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A large number of temperature profiles of the altitude range 80 to 105 km were obtained between 71°S and 54°N latitude from late April to early July 1996. The measurements were performed by a ship‐borne lidar, resolving the Doppler broadening of the K(D1) resonance line. The most notable result of this field campaign is the finding that throughout our observation series the mesopause altitude was located at altitudes of either 100±3 km or 86±3 km. The high “winter” level extended from 71°S to 23°N, the low “summer” level from 24°N until the end of the field observations at 54°N. Our latitudinally distributed observations indicate strongly a worldwide bimodal character of the mesopause altitude. Furthermore, our mesopause temperatures are generally lower than previonsly measured in the northern hemisphere.

Abstract. Observations of the nighttime atmospheric potassium layer were performed on the German research vessel Polarstem from March to June 1996. K density profiles were obtained between 71øS and 45øN. The nightly mean peak densities ranged from 140 cm -3 in the equatorial region to 10 cm -3 in the Antarctic, and the column abundances decreased from 1.2 x 108 to 1.3 x 107 cm -2 going from low to high latitudes. High peak densities and column abundances were also commonly observed together with sporadic K layers. The global mean peak height of the normal (background) K layer was found to be 88.3 km. After the Polarstern campaign, observations were continued at Kfihlungsborn (54øN). The summer and winter K layers, observed during July 1996 and January 1997, were quite different in shape but had similar peak densities and column abundances. A one-dimensional model of the K layer was developed which includes meteoric deposition, vertical transport through eddy diffusion, and a full chemical scheme. This model was able to reproduce very satisfactorily the seasonal behavior of the K layer at 54øN if the wintertime deposition flux of the metal was reduced by 30% compared to the summer. The midlatitude ratio of K to Na was about 1%, much less than either the chondritic or cosmic ratios of the two metals (•8 or 6%, respectively). The most likely reason is that potassium vaporizes less efficiently from meteoroids than sodium, in agreement with a thermodynamic model of a nonideal chondritic magma and observations in the exosphere of Mercury. Finally, the model was generally very successful in reproducing the latitudinal variations in the K layer. IntroductionThe deposition of extraterrestrial material in the Earth's upper atmosphere gives rise to layers of free neutral metal atoms in the altitude range from 80 to 110 km [Plane, 1991]. Investigations of the K layer were initiated by Lytle and Hunten [1959], who were able to estimate the ratio of potassium to sodium. Some years later, Sullivan and Hunten [1962, 1964] determined the peak density and column abundance from weak twilight emission data by using a birefringent filter photometer. These measurements were continued by Gault and Rundle [1969] The first model of mesospheric potassium was developed by Swider [1987] from an existing model of sodium chemistry. He proposed that the seasonal differences between Na and K might be explained by the kinetics of key potassium reactions having smaller temperature dependences than their sodium analogues. However, the sodium model that he used was quite 17,173

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