High precision measurements of photometric variability among solar type stars have now been made since 1980. These observations clearly show that year-to-year brightness variations connected with magnetic activity are a widespread phenomenon among such stars. They also suggest that the Sun’s potential for long-term white light variability may be significantly understated by measurements of solar total irradiance during the 1980s.

Ground-based calorimetry and photometry of the Sun have been carried out for many years. Following the early years, ground-based photometry has largely replaced ground-based calorimetry, in part due to the advent of airborne and spaceborne detector systems for the broad-band measurement of the solar irradiance and the realization of the difficulty of correcting calorimetry measurements for the effects of the terrestrial atmosphere. Even from spacecraft, recent measurements of the total solar irradiance range from about 1367 to 1374 W/m2. Most of this difference can be ascribed to differences in instrumental scales, while a variation of about 1 to 2 W/m2 appears to be due to solar variability. The quiet Sun may also change, globally, over longer time scales. Using disparate data to understand solar variability will require cooperation between a number of current groups, supported by various governments, covering several zones of longitude.

There are few solar surface brightness observations with sufficient accuracy to have direct bearing on the question of the origin of the Sun’s bolometric luminosity and irradiance changes (the correlative observations have been reviewed elsewhere in these proceedings). This paper contrasts these observations with a model for the solar variability and illustrates our clear need for high precision surface photometry, in combination with helioseismic data and numerical experiments.

The Sun’s magnetic field varies on time scales of minutes, months, decades, and centuries. These changes drive variations in solar irradiance. Characteristics of the changing magnetic field reveal, at some level, how the solar cycle works. The basic 22-year magnetic cycle and corresponding 11-year activity cycle of the Sun are driven by the emergence of active regions. The frequencies of solar oscillations that probe the convection zone vaxy on the same time scale. While plausible explanations for the statistical properties of solar activity can be made, the details of flux emergence cannot be predicted. Global activity varies on other intermediate time scales as well.

Defining elements of a cycle as those sharing a common bipolar field orientation, the first signs of a cycle can appear at high latitudes up to 5 years before solar minimum and the final elements disappear at the equator several years after the following minimum - a span as long as 18 years. At a particular latitude waves of activity are spaced by 11 years. The torsional oscillation in solar rotation rate has similar characteristics.

The spatial distributions of total flux and large-scale polarity evolve quite differently: the total flux map resembles the butterfly diagram, moving from mid latitudes toward the equator during the cycle, while the net zonal flux has two maxima in each hemisphere that expand away from mid latitudes.

While each solar cycle shares these basic features, fixed by the action of the solar dynamo working within the convection zone, the details of each cycle are determined by the apparently random timing and placement of emerging flux and by the changing convective flow patterns.

I review information from a variety of sources, including magnetic field measurements, Doppler imaging, and various photometric and spectroscopic survey and monitoring programs for clues concerning the physical properties, spatial structure, variability and evolution of magnetic regions and associated activity on cool stars. Based on these data, I sketch a scenario of what the surfaces of cool stars look like as a function of spectral type and age/rotation/activity level.

The coronal features observed in X-ray emission, varying from the small-scale, short-lived bright points to the large-scale, long-lived coronal holes, are closely associated with the coronal magnetic field and its topology, and their variability depends strongly on the solar cycle. Here we discuss the spatial distribution of the coronal structures, the frequency distribution of the brightness variations in active regions, and the role of magnetic reconnection in determining the variability of the coronal features, on the basis of the new observations of the soft X-ray emission recently obtained with the Yohkoh satellite and the NIXT experiment.

This paper reviews observations of the variation over long and short time scales of solar coronal flux as observed from ground-based optical observatories. This includes data from Fe X, Fe XIV, Ca XV and the white-light electron corona. The various parameters are compared and contrasted as indicators of global coronal structure and output. The overall envelope of Ca XV minimizes before and during sunspot minimum, begins its slow rise in phase with the solar cycle, but maximizes late with strong 150-day oscillations resulting in values of zero at the local minima. Within the sensitivity of the observations, the increase of the envelope from Cycle minimum to maximum is over a factor of 30. Excluding possible secondary maxima and ignoring strong 150-day oscillations, the general envelope of Fe X appears to indicate a slow decline in intensity following sunspot maximum. Earlier observations indicate a Cycle minimum-to-maximum variation of a factor of 2 to 3. Later observations indicate a factor of 4 to 5. The overall picture for Fe XIV indicates that it varies in phase with the sunspot cycle. However, multiple maxima occur. The amplitudes of all observed Cycles are probably approximately equal, and the increase from Cycle minimum to maximum is approximately a factor of 10. The variation of the polarized and unpolarized white light corona is in phase with the sunspot cycle, although it may have a slower decline to minimum than the sunspot number. The increase from Cycle minimum to maximum is a factor of 2 to 3. Notable periodicities seen in the emission-line corona are (approximately) 27, 150, 180, 220 and 340 days and 3.4 years.

