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Adv. Space Res. Vol. 29, No. 10, pp. 1417-1426,2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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A DISCUSSION OF RECENT EVIDENCE FOR SOLAR IRRADIANCE VARIABILITY AND CLIMATE

Judit Pap,’ Claus Frijhlich,2 Jeff Kuhn,3 Sabatino Sofia,4 and Roger Ulrich’

‘University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1562, USA 2Physikalisch-Meteorologishes Observatorium, Davos, 33 Dorfstrasse, CH-7260, Davos-Dorf, Switzerland 3 University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822, USA 4 Yale University, New Haven, CT 06511, USA

ABSTRACT

One of the over-arching questions, among others, to be addressed by studying Sun-Earth connections is: “Is the climate changing in a way we can understand and predict ?” The Earth’s climate is the result of a complex and incompletely understood system of external inputs and interacting parts. Climate change can occur over a range of time scales, may be driven by natural variability, including solar variability, and/or anthropogenic causes and may be identified through the study of a variety of measurable parameters. Global climate change in response to human influences is one of the pressing threats facing science today. However, many of the external factors that govern our climate, including solar variability, cannot be adequately determined from existing operational observations. Since the Sun is the fundamental source of energy that sustains life on Earth, establishing its radiation environment, controls its temperature and atmospheric composition, the accurate knowledge of the solar radiation received by the Earth and understanding of its variability are critical for environmental science and climate studies. In this paper we point out the necessity of a new strategy, i.e., to study global solar properties, such as solar irradiance, solar shape, shape oscillations, and radius, to better understand the origin of solar-induced climate changes. 0 2002 COSPAR. Published by

Elsevier Science Ltd. All rights reserved.

INTRODUCTION

Study of the Sun’s variability has been of high interest for both astrophysics and solar-terrestrial physics. The Sun, a fairly typical star, has the special advantage of proximity which allows the detailed study of a variety of phenomena important for stellar physics. High precision photometric observations of solar-type stars clearly show that year-to-year brightness variations connected with magnetic activity are a widespread phenomenon among such stars (e.g. Radick, 1994). Measurements of the solar energy flux integrated over the entire spectrum, hence total irradiance, started at the beginning of this century, first from the ground and later on from balloons, aircraft, and rockets. However, these early irradiance measurements could not reveal variations in total irradiance, mainly because of the effect of the selective absorption of the terrestrial atmosphere (Frtihlich, 1977). Continuous space observational programs of solar irradiance (both bolometric and at various wavelengths) started about two decades ago. These irradiance observations have convinced the skeptics that solar irradiance at various wavelengths and in the entire spectrum is changing with the waxing and waning solar activity (e.g. Withbroe and Kalkofen, 1994). Although the overall pattern of solar irradiance variations is similar at various wavelengths, being higher during high solar activity conditions, remarkable differences exist between the magnitude and shape of the observed changes. These differences result from the different physical conditions in the solar atmosphere where the irradiance originates from.

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1418 J. Pap et al.

In addition to the astrophysical significance of solar irradiance variations, their terrestrial implications are equally important,. As the solar energy flux is deposited with different fractions in the Earth’s atmosphere, oceans and land, it controls the heating, ionization, radiative, chemical, and dynamical processes character- izing the terrestrial atmosphere-ocean-land system. The total radiative output of the Sun establishes the Earth’s radiative environment, and it is one of the major natural driving forces for the Earth’s climate. This predominantly continuum radiation originates in the solar photosphere, and its major part reaches the troposphere and the Earth’s surface and oceans. As the solar radiation is the ultimate energy source, any change in the rate of solar energy output may influence the Earth’s climate system on time scales ranging from decades to millenia. Therefore, the accurate knowledge of the solar radiation received by the Earth and its temporal variations, especially over decades, is critical for an understanding of the role of solar variability in climate change and the climate response to increasing greenhouse gas concentrations.

