Venus EUV measurements of hydrogen and helium from Venera 11 and Venera 12

Lyman α and 58.4 nm HeI radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to ⋍ 2,000 km of altitude) the distribution can be described by a classical exospheric distribution with TC = 275 ± 25 K and n = 4−2+3 × 104 atom. cm−3 at 250 km. The integrated number density from 250 to 110 km (the level of CO2 absorption) is 2.1 × 1012 atom. cm−2, a factor of 3 to 6 lower than that predicted by aeronomical models. This number density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a “hot” hydrogen population dominates, which can be simulated by T = 103 K and n = 103 atom. cm−3 at the exobase level. The optical thickness of helium above 141 km (the level of CO2 absorption for 58.4 nm radiation) was determined to be τo = 3, corresponding to a density at 150 km of 1.6 × 106 cm−3. This is about 3 times less than what was obtained with the Bus Neutral Mass Spectrometer of Pioneer Venus, and about twice less than ONMS measurements, but is in agreement with earlier EUV measurement by Mariner 10 (2 ± 1 × 106 cm−3).