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

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1. Introduction

There are abundant references to solar electron density (Ne) diagnostics in the literature, with e.g. emission lines arising from transitions in O IV providing accurate determinations of Ne (Griffiths et al. 1999, Doschek et al. 1998, O'Shea et al. 1998, Spadaro et al. 1994, Dwivedi & Gupta 1991, Hayes & Shine 1987, Feldman & Doschek 1978). For instance, Hayes & Shine (1987) used the ratio of Si IV  1402.8 Å and O IV  1401.2 Å, and found that short-lived bursts typically showed electron density increases coupled with a small line shift to the red. They suggested this might be caused by ` micro-flares '. Cheng (1980), analysing coronal loops in Fe XV & Fe XVI lines, found a density enhancement of [FORMULA] in a loop within 7 minutes, plus a slower variation over a longer time interval. He suggested that this increase in density could be due to mass ejection from lower regions, and the associated dissipation of the electric current associated with the resulting high-density twisted flux strands (Nakagawa & Stenflo 1979) contributing to the coronal heating.

In this paper we use the O IV 2[FORMULA]2p2[FORMULA] [FORMULA] 2s2p4P density-sensitive multiplet around 1400 Å to analyse time-series solar spectra. More precisely, we use the O IV  1399.8 Å and 1401.2 Å lines for our analysis.

The Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument (Wilhelm et al. 1997) on SOHO provides the opportunity to observe the solar atmosphere in the spectral range from [FORMULA]500 to 1600 Å with high spectral and spatial resolution. In first order, the spectral resolution is [FORMULA]43mÅ, while [FORMULA]22mÅ is achieved in second order. The spatial resolution is approximately 1 arc sec in the E-W direction and 2 arc sec along the slit (N-S direction). It should be pointed out that only lines separated by less than 40 Å in first order, and 20 Å in second order, can be observed simultaneously with SUMER due to the size of the CCD. Surprisingly few lines can be used for density diagnostics, due to blending problems, the weakness of some lines, and the fact that possible useful lines cannot be observed simultaneously. The line pair most useful for diagnosing the transition region is probably O IV 1399.8[FORMULA]1401.2 (Wikstol et al. 1997).

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999