Bright Faculae Research Articles

Periodicity of Total Solar Irradiance

We investigate the periodicity in the PMOD composite of the daily total solar irradiance (TSI) from 21 September 1978 to 9 June 2009. Besides the Schwabe cycle period (10.32 years), the quasi-rotation period is found to be statistically significant in TSI, whose value is about 32 days, longer than that in sunspot activity (27 days), and it intermittently appears around the sunspot maximum times. The quasi-rotation period in TSI is inferred to be mainly caused by sunspot activity, but to be modulated by bright features as well. It was previously found that variations of TSI over a Schwabe solar cycle mainly come from the combination of the sunspots’ blocking and the intensification due to bright faculae, plages, and network elements, with a slight dominance of the bright-feature effect during the maximum of the Schwabe cycle. For the sunspot-blocking and the bright-feature effect to contribute to TSI over a Schwabe solar cycle, the former is inferred to lead the latter by 29 days at least.

Photospheric activity, rotation, and radial velocity variations of the planet-hosting star CoRoT-7

The CoRoT satellite has recently discovered the transits of a telluric planet across the disc of a late-type magnetically active star dubbed CoRoT-7, while a second planet has been detected after filtering out the radial velocity (hereafter RV) variations due to stellar activity. We investigate the magnetic activity of CoRoT-7 and use the results for a better understanding of its impact on stellar RV variations. We derive the longitudinal distribution of active regions on CoRoT-7 from a maximum entropy spot model of the CoRoT light curve. Assuming that each active region consists of dark spots and bright faculae in a fixed proportion, we synthesize the expected RV variations. Active regions are mainly located at three active longitudes which appear to migrate at different rates, probably as a consequence of surface differential rotation, for which a lower limit of \Delta \Omega / \Omega = 0.058 \pm 0.017 is found. The synthesized activity-induced RV variations reproduce the amplitude of the observed RV curve and are used to study the impact of stellar activity on planetary detection. In spite of the non-simultaneous CoRoT and HARPS observations, our study confirms the validity of the method previously adopted to filter out RV variations induced by stellar activity. We find a false-alarm probability < 0.01 percent that the RV oscillations attributed to CoRoT-7b and CoRoT-7c are spurious effects of noise and activity. Additionally, our model suggests that other periodicities found in the observed RV curve of CoRoT-7 could be explained by active regions whose visibility is modulated by a differential stellar rotation with periods ranging from 23.6 to 27.6 days.

Discriminant analysis of solar bright points and faculae

While photospheric magnetic elements appear mainly as Bright Points (BPs) at the disk center and as faculae near the limb, high-resolution images reveal the coexistence of BPs and faculae over a range of heliocentric angles. This is not explained by a "hot wall" effect through vertical flux tubes, and suggests that the transition from BPs to faculae needs to be quantitatively investigated. To achieve this, we made the first recorded attempt to discriminate BPs and faculae, using a statistical classification approach based on Linear Discriminant Analysis(LDA). This paper gives a detailed description of our method, and shows its application on high-resolution images of active regions to retrieve a center-to-limb distribution of BPs and faculae. Bright "magnetic" features were detected at various disk positions by a segmentation algorithm using simultaneous G-band and continuum information. By using a selected sample of those features to represent BPs and faculae, suitable photometric parameters were identified in order to carry out LDA. We thus obtained a Center-to-Limb Variation (CLV) of the relative number of BPs and faculae, revealing the predominance of faculae at all disk positions except close to disk center (mu > 0.9). Although the present dataset suffers from limited statistics, our results are consistent with other observations of BPs and faculae at various disk positions. The retrieved CLV indicates that at high resolution, faculae are an essential constituent of active regions all across the solar disk. We speculate that the faculae near disk center as well as the BPs away from disk center are associated with inclined fields.

