The Mariner 10 radio occultation measurements of the ionosphere of Venus

Data from the Mariner 10 radio occultation experiment have been utilized to determine the vertical electron density distribution in the ionosphere of Venus. The ingress measurements, which were made at latitude 1.3 deg N on the nightside of the planet, show two distinct layers. The main layer was located at 142 km altitude and had a peak density of 9000 electrons per cu cm. A secondary layer with a peak density of 7000/cu cm was detected at 124 km altitude. During egress, the ionosphere was probed at latitude 56.0 deg S on the dayside of Venus. The solar zenith angle in this region was 67.0 deg. The dayside ionosphere consisted of a main layer with a peak density of 290,000 per cu cm at 142 km altitude and several minor layers. At the top of the dayside ionosphere, the measurements showed an abrupt drop in the density from 2000 per cu cm at 335 km altitude to below the level of detectability, i.e., less than 200 per cu cm, at 360 km altitude. This abrupt density change may be the ionopause where the solar wind plasma interacts with the ionized components of the atmosphere.