Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations
C. Beck, D. Fabbian, F. Moreno-Insertis, K. G. Puschmann, R. Rezaei
备注：
21 pages, 15 figures + 2 pages Appendix, accepted for publication in A&A; v2 version: corrected for an error in the calculation of stray-light estimates, for details see the Corrigendum to A&A, 2013, 557, 109 (DOI: 10.1051/0004-6361/201321596). Corrected text and numbers are in bold font. Apart from the stray-light estimates, nothing in the rest of the paper was affected by the error
Numerical 3D radiative (M)HD simulations of solar convection are used to understand the physical properties of the solar photosphere. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer. We investigate the realism of 3D convection simulations carried out with the Stagger code. We compared the characteristic properties of several spectral lines in solar disc centre observations with spectra synthesized from the simulations. We degraded the synthetic spectra to the spatial resolution of the observations using the continuum intensity distribution. We estimated the necessary spectral degradation by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly, we used the SIR code to invert the spectra. Most of the line parameters from the observational data are matched well by the degraded simulation spectra. The inversions predict a macroturbulent velocity below 10 m/s for the simulation at full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution of 0.3". The temperature fluctuations in the inversion of the degraded simulation do not exceed those from the observational data (of the order of 100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change of (average) line properties at a spatial scale between 0.13" and 0.3". Up to a spatial resolution of 0.3", we find no indications of the presence of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even better spatial resolution.
Using numerical simulations of the magnetised solar photosphere carried out with the radiative magneto-hydrodynamic code, MURaM, and detailed spectro-polarimetric diagnostics of the simulated photospheric 6302A FeI line, spectro-polarimetric signatures of Alfven waves in magnetised intergranular lanes of the simulated solar photosphere were analysed at different positions at the solar disk. The torsional Alfven waves in the intergranular lanes are horizontal plasma motions, which do not have a thermal perturbation counterpart. We find signatures of Alfven waves as small-scale line profile Dopp…
We investigate the generation mechanisms of MHD Poynting flux in the magnetised solar photosphere. Using radiative MHD modelling of the solar photosphere with initial magnetic configurations that differ in their field strength and geometry, we show the presence of two different mechanisms for MHD Poynting flux generation in simulations of solar photospheric magneto-convection. The weaker mechanism is connected to vertical transport of weak horizontal magnetic fields in the convectively stable layers of the upper photosphere, while the stronger is the production of Poynting flux in strongly mag…
As a step toward a complete theoretical integration of 3D compressible hydrodynamic simulations into stellar evolution, convection at the surface and sub-surface layers of the Sun is re-examined, from a restricted point of view, in the language of mixing-length theory (MLT) . Requiring that MLT use a hydrodynamically realistic dissipation length gives a new constraint on solar models. While the stellar structure which results is similar to that obtained by YREC and Garching models, the theoretical picture differs. A new quantitative connection is made between macro-turbulence, micro-turbulence…
We investigate the fast-rotating (P_orb=P_rot=0.465d), active dwarf binary V405 And (M0V+M5V) using photometric BV(RI)_C and optical spectroscopic data. The light variation is caused by the combined effect of spottedness and binarity with a small eclipse. We estimate the system parameters from the available light and radial velocity curves. Three flare events occurred during the observations: two were found in the spectroscopic data and one was observed photometrically in BV(RI)_C colours. An interesting eruptive phenomenon emerged from the photometric measurements that can be interpreted as a…
The solar photosphere is the visible surface of the Sun, where many bright granules, surrounded by narrow dark intergranular lanes, are observed everywhere. The granular pattern is a manifestation of convective motion at the photospheric level, but its velocity structure in the height direction is poorly understood observationally. Applying bisector analysis to a photospheric spectral line recorded by the Hinode Solar Optical Telescope, we derived the velocity structure of the convective motion in granular regions and intergranular lanes separately. The amplitude of motion of the convective ma…