Photospheric activity and rotation of the planet-hosting star CoRoT-4a

The space experiment CoRoT has recently detected a transiting hot Jupiter in orbit around a moderately active F-type main-sequence star (CoRoT-Exo-4a). This planetary system is of particular interest because it has an orbital period of 9.202 days, the second longest one among the transiting planets known to date. We study the surface rotation and the activity of the host star during an uninterrupted sequence of optical observations of 58 days. Our approach is based on a maximum entropy spot modelling technique extensively tested by modelling the variation of the total solar irradiance. It assumes that stellar active regions consist of cool spots and bright faculae, analogous to sunspots and solar photospheric faculae, whose visibility is modulated by stellar rotation. The modelling of the light curve of CoRoT-Exo-4a reveals three main active longitudes with lifetimes between about 30 and 60 days that rotate quasi-synchronously with the orbital motion of the planet. The different rotation rates of the active longitudes are interpreted in terms of surface differential rotation and a lower limit of 0.057 \pm 0.015 is derived for its relative amplitude. The enhancement of activity observed close to the subplanetary longitude suggests a magnetic star-planet interaction, although the short duration of the time series prevents us from drawing definite conclusions.

Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation?

Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with dark sunspots and bright faculae in active regions and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes and spatially correlated with them. Previous reconstructions of TSI report agreement with radiometry that seems to rule out significant “extra-flux-tube” contributions. We show that these reconstructions are more sensitive to the facular contrasts used than has been generally recognized. Measurements with the Solar Bolometric Imager (SBI) provide the first reliable support for the relatively high, wide-band, disk-center contrasts required to produce 10% rms agreement. Longer term bolometric imaging will be required to determine whether the small but systematic TSI residuals we see here are caused by remaining errors in spot and facular areas and contrasts or by extra-flux-tube brightness structures such as bright rings around sunspots or “convective stirring” around active regions.

Solar-Like Activity and Planetary Transits

Late-type MS stars display optical variability on different time scales due to the presence of photospheric brightness inhomogeneities produced by surface magnetic fields, analogous to cool spots and bright faculae observed in the Sun. We are developing methods to model the optical variability of main-sequence late-type stars to understand the impact of solar-like activity on the detection of planetary transits and to significantly improve the detection efficiency. Our techniques will also allow to map the longitude distribution of active regions on stars that harbor planets and to look for a possible connection between stellar activity and the presence of planets around a star.

Comment on “Variations of total solar irradiance produced by structural changes in the solar interior”

In a recent Eos article, Sofia [2004] argues for the influence on irradiance variation of global changes in the Sun's structure associated with its magnetic dynamo. These changes would act in addition to the relatively well understood modulation by dark sunspots and bright faculae at the surface. His assessment of the present observational evidence for such a global change agrees with our earlier conclusion that it is not widely convincing at the present time [Foukal, 2003]. But Sofia's article also claims (1) that the numerical results obtained by him and his collaborators at Yale disagree with and correct earlier work, and (2) that a hydrostatic approximation is not adequate for variations on the 11‐year solar cycle timescale. These surprising claims are based on the results of recently published hydrostatic models [e.g., Sofia and Li, 2004] using the same mixing length approximation for convective heat transport used in earlier work [Spruit, 1982, 1991; Gilliland, 1988].

Modelling the rotational modulation of the Sun as a star

We analyse the time variability of the total solar irradiance (TSI) as measured by VIRGO/SoHO in order to model the variability of the Sun as a star. Apart from the phases near the minimum at the beginning of activity cycle 23, the period of the rotational modulation is significantly different from the solar synodic period as a consequence of the growth and decay of active regions on time scales shorter than a solar rotation. In order to model the variability of the TSI, we have considered the contributions of discrete active regions and a uniformly distributed background emission. To reproduce the rotational modulation of the TSI, we used three active regions, the areas and coordinates of which were changed every seven days to account for their evolution. The simultaneous presence of dark spots and bright faculae was considered by means of appropriate contrast functions which took into account the observed center-to-limb dependence of their contrast with respect to the unperturbed photosphere. The method proved to be capable of modelling the variability of the TSI on time scales going from 7-10 days up to the solar cycle. The relative amplitude of the residuals was of the order of (1-2) x 10 - 4 with the larger values observed during the phases of maximum solar activity of cycle 23. The application of a similar technique to solar-like stars, such as those that will be observed by the next generation of space-borne photometers, should allow us to minimize the effects of stellar magnetic activity on the detection of planetary transits. Moreover, the availability of long-term highly accurate light curves will allow us to measure stellar rotation period, detect stellar activity cycles, and derive information on the inclination of the stellar rotation axis. The location of the active regions and their irradiance properties can also be retrieved with moderate accuracy from single-band light curves. However, a combination of multi-hand photometry and spectroscopy will allow us to constrain some of the free parameters of the model and improve the mapping of stellar surfaces.

