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Astron. Astrophys. 358, L75-L78 (2000)

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1. Introduction

Quiescent solar prominences require both a support mechanism to keep the heavy dense material high up in the corona and an energy supply which can compensate the radiative cooling. These questions were addressed in a recent paper by Anzer & Heinzel (1999, referred to as AH) who constructed slab models which were in mechanical equilibrium. They studied the radiative properties of these models and their energy balance. They used one-dimensional slab models and subdivided the prominence into two distinct regions: an inner cool region which is optically thick and a prominence-corona transition region (PCTR) which can be treated in the optically thin approximation. For the modelling of the inner region an ad-hoc temperature profile was assumed and on this basis the full radiative transfer problem was solved. From this the net radiative losses occurring in the prominence could be calculated. The energy equilibrium then requires that at each position in the prominence these losses have to be balanced by the appropriate local heating. This heating mechanism was not specified in AH, but the need for efficient heating of the central parts of the prominence became quite evident. In the present Letter we study this aspect and in particular we shall answer the question whether this heating can be provided by the inflow of enthalpy and ionisation energy into the prominence. This type of heating was discussed recently for the case of the chromosphere - corona transition region by Chae et al. (1997). These authors found that the predominant redshifts could be explained by downflows of about 7 km s-1 at a height where the temperature amounts to [FORMULA] K (note: the velocity scales roughly as the temperature T ). Since the transition region between the interior of the prominence and the surrounding corona (PCTR) has similar properties, we expect that this heating mechanism can also work in prominences provided that large enough inflows occur (Poland, private communication). This is also consistent with the siphon mechanism suggested by Pikel'ner (1971).

In this paper we shall not study the optically thin hot parts of the transition region. Energy equilibria for these regions were already given in AH. In this region it is fairly easy to achieve an energy balance. The only problem there is to match the curve for the differential emission measure with the observations (Engvold et al. 1987, and Chiuderi & Chiuderi Drago, 1991). In this paper we take the same 1D slab models as in AH. In Sect. 2 we give the equations describing our model, in Sect. 3 we present the results, in Sect. 4 we discuss the effects of vertical downflows and Sect. 5 gives a discussion of these new results.

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000