Solar granules are bright patterns surrounded by dark channels called intergranular lanes in the solar photosphere and are a manifestation of overshooting convection. Observational studies generally find stronger upflows in granules and weaker downflows in intergranular lanes. This trend is, however, inconsistent with the results of numerical simulations in which downflows are stronger than upflows through the joint action of gravitational acceleration/deceleration and pressure gradients. One cause of this discrepancy is the image degradation caused by optical distortion and light diffraction …
Spectra of the cellular photospheric flows are determined from full-disk Doppler velocity observations acquired by the Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics Observatory (SDO) spacecraft. Three different analysis methods are used to separately determine spectral coefficients representing the poloidal flows, the toroidal flows, and the radial flows. The amplitudes of these spectral coefficients are constrained by simulated data analyzed with the same procedures as the HMI data. We find that the total velocity spectrum rises smoothly to a peak at a wavenumber of …
Three-dimensional (3D) radiative hydrodynamic model atmospheres of metal-poor late-type stars are characterized by cooler upper photospheric layers than their 1D counterparts. This property of 3D models can dramatically affect elemental abundances derived from temperature-sensitive spectral lines. We investigate whether the cool surface temperatures predicted by metal-poor 3D models can be ascribed to the approximated treatment of scattering in the radiative transfer. We use the Bifrost code to test three different ways to handle scattering in 3D model atmospheres of metal-poor stars. First, w…
A putative temporally varying circulation-free magnetic-field configuration is inferred in an equatorial segment of the solar convection zone from the helioseismologically inferred angular-velocity variation, assuming that the predominant dynamics is angular acceleration produced by the azimuthal Maxwell stress exerted by a field whose surface values are consistent with photospheric line-of-sight measurements.
In this paper we seek to understand the timescale on which the photospheric motions on the Sun braid coronal magnetic field lines. This is a crucial ingredient for determining the viability of the braiding mechanism for explaining the high temperatures observed in the corona. We study the topological complexity induced in the coronal magnetic field, primarily using plasma motions extracted from magneto-convection simulations. This topological complexity is quantified using the field line winding, finite time topological entropy and passive scalar mixing. With these measures we contrast mixing …
Signatures of photospheric convection throughout the solar atmosphere: the EVE Sun-as-a-star mHz continuum
SUPA School of Physics & Astronomy, University of Glasgow（格拉斯哥大学物理与天文学学院）；Space Science Laboratory, University of California, Berkeley（加州大学伯克利分校空间科学实验室）；University of Warwick（华威大学）；Rosseland Centre for Solar Physics, University of Oslo（奥斯陆大学罗瑟兰太阳物理中心）
We analyse a sunspot simulation in an effort to understand the origin of the convective instabilities giving rise to the penumbral and umbral distinct regimes. We applied the criterion from Gough \& Tayler (1966), accounting for the stabilising effect of the vertical magnetic field to investigate the convective instabilities in a MURaM sunspot simulation. We find: (1) a highly unstable shallow layer right beneath the surface extending all over the simulation box in which convection is triggered by radiative cooling in the photosphere; (2) a deep umbral core (beneath -5 Mm) stabilised against o…
The Sun provides us with the only spatially well-resolved astrophysical example of turbulent thermal convection. While various aspects of solar photospheric turbulence, such as granulation (one-Megameter horizontal scale), are well understood, the questions of the physical origin and dynamical organization of larger-scale flows, such as the 30-Megameters supergranulation and flows deep in the solar convection zone, remain largely open in spite of their importance for solar dynamics and magnetism. Here, we present a new critical global observational characterization of multiscale photospheric f…
Solar activity is studied using a cluster analysis of the time-fluctuations of sunspot number. In the historic period (1850-1932) the cluster exponent was approximately equal to 0.37 (strong clustering) for the high activity components of the solar cycles. In the modern period (last seven solar cycles: 1933-2007) the cluster exponent was approximately equal to 0.50 (random, white noise-like). Comparing these results with the corresponding data from laboratory experiments on convection it is shown, that in the historic period emergence of sunspots in the solar photosphere was dominated by turbu…