Infrared Photometry of Solar Photospheric Structures. II. Center‐to‐Limb Variation of Active Regions

Simultaneous broadband single images and time series of images have been obtained at λ0.8 and λ1.55 μm, which are at the extremes of the continuum absorption coefficient of the solar atmosphere. Active regions have been observed for different heliocentric angles. Weighted difference images between the two wavelength bands have been used to identify faculae. Center-to-limb variations of facular relative intensity distribution, facular size distribution, and facular size-intensity relations have been obtained. At the spatial resolution of the observations, faculae show no contrast at the disk center at λ0.8 μm, while dark faculae are observed at λ1.55 μm. The transition from dark to bright faculae occurs between μ = 0.6 and 0.5 in λ1.55 μm images. The maximum of the mean facular relative intensity is found at μ = 0.3 for both wavelengths. However, the peak of the facular relative intensity is greater and appears closer to the limb the larger the faculae are. Brightness temperature maps have been computed for the best pairs of images, and temperature difference images have been derived. The temperature difference Tb(1.55 μm) - Tb(0.8 μm) in pores is larger than that in the quiet photosphere at the disk center, but smaller near the limb. Faculae show smaller temperature differences than the quiet photosphere at the disk center, but the temperature differences near the limb are almost equal. Pores are surrounded by ringlike structures of low temperature difference at the disk center. Near the limb these ringlike structures appear bright in the brightness temperature maps and show a temperature difference similar to that of the quiet photosphere.

I.UBVphotometry of selected members of the Pleiades and Alpha Persei clusters in 1999

The rotational brightness variations in magnetically active stars are due to the combined light contribution of cool spots and bright faculae on the stellar photosphere. The amplitude of the rotational brightness variations can be used to probe the dependence of the amount of photospheric magnetic fields associated to such inhomogeneities on global stellar parameters such as the rotation rate. However, the upper envelope of the amplitude values must be considered, since the rotational modulation amplitude represents a lower limit of the spots and faculae coverage, being their visibility modulation reduced by several factors. In order to determine accurately the maximum amplitude upper envelope and its dependence on global stellar parameters a program of photometric monitoring of ultra fast rotating active members of the Pleiades and Alpha Persei star clusters was undertaken in the fall of 1999. Light curves and period determinations of 21 target stars are presented as result from the first observing season. The newly determined values of amplitudes of the brightness variations versus rotation period have values as expected based on previous results.

Detection and parameterization of variations in solar mid‐ and near‐ultraviolet radiation (200–400 nm)

Nimbus 7 and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) spacecraft measurements of solar irradiance both exhibit variability at mid (200–300 nm) and near (300–400 nm) ultraviolet (UV) wavelengths that are attributable to the Sun's 27‐day solar rotation, even though instrument sensitivity drifts obscure longer‐term, 11‐year cycle variations. Competing influences of dark sunspots and bright faculae are the dominant causes of this rotational modulation. Parameterizations of these influences using a newly developed UV sunspot darkening index and the Mg index facular proxy replicate the rotational modulation detected in both the broadband Nimbus 7 filter data (275–360 nm and 300–410 nm) and in SOLSTICE 1‐nm spectra from 200 to 400 nm. Assuming that these rotational modulation influences scale linearly over the solar cycle, long–term databases of sunspot and global facular proxies permit estimation of 11‐year cycle amplitudes of the mid– and near–UV solar spectrum, unmeasured at wavelengths longward of 300 nm because of insufficient long‐term repeatability (relative accuracy) of state‐of‐the‐art solar radiometers at these wavelengths. Reconstructions of UV irradiances throughout the 11‐year solar cycle indicate variabilities of 0.173 W/m2 (1.1%) in the integrated radiation from 200 to 300 nm and 0.24 W/m2 (0.25%) in radiation from 300 to 400 nm. These two UV bands thus contribute about 13% and 18%, respectively, to the 1.34 W/m2 (0.1%) total (spectrally integrated) radiative output solar cycle. The parameterizations allow customization of UV irradiance time series for specific wavelength bands required as inputs to general circulation model simulations of solar cycle forcing of global climate change, and have practical implications regarding the long‐term repeatability required for future solar monitoring.

