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[1] We investigate the relationship between solar irradiance and cosmogenic isotope variations by simulating with a flux transport model the effect of solar activity on the Sun's total and open magnetic flux. As the total amount of magnetic flux deposited in successive cycles increases, the polar fields build up, producing a secular increase in the open flux that controls the interplanetary magnetic field which modulates the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric fields at lower latitudes decay on time scales of less than a year; as a result the total magnetic flux at the solar surface, which controls the Sun's irradiance, lacks an upward trend during cycle minima. This suggests that secular increases in cosmogenic and geomagnetic proxies of solar activity may not necessarily imply equivalent secular trends in solar irradiance. Questions therefore arise about the interpretation of Sun-climate relationships, which typically assume that the proxies imply radiative forcing.

[1] The Sun's Bastille Day flare on July 14, 2000 produced a variety of geoeffective events. This solar eruption consisted of an X-class flare followed by a coronal mass ejection that produced a major geomagnetic storm. We have undertaken a study of this event beginning with an analysis of the effects of the radiative phase of the flare on the dayglow and the ionosphere. The key new enabling work is a novel method of evaluating the X-ray and extreme ultraviolet (EUV) solar spectral irradiance changes associated with the flare. We find that the solar radiative output enhancements modeled during the flare are consistent with measurements of both solar EUV irradiance and far UV Earth thermospheric dayglow. We use the SAMI2 model to predict global ionospheric changes along a magnetic meridian that show significantly different northern and southern effects, suggesting that flares can be used to study ionospheric dynamics.

Satellite measurements are providing solar and atmospheric physicists with the most accurate records to date of the Sun's ultraviolet flux and of changes in this emission caused by variations in solar activity. The Upper Atmosphere Research Satellite (UARS), launched by NASA in September 1991 into a 585 km orbit, started observing the Sun near maximum levels of solar activity (figure 1) using an instrument called the Solar Stellar Irradiance Comparison Experiment (SOLSTICE).

Solar variability is examined in search of implications for global change. The topics covered include the following: solar variation modification of global surface temperature; the significance of solar variability with respect to future climate change; and methods of reducing the uncertainty of the potential amplitude of solar variability on longer time scales.

Using the Baliunas and Jastrow (1990) study of cyclic variability in solar-type stars, we transform existing solar data to the stellar HK irradiance scale and examine the state of the solar chromosphere when a solar-type star shows little cyclic variability and surface magnestis m. To reduce the chromospheric emission to levels for G-type stars showing no chromospheric activity cycles, no only must the sun be free of plages and network; the brightness of the quiet chromosphere in the K line must be reduced to levels seen only in 15% of the quiet Sun area today. In contrast, the present day level of K emission from the sun places it in the class of most active solar-type stars, far removed from a non-cycling state.

By how much does the Sun's radiation vary? Although the Sun has long been an object of immense fascination, fundamental information about the magnitude and variability of its radiative output, sought for over a century, is only now approaching a level satisfactory for geophysical applications. During the past decade, satellite instruments have measured, simultaneously, both the Sun's spectrally integrated radiative output and its ultraviolet spectrum. These data have been analyzed in terms of their relationships to ground-based observations that characterize different aspects of the Sun's 11-year activity cycle, allowing estimates of solar radiative output variations over times scales from days to decades and interpretations of these variations in the broader context of the variable Sun. Uncertainties still remain to be answered by the next generation of solar radiometers, which commenced observations with the launch of the Upper Atmosphere Research Satellite in September 1991, near the maximum of solar cycle 22.

