arXiv subjects

S. Sofia

Publications and source records attributed to S. Sofia.

15 recordsLinked to original sources

arXiv2018

Solar radius and luminosity variations induced by the internal dynamo magnetic fields

Although the occurrence of solar irradiance variations induced by magnetic surface features (e.g., sunspots, faculae, magnetic network) is generally accepted, the existence of intrinsic luminosity changes due to the internal magnetic fields is still controversial. This additional contribution is expected to be accompanied by radius variations, and to be potentially significant for the climate of the Earth. We aim to constrain theoretically the radius and luminosity variations of the Sun that are due to the effect of the variable magnetic fields in its interior associated with the dynamo cycle. We have extended a one-dimensional stellar evolution code to include several effects of the magnetic fields on the interior structure. We investigate different magnetic configurations, based on both observational constraints and on the output of state-of-the-art mean field dynamo models. We explore both step-like and simply periodic time dependences of the magnetic field peak strength. We find that the luminosity and radius variations are in anti-phase and in phase, respectively, with the magnetic field strength. For peak magnetic field strengths of the order of tens of kilogauss, luminosity variations ranging between 10^{-6} and 10^{-3} (in modulus) and radius variations between 10^{-6} and 10^{-5} are obtained. Modest but significant radius variations (up to 10^{-5} in relative terms) are obtained for magnetic fields of realistic strength and geometry, providing a potentially observable signature of the intrinsic variations. Establishing their existence in addition to the accepted surface effects would have very important implications for the understanding of solar-induced long-term trends on climate.

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arXiv2013

Variation of the Diameter of the Sun as Measured by the Solar Disk Sextant (SDS)

The balloon-borne Solar Disk Sextant (SDS) experiment has measured the angular size of the Sun on seven occasions spanning the years 1992 to 2011. The solar half-diameter -- observed in a 100-nm wide passband centred at 615 nm -- is found to vary over that period by up to 200 mas, while the typical estimated uncertainty of each measure is 20 mas. The diameter variation is not in phase with the solar activity cycle; thus, the measured diameter variation cannot be explained as an observational artefact of surface activity. Other possible instrument-related explanations for the observed variation are considered but found unlikely, leading us to conclude that the variation is real. The SDS is described here in detail, as is the complete analysis procedure necessary to calibrate the instrument and allow comparison of diameter measures across decades.

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arXiv2009

2D solar modeling

Understanding the reasons of the cyclic variation of the total solar irradiance is one of the most challenging targets of modern astrophysics. These studies prove to be essential also for a more climatologic issue, associated to the global warming. Any attempt to determine the solar components of this phenomenon must include the effects of the magnetic field, whose strength and shape in the solar interior are far from being completely known. Modelling the presence and the effects of a magnetic field requires a 2D approach, since the assumption of radial symmetry is too limiting for this topic. We present the structure of a 2D evolution code that was purposely designed for this scope; rotation, magnetic field and turbulence can be taken into account. Some preliminary results are presented and commented.

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arXiv2006

2-D Stellar Evolution Code Including Arbitrary Magnetic Fields. I. Mathematical Techniques and Test Cases

A high-precision two-dimensional stellar evolution code has been developed for studying solar variability due to structural changes produced by varying internal magnetic fields of arbitrary configurations. Specifically, we are interested in modeling the effects of a dynamo-type field on the detailed internal structure and on the global parameters of the Sun. The high precision is required both to model very small solar changes (of order of $10^{-4}$) and short time scales (or order of one year). It is accomplished by using the mass coordinate to replace the radial coordinate, by using fixed and adjustable time steps, a realistic stellar atmosphere, elements diffusion, and by adjusting the grid points. We have also built into the code the potential to subsequently include rotation and turbulence. The current code has been tested for several cases, including its ability to reproduce the 1-D results.

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arXiv2005

A flux tube solar dynamo model based on the competing role of buoyancy and downflows

A magnetic flux tube may be considered both as a separate body and as a confined field. As a field, it is affected both by differential rotation ($Ω$-effect) and cyclonic convection ($α$-effect). As a body, the tube experiences not only a buoyant force, but also a dynamic pressure due to downflows above the tube. These two competing dynamic effects are incorporated into the $α$-$Ω$ dynamo equations through the total magnetic turbulent diffusivity, leading to a flux tube dynamo operating in the convection zone. We analyze and solve the extended dynamo equations in the linear approximation by adopting the observed solar internal rotation and assuming a downflow effect derived from numerical simulations of solar convection zone. The model reproduces: the 22-year cycle period; the extended butterfly diagram with the confinement of strong activity to low heliographic latitudes $|Φ|\le 35^\circ$; the evidence that at low latitudes the radial field is in an approximately $π$ phase lag compared to the toroidal field at the same latitude; the evidence that the poleward branch is in a $π/2$ phase lag with respect to the equatorward branch; and the evidence that most of the magnetic flux is present in an intermittent form, concentrated into strong flux tubes.

