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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1981

The Sun as a star

Solar physics was reviewed in the context of the solar atmoshere. The understanding of the solar atmosphere is linked to stellar atmospheric research. Topics covered include: the existence of the chromosphere, the corona, and the solar wind; the interactive complex of convection, differential rotation, magnetic field generation and concentration, and the activity cycle; phenomena such as granulation, supergranulation, the 5 minute oscillation, filigree, faculae, sunspots, spicules, prominences, surges, and the spectacular flares.

Jordan, S. D.↗
NASA NTRS1981

A mechanism for solar ultraviolet flux variability

Following an examination of solar UV emission observed by a filter photometer on Nimbus IV from 1969 to 1973 in an attempt to understand the 27-day and secular variability, two models are proposed to account for the variations: (1) a calcium plage model and (2) a chromospheric network faculae and spicule structure model. An association between UV brightenings and the large scale magnetic field has been found to be consistent with the network model. An increase in the UV emittance can be achieved by raising the effective chromospheric temperature closer to a photospheric level. If the sun's luminosity is constant on these time intervals, the enhanced UV radiation could be partially offset by an overall decrease in photospheric temperature such as that measured by Livingston (1978) in visible photospheric profiles.

Schatten, K. H.↗
NASA NTRS1981

Indirect methods for measuring variations of the solar constant

The difference between the methods that measure delta S, and those that measure variations in the solar luminosity, delta L, is discussed. It is shown that the past practice of simply relating delta S to delta L by geometrical arguments is not valid because of anisotropy of the solar radiation. It is concluded that direct techniques prove the existence of short term variability that is fully explainable in terms of the passage of active regions (spots and faculae) on the face of the Sun. The obervations of changes in the solar diameter support the existence of structurally induced variations of the solar luminosity on timescales of tens of years, which are significant in the understanding of climatic variations.

Sofia, S.↗
NASA NTRS1981

The overall structure and evolution of active regions

The evolutionary characteristics and structure of the magnetic fields and atmospheric phenomena associated with the development of a solar magnetic region are discussed. Bipolar magnetic regions are introduced as the source of all solar magnetic fields, formed as bundles of magnetic flux rise to and break through the solar surface and spread throughout the photosphere, chromosphere and corona. The photospheric magnetic region is shown to be characterized by bipolar flux regions of various sizes emerging rapidly for a few days, then expanding and decreasing in flux for several months and fragmenting. Sunspots and faculae are considered as tracers of the magnetic regions in the upper photosphere or lower chromosphere, while chromospheric tracers include arch filaments, field transition arches, long chromospheric threads, disk filaments and dark fibrils in chromospheric lines. The transition region and lower corona exhibit a multithermal plasma distribution, with low-temperature plasmas confined to the footprints and legs of magnetic field lines and high-temperature plasmas originating in loops or systems of unresolved loops. The bipolar magnetic region is also shown to interact with its surroundings to produce an interconnected field line pattern.

Sheeley, N. R., Jr.↗
NASA NTRS1982

Solar irradiance variations due to active regions

Questions regarding changes in solar irradiance due to activity are important, since such changes may have a significant effect on the earth's climate. Solar irradiance measurements conducted outside the earth's atmosphere and, therefore, not affected by it have become possible by utilizing for such measurements satellites, such as the Nimbus 7 and the Solar Maximum Mission satellite (ACRIM experiment). The present investigation has the objective to show that a combination of sunspots and faculae is capable of representing the observed variations in the values of ACRIM data to a satisfactory degree, given the current level of uncertainties in the ground-based data.

Oster, L.↗
NASA NTRS1981

Observations of the wavelength dependence of the average contrast of sunspots

The Extreme Limb Photometer has been used to observe the contrast of sunspots and faculae in conjunction with the Active Cavity Radiometer Irradiance Monitor on the SMM spacecraft. Some of these observations were obtained at five wavelengths from 0.43 to 1.01 micron. The largest average contrast at 0.52 micron was -17% over an area 38.5 x 51 in covering only the largest spot in Boulder AR no. 2684. It was found, for five sunspots, far from the limb, that the wavelength dependence of the contrast, averaged over the entire sunspot, followed a 1/lambda-law. No evidence was found for localized bright emission around the sunspots with an upper limit of about 3%, a limit set by the granular intensity rms variation of 1.3% at 0.52 micron.

