Showing papers on "Solar constant published in 1994"

Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements

Abstract: The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 µm) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees hat the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.

Modelling solar irradiance variations with an area dependent photometric sunspot index

Abstract: The He 1083 nm line equivalent width and the 10.7 cm radio flux are employed to model the total solar irradiance corrected for sunspot deficit. A new “area dependent photometric sunspot index” (APSI) based on sunspot photometry by Steinegger et al. (1990) is used to correct the irradiance data for sunspot deficits. Two periods of time are investigated: firstly, the 1980–1989 period between the maxima of solar cycles 21 and 22; this period is covered by ACRIM I irradiance data. Secondly, the 1978–92 period which includes both maxima; here, the revised Nimbus-7 ERB data are used.

A Review of the Nimbus-7 ERB Solar Dataset

Abstract: Fourteen years (November 16, 1978 through January 24, 1993) of Nimbus-7 total solar irradiance measurements have been made. The measured mean annual solar energy just outside of the Earth's atmosphere was about 0.1% (1.4 W/m2) higher in the peak years of 1979 (cycle 21) and 1991 (cycle 22) than in the quiet Sun years of 1985/86. Comparison with shorter, independent solar measurement sets and with empirical models qualitatively confirms the Nimbus-7 results. But these comparisons also raise questions of detail for future studies: in which years did the peaks actually occur and just how accurate are the models and the measurements?

Solar total irradiance variability from SOVA 2 on board EURECA

Abstract: The solar total irradiance has been measured during the EURECA (EUropean Retrievable CArrier) mission by the radiometers PM06 of the experiment SOVA 2 (SOlar VAriability, Experiment 2). The instruments are of the active cavity type with a sampling of 99 s. Their specification and behavior in space are described. The time series of total irradiance gathered with the radiometers covers 9 months, staating in August 1992. Solar variability on time scales from minutes to the mission duration except for the periods close to the orbit around the Earth is observed. The results are correlated with the Photometric Sunspot Index (PSI) and compared with results from other experiments.

Sensitivity of a coupled atmosphere‐dynamic upper ocean GCM to variations of CO2, solar constant, and orbital forcing

Abstract: Sensitivity of a coupled atmosphere-dynamic upper ocean GCM to varying CO2, solar constant, and orbital forcing was examined. Response to atmospheric CO2 concentrations ranging from 100–3500 ppm is logarithmic at all latitudes and seasons, with highest sensitivity at high latitudes, during the winter season. Solar constant response is approximately linear over the range of values +2%, but the sensitivity at high latitudes is less than for equivalent CO2 forcing. Sensitivity to “cold northern summer” orbital forcing, which occurred at the start of the last glacial cycle, is strongly affected by CO2. For CO2 at or below the present level, perennial snow cover in the northern hemisphere expands dramatically with “cold summer” orbital forcing, but this effect becomes very small for CO2 levels in the range 410–460 ppm. This result suggests that the Quaternary “ice age” mode of climatic behaviour may have been initiated by an atmospheric CO2 decrease below a critical value, probably around 350–450 ppm.

Long-Term Variations in Total Solar Irradiance

Abstract: For more than a decade total solar irradiance has been monitored simultaneously from space by different satellites. The detection of total solar irradiance variations by satellite-based experiments during the past decade and a half has stimulated modeling efforts to help identify their causes and to provide estimates of irradiance data, using ‘proxy’ indicators of solar activity, for time intervals when no satellite observations exist. In this paper total solar irradiance observed by the Nimbus-7/ERB, SMM/ACRIM I, and UARS/ACRIM II radiometers is modeled with the Photometric Sunspot Index and the Mg II core-to-wing ratio. Since the formation of the Mg II line is very similar to that of the Ca II K line, the Mg core-to-wing ratio, derived from the irradiance observations of the Nimbus-7 and NOAA9 satellites, is used as a proxy for the bright magnetic elements. It is shown that the observed changes in total solar irradiance are underestimated by the proxy models at the time of maximum and during the beginning of the declining portion of solar cycle 22 similar to behavior just before the maximum of solar cycle 21. This disagreement between total irradiance observations and their model estimates is indicative of the fact that the underlying physical mechanism of the changes observed in the solar radiative output is not well-understood. Furthermore, the uncertainties in the proxy data used for irradiance modeling and the resulting limitation of the models should be taken into account, especially when the irradiance models are used for climatic studies.

