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6—
Understanding the Variable Sun
Background
Changes in the energy from the Sun potentially could influence
global change directly by modifying the Earth's surface temperature
(Chapter 2) and by creating and destroying atmosphere ozone at
variable rates (Chapter 3). Solar variability may also influence
global change indirectly, by modifying the middle atmosphere, which
is connected chemically, dynamically, and radiatively with the
troposphere/biosphere (Chapter 3). In the upper layers of the
Earth's atmosphere, and in the geospace environment, solar
variations cause dramatic changes that are critical for
understanding the processes within those regions, although the
extent to which these changes couple to lower atmospheric layers
(Chapters 4 and 5) is uncertain.
Observations over the past decade have provided an exciting
perspective on how the Sun's energy inputs to the Earth change with
time. In this period were obtained the first long term records from
space of the solar radiative energy inputs to the Earth that are
critical for studying solar influences on global change: total
solar irradiance and the solar UV spectral irradiance, as well as
fluxes of energetic protons and electrons. Ground based
measurements were also made of solar observables closely related to
the energy inputs measured from space. Physical associations
between open field regions on the Sun, high speed solar wind
streams, coronal mass ejections, and geomagnetic activity were
established through
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a variety of space missions. Taken together, these observations
have revealed new insights into how solar magnetic activity
modulates terrestrial solar energy inputs and how magnetized plasma
from the Sun evolves as its flows to the Earth. These observations
have established beyond doubt that the Sun's energy output varies
continuously on all observed time scales.
Predicting, understanding, and monitoring global change are the
ultimate objectives of the USGCRP (Chapter 1). Yet contemporary
measurements of solar energy inputs alone reveal little about
future solar variability nor of past solar variations that might
have influenced the paleoclimate record, which is the focus of the
Earth System History science element of the USGCRP. To begin to
understand how the Sun varied in the past and how it might vary in
the future, we must first understand why the Sun varies at all.
The fundamental physical processes that generate the variations
observed in solar energy production are associated with the 22-year
magnetic cycle of the Sun. The sunspot number time series remains
the principal historical indicator of this cycle, and it is shown
in Figure 6.1. This is the record of solar activity that was
compared with the 14C and
temperature time series in Figure 1.3 and with surface temperature
anomalies in Figure 2.4. Recent monitoring from space indicates
that both the total solar irradiance (Figure 2.1) and the UV
irradiances (Figure 3.2) increase near the peak of the sunspot
cycle and decrease during times of few sunspots. Likewise, the flow
of energy, plasma, and magnetic fields from the Sun into the
Earth's environment depends on the magnetic cycle. Fundamental to
understanding the Sun's behavior as a variable star is
understanding how variations in its emitted energy are generated
from the magnetic activity cycle.
Origins of Solar Variability
The 22-year magnetic cycle of the Sun manifests itself as the
familiar 11-year sunspot cycle, the 22-year cycle being simply two
11-year cycles having reversed magnetic field polarities.
Physically, the sunspot cycle is a roughly periodic emergence,
approximately every 11.1 years, of strong magnetic flux tubes at
the solar surface in the form of sunspots. More
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Figure 6.1 Solar activity variations during the
past four centuries, as indicated by monthly means since 1750 of
the sunspot number (solid line) with yearly means from 1610 to 1750
(from the National Geophysical Data Center, dashed line, and Eddy,
1976, crosses). Prominent in the record is a cycle of about 11
years. According to Eddy, the 11-year sunspot cycle was severely
depressed between 1645 and 1715 (see also Ribes et al., 1989).
