Structural elements of filaments

Filaments are cool structures ( 7000 K) embedded in the hot corona. Observations in the optical wavelength range (H, H, Ca ii K) show that filaments consist principally of a main body with the footpoints tied in the photosphere, generally at the boundaries of the supergranules. New instrumentation (Big Bear, Sacramento Peak, Canary Islands) reveals that filaments are highly structured. The dimension of the fine structures is limited by the spatial resolving power of the instruments ( 350 km). These fine structures may be caused by filamentation along the magnetic field or by a radiative instability during the formation of the prominences. New attempts based on radiative transfer calculations, density diagnostics, or dynamics are being developed in order to estimate the number of threads integrated along the line of sight, 2 to 200 according to various authors. Plasma parameter values are derived from these studies. While the models of prominences take into account the magnetic field observations, the fine structures and the dynamics are ignored most of the time.The Prominence-Corona Transition Region (PCTR) is mainly observed at radio wavelengths and in EUV lines. It seems that the fine structures, which have different temperatures spread over a large interval, co-exist spatially. Progress on PCTR models is discussed. The existence of coronal cavities around prominences is still controversial.Invited paper presented at the IAU Commission 10 Meeting on Dynamics and Structure of Prominences in Buenos Aires.