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Author name code: metcalf
ADS astronomy entries on 2022-09-14
author:"Metcalf, Thomas R." OR author:"Metcalf, Tomas" OR author:"Metcalf, Tom"
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Title: On Friederich's New Fine-Tuning Argument
Authors: Metcalf, Thomas
2021FoPh...51...31M Altcode:
The most common objection to the Fine-Tuning Argument for the Multiverse
is that the argument commits the Inverse Gambler's Fallacy. Simon
Friederich has recently composed an interesting version of this
fine-tuning argument that avoids this fallacy and better-matches
important scientific instances of anthropic reasoning. My thesis in
this paper is that this new argument, while it may avoid the fallacy,
contains a disputable premise concerning the prior probabilities of
the hypotheses at issue. I consider various ways to modify the argument
to avoid this problem, but I argue that plausible replacements render
other lines unjustified. I also briefly compare `indexical' fine-tuning
arguments such as Friederich's, according to which our universe permits
life, to `existential' fine-tuning arguments, according to which some
universe or other permits life. I conclude that while Friederich is
correct that the new fine-tuning argument avoids the Inverse Gambler's
Fallacy, the argument nevertheless depends on an unjustified premise,
and this is further reason for proponents of fine-tuning arguments for
the multiverse to employ existential arguments rather than indexical
arguments.
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Title: The Imaging Vector Magnetograph at Haleakalā IV: Stokes
Polarization Spectra in the Sodium D<SUB>1</SUB> 589.6 nm Spectral
Line
Authors: Leka, K. D.; Mickey, Donald L.; Uitenbroek, Han; Wagner,
Eric L.; Metcalf, Thomas R.
2012SoPh..278..471L Altcode:
The Imaging Vector Magnetograph (IVM) at the Mees Solar Observatory,
Haleakalā, Maui, Hawai'i, obtained many years of vector magnetic-field
data in the photospheric Fe I 630.25 nm line. In the latter period
of its operation, the IVM was modified to allow routine observations
in the chromospheric Na I D<SUB>1</SUB> line, as well as the Fe I
line. We describe the sodium observational data in detail, including the
data-reduction steps that differ from those employed for the Fe I 630.25
nm line, to obtain calibrated Stokes polarization spectra. We have
performed a systematic comparison between the observational data and
synthetic NLTE Na I D<SUB>1</SUB> Stokes spectra derived for a variety
of solar-appropriate atmospheric and magnetic configurations. While
the Na I D<SUB>1</SUB> Stokes polarization signals from the solar
atmosphere are expected to be weak, they should generally be within the
IVM capability. A comparison between synthetic spectra and observational
data indicates that this is indeed the case.
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Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with
the Imaging Vector Magnetograph II: The Magnetic Maps
Authors: Derouich, M.; Leka, K. D.; Mickey, D. L.; Uitenbroek, H.;
Metcalf, T. R.
2010shin.confE...5D Altcode:
Following Poster I (Leka et al), we focus here on recent progress
regarding the inversion algorithms and interpretation of Zeeman
Na D1 589.6nm Stokes Spectra observed using the Imaging Vector
Magnetograph. We present systematic tests of the inversion procedures
and our interpretation of the results, relying on synthetic Na D1
589.6nm Stokes Spectra generated using known magnetic and atmospheric
models described in Poster I. In this second poster, we present the
results of "hare and hound" exercises which focus on (1) the optimal
fitting and inversion algorithm for the Na D1 589.6nm Stokes spectra,
and (2), the interpretation of the resulting active magnetic field
"maps", especially as a function of inferred height of the solar
atmosphere sampled by these Zeeman-polarization spectra. <P />This
work has been supported by AFOSR contract F49620-03-C-0019, NSF space
weather program grant ATM-0519107 and NSF SHINE grant ATM-0454610.
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Title: Observing and Interpreting Na D1 589.6nm Stokes Spectra with
the Imaging Vector Magnetograph I: Polarization Spectra
Authors: Leka, K. D.; Mickey, D. L.; Uitenbroek, H.; Derouich, M.;
Metcalf, T. R.
2010shin.confE...4L Altcode:
We report on progress made recently on validating and interpreting
Stokes spectra from the Na D1 589.6nm line observed using the Imaging
Vector Magnetograph at Mees Solar Observatory, Haleakala, Maui. While
preliminary results from the dataset (which comprises daily observations
of active regions spanning 2000 - 2005, plus a few additional special
observation campaigns) have been reported previously (e.g., Leka
& Metcalf 2003; Metcalf Leka & Mickey 2005), we focus here
on systematic tests of the observed data and our interpretation of
them. In this first poster, we present Non-LTE synthetic Na D1 589.6nm
spectra, computed using known underlying magnetic and atmospheric
models, which form the basis for various "hare and hound" exercises
to test (1) the observed degree of polarization in (2) the context
of the instrument response and photon noise. While we find generally
excellent agreement, there are some systematic differences between
the synthetic and observational data. We summarize our understandings
of these differences and attempts to mitigate their effects in the
context of the inversion to a magnetic field map (see Poster II,
Derouich et al). <P />This work has been supported by AFOSR contract
F49620-03-C-0019, NSF space weather program grant ATM-0519107 and NSF
SHINE grant ATM-0454610.
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Title: Resolving the 180° Ambiguity in Solar Vector Magnetic
Field Data: Evaluating the Effects of Noise, Spatial Resolution,
and Method Assumptions
Authors: Leka, K. D.; Barnes, Graham; Crouch, A. D.; Metcalf, Thomas
R.; Gary, G. Allen; Jing, Ju; Liu, Y.
2009SoPh..260...83L Altcode:
The objective testing of algorithms for performing ambiguity
resolution in vector magnetic field data is continued, with an
examination of the effects of noise in the data. Through the use
of analytic magnetic field models, two types of noise are "added"
prior to resolving: noise to simulate Poisson photon noise in the
observed polarization spectra, and a spatial binning to simulate the
effects of unresolved structure. The results are compared through
the use of quantitative metrics and performance maps. We find that
while no algorithm severely propagates the effects of Poisson noise
beyond very local influences, some algorithms are more robust against
high photon-noise levels than others. In the case of limited spatial
resolution, loss of information regarding fine-scale structure can
easily result in erroneous solutions. Our tests imply that photon
noise and limited spatial resolution can act so as to make assumptions
used in some ambiguity resolution algorithms no longer consistent
with the observed magnetogram. We confirm a finding of the earlier
comparison study that results can be very sensitive to the details of
the treatment of the observed boundary and the assumptions governing
that treatment. We discuss the implications of these findings, given the
relative sensitivities of the algorithms to the two sources of noise
tested here. We also touch on further implications for interpreting
observational vector magnetic field data for general solar physics
research.
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Title: Forward Modeling of Active Region Coronal
Emissions. II. Implications for Coronal Heating
Authors: Lundquist, L. L.; Fisher, G. H.; Metcalf, T. R.; Leka, K. D.;
McTiernan, J. M.
2008ApJ...689.1388L Altcode:
In Paper I, we introduced and tested a method for predicting solar
active region coronal emissions using magnetic field measurements and
a chosen heating relationship. Here, we apply this forward-modeling
technique to 10 active regions observed with the Mees Solar Observatory
Imaging Vector Magnetograph and the Yohkoh Soft X-ray Telescope. We
produce synthetic images of each region using four parameterized heating
relationships depending on magnetic field strength and geometry. We
find a volumetric coronal heating rate (dE<SUB>H</SUB>/dV, not to be
confused with dE<SUB>H</SUB>/dA quoted by some authors) proportional
to magnetic field and inversely proportional to field-line loop
length (BL<SUP>-1</SUP>) best matches observed coronal emission
morphologies. This parameterization is most similar to the steady-state
scaling of two proposed heating mechanisms: van Ballegooijen's "current
layers" theory, taken in the AC limit, and Parker's "critical angle"
mechanism, in the case where the angle of misalignment is a twist
angle. Although this parameterization best matches the observations,
it does not match well enough to make a definitive statement as
to the nature of coronal heating. Instead, we conclude that (1) the
technique requires better magnetic field measurement and extrapolation
techniques than currently available, and (2) forward-modeling methods
that incorporate properties of transiently heated loops are necessary
to make a more conclusive statement about coronal heating mechanisms.
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Title: Preprocessing of Hinode/SOT Vector Magnetograms for Nonlinear
Force-Free Coronal Magnetic Field Modeling
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
2008ASPC..397..198W Altcode: 2008arXiv0801.2884W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown
to be useful in extrapolating the coronal field from underlying vector
boundary data (for an overview see Schrijver et al. (2006)). However,
we can only measure the magnetic field vector routinely with high
accuracy in the photosphere with, e.g., Hinode/SOT, and unfortunately
these data do not fulfill the force-free consistency condition as
defined by Aly (1989). We must therefore apply some transformations
to these data before nonlinear force-free extrapolation codes can be
legitimately applied. To this end, we have developed a minimization
procedure that uses the measured photospheric field vectors as input
to approximate a more chromospheric like field (The method was dubbed
preprocessing. See Wiegelmann et al. (2006) for details). The procedure
includes force-free consistency integrals and spatial smoothing. The
method has been intensively tested with model active regions (see
Metcalf et al. 2008) and been applied to several ground based vector
magnetogram data before. Here we apply the preprocessing program to
photospheric magnetic field measurements with the Hinode/SOT instrument.
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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.
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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.
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Title: Can We Improve the Preprocessing of Photospheric Vector
Magnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
2008SoPh..247..249W Altcode: 2008arXiv0801.2707W; 2008SoPh..tmp...27W
The solar magnetic field is key to understanding the physical processes
in the solar atmosphere. Nonlinear force-free codes have been shown to
be useful in extrapolating the coronal field upward from underlying
vector boundary data. However, we can only measure the magnetic
field vector routinely with high accuracy in the photosphere, and
unfortunately these data do not fulfill the force-free condition. We
must therefore apply some transformations to these data before nonlinear
force-free extrapolation codes can be self-consistently applied. To
this end, we have developed a minimization procedure that yields a more
chromosphere-like field, using the measured photospheric field vectors
as input. The procedure includes force-free consistency integrals,
spatial smoothing, and - newly included in the version presented here
- an improved match to the field direction as inferred from fibrils
as can be observed in, for example, chromospheric Hα images. We test
the procedure using a model active-region field that included buoyancy
forces at the photospheric level. The proposed preprocessing method
allows us to approximate the chromospheric vector field to within a few
degrees and the free energy in the coronal field to within one percent.
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Title: Nonlinear Force-Free Modeling of Coronal Magnetic
Fields. II. Modeling a Filament Arcade and Simulated Chromospheric
and Photospheric Vector Fields
Authors: Metcalf, Thomas R.; De Rosa, Marc L.; Schrijver, Carolus J.;
Barnes, Graham; van Ballegooijen, Adriaan A.; Wiegelmann, Thomas;
Wheatland, Michael S.; Valori, Gherardo; McTtiernan, James M.
2008SoPh..247..269M Altcode: 2008SoPh..tmp...17M
We compare a variety of nonlinear force-free field (NLFFF) extrapolation
algorithms, including optimization, magneto-frictional, and Grad -
Rubin-like codes, applied to a solar-like reference model. The model
used to test the algorithms includes realistic photospheric Lorentz
forces and a complex field including a weakly twisted, right helical
flux bundle. The codes were applied to both forced "photospheric" and
more force-free "chromospheric" vector magnetic field boundary data
derived from the model. When applied to the chromospheric boundary data,
the codes are able to recover the presence of the flux bundle and the
field's free energy, though some details of the field connectivity are
lost. When the codes are applied to the forced photospheric boundary
data, the reference model field is not well recovered, indicating
that the combination of Lorentz forces and small spatial scale
structure at the photosphere severely impact the extrapolation of the
field. Preprocessing of the forced photospheric boundary does improve
the extrapolations considerably for the layers above the chromosphere,
but the extrapolations are sensitive to the details of the numerical
codes and neither the field connectivity nor the free magnetic energy in
the full volume are well recovered. The magnetic virial theorem gives
a rapid measure of the total magnetic energy without extrapolation
though, like the NLFFF codes, it is sensitive to the Lorentz forces in
the coronal volume. Both the magnetic virial theorem and the Wiegelmann
extrapolation, when applied to the preprocessed photospheric boundary,
give a magnetic energy which is nearly equivalent to the value derived
from the chromospheric boundary, but both underestimate the free
energy above the photosphere by at least a factor of two. We discuss
the interpretation of the preprocessed field in this context. When
applying the NLFFF codes to solar data, the problems associated with
Lorentz forces present in the low solar atmosphere must be recognized:
the various codes will not necessarily converge to the correct, or
even the same, solution.
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Title: Can we Improve the Preprocessing of Photospheric
Vectormagnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
M. L.; Metcalf, T. R.
2007AGUFMSH51C..02W Altcode:
The solar magnetic field is key to understanding the physical
processes in the solar atmosphere. Unfortunately, we can measure
the magnetic field vector routinely with high accuracy only in the
photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These
measurements are extrapolated into the corona under the assumption
that the field is force-free. That condition is not fulfilled in the
photosphere, but is in the chromosphere and corona. In order to make
the observed boundary data consistent with the force-free assumption,
we therefore have to apply some transformations before nonlinear
force-free extrapolation codes can be legitimately applied. We develop
a minimization procedure that uses the measured photospheric field
vectors as input to approximate a more chromospheric-like field. The
procedure includes force-free consistency integrals, spatial smoothing,
and - newly included in the version presented here - an improved match
to the field direction as inferred from fibrils as can be observed in,
e.g., chromospheric H-alpha images. We test the procedure using a model
active-region field that included buoyancy forces at the photospheric
level. We apply the combined preprocessing and nonlinear force-free
extrapolation method to compute the coronal magnetic field in an active
region measured with the Hinode/SOT instrument.
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Title: Energy Deposition in White Light Flares with TRACE and RHESSI
Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R.
2007ASPC..368..423F Altcode:
In Fletcher et al. (2007) we investigated the white light (WL) continuum
during solar flares and its relationship to energy deposition by
electron beams. In 9 flare events, spanning GOES classifications from
C4.8 to M9.1, we have high cadence TRACE WL and RHESSI hard X-ray
observations, and compare the WL radiative power output with that
provided by flare electrons. Under the thick--target model assumptions,
we find that the electron beam must extend down to 15--20 keV, and the
energy input to the chromosphere should occur within the collisional
stopping depth of these electrons - approximately 2× 10<SUP>-4</SUP>
g cm<SUP>-2</SUP>. In this short paper, we discuss some ideas on flare
WL emission, summarise the results of the Fletcher et al. (2007)
study and discuss their implications for chromospheric heating and
white light flare emission.
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Title: Evidence For Reconnection In A Flare Observed With The X-ray
Telescope On Hinode
Authors: Reeves, Kathy; Metcalf, T.; Ishibashi, B.; Weber, M.
2007AAS...210.3703R Altcode: 2007BAAS...39..151R
A long duration flare observed at the limb on December 17th with the
X-Ray Telescope (XRT) on Hinode exhibits many features characteristic
of reconnection. Cusp-shaped loops are clearly resolved by XRT,
and these loops show convincing evidence of field line shrinkage. In
addition, the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) detects a hard X-ray loop top source that extends into
the cusp structure visible in the XRT images. The implications of
these observations for reconnection theories of solar flares will be
discussed. <P />The US XRT team is supported by a contract from NASA
to SAO. Hinode is an international project supported by JAXA, NASA,
PPARC and ESA. We are grateful to the Hinode team for all their efforts
in the design, development and operation of the mission.
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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.
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Title: Is Coronal X-ray Emission Energized By Electric Currents?
Authors: Ishibashi, Kazunori; Metcalf, T.; Lites, B.
2007AAS...210.9421I Altcode: 2007BAAS...39..220I
We examine the spatial correlation between coronal X-ray emission
observed with the Hinode X-Ray Telescope and electric currents observed
with the Hinode Solar Optical Telescope Spectro-polarimeter. We
determine to what extent the X-ray brightness is correlated with
electric current density and hence to what extent the hot corona is
energized by electric currents which flow through the photosphere. We
will also consider whether the currents reach the corona to heat the
coronal plasma or whether they predominantly close below the corona. <P
/>Hinode is an international project supported by JAXA, NASA, PPARC
and ESA. We are grateful to the Hinode team for all their efforts in
the design, development and operation of the mission.
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Title: A TRACE White Light and RHESSI Hard X-Ray Study of Flare
Energetics
Authors: Fletcher, L.; Hannah, I. G.; Hudson, H. S.; Metcalf, T. R.
2007ApJ...656.1187F Altcode:
In this paper we investigate the formation of the white-light (WL)
continuum during solar flares and its relationship to energy deposition
by electron beams inferred from hard X-ray emission. We analyze nine
flares spanning GOES classifications from C4.8 to M9.1, seven of which
show clear cospatial RHESSI hard X-ray and TRACE WL footpoints. We
characterize the TRACE WL/UV continuum energy under two simplifying
assumptions: (1) a blackbody function, or (2) a Paschen-Balmer
continuum model. These set limits on the energy in the continuum,
which we compare with that provided by flare electrons under the
usual collisional thick-target assumptions. We find that the power
required by the white-light luminosity enhancement is comparable to
the electron beam power required to produce the HXR emission only if
the low-energy cutoff to the spectrum is less than 25 keV. The bulk
of the energy required to power the white-light flare (WLF) therefore
resides at these low energies. Since such low-energy electrons cannot
penetrate deep into a collisional thick target, this implies that the
continuum enhancement is due to processes occurring at moderate depths
in the chromosphere.
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Title: An Overview of Existing Algorithms for Resolving the
180<SUP>°</SUP> Ambiguity in Vector Magnetic Fields: Quantitative
Tests with Synthetic Data
Authors: Metcalf, Thomas R.; Leka, K. D.; Barnes, Graham; Lites,
Bruce W.; Georgoulis, Manolis K.; Pevtsov, A. A.; Balasubramaniam,
K. S.; Gary, G. Allen; Jing, Ju; Li, Jing; Liu, Y.; Wang, H. N.;
Abramenko, Valentyna; Yurchyshyn, Vasyl; Moon, Y. -J.
2006SoPh..237..267M Altcode: 2006SoPh..tmp...14M
We report here on the present state-of-the-art in algorithms used
for resolving the 180° ambiguity in solar vector magnetic field
measurements. With present observations and techniques, some assumption
must be made about the solar magnetic field in order to resolve
this ambiguity. Our focus is the application of numerous existing
algorithms to test data for which the correct answer is known. In
this context, we compare the algorithms quantitatively and seek to
understand where each succeeds, where it fails, and why. We have
considered five basic approaches: comparing the observed field to a
reference field or direction, minimizing the vertical gradient of the
magnetic pressure, minimizing the vertical current density, minimizing
some approximation to the total current density, and minimizing some
approximation to the field's divergence. Of the automated methods
requiring no human intervention, those which minimize the square of
the vertical current density in conjunction with an approximation for
the vanishing divergence of the magnetic field show the most promise.
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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.
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Title: Non-linear Force-free Modeling: Applications To Solar Data
Authors: De Rosa, Marc L.; Schrijver, C. J.; Metcalf, T. R.; NLFFF Team
2006SPD....37.1805D Altcode: 2006BAAS...38..247D
Understanding the conditions under which solar magnetic fields can
destabilizeto cause flares and other eruptive events requires a
quantitativeunderstanding of the coronal magnetic field and of the
currents that itcarries. Because no direct measurements of magnetic
fields and current withincoronal volumes exist, the coronal field is
typically modeled usinginformation contained in photospheric vector
magnetograms, to be compared toH-alpha images of the chromosphere
and EUV and X-ray imagery of the corona.We report on recent results
of a team effort to further understand theintricacies of non-linear
force-free extrapolations of the coronal magneticfield, presenting
results from several solar and solar-like test cases. Wealso consider
the use of such coronal field modeling in the upcoming Solar-Band
SDO missions.
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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.
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Title: Nonlinear Force-Free Modeling of Coronal Magnetic Fields Part
I: A Quantitative Comparison of Methods
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Metcalf, Thomas R.;
Liu, Yang; McTiernan, Jim; Régnier, Stéphane; Valori, Gherardo;
Wheatland, Michael S.; Wiegelmann, Thomas
2006SoPh..235..161S Altcode:
We compare six algorithms for the computation of nonlinear force-free
(NLFF) magnetic fields (including optimization, magnetofrictional,
Grad-Rubin based, and Green's function-based methods) by evaluating
their performance in blind tests on analytical force-free-field models
for which boundary conditions are specified either for the entire
surface area of a cubic volume or for an extended lower boundary
only. Figures of merit are used to compare the input vector field to
the resulting model fields. Based on these merit functions, we argue
that all algorithms yield NLFF fields that agree best with the input
field in the lower central region of the volume, where the field and
electrical currents are strongest and the effects of boundary conditions
weakest. The NLFF vector fields in the outer domains of the volume
depend sensitively on the details of the specified boundary conditions;
best agreement is found if the field outside of the model volume is
incorporated as part of the model boundary, either as potential field
boundaries on the side and top surfaces, or as a potential field in
a skirt around the main volume of interest. For input field (B) and
modeled field (b), the best method included in our study yields an
average relative vector error E<SUB>n</SUB> = « |B−b|»/« |B|» of
only 0.02 when all sides are specified and 0.14 for the case where only
the lower boundary is specified, while the total energy in the magnetic
field is approximated to within 2%. The models converge towards the
central, strong input field at speeds that differ by a factor of one
million per iteration step. The fastest-converging, best-performing
model for these analytical test cases is the Wheatland, Sturrock, and
Roumeliotis (2000) optimization algorithm as implemented by Wiegelmann
(2004).
