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3—
Solar Variations, Ozone, and the Middle
Atmosphere
Background
Weather and climate are experienced in the troposphere, which
extends upward to the tropopause at about 15 km. The Earth's middle
atmosphere (the stratosphere and the mesosphere) extends from the
tropopause to approximately 90 km (Figure 1.2). Understanding the
middle atmosphere is crucial for global change studies for two
primary reasons. Firstly, this is where approximately 90 percent of
the Earth's ozone shield resides and, secondly, the region
constitutes the upper boundary of the troposphere. Changes in the
ozone layer modulate the amount of ultraviolet (UV) radiation
reaching the biosphere and are thus of direct concern for life on
Earth. A decrease in column ozone of 1 percent causes the UV dose
in the spectral region damaging to deoxyribonucleic acid (DNA) to
increase by about 2 percent. Changes in ozone, temperature, and
other trace gases have been widely implicated in a variety of the
mechanisms by which changes in the middle atmosphere might
influence the biosphere by physically coupling to the troposphere
and modifying the climate (NAS, 1982; Lacis et al., 1990;
Schwarzkopf and Ramaswamy, 1993; Hauglustaine et al., 1994; Rind
and Balachandran, 1994). Crucial to these mechanisms is ozone,
which is highly responsive to, and also has a controlling influence
on, the state of the middle atmosphere. Ozone is
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Representative terms from entire chapter:
solar cycle
Page 50
directly influenced by changes in solar radiative and energetic
particle inputs to this region.
All of the Sun's ultraviolet energy input at wavelengths between
about 150 and 300 nm is deposited in the Earth's middle atmosphere.
This energy plays an essential role in the chemistry, radiation,
and dynamics of the region. Figure 1.2 shows the altitude at which
solar radiation (from an overhead Sun) reaches 1/e of its original
intensity and gives an approximate measure of the penetration depth
for different wavelengths. The inversion in the atmospheric
temperature profile at about 15 km (Figure 1.2) that defines the
tropopause is a direct consequence of heating by solar UV energy
abosorbed by ozone in the middle atmosphere. The only significant
solar radiation of shorter wavelength reaching the middle
atmosphere is a small and highly sporadic contribution from X-rays,
and the strong H I Lyman 
line at 121.6 nm.
The ozone layer exists because of the interaction of solar UV
radiation with the constituents of the middle atmosphere.
Photodissociation of molecular oxygen by solar UV radiation at
wavelengths from 170 to 242 nm (in the O2 Schumann-Runge bands and Herzberg
continuum) is the chief source of atomic oxygen and hence ozone
production. Formed by combination of atomic and molecular oxygen,
ozone is in turn photodissociated, mainly by solar UV radiation at
wavelengths between 240 and 300 nm (in the strong O3 Hartley bands and continuum), but also
by longer wavelength visible solar radiation. Ozone's strong
absorption in the UV region of the spectrum serves the dual role of
heating the middle atmosphere and protecting the surface of the
Earth from damaging doses of ultraviolet radiation. Solar UV
radiation also creates other important trace constituents, such as
chlorine (Cl) and hydroxyl (OH), that participate in catalytic
reactions that destroy ozone. Current understanding of the
processes affecting ozone and other trace constituents of the
middle atmosphere has been reviewed in a number of reports (e.g.,
WMO, 1988), and is not repeated here.
Stratospheric ozone has received intensive study in recent
years, and is the subject of research under the Biogeochemical
Dynamics science element of the USGCRP. Ozone is known to be
influenced by human related sources such as chlorofluorocarbons
(CFCs), carbon dioxide (CO2), and
methane (CH4) and by natural
occurrences such as volcanoes and solar variability. As summarized
in Figure 3.1, definite changes in the total
Page 51
amount of stratospheric ozone are thought to have occurred
throughout the past few decades. Long term changes in stratospheric
temperatures have also been found (WMO, 1988; Randel and Cobb,
1994). A significant fraction of the long term changes in ozone and
temperature is thought to be caused by human related emissions (of
CFCs and other gases). Recent analysis of data from the Total Ozone
Mapping Spectrometer (TOMS) on board the Nimbus 7 satellite
indicates a global (65 N to 65 S l latitude) ozone decrease of 0.27
percent ± 0.14 percent per year since 1978 (Stolarski et
al., 1991; Hood and McCormack, 1992). This is thought to arise from
anthropogenic effects.
A significant fraction of the ozone variance during the past
decade is also due to solar forcing (Figure 3.1). The ozone content
of the middle atmosphere varies with the 11-year solar cycle
because the solar radiation and particle environment responsible
for creating and destroying ozone varies. Superimposed on the long
term downward ozone trend deduced from TOMS data during the past 11
years is a solar cycle variation whose amplitude is estimated to be
1.8 percent ± 0.3 percent (Hood and McCormack, 1992). These
solar-related changes in ozone exacerbated the downward
anthropogenic trend from 1982 to 1986 (the descending phase of the
solar activity cycle) and masked it almost completely during
1988–1991 (the ascending phase of the cycle).
