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

5. Discussion

This investigation shows that only prominences with a low column mass can be heated sufficiently by the inflow of enthalpy and of ionisation energy from the surrounding corona. This result holds if the central temperature in the prominence is around 8 000 K. If the central temperature becomes sufficiently low then also more massive prominence could be heated in this way. In particular Heasley & Mihalas (1976) have found that if the model is in radiative equilibrium,the central temperature reaches some 4600 K and no heating is needed in these regions. But the value of this equilibrium temperature is so low that it seems very implausible that quiescent prominences are in such an "equilibrium state".

There are still some other unsolved problems related to this heating mechanism.

1. As can be seen from the figures presented in AH the gradients of the temperature and the ionisation degree go to zero in the central parts of the prominence. Therefore the inflow of energy into these regions will also vanish and our heating mechanism does not work there. On the other hand one sees that the radiative loss curves have a maximum in the mid-plane of our slab models because of the density peak. Therefore some additional heating will still be required in the center.

2. Our simple model is not fully self-consistent: as long as there is no flow across magnetic field lines the inflowing plasma has to pile up in the central regions of the prominence. Therefore the prominence mass would grow infinitely. For typical prominence parameters one would have a systematic doubling of the mass within a time of several hours. This obviously is in disagreement with the observations. For this reason one is forced to postulate that in the central regions the plasma can slowly move across the field lines and diffuse downward to leave the prominence at this bottom. But at present it is not clear how this diffusion could occur.

   However if this systematic downflow actually occurs it represents an additional source of energy. For our models the energy associated with this downflow is typically twice as large as that of the inflow of enthalpy and ionisation energy. Therefore it could be a powerful heating source. But we have not yet a model describing the energy conversion into heat for this downflow.

   The investigation presented in this paper only deals with the global energy balance. It does not solve the local heating problem. Such a detailed modelling of the local energetics will be the subject of a forthcoming paper and will require the simultaneous solution of the equation for the flux divergence and the full non-LTE radiative transfer equations.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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