Facular Regions Research Articles

WAVE PROPAGATION AND SHOCK FORMATION IN DIFFERENT MAGNETIC STRUCTURES

Velocity oscillations "measured" simultaneously at the photosphere and the chromosphere—from time series of spectropolarimetric data in the 10830 Å region—of different solar magnetic features allow us to study the properties of wave propagation as a function of the magnetic flux of the structure (i.e., two different-sized sunspots, a tiny pore, and a facular region). While photospheric oscillations have similar characteristics everywhere, oscillations measured at chromospheric heights show different amplitudes, frequencies, and stages of shock development depending on the observed magnetic feature. The analysis of the power and the phase spectra, together with simple theoretical modeling, lead to a series of results concerning wave propagation within the range of heights of this study. We find that, while the atmospheric cutoff frequency and the propagation properties of different oscillating modes depend on the magnetic feature, in all the cases the power that reaches the high chromosphere above the atmospheric cutoff comes directly from the photosphere by means of linear vertical wave propagation rather than from nonlinear interaction of modes.

Spot activity in the RS CVn binary DM Ursae Majoris

We present extensive UBVRI photometry of the active RS CVn binary DM UMa obtained at two different observatories over 1988-2008. We find the light curve of the star to be highly variable. The long-term photometry shows no evidence of any cyclic spot activity. On one occasion, the amplitude of modulation in Vband was 0.30 mag, a rather high value for an active star in a binary seen at a comparatively low orbital inclination. The mean V magnitude of DM UMa during 1993-94 was brighter than that over 1979-85 by about 0.35 mag. We interpret the monotonic increase in mean brightness from 1984 onwards as due to the steady disappearance of star-spots in the near-polar latitudes of the star. The V-band data clearly show evidence for the presence of two distinct regions of enhanced spot activity on the visible primary, which are fixed in the orbital frame of reference, one facing the companion star and the other away from it. It appears that the nearby companion star suppresses the natural tendency of the active star for differential rotation and modifies the physical processes leading to the formation of spots, besides spinning it up by locking its rotation with the orbital motion. The variations in (V -R) and (V - I) colours with the V magnitudes clearly indicate that star-spots about 800 K cooler than the unspotted photosphere are responsible for the rotational modulation of light in DM UMa. The spot-filling factors derived from TiO-band strengths, which are available in the literature, are found to show the expected anticorrelation with the V magnitudes of the star. Apparently, there is excess flux in U and B bands, which increases as the star becomes fainter, partly compensating for the reduction of flux in those bands due to spot activity. The excess flux, probably, originates from plages or facular regions associated with the spot activity, and indicates that the simple two-component spot model with spotted and unspotted photospheric regions is not adequate to represent the star-spot activity in DM UMa.

Recovering facular areas through Doppler imaging

Similar to the Sun, stellar active regions appear to consist of dark spots in combination with surrounding bright facular regions. The present study uses the Doppler imaging technique to recover the stellar surface structures from simu- lated spectral observations obtained from a given theoretical stellar surface. The objective of the study is to investigate how the recovered surface structure depends on observing parameters such as rotational phase coverage, choice of spectral lines, signal-to-noise ratio, and projected equatorial rotational velocity. The inversions demonstrate that those stellar surface active regions with high temperature contrast in both longitudinal and latitudinal directions can be satisfactorily reproduced using Doppler imaging. Observations must, however, fulfill such strict conditions that a complete reconstruction is somewhat unreal- istic. Nevertheless, in a more realistic example, the hot facular areas result in quite distinct and easily recognizable features on the stellar surface map, even though they are not completely reconstructed.

