wang-haimin

explanation blue bibcodes open ADS page with paths to full text

Author name code: wang-haimin
ADS astronomy entries on 2022-09-14
=author:"Wang, Haimin"

---------------------------------------------------------
Title: Solar Chromospheric Network as a Source for Solar Wind
Switchbacks
Authors: Lee, Jeongwoo; Yurchyshyn, Vasyl; Wang, Haimin; Yang, Xu;
Cao, Wenda; Carlos Martínez Oliveros, Juan
2022ApJ...935L..27L Altcode:
Recent studies suggest that the magnetic switchbacks (SBs) detected
by the Parker Solar Probe carry information on the scales of solar
supergranulation (large scale) and granulation (medium scale). We test
this claim using high-resolution Hα images obtained with the visible
spectropolarimeters of the Goode Solar Telescope in Big Bear Solar
Observatory. As possible solar sources, we count all the spicule-like
features standing along the chromospheric networks near the coronal
hole boundary visible in the Hα blue-wing but absent in the red-wing
images and measure the geometric parameters of dense sections of
individual flux tubes. Intervals between adjacent spicules located
along the chromospheric networks are found in the range of 0.4-1.5
Mm (0.°03-0.°12) tending to be smaller than the medium scale of
SBs. Interdistances between all pairs of the flux tubes are also counted
and they appear in a single peak distribution around 0.7 Mm (0.°06)
unlike the waiting-time distribution of SBs in a scale-free single
power-law form. The length-to-diameter ratio of the dense section
of flux tubes is as high as 6-40, similar to the aspect ratio of
SBs. The number of spicules along a network can be as high as 40-100,
consistent with numerous SBs within a patch. With these numbers, it is
argued that the medium scale of SBs can be understood as an equilibrium
distance resulting from a random walk within each diverging magnetic
field funnel connected to the chromospheric networks.

---------------------------------------------------------
Title: A Data-based Magnetohydrodynamic Simulation of the X1.0 Solar
Flare of 2021 October 28
Authors: Yamasaki, Daiki; Wang, Haimin; Inoue, Satoshi
2022cosp...44.2420Y Altcode:
The solar active region NOAA 12887 produced an X1.0 flare on
2021 October 28. During the event, an X-shaped flare ribbon and
a circular-shaped filament eruption were observed. In this study,
we aim to understand the formation and the erupting process of the
magnetic flux rope (MFR), which is associated with the circular-shaped
filament. We performed a data-constrained magnetohydrodynamic simulation
using a nonlinear force-free field as an initial condition. In
our simulation, we successfully reproduced the MFR eruption in good
agreement with the filament eruption observed in H$\alpha$ and SDO/AIA
304A images. From the simulation results, we found two possibilities as
the driving mechanism of the MFR eruption; one is the torus instability,
and the other is a push-up motion driven by newly created large magnetic
loops below the pre-existing MFR via continuous magnetic reconnection
between two sheared magnetic arcades. In order to quantitatively
understand the scenario, we performed a hypothetical experiment in which
the velocity is halted at the strong current density region. Therefore,
the reconnection is halted forcibly there, which inhibits the formation
of the large loops that push out the MFR. Consequently, we found
that in the early phase of the eruption in our simulations the upward
velocity of the pre-existing MFR associated with reconnection below is
twice larger than that without reconnection, i.e., reconnection could
help to accelerate the erupting MFR further. Since four footpoints
of the MFR and the newly formed loops well anchored to each part of
X-shaped flare ribbons observed in SDO/AIA 1600A images, we concluded
that reconnection below the MFR played a key role in accelerating the
erupting filament leading to the X1.0 flare.

---------------------------------------------------------
Title: Multi-instrument Comparative Study of Temperature, Number
Density, and Emission Measure during the Precursor Phase of a
Solar Flare
Authors: Liu, Nian; Jing, Ju; Xu, Yan; Wang, Haimin
2022ApJ...930..154L Altcode:
We present a multi-instrument study of the two precursor brightenings
prior to the M6.5 flare (SOL2015-06-22T18:23) in the NOAA Active
Region 12371, with a focus on the temperature (T), electron number
density (n), and emission measure (EM). The data used in this study
were obtained from four instruments with a variety of wavelengths,
i.e., the Solar Dynamics Observatory's Atmospheric Imaging Assembly
(AIA), in six extreme ultraviolet (EUV) passbands; the Expanded Owens
Valley Solar Array (EOVSA) in microwave (MW); the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays (HXR);
and the Geostationary Operational Environmental Satellite (GOES) in
soft X-rays (SXR). We compare the temporal variations of T, n, and EM
derived from the different data sets. Here are the key results. (1)
GOES SXR and AIA EUV have almost identical EM variations (1.5-3 ×
1048 cm-3) and very similar T variations, from
8 to 15 million Kelvin (MK). (2) Listed from highest to lowest, EOVSA
MW provides the highest temperature variations (15-60 MK), followed
by RHESSI HXR (10-24 MK), then GOES SXR and AIA EUV (8-15 MK). (3)
The EM variation from the RHESSI HXR measurements is always less than
the values from AIA EUV and GOES SXR by at most 20 times. The number
density variation from EOVSA MW is greater than the value from AIA
EUV by at most 100 times. The results quantitatively describe the
differences in the thermal parameters at the precursor phase, as
measured by different instruments operating at different wavelength
regimes and for different emission mechanisms.

---------------------------------------------------------
Title: Predicting Solar Energetic Particles Using SDO/HMI Vector
Magnetic Data Products and a Bidirectional LSTM Network
Authors: Abduallah, Yasser; Jordanova, Vania K.; Liu, Hao; Li, Qin;
Wang, Jason T. L.; Wang, Haimin
2022ApJS..260...16A Altcode: 2022arXiv220314393A
Solar energetic particles (SEPs) are an essential source of space
radiation, and are hazardous for humans in space, spacecraft, and
technology in general. In this paper, we propose a deep-learning
method, specifically a bidirectional long short-term memory (biLSTM)
network, to predict if an active region (AR) would produce an SEP
event given that (i) the AR will produce an M- or X-class flare and
a coronal mass ejection (CME) associated with the flare, or (ii)
the AR will produce an M- or X-class flare regardless of whether or
not the flare is associated with a CME. The data samples used in this
study are collected from the Geostationary Operational Environmental
Satellite's X-ray flare catalogs provided by the National Centers
for Environmental Information. We select M- and X-class flares with
identified ARs in the catalogs for the period between 2010 and 2021,
and find the associations of flares, CMEs, and SEPs in the Space
Weather Database of Notifications, Knowledge, Information during the
same period. Each data sample contains physical parameters collected
from the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory. Experimental results based on different performance metrics
demonstrate that the proposed biLSTM network is better than related
machine-learning algorithms for the two SEP prediction tasks studied
here. We also discuss extensions of our approach for probabilistic
forecasting and calibration with empirical evaluation.

---------------------------------------------------------
Title: Observations of Extremely Strong Magnetic Fields in Active
Region NOAA 12673 Using GST Magnetic Field Measurement
Authors: Lozitsky, Vsevolod; Yurchyshyn, Vasyl; Ahn, Kwangsu; Wang,
Haimin
2022ApJ...928...41L Altcode:
We present a detailed study of very strong magnetic fields in the
NOAA Active Region (AR) 12673, which was the most flare productive
AR in solar cycle 24. It produced four X-class flares including the
X9.3 flare on 2017 September 6 and the X8.2 limb event on September
10. Our analysis is based on direct measurements of full Zeeman
splitting of the Fe I 1564.85 nm line using all Stokes I, Q, U, and
V profiles. This approach allowed us to obtain reliable estimates
of the magnitude of magnetic fields independent of the filling
factor and atmosphere models. Thus, the strongest fields up to 5.5
kG were found in a light bridge (LB) of a spot, while in the dark
umbra magnetic fields did not exceed 4 kG. In the case of the LB,
the magnitude of the magnetic field is not related to the underlying
continuum intensity, while in the case of umbral fields we observed
a well-known anticorrelation between the continuum intensity and the
field magnitude. In this study, the LB was cospatial with a polarity
inversion line of δ-sunspot, and we speculate that the 5.5 kG strong
horizontal fields may be associated with a compact twisted flux rope
at or near the photosphere. A comparison of the depth of the Zeeman
π and σ components showed that in the LB magnetic fields are, on
average, more horizontal than those in the dark umbra.

---------------------------------------------------------
Title: Revisiting the Solar Research Cyberinfrastructure Needs:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Ahmadzadeh, Azim; Criscuoli, Serena;
Davey, Alisdair; Gary, Dale; Georgoulis, Manolis; Hurlburt, Neal;
Kitiashvili, Irina; Kempton, Dustin; Kosovichev, Alexander; Martens,
Piet; McGranaghan, Ryan; Oria, Vincent; Reardon, Kevin; Sadykov,
Viacheslav; Timmons, Ryan; Wang, Haimin; Wang, Jason T. L.
2022arXiv220309544N Altcode:
Solar and Heliosphere physics are areas of remarkable data-driven
discoveries. Recent advances in high-cadence, high-resolution
multiwavelength observations, growing amounts of data from realistic
modeling, and operational needs for uninterrupted science-quality data
coverage generate the demand for a solar metadata standardization and
overall healthy data infrastructure. This white paper is prepared as
an effort of the working group "Uniform Semantics and Syntax of Solar
Observations and Events" created within the "Towards Integration of
Heliophysics Data, Modeling, and Analysis Tools" EarthCube Research
Coordination Network (@HDMIEC RCN), with primary objectives to discuss
current advances and identify future needs for the solar research
cyberinfrastructure. The white paper summarizes presentations and
discussions held during the special working group session at the
EarthCube Annual Meeting on June 19th, 2020, as well as community
contribution gathered during a series of preceding workshops and
subsequent RCN working group sessions. The authors provide examples
of the current standing of the solar research cyberinfrastructure, and
describe the problems related to current data handling approaches. The
list of the top-level recommendations agreed by the authors of the
current white paper is presented at the beginning of the paper.

---------------------------------------------------------
Title: A High-resolution Study of Magnetic Field Evolution and
Spicular Activity around the Boundary of a Coronal Hole
Authors: Wang, Jiasheng; Lee, Jeongwoo; Liu, Chang; Cao, Wenda;
Wang, Haimin
2022ApJ...924..137W Altcode:
In this study, we analyze high-spatial-resolution (0.″24)
magnetograms and high-spatial-resolution (0.″10) Hα off-band (±
0.8 Å) images taken by the 1.6 m Goode Solar Telescope to investigate
the magnetic properties associated with small-scale ejections in a
coronal hole boundary region from a statistical perspective. With one
and a half hours of optical observations under excellent seeing, we
focus on the magnetic structure and evolution by tracking the magnetic
features with the Southwest Automatic Magnetic Identification Suite
(SWAMIS). The magnetic field at the studied coronal hole boundary is
dominated by negative polarity with flux cancellations at the edges
of the negative unipolar cluster. In a total of 1250 SWAMIS-detected
magnetic cancellation events, ~39% are located inside the coronal
hole with an average flux cancellation rate of 2.0 × 1018
Mx Mm-2 hr-1, and ~49% are located outside
the coronal hole with an average flux cancellation rate of 8.8 ×
1017 Mx Mm-2 hr-1. We estimated
that the magnetic energy released due to flux cancellation inside the
coronal hole is six times more than that outside the coronal hole. Flux
cancellation accounts for ~9.5% of the total disappearance of magnetic
flux. Other forms of its disappearance are mainly due to fragmentation
of unipolar clusters or merging with elements of the same polarity. We
also observed a number of significant small-scale ejections associated
with magnetic cancellations at the coronal hole boundary that have
corresponding EUV brightenings.

---------------------------------------------------------
Title: Multi-passband Observations of a Solar Flare over the He I
10830 Å line
Authors: Xu, Yan; Yang, Xu; Kerr, Graham S.; Polito, Vanessa; Sadykov,
Viacheslav M.; Jing, Ju; Cao, Wenda; Wang, Haimin
2022ApJ...924L..18X Altcode: 2021arXiv211209949X
This study presents a C3.0 flare observed by the Big Bear Solar
Observatory/Goode Solar Telescope (GST) and Interface Region Imaging
Spectrograph (IRIS) on 2018 May 28 around 17:10 UT. The Near-Infrared
Imaging Spectropolarimeter of GST was set to spectral imaging mode to
scan five spectral positions at ±0.8, ±0.4 Å and line center of He I
10830 Å. At the flare ribbon's leading edge, the line is observed to
undergo enhanced absorption, while the rest of the ribbon is observed
to be in emission. When in emission, the contrast compared to the
preflare ranges from about 30% to nearly 100% at different spectral
positions. Two types of spectra, "convex" shape with higher intensity at
line core and "concave" shape with higher emission in the line wings,
are found at the trailing and peak flaring areas, respectively. On the
ribbon front, negative contrasts, or enhanced absorption, of about
~10%-20% appear in all five wavelengths. This observation strongly
suggests that the negative flares observed in He I 10830 Å with
mono-filtergram previously were not caused by pure Doppler shifts of
this spectral line. Instead, the enhanced absorption appears to be a
consequence of flare-energy injection, namely nonthermal collisional
ionization of helium caused by the precipitation of high-energy
electrons, as found in our recent numerical modeling results. In
addition, though not strictly simultaneous, observations of Mg II
from the IRIS spacecraft, show an obvious central reversal pattern
at the locations where enhanced absorption of He I 10830 Å is seen,
which is consistent with previous observations.

---------------------------------------------------------
Title: Improving the Spatial Resolution of Solar Images Using
Generative Adversarial Network and Self-attention Mechanism
Authors: Deng, Junlan; Song, Wei; Liu, Dan; Li, Qin; Lin, Ganghua;
Wang, Haimin
2021ApJ...923...76D Altcode:
In recent years, the new physics of the Sun has been revealed
using advanced data with high spatial and temporal resolutions. The
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic
Observatory has accumulated abundant observation data for the
study of solar activity with sufficient cadence, but their spatial
resolution (about 1″) is not enough to analyze the subarcsecond
structure of the Sun. On the other hand, high-resolution observation
from large-aperture ground-based telescopes, such as the 1.6 m Goode
Solar Telescope (GST) at the Big Bear Solar Observatory, can achieve a
much higher resolution on the order of 0.″1 (about 70 km). However,
these high-resolution data only became available in the past 10 yr,
with a limited time period during the day and with a very limited
field of view. The Generative Adversarial Network (GAN) has greatly
improved the perceptual quality of images in image translation tasks,
and the self-attention mechanism can retrieve rich information from
images. This paper uses HMI and GST images to construct a precisely
aligned data set based on the scale-invariant feature transform
algorithm and t0 reconstruct the HMI continuum images with four times
better resolution. Neural networks based on the conditional GAN and
self-attention mechanism are trained to restore the details of solar
active regions and to predict the reconstruction error. The experimental
results show that the reconstructed images are in good agreement with
GST images, demonstrating the success of resolution improvement using
machine learning. *Released on 2021 March 1st.

---------------------------------------------------------
Title: Stokes Inversion with Stacked Deep Neural Networks
Authors: Jiang, Haodi; Li, Qin; Xu, Yan; Ahn, Kwangsu; Cao, Wenda;
Wang, Jason T. L.; Wang, Haimin
2021AGUFMNG45B0583J Altcode:
Prediction of solar eruptive events is an important topic in space
weather research. Acquiring high-quality magnetic and velocity fields
through Stokes inversion is crucial for accurate solar eruption
prediction. We present here a new deep learning method, dubbed Stacked
Deep Neural Networks (SDNN), for Stokes inversion. We apply SDNN to
inferring line-of-sight (LOS) velocities and Doppler widths in addition
to vector magnetic fields from Stokes profiles of the Near InfraRed
Imaging Spectropolarimeter on the 1.6 m Goode Solar Telescope at the Big
Bear Solar Observatory. Experimental results show that SDNN is faster,
while producing smoother and cleaner LOS velocity maps and Doppler
width maps, than the widely used Milne-Eddington method. Furthermore,
SDNN outperforms three related machine learning algorithms, including
a convolutional neural network, multiple support vector regression
model, and multilayer perceptrons model, on different datasets. Thus,
the proposed deep learning based SDNN tool can be considered as an
alternative and efficient method for Stokes inversion.

