11 research outputs found
Solar magnetoconvection
In recent years the study of how magnetic fields interact with thermal convection in the Sun has made significant advances. These are largely due to the rapidly increasing computer power and its application to more physically relevant parameters regimes and to more realistic physics and geometry in numerical models. Here we present a survey of recent results following one line of investigations and discuss and compare the results of these with observed phenomena
Development of structure in pores and sunspots: flows around axisymmetric magnetic flux tubes
Flux elements, pores and sunspots form a family of magnetic features observed at the solar surface. As a first step towards developing a fully nonlinear model of the structure of these features and of the dynamics of their interaction with solar convection, we conduct numerical experiments on idealised axisymmetric flux tubes in a compressible convecting atmosphere in cylindrical boxes of radius up to eight times their depth. We find that the magnetic field strength of the flux tubes is roughly independent of both distance from the centre and the total flux content of the flux tube, but that the angle of inclination from the vertical of the field at the edge of the tube increases with flux content. In all our calculations, fluid motion converges on the flux tube at the surface. The results compare favourably with observations of pores; in contrast, large sunspots lie at the centre of an out-flowing moat cell. We conjecture that there is an inflow hidden beneath the penumbrae of large spots, and that this inflow is responsible for the remarkable longevity of such features
Complete Models of Axisymmetric Sunspots: Magnetoconvection with Coronal Heating
We present detailed results of numerical experiments into the nature of complete sunspots. The models
remain highly idealized but include fully nonlinear compressible magnetoconvection in an axisymmetric layer
that drives energy into an overlying, low-B plasma. We survey a range of parameters in which the resulting
magnetoconvection displays the formation of pore- and sunspot-like behavior and assess the coronal signatures
resulting from the energy generated by the magnetoconvection. The coronal heating is assumed to be a
result of the dissipation by an unspecified means of a fraction of the Poynting flux entering the corona. The
expected signatures in the EUV and soft X-ray bandpasses of the Transition Region and Coronal Explorer
and Yohkoh/SXT, respectively, are examined. This ad hoc coupling of the corona to the subphotospheric
region results in a dynamical behavior that is consistent with recent observational results. This agreement
demonstrates that even simple coupled modeling can lead to diagnostics for investigations of both subphotospheric
sunspot structures and coronal heating mechanisms
Turbulent Compressible Convection with Rotation - Penetration above a Convection Zone
We perform Large eddy simulations of turbulent compressible convection in
stellar-type convection zones by solving the Navi\'{e}r-Stokes equations in
three dimensions. We estimate the extent of penetration into the stable layer
above a stellar-type convection zone by varying the rotation rate
({\boldmath}), the inclination of the rotation vector () and
the relative stability () of the upper stable layer. The computational
domain is a rectangular box in an f-plane configuration and is divided into two
regions of unstable and stable stratification with the stable layer placed
above the convectively unstable layer. Several models have been computed and
the penetration distance into the stable layer above the convection zone is
estimated by determining the position where time averaged kinetic energy flux
has the first zero in the upper stable layer. The vertical grid spacing in all
the model is non-uniform, and is less in the upper region so that the flows are
better resolved in the region of interest. We find that the penetration
distance increases as the rotation rate increases for the case when the
rotation vector is aligned with the vertical axis. However, with the increase
in the stability of the upper stable layer, the upward penetration distance
decreases. Since we are not able to afford computations with finer resolution
for all the models, we compute a number of models to see the effect of
increased resolution on the upward penetration. In addition, we estimate the
upper limit on the upward convective penetration from stellar convective cores.Comment: Accepted for Publication in Asttrophysics & Space Scienc
Can we Determine Electric Fields and Poynting Fluxes from Vector Magnetograms and Doppler Measurements?
