  We investigate the dynamics of negative surface discharges in air through
numerical simulations with a 2D fluid model. A geometry consisting of a flat
dielectric embedded between parallel-plate electrodes is used. Compared to
negative streamers in bulk gas, negative surface streamers are observed to have
a higher electron density, a higher electric field and higher propagation
velocity. On the other hand, their maximum electric field and velocity are
lower than for positive surface streamers. In our simulations, negative surface
streamers are slower for larger relative permittivity. Negative charge
accumulates on a dielectric surface when a negative streamer propagates along
it, which can lead to a high electric field inside the dielectric. If we
initially put negative surface charge on the dielectric, the growth of negative
surface discharges is delayed or inhibited. Positive surface charge has the
opposite effect.
