  The passage of sungrazing comets in the solar corona can be a powerful tool
to probe the local plasma properties. Here, we carry out a study of the striae
pattern appearing in the tail of sungrazing Comet Lovejoy, as observed by the
Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO)
during the inbound and outbound phases of the comet orbit. We consider the
images in EUV in the 171 {\AA} bandpass, where emission from oxygen ions
O$^{4+}$ and O$^{5+}$ is found. The striae are described as due to a beam of
ions injected along the local magnetic field, with the initial beam velocity
decaying because of collisions. Also, ion collisional diffusion contributes to
ion propagation. Both the collision time for velocity decay and the diffusion
coefficient for spatial spreading depend on the ambient plasma density. A
probabilistic description of the ion beam density along the magnetic field is
developed, where the beam position is given by the velocity decay and the
spreading of diffusing ions is described by a Gaussian probability
distribution. Profiles of emission intensity along the magnetic field are
computed and compared with the profiles along the striae observed by AIA,
showing a good agreement for most considered striae. The inferred coronal
densities are then compared with a hydrostatic model of the solar corona. The
results confirm that the coronal density is strongly spatially structured.