Longer-term variations related to solar variability are clearly
possible. To develop a reliable understanding of true anthropogenic
effects on ozone therefore requires that its natural variability in
response to solar influences be fully defined; knowledge of
solar-induced changes will be particularly important in the future
in verifying whether an observed slowing of the ozone depletion
rate is the result of limits on anthropogenic sources or a response
to changing solar energy inputs.
As mentioned in Chapter 2, solar-induced variations in
stratospheric temperature structure may be affecting the
troposphere through modulation of the Hadley circulation in the
tropics or planetary wave genration in the extratropics. Change in
the latitudinal temperature gradient in the stratosphere and
planetary wave generation in the extratropics affect stratospheric
wind speeds and thus the ability of long-wave energy to propagate
out of the troposphere, further altering tropospheric dynamical
patterns and subsequently radiative parameters (Rind and
Balachandran, 1994).
Page 52
Figure 3.1 In the upper panel are compared
changes in global column ozone concentrations in a vertical column
above the Earth's surface during the 11-year solar cycle and the
long term decrease derived from statistical analysis of the TOMS
ozone measurements (Hood and McCormack, 1992). During decreasing
solar activity from 1982 to 1986, solar forcing of ozone was
approximately equivalent to the anthropogenic forcing, causing
ozone depletion double that of the anthropogenic rate determined
from the TOMS data, whereas from 1986 to 1991 the solar-induced
changes approximately canceled the downward trend. Ozone changes
were extended to solar cycles 22 and 23 by assuming equal levels of
solar activity in all three cycles. Compared in the lower panels
are altitude profiles of the mid-latitude Northern Hemisphere ozone
decadal trend (left) and solar cycle variations (right) for a solar
cycle with 150 units change of 10.7 cm flux (typical of cycles 21
and 22), deduced independently by Reinsel et al. (1994) from Umkehr
data. Courtesy of J. Lean.
Page 53
Solar Ultraviolet Radiation
Whereas total solar irradiance (discussed in Chapter 2) was,
until recently, assumed to be invariant, solar emissions at shorter
UV wavelengths are known to vary in phase with the solar activity
cycle, with the shortest wavelengths varying the most (see Figure
1.1). The reason is that, compared with the visible radiation that
contributes most to the total solar irradiance, the ultraviolet
radiation is formed higher in the Sun's atmosphere and is more
susceptible to the impact of magnetic fields that erupt into the
solar atmosphere during times of enhanced solar activity. Accurate
knowledge of the amplitude and temporal structure of solar UV
spectral irradiance variations is clearly critical for studies of
solar forcing of the middle atmosphere. The true extent of the UV
irradiance changes over the 11-year solar activity cycle has long
been debated and remains somewhat uncertain.
Estimates of the amplitude of the solar spectral irradiance
variations shown in Figure 1.1 were derived from a combination of
direct measurements and inferences from empirical variability
models. Ultraviolet radiation is currently thought to vary over the
11-year solar cycle by 5 percent to 10 percent at 200 nm, 50 to 100
percent at H I Lyman , and more than a factor of two
at the shortest extreme ultraviolet (EUV) wavelengths, reaching
maximum values during times of activity maximum, as indicated by
the sunspot number. The solar cycle variations at wavelengths from
125 to 300 nm shown in Figure 1.1 are significantly less at all
wavelengths than inferred directly from the rocket and satellite
data base (Lean, 1987; Rottman, 1988), but still cannot be
considered entirely reliable. Uncertainties remain even for the H I
Lyman line, whose variability is considered to be better
known than for any other portion of the solar spectrum.
Superimposed on the 11-year UV irradiance cycle is a 27-day
rotational modulation whose amplitude near high activity levels may
as much as half the 11-year cycle amplitude.
Measurements of Solar UV Spectral
Irradiance
Various instruments have been launched on rockets, satellites,
and the Space Shuttle during the past 20 years (about two solar
activity cyles) with thegoal of acquiring knowledge of the Sun's
ultraviolet spectral
Page 54
irradiance at wavelengths less than 400 nm. Solar Backscatter
Ultraviolet (SBUV)-type instruments have been flown since November
1978, initially on the Nimbus 7 satellite (Health and Schlesinger,
1986) and subsequently on a series of National Oceanic and
Atmospheric Administration (NOAA) satellites (Donnelly, 1988;
Cebula et al., 1992). Coincident with these observations from 1982
to 1989 are measurements by the solar spectrometer on board the
Solar Mesosphere Explorer (SME) (R (Rottman, 1988; London and
Rottman, 1990). Two solar spectral irradiance monitors for the
region from 115 to 415 nm are currently operating on board the
Upper Atmosphere Research Sateellite (UARS), launched in September
1991 near the maximum of activity in solar cycle 22 (Rottman et
al., 1993; Brueckner et al., 1993). Other solar UV
spectroradiometers have been flown intermittently on rockets and
balloons, with half a dozen recent measurements from the Space
Shuttle.
