Dense and Cold Filaments in the Solar Corona

Abstract

The structure of a solar coronal filament with plasma parameters typical for quiescent solar prominences (particle concentration \(10^{10}{-}10^{12}\) cm\({}^{-3}\) and temperature in the coldest part ranging from 4000 to 20 000 K), located horizontally in the corona at heights of several tens of thousands of kilometers has been calculated. The filament is considered as the upper part of a slightly curved twisted magnetic loop, the legs of which are anchored in the photosphere. The magnetic field of the filament is helical (twisted). To maintain transverse equilibrium, an external longitudinal field is necessary. There is also a weak transverse magnetic field, which has a significant effect on the distribution of pressure and gas density in the filament. A characteristic feature of the model, reproduced theoretically for the first time, is the presence of a rarefaction (cavity) outside the filament. The fine filamentary structure of the prominence, consisting of dense and cold fibrils immersed in a weak uniform horizontal magnetic field, has been modeled. It is shown that the observed vertical movements of plasma elements in the prominence, usually interpreted as manifestations of ‘‘thermal convection’’ in its body, can be due to vertical mechanical displacements of individual magnetic fibrils that make up the body of the prominence. Shear (slip) plasma movements on the photosphere can change the sign of the weak transverse field and thus reduce the plasma density on the filament axis, i.e., bring the density of the electric current on its axis closer to the critical value at which the drift velocity of electrons equals the ion sound speed.