  High-quality white-light images from the SECCHI/HI-1 telescope onboard
STEREO-B reveal high-velocity evanescent clumps [HVECs] expelled from the coma
of the C/2011 L4 [Pan-STARRS] comet. Animated images provide evidence of highly
dynamic ejecta moving near-radially in the anti-sunward direction. The bulk
speed of the clumps at their initial detection in the HI1-B images range from
$200-400$ km s$^{-1}$ followed by an appreciable acceleration up to speeds of
$450-600$ km s$^{-1}$, which are typical of slow to intermediate solar wind
speeds. The clump velocities do not exceed these limiting values and seem to
reach a plateau. The images also show that the clumps do not expand as they
propagate. Order of magnitude calculations show that ionized single atoms or
molecules accelerate too quickly compared to observations, while dust grains
micron sized or larger accelerate too slowly. We find that neutral Na, Li, K,
or Ca atoms with $\beta>50$ could possibly fit the observations. Just as
likely, we find that an interaction with the solar wind and the heliospheric
magnetic field (HMF) can cause the observed clump dynamical evolution,
accelerating them quickly up to solar wind velocities. We thus speculate that
the HVECs are composed of charged particles (dust particles) or neutral atoms
accelerated by radiation pressure at $\beta>50$ values. In addition, the data
suggest that clump ejecta initially move along near-radial, bright structures,
which then separate into HVECs and larger dust grains that steadily bend
backwards relative to the comet's orbital motion due to the effects of solar
radiation and gravity. These structures gradually form new striae in the dust
tail. The near-periodic spacing of the striae may be indicative of outgassing
activity modulation due to the comet nucleus' rotation. It is, however, unclear
whether all striae are formed as a result of this process.
