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Astron. Astrophys. 338, 1102-1108 (1998)

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1. Introduction

The SOHO mission, launched in December 1995, contains an array of instruments for studying the solar atmosphere. Of these, only the Coronal Diagnostic Spectrometer (CDS) (Harrison et al. 1995) and Solar Ultraviolet Measurement of Emitted Radiation (SUMER) (Wilhelm et al. 1995, 1997) are suitable for observing a range of spectral lines, and hence the derivation of electron densities. Using joint observations obtained from these two instruments, we have determined the electron density in the transition region and corona, in order to evaluate whether the solar transition region is magnetically and thermally decoupled from the over-lying corona. Before describing the observations in Sect. 3, we will briefly review the background to this work.

It has been found that even with very good atomic physics data it is not always possible to get consistent results between theoretical and observed intensity ratios. For example emission lines from Be and Mg-like ions, such as Ca XVII and Fe XV , show anomalous results. It has been suggested that the reason for this difference is due to the fact that the plasma is not in equilibrium but is subject to compression by frequent small explosive events or bursts (Laming & Feldman 1992, Feldman et al. 1992, Feldman & Laming 1993). The suggestion is that the plasma is compressed on time-scales which do not allow equilibrium conditions to prevail. This idea was first developed as a way of explaining why the emission lines of neutral and singly ionized helium have larger intensities than would be expected from calculations. Laming & Feldman (1992) noted that this situation would be observed if the temperature of the line excitation was raised while the ionization balance was kept close to that of a lower quiescent temperature. They proposed that the chromosphere is heated by frequent small explosive events or bursts, rising the temperature of small regions of the plasma in time-scales short compared to the ionization equilibrium time.

Similarly, Feldman (1992) proposed that the discrepancy in the electron density determination obtained from Fe IX line ratios, when compared to other line ratios, could also be explained by a burst model. Fe IX ions whose lifetimes in the burst were shorter than the lifetime of the excited level from which the diagnostic radiation is emitted should have that particular line intensity reduced. It has been further proposed that if the transition region is continuously heated by these small bursts, then the notion that this region obeys a constant electron pressure law with the higher temperature atmosphere is incorrect, and that a constant electron density more closely fits the observations (Feldman & Laming 1993). Here, we use data obtained in July 1996 in a joint CDS/SUMER observational campaign to test the constant electron density hypothesis.

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998