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Page 73
4—
Solar Variations and the Upper Atmosphere
Background
Extending from the top of the middle atmosphere to some hundreds
of kilometers into space is the Earth's upper atmosphere (Figure
1.2) and its embedded ionosphere. This tenuous layer of neutral and
charged particles shields the human habitat from high energy solar
radiation and particles, enables part of the extensive
communication network on which society increasingly relies, and is
the medium in which thousands of spacecraft now orbit. Unlike the
relatively placid lower atmosphere, the upper atmosphere is a
region of extreme spatial and temporal variability, constantly
agitated by solar radiative and auroral forcings. Driving the
processes that at any instant define the physical state of the
upper atmosphere and ionosphere is the solar radiation at
wavelengths less than about 180 nm. Many of the region's
continually changing physical phenomena derive directly or
indirectly from changes in this radiation and from the impact of
energetic particles channeled into the upper atmosphere at high
latitudes via the Earth's magnetic field.
While solar variability exerts a dominating influence on the
Earth's upper atmosphere, any direct effect on the biosphere
appears to be more subtle than that exerted by solar forcing of the
middle and lower atmospheres. The fact that the highly variable
upper atmosphere is coupled to the middle atmosphere through
chemical, radiative, and dynamical
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OCR for page 73
Page 73
4—
Solar Variations and the Upper Atmosphere
Background
Extending from the top of the middle atmosphere to some hundreds
of kilometers into space is the Earth's upper atmosphere (Figure
1.2) and its embedded ionosphere. This tenuous layer of neutral and
charged particles shields the human habitat from high energy solar
radiation and particles, enables part of the extensive
communication network on which society increasingly relies, and is
the medium in which thousands of spacecraft now orbit. Unlike the
relatively placid lower atmosphere, the upper atmosphere is a
region of extreme spatial and temporal variability, constantly
agitated by solar radiative and auroral forcings. Driving the
processes that at any instant define the physical state of the
upper atmosphere and ionosphere is the solar radiation at
wavelengths less than about 180 nm. Many of the region's
continually changing physical phenomena derive directly or
indirectly from changes in this radiation and from the impact of
energetic particles channeled into the upper atmosphere at high
latitudes via the Earth's magnetic field.
While solar variability exerts a dominating influence on the
Earth's upper atmosphere, any direct effect on the biosphere
appears to be more subtle than that exerted by solar forcing of the
middle and lower atmospheres. The fact that the highly variable
upper atmosphere is coupled to the middle atmosphere through
chemical, radiative, and dynamical
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mechanisms, and to the troposphere through the global electric
circuit, cannot be ignored. Understanding how the upper atmosphere
varies naturally, and how it may be affected by human activities,
is necessary from a societal and economic perspective because of
the critical role played by the upper atmosphere in communications,
navigation, national defense, and a wide assortment of space
related endeavors, including the presence of humans in space.
Furthermore, current modeling studies indicate that the upper
atmosphere may itself be sensitive to global change caused by human
activities.
Solar EUV and UV Radiation
The Sun's ultraviolet radiation at wavelengths less than about
180 nm varies considerably more than does the UV radiation that is
absorbed in the middle atmosphere and the visible radiation that
penetrates to the Earth's surface (see Figure 1.1). Solar cycle
changes of 100 percent are typical in solar radiation at
wavelengths from 10 to 100 nm; the soft X-rays (1 to 10 nm) that
penetrate to the lowest layers of the upper atmosphere vary by an
order of magnitude. This highly variable energy from the Sun is
deposited entirely in the terrestrial upper atmosphere via
absorption of the primary constituents, O2, N2, and
O. Without heating from the absorption of solar extreme ultraviolet
(EUV) and UV radiation, the thermosphere and the ionosphere would
not exist at all. This heating, which varies with solar activity,
is responsible for the increase of temperature with height above
about 100 km (see Figure 1.2) and for driving most of the bulk
motions of the gases within the entire region. Large variability in
the basic properties of both the thermosphere and ionosphere is the
direct result of the variability in the solar EUV and UV input (as
illustrated in Figure 1.2 by the change in the temperature profile
from minimum to maximum solar activity).
