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The study of the chemistry and physics of the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has primarily been the domain of meteorology.

The chemical and physical properties of the atmosphere and the changes that result from external and internal forces impact all altitudes and global distributions of atoms, molecules, ions, and electrons, both in composition and in density. Dynamical effects are seen in vertical and horizontal atmospheric motion, and energy is transferred through radiation, chemistry, conduction, convection, and wave propagation.

The atmosphere of the Earth is separated into regions defined by the variation of temperature with height. In the middle atmosphere, that region of the atmosphere between the tropopause and the mesopause (10–100 km or 6–60 mi), the temperature varies from 250 K (−9.7°F) at the tropopause to 300 K (80°F) at the stratopause and back down to 200 K (−100°F) at the mesopause . These temperatures are average values, and they vary with season and heat and winds due to the effect of the Sun on the atmosphere. Over this same height interval the atmospheric density varies by over five orders of magnitude. Although there is a constant mean molecular weight over this region, that is, a constant relative abundance of the major atmospheric constituents of molecular oxygen and nitrogen, there are a number of minor constituents that have a profound influence on the biosphere and an increasing influence on the change in the atmosphere below the tropopause associated with the general topic of global change. These constituents, called the greenhouse gases (water vapor, ozone, carbon dioxide, methane, chlorine compounds, nitrogen oxides, chlorofluorocarbons, and others), are all within the middle atmosphere.Greenhouse effect Mesosphere Stratosphere Troposphere

Understanding the aeronomy of the middle atmosphere requires the study of the physical motion of the atmosphere. The particular composition and chemistry at any given time, location, or altitude depends on how the various constituents are transported from one region to another. Thus, global circulation models for both horizontal and vertical motions are needed to completely specify the chemical state of the atmosphere. In understanding the dynamics of the middle atmosphere, internal gravity waves and acoustic gravity waves play significant roles at different altitudes, depending on whether the particle motion associated with the wave is purely transverse to the direction of propagation or has some longitudinal component. These waves originate primarily in meteorological events such as wind shears, turbulent storms, and weather fronts; and their magnitude can also depend on orographic features on the Earth's surface. Atmospheric chemistry

The upper atmosphere is that region above the middle atmosphere that extends from roughly 100 km (60 mi) to the limit of the detectable atmosphere of the planet. This region is characterized by an increasing temperature until it reaches a constant exospheric temperature. There is a slow transition from the region of constant mean molecular weight associated with the middle atmosphere to that of almost pure atomic hydrogen at high altitudes of the exosphere. This is also the region of transition between transport dominated by collision and diffusion, and transport influenced by plasma convection in the magnetic field. The neutral density varies by over ten orders of magnitude from one end to the other and is dominated by molecular processes in the high-density region and an increasing importance of atomic, electron, and ion processes as the density decreases with altitude.

As the Sun sets on the atmosphere, dramatic changes due to the loss of the solar radiation occur. The excitation of the atmosphere declines, and chemical reactions between the constituents become more and more dominant as the night progresses. When observed with very sensitive instruments, the night sky appears to glow at various colors of light. Prominent green and red atomic emissions due to oxygen at 555.7 and 630 nm, respectively, and yellow sodium light at 589 nm appear, while molecular bands of the hydroxyl radical and molecular oxygen even further in the red collectively contribute most of the total intensity of the nightglow spectrum.

The aurora that appears in the southern and northern polar regions is the optical manifestation of the energy loss of energetic particles precipitating into the atmosphere. The region of highest probability of occurrence is called the auroral oval. At high altitudes, electrons and ions present in the magnetosphere are accelerated along magnetic field lines into the atmosphere at high polar latitudes.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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