Modelling the colour‐brightness relation of chromospherically active stars

AbstractThe photometric flux of chromospherically active, spotted stars generally becomes redder and fainter when large starspots rotate into view on the stellar disc. However, some of the most active RS CVn systems show a bluer flux as they get fainter. Modelling of one such system (UX Ari) has shown that hot, bright, photospheric facular regions accompanying the cool, dark spots on the cooler, more active component are a possible explanation. The bluer flux of the hotter, less active component does not appear to be sufficient to explain the observed behaviour. We have begun a search for additional chromospherically active stars with a similar relation between colour and brightness, to investigate whether these relations can be explained in the same way. Our results for V711 Tau and RS CVn are presented here, and we conclude that the photospheric faculae remain the most probable explanation for the observed behaviour. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

On the Facular Area Surrounding Decaying Sunspots

The change in facular area around decaying sunspots is investigated. The data are from full-disk photometric images from CFDT1 (5 arc sec pixels) obtained at the Ca ii K-line λ = 393.4 nm. The 31 active regions are from a list previously used to study the decay rate of sunspots. We find a weak, marginally significant relation between spot decay rate and growth of the surrounding facular region (r2 = 0.1255). We conclude that, for this group of decaying sunspots, the growth or decay of the surrounding facular region was not clearly related to the decay rate of an active region’s sunspots.

Excess facular emission from an isolated active region during solar minimum: the example of NOAA AR 7978

The facular contribution to solar irradiance variations on the short time scale is studied by analyzing a simple case of an isolated active region that crossed the solar disk during the 1996 minimum of activity, NOAA AR 7978. Its passage during several Carrington rotations, specifically from rotation 1911 to 1916, allows us to analyze the evolution of the angular distribution of the excess radiance of the facular region using SOHO/VIRGO and MDI data. We associate this evolution with the evolution of the extent corresponding to the isolated active region as well as with the aging of the region itself. Finally, we evaluate the total (i.e. in all directions) emission of this facular region and its spectral and temporal evolution.

The Contribution of Faculae and Network to Long‐Term Changes in the Total Solar Irradiance

A new database of individual solar features has been compiled from the full-disk photometric Ca II K images taken at the San Fernando Observatory (SFO) during solar cycle 22. The distribution of facular region sizes differs at different phases of the solar cycle; the area coverage of large active regions is reduced by a factor of about 20 at solar minimum compared to solar maximum, while the smaller regions cover about half as much area at minimum as at maximum. The irradiance contribution of large features is about 10 times greater at maximum than at minimum, while that of small features is about twice as large. We have used this data set to model the fraction of variation in the total solar irradiance S that is due to solar features of various sizes. The data show that large-scale bright solar features, i.e., faculae, dominate the ~0.1% change in S between solar maximum and solar minimum. Using a variety of data sets, we conclude that large active regions produce about 80% of the total change.

A Statistical Analysis of the Characteristics of Sunspots and Faculae

We present results from a study of sunspots and faculae on continuum and Ca ii K images taken at the San Fernando Observatory (SFO) during 1989–1992; a total of approximately 800 images in each bandpass were used. About 18 000 red sunspots, 147 000 red faculae, and 800 000 Ca ii K faculae were identified based on their contrasts. In addition, we computed the contrasts of pixels on the red images cospatial with Ca ii K faculae. Sunspot contrasts show a strong dependence on size but no dependence on heliocentric angle. There are continuous but systematic differences among facular regions. We find that the contrast of Ca ii K faculae is relatively insensitive to heliocentric angle, but is a strong function of facular size, in the sense that larger Ca ii K faculae are always brighter. The contrast of red faculae is a function of both heliocentric angle and size: the contrast functions show that larger regions contain larger flux tubes, contain deeper flux tubes, and have larger filling factors than small facular regions. Comparisons of cospatial pixels on red and Ca ii K images show a tight correlation between the average contrast of a region in the continuum and its size and heliocentric angle in the Ca ii K images. The average contrast of all facular regions is positive everywhere on the disk, even though the largest regions contain flux tubes which appear dark at disk center.

Global inertial waves in the solar interior and small deformations of the solar figure

Abstract The features of the background magnetic field evolution, such as the dipole–quadrupole cycle, the four-sectoral structure of the magnetic field polarity migration, and the sectoral structure of the polar facular and active region concentrations (Mikhailutsa and Makarova, 1994; 1997; Mikhailutsa, 1995) have been used for developing the qualitative characteristics of the supposed global inertial wave circulation in the deep interior of the Sun. An important observable consequence of that wave is a slight deformation of the solar figure. Based on recent results of measurements of the solar diameter at Tenerife and Locarno the expected small differences (of about 0.2″) in the values of the solar semidiameter have been found.

