  We report on the first X-ray proper-motion measurements of the
nonthermally-dominated forward shock in the northeastern limb of SN 1006, based
on two Chandra observations taken in 2000 and 2008. We find that the proper
motion of the forward shock is about 0.48 arcsec/yr and does not vary around
the rim within the ~10% measurement uncertainties. The proper motion measured
is consistent with that determined by the previous radio observations. The mean
expansion index of the forward shock is calculated to be ~0.54 which matches
the value expected based on an evolutionary model of a Type Ia supernova with
either a power-law or an exponential ejecta density profile. Assuming pressure
equilibrium around the periphery from the thermally-dominated northwestern rim
to the nonthermally-dominated northeastern rim, we estimate the ambient density
to the northeast of SN 1006 to be about 0.085/cm^3.
