Note on the semiannual effect in the thermosphere.

There are two external heat sources known that oscillate with a period of half a year: (1) the solar heat input associated with the semiannual migration of the subsolar point between both hemispheres, which peaks at the equator and which is small in comparison with that in the annual component, and (2) the auroral heat input associated with the semiannual component in the occurrence of magnetic storms. An attempt is made to show that a number of apparent conflicts in the description and interpretation of the semiannual effect can be resolved by considering some of the dynamic properties of the thermosphere.