261 research outputs found
Excitation of acoustic waves by vortices in the quiet Sun
Five-minutes oscillations is one of the basic properties of solar convection.
Observations show mixture of a large number of acoustic wave fronts propagating
from their sources. We investigate the process of acoustic waves excitation
from the point of view of individual events, by using realistic 3D radiative
hydrodynamic simulation of the quiet Sun. The results show that the excitation
events are related to dynamics vortex tubes (or swirls) in the intergranular
lanes. These whirlpool-like flows are characterized by very strong horizontal
velocities (7 - 11 km/s) and downflows (~ 7 km/s), and are accompanied by
strong decreases of the temperature, density and pressure at the surface and in
a ~ 0.5-1 Mm deep layer below the surface. High-speed whirlpool flows can
attract and capture other vortices. According to our simulation results, the
processes of the vortex interaction, such as vortex annihilation, can cause the
excitation of acoustic waves.Comment: 10 pages, 5 figure, submitted to ApJ
Explanation of the sea-serpent magnetic structure of sunspot penumbrae
Recent spectro-polarimetric observations of a sunspot showed the formation of
bipolar magnetic patches in the mid penumbra and their propagation toward the
outer penumbral boundary. The observations were interpreted as being caused by
sea-serpent magnetic fields near the solar surface (Sainz Dalda & Bellot Rubio
2008). In this Letter, we develop a 3D radiative MHD numerical model to explain
the sea-serpent structure and the wave-like behavior of the penumbral magnetic
field lines. The simulations reproduce the observed behavior, suggesting that
the sea-serpent phenomenon is a consequence of magnetoconvection in a strongly
inclined magnetic field. It involves several physical processes: filamentary
structurization, high-speed overturning convective motions in strong, almost
horizontal magnetic fields with partially frozen field lines, and traveling
convective waves. The results demonstrate a correlation of the bipolar magnetic
patches with high-speed Evershed downflows in the penumbra. This is the first
time that a 3D numerical model of the penumbra results in downward directed
magnetic fields, an essential ingredient of sunspot penumbrae that has eluded
explanation until now.Comment: 9 pages, 3 figures, submitted to ApJ Letter
Dynamics of Magnetized Vortex Tubes in the Solar Chromosphere
We use 3D radiative MHD simulations to investigate the formation and dynamics
of small-scale (less than 0.5 Mm in diameter) vortex tubes spontaneously
generated by turbulent convection in quiet-Sun regions with initially weak mean
magnetic fields. The results show that the vortex tubes penetrate into the
chromosphere and substantially affect the structure and dynamics of the solar
atmosphere. The vortex tubes are mostly concentrated in intergranular lanes and
are characterized by strong (near sonic) downflows and swirling motions that
capture and twist magnetic field lines, forming magnetic flux tubes that expand
with height and which attain magnetic field strengths ranging from 200 G in the
chromosphere to more than 1 kG in the photosphere. We investigate in detail the
physical properties of these vortex tubes, including thermodynamic properties,
flow dynamics, and kinetic and current helicities, and conclude that magnetized
vortex tubes provide an important path for energy and momentum transfer from
the convection zone into the chromosphere.Comment: Submitted to the Astrophysical Journal Letters, 14 pages, 5 figure
Prediction of Sunspot Cycles by Data Assimilation Method
Despite the known general properties of the solar cycles, a reliable forecast
of the 11-year sunspot number variations is still a problem. The difficulties
are caused by the apparent chaotic behavior of the sunspot numbers from cycle
to cycle and by the influence of various turbulent dynamo processes, which are
far from understanding. For predicting the solar cycle properties we make an
initial attempt to use the Ensemble Kalman Filter (EnKF), a data assimilation
method, which takes into account uncertainties of a dynamo model and
measurements, and allows to estimate future observational data. We present the
results of forecasting of the solar cycles obtained by the EnKF method in
application to a low-mode nonlinear dynamical system modeling the solar
-dynamo process with variable magnetic helicity. Calculations of
the predictions for the previous sunspot cycles show a reasonable agreement
with the actual data. This forecast model predicts that the next sunspot cycle
will be significantly weaker (by ) than the previous cycle,
