47 research outputs found
Modeling the Subsurface Structure of Sunspots
While sunspots are easily observed at the solar surface, determining their
subsurface structure is not trivial. There are two main hypotheses for the
subsurface structure of sunspots: the monolithic model and the cluster model.
Local helioseismology is the only means by which we can investigate
subphotospheric structure. However, as current linear inversion techniques do
not yet allow helioseismology to probe the internal structure with sufficient
confidence to distinguish between the monolith and cluster models, the
development of physically realistic sunspot models are a priority for
helioseismologists. This is because they are not only important indicators of
the variety of physical effects that may influence helioseismic inferences in
active regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In this paper,
we provide a critical review of the existing sunspot models and an overview of
numerical methods employed to model wave propagation through model sunspots. We
then carry out an helioseismic analysis of the sunspot in Active Region 9787
and address the serious inconsistencies uncovered by
\citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find
that this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model) and that
travel-time measurements are consistent with a horizontal outflow in the
surrounding moat.Comment: 73 pages, 19 figures, accepted by Solar Physic
Quenching of Meridional Circulation in Flux Transport Dynamo Models
Guided by the recent observational result that the meridional circulation of
the Sun becomes weaker at the time of the sunspot maximum, we have included a
parametric quenching of the meridional circulation in solar dynamo models such
that the meridional circulation becomes weaker when the magnetic field at the
base of the convection zone is stronger. We find that a flux transport solar
dynamo tends to become unstable on including this quenching of meridional
circulation if the diffusivity in the convection zone is less than about 2 *
10^{11} cm^2/s. The quenching of alpha, however, has a stabilizing effect and
it is possible to stabilize a dynamo with low diffusivity with sufficiently
strong alpha-quenching. For dynamo models with high diffusivity, the quenching
of meridional circulation does not produce a large effect and the dynamo
remains stable. We present a solar-like solution from a dynamo model with
diffusivity 2.8 * 10^{12} cm^2/s in which the quenching of meridional
circulation makes the meridional circulation vary periodically with solar cycle
as observed and does not have any other significant effect on the dynamo.Comment: Consistent with the published version. Solar Physics, in pres
Radiative Cooling in MHD Models of the Quiet Sun Convection Zone and Corona
We present a series of numerical simulations of the quiet Sun plasma threaded
by magnetic fields that extend from the upper convection zone into the low
corona. We discuss an efficient, simplified approximation to the physics of
optically thick radiative transport through the surface layers, and investigate
the effects of convective turbulence on the magnetic structure of the Sun's
atmosphere in an initially unipolar (open field) region. We find that the net
Poynting flux below the surface is on average directed toward the interior,
while in the photosphere and chromosphere the net flow of electromagnetic
energy is outward into the solar corona. Overturning convective motions between
these layers driven by rapid radiative cooling appears to be the source of
energy for the oppositely directed fluxes of electromagnetic energy.Comment: 20 pages, 5 figures, Solar Physics, in pres
As(III) Sorption on Nanostructured Rutile, Prepared by High-Energy Milling
In this work, we studied the sorption properties of nanostructured TiO2 of rutile modification before and after high-energy milling with respect to As(III) ions without irradiation and under the action of UV radiation.Исследование поддержано проектом РНФ 21-73-20039
Large-Eddy Simulations of Magnetohydrodynamic Turbulence in Heliophysics and Astrophysics
We live in an age in which high-performance computing is transforming the way we do science. Previously intractable problems are now becoming accessible by means of increasingly realistic numerical simulations. One of the most enduring and most challenging of these problems is turbulence. Yet, despite these advances, the extreme parameter regimes encountered in space physics and astrophysics (as in atmospheric and oceanic physics) still preclude direct numerical simulation. Numerical models must take a Large Eddy Simulation (LES) approach, explicitly computing only a fraction of the active dynamical scales. The success of such an approach hinges on how well the model can represent the subgrid-scales (SGS) that are not explicitly resolved. In addition to the parameter regime, heliophysical and astrophysical applications must also face an equally daunting challenge: magnetism. The presence of magnetic fields in a turbulent, electrically conducting fluid flow can dramatically alter the coupling between large and small scales, with potentially profound implications for LES/SGS modeling. In this review article, we summarize the state of the art in LES modeling of turbulent magnetohydrodynamic (MHD) ows. After discussing the nature of MHD turbulence and the small-scale processes that give rise to energy dissipation, plasma heating, and magnetic reconnection, we consider how these processes may best be captured within an LES/SGS framework. We then consider several special applications in heliophysics and astrophysics, assessing triumphs, challenges,and future directions
The Origin, Early Evolution and Predictability of Solar Eruptions
Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt
Nonstoichiometric transition metal carbides - synthesis, properties and potential areas of application
In this paper the differences of properties of stoichiometric and nonstoichiometric IVB- and VB-Group carbides will be discussed. On the base of newer investigations of their physical and chemical properties it seems to be possible to improve material properties by using them instead of stoichiometric ones in some applications