Solar UV Variability Research Articles

Investigation of the Long-term Variation of Solar Ca ii K Intensity. II. Reconstruction of Solar UV Irradiance

Abstract Reconstruction of long-term solar UV variations during the entire 20th century is reported. The sunspot number has been used for this purpose so far. By using the full-disk Ca K intensity as an additional solar UV proxy, the range of allowed values for the reconstructed UV irradiance becomes more restricted. We use long-term archival data of the photographic Ca K plates digitized at the Kodaikanal Solar Observatory. The photographic calibration method developed in our previous paper (Paper I) is applied. Various long-term proxy data of solar activity have been used to estimate past UV irradiance. In light of this context, some issues using the historical Ca K data are commented on.

The Correlation of Synthetic UV Color versus Mg ii Index along the Solar Cycle

Abstract UV solar irradiance strongly affects the chemical and physical properties of the Earth’s atmosphere. UV radiation is also a fundamental input for modeling the habitable zones of stars and the atmospheres of their exoplanets. Unfortunately, measurements of solar irradiance are affected by instrumental degradation and are not available before 1978. For other stars, the situation is worsened by interstellar medium absorption. Therefore, estimates of solar and stellar UV radiation and variability often rely on modeling. Recently, Lovric et al. used Solar Radiation and Climate Experiment (SORCE)/Stellar Irradiance Comparison Experiment (SOLSTICE) data to investigate the variability of a color index that is a descriptor of the UV radiation that modulates the photochemistry of planets’ atmospheres. After correcting the SOLSTICE data for residual instrumental effects, the authors found the color index to be strongly correlated with the Mg ii index, a solar activity proxy. In this paper, we employ an irradiance reconstruction to synthetize the UV color and Mg ii index with the purpose of investigating the physical mechanisms that produce the strong correlation between the color index and the solar activity. Our reconstruction, which extends back to 1989, reproduces very well the observations, and shows that the two indices can be described by the same linear relation for almost three cycles, thus ruling out an overcompensation of SORCE/SOLTICE data in the analysis of Lovric et al. We suggest that the strong correlation between the indices results from the UV radiation analyzed originating in the chromosphere, where atmosphere models of quiet and magnetic features present similar temperature and density gradients.

Delayed response of the ionosphere to solar EUV variability

Abstract. Physical and chemical processes in the ionosphere are driven by complex interactions with the solar radiation. The ionospheric plasma is in particular sensitive to solar EUV and UV variations with a time delay between one and two days. This delay is assumed to be related to thermospheric transport processes from the lower ionosphere to the F region. In previous analyses, the delay has been investigated using the F10.7 index. Here we present preliminary results of the ionospheric delay based on a comprehensive and reliable database consisting of GNSS TEC Maps and EUV spectral flux data. We plan to specify the various dependencies from geographic/geomagnetic location, altitude, season, local time, geophysical and solar radiation conditions such as the solar activity level. The first results for dependencies from seasons and wavelengths regions of the EUV are presented in this paper. These results can provide more insight into ionospheric processes and are of interest for applications dependent on reliable ionospheric weather forecasts, e.g. GNSS error analyses, prediction and mitigation.

Sensitivity of the tropical stratospheric ozone response to the solar rotational cycle in observations and chemistry–climate model simulations

Abstract. The tropical stratospheric ozone response to solar UV variations associated with the rotational cycle (∼ 27 days) is analyzed using MLS satellite observations and numerical simulations from the LMDz-Reprobus chemistry–climate model. The model is used in two configurations, as a chemistry-transport model (CTM) where dynamics are nudged toward ERA-Interim reanalysis and as a chemistry–climate model (free-running) (CCM). An ensemble of five 17-year simulations (1991–2007) is performed with the CCM. All simulations are forced by reconstructed time-varying solar spectral irradiance from the Naval Research Laboratory Solar Spectral Irradiance model. We first examine the ozone response to the solar rotational cycle during two 3-year periods which correspond to the declining phases of solar cycle 22 (October 1991–September 1994) and solar cycle 23 (September 2004–August 2007), when the satellite ozone observations of the two Microwave Limb Sounders (UARS MLS and Aura MLS) are available. In the observations, during the first period, ozone and UV flux are found to be correlated between about 10 and 1 hPa with a maximum of 0.29 at ∼ 5 hPa; the ozone sensitivity (% change in ozone for 1 % change in UV) peaks at ∼ 0.4. Correlation during the second period is weaker and has a peak ozone sensitivity of only 0.2, possibly due to the fact that the solar forcing is weaker during that period. The CTM simulation reproduces most of these observed features, including the differences between the two periods. The CCM ensemble mean results comparatively show much smaller differences between the two periods, suggesting that the amplitude of the rotational ozone signal estimated from MLS observations or the CTM simulation is strongly influenced by other (non-solar) sources of variability, notably dynamics. The analysis of the ensemble of CCM simulations shows that the estimation of the ensemble mean ozone sensitivity does not vary significantly either with the amplitude of the solar rotational fluctuations or with the size of the time window used for the ozone sensitivity retrieval. In contrast, the uncertainty of the ozone sensitivity estimate significantly increases during periods of decreasing amplitude of solar rotational fluctuations (also coinciding with minimum phases of the solar cycle), and for decreasing size of the time window analysis. We found that a minimum of 3- and 10-year time window is needed for the 1σ uncertainty to drop below 50 and 20 %, respectively. These uncertainty sources may explain some of the discrepancies found in previous estimates of the ozone response to the solar rotational cycle.