Solar Variability and the Relation of Facular to Sunspot Areas during Solar Cycle 22

The total irradiance of the Sun has been found to vary mostly because of changes in the areas of dark sunspots and bright faculae. Improved observations, such as those discussed in this paper, are needed to understand better the interplay between these two competing features. In this paper, faculae are determined by observations using a filter centered at the Ca II K line (393.4 nm) with a bandpass of 0.9 nm. This filter allows the detection of faculae across the entire solar disk rather than just at the limb, as is the case for white-light faculae. Sunspots are detected with a filter at 672.3 nm with a bandpass of 9.7 nm. The mean ratio of facular to sunspot area was found to be 16.7 ± 0.51 for a 7½ year period during solar cycle 22 but showed a significant increase as the solar cycle progressed. This ratio suggests that the irradiance excess associated with faculae outweighs the irradiance deficit associated with sunspots by about 50%. The facular area also exhibited a quadratic dependence on sunspot area, as suggested by Foukal, but there is no clear evidence of a turnover in facular area at large sunspot areas. Lagged cross-correlations between facular and sunspot areas showed a clear rotational modulation extending to lags of five to six rotations when spots led faculae. Lags in the opposite direction, however, showed the rotational modulation falling abruptly after about two rotations.

The 10‐year time delay of the solar cycle luminosity modulation of the Sun behind the solar cycle magnetic oscillation

AbstractWe found an evidence that the solar cycle luminosity modulation of the Sun deduced from the total irradiance modulation which was measured by the Earth Radiation Budget (ERB) experiment on board of Nimbus 7 from November 16, 1978 to December 13, 1993 was not in phase with the solar cycle magnetic oscillation when we used the sunspot relative number as its index. The modulation was delayed in time behind the solar cycle magnetic oscillation by an amount of about 10.3 years on the order of length of one solar cycle. In order to quantitatively evaluate the correlation between the two quantities, we devised a method to extract characteristics which were proper to a particular solar cycle by defining a new index of the correlation called multiplied correlation index (MCI). We found that the characteristics of the ERB data time profile between solar cycles 21 and 22 were more similar to those of the solar cycle magnetic oscillation between solar cycles 20 and 21 than those between solar cycles 21 and 22 and thus the time profile of the luminosity modulation from the maximum phase of solar cycle 21 to the declining phase of the solar cycle 22 corresponded to the solar cycle magnetic oscillation from the maximum phase of solar cycle 20 to the declining phase of solar cycle 21. We interpret this phenomenon as an evidence that main features of the modulation is not caused by dark sunspots and bright faculae and plages on the surface of the Sun that should instantaneously affect the luminosity modulation but is caused by time‐delayed modulation of global convection by the Lorentz force of the magnetic field of the solar cycle. The delay time of about 10.3 years is the time needed for the force to modify the flows of the convection and to modulate heat flow. Thus the delay time is a function of the strength of the magnetic field oscillation of the solar cycle which is represented by amplitude of the solar cycle. Accordingly, the delay time for other time intervals of the solar cycle magnetic oscillation with different amplitudes can be different from 10.3 years for the interval of the present analysis.

Estimating Solar Forcing of Climate Change during the Maunder Minimum

Solar variability models which account for contemporary irradiance variations in terms of modulation by dark sunspots and bright faculae on the solar disk are used to estimate solar radiative output during the Maunder Minimum by postulating the absence of such sources, and additional reduction in the basal solar emission. When the resultant irradiance reduction of 0.25% relative to contemporary mean levels is input to the GISS GCM, the global mean temperature is reduced by 0.46°C. However, as a result of differential heating of the land and oceans, some regions may cool, and others warm, by as much 1°C, in response to the reduced solar irradiance.