Thus, during the contemporary epoch when the Sun has been observed
most intensely, its activity has been at relatively high levels
compared with the past 300 years. This is confirmed by cosmogenic
isotope records (McHargue and Damon, 1991). The Sun is likewise at
high activity levels compared with other Sun-like stars (Figure
2.3). Recent analysis of sunspot observations in the 18th century
indicate that the sunspot number may overestimate solar activity
levels in that period (Hoyt et al., 1994). From J. Lean, Reviews of
Geophysics, 29, 506, 1991, copyright by the American Geophysical
Union.
generally, the solar activity cycle pertains to the periodic
emergence ofmagnetic flux that generates not just sunspots, which
are dark, but avariety of phenomena, especially bright regions
known as plages andfaculae that radiate strongly at UV and EUV
wavelengths. The darksunspots and bright plages and faculae modify
the radiation from the solardisk, thereby generating the variations
observed by spaceborne solarradiometers. Also, changes in the Sun's
magnetic field topology, due to
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both flux tube emergence and latitudinally differential rotation
of the solar atmosphere, generate field configurations that lead to
transients such as solar flares and coronal mass ejections, and
longer lived features such as coronal holes. These latter phenomena
affect the Earth through input of high energy particles and plasma
into the geospace environment.
Solar flares and solar global oscillations are prominent
examples of solar energy variations on time scales of seconds to
hours. Flares, although very energetic, occur over sufficiently
small fractions of the solar hemisphere that even the largest and
most energetic of them do not enhance total solar irradiance more
than a hundredth of a percent. Nevertheless, enhancements in high
energy emissions, such as EUV and X-rays, can be dramatic. The
solar eruptions with which flares are associated also produce
significant fluxes of energetic particles (electrons and protons
and other nuclei). The enhanced EUV and particle fluxes from flares
and coronal mass ejections can significantly alter the ionization
state of the Earth's middle and upper atmosphere. At times when
many eruptions occur in succession, the effects may persist for
many months, as demonstrated by the semicontinuous solar proton
events during 1989–1990 (Reid et al., 1991), discussed in
Chapter 3.
Like flares, global oscillations of the Sun, such as the five
minute p-mode oscillations, have minimal effect on total solar
irradiance — on the order of 3 parts per million of the total
flux (e.g., Hudson, 1987). The significance of the solar
oscillations is that they provide a unique technique for sensing
physical properties and variations of the solar interior (Leibacher
et al., 1985; Gough and Toomre, 1991), thus providing crucial
insight into the mechanisms of the solar cycle.
The Sun's 27-day rotation modulates the steady radiative and
plasma outputs from the Sun because at times (especially near
activity maxima) the sources of these outputs are localized in
narrow heliocentric longitudinal bands and are sufficiently
long-lived that they reappear on successive rotations (see Lean,
1987, for examples), Rotational modulation is clearly seen in the
total solar irradiance, with a rather complex waveform that
reflects the competing effects of dark spots and bright faculae.
Although 27-day periodicity is attributable to the Sun's rotation,
no physical models explain the emergence, evolution, and decay of
the magnetic active regions on the solar surface that cause changes
in the amplitude and phase of this cycle. Since the 27-day rotation
modulates solar irradiance by changing
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the amount of active region emission seen at the Earth,
observations and investigations of this periodicity aid in
understanding the origins of the geomagnetic and photon flux
variations that are important for the terrestrial environment
(e.g., Lean, 1987).
Nor is the emergence of magnetic activity with an 11-year
periodicity properly understood. Conceptually, a dynamo process is
thought to underlie the Sun's magnetic cycle (e.g., DeLuca and
Gilman, 1991), and models have been constructed to provide insight
into the specific basic solar characteristics that must change to
produce the periodic magnetic flux tube emergence that is
responsible for solar energy input variations to the Earth. These
models are necessarily theoretical, based on the internal
interactions of a plasma rotating differentially within a
convective envelope. No widely accepted solar dynamo model can
reproduce all of the observed features of the solar cycle. A
particular problem is the models' inconsistency with
helioseismologic observations of the internal rotation of the Sun
(e.g., Leibacher et al., 1985), an essential component of any model
of flux emergence.
