U.S. scientists interested in the physics and chemistry of the
upper atmosphere (i.e., that region above the Earth's stratosphere,
encompassing the mesosphere [
50-100km], the mesopause region
[
85-100 km], the lower thermosphere [
100-180km], and
the upper thermosphere [
180-1000km]) have increasingly come
to recognize the importance of atmospheric waves of all types.
Gravity/buoyancy waves, planetary waves, and tides pervade the
upper atmosphere and are, in fact, ubiquitous features of all
planetary atmospheres. Upper atmospheric waves generated by
dynamical processes propagate vertically and horizontally,
dissipate, interact non-linearly, and profoundly influence the
flows of momentum, energy, and constituents on a global basis.
These waves also pass vertically through the various regions or
``spheres'' (troposphere, stratosphere, etc.) of the atmosphere
and, in so doing, impel scientists in previously compartmentalized
subdisciplines to assess the effects of the processes that couple
these regions. For example, tropospheric gravity waves generated by
air flow over topographic features can propagate vertically through
the stratosphere and dissipate within the mesosphere [e.g.,
Fritts, 1994]. These ``breaking'' waves alter the global thermal
and wind structures, producing a large mesopause temperature
anomaly (cold summer pole, warm winter pole). The mesopause
turbulence generated by breaking gravity waves on Earth is
substantial and has been noted by astronauts during the gentle
buffeting of the re-entering Space Shuttle [A. England,
private communication, 1994].
A more complete understanding of wave formation, propagation and dissipation is a major goal of present-day upper atmospheric science and is a driving motivation behind various ongoing and planned U.S. science initiatives. Two such programs are the National Science Foundation (NSF) Coupling, Energetics, and Dynamics of Atmospheric Regions (CEDAR) initiative and the National Aeronautics and Space Administration (NASA) Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) mission, planned for launch in 1998.
The mesosphere--lower thermosphere (MLT), between
50--180 km altitude, is a region of transition where wave and
wave-dissipative processes are particularly important drivers for
atmospheric energetics and dynamics. In the MLT, upward-propagating
tides and gravity waves dissipate and deposit their energy and
momentum into the ambient air. Also, highly variable energy
deposition rates due to solar extreme ultraviolet (EUV) insolation
and magnetospheric particle and electrical heating first maximize
and then fall off abruptly. The MLT is a key transition region
between the layers of the atmosphere associated with weather and
those associated with space flight; the complex dynamics and
energetics of the MLT are controlled by coupling processes that are
highly variable in both time and space.
While our understanding of the MLT region is still quite limited (compared with, for example, the troposphere and the upper thermosphere), recent coordinated experimental and theoretical work is beginning to bear important fruit. Major experimental efforts include: (1) optical wind observations from the Upper Atmosphere Research Satellite (UARS) [ Morton et al., 1993; Wu et al., 1993; Lieberman et al., 1993; Hays et al., 1994]; (2) ground radar measurements using Mesosphere-Stratosphere (MST) radar, medium/high frequency (MF/HF) radar, and/or incoherent scatter radar measurements of winds, temperatures, and neutral densities in the mesosphere and lower thermosphere with comparisons to theoretical modeling results [ Clark and Salah, 1991; Johnson, 1991; Johnson and Virdi, 1991; Manson et al., 1991a; Reese et al., 1991; Salah et al., 1991; Fritts and Isler, 1992; Franke and Thorsen, 1993; Johnson and Luhmann, 1993; Clark et al., 1994; Forbes et al., 1994; Salah, 1994; Salah et al., 1994]; (3) optical measurements of mesopause emissions and the theoretical interpretation of these emissions, including the effects of gravity waves [ e.g., Hecht et al., 1991; Taylor and Hill, 1991; Ross et al., 1992; Murti et al., 1993; Hickey et al., 1992; 1993; Hecht et al., 1994; Mende et al., 1994; Walterscheid et al., 1994]; and (4) lidar measurements of the mesopause region using backscattering from the sodium layer [ Bills and Gardner, 1993; She et al., 1991; 1993].
Theoretical modeling approaches have been further developed and refined over the quadrennium to deal with various aspects of the MLT, including the tides and planetary waves [e.g., Forbes and Salah, 1991 ; Forbes and Vial, 1991; Hagan et al., 1993; Forbes et al., 1993; Forbes, 1994], gravity wave processes [e.g., Hines, 1991; Fritts, 1994; Dewan, 1994; Palmer et al., 1994; Gardner, 1994; Fuller-Rowell, 1994b], and the general circulation [e.g., Roble and Ridley, 1994; Fuller Rowell et al., 1991; Mikkelsen and Larsen, 1991; 1993; Chan et al., 1994].
The following sections highlight some of the recent research efforts performed by U.S. scientists in the area of wave and wave-dissipative processes in the upper atmosphere. Section 2 sets the stage by summarizing some of the key recent results on the global energetics and dynamics of the MLT region. Section 3 describes progress in the area of tidal specification and interactions. Section 4 discusses recent work related to gravity waves and their methods of observation. Section 5 outlines some of the efforts to convert our present theoretical understanding of the Earth's upper atmosphere for use in an operational setting (e.g., for predictions of satellite drag). Finally, section 6 provides a brief summary and discussion of future efforts.