Continuum photometry of solar white-light faculae

We have determined absolute continuum intensities and brightness temperatures of individual facular grains at a spatial resolution limited by the φ=50 cm aperture of the SVST on La Palma. A facular region at θ≈57° was observed simultaneously in three narrow continuum windows at 450.5, 658.7, and 863.5 nm. We corrected for image degradation by the Earth's atmosphere using the speckle masking method. The brightness temperatures do not exactly follow the Planck law. The differences of Tblue−Tred=220 K and Tir−Tred=−42 K reflect the wavelength dependence of the continuum formation depth. The (red) temperatures of 250 facular grains show excesses between 250 and 450 K above their undisturbed neighborhood. The wavelength dependence of the relative intensity ratios Cλ= [Ifac/Iphot] λshow a large scatter around mean values of Cblue/Cred=1.075 and Cir/Cred=0.98. We determined the center-to-limb variation of the 863.5 nm continuum contrast for 0.17>cosθ>0.39 by measuring 270 grains in reconstructed facular images. The upper envelope of the data points increases linearly to 1.5 at cos θ=0.17. Application of the mean color dependence yields green contrasts up to C550=1.7, which is far higher than previously observed values. The behaviour for cos θ>0.17 is estimated from (unreconstructed) frame-selected best images taken over a time interval of 7 hours. Six distinct facular regions clearly discernible during the whole time interval indicate a slight contrast decrease towards the extreme limb. The observed quantities are useful for an adjustment of model calculations and for a discrimination of competing models.

On the Energy Balance of Solar Active Regions

view Abstract Citations (47) References (92) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Energy Balance of Solar Active Regions Steinegger, Michael ; Vazquez, Manuel ; Bonet, Jose A. ; Brandt, Peter N. Abstract Ground-based observations of the sunspot irradiance deficit Ψ and the excess by facular regions in the period 1989 June 3-15 are analyzed using photometric techniques. The results are compared with solar irradiance measurements taken by the Nimbus 7 satellite. The photographic sunspot data were obtained at the Vakuum Newton Teleskop, Observatorio del Teide (Tenerife), and the information on facular emission was derived by means of a "proxy" method from the analysis of full-disk Ca II K spectroheliograms taken at Sacramento Peak Observatory. The first part of the observing period was characterized by a rather constant activity with sunspot groups of small and medium size, whereas the second part (from June 8 onward) exhibited a drastic increase of activity, dominated by the appearance of the large group NOAA 5528. By increasing the pixel size of the digitized photographic data, the influence of the variable spatial resolution on the measurements is simulated. Above a certain threshold, the bolometric sunspot contrast falls and the spot area grows; however, the resulting influence on the sunspot deficit Ψ and the corresponding umbral parameters (area, contrast, and deficit) is negligible. Data of areas of sunspot groups (total and umbral) from different observatories are compared with our measurements. Total areas show similar values, but umbral areas from our measurements are clearly larger than those estimated from full-disk images, like those from the Debrecen and San Fernando Observatories. Analyzing the data of single spots, we confirm earlier findings, namely, that the bolometric contrast is lower than 0.32, the standard value used in the calculation of the photometric sunspot index, and we report that small spots exhibit smaller bolometric contrasts than larger ones. The intensity minimum of the spots is a very appropriate parameter to describe their global thermal properties; it is linearly proportional to the total (α) and umbral (αμ) bolometric contrasts and also correlates with the maximum strength of the magnetic field of the spots and with the spot's size. The energy balance of individual active regions seems to depend mainly on the area ratio of the plage and the corresponding sunspot group. The different rates at which these areas change produces the temporal evolution of the energy balance. The daily variations of the global activity indices radio flux at 10.7 cm, Mg II core-to-wing ratio, and the average magnetic field are well correlated with the fluctuations of irradiance and the spot deficit Ψ. But on the other hand, the equivalent width of the He I λ10830 line and the coronal index do not follow this behavior. The irradiance fluctuations, as measured by Nimbus 7, can be modeled to within 60 parts per million (ppm) using only our ground-based determination of Ψ and Φ. By adding the sunspot deficit Ψ to the irradiance fluctuations, the result is modeled to within 25 ppm using an empirical law that combines our Φ with a set of global activity indices. After correcting the irradiance measurements for the facular Φ and spot Ψ contributions, a residual remains with an apparent periodicity of 6 days. Different sources to explain this variability are discussed and the importance of an adequate determination of the threshold of Ca II K brightness for the calculation of Φ is explained. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177075 Bibcode: 1996ApJ...461..478S Keywords: SUN: SUNSPOTS; SUN: ACTIVITY; SUN: FACULAE; PLAGES full text sources ADS |

