54 research outputs found
Structure and Evolution of Giant Cells in Global Models of Solar Convection
The global scales of solar convection are studied through three-dimensional
simulations of compressible convection carried out in spherical shells of
rotating fluid which extend from the base of the convection zone to within 15
Mm of the photosphere. Such modelling at the highest spatial resolution to date
allows study of distinctly turbulent convection, revealing that coherent
downflow structures associated with giant cells continue to play a significant
role in maintaining the strong differential rotation that is achieved. These
giant cells at lower latitudes exhibit prograde propagation relative to the
mean zonal flow, or differential rotation, that they establish, and retrograde
propagation of more isotropic structures with vortical character at mid and
high latitudes. The interstices of the downflow networks often possess strong
and compact cyclonic flows. The evolving giant-cell downflow systems can be
partly masked by the intense smaller scales of convection driven closer to the
surface, yet they are likely to be detectable with the helioseismic probing
that is now becoming available. Indeed, the meandering streams and varying
cellular subsurface flows revealed by helioseismology must be sampling
contributions from the giant cells, yet it is difficult to separate out these
signals from those attributed to the faster horizontal flows of
supergranulation. To aid in such detection, we use our simulations to describe
how the properties of giant cells may be expected to vary with depth, how their
patterns evolve in time, and analyze the statistical features of correlations
within these complex flow fields.Comment: 22 pages, 16 figures (color figures are low res), uses emulateapj.cls
Latex class file, Results shown during a Press release at the AAS meeting in
June 2007. Submitted to Ap
Solar Multi-Scale Convection and Rotation Gradients Studied in Shallow Spherical Shells
The differential rotation of the sun, as deduced from helioseismology,
exhibits a prominent radial shear layer near the top of the convection zone
wherein negative radial gradients of angular velocity are evident in the low-
and mid-latitude regions spanning the outer 5% of the solar radius.
Supergranulation and related scales of turbulent convection are likely to play
a significant role in the maintenance of such radial gradients, and may
influence dynamics on a global scale in ways that are not yet understood. To
investigate such dynamics, we have constructed a series of three-dimensional
numerical simulations of turbulent compressible convection within spherical
shells, dealing with shallow domains to make such modeling computationally
tractable. These simulations are the first models of solar convection in a
spherical geometry that can explicitly resolve both the largest dynamical
scales of the system (of order the solar radius) as well as smaller-scale
convective overturning motions comparable in size to solar supergranulation
(20--40 Mm). We find that convection within these simulations spans a large
range of horizontal scales, and that the radial angular velocity gradient in
these models is typically negative, especially in low- and mid-latitude
regions. Analyses of the angular momentum transport indicates that such
gradients are maintained by Reynolds stresses associated with the convection,
transporting angular momentum inward to balance the outward transport achieved
by viscous diffusion and large-scale flows in the meridional plane. We suggest
that similar mechanisms associated with smaller-scale convection in the sun may
contribute to the maintenance of the observed radial shear layer located
immediately below the solar photosphere.Comment: 45 pages, 17 figures, ApJ in press. A preprint of paper with hi-res
figures can be found at
http://www-lcd.colorado.edu/~derosa/modelling/modelling.htm
Inferring Maps of the Sun's Far-side Unsigned Magnetic Flux from Far-side Helioseismic Images using Machine Learning Techniques
Accurate modeling of the Sun's coronal magnetic field and solar wind
structures require inputs of the solar global magnetic field, including both
the near and far sides, but the Sun's far-side magnetic field cannot be
directly observed. However, the Sun's far-side active regions are routinely
monitored by helioseismic imaging methods, which only require continuous
near-side observations. It is therefore both feasible and useful to estimate
the far-side magnetic-flux maps using the far-side helioseismic images despite
their relatively low spatial resolution and large uncertainties. In this work,
we train two machine-learning models to achieve this goal. The first
machine-learning training pairs simultaneous SDO/HMI-observed magnetic-flux
maps and SDO/AIA-observed EUV 304 images, and the resulting model can
convert 304 images into magnetic-flux maps. This model is then applied
on the STEREO/EUVI-observed far-side 304 images, available for about 4.3
years, for the far-side magnetic-flux maps. These EUV-converted magnetic-flux
maps are then paired with simultaneous far-side helioseismic images for a
second machine-learning training, and the resulting model can convert far-side
helioseismic images into magnetic-flux maps. These helioseismically derived
far-side magnetic-flux maps, despite their limitations in spatial resolution
and accuracy, can be routinely available on a daily basis, providing useful
magnetic information on the Sun's far side using only the near-side
observations.Comment: Accepted by Ap
Implications of Different Solar Photospheric Flux-Transport Models for Global Coronal and Heliospheric Modeling
The concept of surface-flux transport (SFT) is commonly used in evolving
models of the large-scale solar surface magnetic field. These photospheric
models are used to determine the large-scale structure of the overlying coronal
magnetic field, as well as to make predictions about the fields and flows that
structure the solar wind. We compare predictions from two SFT models for the
solar wind, open magnetic field footpoints, and the presence of coronal
magnetic null points throughout various phases of a solar activity cycle,
focusing on the months of April in even-numbered years between 2012 and 2020,
inclusive. We find that there is a solar cycle dependence to each of the
metrics considered, but there is not a single phase of the cycle in which all
the metrics indicate good agreement between the models. The metrics also reveal
large, transient differences between the models when a new active region is
rotating into the assimilation window. The evolution of the surface flux is
governed by a combination of large scale flows and comparatively small scale
motions associated with convection. Because the latter flows evolve rapidly,
there are intervals during which their impact on the surface flux can only be
characterized in a statistical sense, thus their impact is modeled by