Measurements from rockets (some of the more than 50) and
spacecraft of the solar Lyman irradiance are shown in Figure
3.2. The primary satellite data bases are the Orbiting Solar O
Observatory-5 (OSO-5), from 1970 to 1974 (Vidal-Madjar 1975;
Vidal-Madjar and Phissamay, 1980), Atmospheric Explorer-E (AE-E)
from 1977 to 1980 (Hinteregger et al., 1981), SME from 1982 to 1989
(Rottman, 1988; Barth et al, 1990; White et al., 1990) and UARS,
from late 1991 to the present (London et al., 1993); none
overlap.
Figure 3.2 typifies, in many respects, the current state of the
entire data base of solar irradiance at wavelengths shorter than of
400 nm. No portion of the spectrum has been monitored continuously
for even one 11-year solar activity cycle. Instrumental effects
contribute significantly to the scatter of the data. Absolute
accuracies are typically quoted to be 10 to 50 percent, but
inconsistencies between measurements made by different experimental
groups are clearly apparent and signify larger systematic
uncertainties. Information about instrument responsivity drifts
during spacecraft missions is absent (in the case of AE-E), limited
(in the case of SME), or speculative (in the case of SBUV). The
SBUV diffuser reflectivity is thought to have degraded over seven
years by about 30 percent at 250 nm (Herman et al., 1990). Although
the SME instrument included a stored diffuser for periodic
reference, this instrument suffered from problems associated with t
temperature-dependent wavelength calibration drifts. Pronounced
long term drifts are also suspected in the AE-E data.
Page 55
Figure 3.2 Contemporary solar activity
variations as indicated by the sunspot number (top panel) and
changes in the solar UV irradiance at Lyman (bottom
panel). The data (dots) are from OSO Vidal-Madjar, 1975), AE-E
(Hinteregger et al., 1981), SME (from 1982 to 1989, Rottman, 1988),
and UARS/ SOLSTICE (London et al., 1993 courtesy of G. Rottman) and
from rockets and the Space Shuttle (see Lean and Skumanich, 1983,
for details). Also shown as a solid line is an extension o fthe SME
data obtained from a linear relationship with the ground based He I
EW (Figure 6.3). Note that for the Lyman
irradiance, 1x1011 photons/cm2/sec is equivalent to about 1.63
mW/m2. Courtesy of J. Lean.
Only the two UARS solar UV radiometers have the capability for
end-to-end, in-flight sensitivity monitoring using, in the case of
the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), a bank
of four deuterium lamps and redundant optics, and for the Solar
Stellar Irradiance Comparison
Page 56
Experiment (SOLSTICE), the UV fluxes from a collection of bright
stars (Woods et al., 1993).
Despite the inadequacies in the experimental data base, there is
no doubt that the Sun's full disk UV spectral irradiance varies.
Changes of a few percent or more occur in response to the
modulation of solar activity by the 27-day solar rotation and have
been reliably measured at wavelengths as long as 250 nm. The
amplitude of the rotational modulation is largest at the shortest
wavelengths and can be as much as 30 to 40 percent at H I Lyman
(e.g., Lean,
1991). It has proven more difficult to extract from the data base
reliable information about the magnitude of solar irradiance
variations over the 11-year activity cycle. These variation are
expected to be somewhat larger than those associated with solar
rotation (Lean et al., 1992b), but are uncertain because of the
degradation of instrument sensitivies during long-duration space
flight, and because of the difficulty in comparing measurements by
different instruments whose inacurracies typically exceed the
expected amplitude of the spectral irradiance variability.
Irradiance Variability
Parameterizations
In lieu of an adequate experimental data base, information about
the Sun's UV spectral irradiance variations has been acquired by
critically analyzing the available data in conjunction with
knowledge of solar activity derived independently, often from
ground based observations (Lean et al., 1992b; DeLand and Cebula,
1993). In concert with rising solar activity during the 11-year
cycle is an overall increase in the solar ultraviolet spectral
irradiance because of increased emission from magnetic active
regions whose radiation is enhanced at these wavelengths.