Empirical models of solar EUV irradiance variations developed by Hinteregger et al. (1981) and Tobiska and Barth (1990) are compared over time scales of the 27-day solar rotation and the 11-year solar cycle. Relative to the Hinteregger et al. model, the Tobiska and Barth model predicts the variations of the longest-wavelength EUV emissions (λ > 80 nm), which are entirely chromospheric, to be somewhat less during solar rotation and considerably less over the solar cycle. In contrast, emissions at the shortest EUV wavelengths (λ < 20 nm), which are predominantly of coronal origin, are predicted by the Tobiska and Barth model to vary considerably more during solar rotation and somewhat more during the solar cycle than by the Hinteregger et al. model. Within the wavelength region 20–80 nm, chromospheric emission variations are generally underpredicted on all time scales by the Tobiska and Barth model compared with that of Hinteregger et al., with differences between the predicted variations in the strongest chromospheric emission lines (He II 30.378 nm, He I 58.433 nm, and H I Lβ 102.572 nm) exceeding a factor of 2. Neither models nor measurements yet provide a consistent picture of long-term variability in the EUV portion of the sun's spectrum. This is related to discrepancies of apparent instrumental origin which are identified between the rocket measurements of the EUV spectrum in August 1979 and in November 1988, to which the models are each independently tied. It is demonstrated that simple calculations of active region emission can provide useful constraints on empirical variability models, at least for chromospheric emissions.

Empirical models which can provide physical insights to the origins of solar UV irradiance variations are discussed. Calculations using the WDC/NOAA plage records and independently measured UV plage contrasts show that bright, contiguous active regions cause much of the observed variability in the full disc UV irradiances over time scales of solar rotation (≈ 27 days) and active region evolution (≈ 4 to 9 months), but can account for only about half variability observed over the 11-year solar cycle. This is demonstrated for the HI Lyman α emission, the 205 nm emission and also for the spectrally integrated facular emission which causes variations in the total radiative output. These results confirm the need for an additional irradiance component whose magnitude is at least as great as that associated with variations in active region emission alone, and which affects the solar atmosphere from the base of the photosphere to heights beyond the chromosphere on an 11-year time scale. Possible scenarios for this additional irradiance component are considered.

As climate change accelerates, societies and climate-sensitive socioeconomic sectors cannot continue to rely on the past as a guide to possible future climate hazards. Operational decadal predictions offer the potential to inform current adaptation and increase resilience by filling the important gap between seasonal forecasts and climate projections. The World Meteorological Organization (WMO) has recognized this and in 2017 established the WMO Lead Centre for Annual to Decadal Climate Predictions (shortened to “Lead Centre” below), which annually provides a large multimodel ensemble of predictions covering the next 5 years. This international collaboration produces a prediction that is more skillful and useful than any single center can achieve. One of the main outputs of the Lead Centre is the Global Annual to Decadal Climate Update (GADCU), a consensus forecast based on these predictions. This update includes maps showing key variables, discussion on forecast skill, and predictions of climate indices such as the global mean near-surface temperature and Atlantic multidecadal variability. it also estimates the probability of the global mean temperature exceeding 1.5°C above preindustrial levels for at least 1 year in the next 5 years, which helps policy-makers understand how closely the world is approaching this goal of the Paris Agreement. This paper, written by the authors of the GADCU, introduces the GADCU, presents its key outputs, and briefly discusses its role in providing vital climate information for society now and in the future. LH and AS were supported by the EUCP project funded by the European Commission’s Horizon 2020 programme, Grant Agreement 776613 and supported by the U.K.–China Research and Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund. AS, DS, and MS were supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. The EC-Earth simulations at SMHI were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at PDC and NSC and have been performed as part of the NordForsk-funded Nordic Centre of Excellence project (Award 76654) Arctic Climate Predictions: Pathways to Resilient, Sustainable Societies (ARCPATH). SY and TT were supported by the ARCPATH (NordForsk Award 76654) and the Danish state-funded National Centre for Climate Research [Nationalt Center for Klimaforskning (NCFK)]. SY was also partly supported by the EUCP project (Horizon 2020 Grant Agreement 776613). The EC-Earth simulations at BSC were performed using resources from the Red Española de Supercomputación. HP and WM received funding from the German Federal Ministry of Education and Research (BMBF) project MiKlip (FKZ 01LP1519A). Takahito Kataoka and Hiroaki Tatebe were supported by the Integrated Research Program for Advancing Climate Models (TOUGOU) Grant JPMXD0717935457 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. MIROC simulations were performed on the Earth Simulator at JAMSTEC. NK, YW, FC, and IB were supported by the Trond Mohn Foundation (Grant BFS2018TMT01; Bjerknes Climate Prediction Unit). Peer Reviewed "Article signat per 45 autors/es: Leon Hermanson, Doug Smith, Melissa Seabrook, Roberto Bilbao, Francisco Doblas-Reyes, Etienne Tourigny, Vladimir Lapin, Viatcheslav V. Kharin, William J. Merryfield, Reinel Sospedra-Alfonso, Panos Athanasiadis, Dario Nicoli, Silvio Gualdi, Nick Dunstone, Rosie Eade, Adam Scaife, Mark Collier, Terence O’Kane, Vassili Kitsios, Paul Sandery, Klaus Pankatz, Barbara Früh, Holger Pohlmann, Wolfgang Müller, Takahito Kataoka, Hiroaki Tatebe, Masayoshi Ishii, Yukiko Imada, Tim Kruschke, Torben Koenigk, Mehdi Pasha Karami, Shuting Yang, Tian Tian, Liping Zhang, Tom Delworth, Xiaosong Yang, Fanrong Zeng, Yiguo Wang, François Counillon, Noel Keenlyside, Ingo Bethke, Judith Lean, Jürg Luterbacher, Rupa Kumar Kolli, and Arun Kumar"

