Winter bulge and diurnal variations in hydrogen inferred from AE‐C composition measurements

The atomic hydrogen distribution at 250 km during December 1974 solstice has been inferred, considering charge exchange equilibrium, from Atmosphere Explorer‐C measurements of n (H + ), n (O + ), and n (O). An empirical model, derived from the observations by least square analysis in terms of spherical harmonics, has the following characteristics: (a) n (H) increases by as much as a factor of two between the summer and winter hemispheres, (b) the n (H) diurnal variation is largest at the equator where n max/ n min = 3.2, the maximum occurring at 03 h LT, and (c) the diurnal variation is larger in the winter hemisphere ( n max/ n min = 2.6 at +40°) than in the summer ( n max/ n min = 2.3 at −40°). Similar analysis of the gas temperature derived from n (N 2 ) measurements reveals that all n (H) and T g spherical harmonic coefficients are anticorrelated. Both the diurnal and latitudinal (annual) n (H) and T g amplitudes are in substantial agreement with the “zero flux condition,” in which exospheric flow dominates the hydrogen distribution. The observed diurnal phase lag of n (H) with respect to T g is about one hour, agreeing with theory. During the period of measurement the observed mean global values of T g and n (H) were 800K and 3.6 × 10 5 cm −3 , respectively, the latter exceeding the Kockarts‐Nicolet model concentration by a factor of two.