Observation of a thermospheric descending layer of neutral K over Arecibo

We report on the first observation of a descending layer of atomic potassium (K) in the thermosphere. This observation was made with the K Doppler lidar at the Arecibo Observatory in Puerto Rico (18.35 degrees N; 66.75 degrees W) on 12 March 2005. The layer was first observed before 08:00 UT (04:00 AST) centered near 145 km with the vertical extent up to similar to 155 km, and then it descended to near 126 km just over 2 h later at dawn. The descent rate of 2.56 +/- 0.38 m/s matches the vertical phase speed of the GSWM09-computed semidiurnal tide between 120 and 150 km. This also matches the descent rates of the thermospheric semidiurnal tides measured at Arecibo. Although the K density above 120 km remains less than 1 cm(-3), its presence is unequivocal and has strong similarities to the neutral iron (Fe) layers in the thermosphere over 155 km recently discovered by lidar observations at McMurdo, Antarctica. The thermospheric K layer is plausibly explained by radiative electron recombination with K+ within a tidal ion layer, which descends with the downward phase progression of the semidiurnal tide. Based on the production rate of K atoms and using current knowledge of tidal ion layer composition, we calculate an electron density of near 5x10(4) cm(-3) and K+ concentration of 650 cm(-3) at 135 km immediately prior to the layer formation. This discovery of a thermospheric K layer, coupled with the McMurdo discovery of similar Fe layers, may lead to a new approach to studying the thermosphere in the altitude range of similar to 100-150 km with resonance fluorescence lidars. (C) 2013 Elsevier Ltd. All rights reserved.