Richard P. Cebula

Validation of the UARS solar ultraviolet irradiances: Comparison with the ATLAS 1 and 2 measurements

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.

Rotational Raman scattering (Ring effect) in satellite backscatter ultraviolet measurements

A detailed radiative transfer calculation has been carried out to estimate the effects of rotational Raman scattering (RRS) on satellite measurements of backscattered ultraviolet radiation. Raman-scattered light is shifted in frequency from the incident light, which causes filling in of solar Fraunhofer lines in the observed backscattered spectrum (also known as the Ring effect). The magnitude of the rotational Raman scattering filling in is a function of wavelength, solar zenith angle, surface reflectance, surface pressure, and instrument spectral resolution. The filling in predicted by our model is found to be in agreement with observations from the Shuttle Solar Backscatter Ultraviolet Radiometer and the Nimbus-7 Solar Backscatter Ultraviolet Radiometer.

Composite Mg II solar activity index for solar cycles 21 and 22

The Mg II core-to-wing index was first developed for the Nimbus 7 solar backscatter ultraviolet spectrometer (SBUV) instrument as an indicator of solar middle ultraviolet activity that is independent of most instrument artifacts. This index is defined as the ratio of the irradiance in the core of the unresolved Mg II doublet at 280 nm to the nearby continuum irradiance and measures solar variability on both rotational and solar cycle time scales. Mg II index data sets have also been derived for the NOAA 9 and NOAA 11 SBUV/2 instruments. The combined Mg II index data record from the Nimbus 7, NOAA 9, and NOAA 11 instruments presented in this paper extends from November 1978 to January 1992. Differences in the absolute value of the Mg II index and long-term response to solar variations due to differences in wavelength scale and band pass among the three instruments require the use of linear regression fits to create a single composite Mg II index data set which includes more than 13 years of data. This paper documents version 1.0 of the composite Mg II index data set, which has been widely distributed on CD-ROM. Using this composite data set, the change in 27-day running average of the Mg II index from solar maximum to solar minimum is approximately 8% for solar cycle 21 and approximately 9% for solar cycle 22 through January 1992. This difference is not statistically significant when the errors in the linear regression fits used to construct the composite Mg II index are considered. Scaling factors based on the short-term variations in the Mg II index and solar irradiance data sets are developed for each instrument to estimate solar variability at mid-ultraviolet and near-ultraviolet wavelengths. A set of composite scale factors are derived for use with the composite Mg II index presented here. Near 205 nm, where solar irradiance variations are important for stratospheric photochemistry, the estimated change in irradiance during solar cycle 22 is approximately 10(±1)% using the composite Mg II index (version 1.0) and scale factors. However, the actual magnitude of ΔF205 is probably closer to 9% due to unconnected SBUV/2 wavelength scale drift in the current composite Mg II index data set.

Calibration of the NOAA 11 solar backscatter ultraviolet (SBUV/2) ozone data set from 1989 to 1993 using in‐flight calibration data and SSBUV

Total ozone and ozone profiles are currently being measured by solar backscatter ultraviolet (SBUV/2) instruments onboard NOAA polar orbiting spacecraft using the backscattered ultraviolet technique. The NOAA 11 SBUV/2 operational data set was reprocessed from January 1989 to May 1993 and is now called version 6. The version 6 data include an updated algorithm and revised prelaunch and postlaunch calibrations of the geometrical albedo observations used to derive ozone values. Only the calibration revisions are described in this paper. The postlaunch revisions remove time dependent errors in the ozone amounts due to instrument drift, while the revised prelaunch calibration corrects the absolute value of retrieved ozone. The prelaunch corrections are a result of calibration checks from in-orbit comparisons of ultraviolet geometric albedos measured by shuttle SBUV (SSBUV) and the NOAA 11 SBUV/2. Geometric albedo comparison data are further corrected using a radiative transfer code to account for the small difference in observing conditions between the two spacecraft. The postlaunch corrections rely on in-flight calibration and solar irradiance data to account for time dependent changes in instrument gain, thermal response, and instrument diffuser degradation over time. Comparison of data from three SSBUV flights, which occurred about one year apart, with concurrent SBUV/2 data provided an independent check of the time dependent change derived from the in-flight calibration data. Time independent corrections result in an increase of about 1% for total ozone, 5% for ozone at 1 mbar, and near 0% at 15 mbar. The time dependent corrections amount to an increase of 2% for total ozone, 10% for ozone near 1 mbar, and 3% at 15 mbar at the end of the current record in May 1993. Recent laboratory studies indicate that the absolute radiance calibrations may still be in error by a few percent which results in ozone profile values that are too low. The SBUV/2 total and ozone profile data are compared to the Nimbus SBUV data during the period when the data overlapped. Total ozone values agree to about 1%, while ozone profile differences range from −4% to +6%, depending on latitude and altitude, relative to SBUV. These differences are not statistically significant given the uncertainties of the two data sets.

