N. A. Krivova

Effect of spatial resolution on estimating the Sun's magnetic flux

A critical question related to a possible secular trend in the Suns total magnetic flux and consequently in solar irradiance is the total amount of magnetic flux present on the Sun and how it is distributed between active regions and the quiet Sun. NSO/Kitt Peak synoptic charts have in the past been used to estimate the total flux and the fraction of the flux in active regions and in the quiet Sun. Since a single pixel of these synoptic charts is much bigger than individual small-scale magnetic elements and opposite polarities may he present within the same pixel, some magnetic flux escaped notice. Here we estimate the fraction of the magnetic flux escaping detection in Kitt Peak synoptic charts. By artificially reducing the spatial resolution of MDI full-disc and high-resolution magnetograms we study the influence of the resolution on the measured total magnetic flux. Noise in the data poses the main difficulty to this approach and is carefully studied. It is concluded that at least half of the magnetic flux in the quiet Sun remains undetected in Kitt Peak synoptic charts and that the total flux present on the solar surface at maxima of activity is around twice the flux present at activity minima.

Intensity contrast of solar network and faculae

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.

The Disk of ? Pictoris in the Light of Polarimetric Data

We model the linear polarization of the radiation of b Pic scattered by dust particles in the circumstel- lar disk. The observed spatial distribution and the wavelength dependence of the polarization together with the colors of the b Pic disk require that particles in a wide size range be present in the disk, with the grains smaller than a few microns in size being somewhat depleted but still of importance for the polarization and colors. The inferred size distribution is consistent with the production and loss mecha- nisms: the sourcespresumably collisions and evaporation of large bodiescontinuously produce dust with a power-law size distribution with the exponent D3.5 over a broad range of sizes, but the particles smaller than a few microns are blown away by the radiation pressure, which shortens the time they spend in the disk and decreases their number densities. Compact (or slightly porous) silicates are found to give better agreement with the observations, although other materials are still not ruled out and a high —uffiness of the large particles is possible. The observed asymmetry in the polarization of two wings can be explained if more small grains (by 20%¨30%) are present on the northeast side of the disk. We show that such an asymmetry in the size distributions in two wings might be caused by an in—uence of the interstellar medium; a required amount of small grains could be produced by destructive collisions of interstellar grains with the circumstellar dust particles. Subject headings: circumstellar matterpolarizationstars: individual (b Pictoris)

Point spread function of SDO/HMI and the effects of stray light correction on the apparent properties of solar surface phenomena

Aims. We present a point spread function (PSF) for the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) and discuss the effects of its removal on the apparent properties of solar surface phenomena in HMI data. Methods. The PSF was retrieved from observations of Venus in transit by matching it to the convolution of a model of the Venusian disc and solar background with a guess PSF. We described the PSF as the sum of five Gaussian functions, the amplitudes of which vary sinusoidally with azimuth. This relatively complex functional form was required by the data. Observations recorded near in time to the transit of Venus were corrected for instrumental scattered light by the deconvolution with the PSF. We also examined the variation in the shape of the solar aureole in daily data, as an indication of PSF changes over time. Results. Granulation contrast in restored HMI data is greatly enhanced relative to the original data and exhibit reasonable agreement with numerical simulations. Image restoration enhanced the apparent intensity and pixel averaged magnetic field strength of photospheric magnetic features significantly. For small-scale magnetic features, restoration enhanced intensity contrast in the continuum and core of the Fe I 6173 A line by a factor of 1.3, and the magnetogram signal by a factor of 1.7. For sunspots and pores, the enhancement varied strongly within and between features, being more acute for smaller features. Magnetic features are also rendered smaller, as signal smeared onto the surrounding quiet Sun is recovered. Image restoration increased the apparent amount of magnetic flux above the noise floor by a factor of about 1.2, most of the gain coming from the quiet Sun. Line-of-sight velocity due to granulation and supergranulation is enhanced by a factor of 1.4 to 2.1, depending on position on the solar disc. The shape of the solar aureole varied, with time and between the two CCDs. There are also indications that the PSF varies across the FOV. However, all these variations were found to be relatively small, such that a single PSF can be applied to HMI data from both CCDs, over the period examined without introducing significant error. Conclusions. Restoring HMI observations with the PSF presented here returns a reasonable estimate of the stray light-free intensity contrast. Image restoration affects the measured radiant, magnetic and dynamic properties of solar surface phenomena sufficiently to significantly impact interpretation.

The role of the Fraunhofer lines in solar brightness variability

The solar brightness varies on timescales from minutes to decades. A clear identification of the physical processes behind such variations is needed for developing and improving physics-based models of solar brightness variability and reconstructing solar brightness in the past. This is, in turn, important for better understanding the solar-terrestrial and solar-stellar connections. nWe estimate the relative contributions of the continuum, molecular, and atomic lines to the solar brightness variations on different timescales. nOur approach is based on the assumption that variability of the solar brightness on timescales greater than a day is driven by the evolution of the solar surface magnetic field. We calculated the solar brightness variations employing the solar disc area coverage of magnetic features deduced from the MDI/SOHO observations. The brightness contrasts of magnetic features relative to the quiet Sun were calculated with a non-LTE radiative transfer code as functions of disc position and wavelength. By consecutive elimination of molecular and atomic lines from the radiative transfer calculations, we assessed the role of these lines in producing solar brightness variability. nWe show that the variations in Fraunhofer lines define the amplitude of the solar brightness variability on timescales greater than a day and even the phase of the total solar irradiance variability over the 11-year cycle. We also demonstrate that molecular lines make substantial contribution to solar brightness variability on the 11-year activity cycle and centennial timescales. In particular, our model indicates that roughly a quarter of the total solar irradiance variability over the 11-year cycle originates in molecular lines. The maximum of the absolute spectral brightness variability on timescales greater than a day is associated with the CN violet system between 380 and 390 nm.

