Semi-annual Variation in Thermospheric Density

Abstract

KING-HELE¹ has reported a semi-annual variation in thermospheric density at about 190 km with a maximum to minimum ratio of 1.45. Cook's hypothesis² that the semi-annual variation is caused by the modulation of the helium content of the lower thermosphere, originally coined to account for the variation at 1,100 km, clearly cannot be applied to explain changes at 190 km. The reconciliation between theory and observation is in essentially the same state as it was a year ago. There is the hypothesis of Anderson³ that the semi-annual variation results from a migration of the perigee points of the satellites studied with latitude in such a manner that the drag at perigee is small when over the winter pole which is supposed to be cold (and of lower density than the equatorial regions). Debate is continuing about Anderson's hypothesis^4,5 and no doubt the issue can be resolved when sufficient values of density for a wide range of perigee latitudes and over all seasons are available, such as the simultaneous changes at 190 and 1,100 km. Anderson quotes as support my colleagues' computations⁶, which show a cold winter pole. The influence of motions, however, was neglected in these computations and it has recently been shown that the influence of vertical motions is quite substantial (ref. 7 and unpublished work of R. E. Dickinson, C. P. Lagos and R. E. Newell). Satellite drag observations by Jacchia⁸ and Keating and Prior⁹ show higher densities over the winter pole than over the summer pole. Jacchia and Slowey¹⁰ could reduce—but not eliminate—the effect by manipulating parameters in their model; indeed, the effect was predicted before observation¹¹ and seems to increase with altitude as expected from theory.