total solar and spectral irradiance variations from solar cycles 21 to 23

Pergamon Adv. Space Res. Vol. 29, No. 12, pp. 1923-1932.2002

0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

www.elsevier.com/locate/asr PII: SO273-1177(02)00237-5

0273-l 177/02 $22.00 + 0.00

TOTAL SOLAR AND SPECTRAL IRRADIANCE VARIATIONS FROM SOLAR CYCLES 21 TO 23

J.M. Pap’, M. Turmon2, L. Floyd3, C. FrBhlich4, and Ch. Wehrli 4 .

1 University of California, Los Angeles, 405, Hilgard Ave., L.os Angeles, CA 87095 USA 2Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 USA

“Interferometrics Inc., 14120 Parke Long Ct., Chantilly, VA 20151 USA 4Physikalisch-Meteorologishes Observatorium, Davos, 33 Dorfstrasse, CH-7260, Davos-Dor$ Switzerland

ABSTRACT

Total solar and UV irradiances have been measured from various space platforms for more than two decades. More recently, observations of the “Variability of solar IRradiance and Gravity Oscillations” (VIRGO) experiment on SOHO provided information about spectral irradiance variations in the near-UV at 402 nm, visible at 500 nm, and near-IR at 862 nm. Analyses based on these space-borne irradiance measurements have convinced the skeptics that solar irradiance at various wavelengths and in the entire spectrum is changing with the waxing and waning solar activity. The main goal of this paper is to review the short- and long-term variations in total solar and spectral irradiances and their relation to the evolution of magnetic fields from solar cycles 21 to 23. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

INTRODUCTION The multi-decade long measurements of total solar and spectral irradiance established conclusively that the Sun’s

radiative output varies on various time scales: from minutes to the 1 l-year solar cycle (Willson and Hudson, 1988). On time scales of minutes to hours, the effect of granulation, meso-, and supergranulation has been recognized in solar irradiance, whereas the rapid irradiance fluctuations in the 5-minute range are due to the p -mode oscillations (e.g. Frijhlich et al, 1997). On time scales of days to months, the evolution of active regions plays a dominant role in irradiance changes. In the case of total irradiance, the short-term irradiance changes are attributed to the combined effect of sunspots and faculae (Willson et al., 1981). Similar results have been obtained by studying the VIRGO spectral irradiances at 402, 500, and 862 nm (Frohlich et al., 1997; Pap et al.; 1999). The main causes of the short- term variations in UV irradiance are the plages as they evolve and move across the solar disk, although the effect of the network can also be identified in the short-term UV irradiance changes (Pap, 1992; Woods et al., 2000). The longer term irradiance variations on time scales of years to decades are attributed to the changing emission of faculae and the magnetic network (e.g., Foukal and Lean, 1988). Since variations in the solar energy flux that persist over long periods of time may trigger climate changes (e.g. Hansen et al., 1993; Reid, 1997), it is important to identify the causes of irradiance variations and thus the possibilities for a solar forcing of climate on time scales of decades and centuries.

Although considerable information exists on irradiance variations, we still lack the understanding of the underlying physical mechanisms. Correlative studies indicate that a major portion of irradiance changes is related to the surface manifestations of solar activity, such as sunspots and faculae (Foukal and Lean, 1988; Frijhlich and Lean, 1998). However, there is growing evidence that the current empirical models, solely based on the effect of sunspot darkening and faculae brightening, cannot explain all the aspects of the observed irradiance variations (Frohlich and Pap, 1989; Kuhn, 1996; Frijhlich et al., 1997; Wehrli et al., 1998; Kuhn et al., 1998) i.e., there is remaining variability in solar irradiance after removing the effect of sunspots and faculae. Identification of this residual variation is a difficult problem since global effects, such as temperature and radius changes (Delache et al., 1986; Kuhn et al., 1998; Pap et al., 2001a), large scale convective cells or mixing flows (Ribes et al., 1985; Fox and Sofia, 1994), differential rotation in the solar interior (Kuhn, 1996) may also produce irradiance changes.

In this paper we study the effect of solar magnetic fields on solar total and spectral irradiances. For this purpose

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we use the composite total irradiance compiled by Frohlich and Lean (1998). the Mg II h & k core-to-wing ratio (hereafter Mg c/w) compiled by Floyd et al. (2001), the VIRGO spectral irradiance data, Photometric Sunspot Index (PSI), and the magnetic field strength measurements performed at the National Solar Observatory at Kitt Peak and by the “Michelson Doppler Image? (MDI) experiment on SOHO.