The dependence of the [FUV-MUV] colour on solar cycle

Solar UV variability is extremely relevant for the stratospheric ozone. It has an impact on Earth's atmospheric structure and dynamics through radiative heating and ozone photochemistry. Our goal is to study the slope of the solar UV spectrum in two UV bands important for the stratospheric ozone production. In order to investigate the solar spectral variability, we use SOLSTICE (the Solar Stellar Irradiance Comparison Experiment) data onboard Solar Radiation and Climate Experiment (SORCE) satellite. Data sets used are far UV (115-180nm) and middle UV (180-310nm), as well as the Mg II index (the Bremen composite). We introduce the SOLSTICE [FUV - MUV] colour to study the solar spectral characteristics, as well as analysis of the colour versus Mg II index. To isolate the 11-year scale variation, we used the Empirical Mode decomposition (EMD) on the data sets. The [FUV - MUV] colour strongly correlates with the Mg II index. More in detail, the [FUV - MUV] colour shows a time dependent behavior when plotted versus Mg II index. To explain this dependence we hypothesize an efficiency reduction of SOLSTICE FUV irradiance using an exponential aging law.

Solar cycle influence on troposphere and middle atmosphere via ozone layer in the presence of planetary waves: Simulation with ARM

AbstractGlobal circulation model of the Troposphere‐Middle Atmosphere‐Lower Thermosphere ARM (Atmospheric Research Model) is used to simulate the thermal and wind response to solar cycle‐induced UV variations. ARM covers altitudes from 1 to 135 km and has corresponding spatial resolution: 1 km in altitude; 11.25° in longitude; 5° in latitude. Internal Gravity Waves parameterization and planetary waves (PWs) structure on the basis of observations are determined at the lower boundary of the model. Changes in UV radiation, which is absorbed by ozone and molecular oxygen, are introduced into the model to find the corresponding global wind and temperature response. Stationary PWs with zonal wave numbers 1–3 are included at lower boundary in model runs. The simulations show that atmospheric response to solar cycle has a visible nonzonal character with the amplitude of about 5 K in the troposphere for the winter season. The effect is rather smaller for summer due to the trapping PWs at lower altitudes. So, in accordance with the results of simulations, the link between the solar UV variability and the middle and low atmosphere strongly depends on the ozone and PWs activity.

Long-term response of stratospheric ozone and temperature to solar variability

Abstract. The long-term variability in stratospheric ozone mass mixing ratio (O3) and temperature (T) from 1979 to 2013 is investigated using the latest reanalysis product delivered by the European Centre for Medium-Range Weather Forecasts (ECMWF), i.e., ERA-Interim. Moreover, using the Mg II index time series for the same time period, the response of the stratosphere to the 11-year Schwabe solar cycle is investigated. Results reveal the following features: (i) upward (downward) trends characterize zonally averaged O3 anomalies in the upper (middle to lower stratosphere) stratosphere, while prevailing downward trends affect the T field. Mg II index data exhibit a weaker 24th solar cycle (though not complete) when compared with the previous two; (ii) correlations between O3 and Mg II, T and Mg II, and O3 and T are consistent with photochemical reactions occurring in the stratosphere and large-scale transport; and (iii) wavelet cross-spectra between O3 and Mg II index show common power for the 11-year period, particularly in tropical regions around 30–50 hPa, and different relative phase in the upper and lower stratosphere. A comprehensive insight into the actual processes accounting for the observed correlation between ozone and solar UV variability would be gained from an improved bias correction of ozone measurements provided by different satellite instruments, and from the observations of the time behavior of the solar spectral irradiance.

