A STUDY OF SEMI-ANNUAL DENSITY VARIATION IN UPPER ATMOSPHERE FROM 1958 TO 1966 BASED ON SATELLITE DRAG ANALYSIS

The semi-annual variation has been isolated in the drag data of six artificial satellites with perigee heights ranging from 250 to 658 km in the interval from 1958 through 1966 by suppression of all other known types of upper atmosphere variation with the help of empirical formulas. Although the shape of the curve is somewhat variable from year to year, the semi-annual oscillation is a very stable feature and can be followed without any major change in phase throughout the 8 yr covered by the observations. The temperature curves obtained from each of the six satellites are strictly in phase and show the same amplitude, irrespective of perigee height; peculiarities of the variation, such as an unusually broad minimum or a late maximum, are easily recognizable in each of the individual satellite curves. This shows that the semi-annual variation is worldwide and that the observed density variations are the result of temperature variations at essentially the same atmospheric level as those arising from the solar-activity effect. All previously known features of the semi-annual variation, such as the systematic inequality in the maxima and minima and the proportionality between the amplitude of the temperature variation and the 10.7-cm solar flux, are confirmed. © 1969.