Showing papers on "Solar constant published in 1998"

Accurate Determination of the Solar Photospheric Radius

Abstract: The Solar Diameter Monitor measured the duration of solar meridian transits during the 6 years 1981-1987, spanning the declining half of solar cycle 21. We have combined these photoelectric measurements with models of the solar limb-darkening function, deriving a mean value for the solar near-equatorial radius of 695.508±0.026 Mm. Annual averages of the radius are identical within the measurement error of ±0.037 Mm.

First‐year continuous solar EUV irradiance from SOHO by the CELIAS/SEM during 1996 solar minimum

Abstract: The CELIAS/SEM photodiode spectrometer aboard SOHO continuously monitors the full-disk EUV solar irradiance in an 8-nm wavelength band centered at 30.4 nm (first order), and in a broad wavelength band between 0.1 and 77 nm (central order). We present the absolute solar EUV irradiances for the 1996 solar EUV minimum year at 1 AU. The uncertainty in absolute flux for each channel is approximately ± 14%. The accuracy and stability of the instrument make the data extremely useful in modeling the upper terrestrial atmosphere during this period of “low” solar activity. The data show evidence of persistent solar EUV/soft X ray active regions throughout this solar minimum period which give rise to both 27-day and short-term (minutes to hours) solar EUV irradiance variations. The lowest value of solar flux in the first order 30.4-nm band occurred on November 6, 1996, with a photon flux of 9.8 × 109 cm−2 s−1. Using previously obtained solar spectra, we infer a photon flux of 4.7 × 109 cm−2 s−1 within a 1-nm bandpass centered on the solar He II 30.4-nm emission line at this time. The irradiance variation of the first order channel was between +15% and −10% as measured from a smoothed quadratic least squares fit to the entire first-order channel database for 1996. The lowest central-order EUV photon flux occurred on the same day (November 6, 1996) with an absolute flux of 2.2 × 1010 cm−2 s−1. When sharp increases of short-term flux variability are ignored, a variation between +45% and −30% from the smoothed least squares fit to the central-order database is obtained. The long-term solar cycle valuation during the 12-month smoothed data in both channels indicates that the solar EUV minimum was reached during mid 1996. Large short-term sudden increases monitored by both channels correspond to solar flares observed from the ground and from the GOES satellites. New data for two isolated flares obtained from both CELIAS/SEM channels are also presented and compared with GOES 0.1 to 0.8-nm soft X ray data.

Spatial dependence of solar-cycle changes in the Sun's luminosity

Abstract: We report observations of the large-scale spatial dependence of the Sun's luminosity variations over the period 1993–1995. The measurements were made using a new scanning disk solar photometer at Big Bear Solar Observatory, specially designed to measure large-scale brightness variations at the 10−4 level. Since the level of solar activity was very low for the entire observation period, the data show little solar cycle variation. However, the residual brightness signal ΔI/I (after subtracting the mean, first, and second harmonics) does show a strong dependence on heliocentric angle, peaking near the limb. This is as one would expect if the residual brightness signal (including the excess brightness coming from the active latitudes) were primarily facular in origin. Additional data over the next few years, covering the period from solar minimum to maximum, should unambiguously reveal the large-scale spatial structure of the solar cycle luminosity variations.

Pioneer Venus Orbiter Measurements of Solar EUV Flux During Solar Cycles 21 and 22

Abstract: The Pioneer Venus Orbiter (PVO) had on board the electron temperature probe experiment which measured temperature and concentration of electrons in the ionosphere of Venus. When the probe was outside the Venus ionosphere and was in the solar wind, the probe current was entirely due to solar photons striking the probe surface. This probe thus measured integrated solar EUV flux (Ipe) over a 13-year period from January 1979 to December 1991, thereby covering the declining phase of solar cycle 21 and the rising phase of solar cycle 22. In this paper, we examine the behavior of Ipe translated to the solar longitude of Earth (to be called EIpe) during the two solar cycles. We find that total EUV flux changed by about 60% during solar cycle 21 and by about 100% in solar cycle 22. We also compare this flux with other solar activity indicators such as F10.7, Lα, and the solar magnetic field. We find that while the daily values of EIpe are highly correlated with F10.7 (correlation coefficient 0.87), there is a large scatter in EIpe for any value of this Earth-based index. A comparison of EIpe with SME and UARS SOLSTICE Lα measurements taken during the same period shows that EIpe tracks Lα quite faithfully with a correlation coefficient of 0.94. Similar comparison with the solar magnetic field (Bs) shows that EIpe correlates better with Bs than with F10.7. We also compare EIpe with total solar irradiance measured during the same period.

