Data obtained during elevation scans with the Chatanika incoherent scatter radar have revealed the presence of ionization enhancements in the nightside auroral F region with scale sizes ranging from tens of kilometers (medium scale) to hundreds of kilometers (large scale). We have attempted to identify the sources of these enhancements using the radar data and two models. One model combines a prespecified ionization source region with a realistic high-latitude convection model so that the plasma distribution produced by various source regions can be compared with that mapped out by the radar measurements. The second model computes the distortions of drifting plasma blobs produced by convection in the absence of ionization sources and losses. The data and modeling indicate that the Harang discontinuity is an important source region for large- and medium-scale ionization enhancements and that distortion by convection of large-scale ionization enhancements can produce smaller-scale enhancements even in the absence of plasma instabilities or other structuring mechanisms.