Solar signals in global climatic change

Abstract: Based on the physical background that varying solar activity should lead to variations of the ‘solar constant’ and that the climate system may respond sensitively even to small solar variations, a correlation analysis is performed where hemispheric and global averages of the annual mean surface air temperature are compared with the variations of a variety of solar forcing parameters: sunspots, related hypotheses including variations of the quasi-eleven-year solar cycle length, solar diameter variations and gravitational effects. This analysis is based on the 1881–1988 period, for the northern hemisphere including proxy data 1671–1988. Cross correlations and correlations moving in time reveal some instability effects which are hard to interpret. The temperature variance components which may be hypothetically explained by solar forcing are small. Similarly, a seasonal and regional signal and signal-to-noise analysis based on a gridded temperature time series 1890–1985 reveals small signals which do not exceed roughly 1.5 K in the arctic winter (maximum) or 0.2-0.3 K on a global average.

Solar Cycle Variation of the Microwave Spectrum and Total Irradiance

Abstract: We have extended the proxy relationship between irradiance and microwaves by using the daily solar fluxes from Toyokawa Observatory at 1000, 2000, 3750 and 9400 MHz in addition to the Ottawa 2800 MHz flux for the years 1980–1989 It turns out that the flux at 1000 MHz is better correlated with irradiance than the flux at higher frequencies-an unexpected result We have also found that the spectrum of the flux shows shape changes that are related to the number and type of active regions Because of this the five-frequency spectral measurements of microwave flux allow one to separate the sunspot and coronal features, providing an improved proxy of solar variability

On the cause of total irradiance variations observed by the CCD solar surface photometer

Abstract: Spatially-resolved precise photometric observations of the whole Sun at wavelengths of 545nm (FWHM 40nm) were carried out by using the CCD solar surface photometer. Bright parts of photospheric network have contrast of several tenths of percent, and their contribution to the total irradiance is approximately half that of active region faculae. The solar irradiance variations estimated from sunspots, faculae and active network (contrast>0.3%) agreed with the ACRIM data. The quiet Sun irradiance used in the present results was different from the total irradiance at the solar minimum observed by the ACRIM, which indicates unmeasured components (contrast>0.1%) cause the 11-year cycle irradiance variation.

Selection of solar simulator for solar dynamic ground test

Abstract: The 2 kWe Solar Dynamic (SD) Ground Test Demonstration (GTD) experiment will be conducted in 1995 at NASA Lewis Research Center (LeRC). This solar dynamic power system test will be conducted in a simulated space environment and will require an artificial sun. To address the solar simulator requirements for the GTD, Arnold Engineering Development Center (AEDC) was hired under contract to review and visit four existing solar simulator facilities. The four facilities included, AEDC's Mark 1 Chamber, NASA-JSC Chamber A, AEDC's 12V Chamber, and NASA-JPL Space Simulator Chamber. Two design concepts were considered following several months of evaluating existing solar simulator facilities throughout the United States. To satisfy system requirements for the SD GTD experiment the solar simulator needs to provide a uniform light flux to the SD concentrator, provide the light within a subtense angle of one degree, and provide an intensity of one solar constant (1.37 kW/sq m) at airmass zero. Most solar simulators are designed for supplying heat loads to spacecraft where a cone angle as large as 3 degrees is acceptable. It was also concluded that a solar simulator, such like these considered in the AEDC study, would require major facility modifications for NASA LeRC and result in significant impacts to the program. The advanced solar simulator concept developed by NASA LeRC will meet the system requirements for the SD GTD experiment Since SD GTD solar simulator requirements could not be addressed by existing simulator, an advanced concept was considered.

