explanation blue bibcodes open ADS page with paths to full text
Author name code: barnes
ADS astronomy entries on 2022-09-14
=author:"Barnes, G." -aff:"NASA"
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Title: What do pre-event conditions of the upper solar atmosphere
tell us about potential flaring of active regions?
Authors: Dissauer, K.; Leka, K. D.; Barnes, G.; Wagner, E.
2021AAS...23812713D Altcode:
Although solar energetic events are powered by the evolution of the
underlying magnetic field, it is still impossible to deterministically
predict when an active region will flare or not solely based on this
information. Observational case studies of the solar chromosphere and
corona reveal increased levels of magnetic reorganization, dynamics and
temperature variation prior to solar energetic events, however whether
these activities play a role in event initiation is still unclear. <P
/>In order to investigate this question, we statistically analyze the
coronal and chromospheric conditions prior to solar flares and during
flare-quiet periods using data from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO). <P />We create
and use AIA Active Region Patches (AARPs), region-targeted extractions
of AIA time-series data in (extreme-) ultraviolet, matched to the HMI
Active Region Patches (HARPs), for 2010-2018. The pre-event dynamics
and heating of the upper solar atmosphere is characterized using
high-order moments to parameterize brightness images, running-difference
images as well as emission measure, temperature, and density images,
derived from Differential Emission Measure (DEM) analysis. The temporal
behavior is captured by the slope and intercept of a linear fit over
a 7hr time-series of each parameter. <P />The NWRA Classification
Infrastructure (NCI), a well-established statistical classifier system
based on Non-Parametric Discriminant Analysis, and standard skill
scores are used to statistically evaluate if parameters describing
the pre-event conditions significantly differ for flaring-imminent
vs. flare-quiet populations. Early results and their physical
implications will be presented. <P />We note that AARPs present a
newly developed AIA data product which will be freely available to the
scientific community later in 2021. AARPs are presently constructed
daily, from 15:48-21:48 UT in 13 min intervals each hour with a time
cadence of 72 s, suitable for DEM Analysis. AARPs will be available
with the study's publication and at www.nwra.com/AARP
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Title: Enhancements to Hinode/SOT-SP Vector Magnetic Field Data
Products
Authors: DeRosa, M. L.; Leka, K. D.; Barnes, G.; Wagner, E.; Centeno,
R.; De Wijn, A.; Bethge, C.
2021AAS...23821305D Altcode:
The Solar Optical Telescope Spectro-Polarimeter (SOT-SP), on board the
Hinode spacecraft (launched in 2006), is a scanning-slit spectrograph
that continues to provide polarization spectra useful for inferring the
vector (three-component) magnetic field at the solar photosphere. SOT-SP
achieves this goal by obtaining line profiles of two magnetically
sensitive lines, namely the Fe I 6302 Angstrom doublet, using a
0.16"×164" slit as it scans a region of interest. Once the data are
merged, a Milne-Eddington based spectropolarimetric inversion scheme is
used to infer multiple physical parameters in the solar photosphere,
including the vector magnetic field, from the calibrated polarization
spectra. All of these data are publicly available once the processing
has occurred. <P />As of this year, the Hinode/SOT team is also making
available the disambiguated vector magnetic field and the re-projected
heliographic components of the field. In making the disambiguated vector
field data product, the 180° ambiguity in the plane-of-sky component
of the vector magnetic field inherent in the spectropolarimetric
inversion process has been resolved. This ambiguity is resolved
using the Minimum-Energy algorithm, which is the same algorithm used
within the pipeline producing the vector-magnetogram data product
for the Helioseismic and Magnetic Imager aboard the Solar Dynamics
Observatory. The heliographic field components (B<SUB>phi</SUB>,
B<SUB>theta</SUB>, B<SUB>r</SUB>) on the same grid as the inverted data
are also now provided. This poster provides more details about these
data product enhancements, and some examples on how the scientific
community may readily obtain these data.
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Title: On Measuring Divergence for Magnetic Field Modeling
Authors: Gilchrist, S. A.; Leka, K. D.; Barnes, G.; Wheatland, M. S.;
DeRosa, M. L.
2020ApJ...900..136G Altcode: 2020arXiv200808863G
A physical magnetic field has a divergence of zero. Numerical error
in constructing a model field and computing the divergence, however,
introduces a finite divergence into these calculations. A popular metric
for measuring divergence is the average fractional flux $\left\langle
| {f}_{i}| \right\rangle $ . We show that $\left\langle | {f}_{i}|
\right\rangle $ scales with the size of the computational mesh, and
may be a poor measure of divergence because it becomes arbitrarily
small for increasing mesh resolution, without the divergence actually
decreasing. We define a modified version of this metric that does
not scale with mesh size. We apply the new metric to the results of
DeRosa et al., who measured $\left\langle | {f}_{i}| \right\rangle
$ for a series of nonlinear force-free field models of the coronal
magnetic field based on solar boundary data binned at different spatial
resolutions. We compute a number of divergence metrics for the DeRosa et
al. data and analyze the effect of spatial resolution on these metrics
using a nonparametric method. We find that some of the trends reported
by DeRosa et al. are due to the intrinsic scaling of $\left\langle |
{f}_{i}| \right\rangle $ . We also find that different metrics give
different results for the same data set and therefore there is value
in measuring divergence via several metrics.
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Title: Pathways to Coronal Magnetic Energy Storage in The NOAA AR11283
Authors: Cavins, A.; Barnes, G.; Leka, K. D.; Gilchrist, S. A.
2019AGUFMSH31D3332C Altcode:
At the extreme end of the variability spectrum, powerful events
we call solar flares produce orders-of-magnitude increases
in the shorter-wavelength luminosity output on millisecond
time-scales. Although it is generally accepted that solar flares
occur through the release of energy stored in the coronal magnetic
field above an active region it is not well understood how much of
the stored energy will be released in a single event. When examined
with a large sample size, solar flares generally follow a power-law
distribution in size, although it should be noted that this may not
be the case for any individual active region. Such is the case for
NOAA AR11283 (at central meridian on 2011.09.06), which produced
multiple M and X-class flares with comparatively few smaller C class
flares. The objective of the ongoing research on this region is to
compare estimates of the magnetic energy stored by individual current
systems with the region's flaring history. The investigation heavily
focuses on studying energy of sub volumes in the region, identified
from spherical nonlinear force-free modelling, rather than the whole
region in an attempt to better understand the magnitude of single
re-connection events. A small total current along shorter field
lines generally does not store a large amount of magnetic energy,
but either current along longer field lines or a larger total amount
of current present in the individual system can lead to more magnetic
energy storage. These different situations of current size versus
current loop length can produce different distributions of energy
throughout the region. This material is based upon work supported by
the US National Science Foundation under Grant No. 1630454 and the REU
Program Award No. 1659878. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the authors and
do not necessarily reflect the views of the National Science Foundation.
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Title: The How and Why of Big Solar Flares
Authors: Isola, B.; Barnes, G.; Leka, K. D.; Gilchrist, S. A.
2019AGUFMSH31D3336I Altcode:
It is generally understood that the peak soft X-ray flux of solar
flares emanating from active regions follows a power-law spectrum of
magnitudes; however, it is not understood why the flares from some
active regions do not obviously exhibit this distribution. We take
here an approach to understand why this occurs, by combining modeling
and observation to study the energy reservoirs within a solar active
region and the pathway the energy takes to produce solar events. We
consider a complex active region, NOAA AR 11793 from July 19th, 2013,
that was expected to produce larger flares than the actual C-flares
observed. We modeled the coronal magnetic field using the CFITS
nonlinear force-free extrapolation code, then identified individual
current systems by starting from photospheric concentrations of current
and propagating those through the extrapolation volume. We estimated the
energy-release prospects of each current system as a measure of how much
energy might be released in a single reconnection event. We investigated
different ways of determining the current systems to investigate the
sensitivity of the results to the choice of current systems. We present
here results comparing the energy associated with the individual
current systems with the magnitude of the flares originating from
our region. <P />This material is based upon work supported by the
US National Science Foundation REU program under Award No. 1659878,
and NSF Grant No. 1630454. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the authors and
do not necessarily reflect the views of the National Science Foundation.
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Title: Effects of particular smoothing processes for global synoptic
maps on PFSS solutions
Authors: Hayashi, Keiji; Leka, K. D.; Barnes, G.
2019shin.confE.135H Altcode:
Recent space-based and ground-based solar observations produce
high-resolution synoptic maps that capture magnetic features at
various spatial scales, such as small magnetic elements, plages,
sunspot regions, as well as large-scale unipolar magnetic regions
(UMRs). Because high-resolution PFSS solutions are computationally
expensive and because the lifetimes of small-scale magnetic features
are much shorter than one Carrington rotation period, it is a common
practice to reduce the spatial resolution of synoptic maps to employ
lower-order PFSS solutions. <P />As demonstrated in our earlier
work [Hayashi et al., 2016], different size-reducing (smoothing)
methods often alter the PFSS solution substantially. For example, a
simple box-car averaging can suffer the so-called aliasing effect: A
averaging box can contain substantially different amount of total signed
flux than would the same-sized box but shifted by a few degrees in
longitude, in particular for regions in and near pairs of strong-field
sunspots. Such small-scale systematic differences in smoothed/resized
map can cause substantial differences in the resulting global-scale
PFSS solutions, such as the position and shape of the heliospheric
current sheet (HCS). The Gaussian-type smoothing method mitigates
such differences, although it eliminates several of the advantages
gained by using high-resolution observations in the first place. In
addition, the averaging/smoothing can alter the total unsigned fluxes
and horizontal gradients, in particular, in the strong-field sunspot
regions that are crucial for studies on energy build-up processes
and data-driven modeling. <P />We examine differences among the PFSS
solutions of the global solar corona with down-sampling methods
(primarily boxcar-averaging and Gaussian-function smoothing) and
that obtained with high-order PFSS solution using no down-sampling
or smoothing applied to the input Br map, for the target area of
the session. The advantages and disadvantages of different smoothing
methods will be evaluated and discussed. <P />This work is partially
supported by NASA HSWO2R Grant 80NSSC19K0007.
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Title: Photospheric Magnetic Field Properties of Flaring
vs. Flare-quiet active regions, V: Results from HMI
Authors: Leka, KD; Barnes, G.
2018csc..confE..87L Altcode:
What constitutes the difference between those solar active regions
that produce energetic events and those that do not? The answer
no doubt lies in the state and ongoing evolution of the magnetic
field. Extending this series of studies of the photospheric magnetic
field as related to flare imminency, we consider daily evaluations
of almost all HMI Active Region Patches (HARPS), including temporal
evolution. Using the NWRA Classification Infrastructure based on
NonParametric Discriminant Analysis, we evaluate not only the static
characterization of the photospheric field (extending well beyond
the SHARP parameters) but include coronal topology and time-series
considerations, as well. Additionally, we extend the analysis beyond
"global" parametrizations to describe sub-area sites which may play
roles in coronal energization and event triggering. We report here on
those parametrizations which best distinguish imminent flaring from
imminent quiet sunspot groups.
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Title: Predicting the Where and the How Big of Solar Flares
Authors: Leka, K. D.; Barnes, G.; Gilchrist, S.; Wheatland, M.
2017shin.confE..87L Altcode:
The approach to predicting solar flares generally characterizes global
properties of a solar active region, for example the total magnetic flux
or the total length of a sheared magnetic neutral line, and compares new
data (from which to make a prediction) to similar observations of active
regions and their associated propensity for flare production. We take
here a different tack, examining solar active regions in the context
of their energy storage capacity. Specifically, we characterize not
the region as a whole, but summarize the energy-release prospects
of different sub-regions within, using a sub-area analysis of the
photospheric boundary, the CFIT non-linear force-free extrapolation
code, and the Minimum Current Corona model. We present here early
results from this approach whose objective is to understand the
different pathways available for regions to release stored energy, thus
eventually providing better estimates of the 'where' (what sub-areas
are storing how much energy) and the 'how big' (how much energy is
stored, and how much is available for release) of solar flares.
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Title: VizieR Online Data Catalog: Statistical analysis of solar
active regions (Barnes+, 2014)
Authors: Barnes, G.; Birch, A. C.; Leka, K. D.; Braun, D. C.
2017yCat..17860019B Altcode:
In brief, samples from two populations are considered: "pre-emergence"
targets (PE) that track a 32°x32° patch of the Sun prior to the
emergence of a NOAA-numbered AR and "non-emergence" targets (NE)
selected for lack of emergence and lack of strong fields in the
central portions of the tracked patch. The PE sample size comprises
107 targets obtained between 2001 and 2007, matched to 107 NE targets
drawn from an initially larger sample and selected further to match
the PE distributions in time and observing location on the disk. <P
/>(2 data files).
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Title: Evaluating (and Improving) Estimates of the Solar Radial
Magnetic Field Component from Line-of-Sight Magnetograms
Authors: Leka, K. D.; Barnes, G.; Wagner, E. L.
2017SoPh..292...36L Altcode: 2017arXiv170104836L
Although for many solar physics problems the desirable or meaningful
boundary is the radial component of the magnetic field B<SUB>r</SUB>,
the most readily available measurement is the component of the magnetic
field along the line of sight to the observer, B<SUB>los</SUB>. As
this component is only equal to the radial component where the
viewing angle is exactly zero, some approximation is required to
estimate B<SUB>r</SUB> at all other observed locations. In this
study, a common approximation known as the "μ -correction", which
assumes all photospheric field to be radial, is compared to a method
that invokes computing a potential field that matches the observed
B<SUB>los</SUB>, from which the potential field radial component,
B<SUB>r</SUB><SUP>pot</SUP> is recovered. We demonstrate that in
regions that are truly dominated by a radially oriented field at the
resolution of the data employed, the μ -correction performs acceptably
if not better than the potential-field approach. However, it is also
shown that for any solar structure that includes horizontal fields,
i.e. active regions, the potential-field method better recovers both the
strength of the radial field and the location of magnetic neutral line.