---------------------------------------------------------
Title: Solar Filament Segmentation Based on Improved U-Nets
Authors: Liu, Dan; Song, Wei; Lin, Ganghua; Wang, Haimin
2021SoPh..296..176L Altcode:
To detect, track and characterize solar filaments more accurately, novel
filament segmentation methods based on improved U-Nets are proposed. The
full-disk Hα images from the Huairou Solar Observing Station of the
National Astronomical Observatory and the Big Bear Solar Observatory
were used for training and verifying the effectiveness of different
improved networks' filament segmentation performance. Comparative
experiments with different solar dataset sizes and input image
quality were performed. The impact of each improvement method on the
segmentation effect was analyzed and compared based on experimental
results. In order to further explore the influence of network depth
on filament-segmentation accuracy, the segmentation results produced
by Conditional Generative Adversarial Networks (CGAN) were obtained
and compared with improved U-nets. Experiments verified that U-Net
with an Atrous Spatial Pyramid Pooling Module performs better for
high-quality input solar images regardless of dataset sizes. CGAN
performs better for low-quality input solar images with large dataset
size. The algorithm may provide guidance for filament segmentation
and more accurate segmentation results with less noise were acquired.

---------------------------------------------------------
Title: Solar Chromospheric Ejections and Solar Wind Switchbacks
Authors: Lee, Jeongwoo; Yang, Xu; Yurchyshyn, Vasyl; Wang, Haimin
2021AGUFMSH33B..01L Altcode:
Small-scale jet-like ejections in the solar atmosphere are of renewed
interest to NASA's Parker Solar Probe (PSP) mission, because they may
develop into the mysterious solar wind structure of plasma and magnetic
field called switchbacks detected by the PSP. We investigate high
resolution H-alpha images and magnetograms obtained with the visible
(VIS) and near-infrared (NIRIS) spectro-polarimeters of the Goode
Solar Telescope (GST) in Big Bear Solar Observatory (BBSO) to explore
solar origin for the switchbacks. In this effort, we measure spatial
dimensions, orientation, eruption speed of tiny magnetic arcades,
mini-filaments, and plumelets for comparison with the properties of
the switchbacks established from the PSP observations. The spatial
dimensions of these objects range from tens of arcsecond down to sub
arcsecond (200 km), small enough to produce short duration switchbacks
in the PSP locations. However, the orientation combined with the
speed can be tested against the surprisingly high aspect ratio of
the switchbacks, thus providing a strong constraint on the possible
candidates. Other properties of the switchbacks such as clustering and
one-sidedness make this search a narrower choice. We argue that the most
ideal solar candidate should be the tiny upright flux tubes that can
develop into an S-shape after ejection, and that the KelvinHelmholtz
instability arising from the density inhomogeneity along the flux
tube could be a plausible mechanism for transforming the chromospheric
structure into the solar wind switchbacks.

---------------------------------------------------------
Title: Understanding the Initiation of the M2.4 Flare on 2017 July 14
Authors: Jing, Ju; Inoue, Satoshi; Lee, Jeongwoo; Li, Qin; Nita,
Gelu M.; Xu, Yan; Liu, Chang; Gary, Dale E.; Wang, Haimin
2021ApJ...922..108J Altcode:
We present both the observation and the magnetohydrodynamics (MHD)
simulation of the M2.4 flare (SOL2017-07-14T02:09) of NOAA active
region (AR) 12665 with a goal to identify its initiation mechanism. The
observation by the Atmospheric Image Assembly (AIA) on board the Solar
Dynamics Observatory (SDO) shows that the major topology of the AR
is a sigmoidal configuration associated with a filament/flux rope. A
persistent emerging magnetic flux and the rotation of the sunspot in
the core region were observed with Magnetic Imager (HMI) on board the
SDO on the timescale of hours before and during the flare, which may
provide free magnetic energy needed for the flare/coronal mass ejection
(CME). A high-lying coronal loop is seen moving outward in AIA EUV
passbands, which is immediately followed by the impulsive phase of the
flare. We perform an MHD simulation using the potential magnetic field
extrapolated from the measured pre-flare photospheric magnetic field
as initial conditions and adopting the observed sunspot rotation and
flux emergence as the driving boundary conditions. In our simulation,
a sigmoidal magnetic structure and an overlying magnetic flux rope
(MFR) form as a response to the imposed sunspot rotation, and the MFR
rises to erupt like a CME. These simulation results in good agreement
with the observation suggest that the formation of the MFR due to the
sunspot rotation and the resulting torus and kink instabilities were
essential to the initiation of this flare and the associated coronal
mass ejection.

---------------------------------------------------------
Title: Geospace Response to Small-scale Magnetic Flux Ropes in the
Solar Wind
Authors: Shin, Youra; Noh, Sungjun; Huang, Nengyi; Bizos, George;
Farooki, Hameedullah; Desai, Manal; Choi, Kyung-Eun; Wang, Haimin;
Kim, Hyomin
2021AGUFMSM15D1996S Altcode:
Small-scale magnetic flux ropes (SMFRs), a bundle of twisted field
lines, are frequently observed in the solar wind. However, their origin
is still controversial because there have been a number of reports that
SMFRs are observed not only near the sun but also even near the nose of
the Earths bow shock. Since it is widely known that rotating magnetic
field structure is favorable for build-up, release and transport of free
energy, the interaction between SMFRs and the Earths magnetosphere may
lead to transient energy transfer near the boundary (e.g., flux transfer
event or FTE) via magnetic reconnection. However, their effect on the
geospace environment has not yet been extensively studied. We report
on geospace responses to SMFRs by comparing SMFR signatures from the
spacecraft in the solar wind between Sun and Earth (e.g., ACE and Wind)
with field and particle data from the several magnetospheric missions
in various orbits (e.g., MMS, Cluster, GOES and Van Allen Probes)
to investigate geomagnetic activity and plasma characteristics during
passages of SMFRs. Responses in the ionosphere at high latitudes are
also reported using data from magnetometers and SuperDARN radars.

---------------------------------------------------------
Title: Coronal Magnetic Field Measurements along a Partially Erupting
Filament in a Solar Flare
Authors: Wei, Yuqian; Chen, Bin; Yu, Sijie; Wang, Haimin; Jing, Ju;
Gary, Dale E.
2021ApJ...923..213W Altcode: 2021arXiv211006414W
Magnetic flux ropes are the centerpiece of solar eruptions. Direct
measurements for the magnetic field of flux ropes are crucial for
understanding the triggering and energy release processes, yet
they remain heretofore elusive. Here we report microwave imaging
spectroscopy observations of an M1.4-class solar flare that occurred
on 2017 September 6, using data obtained by the Expanded Owens Valley
Solar Array. This flare event is associated with a partial eruption
of a twisted filament observed in Hα by the Goode Solar Telescope
at the Big Bear Solar Observatory. The extreme ultraviolet (EUV)
and X-ray signatures of the event are generally consistent with the
standard scenario of eruptive flares, with the presence of double
flare ribbons connected by a bright flare arcade. Intriguingly, this
partial eruption event features a microwave counterpart, whose spatial
and temporal evolution closely follow the filament seen in Hα and
EUV. The spectral properties of the microwave source are consistent
with nonthermal gyrosynchrotron radiation. Using spatially resolved
microwave spectral analysis, we derive the magnetic field strength
along the filament spine, which ranges from 600 to 1400 Gauss from
its apex to the legs. The results agree well with the nonlinear
force-free magnetic model extrapolated from the preflare photospheric
magnetogram. We conclude that the microwave counterpart of the erupting
filament is likely due to flare-accelerated electrons injected into
the filament-hosting magnetic flux rope cavity following the newly
reconnected magnetic field lines.

---------------------------------------------------------
Title: Global solar flows and their impact on magnetic activity
Authors: Dikpati, Mausumi; Braun, Douglas; Featherstone, Nicholas;
Hindman, Bradley; Komm, Rudolf; Liu, Yang; Scherrer, Philip; Upton,
Lisa; Wang, Haimin
2021AGUFMSH55D1872D Altcode:
This poster presents the second year progress report of the LWS
focused-science team-4 of 2019. The main science objective is to
jointly develop the most comprehensive, dynamically consistent picture
of solar flows at the surface, in the convection zone and tachocline,
and determine the MHD effects induced by these motions. Our major
team-achievements in the second year include: (i) consensus about active
regions' flow and their contributions in modifying the global flow;
(ii) long-term global flow map from various magnetograms, and their
specific properties as function of cycle phase, (iii) impacts of the
flows in polar field evolutions, (iv) simulations of global flows with
various solar-like interior conditions, (v) roles of simulated flows
in driving the nonlinear dynamics of spot-producing magnetic fields
and producing their spatio-temporal patterns, which are compared with
that manifested as active regions patterns in surface magnetograms. We
will describe in detail how these observationally constrained local and
global flows are leading us to improved simulations of model-outputs of
magnetic activity and flows. In turn, these outputs can reliably be used
as inputs to heliospheric models, for example, for simulating properties
of reconnection of active regions' magnetic fields, high-speed streams,
sector passages, all of which have profound influence on various
aspects of space weather and impact on terrestrial atmosphere.

---------------------------------------------------------
Title: An investigation of the causal relationship between sunspot
groups and coronal mass ejections by determining source active regions
Authors: Raheem, Abd-ur; Cavus, Huseyin; Coban, Gani Caglar; Kinaci,
Ahmet Cumhur; Wang, Haimin; Wang, Jason T. L.
2021MNRAS.506.1916R Altcode: 2021MNRAS.tmp.1624R
Although the source active regions of some coronal mass ejections
(CMEs) were identified in CME catalogues, vast majority of CMEs do
not have an identified source active region. We propose a method
that uses a filtration process and machine learning to identify the
sunspot groups associated with a large fraction of CMEs and compare the
physical parameters of these identified sunspot groups with properties
of their corresponding CMEs to find mechanisms behind the initiation
of CMEs. These CMEs were taken from the Coordinated Data Analysis
Workshops (CDAW) data base hosted at NASA's website. The Helioseismic
and Magnetic Imager (HMI) Active Region Patches (HARPs) were taken from
the Stanford University's Joint Science Operations Center (JSOC) data
base. The source active regions of the CMEs were identified by the help
of a custom filtration procedure and then by training a long short-term
memory network (LSTM) to identify the patterns in the physical magnetic
parameters derived from vector and line-of-sight magnetograms. The
neural network simultaneously considers the time series data of these
magnetic parameters at once and learns the patterns at the onset of
CMEs. This neural network was then used to identify the source HARPs
for the CMEs recorded from 2011 till 2020. The neural network was able
to reliably identify source HARPs for 4895 CMEs out of 14 604 listed
in the CDAW data base during the aforementioned period.

---------------------------------------------------------
Title: Tracing Hα Fibrils through Bayesian Deep Learning
Authors: Jiang, Haodi; Jing, Ju; Wang, Jiasheng; Liu, Chang; Li, Qin;
Xu, Yan; Wang, Jason T. L.; Wang, Haimin
2021ApJS..256...20J Altcode: 2021arXiv210707886J
We present a new deep-learning method, named FibrilNet, for tracing
chromospheric fibrils in Hα images of solar observations. Our method
consists of a data preprocessing component that prepares training
data from a threshold-based tool, a deep-learning model implemented
as a Bayesian convolutional neural network for probabilistic image
segmentation with uncertainty quantification to predict fibrils,
and a post-processing component containing a fibril-fitting
algorithm to determine fibril orientations. The FibrilNet tool is
applied to high-resolution Hα images from an active region (AR
12665) collected by the 1.6 m Goode Solar Telescope (GST) equipped
with high-order adaptive optics at the Big Bear Solar Observatory
(BBSO). We quantitatively assess the FibrilNet tool, comparing its
image segmentation algorithm and fibril-fitting algorithm with those
employed by the threshold-based tool. Our experimental results and
major findings are summarized as follows. First, the image segmentation
results (i.e., the detected fibrils) of the two tools are quite similar,
demonstrating the good learning capability of FibrilNet. Second,
FibrilNet finds more accurate and smoother fibril orientation angles
than the threshold-based tool. Third, FibrilNet is faster than the
threshold-based tool and the uncertainty maps produced by FibrilNet
not only provide a quantitative way to measure the confidence on each
detected fibril, but also help identify fibril structures that are not
detected by the threshold-based tool but are inferred through machine
learning. Finally, we apply FibrilNet to full-disk Hα images from other
solar observatories and additional high-resolution Hα images collected
by BBSO/GST, demonstrating the tool's usability in diverse data sets.

---------------------------------------------------------
Title: DeepSun: machine-learning-as-a-service for solar flare
prediction
Authors: Abduallah, Yasser; Wang, Jason T. L.; Nie, Yang; Liu, Chang;
Wang, Haimin
2021RAA....21..160A Altcode: 2020arXiv200904238A
Solar flare prediction plays an important role in understanding
and forecasting space weather. The main goal of the Helioseismic
and Magnetic Imager (HMI), one of the instruments on NASA's Solar
Dynamics Observatory, is to study the origin of solar variability
and characterize the Sun's magnetic activity. HMI provides continuous
full-disk observations of the solar vector magnetic field with high
cadence data that lead to reliable predictive capability; yet, solar
flare prediction effort utilizing these data is still limited. In this
paper, we present a machine-learning-as-a-service (MLaaS) framework,
called DeepSun, for predicting solar flares on the web based on HMI's
data products. Specifically, we construct training data by utilizing
the physical parameters provided by the Space-weather HMI Active Region
Patch (SHARP) and categorize solar flares into four classes, namely
B, C, M and X, according to the X-ray flare catalogs available at the
National Centers for Environmental Information (NCEI). Thus, the solar
flare prediction problem at hand is essentially a multi-class (i.e.,
four-class) classification problem. The DeepSun system employs several
machine learning algorithms to tackle this multi-class prediction
problem and provides an application programming interface (API) for
remote programming users. To our knowledge, DeepSun is the first MLaaS
tool capable of predicting solar flares through the internet.

---------------------------------------------------------
Title: Deep Learning Based Reconstruction of Total Solar Irradiance
Authors: Abduallah, Yasser; Wang, Jason T. L.; Shen, Yucong; Alobaid,
Khalid A.; Criscuoli, Serena; Wang, Haimin
2021arXiv210711042A Altcode:
The Earth's primary source of energy is the radiant energy generated
by the Sun, which is referred to as solar irradiance, or total solar
irradiance (TSI) when all of the radiation is measured. A minor
change in the solar irradiance can have a significant impact on the
Earth's climate and atmosphere. As a result, studying and measuring
solar irradiance is crucial in understanding climate changes and solar
variability. Several methods have been developed to reconstruct total
solar irradiance for long and short periods of time; however, they
are physics-based and rely on the availability of data, which does not
go beyond 9,000 years. In this paper we propose a new method, called
TSInet, to reconstruct total solar irradiance by deep learning for
short and long periods of time that span beyond the physical models'
data availability. On the data that are available, our method agrees
well with the state-of-the-art physics-based reconstruction models. To
our knowledge, this is the first time that deep learning has been used
to reconstruct total solar irradiance for more than 9,000 years.

---------------------------------------------------------
Title: He I 10830 Å Dimming during Solar Flares. I. The Crucial
Role of Nonthermal Collisional Ionizations
Authors: Kerr, Graham S.; Xu, Yan; Allred, Joel C.; Polito, Vanessa;
Sadykov, Viacheslav M.; Huang, Nengyi; Wang, Haimin
2021ApJ...912..153K Altcode: 2021arXiv210316686K
While solar flares are predominantly characterized by an intense
broadband enhancement to the solar radiative output, certain
spectral lines and continua will, in theory, exhibit flare-induced
dimmings. Observations of transitions of orthohelium He I λλ 10830
Å and the He I D3 lines have shown evidence of such dimming, usually
followed by enhanced emission. It has been suggested that nonthermal
collisional ionization of helium by an electron beam, followed by
recombinations to orthohelium, is responsible for overpopulating
those levels, leading to stronger absorption. However, it has not been
possible observationally to preclude the possibility of overpopulating
orthohelium via enhanced photoionization of He I by EUV irradiance from
the flaring corona followed by recombinations. Here we present radiation
hydrodynamics simulations of nonthermal electron-beam-driven flares
where (1) both nonthermal collisional ionization of helium and coronal
irradiance are included, and (2) only coronal irradiance is included. A
grid of simulations covering a range of total energies deposited by
the electron beam and a range of nonthermal electron-beam low-energy
cutoff values were simulated. In order to obtain flare-induced dimming
of the He I 10830 Å line, it was necessary for nonthermal collisional
ionization to be present. The effect was more prominent in flares with
larger low-energy cutoff values and longer lived in weaker flares and
flares with a more gradual energy deposition timescale. These results
demonstrate the usefulness of orthohelium line emission as a diagnostic
of flare energy transport.