Superimposed on this 11-year cycle are intermediate term variations
over time scales of three to nine months that reflect the birth,
growth, and decay of individual active regions associated with
surges of activity. In addition, the UV irradiance is modulated by
the Sun's 27-day rotation, which brings active regions onto the
face of the Sun viewed from the Earth (e.g., Lean, 1987).
Irradiance variability parameterizations range from linear
transformations of solar activity surrogates to detailed
representations of active region source of enhanced UV emission.
Certain solar emissions that can be measured from the ground, such
as the emission core of the Ca II K
Page 57
Fraunhofer line, behave similarly to the UV irradiances in their
responses to solar magnetic activity. These ground based data
reflect the UV enhancement in active regions, exhibiting variations
throughout the 11-year solar cycle in response to active region
evolution and during the 27-day solar cycle. Estimates of solar
cycle UV irradiance variability have been obtained either by
extrapolating known 27-day changes, assuming a linear relationship
with a suitable solar activity indicator (Health and Schlesinger,
1986; Cebula et al., 1992; Lean et al., 1992b) or, in the case of
Lyman , by
linearly regressing extant solar UV irradiance data against
full-disk emission surrogates (e. g., White et al., 1990).
Calculations of the UV spectral irradiance directly from ground
based, spatially resolved Ca II K observations of the areas,
locations, and spectral brightness of active regions have also been
attempted with the purpose of elucidating the origins of the UV
irradiance variations in terms of solar magnetic activity (Cook et
al., 1980; Lean et al., 1982).
Inferences about the amplitude of the 11-year solar cycle
variation in the UV spectral irradiance from 120 to 300 nm derived
from measurements of the rotational modulation (Cebula et al.,
1992; Lean et al., 1992b) are in general agreement with the
variations reported from the SME observations (Rottman, 1988;
London and Rottman, 1990). From 210 to 250 nm the solar cycle
variation appears to be of the order of 3 to 5 percent, increasing
to 7 to 9 percent at 200 nm. In the region of 120 to 200 nm the UV
spectral emission lines vary significantly more than does the
underlying continuum. For example, 1 nm spectral regions dominated
by the H I Lyman and O I 133.5 nm lines are estimated to vary by about
50 percent, with the underlying continuum varying by about half
this amount.
However, parameterizations of solar UV irradiance variability
derived from auxiliary solar data remain to be verified by more
reliable and extended long term observations than SME, made by
instruments such as those on UARS having complete in-flight
sensitivity monitoring. This is true even for the Sun's strongest
emission feature, the H I Lyman line. Although its irradiance
has been measured over more than three solar cycles, and far more
frequently than observations of any other portion of the solar UV
spectrum, unequivocal confirmation of Lyman 's true 11-year cycle
variability has still proven difficult, confounded by apparent
inconsistencies between different data sets and the suspicion of
instrument
Page 58
artifacts in the data base. As demonstrated in Figure 3.2, none
of the four Lyman data bases overlap, which severely impedes the
separation of the data into solar and instrumental effects. Data
from the solar spectrometer on the SME satellite have been widely
used for aeronomic applications, either directly for the period
from 1982 to 1989, or over longer periods via parameterizations
with the Ca II K (White et al., 1990) and He I 1083 nm EW (Lean,
1990) ground based indicators of chromospheric variability.
However, measurements made by the AE-E (Fukui, 1990) and, more
recently, the SOLSTICE on UARS imply significantly higher Lyman
irradiances
(Figure 3.2) and greater variability than is estimated from
parameterizations based on rotational modulation or SME
observations. Pioneer Venus Orbiter Langmuir probe observations of
the integrated solar EUV and Lyman fluxes also provide indirect
evidence for a Lyman 11-year irradiance cycle in excess of 50 percent and
poorly correlated with the Ca II and He I indices near the maximum
of solar cycle 22 (Hoegy et al., 1993). Persistent inconsistencies
among the measurements themselves and with parameterizations
derived from solar activity surrogates caution against supposing
that all is known even about the Sun's photon output variation at H
I Lyman or at
any other UV wavelength.
Energetic Particles
Solar Proton Events
Eruptions of activity on the Sun, such as solar flares and
coronal mass ejections, frequently give rise to fluxes of energetic
particles. Interaction of these particles with the Earth's
environment depends on the site of the eruption on the solar disk
and on the particles' transport to the Earth via the solar wind and
the Earth's near-space environment (discussed in Chapter 5).
Typically, at 10 million electron volts (MeV), a large solar
energetic-particle event reaches its peak at the Earth a few hours
to a day after the initiating eruption and may persist for one to a
few days. The particles are mainly protons and alpha particles with
a small component of medium-mass and heavy ions. Fluxes at energies
> 10 MeV can reach 100,000 cm-2sec
-1sr-1.