The two-decade long space-based irradiance observations have demonstrated that solar total irradiance varies on all time scales between minutes to decades. The effect of granulation and supergranulation has been identified in the power spectrum of total irradiance on time scales of minutes to hours (Frahlich et al, 1997). The variations on the 5-minute time scale are due to the p-mode oscillations with amplitudes of a few parts per million (Woodard and Hudson, 1983). On time scales of days to months, the evolution of active regions causes changes in solar irradiance via the combined effect of sunspots, faculae, and other photospheric changes (Friihlich and Pap, 1989). The most striking events in the short-term total irradiance variations are the sunspot-related irradiance dips with amplitudes up to 0.3% (Willson et al., 1981). The most important discovery of the space-based irradiance measurements is that total irradiance varies with about 0.1% over the solar cycle, being higher during maximum activity conditions (Willson and Hudson, 1988). Since even small variations in total irradiance over long time scales may lead to climate changes (e.g., Eddy, 1977; Hansen et al., 1993; Lean et al., 1995), it is extremely important to understand the underlying physical mechanisms. On one hand, it has been suggested that the solar-cycle-related long-term variation may be due to global changes in the photospheric energy output (Kuhn et al., 1988, Lydon and Sofia, 1995; Ulrich, 1998). On the other hand, long-term variations in small scale photospheric features may also account for some of the global solar cycle changes (Foukal and Lean, 1988). The fundamental question is to what extent global changes and to what extent photospheric magnetic features contribute to the observed long-term irradiance variations.

SOLAR VARIABILITY AND CLIMATE

Although the existence of possible global changes based on the changing solar output had been doubted and debated for a long time, the results of various space experiments for monitoring solar total and spectral irradiancc opened an exciting new era in both solar and atmospheric physics. It has been established conclusively that the Earth’s climate, radiative environment, and upper atmospheric chemistry are influenced by the varying solar energy flux. The principal question in climate research is the path of the future climatic change. The Earth’s climate is the result of a complex and incompletely understood system of external inputs and interacting internal parts. Climate change can occur over a range of time scales and may be driven by natural variability and/or anthropogenic causes. Observations of steadily increasing concentrations of greenhouse gases --primarily man-made- in the Earth’s atmosphere have led to the expectation of global warming during the coming decades (e.g. Hansen et al., 1993). It has been shown that the global mean temperature increased by about 0.55’C from 1860 to 1990 and the anthropogenic greenhouse gases now cause a global climate forcing of 2-2.5 Wm-’ (e.g. Hansen and Lacis, 1990). More than half of this forcing has been added in the past three decades. Computer simulations show that if the emission of these gases contimles at, the present or increased rates, greenhouse forcing may reach a level of 4-5 Wme2 by the middle of t,hc next century (Hansen and Lacis, 1990). However, the greenhouse effect is in competition with other mechanisms for climate change, such as solar variability, cosmic ray flux changes, and change in the atmospheric aerosols, both natural and man-made. Indeed. the surface temperature increases within the last 130 years may be part of a longer warming trend, which started in the seventeenth century, prior to the industrial period (Bradley and -Jones, 1993).

Evidence for Solar Irradiance Variability and Climate 1419

An important issue in the field of climate change is the degree to which various causal agents may affect climate. Although the Sun supplies most of the energy for the Earth’s atmospheric and climate system, the measured 0.1% level of the long-term total irradiance variations is considered to be too small to cause changes in the Earth’s climate above its intrinsic noise. However, some evidence in the climate record does point to a solar influence on time scales of decades and centuries. For example there is a correlation between global sea surface temperature anomaly and the envelope of the sunspot number record over the last I50 years (Reid, 1991). Friis-Christensen and Lassen (1991) found a direct correlation between the Northern Hemisphere temperature anomalies and the length of the solar cycle, suggesting that a significant portion of the temperature increase may be caused by the Sun. In contrast, Kelly and Wigley (1992), using an energy- balance climate model and the length of the solar cycle as parameter for the solar radiative forcing, have pointed out that greenhouse forcing is the dominant cause of global warming, although the solar contribution is not negligible. Schlesinger and Ramankutty (1992) confirmed the results of Kelly and Wigley (1992), using a simple climate-ocean model. Their analysis demonstrates that although the greenhouse gases and not the solar irradiance have been the dominant contributors to the observed temperature changes, there is strong circumstantial evidence that there have been variations in solar irradiance which have contributed to the observed temperature changes since 1856 (see also Crowley and Kim, 1996).

