  We present new spectral (FPI and long-slit) data on the Eastern optical
filament of the well known radionebula W50 associated with SS433. We find that
on sub-parsec scales different emission lines are emitted by different regions
with evidently different physical conditions. Kinematical properties of the
ionized gas show evidence for moderately high (V ~ 100 km/s) supersonic
motions. [OIII]5007 emission is found to be multi-component and differs from
lower-excitation [SII]6717 line both in spatial and kinematical properties.
Indirect evidence for very low characteristic densities of the gas (n ~
0.1cm^{-3}) is found. We propose radiative (possibly incomplete) shock waves in
low-density, moderately high metallicity gas as the most probable candidate for
the power source of the optical filament. Apparent nitrogen over-abundance is
better understood if the location of W50 in the Galaxy is taken into account.