Measurements of the total microwavef emission from the solar disc at single and multiple wavelengths provide a useful indicator of solar activity, and correlate well with other indices of solar activity and with solar luminosity.

Since activity in the solar atmosphere is driven by the emergence, redistribution and disappearance of magnetic flux, the close and linear correspondence between the microwave flux densities, as observed at 10.7 and 21 cm wavelengths, provide a convenient and objective indicator of solar magnetic activity.

Microwave emissions from the “non-flaring” solar corona originate in a wide range of sources. These generally lie in active regions, and comprise two main types: bright, compact sources a few arc-seconds across and brightness temperatures of the order of 106K, and weaker, diffuse sources arc-minutes in size and having brightness temperatures of about 104K. There is also evidence of a weaker, widespread emission covering large areas of the solar disc. The compact sources are probably due to gyroresonance, although some could be non-thermal. The diffuse widespread emission are almost certainly free-free thermal emission from plasma concentrations in the corona.

The strong correlation between the 10.7 cm flux and other activity indices, even when they are produced by disimilar processes and at different heights in the solar atmosphere, may at least partially be due to the overiding role played by magnetic activity in dictating the structure of the chromosphere and corona. However, the consistent correspondences over wide ranges of activity suggest dependences upon relatively few quantities and that the microwave emission from the non-flaring Sun is dominated by a single process, which is closely and simply-related to density and temperature, such as free-free (bremsstrahlung) emission from plasma concentrations trapped in coronal and chromospheric magnetic structures. Attempts to “budget” the total microwave emission at 2.8, 10.7 and 21 cm wavelengths suggest that this is the case.

Where ambient magnetic fields exceed 3000/λ Gauss, gyroresonance can produce much higher optical depths than free-free emission, and accounts for the bright emission observed in the vicinity of sunspots. Although not the dominant contributor to the total emission from the solar disc, these sources stand out brightly in high-resolution maps of active regions.

To examine the assumptions of dominance of free-free thermal emission, and simple dependences upon few parameters, we compute the spectrum for a range of coronal density models, and compare the results with observations. The agreement is good, suggesting that the total disc flux is dominated by thermal, free-free emission, and that the intensity is controlled mainly by the density at the base of the corona, but is otherwise fairly tolerant of the density model used.

The observations from the ACRIM instrument on board the Solar Maximum Mission, and other recent experiments, have allowed us to study the time variations of solar irradiance. These observations identified several mechanisms, ranging from sunspots to global oscillations produced by waves trapped in the solar interior (the p-modes) that cause variations in the total irradiance. In all cases the variations have small amplitudes in integrated light from the visible photosphere. The SXT instrument on board Yohkoh has provided a new data set that can resolve some of these variations spatially. Most of these data are in the form of whole-Sun images: rate about 5 per day; effective pixel size of 4.92 arc sec; passband 28Å centered at 4310Å; data interval beginning in September, 1991 and ending in October, 1992. These observations have several advantages over ground-based observations for the characterization of solar global variability, namely the lack of atmospheric effects and the long-term stability of the instrumentation. We present an assessment of these data in the context of the existing total irradiance data, and also discuss their application to determination of the figure of the Sun, particularly measurements of the variations of the solar radius.

We report on results of a program of coordinated multiwavelength observations of single G stars. While the spectral types are restricted to lie within a narrow range of the Sun’s, they have ages ranging from ∼ 70 Myr to ∼ 9 Gyr, which makes them suitable proxies for the Sun at several stages of its life history from the ZAMS to the very late main-sequence phase.

A method to separate the active region and quiet network components of the magnetic fields in the photosphere is described and compared with the corresponding measurements of the He I λ 10830 absorption. The relation between the total He I absorption and total magnetic flux in active regions is roughly linear and differs between cycles 21 and 22. There appears to no relation between these two quantities in areas outside of active regions. The total He I absorption in the quiet Sun (comprised of network, filaments, and coronal holes) exceeds that in active regions at all times during the cycle. As a whole, active regions of cycle 22 appear to be less complex than the active regions of cycle 21, hinting at one possible cause for a differing relation between spectral-irradiance variations and the underlying magnetic flux for these two cycles.

Small-scale magnetic fields affect the solar luminosity mainly on long time scales. To understand their contribution to solar luminosity variations we must know and understand the contribution of a typical small-scale magnetic feature. In this review I briefly outline our theoretical understanding of the processes leading to the enhancement (or reduction) of the brightness of flux tubes. I also present a brief overview of our observational knowledge.

Solar variability models which account for contemporary irradiance variations in terms of modulation by dark sunspots and bright faculae on the solar disk are used to estimate solar radiative output during the Maunder Minimum by postulating the absence of such sources, and additional reduction in the basal solar emission. When the resultant irradiance reduction of 0.25% relative to contemporary mean levels is input to the GISS GCM, the global mean temperature is reduced by 0.46°C. However, as a result of differential heating of the land and oceans, some regions may cool, and others warm, by as much 1°C, in response to the reduced solar irradiance.