In order to estimate the full range of possible irradiance variations exhibited by the Sun, it is important to reconstruct solar irradiance changes back to the time of the Maunder Minimum (about 1645 - 1705), when the exceptionally low sunspot activity of the Sun coincided with one of the coldest periods of the Little Ice Age in Europe and the Atlantic region (e.g. Eddy, 1977). A second major cold episode of the Little Ice Age occurred in the first two decades of the 19th century, in rough coincidence with the less-pronounced Dalton Minimum of solar activity, while the global warming since the mid-19th century, that marked the termination of the Little Ice Age, coincided with the general increase in solar magnetic activity over the same period. The range of total irradiance variability necessary to cause Maunder-type climate anomalies has been established by several authors. Irradiance models based on contemporary solar and stellar monitoring (e.g. Lean et al., 1995) suggest that the Sun was approximately 0.2% darker at the time of the Maunder Minimum than currently (see also Hoyt and Schatten, 1993). Analyses carried out by Nesme-Ribes et al. (1993, 1994), using long-term observations of the solar diameter and rotation rate, suggest that the Sun could have been 0.6% darker at the time of the Maunder Minimum than at present. Zhang et al. (1994), using brightness observations of solar type stars, estimated that the solar brightness increased from the Maunder Minimum to the 1980s by about 0.4 & 0.2%.

The Lean et al. (1995) model, based on solar magnetic surrogates, such as sunspots and faculae, and stellar monitoring, also indicates that the reconstructed annual solar irradiance accounts for about 74% of the variance of the Northern Hemisphere surface temperature anomalies in the pre-industrial period from 1610 to 1800, implying a predominant solar influence. In contrast, this model suggests that only about half of the 0.55 OC surface warming since 1860 and about a third of the warming since 1970 may be related to solar effects. Recent analysis of Solanki and Fligge (1998) indicates that the reconstructed solar irradiance correlates rather well with global temperature prior to 1975 and solar irradiance does not lag behind climate. However, this model implies that since 1975 the air temperature increased by 0.2 K, whereas solar irradiance has risen by a disproportionally small amount. White et al. (1997) indicate that sea-surface temperatures and upper-ocean heat contents in each major ocean basin have responded coherently and with similar sensitivities to solar irradiance variations on ll-yr, 21-yr, and secular time scales. Furthermore, Hansen et al. (1981) point out that an increase of solar irradiance by only about 0.3-0.4%, just within the range of possible irradiance changes, is one conceivable explanation of the observed global warmth of the 1930 and 1940s.

In addition to the changes in the solar radiative output, it has recently been shown that the effect of changes in the cosmic ray flux may be crucial for the cloud formation in the Earth’s atmosphere, thus they may also play an important role in climate changes (Swensmark and Friis-Christensen, 1997). The form of the solar impact on this process is completely due to the fact that the solar and galactic cosmic rays are modulated

1420 J. Pap er al.

Days (Epoch Jan 0. 1980)

0 2000 4000 6000 I’ I I I

_z p, 1369

-- 1368

1367

1366

1365

1364

13631, , , , , , / , , , , , , , , / , , , , , , 7879808182838485868788899091 929394959697989900

Year

Figure 1: Total solar irradiance measurements from Nimbus-7/ERB (HF), SMM/ACRIM I, UARS/ACRIM II, EURECA/SOVA, ERBS, and SOHO/VIRGO (upper panel). The composite total irra- diance reconstructed from these data is shown on the lower panel (updated from Frohlich, 2000).

differently by solar activity. While the correlation is positive between the changes of the solar-originated cosmic ray flux and solar activity, there is an anticorrelation between the changes in the more energetic galactic cosmic ray flux and solar activity. The existence of this form of Sun-Earth interaction together with the direct energy transfer effects of irradiance variations emphasizes the importance of a new approach to study solar variability and its impact on the Earth’s atmosphere and climate system.

