-
[show abstract]
[hide abstract]
ABSTRACT: The objective testing of algorithms for performing ambiguity resolution in vector magnetic field data is continued, with an
examination of the effects of noise in the data. Through the use of analytic magnetic field models, two types of noise are
“added” prior to resolving: noise to simulate Poisson photon noise in the observed polarization spectra, and a spatial binning
to simulate the effects of unresolved structure. The results are compared through the use of quantitative metrics and performance
maps. We find that while no algorithm severely propagates the effects of Poisson noise beyond very local influences, some
algorithms are more robust against high photon-noise levels than others. In the case of limited spatial resolution, loss of
information regarding fine-scale structure can easily result in erroneous solutions. Our tests imply that photon noise and
limited spatial resolution can act so as to make assumptions used in some ambiguity resolution algorithms no longer consistent
with the observed magnetogram. We confirm a finding of the earlier comparison study that results can be very sensitive to
the details of the treatment of the observed boundary and the assumptions governing that treatment. We discuss the implications
of these findings, given the relative sensitivities of the algorithms to the two sources of noise tested here. We also touch
on further implications for interpreting observational vector magnetic field data for general solar physics research.
KeywordsData analysis-Polarimetry-Sun: magnetic field-Sun: observations
Solar Physics 04/2012; 260(1):83-108. · 2.78 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We address points recently discussed in Georgoulis (2011) in reference to
Leka et al. (2009b). Most importantly, we find that the results of Georgoulis
(2011) support a conclusion of Leka et al. (2009b): that limited spatial
resolution and the presence of unresolved magnetic structures can challenge
ambiguity- resolution algorithms. Moreover, the findings of both Metcalf et al.
(2006) and Leka et al. (2009b) are confirmed in Georgoulis (2011): a method's
performance can be diminished when the observed field fails to conform to that
method's assumptions. The implication of boundaries in models of solar magnetic
structures is discussed; we confirm that the distribution of the field
components in the model used in Leka et al. (2009b) is closer to what is
observed on the Sun than what is proposed in Georgoulis (2011). It is also
shown that method does matter with regards to simulating limited spatial
resolution and avoiding an inadvertent introduction of bias. Finally, the
assignment of categories to data- analysis algorithms is revisited; we argue
that assignments are only useful and elucidating when used appropriately.
10/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate the evolution of coronal loop emission in the context of the coronal magnetic field topology. New modeling techniques allow us to investigate the magnetic field structure and energy release in active regions. Using these models and high resolution multi-wavelength coronal observations from the Transition Region and Coronal Explorer (TRACE) and the X-ray Telescope (XRT) on Hinode, we are able to establish a relationship between the light curves of coronal loops and their associated magnetic topologies for NOAA Active Region 10963. We examine loops that show both transient and steady emission, and we find that loops that show many transient brightenings are located in domains associated with a high number of separators. This topology provides an environment for continual impulsive heating events through magnetic reconnection at the separators. A loop with relatively constant X-ray and EUV emission, on the other hand, is located in domains that are not associated with separators. This result implies that larger-scale magnetic field reconnections are not involved in heating plasma in these regions, and the heating in these loops must come from another mechanism, such as small-scale reconnections (i.e., nanoflares) or wave heating. Additionally, we find that loops that undergo repeated transient brightenings are associated with separators that have enhanced free energy. In contrast, we find one case of an isolated transient brightening that seems to be associated with separators with a smaller free energy. Comment: 36 pages, 11 figures, To appear in Astrophysical Journal
09/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We investigate how the divergence-free property of magnetic fields can be exploited to resolve the azimuthal ambiguity present in solar vector magnetogram data, by using line-of-sight and horizontal heliographic derivative information as approximated from discrete measurements. Using synthetic data we test several methods that each make different assumptions about how the divergence-free property can be used to resolve the ambiguity. We find that the most robust algorithm involves the minimisation of the absolute value of the divergence summed over the entire field of view. Away from disk centre this method requires the sign and magnitude of the line-of-sight derivatives of all three components of the magnetic field vector. Comment: Solar Physics, in press, 20 pages, 11 figures
11/2009;
-
Marc L. DeRosa,
Carolus J. Schrijver, Graham Barnes,
K. D. Leka,
Bruce W. Lites,
Markus J. Aschwanden,
Tahar Amari,
Aurelien Canou,
James M. McTiernan,
Stephane Regnier,
Julia K. Thalmann,
Gherardo Valori,
