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Some of the most intense solar flares measured in 0.1 to 0.8 nm x‐rays in recent history occurred near the end of 2003. The Nov 4 event is the largest in the NOAA records (X28) and the Oct 28 flare was the fourth most intense (X17). The Oct 29 flare was class X7. These flares are compared and contrasted to the July 14, 2000 Bastille Day (X10) event using the SOHO SEM 26.0 to 34.0 nm EUV and TIMED SEE 0.1–194 nm data. High time resolution, ∼30s ground‐base GPS data and the GUVI FUV dayglow data are used to examine the flare‐ionosphere relationship. In the 26.0 to 34.0 nm wavelength range, the Oct 28 flare is found to have a peak intensity greater than twice that of the Nov 4 flare, indicating strong spectral variability from flare‐to‐flare. Solar absorption of the EUV portion of the Nov 4 limb event is a possible cause. The dayside ionosphere responds dramatically (∼2.5 min 1/e rise time) to the x‐ray and EUV input by an abrupt increase in total electron content (TEC). The Oct 28 TEC ionospheric peak enhancement at the subsolar point is ∼25 TECU (25 × 1012 electrons/cm2) or 30% above background. In comparison, the Nov 4, Oct 29 and the Bastille Day events have ∼5–7 TECU peak enhancements above background. The Oct 28 TEC enhancement lasts ∼3 hrs, far longer than the flare duration. This latter ionospheric feature is consistent with increased electron production in the middle altitude ionosphere, where recombination rates are low. It is the EUV portion of the flare spectrum that is responsible for photoionization of this region. Further modeling will be necessary to fully understand the detailed physics and chemistry of flare‐ionosphere coupling.

The measurements of the solar ultraviolet spectral irradiance made by the two Upper Atmosphere Research Satellite (UARS) solar instruments, Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and SOLar STellar Irradiance Comparison Experiment (SOLSTICE), are compared with same‐day measurements by two solar instruments on the shuttle ATmospheric Laboratory for Applications and Science (ATLAS) missions, ATLAS SUSIM and Shuttle Solar Backscatter UltraViolet (SSBUV) experiment. These measurements from the four instruments agree to within the 2σ uncertainty of any one instrument, which is 5 to 10% for all wavelengths above 160 nm and for strong emission features below 160 nm. Additionally, the long‐term relative accuracy of the two UARS data sets is better than the original 2% goal, especially at wavelengths greater than 160 nm. This level of agreement is credited to accurate preflight calibrations coupled with comprehensive inflight calibrations to track instrument degradation. Two solar irradiance spectra, 119 to 410 nm, are presented; the first combines observations from UARS SUSIM and UARS SOLSTICE taken on March 29, 1992, during the ATLAS 1 mission, and the second combines spectra for April 15, 1993, during the ATLAS 2 mission. The ATLAS 1 mission coincided with the initial decline from the maximum of solar cycle 22 when solar activity was relatively high. The ATLAS 2 mission occurred somewhat later during the declining phase of the solar cycle 22 when solar activity was more moderate.

Solar irradiance variations show a strong wavelength dependence. Whereas the total solar irradiance varies by about 0.1% during the course of the solar cycle, variations at the wavelengths around the Ly-α emission line near 121.6 nm range up to 50−100%. These variations may have a significant impact on the Earth's climate system. Being almost completely absorbed in the upper atmosphere, solar UV radiation below 300 nm affects stratospheric chemistry and controls production and destruction of ozone. Models of the solar UV irradiance remain far from perfect, even though considerable progress has been made in modelling the irradiance variations longwards of about 200−300 nm. We show that after correcting for the exposure dependent degradation of the SUSIM channels sampling irradiance at λ > 240 nm (making use of the Mg II core-to-wing ratio) the agreement between model and measurement is significantly improved. At shorter wavelengths the LTE approximation usually made in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. The model reproduces observed solar cycle variations of the irradiance at wavelengths down to 115 nm and indicates an important role of UV irradiance variability: up to 60% of the total irradiance variations over the solar cycle might be produced at wavelengths below 400 nm.

[1] The Mg II core-to-wing ratio is a measure of solar chromospheric variability. The Mg II Index, formed by combining various Mg II core-to-wing data sets, has been used in EUV, UV, and total solar irradiance models. It is one of the longest records of solar variability reaching back nearly 25 years. We present a single, continuous time series of the Mg II core-to-wing ratio extending from November 1978 to the present. The Mg II core-to-wing ratio is a measurement that is well suited to the creating of a single time series despite the fact that the seven different instruments measuring the solar flux near 280 nm have different spectral resolutions and sample rates. The Upper Atmosphere Research Satellite (UARS) Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), UARS Solar Stellar Irradiance Comparison Experiment (SOLSTICE), ERS-2/Global Ozone Monitoring Experiment (GOME) and five NOAA solar backscatter ultraviolet data sets were used. Initially, the best data sets were selected to create a time series spanning from 1978 to the present. Then the gaps in the record were filled with data from various other Mg II data sets. Where no alternate data were available, a cubic spline function was used to bridge the missing data. In some cases the data gaps were too long for reasonable spline fits (more than 5 days), and for these gaps the F10.7 cm flux data were scaled to fill the gaps. Thus a continuous, uninterrupted time series of the Mg II core-to-wing ratio was created. The final Mg II time series is compared with other solar activity indices, such as the F10.7, He I 1083, and Sunspot number, to look for trends in the Mg II data.

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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.