A medium frequency interferometer (MFI) has been established in Sondrestrom, Greenland, to measure the direction of arrival (DOA) of auroral roar emissions at 2.5--3.0 MHz. Combined with electron density data from the Sondrestrom incoherent scatter radar (ISR), these DOA data provide a powerful test of the most-cited generation mechanism of auroral roar emissions. This mechanism requires that the relationship between the electron plasma frequency (fpe) and the electron gryofrequency (fce) in the source regions is fpe2=(n2-1)fce2, where n=2,3 is the harmonic number of the emission. For each of five auroral roar events studied, to within experimental uncertainty this frequency-matching condition is met throughout the source region of the emissions inferred from the MFI and ISR data. Furthermore, ray-tracing analysis indicates that signals at the roar frequency are able to propagate from the source region to the location of the MFI receiver. ¿ 2001 American Geophysical Union