VARIATIONS IN THE SOLAR ENERGY FLUX

Although the ultimate source of the solar energy is the nuclear reactions taking place in the center of the Sun, the immediate source of the energy is the solar surface. While the nuclear reactions are almost certainly steady on time scales shorter than millions of years, the mechanism which carries the energy to the solar surface may not be. Indeed, observations of total irradiance, helioseismic and precise solar photometric measurements all have shown that the Sun varies as a star during the course of an 11-year solar cycle. If the central energy source remains constant while the rate of energy emission from the surface varies, there must be an intermediate reservoir, where the energy can be stored or released depending on the variable rate of energy transport. The gravitational field is such a reservoir. If the energy is stored in this reservoir, it will result in a change of the solar radius. Therefore, a careful determination of the solar radius can provide a constraint on variations of total irradiance (e.g. Lydon and Sofia, 1995; Sofia, 1998). For example, a radius change of 0.06” would be sufficient to explain a long-term 0.1% variation in total irradiance.

To understand the physical mechanisms of the magnetic cycle in the solar interior, we must learn how to measure the energy flow through the solar interior and the tiny changes it produces in the Sun’s global properties, like its radiative flux, internal temperature distribution, and surface luminosity (Ulrich, 1998). Continuous observational programs of total solar irradiance to detect its variability started about two decades ago from several space platforms. The various space observations of total irradiance are plotted in Figure 1. Analysis of these space-borne total irradiance measurements has revealed variations from minutes to the 11-year solar activity cycle (e.g. Frohlich, 1998). In order to study the climate impact of total irradiance

Evidence for Solar Irradiance Variability and Climate 1421

ACRIM TOTAL SOLAR IRRADIANCE MONITORING Scaling Factor to Adjust UARWACRIM II to SMM/ACRIM I Level: 1.001621

1370.0 I r 1

Ki 1369.0 E r 1368.0 g 5 5 1367.0

m 2 3 1366.0

rz z 1365.0

z

k 1364.0

z 8

~ SMMIACRIM I 1363.0 ~ UARS/ACRIM II on ACRIM I Scale

UARWACRIM II Unscaled i 0.0 1000.0 2000.0 3000.0 4000.0 5000.0 6000.0

Days from February 16, 1980 to December 31,1996

Figure 2: The solid line shows the ACRIM I and ACRIM II data on the same scale (Willson, 1997), the dashed line shows the measured, unscaled ACRIM II data.

variability, the most important task is to establish the amplitude of the change in total irradiance between solar maximum and minimum, and from one cycle to another. Since the absolute radiometric accuracy of the various satellite experiments is only about ztO.Z%, which causes the offsets between data sets in Figure 1 and is larger than the &O.l% solar cycle variations we are measuring, to detect and study the small relative changes in solar total irradiance, continuous and overlapping measurements are necessary.