Midlatitude atmospheric OH response to the most recent 11-y solar cycle.

The hydroxyl radical (OH) plays an important role in middle atmospheric photochemistry, particularly in ozone (O(3)) chemistry. Because it is mainly produced through photolysis and has a short chemical lifetime, OH is expected to show rapid responses to solar forcing [e.g., the 11-y solar cycle (SC)], resulting in variabilities in related middle atmospheric O(3) chemistry. Here, we present an effort to investigate such OH variability using long-term observations (from space and the surface) and model simulations. Ground-based measurements and data from the Microwave Limb Sounder on the National Aeronautics and Space Administration's Aura satellite suggest an ∼7-10% decrease in OH column abundance from solar maximum to solar minimum that is highly correlated with changes in total solar irradiance, solar Mg-II index, and Lyman-α index during SC 23. However, model simulations using a commonly accepted solar UV variability parameterization give much smaller OH variability (∼3%). Although this discrepancy could result partially from the limitations in our current understanding of middle atmospheric chemistry, recently published solar spectral irradiance data from the Solar Radiation and Climate Experiment suggest a solar UV variability that is much larger than previously believed. With a solar forcing derived from the Solar Radiation and Climate Experiment data, modeled OH variability (∼6-7%) agrees much better with observations. Model simulations reveal the detailed chemical mechanisms, suggesting that such OH variability and the corresponding catalytic chemistry may dominate the O(3) SC signal in the upper stratosphere. Continuing measurements through SC 24 are required to understand this OH variability and its impacts on O(3) further.

Solar UV variations during the decline of Cycle 23

Previous satellite measurements of solar UV variability show consistent solar cycle irradiance changes within instrumental uncertainties, and also show consistent spectral dependence for both rotational and solar cycle variations. Empirical solar irradiance models produce solar UV variations that agree well with observational data. Recent UV irradiance data from the Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instruments longward of 170nm covering the declining phase of Cycle 23 show solar variations that greatly exceed both previous measurements and predicted irradiance changes over this period. The spectral dependence of the SIM and SOLSTICE variations differs from previous results. However, short-term solar variability derived from SIM and SOLSTICE UV irradiance data agrees with other concurrent solar UV measurements and previous results, suggesting no change in solar physics. The SORCE long-term UV results can be explained by undercorrection of instrument response changes during early on-orbit measurements.

A new approach to the long-term reconstruction of the solar irradiance leads to large historical solar forcing

The variable Sun is the most likely candidate for natural forcing of past climate change on time scales of 50 to 1000 years. Evidence for this understanding is that the terrestrial climate correlates positively with solar activity. During the past 10,000 years, the Sun has experienced substantial variations in activity and there have been numerous attempts to reconstruct solar irradiance. While there is general agreement on how solar forcing varied during the last several hundred years --- all reconstructions are proportional to the solar activity --- there is scientific controversy on the magnitude of solar forcing. We present a reconstruction of the Total and Spectral Solar Irradiance covering 130 nm--10 $\mu$m from 1610 to the present with annual resolution and for the Holocene with 22-year resolution. We assume that the minimum state of the quiet Sun in time corresponds to the observed quietest area on the present Sun. Then we use available long-term proxies of the solar activity, which are $^{10}$Be isotope concentrations in ice cores and 22-year smoothed neutron monitor data, to interpolate between the present quiet Sun and the minimum state of the quiet Sun. This determines the long-term trend in the solar variability which is then superposed with the 11-year activity cycle calculated from the sunspot number. The time-dependent solar spectral irradiance from about 7000 BC to the present is then derived using a state-of-the-art radiation code. We derive a total and spectral solar irradiance that was substantially lower during the Maunder minimum than observed today. The difference is remarkably larger than other estimations published in the recent literature. The magnitude of the solar UV variability, which indirectly affects climate is also found to exceed previous estimates. We discuss in details the assumptions which leaded us to this conclusion.