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Title: White-Light Flares: A TRACE/RHESSI Overview
Authors: Hudson, H. S.; Wolfson, C. J.; Metcalf, T. R.
2006SoPh..234...79H Altcode:
The Transition Region and Coronal Explorer (TRACE) instrument includes
a "white light" imaging capability with novel characteristics. Many
flares with such white-light emission have been detected, and this paper
provides an introductory overview of these data. These observations
have 0.5″ pixel size and use the full broad-band response of the
CCD sensor; the images are not compromised by ground-based seeing and
have excellent pointing stability as well as high time resolution. The
spectral response of the TRACE white-light passband extends into the
UV, so these data capture, for the first time in images, the main
radiative energy of a flare. This initial survey is based on a sample
of flares observed at high time resolution for which the Reuven Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI) had complete data
coverage, a total of 11 events up to the end of 2004. We characterize
these events in terms of source morphology and contrast against the
photosphere. We confirm the strong association of the TRACE white-light
emissions - which include UV as well as visual wavelengths - with hard
X-ray sources observed by RHESSI. The images show fine structure at
the TRACE resolution limit, and often show this fine structure to
be extended over large areas rather than just in simple footpoint
sources. The white-light emission shows strong intermittency both in
space and in time and commonly contains features unresolved at the
TRACE resolution. We detect white-light continuum emission in flares
as weak as GOES C1.6. limited by photon statistics and background
solar fluctuations, and support the conclusion of Neidig (1989) that
white-light continuum occurs in essentially all flares.
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Title: An Improved Virial Estimate of Solar Active Region Energy
Authors: Wheatland, M. S.; Metcalf, Thomas R.
2006ApJ...636.1151W Altcode: 2005astro.ph..9652W
The MHD virial theorem may be used to estimate the magnetic energy of
active regions on the basis of vector magnetic fields measured at the
photosphere or chromosphere. However, the virial estimate depends on
the measured vector magnetic field being force-free. Departure from the
force-free condition leads to an unknown systematic error in the virial
energy estimate and an origin dependence of the result. We present a
method for estimating the systematic error by assuming that magnetic
forces are confined to a thin layer near the photosphere. If vector
magnetic field measurements are available at two levels in the low
atmosphere (e.g., the photosphere and the chromosphere), the systematic
error may be directly calculated using the observed horizontal and
vertical field gradients, resulting in an energy estimate that is
independent of the choice of origin. If (as is generally the case)
measurements are available at only one level, the systematic error
may be approximated using the observed horizontal field gradients
together with a simple linear force-free model for the vertical
field gradients. The resulting “improved” virial energy estimate
is independent of the choice of origin but depends on the choice of
the model for the vertical field gradients, i.e., the value of the
linear force-free parameter α. This procedure is demonstrated for
five vector magnetograms, including a chromospheric magnetogram.
---------------------------------------------------------
Title: High Spectral Resolution Cryogenic Imaging Detectors for
Solar Physics
Authors: Stern, R.; Martinez-Galarce, D.; Rausch, A.; Shing, L.;
Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S.; Brink,
P.; Irwin, K.; Ullom, J.; Alexander, D.
2005AGUFMSH41B1130S Altcode:
X-ray microcalorimeters have the potential to substantially
improve our understanding of magnetic reconnection in the early
phases of flares or during microflare events. Operating in the
sub-Kelvin temperature range, they provide high non-dispersive
energy resolution at ~ keV energies (e.g, E/Δ E ~ 1500 at 6 keV),
coupled with high (msec or better) time resolution. Coupled with
grazing-incidence X-ray optics, microcalorimeter arrays or position
sensitive readout schemes will form the basis for a new generation of
solar imaging spectrometers. An Explorer-class solar mission within
the next 5-10 years, based upon these detectors, is rapidly becoming
technically feasible. LMSAL currently has both internally and NASA
funded laboratory research programs to investigate TES (Transition
Edge Sensor) microcalorimeters. In this presentation, we will discuss
the current status of these programs and their applicability to future
Explorer missions and Roadmap missions such as RAM.
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Title: TRACE white light and RHESSI hard X-rays
Authors: Fletcher, L.; Allred, J.; Hannah, I.; Hudson, H.; Metcalf, T.
2005AGUFMSH13A0286F Altcode:
We study the energetics of RHESSI and TRACE observations for a
sample of 11 solar flares well observed in the TRACE “white light"
channel. In general, the data show excellent correlations between hard
X-rays and white light. We discuss the energetics of this relationship
based upon simple models for the visible/UV continuum, namely (i)
a non-thermal approximation (Balmer and Paschen continuum), (ii)
a thermal approximation (blackbody), and (iii) model spectra derived
from radiation-hydrodynamic modeling. We relate the white-light energy
and the low-energy cutoff energy of the primary electron spectrum
required for energetic equivalence. This comparison will be made in
the context of the flare WL morphology and in comparison with RHESSI
hard X-ray images.
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Title: The Nonpotentiality of Active-Region Coronae and the Dynamics
of the Photospheric Magnetic Field
Authors: Schrijver, Carolus J.; De Rosa, Marc L.; Title, Alan M.;
Metcalf, Thomas R.
2005ApJ...628..501S Altcode:
The magnetic field in the solar photosphere frequently carries strong
electric currents, even though the global coronal configuration often
resembles a potential field ringed by the heliospheric current sheet. To
understand this, we compare TRACE EUV images of active-region coronae
and potential-field source-surface extrapolations based on SOHO MDI
magnetograms for 95 active regions. We conclude that significant
nonpotentiality of the overall active-region coronal field occurs
(1) when new flux has emerged within or very near a region within
the last ~30 hr, resulting in complex polarity separation lines, or
(2) when rapidly evolving, opposite-polarity concentrations are in
contact at 4" resolution. If these criteria are met by more than 15%
of the region's flux, they correctly identify the (non) potentiality of
active-region coronae in 88% of the cases. Flares are found to occur
2.4 times more frequently in active regions with nonpotential coronae
than in near-potential regions, while their average X-ray peak flare
brightness is 3.3 times higher. We suggest that the currents associated
with coronal nonpotentiality have a characteristic growth and decay
timescale of ~10-30 hr. We find that shear flows drive enhanced flaring
or coronal nonpotentiality only if associated with complex and dynamic
flux emergence within the above timescale. We discuss the implications
of this finding for the modeling of the coronal-heliospheric coupling.
---------------------------------------------------------
Title: Cryogenic 3-D Detectors for Solar Physics
Authors: Stern, R. A.; Martinez-Galarce, D.; Rausch, A.; Shing, L.;
Deiker, S.; Boerner, P.; Metcalf, T.; Cabrera, B.; Leman, S. W.;
Brink, P.; Irwin, K.; Alexander, D.
2005AGUSMSP12A..02S Altcode:
Cryogenic microcalorimeters operating in the sub-Kelvin temperature
range provide non-dispersive energy resolution at optical through
gamma ray energies (e.g, E/Δ E ~ 1500 at 6 keV). Microcalorimeters
also have high time resolution (msec or better), and can be made into
imaging arrays through SQUID multiplexing of individual pixels or
employing position sensitive detector structures. The application
of such "3-D" detector technology to solar physics will lead to
significant advances in our understanding of magnetic reconnection
in the Sun, including X-ray jet phenomena, and active region heating
and dynamics. An Explorer-class solar mission within the next 5-10
years, based upon these detectors, is rapidly becoming technically
feasible. LMSAL currently has an internally funded laboratory research
program to investigate TES (Transition Edge Sensor) microcalorimeters;
we recently saw our first X-ray photons using TES detectors supplied
by NIST. In addition, we have recently been funded by NASA to begin
work with NIST on position-sensitive X-ray strip detectors for solar
physics applications. Finally, we are collaborating with with Stanford
and NIST on a solar sounding rocket. In this presentation, we will
discuss the current status of these programs and their applicability
to future Explorer missions and Roadmap missions such as RAM.
---------------------------------------------------------
Title: Non-linear force-free field modeling: model techniques,
boundary conditions, hares, and hounds
Authors: Schrijver, C. J.; De Rosa, M. L.; Metcalf, T.
2005AGUSMSH31A..05S Altcode:
Understanding the conditions under which solar magnetic fields can
destabilize to erupt in flares and coronal mass ejections requires
a quantitative understanding of the coronal magnetic field and of
the currents that it carries. The increased availability of vector
magnetograms, together with EUV and X-ray coronal images, should
provide adequate constraints to model the coronal field, and thus to
visualize its 3D geometry and to measure the available free energy
and helicity. Non-linear force-free fields (NLFFF) are likely a useful
model to use when extrapolating the solar surface field upward into the
coronal volume. It may even be possible to use the observed trajectories
of coronal loops, evident in EUV images of the corona, as a further
constraint. We present initial results of a team effort to understand
the intricacies of NLFFF modeling: we discuss and evaluate comparisons
of NLFFF models computed with different models and applications of
boundary conditions, and look ahead to full coronal field modeling
for the upcoming Solar-B and SDO missions.
---------------------------------------------------------
Title: Microcalorimeter X-ray Detectors for Solar Physics
Authors: Deiker, S.; Boerner, P.; Martinez-Galarce, D.; Metcalf, T.;
Rausch, A.; Shing, L.; Stern, R.; Irwin, K.; William, D.; Reintsema,
C.; Ullom, J.; Cabrera, B.; Lehman, S.; Brink, P.
2005AGUSMSP12A..03D Altcode:
Cryogenic X-ray microcalorimeters provide high spectral resolution
over a large bandwidth. They have achieved < 3 eV resolution at
5.9 keV, and can produce this performance simultaneously from 0.25
to 10 keV. Although they operate at low (< 0.1 K) temperatures,
such temperature are now easily produced. Microcalorimeters cooled
by adiabatic demagnetization refrigerators have already flown on
sounding rocket flights to study the soft X-ray background of the
interstellar medium, and will soon be launched on the ASTRO-E II
satellite. Microcalorimeters based on superconducting transition
edge sensors are multiplexable and may be fabricated using
standard photolithographic techniques. This makes large arrays of
microcalorimeters feasible. Each pixel of such an array detects the
arrival time of each photon to within < 0.01 ms. Such an instrument
would offer simultaneous spatial, temporal and energy resolution,
bringing a wealth of new information about solar processes. Current
design and performance of microcalorimeters will be presented. Future
improvements required to optimize microcalorimeters for solar physics
applications will also be discussed.
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Title: Tracing Coronal Waves Back to the Photosphere
Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T.
2005AGUSMSH11C..03D Altcode:
There are now many observations of waves with periods around 5 minutes
in the outer atmosphere of the Sun. We provide an observational
overview of 5 minute periodicity in chromospheric spicules in active
region plage, upper transition region moss and the low legs of coronal
loops. Using a numerical model, we show that all of these phenomena
are connected: normally evanescent photospheric oscillations can
propagate into the low atmosphere as long as they are guided along
magnetic field lines that are inclined away from the vertical. The
leaked photospheric oscillations develop into shocks and lead to
periodic upward chromospheric flows, which we have identified as
active region spicules. These shocks continue upwards and enter
into the corona. We suggest that TRACE observations of propagating
acoustic waves in the corona are shocked and tunneled photospheric
oscillations. Using SOHO/MDI, TRACE and Imaging Vector Magnetograph
(Hawaii) data we explore how these coronal waves can be exploited to
determine the connectivity between photosphere and corona,and thus
allow seismology of the lower solar atmosphere.
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Title: TRACE and RHESSI observations of white-light flares
Authors: Hudson, H. H.; Metcalf, T.; Wolfson, J.; Fletcher, L.;
Khan, J.
2005AGUSMSP51C..03H Altcode:
TRACE gives a photometrically stable, high-resolution view of the
visible and UV emissions of solar flares, with enough diagnostic power
to distinguish photospheric from chromospheric contributions. These
emissions dominate the radiant energy of a flare and correspond
well with hard X-ray emission in the impulsive phase. We survey
these data with reference to well-observed events of 4~October~2002
and 4~November~2003, on the disk (M4; S19W09) and on the limb (X17;
S19W83) respectively. We analyze the spatial and temporal properties
of the footpoint sources in these two events, characterizing their
physical parameters (contrast, image gradients, apparent motions). The
brightest kernels of the 4~October~2002 event have intensities twice
that of the quiet photosphere, and have sizes close to the Nyquist
limit of the TRACE pixels (0.5~arc~sec). The white-light footpoint
sources can be considered as a proxy for the target region of fast
electrons studied via hard X-ray bremsstrahlung. We compare RHESSI
hard X-ray footpoint sources with their white-light counterparts and
estimate filling factors for the hard X-ray emission itself.
---------------------------------------------------------
Title: Predicting Coronal Emissions with Multiple Heating Rates
Authors: Lundquist, L. L.; Fisher, G. H.; Leka, K. D.; Metcalf, T. R.;
McTiernan, J. M.
2005AGUSMSP14A..02L Altcode:
A variety of proposed coronal heating mechanisms remain prominent
in the literature, with insufficient observational constraints to
distinguish between them. In an attempt to add further constraints, we
create predicted coronal emission maps of several active regions using
simple parametrizations of differing theoretical heating mechanisms and
compare the results to the observed coronal emissions. The results are
interpolated to a 3-d grid, convolved with instrument response function,
and integrated over line of sight to simulate satellite observation
of the modeled loops. We also discuss those factors which dominate
the differences in observed and predicted coronal emission.
---------------------------------------------------------
Title: Another Piece of the Elephant: Chromospheric Vector Field
Observations
Authors: Leka, K. D.; Metcalf, T. R.; Mickey, D. L.
2005AGUSMSH13C..10L Altcode:
As with most solar observational questions, investigating the
structure and role of the chromosphere is one of remote sensing: many
investigations describing their "piece of the elephant". The goal is,
of course, to form a coherent picture of the state of the magnetized
plasma which resides there (or passes through). In this presentation,
recent efforts to understand the chromospheric magnetic field structure
via direct observation, i.e. chromospheric vector magnetograms, will
be presented. 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, with the focus being the structure, free energy
storage and evolution at that low-chromospheric layer. I will present
salient aspects of the observed chromospheric magnetic field structure,
to compare and contrast with the picture formed by the other methods
in this session.
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Title: TRACE and SOHO/MDI Observations of Five X-Flares from AR10720,
the "Fireworks Active Region", on January 15-20, 2005
Authors: Nightingale, R. W.; Frank, Z. A.; Metcalf, T. R.; Kang, C. A.
2005AGUSMSP21A..10N Altcode:
Both TRACE and SOHO/MDI observed the five X-flares that were produced
by AR10720 during January 15-20, 2005. This active region appears
to be "special" because it is occurring so late in the solar cycle,
about two-thirds of the way toward solar minimum. Also its structure
is initially one large penumbral region with several umbral spots that
are being stretched out east to west with rotating sunspots emerging
in leading and trailing positions during the transit across the solar
disk, similar to fireworks with pin-wheels attached. With a neutral line
running almost east-west in between two of the stretched umbral regions
and with opposite magnetic polarities on both sides of the neutral line
adjacent to each other, the beta-delta region starts the "fireworks"
with the first X-flare on January 15, continuing every one to one and
one-half days over the next five days. Images and movies will be shown
of these eruptive events in white light and 1600A from TRACE, and as
magnetograms from MDI. Two of the three rotating sunspots also change
their direction of rotation during this period. These X-flares add to
those observed by TRACE since its launch in April 1998, all of which
have been associated with rotating sunspots in the associated active
regions. These X-flares consist of over half of the X-flares observed by
GOES since April 1998 and many are associated with geo-effective CMEs
in the form of solar energetic particle events. Rotating sunspots may
be a good predictor of solar eruptions. This work has been supported
by NASA, in part under the TRACE project at LMSAL (contract NAS5-38099)
and in part under the MDI/SOHO project (NAG5-13261).
---------------------------------------------------------
Title: Magnetic Free Energy in NOAA Active Region 10486 on 2003
October 29
Authors: Metcalf, Thomas R.; Leka, K. D.; Mickey, D. L.
2005ApJ...623L..53M Altcode:
We calculate the total and the free magnetic energy for solar NOAA
active region 10486 on 2003 October 29 using chromospheric vector
magnetograms observed with the Imaging Vector Magnetograph at Mees Solar
Observatory in the Na I λ5896 spectral line. The magnetic energy is
derived from the magnetic virial theorem using observations spanning the
X10 flare that occurred at 20:39 UT. Although poor atmospheric seeing
prevented us from discerning changes in the free energy associated
with the flare, there was an unusually large amount of free magnetic
energy in NOAA AR 10486: (5.7+/-1.9)×10<SUP>33</SUP> ergs, which is
consistent with the very high level of activity observed in this active
region. It is thus plausible that the extreme activity was powered by
the magnetic free energy.
---------------------------------------------------------
Title: TRACE Observations of Many Active Regions with X-flares and
Rotating Sunspots in the Current Solar Cycle
Authors: Nightingale, R. W.; Metcalf, T. R.
2004AGUFMSH13A1137N Altcode:
The TRACE instrument in its 7 UV and EUV wavelength channels has
observed approximately 55 X-flares throughout its mission so far in
solar cycle 23 from launch in April 1998 through August 2004. About 40
of those X-flares (73%) can be associated with 17 solar active regions
that contain rotating sunspots, which have been observed by TRACE in
its white light channel. These sunspots, rotating about the center of
their umbras, appear to be manifestations of the twist and/or writhe,
i.e., the helicity, instilled in the rising active region omega loops
as they travel through the convection region below the photosphere. Of
the 40 X-flaring active regions observed, 26 (65%) can be further
associated with large, fast Coronal Mass Ejections (CMEs) leaving
the sun, as recorded by the LASCO instrument on SoHO. Furthermore, 24
of these 26 events appear to be geo-effective because the energetic
proton fluxes, observed by instruments on the GOES geosynchronous
spacecraft, increased by several orders of magnitude shortly after
the solar events occurred. The data and the importance of such a high
percentage of rotating sunspots being associated with X-flaring active
regions and CMEs will be discussed. This work was supported by NASA
under the TRACE contract NAS5-38099.
---------------------------------------------------------
Title: Photospheric Oscillations in the Solar Atmosphere: Driving
Chromospheric Spicules and Coronal Waves
Authors: De Pontieu, B.; Erdelyi, R.; De Moortel, I.; Metcalf, T.
2004AGUFMSH13A1142D Altcode:
There are now many observations of oscillations and waves with periods
around 5 minutes in the solar transition region and corona. We provide
an observational overview of 5 minute periodicity in upper transition
region moss, the low legs of coronal loops, and chromospheric spicules
in active region plage. The source of the 5 minute periodicity is
unclear, since photospheric p-modes are evanescent in the upper
photosphere which should prevent them from propagating into the
chromosphere, transition region and corona. Using a numerical model
we show that photospheric oscillations can propagate into the low
atmosphere as long as they are guided along a magnetic flux tube that is
inclined away from the vertical. The leaked photospheric oscillations
develop non-linearly into shocks at low chromospheric heights because
of the density decrease with height. The upward traveling shocks and
resulting rebound shocks of the chromosphere lead to periodic upward
chromospheric flows, which in a recent paper we have identified as
the periodic spicules that we observe in active region plage. After
passage through the spicule, these shocked photospheric oscillations
propagate into the corona. We suggest that TRACE observations of
propagating acoustic waves in the corona are shocked and tunneled
photospheric oscillations. We also explore whether these coronal waves
can be exploited to determine the connectivity between photosphere
and corona, and thus perform seismology of the lower solar atmosphere.
---------------------------------------------------------
Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images
Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser,
J.; Alexander, D.
2004AGUFMSH22A..02A Altcode:
We simulate 3D models of EUV images of flare and CME events, using
TRACE EUV movies. TRACE movies show 2D images in projection along a
particular line-of-sight. We simulate 3D models of erupting filaments,
flare loops, and postflare loops using: (1) a “finger printing”
technique to trace linear structures in 2D images; (2) geometric 3D
models based on force-free fields and curvature radius maximization
of flare loop and flux rope structures; (3) conservation of velocity
and acceleration parameters; (4) multi-temperature plasma filling
according to hydrodyamic scaling laws; and (5) 2D projections from
secondary line-of-sights that correspond to viewpoints of the secondary
STEREO spacecraft. From such simulations we envision to illustrat
3D time-dependent models, what would be observed at the two STEREO
spacecraft positions as well as from a near-Earth spacecraft such as
SoHO. These simulations are used to test STEREO analysis software and
to investigate what physical parameters and geometric 3D reconstructions
can be retrieved from STEREO/SECCHI data.
