An enhancement in the plasma density occurs in more than half of the ISEE-1 and 2 magnetosheath passes near the stagnation streamline just in front of the magnetopause. On average, this structure is about 0.4 Re thick and clearly separated from the magnetopause. The average density enhancement is about 44% above the ambient magnetosheath density. The anticorrelation of the magnetic field and density is that expected for the MHD slow mode. Strong fluctuations in the density within the structure are usually accompanied by fluctuations in velocity. ¿American Geophysical Union 1990