The Experts below are selected from a list of 261 Experts worldwide ranked by ideXlab platform
S K Solanki - One of the best experts on this subject based on the ideXlab platform.
-
how Faculae and network relate to sunspots and the implications for solar and stellar brightness variations
arXiv: Solar and Stellar Astrophysics, 2020Co-Authors: S K Solanki, N A KrivovaAbstract:How global Faculae and network coverage relates to that of sunspots is relevant to the brightness variations of the Sun and Sun-like stars. We extend earlier studies that found the facular-to-sunspot-area ratio diminishes with total sunspot coverage. Chromospheric indices and the total magnetic flux enclosed in network and Faculae, referred to here as 'facular indices', are modulated by the amount of facular and network present. We probed the relationship between various facular and sunspot indices through an empirical model that takes into account how active regions evolve. This model was incorporated into a total solar irradiance (TSI) model. The model presented here replicates most of the observed variability in the facular indices, and is better at doing so than earlier models. Contrary to recent studies, we found the relationship between the facular and sunspot indices to be stable over the past four decades. The model indicates that, like the facular-to-sunspot-area ratio, the ratio of the variation in chromospheric emission and total network and facular magnetic flux to sunspot area decreases with the latter. The TSI model indicates the ratio of the TSI excess from Faculae and network to the deficit from sunspots also declines with sunspot area, with the consequence being that TSI rises with sunspot area more slowly than if the two quantities were linearly proportional to one another. The extrapolation of the TSI model to higher activity levels indicates that in the activity range where Sun-like stars are observed to switch from growing brighter with increasing activity to becoming dimmer instead, the activity-dependence of TSI exhibits a similar transition as sunspot darkening starts to rise more rapidly with activity than facular brightening. This bolsters the interpretation of this behavior of Sun-like stars as the transition from a Faculae-dominated to a spot-dominated regime.
-
spectral variability of photospheric radiation due to Faculae i the sun and sun like stars
Astronomy and Astrophysics, 2017Co-Authors: Charlotte M Norris, S K Solanki, Benjamin Beeck, Y C Unruh, Natalie A KrivovaAbstract:Context. Stellar spectral variability on timescales of a day and longer, arising from magnetic surface features such as dark spots and bright Faculae, is an important noise source when characterising extra-solar planets. Current 1D models of Faculae do not capture the geometric properties and fail to reproduce observed solar facular contrasts. Magnetoconvection simulations provide facular contrasts accounting for geometry. Aims. We calculate facular contrast spectra from magnetoconvection models of the solar photosphere with a view to improve (a) future parameter determinations for planets with early G type host stars and (b) reconstructions of solar spectral variability. Methods. Regions of a solar twin (G2, log g = 4.44) atmosphere with a range of initial average vertical magnetic fields (100 to 500 G) were simulated using a 3D radiation-magnetohydrodynamics code, MURaM, and synthetic intensity spectra were calculated from the ultraviolet (149.5 nm) to the far infrared (160 000 nm) with the ATLAS9 radiative transfer code. Nine viewing angles were investigated to account for facular positions across most of the stellar disc. Results. Contrasts of the radiation from simulation boxes with different levels of magnetic flux relative to an atmosphere with no magnetic field are a complicated function of position, wavelength and magnetic field strength that is not reproduced by 1D facular models. Generally, contrasts increase towards the limb, but at UV wavelengths a saturation and decrease are observed close to the limb. Contrasts also increase strongly from the visible to the UV; there is a rich spectral dependence, with marked peaks in molecular bands and strong spectral lines. At disc centre, a complex relationship with magnetic field was found and areas of strong magnetic field can appear either dark or bright, depending on wavelength. Spectra calculated for a wide variety of magnetic fluxes will also serve to improve total and spectral solar irradiance reconstructions.
