The Astrophysical Journal · 1960 · 66 citations · 0 references
The source of the Lyman-a radiation incident upon the ionospheric E region at night is here identified as scattering of solar Lyman-a radiation by hydrogen atoms in a corona surrounding the earth and constituting the outer fringe of the neutral component of the earth's atmosphere. Doppler shifts, which would be expected if the Lyman-a radiation originated in a cloud of hydrogen atoms in interplanetary space, provide a compelling reason for believing that the radiation must originate in a cloud which is an extension of the earth's atmosphere. The details of the observed nighttime Lyman-a intensity distribution make it possible to determine the optical thickness of the hydrogen corona around the earth Knowledge of the temperature of the earth's exosphere is required to calculate the vertical distribution of hydrogen, and this temperature is obtained primarily from the observed rate of orbital decay of the Vanguard I satellite, 1958p2. The hydrogen concentration is found to fall from 4 X 10 atoms at an altitude of 550 km to 10 atoms at 7000 km and 10 atoms at 36000 km. The optical thickness of the hydrogen above 120 km is slightly over unity.