Even though the 11-year solar activity cycle appears to be the
fundamental cause of changes in terrestrially sensed energy on
decadal time scales, the possibility that long term changes may
also be occurring in the Sun because of non magnetic mechanisms
cannot be ignored. Short (five-minute) p-mode oscillations arising
from a natural cavity resonance inside the Sun are well
established. These oscillations are a property of the global Sun, a
star, as opposed to localized sources of fluctuation such as
sunspots and plages/faculae. Other solar features, such as
ephemeral magnetic regions and the chromospheric grains seen in Ca
IIK, contribute to the energy output and are widely distributed
over the Sun.
Over times much longer than the 11-year activity cycle, there is
the possibility of superimposed slow secular change in the Sun's
energy inputs to the Earth. The Maunder Minimum (Eddy, 1976), seen
clearly in the sunspot record in Figure 6.1, is evidence for such
change and the only example in the contemporary solar record. The
absence of sunspots and possible supression of the 11-year cycle
during this extended period suggests that the flux emergence
process may have stopped completely for several decades. Relative
to the present, the Sun's internal circulation may have been
substantially different (Eddy et al., 1976), and its diameter
larger (Nesme-Ribes et al., 1993). Indirect evidence for many
similar
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episodes in solar behavior comes from the radiocarbon and
auroral records (Eddy, 1976), but only very recently has it been
recognized that substantial decreases in the Sun's radiative output
might have accompanied these episodes (White et al., 1992; Lean et
al., 1992a). The 14 C data also
suggest that solar activity was very high in the twelfth century,
an epoch corresponding to the Medieval Warm Period of approximately
300 years (Figure 1.3). Eddy (1976, 1977) suggests that the total
irradiance may follow the envelope of the sunspot cycle curve,
waxing and waning on century time scales. These findings indeed
suggest a relationship between the Sun and the climate, but a solar
variability model that describes the Sun's energy inputs to Earth
at times in the past has yet to be developed.
Because the Sun is apparently a normal star, insights into solar
variability can be gleaned from observations of Sun-like phenomena
in stars with mass, age, and rotation rates similar to the Sun's
(Baliunas, 1991). In particular, comparative solar and stellar Ca
II emission measurements indicate that the activity levels of the
contemporary Sun correspond to the highest levels observed in other
stars (White et al., 1992). These observations also suggest that
times of arrested activity, as exemplified on the Sun by the
Maunder Minimum, may be common in other stars, and that such times
appear to be accompanied by reduced energy output (Baliunas and
Jastrow, 1990; Lean et al., 1992a).
Relationship Between Solar Surface
Structure
and Energy From the Sun-as-a-Star
To better understand the relationships between changes on the
Sun and changes in the Earth's atmosphere, it is important to
understand how measurements made at the Earth at 1 astronomical
unit (AU) project back to structures on the solar surface.
Radiation
The transmission path of solar radiative input to the Earth is
line-of-sight. Active regions (sunspots, plages and faculae,
filaments, coronal holes, etc) modify the local intensity of the
solar surface, resulting in an inhomogeneous solar disk whose
radiation field is variable in both
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time and direction. As shown in Figure 6.2, these surface
features evolve continuously throughout the solar activity cycle,
and they have the largest effect near times of activity maxima. The
radiative energy input to the Earth is the integral of the radiance
from the entire solar hemisphere visible at the Earth--that is, the
irradiance. As a result of this integration, the interpretation of
variations observed in the total and spectral solar irradiances
involves constructing irradiance time series by combining the
contributions from sunspots, plages, the bright magnetic network,
and internetwork regions. In this way, variations in both total and
ultraviolet spectral irradiance can be traced to the changing
populations of active regions on the solar disk.
Ground based observatories measure the position, brightness, and
area of sunspots, plages, and faculae daily (e.g., Beck and
Chapman, 1993). These data have been combined in simple empirical
models to reconstruct the measured irradiances (Cook et al., 1980;
Lean et al., 1982; Foukal and Lean, 1988; Willson and Hudson,
1991). Comparisons of the model calculations with the measured
irradiances show that the principal contributors to radiative
variability are sunspots, plages/faculae, and chromospheric
network, both on solar rotation time scales as well as over the
decadal scale of the solar cycle. Residuals between the
measurements and reconstructions are analyzed for the possibility
that they arise from experimental error, incorrect assumptions in
the models, or missing energy in storage deep in the Sun.