-
are solar brightness variations Faculae or spot dominated
Astronomy and Astrophysics, 2016Co-Authors: A I Shapiro, S K Solanki, N A Krivova, W SchmutzAbstract:Context. Regular spaceborne measurements have revealed that solar brightness varies on multiple timescales, variations on timescales greater than a day being attributed to a surface magnetic field. Independently, ground-based and spaceborne measurements suggest that Sun-like stars show a similar, but significantly broader pattern of photometric variability. Aims. To understand whether the broader pattern of stellar variations is consistent with the solar paradigm, we assess relative contributions of Faculae and spots to solar magnetically-driven brightness variability. We investigate how the solar brightness variability and its facular and spot contributions depend on the wavelength, timescale of variability, and position of the observer relative to the ecliptic plane. Methods. We performed calculations with the SATIRE model, which returns solar brightness with daily cadence from solar disc area coverages of various magnetic features. We took coverages as seen by an Earth-based observer from full-disc SoHO/MDI and SDO/HMI data and projected them to mimic out-of-ecliptic viewing by an appropriate transformation. Results. Moving the observer away from the ecliptic plane increases the amplitude of 11-year variability as it would be seen in Stromgren ( b + y )/2 photometry, but decreases the amplitude of the rotational brightness variations as it would appear in Kepler and CoRoT passbands. The spot and facular contributions to the 11-year solar variability in the Stromgren ( b + y )/2 photometry almost fully compensate each other so that the Sun appears anomalously quiet with respect to its stellar cohort. Such a compensation does not occur on the rotational timescale. Conclusions. The rotational solar brightness variability as it would appear in the Kepler and CoRoT passbands from the ecliptic plane is spot-dominated, but the relative contribution of Faculae increases for out-of-ecliptic viewing so that the apparent brightness variations are Faculae-dominated for inclinations less than about i = 45°. Over the course of the 11-year activity cycle, the solar brightness variability is Faculae-dominated shortwards of 1.2 μ m independently of the inclination.
-
discriminant analysis of solar bright points and Faculae ii contrast and morphology analysis
arXiv: Solar and Stellar Astrophysics, 2014Co-Authors: Philippe Kobel, J Hirzberger, S K SolankiAbstract:Taken at a high spatial resolution of 0.1 arcsec, Bright Points (BPs) are found to coexist with Faculae in images and the latter are often resolved as adjacent striations. Understanding the properties of these different features is fundamental to carrying out proxy magnetometry. To shed light on the relationship between BPs and Faculae, we studied them separately after the application of a classification method, developed and described in a previous paper) on active region images at various heliocentric angles. In this Paper, we explore different aspects of the photometric properties of BPs and Faculae, namely their G-band contrast profiles, their peak contrast in G-band and continuum, as well as morphological parameters. We find that: (1) the width of the contrast profiles of the classified BPs and Faculae are consistent with studies of disk center BPs at and limb Faculae, which indirectly confirms the validity of our classification, (2) the profiles of limb Faculae are limbward skewed on average, while near disk center they exhibit both centerward and limbward skewnesses due to the distribution of orientations of the Faculae, (3) the relation between the peak contrasts of BPs and Faculae and their apparent area discloses a trend reminiscent of magnetogram studies. The skewness of facular profiles provides a novel constraint for 3D MHD models of Faculae. As suggested by the asymmetry and orientation of their contrast profiles, Faculae near disk center could be induced by inclined fields, while apparent BPs near the limb seem to be in fact small Faculae misidentified. The apparent area of BPs and Faculae could be possibly exploited for proxy magnetometry.
-
intensity contrast of solar network and Faculae
Astronomy and Astrophysics, 2013Co-Authors: S K Solanki, N A KrivovaAbstract:Context. Aims. This study aims at setting observational constraints on the continuum and line core intensity contrast of network and Faculae, specifically, their relationship with magnetic field and disc position. Methods. Full-disc magnetograms and intensity images by the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO) were employed. Bright magnetic features, representing network and Faculae, were identified and the relationship between their intensity contrast at continuum and line core with magnetogram signal and heliocentric angle examined. Care was taken to minimize the inclusion of the magnetic canopy and straylight from sunspots and pores as network and Faculae. Results. In line with earlier studies, network features, on a per unit magnetic flux basis, appeared brighter than facular features. Intensity contrasts in the continuum and line core di er considerably, most notably, they exhibit opposite centre-to-limb variations. We found this di erence in behaviour to likely be due to the di erent mechanisms of the formation of the two spectral components. From a simple model based on bivariate polynomial fits to the measured contrasts we confirmed spectral line changes to be a significant driver of facular contribution to variation in solar irradiance. The discrepancy between the continuum contrast reported here and in the literature was shown to arise mainly from di erences in spatial resolution and treatment of magnetic signals adjacent to sunspots and pores. Conclusions. HMI is a source of accurate contrasts and low-noise magnetograms covering the full solar disc. For irradiance studies it is important to consider not just the contribution from the continuum but also from the spectral lines. In order not to underestimate long-term variations in solar irradiance, irradiance models should take the greater contrast per unit magnetic flux associated with magnetic features with low magnetic flux into account.
