中高层大气物理学第一章Horizontalstratification水平分层.ppt

LOGO 2011 PHYSICS OF THE MIDDLE AND UPPER ATMOSPHERE中高层大气物理学Horizontal stratification Recall from last lecture The boundary at altitude of 100 km Homosphere 均匀层 Heterosphere 非均匀层 Scale height H Mixed atmosphere (below ~100 km) m is the mean molecular weight of atmospheric gases Diffusively separating atmosphere ( above ~100 km) mi is the molecular weight of individual species Upper boundary Exobase Homosphere Homosphere (turbosphere) Composed of N2 , O2 and 1% Ar Mixed by large scale wind and small scale turbulence (eddy diffusion 涡流扩散) Homopause (turbopause) Where eddy diffusion and molecular diffusion rates are equal ~100 km Heterosphere Heterosphere Molecules start to dissociate, O2-O Temperature increases with height and the atmosphere is stabilized. Frictional forces between various gases vanish. Different constituents behaving independently. Lighter species (H, He) get more and more with altitude. Layer of He and H+ Diffusive separation Barosphere Barosphere (气压层) In hydrostatic state, particle distribution is determined by gravitational binding and molecular diffusion. Scale height Sufficient collisions for a Maxwellian velocity distribution Equilibrium between gravitational force and pressure gradient Exobase (500-600 km) Where mean free path and scale height are equal , cross-section of collisions 碰撞截面 , most probable velocity 最可几速率 Exosphere Exosphere (散逸层, 外层) Where escape (evaporation) of gas particles is possible Collision frequency of an atmospheric molecule: ~105 s-1 (at 80 km), ~10-7 s-1 (at the exobase) Mean free path of the molecules: ~10-2 m (at 80 km), several 10s km (at the exobase) A molecule moving upward without experiencing collisions With enough energy (escape velocity) H与O哪种成分更容易逃逸？ Hydrostatic equation validation In exosphere, the atmosphere has to be regarded as an assembly of individual molecules or atoms, instead of as a single fluid. There is significant loss of faster molecules which changes the M