Michael S. Wheatland,
Thomas Wiegelmann,
Mark C. M. Cheung,
Paul A. Conlon,
Marcel Fuhrmann,
Bernd Inhester,
Tilaye Tadesse
[show abstract]
[hide abstract]
ABSTRACT: Nonlinear force-free field (NLFFF) models are thought to be viable tools for
investigating the structure, dynamics and evolution of the coronae of solar
active regions. In a series of NLFFF modeling studies, we have found that NLFFF
models are successful in application to analytic test cases, and relatively
successful when applied to numerically constructed Sun-like test cases, but
they are less successful in application to real solar data. Different NLFFF
models have been found to have markedly different field line configurations and
to provide widely varying estimates of the magnetic free energy in the coronal
volume, when applied to solar data. NLFFF models require consistent, force-free
vector magnetic boundary data. However, vector magnetogram observations
sampling the photosphere, which is dynamic and contains significant Lorentz and
buoyancy forces, do not satisfy this requirement, thus creating several major
problems for force-free coronal modeling efforts. In this article, we discuss
NLFFF modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process illustrate
the three such issues we judge to be critical to the success of NLFFF modeling:
(1) vector magnetic field data covering larger areas are needed so that more
electric currents associated with the full active regions of interest are
measured, (2) the modeling algorithms need a way to accommodate the various
uncertainties in the boundary data, and (3) a more realistic physical model is
needed to approximate the photosphere-to-corona interface in order to better
transform the forced photospheric magnetograms into adequate approximations of
nearly force-free fields at the base of the corona. We make recommendations for
future modeling efforts to overcome these as yet unsolved problems.
The Astrophysical Journal 02/2009; 696(2). · 6.02 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We compare a variety of nonlinear force-free field (NLFFF) extrapolation algorithms, including optimization, magneto-frictional,
and Grad – Rubin-like codes, applied to a solar-like reference model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly twisted, right helical flux bundle. The codes were applied
to both forced “photospheric” and more force-free “chromospheric” vector magnetic field boundary data derived from the model.
When applied to the chromospheric boundary data, the codes are able to recover the presence of the flux bundle and the field’s
free energy, though some details of the field connectivity are lost. When the codes are applied to the forced photospheric
boundary data, the reference model field is not well recovered, indicating that the combination of Lorentz forces and small
spatial scale structure at the photosphere severely impact the extrapolation of the field. Preprocessing of the forced photospheric
boundary does improve the extrapolations considerably for the layers above the chromosphere, but the extrapolations are sensitive
to the details of the numerical codes and neither the field connectivity nor the free magnetic energy in the full volume are
well recovered. The magnetic virial theorem gives a rapid measure of the total magnetic energy without extrapolation though,
like the NLFFF codes, it is sensitive to the Lorentz forces in the coronal volume. Both the magnetic virial theorem and the
Wiegelmann extrapolation, when applied to the preprocessed photospheric boundary, give a magnetic energy which is nearly equivalent
to the value derived from the chromospheric boundary, but both underestimate the free energy above the photosphere by at least
a factor of two. We discuss the interpretation of the preprocessed field in this context. When applying the NLFFF codes to
solar data, the problems associated with Lorentz forces present in the low solar atmosphere must be recognized: the various
codes will not necessarily converge to the correct, or even the same, solution.
Solar Physics 01/2008; 247(2):269-299. · 2.78 Impact Factor
-
Thomas R. Metcalf,
K. D. Leka, Graham Barnes,
Bruce W. Lites,
Manolis K. Georgoulis,
A. A. Pevtsov,
K. S. Balasubramaniam,
G. Allen Gary,
Ju Jing,
Jing Li,
Y. Liu,
H. N. Wang,
Valentyna Abramenko,
Vasyl Yurchyshyn,
Y.-J. Moon
[show abstract]
[hide abstract]
ABSTRACT: We report here on the present state-of-the-art in algorithms used for resolving the 180° ambiguity in solar vector magnetic
field measurements. With present observations and techniques, some assumption must be made about the solar magnetic field
in order to resolve this ambiguity. Our focus is the application of numerous existing algorithms to test data for which the
correct answer is known. In this context, we compare the algorithms quantitatively and seek to understand where each succeeds,
where it fails, and why. We have considered five basic approaches: comparing the observed field to a reference field or direction,
minimizing the vertical gradient of the magnetic pressure, minimizing the vertical current density, minimizing some approximation
to the total current density, and minimizing some approximation to the field's divergence. Of the automated methods requiring
no human intervention, those which minimize the square of the vertical current density in conjunction with an approximation
for the vanishing divergence of the magnetic field show the most promise.
Solar Physics 08/2006; 237(2):267-296. · 2.78 Impact Factor
