Initial speed of knots in the plasma tail of C/2013 R1(Lovejoy)

We report short-time variations in the plasma tail of C/2013 R1(Lovejoy). A series of short (2-3 minutes) exposure images with the 8.2 m Subaru telescope shows faint details of filaments and their motions over a 24 minute observing duration. We identified rapid movements of two knots in the plasma tail near the nucleus (∼3 × 10⁵ km). Their speeds are 20 and 25 km s^-1 along the tail and 3.8 and 2.2 km s^-1 across it, respectively. These measurements set a constraint on an acceleration model of plasma tail and knots as they set the initial speed just after their formation. We also found a rapid narrowing of the tail. After correcting the motion along the tail, the narrowing speed is estimated to be ∼8 km s^-1. These rapid motions suggest the need for high time-resolution studies of comet plasma tails with a large telescope.