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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 NTRS1974

A facular model based on the wings of the Ca II lines

An approach is shown for using the extensive damping wings of the Ca II lines in a direct analysis of photospheric temperature structures. Questions concerning the formation of the Ca II wings are considered. The low angular resolution photoelectric observations reported by Shine and Linsky (1972) are used to derive the spatially averaged differences between faculae and the quiet sun. Data regarding a photospheric facula model are presented in a table.

Shine, R. A.↗
NASA NTRS1974

A three-component concept of the chromosphere and transition region

It is proposed that present observations of the chromosphere and transition region in EUV, optical and mm wavelengths are best described by a three-component concept. The three components are taken to be the interiors of supergranular cells, the hot plagettes overlying faculae, and the cooler, transient mottles which surround them in the network boundaries. The enhanced emission of the hot plagettes in transition ions is interpreted as a direct result of the increased pressure scale height over faculae relative to the cell interiors.

Foukal, P.↗
NASA NTRS1984

Photometric Studies of Heat Flow at the Photosphere

Continuum photometry is carried out and the results of comparing these observations with models of photospheric heat flow are described. The main results are: (1) a possible detection of weak bright rings around some spot penumbrae; (2) no evidence is found for large scale photospheric brightness inhomogeneities exceeding 2-3 K which places tighter constraints on models of global scale convection; (3) supergranular scale continuum structures observed across the photosphere appear mainly due to random clumping of granules; (4) the one case observed of a sunspot emergence shows no thermal shadow exceeding 1.5 K rms one day prior to umbra appearance; (5) network and faculae are found to show a small excess brightness even at mu = 1, so detection of faculae at mu = 1 by differential photometry indicates a gentler temperature gradient near tau = 1 in the facular (relative to cell) atmosphere; (6) the limb darkening study shows no significant global variations to within 0.1% rms.

Foukal, P.↗
NASA NTRS1984

Modulation of solar irradiance by active regions

Hoyt and Eddy (1983) have modeled solar irradiance by accounting for active regions. It is presently noted, however, in contradiction to this model, that in active region modeling of solar irradiance care should be taken to not merely employ average contrasts of faculae. This caveat is especially applicable if the model proceeds to ignore the active region facular areas when their contrasts are low, since this doubly devalues the faculae contribution relative to that of sunspots.

Schatten, K. H.↗
NASA NTRS1985

On the maintenance of sunspots - An ion hurricane mechanism

Inward and downflow motions below the photosphere are considered to be a means of cooling and powering sunspot dynamics. Parker's superadiabatic effect is examined with attention focused on the energetics involved in ionization advection. The current analysis enhances Parker's mechanism by allowing for a 2 m/s downflow velocity at 2000 km depth to significantly reduce the photospheric irradiance to near umbral intensities. Similarities between sunspots and terrestrial hurricanes are noted, and it was found that ionization energy plays the same role in sunspots as latent energy plays in terrestrial weather systems. Based on the hypothesis that magnetic fields are important in organizing the motions on the sun, some understanding is provided of: (1) the instability which drives the cooling mechanism for sunspots, (2) the low latitude appearance of sunspots, (3) active region development with faculae following sunspot growth, (4) the role of the fluid in maintaining the magnetic field and the role of the field as a focal point for the fluid downflow, (5) the heating mechanism and structure of faculae, (6) energy balance in active regions, and (7) the behavior difference of pores and ephemeral active regions in relation to ordinary sunspots and active regions.

Schatten, K. H.↗
NASA NTRS1986

Solar irradiance variations from photometry of active regions

The Extreme Limb Photometer (ELP) has been used to measure the irradiance fluctuation of the sun due to selected active regions. Forty-five active regions that were completely scanned at various disk positions are included in the analysis. The contribution of these active regions to a global solar irradiance fluctuation has been correlated with photometric sunspot and facular indices (PSI and PFI) using published values of sunspot and calcium plage areas. The measured ELP fluctuations are converted to a global brightness fluctuation, Delta B/B. The sunspot component of Delta B/B correlates with PSI with r = 0.95. The facular component of Delta B/B correlates with PFI with r = 0.72. The expression for PFI is important to the question of energy balance between sunspots and faculae and the results presented here are not incompatible with energy balance between the two phenomena; that is the energy deficit of sunspots may be balanced by the energy excess of faculae.

