*Astron. Astrophys. 358, L75-L78 (2000)*
## 4. Vertical flows
Our models give not only an inflow of energy but they also produce
an inflow of mass at a rate of
For our models M1 to M3 this gives
g cm^{-2}s^{-1}. If
this mass is accumulated inside the prominence it would grow very
rapidly, its mass would be doubled wihtin
s for model M1, within
s for model M2 and
s for model M3. Since such a rapid
steady growth of the prominence as a whole is not observed prominence
material has to leave the prominence at a similar rate. (Note:
prominence fine structures can form and disappear on slower time
scales, but the quiescent prominence as a whole will be rather
stationary). Mass losses of the required magnitude could be achieved
by a systematic downflow of cool material in the center of the
prominence. However this downflow cannot be modelled in our 1D slab
configuration. For this reason will shall give here only some order of
magnitude estimates for the flow. If we assume that the prominence
extends over a height *h* and that the vertical outflow at the
bottom is , whereas there is no
inflow at the top then the condition of mass conservation gives
where *d* is the width of the downflow region,
its hydrogen density.
Such systematic downflows can provide additional energy at a rate
of , as has been proposed by Heasley
& Mihalas (1976) and this could lead to an additional heating of
the central parts of prominences. The mean heating rate will be given
by
For cm we then get
The heating by enthalpy and ionisation energy inflow from both
sides amounts to . These numbers show
that for the parameters chosen the gravitational energy release is
twice as large as the enthalpy and ionisation energy flow. Therefore
such a mechanism could be an important heat source for the central
parts of prominences. There are, however, some basic problems with
this scenario. Since the magnetic field in prominences is
predominantly horizontal this downflow has to occur perpendicular to
the field. Even for ionisation degrees as low as 0.2 the flow of
neutral atoms across the field lines will be only of the order of
cm s^{-1} (Mercier &
Heyvaerts, 1977). Such flows are therefore only possible if very
efficient reconnection occurs in the cool part of the prominence. An
additional requirement for the reconnection mechanism is that the
fields are stretched sufficiently downward to lead to the right
magnetic field topology. This reconnection could then result in the
required effective resistivity of the prominence plasma. But when the
prominence material starts moving downward it also has to convert its
kinetic energy into heat. The question how this can be achieved is
also open at present. Therefore we think that this mechanism looks
promising, but there are still many details which will have to be
worked out.
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000
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