Validation of 1985-1997 active cavity radiometer spacecraft measurements of total solar irradiance variability

Abstract: Since 1978, long-term variations in the total solar irradiance (solar constant) have been monitored using spacecraft radiometers, at the 0.01% precision level. The irradiance measurements were performed from the Earth Radiation Budget Satellite [ERBS], Nimbus-7, Solar Maximum Mission [SMM], Upper Atmosphere Research Satellite [UARS], European Retrievable Carrier (EURECA), Solar and Heliospheric Observatory [SOHO], and the Space Shuttle Atmospheric Laboratory for Applications and Science [ATLAS] spacecraft platforms. Radiometer responses can drift or shift at precision levels of a few hundreds of a percent. In-flight calibration sources are not available to detect radiometer response changes at radiometric accuracy or precision levels near the 0.01% (0.1 W/sq m) level. Inconsistent trends among the sets were used to identify possible instrumental drifts or shifts which may be incorrectly interpreted as solar irradiance changes while consistent trends among the different measurement sets were used to detect long-term irradiance variability components. In this paper, 1991-1998 corresponding ERBS, UARS, SOHO, and ATLAS irradiance measurements are inter-compared with each other as well as with the ERBS empirical irradiance fit. The empirical irradiance fit is based upon 10.7-cm solar radio flux (F10) and photometric sunspot index (PSI), indices of solar magnetic activity. Analyses of recent data sets identified no long-term shifts and drifts in the ERBS, SOHO, or UARS data sets. The typical value of the total solar irradiance is approximately 1365 Watts per meter squared (W/sq m).

Variations of Solar Irradiance, 10.7 cm Radio Flux, He I 10830 Å Equivalent Width, and Global Magnetic Field Intensity and their Relation to Large-Scale Solar Magnetic Field Structure

Abstract: Variations of total solar irradiance, 10.7 cm radio emission, the He I 10830 A equivalent width and the solar magnetic field flux measured for the entire Sun are compared with variations of the energy index of the global solar magnetic field and the index of the effective solar multipole for years 1979–1992. It is shown that photospheric radiation and that generated in the upper chromosphere and corona display different relationships with the global magnetic field of the Sun, and that interaction between the magnetic field and the solar irradiance is much more complicated than the traditional blocking effect.

Solar Total Irradiance: A Reference Value for Solar Minimum

Abstract: Simultaneous solar total irradiance observations performed by absolute radiometers on board satellites during the quiet-Sun period between solar cycles 21 and 22 (1985–1987), are analyzed to determine the solar total irradiance at 1 AU for the solar minimum. During the quiet-Sun period the total solar irradiance, UV irradiance, and the various solar activity indices show very little fluctuation. However, the absolute value of the solar total irradiance derived from the observations differ within the accuracy of the radiometers used in the measurements. Therefore, the question often arises about a reference value of the solar total irradiance for use in climate models and for computation of geophysical, and atmospheric parameters. This research is conducted as a part of the Solar Electromagnetic Radiation Study for Solar Cycle 22 (SOLERS22). On the basis of the study we recommended a reference value of 1367.0 ± 0.04 W m-2 for the solar total irradiance at 1 AU for a truly quiet Sun. We also find that the total solar irradiance data for the quiet-Sun period reveals strong short-term irradiance variations.

Azimuth Dependence of Solar Radiation on Inclined Surfaces

Abstract: In general, the daily variation pattern of global solar radiation on an inclined surface is estimated by assuming the symmetrical daily variation of global solar radiation. The author carried out an experiment to examine the azimuth dependences of solar radiation on an inclined surface. In the experiment, the amount of solar radiation was found to be greater on a westward-facing surface than on an eastward-facing surface throughout the year.

Side pumping technique model for laser pumped by solar energy

Abstract: A model of side pumping technique for a system of solar concentrators was performed for laser generation. Measurements for the total solar radiation in the hot desert area near Helwan which is south of Cairo by 30 km, of latitude 30°N and longitude 31°E were carried out. The instruments were installed on the rooftop of the National Research Institute of Astronomy and Geophysics (NRIAG). The measurements indicate that the annual average of the global solar radiation is 5·21 kWh/m2/day, direct solar radiation is 6·26 kWh/m2/day, diffuse solar radiation is 1·86 kWh/m2/day, the clearness index Kt is 0·61, and the diffuse fractionKD is 0·37. The performance of the laser rod and the performance of the system as a whole were tested. Finally, the measurements of solar radiation were applied to the model to obtain the behaviour of the laser output in different seasons, assuming that the concentrator system tracks the sun.

High-Temperature, High-Flux Multifoil Shield Developed for Space Applications

Abstract: Spacecraft employing solar dynamic power systems typically use parabolic, point focus concentrators to collect solar power and direct it to the aperture of a heat receiver. Solar fluxes several thousand times the intensity of one solar constant are typically produced in the focal plane of such concentrators. Under heat loading this severe, passively cooled surfaces constructed of most engineering materials would rapidly melt. Therefore, high-temperature shielding is required to protect heat receiver surfaces and other spacecraft surfaces that may be exposed to high flux. To meet this challenge for the joint U.S./Russian Solar Dynamic Flight Demonstration Program, AlliedSignal Aerospace and the NASA Lewis Research Center developed a high-temperature, high-flux multifoil shield tolerant of extreme heat loading conditions in a vacuum environment. The shield is passively cooled, obviating the need for pumped fluid loops and/or heat pipe cooling systems with their attendant cost, mass, complexity, and reliability issues.