However, one of the largest obstacles of studying long-term changes in total irradiance is the existence of the nearly two-year gap between the SMM/ACRIM I and UARS/ACRIM II data sets. Because of this, the ACRIM II data have to be scaled to the ACRIM I level via the intercomparison of the ACRIM data with measurements of “third party” instruments. We note that Willson (1997) has scaled the ACRIM II data to the ACRIM I level via the mutual intercomparison of the overlapping ACRIM I and Nimbus-7/ERB and ACRIM II and ERB measurements. The solid line in Figure 2 shows the ACRIM I and ACRIM II total irradiances on the same scale (i.e., the ACRIM I scale) using Willson’s (1997) scaling factor of 1.001621. The dashed line shows the UARS/ACRIM II irradiance without scaling. Based on this scaling, Willson (1997) concluded that the Sun was brighter by about 0.03% during the minimum of cycle 22 than during the minimum of cycle 21 and has pointed to its potential significance for climate change. In contrast, Pap et al. (1997), Frohlich and Lean (1998), and Pap and Frohlich (1998) have pointed out that Willson’s (1997) scaling factor may be too high since the upward shifts in the ERB data after 1989 (see Lee et al., 1995; Chapman et al., 1996) have not been taken into account in Willson’s (1997) scaling procedure. Note that the ACRIM II data, shown in Figure 1, have been scaled to the ACRIM I level by Frohlich (1998) taking into account the drifts in the Nimbus-7/ERB data. Furthermore, Frohlich (1998) has used both the Nimbus- 7/ERB and ERBS total irradiance data to derive the scaling factor. As shown in Figure 1, this scaling yield a lower factor (1.001180) than the one derived by Willson (1997). In order to create a homogeneous long- term total irradiance time series, Friihlich and Lean (1998) have compiled the so-called “composite” total irradiance from the Nimbus-7/ERB, SMM/ACRIM I, UARS/ACRIM II, and SOHO/VIRGO measurements, taking into account all possible corrections, including the upward shifts in the ERB data. This composite total irradiance has been shown in Figure 3, and as can be seen, no apparent differences occur between the maximum and minimum levels of solar cycles 21 and 22. However, this exercise demonstrates the difficulty for adjusting data sets with gaps through the observations of a third party experiment and emphasizes the necessity of continuous and homogeneous measurements of total irradiance to study the effect of irradiance

1422 J. Pap et al.

Figure 3: The composite total solar irradiance after Friihlich and Lean (1998).

changes on climate.

When studying irradiance variations, the first distinct question to ask is “how many solar phenomena contribute to the observed irradiance variations?“. It has been demonstrated that the irradiance changes are associated with the evolution of solar magnetic fields (e.g. Harvey, 1994). However, on time scales of days to weeks the observed irradiance changes anticorrelate with the surface magnetic flux, whereas on time scales longer than a month the correlation is positive. This indicates that at least two physical components contribute to irradiance changes: the strong magnetic fields of sunspots cause negative excursions, whereas the weaker magnetic fields of faculae induce enhancements in total irradiance. Foukal and Lean (1988) have shown that a third component, the so-called “active network ” is necessary to explain the long-term irradiance changes. Kuhn et al. (1988) and Kuhn and Libbrecht (1991) have performed broad-band, two-color photometric measurements of the brightness distribution just inside the limb to study long-term irradiance variations. These results have shown that the facular signal alone fails by more than a factor of two in explaining long-term irradiance variations (Kuhn et al., 1988). These brightness observations indicate that t,he long-term irradiance changes may also be related to variations in the photospheric temperature (Kuhn et al., 1988); although it is not clear whether these temperature changes can be linked with the network component. It is interesting to note that former comparisons of the Swiss infrared measurements (Miiller et al., 1975) with visible data have also indicated that there is a latitudional dependence of the effective t,emperaturc of the Sun which may explain the long-t,erm irradiance variations (Pecker, 1994).

Since we observe the Sun’s irradiance from one direction in space, we have difficulty in determining whether the observed irradiance variations represent changes in the Sun’s irradiance in all directions, i.e., true luminosity changcs~ or are simply a result of a change in the angular dependence of the radiation field emerging from the photosphere. It has been shown that the solar cycle related long-term irradiance changes represent real luminosit,y changes (Kuhn and Libbrecht, 1991; Kuhn, 1996). Another question is whet,her the short-term changes in tot,al irradiance represent (1) real luminosity changes, which assumes temporary storage of the missing energy in the sunspot-related irradiancc dips (e.g. Spruit, 1982; Fox et al., 1991; Steinegger et al.: 1996) or (2) a simple re-distribution of the solar irradiance by sunspots and active regions faculae (Kuhn, 1996) or (3) part of the missing energy could be transported to the higher layers of the solar atmosphere contributing t,o t,heir heating (Ulrich, 1996). Although it is irrelevant in the context of

Evidence for Solar Irradiance Variability and Climate 1423

the Earth whether the observed irradiance changes are luminosity changes or results of energy distribution by active regions, this is an important and not yet solved problem in solar physics. Since variations in the solar energy flux - persistent over long periods of time - may trigger climate changes, it is fundamental to understand the underlying physical mechanisms and thus the possibilities for a solar forcing of climate on time scales of decades to centuries.

