HALOE OBSERVATIONS OF A SLOWDOWN IN THE RATE OF INCREASE OF HF IN THELOWER MESOSPHERE

A statistical analysis of monthly averaged HF measurements from HALOE shows definite signs of a slowdown of the increase in HF at 55 km. The data used in this study are monthly averaged HF data from a latitude band from 50N to 50S at an altitude of 55 km. We find that the combina tion of a third-order polynomial, an annual and semi-annual cycle, and a 900-day cycle provides a good model of the data. The changes in the HF trend are consistent with the trends of CFC-11 and CFC-12 in the t roposphere in the early 1990's, adjusted by an estimated lag time to a ccount for transport from the troposphere up to 55 km.