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arXiv2004

New Quasars Detected via Variability in the QUEST1 Survey

By observing the high galactic latitude equatorial sky in drift scan mode with the QUEST (QUasar Equatorial Survey Team) Phase 1 camera, multi-bandpass photometry on a large strip of sky, resolved over a large range of time scales (from hourly to biennially) has been collected. A robust method of ensemble photometry revealed those objects within the scan region that fluctuate in brightness at a statistically significant level. Subsequent spectroscopic observations of a subset of those varying objects easily discriminated the quasars from stars. For a 13-month time scale, 38% of the previously known quasars within the scan region were seen to vary in brightness and subsequent spectroscopic observation revealed that approximately 7% of all variable objects in the scan region are quasars. Increasing the time baseline to 26 months increased the percentage of previously known quasars which vary to 61% and confirmed via spectroscopy that 7% of the variable objects in the region are quasars. This reinforces previously published trends and encourages additional and ongoing synoptic searches for new quasars and their subsequent analysis. During two spectroscopic observing campaigns, a total of 30 quasars were confirmed, 11 of which are new discoveries and 19 of which were determined to be previously known. Using the previously cataloged quasars as a benchmark, we have found it possible to better optimize future variability surveys. This paper reports on the subset of variable objects which were spectroscopically confirmed as quasars.

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arXiv2002

3D convection simulations of the outer layers of the Sun using realistic physics

This paper describes a series of 3D simulations of shallow inefficient convection in the outer layers of the Sun. The computational domain is a closed box containing the convection-radiation transition layer, located at the top of the solar convection zone. The most salient features of the simulations are that: i)The position of the lower boundary can have a major effect on the characteristics of solar surface convection (thermal structure, kinetic energy and turbulent pressure). ii)The width of the box has only a minor effect on the thermal structure, but a more significant effect on the dynamics (rms velocities). iii)Between the surface and a depth of 1 Mm, even though the density and pressure increase by an order of magnitude, the vertical correlation length of vertical velocity is always close to 600 km. iv) In this region the vertical velocity cannot be scaled by the pressure or the density scale height. This casts doubt on the applicability of the mixing length theory, not only in the superadiabatic layer, but also in the adjacent underlying layers. v) The final statistically steady state is not strictly dependent on the initial atmospheric stratification.

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arXiv2002

A Large Area CCD Camera for the Schmidt Telescope at the Venezuelan National Astronomical Observatory

We have designed, constructed and put into operation a large area CCD camera that covers a large fraction of the image plane of the 1 meter Schmidt telescope at Llano del Hato in Venezuela. The camera consists of 16 CCD devices arranged in a 4 x 4 mosaic covering 2.3 degrees x 3.5 degrees of sky. The CCDs are 2048 x 2048 LORAL devices with 15 micron pixels. The camera is optimized for drift scan photometry and objective prism spectroscopy. The design considerations, construction features and performance parameters are described in the following article.

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arXiv2001

Inclusion of turbulence in solar modeling

The general consensus is that in order to reproduce the observed solar p-mode oscillation frequencies, turbulence should be included in solar models. However, until now there has not been any well-tested efficient method to incorporate turbulence into solar modeling. We present here two methods to include turbulence in solar modeling within the framework of the mixing length theory, using the turbulent velocity obtained from numerical simulations of the highly superadiabatic layer of the sun at three stages of its evolution. The first approach is to include the turbulent pressure alone, and the second is to include both the turbulent pressure and the turbulent kinetic energy. The latter is achieved by introducing two variables: the turbulent kinetic energy per unit mass, and the effective ratio of specific heats due to the turbulent perturbation. These are treated as additions to the standard thermodynamic coordinates (e.g. pressure and temperature). We investigate the effects of both treatments of turbulence on the structure variables, the adiabatic sound speed, the structure of the highly superadiabatic layer, and the p-mode frequencies. We find that the second method reproduces the SAL structure obtained in 3D simulations, and produces a p-mode frequency correction an order of magnitude better than the first method.

astro-ph↗

arXiv2001

A convection zone dynamo including the effects of magnetic buoyancy and downward flows

A magnetic flux tube may be considered both as a separate body and as a confined field. As a field, it is affected both by the cyclonic convection ($α$-effect) and differential rotation ($Ω$-effect). As a body, the tube experiences not only a buoyant force, but also a dynamic pressure due to downflows above the tube. When these two dynamic effects are incorporated into the $αΩ$ dynamo equations, we obtain a dynamo operating in the convection zone. We analyze and solve the extended dynamo equations in the linear approximation by using observed solar internal rotation and assuming a downflow suggested by numerical simulations of the solar convection zone. The results produce: (i) the 22-year cycle period; (ii) the extended butterfly diagram; (iii) the confinement of strong activity to low heliographic latitudes $|Φ|\le 35^\circ$; (iv) at low latitudes the radial field is in an approximately $π$ phase lag compared to the toroidal field at the same latitude; (v) the poleward branch is in a $π/2$ phase lag with respect to the equatorward branch; (vi) most of the magnetic flux is present in a strongly intermittent form, concentraed into strong flux tubes; (vii) the magnetic field peaks at a depth of $r=0.96 R_{\sun}$; (viii) total solar irradiance varies in phase with the solar cycle activity, having an amplitude of 0.1%; (ix) solar effective temperature varies in phase with the solar cycle activity, having an amplitude of 1.5 $^\circ C$; and (x) solar radius also varies in phase with the solar cycle activity, having an amplitude of 20 mas. All these results are in agreement with the corresponding observations.

