  Context. The occultation of a radio source by the plasma tail of a comet can
be used to probe structure and dynamics in the tail. Such occultations are
rare, and the occurrence of scintillation, due to small-scale density
variations in the tail, remains somewhat controversial. Aims. A detailed
observation taken with the Low-Frequency Array (LOFAR) of a serendipitous
occultation of the compact radio source 3C196 by the plasma tail of comet
C/2020 F3 (Neowise) is presented. 3C196 tracked almost perpendicularly behind
the tail, providing a unique profile cut only a short distance downstream from
the cometary nucleus itself. Methods. Interplanetary scintillation (IPS) is
observed as the rapid variation of the intensity received of a compact radio
source due to density variations in the solar wind. IPS in the signal received
from 3C196 was observed for five hours, covering the full transit behind the
plasma tail of comet C/2020 F3 (Neowise) on 16 July 2020, and allowing an
assessment of the solar wind in which the comet and its tail are embedded.
Results. The results reveal a sudden and strong enhancement in scintillation
which is unequivocally attributable to the plasma tail. The strongest
scintillation is associated with the tail boundaries, weaker scintillation is
seen within the tail, and previously-unreported periodic variations in
scintillation are noted, possibly associated with individual filaments of
plasma. Furthermore, contributions from the solar wind and comet tail are
separated to measure a sharp decrease in the velocity of material within the
tail, suggesting a steep velocity shear resulting in strong turbulence along
the tail boundary