Chapman, G. A.↗
NASA NTRS1987

Physical interpretation of variations in total solar irradiance

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 percent on time scales of weeks. The rotation and evolution of dark spots and bright faculae across the sun's disk 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 the interpretation and prediction of these changes.

Foukal, P.↗
NASA NTRS1987

Active region influences upon the solar constant

The influence that active regions have upon the solar constant is discussed. 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 completely understood.

Schatten, Kenneth H.↗
NASA NTRS1990

A Program of Photometric Measurements of Solar Irradiance Fluctuations from Ground-based Observations

Photometric observations of the sun have been carried out at the San Fernando Observatory since early 1985. Since 1986, observations have been obtained at two wavelengths in order to separately measure the contributions of sunspots and bright facular to solar irradiance variations. Researchers believe that the contributions of sunspots can be measured to an accuracy of about plus or minus 30 ppm. The effect of faculae is much less certain, with uncertainties in the range of plus or minus 300 ppm. The larger uncertainty for faculae reflects both the greater difficulty in measuring the facular area, due to their lower contrast compared to sunspots, and the greater uncertainty in their contrast variation with viewing angle on the solar disk. Recent results from two separate photometric telescopes will be compared with bolometric observations from the active cavity radiometer irradiance monitor (ACRIM) that was on board the Solar Max satellite.

Chapman, G. A.↗
NASA NTRS1990

An Estimate of Changes in the Sun's Total Irradiance Caused by UV Irradiance Variations from 1874 to 1988

Enhanced emission from bright solar faculae is a source of significant variation in the sun's total irradiance. Relative to the emission from the quiet sun, facular emission is known to be considerably greater at UV wavelengths than at visible wavelengths. Determining the spectral dependence of facular emission is of interest for the physical insight this may provide to the origin of the sun's irradiance variations. It is also of interest because solar radiation at lambda less than 300 nm is almost totally absorbed in the Earth's atmosphere. Depending on the magnitude of the UV irradiance variations, changes in the sun's irradiance that penetrates to the Earth's surface may not be equivalent to total irradiance variations measured above the Earth's atmosphere. Using an empirical model of total irradiance variations which accounts separately for changes caused by bright faculae from those associated with dark sunspots, the contribution of UV irradiance variations to changes in the sun's total irradiance is estimated during solar cycles 12 to 21.

Lean, J.↗
NASA NTRS1990

Solar Cycle Modulation of Total Irradiance: an Empirical Model from 1874 to 1988

Evidence acquired during the past decade indicates that over time scales of the solar cycle, enhanced emission from bright solar faculae cause significant variations in the sun's total irradiance even though, on shorter time scales, the most pronounced variations are those resulting from the passage of dark sunspots across the solar disc. An empirical model which accounts for the competing effects of dark sunspots and bright faculae has been developed from the available radiometry in cycle 21, and extended back to the beginning of solar cycle 12. According to this model, the largest 11-year modulation of total irradiance during the C20th occurred in the most recent cycle 21.

Lean, J.↗
NASA NTRS1991

On the contrast of facular hillocks in the infrared

Faculae appear increasingly bright toward the limb of the sun. Recent observations also have shown that, in the infrared, these features appear dark at disk center. It is demonstrated that the hillock model of Schatten et al. (1986) can describe the essential features of these observations. In light of this model, a simple explanation is given for the contrast behavior of faculae. The observations are discussed in the context of the alternative well model.