V I Makarov - One of the best experts on this subject based on the ideXlab platform.
-
Millimeter-radio, SOHO/EIT 171 Å features and the polar Faculae in the polar zones of the Sun
Astronomy and Astrophysics, 2001Co-Authors: A Riehokainen, V I Makarov, S Urpo, E Valtaoja, L V Makarova, A TlatovAbstract:In this work we study different manifestations of activity in the polar zones of the Sun in order to gain understanding on the phenomenon of enhanced radio temperature regions (ETR) at high solar latitudes. We have obtained simultaneous radio and optical data during 9 days in 1997. The radio data from the Metsahovi radio telescope, Finland, consisted of 37 GHz and 87 GHz solar maps. White light observations at the Kislovodsk solar mountain station in Russia were used to measure the coordinates of polar Faculae groups and diffuse bright structures. We also compared our data with the 171 AEUV SOHO/EIT images for the same time periods. We find the ETRs in general coincide with the relatively dark areas seen in the SOHO/EIT images. Bright structures in the SOHO/EIT maps are, in general, encircled by the polar Faculae groups and diffuse bright structures visible in white light. Some of the EUV bright structures appear to be bases of solar plumes. Connections between ETRs and polar Faculae are complicated; sometimes polar Faculae groups and diffuse bright structures are situated around the ETR maxima or at their borders, sometimes we see the polar Faculae groups distributed over the whole ETR area. Some faint ETRs appear to have no associated polar Faculae. However, in general there is a correlation between the ETRs, the polar Faculae groups, and the bright structures (bases of the plumes and some other features), indicating that they are different manifestations of the same underlying activity. It is possible that magnetic loops in the active areas, traced by the polar Faculae, are responsible for the observed radio enhancements.
-
millimeter radio soho eit 171 a features and the polar Faculae in the polar zones of the sun
Astronomy and Astrophysics, 2001Co-Authors: A Riehokainen, V I Makarov, S Urpo, E Valtaoja, L V Makarova, A TlatovAbstract:In this work we study different manifestations of activity in the polar zones of the Sun in order to gain understanding on the phenomenon of enhanced radio temperature regions (ETR) at high solar latitudes. We have obtained simultaneous radio and optical data during 9 days in 1997. The radio data from the Metsahovi radio telescope, Finland, consisted of 37 GHz and 87 GHz solar maps. White light observations at the Kislovodsk solar mountain station in Russia were used to measure the coordinates of polar Faculae groups and diffuse bright structures. We also compared our data with the 171 AEUV SOHO/EIT images for the same time periods. We find the ETRs in general coincide with the relatively dark areas seen in the SOHO/EIT images. Bright structures in the SOHO/EIT maps are, in general, encircled by the polar Faculae groups and diffuse bright structures visible in white light. Some of the EUV bright structures appear to be bases of solar plumes. Connections between ETRs and polar Faculae are complicated; sometimes polar Faculae groups and diffuse bright structures are situated around the ETR maxima or at their borders, sometimes we see the polar Faculae groups distributed over the whole ETR area. Some faint ETRs appear to have no associated polar Faculae. However, in general there is a correlation between the ETRs, the polar Faculae groups, and the bright structures (bases of the plumes and some other features), indicating that they are different manifestations of the same underlying activity. It is possible that magnetic loops in the active areas, traced by the polar Faculae, are responsible for the observed radio enhancements.
-
spectro polarimetry of polar Faculae
Solar Physics, 1997Co-Authors: T Homann, F Kneer, V I MakarovAbstract:This contribution deals with the properties of small-scale magnetic elements at the polar caps of the Sun. Spectro-polarimetric observations, obtained with high spatial resolution with the Gregory Coude Telescope at the Observatorio del Teide on Tenerife, were analysed. We find, though with limited data sets, that polar Faculae differ in two aspects from Faculae of the network in non-active regions near the equator (equatorial Faculae): (1) Polar Faculae appear to have the same magnetic polarity as the general polar magnetic field. Presumably, the latter is rooted in the small-scale Faculae. The equatorial Faculae show both magnetic polarities. (2) Polar Faculae, with a size of 3.5′′ ± 1.3′′, are larger than equatorial Faculae with 2.1′′ ± 0.4′′. Yet as for equatorial Faculae, polar Faculae possess kilogauss magnetic fields.