Measurements of Solar EUV Spectral
Irradiance
Current knowledge of the magnitude and variability of the solar
EUV energy deposited in the upper atmosphere is based almost
entirely on a brief four-year period of measurements made by the
Atmosphere Explorer
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(AE-E) spacecraft about 15 years ago. These measurements
revealed a considerable increase in the solar EUV flux during the
ascending phase of solar cycle 21. Some emissions at wavelengths
shorter than 30 nm increased by factors of 10 to 100 between solar
minimum conditions in 1976 and maximum activity in 1980. These
emissions emanate from the highest, hottest layers of the Sun's
atmosphere (the solar corona). Radiation at wavelengths between 30
and 120 nm, formed lower in the solar atmosphere (the
chromosphere), varied somewhat less, by factors of two to three
from the minimum to the maximum of activity in solar cycle 21. At
still longer UV wavelengths, solar cycle variability decreases from
a factor of two near 100 nm to about 10 percent near 200 nm. In
addition to the overall change in solar radiation between solar
minimum and maximum, the AE-E data showed shorter term fluctuations
on a monthly, daily, and even an hourly basis, with the coronal
emissions being much more variable than the chromospheric
emissions.
Essentially all interpretive studies of upper atmosphere
phenomena now use scenarios of solar variability derived from the
AE-E data base. However, AE-E did not monitor the highly variable
soft X-rays, nor do the AE-E data agree with earlier rocket
measurements about either the magnitude or the variability of the
EUV irradiance (Lean, 1988). Concerns about the validity and
limitations of the AE-E data base continue to be raised. AE-E's
absolute irradiance calibration was derived from two Air Force
Geophysics Laboratory rocket measurements, one during 1974 (which
preceded the AE-E data) and another in 1979 (Heroux and
Hinteregger, 1978; L. Heroux, private communication, 1981).
Possible changes in the sensitivity of the AE-E instruments
throughout the mission are unknown, since no provision was made for
in-flight calibration. A comparison of the 1979 rocket measurement
used for the AE-E calibration with a recent rocket measurement
(Woods and Rottman, 1990) indicates significant inconsistencies in
that only the strongest emission lines were enhanced in the 1979
spectrum, for which solar activity levels were higher. This
contradicts current understanding of the origin of the EUV
irradiance variations, which predicts that solar activity causes an
increase in the EUV radiation at all wavelengths. The discrepancy
is most likely the result of instrumental effects (see Lean, 1990
for details).
AE-E ceased operation at the end of 1980. In the ensuing decade
only a few isolated measurements of the solar EUV spectral
irradiance were
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made -- from the San Marco satellite for nine months in 1988
(Schmidtke et al., 1992) and from a few rockets (Woods and Rottman,
1990). Some additional measurements of the EUV radiation integrated
over very wide spectral bands have also been made from rockets
(Feng et al., 1989; Ogawa et al., 1990). None of these observations
has succeeded in clarifying the true amplitude and variability of
the solar EUV radiation. Like the AE-E observations, they are
compromised by an assortment of instrumental effects that make it
extremely difficult to extract true solar spectral variability.
Since its launch in mid-1991, the Yohkoh satellite (Petersen et
al., 1993) has monitored the soft X-ray flux from the Sun for most
of the descending portion of solar cycle 22, providing uniquely
valuable data about a highly uncertain region of the solar
spectrum. Continuing these observations into the upcoming solar
minimum and subsequent activity maximum will contribute to improved
understanding of solar forcing of lower thermospheric NO
concentrations, and the possible transfer of chemical energy
between the upper and middle atmospheres.
Irradiance Variability
Parameterizations
The absence of continuous, reliable observations of the solar
EUV spectral irradiance has forced reliance on empirical
variability models based on solar activity surrogates to estimate
EUV spectral irradiances for use in upper atmosphere research and
in operational applications. The AE-E solar irradiance data have
been used to construct parameterizations of the solar EUV flux
variations as a function of primarily the solar 10.7 cm radio
emission that can be measured from the ground (Hinteregger et al.,
1981; Tobiska, 1991). The measured solar EUV flux values cover the
period from 1976 to 1980, but the solar flux models have been used
to represent the solar EUV and UV fluxes for other periods,
generating values that are typically inconsistent with earlier data
(Figure 4.1). Furthermore, different empirical models developed
from ostensibly the same data base can predict quite different EUV
spectral irradiances (Lean, 1990).
