Gary J. Rottman

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.

On the Failure of Standard Emission Measure Analysis for Solar Extreme-Ultraviolet and Ultraviolet Irradiance Spectra

We perform emission measure analysis of new and accurate UV (λ > 1200 A) and extreme-ultraviolet (EUV) (λ ≤ 1200 A) irradiance (Sun-as-a-star) emission-line spectra of the Sun. Our data consist of (1) daily averaged UV irradiances from the SOLSTICE on the UARS spacecraft and (2) EUV irradiances obtained on the same date from a m spectrograph flown on a sounding rocket. Both instruments have a spectral resolution of roughly 1 A. The absolute uncertainties in these data are at most ±15% ( ± 2 σ), one of the highest photometric accuracies yet achieved. We find large, highly significant and systematic discrepancies in the emission measure analysis of transition region lines which can only be accounted for by a breakdown of one or more standard assumptions. All strong lines above 1000 A, which are from the Li and Na isoelectronic sequences, are too strong by factors of between 2.5 and 7 compared with their counterparts in the EUV region. Previous studies were tantalizingly close to finding these discrepancies, but those data lacked the wavelength coverage and relative photometric precision necessary for definitive conclusions. We argue that either dynamical effects, inaccurate treatments of atomic processes, and/or Lyman continuum absorption are the culprits. However, we favor the former explanation. In any event, this study should have implications for models of the solar transition region, for observing programs with the CDS and SUMER instruments on SOHO, and for analysis of UV spectra for stars across the cool half of the H-R diagram. Finally, the discrepancy is not seen for the coronal Li-like ions.

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.

Time variations of solar UV irradiance as measured by the SOLSTICE (UARS) instrument

An analysis is presented of solar ultraviolet irradiance measurements made by the SOLSTICE spectrometers on the Upper Atmosphere Research Satellite (UARS). Reported observations cover the wavelength interval 119–420 nm, and the analysis discussed here is for the time period 26 Nov 1991 to 31 Dec 1992, during which time solar activity decreased in intensity. At the time of peak activity, the average 27-day variation had a relative amplitude of ∼8% at Ly-α, tailing off to about 0.6% at 260 nm. It is shown that over the spectral interval 119–260 nm, the relative 27-day harmonic was about a factor of two larger during the strongly disturbed as compared with the moderately disturbed period.

Scattered-light properties of diffraction gratings

One of the many calibrations performed for a scientific-quality spectrometer is the characterization of its scattered-light properties. The scattered light can arise from any optical surface, and light leaks or scattering from baffles can also contribute to the instrumental stray-light level. For a diffraction-grating spectrometer the primary contribution to instrumental scattered light has been found to be the scattered light from the grating. The results from measuring the scattered-light properties of 10 diffraction gratings are discussed along with the application of these results in analyzing the total scattered light measured for three spectrometers. It has been found from these measurements that there are two components of the grating scattered light: a Lorentzian-type component and a constant background component. The Lorentzian component is predicted from the diffraction theory for a grating, and the constant background component is consistent with Rayleigh scattering from the microscopic surface im erfections. It was also discovered that multiple replicas of gratings from the same master grating exhibit significantly more scattered light than the preceding replica by factors of 1.1-2.

Solar EUV irradiance from the San Marco ASSI - A reference spectrum

The only satellite measurement of the solar EUV irradiance during solar cycle 22 has been obtained with the Airglow Solar Spectrometer Instrument (ASSI) aboard the San Marco 5 satellite flown in 1988. The ASSI in-flight calibration parameters are established by using the internal capabilities of ASSI and by comparing ASSI results to the results from other space-based experiments on the ASSI calibration rocket and the Solar Mesospheric Explorer (SME). A solar EUV irradiance spectrum derived from ASSI observations on November 10, 1988 is presented as a reference spectrum for moderate solar activity (F10.7≈ 150) for the aeronomy community. This ASSI spectrum should be considered as a refinement and extension of the solar EUV spectrum published for the same day by Woods and Rottman [1990]. This new reference solar spectrum covers the wavelength range from 20 to 200 nm and is presented in a format similar to the 37 wavelength intervals defined by Torr and Torr [1985].

Solar UV irradiance variability during the declining phase of the solar cycle 22

The SUSIM (Solar Ultraviolet Spectral Irradiance Monitor) and the SOLSTICE (Solar Stellar Irradiance Comparison Experiment) instruments on the UARS (Upper Atmosphere Research Satellite) have been making continuous measurements of the solar UV flux in the spectral range 115–420 nm since October 1991. This period, characterized as the declining phase of solar cycle 22, shows a transition from near maximum to near minimum solar activity levels. During this period, the solar UV flux at Lyman α decreased by about 45% from a mean solar maximum value of about 9 mW/m², and the integrated solar flux between 200–205 nm decreased by about 5% from a mean value of about 47 mW/m². Using the MgII index as a proxy of solar UV irradiance variability, it is shown that the temporal relationship of the UARS solar Lyman α irradiance and the MgII index during solar cycle 22 is significantly different than during solar cycle 21, inferred from the SME (Solar Mesosphere Explorer) Lyman α measurements. Moreover, during solar cycle 22, the scale factor for solar Lyman α irradiance (% change for 1% change in MgII index) is about 1.5 times larger for long term changes than for changes over the time scale of a solar rotation. Unlike Lyman α, the scale factor for the UV flux in the 200–205 nm wavelength range, is close to unity both for the rotational and longer time scales. The spectral dependence of the two scale factors in the 120–200 nm range is derived from the SOLSTICE data which may be used with the MgII index to estimate the UV irradiance variability in this spectral range for both short and long time scales.

Solar Total Irradiance Monitor (TIM)

The Total Irradiance Monitor (TIM), an active-cavity solar radiometer, is to be launched in 2002 on the SORCE mission of the Earth Observing System (EOS). The relative uncertainty (1 σ) will be better than 10−4, i.e. 100 parts per million (ppm) (1 σ) with a noise level of < 1 ppm each 500 s. Sunlight passes through a shutter, a 50 mm2 aperture, and is then absorbed into a silver cavity blackened inside with nickel phosphorus. High thermal-conductivity diamond insulators at the electrical terminals help to localize thermal nodes. Four cavities are aligned side-by-side at the rear of a 2 kg heat sink. The flight standard digital watt is derived from a high-precision voltage and a pulse-width-modulator; this eliminates the need for a square root in the servo loop. We determine the irradiance from the in-phase sinusoidal component at the shutter frequency. This phase-sensitive detection allows more accurate characterization than traditional time-domain methods. We calibrate the aperture transmission integral over area against a chrome-on-quartz ruling, transferring by charge-coupled device (CCD) images. In flight, we recalibrate the servo-loop gain, pointing variations, and dark signal. We determine the equivalence ratio from models and by laser measurements of the cavity parameters.

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.