---------------------------------------------------------
Title: “Transition-Edge Sensors for Solar X-ray Spectral Observations
- An Update”
Authors: Martinez-Galarce, D. S.; Stern, R.; Cabrera, B.; Deiker,
S.; Alexander, D.; Metcalf, T.; Irwin, K.; Brink, P.; Boerner,
P. F.; Morse, K.; Leman, S.; Shing, L.; Rausch, A.; Nichols, T. D.;
Chakraborty, S.
2004AGUFMSH13A1147M Altcode:
ABSTRACT: The advent of cryogenic microcalorimeters (operating at
temperatures of ∼ 0.1 K) in ground-based and space-based astronomy
promises a revolution of new discoveries. Particularly, Transition-Edge
Sensors (TES) have demonstrated high-energy resolution measurements of
soft X-rays of up to E/Δ E ∼ 1500 (at 6keV) and with high temporal
resolution of a msec or less in photon pulse detections. Fabricated
into multiplexed arrays of single detectors, or position sensing
macropixels, and placed at the focus of a Wolter optic would further
yield high spatial resolution capability of 2 arcsec or less, thus
producing unprecedented "3-D" solar observations. We report herein,
on progress to date in the development of these detectors and
particularly, with on-going work with the sounding rocket payload,
the Advanced Technology Solar Spectroscopic Imager, which will debut
a TES instrument operating in the 300eV - 1.5keV range to study active
region magnetic reconnection. Furthermore, as part of our larger effort,
we discuss also current technical developments and plans at the Lockheed
Martin Solar & Astrophysics Laboratory to design a TES instrument
(3 - 8keV range) for realization into a dedicated Explorer-class solar
observatory in the next 5 - 10 years.
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Title: Energy partition in two solar flare/CME events
Authors: Emslie, A. G.; Kucharek, H.; Dennis, B. R.; Gopalswamy, N.;
Holman, G. D.; Share, G. H.; Vourlidas, A.; Forbes, T. G.; Gallagher,
P. T.; Mason, G. M.; Metcalf, T. R.; Mewaldt, R. A.; Murphy, R. J.;
Schwartz, R. A.; Zurbuchen, T. H.
2004JGRA..10910104E Altcode:
Using coordinated observations from instruments on the Advanced
Composition Explorer (ACE), the Solar and Heliospheric Observatory
(SOHO), and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI),
we have evaluated the energetics of two well-observed flare/CME events
on 21 April 2002 and 23 July 2002. For each event, we have estimated
the energy contents (and the likely uncertainties) of (1) the coronal
mass ejection, (2) the thermal plasma at the Sun, (3) the hard X-ray
producing accelerated electrons, (4) the gamma-ray producing ions,
and (5) the solar energetic particles. The results are assimilated
and discussed relative to the probable amount of nonpotential magnetic
energy available in a large active region.
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Title: Hard X-ray Footpoint Asymmetries in Solar Flares
Authors: Chollet, E.; Alexander, D.; Metcalf, T. R.; Pollack, L.
2004AAS...204.5406C Altcode: 2004BAAS...36R.758C
RHESSI observations of a M4.0 solar flare on 17 March, 2002 are
presented. This flare exhibits footpoint asymmetries in hard X-ray
emission, allowing us to study the energy and time dependence of
particle transport in this flare. The data, originally presented in
Alexander and Metcalf (2002), is reanalyzed with new calibrations,
shorter integration times and better spatial resolution, allowing
for more accurate determination of the time and energy dependence
of the asymmetries. While the previous work suggested that this
was a simple flare with two footpoints, the new analysis suggests a
more complex structure with three or more footpoints, confirmed by
force free magnetic field extrapolations. The time development of
the evolving flare indicates a transition from one flaring structue
to another which significantly complicates the interpretation of the
measured asymmetry. The implications of this new interpretation for the
previously reported energy dependence will be discussed. We gratefully
acknowledge support from NASA (NAS5-02048).
---------------------------------------------------------
Title: Non-Relativistic Electron Beam Stability in Solar Flares
Authors: Daou, A. G.; Alexander, D.; Metcalf, T. R.
2004AAS...204.0203D Altcode: 2004BAAS...36..667D
The thick-target electron beam model has been used for decades
as a viable description for the production of solar flare hard
X-ray emission. The required very rapid transport of energy to the
footpoints during solar flares is achieved in this model by fast
electrons traversing the loop to deposit their energy in the dense
chromosphere. For some of the largest flares the currents (up to
10<SUP>17</SUP> Amps or 10<SUP>36</SUP> electrons/sec) inferred
can significantly exceed the Alfven-Lawson limit suggesting that
the assumed electron beam is inherently unstable. In this paper, we
use the spectral and spatial resolution of RHESSI to explore whether
the conditions for a stable non-thermal electron beam exist in large
solar flares. The incident electron spectra at flaring footpoints are
derived from the RHESSI photon spectra while an upper limit to the
footpoint area is detremined form the hard X-ray images. We determine
the electron beam density needed to produce the hard X-ray emission for
two large flares, July 17, 2002 (M8.5) and October 28, 2003 (X17.2)
and determine whether such beams are viable in these cases. <P />We
gratefully acknowledge support from NASA (NAS5-02048).
---------------------------------------------------------
Title: TRACE Observations of a Torsionally Rotating Sunspot Reversing
Direction
Authors: Nightingale, R. W.; Metcalf, T. R.; Ullman, J. R.
2004AAS...204.3716N Altcode: 2004BAAS...36..711N
For almost a week, starting on July 30, 2003, TRACE observed AR10421
in white light (WL), ultra-violet (UV), and extreme ultra-violet
(EUV) passbands. Analysis of the photospheric WL images shows that the
leading sunspot was torsionally rotating about the center of its umbra,
in a clockwise direction on and before July 31 and counter-clockwise
after a reversal of direction on August 1st. The sunspot appears to
be fragmenting with clumps of small pores moving away toward the east
during the first 2 days, followed by another clump that appears to be
drawn into the sunspot after the rotational direction change on August
1st. Twisted coronal fans can be seen above the rotating sunspot in
the EUV images on and after August 1st. Movies of this rather unique
oscillatory behavior will be shown in 2 or more passbands, along with
corresponding magnetogram data, as observed by SOHO/MDI. <P />This
work has been supported by NASA, in part under the TRACE project at
LMSAL (contract NAS5-38099) and in part under the MDI/SOHO project
(NAG5-13261).
---------------------------------------------------------
Title: Nonlinear Force Free Field Models of AR 0486
Authors: McTiernan, J. M.; Hudson, H. S.; Metcalf, T. R.
2004AAS...204.0204M Altcode: 2004BAAS...36Q.668M
In this work we present nonlinear force free field (NLFFF)
extrapolations from vector magnetograms of AR0486 taken before, during
and after the X11 flare of 29 October 2003. The extrapolations will be
used to interpret the different source patterns and motions seen in
RHESSI and TRACE images of the flare. In particular we will examine
the footpoint motions observed in the flare by RHESSI, and also the
expansion and other changes in the high temperature thermal sources
visible in low energy (less than 20 keV) RHESSI images and TRACE 195
A images. <P />This work is funded by NASA contract NAS 5-98033.
---------------------------------------------------------
Title: The Magnetic Free Energy in AR0486
Authors: Metcalf, T. R.; Leka, K. D.; Mickey, D. L.
2004AAS...204.0208M Altcode: 2004BAAS...36S.668M
During October/November 2003, the dramatic active region 0486 traversed
the solar disk and produced many large solar flares. During this time,
we obtained chromspheric vector magnetic field data for AR0486 using
the Imaging Vector Magnetograph (IVM) at Mees Solar Observatory,
Haleakala, Hawaii. We will describe these vector field data and will
use them to compute the magnetic free energy, and its time variation,
for AR0486. This large, complex active region contained an unusually
large amount of free magnetic energy, not surprising considering the
level of activity it produced. <P />This work was supported by NASA
grant NAG5-12466.
---------------------------------------------------------
Title: On the Resolution of the Azimuthal Ambiguity in Vector
Magnetograms of Solar Active Regions
Authors: Georgoulis, Manolis K.; LaBonte, Barry J.; Metcalf, Thomas R.
2004ApJ...602..446G Altcode:
We introduce a “structure minimization” technique to resolve the
azimuthal ambiguity of 180°, intrinsic in solar vector magnetic
field measurements. We resolve the 180° ambiguity by minimizing the
inhomogeneities of the magnetic field strength perpendicular to the
magnetic field vector. This relates to a minimization of the sheath
currents that envelope the solar magnetic flux tubes, thus allowing
for more space-filling and less complex magnetic fields. Structure
minimization proceeds in two steps: First, it derives a local solution
analytically, by means of a structure minimization function. Second,
it reaches a global solution numerically, assuming smoothness of the
magnetic field vector. Structure minimization (i) is disentangled from
any use of potential or linear force-free extrapolations and (ii)
eliminates pixel-to-pixel dependencies, thus reducing exponentially
the required computations. We apply structure minimization to four
active regions, located at various distances from disk center. The
minimum structure solution for each case is compared with the
“minimum energy” solution obtained by the slower simulated
annealing algorithm. We find correlation coefficients ranging from
significant to excellent. Moreover, structure minimization provides
an ambiguity-free vertical gradient of the magnetic field strength
that reveals the variation of the magnetic field with height. The
simplicity and speed of the method allow a near real-time processing
of solar vector magnetograms. This task was not possible in the past
and may be of interest to both existing and future solar missions and
ground-based magnetographs.
---------------------------------------------------------
Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy
Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm;
Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J.
2004SoPh..219..149A Altcode: 2003astro.ph..9499A
We compare the photometric accuracy of spectra and images in flares
observed with the Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI) spacecraft. We test the accuracy of the photometry by
comparing the photon fluxes obtained in different energy ranges from the
spectral-fitting software SPEX with those fluxes contained in the images
reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit
algorithms. We quantify also the background fluxes, the fidelity of
source geometries, and spatial spectra reconstructed with the five image
reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field of view, and time intervals on the quality of image
reconstruction. The detailed parameters and statistics are provided
in an accompanying CD-ROM and web page. We find that Forward-fit,
Pixon, and Clean have a robust convergence behavior and a photometric
accuracy in the order of a few percent, while MEM does not converge
optimally for large degrees of freedom (for large field of view and/or
small pixel sizes), and MEM-Vis suffers in the case of time-variable
sources. This comparative study documents the current status of the
RHESSI spectral and imaging software, one year after launch.
---------------------------------------------------------
Title: Radiative Hydrodynamic Models of Solar White Light Flares
Authors: Allred, J. C.; Hawley, S. L.; Abbett, W. P.; Fisher, G. H.;
Hudson, H. S.; Metcalf, T. R.
2003AGUFMSH22A0175A Altcode:
We report on theoretical radiative hydrodynamic simulations of solar
white light flares. The solar atmosphere is modeled in detail from
the transition region to the photosphere. The coronal pressure and
X-ray backheating are included self-consistently. Flare heating is
assumed to be from an electron beam which is modeled for several
white light flares using data from RHESSI, TRACE and Yohkoh. We also
investigate the possibility that the 511 keV line width is produced
from a significant column depth of atmosphere at transition region
temperatures. We compare our new solar flare models to previous results,
and to models of M dwarf stellar flares.
---------------------------------------------------------
Title: The Relationship Between X-Ray Radiance and Magnetic Flux
Authors: Pevtsov, Alexei A.; Fisher, George H.; Acton, Loren W.;
Longcope, Dana W.; Johns-Krull, Christopher M.; Kankelborg, Charles
C.; Metcalf, Thomas R.
2003ApJ...598.1387P Altcode:
We use soft X-ray and magnetic field observations of the Sun (quiet
Sun, X-ray bright points, active regions, and integrated solar disk)
and active stars (dwarf and pre-main-sequence) to study the relationship
between total unsigned magnetic flux, Φ, and X-ray spectral radiance,
L<SUB>X</SUB>. We find that Φ and L<SUB>X</SUB> exhibit a very nearly
linear relationship over 12 orders of magnitude, albeit with significant
levels of scatter. This suggests a universal relationship between
magnetic flux and the power dissipated through coronal heating. If the
relationship can be assumed linear, it is consistent with an average
volumetric heating rate Q~B/L, where B is the average field strength
along a closed field line and L is its length between footpoints. The
Φ-L<SUB>X</SUB> relationship also indicates that X-rays provide a
useful proxy for the magnetic flux on stars when magnetic measurements
are unavailable.
---------------------------------------------------------
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: The Relation Between Coronal Mass Ejections and Outward
Motions in the Low Corona
Authors: Metcalf, T. R.
2003AGUFMSH22B..02M Altcode:
Outward motions are frequently observed in the low corona, especially
in the early phase of solar flares. Some of these motions, observed in
X-ray, EUV, and radio wavelengths, appear to be intimately linked to
coronal mass ejections (CMEs) as observed by white-light coronagraphs,
typically above 2 Rsun. In principle, a combination of X-ray/EUV/radio
and coronagraph data should give many constraints on models of CME
initiation and early propagation. However, such attempts are not
straightforward because of the diversity of low coronal motions,
which often have unclear relationships, if any, to the CME. In this
presentation, we compare many examples of low coronal motions observed
by TRACE and Yohkoh with CMEs observed by LASCO, and discuss what we can
objectively conclude from the data. Errors resulting from the inclusion
of different and possibly incompatible data in a single so-called
height-time plot are discussed. We also study the reality of the flare
association of CMEs, which, according to the height-time plots, precede
the flare and hence the low coronal motions by more than an hour.
---------------------------------------------------------
Title: Cryogenic 3-D Detectors for Solar Physics Using Position
Sensitive Transition-Edge Sensors
Authors: Stern, R. A.; Martinez-Galarce, D.; Metcalf, T.; Lemen, J.;
Cabrera, B.; Brink, P.; Leman, S.; Deiker, S.; Irwin, K.; Alexander, D.
2003AAS...203.1805S Altcode: 2003BAAS...35Q1237S
Space and ground-based astronomy is currently undergoing a revolution
in detector technology with the advent of cryogenic sensors operating in
the sub-Kelvin temperature range. These detectors provide non-dispersive
energy resolution at optical through gamma ray energies (e.g, E/Δ E ∼
1500 at 6 keV), high time resolution (msec or better), and can be made
into imaging arrays through SQUID multiplexing of individual pixels or
employing macropixels with position sensing capability. The application
of such “3-D” detector technology to solar physics will lead to
significant advances in our understanding of magnetic reconnection
phenomena in the Sun, including flares and microflares, X-ray jets,
and active region dynamics. An Explorer-class solar mission based upon
these detectors is a distinct possibility within the next 5-10 years. In
this poster, we will describe some of the recent advances in cryogenic
detector technology with particular applicability to solar physics,
and future technical developments required to make such a mission a
reality. <P />This work was supported in part by the Lockheed Martin
Independent Research Program.
---------------------------------------------------------
Title: Data inversion for the Multi-Order Solar Extreme-Ultraviolet
Spectrograph
Authors: Fox, J. Lewis; Kankelborg, Charles C.; Metcalf, Tomas R.
2003SPIE.5157..124F Altcode:
The Multi-Order Solar Extreme Ultraviolet Spectrograph (MOSES) is
a high resolution, slitless imaging spectrometer that will observe
the Sun in extreme ultraviolet near 304A. MOSES will fly on a NASA
sounding rocket launch in spring 2004. The instrument records spatial
and spectral information into images at three spectral orders. To
recover the source spectrum, an ill-posed inversion must be performed
on these data. We will explore two of the techniques by which this may
be accomplished: Fourier backprojection and Pixons, constrained by the
spatially integrated spectrum of the Sun. Both methods produce good
results, including doppler shifts measured to 1/3-pixel accuracy. The
Pixon code better reproduces the line widths.
---------------------------------------------------------
Title: TRACE and Yohkoh Observations of a White-Light Flare
Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.;
Longcope, Dana W.
2003ApJ...595..483M Altcode:
We present observations of a large solar white-light flare observed
on 2001 August 25, using data from the Transition Region and Coronal
Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are
consistent with the classic type I white-light flare mechanism, and we
find that the enhanced white-light emission observed by TRACE originates
in the chromosphere and temperature minimum regions via nonequilibrium
hydrogen ionization induced by direct collisions with the electron beam
and by back-warming of the lower atmosphere. The three flare kernels
observed in hard X-rays and white light are spatially associated with
magnetic separatrices, and one of the kernels is observed to move along
a magnetic separatrix at 400 km s<SUP>-1</SUP>. This is evidence in
favor of particle acceleration models, which energize the electrons
via magnetic reconnection at magnetic separators.
---------------------------------------------------------
Title: Observations of Rotating Sunspots from TRACE
Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver,
C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
2003SoPh..216...79B Altcode:
Recent observations from TRACE in the photospheric white-light channel
have shown sunspots that rotate up to 200° about their umbral centre
over a period of 3-5 days. The corresponding loops in the coronal fan
are often seen to twist and can erupt as flares. In an ongoing study,
seven cases of rotating sunspots have been identified, two of which
can be associated with sigmoid structures appearing in Yohkoh/SXT and
six with events seen by GOES. This paper analyzes the rotation rates
of the sunspots using TRACE white-light data. Observations from AR
9114 are presented in detail in the main text and a summary of the
results for the remaining six sunspots is presented in Appendixes
A-F. Discussion of the key results, particularly common features,
are presented, as well as possible mechanisms for sunspot rotation.
---------------------------------------------------------
Title: A Statistical Study of Limb Flares Observed by RHESSI: Imaging
Authors: Liu, W.; Jiang, Y. W.; Petrosian, V.; Metcalf, T. R.
2003SPD....34.1802L Altcode: 2003BAAS...35..839L
Hard X-ray observations by the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) provide unprecedented opportunities
to understand the underlying physics driving solar flares. We have
embarked on an investigation of a sample of \it limb flares observed by
RHESSI. RHESSI flare images, combined with TRACE EUV observations and
SoHO/MDI magnetograms, are used to infer flaring loop structures and
to distinguish between footpoint, loop top and other possible types of
sources. We will present an analysis of the lightcurves and images for
these features and will show statistics on the occurrence of loop top
and footpoint sources. These will be compared with previous studies
utilizing YOHKOH/HXT observations, and the implications for particle
acceleration processes and other theoretical aspects of flares will
be discussed. <P />The work at Stanford is supported by NASA grants
NAG5-12111, NAG5 11918-1, and SUB2001-402-01 through the University
of Alabama in Huntsville (PI: J. Miller). T. Metcalf would like to
acknowledge support from grant NAS-98033-05/03.
---------------------------------------------------------
Title: RHESSI Observations of Electron Beam Stability
Authors: Metcalf, T. R.; Alexander, D.
2003SPD....34.1805M Altcode: 2003BAAS...35..840M
The standard thick-target electron beam model for solar flare hard
X-ray production typically requires electron beams with a current
of 10<SUP>17</SUP> Amperes in order to explain the observed hard
X-ray emission. Such high beam currents can exceed the Alfven-Lawson
limit causing the beam to essentially 'self-destruct'. One way of
circumventing this problem is to impose a converging field geometry
which reduces the area in the hard X-ray production region relative to
that in the electron acceleration region. In this paper, we use the
highest spatial resolution RHESSI observations of flare footpoints,
with inferred electron spectral and flux properties, to test whether
the conditions for stability of the electron beam are met.
---------------------------------------------------------
Title: The role of magnetic separators in solar flare particle
acceleration
Authors: Alexander, D.; Metcalf, T. R.
2003SPD....34.0418A Altcode: 2003BAAS...35..813A
In recent years, the development of theoretical models of separatrices,
separators, and quasi-separatrix layers, coupled to observations of
flare brightenings, have led to the notion that these topological
structures are the natural locations for current sheets to form and
magnetic reconnection to occur. We report on correlated hard X-ray
and vector magnetic field data from RHESSI and Mees IVM which show
the detailed spatial and temporal relationships between the magnetic
topology and the expected sites of reconnection.
---------------------------------------------------------
Title: Transition-Edge Sensors for Solar Atmospheric Observations
A Technology Overview &Update
Authors: Martinez-Galarce, D.; Stern, R.; Alexander, D.; Metcalf,
T.; Cabrera, B.; Brink, P.; Deiker, S.; Irwin, K.
2003EAEJA....14311M Altcode:
Transition-Edge Sensors (TES) are a novel class of quantum
microcalorimeters that promise to revolutionize astrophysics. Presently
they are being developed for use as the next generation detectors
for many classes of astrophysical observations. Singe pixel X-ray
measurements at the Al-Kα line (1.5keV) have been measured with 2.1
eV precision, rivaling Bragg Crystal Spectrometers resolution. As these
detectors are inherent spectrometers (with near 100 quantum efficiency
in X-ray energies), configured in an array, they promise to provide
near-perfect 4-dimensional (space, time and energy) observations of
astrophysical phenomena. We discuss the current level of advancement
of this technology and as an example we discuss some of the technical
challenges faced to design and fabricate an Explorer-class satellite
to observe the solar atmosphere.