Aeronomic studies of thermospheric and ionospheric properties
indicate, not surprisingly, that existing solar EUV irradiance
variability models are inadequate for many geophysical
applications. While considerable
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Figure 4.1 Comparison of measured solar EUV
irradiance variations with the empirical model calculations of
Hinteregger et al. (1981) based on the 10.7 cm flux. The variations
are shown for a) the coronal emission at 33.54 nm, b) the primarily
chromospheric emission at 30.38 nm, which is the singularly most
important solar emission line for heating the Earth's upper
atmosphere, and c) the chromospheric line at 102.57 nm. The model
calculations (solid line) are based on the AE-E data (dots) in
solar cycle 21 and do not show very good agreement with rocket
measurements (asterisks) during the previous solar activity cycle.
Adapted from J. Lean, Advances in Space Research, 8, (5)263, 1988,
with permission from Elsevier Science Ltd, Pergamon Imprint, The
Boulevard, Langford Lane, Kidlington 0X5 1GB, UK, and J. Lean, J.
Geophs. Res., 95, 11939, 1990, copyright by the American
Geophysical Union.
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progress has been made during the past decade in developing
sophisticated models of thermospheric and ionospheric aeronomic
processes and global dynamics, there has been little improvement in
the (currently inadequate) empirical parameterizations of solar EUV
and UV radiation that these models use. This has led to the
application of various correction factors to the original data to
achieve agreement between model predictions and aeronomic
observations. For example, the solar EUV flux below 20 nm has been
doubled to force agreement with observed photoelectron spectra
(Richards and Torr, 1988; Winningham et al., 1989) and the flux
below 5 nm has also been scaled upwards to account for a
measurement of thermospheric NO at maximum levels of solar activity
(Siskind et al., 1990).
Improved solar irradiance variability models are needed not just
for aeronomic research but, increasingly, for operational
applications such as forecasting ionospheric conditions (Balan et
al., 1994) and for predicting thermospheric density for satellite
orbital and point density determinations. The lifetime and utility
of an Earth-orbiting object depend on the density structure of the
upper atmosphere, which is controlled by solar EUV radiation and
consequently varies over time scales from hours to decades (White
et al., 1994). Desired accuracies of 5 percent for thermospheric
densities for operational purposes require a similar accuracy in
knowledge of the solar EUV and UV spectral irradiance. The need for
this knowledge is demonstrated in Figure 4.2, where the orbital
decay rate of the SMM satellite can be seen to track changes in
solar activity (as indicated by the 10.7 cm radio flux). SMM's
reentry into the Earth's atmosphere is thought to have been
accelerated by the progressively increasing solar activity in 1989,
the ascending phase of solar cycle 22. High uncertainty surrounded
the launch of the Hubble Space Telescope because of insufficient
knowledge of the atmospheric drag that it would experience when
launched at a time near maximum solar activity (Withbroe,
1989).
Auroral Particle And Electric Field
Inputs
The Earth's thermosphere and ionosphere system responds not only
to changes in the solar EUV and UV radiative input but also to
particulate inputs of solar energy and momentum at high latitudes
associated with auroral processes. Although these inputs are
dominant at high latitudes
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Figure 4.2 Shown in the upper panel is the
decreasing altitude of the Solar Maximum Mission (SMM) satellite
(D. Messina and G. Share, private communication, 1990) as a
function of time just prior to its reentry into the Earth's
atmosphere in December 1989, coincident with increasing solar
activity as indicated by the Sun's 10.7 cm radio flux, F10.7. The lower panel shows that the
orbital decay rate (solid line), determined as the change per day
in the altitude, is strongly influenced by variations in solar
energy input, as indicated by the daily F10.7 solar activity proxy (linearly
transformed to an equivalent decay rate, dashed line). The cycle of
about 27 days occurs because the Sun's rotation causes active
regions to move across the face of the solar disk seen at the
Earth, modulating its output of UV and EUV radiation. When the
Sun's radiation is brightest, the Earth's atmosphere expands
outwards and the rate of decay of the satellite orbit increases.
Active regions that cause enhancements of the UV radiative output
also modify the 10.7 cm radio flux. From J. Lean, Reviews of
Geophysics, 29, 511, 1991, copyright by the American Geophysical
Union.