Page 59
Solar energetic particles are excluded from low magnetic
latitudes by the Earth's magnetic field, but the polar regions are
exposed to the full interplanetary flux. Figure 3.3 shows the
regions in the two hemispheres (the polar caps) within which the
effects are particularly felt, although during intense geomagnetic
storms these boundaries can expand to considerably lower latitudes
for brief periods. The depth in the atmosphere to which the
particles penetrate depends on their initial energy; the energy
spectrum of the particles is highly variable from event to
event.
At times, solar protons can bombard the middle atmosphere at
high latitudes for periods ranging from hours to days at a time.
Penetration into the stratosphere (i.e., altitudes below about 50
km) requires energies in excess of 30 MeV for a proton, while
direct penetration into the high-latitude troposphere requires
energies greater than about 1 billion electron volts (GeV). Events
with significant fluxes above this latter threshold are rare, but
ground level neutron monitors, which detect thermal neutrons
generated as secondary particles by the interaction of very
energetic solar protons (several hundred MeV) with atmospheric
constituents, have recorded many events.
The energy loss mechanism for solar protons in the Earth's
atmosphere is almost exclusively through ionization. The impact
between a solar particle and an atmospheric molecule releases a
secondary electron with energy in the range of a few hundred
electron volts that goes on to produce more ionization. Primary and
secondary electrons both dissociate atmospheric nitrogen
efficiently, while subsequent positive-ion reactions dissociate
water vapor. The net result is the production of a wide range of
odd-nitrogen (NOx) and odd-hydrogen
(HOx) compounds, primarily in the
middle atmosphere between heights of roughly 20 to 100 km. At
heights of 60 to 70 km, where the solar energetic particle
ionization reaches its maximum, it can exceed the normal background
ionization rate by a factor of a million. These relatively large
ionization rates give rise to significant increases in the
concentration of free electrons and consequent severe disturbances
in radio propagation at polar latitudes. In fact, the radio
propagation effects provided much of the early information on solar
energetic particle events before direct spacecraft measurements
became available.
Perhaps one of the most surprising aspects of solar terrestrial
physics to emerge in the past decade has been the number and
intensity of major
Page 62
FIGURE 3.4 The integrated proton flux of high
energy solar protons typical of ordinary flares is compared with
two of the largest disturbances seen in the spacecraft era, in
August 1972 and September 1989. Heath et al. (1977) reported
significant changes in ozone composition as a result of the August
1972 event. Reid et al. (1991) have analyzed the effects on the
middle atmosphere of solar proton events during August-December
1989. Courtesy of D. Baker.
penetrates to about 100 km. Thus, if there is a significant
population andflux of trapped high-energy relativistic electrons
(energy > 700 KeV) in themagnetosphere, precipitation in auroral
and subauroral latitudes may occur, withenergy deposition and
resulting production of hydrogen oxides
Page 63
and nitrogen oxides in the lower mesosphere and upper
stratosphere. If the energetic events are sufficiently frequent,
the downward transport of nitrogen oxide molecules during the
winter and spring could result in a significant effect on the
budget of odd-nitrogen in the lower stratosphere (e.g., Dahe et
al., 1991).
Measurements from the Spectrometer for Energetic Electrons (SEE)
on spacecraft 1979-053 and 1982-019 do provide evidence for large
fluxes of relativistic electrons at 6.6 Earth radii (RE) that undergo variations on 27-day and
11-year time scales (Baker, et al., 1987). Precipitation of 30 to
40 percent of these electrons from the largely trapped trajectories
seen at synchronous orbit into the atmosphere could have
significant effects on middle atmosphere chemistry (Baker et al.,
1993). However, measurements of the energy deposited into the
middle atmosphere made by sounding rockets (e.g., Goldberg et al.,
1984) have typically indicated much smaller fluxes of relativistic
electrons than measured by the SEE instrument. Figure 3.5 shows a
comparison of a typical energy deposition rate from an REP event
measured by Goldberg et al. (1984) with energy deposition rates of
EUV and galactic cosmic rays. In the lower mesosphere and
stratosphere the energy deposition rates are almost two orders of
magnitude larger for the Baker et al. REP event than for the
Goldberg et al. event. On the other hand, recent rocket
observations (Herrero et al., 1991) indicate that the fraction of
the high altitude flux of relativistic electrons that reaches the
middle atmosphere near noon (when daytime REP fluxes peak) may
actually be significantly larger, and penetrate deeper, than the
nighttime REP events measured by earlier rockets (Goldberg et al.,
1984). At about 100 km, electron spectra were estimated to be 5 to
25 percent of the geostationary orbit fluxes at the same local
time, and Baker et al. (1993) argue that precipitated fluxes of
electrons with energies > 1 MeV could be as much as 30 to 50
percent of the trapped flux levels. New measurements being made by
the Solar, Anomalous, and Magnetospheric Particles Explorer
(SAMPEX) may help to resolve these differences and determine
whether REPs are as important for long term ozone variability as
has been inferred from some studies (Callis et al., 1991).