-
polar Faculae and sunspot cycles
Solar Physics, 1996Co-Authors: V I Makarov, V V MakarovaAbstract:The monthly number of polar Faculae of the Sun were determined from white-light images at spectral band λ(eff) = (4100 ± 200) A obtained at the Kislovodsk Solar Station during 1960–1994. Corrected monthly numbers were obtained with the help of the visibility function. The level of polar activity larger than 1σ above the monthly running mean was calculated, and the relation between the polar Faculae and sunspot cycle was studied. We confirmed earlier results (Makarov and Makarova, 1987) that the monthly number of polar Faculae, NPFm(t) correlates with the monthly sunspot area Am(Sp)(t + T) with a time shift T ≈ 6 yr. The new polar Faculae cycle began in the middle of 1991. Peculiarities of the first part of sunspot cycle 23 are discussed.
A Tlatov - One of the best experts on this subject based on the ideXlab platform.
-
Millimeter-radio, SOHO/EIT 171 Å features and the polar Faculae in the polar zones of the Sun
Astronomy and Astrophysics, 2001Co-Authors: A Riehokainen, V I Makarov, S Urpo, E Valtaoja, L V Makarova, A TlatovAbstract:In this work we study different manifestations of activity in the polar zones of the Sun in order to gain understanding on the phenomenon of enhanced radio temperature regions (ETR) at high solar latitudes. We have obtained simultaneous radio and optical data during 9 days in 1997. The radio data from the Metsahovi radio telescope, Finland, consisted of 37 GHz and 87 GHz solar maps. White light observations at the Kislovodsk solar mountain station in Russia were used to measure the coordinates of polar Faculae groups and diffuse bright structures. We also compared our data with the 171 AEUV SOHO/EIT images for the same time periods. We find the ETRs in general coincide with the relatively dark areas seen in the SOHO/EIT images. Bright structures in the SOHO/EIT maps are, in general, encircled by the polar Faculae groups and diffuse bright structures visible in white light. Some of the EUV bright structures appear to be bases of solar plumes. Connections between ETRs and polar Faculae are complicated; sometimes polar Faculae groups and diffuse bright structures are situated around the ETR maxima or at their borders, sometimes we see the polar Faculae groups distributed over the whole ETR area. Some faint ETRs appear to have no associated polar Faculae. However, in general there is a correlation between the ETRs, the polar Faculae groups, and the bright structures (bases of the plumes and some other features), indicating that they are different manifestations of the same underlying activity. It is possible that magnetic loops in the active areas, traced by the polar Faculae, are responsible for the observed radio enhancements.
-
millimeter radio soho eit 171 a features and the polar Faculae in the polar zones of the sun
Astronomy and Astrophysics, 2001Co-Authors: A Riehokainen, V I Makarov, S Urpo, E Valtaoja, L V Makarova, A TlatovAbstract:In this work we study different manifestations of activity in the polar zones of the Sun in order to gain understanding on the phenomenon of enhanced radio temperature regions (ETR) at high solar latitudes. We have obtained simultaneous radio and optical data during 9 days in 1997. The radio data from the Metsahovi radio telescope, Finland, consisted of 37 GHz and 87 GHz solar maps. White light observations at the Kislovodsk solar mountain station in Russia were used to measure the coordinates of polar Faculae groups and diffuse bright structures. We also compared our data with the 171 AEUV SOHO/EIT images for the same time periods. We find the ETRs in general coincide with the relatively dark areas seen in the SOHO/EIT images. Bright structures in the SOHO/EIT maps are, in general, encircled by the polar Faculae groups and diffuse bright structures visible in white light. Some of the EUV bright structures appear to be bases of solar plumes. Connections between ETRs and polar Faculae are complicated; sometimes polar Faculae groups and diffuse bright structures are situated around the ETR maxima or at their borders, sometimes we see the polar Faculae groups distributed over the whole ETR area. Some faint ETRs appear to have no associated polar Faculae. However, in general there is a correlation between the ETRs, the polar Faculae groups, and the bright structures (bases of the plumes and some other features), indicating that they are different manifestations of the same underlying activity. It is possible that magnetic loops in the active areas, traced by the polar Faculae, are responsible for the observed radio enhancements.
G A Chapman - One of the best experts on this subject based on the ideXlab platform.
-
the contribution of Faculae and network to long term changes in the total solar irradiance
The Astrophysical Journal, 2003Co-Authors: Stephen R Walton, Dora G Preminger, G A ChapmanAbstract: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
Solar Physics, 2003Co-Authors: S R Walton, D G Preminger, G A ChapmanAbstract: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.