Sunspot areas and tilt angles for solar cycles 7-10

Extending the knowledge about the properties of solar cycles into the past is essential for understanding the solar dynamo. This paper aims at estimating areas of sunspots observed by Schwabe in 1825-1867 and at calculating the tilt angles of sunspot groups. The sunspot sizes in Schwabes drawings are not to scale and need to be converted into physical sunspot areas. We employed a statistical approach assuming that the area distribution of sunspots was the same in the 19th century as it was in the 20th century. Umbral areas for about 130,000 sunspots observed by Schwabe were obtained, as well as the tilt angles of sunspot groups assuming them to be bipolar. There is, of course, no polarity information in the observations. The annually averaged sunspot areas correlate reasonably with sunspot number. We derived an average tilt angle by attempting to exclude unipolar groups with a minimum separation of the two alleged polarities and an outlier rejection method which follows the evolution of each group and detects the moment it turns unipolar at its decay. As a result, the tilt angles, although displaying considerable scatter, place the leading polarity on average 5.85+-0.25 closer to the equator, in good agreement with tilt angles obtained from 20th-century data sets. Sources of uncertainties in the tilt angle determination are discussed and need to be addressed whenever different data sets are combined. The sunspot area and tilt angle data are provided online.

A stream of particles from the β Pictoris disc: A possible ejection mechanism

Recently, a stream of particles originating from the direction of β Pictoris, a young main sequence star surrounded by a dust disc, has been reported (Baggaley 2000). Standard mechanisms of particle ejection from a disc fail to reproduce the properties of this stream. We find that scattering by a giant proto-planet with properties taken from the literature is consistent with the observations. The fact that a straightforward ejection mechanism reproduces the data supports the identification of the particle streams source with β Pic. Our work also indicates that protoplanetary dust discs form a potentially rich source of large interstellar grains, as widely detected in the Solar System.

Towards understanding the β Pictoris dust stream

The recent radar detection by Baggaley (2000) of a collimated stream of interstellar meteoroids postulated to be sourced at β Pictoris, a nearby star with a prominent dust disk, presents a challenge to theoreticians. Two mechanisms of possible dust ejection from β Pic have been proposed: ejection of dust by radiation pressure from comets in eccentric orbits and by gravity of a hypothetical planet in the disk. Here we re-examine observational data and reconsider theoretical scenarios, substantiating them with detailed modeling to test whether they can explain quantitatively and simultaneously the masses, speeds, and fluxes. Our analysis of the stream geometry and kinematics confirms that β Pic is the most likely source of the stream and suggests that an intensive dust ejection phase took place ∼0.7 Myr ago. Our dynamical simulations show that high ejection speeds retrieved from the observations can be explained by both planetary ejection and radiation pressure mechanisms, providing, however, several important constraints. In the planetary ejection scenario, only a hot Jupiter-type planet with a semimajor axis of less than 1 AU can be responsible for the stream, and only if the disk was dynamically heated by a more distant massive planet. The radiation pressure scenario also requires the presence of a relatively massive planet at several AU or more, that had heated the cometesimal disk before the ejection occurred. Finally, the dust flux measured at Earth can be brought into reasonable agreement with both scenarios, provided that β Pics protoplanetary disk recently passed through an intensive short-lasting (∼0.1 Myr) clearance stage by nascent giant planets, similar to what took place in the early solar system.

Dust around Herbig Ae/Be stars: modelling of observational data

Herbig Ae/Be (HAeBe) stars, young stars surrounded by dust shells, are believed to be precursors of β Pic-like stars, and the dust around them is thought to be a possible source material for the formation of planets. A group of the HAeBe stars (UX Ori, WW Vul, etc.) shows large brightness variations. The dust in the vicinity of these stars is responsible not only for their excess emission in the infrared, anomalous extinction in the ultraviolet and visual, and specific spatial distributions of the intensity and polarization, but also for the “blueing” effect in the colour-magnitude diagrams and the intrinsic polarization increase observed in deep minima.In contrast to the previous studies, we take advantage of a simultaneous modelling of all the observational data mentioned. Monte-Carlo simulations of polarized radiation transfer in the shells with a spheroidal density distribution have been performed for different dust grain models. The results are compared with observations of a typical HAeBe star WW Vul. Some effects related to a possible porosity of the circumstellar grains are considered.

From solar to stellar brightness variations. The effect of metallicity

Context. Comparison studies of Sun-like stars with the Sun suggest an anomalously low photometric variability of the Sun compared to Sun-like stars with similar magnetic activity. Comprehensive understanding of stellar variability is needed, to find a physical reasoning for this observation. Aims. We investigate the effect of metallicity and effective temperature on the photometric brightness change of Sun-like stars seen at different inclinations. The considered range of fundamental stellar parameters is sufficiently small so the stars, investigated here, still count as Sun-like or even as solar twins. Methods. To model the brightness change of stars with solar magnetic activity, we extend a well established model of solar brightness variations, SATIRE (which stands for Spectral And Total Irradiance Reconstruction), which is based on solar spectra, to stars with different fundamental parameters. For that we calculate stellar spectra for different metallicities and effective temperature using the radiative transfer code ATLAS9. Results. We show that even a small change (e.g. within the observational error range) of metallicity or effective temperature significantly affects the photometric brightness change compared to the Sun. We find that for Sun-like stars, the amplitude of the brightness variations obtained for Stromgren (b + y)/2 reaches a local minimum for fundamental stellar parameters close to the solar metallicity and effective temperature. Moreover, our results show that the effect of inclination decreases for metallicity values greater than the solar metallicity. Overall, we find that an exact determination of fundamental stellar parameters is crucially important for understanding stellar brightness changes.