Scale height in the upper atmosphere, derived from changes in satellite orbits

The 'density scale height’ H in the upper atmosphere is a measure of the rate at which air density ρ varies with height y, being given by H = — ρ/(dρ/dy). The value of H , although important because (with the molecular weight of the air) it determines the air temperature, has not as yet been well determined at heights above 200 km. This paper develops methods for finding H from the decrease in a satellite’s perigee height and from the decrease in the orbital period of a satellite in a small-eccentricity orbit. These methods are then applied to all the 14 satellites found suitable for the purpose. The 44 values of H obtained, for heights of 200 to 450 km, represent an average over day and night and probably have errors (s.d.) of 5 to 10 %. It is found that, as solar activity declined between 1957 and 1961, H decreased greatly: e.g. at height 275 km, H decreased from 60 km in early 1958 to 40 km in 1960-61. The results also show that the increase of H with height becomes much less rapid above 350 km, and are consistent with the supposition that H had low values, near 35 km, at heights near 250 km, for 1959-61, The results could be greatly extended in scope and improved in accuracy if more accurate orbits were available for short-lifetime satellites.