---------------------------------------------------------
Title: Active-Region Magnetic Structure Observed in the Photosphere
and Chromosphere
Authors: Leka, K. D.; Metcalf, Thomas R.
2003SoPh..212..361L Altcode:
The full magnetic vector has been measured in both the photosphere and
chromosphere across sunspots and plage in NOAA Active Region 8299. We
investigate the vertical magnetic structure above the umbral, penumbral
and plage regions using quantitative statistical comparisons of the
photospheric and chromospheric magnetic data. The results include: (1)
a general decrease in average magnetic flux density with height; (2)
the direct detection of the superpenumbral canopy in the chromosphere;
(3) values for dB/dz which are consistent with earlier investigations
when derived from a straight difference between the two measurements,
but which are somewhat small when derived from the ∇⋅B=0 condition,
(4) a monolithic structure in the umbrae which extends well into
the upper chromosphere, with a very complex and varied structure in
penumbrae and plage, as evidenced by (5) a uniform magnetic scale height
in the umbrae with an abrupt jump to widely varying scale heights in
penumbral and plage regions. Further, we find (6) evidence that field
extrapolations using the photospheric flux as the boundary may not
agree with expectations or with observed coronal structures as well as
those which use the chromospheric magnetic flux as the extrapolation
starting point.
---------------------------------------------------------
Title: Mapping RHESSI footpoints with potential-field models
Authors: Fletcher, L.; Hudson, H. S.; Metcalf, T. R.
2002AGUFMSH52A0452F Altcode:
RHESSI hard X-ray observations help us to identify the locations of
magnetically conjugate footpoints, and to study their apparent motions
during the evolution of the impulsive phase of a flare. We put this
information into the context of an M-class flare that occurred 2002
March 14 01:50 UT (GOES peak time) at S12, E23 (NOAA region 9866) by
making potential-field mappings of the coronal magnetic structure. In
principle the hard X-ray sources (plus the mapping) constrain the
site of magnetic energy release, and the maps reveal the location
of the stored energy. The RHESSI source centroids can be determined
to better than 1” (rms) for an M-class flare. This analysis is an
exploration of the feasibility of such an approach, since full success
would require understanding the magnetic restructuring in detail. If
suitable X-class RHESSI flares occur we will be able to present data
with better precision.
---------------------------------------------------------
Title: The RHESSI Imaging Concept
Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway,
A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull,
C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato,
J.; Smith, D. M.
2002SoPh..210...61H Altcode:
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV
with spatial resolution as high as 2.3 arc sec. Instead of focusing
optics, imaging is based on nine rotating modulation collimators that
time-modulate the incident flux as the spacecraft rotates. Starting from
the arrival time of individual photons, ground-based software then uses
the modulated signals to reconstruct images of the source. The purpose
of this paper is to convey both an intuitive feel and the mathematical
basis for this imaging process. Following a review of the relevant
hardware, the imaging principles and the basic back-projection method
are described, along with their relation to Fourier transforms. Several
specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable
to RHESSI imaging are briefly described. The characteristic strengths
and weaknesses of this type of imaging are summarized.
---------------------------------------------------------
Title: Energy dependence of electron trapping in a solar flare
Authors: Alexander, David; Metcalf, Thomas R.
2002SoPh..210..323A Altcode:
Observations of an energy-dependent asymmetry in footpoint hard X-ray
emission by RHESSI for the M4.0 solar flare of 17 March 2002 allows us
to probe the dynamics of particle transport with energy and time. The
presence of such an asymmetry is most readily explained by the effects
of a converging magnetic field with different rates of convergence at
the different footpoints, as would be expected from realistic surface
field distributions. Such a geometry has been discussed in the context
of a trap-plus-precipitation model where the transport of energetic
particles in the flare is governed by the precipitation out of the
coronal trap via collisions, wave-particle interactions or some other
scattering process, into the high-density chromosphere. Comparison
of RHESSI observations with a trap-plus-precipitation model allows us
to use the energy dependence of the asymmetry and the observed ratio
of footpoint to coronal emission at the different energies to assess
the role of the trapping in the transport of energetic electrons and
to probe the nature of the particle precipitation process inside the
loss cone.
---------------------------------------------------------
Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI)
Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.;
Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.;
Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.;
Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.;
Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii,
M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone,
D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.;
Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey,
K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck,
R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner,
Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson,
H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie,
A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.;
Metcalf, T.; Conway, A.
2002SoPh..210....3L Altcode:
RHESSI is the sixth in the NASA line of Small Explorer (SMEX)
missions and the first managed in the Principal Investigator mode,
where the PI is responsible for all aspects of the mission except
the launch vehicle. RHESSI is designed to investigate particle
acceleration and energy release in solar flares, through imaging and
spectroscopy of hard X-ray/gamma-ray continua emitted by energetic
electrons, and of gamma-ray lines produced by energetic ions. The
single instrument consists of an imager, made up of nine bi-grid
rotating modulation collimators (RMCs), in front of a spectrometer
with nine cryogenically-cooled germanium detectors (GeDs), one behind
each RMC. It provides the first high-resolution hard X-ray imaging
spectroscopy, the first high-resolution gamma-ray line spectroscopy,
and the first imaging above 100 keV including the first imaging of
gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec
with a full-Sun (≳ 1°) field of view, and the spectral resolution
is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV)
to gamma-rays (17 MeV). An automated shutter system allows a wide
dynamic range (>10<SUP>7</SUP>) of flare intensities to be handled
without instrument saturation. Data for every photon is stored in a
solid-state memory and telemetered to the ground, thus allowing for
versatile data analysis keyed to specific science objectives. The
spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼
0.2° and operates autonomously. RHESSI was launched on 5 February
2002, into a nearly circular, 38° inclination, 600-km altitude orbit
and began observations a week later. The mission is operated from
Berkeley using a dedicated 11-m antenna for telemetry reception and
command uplinks. All data and analysis software are made freely and
immediately available to the scientific community.
---------------------------------------------------------
Title: Observations of rotating sunspots and their effect in the
corona
Authors: Brown, D. S.; Nightingale, R. W.; Alexander, D.; Schrijver,
C. J.; Metcalf, T. R.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
2002ESASP.505..261B Altcode: 2002IAUCo.188..261B; 2002solm.conf..261B
Recent observations from TRACE have seen sunspots, in the photospheric
white light filter, rotate up to 180 degrees about their umbral
centre. The corresponding loops in the coronal fan are seen to twist
and can erupt. In an ongoing study, five cases of rotating sunspots
have been identified, three of which can be identified with sigmoid
structures appearing in Yohkoh/SXT. This paper will present images from
one of these events, showing the coupling between the photosphere and
the corona, and observational analysis deducing the rotation speeds
and how they change through time and with radius of the sunspot. In
particular, the paper will focus on the best example of a rotating
sunspot observed so far, that of AR 9114 which occurred over 8-10 August
2000 and was observed by TRACE, SoHO/MDI and Yohkoh/SXT. The sunspot
rotated 150 degrees within this time and is associated with a sigmoid.
---------------------------------------------------------
Title: Helicity driven sigmoid evolution
Authors: Alexander, D.; Nightingale, R.; Metcalf, T. R.; Brown, D.
2002AAS...200.3608A Altcode: 2002BAAS...34..694A
Recent observations of rotating sunspots in TRACE white light images
and their apparent association with soft X-ray sigmoids have led to the
intriguing possibility that the sunspot rotation serves as the driver
for both sigmoid formation and their potential eruption. We discuss
the energization of the corona resulting from currents generated by
the vortex motions of the rotating sunspot. We will present data from
events for which we have good white light coverage of the sunspot, an
evolving sigmoid and an associated CME (in those cases where the sigmoid
erupts). We investigate the relationship between the sunspot rotation
and the evolution of the sigmoid structure and attempt to determine
the key physical conditions which result in a sigmoid destabilizing
and ultimately producing a CME.
---------------------------------------------------------
Title: The Electron Number Problem Revisited with RHESSI Flare
Observations
Authors: Aschwanden, M. J.; Alexander, D.; Metcalf, T.; Nitta, N.
2002AAS...200.7608A Altcode: 2002BAAS...34..776A
The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observed
a number of solar flares since its launch on February 5, 2002. We
analyze first images and spectra from this new mission, using a
variety of image reconstruction methods, such as Clean, Pixon, Maximum
Entropy (MEM), and Forward-Fitting. We obtain context images of the
flare regions in EUV from SoHO/EIT and TRACE, and soft X-ray light
curves from GOES. From RHESSI we reconstruct hard X-ray images with
full uv-coverage in time intervals of the spin period ( 4 s). RHESSI
provides a substantially higher spatial resolution ( 2") and spectral
resolution than previous data from Yohkoh/HXT. A longstanding problem
in solar flare physics is the so-called electron number problem,
which challenges a reconciliation between (1) the rate of injected
electrons inferred from the observed hard X-ray photon spectrum in
the framework of the thick-target model and the (2) the replenishment
rate of electrons in the coronal acceleration volume. Thanks to the
high spectral resolution of RHESSI we can for the first time properly
separate the thermal and the non-thermal electron spectral components,
and this way determine the injection rate of nonthermal electrons
more accurately. In addition, imaging in many energy bands allows us
to trace propagation and energy loss between the coronal acceleration
site and the chromospheric thick-target site with higher accuracy,
taking into account the partial energy loss of trapped and precipitating
electrons. The goal of this investigation is to quantify the geometry,
density, and inflows in the coronal acceleration region, which provides
crucial constraints for the underlying magnetic reconnection process.
---------------------------------------------------------
Title: Hard X-ray and White Light Observations of the August 25,
2001 X Flare
Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. S.; Longcope, D.;
Myers, D.
2002AAS...200.6803M Altcode: 2002BAAS...34..755M
An X5.3 flare occurred about 16:31 UT on 2001 August 25 and was well
observed by the Yohkoh and TRACE spacecraft. The flare showed gamma-ray
emission, nuclear lines, and was a dramatic white light flare seen in
TRACE data. A preliminary analysis of the hard X-ray images from the
Yohkoh/HXT instrument shows two clear footpoints and a moving HXR
source in this very energetic flare. The moving hard X-ray source
appears to move along a magnetic separatrix at 400 km/sec. We will
discuss the hard X-ray and white light structure of this flare and
discuss the energetics and possible mechanisms for the formation of
the TRACE white light emission.
---------------------------------------------------------
Title: Fast acceleration of a CME-related X-ray structure in the
low solar corona
Authors: Alexander, David; Metcalf, Thomas R.; Nitta, Nariaki V.
2002GeoRL..29.1403A Altcode: 2002GeoRL..29j..41A
We report on a unique observation of the early and rapid acceleration of
a fast CME in the low solar corona. The coronal disturbance associated
with a LASCO CME and concurrent X1.2 flare on 1998 April 23 was
well-observed by the Yohkoh Soft X-ray Telescope. The X-ray observations
clearly show an accelerating structure, reaching 800-1100 km/s in ~500s
at a de-projected height of only 280 Mm, indicating an acceleration
well in excess of 1700 m/s<SUP>2</SUP>. The soft X-ray observations
are coincident with the onset of the CME, which ultimately attained
a velocity of 1390 km/s and generated a number of interplanetary
radio signatures, frequently associated with fast events. Assuming a
constant acceleration, a<SUB>0</SUB>, throughout the duration of the
X-ray observations, we determine a best-fit value of a<SUB>0</SUB> =
1756 m/s<SUP>2</SUP>, while application of a power-law acceleration
model yields a better fit to the data with a peak acceleration of
~4865 m/s<SUP>2</SUP> within the first 280 Mm of the solar corona.
---------------------------------------------------------
Title: The Magnetic Free Energy in Active Regions
Authors: Metcalf, Tom
2002smra.progE..16M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Concurrent Rotating Sunspots, Twisted Coronal Fans, Simgoid
Structures and Coronal Mass Ejections
Authors: Nightingale, R. W.; Brown, D. S.; Metcalf, T. R.; Schrijver,
C. J.; Shine, R. A.; Title, A. M.; Wolfson, C. J.
2002mwoc.conf..149N Altcode:
In an on-going study, several sunspots, in apparent rotation, have
been identified in TRACE photospheric white light (WL) images with
accompanying twisting of coronal fans in the corresponding EUV (171,
195 AA) images. These observations can also be temporally and spatially
associated with S or inverse-S shaped regions (sigmoid structures)
appearing in Yohkoh SXT images and with concurrent coronal mass
ejections (CMEs) and/or flares. We have determined the rotational
speed of the apparently rotating sunspot in AR 9114 over 8-10 August
2000, established the inverse S shape observed in the SXT data, and
viewed a rapid, bright flash of possible reconnection in a TRACE
EUV movie. A CME was observed during the 15-18 August 1999 event,
which also included an inverse S shaped region in the SXT data, and
a rotating sunspot and twisting coronal fans in the TRACE data. The
large Bastille Day CME event of 14 July 2000 was accompanied by one
or more apparently rotating sunspots as observed in TRACE WL and by
an inverse S shaped region as seen in a difference SXT image. Movies
and plots of some of these data will be shown along with flow maps and
a list of the pertinent parameters for several rotating sunspots. We
will report on our attempt to determine the vertical electric current
flowing through the 8 August 2000 sunspot utilizing the Mees vector
magnetograph data in order to better understand the apparent rotation
"driver". These observations display the coupling of the solar magnetic
field from the photosphere into the corona.
---------------------------------------------------------
Title: How the corona depends on magnetic variables
Authors: Metcalf, Thomas
2002ocnd.confE..21M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Yohkoh Public Outreach Project
Authors: Larson, M. B.; Slater, T.; McKenzie, D.; Acton, L.; Alexander,
D.; Lemen, J.; Freeland, S.; Metcalf, T.
2002mwoc.conf..117L Altcode:
The NASA funded Yohkoh Public Outreach Project (YPOP) began in 1995
with the goal of providing public access to high quality Yohkoh
SXT data via the World Wide Web. The project utilizes the intrinsic
excitement of the SXT data, and in particular the SXT movies, to develop
science learning tools and classroom activities. The WWW site at URL:
http://www.lmsal.com/YPOP/ uses a movie theater theme to highlight
available Yohkoh movies in a format that is entertaining and inviting
to non-scientists and well received by scientists. We will discuss
the wide range of people YPOP has reached over the past six years,
as well as lessons learned during the development of the project.
---------------------------------------------------------
Title: Yohkoh: A Decade of Discovery [Invited]
Authors: Alexander, D.; Metcalf, T. R.
2002mwoc.conf..103A Altcode:
Data from the Yohkoh satellite has led the way in the dissemination of
scientific knowledge to the general public over the last decade. This
talk will discuss the role that solar physics in general, and Yohkoh in
particular, have played in stimulating a public interest in science and
the effect they have had in the teaching of science in the classroom. To
highlight Yohkoh we will discuss the extremely successful Yohkoh Public
Outreach Project and its daughter Solar Week with particular emphasis
on the role played by scientists in the effective dissemination of
the solar science.
---------------------------------------------------------
Title: The Magnetic Free Energy and a CME in Active Region 8299
Authors: Metcalf, T. R.; Mickey, D. L.; Labonte, B. J.; Ryder, L. A.
2002mwoc.conf..249M Altcode:
We calculate the magnetic free energy as a function of time for
NOAA active region 8299 on 1998 August 11 using vector magnetic
field measurements in the ion Na 1 5896Å sepctral line observed
with the Imaging Vector Magnetograph at Mees Solar Observatory. The
free energy in this active region is significant with a magnitude
of about 10<SUP>33</SUP> ergs, though the active region was not
flare productive. This amount of free energy is more than enough to
explain the enhanced heating of the active region corona. The free
energy dipped to a value consistent with zero for one hour during
the observation. Yohkoh/SXT images reveal that during this dip in the
free energy, the coronal structure of AR 8299 and the nearby AR8297
changed significantly. SXT observed the brightening of a coronal loop
connecting AR 8299 and AR 8297 and observed coronal dimming and the
formation of a cusp structure in AR 8297, suggesting that a gradual CME
was launched as the magnetic energy dipped. Unfortunately, LASCO data
were not available to confirm the existence of a halo CME. However,
the circumstantial evidence points to the magnetic free energy as the
energy source for the postulated CME.
---------------------------------------------------------
Title: The Yohkoh Public Outreach Project: A Space Science Resource
for Formal and Informal Education
Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.;
Acton, L. W.; Larson, M.; McKenzie, D.; Slater, T.
2001AGUFMED12A0160L Altcode:
The Yohkoh Public Outreach Project (YPOP) is a NASA-funded web site
maintained by scientists and educators at Lockheed Martin Solar and
Astrophysics Lab. and Montana State University. YPOP includes a range
of activities for youngsters, parents, teachers and anyone interested
in learning more about the Sun. YPOP utilizes a number of approaches
to the dissemination of solar data which incorporates elements of both
formaleducation, via a number of lesson plans and classroom activities,
and informal education, via access to the latest solar images, a
solar tour, and updated movies. This combination has proved extremely
effective in providing quality access to scientific data for a broad
audience with a wide range of interests. The Yohkoh Public Outreach
Project can be found at http://www.LMSAL.com/YPOP.
---------------------------------------------------------
Title: Energization of Rotating Sunspots, Twisted Coronal Fans,
Sigmoid Structures, and Coronal Mass Ejections
Authors: Nightingale, R. W.; Alexander, D.; Brown, D. S.; Metcalf,
T. R.
2001AGUFMSH11C0724N Altcode:
In an on-going study, several sunspots, rotating about their umbral
centers, have been identified in TRACE photospheric white light
(WL) images with accompanying twisting of coronal fans connected to
penumbral magnetic field lines in the corresponding EUV (171, 195
Å) images. These observations can also be temporally and spatially
associated with S or inverse-S shaped regions (sigmoid structures)
appearing in Yohkoh SXT images and with concurrent coronal mass
ejections (CMEs) and/or flares. We have determined the rotational
speed of the sunspot in AR9114 over 8-10 August 2000, established the
inverse-S shape observed in the SXT data, and viewed a rapid flash of
possible reconnection in a TRACE EUV movie. A CME was observed in AR8667
during the 15-18 August 1999 event, which also included an inverse-S
shaped region in the SXT data, and a rotating sunspot and twisting
coronal fans in the TRACE data. The large Bastille Day CME event of 14
July 2000 in AR9077 was accompanied by one or more rotating sunspots
as observed in TRACE WL and by an inverse-S shaped region as seen in
a difference SXT image. We will utilize these data to estimate the
energization of the twisted coronal fans resulting from the sunspot
rotation and compare this with the temporal evolution of the sigmoid
structures. We will investigate the possibility of a direct role of
the observed sunspot rotation and the potential for a sigmoid to erupt
as a CME. This work was supported by NASA under contract NAS5-38099.
---------------------------------------------------------
Title: Hard X-ray Observations of the August 25, 2001 X Flare
Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. H.
2001AGUFMSH42A0776M Altcode:
The X5.3 flare which occurred about 16:31 UT on 2001 August 25 was
well observed by the Yohkoh spacecraft. The flare showed gamma-ray
emission, nuclear lines and was a dramatic white light flare. A
preliminary analysis of the hard X-ray images from the Yohkoh/HXT
instrument show two clear footpoints to this very energetic flare,
one nearly stationary and the other apparently moving as the flare
progresses. We will discuss the hard X-ray and soft X-ray structure
of this flare and compare the hard X-ray images to the TRACE and MDI
white light data both spatially and temporally.
---------------------------------------------------------
Title: Evidence for the Flare Trigger Site and Three-Dimensional
Reconnection in Multiwavelength Observations of a Solar Flare
Authors: Fletcher, L.; Metcalf, T. R.; Alexander, D.; Brown, D. S.;
Ryder, L. A.
2001ApJ...554..451F Altcode:
Based on a multiwavelength data set and a topological model for
the magnetic field, we argue that a M1.9 flare which occurred on
1993 May shows evidence of three-dimensional coronal reconnection
in a spine-fan configuration. Images from the Transition Region and
Coronal Explorer allow the detailed examination of the structures
involved in the flare and preflare in the 171 Å (1 MK) EUV passband
and the Lyα (10,000-20,000 K) passband. Yohkoh Hard X-ray Telescope
maps the position of nonthermal electron precipitation and the
Soft X-ray Telescope reveals preflare and flare heating on large
and small scales. While the flare appears to be driven by changes in
small-scale field close to the photosphere, near the interface between
strong opposite magnetic polarities, the result is the disruption of
large-scale field. We demonstrate how this observed activity on large
and small scales, along with many other aspects of the flare, suggests
a qualitative explanation in the three-dimensional reconfiguration of
coronal magnetic field, following a small-scale flux cancellation at
the photosphere.
---------------------------------------------------------
Title: A Comparison of the Active Region Magnetic Field in the
Photosphere and Chromosphere
Authors: Metcalf, T. R.; Leka, K. D.