Atmospheric production of nitrous oxide from excited ozone and its potentially important implications for global change studies

Nitrous oxide (N2O) is a greenhouse gas included in the Kyoto Protocol. Its production from excited ozone (O3) may potentially influence inverse modeling, future growth projection, and the use of mass‐independent Δ17O anomaly of N2O for probing paleoatmospheric O3. On the basis of the three‐component model of N2O quantum yield in photolysis of O3 in air, the globally averaged atmospheric production of N2O from O3 electronically excited by the Hartley‐Huggins band and from highly vibrationally excited ground‐state O3 are 1.01 and 0.26 Tg N a−1, respectively. The sum of the two productions is 9.4 and 7.7%, respectively, of the N2O from microbial and anthropogenic activities estimated by Global Emissions Inventory Activity and by the Intergovernmental Panel on Climate Change (2001). Uncertainties in these results are discussed. Subject to those uncertainties, inverse modeling of N2O that neglects productions from O3 could yield artificially magnified (by about 7%) globally averaged emission of N2O from microbial and anthropogenic activity and introduce distortion in the regional and seasonal pattern in that emission. Experiments that could narrow the uncertainties are discussed. Production from highly vibrationally excited O3 reduces the steepness in the decrease of N2O volume‐mixing ratios (VMR) above 35 km. Modeled and observed VMR comparisons show latitude‐ and season‐dependent overestimation and underestimation of the N2O VMR by models. Globally averaged comparison suggests possible N2O source deficit in the stratosphere. Limitations, uncertainties, and need for experiments associated with this possibility are also discussed. If proven real, the possible missing N2O source could influence the atmospheric affects of solar UV variability, subject to conditions that are discussed.

Solar‐QBO interaction and its impact on stratospheric ozone in a zonally averaged photochemical transport model of the middle atmosphere

We investigate the solar cycle modulation of the quasi‐biennial oscillation (QBO) in stratospheric zonal winds and its impact on stratospheric ozone with an updated version of the zonally averaged CHEM2D middle atmosphere model. We find that the duration of the westerly QBO phase at solar maximum is 3 months shorter than at solar minimum, a more robust result than in an earlier CHEM2D study due to reduced Rayleigh friction drag in the present version of the model. The modeled solar cycle ozone response, determined via multiple linear regression, is compared with observational estimates from the combined Solar Backscattered Ultraviolet (SBUV/2) data set for the period 1979–2003. We find that a model simulation including imposed solar UV variations, the zonal wind QBO, and an imposed 11‐year variation in planetary wave 1 amplitude produces a lower stratospheric ozone response of ∼2.5% between 0 and 20°S and an upper stratospheric ozone response of ∼1% between 45 and 55 km, in good agreement with the SBUV‐derived ozone response. This simulation also produces an (enhancement/reduction) in the (lower/upper) stratospheric temperature response at low latitudes compared to the effects of solar UV variations alone, which are consistent with model vertical velocity anomalies produced by the solar‐modulated QBO and imposed changes in planetary wave forcing.

The QBO as potential amplifier and conduit to lower altitudes of solar cycle influence

Abstract. In several papers, the solar cycle (SC) effect in the lower atmosphere has been linked observationally to the Quasi-biennial Oscillation (QBO) of the zonal circulation. Salby and Callaghan (2000) in particular analyzed the QBO wind measurements, covering more than 40 years, and discovered that they contain a large SC signature at 20 km. We present here the results from a study with our 3-D Numerical Spectral Model (NSM), which relies primarily on parameterized gravity waves (GW) to describe the QBO. In our model, the period of the SC is taken to be 10 years, and the relative amplitude of radiative forcing varies exponentially with height, i.e., 0.2% at the surface, 2% at 50 km, and 20% at 100 km and above. Applying spectral analysis to identify the SC signature, the model generates a relatively large modulation of the QBO, which reproduces the observations qualitatively. The numerical results demonstrate that the QBO modulation, closely tracking the phase of the SC, is robust and persists at least for 70 years. The question is what causes the SC effect, and our analysis shows that four interlocking processes are involved: (1) In the mesosphere at around 60 km, the solar UV variations generate in the zonal winds a SC modulation of the 12-month annual oscillation, which is hemispherically symmetric and confined to equatorial latitudes like the QBO. (2) Although the amplitude of this equatorial annual oscillation (EAO) is relatively small, its SC modulation is large and extends into the lower stratosphere under the influence of, and amplified by, wave forcing. (3) The amplitude modulations of both EAO and QBO are essentially in phase with the imposed SC heating for the entire time span of the model simulation. This indicates that, due to positive feedback in the wave mechanism, the EAO apparently provides the pathway and pacemaker for the SC modulation of the QBO. (4) Our analysis demonstrates that the SC modulations of the QBO and EAO are amplified by tapping the momentum from the upward propagating gravity waves. Influenced and amplified by wave processes, the QBO thus acts as conduit to transfer to lower altitudes the larger SC variations in the UV absorbed in the mesosphere. Our model produces in the temperature variations of the QBO and EAO measurable SC modulations at polar latitudes near the tropopause. The effects are apparently generated by the meridional circulation, and planetary waves presumably, which redistribute the energy from the equatorial region where the waves are very effective in amplifying the SC influence.