Measurements of the total solar irradiance (TSI) during the last 18 years from spacecraft are reviewed. Corrections are determined for the early measurements made by the HF radiometer within the ERB experiment on NIMBUS7 and the factor to refer ACRIM II to the ACRIM I irradiance scale. With these corrections a composite TSI is constructed for the period from 1978-1997. This time series is compared with a model that combines a magnetic brightness proxy with observed sunspot darkening and explains nearly 90% of the observed short and longterm variance. Possible, but still unverified degradation of the radiometers hampers conclusions about irradiance changes on decadal time scales and longer.

Nocturnal ozone densities between 32 and 72 km have been derived from observations of the full moon, made by three optical ozone detectors, each comprising three ultraviolet photometers. The three similar detectors were carried on a single spinning rocket, launched at Wallops Island on November 4, 1979. The mean of the three measured ozone profiles is consistent with other nighttime ozone profiles obtained from the LRIR satellite experiment and also from ground based millimeter wave observations. The relative standard error in the mean ozone profile is approximately 5%. At 70 km the ozone concentration of 1.5×109 cm−3 is a factor of three greater than the U.S. Standard Atmosphere, 1976 mid latitude daytime model.

A composite record of the Sun's total irradiance compiled from measurements made by five independent space-based radiometers since 1978 exhibits a prominent 11-year cycle with similar levels during 1986 and 1996, the two most recent minimum epochs of solar activity. This finding contradicts recent assertions of a 0.04% irradiance increase from the 1986 to 1996 solar minima and suggests that solar radiative output trends contributed little of the 0.2°C increase in the global mean surface temperature in the past decade. Nor does our 18-year composite irradiance record support a recent upward irradiance trend inferred from solar cycle length, a parameter used to imply a close linkage in the present century between solar variability and climate change.

Observations of the solar ultraviolet irradiance during 1979 (solar maximum) by the Solar Backscatter Ultraviolet Experiment on the Nimbus 7 satellite have established that the irradiance at wavelengths between 200 and 300 nm is modulated by solar rotation. This short-term variability, which is of the order of 3% at 200 nm and 1% at 250 nm, is associated with the evolution and rotation on the solar disc of plage areas which are enhanced in ultraviolet emission. By using ground-based Ca II K observations of the fraction of the disc covered by plage during 1979, the intensity ratios for plage to quiet sun emission at these wavelengths have been derived. During the 11-year solar cycle the number of plage regions on the solar disc increases. There is also an accumulation of active network arising from the remnants of decayed magnetic features. A three-component model of the solar irradiance is used to estimate the temporal behavior of the ultraviolet emission between 200 and 300 nm, resulting from changes throughout the solar cycle in the active features on the disc. These calculations suggest that for solar cycle 21 the flux variability at 200 nm is 25%, decreasing to 10% at wavelengths between 210 and 250 nm, and to 2% at 300 nm. Ultraviolet flux variability of this magnitude corresponds to an average increase in the total solar irradiance from 1976 to 1979 of about 0.05%. For comparison, the average reduction in the total solar irradiance from 1976 to 1979 due to sunspot blocking is 0.07%. Because of the enhanced ultraviolet emission from solar active features, the variability of the total solar irradiance during cycle 21 is predicted to be less than is calculated by models which consider only the effects of sunspot blocking. This result is consistent with the need for an added faculae brightening term in total solar irradiance models.