---------------------------------------------------------
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
2021SoPh..296...70R Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.

---------------------------------------------------------
Title: Migration of Solar Polar Crown Filaments in the Past 100 Years
Authors: Xu, Yan; Banerjee, Dipankar; Chatterjee, Subhamoy; Pötzi,
Werner; Wang, Ziran; Ruan, Xindi; Jing, Ju; Wang, Haimin
2021ApJ...909...86X Altcode:
Polar crown filaments (PCFs) are formed above the polarity inversion
line, which separates unipolar polar fields and the nearest dispersed
fields. They are important features in studying solar polar fields and
their cyclical variations. Due to the relatively weak field strength and
projection effects, measuring polar magnetic fields is more difficult
than obtaining the field strengths concentrated in active regions at
lower latitudes. "Rush-to-the-pole" of PCFs represent the progress of
unipolar polar fields from the previous solar cycle being canceled by
the dispersed fields generated in the current cycle. Such progress
is a good indicator of the polarity reversal in the polar areas and
a precursor for the solar maximum. In this study, PCFs are identified
from a 100 yr archive, covering cycles 16-24. This archive consists of
full-disk Hα images obtained from the Kodaikanal Solar Observatory of
the Indian Institute of Astrophysics, Kanzelhöhe Solar Observatory,
and Big Bear Solar Observatory. The poleward migration speeds are
measured and show an obvious asymmetry in the northern and southern
hemispheres. In addition, our results show that the PCFs usually reach
their highest latitudes first in the northern hemisphere, except cycle
17. Similarly, previous studies show that the magnetic field reversed
first at the north pole in six out of nine cycles. We also compare the
temporal variations of PCF migration and the latitude gradient factor
of the differential rotation, which shows a trend in the southern
hemisphere. Moreover, the migration speed of PCFs does not seem to be
well correlated with the maximum sunspot numbers.

---------------------------------------------------------
Title: Identifying and Tracking Solar Magnetic Flux Elements with
Deep Learning
Authors: Jiang, Haodi; Wang, Jiasheng; Liu, Chang; Jing, Ju; Liu,
Hao; Wang, Jason T. L.; Wang, Haimin
2020ApJS..250....5J Altcode: 2020arXiv200812080J
Deep learning has drawn significant interest in recent years due to
its effectiveness in processing big and complex observational data
gathered from diverse instruments. Here we propose a new deep learning
method, called SolarUnet, to identify and track solar magnetic flux
elements or features in observed vector magnetograms based on the
Southwest Automatic Magnetic Identification Suite (SWAMIS). Our method
consists of a data preprocessing component that prepares training
data from the SWAMIS tool, a deep learning model implemented as a
U-shaped convolutional neural network for fast and accurate image
segmentation, and a postprocessing component that prepares tracking
results. SolarUnet is applied to data from the 1.6 m Goode Solar
Telescope at the Big Bear Solar Observatory. When compared to the
widely used SWAMIS tool, SolarUnet is faster while agreeing mostly with
SWAMIS on feature size and flux distributions and complementing SWAMIS
in tracking long-lifetime features. Thus, the proposed physics-guided
deep learning-based tool can be considered as an alternative method
for solar magnetic tracking.

---------------------------------------------------------
Title: High-resolution Observations of Small-scale Flux Emergence
by GST
Authors: Wang, Jiasheng; Liu, Chang; Cao, Wenda; Wang, Haimin
2020ApJ...900...84W Altcode: 2020arXiv200906717W
Recent observations demonstrated that emerging flux regions, which
constitute the early stage of solar active regions, consist of
emergence of numerous small-scale magnetic elements. They in turn
interact, merge, and form mature sunspots. However, observations of
fine magnetic structures on photosphere with subarcsecond resolution
are very rare due to limitations of observing facilities. In this
work, taking advantage of the high resolution of the 1.6 m Goode Solar
Telescope, we jointly analyze vector magnetic fields, continuum images,
and Hα observations of NOAA AR 12665 on 2017 July 13, with the goal
of understanding the signatures of small-scale flux emergence, as well
as their atmospheric responses as they emerge through multiple heights
in the photosphere and chromosphere. Under such a high resolution of
0"1-0"2, our results confirm two kinds of small-scale flux emergence:
magnetic flux sheet emergence associated with the newly forming
granules, and the traditional magnetic flux loop emergence. With
direct imaging in the broadband TiO, we observe that both types of
flux emergence are associated with darkening of granular boundaries,
while only flux sheets elongate granules along the direction of
emerging magnetic fields and expand laterally. With a life span of
10 ∼ 15 minutes, the total emerged vertical flux is on the order
of 1018 Mx for both types of emergence. The magnitudes of
the vertical and horizontal fields are comparable in the flux sheets,
while the former is stronger in flux loops. Hα observations reveal
transient brightenings in the wings in the events of magnetic loop
emergence, which are most probably the signatures of Ellerman bombs.

---------------------------------------------------------
Title: An Eruptive Circular-ribbon Flare with Extended Remote
Brightenings
Authors: Liu, Chang; Prasad, Avijeet; Lee, Jeongwoo; Wang, Haimin
2020ApJ...899...34L Altcode: 2020arXiv200714428L
We study an eruptive X1.1 circular-ribbon flare on 2013 November
10, combining multiwavelength observations with a coronal field
reconstruction using a non-force-free field method. In the first stage,
a filament forms via magnetic reconnection between two mildly twisted
sheared arcades, which are embedded under the fan dome associated with
a null point (NP). This reconnection seems to be driven by photospheric
shearing and converging flows around the inner two arcade footpoints,
consistent with the flare-related changes of the transverse field. The
southern portion of the filament rises upward due to torus instability
and pushes against the NP. The induced NP reconnection then generates
the circular ribbon and the initial remote brightening in the west, as
accelerated electrons precipitate along the fan and propagate outward
along quasi-separatix surfaces with high values of the squashing factor
(Q) in the envelope fields, which have a curtain-like shape here. In
the second stage, the southern end of the flux rope breaks away from
the surface, sequentially disrupts the dome and overlying fields,
and erupts in a whipping-like fashion to become a partial halo coronal
mass ejection. This leads to an enhanced flare emission and fast-moving
remote brightenings at the footpoints of the magnetic curtain, which
span a remarkably broad region and are also associated with coronal
dimmings. This is a rare example of eruptive circular-ribbon flares,
in which the evolution of a flux rope from its formation to successful
eruption out of the dome and the resulting unusually extended remote
brightenings are completely observed.

---------------------------------------------------------
Title: Comparison of Enhanced Absorption in He I 10830 Šin
Observations and Modeling during the Early Phase of a Solar Flare
Authors: Huang, Nengyi; Sadykov, Viacheslav M.; Xu, Yan; Jing, Ju;
Wang, Haimin
2020ApJ...897L...6H Altcode: 2020arXiv200610218H
The He I 10830 Å triplet is a very informative indicator of
chromospheric activities as the helium is the second most abundant
element in the solar atmosphere. Taking advantage of the high resolution
of the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory,
previous observations have shown clear evidence of the enhanced
absorption, instead of typically observed emission, for two M-class
flares. In this study, we analyze the evolution of the He I 10830
Å emission in numerical models and compare it with observations. The
models represent the RADYN simulation results obtained from the F-CHROMA
database. We consider the models with the injected electron spectra
parameters close to observational estimates for the 2013 August 17
flare event (δ = 8, ${E}_{c}=\{15,20\}\,\,\mathrm{keV}$ , $F=\{1\times
{10}^{11},3\times {10}^{11}\}\,\mathrm{erg}\,{\mathrm{cm}}^{-2}$ ) in
detail, as well as other available models. The modeling results agree
well with observations, in the sense of both the maximum intensity
decrease (-17.1%, compared with the observed value of -13.7%) and the
trend of temporal variation (initial absorption phase followed by the
emission). All models demonstrate the increased number densities and
decreased ratio of the upper and lower level populations of the He I
10830 Å transition in the initial phase, which enhances the opacity
and forms an absorption feature. Models suggest that the temperatures
and free electron densities at heights of 1.3-1.5 Mm should be larger
than ∼104 K and 6 × 1011 cm-3
thresholds for the line to start being in emission.

---------------------------------------------------------
Title: A New Comprehensive Data Set of Solar Filaments of 100 yr
Interval. I.
Authors: Lin, GangHua; Zhu, GaoFei; Yang, Xiao; Song, YongLiang;
Zhang, Mei; Liu, Suo; Wang, XiaoFan; Su, JiangTao; Zheng, Sheng; Zhang,
JiaFeng; Tao, DongYi; Zeng, ShuGuang; Wang, HaiMin; Liu, Chang; Xu, Yan
2020ApJS..249...11L Altcode: 2020arXiv200609082L
Filaments are very common physical phenomena on the Sun and are often
taken as important proxies of solar magnetic activities. The study of
filaments has become a hot topic in the space weather research. For
a more comprehensive understanding of filaments, especially for an
understanding of solar activities of multiple solar cycles, it is
necessary to perform a combined multifeature analysis by constructing
a data set of multiple solar cycle data. To achieve this goal, we
constructed a centennial data set that covers the Hα data from five
observatories around the world. During the data set construction,
we encountered varieties of problems, such as data fusion, accurate
determination of the solar edge, classifying data by quality, dynamic
threshold, and so on, which arose mainly due to multiple sources and
a large time span of data. But fortunately, these problems were well
solved. The data set includes seven types of data products and eight
types of feature parameters with which we can implement the functions of
data searching and statistical analyses. It has the characteristics of
better continuity and highly complementary to space observation data,
especially in the wavelengths not covered by space observations, and
covers many solar cycles (including more than 60 yr of high-cadence
data). We expect that this new comprehensive data set as well as the
tools will help researchers to significantly speed up their search for
features or events of interest, for either statistical or case study
purposes, and possibly help them get a better and more comprehensive
understanding of solar filament mechanisms.

---------------------------------------------------------
Title: Machine Learning in Heliophysics and Space Weather Forecasting:
A White Paper of Findings and Recommendations
Authors: Nita, Gelu; Georgoulis, Manolis; Kitiashvili, Irina; Sadykov,
Viacheslav; Camporeale, Enrico; Kosovichev, Alexander; Wang, Haimin;
Oria, Vincent; Wang, Jason; Angryk, Rafal; Aydin, Berkay; Ahmadzadeh,
Azim; Bai, Xiaoli; Bastian, Timothy; Filali Boubrahimi, Soukaina; Chen,
Bin; Davey, Alisdair; Fereira, Sheldon; Fleishman, Gregory; Gary, Dale;
Gerrard, Andrew; Hellbourg, Gregory; Herbert, Katherine; Ireland,
Jack; Illarionov, Egor; Kuroda, Natsuha; Li, Qin; Liu, Chang; Liu,
Yuexin; Kim, Hyomin; Kempton, Dustin; Ma, Ruizhe; Martens, Petrus;
McGranaghan, Ryan; Semones, Edward; Stefan, John; Stejko, Andrey;
Collado-Vega, Yaireska; Wang, Meiqi; Xu, Yan; Yu, Sijie
2020arXiv200612224N Altcode:
The authors of this white paper met on 16-17 January 2020 at the New
Jersey Institute of Technology, Newark, NJ, for a 2-day workshop that
brought together a group of heliophysicists, data providers, expert
modelers, and computer/data scientists. Their objective was to discuss
critical developments and prospects of the application of machine and/or
deep learning techniques for data analysis, modeling and forecasting
in Heliophysics, and to shape a strategy for further developments in
the field. The workshop combined a set of plenary sessions featuring
invited introductory talks interleaved with a set of open discussion
sessions. The outcome of the discussion is encapsulated in this white
paper that also features a top-level list of recommendations agreed
by participants.

---------------------------------------------------------
Title: Three-Dimensional Magnetic and Velocity Structures of Active
Region 12673
Authors: Wang, Haimin
2020EGUGA..22.2234W Altcode:
We study the Solar Active Region (AR) 12673 in September 2017, which
is the most flare productive AR in the solar cycle 24. Observations
from Goode Solar Telescope (GST) show the strong photospheric magnetic
fields (nearly 6000 G) in polarity inversion line (PIL) and apparent
photospheric twist on September 6, the day of X9.3 flare. Corresponding
to the strong twist, upflows are observed to last one day at the
center part of that section of PIL; down flows are observed in two
ends. Transverse velocity fields are derived from flow tracking. Both
Non-Linear Force-Free Field (NLFFF) and Non-Force-Free Field (NFFF)
extrapolations are carried out and compared to trace 3-D magnetic
fields in corona. Combining with EOVSA, coronal magnetic fields
between 1000 and 2000 gauss are found above the flaring PIL at the
height range between 8 and 4Mm, outlining the structure of a fluxrope
with sheared arcade. The above magnetic and velocity fields, as well
as thermal structure of corona, provide initial condition for further
data-driven MHD simulation.

---------------------------------------------------------
Title: Inferring Vector Magnetic Fields from Stokes Profiles of
GST/NIRIS Using a Convolutional Neural Network
Authors: Liu, Hao; Xu, Yan; Wang, Jiasheng; Jing, Ju; Liu, Chang;
Wang, Jason T. L.; Wang, Haimin
2020ApJ...894...70L Altcode: 2020arXiv200503945L
We propose a new machine-learning approach to Stokes inversion based
on a convolutional neural network (CNN) and the Milne-Eddington (ME)
method. The Stokes measurements used in this study were taken by the
Near InfraRed Imaging Spectropolarimeter (NIRIS) on the 1.6 m Goode
Solar Telescope (GST) at the Big Bear Solar Observatory. By learning
the latent patterns in the training data prepared by the physics-based
ME tool, the proposed CNN method is able to infer vector magnetic
fields from the Stokes profiles of GST/NIRIS. Experimental results
show that our CNN method produces smoother and cleaner magnetic maps
than the widely used ME method. Furthermore, the CNN method is four
to six times faster than the ME method and able to produce vector
magnetic fields in nearly real time, which is essential to space
weather forecasting. Specifically, it takes ∼50 s for the CNN method
to process an image of 720 × 720 pixels comprising Stokes profiles
of GST/NIRIS. Finally, the CNN-inferred results are highly correlated
to the ME-calculated results and closer to the ME's results with the
Pearson product-moment correlation coefficient (PPMCC) being closer
to 1, on average, than those from other machine-learning algorithms,
such as multiple support vector regression and multilayer perceptrons
(MLP). In particular, the CNN method outperforms the current best
machine-learning method (MLP) by 2.6%, on average, in PPMCC according
to our experimental study. Thus, the proposed physics-assisted
deep learning-based CNN tool can be considered as an alternative,
efficient method for Stokes inversion for high-resolution polarimetric
observations obtained by GST/NIRIS.

---------------------------------------------------------
Title: Heating and Eruption of a Solar Circular-ribbon Flare
Authors: Lee, Jeongwoo; Karpen, Judith T.; Liu, Chang; Wang, Haimin
2020ApJ...893..158L Altcode: 2020arXiv200805020L
We studied a circular-ribbon flare, SOL2014-12-17T04:51, with emphasis
on its thermal evolution as determined by the differential emission
measure (DEM) inversion analysis of the extreme ultraviolet (EUV)
images of the Atmospheric Imaging Assembly instrument on board the
Solar Dynamics Observatory. Both temperature and emission measure
start to rise much earlier than the flare, along with an eruption and
formation of a hot halo over the fan structure. In the main flare phase,
another set of ribbons forms inside the circular ribbon, and expands
as expected for ribbons at the footpoints of a postflare arcade. An
additional heating event further extends the decay phase, which is also
characteristic of some eruptive flares. The basic magnetic configuration
appears to be a fan-spine topology, rooted in a minority-polarity patch
surrounded by majority-polarity flux. We suggest that reconnection at
the null point begins well before the impulsive phase, when the null
is distorted into a breakout current sheet, and that both flare and
breakout reconnection are necessary in order to explain the subsequent
local thermal evolution and the eruptive activities in this confined
magnetic structure. Using local DEMs, we found a postflare temperature
increase inside the fan surface, indicating that the so-called EUV
late phase is due to continued heating in the flare loops.