2001AGUSM..SP41B07M Altcode:
During the Whole Sun Fortnight, the Imaging Vector Magnetograph at Mees
Solar Observatory obtained vector magnetic field maps of AR 8299 in the
Na-D line (the core of which is formed in the lower chromosphere). At
almost the same time, the HAO/NSO Advanced Stokes Polarimeter
obtained data in the Fe-I line, formed in the photosphere. We present
a comparison of the structures observed in AR 8299's main sunspots on
18 August 1998 at the different atmospheric heights. The data suggest
that the Na-D magnetic field maps are formed 2.5 Mm above the Fe-I
magnetic field maps. At this height in the atmosphere, the magnetic
field is force-free and we will explore the implications of this for
the extrapolation of the magnetic field in the corona.
---------------------------------------------------------
Title: Analysis of X-ray counterparts for Fast and Slow Halo CMEs
Authors: Alexander, D.; Metcalf, T. R.; Nitta, N.
2001AGUSM..SH22B05A Altcode:
In many cases, coronal mass ejections exhibit a strong counterpart in
the X-ray corona, particularly in flare-related events. Observations
by the Soft X-ray Telescope on Yohkoh have exhibited a number of
manifestations in association with CME eruptions. These include the
well-known dimming events, post-event arcades and expanding loop
systems. We examine the X-ray signatures of a number of fast and
slow halo CME events for evidence of two-classes of CME eruption in
accord with the observed velocity profiles determined from white-light
data. Flare-associated CMEs, which have a tendency to exhibit constant
velocity, necessarily undergo a rapid acceleration in the low corona
and should exhibit enhanced heating of the ambient corona to X-ray
emitting temperatures. Slow CMEs, on the other hand, are expected to
display a more subtle, if any, effect in the hot corona. We examine
the Yohkoh database for evidence of a dichotomy in the X-ray signatures
of halo CMEs.
---------------------------------------------------------
Title: Instrumental Effects and their Removal from TRACE Images
Authors: Tarbell, T. D.; Nightingale, R. N.; Metcalf, T. R.; Frank,
Z. A.; TRACE Team
2000SPD....31.0291T Altcode: 2000BAAS...32..827T
TRACE raw images often show fine structures at such high contrast
that compensation for instrumental effects is not necessary to study
their morphology and evolution. Nevertheless, TRACE team members have
gradually been developing the techniques and calibrations necessary
to understand and (sometimes) remove the principal instrumental
degradations. These degradations include CCD pedestal variation
with temperature and time, flat-field response, CCD sensitivity loss
varying with position and wavelength, electrical interference during
CCD readout, permanent dim pixels, intermittent hot pixels, overall
system point spread function, diffraction of EUV radiation by the front
entrance filters, scattered light at off-limb pointings, tracks of
electrons and protons from the radiation belts, and artifacts from JPEG
compression of solar strucures and the above defects. Characterization
and removal of some of these will be presented in the poster, such
as the following. A deconvolution routine can partially compensate
for the EUV diffraction, which is described in the adjacent poster by
Frank et al. White light flat fields are derived using the Kuhn-Lin
algorithm. CCD sensitivity degradation (presumably lumogen damage) is
measured both from crude UV and EUV flat fields and from mission-long
analysis of synoptic disk center images; combining these results with
the WL flats yields flat fields at all wavelengths. Some information on
scattered light and point spread functions are obtained from the August,
1999, eclipse observations and the Mercury transit. SSW routines for
dealing with some of these degradations will be identified. This work
is supported by the TRACE project at LMSAL (contract NAS5-38099).
---------------------------------------------------------
Title: Coronal Heating and Sheared Neutral Line Length in Solar
Active Regions
Authors: Griffiths, N. W.; Fisher, G. H.; Metcalf, T. R.; Falconer,
D. A.
2000SPD....31.0209G Altcode: 2000BAAS...32..812G
We present a comparison of X-ray and magnetic data from a sample of
several hundred solar active regions, with the aim of finding which
global magnetic properties are most strongly related to observed levels
of coronal heating. The study includes SFD images from several different
bandpasses of the Soft X-ray Telescope on the Yohkoh spacecraft, and
vector magnetogram data from the Haleakala Stokes Polarimeter at the
University of Hawaii's Mees Solar Observatory. Following upon earlier
work by Falconer, we will examine the role that sheared neutral lines
(as well as other magnetic quantities, such as magnetic flux), might
play in determining the level of coronal heating. In particular, we
will test whether sheared neutral line length within active regions
provides a better correlation with X-ray luminosity than the established
relationship between unsigned magnetic flux and X-rays.
---------------------------------------------------------
Title: Evidence for the Flare Trigger Site and 3-D Reconnection in
Multi-Wavelength Observations of a Solar Flare
Authors: Metcalf, T. R.; Fletcher, L.; Alexander, D.; Brown, D. S.;
Ryder, L. A.
2000SPD....31.0261M Altcode: 2000BAAS...32..822M
Based on a multi-wavelength data set and a topological model for the
magnetic field, we argue that a M1.9 flare which occurred on 3-May-99
shows evidence of 3-d reconnection through a coronal null. Images
from the Transition Region and Coronal Explorer allow the detailed
examination of the structures involved in the flare and pre-flare in
the 171 Angstroms (1MK) EUV passband, and the Lyman α (10,000-20,000K)
passband. Yohkoh Hard X-ray Telescope maps the position of non-thermal
electron precipitation and the Soft X-ray Telescope reveals pre-flare
and flare heating on large and small-scales. While the flare appears
to trigger in small scale field close to the photosphere, near the
interface between strong opposite magnetic polarities, the large-scale
field is directly involved in or affected by all phases of the flare. We
demonstrate how this observed activity on large and small scales, along
with many other aspects of the flare find a qualitative explanation
in the three-dimensional reconfiguration of coronal magnetic field,
following a small-scale flux cancelation at the photosphere. This
work is supported by the TRACE and SXT projects at LMSAL (contract
NAS5-38099 and NAS8-40801).
---------------------------------------------------------
Title: Observations of H-alpha Polarization in Flares
Authors: Mickey, D. L.; Metcalf, T. R.; Fletcher, L.
2000SPD....31.0255M Altcode: 2000BAAS...32R.820M
Max Millennium Coordinated Observing Plan #006 has as its goal the
measurement of H-alpha linear polarization, a possible signature of
low-energy proton beams in solar flares. Such observations have been
attempted in the past, but the results were inconclusive. A campaign
involving Mees Solar Observatory and Big Bear Solar Observatory,
together with Yohkoh, SOHO and TRACE spacecraft, was carried
out during the latter half of March 2000. We present preliminary
results from the observations with the Imaging Vector Magnetograph
at Mees Solar Observatory. The IVM was operated in a standard mode,
except that the spectral scan was limited to one point in the core
of H-alpha and one in the blue wing. This limited spectral sampling,
together with a recently upgraded data acquisition system, permitted
a complete measurement of Stokes vectors every 3.5 seconds. The field
of view was 280 arc sec square, with one arc sec pixels. A second
camera, exposed simultaneously but with a broad-band filter, provides
images which allow compensation for relative image motion and stretch
between exposures. Approximately two dozen flares were observed by
the IVM during the campaign, including one X-class and one M-class
flare. We present samples of the observations, including relevant
spacecraft observations, and discuss the sensitivity of the IVM to
linear polarization under these conditions. This work was supported
in part by the SXT project at LMSAL (contract NAS5-38099) and by NASA
grant NAG5-4941.
---------------------------------------------------------
Title: Anticipating HESSI's Spatially Resolved View of Spectral
Evolution
Authors: Newton, E.; Giblin, T.; Metcalf, T.
2000ASPC..206..272N Altcode: 2000hesp.conf..272N
No abstract at ADS
---------------------------------------------------------
Title: Coronal Trapping of Energetic Flare Particles: Yohkoh/HXT
Observations
Authors: Metcalf, Thomas R.; Alexander, David
2000ASPC..206..233M Altcode: 2000hesp.conf..233M
No abstract at ADS
---------------------------------------------------------
Title: Coronal Trapping of Energetic Flare Particles:Yohkoh/HXT
Observations
Authors: Metcalf, Thomas R.; Alexander, David
1999ApJ...522.1108M Altcode:
We examine spectroscopic data from the Yohkoh Hard X-Ray Telescope in
a search for spectral evidence of the coronal trapping of energetic
particles during solar flares. Two distinct particle populations with
significantly different spectral properties are found to be present in
three of the six flares studied; the first population is trapped in the
corona, where it encounters a “thick-thin” target, while the second
population precipitates directly to the footpoints. In the remaining
three events, a single population of energetic particles appear to be
responsible for the observed hard X-ray emission, either via thermal
bremsstrahlung (one case) or nonthermal thin-target emission (two
cases). For the three events in which a trapped population is observed,
the spectroscopic observations imply first that there is likely
to be a single acceleration mechanism for both the trapped and the
precipitating populations and second that the magnetic field geometry
in these flares is conducive to trapping in a confined region high in
the corona, above the soft X-ray loops. Both conditions are consistent
with magnetic reconnection models of flares in which energetic particles
are trapped between MHD slow-mode shocks attached to the reconnection
region and a fast-mode shock formed by the reconnection outflow jet.
---------------------------------------------------------
Title: Making YOHKOH SXT Images Available to the Public: The YOHKOH
Public Outreach Project
Authors: Larson, M. B.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
1999AAS...194.7024L Altcode: 1999BAAS...31..941L
The NASA funded Yohkoh Public Outreach Project (YPOP) provides public
access to high quality Yohkoh SXT data via the World Wide Web. The
products of this effort are available to the scientific research
community, K-12 schools, and informal education centers including
planetaria, museums, and libraries. The project utilizes the intrinsic
excitement of the SXT data, and in particular the SXT movies, to develop
science learning tools and classroom activities. The WWW site at URL:
http://solar.physics.montana.edu/YPOP/ uses a movie theater theme to
highlight available Yohkoh movies in a format that is entertaining and
inviting to non-scientists. The site features informational tours of
the Sun as a star, the solar magnetic field, the internal structure
and the Sun's general features. The on-line Solar Classroom has proven
very popular, showcasing hand-on activities about image filtering,
the solar cycle, satellite orbits, image processing, construction of a
model Yohkoh satellite, solar rotation, measuring sunspots and building
a portable sundial. The YPOP Guestbook has been helpful in evaluating
the usefulness of the site with over 300 detailed comments to date.
---------------------------------------------------------
Title: The Magnetic Free Energy in Active Regions
Authors: Metcalf, T. R.; Mickey, D. L.
1999AAS...194.9407M Altcode: 1999BAAS...31..992M
The energy source for the heating of the solar corona and for solar
flares is widely believed to be the solar magnetic field. While most
observations of the solar magnetic field are of the photospheric
magnetic field, there are a number of advantages to observing the
magnetic field in the chromosphere. Most of these derive from the fact
that the magnetic field in the chromosphere is force-free more than a
few hundred km above the photosphere. This was first demonstrated by
Metcalf et al. (ApJ, 439, 474, 1995) using NaI D-line observations from
the Haleakala Stokes Polarimeter. When analyzing the magnetic field in
an active region, the field is usually assumed to be force-free. The
fact that it is not restricts the usefulness of such analyses. For
example, the magnetic field is universally assumed to be force-free
when extrapolations of the field into the corona are computed. Clearly,
if the measured field really is force-free, such extrapolations are
more robust. Also, when the field can be shown to be force-free,
new analysis tools are available. For example, the free energy in the
magnetic field can be measured using the magnetic virial theorem. This
is impossible with photospheric measurements of the magnetic field
(Metcalf et al., 1995). The objective of this project is to measure the
solar chromospheric magnetic field using the NaI D line observed with
the University of Hawaii's Imaging Vector Magnetograph (IVM). Since the
magnetic field observed in the chromosphere is known to be force-free,
we plan to use the data to measure the magnetic free energy in active
regions. This data set will enable us to look for relationships between
the free energy and coronal heating rates and flare rates. This work
is supported by NASA contract NAG5-7438.
---------------------------------------------------------
Title: High Cadence EUV Imaging Spectrograph for the 2010 Solar
Maximum
Authors: Wuelser, J. -P.; Metcalf, T. R.
1999AAS...194.7616W Altcode: 1999BAAS...31..959W
Recent observations by TRACE clearly show that the solar corona is
both highly structured and highly time variable. To understand the
physics behind the dynamical coronal processes, future observations
should determine not only the morphology, but also the physical
condition of the plasma. This task is best accomplished by imaging
spectroscopy of the corona. However, currently planned and operating
instruments fail to provide the necessary measurements at the relevant
spatial and temporal scales. This paper presents the concept of an
EUV imaging spectrograph that overcomes many of the limitations of
current designs. The concept focusses on high sensitivity over a
limited spectral range, and achieves spectroscopy with full spatial
coverage at TRACE-like resolution, cadence, and signal-to-noise ratio.
---------------------------------------------------------
Title: Coronal Heating in Active Regions as a Function of Global
Magnetic Variables
Authors: Fisher, George H.; Longcope, Dana W.; Metcalf, Thomas R.;
Pevtsov, Alexei A.
1998ApJ...508..885F Altcode:
A comparison of X-ray images of the Sun and full disk magnetograms
shows a correlation between the locations of the brightest X-ray
emission and the locations of bipolar magnetic active regions. This
correspondence has led to the generally accepted idea that magnetic
fields play an essential role in heating the solar corona. <P />To
quantify the relationship between magnetic fields and coronal
heating, the X-ray luminosity of many different active regions
is compared with several global (integrated over entire active
region) magnetic quantities. The X-ray measurements were made with
the SXT Telescope on the Yohkoh spacecraft; magnetic measurements
were made with the Haleakala Stokes Polarimeter at the University
of Hawaii's Mees Solar Observatory. <P />The combined data set
consists of 333 vector magnetograms of active regions taken between
1991 and 1995; X-ray luminosities are derived from time averages
of SXT full-frame desaturated (SFD) images of the given active
region taken within +/-4 hours of each magnetogram. Global magnetic
quantities include the total unsigned magnetic flux Φ<SUB>tot</SUB>
≡ \smallint dA|B<SUB>z</SUB>|, B<SUP>2</SUP><SUB>z,tot</SUB>≡
dAB<SUP>2</SUP><SUB>z</SUB>, J<SUB>tot</SUB> ≡ \smallint
dA|J<SUB>z</SUB>|, and B<SUP>2</SUP><SUB>⊥,tot</SUB>≡
dAB<SUP>2</SUP><SUB>⊥</SUB>, where J<SUB>z</SUB> is the vertical
current density and B<SUB>z</SUB> and B<SUB>⊥</SUB> are the vertical
and horizontal magnetic field amplitudes, respectively. <P />The
X-ray luminosity L<SUB>X</SUB> is highly correlated with all of the
global magnetic variables, but it is best correlated with the total
unsigned magnetic flux Φ<SUB>tot</SUB>. The correlation observed
between L<SUB>X</SUB> and the other global magnetic variables
can be explained entirely by the observed relationship between
those variables and Φ<SUB>tot</SUB>. In particular, no evidence
is found that coronal heating is affected by the current variable
J<SUB>tot</SUB> once the observed relationship between L<SUB>X</SUB>
and Φ<SUB>tot</SUB> is accounted for. A fit between L<SUB>X</SUB>
and Φ<SUB>tot</SUB> yields the relationship L<SUB>X</SUB> ~= 1.2 ×
10<SUP>26</SUP> ergs s<SUP>-1</SUP>(Φ<SUB>tot</SUB>/10<SUP>22</SUP>
Mx)<SUP>1.19</SUP>. <P />The observed X-ray luminosities are compared
with the behavior predicted by several different coronal heating
theories. The Alfvén wave heating model predicts a best relationship
between L<SUB>X</SUB> and Φ<SUB>tot</SUB>, similar to what is found,
but the observed relationship implies a heating rate greater than the
model can accommodate. The “Nanoflare Model” of Parker predicts a best
relationship between L<SUB>X</SUB> and B<SUP>2</SUP><SUB>z,tot</SUB>
rather than Φ<SUB>tot</SUB>, but the level of heating predicted by the
model can still be compared to the observed data. The result is that
for a widely used choice of the model parameters, the nanoflare model
predicts 1.5 orders of magnitude more heating than is observed. The
“Minimum Current Corona” model of Longcope predicts a qualitative
variation of L<SUB>X</SUB> with Φ<SUB>tot</SUB> that agrees with
what is observed, but the model makes no quantitative prediction
that can be tested with the data. A comparison between L<SUB>X</SUB>
and the magnetic energy E<SUB>mag</SUB> in each active region leads
to a timescale that is typically 1 month, or about the lifetime of
an active region, placing an important observational constraint on
coronal heating models. <P />Comparing the behavior of solar active
regions with nearby active stars suggests that the relationship observed
between L<SUB>X</SUB> and Φ<SUB>tot</SUB> may be a fundamental one that
applies over a much wider range of conditions than is seen on the Sun.
---------------------------------------------------------
Title: Microwave and Hard X-Ray Observations of an X-Class Limb Flare
Authors: Nakajima, H.; Fujiki, K.; Metcalf, T. R.; Kane, S. R.;
Akioka, M.
1998ASSL..229..295N Altcode: 1998opaf.conf..295N
No abstract at ADS
---------------------------------------------------------
Title: A Spectral Analysis of the Masuda Flare Using Yohkoh Hard
X-Ray Telescope Pixon Reconstruction
Authors: Alexander, David; Metcalf, Thomas R.
1997ApJ...489..442A Altcode:
Masuda's discovery of a compact hard X-ray impulsive source at the apex
of a flaring coronal loop has received a great deal of recent attention
in the solar physics community. The Masuda flare, which occurred on 1992
January 13, exhibited evidence of energy deposition in a compact region
some distance above the soft X-ray loop, suggesting, to some authors,
a flare process similar to the classical model for two-ribbon flares
proposed by Shibata et al. These conclusions were made on the basis
of a maximum entropy method (MEM) reconstruction of the Yohkoh Hard
X-Ray Telescope (HXT) observations. Recently, a new approach has been
developed for reconstructing the spatial information from the HXT: that
of pixon reconstruction, proposed by Metcalf et al. <P />In this paper,
we apply the pixon reconstruction technique to the event of 1992 January
13 and determine the temporal and spectral characteristics of the
loop-top source. While our emphasis here is on the spectral properties
of the Masuda flare, we also provide a brief comparison between the
pixon reconstruction and that of MEM for the hard X-ray loop top. In
carrying out the comparison between the methods, we have applied recent
improvements to the instrument response functions and reconstruction
algorithms. We have also identified a previously unknown effect of weak
source suppression that was inherent in previous analyses and that
significantly compromised the ability to study weak sources of hard
X-ray emission in the presence of strong sources. The improved response
functions and the better flux estimation used in this paper reduce (but
do not eliminate) the effects of this suppression, and consequently,
it should be noted that the MEM analysis presented in this paper is
quite distinct from any that have been carried out previously. <P />Our
conclusions are that (a) a compact loop-top hard X-ray source exists
with an impulsive temporal profile spanning the peak of the flare; (b)
the loop-top source is nonthermal in nature at the peak of the flare;
(c) there is a distinct dearth of HXT LO channel emission, relative
to the higher energy channels, from the loop-top region, indicating
either a very hard spectrum or the presence of a low-energy cutoff
in the energetic electron spectrum; (d) the footpoint and loop-top
emission during the impulsive phase of the flare are produced by
two distinct particle populations; (e) following the main phase of
this flare, the loop top is clearly thermal in nature with a peak
temperature of ~40 MK that decreases with time as the event proceeds;
and (f) the disparity between the present pixon results and previous
MEM results is primarily due to the intrinsically better photometry
achieved by the pixon method and the avoidance of suppression effects
in the present analysis. These conclusions therefore support, in part,
those made in previous works, confirming the existence of an impulsive
source of hard X-rays in the corona above a flaring loop. Our analysis
does, however, allow for a more comprehensive understanding of the
temporal and spectral development of this event in the context of an
alternative reconstruction technique.
---------------------------------------------------------
Title: What Is the Spatial Relationship between Hard X-Ray Footpoints
and Vertical Electric Currents in Solar Flares?
Authors: Li, Jing; Metcalf, Thomas R.; Canfield, Richard C.; Wülser,
Jean-Pierre; Kosugi, Takeo
1997ApJ...482..490L Altcode:
We examine the spatial relationship between solar hard X-ray sources
observed with the Hard X-Ray Telescope aboard Yohkoh and photospheric
electric currents observed at Mees Solar Observatory. In 1993, Canfield
et al. concluded that energetic electron precipitation tends to occur
at the edge of sites of high vertical current. They did not, however,
have a direct diagnostic of particle precipitation; they used Hα
Stark-wing emission as a proxy. In this paper, we analyze hard X-ray
images and vector magnetograms of six flares of M/X X-ray class to reach
two basic conclusions. First, we confirm that electron precipitation
avoids sites of high vertical current density at photospheric levels,
preferentially occurring adjacent to these current channels. Hence, we
conclude that our observations rule out flare models in which nonthermal
electrons are accelerated within the large-scale active-region current
systems that are observed by present vector magnetographs. Second,
at conjugate magnetic footpoints the stronger hard X-ray emission is
associated with smaller vertical current density and weaker magnetic
field. This result is consistent with a “cornucopia”-shaped magnetic
morphology in which precipitating electrons are preferentially deflected
away from the narrower footpoint by magnetic mirroring.