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Title: A Fixed-point Scheme for the Numerical Construction of
Magnetohydrostatic Atmospheres in Three Dimensions
Authors: Gilchrist, S. A.; Braun, D. C.; Barnes, G.
2016SoPh..291.3583G Altcode: 2016arXiv160900733G; 2016SoPh..tmp..182G
Magnetohydrostatic models of the solar atmosphere are often based on
idealized analytic solutions because the underlying equations are too
difficult to solve in full generality. Numerical approaches, too, are
often limited in scope and have tended to focus on the two-dimensional
problem. In this article we develop a numerical method for solving the
nonlinear magnetohydrostatic equations in three dimensions. Our method
is a fixed-point iteration scheme that extends the method of Grad and
Rubin (Proc. 2nd Int. Conf. on Peaceful Uses of Atomic Energy31, 190,
1958) to include a finite gravity force. We apply the method to a test
case to demonstrate the method in general and our implementation in
code in particular.
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Title: A Comparison of Flare Forecasting Methods. I. Results from
the “All-Clear” Workshop
Authors: Barnes, G.; Leka, K. D.; Schrijver, C. J.; Colak, T.;
Qahwaji, R.; Ashamari, O. W.; Yuan, Y.; Zhang, J.; McAteer, R. T. J.;
Bloomfield, D. S.; Higgins, P. A.; Gallagher, P. T.; Falconer, D. A.;
Georgoulis, M. K.; Wheatland, M. S.; Balch, C.; Dunn, T.; Wagner, E. L.
2016ApJ...829...89B Altcode: 2016arXiv160806319B
Solar flares produce radiation that can have an almost immediate effect
on the near-Earth environment, making it crucial to forecast flares
in order to mitigate their negative effects. The number of published
approaches to flare forecasting using photospheric magnetic field
observations has proliferated, with varying claims about how well
each works. Because of the different analysis techniques and data
sets used, it is essentially impossible to compare the results from
the literature. This problem is exacerbated by the low event rates of
large solar flares. The challenges of forecasting rare events have long
been recognized in the meteorology community, but have yet to be fully
acknowledged by the space weather community. During the interagency
workshop on “all clear” forecasts held in Boulder, CO in 2009,
the performance of a number of existing algorithms was compared
on common data sets, specifically line-of-sight magnetic field and
continuum intensity images from the Michelson Doppler Imager, with
consistent definitions of what constitutes an event. We demonstrate
the importance of making such systematic comparisons, and of using
standard verification statistics to determine what constitutes a good
prediction scheme. When a comparison was made in this fashion, no one
method clearly outperformed all others, which may in part be due to the
strong correlations among the parameters used by different methods to
characterize an active region. For M-class flares and above, the set
of methods tends toward a weakly positive skill score (as measured
with several distinct metrics), with no participating method proving
substantially better than climatological forecasts.
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Title: Fractionated (Martian) Noble Gases — EFA, Experiments
and Meteorites
Authors: Schwenzer, S. P.; Barnes, G.; Bridges, J. C.; Bullock,
M. A.; Chavez, C. L.; Filiberto, J.; Herrmann, S.; Hicks, L. J.;
Kelley, S. P.; Miller, M. A.; Moore, J. M.; Ott, U.; Smith, H. D.;
Steer, E. D.; Swindle, T. D.; Treiman, A. H.
2016LPICo1921.6099S Altcode:
Noble gases are tracers for physical processes, including adsorption,
dissolution and secondary mineral formation. We examine the Martian
fractionated atmosphere through literature, terrestrial analogs and
experiments.
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Title: Lessening the Effects of Projection for Line-of-Sight Magnetic
Field Data.
Authors: Leka, K. D.; Barnes, G.; Wagner, E. L.
2016shin.confE.147L Altcode:
A method for treating line-of-sight magnetic field data (B_los)
is developed for the goal of reconstructing the radially-directed
component (B_r) of the solar photospheric magnetic field. The latter
is generally the desired quantity for use as a boundary for modeling
efforts and observational interpretation of the surface field, but the
two are only equivalent where the viewing angle is exactly zero. A
common approximation known as the 'μ-correction', which assumes
all photospheric field to be radial, is compared to a method which
invokes a potential field constructed to match the observed B_los
(Alissandrakis 1981; Sakurai 1982), from which the potential field
radial field component is recovered.
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Title: The Influence of Spatial resolution on Nonlinear Force-free
Modeling
Authors: DeRosa, M. L.; Wheatland, M. S.; Leka, K. D.; Barnes, G.;
Amari, T.; Canou, A.; Gilchrist, S. A.; Thalmann, J. K.; Valori,
G.; Wiegelmann, T.; Schrijver, C. J.; Malanushenko, A.; Sun, X.;
Régnier, S.
2015ApJ...811..107D Altcode: 2015arXiv150805455D
The nonlinear force-free field (NLFFF) model is often used to
describe the solar coronal magnetic field, however a series of
earlier studies revealed difficulties in the numerical solution of the
model in application to photospheric boundary data. We investigate
the sensitivity of the modeling to the spatial resolution of the
boundary data, by applying multiple codes that numerically solve the
NLFFF model to a sequence of vector magnetogram data at different
resolutions, prepared from a single Hinode/Solar Optical Telescope
Spectro-Polarimeter scan of NOAA Active Region 10978 on 2007 December
13. We analyze the resulting energies and relative magnetic helicities,
employ a Helmholtz decomposition to characterize divergence errors, and
quantify changes made by the codes to the vector magnetogram boundary
data in order to be compatible with the force-free model. This study
shows that NLFFF modeling results depend quantitatively on the spatial
resolution of the input boundary data, and that using more highly
resolved boundary data yields more self-consistent results. The
free energies of the resulting solutions generally trend higher
with increasing resolution, while relative magnetic helicity values
vary significantly between resolutions for all methods. All methods
require changing the horizontal components, and for some methods also
the vertical components, of the vector magnetogram boundary field in
excess of nominal uncertainties in the data. The solutions produced
by the various methods are significantly different at each resolution
level. We continue to recommend verifying agreement between the modeled
field lines and corresponding coronal loop images before any NLFFF
model is used in a scientific setting.
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Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: Optimization of the Spectral Line Inversion Code
Authors: Centeno, R.; Schou, J.; Hayashi, K.; Norton, A.; Hoeksema,
J. T.; Liu, Y.; Leka, K. D.; Barnes, G.
2014SoPh..289.3531C Altcode: 2014SoPh..tmp...44C; 2014arXiv1403.3677C
The Very Fast Inversion of the Stokes Vector (VFISV) is a
Milne-Eddington spectral line inversion code used to determine the
magnetic and thermodynamic parameters of the solar photosphere from
observations of the Stokes vector in the 6173 Å Fe I line by the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO). We report on the modifications made to the original
VFISV inversion code in order to optimize its operation within
the HMI data pipeline and provide the smoothest solution in active
regions. The changes either sped up the computation or reduced the
frequency with which the algorithm failed to converge to a satisfactory
solution. Additionally, coding bugs which were detected and fixed in
the original VFISV release are reported here.
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Title: The Helioseismic and Magnetic Imager (HMI) Vector Magnetic
Field Pipeline: SHARPs - Space-Weather HMI Active Region Patches
Authors: Bobra, M. G.; Sun, X.; Hoeksema, J. T.; Turmon, M.; Liu,
Y.; Hayashi, K.; Barnes, G.; Leka, K. D.
2014SoPh..289.3549B Altcode: 2014arXiv1404.1879B; 2014SoPh..tmp...68B
A new data product from the Helioseismic and Magnetic Imager (HMI)
onboard the Solar Dynamics Observatory (SDO) called Space-weather
HMI Active Region Patches (SHARPs) is now available. SDO/HMI is the
first space-based instrument to map the full-disk photospheric vector
magnetic field with high cadence and continuity. The SHARP data series
provide maps in patches that encompass automatically tracked magnetic
concentrations for their entire lifetime; map quantities include the
photospheric vector magnetic field and its uncertainty, along with
Doppler velocity, continuum intensity, and line-of-sight magnetic
field. Furthermore, keywords in the SHARP data series provide several
parameters that concisely characterize the magnetic-field distribution
and its deviation from a potential-field configuration. These indices
may be useful for active-region event forecasting and for identifying
regions of interest. The indices are calculated per patch and are
available on a twelve-minute cadence. Quick-look data are available
within approximately three hours of observation; definitive science
products are produced approximately five weeks later. SHARP data are
available at jsoc.stanford.edu and maps are available in either of
two different coordinate systems. This article describes the SHARP
data products and presents examples of SHARP data and parameters.
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Title: Studies on Forecasting Solar Flares
Authors: Leka, K. D.; Barnes, G.; Braun, D. C.; Wagner, E. L.
2014shin.confE.171L Altcode:
Forecasting solar flares is a challenge from various scientific
perspectives; major solar flares are inherently rare events, and all
observations available with which to evaluate the flare-readiness of the
Sun are remote, with inferences about the physical state rather than
direct measurements. We report on efforts to improve forecasts, using
data from the Helioseismic and Magnetic Imager on the Solar Dynamics
Observatory using magnetic field and helioseismic parametrization,
magnetic charge topology and Discriminant Analysis. We report on
preliminary results of the performance, including the temporal
variations of the parametrizations. <P />This work is supported by
NASA contract NNH12CG10C and NOAA Contract WC-133R-13-CN-0079
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Title: Helioseismology of Pre-emerging Active
Regions. III. Statistical Analysis
Authors: Barnes, G.; Birch, A. C.; Leka, K. D.; Braun, D. C.
2014ApJ...786...19B Altcode: 2013arXiv1307.1938B
The subsurface properties of active regions (ARs) prior to their
appearance at the solar surface may shed light on the process of
AR formation. Helioseismic holography has been applied to samples
taken from two populations of regions on the Sun (pre-emergence and
without emergence), each sample having over 100 members, that were
selected to minimize systematic bias, as described in Paper I. Paper
II showed that there are statistically significant signatures in
the average helioseismic properties that precede the formation of
an AR. This paper describes a more detailed analysis of the samples
of pre-emergence regions and regions without emergence based on
discriminant analysis. The property that is best able to distinguish
the populations is found to be the surface magnetic field, even
a day before the emergence time. However, after accounting for the
correlations between the surface field and the quantities derived from
helioseismology, there is still evidence of a helioseismic precursor
to AR emergence that is present for at least a day prior to emergence,
although the analysis presented cannot definitively determine the
subsurface properties prior to emergence due to the small sample sizes.
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Title: AMBIG: Automated Ambiguity-Resolution Code
Authors: Leka, K. D.; Barnes, G.; Crouch, A.
2014ascl.soft04007L Altcode:
AMBIG is a fast, automated algorithm for resolving the 180°
ambiguity in vector magnetic field data, including those data from
Hinode/Spectropolarimeter. The Fortran-based code is loosely based
on the Minimum Energy Algorithm, and is distributed to provide
ambiguity-resolved data for the general user community.
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Title: The Second NWRA Flare-Forecasting Comparison Workshop:
Methods Compared and Methodology
Authors: Leka, K. D.; Barnes, G.; Flare Forecasting Comparison Group
2013SPD....44...81L Altcode:
The Second NWRA Workshop to compare methods of solar flare forecasting
was held 2-4 April 2013 in Boulder, CO. This is a follow-on to the
First NWRA Workshop on Flare Forecasting Comparison, also known as the
“All-Clear Forecasting Workshop”, held in 2009 jointly with NASA/SRAG
and NOAA/SWPC. For this most recent workshop, many researchers who are
active in the field participated, and diverse methods were represented
in terms of both the characterization of the Sun and the statistical
approaches used to create a forecast. A standard dataset was created
for this investigation, using data from the Solar Dynamics Observatory/
Helioseismic and Magnetic Imager (SDO/HMI) vector magnetic field HARP
series. For each HARP on each day, 6 hours of data were used, allowing
for nominal time-series analysis to be included in the forecasts. We
present here a summary of the forecasting methods that participated
and the standardized dataset that was used. Funding for the workshop
and the data analysis was provided by NASA/Living with a Star contract
NNH09CE72C and NASA/Guest Investigator contract NNH12CG10C.
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Title: Solar Flare Forecasting: a "State of the Field" Report for
Researchers
Authors: Leka, K. D.; Barnes, G.
2013SPD....44...82L Altcode:
It can be argued that the most stringent test of understanding a
deterministic system is to be able to forecast an outcome based on
observable particulars. It can also be argued that (1) solar flares may
not be deterministic , and even if they were, our present understanding
is nowhere close to being able to predict the time and location of
a solar flare with any certainty. Still, solar flare prediction is a
needed component of our national space weather infrastructure, and many
groups around the world are investigating ways to improve forecasting
methods, especially in light of new observational data available,
such as from the Solar Dynamics Observatory. We present a (very) brief
report of the "state of the field", summarizing insights gained from
workshops (held in 2009 and 2013) aimed at head-to-head comparisons
of flare forecasting methods in specific contexts. In summary,
today's methods combine sophisticated data analysis with statistical
or computer-learning algorithms generally result in probabilistic
forecasts. It is unclear whether any of the presently developed
methods clearly outperforms the others, as measured using standard
skill scores applied to the careful comparisons that participating
researchers engaged in at the workshops. However, it is also clear that
new insights into flare triggering mechanisms, especially as afforded
by modern analysis of high-cadence, high-quality data such as from
SDO, have yet to be fully exploited. Funding for the workshops and
subsequent analysis was provided by NASA/Living with a Star contract
NNH09CE72C and NASA/Guest Investigator contract NNH12CG10C.