Magnetic activity cycles for solar-type stars are believed to originate from non-uniform internal rotation. To determine this depthwise angular velocity distribution, helioseismology is a valuable source of information. Surface rotation, as traced by sunspot motion, is a well-observed parameter with data going back to the beginning of the telescopic era. This long sunspot series can be used in understanding the behaviour of the Sun’s surface rotation, the connection with its internal rotation, and thereby its magnetic activity. Apparent solar diameter is another important parameter. This is related to the structure of the convective envelope and how it reacts to the presence of magnetic fields. Both these parameters are related to the solar output, and can provide a surrogate for total solar irradiance, by way of a theoretical modeling of the response of the convective zone to the emergence of periodic magnetic fields. The impact of solar variability on the terrestrial climate is also addressed.

Dynamical and thermal modulation of the internal structure of the Sun can be manifest at the surface as changes in irradiance and radius. The relative magnitudes of these changes could provide a diagnostic of at least the location of the primary modulation, if only the mechanism were known. Variations of the frequencies of solar oscillations offer additional potentially valuable diagnostics, but unfortunately at present the mechanisms causing those variations and their relation to the structural and irradiance changes are not yet understood. In this lecture I shall review some of the theoretical conjectures that have been put forward to explain the observations.

Magnetic fields generated and driven by thermal convection are the primary cause of solar activity. There are many facets of the activity, such as plages, flares, sunspots, coronal heating, and the variation of solar luminosity or irradiance whose nature and cause are understood only partially or not at all, although detailed superficial observational descriptions are available. It is suggested that the inferred 105 gauss azimuthal field bundles may be a direct result of the emergence of Ω-loops to form bipolar magnetic regions on the surface in association with an increase in solar irradiance.

The main cause of variability of solar type stars are their varying magnetic fields. To compute irradiance variations one has to compute the magnetic field (the dynamo problem), and from this the irradiance effects. The second problem is considered here. The theoretical work of the past decade has shown that the dominant effect of magnetic fields is a surface effect: a change of effective emissivity of the magnetic parts of the surface while the nonmagnetic part of the surface contributes very little to the irradiance variation on almost all time scales. No other processes have yet been found that would cause variations exceeding (at the current level of magnetic activity) the observed 0.1% irradiance fluctuation of the Sun. This implies that a knowledge of the surface magnetic fields [separated into its bright small scale (faculae, network) and dark large scale (spots) components] is sufficient for pre- or postdicting the solar irradiance. It is hypothesized that the discrepancy remaining between the measured irradiance variations and values reconstructed from proxies is due to the difficulty of finding a proxy that accurately correlates with the continuum contrast of a dispersed small scale magnetic field. Stellar structure theory predicts that the variations in the solar radius associated with magnetic activity are quite small. For stars, color and brightness variations should primarily be interpreted in terms of variations in the fraction of the surface covered by magnetic patches. Their (long term) displacement from the main sequence is not very large.

In the outer layers of the Sun (≈ 30% by radius), energy is transported by convection. The nature of the highly stratified and compressible convective flow is determined from the components of the energy flux (internal, kinetic, viscous, magnetic and radiative). Local suppressions or enhancements of any of these components may give rise to measurable changes in the emergent radiation.

On the solar surface there is direct evidence for modulation of the emerging heat flux covering a large range in spatial and temporal scales, particularly associated with concentrated magnetic fields (e.g. sunspots, plages). Associated with these surface features is the observation that the characteristics of convective motions are also modified. In the deeper layers, the interaction of convection and magnetic fields will play an important role in readjusting the local emerging heat flux and thus should contribute to the modulation of the total solar irradiance.

The task of calculating the response of the convection zone structure to developing active regions, and the solar activity cycle in general is difficult and complex due to the highly non-linear nature of the interaction of convection and magnetic fields. Theoretical work has ranged from empirical and global structure models, all the way to fine scale compressible convection simulations. This paper will highlight some recent theoretical advances that may have a direct bearing on the understanding of solar luminosity and irradiance variations and outline the important problems that must be addressed and what observational constraints may be used.

10Be has been measured in a 300 m long ice core from Greenland. After a general analysis of the 10Be data the potential and the limitations of 10Be records in ice to reconstruct solar variability are discussed. Special emphasis is given to periods of low solar activity (Maunder Minimum etc.). The ‘clock problem’ is addressed, also the question whether the Schwabe cycle maintains its phase. Finally the question of what 10Be can tell us about changes of the solar irradiance in the past is explored.

Using present global warming and paleoclimatic records from climatically sensitive regions as a frame of reference, we infer that global temperature changes did not exceed ±0.5°C during the current interglacial or ±2.0°C during the last glacial period. In order to completely explain such fluctuations by solar irradiance changes, all solar and terrestrial factors must be optimized. Since this is unlikely, we conclude that solar forcing of pre-anthropogenic climate change is a significant and perhaps dominant factor but other processes must also be significant. Solar irradiance changes alone cannot result in the temperature decrease required to change from interglacial to glacial as suggested by Opik (1965).