Dianne K. Prinz

Solar Cycle 22 UV Spectral Irradiance Variability: Current Measurements by SUSIM UARS

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) has measured the solar spectral irradiance for wavelengths 115–410 nm on a daily basis since October 11, 1991. The absolutely calibrated solar UV irradiances through January 8, 1996 have been produced. Their time-dependent behavior is similar to that of the Mg II index as measured both by NOAA-9 SBUV and by SUSIM itself. The maximum long-term variation observed by SUSIM is at La and is measured to be in excess of a factor of 2. This maximum variation decreases with increasing wavelength until about 300 nm where no significant long-term variation is directly measured above SUSIM’s estimated 1 – 2% relative accuracy. The wavelength dependence of the measured UV variability is found to roughly correspond to the mean emission height given by solar atmospheric radiative transfer models. Because SUSIM observations began when solar activity was near its peak and now extend to very near its minimum, estimates of the solar cycle 22 U V variability are generated from a combination of these measurements and solar activity proxy indices.

SUSIM/UARS observations of the 120 to 300 nm flux variations during the maximum of the solar cycle: Inferences for the 11‐year cycle

Since October 1991, the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on board the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 120 to 400 nm with low (5 nm) and moderate (1.1 nm) resolution on a daily basis. By scaling the modulation measured over four solar rotations to proxy indicators of solar activity, we estimate the amplitude of the solar ultraviolet irradiance variation during the 11-year activity cycle, extending the wavelength coverage of existing empirical variability models to 120 nm and distinguishing the variability of the emission lines from that of the underlying solar continuum. These estimates are compared with results from another empirical variability model and from direct measurements.

Solar UV irradiance variation during cycles 22 and 23

Abstract The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring solar UV irradiances since October 1991, a period which includes the decline of solar cycle 22 followed by the rise of cycle 23. Daily solar measurements include scans over the wavelength range 115–410 nm at 1.1 nm resolution. As expected, the measured time series of UV irradiances exhibit strong periodicities in solar cycle and solar rotation. For all wavelengths, the UV irradiance time series are similar to that of the Mg II core-to-wing ratio. During solar cycle 22, the irradiance of the strong Ly-α line varied by more than a factor of two. The peak-to-peak irradiance variation declined with increasing wavelength, reaching ∼10% just below the Al edge at 208 nm. Between the Al edge and 250 nm the variation was ∼6–7%. Above 250 nm, the variation declines further until none is observed above ∼290 nm. Preliminary results for the first portion of cycle 23 indicate that the far UV below the Al edge is rising at about the same rate as the Mg II index while the irradiances in the Ly-α emission line and for wavelengths longer than the Al edge are rising more slowly — even after accounting for the lower level of activity of cycle 23.

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.

The center-to-limb behavior of solar active regions at ultraviolet wavelengths

The time series of solar ultraviolet irradiances measured by the Solar Ultraviolet Spectral Irradiance Monitor on the Upper Atmosphere Research Satellite have been analyzed to describe the center-to-limb behavior of the excess surface brightness of solar active regions over the wavelength range 142-265 nm. Comparison of these results with the analysis by Worden, Woods, and Bowman (2001, ApJ, 560, 1020) of the time series produced by the Solar-Stellar Irradiance Comparison Experiment on the same spacecraft over the wavelength range 120-170 nm shows excellent agreement. For 170-265 nm, we present new results on the center-to-limb behavior of the surface brightness of solar active regions. Comparisons with previous results for the quiet disk show roughly similar behaviors at wavelengths below 168 nm and above 210 nm, where both exhibit weak limb brightening and darkening, respectively. At intermediate wavelengths, 168-210 nm, active regions exhibit much stronger limb darkening than does the quiet disk. Our Fourier analysis and the multi-component modeling of Worden et al. (2001, ApJ, 560, 1020) are found to be complementary and could be productively combined in future work. We also compare our results with a similar analysis based upon the semi-empirical model atmospheres of Fontenla et al. (1999, ApJ, 518, 480), further improved by Avrett. We compare the measurement- and model-based analyses and suggest the direction of improvements needed in the model atmospheres.