Chapman, G. A.↗
NASA NTRS1983

Sunspot bright rings and the thermal diffusivity of solar convection

Raster-scan observations of 10 sunspots, made in 1980 and 1981 with the 512-channel diode array and vacuum telescope at the Kitt Peak National Observatory, are reported. Data from several 10-min scans of 0.25-A passbands of clean continuum were summed to give an rms noise level of 0.25 percent, corrected by applying a limb-darkening curve, and analyzed to determine the average intensity for each of eight segments of a series of concentric rings around each sunspot. Faculae and pores were identified and discarded in constructing radial intensity profiles. Marginally significant bright symmetric rings (peak amplitude 0.1-0.3 percent) not attributable to residual facular signal or instrumental effects were observed around 6 of 10 sunspots. No evidence of more intense bright rings was found. These findings are discussed in terms of thermal-diffusion models proposed to explain the fate of the radiative flux blocked by sunspots.

Duvall, T., Jr.↗
NASA NTRS1984

Ground-based Measurements of Solar Irradiance Variations

Observations and analysis programs being carried out at the San Fernando Observatory are reviewed. A digital analysis of sunspot areas from full disk photographs shows an especially good correlation with areas published in the Solar Geophysical Data Bulletin with scale factor near unity. Results are presented from photoelectric photometry of active regions using the Extreme Limb Photometer. These results suggest energy balance between sunspots and faculae. Preliminary results are presented from a new program of photoelectric photometry using a linear array of diodes. Results are presented for the August 1982 passage of a large active region. This active region caused a maximum dip in the quiet Sun irradiance of about 800 parts per million.

Chapman, G. A.↗
NASA NTRS1984

Energy Flow Continuity in Solar Active Regions

The models for sunspots are combined into an active region model with consideration for the energy flow beneath active regions. An apparent average energy balance exists between the sunspot deficit and the facular excess, i.e., no 11 year variations in solar luminosity associated with the activity centers. This is seen as a consequence of the upper convection zone's inability to store these significant amounts of energy for periods greatly in excess of weeks. This view is supported by observed active region behavior and detailed numerical modelling. Increases in facular and spot brightness are nearly commensurate, with the faculae outlasting the spots on time scales of the order of weeks to a couple of months. Foukal finds the radiation (deficit from a sunspot blocking model) recovers slowly on a timescale of approximately 83 days.

Schatten, K. H.↗
NASA NTRS1984

Limb Darkening Variations

Variations of the solar limb darkening as measured in the line wing of the Fe I line at wavelength 5,250 are observed. The measurements are made over the visible solar disk excluding those points where the magnetic field strength exceeded five Gauss. This exclusion of magnetic points should reduce the effects of faculae upon the derived limb darkening curve. The observations cover 160 days and show evidence of variations of 0.002 I(0) over timescales of thirty days.

Bruning, D. H.↗
NASA NTRS1984

The Solar Constant, Climate, and Some Tests of the Storage Hypothesis

Activity related modulation of the solar constant can have practical consequences for climate only if storage is involved, as opposed to a detailed balance between sunspot blocking and facular reemission. Four empirical tests are considered that might distinguish between these opposing interpretations: monochromatic measurements of positive and negative flux; comparison of modelled and measured irradiance variations; the interpretation of secular trends in irradiance data; and the direct test of an anticipated signal in climate records of surface air temperature. The yet unanswered question of the role of faculae as possible reemitters of blocked radiation precludes a definitive answer, although other tests suggest their role to be minor, and that storage and an 11 year modulation is implicated. A crucial test is the behavior of the secular trend in irradiance in the declining years of the present activity cycle.

Eddy, J. A.↗
NASA NTRS1985

The importance of improved facular observations in understanding solar constant variations

A new study of solar irradiance modeling has been undertaken to improve the previous modeling efforts and perhaps to resolve the energy-balance question. In the present study, the daily sunspot and facular areas (using plages as a proxy measure of faculae) have been utilized, as well as a plage intensity index to examine brightness variations. It is noted that the reported plage areas changed by a factor of 2 near the end of 1979. Although this can be partially modeled because a commensurate change in plage brightness occurs, it leads to the conclusion that facular areas and brightness uncertainties prevent a definitive answer to the energy-balance question with this technique.