Bifurcation in the low chromosphere

Bright chromospheric faculae of diameter d(c) covering an area of the solar disk f(c) smaller than 1 yield a predictable level of limb brightening. For a resolution diameter d(r), the combination of f(c) and d(c) leads, also, to a prediction of the fraction, phi(0), of observing pixels (resolution areas) that are free of facular emission. Data from the first flight of the High Resolution Telescope and Spectrograph of the U.S. Naval Research Laboratory with d(r) about 1 arcsec are used to measure both the level of limb brightening and phi(0) in EUV emission lines of O I, C I, and Fe II. The results favor f(c) about 0.9 and d(c) greater than about 2.5 arcssec. It is concluded that the chromospheric temperature rise is present over about 90 percent of the solar surface. 15 refs.

Solar luminosity variations over timescales of days to the past few solar cycles

Dips in the total irradiance of up to 0.2 % and lasting typically 10 days are now well known to be caused by the transit of dark sunspots across the photospheric disc. The large bright magnetic faculae usually associated with spots cause irradiance increases of comparable magnitude although the form of their signal is more subtle. Radiometry from five satellites beginning in late 1978 indicates a minimum in irradiance at the epoch of lowest magnetic activity between solar cycles 21 and 22. Analysis of these radiometric measurements indicates that this irradiance decline between about 1981 and 1986 was caused mainly by decay in the excess radiation of bright faculae in the magnetic network outside of active regions. Empirical models of irradiance modulation extending back to 1874 indicate that the Sun is typically about 0.05 % brighter at activity maximum than at minimum.

Infrared imaging of faculae at the deepest photospheric layers

Abstract : We have used the NOAO 58 x 62 InSb array and the National Solar Observatory McMath telescope to image the deepest photospheric layers of three active regions at the 1.63 micron opacity minimum. At 1.63 microns, faculae are darker than the photosphere, with a measured contrast of at least 2% at positions on the disk between approx. 0.75<mu<1.0. Near the limb, at mu <0.5, they are brighter than the photosphere, as in the visible. At intermediate 0.5< or = mu < or = 0.75, they are difficult to detect at 1.63 microns. The observation that faculae and their immediate surroundings exhibit a clear deficit of brightness temperature near disk center at 1.63 microns seems to rule out the 'hillock' model put forward to explain their center-to-limb contrast variation. The transition from bright faculae at mu = approx. 1 in visible continuum, to dark faculae at 1.63 microns seems to be explained by existing 'hot wall' models of these structures. We also discuss the imperfect spatial correspondence found between dark faculae and bright Ca K plage in terms of possible differences in size between photospheric flux tubes that give rise to Ca K plages.

Solar constant secular changes

Our recent model for solar constant secular changes suggests that over extended time intervals, a positive correlation of the solar constant with solar activity results. The positive correlation in this model is not associated with the direct result of active region, which may be in a close ‘energy balance’ over their lifetimes, but rather is associated with global features - bright global faculae. These exist as both polar and network features on the solar disk. The high latitude faculae enable the solar constant to peak prior to sunspot maximum. Recent solar constant observations support the model's general trend. Using this model, we now calculate a ‘proxy’ solar constant for: (1) the past four centuries, based upon the sunspot record, (2) the past nine centuries, based upon 14C observations and their relation to solar activity, and (3) the next decade, based upon a dynamo theory model for the solar cycle. The proxy solar constant data is tabulated as it may be useful for climate modelers studying global climate changes. This could be helpful in disentangling possible solar influences from any anthropogenic changes associated with trace gas increases in the terrestrial atmosphere. An important point for climate modelers to consider in fixing climate model parameters, is that the Sun has been relatively active and thus bright in the latter half of the 20th century, compared with the past few centuries.

A Model of Solar Luminosity Modulation by Magnetic Activity Between 1954 and 1984

A simple model based on the changes in excess radiation from bright magnetic faculae and on changes in reduced radiation from dark spots is remarkably successful in matching the slow variations of total solar irradiance measured simultaneously by the ERB and ACRIM satellite radiometers between 1981 and 1984. This model was extended back to 1954 to reconstruct the modulation of irradiance by magnetic activity during the past three 11-year solar cycles. The model predicts that the sun is consistently brighter at activity maximum than at minimum. The 0.07 percent brightening at the peak of the last cycle in 1980 was more pronounced than the brightenings found for either of the two previous cycles, even though cycle 19, which peaked around 1957, had the largest sunspot number amplitude in the history of reliable sunspot records.

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% 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 changes.