Anomalos behavior of the total hydrogen mixing ratio around the Turbopause
Abstract
More than a quarter of century ago Hunten and Strobel [1974] showed, under the condition of a hydrogen flux through the middle atmosphere, that the total hydrogen mixing ratio (THMR) decreases increasingly monotonically with height when molecular diffusion is neglected. On the basis of a simplified 1D-model the authors also found a small reduction of the THMR when molecular diffusion is considered, but they stated that the total mixing ratio of hydrogen atoms remains nearly the same within the model7-2 -1 domain. Among other things Hunten and Strobel took only a small value of 5×10 cm s for the hydrogen escape flux according to the thermal escape flux into calculation. Based on a global 3D-model of the dynamics and chemistry of the middle atmosphere×8-2 - 1 using an essentially higher value of the hydrogen escape flux of 3 10 cm s corresponding to the most recent results, we show that the THMR decreases considerably. The effect especially occurs above the mesopause, reaching a marked minimum at about 108 km. The cause of this seemingly paradoxical behavior is that under the condition of an escape flux just in the domain of transition of predominant turbulent diffusion to prevailing molecular diffusion, a strong decomposition of the heavier H2O component into the light hydrogen constituents H and H2 takes place. In addition to these conditions, the temperature increases strongly with height above the mesopause, influencing the molecular diffusion coefficient. The distinct decrease of the THMR within the extended mesopause region is exceedingly important for questions regarding the chemistry and physics of this domain. Such queries address problems such as long- term trends in this domain, investigations of NLCs and PMSEs, calculation of the* chemical heating rates, exploration of the OH -layer, or the upper boundary conditions of models which have their upper border just at this critical height. Hunten, D. M. and D.F. Strobel, Production and escape of terrestrial hydrogen, J. Atmos. Sci., 31, 305-317, 1974.
- Publication:
-
34th COSPAR Scientific Assembly
- Pub Date:
- 2002
- Bibcode:
- 2002cosp...34E1355S