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(Figure 3.3), they have a great variability that
affects the basic structure and dynamics of the entire upper
atmosphere system. At times, such as in brief periods during
intense geomagnetic storms, energized electrons bombard the upper
atmosphere, colliding with atmospheric constituents and
transferring their energy, resulting in visual displays of auroral
phenomena. The energy deposited at high latitudes in the aurora can
increase by as much as two orders of magnitude relative to
geomagnetically quiet conditions, locally exceeding the energy
deposited from solar EUV radiation. Auroral energy inputs are known
to have a significant effect on aeronomic processes and dynamics of
the ionosphere, thermosphere, and mesosphere and perhaps indirectly
(via couplings to the stratosphere) on the troposphere, even though
the physical couplings are not understood.
In addition to knowledge of radiative energy inputs, global
dynamic models of the thermosphere and ionosphere system require
knowledge of the global distributions of auroral particle
precipitation, electric fields, and currents. During the past
decade, spacecraft such as the Atmospheric Explorer and Dynamics
Explorer, as well as various ground based programs, have provided a
good first order understanding of the energy inputs to the
thermosphere and ionosphere. Some information on global particle
inputs has been derived from satellite images of UV and visible
auroral airglow. However, many unresolved questions remain about
the variability of the fundamental energy inputs and the global
distribution of electric fields and currents.
Many questions also remain about the impact of auroral processes
on global change. For example, how are the atmospheric chemical
species such as NO that are produced by auroral processes
transported globally? How might they be transported to the lower
atmosphere, where they may influence global atmospheric properties?
How do the enhanced currents and fields produced during geomagnetic
storms influence properties of the troposphere and at the ground by
coupling into the Earth's global electric circuit?
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Global Currents and Electric Field
Couplings
The Earth and its atmosphere are almost permanently electrified.
The Earth's surface has a net negative charge, and an equal
positive charge is distributed throughout the atmosphere. The
atmospheric region above about 60 km is generally considered the
upper conducting boundary of the global electric circuit, which
includes electrical interactions between all atmospheric regions.
This upper conducting boundary is formed by solar ionization of
atmospheric constituents.
Global Circuit Processes
Three main generators operate in the Earth's global electric
circuit (Figure 4.3): (1) thunderstorms, (2) the ionospheric wind
dynamo, and (3) the solar wind/magnetosphere dynamo. These
processes are reviewed in NAS (1986) and are only briefly described
here.
Thunderstorms are electrical generators whose global activity
provides a current output that maintains a vertical potential
difference of about 300 kV between the ground and ionosphere, with
a total current flow between the two of about 103 A. The variability of thunderstorm
activity results in diurnal, seasonal, and interannual variations
in the potential differences and currents in the circuit. The
electrical processes associated with thunderclouds are many and
complex. In general, a conduction current flows from the top of the
cloud toward the ionosphere and into the global circuit. Beneath
the thundercloud a number of complex currents flow. The total
Maxwell current, defined as the sum of all of these current systems
plus the displacement current, has been shown to vary slowly over
the storm's history, suggesting that this electrical quantity is
coupled to the meteorological structure of the storm. Recent
aircraft measurements show that Maxwell current output from
thunderstorms is related to the lightning flash rate.
The ionospheric wind dynamo is produced in the region of the
atmosphere where neutral winds can have the effect of moving an
electrical conductor (the weakly ionized plasma) through the
Earth's geomagnetic field. This produces an electromotive force
that generates potential differences of 5 to 10 kV with a total
current of 105 A extending over
thousands of kilometers and flowing primarily on the dayside of the
Earth.
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Figure 4.3 Schematic depiction of various
electrical processes that make up the global electric circuit,
illustrating how coupling between the different atmospheric regions
connects the Earth's upper atmosphere to the biosphere. The
ionosphere exists because of ionization by solar extreme
ultraviolet radiation. From Studies in Geophysics: The Earth's
Electical Environment, (NAS, 1986).
This current system is highly variable because of the changing
tides and other disturbances propagating into the dynamo region
from the middle atmosphere. Solar and auroral variability alter
thermospheric winds and ionospheric electrical conductivities and
thus influence the currents and fields in the dynamo region.
The flow of solar wind around and partly into the magnetosphere
produces the solar wind/magnetosphere dynamo, which sets up plasma
motion in the magnetosphere as well as producing electric fields
and currents. This interaction is highly variable, but typically
generates horizontal dawn-to-dusk potential drops of 40 to 150 kV
across magnetic conjugate polar caps. The solar wind/magnetosphere
dynamo is associated with a current flow of about 106 A between the magnetosphere and
ionosphere. This generator depends on the properties of the solar
wind flowing past the Earth's magnetosphere as well as on any
internal current
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flows within the magnetosphere. The magnetosphere and the
Earth's near-space environment are more fully discussed in Chapter
5.
