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Recent measurements of the positron/electron ratio in the cosmic ray (CR)flux exhibits an apparent anomaly, whereby this ratio increases between 10 and100 GeV. We show that inhomogeneity of CR sources on a scale of order a kpc,can naturally explain this anomaly. If the nearest major CR source is about akpc away, then low energy electrons ($\sim 1$ GeV) can easily reach us. Athigher energies ($\gtrsim 10$ GeV), the source electrons cool via synchrotronand inverse-Compton before reaching Earth. Pairs formed in the local vicinitythrough the proton/ISM interactions can reach Earth also at high energies, thusincreasing the positron/electron ratio. A natural origin of sourceinhomogeneity is the strong concentration of supernovae in the galactic spiralarms. Assuming supernova remnants (SNRs) as the sole primary source of CRs, andtaking into account their concentration near the galactic spiral arms, weconsistently recover the observed positron fraction between 1 and 100 GeV.ATIC's electron excess at $\sim 600$ GeV is explained, in this picture, as thecontribution of a few known nearby SNRs. The apparent coincident similaritybetween the cooling time of electrons at 10 GeV (where the positron/electronratio upturn), $\sim 10$ Myr, and the CRs protons cosmogenic age at the sameenergy is predicted by this model.

Inhomogeneity in the Supernova Remnant Distribution as the Origin of the PAMELA Anomaly