RADIUS AND HELIOSEISMIC MEASUREMENTS

Simultaneous studies of total irradiance, p-mode oscillations and radius changes are essential to better understand the underlying physical mechanisms of irradiance variations and to predict the solar-induced climate changes. In order to measure the solar radius, one needs to establish the position of the solar limb with high accuracy. The solar limb is potentially a sharp spatial reference with which we can hope to detect the effect of solar oscillations (both p-mode and g-mode); the gravitational quadrupole moment (or the solar oblateness); and changes in the solar radius (e.g. as a diagnostic for int,erior changes due to the solar luminosity cycle) (Kuhn et al. 1997; 1998). Ground-based measurements of the solar radius exist over the last 300 years (e.g. Ribes et al., 1991), however the results are very controversial. Historical radius data show that the Sun’s radius may have been larger during the Maunder Minimum (Ribes et al., 1991). These results are confirmed by the French CERGA astrolabe radius measurements. i.e.! the solar radius has been found larger during solar minimum (e.g. Lacrare et al., 1996). In contrast, the astrometric observations of the Sun with the modified Danjon astrolabe of Santiago, Chile indicate that during the time interval of 1990 to 1995 the solar radius changed in parallel with the solar activity cycle (Noel, 1997). Ulrich and Bertello (1995) also showed a positive correlation between the apparent radius changes and the solar activity cycle. While there are also hints of periodic solar radius variations over time scales of 1,000 days to 80 years (as determined from the CERGA data), the measurements arc generally neither consistent nor conclusive (Parkinson et al., 1980; Brown, 1987; Ribes et al., 1991). These controversial results underscore the necessity of further efforts to measure the solar radius, using more sophisticated techniques which are also free from the atmospheric seeing effects. The difficulty of ground-based observations of the solar radius due to the distorting effects of the Earth’s atmosphere has also been recognized by Sofia and his collaborators in their development of the Solar Disk Sextant (Sofia et ~1.; 1994).

Recently, Kuhn et al. (1997; 1998) reported results of solar limb measurements using images taken by the Michelson Doppler Imager (MDI) experiment on SOHO. The MD1 experiment was primarily designed as a helioseismic experiment to provide full disk Doppler images of the Sun with l-min cadence. MD1 has a 1024 x 1024 CCD camera to obtain full disk and higher resolution Doppler images of the Sun near the 676.8 nm Ni I line (Scherrer et al., 1995). Although the SOHO/MDI experiment was not designed for astrometric imaging or photometric observations, it has been proven to be effective for measuring small changes in the solar limb position (Kuhn et al.! 1998). It has been shown that when the SOHO spacecraft is “rolled” for calibration, it is possible to measure solar limb shape and size changes at, t,he 1O-6 pixel level for oscillations, and at the lo-” level for static shape and size changes. The left-side panel of Figure 4 shows the residual solar shape (which rotates as the SOHO spacecraft rotates). This analysis was done with approximately 8-min of data and a non-optimum roll sequence carried out in 1996. These data indicate a larger equatorial radius than polar radius. A record of the solar “radius” changes from one day of MD1 data is presented in the right-side of Figure 4. The apparent lipward spikes of l-2 millipixcls in the radius are due to small temperature changes of the optics when the observations are interrupted every 96 minutes for magnetogram acquisition. This temperature change is enough to change the effective focal length of the optic (there are literally dozens of optical surfaces in the MD1 optical train). As the preliminary results on radius variations from the MD1 measurements indicate, we can measure the solar radius with the necessary precision to reveal its variability. The MD1 astrometric measurements noise appears to be limited by photometric calibration noise and small changes in the transmission through the complicated optical path of the instrument. One can imagine what a new concept for solar shape, radius: and high precision photometry could do on a dedicated Solar Astrometric Experiment,.