TOTAL SOLAR AND SPECTRAL IRRADIANCE RECORD Continuoues measurements of total solar and UV irradiances from various space platforms have been conducted on

daily bases since late November of 1978 (see details by DeLand and debula, 1998; Frohlich, 2000; Floyd et al., 2001). The various total irradiance time series are presented on the left-side of Figure 1 (upper panel), while the lower panel shows the composite total irradiance compiled from the measurements of the ERR, ACRIM I, ACRIM II, and VIRGO experiments (see details by Friihlich and Lean, 1998). As can be seen, the scale of the measurements varies from one experiment to the other, resulting from the limited absolute accuracy (&0.2%) of the calibration of the individual measurements. However, the precision and stability of the measurements is much better - making it possible to study the relative variations in total irradiance as a function of solar cycle and from one cycle to the other.

To illustrate the variations in UV irradiance, the long-term Mg c/w is shown on the right-side of Figure 1. This Mg c/w composite has been computed from the Nimbus-7, NOAA9, and UARSBUSIM measurements using their overlapping time intervals (Floyd et al., 2001). The advantage of using the Mg c/w is that this ratio is calculated from the irradiance in the core of the Mg 280 nm line, which is highly variable, to the less variable irradiance at neighboring continuum wavelengths. Therefore, the Mg c/w is relatively insensitive to instrumental effects and it is used as a reasonably good indicator of UV irradiance variations between 200 and 300 nm (e.g. DeLand and Cebula, 1998). As can be seen, both total irradiance and the Mg c/w vary in parallel with the solar cycle, being higher during solar maximum.

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Fig. 1. The various total irradiance time series are presented on the left-side panel (upper part), the composite total solar irradiance, derived from the Nimbus-7’IERB (HF), SMM/ACRIM I, UAFWACRIM II and SOHO/VIRGO (version 3.50) is shown on the lower part (updated from Frohlich, 2000). The composite Mg core-to-wing ratio index, formed from the Nimbus-7/SBUV, NOAASKBUV2 and SUSIM V19r3 indices, is given on the right side panel (updated from Floyd et al., 2001).

While considerable information exists about the variations in total solar and UV h-radiances, variations in the visible and infrared parts of the spectrum are less understood, mainly because of the lack of long-term space-based measurements. The Sunphotometers (SPM) of VIRGO have provided the first information about the changes in the near-UV, visible and near-infrared spectral ranges at selected wavelengths at 402, 500, and 862 nm. However,

Total Solar and Spectral Irradiance Variations 1925

evaluation of the VIRGO spectral data is a difficult task, especially because of the degradation of the SPM instrument (see details by Friihlich et al., 1997). Since we cannot correct properly for the degradation of the SPM instrument, one has to find other ways to remove instrumental trends, i.e., using quite Sun periods when irradiance was not influenced by active regions (Frbhlich et al., 1997) or statistical methods (Pap et al., 1999). Unfortunately, the solar-cycle-related trend is also removed this way from the data - making it possible to study only the effect of active regions on spectral irradiance. The VIRGO SPM data, after removing the instrumental and longer term trends, as described by Pap et al. (1999), are shown on the left-side panel of Figure 2 for the time interval of 1996 to 1998. The SPM spectral irradiance at 402 nm, together with PSI and the Mg c/w ratio is plotted on the right side panel. We note that PSI is computed from the area, position, and contrast of sunspots (Frijhlich et al., 1994) and is used to model the effect of sunspots on irradiance, while Mg c/w is used as a proxy for the changing emission of faculae. As can be seen, the VIRGO 402, 500, and 862 nm spectral data vary in phase and the effect of active regions is clearly resolved in the VIRGO spectral irradiances as shown on the right-side panel of Figure 2. We note that the short-term variations in the VIRGO spectral and total irradiance data correlate well with each other (Pap et al., 1999).

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0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 Data behveen May1,1996andFebruary21,1998 Databetween May1,1996and February21,1998

Fig. 2. The solid (black) line on the left-side panel shows the near-UV irradiance reconstructed without the instrumental trend components, the dashed grey line shows the same for the visible and the long- dashed gray line shows the same for the infrared. The SPM near-UV spectral irradiance together with the Photometric Sunspot Index and the M c/w ratio are presented on the right-side panel (updated from Pap et al., 1999.)

