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We examine the \alpha-element abundance ratio, [\alpha/Fe], of 5620 stars,observed by the Sloan Extension for Galactic Understanding and Explorationsurvey in the region 6 kpc < R < 16 kpc, 0.15 kpc < |Z| < 1.5 kpc, as afunction of Galactocentric radius R and distance from the Galactic plane |Z|.Our results show that the high-\alpha\ thick disk population has a short scalelength (L_thick ~ 1.8 kpc) compared to the low-\alpha population, which istypically associated with the thin disk. We find that the fraction ofhigh-\alpha\ stars in the inner disk increases at large |Z|, and thathigh-\alpha\ stars lag in rotation compared to low-\alpha\ stars. In contrast,the fraction of high-\alpha\ stars in the outer disk is low at all |Z|, andhigh- and low-\alpha\ stars have similar rotational velocities up to 1.5 kpcfrom the plane. We interpret these results to indicate that different processeswere responsible for the high-\alpha\ populations in the inner and outer disk.The high-\alpha\ population in the inner disk has a short scale length andlarge scale height, consistent with a scenario in which the thick disk formsduring an early gas-rich accretion phase. Stars far from the plane in the outerdisk may have reached their current locations through heating by minor mergers.The lack of high-\alpha\ stars at large R and |Z| also places strictconstraints on the strength of radial migration via transient spiral structure.

A Short Scale Length for the \alpha-Enhanced Thick Disk of the Milky Way: Evidence from Low-Latitude SEGUE Data