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Title: A Search for Pre-Emergence Helioseismic Signatures of Active
Regions: Study Design and some Average Results
Authors: Leka, K. D.; Birch, A.; Barnes, G.; Braun, D.; Javornik,
B.; Gonzalez-Hernandez, I.; Dunn, T.
2013SPD....44...91L Altcode:
Helioseismology can be an important tool for understanding the
formation of active regions. This poster describes the design of a
recently completed study, testing whether pre-appearance signatures
of solar magnetic active regions were detectable using various tools
of local helioseismology. We provide details of the data selection
and preparation of samples, each containing over 100 members, of
two populations: regions on the Sun which produced a numbered NOAA
active region, and a "control" sample of areas which did not. The
seismology is performed on data from the GONG network; accompanying
magnetic data from the Michelson Doppler Imager aboard SoHO are used
for co-temporal analysis of the surface magnetic field. Samples are
drawn from 2001--2007, and each target is analyzed for 27.7hr prior
to an objectively determined time of emergence. We describe known
sources of bias and the approaches used to mitigate them. Examining the
average ensemble differences between the two populations, we describe
significant and surprising differences between our samples in both
quantities determined from helioseismology and from surface magnetic
fields. This work was supported by NASA contract NNH07CD25C.
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Title: Making global map of the solar surface B<SUB>r</SUB> from
the HMI vector magnetic field observations
Authors: Hayashi, K.; Liu, Y.; Sun, X.; Hoeksema, J. T.; Centeno,
R.; Barnes, G.; Leka, K. D.
2013JPhCS.440a2036H Altcode:
The Helioseismic Magnetic Imager (HMI) has made full-disk vector
magnetic field measurements of the Sun with cadence of 12 minutes. The
three-component solar surface magnetic field vector data are from
the HMI observations with the data process pipeline modules, VFISV
(Very Fast Inversion of the Stokes Vector, Borrero et al., 2011)
for Milne-Eddington inversion and the minimum-energy disambiguation
algorithm (Metcalf 1994, Leka et al, 2009). The models of the global
corona and solar wind, such as the PFSS (potential-field source-surface)
model and the MHD simulations, often use the maps of solar surface
magnetic field, especially the radial component (B<SUB>r</SUB>) as the
boundary condition. The HMI observation can provide new B<SUB>r</SUB>
data for these model. Because of weak magnetic signals at the quiet
regions of the Sun, the limb darkening, and geometric effects near solar
poles, we need to apply an assumption to make a whole-surface map. In
this paper, we tested two assumptions for determining B<SUB>r</SUB>
at weak-field regions. The coronal structures calculated by the PFSS
model with the vector-based B<SUB>r</SUB> are compared with those with
the magnetogram-based B<SUB>r</SUB> and the corona observed by the
SDO/AIA (Atmospheric Imaging Assembly). In the tested period, CR 2098,
the vector-based B<SUB>r</SUB> map gives better agreements than the
line-of-sight magnetogram data, though we need further investigation
for evaluation.
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Title: Helioseismology of Pre-emerging Active Regions. II. Average
Emergence Properties
Authors: Birch, A. C.; Braun, D. C.; Leka, K. D.; Barnes, G.;
Javornik, B.
2013ApJ...762..131B Altcode: 2013arXiv1303.1391B
We report on average subsurface properties of pre-emerging active
regions as compared to areas where no active region emergence was
detected. Helioseismic holography is applied to samples of the two
populations (pre-emergence and without emergence), each sample having
over 100 members, which were selected to minimize systematic bias,
as described in Leka et al. We find that there are statistically
significant signatures (i.e., difference in the means of more than a few
standard errors) in the average subsurface flows and the apparent wave
speed that precede the formation of an active region. The measurements
here rule out spatially extended flows of more than about 15 m
s<SUP>-1</SUP> in the top 20 Mm below the photosphere over the course
of the day preceding the start of visible emergence. These measurements
place strong constraints on models of active region formation.
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Title: Helioseismology of Pre-emerging Active Regions. I. Overview,
Data, and Target Selection Criteria
Authors: Leka, K. D.; Barnes, G.; Birch, A. C.; Gonzalez-Hernandez,
I.; Dunn, T.; Javornik, B.; Braun, D. C.
2013ApJ...762..130L Altcode: 2013arXiv1303.1433L
This first paper in a series describes the design of a study testing
whether pre-appearance signatures of solar magnetic active regions
were detectable using various tools of local helioseismology. The
ultimate goal is to understand flux-emergence mechanisms by setting
observational constraints on pre-appearance subsurface changes, for
comparison with results from simulation efforts. This first paper
provides details of the data selection and preparation of the samples,
each containing over 100 members, of two populations: regions on
the Sun that produced a numbered NOAA active region, and a "control"
sample of areas that did not. The seismology is performed on data from
the GONG network; accompanying magnetic data from SOHO/MDI are used
for co-temporal analysis of the surface magnetic field. Samples are
drawn from 2001-2007, and each target is analyzed for 27.7 hr prior
to an objectively determined time of emergence. The results of two
analysis approaches are published separately: one based on averages of
the seismology- and magnetic-derived signals over the samples, another
based on Discriminant Analysis of these signals, for a statistical test
of detectable differences between the two populations. We include
here descriptions of a new potential-field calculation approach
and the algorithm for matching sample distributions over multiple
variables. We describe known sources of bias and the approaches used
to mitigate them. We also describe unexpected bias sources uncovered
during the course of the study and include a discussion of refinements
that should be included in future work on this topic.
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Title: A First Look at Magnetic Field Data Products from SDO/HMI
Authors: Liu, Y.; Scherrer, P. H.; Hoeksema, J. T.; Schou, J.; Bai,
T.; Beck, J. G.; Bobra, M.; Bogart, R. S.; Bush, R. I.; Couvidat,
S.; Hayashi, K.; Kosovichev, A. G.; Larson, T. P.; Rabello-Soares,
C.; Sun, X.; Wachter, R.; Zhao, J.; Zhao, X. P.; Duvall, T. L., Jr.;
DeRosa, M. L.; Schrijver, C. J.; Title, A. M.; Centeno, R.; Tomczyk,
S.; Borrero, J. M.; Norton, A. A.; Barnes, G.; Crouch, A. D.; Leka,
K. D.; Abbett, W. P.; Fisher, G. H.; Welsch, B. T.; Muglach, K.;
Schuck, P. W.; Wiegelmann, T.; Turmon, M.; Linker, J. A.; Mikić,
Z.; Riley, P.; Wu, S. T.
2012ASPC..455..337L Altcode:
The Helioseismic and Magnetic Imager (HMI; Scherrer & Schou 2011)
is one of the three instruments aboard the Solar Dynamics Observatory
(SDO) that was launched on February 11, 2010 from Cape Canaveral,
Florida. The instrument began to acquire science data on March 24. The
regular operations started on May 1. HMI measures the Doppler velocity
and line-of-sight magnetic field in the photosphere at a cadence of
45 seconds, and the vector magnetic field at a 135-second cadence,
with a 4096× 4096 pixels full disk coverage. The vector magnetic
field data is usually averaged over 720 seconds to suppress the p-modes
and increase the signal-to-noise ratio. The spatial sampling is about
0".5 per pixel. HMI observes the Fe i 6173 Å absorption line, which
has a Landé factor of 2.5. These data are further used to produce
higher level data products through the pipeline at the HMI-AIA Joint
Science Operations Center (JSOC) - Science Data Processing (Scherrer et
al. 2011) at Stanford University. In this paper, we briefly describe the
data products, and demonstrate the performance of the HMI instrument. We
conclude that the HMI is working extremely well.
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Title: Modeling and Interpreting the Effects of Spatial Resolution
on Solar Magnetic Field Maps
Authors: Leka, K. D.; Barnes, G.
2012SoPh..277...89L Altcode: 2011arXiv1106.5024L
Different methods for simulating the effects of spatial resolution on
magnetic field maps are compared, including those commonly used for
inter-instrument comparisons. The investigation first uses synthetic
data, and the results are confirmed with Hinode/SpectroPolarimeter
data. Four methods are examined, one which manipulates the Stokes
spectra to simulate spatial-resolution degradation, and three
"post-facto" methods where the magnetic field maps are manipulated
directly. Throughout, statistical comparisons of the degraded maps
with the originals serve to quantify the outcomes. Overall, we find
that areas with inferred magnetic fill fractions close to unity may
be insensitive to optical spatial resolution; areas of sub-unity fill
fractions are very sensitive. Trends with worsening spatial resolution
can include increased average field strength, lower total flux, and
a field vector oriented closer to the line of sight. Further-derived
quantities such as vertical current density show variations even in
areas of high average magnetic fill fraction. In short, unresolved maps
fail to represent the distribution of the underlying unresolved fields,
and the "post-facto" methods generally do not reproduce the effects of
a smaller telescope aperture. It is argued that selecting a method in
order to reconcile disparate spatial resolution effects should depend
on the goal, as one method may better preserve the field distribution,
while another can reproduce spatial resolution degradation. The results
presented should help direct future inter-instrument comparisons.
---------------------------------------------------------
Title: Spectropolarimetry in the Sodium 589.6nm D1 line: Evaluating
the Resulting Chromospheric (?) Vector Field Maps.
Authors: Leka, K. D.; Barnes, G.; Stockwell, R. G.; Wagner, E. L.;
Uitenbroek, H.; Derouich, M.
2012decs.confE..79L Altcode:
Pioneering work by T. R. Metcalf almost two decades ago pointed to
the Na 589.6nm D1 line as a contender for providing chromospheric
vector magnetic field measurements (using the Zeeman effect). We
report here on a systematic examination of what can be expected from
Sodium 589.6nm spectropolarimetry, with respects to polarization-signal
amplitudes and retrieval, and the implementation of the inversion for
this line based on the Jeffries, Lites & Skumanich Weak-Field
Approximation algorithm. The analysis is performed using both
synthetic data and observations from the Imaging Vector Magnetograph,
for which a large dataset of Sodium 589.6nm vector spectropolarimetry
exists. The synthetic data are based on a 3-D field extrapolated from
photospheric vector magnetograms of two active regions, four distinct
model atmospheres coupled with NLTE synthesis of the emergent NaI
D1 Stokes polarization spectra, computed for a variety of viewing
angles. In this manner, a broad representation of active-region
features, field strengths and observing angles are tested using “hare
& hound” approaches, including evaluating algorithm performance in
the presence of noise and instrumental effects. We compare retrieval
algorithms for the very weak (as expected) polarization signals, and
evaluate the retrieved vector magnetic field at a range of inferred
heights. Finally, we provide an example from the IVM and discuss the
prospects for obtaining and interpreting chromospheric vector magnetic
field maps. Support for this work comes from NASA NAG5-12466, NASA
NNH09CE60C, AFOSR F49620-03-C-0019, NSF/NSWP ATM-0519107, NSF/SHINE
ATM-0454610, and NSF CRG ATM-0551055.
---------------------------------------------------------
Title: A Comparison of Methods for Manipulating SpectroPolarimetric
and Magnetic Field Data for Heliospheric Models, Data Comparisons,
and Physical Interpretation
Authors: Leka, K. D.; Barnes, G.
2011shin.confE..22L Altcode:
Heliospheric modeling efforts often begin with boundary data, and
those boundary data are either observed or simulated photospheric
magnetic field maps. Oftentimes, the available boundary data just
aren't compatible with what is needed, or what can be handled by
subsequent code. But how well do rebinned/remapped/averaged magnetic
maps represent the underlying field? We address this question using
model fields, where the true field is known, but the
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Title: Topology of Coronal Fields from Potential Field Models
Authors: DeRosa, Marc L.; Schrijver, C. J.; Barnes, G.
2011SPD....42.1810D Altcode: 2011BAAS..43S.1810D
The topology of the solar coronal magnetic field has been the subject of
much recent interest, due to its apparent importance in determining (for
example) the sources of the solar wind, the evolution of coronal hole
boundaries, and whether the configurations of coronae overlying active
regions are unstable and thus possibly eruption-prone. We identify
the topological skeleton (null points, spline lines, separators, and
separatrix surfaces) for a selection of dates of interest from the
database of potential-field source-surface models available through
the “PFSS” SolarSoft package. Several features of interest have been
identified by recent studies (e.g., Antiochos et al. 2007, Parnell et
al. 2010, Titov et al. 2011), including exceedingly narrow channels of
open field or separators associated with inferred reconnection sites. We
find that these features of interest occur frequently in the topologies
of even potential-field models of the magnetic corona. The actual solar
corona is of course likely to involve even more complex topologies,
especially as its dynamics and evolution are taken into account.
---------------------------------------------------------
Title: Subsurface Vorticity of Flaring versus Flare-Quiet Active
Regions
Authors: Komm, R.; Ferguson, R.; Hill, F.; Barnes, G.; Leka, K. D.
2011SoPh..268..389K Altcode: 2010SoPh..tmp...78K
We apply discriminant analysis to 1023 active regions and their
subsurface-flow parameters, such as vorticity and kinetic helicity
density, with the goal of distinguishing between flaring and non-flaring
active regions. We derive synoptic subsurface flows by analyzing GONG
high-resolution Doppler data with ring-diagram analysis. We include
magnetic-flux values in the discriminant analysis derived from NSO
Kitt Peak and SOLIS synoptic maps binned to the same spatial scale
as the helioseismic analysis. For each active region, we determine
the flare information from GOES and include all flares within 60°
central meridian distance to match the coverage of the ring-diagram
analysis. The subsurface-flow characteristics improve the ability to
distinguish between flaring and non-flaring active regions. For the C-
and M-class flare category, the most important subsurface parameter
is the so-called structure vorticity, which estimates the horizontal
gradient of the horizontal-vorticity components. The no-event skill
score, which measures the improvement over predicting that no events
occur, reaches 0.48 for C-class flares and 0.32 for M-class flares, when
the structure vorticity at three depths combined with total magnetic
flux are used. The contributions come mainly from shallow layers within
about 2 Mm of the surface and layers deeper than about 7 Mm.