There is considerable variability in tropospheric electrical
parameters that influence the properties of the global circuit.
These parameters include global cloudiness as it affects the
electrical conductivity; turbulence in the planetary boundary
layer; aerosols; pollution; radioactive ion production near the
Earth's surface; fog; and surface processes in grasslands, forests,
deserts, and ocean spray. Many of these processes have been studied
in isolation, but their combined impact on the global electric
circuit has not been properly evaluated.
Important processes in the middle atmosphere also have
implications for variability in the global circuit. Aerosols
produced by volcanoes can affect the electrical conductivities and
electric fields. Rocket measurements of electric fields in the
mesosphere indicate strong departures from Ohm's law, suggesting
the presence of an as yet unidentified generator operating at
mesospheric heights. Electrical conductivity enhancements in polar
regions associated with energetic particle precipitation, solar
proton events, and Forbush decreases in cosmic ray fluxes following
solar eruptions all influence the properties of the global circuit
in ways that are not well understood at present.
Electrical Couplings Between the Upper
and Lower Atmospheres
Large horizontal electric fields (100 to 1000 km) generated
within the ionosphere project downward in the direction of
decreasing electrical conductivity, effectively down to the ground.
Small horizontal electric fields (1 to 10 km) are rapidly damped as
they map downward into the atmosphere from ionospheric heights.
Since the electrical conductivity of the Earth's surface is large,
horizontal electric fields cannot be maintained there, and a
vertical electric field variation results to accommodate horizontal
variations of ionospheric potential. Calculations show that the
solar wind/magnetosphere generator can increase or decrease by up
to 20 percent the air-Earth current and ground electric field at
high latitudes during geomagnetic quiet times, with larger
perturbations during geomagnetic storms. The magnitude of the
ground variations also depends on the alignment of the potential
pattern over the ground, being much enhanced by mountainous terrain
in the polar region.
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While electric and magnetic field coupling between the upper and
lower atmospheres is well established physically, the impact on
processes in the troposphere and biosphere and on other processes
important for global change is unknown. Furthermore, human
activities are slowly changing the atmospheric structure,
atmospheric and ionospheric composition, aerosol loading, land
surface properties, and other tropospheric variables, all of which
will probably have some influence on the properties of the global
electric circuit (Price and Rind, 1994).
Solar Forcing and Global Change Within
the Upper Atmosphere
That the Earth's upper atmosphere is forced directly by variable
solar energy inputs on all time scales is well established. Global
mean temperature, global wind circulation, and constituent particle
densities change continuously, in response to changing solar
activity throughout the 11-year cycle (e.g., Evans, 1982), with the
extent of the changes generally increasing with altitude. From the
minimum to the maximum of the Sun's activity cycle, upper
atmosphere temperatures increase by many hundreds of degrees
(Figure 1.2), a direct consequence of solar EUV and UV heating.
Greenhouse gases such as carbon dioxide and methane contribute to
the radiative balance of the Earth's upper atmosphere as well as
the lower atmosphere. Carbon dioxide cooling is the dominant
cooling mechanism in the atmospheric region between 70 and 200 km;
infrared cooling by CO2 is largely
responsible for the temperature minimum near 80 km shown in Figure
1.2.
Most studies of the climate change anticipated from the
anthropogenic loading of greenhouse gases focus on the effects on
the troposphere and middle atmosphere (Rind et al., 1990; Hansen et
al., 1993). Recognition that trace gases released into the Earth's
atmosphere from human activity could perturb the climate of the
Earth provided much of the motivation for the USGCRP. As discussed
in Chapter 2, these studies suggest that the troposphere will warm
and the middle atmosphere will cool as trace gas concentrations
increase into the twenty-first century. In the upper atmosphere, as
in the middle and lower atmospheres, increases in anthropogenic
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gases are expected to affect the energy balance between solar
heating and infrared cooling.
Recent studies have shown that trace greenhouse gases could
effect considerable change in the structure of the Earth's upper
atmosphere (Roble and Dickinson, 1989; Rishbeth, 1990). The global
mean temperature of the upper atmosphere has been projected to cool
by 10 K at altitudes near 70 km, and by 50 K around 150 km, in
response to doublings of CO2 and
CH4 concentrations (Figure 4.4).