A simple model of solar variability influence on climate

We present a simple dynamic model of solar variability influence on climate, which is truncated from the stratospheric wave-zonal flow interaction dynamics over a β-plane. The model consists of three ordinary differential equations controlled by two parameters: the initial amplitude of planetary waves and the vertical gradient of the zonal wind. The changes associated with the solar UV variability, as well as with seasonal variations, are introduced as periodic modulations of the zonal wind gradient. Influence of the Quasi-Biennial Oscillation is included as a periodic change of the width of the latitudinal extent of the β-plane. The major climate response to these changes is seen through modulation of the number of cold and warm winters.

Solar uv irradiance, its variation, and its relevance to the earth

Originating in the sun's upper photosphere, chromosphere, transition zone, and corona, solar UV (including EUV) emissions have a profound effect on Earth's ionosphere, thermosphere, mesosphere, and stratosphere. Through ionization, dissociation and excitation processes, solar UV is the primary source of energy input to the atmosphere and, as a result, it plays a central role in the atmosphere's vertical, thermal, and electronic structure. Further, the solar UV irradiance is a principal driver of the strong dynamics in the atmosphere and its cycles of chemical species, especially those of nitrogen and oxygen. Over the past three decades, numerous space-based measurements of solar UV light have been made in order to better understand its nature and effects. They show that the UV irradiance does varies through time, primarily on 27-day solar rotation and 11-year solar cycle time scales, and that this wavelength-dependent variation is primarily associated with the emergence, growth, and decay of active regions on the solar disk. Numerous models and proxies have been developed with some success to better understand and predict solar UV variations and their wavelength dependence. However, in spite of considerable progress, the present estimates of solar variability together with atmospheric models still do not provide complete and accurate descriptions of atmospheric phenomena. Acordingly, we cannot be sure that the accuracy of current measurements is sufficient. Moreover, in some cases, correlation studies of both atmosphere and climate, imply that solar variations are more important than physics-based models would indicate. We discuss the current international UV irradiance measurement and modeling program and make recommendations about its continuation. Also, we outline some evidence for and possible causes of a solar connection in the evolution of atmosphere and climate.

Possible new atmospheric sources and sinks of odd nitrogen: 2. A revisit with O2(B) and discussions of other possibilities

Recently, modelers have expressed a concern that the currently known chemistry of atmospheric NOy is deficient. It is therefore necessary to explore possible sources and sinks of atmospheric NOx that may have been overlooked. In this context, it is noteworthy that the experimentally observed, four-center, Woodward-Hoffman forbidden, reaction 02(B 3Σ) + N2 → NO(X) + NO (X) is atmospherically significant. In the 20 to 30 km region NOx production from this reaction may potentially exceed the production from the “classical” N20 + O(1D) reaction, and may provide a new mechanism to couple the solar UV variability and stratospheric ozone. The avoidance of the non-conservation of the orbital symmetry via the production of one NO in the excited electronic state being endothermic, it appears that the interaction of 02(B 3Σ) with the adjoining 1Λ, 3Λ and 3Σu+ states might be responsible for the reaction. Experimental studies of the reaction as a function of the vibrational levels of the B-state, temperature and pressure are needed for reliable atmospheric applications of this reaction. At altitudes greater than about 50 km the production of NO from 02(B) begins to decrease rapidly. The NO production from 02 (A 3Σ++) + N2 → NO + NO reaction may become important here, if the reaction is confirmed by experiments. These new sources of NOx call for new sinks of this species. In the upper stratosphere and mesosphere the chemical acceleration of NO dissociation via the reactions of electronically and vibrationally excited NO with 02 may help. In the lower atmosphere there is a possibility of the annihilation of NO, N02pair leading to the recreation of a stable NN bond. This might happen if N203 from NO and N02 recombination may photodissociate as N20 + 02. Unfortunately the requirements are stringent, and only experiments can tell whether or not this mechanism operates in the atmosphere.