In making the connection between fluctuations in irradiances and
solar surface inhomogeneities, two types of ground based data have
been crucial. One is photometric data on sunspots, faculae, and
plages, and the other is spectral irradiance data; both are
measured in appropriate regions of the solar spectrum. The
photometric data provide records of the individual active regions
that generate irradiance variations (Figure 6.2). Sunspots are
identified most clearly in white-light solar spectroheliograms,
whereas images in the Ca II K line remain the principal source of
photometric data on plages and the network, features that are also
seen clearly in solar images in other spectral lines, such as the
He I 1083 nm line. The spectral irradiance (Sun-as-a-star) data
measure the integrated effects of these active regions on layers of
the Sun from which the total and UV radiations are also emitted.
Figure 6.3 shows three of the most informative solar records: the
Sun-as-a-star chromospheric Ca II and He I indices (White
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Figure 6.2 Active regions on the Sun are seen in
spectroheliograms of the photosphere and chromosphere, taken in the
wings and at the center, respectively, of the Ca II K line during
solar minimum (1975) and solar maximum (1979). Surface
inhomogeneities associated with magnetic active regions can been
seen as regions of enhanced brightness (plage and faculae) and also
as small dark regions (sunspots). Both the area and the number of
such magnetic regions increase at times of solar activity maxima,
relative to activity minima. Spectroheliograms from R. Howard. From
J. Lean, J. Geophys. Res., 92, 843, 1987, copyright by the
AmericanGeophysical Union.
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Figure 6.3 Variations during solar cycles 21 and
22 of ground-based solar indices of high relevance for interpreting
solar irradiance variations. In the upper panel is the
chromospheric Ca II K index (ratio of the core to wing emission of
the Fraunhofer line at 393.37 nm) (White et al., 1990). In the
middle panel is the primarily chromospheric He I 1083 nm line
equivalent width (Harvey, 1984; Harvey and Livingston, 1994) and in
the lower panel is the sunspot blocking function (calculated
according to Foukal, 1981). In each figure , the solid lines are
the variations smoothed over approximately three 27-day solar
rotations. Both the Ca II and He I time series are useful proxies
for solar radiative output from bright faculae while the sunspot
blocking is a parameterization of the emission deficit caused by
dark sunspots. Based on J. Lean, Reviews of Geophysics, 29, 513,
1991, copyright by the American Geophysics Union.
and Livingston, 1981; Harvey, 1984; Livingston et al., 1988;
White et al.,1990) and the sunspot blocking. In this regard, the
solar 10.7 cm radioflux is also important, especially in an
historical context, since it was theonly Sun-as-a-star indicator
measured during solar cycles 19 and 20.
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Figure 6.4 shows a comparison of the total solar irradiance
measured by ACRIM I (on SMM) and by Nimbus 7/ERB with models
developed from ground based data for the respective data sets. The
rotational modulation data during 1982 illustrate that the
day-to-day variation that arise from the competing effects of dark
sunspots and bright faculae are well reproduced by the Foukal and
Lean (1990) model. Much of the longer term variability during solar
cycle 21 and the ascending phase of cycle 22 is also reproduced by
this model, with the exception of the first years of the record.
The longer term solar cycle changes occur because of a brightness
component in addition to the sunspots and the brightest faculae
associated with magnetic active regions. While the existence of
this additional 11-year variability component has not been verified
by direct observation, it is thought to reside, at least in part,
in the network of bright emission that surrounds the large active
regions (Foukal and Lean, 1988; Foukal et al., 1991). It may also
have a global (i.e., non magnetic) component (Kuhn et al., 1988;
Kuhn and Libbrecht, 1991). Discrepancies between the measurements
and the model during 1979–1980 may be instrumental in origin.
