Motions of loop prominence knots have been studied on Ha-line filtergrams. By making use of contours of equal photographic density for entire cinegram it has been possible to significantly decrease the error in determining the locations of the knots. The method of mean velocities has been developed, which has permitted for the first time accurate determination of the laws of knot motion with sufficient accuracy. Two types of falling knots are distinguished: (1) those with a constant acceleration that is always below the gravitational acceleration, and (2) those with a constant velocity. The iLnitial velocity of the falling knots is always different from zero. The gasdynamic theory has been developed to explain the deceleration of the two types of knots due to: the work done against the pressure force; pileup plasma raking; and shock-wave generation. The shock mechanism imparts a constant velocity to the motion. The temperature along the knot trajectories has been estimated. The ratio of the densities in the knots to the surrounding medium has been found.
The aim of the present work is to gain new insight into the known observations of the motions in active prominences, in particular in the loop prominences, and to understand the reasons for the observed motions.
A number of studies of prominence knot motions have been made using filter photographs (cinefilms). It seems to us, however, that the velocities (and especially acceleration) of individual knots have been determined within insufficient accuracy. It will be noted first of all that values of V(t) have been determined with a low accuracy by some authors even for high velocities (V~ 100 km s-l). In fact, to achieve at least 10% accuracy in determining V by comparing two photographs obtained with a 30 s interval, it is necessary to measure the location of knots to 0'.'5 accuracy. The problem is the more complicated as the size of the knots can often reach 5" x 10". This is why the temporal dependence of the velocity of prominence knot motion is represented with a complicated curve.