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Astron. Astrophys. 357, 743-756 (2000)

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1. Introduction

A seminal paper by Gabriel (1976) provided a model for the solar atmosphere which took account of the magnetic field geometry resulting from supergranular convective flux. This model has become a benchmark for theoretical models of the temperature and density structure in the quiet Sun. Recently, Gallagher et al. (1999) have studied quiet Sun observations obtained with the Coronal Diagnostic Spectrometer (CDS, Harrison et al. 1995) and find that they broadly support the model of Gabriel (1976). A heated debate has been raging on the nature of the transition region, at a temperature of around 105 K, which separates the chromosphere and corona. Early models (cf Gabriel & Mason, 1982) were based on a plane parallel atmosphere and are no longer appropriate to describe the dynamic and inhomogeneous nature of the transition region.

Analyses of Skylab observations led to the proposal by Feldman (1983) that the transition region is composed of Unresolved Fine Structures (UFSs). Feldman & Laming (1994) provided evidence to suggest that most of the low temperature transition region ([FORMULA] K) and the coronal emission ([FORMULA] K) is disconnected, that is not part of continuous atmospheric structures.

Observations of the transition region in UV emission from instruments such as the High Resolution Telescope and Spectrometer (HRTS) (Brueckner & Bartoe, 1983) showed inhomogeneities on a scale at the spatial resolution of the instrument (1"). The electron density values derived from the O iv emission lines indicated the existence of sub-resolution filamentary structures, with fill factors of 10[FORMULA] (Dere et al., 1987). Dere and co-authors concluded that the macroscopic structures seen in C iv emission were probably composed of discrete, subresolution structures.

Antiochos & Noci (1986) proposed the co-existence of hot and cool loops to explain a different puzzle, the upturn in the emission measure towards the lower temperatures. Dowdy et al. (1986) went one step further to describe the transition region as a magnetic junkyard , comprising cool loops (around 105 K) of varying sizes and hotter (around 106 K) large loops or funnels .

Wikstol et al. (1998) re-examined the evidence for UFSs, which are magnetically and thermally disconnected from the corona, and the Classical Transition Region (CTR) models which provide continuity between the cool and hot plasma. They were very critical of the empirical approach adopted by Feldman and co-workers and suggest that a forward approach to modelling the solar atmosphere should be adopted. They stressed the non-uniqueness of inferring physical plasma properties from observations. In particular, they were concerned that plasma processes in the solar atmosphere are dynamic and time-dependent.

Recently, Feldman et al. (1999) have re-examined the observational evidence for the morphology of the quiet solar upper atmosphere. They review data from the Skylab S082a spectroheliograph, the Transition Region and Coronal Explorer (TRACE), YOHKOH and SOHO. The coronal emission (T between 106 K and [FORMULA] K) in the quiet Sun clearly originates from a large number of densely packed loop-like structures. From SUMER and Skylab observations, Feldman et al. confirm that at lower temperatures ([FORMULA] K) the emission originates predominantly in cool loop-like structures, which are brightest along the chromospheric network.

Spectral line shifts and broadenings provide a diagnostic tool for probing the dynamics of the solar atmosphere. Systematic redshifts in transition region lines have been observed in both solar and stellar spectra. A persistent redshift is observed in the quiet Sun transition region lines. Brekke et al. (1997), Chae et al. (1998b), Peter (1999) give useful summaries of recent observations with SUMER. Several authors have studied the spectral line broadening of transition region and coronal emission from Skylab, HRTS and more recently SUMER. The non-thermal broadening can provide a useful constraint on the possible heating mechanisms for the solar plasma (Doyle et al., 1997, Erdelyi et al., 1998a). The correlation between the line profile and redshift for active and quiet regions is discussed by Brynildsen et al. (1998) and Teriaca et al. (1999a).

The HRTS observations of C iv emission showed a dynamic transition region with a multitude of explosive events (Brueckner & Bartoe, 1983, Dere et al., 1989, Dere, 1994). The average extent of these explosive events was found to be 1500 km, with Doppler shifts giving typical maximum velocities of 150 km s-1 in both the blue and red wings, with an average lifetime of 60 s. Innes et al. (1997) reported the observation of similar bi-directional jets in Si iv using SUMER. Chae et al. (1998a) found a strong tendancy for explosive events to occur repeatedly in bursts. These could be sites of magnetic energy release. Small scale brightenings have also been observed with CDS (blinkers , Harrison, 1997, Harrison et al., 1999) and EIT (Berghmans et al., 1998). All these events seem to be located at or near the network boundaries and could be related to the nano-flare activity proposed by Parker (1998). Krucker & Benz (1998) and Benz & Krucker (1999), using data from SOHO and radio observations, have shown that intensity variations in chromospheric/transition region emission in small scale brightenings correlate with coronal variations in the same way as in solar flares. They also show that such events, if interpreted as microflares and nanoflares, can be responsible at least for a substantial part of heating in quiet solar corona.

In this paper we re-examine the nature of the transition region using the impressive spatial and spectral capability of the SUMER instrument on SOHO. Preliminary results were presented at the SOHO 7 workshop in September, 1998 (Landi et al., 1999a). In Sect. 2 we describe the SUMER instrument, observations and data reduction. In Sect. 3 we present intensity maps of transition region, coronal and chromospheric lines over a large spatial area. In Sect. 4 we analyse the spectral line shifts, widths and the electron density and temperature distribution for the quiet features. In Sect. 5, we discuss the more active features, explosive events, which have broader line profiles, together with a jet-like structure. In Sect. 6, we summarise our results and the implications which these have for transition region models.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000