---------------------------------------------------------
Title: Coronal Loop Evolution and Inferred Coronal Magnetic Energy
in a Quiet Active Region
Authors: Lee, Jin-Yi; Barnes, G.; Leka, K.; Reeves, K. K.; Korreck,
K. E.; Golub, L.
2010AAS...21640514L Altcode: 2010BAAS...41R.891L
We investigate changes in the properties of the coronal magnetic field
in the context of different emission of coronal loops. Observations by
the Transition Region and Coronal Explorer (TRACE), the Hinode/X-ray
Telescope (XRT), and the SOHO/Michelson Doppler Imager (MDI), the
X-ray and EUV light curves as well as the photospheric magnetic flux
of NOAA active region 10963 are utilized to compare the coronal and
photospheric magnetic fields. A Magnetic Charge Topology (MCT) model
is used to establish potential magnetic field connectivity of the
surface magnetic flux distribution. A Minimum Current Corona (MCC)
model is applied to determine the coronal currents and quantify the
energy build-up. The results of the MCC analysis are compared to the
evolution of the coronal loop brightness, comparing areas of steady
emission, transient emission, and temporal patterns of emission which
imply coronal cooling.
---------------------------------------------------------
Title: Subsurface Flow Properties of Flaring versus Flare-Quiet
Active Regions
Authors: Ferguson, R.; Komm, R.; Hill, F.; Barnes, G.; Leka, K. D.
2009ASPC..416..127F Altcode:
We apply discriminant analysis to 1009 active regions and their
subsurface flow parameters, such as vorticity and kinetic helicity
density, with the goal of distinguishing between flaring and non-flaring
active regions. Flow and flux variables lead to better classification
rates than a no-event prediction. The Heidke skill score, which measures
the improvement over predicting that no events occur, increases by
about 25% and 50% for C- and M-class flares when several subsurface
characteristics are included compared to using a single magnetic
flux measure.
---------------------------------------------------------
Title: Resolving the Azimuthal Ambiguity in Vector Magnetogram Data
with the Divergence-Free Condition: Application to Discrete Data
Authors: Crouch, A. D.; Barnes, G.; Leka, K. D.
2009SoPh..260..271C Altcode: 2009arXiv0911.0711C
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.
---------------------------------------------------------
Title: An Automated Ambiguity-Resolution Code for Hinode/SP Vector
Magnetic Field Data
Authors: Leka, K. D.; Barnes, G.; Crouch, A.
2009ASPC..415..365L Altcode:
A fast, automated algorithm is presented for use in resolving
the 180° ambiguity in vector magnetic field data, including
those data from Hinode/Spectropolarimeter. The Fortran-based
code is loosely based on the Minimum Energy Algorithm, and is
distributed to provide ambiguity-resolved data for the general user
community. Here we generally describe the released code (available
at http://www.cora.nwra.com/AMBIG), examples of its performance and
usage for Hinode/SP data.
---------------------------------------------------------
Title: Evolution of Magnetic Properties for Two Active Regions
Observed by Hinode/XRT and TRACE
Authors: Lee, J. -Y.; Leka, K. D.; Barnes, G.; Reeves, K. K.; Korreck,
K. E.; Golub, L.
2009ASPC..415..279L Altcode:
We investigate two active regions observed by the Hinode X-ray Telescope
(XRT) and the Transition Region and Coronal Explorer (TRACE). One active
region shows constant brightness in both XRT and TRACE observations. The
other active region shows a brightening in the TRACE observation
just after a decrease in X-ray brightness indicating the cooling of a
coronal loop. The coronal magnetic topology is derived using a magnetic
charge topology (MCT) model for these two active regions applied to
magnetograms from the Michelson Doppler Imager (MDI) on board the Solar
and Heliospheric Observatory (SOHO). We discuss the results of the MCT
analysis with respect to the light curves for these two active regions.
---------------------------------------------------------
Title: Magnetic energy build-up and coronal brightness evolution
Authors: Lee, J.; Barnes, G.; Leka, K. D.; Reeves, K. K.; Korreck,
K. E.; Golub, L.
2009AGUFMSH41B1664L Altcode:
We have investigated changes in the properties of the coronal magnetic
field in the context of different emission behaviors of coronal
loops. Using observations by the Transition Region and Coronal Explorer
(TRACE), the Hinode/X-ray Telescope (XRT), and the SoHO/Michelson
Doppler Imager (MDI), NOAA active region 10963 has been analyzed
in depth as to how different coronal signatures compare to inferred
coronal energy build-up. A Magnetic Charge Topology (MCT) model was
used to establish potential magnetic field connectivity of the surface
magnetic flux distribution, and a Minimum Current Corona (MCC) model was
applied to quantify the energy build-up along separator field lines. The
results of the MCC analysis are compared to the evolution of the coronal
brightness, comparing areas of steady emission, very transient emission,
and temporal patterns of emission which imply coronal cooling.
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
Inhester, B.; Tadesse, T.
2009SPD....40.3102D Altcode:
Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have failed to arrive at consistent solutions when
applied to (thus far, two) cases using the highest-available-resolution
vector magnetogram data from Hinode/SOT-SP (in the region of the
modeling area of interest) and line-of-sight magnetograms from
SOHO/MDI (where vector data were not available). One issue is that
NLFFF models require consistent, force-free vector magnetic boundary
data, and vector magnetogram data sampling the photosphere do not
satisfy this requirement. Consequently, several problems have arisen
that are believed to affect such modeling efforts. We use AR 10953
to illustrate these problems, namely: (1) some of the far-reaching,
current-carrying connections are exterior to the observational field
of view, (2) the solution algorithms do not (yet) incorporate the
measurement uncertainties in the vector magnetogram data, and/or (3)
a better way is needed to account for the Lorentz forces within the
layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
---------------------------------------------------------
Title: Detecting, Selecting, And Controlling For Emerging
ActiveRegions In The Search For Helioseismic Pre-emergence Signatures.
Authors: Leka, K. D.; Dunn, T.; Gonzalez-Hernandez, I.; Barnes, G.;
Braun, D.; Birch, A.
2009SPD....40.0708L Altcode:
Helioseismology is potentially capable of predicting the emergence of
solaractive regions. As part of a search for statistically significant
helioseismic predictors of active region emergence, we have developed
methods for the automatic determination of emergence times based on the
NOAA/NGDC active region catalog and MDI/SOHO 96 minute magnetograms. We
demonstrate the application of this method and its sister task of
selecting an appropriate control sample. We show first results from
a statistical study investigating the pre-emergence signatures of
Solar Active Regions using GONG data. This work was supported by NASA
contract NNH07CD25C.
---------------------------------------------------------
Title: A Search for Pre-Emergence Signatures of Active Regions
Authors: Birch, Aaron; Braun, D. C.; Leka, K. D.; Barnes, G.; Dunn,
T. L.; González Hernández, I.
2009SPD....40.0402B Altcode:
Prediction of solar active region emergence is an important goal
for helioseismology. As a first step towards developing prediction
methods, we are carrying out a search for helioseismic pre-emergence
signatures. Using GONG data, we have applied helioseismic holography
to about 150 pre-emergence active regions and a control sample of 450
quiet-Sun regions. We will show preliminary results of this study. <P
/>This work was supported by NASA contract NNH07CD25C
---------------------------------------------------------
Title: Subsurface Flow Properties of Flaring Versus Flare-quiet
Active Regions
Authors: Ferguson, Ryan M.; Komm, R.; Hill, F.; Barnes, G.; Leka, K. D.
2009SPD....40.1908F Altcode:
Previous studies have shown that the flare activity of active regions is
intrinsically linked with the vorticity of subsurface flows on temporal
and spatial scales comparable to the size and lifetime of active
regions. We begin to address the question whether the measured vorticity
of subsurface flows associated with active regions can help to improve
flare forecasting. For this purpose, we apply statistical tests based
on discriminant analysis to several subsurface flow parameters with the
goal to differentiate between flaring and non-flaring active regions. <P
/>We will present the latest results. This work is carried out through
the National Solar Observatory Research Experiences for Undergraduate
(REU) site program, which is co-funded by the Department of Defense
in partnership with the National Science Foundation REU Program.
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Title: Resolving the Azimuthal Ambiguity in Vector Magnetogram Data
with the Divergence-Free Condition
Authors: Crouch, Ashley D.; Leka, K.; Barnes, G.
2009SPD....40.0915C Altcode:
We demonstrate how the divergence-free property of magnetic fields can
be exploited to resolve the azimuthal ambiguity that is present in
solar vector magnetogram data by using line-of-sight and horizontal
heliographic derivative information. Using synthetic data at two
heights we objectively test several methods that each make a different
assumption about how the divergence-free property can be used to resolve
the ambiguity. We investigate how the different approaches respond to
various effects, including the presence of noise and limited spatial
resolution. This work was supported by funding from NASA under contracts
NNH05CC49C/NNH05CC75C and NNH09CE60C.
---------------------------------------------------------
Title: Magnetic Topology and Coronal Brightness Evolution: A Case
Study
Authors: Lee, Jin-Yi; Barnes, G.; Leka, K.; Reeves, K. K.; Korreck,
K. E.; Golub, L.
2009SPD....40.1209L Altcode:
We have applied a Magnetic Charge Topology model to investigate
what changes in the properties of the magnetic field are responsible
for different coronal emission behavior of the coronal loops in two
different active regions. Observations from the X-ray Telescope (XRT)
on board Hinode and the Transition Region and Coronal Expolorer (TRACE)
were used, along with time-series of magnetograms for 24 hours from
the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric
Observatory (SOHO). The magnetic connectivity and separator field
lines were established by potential field extrapolation of the observed
surface magnetic flux distribution. We present the evolution for the
two active regions in terms of the changes in both the connections and
in the separator flux, the latter indicative of locations of possible
energy deposit or release.
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Title: UVNS: An UVvis-NIR Spectrometer for Mars airglow.
Authors: McConnell, J. C.; Barnes, G.; McDade, I.; Solheim, B.;
Llewellyn, T.; Bourassa, A.; Daerden, F.; Friberg, D.; Blaxley, S.;
Marchand, P.; Proulx, P.; Donovan, E.; Sioris, C.; McLinden, C.;
Siskind, D.; Stevens, M.; Murtagh, D.; Smith, K.; Kabin, K.
2009AGUSM.P23A..01M Altcode:
Airglow from a planetary atmosphere can yield important information
on composition and dynamics. In this poster we examine the scientific
return for the Mars Science Orbiter goals using a set of compact light
instruments with a proven heritage (OSIRIS, SOIR, and SHOW) which span
the UV-vis-NIR-SWIR part of the spectrum measuring scattered light
and airglow from the limb and also stellar and solar occultation. A
nadir viewing option is also a possibility for the UV spectrometer. The
prime instruments consist of (a) a NIR- tomographic Ox imager (NTOI)
(provenance OSIRIS) and (b) a high resolution IR spectrometer provenance
SOIR) operating in solar and stellar occultation modes. By imaging the
O2(1Δ) with the NTOI in the vertical it should be possible to derive
a 2D structure for ozone during the daytime and O at night, providing
chemical structure in the 50-80 km height range. The observed structure,
analysed using 3D chemistry climate models, will also provide much
needed information on dynamics and possibly the role of gravity
waves and thermal tides. The HRIRS/SOIR occultation instrument,
with a sensitivity of 2 ppbv at 3.3 microns will provide a unique
opportunity to detect methane and measure any spatial variability. But
other species such as CO should be detectable. A third instrument
(c) uses the spatial heterodyne method of the SHOW instrument and it
should be possible to measure water vapour and SO2 and perhaps tune the
instrument for other species. (d) The UV-Vis spectrometer (UVS) would
be based on the OSIRIS spectrometer but tuned for 200-600 nm to measure
NO airglow and aurorae ∼ 200 nm, Herzberg II bands, ozone column
in the Herzberg continuum at ∼ 250 nm with nadir viewing. Rayleigh
scattering would provide temperature and pressure. Thus measurements
of ozone column and water vapour will continue the climatology of these
species initiated by MAWD on the Viking orbiters, TES on MGS and PFS on
Mars Express. The unambiguous detection of methane and its distribution
would be a major achievement and contribute to the question of its
source. The UVNS will also measure aerosol (dust, water ice, and CO2
ice) optical depth and with the inclusion of infrared channels [SOIR]
will provide improved information on the size distribution (Bourassa et
al., 2008). A secondary objective would be to use the airglow data that
would be observed to extend the MAVEN science mission and characterise
the interaction of the solar wind with the Martian upper atmosphere
by means of detailed airglow measurements.
---------------------------------------------------------
Title: Effects of Partitioning and Extrapolation on the Connectivity
of Potential Magnetic Fields
Authors: Longcope, D. W.; Barnes, G.; Beveridge, C.
2009ApJ...693...97L Altcode: 2008arXiv0811.1241L
Coronal magnetic field may be characterized by how its field
lines interconnect regions of opposing photospheric flux—its
connectivity. Connectivity can be quantified as the net flux connecting
pairs of opposing regions, once such regions are identified. One
existing algorithm will partition a typical active region into a
number of unipolar regions ranging from a few dozen to a few hundred,
depending on algorithmic parameters. This work explores how the
properties of the partitions depend on some algorithmic parameters,
and how connectivity depends on the coarseness of partitioning for
one particular active region magnetogram. We find the number of
connections among them scales with the number of regions even as
the number of possible connections scales with its square. There
are several methods of generating a coronal field, even a potential
field. The field may be computed inside conducting boundaries or over
an infinite half-space. For computation of connectivity, the unipolar
regions may be replaced by point sources or the exact magnetogram may
be used as a lower boundary condition. Our investigation shows that the
connectivities from these various fields differ only slightly—no more
than 15%. The greatest difference is between fields within conducting
walls and those in the half-space. Their connectivities grow more
different as finer partitioning creates more source regions. This also
gives a quantitative means of establishing how far away conducting
boundaries must be placed in order not to significantly affect
the extrapolation. For identical outer boundaries, the use of point
sources instead of the exact magnetogram makes a smaller difference in
connectivity: typically 6% independent of the number of source regions.
