A kinematic spiral arm shock signature: "Ringing" in the vertical motion   of stars
  We analyze the motion of stars in the direction perpendicular to the galactic plane following a spiral arm passage. We show that the fast change in the vertical galactic potential causes a thermalized distribution to develop a distinctive "ringing"-like non-thermal signature. We use A and F-stars from the extended Hipparocos catalogue to show that a spiral arm passage (or passages), with an amplitude (or randomly combined amplitudes) of at least {\delta}{\rho} / {\rho} > ~0.15 must have taken place in the past (with {\rho} being the total background density). Presently, the local stellar distribution within ~100 pc of the plane, appears (at the 2.5{\sigma} level) to be contracting towards it. 
Nir J. Shaviv
astro-ph.GA