Schatten, K. H.↗
NASA NTRS1986

Energy balance in solar active regions - The dip of April, 1985

The presence of a solar active region affects the luminosity of the sun. Sunspots directly produce 'dips' in the total solar irradiance approximately proportionally to their projected area, while faculae produce excess energy. These effects were discovered during the solar maximum period of 1980, and the sunspot effect during solar minimum is examined. The 'dip' due to an active region in April, 1985, as observed in the total solar irradiance by the ACRIM instrument on the Solar Maximum Mission is examined. These data (obtained after the spacecraft repair in May, 1984) have simple variations, relative to those observed in 1980, because of the reduced level of activity approaching solar minimum. It is found that the PSI index of projected sunspot area as defined in 1980 appears to describe this 'dip' satisfactorily.

Hudson, H. S.↗
NASA NTRS1987

Solar flare discovery

This paper considers the discoveries that have appreciably changed our understanding of the physics of solar flares. A total of 42 discoveries from all disciplines, ranging from Galileo's initial observation of faculae to the recent discovery of strong limb brightening in 10-MeV gamma-radiation, are identified. The rate of discovery increased dramatically over the past four decades as new observational tools became available. The assessment of significance suggests that recent discoveries -though more numerous - are individually less significant; perhaps this is because the minor early discoveries tend to be taken for granted.

Hudson, Hugh S.↗
NASA NTRS1988

Observed variability of the solar luminosity

Data on solar luminosity variations are compiled in graphs and analyzed, with a focus on the SMM satellite bolometric measurements and other observations made since 1983. Consideration is given to total irradiance measurements in integrated light; emission-line variability in the visible, UV, and IR; mechanisms contributing to the solar variability (sunspots, faculae, rotation, oscillations, granulation, and active networks); the role of the solar magnetic cycle; and other observable parameters indicating variability. Also discussed are the effects of solar variations on the earth climate and the implications of the observations for theoretical models of convective-envelope processes (thermal diffusion near the surface and magnetic pressure in the interior).

Hudson, H. S.↗
NASA NTRS1988

Observations from 1982 of facular limb darkening and excess solar oblateness

Observations of facular regions on 35 days during 1982 obtained with the Extreme Limb Photometer are reported. The data were obtained at a wavelength of 0.53 microns with two apertures, No. 1 covering 36 arcsec and No. 2 covering 11 arcsec, inwards from the limb. The mean contrast of the faculae closer to the limb (aperture 2) is 1.51 + or - 0.23 times that from aperture No. 1. This contrast ratio can be fit to a 1/ms curve. These results are consistent with those from 1975 and 1979 observations and may be consistent with the facular limb-darkening function determined by Libbrecht and Kuhn (1984, 1985) if the data are normalized by the area of the solar surface. However, no calibrations or corrections are required to obtain the mean facular contrast presented here.

Chapman, G. A.↗
NASA NTRS1988

Modelling of total solar irradiance variability - An overview

Several components contribute to the observed variations of the total solar irradiance. There has been considerable effort expended on building empirical models for specific components, especially for sunspots and faculae. These models typically use time series of ground-based data as a means of representing the total-irradiance variability. The models may help to identify the physical cause of a variation; the parameters of a model (e.g. the effective temperature of a sunspot) may be determinable via correlation with the total-irradiance observations; the models may be used as proxy representations for total-irradiance variability during periods of no data; finally, the models in principle can be used as a basis for 'correcting' the total-irradiance data, as a means for better identification of additional components of variability.

Hudson, Hugh S.↗
NASA NTRS1989

Photometric measurements of solar irradiance variations due to sunspots

A photometric telescope constructed to obtain photometric sunspot areas and deficits on a daily basis is described. Data from this Cartesian full disk telescope (CFDT) are analyzed with attention given to the period between June 4 and June 17, 1985 because of the availability of overlapping sunspot area and irradiance deficit data from high-resolution digital spectroheliograms made with the San Fernando Observatory 28 cm vacuum solar telescope and spectroheliograph. The CFDT sunspot deficits suggest a substantial irradiance contribution from faculae and active region plage.

Chapman, G. A.↗