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar
Corona: A Critical Assessment
Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.;
Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung,
M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T.
2008AGUFMSH41A1604D Altcode:
Nonlinear force-free field (NLFFF) modeling promises to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have so far failed to arrive at consistent solutions
when applied to cases using the highest-available-resolution vector
magnetogram data from Hinode/SOT-SP (in the region of the modeling
area of interest) and line-of-sight magnetograms from SOHO/MDI (where
vector data were not been available). It is our view that the lack of
robust results indicates an endemic problem with the NLFFF modeling
process, and that this process will likely continue to fail until (1)
more of the far-reaching, current-carrying connections are within the
observational field of view, (2) the solution algorithms incorporate
the measurement uncertainties in the vector magnetogram data, and/or
(3) a better way is found to account for the Lorentz forces within
the layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
---------------------------------------------------------
Title: Evaluating the Performance of Solar Flare Forecasting Methods
Authors: Barnes, G.; Leka, K. D.
2008ApJ...688L.107B Altcode:
The number of published approaches to solar flare forecasting using
photospheric magnetic field observations has proliferated recently, with
widely varying claims about how well each works. As different analysis
techniques and data sets were used, it is essentially impossible to
directly compare the results. A systematic comparison is presented
here using three parameters based on the published literature that
characterize the photospheric magnetic field itself, plus one that
characterizes the coronal magnetic topology. Forecasts based on the
statistical method of discriminant analysis are made for each of these
parameters, and their ability to predict major flares is quantified
using skill scores. Despite widely varying statements regarding their
forecasting utility in the original studies describing these four
parameters, there is no clear distinction in their performance here,
thus demonstrating the importance of using standard verification
statistics.
---------------------------------------------------------
Title: Statistical Prediction of Solar Flares Using Magnetic Field
Data: A Status Report
Authors: Leka, K.; Barnes, G.; Knoll, J.; Tessein, J. A.
2008AGUFMSA51A1535L Altcode:
The energy to power solar flares is undoubtedly stored in the
concentrated magnetic field structures of solar active region
atmospheres. Exactly how to make use of observations of the solar
magnetic field for predicting the occurrence of solar energetic events
is, however, a great challenge. Building upon our prior work of "daily"
forecasts using a dataset of photospheric magnetic vector field maps,
we examine here questions of forecasting ability in light of data
source and the target temporal window. We will discuss the benefits and
problems of relying upon line-of-sight magnetic field data (vs. vector
photospheric magnetic field maps). In addition, we begin to examine
changes in forecasting ability, as measured by standard validation
statistics, that result from considering different forecasting windows.
---------------------------------------------------------
Title: Non-Linear Force-Free Field Modeling of a Solar Active Region
Around the Time of a Major Flare and Coronal Mass Ejection
Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes,
G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann,
T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J.
2008AGUSMSP31A..06D Altcode:
Solar flares and coronal mass ejections are associated with rapid
changes in coronal magnetic field connectivity and are powered by
the partial dissipation of electrical currents that run through
the solar corona. A critical unanswered question is whether the
currents involved are induced by the advection along the photosphere
of pre-existing atmospheric magnetic flux, or whether these currents
are associated with newly emergent flux. We address this problem by
applying nonlinear force-free field (NLFFF) modeling to the highest
resolution and quality vector-magnetographic data observed by the
recently launched Hinode satellite on NOAA Active Region 10930 around
the time of a powerful X3.4 flare in December 2006. We compute 14
NLFFF models using 4 different codes having a variety of boundary
conditions. We find that the model fields differ markedly in geometry,
energy content, and force-freeness. We do find agreement of the best-fit
model field with the observed coronal configuration, and argue (1)
that strong electrical currents emerge together with magnetic flux
preceding the flare, (2) that these currents are carried in an ensemble
of thin strands, (3) that the global pattern of these currents and
of field lines are compatible with a large-scale twisted flux rope
topology, and (4) that the ~1032~erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection. We discuss the relative merits of
these models in a general critique of our present abilities to model
the coronal magnetic field based on surface vector field measurements.
---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
around the Time of a Major Flare and Coronal Mass Ejection
Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.;
Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.;
Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
M.; Kusano, K.; Régnier, S.; Thalmann, J. K.
2008ApJ...675.1637S Altcode: 2007arXiv0712.0023S
Solar flares and coronal mass ejections are associated with rapid
changes in field connectivity and are powered by the partial dissipation
of electrical currents in the solar atmosphere. A critical unanswered
question is whether the currents involved are induced by the motion of
preexisting atmospheric magnetic flux subject to surface plasma flows or
whether these currents are associated with the emergence of flux from
within the solar convective zone. We address this problem by applying
state-of-the-art nonlinear force-free field (NLFFF) modeling to the
highest resolution and quality vector-magnetographic data observed
by the recently launched Hinode satellite on NOAA AR 10930 around
the time of a powerful X3.4 flare. We compute 14 NLFFF models with
four different codes and a variety of boundary conditions. We find
that the model fields differ markedly in geometry, energy content,
and force-freeness. We discuss the relative merits of these models in
a general critique of present abilities to model the coronal magnetic
field based on surface vector field measurements. For our application
in particular, we find a fair agreement of the best-fit model field
with the observed coronal configuration, and argue (1) that strong
electrical currents emerge together with magnetic flux preceding the
flare, (2) that these currents are carried in an ensemble of thin
strands, (3) that the global pattern of these currents and of field
lines are compatible with a large-scale twisted flux rope topology,
and (4) that the ~10<SUP>32</SUP> erg change in energy associated with
the coronal electrical currents suffices to power the flare and its
associated coronal mass ejection.
---------------------------------------------------------
Title: Resolving the Azimuthal Ambiguity in Vector Magnetogram Data
with the Divergence-Free Condition: Theoretical Examination
Authors: Crouch, A. D.; Barnes, G.
2008SoPh..247...25C Altcode:
We demonstrate that the azimuthal ambiguity that is present in
solar vector magnetogram data can be resolved with line-of-sight
and horizontal heliographic derivative information by using the
divergence-free property of magnetic fields without additional
assumptions. We discuss the specific derivative information that
is sufficient to resolve the ambiguity away from disk centre, with
particular emphasis on the line-of-sight derivative of the various
components of the magnetic field. Conversely, we also show cases where
ambiguity resolution fails because sufficient line-of-sight derivative
information is not available. For example, knowledge of only the
line-of-sight derivative of the line-of-sight component of the field
is not sufficient to resolve the ambiguity away from disk centre.
---------------------------------------------------------
Title: A Comparison of Flare Forecasting Parameters Derived From
Photospheric Magnetograms
Authors: Barnes, G.; Leka, K.
2007AGUFMSM41A0314B Altcode:
A variety of researchers have proposed parameters for use in
forecasting of solar flares. However, the parameters have been
calculated from different data sources, and their performance has
been judged based on various different criteria. We present here a
systematic comparison of a small number of parameters which can be
derived from the photospheric magnetic field, some of which characterize
the photospheric field itself, and some which characterize the coronal
magnetic topology. We compute the parameters for a collection of
over 1200 vector magnetograms from the Imaging Vector Magnetograph
at Haleakala, and judge their ability to forecast flares based on
discriminant analysis, climatological skill scores, and the ability
to provide an "all-clear" forecast.
---------------------------------------------------------
Title: On the Relationship between Coronal Magnetic Null Points and
Solar Eruptive Events
Authors: Barnes, G.
2007ApJ...670L..53B Altcode:
One mechanism that has been proposed for initiating coronal mass
ejections (CMEs) is the “breakout” model. For this model to
account for CMEs, a coronal null point must be present prior to the
eruption. The relationship between the existence of coronal null
points and eruptive events is investigated using a collection of
over 1800 vector magnetograms from the Imaging Vector Magnetograph at
Haleakalā. Each magnetogram is subjected to magnetic charge topology
analysis, including determining the presence of coronal null points. It
is found that the majority of events originate in regions above which no
null point is found. However, a much larger fraction of active regions
for which a coronal null point was found were the source of an eruption
than active regions for which no null was found. The implications of
these results for the breakout model are discussed.
---------------------------------------------------------
Title: Determining the Source of Coronal Helicity through Measurements
of Braiding and Spin Helicity Fluxes in Active Regions
Authors: Longcope, D. W.; Ravindra, B.; Barnes, G.
2007ApJ...668..571L Altcode:
Magnetic helicity has become a valuable tool for understanding the
energetics and dynamics of coronal magnetic fields. Recently, long
time sequences of magnetograms have been used to measure the flux of
helicity into active region coronae. We demonstrate how this helicity
flux can be usefully decomposed into contributions of differing
origin, called “spin” helicity and “braiding” helicity. These
contributions could be envisioned to come at the expense of twist
and writhe helicity, respectively, of a subphotospheric flux tube
anchored to the regions. In order to effect this decomposition, each
magnetogram is partitioned into a set of unipolar regions. We present
a method of defining such regions so that they persist through the
sequences and track the photospheric flow. The spin helicity of a
given region quantifies the mean rotation rate of motions internal
to that region, while braiding helicity is injected by the motions
of whole regions about one another. Applying the method to six active
regions shows cases where either spin or braiding dominates, and where
they have the same signs and opposite signs. Thus, it would seem that
no general statement can be made regarding the dominance of twist or
writhe in supplying helicity to the corona. In one particular case,
spin and braiding helicity follow different time histories but inject
equal and opposite net helicities. This suggests that the spinning and
braiding are driven by a kink instability in the submerged flux tube.
---------------------------------------------------------
Title: Probabilistic forecasting of solar flares from vector
magnetogram data
Authors: Barnes, G.; Leka, K. D.; Schumer, E. A.; Della-Rose, D. J.
2007SpWea...5.9002B Altcode:
Discriminant analysis is a statistical approach for assigning a
measurement to one of several mutually exclusive groups. Presented here
is an application of the approach to solar flare forecasting, adapted
to provide the probability that a measurement belongs to either group,
the groups in this case being solar active regions which produced
a flare within 24 hours and those that remained flare quiet. The
technique is demonstrated for a large database of vector magnetic
field measurements obtained by the University of Hawai'i Imaging Vector
Magnetograph. For a large combination of variables characterizing the
photospheric magnetic field, the results are compared to a Bayesian
approach for solar flare prediction, and to the method employed by the
U.S. Space Environment Center (SEC). Although quantitative comparison
is difficult as the present application provides active region (rather
than whole-Sun) forecasts, and the present database covers only part
of one solar cycle, the performance of the method appears comparable
to the other approaches.
---------------------------------------------------------
Title: Resolving The Azimuthal Ambiguity In Vector Magnetograms Away
From Disk Centre With The Solenoidal Condition
Authors: Crouch, Ashley D.; Barnes, G.
2007AAS...210.5304C Altcode: 2007BAAS...39..164C
We employ the divergence-free property of magnetic fields to resolve
the azimuthal ambiguity in solar vector magnetograms. We show that
the ambiguity can be resolved away from disk centre if one knows
the line-of-sight derivative of the magnetic field components in
the directions parallel and transverse to the line-of-sight. However,
knowing only the line-of-sight derivative of the line-of-sight component
of the magnetic field is not sufficient except at disk centre. Thus,
multi-height vector magnetogram data can be used to resolve the
ambiguity provided that all the line-of-sight derivatives can be
determined reliably. We use a simple theoretical example, consisting of
two submerged magnetic point sources, to demonstrate our findings. This
work was supported by funding from NASA/LWS under contract NNH05CC75C.
---------------------------------------------------------
Title: Active Region Magnetic Field Line Twist and Source of Coronal
Magnetic Helicity.
Authors: Belur, Ravindra; Longcope, D.; Barnes, G.; Nandy, D.
2007AAS...210.2401B Altcode: 2007BAAS...39..128B
Magnetic helicity is an important quantity which measures how the
magnetic field lines are twisted and sheared. Recently it has become
possible to measure the flux of magnetic helicity in active regions
using the observational data. These observed helicity fluxes may
arise due to the twist in the emerging active region flux tubes or it
may come from the photospheric shearing motion. Here, we decompose
the helicity flux into two different contributions called spin
and braiding. These components typically come from twist and writhe
helicity of a sub-photospheric flux tube anchored to the regions. The
spin helicity of a given region quantifies the mean rotation rate of
motion internal to that region and braiding helicity is injected by the
motions of whole <P />regions about one another. The injected helicity
flux due to spin and braiding motion leads to the coronal magnetic
field line twist. The twist determined from vector magnetograms can be
used to estimate the total helicity content of the coronal field at one
time. The rate of change of this helicity estimate can be compared to
the total helicity flux as well as its spin and braiding component. We
make such a comparison for several active regions.
---------------------------------------------------------
Title: Non-linear Force-free Modeling Of Coronal Magnetic Fields
Authors: Metcalf, Thomas R.; De Rosa, M. L.; Schrijver, C. J.; Barnes,
G.; van Ballegooijen, A.; Wiegelmann, T.; Wheatland, M. S.; Valori,
G.; McTiernan, J. M.