---------------------------------------------------------
Title: Predicting Coronal Mass Ejections Using SDO/HMI Vector Magnetic
Data Products and Recurrent Neural Networks
Authors: Liu, Hao; Liu, Chang; Wang, Jason T. L.; Wang, Haimin
2020ApJ...890...12L Altcode: 2020arXiv200210953L
We present two recurrent neural networks (RNNs), one based on gated
recurrent units and the other based on long short-term memory,
for predicting whether an active region (AR) that produces an M-
or X-class flare will also produce a coronal mass ejection (CME). We
model data samples in an AR as time series and use the RNNs to capture
temporal information on the data samples. Each data sample has 18
physical parameters, or features, derived from photospheric vector
magnetic field data taken by the Helioseismic and Magnetic Imager on
board the Solar Dynamics Observatory. We survey M- and X-class flares
that occurred from 2010 to 2019 May using the Geostationary Operational
Environmental Satellite's X-ray flare catalogs provided by the National
Centers for Environmental Information (NCEI), and select those flares
with identified ARs in the NCEI catalogs. In addition, we extract the
associations of flares and CMEs from the Space Weather Database of
Notifications, Knowledge, Information. We use the information gathered
above to build the labels (positive versus negative) of the data samples
at hand. Experimental results demonstrate the superiority of our RNNs
over closely related machine learning methods in predicting the labels
of the data samples. We also discuss an extension of our approach to
predict a probabilistic estimate of how likely an M- or X-class flare
is to initiate a CME, with good performance results. To our knowledge
this is the first time that RNNs have been used for CME prediction.

---------------------------------------------------------
Title: The Eruption of Outer Spine-like Loops Leading to a
Double-stage Circular-ribbon Flare
Authors: Liu, Chang; Lee, Jeongwoo; Wang, Haimin
2019ApJ...883...47L Altcode:
Circular-ribbon flares occur in a confined magnetic structure, but can
also be associated with coronal mass ejections (CMEs) when a filament
embedded under the fan erupts. Here we study an M8.7 circular-ribbon
flare (SOL2014-12-17T04:51), which is accompanied by a CME yet without
a clear indication of filament eruption. Using a nonlinear force-free
field model, we find that the outer spine-like loops form a magnetic
flux rope (FR1) rooted at the edge of the fan, and that there is another
flux rope (FR2) at the main magnetic polarity inversion line (PIL)
under a fan-like flux rope FR3. We divide the event evolution into
two stages by combining modeling results with EUV observations. The
onset stage is featured with bidirectional jets that occurred between
a filament and FR1, immediately followed by an upward motion of the
latter. During this first stage, the inner/outer spine-related ribbons
and the circular ribbon begin to brighten up. After about 10 minutes,
another ejection stems from the main PIL region. In this second stage,
all ribbons are significantly enhanced, and the twist of FR2 footpoints
is decreased. We discuss these results in favor of a scenario where
the initial reconnection between the filament and FR1 activates the
latter to reconnect with FR3 with opposite twist. This produces larger
scale erupting loops and consequently causes a weakening of FR3, which
induces another eruption of FR2 from below. This event thus represents
a new type of eruptive circular-ribbon flare caused by unstable outer
spine-like loops.

---------------------------------------------------------
Title: Intelligent Recognition of Time Stamp Characters in Solar
Scanned Images from Film
Authors: Zhang, Jiafeng; Lin, Guangzhong; Zeng, Shuguang; Zheng,
Sheng; Yang, Xiao; Lin, Ganghua; Zeng, Xiangyun; Wang, Haimin
2019AdAst2019E..28Z Altcode: 2019arXiv190900316Z
Prior to the availability of digital cameras, the solar observational
images are typically recorded on films, and the information such
as date and time were stamped in the same frames on film. It is
significant to extract the time stamp information on the film so
that the researchers can efficiently use the image data. This paper
introduces an intelligent method for extracting time stamp information,
namely, the Convolutional Neural Network (CNN), which is an algorithm in
deep learning of multilayer neural network structures and can identify
time stamp character in the scanned solar images. We carry out the
time stamp decoding for the digitized data from the National Solar
Observatory from 1963 to 2003. The experimental results show that the
method is accurate and quick for this application. We finish the time
stamp information extraction for more than 7 million images with the
accuracy of 98\%.

---------------------------------------------------------
Title: High-resolution Observations of Dynamics of Superpenumbral
Hα Fibrils
Authors: Jing, Ju; Li, Qin; Liu, Chang; Lee, Jeongwoo; Xu, Yan; Cao,
Wenda; Wang, Haimin
2019ApJ...880..143J Altcode:
We present high-resolution Hα observations of a small solar pore in
NOAA active region 12661, using the 1.6 m Goode Solar Telescope equipped
with high-order adaptive optics at Big Bear Solar Observatory. The
observations reveal copious fine-scale chromospheric superpenumbral
fibrils (with an average cross-sectional width of ∼0.″17 ±
0.″03), along with associated transit and intermittent flows with
apparent speeds of 5-14 km s-1. Wavelet analysis and
the spatio-temporal pattern of superpenumbral fibrils suggest that
the observed flows along fibrils are not likely an oscillation/wave
phenomenon. Based on our pseudo-Dopplergrams, we suggest that the
observed flows may be a phenomenon similar to inverse Evershed flows in
the chromosphere. The three-dimensional potential field model indicates
that the pore and the surrounding fibrils are enclosed by fan-field
lines forming a separatrix dome configuration. Such a magnetically
confined configuration may help to maintain the steadfastness of the
superpenumbral fibril dynamics.

---------------------------------------------------------
Title: GST high-resolution observations of small-scale flux emergence
in multiwavelengths
Authors: Wang, Jiasheng; Liu, Chang; Wang, Haimin
2019AAS...23421701W Altcode:
The near-infrared imaging spectropolarimeter (NIRIS) of the 1.6 m Goode
Solar Telescope (GST) at Big Bear Solar Observatory (BBSO) produces
vector magnetograms at 0.24" resolution and up to 30 s cadence. These
unprecedented high spatiotemporal resolution data provide us a unique
opportunity to study the small-scale magnetic flux emergence and
cancellation events, which can advance our understanding of the
structural evolution of photospheric fields and related dynamic
activities spanning the solar atmosphere. In our previous study
using NIRIS observations, we detected that during flux emergence
there appears two components of magnetic fluxes, the central diffused
magnetic structure with enhancing horizontal field and concentrated
opposite-polarity fluxes at the two ends. In this work, we jointly
analyze NIRIS and other BBSO/GST observations of NOAA AR 12665 on 2017
July 13, with the goal of shedding light on the signatures and responses
of small-scale flux emergence in multiple heights. We focus on case
studies of flux emergence events near the magnetic polarity inversion
line. With direct imaging in broadband TiO (a proxy for continuum
in photosphere at 705.7 nm) at 0.1" resolution and 15 s cadence, we
observe that magnetic flux emergence is associated with darkening of
granular boundaries and elongation of granules. When newly emerged
magnetic elements cancel with the existing opposite-polarity fields,
bright point features are seen and they travel along the intergranular
dark lanes at a speed of about 4 km/s. The canceled flux is in the
order of 4x1016 Mx. In Hα observations (0.1 resolution
and 34 s cadence) at several line positions (±1.0, ±0.6, ±0.4, and
line center), we see transient brightenings in the red and blue wings,
which are most probably Ellerman bombs. In a particularly well observed
event, we find brightenings in Hα red-wing(0.4 Å) around the central
diffused emerging region, where horizontal flux density increases and
peaks at 250 Mx/cm2; when it restores to its initial state,
the horizontal flux density increases at the concentrated flux regions
at the two ends. In the mean time, brightenings are also observed at
concentrated flux footpoints in Hα far red-wing (0.6 Å, 1.0 Å). We
suggest that these observations may reflect the reconnection process
between the emerging flux and overlying field.

---------------------------------------------------------
Title: Joint GST and EOVSA Observation of an M1.4 Flare on 2017
September 6
Authors: Wei, Yuqian; Chen, Bin; Yu, Sijie; Wang, Haimin
2019AAS...23421004W Altcode:
We study an M1.4 class flare occurred shortly after the X9.3-class
flare on 2017 September 6 in NOAA active region (AR) 12673. The flare
was well observed by the Expanded Owens Valley Solar Array (EOVSA),
the Goode Solar Telescope (GST) at the BBSO, Hinode, and RHESSI. We
find a partial eruption of a flux rope in GST H-α images during the
flare, with coronal counterparts seen in SDO/AIA images. Microwave
spectral imaging observations from EOVSA provide unique measurements of
the coronal magnetic field and energy distribution of the nonthermal
electrons accelerated to mildly relativistic energies. We compare the
location and morphology of the microwave source with H-α, EUV, and
X-ray data, and discuss implications for energy release and electron
acceleration in this flare.

---------------------------------------------------------
Title: The Eruption of Outer Spine-like Loops Leading to a
Double-Stage Circular-Ribbon Flare
Authors: Liu, Chang; Lee, Jeongwoo; Wang, Haimin
2019AAS...23420405L Altcode:
Studying circular-ribbon flares (CRFs) and the related magnetic
structure/topology can advance our knowledge of solar eruptions in
general. Although they are usually confined events, CRFs can also be
associated with coronal mass ejections (CMEs) when a filament embedded
under the fan dome erupts. Here we study the M8.7 CRF occurred in
NOAA AR 12242 (SOL2014-12-17T04:51), which is accompanied by a CME
but the active region filaments remain intact. Using coronal magnetic
field reconstructed under the nonlinear force-free field assumption, we
find that the outer spine-like loops form a twisted magnetic flux rope
(MFR1) rooted at the edge of the fan-dome field with opposite twist, and
that there is a second flux rope (MFR2) lying above the main polarity
inversion line (PIL). We divide the event evolution into two main stages
based on EUV observations. (1) The event onset is featured with small
bidirectional jetting activities between the sheared filament field
and MFR1, immediately followed by an upward motion of MFR1. During this
first stage, the inner/outer and the circular ribbons begin to brighten
up. (2) After about 10 minutes, another ejection stems from the main
PIL region. This is most probably related to an eruption of MFR2, as
the twist of MFR2 footpoints exhibits a clear decrease subsequently. In
this second stage, all ribbons are significantly enhanced. We discuss
these results in favor of a scenario where the initial reconnection
between the sheared filament field and MFR1 triggers the latter to
erupt, then the erupting MFR1 interacts with the overlying fan-like
field to allow a second eruption of MFR2. This event thus represents
a new type of eruptive CRFs caused by an unstable outer spine-like
loops without involving a filament.

---------------------------------------------------------
Title: Chromospheric Evaporation Observed in an Isolated Magnetic
Flux Tube
Authors: Xu, Yan; Huang, Nengyi; Jing, Ju; Wang, Haimin
2019AAS...23431101X Altcode:
We study chromospheric evaporation of an isolated magnetic flux tube,
with IRIS, SDO and BBSO/GST observations. During the initial phase of
an M6.5 flare on 2015-06-22, two isolated emission patches are visible
prior merging with the main flare ribbons. These two point sources are
characterized with central reversals and significantly broadened Mg II
k/h and C II Å lines, (with a FWHM of 1.6 Å and 0.8 Å, respectively),
respectively, likely due to turbulences. The Doppler shifts of optically
thin lines, Si IV 1402.77 Å and Mg II 2791.59 Å, suggest upflows about
26 km/s and 2 km/s, respectively. A stand-alone EUV loop, connecting the
two stationary flare footpoints, is identified by SDO/AIA images and
confirmed by the coronal magnetic field extrapolated from the SDO/HMI
magnetograms. The temporal variation of the intensity and differential
emission measure of the two brightenings show obvious delay, about 2-3
minutes, indicating localized reconnection and heating. In contrast,
on the main flare ribbon, the Si IV line shows obvious downflows with
a speed of 38 km/s. The different behavior of the flare ribbon and
isolated footpoints is possibly due to the different heating magnitudes
and altitudes.

---------------------------------------------------------
Title: High-resolution Observations of Dynamics of Superpenumbral
Hα Fibrils
Authors: Jing, Ju; Li, Qin; Liu, Chang; Lee, Jeongwoo; Xu, Yan; Cao,
Wenda; Wang, Haimin
2019AAS...23430502J Altcode:
We present unprecedented high-resolution Hα observations of a small
solar pore in NOAA Active Region 12661, using the 1.6 m Goode Solar
Telescope (GST) equipped with high-order adaptive optics at Big Bear
Solar Observatory (BBSO). The observations reveal copious fine-scale
chromospheric superpenumbral fibrils (with a cross-sectional width of
0.15") around the pore, along with associated transit and intermittent
flows with apparent speeds of 5-20 km s-1. The wavelet
analysis suggests that the observed flows along fibrils are not likely
a manifestation of oscillations, but rather collections of actual
counterstreaming mass motions. The observed flow is interpreted as
siphon flow. In addition, the three-dimensional potential field model
indicates that the pore and the surrounding fibrils are enclosed by
fan field lines forming a separatrix dome configuration. We suggest
that such a magnetically confined configuration may help to maintain
the steadfastness of the superpenumbral fibril dynamics.

---------------------------------------------------------
Title: Predicting Solar Flares Using a Long Short-term Memory Network
Authors: Liu, Hao; Liu, Chang; Wang, Jason T. L.; Wang, Haimin
2019ApJ...877..121L Altcode: 2019arXiv190507095L
We present a long short-term memory (LSTM) network for predicting
whether an active region (AR) would produce a ϒ-class flare within
the next 24 hr. We consider three ϒ classes, namely ≥M5.0 class,
≥M class, and ≥C class, and build three LSTM models separately,
each corresponding to a ϒ class. Each LSTM model is used to make
predictions of its corresponding ϒ-class flares. The essence of our
approach is to model data samples in an AR as time series and use
LSTMs to capture temporal information of the data samples. Each data
sample has 40 features including 25 magnetic parameters obtained from
the Space-weather HMI Active Region Patches and related data products
as well as 15 flare history parameters. We survey the flare events
that occurred from 2010 May to 2018 May, using the Geostationary
Operational Environmental Satellite X-ray flare catalogs provided by
the National Centers for Environmental Information (NCEI), and select
flares with identified ARs in the NCEI flare catalogs. These flare
events are used to build the labels (positive versus negative) of the
data samples. Experimental results show that (i) using only 14-22 most
important features including both flare history and magnetic parameters
can achieve better performance than using all 40 features together;
(ii) our LSTM network outperforms related machine-learning methods in
predicting the labels of the data samples. To our knowledge, this is
the first time that LSTMs have been used for solar-flare prediction.

---------------------------------------------------------
Title: Multi-wavelength Multi-height Study of Super Strong Surface
and Coronal Magnetic Fields in Active Region 12673
Authors: Wang, Haimin; Chen, Bin; Jing, Ju; Yu, Sijie; Liu, Chang;
Yurchyshyn, Vasyl B.; Ahn, Kwangsu; Okamoto, Takenori; Toriumi, Shin;
Cao, Wenda; Gary, Dale E.
2019AAS...23440205W Altcode:
Using the joint observations of Goode Solar telescope (GST), Expanded
Owens Valley Solar Array (EOVSA), Solar Dynamics Observatory (SDO)
and Hinode, we study the Solar Active Region (AR) 12673 in September
2017, which is the most flare productive AR in the solar cycle 24. GST
observations show the strong photospheric magnetic fields (nearly
6000 G) in polarity inversion line (PIL) and apparent photospheric
twist. Consistent upward flows are also observed in Dopplergrams
of Hinode, HMI and GST at the center part of that section of PIL,
while the down flows are observed in two ends, indicating that the
structure was rising from subsurface. Combining Non-Linear Force Free
Extrapolation and EOVSA microwave imaging spectroscopy, we also look
into the coronal structure of magnetic fields in this unusual AR,
including the evolution before and after the X9.3 flare on September
6, 2017. Coronal fields between 1000 and 2000 gauss are found above
the flaring PIL at the height range between 8 and 4Mm, outlining the
structure of a fluxrope or sheared arcade.

---------------------------------------------------------
Title: High Resolution Post-flare Loop Observations by GST and IRIS
Authors: Huang, Nengyi; Xu, Yan; Jing, Ju; Wang, Haimin
2019AAS...23420603H Altcode:
We present the study of post-flare loops, during the gradual phase
of the 2015-06-22 M6.5 two-ribbon flare, which was observed by Goode
Solar Telescope (GST) and Interface Region Imaging Spectrograph
(IRIS). Imaging spectroscopic data in UV and Hα lines are analyzed,
with supplementary data of magnetograms and EUV images. We compare the
Doppler signals derived from Mg II 2791.59 Å, Si IV 1402.77 Å with
those derived from Hα line to study the mass motions in the transition
region and chromosphere. In addition, Hα images are used for tracing
the structure and mass motion in loops. Furthermore, we investigate the
oscillations of flare loops in Fe XXI 1354.08 Å and Hα lines. Finally,
we compare our diagnostics with the results of flare loop modeling.

