Coordinated observations of the incoherent scatter radar at Arecibo, Puerto Rico, and total electron content/scintillation measurements made by using the 137-MHz transmissions from the geostationary satellite SMS 1 from the nearby station of Ramey are studied to determine the background ionospheric conditions necessary for the generation of kilometer scale irregularities in the midlatitude ionosphere. It is found tha the well-known midnight descent or collpse of the F region known to occur in this part of the world is sometimes associated with increases in the bottomside electron content, as well as large scintillation events (~10 dB). From a measurement of vector ion velocities and electron densities during such events, it is determined that the scintillations occur in a region of eastward and northward electric fields coupled with a northward directed density gradient caused by the northward propagation of the midnight collapse. The role of possible plasma instability mechanisms, specifically the E¿B gradient drift instability and the Perkins instability, is discussed in the formation of the kilometer scale irregularities giving rise to VHF scintillations.