Matthew T. DeLand

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.

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.

Updated PMC trends derived from SBUV data

Previous analysis of polar mesospheric clouds (PMCs) observed by Solar Backscatter Ultraviolet (SBUV) instruments found that long-term variations in PMC brightness and occurrence frequency were anticorrelated with solar activity and that an increasing secular trend was present at most latitudes. In this paper, long-term PMC variations are presented in terms of ice water content (IWC), a physically based variable which is easier to interpret than previously reported UV albedo values. This model-based conversion from albedo to IWC removes most scattering angle effects. The derived long-term PMC variations in the SBUV data set are qualitatively the same using either an empirically derived adjustment for local time effects or no adjustment (i.e., assuming cancelation of interannual variations in tidally induced amplitude and/or phase). When we use stratospheric ozone variations as a proxy for mesospheric temperature changes, as suggested by recent model studies, we can explain more of the long-term IWC variability than if we use a linear trend. These results show that PMC ice water content in bright clouds increased rapidly from 1979 through the late 1990s and has been approximately constant from the late 1990s through 2013. The numerical value and sign of this trend during the last 15 years depend on the choice of end points and latitude band. Simultaneously, the solar response of IWC observed by SBUV has weakened during the most recent cycle in the Northern Hemisphere, but increased in the Southern Hemisphere.

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.

NOAA 11 Solar Backscattered Ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989–1994: 1. Observations and long‐term calibration

Measurements of solar ultraviolet (UV) irradiance are a valuable diagnostic of physical processes in the solar atmosphere and a key component in characterizing the external forcing of the Earths atmosphere. However, the deleterious effects of solar UV radiation on satellite instrument components have complicated efforts to determine the magnitude of long-term solar variations for almost 30 years. The NOAA 11 Solar Backscatter Ultraviolet, model 2 (SBUV/2) instrument, primarily designed to measure stratospheric ozone, also made daily spectral scan measurements of solar UV irradiance in the 160–405 nm region from February 1989 to October 1994. An onboard calibration system and comparisons with coincident Shuttle SBUV (SSBUV) measurements were used to correct for long-term NOAA 11 instrument sensitivity changes. Time series of the NOAA 11 solar irradiance data indicate a long-term accuracy of approximately ±0.9–2.3% (2σ) over the 5.5-year data record. Long-term solar irradiance changes of approximately −3% are observed between 215 and 250 nm, increasing in magnitude to approximately −7% at 200–205 nm. Additional discussion of solar variations observed by NOAA 11 SBUV/2 are presented by DeLand and Cebula [this issue].

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 1 1 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 1 1 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.

Accuracy of total ozone retrieval from NOAA SBUV/2 measurements: Impact of instrument performance

The National Oceanic and Atmospheric Administration/National Environmental Satellite Data and Information Service (NOAA/NESDIS) has been collecting and evaluating the solar backscattered ultraviolet (SBUV/2) instrument data from NOAA 9 and NOAA 11 spacecraft since March 1985. Over 5 years (March 1985 to October 1990) of NOAA 9 (version 5.0) and over 4 years (January 1989 to June 1993) of NOAA 11 (version 6.0) reprocessed data are now available to the scientific community to study geophysical phenomena involving ozone. This paper examines the impact of the instrument performance on total ozone retrieval from the two instruments. We estimate that at the end of October 1990 the total postlaunch error for NOAA 9 due to instrument alone is -2.2%. A significant fraction of this error (-1.9%) is due to diffuser degradation which is not accounted for in the version 5 reprocessing. The estimate for NOAA 11 total postlaunch instrument error, at the end of June 1993, is -0.4%.

Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II Solar Activity Proxy Indexes

A NOAA-11 SBUV/2 Mgii solar activity proxy index has been created for the period February 1989 through October 1994 from the daily discrete mode solar irradiance data using an algorithm that utilizes a thorough instrument characterization. This product represents a significant improvement over the previously released NOAA-11 SBUV/2 sweep mode-based Mgii data set. As measured by the NOAA-11 Mgii index, the amplitude of solar rotational activity declined from approximately 4–7% peak-to-peak near the maximum of solar cycle 22 in 1989–1991 to roughly 1% peak-to-peak by late-1994. Corresponding to this decrease, the 27-day averaged NOAA-11 Mgii index decreased by 5.8% over this period. The NOAA-11 Mgii data set is compared with coincident data sets from the UARS SOLSTICE and SUSIM instruments. The impact of differences in instrument resolution and observation platform are examined with respect to both the absolute value and temporal variations of the Mgii index. Periodograms of the three indexes demonstrate comparable solar variation tracking. Between October 1991 and October 1994 predominate power occurs near 27 days, with secondary maxima in the power spectra near 29 and 25 days. Overall, there is low power near 13.5 days during this period. Dynamic power spectral analysis reveals the quasi-periodic and quasi-stationary nature of the middle UV variations tracked by the Mgii index, and periods of significant power near 13.5 days in mid-1991 and late-1994 through mid-1995.

Measurements and products from the Solar Backscatter Ultraviolet (SBUV/2) and Ozone Mapping and Profiler Suite (OMPS) instruments

This paper presents an overview of the state of the National Oceanic and Atmospheric Administration (NOAA) satellite ozone programme including assessments of the current Solar Backscatter Ultraviolet (SBUV/2) and the future Ozone Mapping and Profiler Suite (OMPS) instruments and products. It provides evaluation of the quality of the NOAA Polar-orbiting Operational Satellites (POES) SBUV/2 measurements and the Version 8 algorithm products, both for operational and reprocessed data records. The presentation summarizes work on the instrument calibration and characterization; the algorithm theory and implementation; and the information content, quality and validation of the ozone estimates. This is followed by similar information on the measurements and products expected from the OMPS on the National Polar Operational Environmental Satellite System (NPOESS) and NPOESS Preparatory Project (NPP) beginning in 2011.