2007AAS...210.9102M Altcode: 2007BAAS...39..204M
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 <P />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 Lorentz
forces on the photosphere severely impact the extrapolation of the
field. Preprocessing of the photospheric boundary does improve the
extrapolations considerably, although the results depend sensitively
on the details of the numerical codes. 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.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring versus
Flare-quiet Active Regions. IV. A Statistically Significant Sample
Authors: Leka, K. D.; Barnes, G.
2007ApJ...656.1173L Altcode:
Statistical tests based on linear discriminant analysis are applied
to numerous photospheric magnetic parameters, continuing toward
the goal of identifying properties important for the production of
solar flares. For this study, the vector field data are University
of Hawai`i Imaging Vector Magnetograph daily magnetograms obtained
between 2001 and 2004. Over 1200 separate magnetograms of 496
numbered active regions comprise the data set. At the soft X-ray
C1.0 level, 359 magnetograms are considered “flare productive”
in the 24 hr postobservation. Considering multiple photospheric
variables simultaneously indicates that combinations of only a few
familiar variables encompass the majority of the predictive power
available. However, the choice of which few variables is not unique,
due to strong correlations among photospheric quantities such as total
magnetic flux and total vertical current, two of the most powerful
predictors. The best discriminant functions result from combining one
of these with additional uncorrelated variables, such as measures
of the magnetic shear, and successfully classify over 80% of the
regions. By comparison, a success rate of approximately 70% is achieved
by simply classifying all regions as “flare quiet.” Redefining
“flare-productive” at the M1.0 level, parameterizations of excess
photospheric magnetic energy outperform other variables. However, the
uniform flare-quiet classification rate is approximately 90%, while
incorporating photospheric magnetic field information results in at
most a 93% success rate. Using nonparametric discriminant analysis,
we demonstrate that the results are quite robust. Thus, we conclude
that the state of the photospheric magnetic field at any given time
has limited bearing on whether that region will be flare productive.
---------------------------------------------------------
Title: Topological Estimates of Free-Energy Build-up in Active Regions
Authors: Longcope, D. W.; Barnes, G.; Ravindra, B.; Beveridge, C.
2006AGUFMSH31B..02L Altcode:
There is a growing consensus that slow evolution of an active region's
photospheric flux leads to a build-up in the energy of its coronal
field. The anchoring of coronal field lines to the photosphere
defines a connectivity between photospheric footpoints of opposing
polarities. Due to the corona's extremely high electrical conductivity
these connections remain unchanged even as the footpoints move. To
estimate the energy stored this way, we group photospheric footpoints
into unipolar source regions and reduce the pointwise connectivity
map to a matrix of connections between regions. The flux in each such
connection must remain fixed even as the source regions evolve. One
coronal magnetic field, called the flux constrained equilibrium, has the
minimum possible energy for a specified connectivity. The free energy
in this equilibrium provides a lower bound on the free energy in the
actual field. We obtain such a free-energy lower bounds for several
observed active regions. Source regions are defined in a magnetogram
(MDI) time sequence of an active region. As the sources evolve the
connectivity in a potential field will change, however, the actual
connectivity will not. The growing disparity between the two is used to
estimate the free energy stored in the coronal field. Flare reconnection
will release some portion of this stored energy by changing some of
the connectivities. We compare these estimates with observational
signatures of energy release. This work supported by NASA's Living
with a Star Program and by AFOSR.
---------------------------------------------------------
Title: Estimating Active Region Free Energy and Helicity from the
Minimum Current Corona Model
Authors: Barnes, G.; Longcope, D. W.; Beveridge, C.; Ravindra, B.;
Leka, K. D.
2006IAUJD...3E..80B Altcode:
We employ the Minimum Current Corona (MCC) model to estimate the amount
of magnetic free energy and helicity injected into the coronal magnetic
field of an active region. In the MCC model, each concentration of
photospheric magnetic flux is represented by a point source, greatly
simplifying the magnetic topology. Advecting an initial partitioning
of the flux through a long time series of magnetograms results in
a persistent set of sources. We show that the centroid velocity of
a partition compares well with the flux-weighted average over the
partition of the local correlation tracking velocities. Flux domains,
bundles of field lines interconnecting pairs of sources, are surrounded
by separatrix surfaces. The intersection of two separatrices is
a separator field line, which is the site of reconnection in this
model. The evolution of the photospheric field causes the sources
to also evolve, which would lead to changes in the domain fluxes
to maintain a potential field configuration if reconnection could
proceed rapidly. However, in the absence of reconnection, currents
begin to flow to maintain the initial distribution of domain fluxes. The
minimum energy state occurs when currents flow along the separators. The
magnitude of the separator currents can be estimated and combined with
geometrical properties of the separators to give a lower bound to
the magnetic free energy of the system. The motion of sources about
one another adds braiding helicity to the system, while the internal
rotation of a partition adds spin helicity. Starting from an initial
potential field configuration, changes in the free energy are presented
for a time series of data for NOAA AR 8210 on 1 May 1998. This work
was supported by AFOSR, NSF and NASA.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring versus
Flare-quiet Active Regions. III. Magnetic Charge Topology Models
Authors: Barnes, G.; Leka, K. D.
2006ApJ...646.1303B Altcode:
A magnetic charge topology (MCT) model is applied to time series of
photospheric vector magnetic field data for seven active regions divided
into epochs classified as flare-quiet and flare-productive. In an
approach that parallels an earlier study by the authors using quantities
describing the photospheric properties of the vector magnetic field,
we define quantities derived from the MCT analysis that quantify the
complexity and topology of the active region coronal fields. With
the goal of distinguishing between flare-quiet and flare-imminent
magnetic topology, the time series are initially displayed for three
active regions for visual inspection with few clear distinguishing
characteristics resulting. However, an analysis of all 24 epochs
using the discriminant analysis statistical approach indicates that
coronal field topology, derived from the observed photospheric vertical
field, may indeed hold relevant information for distinguishing these
populations, although the small sample size precludes a definite
conclusion. The variables derived from the characterization of coronal
topology routinely result in higher probabilities of being able to
distinguish between the two populations than the analogous variables
derived for the photospheric field.
---------------------------------------------------------
Title: Observations of The Chromospheric Magnetic Field In Solar
Active Regions
Authors: Leka, K. D.; Metcalf, T. R.; Mickey, D. L.; Barnes, G.
2006IAUJD...3E..53L Altcode:
Measuring the magnetic field in solar active regions in all spatial
and temporal dimensions is a long-standing and ambitious goal in
solar physics. As the locations of complex and rapidly evolving
magnetic fields and the source of geo-effective energetic events,
understanding active region magnetic field generation and evolution
is extremely an important goal; however, basic physics presents great
challenges to achieving it. Measuring the chromospheric magnetic field
in active regions is an important first step beyond routine photospheric
measurements; important both for basic understanding of active region
structure but also for the many ramifications coming from chromosphere
being closer to a force-free state than the photosphere. However,
it is also a very difficult measurement. In this talk I will describe
highlights of our group's on-going efforts to understand solar active
region magnetic field structure via direct observation of the vector
chromospheric magnetic field. Since late 2003, the U. Hawai`i/Mees
Solar Observatory's Imaging Vector Magnetograph has routinely
acquired spectropolarimetry measurements of active regions across
the Na-I 589.6nm line; from the polarization at the line's near-wings
approximately 0.007nm from line center we deduce the vector magnetic
field. The data are specific to active regions, specifically the
structure, free energy storage and evolution at that low-chromospheric
layer. I will present recent results from these chromospheric data with
a focus on the differences between the photosphere and chromosphere,
and the free energy storage in solar active regions.
---------------------------------------------------------
Title: Modeling And Measuring The Flux Reconnected By The Two-ribbon
Flare On 2004-11-07
Authors: Longcope, Dana; Beveridge, C.; Qiu, J.; Belur, R.; Barnes, G.
2006SPD....37.0803L Altcode: 2006BAAS...38R.230L
Observations of the large two-ribbon flare on 2004-Nov-7 made using SOHO
and TRACE data are interpreted in terms of a three-dimensional magnetic
field model. This model predicts the amount of flux reconnected during
the flare and the energy it would release. These values are compared to
the flux swept up by the flare ribbons observed by TRACE in 1600 A and
the energy release inferred by the GOES light curves. The helicity of
the model field may be independently compared to the helicty injected
by photospheric motions during the buildup to the flare. The model also
predicts the sequence in which the reconnections should occur. This
in turn provide insight into the conversion of mutual helicity into
self-helicity during the production of a twisted flux rope.This work
is supported by NASA Grant NAG5-10489 and DoD MURI grant.
---------------------------------------------------------
Title: Progress on Determining What Makes a Flare-Producing Active
Region
Authors: Leka, K. D.; Barnes, G.
2006SPD....37.2203L Altcode: 2006BAAS...38R.249L
We present the results of a large effort to investigate what,
if anything,can be determined from observations of solar
photospheric magnetic fieldsconcerning the flare productivity of
active regions. Different aspects ofthis work include examining the
temporal variations of the field prior toflare events, and applying the
Magnetic Charge Topology model in order toquantify the variations of the
coronal topology prior to flare events.A slightly different approach
was also investigated, by dropping thetime-sequence data and using a
statistically significant data-base of"daily" magnetograms. Throughout,
a statistical evaluation based onDiscriminant Analysis was used
to determine how the two populations inquestion (flare-producing
and flare-quiet) could best be differentiated,often using numerous
variables simultaneously. In this presentation,the results from this
project will be summarized in the context offlare-forecasting but also
in the context of applying the results tomodeling efforts.
---------------------------------------------------------
Title: Measuring the Magnetic Free Energy Available for Solar Flares
Authors: Metcalf, Thomas R.; Leka, K. D.; Mickey, D. L.; Barnes, G.
2006SPD....37.0903M Altcode: 2006BAAS...38..236M
In this poster we report on recent progress in the effort to measurethe
magnetic energy available to power solar flares. To directlymeasure the
free magnetic energy using the virial theorem, themagnetic field must
be known at an atmospheric height where it isforce-free, i.e. J x B =
0. In Metcalf, Leka & Mickey (2005) the freeenergy of AR 10486
was determined just prior to the X10 flare at20:39UT on 29 October
2003, using vector magnetic field measurementsobtained in the solar
chromosphere where the field is force-free. Theresults from this study
are expanded here to a wider investigation ofthe magnetic energy storage
in flare- and CME-producing activeregions. With appropriate effort and
instrumentation, directlymeasuring the free energy and its evolution
may provide a powerfulflare-prediction capability. This research was
funded by NASAcontract NAG5-12466 and AFOSR contract F49620-03-C-0019.
---------------------------------------------------------
Title: Quantifying the Performance of Force-free Extrapolation
Methods Using Known Solutions
Authors: Barnes, G.; Leka, K. D.; Wheatland, M. S.
2006ApJ...641.1188B Altcode:
We outline a method for quantifying the performance of extrapolation
methods for magnetic fields. We extrapolate the field for two
model cases, using a linear force-free approach and a nonlinear
approach. Each case contains a different topological feature of
the field that may be of interest in solar energetic events. We are
able to determine quantitatively whether either method is capable
of reproducing the topology of the field. In one of our examples, a
subjective evaluation of the performance of the extrapolation suggests
that it has performed quite well, while our quantitative score shows
that this is not the case, indicating the importance of being able
to quantify the performance. Our method may be useful in determining
which extrapolation techniques are best able to reproduce a force-free
field and which topological features can be recovered.
---------------------------------------------------------
Title: Measuring Braiding and Spin Helicity Fluxes in Active Regions
Authors: Belur, R.; Longcope, D. W.; Barnes, G.
2005AGUFMSH11A0248B Altcode:
Magnetic helicity has become a valuable theoretical tool for
understanding the dynamics of the solar corona. The free energy
stored in the coronal magnetic field can be estimated based on its
helicity content. Furthermore, rapid release of stored energy must be
accomplished while preserving the total magnetic helicity. Recently
long time-sequences of magnetograms have been used to measure the flux
of helicity into active regions. We demonstrate how this helicity flux
can be usefully decomposed into a sum of spin helicity terms and an
overall mutual helicity term. Each magnetogram is partitioned into
a set of unipolar regions. These must persist through the sequence
and track the photospheric flow. The spin helicity of a given region
quantifies the effects of motions internal to that region, while
braiding helicity is injected by the motions of whole regions about
one another. Since the terms themselves can be of different signs
it is possible to re-distribute the coronal helicity by reconnection
without changing the overall helicity content. This decomposition is
demonstrated on active region observations.
---------------------------------------------------------
Title: Implementing a Magnetic Charge Topology Model for Solar
Active Regions
Authors: Barnes, G.; Longcope, D. W.; Leka, K. D.
2005ApJ...629..561B Altcode:
Information about the magnetic topology of the solar corona is
crucial to the understanding of solar energetic events. One approach to
characterizing the topology that has had some success is magnetic charge
topology, in which the topology is defined by partitioning the observed
photospheric field into a set of discrete sources and determining which
pairs are interlinked by coronal field lines. The level of topological
activity is then quantified through the transfer of flux between
regions of differing field line connectivity. We discuss in detail how
to implement such a model for a time series of vector magnetograms,
paying particular attention to distinguishing real evolution of the
photospheric magnetic flux from changes due to variations in atmospheric
seeing, as well as uncorrelated noise. We determine the reliability
of our method and estimate the uncertainties in its results. We then
demonstrate it through an application to NOAA active region 8210,
which has been the subject of extensive previous study.
---------------------------------------------------------
Title: On the Availability of Sufficient Twist in Solar Active
Regions to Trigger the Kink Instability
Authors: Leka, K. D.; Fan, Y.; Barnes, G.