Dynamical jets are generally found on Light bridges (LBs), which are key to studying sunspots decays. So far, their formation mechanism is not fully understood. In this paper, we used state-of-the-art observations from the Goode Solar Telescope, the Interface Region Imaging Spectrograph, the Spectro-Polarimeter on board Hinode and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to analyze the fan shape jets on LBs in detail. Continuous upward motion of the jets in ascending phase is found from the H$α$ velocity, which lasts for 12 minutes and is associated with t…
A magneto-hydrostatic model is constructed with spectropolarimetric properties close to those of solar photospheric magnetic bright points. Results of solar radiative magneto-convection simulations are used to produce the spatial structure of the vertical component of the magnetic field. The horizontal component of magnetic field is reconstructed using the self-similarity condition, while the magneto-hydrostatic equilibrium condition is applied to the standard photospheric model with the magnetic field embedded. Partial ionisation processes are found to be necessary for reconstructing the corr…
Chromospheric and photospheric properties of sunspots as inferred from Stokes inversions under magneto-hydrostatic and non-local-thermodynamic equilibrium
Institut für Sonnenphysik（太阳物理研究所）；Institute for Solar Physics, Department of Astronomy, Stockholm University（斯德哥尔摩大学天文学系太阳物理研究所）；Leibniz-Institut für Astrophysik Potsdam (AIP)（波茨坦阿诺德·爱因斯坦天文 Leibniz 研究所）；ETH Zürich, Institute for Particle Physics and Astrophysics（苏黎世联邦理工学院粒子物理与天体物理研究所）；Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana（意大利瑞士大学信息学院阿尔多和塞莱达科太阳研究所以及研究所）
Massive, hot OB-stars show clear evidence of strong macroscopic broadening (in addition to rotation) in their photospheric spectral lines. This paper examines the occurrence of such "macro-turbulence" in slowly rotating O-stars with strong, organised surface magnetic fields. Focusing on the CIV 5811A line, we find evidence for significant macro-turbulent broadening in all stars except NGC1624-2, which also has (by far) the strongest magnetic field. Instead, the very sharp CIV lines in NGC1624-2 are dominated by magnetic Zeeman broadening, from which we estimate a dipolar field of approximately…
The goal of this study is to explore a novel method for the solar photospheric magnetic field diagnostics using Stokes V widths of different magnetosensitive Fe~I spectral lines. We calculate Stokes I and V profiles of several Fe I lines based on a one-dimensional photospheric model VAL C using the NICOLE radiative transfer code. These profiles are used to produce calibration curves linking the intrinsic magnetic field values with the widths of blue peaks of Stokes V profiles. The obtained calibration curves are then tested using the Stokes profiles calculated for more realistic photospheric m…
Granules observed in solar photosphere are believed to be convective and turbulent, but the physical picture of granular dynamical process remains unclear. Here we performed an investigation of granular dynamical motions of full length scales based on data obtained by the 1-meter New Vacuum Solar Telescope (NVST) and the 1.6-meter Goode Solar Telescope (GST). We developed a new granule segmenting method, which can detect both small faint and large bright granules. A large number of granules were detected and two critical sizes, 265 km and 1420 km, were found to separate the granules into three…
Betelgeuse is an M supergiant with a complex and extended atmosphere, which also harbors spots and giant granules at its surface. A possible magnetic field could contribute to the mass loss and to the heating of the outer atmosphere. We observed Betelgeuse, to directly study and infer the nature of its magnetic field. We used the new-generation spectropolarimeter NARVAL and the least square deconvolution (LSD) method to detect circular polarization within the photospheric absorption lines of Betelgeuse. We have unambiguously detected a weak Stokes V signal in the spectral lines of Betelgeuse, …
We have derived the temporal power spectra of the horizontal velocity of the solar photosphere. The data sets for 14 quiet regions observed with the Gband filter of Hinode/SOT are analyzed to measure the temporal fluctuation of the horizontal velocity by using the local correlation tracking (LCT) method. Among the high resolution (~0.2") and seeing-free data sets of Hinode/SOT, we selected the observations whose duration is longer than 70 minutes and cadence is about 30 s. The so-called k-ω diagrams of the photospheric horizontal velocity are derived for the first time to investigate the tempo…