Effect of Magnetic Fields onLong-Term Irradiance Variations To study the effect of the magnetic fields on long-term irradiance variations, we have used the magnetic field

strength values as computed from the Kitt Peak magnetograms taken at the 868.8 nm spectral line with an 1.14 arc set square pixel resolution. To determine the magnetic field strength values, the averages of the absolute values of all the pixels on a full disk magnetogram are computed and corrected for the noise level. The time series of the Kitt Peak magnetic field strength data is plotted in Figure 3, together with PSI and solar irradiances. The upper panels show the composite total irradiance and Mg c/w, while the lower panels show PSI and the magnetic field strength data. On the top of the daily values, as represented by dots, the 8 1 and 365 day running averages are plotted to better illustrate the long-term variations.

As can be seen from Figure 3, the long-term variations of total irradiance and the Mg c/w are rather similar for solar cycles 21 to 23, showing that their maximum and minimum levels are about the same within their measurement uncertainties. By contrast, there are significant differences between the maximum and minimum levels of the magnetic field strength and PSI bver the examined time interval of 1978 to 2000. While the maximum of total irradiance and the Mg c/w is about the same during solar cycle 23 as during the two previous cycles, both PSI and the magnetic flux show a much lower maximum for the current cycle. In addition, the behavior of the examined time series during the two observed solar minima - i.e., between solar cycles 21 and 22 and cycles 22 and 23, respectively - is quite

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Fig. 3. The upper panels show the composite total irradiance and Mg c/w, while the lower panels show the Photometric Sunspot Index and the absolute values of the averaged magnetic field strength as computed from the Kitt Peak measurements. Dots show the daily values, the heavy lines show the 81 and 365 day running means of data.

different. As seen from Figure 3, there is a two year long flat minimum between solar cycles 21 and 22 in PSI and the magnetic flux, while both total irradiance and Mg c/w show a much shorter minimum. In other words, there is a phase shift between the variations in the magnetic flux and the photospheric and chromospheric irradiances at both the end of the declining portion of cycle 21 and at the beginning of the rising portion of cycle 22 - irradiance leading the magnetic field variations.

To further study the relation between the long-term variations of solar irradiance and magnetic fields, the long-term trends in the multi-decade long data have been separated from the shorter term variations. These trend components have been calculated by Singular Spectrum Analysis (SSA), and are reconstructed from the first two major oscillatory components found in the time series. Further details on the application of SSA to irradiance variations are given by Pap and Frohlich (1998) and Pap et al. (1999; 2OOla). The computed long-term trends are plotted on the left-side of Figure 4 for the time interval of November 1978 to September 2000.

As can be seen from Figure 4, significant differences exist between the computed long-term trends of total irradiance, Mg c/w, magnetic field strength and PSI. These trend components indicate clearly that both total irradiance and Mg c/w started to rise sooner than the magnetic field strength and PSI at the beginning of the ascending phase of solar cycle 22 (see also Vigouroux er al., 1997), and total irradiance was leading the Mg c/w. However, at the


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Fig. 4. The heavy dotted line on the left-side panel shows the reconstructed trend in total irradiance, the solid line shows the Mg c/w trend and the long-dashed line shows the trend in the Kitt Peak magnetic field data. The same is presented on the right-side panel for a shorter time interval (December 1981 to July 2000), where the trend in SC is given by the dot-dashed line.

beginning of 1988, when more and more sunspots were formed, Mg c/w started to rise faster reaching a higher level at the maximum of solar cycle 22 than total irradiance. This brings into question whether the growing number of sunspots is not only capable of reducing total irradiance on time scales of days to weeks, but also whether they affect the averaged total irradiance values around the time of solar maximum, thus controlling of the amplitude of total irradiance variation from solar minimum to maximum.

The correlation between irradiance and magnetic field variations during the minimum between solar cycles 22 and 23 and the rising portion and maximum of cycle 23 is quite different than for the previous two cycles. On one hand, during the minimum between solar cycles 22 and 23, total irradiance reached its minimum level around January 1996 and started to rise around early July 1996. By contrast, Mg c/w, the averaged magnetic field, and PSI (not shown here) all reached their minimum level only in early October 1996, and started to rise in phase in January 1997, about 6 months later than total irradiance. As the trend components show, at the maximum of solar cycle 23, total irradiance rises well above the Mg c/w, magnetic field strength and PSI, while the long-term trend in Mg c/w increases only slightly above the magnetic field strength values.