Solar Irradiance Reference Spectra

The solar spectrum is a key input for the study of the planetary atmospheres. It allows the understanding through theoretical modeling of the atmospheric properties (e.g., composition and variability). Furthermore, a reference model is useful for the preparation of instruments and platforms to be operated in space. New composite solar irradiance spectra are formed from 0.1 to 2400 nm using recent measurements for two distinct time periods during solar cycle 22. These two time periods correspond to the activity levels encountered during the ATmospheric Laboratory for Applications and Science (ATLAS) Space Shuttle missions which were moderately high (ATLAS 1, March 1992) and low (ATLAS 3, November 1994). The two reference times span approximately half of the total solar cycle amplitude in terms of the Mg II and F10.7 indices. The accuracy of the two presented spectra varies from 40% in the X-ray range to a mean of 3% in the UV, visible, and near IR ranges. After integration over all wavelengths, a comparison with the total solar irradiance measured at the same time shows an agreement of the order of 1%.

Observations of the solar irradiance in the 200–350 nm interval during the ATLAS‐1 Mission: A comparison among three sets of measurements‐SSBUV, SOLSPEC, and SUSIM

The SOLSPEC, SSBUV, and SUSIM spectrometers simultaneously observed the solar spectral irradiance during the ATLAS-1 mission flown on board the Space Shuttle Atlantis in March 1992. The three instruments use different methods and means of absolute calibration and were each calibrated preflight and postflight. The three data sets are reported from 200 to 350 nm at 1.1 nm resolution. The method of comparing the three independent data sets is discussed. The importance of a common, precise wavelength scale is shown when comparing the data in wavelength regions of strong Fraunhofer lines. The agreement among the solar irradiance measurements is better than ±5%. The fact that the calibrations of the three instruments were based on three independent standards provides confidence that the absolute solar spectral irradiance in the range 200–350 nm is now known with an accuracy better than ±5%. The mean ATLAS-1 solar spectrum is compared with simultaneous solar observations from the UARS SOLSTICE and UARS SUSIM instruments. The two mean solar spectra agree to within ±3%.

Estimates of Solar Variability Using the Solar Backscatter Ultraviolet (SBUV) 2 Mg II Index From the NOAA 9 Satellite

The Mg II core to wing index was first developed for the Nimbus 7 solar backscatter ultraviolet (SBUV) instrument as an indicator of solar variability on both solar 27-day rotational and solar cycle time scales. This work extends the Mg II index to the NOAA 9 SBUV 2 instrument and shows that the variations in absolute value between Mg II index data sets caused by interinstrument differences do not affect the ability to track temporal variations. The NOAA 9 Mg II index accurately represents solar rotational modulation but contains more day to day noise than the Nimbus 7 Mg II index. Solar variability at other UV wavelengths is estimated by deriving scale factors between the Mg II index rotational variations and at those selected wavelengths. Because radiation near the Mg II line core originates at levels in the solar atmosphere comparable to those giving rise to the continuum near 200 nm, the Mg II index accurately tracks the flux in this photochemically important region. Based on the 27-day average of the NOAA 9 Mg II index and the NOAA 9 scale factors, the solar irradiance change from solar minimum in September 1986 to the beginning of the maximum of solar cycle 22 in 1989 is estimated to be 8.6% at 205 nm, 3.5% at 250 nm, and less than 1% beyond 300 nm.

Northern hemisphere total ozone values from 1989–1993 determined with the NOAA‐11 Solar Backscatter Ultraviolet (SBUV/2) instrument

Determinations of global total ozone amounts have been made from recently reprocessed measurements with the SBUV/2 on the NOAA-11 environmental satellite since January 1989. This data set employs a new algorithm and an updated calibration. Comparisons with total ozone amounts derived from a significant subset of the global network of Dobson spectrophotometers shows a 0.3% bias between the satellite and ground measurements for the period January 1989-May 1993. Comparisons with the data from individual stations exhibit differing degrees of agreement which could be due to the matchup procedures and also to the uncertainties in the Dobson data. The SBUV/2 data set discussed here traces the Northern Hemisphere total ozone from 1989 to the present, showing a marked decrease from the average of those years starting in the summer of 1992 and continuing into 1993, with an apparent returning to more normal levels in late 1993. 17 refs., 21 figs.

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

Abstract Originating in the suns upper photosphere, chromosphere, transition zone, and corona, solar UV (including EUV) emissions have a profound effect on Earths 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 atmospheres 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.

Calibration and postlaunch performance of the Meteor 3/TOMS instrument

Prelaunch and postlaunch calibration results for the Meteor 3/TOMS instrument are presented here. The instrument, launched aboard a Russian spacecraft in 1991, is the second in a series of total ozone mapping spectrometer (TOMS) instruments designed to provide daily global mapping of ozone overburden. Ozone amounts are retrieved from measurements of Earth albedo in the 312- to 380-nm range. The accuracy of albedo measurements is primarily tied to knowledge of the reflective properties of diffusers used in the calibrations and to the instruments wavelength selection. These and other important prelaunch calibrations are presented. Their estimated accuracies are within the bounds necessary to determine column ozone to better than 1%. However, postlaunch validation results indicate some prelaunch calibration uncertainties may be larger than originally estimated. Instrument calibrations have been maintained postlaunch to within a corresponding 1% error in retrieved ozone. Onboard calibrations, including wavelength monitoring and a three-diffuser solar measurement system, are described and specific results are presented. Other issues, such as the effects of orbital precession on calibration and recent chopper wheel malfunctions, are also discussed.