The availability of vector magnetogram sequences with sufficient accuracy and
cadence to estimate the time derivative of the magnetic field allows us to use
Faraday's law to find an approximate solution for the electric field in the
photosphere, using a Poloidal-Toroidal Decomposition (PTD) of the magnetic
field and its partial time derivative. Without additional information, however,
the electric field found from this technique is under-determined -- Faraday's
law provides no information about the electric field that can be derived the
gradient of a scalar potential. Here, we show how additional information in the
form of line-of-sight Doppler flow measurements, and motions transverse to the
line-of-sight determined with ad-hoc methods such as local correlation
tracking, can be combined with the PTD solutions to provide much more accurate
solutions for the solar electric field, and therefore the Poynting flux of
electromagnetic energy in the solar photosphere. Reliable, accurate maps of the
Poynting flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.Comment: Solar Physics, in press. 14 pages, 3 figure
Comparison of large-scale flows on the Sun measured by time-distance helioseismology and local correlation tracking technique
We present a direct comparison between two different techniques time-distance
helioseismology and a local correlation tracking method for measuring mass
flows in the solar photosphere and in a near-surface layer: We applied both
methods to the same dataset (MDI high-cadence Dopplergrams covering almost the
entire Carrington rotation 1974) and compared the results. We found that after
necessary corrections, the vector flow fields obtained by these techniques are
very similar. The median difference between directions of corresponding vectors
is 24 degrees, and the correlation coefficients of the results for mean zonal
and meridional flows are 0.98 and 0.88 respectively. The largest discrepancies
are found in areas of small velocities where the inaccuracies of the computed
vectors play a significant role. The good agreement of these two methods
increases confidence in the reliability of large-scale synoptic maps obtained
by them.Comment: 14 pages, 6 figures, just before acceptance in Solar Physic
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
Are Solar Active Regions with Major Flares More Fractal, Multifractal, or Turbulent than Others?
Multiple recent investigations of solar magnetic field measurements have
raised claims that the scale-free (fractal) or multiscale (multifractal)
parameters inferred from the studied magnetograms may help assess the eruptive
potential of solar active regions, or may even help predict major flaring
activity stemming from these regions. We investigate these claims here, by
testing three widely used scale-free and multiscale parameters, namely, the
fractal dimension, the multifractal structure function and its inertial-range
exponent, and the turbulent power spectrum and its power-law index, on a
comprehensive data set of 370 timeseries of active-region magnetograms (17,733
magnetograms in total) observed by SOHO's Michelson Doppler Imager (MDI) over
the entire Solar Cycle 23. We find that both flaring and non-flaring active
regions exhibit significant fractality, multifractality, and non-Kolmogorov
turbulence but none of the three tested parameters manages to distinguish
active regions with major flares from flare-quiet ones. We also find that the
multiscale parameters, but not the scale-free fractal dimension, depend
sensitively on the spatial resolution and perhaps the observational
characteristics of the studied magnetograms. Extending previous works, we
attribute the flare-forecasting inability of fractal and multifractal
parameters to i) a widespread multiscale complexity caused by a possible
underlying self-organization in turbulent solar magnetic structures, flaring
and non-flaring alike, and ii) a lack of correlation between the fractal
properties of the photosphere and overlying layers, where solar eruptions
occur. However useful for understanding solar magnetism, therefore, scale-free
and multiscale measures may not be optimal tools for active-region
characterization in terms of eruptive ability or, ultimately,for major
solar-flare prediction.Comment: 25 pages, 7 figures, 2 tables, Solar Phys., in pres
Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review)
Mechanisms of the formation and stability of sunspots are among the
longest-standing and intriguing puzzles of solar physics and astrophysics.
Sunspots are controlled by subsurface dynamics hidden from direct observations.
Recently, substantial progress in our understanding of the physics of the
turbulent magnetized plasma in strong-field regions has been made by using
numerical simulations and local helioseismology. Both the simulations and
helioseismic measurements are extremely challenging, but it becomes clear that
the key to understanding the enigma of sunspots is a synergy between models and
observations. Recent observations and radiative MHD numerical models have
provided a convincing explanation to the Evershed flows in sunspot penumbrae.
Also, they lead to the understanding of sunspots as self-organized magnetic
structures in the turbulent plasma of the upper convection zone, which are
maintained by a large-scale dynamics. Local helioseismic diagnostics of
sunspots still have many uncertainties, some of which are discussed in this
review. However, there have been significant achievements in resolving these
uncertainties, verifying the basic results by new high-resolution observations,
testing the helioseismic techniques by numerical simulations, and comparing
results obtained by different methods. For instance, a recent analysis of
helioseismology data from the Hinode space mission has successfully resolved
several uncertainties and concerns (such as the inclined-field and phase-speed
filtering effects) that might affect the inferences of the subsurface
wave-speed structure of sunspots and the flow pattern. It becomes clear that
for the understanding of the phenomenon of sunspots it is important to further
improve the helioseismology methods and investigate the whole life cycle of
active regions, from magnetic-flux emergence to dissipation.Comment: 34 pages, 18 figures, submitted to Solar Physic