  We use the Gaia-TGAS data to compare the transverse velocities in Galactic
longitude (coming from proper motions and parallaxes) in the Milky Way disk for
negative and positive longitudes as a function of distance. The transverse
velocities are strongly asymmetric and deviate significantly from the
expectations for an axisymmetric Galaxy. The value and sign of the asymmetry
changes at spatial scales of several tens of degrees in Galactic longitude and
about 0.5 kpc in distance. The asymmetry is statistically significant at 95%
confidence level for 57% of the region probed, which extends up to ~1.2 kpc. A
percentage of 24% of the region studied shows absolute differences at this
confidence level larger than 5 km/s and 7% larger than 10 km/s. The asymmetry
pattern shows mild variations in the vertical direction and with stellar type.
A first qualitative comparison with spiral arm models indicates that the arms
are unlikely to be the main source of the asymmetry. We briefly discuss
alternative origins. This is the first time that global all-sky asymmetries are
detected in the Milky Way kinematics, beyond the local neighbourhood, and with
a purely astrometric sample.
