obtained by using the CHIANTI results. For the Fe XIII 359/348 ratio, the results for the s3477r01 dataset imply an electron density of Log Ne=10.1 cm-3 and a corresponding coronal electron pressure of 16.3 cm-3 K. For the O IV 1407/1401 ratio the results for the 165749 dataset are Log Ne=11.0 cm-3, giving a transition region electron pressure of Log 16.2 cm-3 K. Again we see that the difference in pressures is only 0.1 dex, indicating a constant electron pressure from the transition region to the corona.
Similarly it is possible to do a comparison between datasets s3479r00 and 173109. As discussed previously region 10 to 55 arc sec in dataset s3479r00 and 4 to 49 arc sec in dataset 173109 observe the same spatial region. Region 10 to 55 arc sec was measured in dataset s3479r00 and found to have an average electron density over the observation time of Log Ne=9.6 cm-3 using the QUB results and Log Ne=10.0 cm-3 using CHIANTI. These correspond to coronal electron pressures of Log 15.8 cm-3 K and Log 16.2 cm-3 K respectively. The average electron density over the observation time in region 4 to 49 arc sec of dataset 173109 was found to be Log Ne=10.9 cm-3 using the Keenan et al. results and Log Ne= 11.0 cm-3 using CHIANTI. These correspond to pressures of Log 16.1 cm-3 K and Log 16.2 cm-3 K respectively in the transition region.
From all these results it is clear that there is little evidence of a constant electron density between the transition region and corona and we must therefore conclude that a constant electron pressure is a more likely explanation. This appears at first to contradict the findings of Feldman & Laming (1993). For the present data we used a coronal line at a temperature of Log Te=6.2 K comparing it to a transition region line at Log Te=5.2 K. It may be that the burst model proposed by Feldman & Laming is relevant for events such as the transition region explosive events (Dere 1994) which are thought to be caused by reconnection of magnetic field lines in a process similar to that thought to be responsible for nanoflares. These occur in a narrow temperature band comprising only transition region plasma. In-fact, the lines used by Feldman & Laming covered only transition region plasma and were for active region plasma conditions.
A constant electron pressure between the transition region and conora would suggest that the transition region is simply a time varying interface between the chromosphere and corona and is not formed in unresolved fine structures that are magnetically and thermally disconnected from the corona.
© European Southern Observatory (ESO) 1998
Online publication: September 17, 1998