introducing a random evolution that reproduces the typical surface flux
evolution. We find that the differences between the predicted properties are
dominated by differences in the model assumptions and implementation, rather
than selection of a particular realization of the random evolution.Comment: Accepted for publication in The Astrophysical Journa
A Method for Data-Driven Simulations of Evolving Solar Active Regions
We present a method for performing data-driven simulations of solar active
region formation and evolution. The approach is based on magnetofriction, which
evolves the induction equation assuming the plasma velocity is proportional to
the Lorentz force. The simulations of active region coronal field are driven by
temporal sequences of photospheric magnetograms from the Helioseismic Magnetic
Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). Under
certain conditions, the data-driven simulations produce flux ropes that are
ejected from the modeled active region due to loss of equilibrium. Following
the ejection of flux ropes, we find an enhancement of the photospheric
horizontal field near the polarity inversion line. We also present a method for
the synthesis of mock coronal images based on a proxy emissivity calculated
from the current density distribution in the model. This method yields mock
coronal images that are somewhat reminiscent of images of active regions taken
by instruments such as SDO's Atmospheric Imaging Assembly (AIA) at extreme
ultraviolet wavelengths.Comment: Accepted to ApJ; comments/questions related to this article are
welcome via e-mail, even after publicatio
Global MHD Simulations of the Time-Dependent Corona
We describe, test, and apply a technique to incorporate full-sun, surface
flux evolution into an MHD model of the global solar corona. Requiring only
maps of the evolving surface flux, our method is similar to that of Lionello et
al. (2013), but we introduce two ways to correct the electric field at the
lower boundary to mitigate spurious currents. We verify the accuracy of our
procedures by comparing to a reference simulation, driven with known flows and
electric fields. We then present a thermodynamic MHD calculation lasting one
solar rotation driven by maps from the magnetic flux evolution model of
Schrijver & DeRosa (2003). The dynamic, time-dependent nature of the model
corona is illustrated by examining the evolution of the open flux boundaries
and forward modeled EUV emission, which evolve in response to surface flows and
the emergence and cancellation flux. Although our main goal is to present the
method, we briefly investigate the relevance of this evolution to properties of
the slow solar wind, examining the mapping of dipped field lines to the
topological signatures of the "S-Web" and comparing charge state ratios
computed in the time-dependently driven run to a steady state equivalent.
Interestingly, we find that driving on its own does not significantly improve
the charge states ratios, at least in this modest resolution run that injects
minimal helicity. Still, many aspects of the time-dependently driven model
cannot be captured with traditional steady-state methods, and such a technique
may be particularly relevant for the next generation of solar wind and CME
models
Large-Scale Spatial Cross-Calibration of Hinode/SOT-SP and SDO/HMI
We investigate the cross-calibration of the Hinode/SOT-SP and SDO/HMI
instrument meta-data, specifically the correspondence of the scaling and
pointing information. Accurate calibration of these datasets gives the
correspondence needed by inter-instrument studies and learning-based
magnetogram systems, and is required for physically-meaningful photospheric
magnetic field vectors. We approach the problem by robustly fitting geometric
models on correspondences between images from each instrument's pipeline. This
technique is common in computer vision, but several critical details are
required when using scanning slit spectrograph data like Hinode/SOT-SP. We
apply this technique to data spanning a decade of the Hinode mission. Our
results suggest corrections to the published Level 2 Hinode/SOT-SP data. First,
an analysis on approximately 2,700 scans suggests that the reported pixel size
in Hinode/SOT-SP Level 2 data is incorrect by around 1%. Second, analysis of
over 12,000 scans show that the pointing information is often incorrect by
dozens of arcseconds with a strong bias. Regression of these corrections
indicates that thermal effects have caused secular and cyclic drift in
Hinode/SOT-SP pointing data over its mission. We offer two solutions. First,
direct co-alignment with SDO/HMI data via our procedure can improve alignments
for many Hinode/SOT-SP scans. Second, since the pointing errors are
predictable, simple post-hoc corrections can substantially improve the
pointing. We conclude by illustrating the impact of this updated calibration on
derived physical data products needed for research and interpretation. Among
other things, our results suggest that the pointing errors induce a hemispheric
bias in estimates of radial current density.Comment: Under revisions at ApJ
First 3D Reconstructions of Coronal Loops with the STEREO A+B Spacecraft: IV. Magnetic Modeling with Twisted Force-Free Fields
The three-dimensional (3D) coordinates of stereoscopically triangulated loops
provide strong constraints for magnetic field models of active regions in the
solar corona. Here we use STEREO/A and B data from some 500 stereoscopically
triangulated loops observed in four active regions (2007 Apr 30, May 9, May 19,
Dec 11), together with SOHO/MDI line-of-sight magnetograms. We measure the
average misalignment angle between the stereoscopic loops and theoretical
magnetic field models, finding a mismatch of for a
potential field model, which is reduced to for a
non-potential field model parameterized by twist parameters. The residual error
is commensurable with stereoscopic measurement errors (). We developed a potential field code that deconvolves a
line-of-sight magnetogram into three magnetic field components , as well as a non-potential field forward-fitting code that determines
the full length of twisted loops ( Mm), the number of twist
turns (median ), the nonlinear force-free -parameter
(median cm), and the current density
(median Mx cm s). All twisted loops are found
to be far below the critical value for kink instability, and Joule dissipation
of their currents is found be be far below the coronal heating requirement. The
algorithm developed here, based on an analytical solution of nonlinear
force-free fields that is accurate to second order (in the force-free parameter
), represents the first code that enables fast forward-fitting to
photospheric magnetograms and stereoscopically triangulated loops in the solar
corona.Comment: The Astrophysical Journal (in press), 37 pages, 14 Figure
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