Early magnetographic observations indicated that magnetic field in the solar photosphere has unresolved small-scale structure. Near-infrared and optical data with extremely high spatial resolution show that these structures have scales of few tens of kilometres, which are not resolved in the majority of solar observations. The goal of this study is to establish the effect of unresolved photospheric magnetic field structure on Stokes profiles observed with relatively low spatial resolution. Ultimately, we aim to develop methods for fast estimation of the photospheric magnetic filling factor and…
We examine the excitation of unstable magnetosonic waves in the radiative envelopes of intermediate- and high-mass stars with a magnetic field of ~kG strength. Wind clumping close to the star and microturbulence can often be accounted for when including small-scale, sub-photospheric density or velocity perturbations. Compressional waves - with wavelengths comparable to or shorter than the gas pressure scale height - can be destabilized by the radiative flux in optically-thick media when a magnetic field is present, in a process called the Radiation-Driven Magneto-Acoustic Instability (RMI). Th…
We present a new determination of the solar nitrogen abundance making use of 3D hydrodynamical modelling of the solar photosphere, which is more physically motivated than traditional static 1D models. We selected suitable atomic spectral lines, relying on equivalent width measurements already existing in the literature. For atmospheric modelling we used the co 5 bold 3D radiation hydrodynamics code. We investigated the influence of both deviations from local thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities (granulation effects) on the resulting abundance. We also co…
We compare the properties of kG magnetic structures in the solar network and in active region plage at high spatial resolution. Our analysis used six SP scans of the solar disc centre aboard Hinode SOT and inverted the obtained spectra of the photospheric 6302 Åline pair using the 2D SPINOR code. Photospheric magnetic field concentrations in network and plage areas are on average 1.5 kG strong with inclinations of 10-20 degrees, and have <400 m/s internal and 2-3 km/s external downflows. At the disc centre, the continuum intensity of magnetic field concentrations in the network are on average …
Stochastically-triggered photospheric light variations reaching $\sim$$40$ mmag peak-to-valley amplitudes have been detected in the O8Iaf supergiant V973 Scorpii as the outcome of two months of high-precision time-resolved photometric observations with the BRIght Target Explorer (BRITE) nanosatellites. The amplitude spectrum of the time series photometry exhibits a pronounced broad bump in the low-frequency regime ($\lesssim$$0.9$ d$^{-1}$) where several prominent frequencies are detected. A time-frequency analysis of the observations reveals typical mode lifetimes of the order of $5-10$ days.…
The recent progress in high-spatial-resolution techniques, spanning wavelengths from the visual to the radio regime, is leading to new valuable insights into the complex dynamical atmospheres of Asymptotic Giant Branch (AGB) stars and their wind forming regions. Striking examples are images of asymmetries and inhomogeneities in the photospheric and dust-forming layers which vary on time-scales of months. These features are probably related to large-scale convective flows predicted by 3D star-in-a-box models. Furthermore, high-resolution observations make it possible to measure dust condensatio…
We model the deepest observable layers of dark sunspot umbral atmospheres in terms of an empirical model which equally describes observed near infrared continuum intensities and line profiles. We use the umbral continuum intensity at 1.67 nm and the three C I lines at 1,6888, 1,7449 and 1,7456 nm to model the deep layers near the minimum of H- absorption. We find that a radiative equilibrium stratification yields the best compromise between continuum and C I line observations. We determine the effective temperature from the umbral and photospheric flux ratio by down-scaling the monochromatic p…
The use of hydrodynamical simulations, the selection of atomic data, and the computation of deviations from local thermodynamical equilibrium for the analysis of the solar spectra have implied a downward revision of the solar metallicity. We are in the process of using the latest simulations computed with the CO5BOLD code to reassess the solar chemical composition. We determine the solar photospheric carbon abundance by using a radiation-hydrodynamical CO5BOLD model, and compute the departures from local thermodynamical equilibrium by using the Kiel code. We measure equivalent widths of atomic…
