  Solar coronal plumes long seemed to possess a simple geometry supporting
spatially coherent, stable outflow without significant fine structure. Recent
high-resolution observations have challenged this picture by revealing numerous
transient, small-scale, collimated outflows ("jetlets") at the base of plumes.
The dynamic filamentary structure of solar plumes above these outflows, and its
relationship with the overall plume structure, have remained largely
unexplored. We analyzed the statistics of continuously observed fine structure
inside a single representative bright plume within a mid-latitude coronal hole
during 2016 July 2-3. By applying advanced edge-enhancement and spatiotemporal
analysis techniques to extended series of high-resolution images from the Solar
Dynamics Observatory's Atmospheric Imaging Assembly, we determined that the
plume was composed of numerous time-evolving filamentary substructures,
referred to as "plumelets" in this paper, that accounted for most of the plume
emission. The number of simultaneously identifiable plumelets was positively
correlated with plume brightness, peaked in the fully formed plume, and
remained saturated thereafter. The plumelets had transverse widths of 10 Mm and
intermittently supported upwardly propagating periodic disturbances with phase
speeds of 190-260 km/s and longitudinal wavelengths of 55-65 Mm. The
characteristic frequency (3.5 mHz) is commensurate with that of solar p-modes.
Oscillations in neighboring plumelets are uncorrelated, indicating that the
waves could be driven by p-mode flows at spatial scales smaller than the
plumelet separation. Multiple independent sources of outflow within a single
coronal plume should impart significant fine structure to the solar wind that
may be detectable by Parker Solar Probe and Solar Orbiter.