By how much does changing radiation from the sun influence the earth’s climate, presently and in the recent past, compared with other natural and anthropogenic processes? Current knowledge of the amplitudes and timescales of solar radiative output variability needed to address this question is described from contemporary solar monitoring and historical reconstructions. The 17-yr observational database of space-based solar monitoring exhibits an 11-yr irradiance cycle with amplitude of about 0.1%. Larger amplitude solar total radiative output changes—of 0.24% relative to present levels—are estimated for the seventeenth-century Maunder Minimum by parameterizing the variability mechanisms identified for the 11-yr cycle, using proxies of solar and stellar variability. The 11- and 22-yr periods evident in solar activity proxies appear in many climate and paleoclimate records, and some solar and climate time series correlate strongly over multidecadal and centennial timescales. These statistical relati...

The state of solar ultraviolet irradiance measurements in 1978, when NASA requested proposals for a new generation of solar ultraviolet monitors to be flown on the Upper Atmosphere Research Satellite (UARS), is described. To overcome the radiometric uncertainties that plagued the measurements at this time, the solar ultraviolet spectral irradiance monitor (SUSIM) instrument design included in-flight calibration light sources and multichannel photometers. Both are aimed at achieving a maximum precision of the SUSIM measurements over a long period of time, e.g., one solar cycle. The design of the SUSIM-UARS instrument is compared with the original design specifications for the UARS instruments. Details including optical train, filters, detectors, and contamination precautions are described. Also discussed are the SUSIM-UARS preflight calibration and characterization, as well as the results of the inflight performance of the instrument during the first 3 months of operation. Finally, flight operations, observation strategy, and data reduction schemes are outlined.

The climate during the Maunder Minimum is compared with current conditions in GCM simulations that include a full stratosphere and parameterized ozone response to solar spectral irradiance variability and trace gas changes. The Goddard Institute for Space Studies (GISS) Global Climate/Middle Atmosphere Model (GCMAM) coupled to a q-flux/mixed-layer model is used for the simulations, which begin in 1500 and extend to the present. Experiments were made to investigate the effect of total versus spectrally varying solar irradiance changes; spectrally varying solar irradiance changes on the stratospheric ozone/climate response with both preindustrial and present trace gases; and the impact on climate and stratospheric ozone of the preindustrial trace gases and aerosols by themselves. The results showed that 1) the Maunder Minimum cooling relative to today was primarily associated with reduced anthropogenic radiative forcing, although the solar reduction added 40% to the overall cooling. There is no obv...

A simple model based on the changes in excess radiation from bright magnetic faculae and on changes in reduced radiation from dark spots is remarkably successful in matching the slow variations of total solar irradiance measured simultaneously by the ERB and ACRIM satellite radiometers between 1981 and 1984. This model was extended back to 1954 to reconstruct the modulation of irradiance by magnetic activity during the past three 11-year solar cycles. The model predicts that the sun is consistently brighter at activity maximum than at minimum. The 0.07 percent brightening at the peak of the last cycle in 1980 was more pronounced than the brightenings found for either of the two previous cycles, even though cycle 19, which peaked around 1957, had the largest sunspot number amplitude in the history of reliable sunspot records.