Ifnot, they raise the possibility of a variability component acting
over time scales longer than the 11-year cycle (Lee III et al.,
1994). The utility of these models, and the need to resolve
discrepancies with the measurements, emphasize the need for high
precision image data in the interpretation of irradiance time
series in the future.
Statistical comparisons indicate that the Ca II and He I indices
provide reconstructions of solar UV irradiance variations superior
to those afforded by the 10.7 cm radio flux, over both solar
rotation and solar cycle time scales (Barth et al., 1990; Bachmann
and White, 1994). Nevertheless, the 10.7 cm flux has been used
extensively for the past few decades as a proxy for EUV and UV
irradiance variations in terrestrial applications. For example,
essentially all analyses of ozone data for the purpose of
extracting long term trends have used the 10.7 cm flux, in lieu of
UV irradiance data, to account for solar forcing of ozone changes
(Stolarski et al., 1991; Hood and McCormack, 1992; Randel; and
Cobb, 1994; Reinsel
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Figure 6.4 Comparison of the SMM/ACRIM I and
Nimbus 7/ERB total solar irradiance data with empirical variability
models constructed from the respective radiometry. In the upper
panel, daily values of a model developed by Foukal and Lean (1990)
show good agreement with the measurements over solar rotation time
scales. In the lower panel 81-day running means during cycle 21 and
cycle 22 of the solar irradiance data and the Foukal and Lean
(1990) model show similar variations over time scales of active
region evolution. Differences between the measurements and models
are largest near maximum solar activity, especially prior to 1981.
In Schatten's (1988) model, maximum total irradiance is postulated
to occur prior to the peak of the activity cycle because of the
role played by faculae at high heliocentric latitudes, which are
thought to occur more frequently during times of minimum solar
activity. This high latitude facular emission was postulated to
help explain the discrepancy, possibly of instrumental origin,
between the Foukal and Lean model and the irrandiance data prior to
1981. Based on J. Lean, Reviews of Geophysics, 29, 520, 1991,
copyright by the American Geophysical Union.
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et al., 1994b) (Chapter 3). With the availability of ground
based data that better reflect the processes that generate solar
radiative output variations, empirical models that predict these
variations can be improved. Knowledge of the relationship between
the 10.7 cm radio flux and solar radiative output nevertheless
remains important for its historical relevance.
Plasma and Particles
Plasma from the solar corona flows radially outward from the Sun
and impacts the Earth as the solar wind. Frozen into this plasma is
the interplanetary magnetic field (the extended field of the Sun).
Solar rotation, combined with the outward motion of the highly
conducting plasma, winds this field into a spiral pattern and
produces high speed and low speed streams. High speed solar wind
streams (velocities of 700 to 850 km/sec) originate in regions
where the magnetic field lines are open and connect to the
interplanetary field; in contrast, the slow streams (velocities of
300 to 400 km/sec) come from regions above sunspot complexes where
the magnetic field is closed to the solar surface. Shocks are
formed when a high speed stream overtakes a low speed stream.
Coronal mass ejections periodically disrupt the quasistationary
pattern of high and low speed flows from coronal holes and
streamers. The projection of solar wind profiles at the Earth back
to their origins on the Sun can therefore be ambiguous, which
emphasizes, again, the importance of knowledge of solar magnetic
field structure in determining the time profile of solar energy
input at 1 AU.
Solar energetic particles carried to the Earth by the solar wind
interact with the Earth's magnetic field in ways that depend on the
spectral distribution of their energy. The particles spiral along
terrestrial magnetic field lines, entering atmosphere primarily in
the auroral zones surrounding the two geomagnetic poles (Figure
3.3). Projection of this complex field interaction backward to the
Sun is aided by solar images in the visible spectrum that locate
the sites of flares that are frequently associated with the
eruptive events that are the sources of these particles. At times
of high solar activity, however, the occurrence of numerous
eruptions makes identification of the source of the particle flux
more difficult.
