Recent observations with the new mid-latitude SuperDARN HF radar located at Wallops Island, Virginia have identified a class of ionospheric irregularities that is prevalent in the nightside sub-auroral ionosphere under low-to-moderate Kp conditions. These irregularities can be observed for many hours and generally exhibit very low Doppler velocities. A recent collaborative experiment using the Wallops radar and the Millstone Hill incoherent scatter radar has determined that these irregularities are located at the ionospheric footprint of the plasmapause and in a region of opposed electron density and electron temperature gradients. We conclude that the irregularities are produced by the temperature gradient instability (TGI) or by turbulent cascade from primary irregularity structures produced from this instability. This is the first experimental confirmation that the TGI is effective in producing decameter-scale ionospheric irregularities.