Chromospheric emission lines are the dominant energy input to the Earth's upper atmosphere, where they create its embedded ionosphere. Knowledge of the Sun's chromospheric radiation is therefore essential for understanding variability in this region, which contributes significantly to space weather. With the goal of obtaining a suitable surrogate of extreme ultraviolet irradiance variability for atmospheric studies, we construct a composite chromospheric index by comparing and cross calibrating available ground- and space-based indices. Since ground-based flux observations of the chromosphere exist only since 1974, we construct 11-year activity cycles of the index since 1950 by using a parameterization of the daily plage index and the 100-day smoothed 10.7-cm flux, and prior to that by an analogous parameterization of the daily arid 100-day smoothed sunspot group numbers. Comparisons of solar and stellar chromospheric indices suggest that long-term changes in the Sun's chromospheric emission since the Maunder minimum may exceed recent solar cycle amplitudes by as much as a factor of 2. We simulate this by adding to the 11-year activity cycles a speculated varying background component derived from 15-year smoothed sunspot group numbers that increases from 1900 to 1950 and remains approximately level in the decades since then.

Sunspot area data during solar cycles 12-21 are examined with both periodogram and evolutionary spectral analysis techniques for evidence of a periodicity near 155 days. This periodicity is found to be present only during epochs of maxium activity, when it modulates the sunspot areas by as much as 15-20 percent of the amplitude of the 11 yr cycle. The 155 day periodicity typically occurs in episodes of from 1 to 3 yr, with sequential episodes often in opposite solar hemispheres; the strengths of these episodes appear to track the general level of solar activity. Comparing the phases of sinusoids fitted to the sunspot area data within individual solar cycles provides evidence for coherency within + or - 8 days of a period at 155 days between solar cycles 19, 20, and 21. However, within any one episode the actual period of the 155 day cycle may drift from 130 to 185 days. 31 refs.

A three-component model of the solar chromosphere, developed from ground based observations of the Ca II K chromospheric emission, is used to calculate the variability of the Lyman alpha flux between 1969 and 1980. The Lyman alpha flux at solar minimum is required in the model and is taken as 2.32×1011 photons/cm²/s. This value occurred during 1975 as well as in 1976 near the commencement of solar cycle 21. The model predicts that the Lyman alpha flux increases to as much as 5×1011 photons/cm²/s at the maximum of the solar cycle. The ratio of the average fluxes for December 1979 (cycle maximum) and July 1976 (cycle minimum) is 1.9. During solar maximum the 27-day solar rotation is shown to cause the Lyman alpha flux to vary by as much as 40% or as little as 5%. The model also shows that the Lyman alpha flux varies over intermediate time periods of 2 to 3 years, as well as over the 11-year sunspot cycle. We conclude that, unlike the sunspot number and the 10.7-cm radio flux, the Lyman alpha flux had a variability that was approximately the same during each of the past three cycles. Lyman alpha fluxes calculated by the model are consistent with measurements of the Lyman alpha flux made by 11 of a total of 14 rocket experiments conducted during the period 1969–1980. The model explains satisfactorily the absolute magnitude, long-term trends, and the cycle variability seen in the Lyman alpha irradiances by the OSO 5 satellite experiment. The 27-day variability observed by the AE-E satellite experiment is well reproduced. However, the magnitude of the AE-E Lyman alpha irradiances are higher than the model calculations by between 40% and 80%. We suggest that the assumed calibration of the AE-E irradiances is in error.

The sun's total irradiance decreased from 1980 to mid-1985, remained approximately constant until mid-1987, and has recently begun to increase. This time interval covered the decrease in solar activity from the maximum of solar cycle 21 to solar minimum and the onset of cycle 22. The sun's ultraviolet irradiance also decreased during the descending phase of cycle 21 and, like the total irradiance, is now increasing concurrently with the increase in cycle 22 activity. Although only 1 percent of the sun's energy is emitted at ultraviolet wavelengths between 200 and 300 nanometers, the decrease in this radiation from 1 July 1981 to 30 June 1985 accounted for 19 percent of the decrease in the total irradiance over the same period.