continuing the trend of low solar activity.Comment: 10 pages, 3 figure
Mechanism of spontaneous formation of stable magnetic structures on the Sun
One of the puzzling features of solar magnetism is formation of long-living
compact magnetic structures; such as sunspots and pores, in the highly
turbulent upper layer of the solar convective zone. We use realistic radiative
3D MHD simulations to investigate the interaction between magnetic field and
turbulent convection. In the simulations, a weak vertical uniform magnetic
field is imposed in a region of fully developed granular convection; and the
total magnetic flux through the top and bottom boundaries is kept constant. The
simulation results reveal a process of spontaneous formation of stable magnetic
structures, which may be a key to understanding of the magnetic
self-organization on the Sun and formation of pores and sunspots. This process
consists of two basic steps: 1) formation of small-scale filamentary magnetic
structures associated with concentrations of vorticity and whirlpool-type
motions, and 2) merging of these structures due to the vortex attraction,
caused by converging downdrafts around magnetic concentration below the
surface. In the resulting large-scale structure maintained by the converging
plasma motions, the magnetic field strength reaches ~1.5 kG at the surface and
~6 kG in the interior; and the surface structure resembles solar pores. The
magnetic structure remains stable for the whole simulation run of several hours
with no sign of decay.Comment: 13 pages, 4 figures, submitted to the Astrophysical Journa
Using Realistic MHD Simulations for Modeling and Interpretation of Quiet-Sun Observations with the Solar Dynamics Observatory Helioseismic and Magnetic Imager
The solar atmosphere is extremely dynamic, and many important phenomena
develop on small scales that are unresolved in observations with the
Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics
Observatory (SDO). For correct calibration and interpretation of the
observations, it is very important to investigate the effects of small-scale
structures and dynamics on the HMI observables, such as Doppler shift,
continuum intensity, spectral line depth, and width. We use 3D radiative
hydrodynamics simulations of the upper turbulent convective layer and the
atmosphere of the Sun, and a spectro-polarimetric radiative transfer code to
study observational characteristics of the Fe I 6173A line observed by HMI in
quiet-Sun regions. We use the modeling results to investigate the sensitivity
of the line Doppler shift to plasma velocity, and also sensitivities of the
line parameters to plasma temperature and density, and determine effective line
formation heights for observations of solar regions located at different
distances from the disc center. These estimates are important for the
interpretation of helioseismology measurements. In addition, we consider
various center-to-limb effects, such as convective blue-shift, variations of
helioseismic travel-times, and the 'concave' Sun effect, and show that the
simulations can qualitatively reproduce the observed phenomena, indicating that
these effects are related to a complex interaction of the solar dynamics and
radiative transfer.Comment: 21 pages, 10 figures, accepted for publication in Ap
Cluster Analysis of IRIS Spectroscopic Line Profiles and SDO/AIA EUV Emission in Observations and RMHD Simulations of the Solar Atmosphere
Spatially-resolved observations from the IRIS and SDO/AIA satellites, especially when coupled with realistic 3D RMHD simulations, are a powerful tool for analysis of processes in the solar chromosphere, transition region, and corona. However, the complexity of the data makes understanding the observations and modeling results difficult. In this work, we apply unsupervised clustering algorithms for analysis of observational and synthetic chromospheric Mg II h&k 2796&2803 and transition region C II 1334&1335 line profiles observed by IRIS, and extreme ultraviolet (EUV) emission observed by SDO/AIA, for various types of problems. The synthetic line profiles are computed for simulations of the quiescent solar atmosphere (using the StellarBox and RH1.5 codes). The K-Means clustering algorithm is applied, and the selection of an optimal number of clusters is supported by the average silhouette width technique. We discuss applications of the line profile clustering method to 1) visualization of computational and observational spectroscopic imaging data; 2) understanding of evolutionary trends and behavior patterns of quiet Sun emission and during solar flares; and 3) recognition of heating events and shock waves
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