We outline our techniques to characterise photospheric granulation as an astrophysical noise source. A four component parameterisation of granulation is developed that can be used to reconstruct stellar line asymmetries and radial velocity shifts due to photospheric convective motions. The four components are made up of absorption line profiles calculated for granules, magnetic intergranular lanes, non-magnetic intergranular lanes, and magnetic bright points at disc centre. These components are constructed by averaging Fe I $6302 \mathrmÅ$ magnetically sensitive absorption line profiles output…
Aims. We use magnetic and non-magnetic 3D numerical simulations of solar granulation and G-band radiative diagnostics from the resulting models to analyse the generation of small-scale vortex motions in the solar photosphere. Methods. Radiative MHD simulations of magnetoconvection are used to produce photospheric models. Our starting point is a non-magnetic model of solar convection, where we introduce a uniform magnetic field and follow the evolution of the field in the simulated photosphere. We find two different types of photospheric vortices, and provide a link between the vorticity genera…
A study of the propagation of magnetoacoustic waves in small-scale magnetic fields using solar photospheric and chromospheric Dopplergrams: HMI/SDO and MAST observations
Several solar-like stars exhibit cyclic magnetic activity similar to the Sun as found in photospheric and chromospheric emission. We want to understand the physical mechanism involved in rotational dependence of these activity cycle periods. We use three-dimensional magnetohydrodynamical simulations of global convective dynamos models of solar-like stars to investigate the rotational dependency of dynamos. We further apply the test-field method to determine the $α$ effect in these simulations. We find dynamo with clear oscillating mean magnetic fields for moderately and rapidly rotating runs. …
The extended wings of the Ca II H and K lines provide excellent diagnostics of the temperature stratification of the photosphere of the Sun and of other cool stars, thanks to their LTE opacities and source functions and their large span in formation height. The aim of this study is to calibrate the usage of the H and K wings in one-dimensional interpretation of spatially averaged spectra and in deriving per-pixel stratifications from resolved spectra. I use multi-dimensional simulations of solar convection to synthesize the H and K wings, derive one-dimensional models from these wings as if th…
Recent ground- and space-based observations reveal the presence of small-scale motions between convection cells in the solar photosphere. In these regions small-scale magnetic flux tubes are generated due to the interaction of granulation motion and background magnetic field. This paper studies the effects of these motions, on magnetohydrodynamic wave excitation from broadband photospheric drivers. Numerical experiments of linear magnetohydrodynamic wave propagation in a magnetic flux tube embedded in a realistic gravitationally stratified solar atmosphere between the photosphere and the low c…
The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two sunrise missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the ph…
We investigate the effect of helium abundance and $α$-element enhancement on the properties of convection in envelopes of solar-like main-sequence stars stars using a grid of 3D radiation hydrodynamic simulations. Helium abundance increases the mean molecular weight of the gas, and alters opacity by displacing hydrogen. Since the scale of the effect of helium may depend on the metallicity, the grid consists of simulations with three helium abundances ($Y=0.1, 0.2, 0.3$), each with two metallicities ($Z=0.001, 0.020)$. We find that changing the helium mass fraction generally affects structure a…
Where does solar flare energy come from? More specifically, assuming that the ultimate source of flare energy is mechanical energy in the convection zone, how is this translated into energy dissipated or stored in the corona? This question appears to have been given relatively little thought, as attention has been focussed predominantly on mechanisms for the rapid dissipation of coronal magnetic energy by way of MHD instabilities and plasma micro instabilities. We consider three types of flare theory: the steady state "photospheric dynamo" model in which flare power represents coronal dissipat…
We study the solar photospheric abundance of singly ionized neodymium (Nd II) using high resolution spectroscopic data obtained by Fourier transform spectrograph (FTS). Based on the Local Thermodynamical Equilibrium (LTE) assumption, a new value of Nd abundance is derived. We succeed to select of 51 solar Nd II lines with accurate transition probabilities measured experimentally by Den Hartog et al. (2003) and with accurate damping parameters are determined from literature. Relying on atomic data of Opacity Distribution Function (ODF), we construct theoretical photospheric solar model. The mea…
