Kinematics and evolution of twist in the eruptive prominence of August 18, 1980

The prominence which erupted at the SE limb on August 18, 1980 is one of the best observed disparition brusque events: high-resolution monochromatic ground-based observations in the H line were supplemented by the SMM and Solwind satellite coronographic observations; the radio wavelength range was well covered by single-frequency and spectral observations, and the prominence magnetic fields were measured two days before the eruption.The prominence showed a helical-like internal structure from the pre-eruptive phase, up to the late phases of eruption. The pitch angles of the helical-like threads were measured at several positions 31 along the prominence axis, and the evolution of twist was followed during the eruption. These measurements provide an estimate of the parameters which are directly comparable with theoretical models. The pitch angles of the helical threads decreased during the eruption. A redistribution of twist along the prominence axis could not be detected within the accuracy of measurements, although there are indications that the twist was partly transformed into an external kink-type screw of the prominence axis. The value of the total twist did not change during the eruption within the accuracy of the measurements.The kinematics of the process was followed, and accompanying events in the radio-range and soft X-rays are listed. Measurements of the magnetic field vector in the prominence are reviewed briefly. The observations were compared with predictions of cylindrical models, considering the forces acting at the prominence summit. Observational implications and constraints are discussed, and the decrease of the axial electric current and the mass loss are inferred.