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arXiv2001

The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo

We have measured the periods and light curves of 148 RR Lyrae variables from V=13.5 to 19.7 from the first 100 sq. degrees of the QUEST RR Lyrae survey. Approximately 55% of these stars belong to the clump of stars detected earlier by the Sloan Digital Sky Survey. According to our measurements, this feature has ~10 times the background density of halo stars, spans at least 37.5 deg by 3.5 deg in right ascension and declination (>=30 by >=3 kpc), lies ~50 kpc from the Sun, and has a depth along the line of sight of ~5 kpc (1 sigma). These properties are consistent with the recent models that suggest it is a tidal stream from the Sgr dSph galaxy. The mean period of the type ab variables, 0.58 d, is also consistent. In addition, we have found two smaller over-densities in the halo, one of which may be related to the globular cluster Pal 5.

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arXiv2000

Discovery of the Bright Trans-Neptunian Object 2000 EB173

We describe the discovery circumstances and photometric properties of 2000 EB173, now one of the brightest trans-Neptunian objects (TNOs) with opposition magnitude m_R=18.9 and also one of the largest Plutinos, found with the drift-scanning camera of the QUEST Collaboration, attached to the 1-m Schmidt telescope of the National Observatory of Venezuela. We measure B-V = 0.99 +/- 0.14 and V-R = 0.57 +/- 0.05, a red color observed for many fainter TNOs. At our magnitude limit m_R = 20.1 +/- 0.20, our single detection reveals a sky density of 0.015 (+0.034, -0.012) TNOs per deg^2 (the error bars are 68% confidence limits), consistent with fainter surveys showing a cumulative number proportional to 10^0.5m_R. Assuming an inclination distribution of TNOs with FWHM exceeding 30 deg, it is likely that one hundred to several hundred objects brighter than m_R=20.1 remain to be discovered.

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arXiv2000

The effect of turbulent pressure on the p-mode frequencies in stellar models

We have constructed models for the sun at three stages of its evolution: a zero-age main sequence model, the present sun, and a subgiant model. For each model, the turbulent pressure and turbulent kinetic energy were calculated from 3-d radiative hydrodynamical simulations (described in the poster by Robinson et al.), and inserted into the 1-d stellar models. We note that in these simulations, the turbulent pressure is not a free parameter, but can be computed from the resulting velocity field. We show the calculated p-mode frequencies for the model of the present sun, with and without turbulent pressure, and compare them to the observed solar frequencies. When the turbulent pressure is included in the models, the calculated frequencies are brought closer to the observed frequencies in the sun by up to two $μHz$, strictly from structural effects. The effect of including turbulent pressure on p-mode frequencies is also shown for the zero-age main sequence model. Our models also suggest that the importance of turbulent pressure increases as the star evolves into the subgiant region. We discuss the importance of also including realistic turbulence as well as radiation in the non-adiabatic calculation of oscillation frequencies.

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arXiv2000

Radiative hydrodynamics in the highly super adiabatic layer of stellar evolution models

We present results of three dimensional simulations of the uppermost part of the sun, at 3 stages of its evolution. Each model includes physically realistic radiative-hydrodynamics (the Eddington approximation is used in the optically thin region), varying opacities and a realistic equation of state (full treatment of the ionization of H and He). In each evolution model, we investigate a domain, which starts at the top of the photosphere and ends just inside the convection zone (about 2400 km in the sun model). This includes all of the super-adiabatic layer (SAL). Due to the different positions of the three models in the $log (g) $ vs $log T_{eff}$ plane, the more evolved models have lower density atmospheres. The reduction in density causes the amount of overshoot into the radiation layer, to be greater in the more evolved models.

astro-ph↗

arXiv2000

Measurements of solar irradiance and effective temperature as a probe of solar interior magnetic fields

We argue that a variety of solar data suggest that the activity-cycle timescale variability of the total irradiance, is produced by structural adjustments of the solar interior. Assuming these adjustments are induced by variations of internal magnetic fields, we use measurements of the total irradiance and effective temperature over the period from 1978 to 1992, to infer the magnitude and location of the magnetic field. Using an updated stellar evolution model, which includes magnetic fields, we find that the observations can be explained by fields whose peak values range from 120k to 2.3k gauss, located in the convection zone between $0.959R_{\sun}$ and $0.997R_{\sun}$, respectively. The corresponding maximal radius changes, are 17 km when the magnetic field is located at $0.959R_{\sun}$ and 3 km when it is located at $0.997R_{\sun}$. At these depths, the $W$ parameter(defined by $Δ\ln R / Δ\ln L$, where $R$ and $L$ are the radius and luminosity) ranges from 0.02 to 0.006. All these predictions are consistent with helioseismology and recent measurements carried out by the MDI experiment on SOHO.

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