Page 64
FIGURE 3.5 The upper figure depicts the
penetration of energetic particles in the middle atmosphere. In the
lower figure, energy deposition due to relativistic electrons in
June 1980 and September 1976 are compared with energy deposition
from extreme ultraviolet radiation and galactic cosmic rays during
minima and maxima of the 11-year activity cycle. Courtsey of
Jackman (1991), adapted from Baker et al. (1987) and Goldberg et
al. (1984). Copyright by the American Geophysical Union.
Page 65
Galactic Cosmic Rays
The Sun is not the only source of energetic particles that
penetrate the Earth's atmosphere. Galactic cosmic rays (GCR), which
are mostly protons and alpha particles, have typically higher
characteristic energies (peaking in the range 0.1– 1 GeV per
nucleon near Earth orbit) but lower fluxes than the solar (cosmic
ray) protons. Carbon, nitrogen, oxygen, and other medium-mass
nuclei are also present, together with a significant heavy ion
population. Like solar energetic protons and relativistic
electrons, galactic cosmic rays are strongly shielded from reaching
the equatorial regions of the Earth's surface by the terrestrial
magnetic field and by the thick neutral atmosphere. This shielding
is energy dependent, deflecting incident particles with energies as
high as 15 GeV at the magnetic equator. However, in the polar
regions of the Earth and at high altitudes (> 5 Earth radii) the
GCR component has direct access to the middle atmosphere.
At energies less than about 1– 2 Gev, the main mechanism
for energy loss in the atmosphere is through ionization, and the
particles stop well above the surface. At higher energies, nuclear
interactions with atmospheric constituents become increasingly more
important, giving rise to energetic secondary particles that can
penetrate to the surface of the Earth. Among the products of these
interactions are neutrons, which are created with relatively high
energies and can subsequently be thermalized as they travel down
through the atmosphere. The fact that these neutrons can be readily
measured at the Earth's surface provides a convenient means of
monitoring cosmic ray fluxes on a routine basis.
There is a relatively strong (5 to 15 percent) solar cycle
modulation near Earth's orbit for galactic cosmic rays. This
modulation, which is also energy dependent and increases at lower
energies (below 1 GeV) to well over a factor of two, is in
antiphase with the 11-year solar activity (sunspot) cycle and
reflects the effects of changes in magnetic structures in the solar
wind on the flow of cosmic rays into the solar system. Significant
cosmic ray (Forbush) decreases also occur during large magnetic
storms associated with coronal mass ejections and usually with
solar flares (see Herman and Goldberg, 1978; Tinsley and Deen,
1991).
Cosmic ray secondary neutrons have an important by-product in
the form of 14 C, or radiocarbon,
produced by the reaction of a neutron with
Page 66
14N, the most abundant
atmospheric isotope. The 14 C
becomes incorporated into the terrestrial carbon cycle and
eventually into the carbon of living organisms. Once created, the
14C decays with a half-life of
5730 years, thereby providing the basis for the radiocarbon dating
technique that has revealed much about biosphere evolution on
millennial time scales. Laboratory measurements of the 14 C:12
C ratio in tree-ring dated wood have provided a chronicle of 14 C input variations and hence of
variations in solar activity that now extend almost 10,000 years
into the past (e.g., Stuiver and Braziunas, 1993).
Solar Forcing of the Middle
Atmosphere
Changes in both solar UV radiation and in the flux of energetic
particles influence the middle atmosphere directly. Determining the
impact of human activities on upper stratospheric ozone and
temperature requires a knowledge of the impact of this solar
forcing background.
Effects from Variations in UV
Irradiance
As discussed previously, variations in the solar flux during the
11-year activity cycle are still not well understood, particularly
at the ultraviolet wavelengths that are responsible for the primary
production and destruction mechanisms of middle atmosphere ozone.
Changes in ozone and temperature resulting from the 27-day solar
irradiance rotation cycle are better understood (Hood, 1987; Hood
and Douglass, 1988; Chandra, 1991; Brasseur, 1993) and have
confirmed the response of the middle atmosphere to changes in solar
UV energy inputs. Ozone changes related to the 11-year irradiance
cycle are expected to be larger than those detected over solar
rotation time scales (Chandra, 1991) because the UV irradiance
variation is larger (Lean, 1991) and the sensitivity of ozone to
longer period forcing is greater (Brasseur, 1993).