---------------------------------------------------------
Title: Multi-instrument Comparative Study of Temperature, Number
Density and Emission Measure during the Precursor Phase of a Solar
Flare
Authors: Liu, Nian; Jing, Ju; Wang, Haimin; Xu, Yan; Cheung, Mark;
Fleishman, G. D.
2019AAS...23420407L Altcode:
The precursor brightenings of solar flares hold valuable clues
concerning the flare triggering and energy release mechanisms, but
have not been well studied. This paper presents a multi-instrument
study of the two precursor brightenings prior to the M6.5 flare
(SOL2015-06-22T18:23) in NOAA active region 12371, with a focus on
the temperature (T), number density (n) and emission measure (EM)
of these two precursors. The multi-instrument data used in this study
were obtained from four instruments with variety of wavelengths, i.e.,
the Solar Dynamics Observatory's Atmospheric Imaging Assembly (AIA)
in six EUV passbands, the Expanded Owens Valley Solar Array (EOVSA)
in microwave, RHESSI in hard X-ray and GOES in soft X-ray. We compare
the temporal variation of T, n and EM derived from different data sets
during the precursor period and discuss the differences in terms of
the sensitivity of the instruments.

---------------------------------------------------------
Title: Thermal and Nonthermal Evolutions of a Circular Ribbon Flare
Authors: Lee, Jeongwoo; Liu, Chang; White, Stephen M.; Wang, Haimin
2019AAS...23431003L Altcode:
We studied the activation and the extended activity of a circular-ribbon
flare, SOL2014-12-17T04:51 by performing the Differential Emission
Measure (DEM) inversion analysis of the extreme ultraviolet (EUV)
images of the Atmospheric Imaging Assembly (AIA) instrument onboard
the Solar Dynamics Observatory (SDO) and analysis of the microwave data
from the Nobeyama Radioheliograph (NoRH). We will present the following
results. 1. The circular ribbon as an indicator of the dome-shaped
separatrix is better visible in the preflare phase in the form of very
narrow channel of enhance temperature. It is activated much earlier
(10-15 min before the impulsive phase at 04:33 UT), and its temporal
correlation with the outer spine temperature suggests the initial
activation of the circular ribbon by magnetic reconnection in the
outer spine. 2. The flare ribbons are active only within a limited
section of the entire circular ribbon, suggesting the hyperbolic
flux tube reconnection rather than null point reconnection. They are
activated 4 min before the impulsive phase together with the inner
spine, and involve nonthermal electrons as evidenced by the NoRH 34
GHz emission. 3. The temperature-high region and the DEM-high region
coincide each other in the impulsive phase, and then separate from each
other, implying that the rising magnetic fields carry hot plasma away
while the high-density cooler plasma stays on the fan surface. 4. An
unusually extended decay phase in this event can be understood as the
maximum DEM in the impulsive phase occurs at a higher temperature and
then gradually shifts to lower temperatures where EUV emission is more
efficient. Namely, the extended EUV activity is due to slow cooling
of the hot plasma within the confined magnetic structure.

---------------------------------------------------------
Title: Spectral Diagnosis of Mg II and Hα Lines during the Initial
Stage of an M6.5 Solar Flare
Authors: Huang, Nengyi; Xu, Yan; Sadykov, Viacheslav M.; Jing, Ju;
Wang, Haimin
2019ApJ...878L..15H Altcode:
Recent studies have shown special spectral properties during the initial
stage of flare emissions, such as the enhanced absorption in He I
10830 Å line and a strong redshift in Hα. Using the high-resolution
imaging spectroscopic data obtained by the Interface Region Imaging
Spectrograph (IRIS), we investigate the Mg II emission lines during
an M6.5 flare (SOL2015-06-22T18:23), which was well covered by the
joint observation of IRIS and the Goode Solar Telescope at Big Bear
Solar Observatory. On the leading edge of the propagating ribbon,
Mg II lines are characterized by blue-wing enhancement and strong
broadening. On the other hand, redshifts in Mg II and Hα are found in
the trailing areas of the flare ribbons. Numerical modeling, produced
by combining RADYN and RH, suggests that the Mg II line broadening is
possibly caused by unresolved turbulence with velocities about 10 to 30
km s-1. The enhanced blue wing is likely due to a decrease
of temperature and an increase of electron density, as consequences
of electron precipitation. Based on the observations and simulation
results, we discuss the possible response of the lower atmosphere to
the electron precipitation, in terms of the evolution of temperature,
electron density, and turbulence velocities.

---------------------------------------------------------
Title: Innovative Deep Learning Methods for Analysis and Prediction
of Solar Activity
Authors: Wang, Jason; Liu, Hao; Xu, Yan; Liu, Chang; Jing, Ju;
Wang, Haimin
2019shin.confE..31W Altcode:
We present new deep learning methods for tackling two important problems
in heliophysics: solar flare prediction and Stokes inversion. First,
we present a long short-term memory (LSTM) network for predicting
whether an active region (AR) would produce a certain class flare
within the next 24 hours. The essence of our approach is to model data
samples in an AR as time series and use LSTMs to capture temporal
information of the data samples. Each data sample has 40 features
(i.e., 40 dimensions) including 25 magnetic field parameters and 15
flare history parameters. We survey the flare events that occurred from
2010 May to 2018 May, using the GOES X-ray flare catalogs provided by
the National Centers for Environmental Information (NCEI), and select
flares with identified ARs in the NCEI flare catalogs. These flare
events are used to build the labels (positive vs. negative) of the
data samples. Experimental results show that (i) using only 14-22 most
important features can achieve better performance than using all the
40 features together; (ii) our LSTM network outperforms related machine
learning methods in predicting the labels of the data samples. Second,
we present a convolutional neural network (CNN) for inverting Stokes
profiles to obtain vector magnetic fields. The magnetograms used in
this study were taken by the Near Infrared Imaging Spectropolarimeter
(NIRIS) on the 1.6 m Goode Solar Telescope (GST) at the Big Bear Solar
Observatory (BBSO). By learning the latent patterns between the Stokes
profiles and the vector magnetic fields, the CNN can infer the vector
magnetic field of each pixel from its Stokes profiles much faster,
and images appear to be much smoother, than traditional physics-based
inversion techniques, such as the Milne-Eddington method. Our CNN method
is well suited for Stokes inversion for high-resolution polarimetric
observations, obtained by GST/NIRIS and the upcoming Daniel K. Inouye
Solar Telescope (DKIST).

---------------------------------------------------------
Title: Flare-productive active regions
Authors: Toriumi, Shin; Wang, Haimin
2019LRSP...16....3T Altcode: 2019arXiv190412027T
Strong solar flares and coronal mass ejections, here defined not only
as the bursts of electromagnetic radiation but as the entire process
in which magnetic energy is released through magnetic reconnection and
plasma instability, emanate from active regions (ARs) in which high
magnetic non-potentiality resides in a wide variety of forms. This
review focuses on the formation and evolution of flare-productive
ARs from both observational and theoretical points of view. Starting
from a general introduction of the genesis of ARs and solar flares,
we give an overview of the key observational features during the
long-term evolution in the pre-flare state, the rapid changes in the
magnetic field associated with the flare occurrence, and the physical
mechanisms behind these phenomena. Our picture of flare-productive
ARs is summarized as follows: subject to the turbulent convection, the
rising magnetic flux in the interior deforms into a complex structure
and gains high non-potentiality; as the flux appears on the surface,
an AR with large free magnetic energy and helicity is built, which is
represented by δ -sunspots, sheared polarity inversion lines, magnetic
flux ropes, etc; the flare occurs when sufficient magnetic energy has
accumulated, and the drastic coronal evolution affects magnetic fields
even in the photosphere. We show that the improvement of observational
instruments and modeling capabilities has significantly advanced our
understanding in the last decades. Finally, we discuss the outstanding
issues and future perspective and further broaden our scope to the
possible applications of our knowledge to space-weather forecasting,
extreme events in history, and corresponding stellar activities.

---------------------------------------------------------
Title: High-resolution Observation of Moving Magnetic Features
Authors: Li, Qin; Deng, Na; Jing, Ju; Liu, Chang; Wang, Haimin
2019ApJ...876..129L Altcode:
Moving magnetic features (MMFs) are small magnetic elements moving
almost radially away from sunspots toward the boundary of moat
regions. They are mostly seen during the decay phase of a sunspot. Here
we present a high-resolution observation of MMFs around a sunspot
in NOAA AR 12565 on 2016 July 14 using the the 1.6 m Goode Solar
Telescope (GST) at Big Bear Solar Observatory. The spectropolarimetric
measurements of photospheric magnetic field are obtained from GST’s
newly commissioned Near Infra-Red Imaging Spectropolarimeter at the
Fe I 1.56 μm line. The statistical study of physical properties
of identified fine-scale MMFs (i.e., size, lifetime, inclination,
horizontal velocity, and flux) is presented. The origin of the
minority polarity flux in the sunspot is determined. Same as the
majority polarity flux, the minority polarity flux can originate
from mid-penumbra with highly inclined fields. The role of MMFs in
both polarities in the flux evolution of the sunspot is speculated,
as they can both contribute to the sunspot evolution.

---------------------------------------------------------
Title: Signatures of Magnetic Flux Ropes in the Low Solar Atmosphere
Observed in High Resolution
Authors: Wang, Haimin; Liu, Chang
2019FrASS...6...18W Altcode:
Magnetic flux ropes (MFRs) are important physical features closely
related to solar eruptive activities with potential space weather
consequences. Studying MFRs in the low solar atmosphere can shed light
on their origin and subsequent magnetic structural evolution. In recent
years, observations of solar photosphere and chromosphere reached a
spatial resolution of 0.1 to 0.2 arcsec with the operation of meter
class ground-based telescopes, such as the 1.6m Goode Solar Telescope
at Big Bear Solar Observatory and the 1m New Vacuum Solar Telescope
at Yunnan Observatory. The obtained chromospheric Halpha filtergrams
with the highest resolution thus far have revealed detailed properties
of MFRs before and during eruptions, and the observed pre-eruption
structures of MFRs are well consistent with those demonstrated by
nonlinear force-free field extrapolations. There are also evidences
that MFRs may exist in the photosphere. The magnetic channel
structure, with multiple polarity inversions and only discernible
in high-resolution magnetograph observations, may be a signature of
photospheric MFRs. These MFRs are likely formed below the surface due
to motions in the convection zone and appear in the photosphere through
flux emergence. Triggering of some solar eruptions is associated with
an enhancing twist in the low-atmospheric MFRs.

---------------------------------------------------------
Title: Flame-like Ellerman Bombs and Their Connection to Solar
Ultraviolet Bursts
Authors: Chen, Yajie; Tian, Hui; Peter, Hardi; Samanta, Tanmoy;
Yurchyshyn, Vasyl; Wang, Haimin; Cao, Wenda; Wang, Linghua; He, Jiansen
2019ApJ...875L..30C Altcode: 2019arXiv190301981C
Ellerman bombs (EBs) are small-scale intense brightenings in Hα
wing images, which are generally believed to be signatures of
magnetic reconnection around the temperature minimum region of the
solar atmosphere. They have a flame-like morphology when observed
near the solar limb. Recent observations from the Interface Region
Imaging Spectrograph (IRIS) reveal another type of small-scale
reconnection event called an ultraviolet (UV) burst, in the lower solar
atmosphere. Though previous observations have shown a clear coincidence
between the occurrence of some UV bursts and EBs, the exact relationship
between these two phenomena is still debated. We investigate the spatial
and temporal relationship between flame-like EBs and UV bursts using
joint near-limb observations between the 1.6 m Goode Solar Telescope
(GST) and IRIS. In total, 161 EBs have been identified from the GST
observations, and ∼20 of them reveal signatures of UV bursts in
the IRIS images. Interestingly, we find that these UV bursts have a
tendency to appear at the upper parts of their associated flame-like
EBs. The intensity variations of most EB-related UV bursts and their
corresponding EBs match well. Our results suggest that some of these
UV bursts and EBs likely form at different heights during a common
reconnection process.

---------------------------------------------------------
Title: Evolution of Photospheric Vector Magnetic Field Associated
with Moving Flare Ribbons as Seen by GST
Authors: Liu, Chang; Cao, Wenda; Chae, Jongchul; Ahn, Kwangsu; Prasad
Choudhary, Debi; Lee, Jeongwoo; Liu, Rui; Deng, Na; Wang, Jiasheng;
Wang, Haimin
2018ApJ...869...21L Altcode: 2018arXiv181011733L
The photospheric response to solar flares, also known as coronal
back reaction, is often observed as sudden flare-induced changes in
the vector magnetic field and sunspot motions. However, it remains
obscure whether evolving flare ribbons, the flare signature closest to
the photosphere, are accompanied by changes in vector magnetic field
therein. Here we explore the relationship between the dynamics of
flare ribbons in the chromosphere and variations of magnetic fields
in the underlying photosphere, using high-resolution off-band Hα
images and near-infrared vector magnetograms of the M6.5 flare on 2015
June 22 observed with the 1.6 m Goode Solar Telescope. We find that
changes of photospheric fields occur at the arrival of the flare ribbon
front, thus propagating analogously to flare ribbons. In general, the
horizontal field increases and the field lines become more inclined
to the surface. When ribbons sweep through regions that undergo a
rotational motion, the fields transiently become more vertical with
decreased horizontal field and inclination angle, and then restore
and/or become more horizontal than before the ribbon arrival. The
ribbon propagation decelerates near the sunspot rotation center,
where the vertical field becomes permanently enhanced. Similar magnetic
field changes are discernible in magnetograms from the Helioseismic and
Magnetic Imager (HMI), and an inward collapse of coronal magnetic fields
is inferred from the time sequence of nonlinear force-free field models
extrapolated from HMI magnetograms. We conclude that photospheric fields
respond nearly instantaneously to magnetic reconnection in the corona.

---------------------------------------------------------
Title: Statistical Analysis of Torus and Kink Instabilities in
Solar Eruptions
Authors: Jing, Ju; Liu, Chang; Lee, Jeongwoo; Ji, Hantao; Liu, Nian;
Xu, Yan; Wang, Haimin
2018ApJ...864..138J Altcode: 2018arXiv180808924J
A recent laboratory experiment of ideal magnetohydrodynamic
instabilities revealed four distinct eruption regimes readily
distinguished by the torus instability (TI) and helical kink instability
(KI) parameters. To establish its observational counterpart, we
collected 38 solar flares (stronger than GOES-class M5 in general)
that took place within 45° of disk center during 2011-2017, 26 of
which are associated with a halo or partial halo coronal mass ejection
(CME; i.e., ejective events), while the others are CME-less (i.e.,
confined events). This is a complete sample of solar events satisfying
our selection criteria detailed in the paper. For each event, we
calculate a decay index n of the potential strapping field above the
magnetic flux rope (MFR) in and around the flaring magnetic polarity
inversion line (a TI parameter) and the unsigned twist number T 
w of the nonlinear force-free field lines forming the same MFR
(a KI parameter). We then construct an n-T w diagram to
investigate how the eruptiveness depends on these parameters. We find
that (1) T w appears to play little role in discriminating
between confined and ejective events; (2) the events with n ≳ 0.8
are all ejective, and all confined events have n ≲ 0.8. However, n
≳ 0.8 is not a necessary condition for eruption because some events
with n ≲ 0.8 also erupted. In addition, we investigate the MFR’s
geometrical parameters, apex height, and distance between footpoints,
as a possible factors for the eruptiveness. We briefly discuss the
difference of the present result for solar eruptions with that of the
laboratory result in terms of the role played by magnetic reconnection.

