B. G. Knapp

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.

Mg II core-to-wing index: Comparison of SBUV2 and SOLSTICE time series

The Mg II core-to-wing index is a ratio of the Mg II chromospheric emission at 280 nm to the photospheric radiation in the line wings and is used as an indicator of solar activity. Since October 1991, the Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) has made daily irradiance measurements in the range 119–420 nm from the Upper Atmosphere Research Satellite (UARS). A new Mg II index, based on the SOLSTICE observations at a spectral resolution of 0.24 nm, is presented and compared to previous measurements. Spectral irradiance measurements of the Mg II doublet at low spectral resolution (≈ 1 nm) have been made by the Solar Backscatter UltraViolet (SBUV) instrument on Nimbus 7 since November 1978 and subsequently by the SBUV2 instruments on NOAA 9 and NOAA 11 satellites. We compare the SOLSTICE data with the Mg II time series derived from SBUV2 data by the groups at the National Oceanic and Atmospheric Administration (NOAA) and at the Goddard Space Flight Center (GSFC). SOLSTICE data are convolved to the lower SBUV2 resolution, and the NOAA and GSFC algorithms are then applied to this data set. The SOLSTICE Mg II indices constructed in this manner simulate the SBUV2 indices and can be used to validate the SBUV2 time series and identify data problems. From our analysis, we conclude that the NOAA Mg II time series is the most consistent during the period 1978–1994. The new GSFC Mg II time series has comparable accuracy for the period starting in 1989. We also derive the linear transformation equations required to put the high-and low-resolution time series onto common scales.

Change in the radiative output of the Sun in 1992 and its effect in the thermosphere

Ground and space measurements of the solar spectral irradiance at radio, visible, UV, and X ray wavelengths show a large decline in the first 6 months of 1992. This sustained drop in the solar output is important in understanding the connection between the emergent magnetic flux on the Sun and the radiative output as well as in understanding the effects of such change in the upper atmosphere of the earth. We present preliminary estimates of the observed changes as the means to spur inquiry into this solar event in the declining phase of solar cycle 22. Typical decreases are 15% in Lyman α and 40% in 10.7-cm radio flux. Mass spectrometer and incoherent scatter model calculations at 600 km in the thermosphere indicate a 30% decrease in the temperature and a 3X decrease in the density of the thermosphere near the altitude where both the Upper Atmosphere Research Satellite (UARS) and Hubble Space Telescope are flying. Decrease of the orbital period of the UARS shows the expected effect of decreasing density at flight altitude. Work in progress indicates that the output change results from the decline in solar magnetic flux to a lower level of activity in the southern hemisphere of the Sun.

In-flight degradation results for the UARS SOLSTICE instrument

The SOLar STellar Irradiance Comparison Experiment (SOLSTICE) is a three-channel spectrometer designed for measuring the solar ultraviolet (UV) irradiance from 119 nm to 420 nm with a spectral resolution of 0.1 nm to 0.3 nm. The SOLSTICE is aboard the NASA Upper Atmosphere Research Satellite (UARS), which was launched on 12 September 1991. The degradation of the SOLSTICE sensitivity is primarily tracked in-flight by measuring a set of bright, early-type stars with the same optics and detectors and by changing only slit sizes and integration times. While the Sun changes by 1% in the near-UV and by as much as a factor of 2 in the far-UV, early-type main-sequence stars are not expected to change by more than 1% in the UV for long time periods. The ensemble average of the SOLSTICE stellar observations indicates that these stars are indeed stable to 2% or better. Since the launch of the UARS, the SOLSTICE sensitivity has decreased by a few percent per year. We attribute the degradation primarily to ageing of the photomultiplier tubes for all three channels and to diffusion of layers in the broadband interference filters for the F and N channels. There appears only minor degradation associated with optical contamination, mainly because of the strict use of low-outgassing materials in the SOLSTICE instrument and maintenance of class 10000 clean rooms and oil-free vacuum systems for all pre-flight testing of the instrument.

Effect of Spectral Resolution on the Mg II Index as a Measure of Solar Variability

The solar Mgii core-to-wing ratio is a useful index of UV variability throughout the solar cycle because it has been measured since 1978 in a series of successive satellite missions: Nimbus 7, Solar Mesosphere Explorer (SME), the NOAA 9–14 series, Upper Atmosphere Research Satellite (UARS), and ERS-2. Eventual construction of a single time series from 1978 to the present by combining these measurements will give a long record of almost daily UV variability to serve as a surrogate for estimating both UV and EUV solar radiation. Here we address the effect of spectral resolution on determination of both long-term and short-term solar variability from this index. We use UARS/SOLSTICE measurements of the Mgii line from October 1991 to December 1996 to study the effect of two spectral resolution regimes characteristic of existing measurements, 0.20 to 0.25 nm and 1.10 to 1.15 nm, on determination of the amplitude of 27-day rotational modulation and the more gradual change in chromospheric radiation in the declining phase of solar cycle 22. The two Mgii indices give solar variations that differ by a scaling factor of ≈ 2× for both the solar cycle change from 1992 to 1997 and the amplitude of 27-day modulation over the same period. Both types of measurements appear to yield solar signal equally well except at solar minimum when the solar changes become quite small.

The SORCE Science Data System

The SORCE Science Data System produces total solar irradiance (TSI) and spectral solar irradiance (SSI) data products on a daily basis, which are formulated using measurements from the four primary instruments onboard the SORCE spacecraft. The Science Data System utilizes raw spacecraft and instrument telemetry, calibration data, and other ancillary information to produce and distribute a variety of data products that have been corrected for all known instrumental and operational effects. SORCE benefits from a highly optimized object-oriented data processing system in which all data are stored in a commercial relational database system, and the software itself determines the versions of data products at run-time. This unique capability facilitates optimized data storage and CPU utilization during reprocessing activities by requiring only new data versions to be generated and stored. This paper provides an overview of the SORCE data processing system, details its design, implementation, and operation, and provides details on how to access SORCE science data products.