Trend studies of middle atmosphere ozone and temperature
observations provide evidence for long term changes associated with
solar cycle variations in UV radiation, albeit with large
uncertainties in the magnitude of the effect, which has thus far
been measured for only one 11-year solar cycle. The results
indicate that over decadal time scales solar cycle forcing
Page 67
is a sizable contributor to trends in global ozone and upper
stratospheric temperatures. This is illustrated in Figure 3.1,
where the solar cycle in global total ozone is compared with the
long term trend deduced from the TOMS observational record. Time
series statistical analyses of the TOMS data (corrected for
instrument degradation) indicate solar cycle changes in globally
averaged total ozone of the order of 1.8 ± 0.3 percent, in
phase with the activity cycle (Stolarski et al., 1991, Chandra,
1991, Hood and McCormack, 1992). Ground based data signify a
slightly smaller effect, in the range 1.2 ± 0.4 percent to
1.5 ± 0.9 percent (Reinsel et al., 1988; Bojkov et al.,
1990). The trends are larger at higher latitudes than in the
tropics (Randel and Cobb, 1994; Reinsel et al., 1994a).
Both spacecraft and ground based observations indicate that
significant solar cycle changes are also occurring in the vertical
ozone profile. Global upper stratosphere (about 45 km) ozone is
estimated from SBUV data to increase by 3 to 4 percent from the
minimum to the maximum of the 11-year solar cycle (Hood et al.,
1993). Similar results were found by Reinsel et al. (1994b) using
northern hemisphere mid-latitude Umkehr data (revised for aerosol
effects). Changes of this magnitude may also be occurring in the
lower stratosphere (about 20 km), with a reduced solar cycle effect
of about 1 percent near 30 km. For comparison, the data indicate
upper stratosphere depletion at northern mid-latitudes of the order
of 6 percent per decade and about 3 percent per decade in the lower
stratosphere (Reinsel et al., 1994b), which thus may be masked by
solar-cycle effects for a number of future solar cycles (Figure
3.1). In addition, solar cycle variations have been reported in
both stratospheric temperatures and winds (Mohanakumar, 1989;
Kodera and Yamazaki, 1990; Chanin and Keckhut, 1991; Hood et al.,
1993).
That changing UV radiation throughout the 11-year solar activity
cycle should evince a measurable effect on upper stratospheric
ozone and temperature has been predicted by atmospheric modeling
studies over the past decade. Assuming an irradiance increase at
205 nm from solar minimum to solar maximum ranging from 6 to 9
percent based on available spacecraft data, and using H I Lyman
and He I EW
surrogates for the variations in UV flux with time, Wuebbles et al.
(1991) determined from a two-dimensional
chemical-radiative-transport model of the global middle atmosphere
a corresponding increase in total ozone of 1.2 to 1.7 percent. The
largest local changes, about 2 to 3 percent, occurred at about
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40 km altitude, while maximum temperature changes, 0.75 to 1 K,
were determined near the stratopause. Contrary to the observations,
significant solar-induced ozone changes in the lower stratosphere
are not simulated by two-dimensional middle atmosphere models;
equally small changes were found in a 2D model study by Huang and
Brasseur (1993). Furthermore, when downward transport of the upper
atmosphere nitric oxide (NO) enhancement at solar cycle maximum is
included in 2D model simulations, there is significant ozone
depletion, resulting in an antiphase global total ozone change,
also contrary to the observations.
Clearly, substantial uncertainties exist in determining the
response of middle atmosphere ozone and temperature to solar
radiative forcing. In part, this is because the magnitude of the
solar flux variations at the important wavelengths (200–300
nm) are still uncertain. Perhaps more importantly, a primary
limitation of the 2D models is that dynamical feedbacks are not
available to amplify initial perturbations of the middle atmosphere
by changing UV energy inputs (Balachandran and Rind, 1994). To
clarify the discrepancies between observed and modeled ozone
response to solar forcing it is essential to continue to monitor
the relationships between variations in the middle atmosphere
parameters and observed solar irradiance variations, both for the
27-day rotation and the 11-year solar cycle, as well as longer term
trends. Because the prime space based global data sets extend over
only one 11-year solar activity cycle, more definite studies of,
for example, the role that might be played by dynamics and by the
QBO in the observed solar cycle variation of ozone, await longer
observational records.
Effects from Solar Proton Events
During a solar proton event, the primary energetic protons
produce secondary electrons by impact on atmospheric molecules, and
these electrons can efficiently dissociate molecular nitrogen,
producing a range of odd-nitrogen species by subsequent chemical
reactions. The odd-nitrogen, in turn, catalytically destroys ozone
in the stratosphere. This affects the Earth's surface environment
directly through an increase in the solar ultraviolet flux and may
have indirect effects via coupling of the middle atmosphere to the
troposphere. Ozone is also depleted at higher altitudes in the
mesosphere as a consequence of the dissociation of water
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vapor, producing odd-hydrogen that destroys ozone at these
altitudesthrough a catalytic reaction sequence.