---------------------------------------------------------
Title: Dark Structures in Sunspot Light Bridges
Authors: Zhang, Jingwen; Tian, Hui; Solanki, Sami K.; Wang, Haimin;
Peter, Hardi; Ahn, Kwangsu; Xu, Yan; Zhu, Yingjie; Cao, Wenda; He,
Jiansen; Wang, Linghua
2018ApJ...865...29Z Altcode: 2018arXiv180900146Z
We present unprecedented high-resolution TiO images and Fe I 1565
nm spectropolarimetric data of two light bridges taken by the 1.6
m Goode Solar Telescope at Big Bear Solar Observatory. In the first
light bridge (LB1), we find striking knot-like dark structures within
the central dark lane. Many dark knots show migration away from the
penumbra along the light bridge. The sizes, intensity depressions,
and apparent speeds of their proper motion along the light bridges of
33 dark knots identified from the TiO images are mainly in the ranges
of 80 ∼ 200 km, 30% ∼ 50%, and 0.3 ∼ 1.2 km s-1,
respectively. In the second light bridge (LB2), a faint central dark
lane and striking transverse intergranular lanes were observed. These
intergranular lanes have sizes and intensity depressions comparable
to those of the dark knots in LB1 and also migrate away from the
penumbra at similar speeds. Our observations reveal that LB2 is made
up of a chain of evolving convection cells, as indicated by patches of
blueshift surrounded by narrow lanes of redshift. The central dark lane
generally corresponds to blueshifts, supporting the previous suggestion
of central dark lanes being the top parts of convection upflows. In
contrast, the intergranular lanes are associated with redshifts and
located at two sides of each convection cell. The magnetic fields are
stronger in intergranular lanes than in the central dark lane. These
results suggest that these intergranular lanes are manifestations of
convergent convective downflows in the light bridge. We also provide
evidence that the dark knots observed in LB1 may have a similar origin.

---------------------------------------------------------
Title: Collective Study of Polar Crown Filaments in the Past Four
Solar Cycles
Authors: Xu, Yan; Pötzi, Werner; Zhang, Hewei; Huang, Nengyi; Jing,
Ju; Wang, Haimin
2018ApJ...862L..23X Altcode: 2018arXiv180711844X
Polar crown filaments (PCFs) form above the magnetic polarity
inversion line, which separates the unipolar polar fields and the
nearest dispersed fields from the trailing part of active regions with
opposite polarity. The statistical properties of PCFs are correlated
with the solar cycle. Therefore, the study of PCFs plays an important
role in understanding the variations of the solar cycle, especially the
prolonged cycle 23 and the current “abnormal” solar cycle 24. In
this Letter, we investigate PCFs using full-disk Hα data from 1973
to early 2018, recorded by Kanzelhöhe Solar Observatory and Big Bear
Solar Observatory, in digital form from 1997 to 2018 and in 35 mm film
(digitized) from 1973 to 1996. PCFs are identified manually because
their segmented shape and close-to-limb location were not handled
well by automatical detections in several previous studies. Our
results show that the PCFs start to move poleward at the beginning
of each solar cycle. When the PCFs approach to the maximum latitude,
the polar field strength reduces to zero followed by a reversal. The
migration rates are about 0.°4 to 0.°7 per Carrington rotation,
with a clear N-S asymmetric pattern. In cycles 21 and 23, the PCFs
in the northern hemisphere migrate faster than those in the southern
hemisphere. However, in the “abnormal” cycle 24, the southern
PCFs migrate faster, which is consistent with other observations of
magnetic fields and radio emission. In addition, there are more days
in cycle 23 and 24 without PCFs than in the previous cycles.

---------------------------------------------------------
Title: Spatial Organization of Seven Extreme Solar Energetic
Particle Events
Authors: Kocharov, Leon; Pohjolainen, Silja; Reiner, Mike J.; Mishev,
Alexander; Wang, Haimin; Usoskin, Ilya; Vainio, Rami
2018ApJ...862L..20K Altcode:
Emission of relativistic protons and helium responsible for extreme
solar particle events (ground level enhancements (GLEs)) is often
structured. We investigate its organization depending on the eruption
stage characterized by the heliocentric height of associated coronal
mass ejections (CMEs). Seven GLEs are considered: events on 1997
November 6, 1998 May 2, 2000 July 14, 2001 December 26, 2003 November
2, 2006 December 13, and 2012 May 17, which are half of the SOlar
and Heliospheric Observatory (SOHO )-era GLEs, excluding very weak
events. Count-rate profiles of the GLEs plotted as a function of the CME
height reveal two types (or two components) of the high-energy particle
emission. The first component rises in a step-like manner during
the CME transit from 2 R ⊙ to 3 R ⊙, when
the CME exits from predominantly closed coronal magnetic structures,
irrespective of the CME speed (type H). This component is of coronal
origin. The second component of the GLE-producing particles starts to
rise when CME is at about 4 R ⊙, achieves its maximum at
6-10 R ⊙, and declines shortly after that (type J). The
type J particle injection into the interplanetary space coincides with
the decametric-hectometric radio burst complex that includes enhanced
emission of type II and concurrent low-frequency type III bursts,
indicative of the CME interaction with a streamer-like structure at
a few solar radii from the Sun. Those could be delayed particles
from the flare region. A possible additional contribution of the
CME-bow-shock acceleration in unstructured solar wind is not large in
the two considered types of events.

---------------------------------------------------------
Title: Predicting the Flare Index of Active Regions Using the Random
Forest Algorithm
Authors: Branca, William Louis; Liu, Chang; Jing, Ju; Li, Qin;
Wang, Haimin
2018shin.confE.149B Altcode:
Solar flares can have severe impacts on the near-Earth environment,
and the resulting geomagnetic disturbances may cause major damage
to the critical infrastructure. The goal of this research is to
use the Random Forest (RF) algorithm to predict the impending flare
production of solar active regions (ARs) over the subsequent 24 hours,
i.e., 24-hr flare index. Here we predict the 24-hr flare index of 843
ARs from 2010 to 2016, using the RF algorithm in the regression-based
prediction mode. The inputs of this mode are 13 physical parameters
(selected by Bobra &amp; Couvidat in 2015) provided by the data
products of the Helioseismic Magnetic Imager (HMI) on the Solar
Dynamic Observatory (SDO). These parameters are believed to be the
most important ones related to the occurrence of solar flares. We then
compared the prediction results with the actual flare index obtained
from the National Oceanic and Atmospheric Administration's Space
Weather Prediction Center, and evaluated the prediction accuracy for
each class of flares.

---------------------------------------------------------
Title: Magnetic reconnection within a multi-flux-rope system during
the precursor phase of a confined flare
Authors: Awasthi, Arun Kumar; Wang, Yuming; Shen, Chenglong; Liu,
Rui; Wang, Haimin
2018cosp...42E.146A Altcode:
The core structure of a solar eruption is believed to be a flux rope
made of helical magnetic field. While the post-eruption "standard"
picture of the flux rope has been largely studied, the flux rope
formation and its evolution toward eruption remain elusive owing to
subtle activity and emission during this phase. Here we investigate the
pre-eruption morphology of a complex ejecta and associated activities
during the precursor phase of a confined C1.1 SOL2015-06-22T16:45
flare exhibiting multiple intermittent brightenings along the polarity
inversion line (PIL). Spectral analysis of X-ray emission recorded
during the precursor phase revealed the presence of non-thermal electron
with a spectral index -6 while the plasma temperature during this phase
reached to ∼20 MK as also confirmed by the DEM maps synthesized using
EUV observations. The coronal magnetic-field topology, derived from the
NLFFF extrapolations revealed a multi-flux-rope system comprising of at
least five branches spanned across ∼25 Mm altitude. A combined map of
magnetic twist-number and squashing factor revealed the release of the
twist after the flare, confirming the reconnection taking place within
the flux-rope system spatially linked with the excursions observed
during the precursor phase. Further, EUV images revealed bi-directional
outflows of the plasma blobs from the reconnection site. In summary,
our investigation of a rare flare precursor with definitive signatures
of magnetic reconnection within a system of multiple braided flux-rope
branches with different degree of coherency signifies the importance
of internal structure and reconnection in understanding CMEs.

---------------------------------------------------------
Title: Formation of Large-scale Coronal Loops Interconnecting Two
Active Regions through Gradual Magnetic Reconnection and an Associated
Heating Process
Authors: Du, Guohui; Chen, Yao; Zhu, Chunming; Liu, Chang; Ge, Lili;
Wang, Bing; Li, Chuanyang; Wang, Haimin
2018ApJ...860...40D Altcode: 2018arXiv180504831D
Coronal loops interconnecting two active regions (ARs), called
interconnecting loops (ILs), are prominent large-scale structures in
the solar atmosphere. They carry a significant amount of magnetic flux
and therefore are considered to be an important element of the solar
dynamo process. Earlier observations showed that eruptions of ILs are an
important source of CMEs. It is generally believed that ILs are formed
through magnetic reconnection in the high corona (&gt;150″-200″),
and several scenarios have been proposed to explain their brightening
in soft X-rays (SXRs). However, the detailed IL formation process has
not been fully explored, and the associated energy release in the corona
still remains unresolved. Here, we report the complete formation process
of a set of ILs connecting two nearby ARs, with successive observations
by STEREO-A on the far side of the Sun and by SDO and Hinode on the
Earth side. We conclude that ILs are formed by gradual reconnection
high in the corona, in line with earlier postulations. In addition,
we show evidence that ILs brighten in SXRs and EUVs through heating
at or close to the reconnection site in the corona (i.e., through
the direct heating process of reconnection), a process that has been
largely overlooked in earlier studies of ILs.

---------------------------------------------------------
Title: Statistical analysis of Torus and Kink Instability Effects
on Solar Eruptions
Authors: Jing, Ju; Ji, Hantao; Liu, Nian; Xu, Yan; Liu, Chang;
Wang, Haimin
2018tess.conf21161J Altcode:
The torus and kink instabilities are related to the strapping and core
field of a magnetic flux rope (FR), respectively, and are thought to
jointly govern the initiation and progression of a FR eruption in the
ideal magnetohydrodynamic model. A recent laboratory experiment of
ideal instability reveals four distinct eruption regimes, ejective,
stable, failed torus and failed kink, which are readily distinguished
by the torus and kink instability parameters (Myers et al. 2015). In an
effort to obtain the observation counterpart of the laboratory results,
we statistically analyze 39 major solar flares (&gt;M5) that took place
within 45º from the disk center during 2011-2017, 26 of
which are associated with a halo or partial halo coronal mass ejection
(CME) (i.e., ejective events), while the others are not associated with
any CME (i.e., confined events). For each event, we calculated (1)
the potential field decay index n (a parameter of torus instability,
Kliem &amp; Török 2006) at the apex of the FR, and (2) the maximum
magnetic twist number Tw (a parameter of kink instability, Berger &amp;
Prior 2006) of the non-linear force-free field (NLFFF) lines in and
around the flaring magnetic polarity inversion line (PIL). While we do
find that the torus-unstable FRs (n&gt;0.5) are predominantly ejective,
the onset criteria of kink instability for distinguishing between
ejective and confined events are inconclusive. The discrepancy between
this observational/analytical and the idealized laboratory results
is probably attributed to the multiple sources of approximations and
errors from both observation and modeling. In addition, based on the
sample of 26 CME-associated events, the CME's kinetic energy shows a
moderate positive relationship with n and with free magnetic energy
Efree contained in an active region.

---------------------------------------------------------
Title: High-resolution Observation of Moving Magnetic Features in
Active Regions
Authors: Li, Qin; Liu, Chang; Jing, Ju; Wang, Haimin
2018tess.conf21705L Altcode:
Moving magnetic features (MMFs) are small photospheric magnetic elements
shredded from the sunspot and move outward toward the boundary of
moat regions mostly during a sunspot decaying phase. In this work,
we present an update of the study of MMFs of a sunspot in NOAA AR
12565 using 6.5 hours high-resolution observations from the 1.6 m
Goode Solar telescope (GST) at Big Bear Solar Observatory (BBSO). The
spectropolarimetric measurements of photospheric magnetic field are
obtained from GST's Near Infra-Red Imaging Spectropolarimeter (NIRIS)
at Fe I 1.56 um line. We focus on the statistical study of physical
properties of identified fine-scale MMFs (i.e., size, lifetime,
inclination, horizontal velocity, and flux) and their relationships to
the partial decaying region of the sunspot. In addition, we determine
the origin of inverse polarity flux in the sunspot and distinguish the
uncombed structure in the penumbral fine structure. We also estimate
the emergence rate of sunspot structure and flux transport rate of
MMFs around it. Finally, we compare our findings to the numerical
sunspot model of Rempel et al. (2012).

---------------------------------------------------------
Title: Evolution of Photospheric Magnetic Field Associated with
Flare Ribbons as Seen by GST
Authors: Liu, Chang; Cao, Wenda; Xu, Yan; Ahn, Kwangsu; Wang, Haimin
2018tess.conf40803L Altcode:
Solar flares produce signatures at various wavelengths and heights,
among which the response of photospheric magnetic and flow fields
to flare reconnection has drawn increasing attention recently. Such
a response has been evidenced as sudden flare-induced photospheric
structural evolution, especially vector magnetic field changes
and sunspot rotations. However, previous studies on this coronal
back-reaction process focused on the magnetic polarity inversion line
region and/or the overall temporal relationship. Here we present
high-resolution H-alpha off-band images and near-infrared vector
magnetograms of a major flare observed with the 1.6 m Goode Solar
Telescope (GST), which allow us to probe the relationship between
motions of flare ribbons and variations of the underlying field. It
is found that changes of photospheric fields occur instantly in a
largely step-wise fashion at the arrival of the flare ribbon front,
thus exhibit a propagation analogous to flare ribbons. In general, the
fields become more inclined to the surface, with enhanced horizontal
field and magnetic shear. When ribbons sweep through regions that
undergo an apparent rotational motion, the fields transiently turn
more vertical with decreased horizontal field and inclination angel,
and then become more horizontal. Interestingly, the ribbon decelerates
as approaching the sunspot rotation center, where the vertical field
becomes persistently enhanced. Overall, NLFFF extrapolation models
based on HMI vector field data suggest an inward collapse of magnetic
fields. These results indicate that photospheric fields may respond
instantaneously and differentially to the coronal field restructuring
due to reconnection of individual flux bundles.

---------------------------------------------------------
Title: Observation and Modeling of Mg II lines during an M6.5 Flare
Authors: Huang, Nengyi; Xu, Yan; Jing, Ju; Sadykov, Viacheslav M.;
Kleint, Lucia; Wang, Haimin
2018tess.conf11403H Altcode:
Recent studies have shown special properties of flare emission at the
precipitating site of electrons, such as the enhanced absorption in He
I lines and strong Doppler shift in H-alpha. Using the high resolution
imaging spectroscopic data obtained by IRIS, we investigate the Mg
II emission lines during an M6.5 flare (SOL2015-06-22T18:23), which
was well covered by the joint observation of IRIS and BBSO/GST. On
the leading edge of the propagating ribbon, Mg II h and k lines are
characterized by strong broadening (~100 km/s) and weak blue shift
(~5km/s), cospatial with red shifted H-alpha spectra. Additionally,
we carry out numerical simulation of the special Mg II lines using RH
code, taking inputs of flare atmosphere calculated by the radiative
hydrodynamic code RADYN. By iterating the parameters such as velocity
fields, plasma temperature and electron density, we investigate the
dominant factors in generating the Mg II broadening and blue shift. Our
preliminary results indicate that nonuniform velocity field and strong
microturbulence can enhance the line broadening, but the cause of blue
shift is not conclusive.

---------------------------------------------------------
Title: Using Random Forest Algorithm to Predict Flare Index of
Active Regions
Authors: Branca, William; Liu, Chang; Jing, Ju; Li, Qin; Wang, Haimin
2018tess.conf22406B Altcode:
Solar flares can have severe impacts on the near-Earth environment,
and the resulting geomagnetic disturbances may cause major damage
to the critical infrastructure. The goal of this research is to
use the Random Forest (RF) algorithm to predict the impending flare
production of solar active regions (ARs) over the subsequent 24 hours,
i.e., 24-hr flare index. Here we predict the 24-hr flare index of 843
ARs from 2010 to 2016, using the RF algorithm in the regression-based
prediction mode. The inputs of this mode are 13 physical parameters
(selected by Bobra &amp; Couvidat in 2015) provided by the data
products of the Helioseismic Magnetic Imager (HMI) on the Solar
Dynamic Observatory (SDO). These parameters are believed to be the
most important ones related to the occurrence of solar flares. We then
compared the prediction results with the actual flare index obtained
from the National Oceanic and Atmospheric Administration's Space
Weather Prediction Center, and evaluated the prediction accuracy for
each class of flares.

---------------------------------------------------------
Title: Statistical Study of Polar Crown Filaments in the Past Four
Solar Cycles
Authors: Xu, Yan; Zhang, Hewei; Huang, Nengyi; Jing, Ju; Wang, Haimin
2018tess.conf22408X Altcode:
Polar Crown Filaments (PCFs) form near the polarity inversion line
that separates the unipolar polar fields and the closest dispersed
fields with opposite polarity. The statistics and variation of PCFs are
well correlated with the solar cycle. Therefore, study of PCFs plays an
important role in understanding the variation of solar cycle, especially
the current "abnormal" solar cycle 24. In this study, we investigate the
full disk H-alpha data from 1973 to early 2018, recorded by Kanzelhöhe
Solar Observatory (KSO), Big Bear Solar Observatory (BBSO) and National
Solar Observatory (NSO), using CCDs and films. PCFs are identified
manually during the 4-cycle period. Here we present the migration and
south-north asymmetric of PCFs, and the correlation with the polar
magnetic fields obtained by NSO Kitt Peak, SOHO/MDI and SDO/HMI.