The reason of these observed differences between long-term irradiance and magnetic field variations is not understood. The obvious questions are: (1) Why total irradiance starts to rise prior to the magnetic flux at the beginning of the rising portions of both solar cycles 22 and 23? (2) Why total irradiance rises higher at the maximum of solar cycle 23 than the examined magnetic indices, which show that cycle 23 was lower than the two previous cycles? On one hand, one can speculate that at the end of solar minimum and the beginning of the rising portion of the solar cycle, the emerging magnetic flux of the new cycle, concentrating mainly in weaker magnetic fields which are distributed over high latitudes and possibly around the poles, could already influence total irradiance. By contrast, the effect of stronger magnetic fields leading to the formation of active regions (sunspots and faculae) becomes evident only later, when the magnetic flux concentration is sufficiently large in the photosphere and extending also to the chromosphere.

On the other hand, it is an important question whether the effect of sunspot darkening influences the variations in total irradiance over time scales longer than days and weeks. As seen from Figures 3 and 4, the activity of the Sun was much lower during the rise and maximum of cycle 23 than during the previous cycles. As shown by PSI (and also by magnetic maps), during solar cycle 23 fewer and smaller sunspots occurred on the Sun causing smaller dips in total irradiance compared to the large dips observed during the two previous cycles when many big and complex sunspot groups were present. To study the effect of sunspot darkening on longer term variations of total irradiance, the long-term trend of total irradiance corrected for sunspot darkening (S, = total irradiance + PSI) has been computed in a fashion similar to the trends plotted in Figure 4. The trends, including the S, trend, are plotted again on the right-side panel of Figure 4 for the shorter time interval of December 1, 1981 to July 18, 2000. We note that S, is available


only for the time interval of December 198 1 to July 2000, since PSI has not been calculated prior to December 198 1 because of the large uncertainties of the measured sunspot areas (see details by Friihlich et al., 1994).

As can be seen from Figure 4, if sunspots were not present on the Sun, the amplitude of the solar cycle variation of total irradiance would be considerably higher than the 0.1% solar-cycle variation observed over the previous two solar cycles. It is interesting to note that the amplitude of total irradiance variation from minimum to maximum seems to be lower when the activity of the Sun is high, indicating that the strong magnetic fields of sunspots are capable to block the energy transported to the photospheric layers on longer time scales. In contrast, the trend in total irradiance appears higher during the weak solar cycle 23, when the sunspot magnetic fields cannot block effectively the energy transport to the photosphere. This peculiar behavior of total irradiance over stronger and weaker solar cycles also indicates that besides magnetic fields, additional mechanisms are also at work to shape the long-term variations in total irradiance, which are missed in the empirical models currently used to study the climate impact of irradiance variations.

Effect of Magnetic Fields on Short-Term Irradiance Variations To study the relation between the short-term variations in solar irradiance and magnetic fields, their residual time

series excluding the solar-cycle-related long-term trends were computed by means of SSA. As a first step, auto- and cross-correlation techniques were applied to reveal the prominent variations present in both magnetic indices and solar irradiance. These results are shown on the upper left-side of Figure 5. The solid line represents the auto-correlation for the magnetic field strength and the dot-dashed line represents the cross-correlation between the magnetic field and Mg c/w. As can be seen, the 27-day rotational period dominates the short-term variations of the magnetic field strength. The cross-correlation between the magnetic field strength and the Mg c/w is the highest at zero-lag (r = 0.68), and also shows the strong correspondence between the rotational variation of the photospheric magnetic fields and the chromospheric UV index derived from the measurements in the vicinity of 280 nm. By contrast, the cross-correlation is low between the magnetic field and total irradiance (represented by dots), being negative at zero-lag and showing that strong magnetic fields reduce total h-radiance. Additional small negative peaks are seen at time-lags of 7, 13.5, and 53 days.

