The midlatitude Super Dual Auroral Radar Network (SuperDARN) radars regularly observe nighttime lowvelocity SubAuroral Ionospheric Scatter (SAIS) from decameterscale ionospheric density irregularities during quiet geomagnetic conditions. To establish the origin of the density irregularities responsible for lowvelocity SAIS, it is necessary to distinguish between the effects of high frequency (HF) propagation and irregularity occurrence itself on the observed backscatter distribution. We compare range, azimuth, and elevation data from the Blackstone SuperDARN radar with modeling results from ray tracing coupled with the International Reference Ionosphere assuming a uniform irregularity distribution. The observed and modeled distributions are shown to be very similar. The spatial distribution of backscattering is consistent with the requirement that HF rays propagate nearly perpendicular to the geomagnetic field lines (aspect angle 1 degrees). For the first time, the irregularities responsible for lowvelocity SAIS are determined to extend between 200 and 300 km altitude, validating previous assumptions that lowvelocity SAIS is an Fregion phenomenon. We find that the limited spatial extent of this category of ionospheric backscatter within SuperDARN radars' fieldsofview is a consequence of HF propagation effects and the finite vertical extent of the scattering irregularities. We conclude that the density irregularities responsible for lowvelocity SAIS are widely distributed horizontally within the midlatitude ionosphere but are confined to the bottomside Fregion.