---------------------------------------------------------
Title: Evolution of a Delta Group in the Photosphere and Corona
Authors: Van Driel-Gesztelti, L.; Csepura, G.; Schmieder, B.; Malherbe,
J. -M.; Metcalf, T.
1997SoPh..172..151V Altcode: 1997ESPM....8..151V
We present a study of the evolution of NOAA AR 7205 in the photosphere
and corona, including an analysis of sunspot motions, and show the
evolutionary aspects of flare activity using full-disc white-light
observations from Debrecen, vector magnetograms from Mees Observatory,
Hawaii, and Yohkoh soft X-ray observations. NOAA AR 7205 was born on
the disc on 18 June, 1992. During the first 3 days it consisted of
intermittent minor spots. A vigorous evolution started on 21 June when,
through the emergence and merging (v ≈ 100-150 m s<SUP>-1</SUP>) of
several bipoles, a major bipolar sunspot group was formed. Transverse
magnetic fields and currents indicated the presence of shear (clockwise
twist) already on 21 June (with α ≈ 0.015 Mm<SUP>-1</SUP>). On 23
June, new flux emerged in the trailing part of the region with the
new negative polarity spot situated very close to the big positive
polarity trailing spot of the main bipole. The secondary bipole seemed
to emerge with high non-potentality (currents). From that time the AR
became the site of recurrent flare activity. We find that all 14 flares
observed with the Yohkoh satellite occurred between the highly sheared
new bipole and the double-headed principal bipole. Currents observed
in the active region became stronger and more extended with time. We
propose that the currents have been (i) induced by sunspot motions and
(ii) increased by non-potential flux emergence leading to the occurrence
of energetic flares (X1.8 and X3.9). This observation underlines the
importance of flare analysis in the context of active region evolution.
---------------------------------------------------------
Title: A Comparison of the MEM and Pixon Algorithms for HXT Image
Reconstruction
Authors: Metcalf, Thomas R.; Alexander, David; Nitta, Nariaki;
Kosugi, Takeo
1997SPD....28.0217M Altcode: 1997BAAS...29Q.896M
Recently a workshop was held in Palo Alto, CA to discuss image
reconstruction for the Hard X-ray Telescope (HXT) on the Yohkoh
satellite. At the workshop, the participants concluded that a
detailed comparison of the primary reconstruction algorithms should
be undertaken. We will report on the results of a comparison of the
Maximum Entropy and Pixon algorithms using pseudo data. The comparison
will check photometric accuracy, speed, and image quality using a number
of test images. The test images utilized in the comparison will examine
a broad range of reconstruction problems, including the ability of the
algorithms to accurately reconstruct single sources, multiple sources
and loop-like features, as well as the ability to reconstruct weak
sources in the presence of spatially distinct bright sources.
---------------------------------------------------------
Title: Coordinated SOHO, Yohkoh, and Magnetogram Observations Of
Transient Loop Brightenings
Authors: Zarro, D. M.; Metcalf, T. R.; Fisher, G. H.; Siegmund, O.;
Longcope, D. W.; Kucera, T.; Griffiths, N. W.
1997SPD....28.0503Z Altcode: 1997BAAS...29..909Z
Transient soft X-ray brightenings occur frequently in solar active
regions, with typical durations of 2-10 minutes. They have been
observed with the Yohkoh Soft X-ray Telescope (SXT) and appear to be
associated primarily with interactions of multiple loops that brighten
initially near their footpoints (Shimuzu et al. 1994, Ap.J., 422,
906). Suggested mechanisms for the production of soft X-ray emission
include: conduction-driven chromospheric evaporation; Alfvenic outflows
from reconnection of colliding field lines; and expulsion of untwisting
loop material in emerging flux tubes (Uchida and Shibata 1988, Solar
Phys., 116, 291). To further study the dynamics of transient soft X-ray
brightenings and their relationship to the lower atmospheric magnetic
field, we have conducted a coordinated SOHO/Yohkoh campaign to observe
soft X-ray brightenings in a small active region at disk center on
1996 June 6. The region was observed simultaneously by Yohkoh SXT, the
Coronal Diagnostic Spectrometer (CDS) on SOHO, and the Imaging Vector
Magnetograph (IVM) at Mees Observatory, Hawaii. In particular, the CDS
instrument obtained Mg X (609 Angstroms) line spectra with 2-3 arcsec
spatial resolution in a 2x2 arcmin field with approximately 8 minute
cadence. The Mg X line is formed in the low corona at approximately
10(6) K. Individual Mg X spectra were obtained with 5 second exposures
per slit position. The SXT and CDS observations show evidence of soft
X-ray brightness variations on timescales of 5-10 minutes. The CDS Mg
X spectra show a mixture of red and blue Doppler shifts (< 100 km
s(-1) ) that are spatially associated with loop footpoints indicated
by the IVM. Based on the observed temporal and spatial variations of
the implied plasma upflows and downflows, we investigate the validity
of different proposed models of transient soft X-ray brightenings.
---------------------------------------------------------
Title: A Reconnection Model for Observed Transient Loop Brightenings
Authors: Longcope, D. W.; Fisher, G. H.; Metcalf, T. R.; Lemen, J.;
Zarro, D. M.; Kucera, T.; Griffiths, N.; Siegmund, O. H. W.
1997SPD....28.0128L Altcode: 1997BAAS...29..884L
Several recent theoretical models explain coronal activity in terms of
magnetic reconnection at “separator” field lines. These are field
lines lying at the boundary between domains of coronal flux with
distinct photospheric origin. Transient brightenings of X-ray loops
(Shimizu et al. 1992) may be the manifestations of such localized events
(Longcope 1996). Their relative simplicity, compared to large events
such as flares, makes them ideal objects for exploring reconnection
models. Toward this end, a campaign of coordinated observations of
loop brightenings in a small active region was undertaken on June 6,
1996. High time cadence observations were made of the transition region
and low corona in EUV (SUMER and CDS) and of the high corona in soft
X-rays (Yohkoh), while high cadence, high resolution vector magnetograms
were obtained with the Imaging Vector Magnetograph at the University of
Hawaii. This series of magnetograms is used to calculate the magnetic
topology of the coronal field, and to locate the separator field
lines. The high time cadence of the magnetograms allows the estimation
of reconnection rates: the rate at which flux must be exchanged between
domains. The "minimum current corona" model (Longcope 1996) is then
used to provide quantitative predictions of energy released on each
separator due to this reconnection. The observational results of the
campaign are described in a companion paper by Zarro et al at this
meeting; we discuss our predictions in the context of their results.
---------------------------------------------------------
Title: Movies of flares observed by YOHKOH/HXT
Authors: Lemen, J. R.; Alexander, D.; Metcalf, T. R.; Freeland, S. L.;
Nitta, N.
1997SPD....28.0169L Altcode: 1997BAAS...29..892L
The Yohkoh Hard X-ray Telescope (HXT) has revealed new information
about solar flares by providing high spatial resolution (5 arcsec)
images in four energy bands covering 14 to 99 keV. The comparison of
images obtained in hard X-rays with those obtained with the Yohkoh Soft
X-ray Telescope (SXT) has led to important interpretations of flare
heating mechanisms (e.g., Masuda et al, 1994, Nature, 374, 495). The
HXT images are formed by reconstructing data obtained from 64 detectors
located behind a bi-grid modulation pattern. Reconstruction algorithms
using Maximum Entropy and PIXON methods have been successfully applied
to the HXT data (Alexander and Metcalf, 1997, ApJ, submitted). At the
recent HXT Coordinated Data Analysis Workshop held in January 1997, new
calibration data for the response of the grids were incorporated and an
error in the application of the reconstruction algorithms was identified
and corrected (see Kosugi et al 1997 in these proceedings). As a result,
it is now possible to reconstruct the HXT images semi-automatically to
produce quick-look movies. We are in the process of generating movies
for all flares obtained with the HXT using an MEM reconstruction. The
automated procedure selects time intervals on the basis of the count
rate statistics. We present a selection of reconstructed images and
movies from this on-going project. We expect that the ability to view
easily time sequences from many flares observed with the HXT will
provide new insights for flare studies.
---------------------------------------------------------
Title: Using the WWW to Make YOHKOH SXT Images Available to the
Public: The YOHKOH Public Outreach Project
Authors: Larson, M.; McKenzie, D.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
1997SPD....28.0231L Altcode: 1997BAAS...29..898L
The Yohkoh Public Outreach Project (YPOP) is funded by NASA as one of
the Information Infrastructure Technology and Applications Cooperative
Agreement Teams to create public access to high quality Yohkoh SXT data
via the World Wide Web. These products are being made available to the
scientific research community, K-12 schools, and informal education
centers including planetaria, museums, and libraries. The project aims
to utilize the intrinsic excitement of the SXT data, and in particular
the SXT movies, to develop science learning tools and classroom
activities. The WWW site at URL: http://www.space.lockheed.com/YPOP/
uses a movie theater theme to highlight available Yohkoh movies in a
non-intimidating and entertaining format for non-scientists. The site
features lesson plans, 'solar' activities, slide shows and, of course,
a variety of movies about the Sun. Classroom activities are currently
undergoing development with a team of scientists and K-12 teachers
for distribution in late 1997. We will display the products currently
online, which include a solar classroom with activities for teachers,
background resources, and a virtual tour of our Sun.
---------------------------------------------------------
Title: Automated Identification of Soft X-ray Coronal Loops
Authors: Alexander, David; Metcalf, Thomas R.
1997SPD....28.0138A Altcode: 1997BAAS...29..886A
The presence of loop-like structures in the solar corona is clearly
evident in soft X-ray images such as those from the Soft X-ray Telescope
on board the Yohkoh satellite. These structures were first discovered
in Skylab data and are thought to represent the enhanced heating
of the coronal plasma confined in magnetic fluxtubes. In many cases
the heating is not confined to a single well defined fluxtube nor is
it always strong enough to dilineate the structure sharply against
the diffuse coronal background, which is itself presumably composed
of loop structures. In these cases it is often extremely difficult
to identify the structures involved in coronal activity. We have
developed a technique which uses a Pixon reconstruction of the soft
X-ray images to search specifically for loop-like structures. This
allows us to pick out faint loops against a strong background and to
identify multiple loop structures in bright regions. We are, therefore,
better able to address the heating of the solar corona both in the
diffuse and active regions.
---------------------------------------------------------
Title: Using the World Wide Web to Make YOHKOH SXT Images Available
to the Public: The YOHKOH Public Outreach Project
Authors: McKenzie, D.; Larson, M. B.; Slater, T.; Acton, L.; Alexander,
D.; Freeland, S.; Lemen, J.; Metcalf, T.
1997ESASP.404..561M Altcode: 1997cswn.conf..561M
No abstract at ADS
---------------------------------------------------------
Title: Pixon-based Multiresolution Image Reconstruction for Yohkoh's
Hard X-Ray Telescope
Authors: Metcalf, Thomas R.; Hudson, Hugh S.; Kosugi, Takeo; Puetter,
R. C.; Pina, R. K.
1996ApJ...466..585M Altcode:
We present results from the application of pixon-based multiresolution
image reconstruction to real and simulated data from Yohkoh's Hard
X-ray Telescope (HXT). The goal of the pixon algorithm is to minimize
the number of degrees of freedom used to describe an image within the
accuracy allowed by the noise. This leads to a reconstruction that is
optimally constrained. We apply the pixon code to two solar flares in
the HXT database and compare the results of the pixon reconstruction to
the results of a direct, linear, smoothed inversion of the HXT Fourier
synthesis data and to a maximum entropy reconstruction. The maximum
entropy reconstruction is vastly better than the direct inversion,
but the pixon reconstruction gives superior noise suppression and
photometry. Further, the pixon reconstruction does not suffer from
overresolution of the images.
---------------------------------------------------------
Title: What is the spatial relationship between hard x-ray footpoints
and vertical electric currents?
Authors: Li, Jing; Metcalf, Thomas R.; Canfield, Richard C.; Wülser,
Jean-Pierre; Kosugi, Takeo
1996AIPC..374..336L Altcode: 1996hesp.conf..336L
We examine the spatial relationship between solar flare hard x-ray
emission sites observed with the HXT aboard Yohkoh and photospheric
electric currents observed at Mees Solar Observatory. Canfield et
al. (3) concluded that nonthermal electron precipitation tends to
occur at the edges of sites of high vertical current, not at their
maxima. They did not, however, have very direct observations of the
electron precipitation, since they used Hα Stark wing emission. In
this work we compare hard x-ray images and vertical current maps in six
large M/X-class flares. Our results confirm that electron precipitation
sites avoid regions of strong photospheric vertical currents in large
flares, and that magnetic mirroring strongly influences the relative
HXR brightness of conjugate footpoints.
---------------------------------------------------------
Title: Microwave and hard X-ray sources in two X-class limb flares
Authors: Nakajima, Hiroshi; Metcalf, Thomas R.
1996AIPC..374..393N Altcode: 1996hesp.conf..393N
We have analyzed two intense, extended microwave and hard X-ray flares
which occurred slightly behind the west limb and were accompanied by
X-class, long-duration events (LDEs) in soft X-rays. We have found
that: (1) Both events have typical soft X-ray properties of LDEs. (2)
A major hard X-ray source at 23-33 keV and 33-53 keV is located in the
high-temperature region, considerably higher than the corresponding
soft X-ray loop, while a 17 GHz microwave source, where electrons
at energy ≥ several hundreds keV mainly contribute to the 17 GHz
emission, nearly coincides with the soft X-ray loop in extent. (3)
The electron spectrum, which is derived from the microwave spectrum,
is significantly harder than the electron spectrum at energy ≤100
keV, which is derived from the hard X-ray data. (4) The time profile
of total intensity at 17 GHz is delayed by about 25 s with respect
to that of the hard X-rays for one of the events. The above results
show evidence that lower-energy (≤100 keV) electrons with a softer
spectrum are accelerated at the top of the cusp, and that higher-energy
(≫100 keV) electrons are accelerated at the top of the soft X-ray
loop and trapped. Probably, the lower-energy electrons accelerated in
the cusp are transferred by a reconnection outflow into the second
acceleration region at the top of the soft X-ray loop, and further
accelerated to higher energy.
---------------------------------------------------------
Title: A Test of a New Flare Loop Scaling Law Using YOHKOH SXT and
GOES Observations
Authors: Metcalf, Thomas R.; Fisher, George H.
1996ApJ...462..977M Altcode:
We compare the lengths of coronal loops observed on the Sun
using the Soft X-Ray Telescope on Yohkoh with those predicted
from the theoretical model of Hawley et al. This model relates
the temporal behavior of coronal emission observed during flares
on active stars to the length of the loops undergoing flaring. In
that model, the footpoint-to-footpoint loop length 2L (measured
in kilometers) should obey the relationship 2L ≃ 0.01Y where Y =
l.25π<SUB>r</SUB><SUP>3/7</SUP>τ<SUB>d</SUB><SUP>4/7</SUP>T<SUB>A</SUB><SUP>½</SUP>,
τ<SUB>r</SUB> is the "rise time" (in seconds) from flare onset to
flare peak, τ<SUB>d</SUB> is the "decay time" (s) from flare peak
to the time when the emission measure is 25% of its peak value, and
T<SUB>A</SUB> (K) is the temperature at the top of the loop at flare
peak. The observations show a strong correlation between 2L and Y, with
the theoretical relationship consistent with observed loop lengths in
most cases. For a few loops, the theoretical model tends to somewhat
over-predict the loop length. When the observed loop lengths are fitted
to a single power-law relationship in Y, we find the data are best
fitted by log 2L = log (0.44±0.06) + (0.75±0.05) log Y. We offer no
quantitative theoretical justification for this relationship, but we
do suggest several reasons why the Hawley et al. model over-predicts
loop lengths in some cases.
---------------------------------------------------------
Title: Coronal Heating in Active Regions as a Function of Global
Magnetic Variables
Authors: Fisher, G. H.; Longcope, D. W.; Metcalf, T. R.; Pevtsov, A. A.
1996AAS...188.3304F Altcode: 1996BAAS...28..868F
A comparison of X-ray images of the Sun and full disk magnetograms shows
a correlation between the locations of the brightest X-ray emission
and the locations of bipolar magnetic regions. This correspondence
has led to the generally accepted idea that magnetic fields play an
essential role in heating the Solar corona. To quantify the relationship
between magnetic fields and coronal heating, we have compared the
X-ray luminosity of many different Active Regions with several global
(integrated over entire active region) magnetic quantities. The X-ray
measurements were made with the SXT Telescope on the Yohkoh spacecraft;
magnetic measurements were made with the Haleakala Stokes Polarimeter
at the University of Hawaii's Mees Solar Observatory. Our combined
dataset consists of 333 vector magnetograms of active regions taken
between 1991 and 1995; SXT luminosities consist of time averages of
SFD images of the given active region taken within +/- 4 hours of each
magnetogram. Global magnetic quantities include the total unsigned
magnetic flux, area integrals of B(2) , J_z(2) (J_z is the vertical
component of the electric current density), and the best-fit alpha
of the linear force-free field for the entire active region (nabla x
B = alphaB ). Our results show clear and unmistakable relationships
between the X-ray luminosity and most of these magnetic variables. The
relationship between total unsigned magnetic flux and X-ray luminosity
is especially compelling, holding over 2 orders magnitude in both
quantities. These measurements provide important contraints on coronal
heating mechanisms. This work was supported in part by NASA grant
NAGW-3429, NSF grant AST-9218085, and Cal Space grant CS-17-95.
---------------------------------------------------------
Title: Pixon Reconstruction and the Masuda Event of 1992 January 13
Authors: Alexander, D.; Metcalf, T.; Hudson, H. S.
1996ASPC..111..253A Altcode: 1997ASPC..111..253A
The data set of localised hard X-ray sources observed by the Yohkoh/HXT
has been analysed using maximum entropy methods (MEM) to reconstruct
HXT images. Recently, an alternative method, that of fractal pixon
reconstruction, has been developed for use with the HXT. The authors
have reanalysed the event of 13 January 1992 (the Masuda event),
comparing the MEM and pixon methods. There are distinct differences
in the two sets of results. The pixon method, favoured by the authors,
indicates a less impulsive coronal source than the MEM reconstruction
and also a relatively weaker coronal/footpoint emission ratio.
---------------------------------------------------------
Title: Evolution of two small solar flares.
Authors: Fludra, A.; Doyle, J. G.; Metcalf, T.; Lemen, J. R.; Phillips,
K. J. H.; Culhane, J. L.; Kosugi, T.
1995A&A...303..914F Altcode:
Data from the YOHKOH satellite have been analysed for two small flares
(GOES class C) of total duration of 10 and 60 minutes. Upflows in S
XV, Ca XIX and Fe XXV lines were compared and the presence of a range
of upflow velocities was found. Emission from flare loop footpoints
corresponding to plasma moving with a typical velocity of 200-400km/s
is seen in soft X-ray images. In one of these events (23 June 1992),
which occurred in sheared loops being part of a sparse magnetic arcade,
with initial energy release taking place near one of the footpoints, a
large proportion of upflowing plasma was seen at least 1 minute before
the first peak in hard X-rays. In the second event (13 July 1992), the
increase of soft X-ray emission began more than 3 minutes before, and
weak mass upflows one minute before the rapid increase of temperature
and the onset of the detectable hard X-ray emission. This event was
probably triggered by emerging magnetic flux and accompanied by heating
and restructuring of two nearby magnetic loops. In both events the
emission measure of upflowing plasma is present simultaneously to,
and is very well correlated in time with the hard X-ray flux in the
14-23keV band. Differential emission measure in the temperature range
5-60x10^6^K was derived from S XV, Ca XIX and Fe XXV line and continuum
fluxes, and from images in two broad band soft X-ray filters, and
used to analyse the thermal contribution to the hard X-ray emission. A
non-thermal component of the hard X-ray emission is found at the peak
of the 23 June 1992 flare. The hard X-ray emission in the 13 July 1992
flare is primarily thermal, however, a possibility of an enhanced tail
of the electron energy distribution above 14keV is also indicated. The
chromospheric evaporation in these flares was driven both by electron
beams and thermal conduction, with conduction predominating during most
of the rise phase of the 13 July 1992 flare. In both events, the soft
X-ray emission measure at flare maximum was a few times 10^48^cm^-3^
with an electron temperature 19 and 24x10^6^K; the estimated lower
limit of the electron density is ~10^11^cm^-3^. The broadening of
Ca XIX spectral lines in the decay phase of these flares indicates
persisting random motions with a velocity of 60km/s, which is very
similar to the non-thermal broadening observed previously by SMM in
M and X class flares.
---------------------------------------------------------
Title: Fractal Pixon Image Reconstruction for Yohkoh's Hard X-Ray
Telescope
Authors: Metcalf, T. R.; Hudson, H. S.; Kosugi, T.; Puetter, R. C.;
Piña, R. K.
