Thermal structure of the Arctic summer mesosphere

In the last 10 years a total of 89 falling spheres (FS) have been launched at high latitudes (similar to 70 degrees N) in the summer season between la te April and late September. From this experimental technique, densities an d temperatures in the mesosphere and upper stratosphere (similar to 95-35 k m) are deduced which represent nearly the entire data set regarding the the rmal structure in the high-latitude summer mesosphere where optical methods have problems to give reliable results because of the large solar photon b ackground. Some of the launches took place at times in the season where no measurements have been performed before. The seasonal variation of the mean temperatures and densities derived from the FS measurements deviates signi ficantly from the latest empirical models, in particular, in the upper meso sphere during summer. For example, at the summer mesopause (88 km) the FS t emperatures are lower by more than 10 K compared to CIRA-1986 in the time p eriod from the beginning of June until the end of August. The thermal struc ture in the upper mesosphere is rather persistent throughout the core summe r months and changes rapidly in the winter-summer transition at mid-May, an d vice versa at mid-August. For example, at typical noctilucent cloud altit udes (82 km) the mean temperature is in the range 153+/-3 K from the beginn ing of June until mid-August but changes by, typically, 5-10 It per week be fore and after this period. A comparison of the FS temperatures with the oc currence probability of noctilucent clouds and polar mesosphere summer echo es suggests that the thermal structure is the main controlling factor for t hese layers, whereas other ingredients required to form aerosol particles, such as water vapor or condensation nuclei, are of secondary importance.