A five chord, high resolution x-ray spectrometer array has been used to measure vertical brightness profiles of heliumlike argon emission from Alcator C-Mod plasmas, out to the last closed flux surface. During standard Alcator C-Mod operation, with the X-point and the ion B x VB drift down, the heliumlike argon brightness is a factor of -8 larger at the top of the plasma than at the bottom, near the plasma edge. In these edge regions, where the electron temperature is low, upper levels of observed transitions are populated by radiative recombination of hydrogenlike argon. This up-down brightness asymmetry can be explained by a factor of ~8 enhancement of hydrogenlike argon at the top of the machine at r/a = .9 . This implies a vertical impurity drift from inside the plasma since the hydrogenlike argon is born near the plasma center. With the ion BxVB drift up, the enhancement switched to the bottom of the machine, but did not change when the X-point was moved up. This asymmetry agrees qualitatively with the predictions of neo-classical parallel impurity transport.
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