1995SPD....26.1314M Altcode: 1995BAAS...27..990M
No abstract at ADS
---------------------------------------------------------
Title: Latitudinal Variation of Helicity of Photospheric Magnetic
Fields
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R.
1995ApJ...440L.109P Altcode:
Using a 1988-1994 data set of original photospheric vector magnetograms
as well as published data, we have studied the average magnetic
helicity of 69 diverse active regions, adopting the linear force-free
field parameter alpha as a measure. This average value was determined
by minimizing the differences between the computed constant-alpha
force-free and observed horizontal magnetic fields. The average
magnetic helicity shows a sign difference at the 2 sigma level in
opposite hemispheres. In our data set, 76% of the active regions in
the northern hemisphere have negative helicity, and 69% in the southern
hemisphere, positive. Although the data show considerable variation from
one active region to the next, the data set as a whole suggest that
the magnitude of the average helicity increases with solar latitude,
starting at zero near the equator, reaches a maximum near 15 deg - 25
deg in both hemispheres, and drops back toward smaller values avove 35
deg - 40 deg. Qualitative comparison with published models shows that
such latitudinal variation of the average magnetic helicity may result
from either turbulent convective motions or differential rotation,
although our studies of rotating sunspots lead us to favor the former.
---------------------------------------------------------
Title: Is the Solar Chromospheric Magnetic Field Force-free?
Authors: Metcalf, Thomas R.; Jiao, Litao; McClymont, Alexander N.;
Canfield, Richard C.; Uitenbroek, Han
1995ApJ...439..474M Altcode:
We use observations of the Na I lambda 5896 spectral line, made with
the Stokes Polarimeter at Mees Solar Observatory, to measure the
chromospheric vector magnetic field in NOAA active region 7216. We
compute the magnetic field from observations of the Stokes parameters
at six wavelengths within this spectral line using a derivative method
and calculate the height dependence of the net Lorentz force in the
photosphere and low chromosphere. We conclude that the magnetic field
is not force-free in the photosphere, but becomes force-free roughly
400 km above the photosphere.
---------------------------------------------------------
Title: Resolving the 180-degree ambiguity in vector magnetic field
measurements: The `minimum' energy solution
Authors: Metcalf, Thomas R.
1994SoPh..155..235M Altcode:
I present a robust algorithm that resolves the 180-deg ambiguity
in measurements of the solar vector magnetic field. The technique
simultaneously minimizes both the divergence of the magnetic
field and the electric current density using a simulated annealing
algorithm. This results in the field orientation with approximately
minimum free energy. The technique is well-founded physically and is
simple to implement.
---------------------------------------------------------
Title: A search for low-energy protons in a solar flare from October
1992: Preliminary results
Authors: Metcalf, T.; Mickey, D.; Canfield, R.; Wülser, J. -P.
1994AIPC..294...59M Altcode: 1994hesp.conf...59M
We give preliminary results from the first use of the University of
Hawaii's new Imaging Vector Magnetograph (IVM) to search for linear
polarization in the H-alpha spectral line during solar flares. Such
polarization has previously been interpreted as impact polarization
from 100 keV protons impacting the chromosphere. The new data set has
several advantages over previous data. First, the field of view is
substantially larger than that used by Metcalf et al., and, second,
the temporal resolution (16 s) is a factor of two better than that
previously obtained. We show a preliminary comparison between the flare
Hα polarization and hard X-rays observed with the Compton Observatory.
---------------------------------------------------------
Title: Temperature Analysis of the Post-Flare Loops of June 25-26,
1992
Authors: Anwer, B.; Hiei, E.; Hudson, H. S.; Acton, L. W.; Lemen,
J.; Metcalf, T. R.
1994kofu.symp..137A Altcode:
We have performed an analysis of temperatures and emission measures
of thermal plasma on a post--flare loop system following an X3.9
flare of June 25, 1992, at 20:14 UT in NOAA active region 7205 near
the west limb (N09, W67). The filter ratio method was applied to the
data sets taken using the Al 0.1 micron (thin Al) and Al 12 micron
(thick Al) filters of the Yohkoh Soft X-ray Telescope (SXT). We found
that the plasma temperature of the top of loops was in the range 5 -
8 x 10^6 K and log emission measure between 44.6 and 46.7 cm^(-3)
for data sets taken from 22:56:57 UT of June 25 to 09:00 UT of June
26. Furthermore, the occurrence of a C1-class flare at the top of the
flare loops increased the plasma temperature from 5.5 x 10^6 K to 6.6
x 10^6 K at 06:57:11 UT. The loops top was much brighter than the legs
and footpoints, with delta_T was about 0.1 x 10^6 K.
---------------------------------------------------------
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
Authors: Nitta, N.; Driel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
1994kofu.symp..385N Altcode:
Active region NOAA 7260 exhibited remarkable flare activity as an
emerging flux region appeared in the following part and evolved into
the delta configuration. While it is difficult to associate an emerging
bipole with a flare both temporally and spatially, there is an overall
correlation of the total darkness integrated over of the sunspot area,
as measured in the Yohkoh/SXT white-light images, with the soft X-ray
flux and flare occurrence. It appears that the flares in the emerging
flux region occurred preferentially at locations close to the spot of
preceding polarity that emerged in the earliest evolution of the region.
---------------------------------------------------------
Title: Electric Currents and Coronal Heating in NOAA Active Region
6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Wulser, J. -P.; Martens, P. C. H.; Tsuneta, S.
1994ApJ...428..860M Altcode:
We examine the spatial and temporal relationship between coronal
structures observed with the soft X-ray telescope (SXT) on board the
Yohkoh spacecraft and the vertical electric current density derived from
photospheric vector magnetograms obtained using the Stokes Polarimeter
at the Mees Solar Observatory. We focus on a single active region:
AR 6952 which we observed on 7 days during 1991 December. For 11
independent maps of the vertical electric current density co-aligned
with non-flaring X-ray images, we search for a morphological
relationship between sites of high vertical current density in the
photosphere and enhanced X-ray emission in the overlying corona. We
find no compelling spatial or temporal correlation between the sites of
vertical current and the bright X-ray structures in this active region.
---------------------------------------------------------
Title: Electron precipitation and mass motion in the 1991 June 9
white-light flare
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.;
Hiei, E.; Sakurai, T.; Ichimoto, K.
1994SoPh..151..389D Altcode:
We use Hα line profiles as a diagnostic of mass motion and
nonthermal electron precipitation in the white-light flare (WLF) of
1991 June 9 01:34 UT. We find only weak downflow velocities (≈10 km
s<SUP>−1</SUP>) at the site of white-light emission, and comparable
velocities elsewhere.We also find that electron precipitation is
strongest at the WLF site. We conclude that continuum emission in this
flare was probably caused by nonthermal electrons and not by dynamical
energy transport via a chromospheric condensation.
---------------------------------------------------------
Title: Patterns of Helicity in Solar Active Regions
Authors: Pevtsov, Alexei A.; Canfield, Richard C.; Metcalf, Thomas R.
1994ApJ...425L.117P Altcode:
Using 46 vector magnetograms from the Stokes Polarimeter of Mees
Solar Observatory (MSO), we studied patterns of local helicity
in three diverse solar active regions. From these magnetograms
we computed maps of the local helicity parameter alpha =
J<SUB>z</SUB>/B<SUB>z</SUB>. Although such maps are noisy, we
found patterns at the level approximately 2 to 3 sigma<SUB>J(sub
z</SUB>), which repeat in successive magnetograms for up to
several days. Typically, the alpha maps of any given active region
contain identifiable patches with both positive and negative values
of alpha. Even within a single sunspot complex, several such alpha
patches can often be seen. We followed 68 alpha patches that could be
identified on at least two successive alpha maps. We found that the
persistence fraction of such patches decrease exponentially, with a
characteristic time approximately 27 hr.
---------------------------------------------------------
Title: Radio Imaging Observations of the Evolution of Thermal and
Nonthermal Sources during a Gradual Solar Burst
Authors: Nishio, Masanori; Nakajima, Hiroshi; Enome, Shinzo; Shibasaki,
Kiyoto; Takano, Toshiaki; Hanaoka, Yoichiro; Torii, Chikayoshi;
Sekiguchi, Hideaki; Bushimata, Takeshi; Kawashima, Susumu; Shinohara,
Noriyuki; Irimajiri, Yoshihisa; Choi, Yong-Seok; Koshiishi, Hideki;
Shiomi, Yasuhiko; Metcalf, Thomas R.; Canfield, Richard C.
1994PASJ...46L..11N Altcode:
With the Nobeyama Radioheliograph, evolution of thermal and nonthermal
radio sources was observed during a gradual solar radio burst. Two
thermal sources were located near the top of an arcade-like soft X-ray
structure observed by the Yohkoh satellite, while a nonthermal source
was observed near the foot points of the arcade-like structure. During
the early phase of the burst, an elongated structure was observed
connecting one of the thermal sources and the nonthermal source. The
elongation started from the location of the nonthermal source and
gradually progressed to the thermal source placed just radially above
the nonthermal source with the velocity of about 170 km s(-1) . These
observations imply that the thermal source and the nonthermal source
are connected by a single magnetic loop, and that the elongation of
the source structure is an indication of chromospheric evaporation by
radio imaging observations. The energetics of the thermal and nonthermal
sources was analyzed using the radio and soft X-ray data. We suggest
that a continuous supply of accelerated electrons for about 1000 s
can maintain the hot plasma observed at the foot points of the loop.
---------------------------------------------------------
Title: Bayesian Timing Analysis of Three Cataclysmic Binary Systems
Authors: Freeman, P. E.; Metcalf, T. H.; Lamb, D. Q.
1994AIPC..313..294F Altcode: 1994sxrc.conf..294F
The identification of periodic signals from cataclysmic binary
systems is crucial for classifying them and interpreting their
behavior. We use the Bayesian timing analysis technique of Gregory
& Loredo to attempt to detect signals in the ROSAT PSPC data of
three such systems. This technique, conceptually similar to, but more
sensitive than, epoch folding, involves directly comparing a model
with no count rate variability with one describing the count rate as a
stepwise distribution with m bins per period of length P, and phase φ
(relative to the start of observation). This model comparison allows
the computation of the odds favoring the more complex model. If the
odds strongly support variability, one then estimates the period of
variation by integrating the likelihood function over all values of m
and φ for each value of P. <P />With this technique, we attempt to
find the signal which Wood et al. interpret as that of the orbital
period, P<SUB>orb</SUB>, of the He white dwarf PG 1346+082. We also
attempt to find the signals at the spin period, P<SUB>spin</SUB>, and
beat period, P<SUB>beat</SUB>, of the DQ Her-type source 1H0542-407
and the possibly DQ Her-type source V426 Oph. Detection of a spin
and beat period in V426 Oph would indicate that the binary system
is not synchronized, aiding the DQ Her classification. We find that
the odds do not favor a variable rate model for PG1346+082. The odds
do strongly favor variable rate models for the latter two sources,
but we cannot conclusively determine periods of variation, because of
insufficient data and large gaps in the observations.
---------------------------------------------------------
Title: Dynamics of a Low Energetic Solar Flare
Authors: Doyle, J. G.; Fludra, A.; Bentley, R. D.; Culhane, J. L.;
Metcalf, T.; Lemen, J. R.; Kosugi, T.
1994ASPC...64..402D Altcode: 1994csss....8..402D
No abstract at ADS
---------------------------------------------------------
Title: The X Flare of 15 November, 1991: Preflare Flux Emergence,
Heating and Filament Eruption
Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf,
T. R.; Reardon, K. P.; Wülser, J. -P.; Acton, L. W.; Kurokawa, H.;
Hirayama, T.
1994xspy.conf..153C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Diagnostics of Twisted Flux Emergence (noaa AR7260)
Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Anwar, B.; Canfield,
R. C.; Hudson, H. S.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.;
Kurokawa, H.
1994xspy.conf...25L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flares in Active Region NOAA 7260
Authors: Nitta, N.; van Driel-Gesztelyi, L.; Leka, K. D.; Sakurai,
T.; Shibata, K.; Ichimoto, K.; Canfield, R. C.; Wülser, J. -P.;
Metcalf, T. R.; Mickey, D. L.
1994xspy.conf..111N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Electric Currents and Coronal Structures in NOAA Active
Region 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Wülser, J. -P.; Tsuneta, S.
1994xspy.conf...51M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The 1992 January 5 Flare at 13.3 UT: Observations from YOHKOH
Authors: Doschek, G. A.; Strong, K. T.; Bentley, R. D.; Brown, C. M.;
Culhane, J. L.; Fludra, A.; Hiei, E.; Lang, J.; Mariska, J. T.;
Phillips, K. J. H.; Pike, C. D.; Sterling, A. C.; Watanabe, T.; Acton,
L. W.; Bruner, M. E.; Hirayama, T.; Tsuneta, S.; Rolli, E.; Kosugi,
T.; Yoshimori, M.; Hudson, H. S.; Metcalf, T. R.; Wuelser, J. -P.;
Uchida, Y.; Ogawara, Y.
1993ApJ...416..845D Altcode:
We discuss X-ray spectra and soft X-ray images of an M1.9 flare that
occurred on 1992 January 5 near 13.3 UT. These data were obtained
with instrumentation on the Japanese Yohkoh spacecraft. They cover
the entire rise phase of the flare. To supplement these data we have
ground-based magnetograms and Hα spectroheliograms. We calculate
the electron temperature and emission measure of the flare as a
function of time during the early rise phase using X-ray spectral
line intensities and line ratios. Using spectral line widths, line
profile asymmetries, and wavelength shifts due to the Doppler effect,
we calculate the dynamical properties of the flare. The time development
of the morphology of the flare, as revealed by the soft X-ray images
and the Hα spectroheliograms, and the physical quantities inferred
from the X-ray spectra, are compared with chromospheric evaporation
models. There is an enhancement of blueshifted emission that is closely
correlated with the hard X-ray bursts. Heating of one loop in the flare
is consistent with a conduction-evaporation model, but heating is found
in several structures that do not appear to be physically associated
with each other. No standard evaporation model can adequately explain
all of the observations.
---------------------------------------------------------
Title: The Morphology of Flare Phenomena, Magnetic Fields, and
Electric Currents in Active Regions. I. Introduction and Methods
Authors: Canfield, Richard C.; de La Beaujardiere, J. -F.; Fan,
Yuhong; Leka, K. D.; McClymont, A. N.; Metcalf, Thomas R.; Mickey,
Donald L.; Wuelser, Jean-Pierre; Lites, Bruce W.
1993ApJ...411..362C Altcode:
Electric current systems in solar active regions and their spatial
relationship to sites of electron precipitation and high-pressure
in flares were studied with the purpose of providing observational
evidence for or against the flare models commonly discussed in the
literature. The paper describes the instrumentation, the data used, and
the data analysis methods, as well as improvements made upon earlier
studies. Several flare models are overviewed, and the predictions
yielded by each model for the relationships of flares to the vertical
current systems are discussed.
---------------------------------------------------------
Title: Electric Currents and Coronal Structures in NOAA Active
Region 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Martens, P. C. H.; Tsuneta, S.
1993BAAS...25.1179M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Hα Impact Polarization Observations in Solar Flares
Authors: Mickey, D. L.; Metcalf, T.; Canfield, R. C.; Wuelser, J. -P.
1993BAAS...25.1223M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Filament Tether Cutting Before a Major Eruptive Flare
Authors: Canfield, R. C.; Blais, K. A.; McClymont, A. N.; Metcalf,
T. R.; Reardon, K. P.; Wuelser, J. -P.; Acton, L. W.; Kurokawa, H.
1993BAAS...25.1188C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Measurement of the Chromospheric Magnetic Field Using
the Na I γ5896Å Spectral Line
Authors: Jiao, L.; Metcalf, T. R.; Uitenbroek, H.
1993BAAS...25.1206J Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evidence for Twisted Emerging Flux: NOAA AR 7260
Authors: Leka, K. D.; van Driel-Gesztelyi, L.; Canfield, R. C.; Anwar,
B.; Metcalf, T. R.; Mickey, D. L.; Nitta, N.
1993BAAS...25R1187L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Full-Disk Magnetogram Cross-correlations at Long Time Lags
Authors: Snodgrass, H. B.; Metcalf, T.; Vandekop, T.
1993BAAS...25.1194S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flares in Active Region NOAA 7260 - Role of Emerging Flux
and Reconnection
Authors: Nitta, N.; Drel-Gesztelyi, L. V.; Leka, K. D.; Mickey, D. L.;
Metcalf, T. R.; Wuelser, J. -P.; Ichimoto, K.; Sakurai, T.; Shibata, K.
1993BAAS...25.1223N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Correlation between X-ray Temporal Variability and Magnetic
Environment in Solar Flares
Authors: Nitta, N.; Harvey, K.; Hudson, H.; Ichimoto, K.; Metcalf,
T.; Mickey, D.; Sakai, J. -I.; Sakao, T.; Sakurai, T.; Takahashi, M.
1992AAS...181.5503N Altcode: 1992BAAS...24.1211N
The X-ray time history of a solar flare can reflect basic processes of
heating and/or acceleration, which in turn may depend on the magnetic
environment of the site. Some flares show a simple rise and fall
temporal behavior, whereas others show more than one peak. Comparisons
of images taken by the Soft X-ray Telescope (SXT) aboard the Yohkoh
spacecraft with ground-based magnetic data (Hawaii, Kitt Peak and
Mitaka) reveal that, at least for a flare-productive active region
(NOAA 7260), flares with double-peaked and single-peaked time profiles
occurred at systematically different locations within the region. We
discuss this result in terms of theoretical models, especially those
of coalescence of two current loops.
---------------------------------------------------------
Title: The X Flare of 1991 November 15: Coordinated Mees/Yohkoh
Observations
Authors: Canfield, Richard C.; Hudson, Hugh S.; Leka, K. D.; Mickey,
Donald L.; Metcalf, Thomas R.; Wuelser, Jean-Pierre; Acton, Loren W.;
Strong, Keith T.; Kosugi, Takeo; Sakao, Taro; Tsuneta, Saku; Culhane,
J. Leonard; Phillips, Andrew; Fludra, Andrzej
1992PASJ...44L.111C Altcode:
This is a preliminary report on two unique new results from coordinated
observations at Mees Solar Observatory and Yohkoh of the X1.5 flare
of 1991 November 15, using vector magnetograms, Hα imaging spectra,
X-ray images, and X-ray spectra. First, we find a close spatial
relationship between Hα redshifts and X-rays from a flare loop and
its footpoints at a time of large X-ray blueshifts. Second, we find
that impulsive-phase hard X-rays originate in regions that are near,
but not coincident with, the peaks of the vertical electrical current
density distribution in AR 6919.
---------------------------------------------------------
Title: Hard X-Ray Imaging Observations by YOHKOH of the 1991 November
15 Solar Flare
Authors: Sakao, Taro; Kosugi, Takeo; Masuda, Satoshi; Inda, Mika;
Makishima, Kazuo; Canfield, Richard C.; Hudson, Hugh S.; Metcalf,
Thomas R.; Wuelser, Jean-P.; Acton, Loren W.; Ogawara, Yoshiaki
1992PASJ...44L..83S Altcode:
We report on hard X-ray imaging observations of the 1991 November 15
flare with the HXT instrument aboard {Yohkoh}. Distributions of the hard
X-ray sources at various stages of the flare, together with an overlay
of the white-light flare, are presented. Attention is concentrated on
the behavior of hard X-ray sources during the impulsive phase. The
hard X-ray source appeared initially as a single source near the
magnetic neutral line, then evolved into a double-source shape with the
separation increasing with time. We believe that this is evidence for a
multiple loop system flaring successively with a rising energy-release
site. At the minima between the individual spikes of the time profile,
the hard X-rays at 20--30 keV were concentrated near the apex of the
flaring loop, whereas the hard X-rays above 30 keV originated from the
footpoints. These observations are compared with the existing models.
---------------------------------------------------------
Title: The X10 Flare of 1991 June 9: White Light, H-alpha, Magnetic
Fields, and Electric Currents
Authors: de La Beaujardiere, J. -F.; Canfield, R. C.; Metcalf, T. R.;
Hiei, E.; Sakurai, T.; Ichimoto, K.
1992AAS...180.4108D Altcode: 1992BAAS...24..795D
We present observations by several instruments of the white-light
flare (WLF) of 1991 June 9 01:34 UT. A white-light image from the
National Astronomical Observatory of Japan indicates the location
of the WLF within the active region (NOAA 6659). Stokes polarimetry
from Mees Solar Observatory (University of Hawai`i) yields a vector
magnetogram and a map of the vertical electric currents. Also from
Mees comes Hα imaging spectroscopy, which is an indicator of such
specific chromospheric processes as nonthermal electron precipitation,
high coronal pressure, and mass motion. Both Mees instruments provide
continuum images, allowing coregistration of the various datasets. The
white-light emission arises from an extended area including both
sunspot umbra and penumbra. The active region magnetic field is strongly
nonpotential and has numerous vertical electric current channels. The
WLF is situated in an area of relatively low current density at the
edge of the strongest current in the active region, and the nearby
magnetic neutral line is highly sheared. The WLF site shows electron
precipitation, but only in its penumbral portion; stronger magnetic
mirroring in the umbra may inhibit precipitation there. Also, the
precipitation is not especially strong (relative to that observed
elsewhere in this flare), suggesting that the electron-beam model
for WLFs is not appropriate in this case. Also, the lack of strong
redshifts argues against a dynamical energy transport model.