Properties of Faculae from Observations Near the Opacity Minimum

Imaging of active regions in continuum around 1.6 μm shows that many facular regions are less bright than the photosphere when observed nearer to disk center than μ = cos θ ~ 0.75. The contrast of these dark faculae increases with magnetic flux above a threshold of approximately 2 × 1018 Mx. This explains why not all faculae are dark at 1.6 μm, since the magnetic flux density in many regions of bright Ca K plage emission falls below this threshold. After correction for blurring, the typical contrast value is about 4-5%, so the brightness temperature deficit is about 130 K. Faculae are brighter than the photosphere at 1.63 μm nearer to the limb than μ ~ 0.5. The negative contrast of dark faculae may arise from cooling of the surrounding photosphere, or from increased visibility of cool layers of the facular flux tube itself. Quantitative comparison of these IR data with MHD models awaits calculation of flux tube contrasts at realistic angular resolution.

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 μm with two apertures, No. 1 covering 36 arc sec and No. 2 covering 11 arc sec, inwards from the limb. The mean contrasts for all regions detected are 1.05 ± 0.12% and 1.59 ± 0.16%, respectively. The mean contrast of the faculae closer to the limb (aperture 2) is 1.51 ± 0.23 times that from aperture No. 1. This contrast ratio can be fit to a μ−1-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 our 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.

Measurements of the limb darkening of faculae near the solar limb

view Abstract Citations (30) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measurements of the limb darkening of faculae near the solar limb Chapman, G. A. ; Klabunde, D. P. Abstract Measurements are presented of the contrast at 525 nm of solar faculae. The measurements were obtained using an extreme limb photometer (ELP) in 1975 and 1979 at the San Fernando Observatory. The mean contrast of active regions was determined from the limb inward to 54 arcsec, averaged over an annulus determined using the slits of the ELP. It is found that the contrast of faculae increases with heliocentric angle theta, from mu approximately 0.25 to mu approximately 0.065, where mu is the cosine of theta. The average value of the mean contrast of facular regions is 2.1%, with a possible upper limit of approximately 5% at mu approximately 0.1, determined from regions with the greatest filling factor. When corrected for a filling factor of 10-15%, this mean contrast implies a contrast for individual faculae of 30-50%. Publication: The Astrophysical Journal Pub Date: October 1982 DOI: 10.1086/160349 Bibcode: 1982ApJ...261..387C Keywords: Faculae; Limb Darkening; Solar Limb; Astronomical Photometry; Image Contrast; Relativity; Solar Oblateness; Solar Physics full text sources ADS |

The wavelength dependence of the facular excess brightness

view Abstract Citations (30) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The wavelength dependence of the facular excess brightness. Chapman, G. A. ; McGuire, T. E. Abstract Observations are presented of the wavelength dependence of photospheric solar faculae obtained during August 1975. Systematic observations with an extreme limb photometer were carried out in five different bands. Plots of the limb brightness of a facular region for each of the five colors are presented in a graph. Another graph shows the wavelength dependence of the facular excess brightness, normalized to unity for the green filter used. The wavelength variation to be expected from the facular contribution to oblateness measurements is listed in a table. Publication: The Astrophysical Journal Pub Date: October 1977 DOI: 10.1086/155611 Bibcode: 1977ApJ...217..657C Keywords: Brightness; Colorimetry; Faculae; Photosphere; Stellar Spectrophotometry; Stellar Structure; Oblate Spheroids; Relativity; Solar Limb; Wavelengths; Solar Physics full text sources ADS |

A Statistical Photometric Analysis of Granulation across the Solar Disk.