Results of SSA-based decomposition of the magnetic field confirm these results. The upper panel on the right-side of Figure 5 shows the 27-day rotational component in the magnetic field strength (RC5), the lower panel shows the same for Mg c/w (RCs4&5). As can be seen, the two rotational components correspond to each other well, correlating with r = 0.98. Resulting from the lower activity of solar cycle 23, the amplitude of the rotational modulations was much smaller during cycle 23 than during the previous two cycles. We note that no corresponding components have been found between total irradiance and magnetic field strength in the vicinity of the 27-day rotational period. These results indicate that the photospheric magnetic fields have a strong control on both the solar-cycle-related long-term trend and the rotational modulation of the Mg c/w representing the chromospheric UV flux. By contrast, the response of total irradiance to the changing magnetic fields is far more complex than that of UV irradiance. This partially arises from the fact that the increased magnetic fields may result either in the appearance of sunspots, which cause dips in total irradiance, or in the form of faculae, which cause temporary enhancements - whereas the chromospheric radiation is always higher above the magnetic fields concentrated in active regions.

To distinguish between the effect of magnetic fluxes concentrated in sunspots and faculae on solar irradiance, we have compared the sunspot and faculae indices derived from the MD1 images for the time interval of July 1, 1996 and September 30, 1997. We note that the MD1 images are taken with a CCD camera near the Ni I 676.8 nm absorption line originating in the mid-photosphere (Scherrer et al., 1995). To extract sunspots and faculae from the MD1 images we used an image processing and analysis technique based on a Bayesian image segmentation method to incorporate simple spatial information about how these activity regions occur. The Bayesian framework allows controlled introduction of physical knowledge of the characteristics of the activity types. To separate sunspots and faculae, we have used a two parameter classification system, which includes both the magnetic field strength and the intensity objects after our finding that small magnetic fields of about f200 Gauss in MD1 units may cause either sunspots or faculae. Detailed description of the image analysis technique is given by Turmon et al. (2001).

Results of the MD1 image analysis are presented in Figure 6. The correlation between the VIRGO total irradiance and the area of sunspots and faculae is shown on the upper left-side of Figure 6. The upper panel on the right side gives the SUSIM Mg c/w (solid line), the dashed gray line shows the faculae area, while the dotted gray line shows the Mg c/w residuals after removing the effect of faculae. As can be seen, there is an excellent correlation between


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Fig. 5. The left-side of the upper panel shows the results of the auto- and cross-correlation between the magnetic field strength, total irradiances and Mg c/w. The 27-day rotational components of the full disk magnetic field strength (RC4) and Mg c/w (RC4&5), established by SSA, are plotted on the right- side panels. The lower part of the left-side panel shows the correlation between the 27-day rotational components identified in the magnetic field and Mg c/w.

the Mg c/w ratio and the MD1 faculae area (r = 0.94), and as the residuals show, only a small variation remains unexplained in Mg c/w. By contrast, a considerable variation remains unexplained in total u-radiance after removing the effect of sunspots and faculae, as the residuals (gray dashed line) show on the lower left-side of Figure 6. Note that both the areas of sunspots and faculae, derived from the MD1 images, sometimes overestimate and sometimes underestimate the observed irradiance changes. The largest discrepancy between the residuals and the measured dips in total irradiance occurs in November 1996 and August 1997. One possible explanation for this relatively large residual variability in case of total irradiance is that we use simple area values of spots and faculae without correcting for their contrast and center-to-limb variations. However, we note that if we use PSI instead of the MD1 faculae area the residual total ii-radiance variation is even larger, despite that in PSI both the center-to-limb variation and the area dependence of the contrast is taken into account (see Frohlich et al., 1994).

Results of the cross-correlation between total irradiance, Mg c/w and the MD1 area and sunspots as well as irradi- ante residuals are presented on the lower right-side of Figure 6. As can be seen, there is a high correlation between the Mg c/w and faculae at the 27-day rotational period, while a very small correlation remains between the Mg c/w and its residuals at 9 and 23 days. As expected, the cross-correlation is low between total irradiance and the MD1


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Fig. 6. The left-side of the upper panel shows the VIRGO total irradiance (black solid line), sunspot (dotted gray line) and faculae areas (long-dashed gray line) derived from MDI. The lower panel shows total irradiance and its residuals (dashed gray line). The SUSIM Mg c/w (black solid line) and MDI faculae area (gray long-dashed line) and the Mg c/w residuals (gray dotted line) are given on the upper panel of the right side. Results of the cross-correlation are shown on the lower panel.

spot areas (r = -0.54). However, a positive correlation exists between total irradiance and the faculae area. Due to the relatively large unexplained variation of total irradiance, the measured VIRGO irradiance and its residual show a relatively strong correlation at zero lag (r = 0.52), and smaller peaks are seen at 10, 35,60, and also at 87 days. These results show, that the MD1 magnetic indices, such as the faculae area as derived from the magnetograms, correlate well with the chromospheric irradiance, while the MD1 magnetic indices leave a relatively large residual variation in total irradiance. We note that similar conclusions can be derived from the comparison of the MD1 indices and the VIRGO spectral irradiances (Pap et al., 2001b).

