中高层大气物理学第一章exoshere外大气层.ppt

LOGO 2011 PHYSICS OF THE MIDDLE AND UPPER ATMOSPHERE中高层大气物理学Exosphere Limit of the atmosphere ρ = 0? An atmosphere in adiabatic equilibrium has a limit. At any level of a planetary atmosphere there will be some particles which move upward with a velocity greater than that required for escape from the gravitational attraction of the planet. These particles are representative of the high velocity tail of the Maxwellian velocity distribution. At high latitudes the collision frequency will be low enough (i.e., the mean free path long enough). This region is called the exosphere. Cone of Escape Obtain the limit of the atmosphere by defining the cone of escape. The probability for a molecule travelling a distance z without a collision is give by σ is the gas-kinetic collision cross-section. The mean free path represents the distance for which this probability is 1/e. Exosphere The base of the exosphere zc (exobase) is usually defined by this probability requiring that Exospheric conditions, are therefore assumed to hold at altitudes (Exobase) where the mean free path is equal to or greater than the local scale height. Numerically, nc[cm?3] ≈ 3×109/H[km] for σ ≈ 3×10-15cm?2 and the total content of an exosphere is The exobase may also be called the baropause. The pressure at this level is Escape (Evaporation) If all this kinetic energy in the vertical direction is converted to potential energy, the molecule must be lifted a height eqaul to the half of the scale height A molecule travelling with the most probable velocity, moving upward would reach a height equal to H A particle’s kinetic energy must be larger than its potential energy to escape times the circular orbital velocity at the Earth’s surface vesc = 11.2km/s at about 2000 km vesc = 9.7km/s Escape Orbits Without collisions, a particle will move freely and describe elliptic, parabolic and hyperbolic paths according to the magnitude of the velocity acquired at the last collision