Linear-array apertures for in-flight dynamic solar calibration of radiometric channels for Earth radiation-budget applications.

Abstract: The zero-frequency gain of nonimaging radiometers used in Earth radiation-budget applications is usually verified by a procedure that allows the instrument to view the Sun through an appropriate attenuating aperture and then equates its response to the known attenuated solar constant. However, channel intercomparison often requires that data from a low-resolution, relatively slow instrument such as an active-cavity radiometer be compared with data from a high-resolution, fast instrument such as a scanning thermistor–bolometer radiometer. In such a case, consideration of the difference in the dynamic responses of the two channels may be important. A novel technique for in-flight measurement of the radiometric transfer function of such instruments is described and then demonstrated through the use of a high-order dynamic model of the total, wide-field-of-view, nonscanning channel of NASA’s Earth Radiation Budget Experiment (ERBE).

Selection of Solar Simulator for Solar Dynamic Ground Test

Abstract: The 2 kWe Solar Dynamic (SD) Ground Test Demonstration (GTD) experiment will be conducted in 1995 at NASA Lewis Research Center (LeRC). This solar dynamic power system test will be conducted in a simulated space environment and will require an artificial sun. To address the solar simulator requirements for the GTD, Arnold Engineering Development Center (AEDC) was hired under contract to review and visit four existing solar simulator facilities. The four facilities included, AEDC's Mark 1 Chamber, NASA-JSC Chamber A, AEDC's 12V Chamber, and NASA-JPL Space Simulator Chamber. Two design concepts were considered following several months of evaluating existing solar simulator facilities throughout the United States. To satisfy system requirements for the SD GTD experiment the solar simulator needs to provide a uniform light flux to the SD concentrator, provide the light within a subtense angle of one degree, and provide an intensity of one solar constant (1.37 kW/sq m) at airmass zero. Most solar simulators are designed for supplying heat loads to spacecraft where a cone angle as large as 3 degrees is acceptable. It was also concluded that a solar simulator, such like these considered in the AEDC study, would require major facility modifications for NASA LeRC and result in significant impacts to the program. The advanced solar simulator concept developed by NASA LeRC will meet the system requirements for the SD GTD experiment Since SD GTD solar simulator requirements could not be addressed by existing simulator, an advanced concept was considered.

The Global Potential of Renewable Energy Sources

Abstract: Despite the considerable progress made in solar energy research and development over the past 20 years, an integrated approach to the energy economy barely exists. Not counting traditional renewabl...

Panel Discussions on Total Solar Irradiance Variations and the Maunder Minimum

Abstract: For more than a decade, total solar irradiance has been monitored from several satellites, namely the Nimbus-7, Solar Maximum Mission (SMM), the NASA ERBS, NOAA9 and NOAA1O, EURECA, and the Upper Atmospheric Research Satellite (UARS) (e.g. Willson and Hudson, 1991; Hoyt et al., 1992; Mecherikunnel et al., 1988; Romero et al., 1994). These observations have revealed variations in total irradiance ranging from minutes to the 11-year solar cycle (Figure 1, from Frohlich 1994). The very small, rapid irradiance fluctuations are due to solar oscillations (Woodard and Hudson, 1983; Frohlich, 1992). The short-term variations (from days to months) are directly related to the evolution of active regions via the combined effect of dark sunspots and bright faculae (Chapman, 1987). The most important discovery of irradiance observations is the 0.1% peak-to-peak variation in total solar irradiance over the solax cycle (Willson and Hudson, 1991). This solar-cycle-related variation of total irradiance is attributed to the changing emission of bright magnetic elements, including faculae and the magnetic network (Foukal and Lean 1988). This solar cycle variability may also be related to changes in the photospheric temperature; however it is not clear as yet whether this change can be linked to the bright network component (Kuhn et al., 1988).