During their main sequence evolution, massive stars can develop convective regions very close to their surface. These regions are caused by an opacity peak associated with iron ionization. Cantiello et al. (2009) found a possible connection between the presence of sub-photospheric convective motions and small scale stochastic velocities in the photosphere of early-type stars. This supports a physical mechanism where microturbulence is caused by waves that are triggered by subsurface convection zones. They further suggest that clumping in the inner parts of the winds of OB stars could be relate…
The solar photospheric magnetic field is highly structured owing to its interaction with the convective flows. Its local structure has a strong influence on the profiles of spectral lines not only by virtue of the Zeeman effect, but also through the modification of the thermodynamical structure (e.g. line weakening in hot small-scale magnetic structures). Many stars harbor surface magnetic fields comparable to or larger than the Sun at solar maximum. Therefore, a strong influence of the field on the surface convection and on spectral line profiles can be expected. We carried out 3D local-box M…
We present results from 2D radiation-hydrodynamical simulations of fully compressible convection for the surface layers of A-type stars with the ANTARES code. Spectroscopic indicators for photospheric convective velocity fields show a maximum of velocities near Teff ~8000 K. In that range the largest values are measured for the subgroup of Am stars. Thus far, no prognostic model, neither theoretical nor numerical, is able to exactly reproduce the line profiles of sharp line A and Am stars in that temperature range. In general, the helium abundance of A stars is not known from observations. Hen…
CONTEXT: In recent years, the solar chemical abundances have been studied in considerable detail because of discrepant values of solar metallicity inferred from different indicators, i.e., on the one hand, the "sub-solar" photospheric abundances resulting from spectroscopic chemical composition analyses with the aid of 3D hydrodynamical models of the solar atmosphere, and, on the other hand, the high metallicity inferred by helioseismology. AIMS: After investigating the solar oxygen abundance using a CO5BOLD 3D hydrodynamical solar model in previous work, we undertake a similar approach studyi…
The atmospheres of cool main-sequence stars are structured by convective flows from the convective envelope that penetrate the optically thin layers and lead to structuring of the stellar atmospheres analogous to solar granulation. The flows have considerable influence on the 3D structure of temperature and pressure and affect the profiles of spectral lines formed in the photosphere. For the set of six 3D radiative (M)HD simulations of cool main-sequence stars described in the first paper of this series, we analyse the near-surface layers. We aim at describing the properties of granulation of …
Relations between temperature, T, and optical depth, tau, are often used for describing the photospheric transition from optically thick to optically thin in stellar structure models. We show that this is well justified, but also that currently used T(tau) relations are often inconsistent with their implementation. As an outer boundary condition on the system of stellar structure equations, T(tau) relations have an undue effect on the overall structure of stars. In this age of precision asteroseismology, we need to re-assess both the method for computing and for implementing T(tau) relations, …
On the basis of observations of solar granulation obtained with the New Solar Telescope (NST) of Big Bear Solar Observatory, we explored proper motion of bright points (BPs) in a quiet sun area, a coronal hole, and an active region plage. We automatically detected and traced bright points (BPs) and derived their mean-squared displacements as a function of time (starting from the appearance of each BP) for all available time intervals. In all three magnetic environments, we found the presence of a super-diffusion regime, which is the most pronounced inside the time interval of 10-300 seconds. S…
Convection plays a central role in the dynamics of any stellar interior, and yet its operation remains largely-hidden from direct observation. As a result, much of our understanding concerning stellar convection necessarily derives from theoretical and computational models. The Sun is, however, exceptional in that regard. The wealth of observational data afforded by its proximity provides a unique testbed for comparing convection models against observations. When such comparisons are carried out, surprising inconsistencies between those models and observations become apparent. Both photospheri…
