Recent measurements made from platforms in space prove beyond question that the radiant energy received from the Sun at the Earth, once called the ‘solar constant’, fluctuates over a wide range of amplitudes and time scales. The source of that variability and its impact on our terrestrial environment pose major challenges for modern science. We are confronted with a tangled web of facts which requires the combined ingenuity of solar, stellar, planetary and atmospheric scientists to unravel. This brief overview draws attention to key developments during the past century which shaped our concepts about sources of solar variability and their connection with solar activity.

Intrinsic variations of total solar irradiance have, after nearly a century of sustained effort, been demonstrated by flight experiments during solar cycles 21 and 22. This accomplishment has been the result of a collaborative effort on behalf of several groups of researchers in both the United States and Europe. The overriding concern at this point of time is whether the integrity of the precision long-term solar total irradiance database, which began at the maximum of solar cycle 21 and continues in the present, will be lost in the late 1990’s, prior to the inception of experiments planned for the NASA Earth Observing System beginning in 2002. Experiments are not currently in place to prevent an unbridgeable discontinuity in the precision total solar irradiance database.

The solar output in radiation and particles varies on an eleven-year time scale, affecting terrestrial temperatures and perhaps even climate. We examine some of the data that support this contention.

Using a nine-year time series of photometric observations of 33 stars similar in temperature and mass to the Sun, but covering a wide range of age and mean chromospheric activity, we found that a majority varies from year to year, some by as much as several percent. We describe the methodology, circumstances of the observations, and photometric results. Stars most similar in mean chromospheric activity to the Sun varied by amounts several times greater than the Sun over a comparable time interval. Thus, the Sun’s present low level of variability, as measured from 1980 to 1989 by the Solar Maximum Mission, appears unusual.

Measurements of the total solar irradiance during the last 14 years from satellites show variations over time scales from minutes to years and decades. The most important variance is in the range from days to several months and is related to the photospheric features of solar activity: decreasing the irradiance during the appearance of sunspots, and increasing it by faculae and the bright magnetic network. Long-term modulation by the 11-year activity cycle is observed conclusively with the irradiance being higher during solar maximum. The accuracy of the determined variability and its interpretation in terms of manifestations of activity related features on the photosphere is discussed. Besides the direct influence of the spots, faculae and magnetic network more profound changes in the thermal transport seem to influence the behaviour of the solar photospheric radiation on the solar cycle and longer time scales.

Requirements and problems in establishing an absolute solar spectrum, which could serve equally well all concerned sciences (e.g. solar physics, atmospheric physics, astrophysics) as a qualified reference, are illustrated by means of a high resolution spectral atlas with absolute intensity and irradiance scales. Also the organizational aspects are pointed out: the desire for one worldwide reference spectrum involves its recommendation by a competent team of scientists and the agreement of the relevant, official organizations.

Sources of new measurements of the solar EUV, UV, and visible spectrum are presented together with discussion of formation of the solar spectrum as a problem in stellar atmospheres. Agreement between the data and a modern synthetic spectrum shows that observed radiative variability is a minor perturbation on a photosphere in radiative equilibrium and LTE. Newly observed solar variability in 1992 defines a magnetic episode on the Sun closely associated with changes in both spectral irradiances and the total irradiance. This episode offers the opportunity to track the relationship between radiation and magnetic flux evolution.

Detection of intrinsic solar variability on the total flux level was made using results from the first Active Radiometer Irradiance Monitor (ACRIM) experiment, launched on the Solar Maximum Mission (SMM) in early 1980. ACRIM I, specifically designed to start the high precision total solar irradiance database as part of the U.S. Climate Research Program, produced high precision results throughout the 9.75 years of the Solar Maximum Mission. The second ACRIM experiment was flown aboard the Space Shuttle as part of the NASA/ESA Spacelab 1 Mission in late 1983. Its primary function has been to provide a comparison with ACRIM I that could be used to relate its observations with future satellite solar monitors, should they and ACRIM I fail to overlap in time. The second ACRIM satellite solar monitoring experiment (ACRIM II) has provided high precision total solar irradiance observations since its launch as part of the Upper Atmosphere Research Satellite (UARS) mission in late 1991 and continues at present. The shuttle ACRIM instrumentation has been flown on the ATLAS 1 and 2 missions in 1992 and 1993, providing comparisons with the UARS/ACRIM II.