---------------------------------------------------------
Title: Fine-scale Emerging Flux Elements Observed by 1.6m Telescope
at BBSO
Authors: Wang, Jiasheng; Liu, Chang; Wang, Haimin
2018tess.conf21703W Altcode:
Magnetic flux emerges in various scales on the Sun, ranging from
about 1016 Mx in quiet Sun to 1022 Mx in active
regions. As it is considered to be in the transport stage, emerging flux
can provide clues about processes, evolution, and structure of magnetic
fields near the solar surface. Also, emerging flux plays an important
role in the build-up of magnetic energy, as it often carries currents
and is thus twisted. Recent data-driven simulation models show that
fragmented small-scale magnetic elements can reach photosphere through
convective upflows and then concentrate. In this work, we use magnetic
field data taken by the near-infrared imaging spectropolarimeter (NIRIS)
of the 1.6 m Goode Solar Telescope at Big Bear Solar Observatory to
study an emerging flux region in NOAA AR 12665 on 2017 July 13. To
understand the processes underlying the elementary flux emergence and
concentration, we focus on investigating the temporal and structural
characteristics of emerging flux in fine scale and compare the results
with simulation. Preliminarily, we detect two components of magnetic
flux near the magnetic polarity inversion line during the continuous
emergence, including confined flux footpoints with opposite polarities
and diffused magnetic structure between them. These components are
found to have a typical line-of-sight magnetic field of about 1000 and
300 G, respectively. We also present the analysis of electric currents
carried by the emerging flux with analysis of vector magnetic field
measurement in high resolution.

---------------------------------------------------------
Title: Study of 3D magnetic Structure Corresponding to Extremely
Strong Photospheric Magnetic Fields in Active Region 12673
Authors: Wang, Haimin; Yurchyshyn, Vasyl; Liu, Chang; Chen, Bin;
Jing, Ju; Ahn, Kwangsu; Toriumi, Shin; Cao, Wenda
2018tess.conf31902W Altcode:
Solar Active Region (AR) 12673 is the most flare productive AR in the
solar cycle 24. It produced four X-class flares including the X9.3 flare
on 06 September 2017 and the X8.2 limb event on 10 September. Sun and
Norton (2017) reported that this region had an unusual high rate of
flux emergence, while Huang et al. (2018) reported that the X9.3 flare
had extremely strong white-light flare emission. Yang et al. (2017)
described the detailed morphological evolution of this AR. In this work,
we first study the unusual behaviors of the light bridge (LB) dividing
the delta configuration of this AR, namely the strong magnetic fields
(above 5500 G) in the LB and the apparent photospheric twist as seen in
observations with a 0.1 arcsec spatial resolution obtained by the 1.6m
Goode Solar Telescope (GST) at the Big Bear Solar Observatory. Using the
newly commissioned Expanded Owens Valley Solar Array (EOVSA), we carried
out diagnoses of magnetic fields in this AR with microwave imaging
spectroscopy from 2.5 to 18 GHz. Finally, we utilized Non-Linear Force
Free Field (NLFFF) extrapolation to reveal 3-D magnetic structure to
gain a physical understanding of GST and EOVSA observations of this AR.

---------------------------------------------------------
Title: Pre-eruptive Magnetic Reconnection within a Multi-flux-rope
System in the Solar Corona
Authors: Awasthi, Arun Kumar; Liu, Rui; Wang, Haimin; Wang, Yuming;
Shen, Chenglong
2018ApJ...857..124A Altcode: 2018arXiv180304088A
The solar corona is frequently disrupted by coronal mass ejections
(CMEs), whose core structure is believed to be a flux rope made of
helical magnetic field. This has become a “standard” picture;
though, it remains elusive how the flux rope forms and evolves toward
eruption. While one-third of the ejecta passing through spacecraft
demonstrate a flux-rope structure, the rest have complex magnetic
fields. Are they originating from a coherent flux rope, too? Here we
investigate the source region of a complex ejecta, focusing on a flare
precursor with definitive signatures of magnetic reconnection, i.e.,
nonthermal electrons, flaring plasma, and bidirectional outflowing
blobs. Aided by nonlinear force-free field modeling, we conclude that
the reconnection occurs within a system of multiple braided flux ropes
with different degrees of coherency. The observation signifies the
importance of internal structure and dynamics in understanding CMEs
and in predicting their impacts on Earth.

---------------------------------------------------------
Title: Statistical Study of Magnetic Topology for Eruptive and
Confined Solar Flares
Authors: Cui, Yanmei; Wang, Haimin; Xu, Yan; Liu, Siqing
2018JGRA..123.1704C Altcode:
Large flares and halo coronal mass ejections (CMEs) can often cause
strong space environment disturbances and sequently a series of space
environment effects. The X-class flares associated with halo CMEs are
particular prone to these effects. In this paper, 58 X-class flares
were collected and studied with the source locations in 30° from
the disk center, which were observed from 1996 to 2015. Among these
events, 48 flares were associated with CMEs and defined as "eruptive"
events. The other 10 flares without CMEs were defined as "confined"
flares. By comparing the properties of flares and associated magnetic
fields for the two sets of samples, we found the following: (1)
magnetic free energy and overlying transverse fields play important
roles in producing solar eruptions. Eruptive flares with high-speed
CMEs tend to occur in active regions with more free energy and larger
decay index. (2) CME speeds are affected by magnetic free energy,
which are described by parameters of the unsigned magnetic flux,
the area of polarity inversion region, and the strength of transverse
fields in the low altitude. These parameters have moderate positive
correlations with CME speeds.

---------------------------------------------------------
Title: Evolution of Photospheric Flow and Magnetic Fields Associated
with the 2015 June 22 M6.5 Flare
Authors: Wang, Jiasheng; Liu, Chang; Deng, Na; Wang, Haimin
2018ApJ...853..143W Altcode: 2018arXiv180103486W
The evolution of photospheric flow and magnetic fields before and after
flares can provide important information regarding the flare triggering
and back-reaction processes. However, such studies on the flow field
are rare due to the paucity of high-resolution observations covering
the entire flaring period. Here we study the structural evolution of
penumbra and shear flows associated with the 2015 June 22 M6.5 flare
in NOAA AR 12371, using high-resolution imaging observation in the
TiO band taken by the 1.6 m Goode Solar Telescope at Big Bear Solar
Observatory, with the aid of the differential affine velocity estimator
method for flow tracking. The accompanied photospheric vector magnetic
field changes are also analyzed using data from the Helioseismic and
Magnetic Imager. As a result, we found, for a penumbral segment in the
negative field adjacent to the magnetic polarity inversion line (PIL),
an enhancement of penumbral flows (up to an unusually high value of
∼2 km s-1) and extension of penumbral fibrils after the
first peak of the flare hard X-ray emission. We also found an area
at the PIL, which is co-spatial with a precursor brightening kernel,
that exhibits a gradual increase of shear flow velocity (up to ∼0.9
km s-1) after the flare. The enhancing penumbral and shear
flow regions are also accompanied by an increase of horizontal field
and decrease of magnetic inclination angle (measured from the solar
surface). These results are discussed in the context of the theory of
back-reaction of coronal restructuring on the photosphere as a result
of flare energy release.

---------------------------------------------------------
Title: Extending Counter-streaming Motion from an Active Region
Filament to a Sunspot Light Bridge
Authors: Wang, Haimin; Liu, Rui; Li, Qin; Liu, Chang; Deng, Na; Xu,
Yan; Jing, Ju; Wang, Yuming; Cao, Wenda
2018ApJ...852L..18W Altcode: 2017arXiv171206783W
We analyze high-resolution observations from the 1.6 m telescope
at Big Bear Solar Observatory that cover an active region
filament. Counter-streaming motions are clearly observed in the
filament. The northern end of the counter-streaming motions extends
to a light bridge, forming a spectacular circulation pattern around a
sunspot, with clockwise motion in the blue wing and counterclockwise
motion in the red wing, as observed in the Hα off-bands. The apparent
speed of the flow is around 10-60 km s-1 in the filament,
decreasing to 5-20 km s-1 in the light bridge. The most
intriguing results are the magnetic structure and the counter-streaming
motions in the light bridge. Similar to those in the filament, the
magnetic fields show a dominant transverse component in the light
bridge. However, the filament is located between opposed magnetic
polarities, while the light bridge is between strong fields of
the same polarity. We analyze the power of oscillations with the
image sequences of constructed Dopplergrams, and find that the
filament’s counter-streaming motion is due to physical mass motion
along fibrils, while the light bridge’s counter-streaming motion
is due to oscillation in the direction along the line-of-sight. The
oscillation power peaks around 4 minutes. However, the section of
the light bridge next to the filament also contains a component of
the extension of the filament in combination with the oscillation,
indicating that some strands of the filament are extended to and rooted
in that part of the light bridge.

---------------------------------------------------------
Title: Relationship between Intensity of White-light Flares and
Proton Flux of Solar Energetic Particles
Authors: Huang, Nengyi; Xu, Yan; Wang, Haimin
2018RNAAS...2....7H Altcode: 2018arXiv180104316H; 2018RNAAS...2a...7H
Solar energetic particles (SEPs), including protons and heavy ions,
are believed to be accelerated either by CME shock or by magnetic
reconnection. The latter also produces solar flares, in which the
white light flares (WLFs) are among the most energetic ones. Magnetic
reconnections occur in the corona and the accelerated particles
propagate both downward and upward along the magnetic loops. The
former is the source of flare emission and the latter is thought
to be SEPs. Therefore, a comparison between flare emission and SEP
events provides valuable constraint in determining the acceleration
site of SEPs. We collect 43 SEP events, observed from 2010 to 2017,
and investigate their correlation with WL emission and SXR flus,
observed by SDO/HMI and GOES, respectively. Our preliminary results
show: 1) Among 47 SEP events, 39 of which do not have detectable
flare emissions in white light and SXR. 2) Most strong WLFs are not
associated with SEPs. 3) No clear correlation is found between the
proton flux and the equivalent area, a quantity that measures the
magnitude of WLF emission. A straightforward speculation is that the
acceleration process could be different for SEPs and the energetic
electrons powering WLFs in the events analyzed.

---------------------------------------------------------
Title: Strong Transverse Photosphere Magnetic Fields and Twist in
Light Bridge Dividing Delta Sunspot of Active Region 12673
Authors: Wang, Haimin; Yurchyshyn, Vasyl; Liu, Chang; Ahn, Kwangsu;
Toriumi, Shin; Cao, Wenda
2018RNAAS...2....8W Altcode: 2018RNAAS...2a...8W; 2018arXiv180102928W
Solar Active Region (AR) 12673 is the most flare productive AR in the
solar cycle 24. It produced four X-class flares including the X9.3 flare
on 06 September 2017 and the X8.2 limb event on 10 September. Sun and
Norton (2017) reported that this region had an unusual high rate of
flux emergence, while Huang et al. (2018) reported that the X9.3 flare
had extremely strong white-light flare emissions. Yang at al. (2017)
described the detailed morphological evolution of this AR. In this
report, we focus on usual behaviors of the light bridge (LB) dividing
the delta configuration of this AR, namely the strong magnetic fields
(above 5500 G) in the LB and apparent photospheric twist as shown in
observations with a 0.1 arcsec spatial resolution obtained by the 1.6m
telescope at Big Bear Solar Observatory.

---------------------------------------------------------
Title: Transient rotation of photospheric vector magnetic fields
associated with a solar flare
Authors: Xu, Yan; Cao, Wenda; Ahn, Kwangsu; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
2018NatCo...9...46X Altcode: 2018arXiv180103171X
As one of the most violent eruptions on the Sun, flares are believed to
be powered by magnetic reconnection. The fundamental physics involving
the release, transfer, and deposition of energy have been studied
extensively. Taking advantage of the unprecedented resolution provided
by the 1.6 m Goode Solar Telescope, here, we show a sudden rotation of
vector magnetic fields, about 12-20° counterclockwise, associated
with a flare. Unlike the permanent changes reported previously,
the azimuth-angle change is transient and cospatial/temporal with
Hα emission. The measured azimuth angle becomes closer to that in
potential fields suggesting untwist of flare loops. The magnetograms
were obtained in the near infrared at 1.56 μm, which is minimally
affected by flare emission and no intensity profile change was
detected. We believe that these transient changes are real and discuss
the possible explanations in which the high-energy electron beams or
Alfve'n waves play a crucial role.

---------------------------------------------------------
Title: Three-dimensional Forward-fit Modeling of the Hard X-Ray and
Microwave Emissions of the 2015 June 22 M6.5 Flare
Authors: Kuroda, Natsuha; Gary, Dale E.; Wang, Haimin; Fleishman,
Gregory D.; Nita, Gelu M.; Jing, Ju
2018ApJ...852...32K Altcode: 2017arXiv171207253K
The well-established notion of a “common population” of the
accelerated electrons simultaneously producing the hard X-ray (HXR)
and microwave (MW) emission during the flare impulsive phase has
been challenged by some studies reporting the discrepancies between
the HXR-inferred and MW-inferred electron energy spectra. The
traditional methods of spectral inversion have some problems that
can be mainly attributed to the unrealistic and oversimplified
treatment of the flare emission. To properly address this problem,
we use a nonlinear force-free field (NLFFF) model extrapolated from an
observed photospheric magnetogram as input to the three-dimensional,
multiwavelength modeling platform GX Simulator and create a unified
electron population model that can simultaneously reproduce the
observed HXR and MW observations. We model the end of the impulsive
phase of the 2015 June 22 M6.5 flare and constrain the modeled
electron spatial and energy parameters using observations made by the
highest-resolving instruments currently available in two wavelengths,
the Reuven Ramaty High Energy Solar Spectroscopic Imager for HXR and the
Expanded Owens Valley Solar Array for MW. Our results suggest that the
HXR-emitting electron population model fits the standard flare model
with a broken power-law spectrum ({E}{break}∼ 200 keV)
that simultaneously produces the HXR footpoint emission and the MW
high-frequency emission. The model also includes an “HXR-invisible”
population of nonthermal electrons that are trapped in a large volume of
magnetic field above the HXR-emitting loops, which is observable by its
gyrosynchrotron radiation emitting mainly in the MW low-frequency range.

---------------------------------------------------------
Title: Thermodynamics of supra-arcade downflows in solar flares
Authors: Chen, Xin; Liu, Rui; Deng, Na; Wang, Haimin
2017A&A...606A..84C Altcode: 2017arXiv170603452C
Context. Supra-arcade downflows (SADs) have been frequently observed
during the gradual phase of solar flares near the limb. In coronal
emission lines sensitive to flaring plasmas, they appear as tadpole-like
dark voids against the diffuse fan-shaped "haze" above, flowing toward
the well-defined flare arcade. Aims: We aim to investigate
the evolution of SADs' thermal properties, and to shed light on the
formation mechanism and physical processes of SADs. Methods:
We carefully studied several selected SADs from two flare events
and calculated their differential emission measures (DEMs) as well as
DEM-weighted temperatures using data obtained by the Atmospheric Imaging
Assembly (AIA) onboard the Solar Dynamic Observatory. Results:
Our analysis shows that SADs are associated with a substantial decrease
in DEM above 4 MK, which is 1-3 orders of magnitude smaller than the
surrounding haze as well as the region before or after the passage
of SADs, but comparable to the quiet corona. There is no evidence for
the presence of the SAD-associated hot plasma (&gt;20 MK) in the AIA
data, and this decrease in DEM does not cause any significant change
in the DEM distribution as well as the DEM-weighted temperature, which
supports this idea that SADs are density depletion. This depression in
DEM rapidly recovers in the wake of the SADs studied, generally within a
few minutes, suggesting that they are discrete features. In addition,
we found that SADs in one event are spatio-temporally associated
with the successive formation of post-flare loops along the flare
arcade. Movies associated to Figs. A.1 and A.2 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629893/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: 1.56 Micron Spectropolarimetry of Umbral Dots and Their
Evolution Associated with a Major Flare
Authors: Deng, Na; Liu, Chang; Xu, Yan; Ahn, Kwangsu; Jing, Ju; Cao,
Wenda; Wang, Haimin
2017SPD....4830003D Altcode:
We present unprecedented high-resolution and high magnetic
sensitivity spectropolarimetric characterization of umbral dots
(UDs), the prevailing fine scale brightness structure manifesting
magneto-convection inside sunspot umbrae where the magnetic fields
are strongest and nearly vertical. This is made available by recent
development of the Near InfraRed Imaging Spectropolarimeter (NIRIS)
using the 1.56 micron FeI line at the 1.6 meter New Solar Telescope
of Big Bear Solar Observatory. Vector magnetograms are obtained after
Milne-Eddington Stokes inversions, 180-degree azimuthal ambiguity
resolution, and correction of projection effects. A βγδ spot in NOAA
AR 12371 was observed for six hours on June 22, 2015 with a cadence of
87 s, which covered an M6.6 flare. The overall umbra is separated into
several smaller umbrae by light bridges. The umbrae are close to the
flaring polarity inversion line and show an average inclination of about
17° and field strength of about 2100 Gauss. The UDs are resolvable in
NIRIS vector magnetograms, especially for peripheral UDs. The measured
field strength is about 3% lower in UDs comparing to umbral cores (UCs)
where the continuum intensity is below the threshold of UDs. The field
is more inclined in UDs by 5% ( ≈ 1°) than that in UCs. One of the
umbrae showed rapid evolution associated with the flare. Its overall
intensity and the number of UDs decrease by at least 7% within two
hours after being swept by the flare ribbon. NIRIS vector magnetograms
indicate that the average field strength of that umbra has a rapid
stepwise increase for about 100 Gauss while the inclination almost
has no change. The decreases of the umbral brightness and the number
of UDs are thus attributed to the increase of the field strength. The
results suggest that the field strength plays the most important role
in constraining convective heat transport in umbra.

