Dynamics of HVECs emitted from comet C/2011 L4 as observed by STEREO

Abstract

AbstractHigh‐quality white‐light images from the Sun‐Earth Connection Coronal and Heliospheric Investigation (SECCHI) HI‐1 telescope on board 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., ∼0.3 AU) during the period 9–16 March 2013. Animated images provide evidence of highly dynamic ejecta moving near radially in the antisunward direction. The bulk speed of the clumps at their initial detection in the HI1‐B images range from 200 to 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 white‐light images do not provide direct insight into the composition of the expelled clumps, which could potentially be composed of fine, submicron 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 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 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 backward 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.