Exospheric densities near solar minimum derived from the orbit of Echo 2

A discussion on orbital analysis - Comparison of air densities obtained from orbital decay and instruments

“…For helium in particular the variations are much too large to be due to changes in exospheric temperature alone and must be attributed to changes in the boundary conditions at the height where diffusive equilibrium begins. This is in accord with the possible explanation (Cook 1967) put forward recently for the large semi-annual variation in exospheric density revealed by analysing the orbit of Echo 2 (Cook & Scott 1966). The daytime helium concentrations given in Fig.…”

“…So far very few experimental measurements of accommodation coefficients have been made in this energy range. Over the past few years, however, a number of theoretical studies have been made and these have indicated the important factors affecting the energy accommodation (Cook 1966. Although, at present, there is perhaps not sufficient reason to abandon the value of 2*2 which has been widely used in recent years for CD ) it must be recognized that this value is subj certainty and may be too low, perhaps by as much as 15%, at the heights where gauge data is available.…”

Semiannual variation in the heterosphere: A reappraisal

“…My more recent models, however [Jacchia, 1970[Jacchia, , 1971], in which the constant boundary conditions were moved to 90 km, also proved inadequate to represent the observed semiannual density variation in the 150-to 180-km region. Cook [1969b] actually found that the semiannual variation, still in phase with that in the thermosphere and exosphere, can be discerned even at 90 km, which is a near-isopycnic level at which all other den-SE•TA•VAT• VART•TTO• T• sity variations nearly vanish [Cole, 1961;Groves, 1970]. The range in density found by Cook at 90 km amounts to about 25%, but my reanalysis of his data by eliminating high-latitude measurements, which are affected by nual (seasonal) variations, reduces the range 15%, which is still a respectable amount.…”

The bakerian lecture, 1974 A view of earth and air

“…This effect occurs at all heights between 100 and 1000 km, and the maximum density exceeds the minimum by a factor of about 1.5 at 200 km, increasing to 2.5 at 500 km and decreasing to about 2 at 1000 km (Cook 1969). The variation is particularly clear at heights near 1000 km when solar activity is low (Cook & Scott 1966), but it is also readily apparent at lower heights when solar activity is higher: figure 11 shows variations at a height of 230 km in 1970-71 (Walker 1974). The strength of the semi-annual variation is not directly related to solar activity, but varies from year to year, perhaps quite irregularly (Voiskovskii et al 1973), or perhaps exhibiting a recur rence period of about 3 years Cook 1972).…”