Stratospheric effects of 27‐day solar ultraviolet variations: The column ozone response and comparisons of solar cycles 21 and 22

Two unresolved observational issues concerning the response of stratospheric ozone to 27‐day solar ultraviolet variations are as follows: (1) the amplitude of the column ozone response and whether it is consistent with the predictions of current two‐dimensional stratospheric models and (2) whether the ozone profile response in the upper stratosphere differed appreciably during the solar cycle 22 maximum period (around 1990) as compared with the solar cycle 21 maximum period (around 1980). To investigate these issues, two separate 4‐year intervals (1979–1982 and 1989–1992) of daily zonal mean Nimbus 7 Total Ozone Mapping Spectrometer, Nimbus 7 solar backscattered ultraviolet (SBUV), and/or NOAA 11 SBUV/2 data for tropical latitudes (30°S to 30°N) are analyzed using cross correlation and cross‐spectral and regression methods. The Mg II core‐to‐wing ratio is employed as a measure of solar UV variations near 200 nm. Results show that the mean tropical column ozone sensitivity (percent change of ozone for a 1% change in solar flux) is 0.09±0.01 at a lag of 4–6 days during both intervals and is approximately consistent with model predictions. Ozone profile sensitivities and phase lags are also in agreement between the two 4‐year intervals when statistical uncertainties and differences in data processing algorithms are considered.

The role of solar forcing upon climate change

Evidence for millennial-scale climate changes during the last 60,000 years has been found in Greenland ice cores and North Atlantic ocean cores. Until now, the cause of these climate changes remained a matter of debate. We argue that variations in solar activity may have played a significant role in forcing these climate changes. We review the coincidence of variations in cosmogenic isotopes ( 14C and 10Be) with climate changes during the Holocene and the upper part of the last Glacial, and present two possible mechanisms (involving the role of solar UV variations and solar wind/cosmic rays) that may explain how small variations in solar activity are amplified to cause significant climate changes. Accepting the idea of solar forcing of Holocene and Glacial climatic shifts has major implications for our view of present and future climate. It implies that the climate system is far more sensitive to small variations in solar activity than generally believed.

Effects of short-term solar uv variability on the stratosphere

Changes in solar ultraviolet flux produce changes in ozone concentration in the upper stratosphere with associated radiative and dynamical effects. At low latitudes, the response of ozone mixing ratio to solar UV variations on the time scale of the solar rotation period is well characterized observationally. In addition, there is some provisional evidence for an ozone response at intermediate periods of 60-80 days. Current two-dimensional stratospheric models simulate the observed 27-day response amplitudes and phase lags with reasonable accuracy in the upper stratosphere. The observed response of total ozone on the 27-day time scale is also in approximate agreement with the same models although observed ozone sensitivities and phase lags are slightly larger than expected theoretically. Future studies of the 27-day response at higher latitudes and altitudes are needed to test more completely our understanding of the direct effects of solar UV variability on the middle atmosphere.

NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 2. Results, validation, and comparisons

Accurately measuring long‐term solar UV variability is an experimental challenge because instrument response degradations are typically large enough to obscure solar change. For satellite instruments, one solution is a series of regular comparisons with a well‐calibrated reference. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument made solar spectral irradiance measurements between 170 and 400 nm from December 1988 to October 1994, covering the maximum and most of the decline of solar cycle 22. The NOAA 11 irradiance data were corrected for long‐term instrument sensitivity changes using comparisons with coincident flights of the Shuttle SBUV (SSBUV) instrument. The NOAA 11 data show a decrease of 7.0(±1.8)% in smoothed 200–208 nm irradiance from Cycle 22 maximum in mid‐1989 to October 1994, near solar minimum. The long‐term decrease in solar irradiance at 250 nm was ∼3.5(±1.8)%. Longward of 300 nm, no solar variations were observed to within the 1% accuracy of the data. The NOAA 11 measurements overlap observations from the Upper Atmosphere Research Satellite (UARS) Solar Stellar Irradiance Comparison Experiment (SOLSTICE) and Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instruments from October 1991 to October 1994, providing the first opportunity to compare three coincident long‐term solar UV irradiance data sets. We find reasonable agreement between the NOAA 11, SOLSTICE, and SUSIM results at all wavelengths in the 170–400 nm region. Power spectral analysis gives consistent results for all three instruments on solar rotational timescales, and reveals the evolution of solar rotation periodicity and strength during a solar cycle. We find significant differences between instruments in both period and spectral location when the spectral irradiance data are analyzed on intermediate (50–250 days) timescales. The NOAA 11 spectral irradiance data provide a valuable complement to the UARS solar data, and capture the entire maximum of solar cycle 22.