These changes will be superimposed on upper atmosphere temperature
variations of many hundreds of degrees generated by changes in
solar energy inputs throughout the 11-year activity cycle (Figure
1.2). Concomitant redistributions of major and minor constituents
should occur throughout the entire atmospheric region. In the
thermosphere, the atmospheric density at a given altitude has been
projected to decrease by as much as 40 percent. The atmospheric
scale heights that govern thermospheric and ionospheric properties
should also be reduced, and the peak height of the ionospheric F2
region may be lowered by 20 km.
As a result of changes in the basic thermal and compositional
structure of the atmosphere, increases in CO2 may also damp the response of the
thermosphere and ionosphere to solar and auroral variability. These
changes also could affect the propagation of atmospheric tides,
gravity waves, and planetary waves into the thermosphere. It is not
clear how changes in the basic atmospheric structure and dynamics
will affect the ionospheric wind dynamo and the coupling of the
ionosphere and magnetosphere with the solar wind, but changes in
thermospheric circulation might result in a changed electrodynamic
structure of the upper atmosphere, and through dynamo action, alter
magnetosphere/ionosphere coupling processes, as well as the entire
terrestrial electric field system.
Couplings of the Upper Atmosphere to
the Lower Atmosphere
The most direct coupling between the upper and lower atmospheres
is electrical. As discussed previously, large horizontal electric
fields map almost unattenuated to the Earth's surface, where they
perturb the vertical electric field maintained by global
thunderstorm activity. In addition, the
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Figure 4.4 Temperature and density changes in
the upper atmosphere as a function of the altitude in km predicted
by the model calculations of Roble and Dickinson (1989) as a
consequence of increasing greenhouse gases released in the lower
atmosphere. Shown on the left are changes in the altitude profile
of the neutral gas temperature for doubled and halved
concentrations of CO2 and CH4. On the right are the corresponding
percent changes in the density profiles of the primary upper
atmosphere constituents. From R. Roble and R. Dickinson, Geophys,
Res. Lett. 16, 1443, 1989, copyright by the American Geophysical
Union.
geomagnetic quiet-time dynamo current and highly variable
auroral current systems induce magnetic currents in the Earth's
crust and alter the ground electric field. These perturbations have
demonstrable impacts on human infrastructure, such as power
networks and oil pipelines (Allen et al., 1989), but as yet unknown
impacts on biological and atmospheric physical processes.
Also suspected are chemical and dynamical couplings between the
upper and lower atmospheres. Nitric oxide is produced by solar soft
X-ray and EUV radiation and auroral particle dissociation of
molecular nitrogen into excited and ground states of atomic
nitrogen that react with molecular oxygen. NO concentrations change
significantly as a result of larger variations in solar soft X-ray
fluxes (Siskind et al., 1990). Nitric oxide is important for
understanding possible radiative and dynamical couplings of solar
variability effects in the thermosphere with the middle atmosphere
(Garcia et al., 1984; Siskind, 1994). For example, if
transported
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downward to the region of 40 km, NO may have an important
influence on the ozone density of the stratosphere (Huang and
Brasseur, 1993). While the catalytic destruction of ozone in this
region is relatively well understood (see Chapter 3), the transport
of thermospheric-generated nitric oxide into the region is not.
Because the photochemical destruction lifetime is on the order of
one day throughout the sunlit mesosphere, NO must move downward
during the polar night to reach the upper stratosphere.
Two-dimensional modeling studies, as well as spacecraft
observations of middle atmospheric ozone abundances, suggest that
this may indeed be a viable coupling mechanism (Garcia et al.,
1984; Solomon and Garcia, 1984). Changes in nitrate content of
antarctic snow associated with solar activity may reflect these
processes (Dreschhoff and Zeller, 1990).
In addition to solar-related processes, the lowest layers of the
upper atmosphere are influenced by turbulent breaking of
atmospheric gravity waves and tides and by sporadic, intermittent
compositional exchanges of atomic oxygen and nitric oxide. These
complex turbulent exchange processes influence long-lived species
such as carbon dioxide, carbon monoxide, water vapor, and atomic
and molecular hydrogen. Although theoretical studies have indicated
that chemical, dynamical, and radiative interactions are a viable
coupling mechanism, the whole question of thermospheric/lower
atmospheric exchange is not well understood, primarily because our
atmosphere between about 50 and 200 km is virtually unexplored on a
global basis.
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Representative terms from entire chapter:
solar euv
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