The middle-latitude ionization trough, the plasma density depletion in the subauroral region, has been extensively studied since its discovery in the 1960s. Our knowledge of the variability of the trough is mostly based on the observations in the northern hemisphere. Recently, the COSMIC observations enabled the investigation of the trough in both hemispheres at all local time. However, the investigation was limited to the period of the low solar activity. In this study, we investigate the variability of the trough location and morphology with local time, solar cycle, magnetic activity, and interplanetary magnetic field. For this purpose, we analyze the DMSP F15 data acquired during 2000-2010.