Abstract. The formation of ice particle density irregularities with a
meter scale in the mesopause region is explored in this paper by developing
a growth and motion model of ice particles based on the motion equation of a
variable mass object. The growth of particles by water vapor adsorption and
the action of gravity and the neutral drag force on particles are considered in
the model. The evolution of the radius, velocity, and number density of ice
particles is then investigated by solving the growth and motion model
numerically. For certain nucleus radii, it is found that the velocity of
particles can be reversed at a particular height, leading to a local gathering
of particles near the boundary layer, which then forms small-scale ice
particle density structures. The spatial scale of the density structures can
be affected by vertical wind speed, water vapor density, and altitude, and
it remains stable as long as these environmental parameters do not change. The
influence of the stable small-scale structures on electron and ion density
is further calculated by a charging model, which considers the production,
loss, and transport of electrons and ions, along with dynamic particle
charging processes. Results show that the electron density is
anti-correlated to the charged ice particle density and ion density for
particles with radii of 11 nm or less due to plasma attachment by particles
and plasma diffusion. This finding is in accordance with most rocket
observations. The small-scale electron density structures created by
small-scale ice particle density irregularities can produce the polar
mesosphere summer echo (PMSE) phenomenon.
The research on small-scale structures of ice particle density and electron density in the mesopause region
