Unlike adjacent portions of the Earth's atmosphere, the mesosphere
is a region where relatively little direct solar energy is
deposited. The EUV component of sunlight is primarily absorbed
above the mesosphere and the UV component is primarily absorbed
below. The mesosphere is heated by: (1) solar absorption in the
Hartley bands of Ozone, between 242 and 310-Å
where ozone efficiently absorbs UV radiation [ Mlynczak and
Solomon, 1991a]; (2) quenching of the O(
D) metastable species
generated by photolysis of ozone and O
; (3) the release of
significant amounts of stored chemical potential energy due to
atomic oxygen (3-body) recombination and to the exothermicity of
the reaction of hydrogen and ozone [ Mlynczak and Solomon,
1991b]; (4) poorly understood dynamical interactions in which
gravity waves and tides dissipate and a portion of this energy is
transferred from macroscopic to microscopic motions [ Gavrilov
and Roble, 1994]; and (5) adiabatic compressional heating due to
vertical motions, some of which is related to tides [ Miyahara
et al., 1991, 1993]. The mesospheric cooling rate is dominated by
radiative processes involving CO
, NO, O and O
[ Mlynczak
and Solomon, 1993], with non-local thermodynamic equilibrium
(NLTE) radiation from CO
playing the dominant role. The global
mean heat balance of the mesosphere has been recently reviewed by
Roble, [1994].
In stark contrast to the mesosphere, the energetics of the lower thermosphere is controlled by several competing, direct heating processes. Here, energy-deposition due to the effects of: (1) EUV absorption; (2) magnetospheric auroral particle precipitation; and (3) dissipation of electric currents in the local ionosphere arising from various sources all maximize. The high variability of the solar EUV flux [ Lean, 1991] means that, at any given time, the lower thermosphere may be dominated by energy coming from the Sun, the magnetosphere, or the lower atmosphere. Of course, magnetospheric inputs often dominate at high latitudes.
While significant progress in modeling the energetics of the MLT has been made, there is a growing body of experimental evidence (particularly from Lidar observations near the mesopause) that the temperature profile is more complicated than typical textbook representations depict [e.g., She et al., 1994] and much theoretical and experimental work remains to be done. R. G. Roble and colleagues at the National Center for Atmospheric Research have further developed two important theoretical tools during the past four years: (1) a global mean energy-balance model containing a detailed specification of the relevant aeronomical processes [ Roble, 1994] and (2) a Thermosphere-Ionosphere-Mesospher e-Electrodynamics General Circulation Model (TIMEGCM) extending from 30 to 500 km altitude [ Roble and Ridley, 1994]. The latter model is an extension (to lower altitudes) of a series of very successful numerical models of the thermosphere and ionosphere. Figures 1a and 1b, illustrate, respectively, calculated major heating and cooling terms operating on the MLT [ Roble, 1994]. Clearly, the energy balance is complex, with numerous competing processes dominating in different altitude regions.