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Representative terms from entire chapter:
solar irradiance
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Cosmic Rays
The diffuse cosmic ray flux that produces 14C in the Earth's atmosphere (see
Chapters 3 and 5) is modulated by solar activity on passing through
the heliosphere (e.g., Lopate and Simpson, 1991). The particle flux
at the Earth is highest when solar activity is at a minimum, so
that the naturally archived 14C
record contains the signature of past solar variability excursions.
Interpreting the 14C record
depends on understanding this modulation process as well as other
effects, such as changes in magnetic field strength and in the
terrestrial carbon cycle (Stuiver and Braziunas, 1993). Until there
is a solid appreciation of the connection between the cosmogenic
isotope variations and the modulation of the energy output from the
Sun by magnetic active region phenomena (sunspots, plages,
network), it will be difficult to construct reliable quantitative
estimates of the strengths of extrema in solar energy, such as
during the Maunder Minimum type episodes that appear to have
occurred commonly (every 200 years or so) in the past.
Requirements For Improved
Understanding
Present
Since relatively reliable knowledge of solar radiative output
variations exists for only the most recent 11-year solar cycle,
much will be learned by the continuation and improvement of the
total and UV spectral irradiance observations from space and of the
ground based indicators of the various solar processes that cause
and reflect their variations. The required ground based data are
primarily white light images of sunspots and the Ca II and He I
indices and images. The images (preferably of a few arc-seconds
resolution or better, with accurate photometric calibration) are
necessary not only for the formal construction of irradiance time
series from ground based surrogates, but also to allow fundamental
descriptions of spots, plages, faculae, and network as they evolve
in time. Continuous daily measurements are needed at least for the
duration of the Sun's magnetic cycle of 22 years, and more ideally
for 100 years or longer, since the 88-year Gleissberg cycle may
represent the dominant forcing factor.
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The basic process that leads to variations in solar energy input
to the Earth is the emergence and evolution of magnetic flux tubes
in the solar atmosphere. The principal observational needs are for
better photometric and positional data pertaining to the flux tubes
that form sunspots, plages, faculae, the magnetic network and their
motions on the solar disk. Theoretical models are needed to
understand the basic energetics in these flux tubes and how the
magnetic field interacts with solar convection to change the energy
transport. Since this affects the brightness of magnetic structures
throughout the solar spectrum, it bears directly on interpretation
of the full disk observations.
An important tool in interpreting measurements of solar
radiative output is the comparison of the measurements with
synthesized solar spectra (Kurucz, 1991; Mitchell and Livingston,
1991; Avrett, 1991). Theoretical reconstructions of the solar
spectrum, although based on one-dimensional thermodynamic and
structure models, include millions of solar spectral lines from EUV
to infrared wavelengths and provide insight into the spectral
composition of the solar irradiance. At present it is not certain
how the spectral energy distribution of the total irradiance
changes over the solar cycle. The theory of the formation of the
solar spectrum also establishes physical connections between
different wavelengths, providing insight into the extent that
variations at one wavelength mimic variations at another.
Consideration of such connections may lead to more efficient
observational programs in the future.
Magnetic flux tube emergence is thought to be associated with a
dynamo lying deep inside the Sun. The goal of stellar dynamo models
is to reproduce the periodic variation of flux tube emergence seen
on the Sun and also to show the systematic latitude and polarity
variations that occur as spots move from high latitudes toward the
equator as the cycle progresses, with the polarity of sunspot pairs
and the polar fields reversing from one cycle to the next. The
necessary empirical boundary conditions for such models come from
detailed knowledge of magnetic flux tubes, their distribution in
both time and position on the Sun, and evolution of their energy
transport seen in observations of radiative output. The most
important constraints lie in the differential rotation structure of
the solar interior and the properties of the interface between the
convection zone and the Sun's radiative core, where the solar
dynamo is thought to lie. Helioseismic observations continue to
play a crucial role in studying the
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dynamo because they provide a way to probe the solar interior,
but they will illuminate the dynamo problem only if measurements
extend over solar cycle time scales.