The total solar irradiance was observed by Differential Dual Absolute Radiometers (DIARAD) within the SOVA (SOlar VAriablity) experiment during the EURECA-I mission (EUropean REtrievable CArrier; August 1992 – June 1993) and within the SOLCON (SOLar CONstant) experiment during the ATLAS-1 and 2 missions (ATmospheric Laboratory for Applications and Sciences; March 1992 and April 1993). The results from these observations are presented and the metrology aspect addressed through comparison of the simultaneaous measurements with the two DIARADs on EURECA-I and ATLAS-2, respectively.

For the ATLAS and EURECA missions, we have used two identical instruments to measure the absolute solar spectral irradiance from 180 to 3200 nm. These instruments are calibrated by use of a blackbody and a set of lamp standards. The measurements are carried out with 1 nm bandpass up to 800 nm and 20 nm above. The instruments and calibration procedures are described by Thuillier et al. (1981). The platform capability of instruments retrieval after measurements allows a post-flight calibration which is essential for accurate measurements. The main results obtained up to now are:

- In the UV, the ATLAS-1 and EURECA-1 solar spectral irradiance are consistent with the SpaceLab 1 data obtained in 1983 (Labs et al. 1987). Figure 1 shows the ATLAS 1 and SL 1 spectra. The origin of the existing differences is presently under investigation.

- In the visible domain, our measurements agree with the solar spectrum from Neckel & Labs (1984) within a few percent difference at certain wavelength.

- In the IR domain, the preliminary processing shows a spectrum close to the one obtained by Thekaekara (1974).

The SUSIM (Solar Ultraviolet Spectral Irradiance Monitor) on board the UARS (Upper Atmosphere Research Satellite) has measured the solar UV output from 120 nm to 400 nm on a daily basis since October 1991. A reference channel records a solar spectrum semi-annually only to reduce the instrument degradation of this channel and to provide long-term stability marks. Four deuterium lamps are used at monthly, semi-annual and annual intervals to provide long term calibration of the instrument. A preliminary analysis of the long term stability of SUSIM-UARS indicates that the precision of the instrument should be better than a few percent. The repeatability of two scans is better than 0.2%. A simplified SUSIM instrument is flying on NASA’s ATLAS Spacelab missions anually to provide calibration points for the SUSIM-UARS.

The Solar/Stellar Irradiance Comparison Experiment (SOLSTICE) is one of the two ultraviolet spectrometers on the Upper Atmosphere Research Satellite (UARS) measuring the full disk solar irradiance. This spectrometer covers the spectral range of 119 to 421 nm and typically makes ten full spectral scans per day. The instrument was designed to monitor changes in the solar radiation with a relative accuracy of better than 1%. To achieve this goal the spectrometer makes repeated observations of a number of bright blue stars; stars that individually should vary by only small fractions of a percent over time periods of thousands of years. The calibration stars are observed with the same optics and detectors used for the solar observations and any drift in the instrument response is directly determined by changes in the ensemble average flux from these calibration stars. The instrument performance and calibration are described and solar data from the first year of the UARS mission are presented. These UARS data show significant solar activity during late 1991 and early 1992 and then clearly mark a decrease as we move off the maximum of solar cycle 22. We are optimistic that an extended UARS mission, perhaps as long as ten years, is now possible.

Solar spectral irradiance measurements from the NOAA-9 and NOAA-11 SBUV/2 instruments and SSBUV are presented and compared. Inter-instrument solar spectral irradiance differences on the order of 5-10% are observed. Internal comparisons of the SBUV/2 solar data suggest instrument sensitivity changes ranging from less than 1% per year near 400 nm to approximately 5% per year near 200 nm.

The mission-to-mission accuracy of the SSBUV solar spectral irradiance measurements is better than ±2%. Initial results of our investigation into the feasibility of using SSBUV data to maintain the long-term calibration of the SBUV/2 instruments for solar spectral irradiance monitoring are reviewed.

Recent findings on the variable structure of the corona by the soft X-ray telescope on board Yohkoh are reviewed, with emphasis upon the structure and evolution of active regions, and the long term evolution of the global structure of the corona.

The variations of solar irradiance can only be measured with a reasonable precision from space for almost all wavelengths. Yet the history of solar irradiance determinations is, with few exceptions, the history of efforts to patch the infrequent or too discontinuous space-based measurements by devising mechanisms for proxy determination of solar irradiance fluxes from ground-based obtained parameters. Although it appears that in the near future the situation will not be too different, there is some increase in the coverage of the spectral range that will be monitored. From gamma-rays to infrared light there are instruments that are now operating (e.g. GOES, Yohkoh, UARS) or are planned to operate in approved space missions (e.g. SOHO, ATLAS), or form part of proposed, not yet approved, missions. New techniques are being developed specifically for the measurement of solar irradiance variations with increased precision (e.g. cryogenic radiometer) in the next generation of space-borne measurements.