---------------------------------------------------------
Title: Electric Currents and Coronal Structures in Two Flare-
Productive Active Regions, AR 6850 and AR 6952
Authors: Metcalf, T. R.; Canfield, R. C.; Hudson, H. S.; Mickey,
D. L.; Strong, K. T.; Tsuneta, S.
1992AAS...180.3004M Altcode: 1992BAAS...24R.775M
In this study, we examine the spatial and temporal relationship between
coronal structures observed with the Soft X-Ray Telescope (SXT) on
board the YOHKOH spacecraft and vertical electric currents derived
from vector magnetograms obtained at the Mees Solar Observatory,
Haleakala, Hawaii. We have focused on two active regions, AR 6850
(October 1991) and AR 6952 (December 1991). In both active regions,
we observed significant current structures which persisted over time
scales of days. The SXR emitting coronal structures, however, changed
on much shorter time scales, indicating that there is no compelling,
direct spatial and temporal relationship between the non-flaring SXR
structures and the long-lived electric current systems. We have seen
at least one case (in AR 6952) where a SXR brightening was associated
spatially with a change in the vertical electric current. In this case,
the the vertical current dissipated between December 8, 00:35 UT and
the next observation at 00:48 UT on December 9, leaving a bright SXR
structure which was observed at 24:27 UT on December 8. Hence, although
more data must be analyzed to make a compelling case, it is possible
that the SXR emission is related more closely to changes in the electric
current systems rather than simply to the presence of these currents.
---------------------------------------------------------
Title: The Haleakala Imaging Vector Magnetograph
Authors: Mickey, D. L.; Canfield, R. C.; Labonte, B. J.; Metcalf, T. R.
1992AAS...180.4005M Altcode: 1992BAAS...24..793M
The Imaging Vector Magnetograph, now in daily use at Mees Solar
Observatory, Haleakala, Hawaii, extends our capabilities in the
measurement of solar vector magnetic fields by providing high
spatial and temporal resolution, together with extended area
coverage. The instrument is based on a fast-readout CCD camera as
its primary detector, a tunable air-spaced Fabry-Perot filter for
spectral selection, and variable nematic liquid-crystal retarders
for polarization selection. A 28 cm aperture reflecting telescope
provides an image the size of a large active region; it is coupled
to the magnetograph in a configuration compact enough to mount on
the Observatory's 3.6 m equatorial spar. The assembly can be pointed
independently of the spar to select a region of interest, so the
telescope is always used on-axis. A tip-tilt image stabilizer corrects
for image displacement due to spar shake or large-scale atmospheric
turbulence. A workstation-based computer control system, incorporating
separate processors for user interface, process management and device
control, permits accurate process timing along with a flexible user
interface. The latter is implemented in an X-windows framework, so that
in fact we have found it quite reasonable to operate the instrument
from an X terminal at a remote location. The raw data images are
stored on 8mm tape for off-line processing, or can be reduced in a
few minutes using the instrument's built-in array processor to provide
near-real-time magnetograms. Instrument operational parameters can be
adjusted in several ways to favor spatial resolution, spatial field,
temporal resolution or magnetic sensitivity, but typically we have
a 4.5 x 4.5 arcminute field, 1 arcsecond spatial sampling, and an
observation interval of five minutes. Initial solar observations
showing current capabilities will be presented.
---------------------------------------------------------
Title: SPAM: A Canned Internet-Accessible Database of Interest to
Solar Flare Researchers
Authors: Canfield, R. C.; Hudson, H. S.; Kiernan, E.; Metcalf, T. R.;
Wulser, J. -P.
1992AAS...180.5103C Altcode: 1992BAAS...24..813C
We have established a searchable database, called SPAM (Spectroscopy
and Polarimetry at Mees), which contains logs of observations made at
Mees Solar Observatory (Haleakala, Maui). Of more general interest,
the database also includes the Events List and Region Report from
the Space Environment Laboratory (Boulder). Logs from YOHKOH are
currently being added. Hence, SPAM can be used to determine, for
example, whether Mees has vector magnetograms of a certain NOAA AR
or whether YOHKOH has certain types of observations in specified time
ranges. As well, it can be used to search the SEL database for flares
with selected attributes. Included logs (and searchable attributes, in
addition to date, day of year, and time) are: Mees Solar Observatory Log
(instrument, NOAA AR, data type, observing setup), SEL Event List (NOAA
AR, X-ray Class), SEL Region Report (NOAA AR), YOHKOH Orbit Summary,
YOHKOH SXT Quiet Mode PFI Observations (latitude, longitude, X-ray and
optical image size), YOHKOH Flare Observations (latitude, longitude,
specific channel counts or ratios). SPAM runs on a Sun workstation at
Mees Solar Observatory, and is available over Internet. Simply access
(e.g., telnet) koa.ifa.hawaii.edu (128.171.167.1) from any vt100,
Sun, or xterm emulator. Log on as spam (lower case); there is no
password. New users are asked to read release notes and hints.
---------------------------------------------------------
Title: Electric Currents and Hard X-ray Images in the X Class Flare
of November 15, 1991
Authors: Metcalf, T. R.; Sakao, T.; Acton, L. W.; Canfield, R. C.;
Hudson, H. S.; Inda, M.; Kosugi, T.; Wulser, J. P.
1992AAS...180.3005M Altcode: 1992BAAS...24..776M
We present co-aligned observations of hard x-rays observed with the
Hard X-ray Telescope (HXT) on board the YOHKOH spacecraft and vertical
electric currents derived from a vector magnetogram obtained at the
Mees Solar Observatory, Haleakala, Hawaii. Previous work comparing
the wings of the Hα line to vertical electric currents has suggested
that electron precipitation in flares occurs at the edges of these
currents. The Stark wings of Hα were interpreted as a signature of
non-thermal electrons penetrating the relatively dense chromosphere and
used as a proxy for direct observation of the non-thermal electrons. The
hard X-rays used in this study provide a direct determination of
the locations of the electron energy losses. In the X class flare
of November 15, 1991, we find the same relation between hard X-ray
emission and vertical electric currents as was found between Hα Stark
wing emission and vertical currents: the hard x-ray emission occurs
predominantly at the edges of the vertical current sites, and not
spatially on top of these currents. Canfield, R. C., de La Beaujardiere,
J., and Leka, K. D., in “The Physics of Solar Flares", ed. Culhane and
Jordan, The Royal Society, London, 1991 Canfield, R. C., Leka, K. D.,
and Wulser,J. P., in “Flare Physics in Solar Activity Maximum 22",
ed. Uchida, Canfield, Watanabe, and Hiei, Springer-Verlag, Berlin, 1991
---------------------------------------------------------
Title: Search for evidence of low energy protons in solar flares.
Authors: Metcalf, Thomas R.; Wuelser, Jean-Pierre; Canfield, Richard
C.; Hudson, Hugh S.
1992NASCP3137..536M Altcode: 1992como.work..536M
We searched for linear polarization in the H alpha line using the
Stokes Polarimeter at Mees Solar Observatory and present observations
of a flare from NOAA active region 6659 which began at 01:30 UT on
14 Jun. 1991. Our dataset also includes H alpha spectra from the Mees
charge coupled device (MCCD) imaging spectrograph as well as hard x ray
observations from the Burst and Transient Source Experiment (BATSE)
instrument on board the Gamma Ray Observatory (GRO). The polarimeter
scanned a 40 x 40 inch field of view using 16 raster points in a 4 x
4 grid. Each scan took about 30 seconds with 2 seconds at each raster
point. The polarimeter stopped 8.5 inches between raster points and
each point covered a 6 inch region. This sparse sampling increased
the total field of view without reducing the temporal cadence. At
each raster point, an H alpha spectrum with 20 mA spectral sampling is
obtained covering 2.6 A centered on H alpha line center. The preliminary
conclusions from the research are presented.
---------------------------------------------------------
Title: Simultaneous EUV, Microwave, and Magnetic Field Observations
of Solar Active Regions
Authors: Brosius, J. W.; Davila, J. M.; Thompson, W. T.; Gopalswamy,
N.; White, S. M.; Jones, H. P.; Metcalf, T. R.
1991BAAS...23.1388B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Derivation of Vector Magnetic Fields from Stokes Profiles:
Derivative versus Least Squares Fitting Techniques
Authors: Metcalf, T. R.; Canfield, R. C.; Mickey, D. L.
1991BAAS...23.1054M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of Linear Hα Polarization in Flares
Authors: Metcalf, T. R.; Wulser, J. P.; Canfield, R. C.
1991BAAS...23.1073M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The derivation of vector magnetic fields from Stokes profiles:
derivative versus least squares fitting techniques.
Authors: Metcalf, T. R.; Canfield, R. C.; Mickey, D. L.; Lites, B. W.
1991sopo.work..376M Altcode:
The authors present a comparison of solar magnetic fields calculated
using the weak field equations of Jefferies, Lites, and Skumanich
and the least squares fitting method of Skumanich and Lites. The two
calculations used Fe I 6302 data from June, 1985, and are found to
agree quite well up to at least 1200 G.
---------------------------------------------------------
Title: Flare Heating and Ionization of the Low Solar
Chromosphere. II. Observations of Five Solar Flares
Authors: Metcalf, Thomas R.; Canfield, Richard C.; Saba, Julia L. R.
1990ApJ...365..391M Altcode:
Two neutral Mg spectral lines formed in the temperature-minimum
region and the low chromosphere, at 4571 and 5173 A, are used to
quantify the changes in the atmospheric structure as a function
of time during five solar flares. Eight proposed flare heating and
ionization mechanisms and predictions of the effects of each on the
temperature minimum region are discussed. Two Mg spectral observations
made at the National Solar Observatory (Sacramento Peak), along with
observations of hard and soft X-rays from the SMM and GOES satellites,
are compared to the predictions of the eight proposed mechanisms. The
initial effects in all five flares are consistent with backwarming by
enhanced Balmer- and Paschen-continuum radiation originating in the
upper chromosphere. Extended heating observed in two of the flares
is most likely due to UV irradiation. In all cases heating by the
dissipation of nonreversed electric currents, collisions with an
electron or proton beam, irradiation by soft X-rays, and dissipation
of Alfven waves are eliminated.
---------------------------------------------------------
Title: Flare Heating and Ionization of the Low Solar Chromosphere
Authors: Metcalf, T. R.; Canfield, R. C.
1990BAAS...22Q.891M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coordinated Optical and Hard X-ray Flare Observations: A GRO
Guest Investigator Proposal
Authors: Metcalf, T. R.; Wülser, J. P.; Canfield, R. C.
1990BAAS...22..791M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flare Heating and Ionization of the Low Solar
Chromosphere. I. Inversion Methods for MG i lambda lambda 4571
and 5173
Authors: Metcalf, Thomas R.; Canfield, Richard C.; Avrett, Eugene H.;
Metcalf, Frederic T.
1990ApJ...350..463M Altcode:
Various methods of inverting solar Mg I 4571 and 5173 spectral line
observations are examined to find the best method of using these lines
to calculate the vertical temperature and electron density structure
around the temperature minimum region. Following a perturbation
analysis by Mein (1971), a Fredholm integral equation of the first
kind is obtained which can be inverted to yield these temperature and
density structures as a function of time. Several inversion methods are
tested and compared. The methods are used to test data as well as to a
subset of observations of these absorption lines taken on February 3,
1986 before and during a solar flare. A small but significant increase
is found in the temperature and a relatively large increase in the
electron density during this flare. The observations are inconsistent
with heating and ionization by an intense beam of electrons and with
ionization by UV photoionization of Si I.
---------------------------------------------------------
Title: Flare Heating and Ionization of the Low Solar Chromosphere.
Authors: Metcalf, Thomas Robert
1990PhDT.........6M Altcode:
In this dissertation, the long-standing question of what mechanisms
are responsible for observations of flare heating at the temperature
minimum region of the solar atmosphere is examined. Two neutral
magnesium spectral lines formed in the temperature minimum region and
the low chromosphere, lambdalambda4571 and 5173 A, were observed at
the Sacramento Peak Observatory in February, 1986, and are used to
quantify the changes in the atmospheric structure as a function of
time. To accomplish this, a technique for inverting the Mg scI lines
to yield the vertical temperature and electron density structure in the
atmosphere is described. The inversion uses a first order perturbation
analysis applied to a model atmosphere to determine the changes in
the model's temperature and electron density structures required to
bring the calculated and observed spectral lines into agreement. Since
small perturbations in the observed lines can yield large changes in
the required perturbation of the model atmosphere, the inversion must
be stabilized and a smoothness constraint is imposed on the solution
to ensure stability. After a discussion of eight proposed heating and
ionization mechanisms and predictions of the effects of each on the
temperature minimum region, this inversion technique is applied to
five separate solar flares, yielding the changes in temperature and
electron density as a function of time and atmospheric depth. These
observations, along with observations of hard and soft X-rays from
the SMM and GOES satellites, are compared to the predictions of the
eight proposed mechanisms to show that the initial effects in all five
flares are consistent with backwarming by enhanced Balmer and Paschen
continuum radiation originating in the upper chromosphere. Extended
heating observed in two of the flares is most likely due to UV
irradiation. In all cases we eliminate heating by the dissipation of
non -reversed electric currents, collisions with an electron or proton
beam, irradiation by soft X-rays, and dissipation of Alfven waves.
---------------------------------------------------------
Title: Momentum Balance in Four Solar Flares
Authors: Canfield, Richard C.; Zarro, Dominic M.; Metcalf, Thomas R.;
Lemen, James R.
1990ApJ...348..333C Altcode:
Solar Maximum Mission soft X-ray spectra and National Solar Observatory
(Sacramento Peak) H-alpha spectra were combined in a study of high-speed
flows during the impulsive phase of four solar flares. In all events,
a blue asymmetry (indicative of upflows) was observed in the coronal Ca
XIX line during the soft X-ray rise phase. In all events a red asymmetry
(indicative of downflows) was observed simultaneously in chromospheric
H-alpha. These oppositely directed flows were concurrent with impulsive
hard X-ray emission. Combining the velocity data with estimates of the
density based on emission measurements and volume estimates, it is shown
that for the impulsive phase as a whole the total momentum of upflowing
soft X-ray plasma equaled that of the downflowing H-alpha plasma, to
within an order of magnitude, in all four events. Only the chromospheric
evaporation model predicts equal total momentum in the upflowing soft
X-ray-emitting and downflowing H-alphba-emitting materials.
---------------------------------------------------------
Title: The Global Energy Balance in Flares
Authors: Simnett, G. M.; Linford, G. A.; Metcalf, T. R.; Henoux,
J. C.; Spicer, D. S.
1989tnti.conf....4S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Determination of the Temperature Structure of the Temperature
Minimum Region during Solar Flares
Authors: Metcalf, T. R.; Metcalf, F. T.; Canfield, R. C.; Avrett, E. H.
1988BAAS...20..688M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Momentum Balance in Four Solar Flares
Authors: Canfield, R. C.; Zarro, D. M.; Metcalf, T. R.; Lemen, J. R.
1988BAAS...20..688C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Explosive Plasma Flows in a Solar Flare
Authors: Zarro, Dominic M.; Canfield, Richard C.; Strong, Keith T.;
Metcalf, Thomas R.
1988ApJ...324..582Z Altcode:
Solar Maximum Mission soft X-ray data and Sacramento Peak Observatory
H-alpha observations are combined in a study of the impulsive phase of
a solar flare. A blue asymmetry, indicative of upflows, was observed
in the coronal Ca XIX line during the soft X-ray rise phase. A red
asymmetry, indicative of downflows, was observed simultaneously in
chromospheric H-alpha emitted from bright flare kernels during the
period of hard X-ray emission. Combining the velocity data with a
measurement of coronal electron density, it is shown that the impulsive
phase momentum of upflowing soft X-ray-emitting plasma equalled that
of the downflowing H-alpha-emitting plasma to within one order of
magnitude. In particular, the momentum of the upflowing plasma was 2 x
10 to the 21st g cm/s while that of the downflowing plasma was 7 x 10
to the 21st g cm/s, with a factor of 2 uncertainty on each value. This
equality supports the explosive chromospheric evaporation model of
solar flares, in which a sudden pressure increase at the footprint of
a coronal loop produces oppositely directed flows in the heated plasma.
---------------------------------------------------------
Title: Coordinated soft X-ray and Hα observations of solar flares
Authors: Zarro, D. M.; Canfield, R. C.; Metcalf, T. R.; Lemen, J. R.
1988AdSpR...8k.149Z Altcode: 1988AdSpR...8..149Z
We have analyzed a unique set of four solar flares for which coordinated
soft X-ray Ca XIX and Hα spectral observations were obtained during
the impulsive phase. In all events, a blue asymmetry (indicative of
upflows) was observed in the coronal Ca XIX line during the soft X-ray
rise phase. In all events, a red asymmetry (indicative of downflows)
was observed simultaneously in chromospheric Hα at spatial locations
associated with enhanced flare heating. Combining the velocity data with
estimates of the evaporated mass based on soft X-ray emission measure,
we demonstrate that the impulsive phase momentum of upflowing soft
X-ray plasma equalled that of the downflowing Hα plasma, to within an
order of magnitude. This equality supports the explosive chromospheric
evaporation model of solar flares - the only model that predicts equal
momentum content in upflowing coronal and downflowing chromospheric
plasmas. <P />under contract with Lockheed Palo Alto Research Laboratory
---------------------------------------------------------
Title: The H alpha Spectral Counterparts of Solar Hard X-Rays
Microflares
Authors: Canfield, Richard C.; Metcalf, Thomas R.
1987ApJ...321..586C Altcode:
X-ray observations have revealed energetically significant numbers
of very small hard X-ray bursts, termed microflares by Lin et al. The
authors have simultaneously observed the Hα counterparts of serveral
of these microflares. It is found that microflares occur in regions
that are also productive of larger flares, suggesting that they may be
components of the larger flare event. All but the weakest miroflares
show pronounced impulsive-phase red asymmetry in Hα. Their energetics,
interpreted using the thick-target nonthermal model of electron
transport, implies that these events are substantially underresolved at
the authors' spatial resolution and have a true area of 10<SUP>15</SUP>-
10<SUP>16</SUP>cm<SUP>-2</SUP>.
---------------------------------------------------------
Title: A novel observational test of momentum balance in a solar flare
Authors: Canfield, Richard C.; Metcalf, Thomas R.; Strong, Keith T.;
Zarro, Dominic M.
1987Natur.326..165C Altcode:
A unique combination of Solar Maximum Mission X-ray spectra and
Sacramento Peak Observatory Hα imaging spectra has enabled us,
for the first time, to measure and compare momentum values of
upflowing and downflowing plasmas during the impulsive phase
of a solar flare. We observed the well-known blue asymmetry of
X-ray spectral lines<SUP>1</SUP>, indicative of upflow, in the
coronal Ca XIX line. We simultaneously observed the red asymmetry
of H<SUP>α</SUP> line profiles, indicative of downflow, in bright Ha
kernels. We find that, to within observational uncertainty, the momentum
transported by the upflowing X-ray plasma was the same as that of the
downflowing H<SUP>α</SUP> material. Of the several physical mechanisms
advanced<SUP>2</SUP> to explain the observed blue asymmetry of X-ray
lines, only explosive chromospheric evaporation predicts oppositely
directed momenta of equal amplitude.
---------------------------------------------------------
Title: Action and Reaction Observed in a Solar Flare
Authors: Zarro, D. M.; Canfield, R. C.; Strong, K. T.; Metcalf, T. R.
1986BAAS...18..966Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evidence for explosive chromospheric evaporation in a solar
flare observed with SMM
Authors: Zarro, D. M.; Strong, K. T.; Canfield, R. C.; Metcalf, T.;
Saba, J. L. R.
1986AdSpR...6f.155Z Altcode: 1986AdSpR...6..155Z
Solar Maximum Mission soft X-ray data and Sacramento Peak Observatory
Hα observations are combined in a study of the impulsive phase
of a solar flare. A blue asymmetry, indicative of upflow motions,
was observed in the coronal Ca XIX line during the soft X-ray rise
phase. Hα redshifts, indicative of downward motions, were observed
simultaneously in bright flare kernels during the period of hard X-ray
emission. We show that, to within observational errors, the impulsive
phase momentum transported by the upflowing soft X-ray plasma is
equivalent to that of the downward moving chromospheric material.
---------------------------------------------------------
Title: Hα Microflares at the Limit of Hard X-ray Detectability
Authors: Metcalf, T. R.; Canfield, R. C.
1985BAAS...17..644M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Hα Spectral Counterpart of Hard X-ray Microflares
Authors: Canfield, R. C.; Metcalf, T. R.
1984BAAS...16..891C Altcode:
No abstract at ADS