The reported investigation of the facular contribution to the total solar irradiance is based on the data provided by the Active Cavity Radiometer Irradiance Monitor (ACRIM) for 1980-1982, and on the Earth Radiation Budget (ERB) radiometry for 1978-1982. It is found that the solar irradiance records from the ACRIM and the ERB radiometers, after subtraction of the calculated sunspot-blocking contribution, exhibit a short-term modulation which is better explained by faculae than be errors in the sunspot-blocking function. The evidence indicates that faculae produce an important contribution to short-term modulation of the total solar irradiance.

Comparison of NIST's SURF-II primary irradiance standard and argon miniarc irradiance standard at 214 nm with an uncertainty of ~3% shows that at this wavelength these irradiance standards agree to within the uncertainties of 1.3 and 7%, respectively, assigned to them by NIST.

Attention is given to two types of temporal variations in the solar UV spectral irradiance caused by solar rotation and active region evolution. It is noted that the first type of dissimilar temporal behavior occurs when concentrations of solar active regions evolve at solar longitudes nearly 180 deg apart. Both the UV observations and modeled UV fluxes based on Ca-K plage data then exhibit pronounced 13-day periodicity, whereas the 10.7-cm solar radio flux and sunspot number exhibit quite dissimilar temporal variations. This type of dissimilarity is related to the modeled UV flux and has a dependence on the solar central meridian distance that is narrower than that for the 10.7-cm radio flux or for sunspot numbers. A second case of marked dissimilarity is seen when major new solar active regions arise and dominate the full-disk fluxes for several rotations. It is found that the strongest peaks in 10.7 cm and sunspot numbers tend to occur on their first rotation, for example, during major dips in the total solar irradiance, whereas the Ca-K plages and UV enhancements peak on the next rotation and then decay more slowly on subsequent rotations.

Variations in the total solar irradiance, solar UV spectral irradiance, and solar soft X-ray emission caused by active region evolution and solar rotation are analyzed by using concurrent measurements from the NIMBUS 7 and GOES satellites. The observations are interpreted by using simple empirical models that relate ground-based observations of the size and location of sunspots and plages to the full-disk temporal variations. It is found that the major dips in the photospheric total solar irradiance S, which are evident in both satellite measurements and model predictions, are usually not accompanied by outstanding enhancements in the chromospheric and upper photospheric UV spectral irradiance or coronal X rays. The main cause of this difference between the variability of S and of the UV flux is that the total chromospheric plage enhancements are not outstanding at those times when the total sunspot are outstanding. X rays are even more variable because of a much wider CMD sensitivity.

Ozone densities in the stratosphere and mesosphere have been derived from broad-band photometer measurements of Hartley band absorption of middle ultraviolet radiation. Seven rockets were launched during October–November 1979 from Wallops Island. Six rockets, each carrying one detector comprising two UV photometers, were launched at different times of the day. A seventh rocket, with three similar detectors each having three UV photometers, was launched at the time of a full moon and provided estimates of the nighttime ozone densities. Results from these rocket flights form a basis for investigating ozone diurnal variations. The number of flights provide greater statistical reliability for the ozone profiles than is generally afforded from in situ measurements with a single rocket. During the night, an enhancement in ozone densities occurred at altitudes above about 50 km. At 70 km, for example, the nighttime ozone was determined to be a factor of 6.4 greater than at sunset. In addition, these experiments suggest that near 40 km the magnitude of the ozone density at noon may be greater by 10–15% than the nighttime concentration.

The variability of the ultraviolet flux between 145 and 200 nm over both the eleven-year cycle and the 27-day solar rotation period is examined in terms of chromospheric activity, as determined from ground-based observations of the CaII K chromosphere. A three-component model of the solar UV flux is developed which includes the contributions to the full disk flux from both plage and active network emission. Solar cycle and solar rotation variations derived from the model are compared with the results of satellite and rocket experiments and with the two-component model of Cook et al (1980). Finally, possible ways of improving the model are discussed.