Condensation of material in quiescent prominences

Calculations by Hildner show that coronal material having temperature in excess of 106 K will cool on the time scale observed in prominences only if the electron number density is considerably greater than 108 cm-3. This density is larger than is usually considered appropriate at the relevant range of heights in the corona. Conditions which favor the onset of condensation can still develop if the magnetic field of the region is disturbed violently by subphotospheric motions, however, because the twisting of flux into braids threading this structure produces magnetic fields with anti-parallel components. Rapid relaxation of this configuration is inhibited by the formation of dense sheets of material between braids; this plasma is maintained at coronal temperatures by the merging magnetic flux. Although the material in the sheets collapses gradually in the gravitational field as the source of magnetic energy becomes depleted, some remains trapped in the essentially horizontal field at the top of the support arches. This forms a nucleus for the condensation process.Estimates are provided for the various parameters, and evidence of braiding in prominence structures is discussed