The chromospheric Evershed flow

High-resolution filtergrams of a sunspot region observed at seven wavelengths in Hα with a resolution of 1/8 Å have been used to investigate the three-dimensional structure of the chromospheric Evershed flow and its time variation. The flow channels have the form of loops whose cross-section alters with height and distance from the spot. The loops probably reach a height of 5000 km. The presence of a compression shock is suggested by the observations. Many velocity channels appear to alter their direction with position in the line, giving the impression of a screw-like motion resembling a Beltrami flow. The flow channel alters with a characteristic time of 8±2 min. It is suggested that this relatively short time constant represents the characteristic time of perturbations of the flow channels and the superpenumbra fibrils. The lifetime of the flow channel istelf is estimated to be 70 min. The observations are discussed in terms of material moving inwards towards the spot along curved flux loops.