The odd-nitrogen and odd-hydrogen effects on ozone have been
observed directly by satellites and rockets during solar proton
events (e.g., Weeks et al., 1972; Crutzen et al., 1975; Heath et
al., 1977; Thomas et al., 1983) and have been studied theoretically
by several groups (e.g., Jackman et al., 1990; Reid et al., 1991).
As demonstrated in Figure 3.4, the August 1972 solar proton event
was one of the largest events in the past 30 years. Substantial
increases in nitrogen oxides were expected to be associated with
this event. Available satellite measurements and modeling studies
indicate ozone depletions >20 percent in the upper stratosphere,
with depletions >15 percent persisting for about two months
after the event, indicating a slower recovery time than is
estimated by two-dimensional modeling studies (Jackman et al.,
1990).
In a sense, each major solar proton event can be looked on as a
controlled experiment, in which a calculable amount of NOx is introduced into the atmosphere, and
the subsequent effects can be monitored. These events are thus a
potentially valuable source of information on the behavior of the
atmosphere, providing a unique means of testing the validity of
model predictions (Jackman and McPeters, 1987; Jackman, 1991).
Also, over the longer time scales of the 11-year solar cycle,
variations in the downward transport of nitrogen oxides produced by
auroral particle precipitation at altitudes in the thermosphere
(Garcia et al., 1984) and other solar particle effects could
further affect stratospheric temperature and ozone. Understanding
these processes, however, requires accurate monitoring of the
particle influx at the top of the atmosphere, since this is the
basic information needed to calculate the NOx production. Since the most relevant
effects are those occurring at the lowest levels in the atmosphere,
the high-energy spectrum must be reliably known beyond energies of
100 MeV. Routine spacecraft monitoring of energetic-particle fluxes
has generally been carried out only for energies much less than
this.
Effects from Relativistic Electron
Precipitation
Various studies have suggested that relativistic electron
precipitation events may be important contributors to the
odd-nitrogen budget of the middle atmosphere (Thorne, 1980; Baker
et al., 1987; Callis et al., 1991).
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Thorne (1980) indicated that the odd-nitrogen and odd-hydrogen
formed in the upper stratosphere and lower mesosphere by the ion
chemistry associated with these precipitation events can lead to
destruction of ozone. Although these studies suggest that most of
the production of the odd-nitrogen from such events occurs at
subauroral latitudes in the lower mesosphere, the downward polar
transport during the winter and spring may provide a source of
stratospheric nitrogen oxides. Callis et al. (1991) have suggested
that this phenomenon may explain a significant fraction of the
measured lower stratospheric ozone decrease in the 1978-1986
period. However, as discussed earlier, there remain many
uncertainties associated with the amount of nitrogen oxides
produced and the resulting effects on lower stratospheric
ozone.
Ultraviolet Radiation Reaching the
Biosphere
The amount of solar ultraviolet radiation reaching the biosphere
is extremely sensitive to the amount of overhead ozone, especially
at the UV-B wavelengths that coincide with a sharp decrease in the
ozone (O3 ) absorption at about 310
nm (Figure 1.2), the edge of the O3
Hartley band. Therefore, the amount of ozone in the middle
atmosphere, and its response to natural and anthropogenic
influences, is extremely important for determining potential
impacts on the UV-sensitive biosphere (Madronich, 1992). Small
reductions in ozone column density are expected to cause an
increase in the incidence of skin cancer (Madronich and de Gruijl,
1993) and in blindness due to UV-B induced cataracts, as well as
decreased agricultural and fisheries production. Biological immune
systems, as well as the oxidizing capacity of the troposphere, are
also expected to be affected.
Although measurements indicate that total ozone has been
decreasing over the past two decades, and theoretical calculations
predict a corresponding increase in the UV radiation reaching the
Earth's surface (Madronich, 1992), experimental evidence for this
increased UV exposure is at present inconclusive. Factors such as
tropospheric ozone, sulfate particles in the atmosphere, and
cloudiness can produce opposite effects (Crutzen, 1992).
Substantial downward, upward, and null trends have all been
reported (Justus and Murphey, 1994). At the same time, there
are
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questions about the adequacy of the primary device used in these
measurements, the Robertson-Berger meter. The measurements require
not only accurate absolute radiometry, but also high wavelength
resolution and accuracy, because the ozone absorption spectrum at
the edge of the Hartley band is so steep. The spectral intensity of
solar radiation received at Earth drops by eight orders of
magnitude in the spectral band from 300 to 280 nm. The data from
the existing ground based network probably lack adequate
calibration stability, limiting their suitability for trend
analyses.