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Astron. Astrophys. 358, 741-748 (2000)

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1. Introduction

The simplest picture of the transition region, formed between temperatures of [FORMULA]104K and 106K, is that it is the thermal interface between the hot corona and the cooler chromosphere, and that it is heated by a process of back heating. Early models of the transition region assumed a plane-parallel geometry. However, following Gabriel (1976) these simple energy balance models were modified to include the effect of a magnetic field that expanded and fanned out at coronal heights. This model and later ones (e.g. Athay, 1981, 1982) implicitly include the assumption that the magnetic field within the network is unipolar over supergranular scales. Dowdy et al. (1986) have argued that the observed fine scale structure of the network, consists of mixed magnetic polarities. This led them to propose an alternative `magnetic junkyard' picture of the transition region, consisting of coronal funnels and small cool network loops.

It has been proposed (Habbal et al. 1990) that bright points in the network boundaries are formed from a complex of loop structures at different temperatures, which are a subset of the small-scale network loops proposed by Dowdy et al. Habbal & Grace (1991) argue that these bright points show two favoured temperature distributions (of closed magnetic structures) in the quiet Sun, one below 3 [FORMULA] 105 K, and one at coronal temperatures, implying that not all structures in the quiet Sun reach coronal temperatures. Dowdy (1993) suggests that bright points without associated coronal brightenings can be interpreted as network loops that are heated internally but which have no strong thermal connection to the large-scale corona. Similarly he suggests that bright points that have associated coronal brightenings, i.e. coronal bright points, are network loops that have been heated to coronal temperatures but which are still insulated from the large-scale corona by their magnetic fields.

Early investigations of the quiet Sun with the Skylab S055 instrument (Reeves, Vernazza & Withbroe, 1976) indicated that the intensity histogram of EUV features follows a distribution in which there is a strong peak at a relatively low intensity and a tail extending to several times the peak intensity. Vernazza, Avrett & Loeser (1981) found that up to six different features such as dark points within the internetwork, bright network elements, average network emission etc. could be characterised from the intensity distribution. Using a number of transition region and coronal lines Gallagher et al. (1998) seperated intensity distributions into internetwork and network regions and examined the variation of different network properties with temperature. They found that the transition region model of Gabriel, i.e. that the network fans out in the form of a funnel at coronal heights, is consistent with their results. Recently Patsourakos et al. (1999) examined the expansion of network boundaries with temperature and found similar results.

In this paper we use Coronal Diagnostic Spectrometer (CDS ) and Michelson Doppler Imager (MDI ) observations to study the internetwork and network emission in the quiet Sun. Following Vernazza, Avrett & Loeser (1981) and expanding on the work of Gallagher et al. (1998), we propose that the quiet Sun intensities can be separated into three distributions; the internetwork, the `normal' network and the bright network. We focus our analysis primarily on the network and bright network emission and it's variation with temperature. We present the results of this analysis in Sect. 3 and the conclusions in Sect. 4.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000