Ariel 4 data (Goodall et al., 1973) of February 4 to May 5 1972 have been used to investigate the cusp boundaries. Data with the same local time (LT) and magnetic local time (MLT) around noontime and at high invariant latitudes (INL) have been used to represent the cusp area. (1) For noontime hours (MLT = 1200 and LT = 1200) high electron density values are observed. (2) The boundary towards the equator of the high density region starts at 76 degrees-77 degrees INL and the poleward boundary is at about 82 degrees INL. Therefore, high electron densities are observed along an INL interval of 5 degrees-6 degrees at the Ariel 4 heights of 500-600 km, agreeing with other investigations.