ISSN:0148-0227    

DOI:10.1029/JD092iD01p00795

年代:1987

数据来源: WILEY

摘要:

Solar irradiance variability has been studied from measurements of total solar irradiance at 1 AU, the solar “constant”S0, since the late 1960s. Power spectra of time series from satellite and balloon experiments show that the irradiance varies, with periods of minutes to several hours and with amplitudes of parts per million to a few hundredths of a percent. Part of this variance is due to coherent pressure and, possibly, gravity oscillations as spherical harmonics of the sun. On time scales of days to months most of the variance is a modulation of the solar irradiance by photospheric features related to solar activity and reaches amplitudes of a few tenths of a percent. Because of its origin the variance in this range changes by up to 1 order of magnitude with the solar cycle, as shown by comparison of the power spectra from 1980 and 1984/1985, respectively. The first evidence of a persistent long‐term change comes from satellite data, which show a downward trend ofS0since 1980 by about 0.02% per year. TheS0determinations between 1967 and 1970, compared with the modern ones show a statistically significant increase during that period of similar magnitude. Both trends together could be part of a 22‐year cycle related to the magnetic cycle of

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00796

年代:1987

数据来源: WILEY

摘要:

Radiometry from the Solar Maximum Mission and Nimbus 7 satellites has demonstrated that the solar constant varies at a peak‐to‐peak level of up to 0.2% on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disc accounts for most of that variation. Reasonable explanations have been put forward to explain how the spot‐blocked heat flow might be stored and to explain the source of the intense radiation that gives rise to the increased irradiance produced by the bright magnetic faculae. Time‐dependent models of the response of the solar convection zone to small perturbations also indicate that slower variations in total solar irradiance of comparable magnitude are likely. More precise observations of the total solar irradiance and radius over long time scales are required to demonstrate the existence of such climatologically relevant changes and to test models that would enable us to interpret and, possibly, to predict these

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00801

年代:1987

数据来源: WILEY

摘要:

Results of photometry of solar active regions and their effect on the solar irradiance in the visible part of the spectrum are presented. The effects of sunspots and faculae are given separately, since the measurement of sunspot irradiance fluctuations is less uncertain. It is argued that energy balance may exist between sunspot deficits and facular excesses. The uncertainty however, is ±15% (1σ). This possible balance also depends on the correct mathematical model for the contrast of faculae as a function of position on the solar disk. Extreme Limb Photometer (ELP) data are presented in such a way as to show that the model for facular limb darkening is consistent with the assumptions inherent in the irradiance modeling. The ELP data support the notion that energy balance between spots and faculae is possible. It is emphasized that even if there is energy balance, there will still be variations in the solar irradiance at the eart

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00809

年代:1987

数据来源: WILEY

摘要:

Published sunspot and calcium plage areas were examined for about 1,700 solar active regions (ARs) between 1971 and 1982. With these data, averaged AR properties, the possibility of energy balance between spot and facular emission over the lifetimes of activity complexes, and changes in AR properties over an 11‐year sunspot cycle were studied. Overall energy balance can neither be established nor ruled out. Apparently, real temporal changes in the nature of ARs, however, imply either that energy balance cannot hold over periods shorter than a few years or that standard models for estimating irradiance fluctuations from spot and/or plage areas must be modifie

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00813

年代:1987

数据来源: WILEY

摘要:

In this paper we discuss the influence that active regions have upon the solar constant. Sunspots appear to lower the solar constant for the few days in which they are located near central meridian. This raises the possibility that an 11‐year, solar‐cycle‐related depression in the solar constant may occur. Recent findings concerning the physics of active regions suggest that sunspots and faculae are largely surface features. Within that surface faculae reradiate, within a few weeks, the “missing energy” associated with sunspots. This is consistent with the observations showing that the solar constant does not have an 11‐year cycle‐related depression that some authors predicted. However, there is a secular variation in the solar constant, whose explanation is not complete

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00818

年代:1987

数据来源: WILEY

摘要:

A program to measure the variability of solar spectrum lines in the visible wavelengths began in 1974 at Kitt Peak Observatory and continues as we approach the minimum between solar cycles 21 and 22. Both photospheric and chromospheric lines are measured on a monthly basis using an optical system that permits observation of the irradiance from the full solar disk as well as a smaller region near the center of the sun's disk. The full disk measurements of the Ca IIKline quantify the increase and subsequent decrease in line flux with the change in solar activity. However, spectra of quiet regions at disk center show little if any variation throughout the cycle. Photospheric lines separate into two classes according to their variability: low‐excitation lines of neutral metals resemble theKline in the shape of their solar cycle variability curve, but weaker lines formed deeper in the photosphere show no variation from 1974 to 1986. All of these findings point to solar activity as the source of the observed spectral variations, but the largest radiometric effects occur in chromospheric lines, with little or no evidence of variability in lines formed deep in the photospher

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00823

年代:1987

数据来源: WILEY

摘要:

Studies made by means of high‐resolution radio telescopes indicate that solar centimetric emission contains contributions from many different sources and involves more than one radiation mechanism. Two emission components have been identified: bright, compact sources and weaker, diffuse emission which is distributed over the plage and surrounding areas of enhanced magnetic field. There is evidence for the occurrence of weaker but widely distributed radiation from elements of the active network. The good correlation between the 10.7‐cm flux and shorter‐wavelength indicators of solar activity, such as Ca II emission, suggests that gyromagnetic processes contribute little to the total centimetric flux. If the 10.7‐cm flux is predominantly thermal (free‐free) emission, its intensity will be a measure of the total amount of magnetic flux emerging through the photosphere and supporting density enhancements in the lower solar a

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00829

年代:1987

数据来源: WILEY

摘要:

Despite the geophysical importance of solar ultraviolet radiation, specific aspects of its temporal variations have not yet been adequately determined experimentally, nor are the mechanisms for the variability completely understood. Satellite observations have verified the reality of solar ultraviolet irradiance variations over time scales of days and months, and model calculations have confirmed the association of these short‐term variations with the evolution and rotation of regions of enhanced magnetic activity on the solar disc. However, neither rocket nor satellite measurements have yet been made with sufficient accuracy and regularity to establish unequivocally the nature of the variability over the longer time of the 11‐year solar cycle. The comparative importance for the long‐term variations of local regions of enhanced magnetic activity and global scale activity perturbations is still being investigated. Solar ultraviolet irradiance variations over both short and long time scales are reviewed, with emphasis on their connection to solar magnetic activity. Correlations with ground‐based measures of solar variability are examined because of the importance of the ground‐based observations as historical proxies of ultraviolet irradiance variations. Current problems in understanding solar ultraviolet irradiance variations are discussed, and the measurements planned for solar cycle 22, which may resolve these problems, are briefly

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00839

年代:1987

数据来源: WILEY

摘要:

The effects of solar activity on the thermosphere are reviewed. The neutral total density, temperature, and electron density increase with solar activity, and their variations are fairly well correlated with variations in solar 10.7‐cm flux. The electron temperature, by contrast, has a complex behavior with solar activity, depending on altitude and season, as increased heat input is balanced by increased cooling provided by the enhanced electron density. Also, individual neutral species do not all increase in step but are influenced by dissociation and dynamical processes. The inceased ion drag at higher solar activities inhibits the neutral wind from increasing in the same proportion as the neutral temperatures and densitie

ISSN:0148-0227    

DOI:10.1029/JD092iD01p00869

年代:1987

数据来源: WILEY