1424 J. Pap et al.

0 100 200 300 -0.004 s I b > 1 I I , I

Central Angle From Equator 50 5.2 5.4 56 55 6.0 Time [days since May 1, 19961

Figure 4: The left-side panel shows the residual solar shape values derived from the SOHO/MDI observations. The solid line shows the spherical harmonic (oblateness and hexadecapole) fit to the limb shape. The dashed line shows the 30” running mean shape, and the dotted line shows the instrumental noise divided by 10. The MD1 ‘?adius” changes within a day in May, 1996 are shown on the right-side panel. (updated from Kuhn et al., 1998).

CONCLUSIONS - FUTURE REQUIREMENTS

The conclusion we can derive from the current irradiance measurements is that the solar radiative flux is anisotropic, a function of latitude, and naturally a function of the time during the migration of solar activity (Pecker, 1994). C urrent results indicate that although a considerable part of the observed irradiance changes is associated with magnetic activity features, such as sunspots, faculae and the magnetic network (e.g. Foukal and Lean, 1988), we can neither explain nor understand all of these changes solely as manifestations of surface solar magnetic activity (e.g. Frohlich and Pap, 1989; Kuhn, 1996; 1998; Frijhlich et al., 1997; Sofia, 1998; Ulrich, 1998). It has been shown that solar irradiance and luminosity vary in response to global solar cycle perturbations, such as photospheric temperature changes (Kuhn et al., 1988), large scale convective cells or mixing flows (Ribes et al., 1985; Fox and Sofia, 1994), and/ or radius changes (Delache et al., 1986; Ulrich and Bertello, 1995; Lydon and Sofia, 1995; Sofia; 1998). An important key to understanding how superficial surface magnetic features and solar interior mechanisms affect the solar irradiance and luminosity is contained in helioseismic data, which provide an excellent tool to probe the solar interior. Recent analyses show that solar cycle variations in global p-modes cannot be explained as a consequence of sunspots and faculae (Friihlich et al., 1997; Kuhn, 1998). Hints from precise photometry and helioseismology (Kuhn, 1989) indicate that the solar interior changes that will lead us to understand and forecast solar irradiance and luminosity variations.

These results demonstrate that we need to develop a new strategy to study solar variability and its effect on climate. Since we cannot wait a century, the time required to collect long enough total irradiance data, to find out what the solar effect is - a necessary step in assessing the seriousness of greenhouse warming - we must seek alternate means to expedite getting this information. Models of solar interior with variable internal fields indicate that all global parameters (luminosity, radius, and effective temperature) vary as a consequence of structural changes. However, models of the solar interior also indicate that the relationships between variations of these global parameters depend on the details of the internal mechanisms (depth,

Evidence for Solar Irradiance Variability and Climate 1425

magnitude of the magnetic field, etc...). This new strategy should concentrate on measuring solar irradiance, radius/shape and solar oscillations concurrently to determine their relationship experimentally. Therefore, a dedicated experiment is required to detect radius and low order shape oscillations and may realize over an order of magnitude improvement in the detection threshold of coherent oscillations. Accurate radius and shape observations will enable a new set of helioseismic and photometric diagnostics of the radiative and convective interior. Concurrent measurements of the solar shape, shape oscillations, radius and total solar irradiance will reveal the mechanisms causing the changes in the solar energy flux. Until we understand the physics of global solar variability, we will remain ignorant of the possibility for larger solar-cycle-related irradiance variations than have been observed over the last two decades.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the past and ongoing efforts of the VIRGO and MD1 teams to produce and interpret the SOHO/VIRGO total irradiance data and MD1 images. SOHO is a mission of international cooperation between ESA and NASA. The Nimbus-7/ERB, SMM/ACRIM I and UARS/ACRIM II as well as the ERBE total irradiance data were taken from the NOAA World Data Center, as published in SGD.

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