Past
In the context of global change, understanding the past behavior
of the Sun may well be essential for unraveling the paleoclimate
record. Coincidences between climate change in the twelfth and
seventeenth centuries and changes in
14C as a proxy for solar activity (corroborated by the 10Be record) suggest that there may be
threshold levels of high and low solar activity at which the Sun
begins to play a significant role in changing global climate (see
Chapter 2). Knowledge of physical conditions on the Sun at these
extrema is primitive because modern scientific observations have
all been made during an era of high solar activity. Galileo's and
others' discovery of sunspots at the beginning of the seventeenth
century came at a time to document an ensuing period of low solar
activity. Without this documentation, the possible role of the Sun
in the Little Ice Age might still not be appreciated. Given the
meager and often discontinuous evidence for solar variability in
the past, inference of the physical state of the Sun at times of
extrema is speculative but necessary. The first steps can be taken
with empirical models now available (e.g., Foukal and Lean, 1990;
White et al., 1992; Lean et al., 1992a; Hoyt and Schatten, 1993;
Nesme-Ribes et al., 1993), but the credibility of such research
would be strengthened immeasurably through development of a
successful physical model of the solar cycle.
An expanded view of solar variability is provided by knowledge
of cyclic behavior in other stars, afforded by long term
measurements of stellar Ca II emissions in Sun-like stars. Baliunas
and Jastrow (1990) present stellar cycle data that may indicate the
presence of Maunder Minimum-type episodes in one-third of the
observed stars, but their sample (13 stars) is so small that this
conclusion must be regarded as speculative at this time. It is,
however, consistent with an independent result from analysis of the
radiocarbon data by Damon (1977). Furthermore, the distribution of
Ca II emission exhibited by the Sun-like stars does appear to be
consistent with the range of Ca II K emission seen in the
present-day Sun (White et al., 1992). By assuming that during the
Maunder Minimum,
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the mix of active region emission that generates the solar Ca II
irradiance was somewhat different than is currently seen in the
Sun, the radiative output of the Sun can be estimated for the
Maunder Minimum (Figure 2.3).
Future
Current ability to predict solar activity is at best primitive.
Statistical methods predict sunspot numbers and the 10.7 cm radio
flux 12 months in the future with moderate success. There are also
precursor methods that predict the strength of the next solar cycle
from the behavior of polar structure on the Sun and geomagnetic
activity in the declining phase of the current cycle (e.g.,
Schatten and Pesnell, 1993; Thompson, 1993). But there is limited
physical understanding of why these precursor methods should be
appropriate except that the magnetic fields and corona near the
solar poles change near solar maximum and hence may herald the
onset of the new cycle before the next generation of sunspots
appears.
On century time scales, the periodicities of 11 and 88 years
identified in the sunspot record, together with the 208 year
periodicity found in the 14C
record, provide limited guidance to future solar behavior, such as
the occurrence of the next Maunder Minimum. The time span of solar
measurement is simply too short for reliable prediction of solar
extrema occurring sporadically every 200 years or so. Nevertheless,
it has been speculated that the concatenations of the 208 and 88
year periods may have contributed to generally increasing solar
activity levels during the twentieth century, with maximum activity
predicted to occur during the first half of the twenty-first
century (Damon and Sonnet, 1991).
Predictive capability will be substantially improved when a
complete understanding is obtained of the mechanisms within the
solar atmosphere that produce the emitted radiation, and form
sunspots and plages. Predicting the Sun-as-a-star energy quantities
needed for global change studies will ultimately require
development of a theory for the solar dynamo that can accommodate
known solar behavior. Yet, the very nature of solar variability,
whether driven by an internal chronometer (Dicke, 1978) or by
stochastic or chaotic processes (Mundt et al., 1991; Morfill et
al., 1991; Kremliovsky, 1994), remains elusive. Solar activity
levels may well defy reliable prediction in the near future.