2005ApJ...626.1091L Altcode:
The question of whether there is sufficient magnetic twist in solar
active regions for the onset of the kink instability is examined
using a “blind test” of analysis methods commonly used to interpret
observational data. “Photospheric magnetograms” are constructed from
a recently developed numerical simulation of a kink-unstable emerging
flux rope with nearly constant (negative) wind. The calculation of the
best-fit linear force-free parameter α<SUB>best</SUB> is applied,
with the goal of recovering the model input helicity. It is shown
that for this simple magnetic structure, three effects combine to
produce an underestimation of the known helicity: (1) the influence of
horizontal fields with lower local α values within the flux rope; (2)
an assumed simple relation between α<SUB>best</SUB> and the winding
rate q does not apply to nonaxis fields in a flux rope that is not
thin; and (3) the difficulty in interpreting the force-free twist
parameter measured for a field that is forced. A different method to
evaluate the magnetic twist in active region flux ropes is presented,
which is based on evaluating the peak α value at the flux rope
axis. When applied to data from the numerical simulation, the twist
component of the magnetic helicity is essentially recovered. Both
the α<SUB>best</SUB> and the new α<SUB>peak</SUB> methods are then
applied to observational photospheric vector magnetic field data of
NOAA AR 7201. The α<SUB>best</SUB> approach is then confounded further
in NOAA AR 7201 by a distribution of α that contains both signs, as
is generally observed in active regions. The result from the proposed
α<SUB>peak</SUB> approach suggests that a larger magnetic twist is
present in this active region's δ-spot than would have been inferred
from α<SUB>best</SUB>, by at least a factor of 3. It is argued that
the magnetic fields in localized active region flux ropes may indeed
carry greater than 2π winds, and thus the kink instability is a
possible trigger mechanism for solar flares and coronal mass ejections.
---------------------------------------------------------
Title: The OSIRIS instrument on the Odin spacecraft
Authors: Llewellyn, E.; Lloyd, N. D.; Degenstein, D. A.; Gattinger,
R. L.; Petelina, S. V.; Bourassa, A. E.; Wiensz, J. T.; Ivanov, E. V.;
McDade, I. C.; Solheim, B. H.; McConnell, J. C.; Haley, C. S.; von
Savigny, C.; Sioris, C. E.; McLinden, C. A.; Griffioen, E.; Kaminski,
J.; Evans, W. F. J.; Puckrin, E.; Strong, K.; Wehrle, V.; Hum, R. H.;
Kendall, D. J. W.; Matsushita, J.; Murtagh, D. P.; Brohede, S.;
Stegman, J.; Witt, G.; Barnes, G.; Payne, W. F.; Piché, L.; Smith,
K.; Warshaw, G.; Deslauniers, D. L.; Marchand, P.; Richardson, E. H.;
King, R. A.; Wevers, I.; McCreath, W.; Kyrölä, E.; Oikarinen, L.;
Leppelmeier, G. W.; Auvinen, H.; Megie, G.; Hauchecorne, A.; Lefevre,
F.; de La Nöe, J.; Ricaud, P.; Frisk, U.; Sjoberg, F.; von Schéele,
F.; Nordh, L.
2004CaJPh..82..411L Altcode:
The optical spectrograph and infrared imager system (OSIRIS) on
board the Odin spacecraft is designed to retrieve altitude profiles
of terrestrial atmospheric minor species by observing limb-radiance
profiles. The grating optical spectrograph (OS) obtains spectra
of scattered sunlight over the range 280-800 nm with a spectral
resolution of approximately 1 nm. The Odin spacecraft performs a
repetitive vertical limb scan to sweep the OS 1 km vertical field of
view over selected altitude ranges from approximately 10 to 100 km. The
terrestrial absorption features that are superimposed on the scattered
solar spectrum are monitored to derive the minor species altitude
profiles. The spectrograph also detects the airglow, which can be
used to study the mesosphere and lower thermosphere. The other part of
OSIRIS is a three-channel infrared imager (IRI) that uses linear array
detectors to image the vertical limb radiance over an altitude range
of approximately 100 km. The IRI observes both scattered sunlight and
the airglow emissions from the oxygen infrared atmospheric band at 1.27
mum and the OH (3-1) Meinel band at 1.53 mum. A tomographic inversion
technique is used with a series of these vertical images to derive the
two-dimensional distribution of the emissions within the orbit plane.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring
vs. Flare-Quiet Active Regions III: Discriminant Analysis of a
Statistically Significant Database
Authors: Leka, K. D.; Barnes, G.
2004AAS...204.3905L Altcode: 2004BAAS...36..715L
Solar active regions are often evaluated for their potential to produce
energetic events based their magnetic morphology. Quantitatively,
this information is available using vector magnetic field information
which is (presently only) routinely gathered from photospheric
observations. Recently we demonstrated a method of parameterizing vector
field information such that variations in the magnetic morphology
and complexity were contained in the statistical description of
(as examples) the vertical current or magnetic shear angles; it was
also demonstrated that no single parameter consistently and uniquely
displayed pre-event variations (Leka & Barnes 2003a). We also
showed that with Discriminant Analysis (Leka & Barnes 2003b), it is
possible to distinguish between an event-imminent photospheric magnetic
state and an event-quiet state -- but only by considering multiple
variables simultaneously. The limitations of that demonstration were
primarily due to small-number statistics given the dataset used. <P
/>In the present work, Discriminant Analysis is applied to a very
different dataset: the daily "survey" magnetograms obtained by the
U. Hawai`i/Mees Solar Observatory Imaging Vector Magnetograph. In
this manner, the problem of small-number statistics is relieved and
advantages available by DA are explored. However, given the daily
temporal cadence, the focus shifts toward detecting parametric
thresholds rather than pre-event-specific evolution. Nonetheless,
the central question remains how to distinguish a region which is
primed for an energetic event, with results applicable to modeling
efforts by providing empirical discriminating information as to the
pre-eruption state of the boundary magnetic field. <P />This effort
is funded by contract F49620-03-C-0019 through the Air Force Office
of Scientific Research.
---------------------------------------------------------
Title: Magnetic Topology, Flux Emergence/Reconnection and Velocities
from a Magnetic Charge Topology Model for Solar Active Regions
Authors: Barnes, G.; Longcope, D. W.; Leka, K. D.
2004AAS...204.3906B Altcode: 2004BAAS...36..715B
Magnetic Charge Topology (MCT) models represent the field in the solar
corona as being due to collection of point magnetic charges located at
or below the photosphere. These models have the advantage of providing a
simple quantitative description of the field topology. We apply MCT to
time series of magnetograms from the U. Hawai`i/Mees Solar Observatory
Imaging Vector Magnetograph (IVM). We first describe the evolution of
the magnetic topology of the region, by calculating such quantities
as the magnetic flux connecting each pair of point sources, and the
number and locations of magnetic separators, which are likely to be
the location of reconnection in the solar corona. Using the changes in
the magnitudes of the point sources, and in the connectivity matrix,
we estimate the rate at which flux is emerging and submerging through
the photosphere, and the rate at which reconnection is happening in
the corona. By tracking the changes in the locations of the sources,
we are also able to estimate the horizontal velocities. <P />This work
was performed under Air Force Office of Scientific Research contracts
F49620-03-C-0019 and F49620-02-C-0191.
---------------------------------------------------------
Title: Observational consequences of a magnetic flux rope topology
Authors: Gibson, S.; Barnes, G.; Demoulin, P.; Fan, Y.; Fisher, G.;
Leka, K.; Longcope, D.; Mandrini, C.; Metcalf, T.
2003AGUFMSH42B0516G Altcode:
We consider the implications of a magnetic flux rope topology for
the interpretation of observations of sigmoidal active regions. A
region of tangential magnetic discontinuities can be identified
using techniques that determine a bald patch (BP) and corresponding
separatrices or a quasi-separatrix layer (QSL) -- for a flux rope this
region can be S-shaped, or sigmoidal. If such a region is physically
driven, current sheets can form yielding conditions appropriate for
reconnective heating. Using a numerical simulation of an emerging
flux rope driven by the kink instability, Fan and Gibson (ApJL, 2003)
showed that current sheets indeed formed a sigmoidal surface. In this
poster we will demonstrate that the current sheets formed on the BP and
BP separatrices. Moreover, we will use the results of the numerical
simulation as proxies for observations: specifically the simulated
field at the photosphere as proxy for the magnetic boundary condition,
the sigmoidal current sheets as proxy for the X-ray active region
emission, and the location of dipped magnetic field lines as proxy
for a filament. We will then consider to what extent such observations
might be used to understand and constrain the basic properties of the
coronal field.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring versus
Flare-quiet Active Regions. I. Data, General Approach, and Sample
Results
Authors: Leka, K. D.; Barnes, G.
2003ApJ...595.1277L Altcode:
Photospheric vector magnetic field data from the University of
Hawai'i Imaging Vector Magnetograph, with good spatial and temporal
sampling, are used to study the question of identifying a preflare
signature unique to flare events in parameters derived from the
magnetic vector field, B. In this first of a series of papers, we
present the data analysis procedure and sample results focusing only
on three active regions (NOAA Active Regions 8636, 8771, and 0030),
three flares (two M class and one X class), and (most importantly) a
flare-quiet epoch in a comparable flare-producing region. Quantities
such as the distribution of the field morphology, horizontal spatial
gradients of the field, vertical current, current helicity, “twist”
parameter α, and magnetic shear angles are parameterized using their
moments and appropriate summations. The time series of the resulting
parameterizations are examined one at a time for systematic differences
in overall magnitude and evolution between the flare and flare-quiet
examples. The variations expected due to atmospheric seeing changes
are explicitly included. In this qualitative approach we find (1)
no obvious flare-imminent signatures from the plain magnetic field
vector and higher moments of its horizontal gradient or from most
parameterizations of the vertical current density; (2) counterintuitive
but distinct flare-quiet implications from the inclination angle
and higher moments of the photospheric excess magnetic energy; (3)
flare-specific or flare-productivity signatures, sometimes weak,
from the lower moments of the field gradients, kurtosis of the
vertical current density, magnetic twist, current helicity density,
and magnetic shear angle. The strongest results are, however, that (4)
in ensuring a flare-unique signature, numerous candidate parameters
(considering both their variation and overall magnitude) are nullified
on account of similar behavior in a flare-quiet region, and hence (5)
considering parameters one at a time in this qualitative manner is
inadequate. To address these limitations, a quantitative statistical
approach is presented in Paper II by Leka & Barnes.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring versus
Flare-quiet Active Regions. II. Discriminant Analysis
Authors: Leka, K. D.; Barnes, G.
2003ApJ...595.1296L Altcode:
We apply statistical tests based on discriminant analysis to the wide
range of photospheric magnetic parameters described in a companion paper
by Leka & Barnes, with the goal of identifying those properties
that are important for the production of energetic events such as solar
flares. The photospheric vector magnetic field data from the University
of Hawai'i Imaging Vector Magnetograph are well sampled both temporally
and spatially, and we include here data covering 24 flare-event and
flare-quiet epochs taken from seven active regions. The mean value
and rate of change of each magnetic parameter are treated as separate
variables, thus evaluating both the parameter's state and its evolution,
to determine which properties are associated with flaring. Considering
single variables first, Hotelling's T<SUP>2</SUP>-tests show small
statistical differences between flare-producing and flare-quiet
epochs. Even pairs of variables considered simultaneously, which do
show a statistical difference for a number of properties, have high
error rates, implying a large degree of overlap of the samples. To
better distinguish between flare-producing and flare-quiet populations,
larger numbers of variables are simultaneously considered; lower error
rates result, but no unique combination of variables is clearly the
best discriminator. The sample size is too small to directly compare the
predictive power of large numbers of variables simultaneously. Instead,
we rank all possible four-variable permutations based on Hotelling's
T<SUP>2</SUP>-test and look for the most frequently appearing variables
in the best permutations, with the interpretation that they are
most likely to be associated with flaring. These variables include
an increasing kurtosis of the twist parameter and a larger standard
deviation of the twist parameter, but a smaller standard deviation of
the distribution of the horizontal shear angle and a horizontal field
that has a smaller standard deviation but a larger kurtosis. To support
the “sorting all permutations” method of selecting the most frequently
occurring variables, we show that the results of a single 10-variable
discriminant analysis are consistent with the ranking. We demonstrate
that individually, the variables considered here have little ability
to differentiate between flaring and flare-quiet populations, but with
multivariable combinations, the populations may be distinguished.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring
vs. Flare-Quiet Active Regions I: Data, General Approach, and
Statistical Results
Authors: Leka, K. D.; Barnes, G.
2003SPD....34.1615L Altcode: 2003BAAS...35R.835L
Photospheric vector magnetic field data from the U. Hawai`i Imaging
Vector Magnetograph are examined for pre-event signatures unique to
solar energetic phenomena. Parameters are constructed from B(x,y) to
describe (for example) the distributions of the field, spatial gradients
of the field, vertical current, current helicity, ”twist” parameter
α and magnetic shear angles. A quantitative statistical approach
employing discriminant analysis and Hotelling's T<SUP>2</SUP>-test is
applied to the magnitude and temporal evolution of parameters from
24 flare-event and flare-quiet epochs from seven active regions. <P
/>We demonstrate that (1) when requiring a flare-unique signature,
numerous candidate parameters are nullified by considering flare-quiet
epochs, (2) a more robust method exists for estimating error rates
than conventional ”truth tables”, (3) flaring and flare-quiet
populations do not necessarily have low error rates for classification
even when statistically distinguishable, and that (4) simultaneous
consideration of a large number of variables is required to produce
acceptable error rates. That is, when the parameters are considered
individually, they show little ability to differentiate between the
two populations; multi-variable combinations can discriminate the
populations and/or result in perfect classification tables. <P />In
lieu of constructing a single all-variable discriminant function to
quantify the flare-predictive power of the parameters considered,
we devise a method whereby all permutations of the four-variable
discriminant functions are ranked by Hotelling's T<SUP>2</SUP>. We
present those parameters (e.g. the temporal increase of the kurtosis
of the spatial distribution of the vertical current density) which
consistently appear in the best combinations, indicating that they may
play an important role in defining a pre-event photospheric state. While
no single combination is clearly the best discriminator, we demonstrate
here the requisite approach: include flare-quiet epochs as a control
group for statistical tests of the null hypothesis. <P />This work
was performed under Air Force Office of Scientific Research contracts
F49620-00-C-0004 and F49620-03-C-0019.
