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Astron. Astrophys. 351, 1139-1148 (1999)

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5. Discussion

Datasets taken in a coronal hole, a`quiet' Sun region at disk center plus an active region show variations in the electron density in the transition region over time periods of a few minutes. Such variations can be as large as a factor of two in [FORMULA]5 minutes, but unfortunately the time resolution of our datasets do not permit us to detect faster variability. Electron density enhancements due to spatial structures of 5-10 arc sec are also clearly visible. In each dataset there are a few locations where the electron density showed periodicities of between 8 and 16 min. There is a remarkable agreement between the scale and temporal variability in the coronal hole and `quiet' Sun datasets, in agreement with a bright point study by Habbal et al. (1990) who found that these were indistinguishable. The above study also showed that bright point detection had two maxima, one at coronal temperatures and the other at [FORMULA] K, i.e. around the formation temperature of O IV .

Numerous studies (e.g. the statistical analyses of the HRTS-3 mission by Dere et al. 1983 or SOHO Chae et al., 1998, Pérez et al. 1999), have shown ultraviolet explosive events occurring in a burst-type manner in the solar transition region. Explosive events have been connected to magnetic reconnection occurring on time-scales of minutes over regions with sizes of few arc sec. The distribution of density increases along the network boundaries, as reported in our present work, is consistent with the predominant location of explosive events as already observed by Dere (1991) and recently by Chae et al. (1998). Moreover, these density enhancements are in good agreement with numerical simulations of explosive events by Sarro et al. (1999), who found increases of a factor of two or three at these temperatures. In the CH dataset we calculate a birthrate of [FORMULA] cm-2 s-1 for these density enhancements, in excellent agreement with that derived by Dere et al. (1983) for the explosive event birthrate as observed in C IV .

Judge et al. (1998) found evidence in support of the ` nanoflare ' picture of coronal heating, that would explain his observations of predominantly downward-propagating compressive waves in the solar transition region. Judge et al. do not rule out other heating mechanisms such as resonant absorption of Alfvén waves as described by Ofman et al. (1998). This mechanism would be consistent as well with a density structure showing filamentary and closely spaced density enhancements up to a factor of two, varying on a time-scale of minutes. Thus, in principle, this could also be the cause of the downward-propagating compressive waves. Nevertheless, other work by Peter & Judge (1999) and Teriaca et al. (1999) recently suggested that nano-flares predominantly occurring around the O VI formation temperature ([FORMULA] K) could account for the redshift observed in the low and middle transition region and for the blue-shift seen in the upper transition region and coronal lines. This idea of nano-flares/explosive events occurring in the high transition region is also in agreement with the present detected electron density enhancements. From this point of view, the larger range of density values detected for the active region could be explained in terms of higher frequency of occurrence and/or energy releases in the active region with respect to the `quiet' Sun or coronal holes. Nevertheless, a preliminary analysis of the line widths variations for the data presented here has not been conclusive. Further numerical work based on this type of model is required.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999