Twenty-five microphotometer scans from Stratoscope granulation photographs distributed across the solar disk have been analyzed statistically to yield amplitude distributions, power spectra, and related quantities. Power spectra have been calculated for twenty additional scans of the center of the disk obtained by Schwarzschild (1959). A difference in calibration procedures in part explains RMS's that are nearly twice those obtained by sChwarzschild. The range of wave numbers k1 for which power-spectra calculations yield quantitative results is limited by uncertainties in low-wave-number filtering to k1 > 0.001 km-' and by what is probably instrumental noise in the photographs to k1 < 0.008 km-'. This upper limit is further reduced to k1 < 0.004 km-' or less by instrumental corrections, using the method of Uberoi (1955b) generalized for foreshortening. These corrections account for the instrumental profile of the Stratoscope (determined from the limb of the solar disk),for focus of the photographs, and for the analyzing spot and focus of the microphotometer. The variation in the relative brightness fluctuation RMS across the solar disk (Figs. 4 and 9) is characterized by an increase from 0.14 for the central regions of the disk (out to 0 20 , to 0.20 at 0 50 and then a sharper decrease, which levels off and reverses itself at 0 70 . Ezamination of limb photographs attributes this reversal to the emerging photometric importance of a large-scale, essentially unforeshortened, pattern which does not seem to be instrumental or telluric aild may not be part of the normal photosphere granulation. Scatter in the RMS of adjacent scans can be attributed to statistical inhomogeneities in the granulation pattern. The amplitude distributions are approximately Gaussian (Fig. 3) with moderate positive skewness. The power spectra (uncorrected, Tables 3 and 4 and Figs. 2 and 5; corrected, Table 8 and Fig. 10) are characterized by a rapid decrease with increasing k,. A steepening of this falling-off toward the limb is probably due to the large-scale pattern. The variation in the brightness fluctuation RMS across the disk has been converted to the RMS of temperature fluctuations as a function of optical depth, using the Eddington-Barbier approximation (Fi . 11). The maximum of brightness RMS at 0 50 becomes a maximum in temperature RMS of 290 K at r 0.65. An inhomogeneous model solar atmosphere based on these temperature fluctuations has been constrncted (Table 9), and, using the&etical results of Ledoux, Schwarzschild, and Spiegel (1961), an associated velocity spectrum and convective transport have been roughly determined. Scans of two special regions have been analyzed: a region at the center of the disk, which is characterized by a low RMS and a flattened spectrum, and a facular region, which shows the normal spectrum and ouly a slightly higher RMS.

On Photospheric Faculae.

view Abstract Citations (41) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Photospheric Faculae. Rogerson, John B., Jr. Abstract A group of photospheric faculae have been photometered on a solar limb exposure taken by a balloon- borne telescope. These faculae are located at a mean sin 0 = 0.99 and are an average 64 per cent brighter than the undisturbed photosphere at the same limb distance. The neighboring photosphere is found to be brighter than the undisturbed photosphere by about 6 per cent. A simple model for a facula is proposed and discussed. The observations are consistent with the picture that in the facular regions the layers above r = 0.3 are approximately 9000 C. hotter than normal. Publication: The Astrophysical Journal Pub Date: September 1961 DOI: 10.1086/147162 Bibcode: 1961ApJ...134..331R full text sources ADS |

Facular Granule Lifetimes Determined with a Seeing?Monitored Photoheliograph

A telescope designed to take exposures automatically at moments of good seeing has been used to obtain a 5! hr sequence of high quality photographs of a facular region near the east limb of the Sun. Individual facular granules are found to be much longer lived than the photospheric granules, a result which agrees with the work of ten Bruggencate (1940) and Waldmeier (1940) but disagrees with the more recent observations of Macris (1953) and of Krat and Goldberg-Rogosinskaja (1956). Fifty per cent. of the facular granules lasL for over 2 he, and 10% last for over 5 hr. In some cases, a facular granule, only 750--1500 km in diameter, occurs as an isolated bright structure, surrounded by normal photosphere and well removed from neighbouring sunspots or faculae. Apart from their greater brightness and much longer lifetimes, the facular granules differ from the photospheric granules in that they do not form awell-defined cellular pattern; these differences suggest different modes of origin