---------------------------------------------------------
Title: Photospheric Magnetic Field Properties of Flaring
vs. Flare-Quiet Active Regions II: A Magnetic Charge Topology Model
and Statistical Results
Authors: Barnes, G.; Leka, K. D.; Longcope, D. W.
2003SPD....34.1616B Altcode: 2003BAAS...35..835B
The complexity of the coronal magnetic field extrapolated from a
Magnetic Charge Topology (MCT) model, is examined for pre-event
signatures unique to solar energetic phenomena. Although extensive
use has been made of quantities measured at the photosphere, it is
important to consider the magnetic field in the corona, where (for
example) the hard X-ray signatures of energy release in solar flares
are observed. By quantifying the inferred coronal magnetic topology we
are no longer limited to considering solely the magnetic state of the
photosphere. <P />MCT is applied to temporally sampled photospheric
magnetic data from the U. Hawai`i Imaging Vector Magnetograph, for
24 flare-event and flare-quiet epochs from seven active regions. We
outline the methodology employed for automating the application of MCT
to large data sets of complex active regions: partitioning the observed
B<SUB>z</SUB> at the photosphere, assigning a charge to each partition,
and using this charge distribution to extrapolate the field in the
corona. From the resulting field we compute the connectivity matrix
ψ <SUB>ij</SUB>, the location of null points and the intersection
of separatrix surfaces, i.e. separator field lines. Parameters are
constructed to describe, for example, the magnetic connectivities, the
magnetic flux in those connections, and the number of separators. <P
/>Examining particular events results in no obvious trends in the
magnitude and temporal evolution of the parameters just prior to
flare events. Thus, we employ the same quantitative statistical
approach outlined in Leka and Barnes [this session], i.e. applying
discriminant analysis and Hotelling's T<SUP>2</SUP>-test, and ranking
all four-variable discriminant functions as a proxy for a single
all-variable discriminant function. We present those parameters which
consistently appear in the best combinations, indicating that they
may play an important role in defining a pre-event coronal state. <P
/>This work was performed under Air Force Office of Scientific Research
contracts F49620-00-C-0004, F49620-03-C-0019 and F49620-02-C-0191.
---------------------------------------------------------
Title: Photospheric Magnetic Fields Complexity Variations and
Solar Flares
Authors: Barnes, G.; Leka, K. D.; Longcope, D. W.
2002AAS...200.6808B Altcode: 2002BAAS...34..756B
Do photospheric magnetic fields show systematic changes which precede
energetic events such as solar flares? The answer has proved elusive. We
address this question by examining vector magnetic flux maps from
the U. Hawai`i Imaging Vector Magnetograph (Mickey et al. 1996),
which obtain full Stokes spectra over entire active regions every
4 minutes on average. We compare numerous parameters derived from
the vector magnetograms of flaring active regions to those from
comparable non-flaring active regions. In addition, we determine
quantitative measurements of the complexity of the field topology
using the Minimum-Current Corona analysis (Longcope 1996). The goal
is to determine quantitative measurements of the complexity of the
field topology, and determine whether variations in those measures
correlate with or precede flare events. This project was funded by
AFOSR contract F49620-00-C-0004.
---------------------------------------------------------
Title: Frequency Dependent Ray Paths in Local Helioseismology
Authors: Barnes, G.; Cally, P. S.
2001PASA...18..243B Altcode:
The surface of the Sun is continually oscillating due to sound waves
encroaching on it from the interior. Measurements of the surface
velocity are used to infer some of the properties of the regions
through which the sound waves have propagated. Traditionally,
this has been done by using a modal decomposition of the surface
disturbances. However, the use of ray descriptions, in the form
of acoustic holography or time-distance helioseismology, provides
an alternative approach which may reveal more detailed information
about the properties of local phenomena such as sunspots and active
regions. Fundamental to any such treatment is determining the correct
ray paths in a given atmosphere. In the simplest approach, the ray
paths are constructed to minimise the travel time between two points
(Fermat's principle). However, such an approach is only valid in the
high frequency limit, ω≫ω<SUP>c</SUP>, N, where ω<SUP>c</SUP>
is the acoustic cut-off and N the Brunt-Väisälä frequency. Although
ω<SUP>c</SUP> is often included in time-distance calculations, and N
occasionally, the same is not true of acoustic holography. We argue
that this raises concerns about image sharpness. As illustrations,
representative ray paths are integrated in a realistic solar model to
show that the Fermat approximation performs poorly for frequencies of
helioseismic interest. We also briefly discuss the importance of the
Brunt-Väisälä frequency to the time-distance diagram.
---------------------------------------------------------
Title: Mode Mixing by a Shallow Sunspot
Authors: Barnes, G.; Cally, P. S.
2000SoPh..193..373B Altcode:
Sunspots are strong absorbers of f and p modes. A possible
absorption mechanism is direct conversion to slow magnetoacoustic
waves. Calculations based on vertical magnetic field models show that
this works well for f modes, but is inadequate for p modes. Using a very
simple `shallow spot' model, in which the effects of the magnetic field
are accounted for solely by a surface condition, we investigate the
possibility that p modes first scatter into f modes inside the spot,
which are then more susceptible to conversion to slow modes. We find
that the coupling between an incident p mode and the internal f mode
is unlikely to be strong enough to account for the observed absorption,
but that the incident modes do couple strongly to the acoustic jacket in
some cases, leading to a region immediately around the sunspot where a
significant fraction of the surface velocity is due to the jacket modes.
---------------------------------------------------------
Title: Magnetic fields and light element depletion in the Sun
Authors: Charbonneau, P.; Barnes, G.; MacGregor, K. B.
2000IAUJD...5E..14C Altcode:
I will first briefly review some important similarities and differences
in models for the spin-down of solar-type stars, with or without
internal magnetic fields in their radiative interior. This will
be followed by a presentation of some simple calculations for
the main-sequence depletion of Lithium and Beryllium in the Sun,
in a regime where magnetic fields provide the chief mechanism for
the internal redistribution of angular momentum. In this model the
transport of light elements still occurs in response to shear-induced
small-scale turbulence, following various commonly used prescriptions
for the transport coefficients. For some (physically reasonable)
values of model parameters, both internal differential rotation and
light element abundances end up solar-like at 4.5Gyr. Within this
framework light element depletion is a sensitive function of the
strength of the assumed internal magnetic field.
---------------------------------------------------------
Title: On the magnetohydrodynamics of a conducting fluid between
two flat plates
Authors: Barnes, G.; MacGregor, K. B.
1999PhPl....6.3030B Altcode:
The time-dependent flow of a viscous, electrically conducting fluid
contained within the space between two parallel, semi-infinite,
perfectly conducting plates is considered. A uniform magnetic field
directed perpendicular to the plate surfaces is assumed to pervade
the fluid. Oscillatory motion of one of the plates in its own plane
is induced through the application of a prescribed acceleration, the
magnitude and direction of which vary sinusoidally in time. For a system
forced in this manner, the resulting flow and transverse field component
are solved for, as well as for the motion of the plate as a function
of time. The magnetic and viscous stresses exerted on the boundary
plate by the contiguous field and fluid are explicitly incorporated
into the treatment of its motion. The physical properties and behavior
of this system are investigated by examining analytic and numerical
solutions obtained for a range of forcing periods, Reynolds numbers,
and plate mass column densities. The relevance of these results to the
interpretation of a model for Alfvénic torsional oscillations in the
solar interior are discussed.
---------------------------------------------------------
Title: Mode Mixing by a Shallow Sunspot
Authors: Barnes, G.; Cally, P. S.
1999soho....9E..35B Altcode:
In a polytropic atmosphere, the oscillation modes are described by
well-known special functions. However, the presence of a magnetic
field inside a sunspot makes the mode structure much more complex,
so that analytic expressions are not available. Recent observations
of sunspots suggest that most of the scattering and absorption due
to the spots occurs in a layer immediately below the surface of the
sun. We have therefore modelled the acoustic modes inside a sunspot by
assuming that the effect of the magnetic field is concentrated right
at the surface. Instead of imposing the conventional upper boundary
condition, that the divergence of the velocity vanish, we require
that the horizontal component of the velocity vanish, which allows
us to write down analytic expressions for the acoustic modes inside
the spot. This may be justified by arguing that a vertical magnetic
field will tend to inhibit horizontal fluid motions. In effect, we are
introducing a purely scattering disk to the surface of the sun. More
realistic models, in which the disk both scatters and absorbs energy are
also possible. We consider the scattering of an incident p-mode off our
"sunspot," matching the pressure and horizontal velocity across the
boundary. The result is a mixing of the incident mode into outgoing
external p-modes and internal p-modes, as well as jacket modes both
inside and outside the spot. We find that the inclusion of the jacket
modes is crucial to satisfying the matching conditions, and we present
results indicating the spectrum of outgoing and internal modes that
are present.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones. III. The Solar Light-Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1999ApJ...511..466B Altcode:
We calculate the depletion of the trace elements lithium and beryllium
within a solar-mass star during the course of its evolution from
the zero-age main sequence to the age of the present-day Sun. In the
radiative layers beneath the convection zone, we assume that these
elements are transported by the turbulent fluid motions that result from
instability of the shear flow associated with internal differential
rotation. This turbulent mixing is modeled as a diffusion process,
using a diffusion coefficient that is taken to be proportional to the
gradient of the angular velocity distribution inside the star. We study
the evolution of the light-element abundances produced by rotational
mixing for models in which internal angular momentum redistribution
takes place either by hydrodynamic or by hydromagnetic means. Since
models based on these alternative mechanisms for angular-momentum
transport predict similar surface rotation rates late in the evolution,
we explore the extent to which light-element abundances make it possible
to distinguish between them. In the case of an internally magnetized
star, our computations indicate that both the details of the surface
abundance evolution and the magnitude of the depletion at solar age can
depend sensitively on the assumed strength and configuration of the
poloidal magnetic field inside the star. For a configuration with no
direct magnetic coupling between the radiative and convective portions
of the stellar interior, the depletion of lithium calibrated to the
solar lithium depletion at the solar age is similar at all ages to
the lithium depletion of a model in which angular-momentum transport
occurs solely by hydrodynamical processes. However, the two models can
be distinguished on the basis of their respective beryllium depletions,
with the depletion of the magnetic model being significantly smaller
than that of the nonmagnetic model.
---------------------------------------------------------
Title: Gravity Waves in a Magnetized Shear Layer
Authors: Barnes, G.; MacGregor, K. B.; Charbonneau, P.
1998ApJ...498L.169B Altcode:
We use the equations governing the propagation of a gravity wave
in the presence of a background flow and magnetic field to derive,
in the Boussinesq approximation, dispersion relations for plane wave
solutions in certain special cases. We show how, under conditions
typical of the interior of the Sun, the addition of a magnetic field
may prevent certain wavevectors from propagating and alter the existence
and position of any critical layer that might absorb the gravity wave.
---------------------------------------------------------
Title: Angular Momentum Transport in Magnetized Stellar Radiative
Zones: The Solar Light Element Abundances
Authors: Barnes, G.; Charbonneau, P.; MacGregor, K. B.
1998ASPC..154..886B Altcode: 1998csss...10..886B
We calculate the depletion of the trace elements lithium and beryllium
within a solar mass star, during the course of its evolution from
the zero-age main sequence to the age of the present-day Sun. In the
radiative layers beneath the convection zone, we assume that these
elements are transported by the turbulent fluid motions that result from
the instability of the shear flow associated with internal differential
rotation. This turbulent mixing is modeled as a diffusive process,
using a diffusion coefficient that is taken to be proportional to the
gradient of the angular velocity distribution inside the star. We study
the evolution of the light element abundances produced by rotational
mixing for models in which internal angular momentum redistribution
takes place either by hydrodynamic or by hydromagnetic means. Since
models based on these alternative mechanisms for angular momentum
transport predict similar surface rotation rates late in the evolution,
we explore the extent to which light element abundances make it possible
to distinguish between them. In the case of an internally magnetized
star, our computations indicate that both the details of the surface
abundance evolution and the magnitude of the depletion at solar age can
depend sensitively on the assumed strength and configuration of the
poloidal magnetic field inside the star. For a configuration with no
direct magnetic coupling between the radiative and convective portions
of the stellar interior, the depletion of lithium as a function of
age is similar to that of a model in which angular momentum transport
occurs solely by hydrodynamical processes. However, the two models can
be distinguished on the basis of their respective beryllium depletions,
with the depletion of the magnetic model being significantly smaller
than that of the non-magnetic model.
---------------------------------------------------------
Title: Mixing in Low-Mass Stars: The Lithium-Rotation Connection
Authors: Balachandran, Suchitra C.; Garcia Lopez, R. J.; Kraft, R. P.;
MacGregor, K. B.; Barnes, G.; Martin, E. L.; Pinsonneault, Marc H.
1998ASPC..154..111B Altcode: 1998csss...10..111B
We have known for over three decades that the Sun has depleted
its surface lithium. During this period it has become increasingly
evident that mixing, unaccounted for by the standard models, occurs
in the stellar interior. There is some conjecture that this mixing
may be driven by rotation and thus be dependent upon the rotational
history of the star. In this discussion session, we will examine the
observational connection between mixing and rotation and critically
evaluate current models.
