Dynamics of High-Velocity Evanescent Clumps (HVECs) Emitted from Comet C/2011 L4 (Pan-STARRS) as Observed by STEREO

We use white-light images from the STEREO/SECCHI/HI-1B to characterize newly discovered high-velocity evanescent clumps (HVECs) expelled from the coma of the C/2011 L4 (Pan-STARRS) comet. The observations were recorded around the comet's perihelion (i.e., ~0.3 AU) during the period 09-16 March 2013. The HVECs are moving near-radially in the anti-sunward direction with bulk speeds at their initial detection ranging 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 HVECs do not show any significant expansion as they propagate. The while-light images do not provide direct insight into the composition of the expelled clumps, which could potentially be composed of fine, sub-micron dust particles, neutral atoms and molecules, and/or ionized atomic/molecular cometary species. Although solar radiation pressure plays a role in accelerating and size sorting of small dust grains, it cannot accelerate them to velocities >200 km s-1 in the observed time interval of a few hours and distance of <106 km. Further, 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 β > 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 atoms accelerated by radiation pressure at β > 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.