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 .
The observations were recorded around the comet ’ s perihelion [ i.e. , \sim~ { } 0.3 AU ] during the period 09 - 16 March 2013 .
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 .
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 can not accelerate them to velocities > 200 km s ^ { -1 } in the observed time interval of a few hours and distance of < 10 ^ { 6 } 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 \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 .