Schatten, Kenneth H.↗
NASA NTRS1991

Solar irradiance variability from modern measurements

Direct measurements from satellites of the solar 'constant' (the total irradiance at mean sun-earth distance) during more than ten years show variations over time scales from minutes to years and decades. At high frequencies, solar oscillations contribute to the variance. The most important influences are related to solar activity: during the passage of active regions on the solar disk (sunspots and faculae) changes of a few 0.1 percent lasting for several days are observed. The effects of spots can be well reproduced by the projected sunspot index, whereas the influence of faculae have to be modeled from proxy data like the Ca-K plage index or the He I index. Long-term trends are detected which are connected to the 11-yr solar activity cycle.

Froehlich, C.↗
NASA NTRS1992

Properties of the smallest solar magnetic elements. I - Facular contrast near sun center

Measurements are presented which indicate that the continuum intensity of facular areas in solar active regions, outside sunspots and pores, is less than that of the quiet sun very near disk center. It is shown that the observed continuum intensity of faculae at disk center near 5000 A is nearly 3 percent less than that of the quiet sun. The continuum contrast increases rapidly away from disk center, reaching +2 percent at 45 deg. The zero-crossing point, where the contrast changes sign, occurs at 20-degree heliocentric angle. This is contrary to many earlier observations. The constraint these observations place on the size of flux tubes depends upon the value of the zero-crossing point. It is proposed that most of the flux tubes in solar faculae may be very small, in the range 50-100 km in diameter, and that inclination from local vertical of about 10 deg at the photosphere is common on the sun. Footpoints of opposite polarity tend to tilt toward one another.

Topka, K. P.↗
NASA NTRS1993

Long-Term Variations in Total Solar Irradiance

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 the time intervals when no satellite observations exist. In this paper the variations in total solar irradiance observed by the Nimbus-7/ERB, SMM/ACRIM I, and UARS/ACRIM II radiometers are compared to the changes in sunspot darkening and the enhanced emission of bright magnetic elements, including faculae and the magnetic network. Quantitative indices of sunspot darkening have been derived from the area and position of sunspots published in the NOAA-WDC Solar Geophysical Data catalog. The Mg core-to-wing ratio, derived from the irradiance observations of Nimbus-7 and NOAA9 satellites, is used as a proxy for the bright magnetic elements. It has been found that a model, calculated from the Mg II core-to-wing ratio underestimates the observed total irradiance at the time of maximum and during the beginning of the declining portion of solar cycle 22 similar in behaviour just before the maximum of solar cycle 21. Similar results are found using He-line equivalent width and 10.7 cm radio flux which indicates that the current irradiance models are in general not capable to reproducing the changes observed in total solar irradiance.

total↗
NASA NTRS1975

Flare model chromospheres and photospheres

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of about 100 K over the temperature distribution in the pre-existing facula at a height of 300 km. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We find that with increasing flare importance the heights of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60-600 increase in pressure in these regions relative to the quiet sun. The Ca II line profiles agree with observations only by assuming a macrovelocity distribution that increases with height. Also, the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.

Machado, M. E.↗
NASA NTRS1979

Infrared continuum observations of the solar atmosphere

The far-infrared wavelengths (10 microns to 1 mm) were used to study the spatial and temporal structure of the solar atmosphere. Observational results were obtained on flares, faculae, sunspots, and on the center-to-limb intensity distribution, as well as on time variability within these regions. A program of precise monitoring of slow variations in the integrated solar luminosity was shown to be feasible, and initial steps to implement observations were completed.

Hudson, H.↗
NASA NTRS1978

Magnetic field rotation at high solar latitudes

Measurements of the rotation rate of polar magnetic features during 1974-76 lead to a significantly slower rotation rate than that found earlier for polar faculae in 1951-54. Similarly, the rotation rate of these features is slower than the Doppler-determined rate at polar latitudes or the rotation rate of polar filaments. It is suggested that the strong latitude rotation gradient in the subsurface magnetic flux tubes which is implied by these results may presage a very active solar maximum for cycle 21.

Howard, R.↗