-
an improved determination of the area ratio of Faculae to sunspots
The Astrophysical Journal, 2001Co-Authors: G A Chapman, A M Cookson, J J Dobias, S R WaltonAbstract:We report new results on the ratio of facular area to sunspot area from a program of continuing photometric observations using the Cartesian Full Disk Telescope No. 1 (CFDT1) at the San Fernando Observatory (SFO). The facular areas are determined from images obtained with a 1 nm bandpass Ca II K line filter, and sunspot areas are determined from red images at 672 nm with a 10 nm bandpass filter. On the K line images Faculae were identified by pixels that had a contrast equal to or greater than 4.8% divided by μ. Previously, we found that the average facular-to-spot area ratio was 16.7 ± 0.5 during the latter part of solar cycle 22 and that there was a small but statistically significant rise in the ratio with time. If we take an average from the beginning of the K line data (mid-1988) until the middle of 1996, excluding days of zero sunspot area, the average ratio is 16.4 ± 0.4. The average ratio from mid-1996 to the end of 1999 November is 12.6 ± 0.5. Including days of zero sunspot area for these same intervals we find average ratios of 16.8 ± 0.5 and 13.2 ± 0.6, respectively. We have recently reprocessed our K line images, which have been photometrically "cleaned." We can now reliably identify facular pixels with a contrast criterion of 2.4%, resulting in an increase in the average facular-to-spot ratio of approximately 3. The average facular and sunspot areas for cycle 23 are significantly lower than for cycle 22.
Peter Foukal - One of the best experts on this subject based on the ideXlab platform.
-
Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation?
Solar Physics, 2008Co-Authors: Peter Foukal, Pietro N. BernasconiAbstract: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.
-
what determines the relative areas of spots and Faculae on sun like stars
The Astrophysical Journal, 1998Co-Authors: Peter FoukalAbstract:We analyze newly digitized Ca K plage area data extending back to 1915, and also the white-light facular area data beginning in 1874, to investigate further our earlier finding that the area ratio of Faculae to spots decreases at increasing activity levels. We find that this ratio decreases in plage as well as facular data, so it cannot be an artifact of the visibility function of limb Faculae. The decrease is also accentuated in daily data, compared to annual means; we explain this as a consequence of the different dependences of facular, plage, and spot lifetimes upon their emergent magnetic flux. From this we show that subphotospheric field properties are more likely to determine this ratio, rather than photospheric field diffusion rates. Systematic, cycle-to-cycle variations in its value suggest an origin in fluctuations of the field generation mechanism; specifically, a mechanism that produces a positive correlation between magnetic flux generation efficiency, and relative power in the spatial spectrum at low frequencies. Our results also suggest that main-sequence stars about 50% more magnetically active than the present Sun might exhibit ratio values an order of magnitude lower than current solar values. This evidence strengthens our earlier argument that a rapid shift toward dark photospheric structures in both active regions and network provides the most likely explanation of the recently reported sharp increase of photometric variability in late-type stars somewhat more active than the Sun.
-
the curious case of the greenwich Faculae
Solar Physics, 1993Co-Authors: Peter FoukalAbstract:We analyze the record of facular areas compiled by the Royal Greenwich Observatory (RGO) from daily white-light observations between 1874 and 1976. Curiously, the relative amplitudes of the three largest sunspot cycles 17, 18, and 19 in this record are reversed when they are ranked by facular area. We show that this negative correlation arises from a general decrease of the ratioA F/A S, of facular to sunspot area, with increasingA S. Within a given cycle,A F/A Sdecreases in active regions of largeA S, butA F/A Sis also lower at allA S, in cycles of higher peak amplitude inA S. This decrease ofA F/A Sin large spot groups is consistent with its decrease in younger, more active solar-mass stars, and it may explain why stars only slightly more magnetically active than the Sun tend to exhibit much greater variability in broad-band photometry. We suggest that the physical explanation is an increased spatial filling factor of magnetic flux, favoring formation of sunspots over Faculae. We also explain why the decrease inA F/ASis not seen in the disc-integrated Ca K plage areas, nor in theF10.7 microwave index, both of which exhibit remarkable linearity when plotted against smoothed sunspot area. This explanation suggests how complementary data on Faculae and plages from RGO and Mt. Wilson could be used to improve empirical models of total irradiance variation, extending back to 1874.