CONCLUSIONS In this paper we have studied the relation between the short- and long-term variations in solar irradiance and

photospheric magnetic fields averaged over the entire disk and separated into sunspot and facular magnetic fields. As our results show: (1) There is a phase shift between total irradiance and the absolute value of the averaged magnetic field strength at the beginning of both solar cycles 22 and 23, total irradiance leading the magnetic field values. (2) While both sunspots (as represented by PSI) and the magnetic field strength data show that the maximum of solar


cycle 23 is lower than the maxima of the two previous cycles, the long-term variation of total irradiance and the Mg II h.& k ratio is rather symmetrical over the last two and half cycles - showing that the maximum level of solar irradiance is higher during solar cycle 23 than the maximum of the magnetic indices. (3) Our results also indicate that while there is a considerably large residual variation in total irradiance after removing the effect of sunspots and faculae, both the short- and long-term variations of the Mg c/w ratio correlate well with the magnetic field variations.

The reason of these discrepancies between the short- and long-term variations of total irradiance and magnetic field indices is not understood. Although we cannot rule out the possibility of instrumental effects, which may influence the long-term precision of irradiance time series, the systematic differences between the multi-decade long total irradiance composite and the magnetic surrogates on both short and long time scales (Frohlich and Pap, 1989; Kuhn, 1996; Vigouroux et al., 1997; Frohlich et al., 1997) point to the direction of the presence of unidentified component(s) which may be missed in the current irradiance models. On one hand, one can raise the question whether the emerging new magnetic fields can cause an early rise in total irradiance at the beginning of the ascending phase of the solar cycle - well before the concentration of the magnetic flux tubes is sufficiently large enough at the photospheric levels to form active regions and extending also to the chromosphere, and thus influencing UV irradiance. As the results of cross-correlation and Singular Spectrum Analysis show, variations of the photospheric magnetic fields control the chromospheric UV it-radiance variations on both short and long time scales. In contrast, the magnetic field variations cannot fully account for either the short- or the long-term variations in total irradiance.

As shown by the long-term irradiance measurements, the amplitude of the peak-to-peak variation of total irradiance is about &O. 1% during solar cycles 21 and 22 (see Friihlich, 2000). However, our results indicate that total irradiance rises slightly higher during the maximum of solar cycle 23 than during the two previous cycles. It remains to be seen whether this relation between the strength of the solar cycle and the amplitude of the solar-cycle-related total irradiance variations is consistent, i.e., the maximum level of total irradiance is higher during weaker cycles and lower during stronger cycles or it may be related to uncalibrated instrumental effects. Since we lack the physical understanding of irradiance variations, we need to continue the uninterrupted space-based observations of total irradiance to further study the relation between the strength of the solar cycle and the maximum activity level of total irradiance. If our current finding stands (i.e., is not a result of instrumental effects), we need to re-evaluate our long-term empirical irradiance models, which assume that total irradiance varies in a fashion similar to the sunspot number - showing larger cycle-to-cyle variations during high activity cycles and smaller variations during weaker cycles.

Further studies are required on this subject and identification of the missing irradiance components is an important issue to better understand the physical processes below the photosphere and to better understand the climate impact of solar variability. For this purpose we need to combine our efforts to identify small scale magnetic features and possible temperature and radius changes which may also contribute to total irradiance variations - in parallel with efforts to provide irradiance measurements with better long-term precision and higher resolution solar images to study the effect of the evolution and distribution of solar magnetic fields on total irradiance.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the past and ongoing efforts of the VIRGO and MD1 teams to produce the

SOHONIRGO irradiance data and MD1 images. SOHO is a mission of international cooperation between ESA and NASA. NSO/Kitt Peak magnetic data used here are produced cooperatively by NSF/NOAO, NASA/GSFC and NOAA/SEC. This research was supported by a grant NAG 5-7877 from the SOHO Office of NASA’s Office of Space Science.

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total solar and spectral irradiance variations from solar cycles 21 to 23

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