The sun as a variable star : solar and stellar irradiance variations : proceedings of the 143rd Colloquium of the International Astronomical Union held in the Clarion Harvest House, Boulder, Colorado, June 20-25, 1993

Abstract: Cryogenic Solar Absolute Radiometer - CSAR. A Review of the Nimbus-y ERB Solar Dataset. Long-Term Variations in Total Solar Irradiance. Solar Total Irradiance Variability from SOVA 2 on Board EURECA. The Periodicity of Solar Activity Cycles. Maximum and Minimum Temperatures at Armagh Observatory 1844-1992, and the Length of the Sunspot Cycle. Photometer "DIFOS" for the Study of Solar Brightness Variations. The Brightness of the Solar Disk in tne Continuum in the Region 1.0-2.4 M. irradiance Observations of the 1-8 A Solar Soft X-Ray Flux from GOES. Comparisons of the Mg II Index Products from the NOAA-9 and NOAA-aa SBUV/2 Instruments. The Solar Ca II K Index and the Mg II Core-to-Wing Ratio. Solar EUV Flux Variations near Activity Maxima. The X-Ray-EUV Spectrum of Optically Thin Plasmas. List of Stars Recommended as Spectrophotometric Standards. Variable Stars with the Hipparcos Satellite. Variation of the Solar Diameter from Solar Eclipse Observations, 1715-1991. Latitudinal Variation of the Solar Limb-Darkening Function. Improvement of the Photometric Sunspot Index and Changes of the Disk-Integrated Sunspot Contrast with Time. Modelling Solar Irradiance Variations with an Area-Dependent Photometric Sunspot Index. On the Cause of Total Irradiance Variations Observed by the CCD Solar Surface Photometer. Latitude and Cycle Variations of the Photospheric Network. Variability of the Solar Chromospheric Network over the Solar Cycle. A YOHKOH Search for "Black-Light Flares". Coronal Index of Solar Activity: Years 1939-1963. Rotational Characteristics of the Green Solar Corona: 1964-1989. Solar Cycle Variation of the Microwave Spectrum and Total Irradiance. Solar Brightness Distribution and its Variability at 3 Millimeter Wavelength. Latitudinal Variability of Large-Scale Coronal Temperature and its Association with the Density and the Global Magnetic Field. Active Region Evolution and Solar Flux Variations. On the Variability of Some Characteristics of Solar Radiative Flux. Lyman-Alpha Line Intensity as a Solar Activity Index in the Far Ultraviolet Range. Modelled Soft X-Ray Solar Irradiances. Cosmic Rays as an Indicator of Solar Activity. Meridional Motions of Sunspot Groups during Eleven Activity Cycles. Radiation-Hydrodynamic Waves and Global Solar Oscillations. Excitation of Solar Gravity Waves. Solar Noise Simulations in Irradiance. Secular Variations in the Spectrum of Solar P-Modes. Solar Cycle Variations in P-Modes and Chromospheric Magnetism. Sunspot Number Uncertainties and parametric Representations of Solar Activity Variations. "Star as a Sun" Observations in Seismology of Distant Stars. Rotating Convection and the Solar Differential Rotation. Variations of the Magnetic Fields of the Sun and the Earth within 7-50 Day Periods. Distinction between the Climatic Effects of the Solar Corpuscular and Electromagnetic Radiation. (Part contents).

A Historical Perspective on Measurements of Solar Irradiance

Abstract: Recent measurements made from platforms in space prove beyond question that the radiant energy received from the Sun at the Earth, once called the ‘solar constant’, fluctuates over a wide range of amplitudes and time scales. The source of that variability and its impact on our terrestrial environment pose major challenges for modern science. We are confronted with a tangled web of facts which requires the combined ingenuity of solar, stellar, planetary and atmospheric scientists to unravel. This brief overview draws attention to key developments during the past century which shaped our concepts about sources of solar variability and their connection with solar activity.