Ozone variability in the upper stratosphere during the declining phase of the solar cycle 22

Recent studies of the solar cycle variation of ozone have shown that the response of ozone in the upper stratosphere to solar UV variation, as inferred from the SBUV (Solar Backscatter Ultraviolet) type measurements, is about a factor of two greater than estimated from 2-D photochemical models. Because of potential errors in accounting for the long term instrument drift in the SBUV type of measurements, the significance of this discrepancy is difficult to quantify. In this paper, ozone measurements from the Microwave Limb Sounder (MLS) and the solar irradiance measurements from the Solar Stellar Irradiance Comparison Experiment (SOLSTICE) and the Solar Ultraviolet Irradiance Monitor (SUSIM) onboard the Upper Atmosphere Research Satellite (UARS) are analyzed to estimate the upper stratosphere ozone response to changes in the solar UV irradiance. During the three year period of UARS measurements, analyzed here for the declining phase of the solar cycle 22, the solar irradiance in the 200–205 nm range decreased by about 5 % from a near solar maximum to a near solar minimum level. During the same period, ozone mixing ratio measured from the MLS instrument decreased by about 2–4% in the 0.7–3 hPa region. In the upper stratosphere, the general characateristics of the MLS time series are similar to those inferred from the NOAA-11 SBUV/2 measurements. The SBUV/2 trends above 1.5 hPa, however, are significantly greater than those derived from the MLS data. The UARS data suggest that the long term solar UV response of ozone in the upper stratosphere is underestimated by 2-D photochemical models as in previous studies based on the SBUV type measurements.

Instrument responsivity evolution of SUSIM UARS

The Solar UV Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring the solar spectral irradiance from 115 nm to 410 nm daily since October 1991. The primary difficulty in maintaining the calibration of these long-term measurements is the correct accounting of degradation in the instruments responsivity. Accordingly, SUSIM was equipped with redundant optical elements and four stable deuterium lamps, any one of which can replace the sun in the optical path. Periodic calibration of the responsivity of the daily used optical paths is accomplished by comparison of the solar signals from these paths with the solar signals from redundant optical paths containing less-frequently used elements which, in turn, are calibrated using the lamps. Measurements of optical responsivity changes during the first 6.3 years of the SUSIM UARS mission are presented in this paper. The degradation is found to be almost entirely dependent on UV exposure. Consequently, most of the measured degradation takes place in optical elements which precede dispersion. Of these, degradation of transmission elements is strong and has a wavelength dependence similar to that of the lens on the calibration lamps on which contaminant layers apparently also form. The time dependence of the degradation also appears to follow a particular functional form for most wavelengths. Degradation in reflective elements is more moderate and, for some wavelengths, has been observed to reverse, i.e. to increase in responsivity. As an example, the standard channel responsivity at Lyman-(alpha) has increased from its minimum by a factor of more than three.

On-orbit performance of deuterium calibration lamps during four years of SUSIM operations on the UARS

The Solar Ultraviolet Spectral Irradiance Monitor on the Upper Atmospheric Research Satellite (UARS) carries four deuterium discharge lamps to assist in tracking inflight degradation of the instrument in the spectral range between 115 nm and 410 nm. The lamps have been in orbit since the launch of UARS on 12 September 1991. In this paper we discuss the performance of the two most-used lamps during the first four years of the UARS mission, and offer recommendations for improving the ability of the lamps to track inflight degradation in future space instruments.

Solar ultraviolet spectral-irradiance observations from the SUSIM-UARS experiment

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) instrument on board the Upper Atmosphere Research Satellite (UARS) has measured solar ultraviolet spectral-irradiance values since October 1991. The calibration and tracking methods are described. The intensity values have a precision of 1% at wavelengths longer than 200 nm. Short-term ultraviolet (UV) variability is correlated with other chromospheric indices at wavelengths shorter than 280 nm. At longer wavelengths, this correlation changes gradually into an anticorrelation. There exists a long-term solar-cycle component at all wavelengths, which seems to be independent of the rotational modulation. The variability in the integrated UV from 110 nm to 300 nm amounts to 33% of the variability in the total solar irradiance.

A high precision Solar Ultraviolet Spectral Irradiance Monitor for the wavelength region 120–400 nm

There exists a growing need to improve the accuracy of measurement of the absolute solar flux within the wavelength range 120–400 nm. Although full-disk solar fluxes and variations thereof in the 120–400 nm region are required to model the solar atmosphere, current increased interest in the measurements arises from their importance in modeling the terrestrial atmosphere. We describe the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment under development at the Naval Research Laboratory (NRL) for flight aboard the Space Shuttle and the Upper Atmospheric Research Satellite (UARS). SUSIM will monitor the solar flux in the 120–400 nm region with high precision, using an in-flight calibration system to reduce absolute error to < 10%, and error relative to the 400 nm continuum to < 1%.