The Sun is replete with magnetic fields, with sunspots, pores and plage regions being their most prominent representatives on the solar surface. But even far away from these active regions, magnetic fields are ubiquitous. To a large extent, their importance for the thermodynamics in the solar photosphere is determined by the total magnetic flux. Whereas in low-flux quiet Sun regions, magnetic structures are shuffled around by the motion of granules, the high-flux areas like sunspots or pores effectively suppress convection, leading to a temperature decrease of up to 3000 K. The importance of m…
Magnetic flux generated and intensified by the solar dynamo emerges into the solar atmosphere, forming active regions (ARs) including sunspots. Existing theories of flux emergence suggest that the magnetic flux can rise buoyantly through the convection zone but is trapped at the photosphere, while its further rising into the atmosphere resorts to the Parker buoyancy instability. To trigger such an instability, the Lorentz force in the photosphere needs to be as large as the gas pressure gradient to hold up an extra amount of mass against gravity. This naturally results in a strongly non-force-…
Observations of solar and stellar flares have revealed the presence of power law dependences between the flare energy and the time interval between flares. Various models have been proposed to explain these dependences, and to explain the numerical value of the power law indices. Here, we propose a model in which convective flows in granules force the foot-points of coronal magnetic loops, which are frozen-in to photospheric gas, to undergo a random walk. In certain conditions, this can lead to a twist in the loop, which drives the loop unstable if the twist exceeds a critical value. The possi…
We study the process of magnetic field annihilation and reconnection in simulations of magnetised solar photosphere and chromosphere with magnetic fields of opposite polarities and constant numerical resistivity. Exact analytical solutions for reconnective annihilations are used to interpret the features of magnetic reconnection in simulations of flux cancellation in the solar atmosphere. We use MURaM high-resolution photospheric radiative magneto-convection simulations to demonstrate the presence of magnetic field reconnection consistent with the magnetic flux pile-up models. Also, a simulate…
Meridional flow is thought to play a very important role in the dynamics of the solar convection zone; however, because of its relatively small amplitude, precisely measuring it poses a significant challenge. Here we present a complete time-distance helioseismic analysis of about two years of ground-based GONG Doppler data to retrieve the meridional circulation profile for modest latitudes, in an attempt to corroborate results from other studies. We use an empirical correction to the travel times due to an unknown center-to-limb systematic effect. The helioseismic inversion procedure is first …
In this work, we present a photospheric solar silicon abundance derived using CO5BOLD model atmospheres and the LINFOR3D spectral synthesis code. Previous works have differed in their choice of a spectral line sample and model atmosphere as well as their treatment of observational material, and the solar silicon abundance has undergone a downward revision in recent years. We additionally show the effects of the chosen line sample, broadening due to velocity fields, collisional broadening, model spatial resolution, and magnetic fields. CO5BOLD model atmospheres for the Sun were used in conjunct…
We model the solar horizontal velocity power spectrum at scales larger than granulation using a two-component approximation to the mass continuity equation. The model takes four times the density scale height as the integral (driving) scale of the vertical motions at each depth. Scales larger than this decay with height from the deeper layers. Those smaller are assumed to follow a Kolomogorov turbulent cascade, with the total power in the vertical convective motions matching that required to transport the solar luminosity in a mixing length formulation. These model components are validated usi…
In the last decade, the photospheric abundances of the Sun had been revised several times by many observers. The standard solar models (SSM) constructed with the new low-metal abundances disagree with helioseismic results and detected neutrino fluxes. The solar model problem has been puzzled some stellar physicists for more than ten years. Rotation, enhanced diffusion, convection overshoot, and magnetic fields are used to reconcile the new abundances with helioseismology. The \textbf{too} low-helium \textbf{subsurface abundance} in enhanced diffusion models can be improved by the mixing caused…
Simulating the photospheric to coronal plasma using magnetohydrodynamic characteristics III: validation including gravity, flux emergence, and an eruption
National Solar Observatory（国家太阳观测台）；NASA Goddard Space Flight Center（美国宇航局戈达德太空飞行中心）；U.S. Naval Research Laboratory（美国海军研究实验室）