---------------------------------------------------------
Title: High-resolution Observations of Photospheric Structural
Evolution Associated with a Flare
Authors: Liu, Chang; Xu, Yan; Ahn, Kwangsu; Jing, Ju; Deng, Na; Cao,
Wenda; Wang, Haimin
2017SPD....4810101L Altcode:
The structural evolution of the photosphere not only play an important
role in contributing to the accumulation of free energy in the corona
that powers solar flares, but also may response to the restructuring
of coronal field as a result of flare energy release. A better
understanding of these issues may be achieved by high-resolution
observations of the photospheric structure covering the entire
flaring period, which are, however, still rare. Here we present
photospheric vector magnetograms and TiO images (at 0.2" and 0.09"
resolution, respectively) from before to after a major flare, taken
by the 1.6 m New Solar Telescope at Big Bear Solar Observatory. In the
pre-flare state, a small-scale magnetic structure of opposite-polarity
configuration is seen near the footpoints of sheared magnetic loops;
its magnetic fluxes and currents enhance till the flare start time and
decline afterwards. During the main phase, as one flare ribbon sweeps
across a sunspot, its different portions accelerate at different times
corresponding to peaks of flare hard X-ray emission. We suggest that
the small-scale flux emergence between the two sheared flux systems
triggers the flare reconnection, and that the sunspot rotation is driven
by the surface Lorentz-force change due to the coronal back reaction.

---------------------------------------------------------
Title: Spectral Analysis Flare ribbons by NST and IRIS
Authors: Huang, Nengyi; Xu, Yan; Wang, Haimin; Jing, Ju
2017SPD....48.0301H Altcode:
As one of the most powerful phenomena of solar activities, flares
have long been observed and studied extensively. Taking advantages
of observing capabilities of modern solar telescopes and focal-plane
instruments such as the Interface Region Imaging Spectrograph (IRIS)
and the 1.6 m New Solar Telescope (NST) at Big Bear Solar observatory
(BBSO), we are able to obtain high resolution imaging spectroscopic
data in UV, visible and near-infrared (NIR) wavelengths. Here we
present the spectral analysis of an M6.5 flare (SOL2015-06-22T18:23)
which was well covered by the joint observation of IRIS and NST. In
the visible wavelengths H-alpha and TiO, we can separate the flare
ribbon into a very narrow leading front and faint trailing component,
of which the former is characterized by the intense emission and
significant Doppler signals. In the IRIS UV spectra, the ribbon front
shows distinct properties, such as the line broadening, Doppler shifts
and central reversal pattern, which are consistent with the visible
observations. These characteristics suggest that the ribbon front to
be the p

---------------------------------------------------------
Title: Predicting Solar Flares Using SDO/HMI Vector Magnetic Data
Product and Random Forest Algorithm
Authors: Liu, Chang; Deng, Na; Wang, Jason; Wang, Haimin
2017SPD....4810814L Altcode:
Adverse space weather effects can often be traced to solar flares,
prediction of which has drawn significant research interests. Many
previous forecasting studies used physical parameters derived
from photospheric line-of-sight field or ground-based vector field
observations. The Helioseismic and Magnetic Imager (HMI) on board the
Solar Dynamics Observatory produces full-disk vector magnetograms with
continuous high-cadence, while flare prediction efforts utilizing
this unprecedented data source are still limited. Here we report
results of flare prediction using physical parameters provided by
the Space-weather HMI Active Region Patches (SHARP) and related
data products. We survey X-ray flares occurred from 2010 May to 2016
December, and categorize their source regions into four classes (B,
C, M, and X) according to the maximum GOES magnitude of flares they
generated. We then retrieve SHARP related parameters for each selected
region at the beginning of its flare date to build a database. Finally,
we train a machine-learning algorithm, called random forest (RF), to
predict the occurrence of a certain class of flares in a given active
region within 24 hours, evaluate the classifier performance using the
10-fold cross validation scheme, and characterize the results using
standard performace metrics. Compared to previous works, our experiments
indicate that using the HMI parameters and RF is a valid method for
flare forecasting with fairly reasonable prediction performance. We
also find that the total unsigned quantities of vertical current,
current helicity, and flux near polarity inversion line are among
the most important parameters for classifying flaring regions into
different classes.

---------------------------------------------------------
Title: Magnetic vector rotation in response to the energetic electron
beam during a flare
Authors: Xu, Yan; Cao, Wenda; Kwangsu, Ahn; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
2017SPD....4810001X Altcode:
As one of the most violent forms of eruption on the Sun, flares are
believed to be powered by magnetic reconnection, by which stored
magnetic energy is released. The fundamental physical processes
involving the release, transfer and deposition of energy in multiple
layers of the solar atmosphere have been studied extensively with
significant progress. Taking advantage of recent developments in
observing facilities, new phenomena are continually revealed, bringing
new understanding of solar flares. Here we report the discovery of
a transient rotation of vector magnetic fields associated with a
flare observed by the 1.6-m New Solar Telescope at Big Bear Solar
Observatory. After ruling out the possibility that the rotation is
caused by line profile changes due to flare heating, our observation
shows that the transverse field rotateded by about 12-20 degrees
counterclockwise, and returned quickly to previous values after the
flare ribbons swept through. More importantly, as a consequence of
the rotation, the flare loops untwisted and became more potential. The
vector magnetograms were obtained in the near infrared at 1560 nm, which
is minimally affected by flare emission and no intensity profile change
was detected. Therefore, we believe that these transient changes are
real, and conclude the high energy electron beams play an crucial role
in the field changes. A straightforward and instructive explanation is
that the induced magnetic field of the electron beam superimposed on
the pre-flare field leads to a transient rotation of the overall field.

---------------------------------------------------------
Title: Witnessing a Large-scale Slipping Magnetic Reconnection along
a Dimming Channel during a Solar Flare
Authors: Jing, Ju; Liu, Rui; Cheung, Mark; Lee, Jeongwoo; Xu, Yan;
Liu, Chang; Zhu, Chunming; Wang, Haimin
2017SPD....4840601J Altcode:
We report the intriguing large-scale dynamic phenomena associated
with the M6.5 flare~(SOL2015-06-22T18:23) in NOAA active region
12371, observed by RHESSI, Fermi, and the Atmospheric Image Assembly
(AIA) and Magnetic Imager (HMI) on the Solar Dynamic Observatory
(SDO). The most interesting feature of this event is a third ribbon
(R3) arising in the decay phase, propagating along a dimming channel
(seen in EUV passbands) towards a neighboring sunspot. The propagation
of R3 occurs in the presence of hard X-ray footpoint emission, and
is broadly visible at temperatures from 0.6 MK to over 10 MK through
the Differential Emission Measure (DEM) analysis. The coronal loops
then undergo an apparent slipping motion following the same path of
R3, after a ~80 min delay. To understand the underlying physics, we
investigate the magnetic configuration and the thermal structure of the
flaring region. Our results are in favor of a slipping-type reconnection
followed by the thermodynamic evolution of coronal loops. In comparison
with those previously reported slipping reconnection events, this
one proceeds across a particularly long distance (~60 Mm) over a long
period of time ~50 min), and shows two clearly distinguished phases:
the propagation of the footpoint brightening driven by nonthermal
particle injection and the apparent slippage of loops governed by
plasma heating and subsequent cooling.

---------------------------------------------------------
Title: Flux rope, hyperbolic flux tube, and late extreme ultraviolet
phases in a non-eruptive circular-ribbon flare
Authors: Masson, Sophie; Pariat, Étienne; Valori, Gherardo; Deng,
Na; Liu, Chang; Wang, Haimin; Reid, Hamish
2017A&A...604A..76M Altcode: 2017arXiv170401450M
Context. The dynamics of ultraviolet (UV) emissions during solar flares
provides constraints on the physical mechanisms involved in the trigger
and the evolution of flares. In particular it provides some information
on the location of the reconnection sites and the associated magnetic
fluxes. In this respect, confined flares are far less understood
than eruptive flares generating coronal mass ejections. Aims:
We present a detailed study of a confined circular flare dynamics
associated with three UV late phases in order to understand more
precisely which topological elements are present and how they constrain
the dynamics of the flare. Methods: We perform a non-linear
force-free field extrapolation of the confined flare observed with the
Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly
(AIA) instruments on board Solar Dynamics Observatory (SDO). From the
3D magnetic field we compute the squashing factor and we analyse its
distribution. Conjointly, we analyse the AIA extreme ultraviolet (EUV)
light curves and images in order to identify the post-flare loops,
and their temporal and thermal evolution. By combining the two analyses
we are able to propose a detailed scenario that explains the dynamics
of the flare. Results: Our topological analysis shows that in
addition to a null-point topology with the fan separatrix, the spine
lines and its surrounding quasi-separatix layer (QSL) halo (typical
for a circular flare), a flux rope and its hyperbolic flux tube (HFT)
are enclosed below the null. By comparing the magnetic field topology
and the EUV post-flare loops we obtain an almost perfect match between
the footpoints of the separatrices and the EUV 1600 Å ribbons and
between the HFT field line footpoints and bright spots observed inside
the circular ribbons. We show, for the first time in a confined flare,
that magnetic reconnection occurred initially at the HFT below the flux
rope. Reconnection at the null point between the flux rope and the
overlying field is only initiated in a second phase. In addition, we
showed that the EUV late phase observed after the main flare episode
is caused by the cooling loops of different length which have all
reconnected at the null point during the impulsive phase. 
Conclusions: Our analysis shows in one example that flux ropes are
present in null-point topology not only for eruptive and jet events,
but also for confined flares. This allows us to conjecture on the
analogies between conditions that govern the generation of jets,
confined flares or eruptive flares. A movie is available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629654/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Extending Counter-Streaming Motion from an Active Region
Filament to Sunspot Light Bridge
Authors: Wang, Haimin; Liu, Rui; Deng, Na; Liu, Chang; Xu, Yan; Jing,
Ju; Wang, Yuming; Cao, Wenda
2017SPD....4810405W Altcode:
In this study, we analyze the high-resolution observations from the 1.6
m New Solar Telescope at Big Bear Solar Observatory that cover an entire
active region filament. The southern end of the filament is well defined
by a narrow lane situated in the negative magnetic polarity, while the
northern end lies in the positive polarity, extending to a much larger
area. Counter-streaming motions are clearly seen in the filament. The
northern end of the counter-streaming motions extends to a light
bridge, forming a spectacular circulation pattern around a sunspot,
with clockwise motion in the blue wing and counterclockwise motion in
the red wing as observed in H-alpha off-band. The apparent speed of the
flow is around 10 km/s. We show that the southern end of the filament
is consistent with that of a flux rope in a NLFFF extrapolation model,
but the northern ends of the modeled flux rope and observed H-alpha
footpoints have a significant spatial mismatch. The most intriguing
results are the magnetic structure and the counter-streaming motions
in the light bridge. Similar to those in the filament, magnetic fields
show a dominant transverse component in the light bridge. However,
the filament is located between opposite magnetic polarities, while
the light bridge is between strong fields of the same polarity. We
studied the correlation coefficients of image sequences of constructed
Dopplergrams, and found that the filament and the section of light
bridge next to it do not show oscillation motions, while a small section
of light bridge shows a prominent oscillation pattern. Therefore, we
conclude that the observed circulating counter-streaming motions are
largely collections of physical mass flows in the transverse direction
from the filament extending to a large section of the light bridge,
rather than a form of periodic oscillatory mass motions in line-of-sight
direction generated by perturbations omnipresent in the chromosphere.

---------------------------------------------------------
Title: Evolution of Photospheric Flow and Magnetic Fields Associated
with the 2015 June 22 M6.5 Flare
Authors: Wang, Jiasheng; Liu, Chang; Deng, Na; Wang, Haimin
2017SPD....4810102W Altcode:
The evolution of photospheric flow and magnetic fields before and after
flares can provide important information regarding the flare triggering
and back reaction processes. However, such studies on the flow field
are rare due to the paucity of high-resolution observations covering
the entire flaring period. Here we study the structural evolution of
penumbra and shear flows associated with the 2015 June 22 M6.5 flare in
NOAA AR 12371, using high-resolution imaging observation in the TiO band
taken by the 1.6 m New Solar Telescope at Big Bear Solar Observatory,
with the aid of the differential affine velocity estimator(DAVE)
method for flow tracking. The accompanied photospheric vector magnetic
field changes are also analyzed using data from the Helioseismic and
Magnetic Imager. As a result, we found, for a penumbral segment in
the negative field adjacent to the magnetic polarity inversion line
(PIL), an enhancement of penumbral flows (up to ~2 km s-1)
and extension of penumbral fibrils after the first peak of the flare
hard X-ray (HXR) emission. We also found a shear flow region at the PIL,
which is co-spatial with a precursor brightening kernel and exhibits a
gradual increase of shear flow velocity (up to ~0.9 km s-1)
after the flare. The enhancing penumbral and shear flow regions are also
accompanied by an increase of horizontal field and decrease of magnetic
inclination angle. These results are discussed in the context of the
theory of back reaction of coronal restructuring on the photosphere
as a result of flare energy release.

---------------------------------------------------------
Title: High-resolution Observation of Moving Magnetic Features in
Active Regions
Authors: Li, Qin; Deng, Na; Jing, Ju; Wang, Haimin
2017SPD....4810103L Altcode:
Moving magnetic features (MMFs) are small photospheric magnetic elements
that emerge and move outward toward the boundary of moat regions mostly
during a sunspot decaying phase, in a serpent wave-like magnetic
topology. Studies of MMFs and their classification (e.g., unipolar
or bipolar types) strongly rely on the high spatiotemporal-resolution
observation of photospheric magnetic field. In this work, we present a
detailed observation of a sunspot evolution in NOAA active region (AR)
12565, using exceptionally high resolution Halpha images from the 1.6
New Solar telescope (NST) at Big Bear Solar Observatory (BBSO) and the
UV images from the Interface Region Imaging Spectrograph (IRIS). The
spectropolarimetric measurements of photospheric magnetic field are
obtained from the NST Near InfraRed Imaging Spectropolarimeter (NIRIS)
at Fe I 1.56 um line. We investigate the horizontal motion of the
classified MMFs and discuss the clustering patterns of the geometry
and motion of the MMFs. We estimate the rate of flux generation by
appearance of MMFs and the role MMFs play in sunspot decaying phase. We
also study the interaction between the MMFs and the existing magnetic
field features and its response to Ellerman bombs and IRIS bombs
respectively at higher layers.

---------------------------------------------------------
Title: The analysis and the three-dimensional, forward-fit modeling
of the X-ray and the microwave emissions of major solar flares
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2017SPD....4840003K Altcode:
It is well known that the time profiles of the hard X-ray (HXR)
emission and the microwave (MW) emission during the impulsive phase of
the solar flare are well correlated, and that their analysis can lead
to the understandings of the flare-accelerated electrons. In this work,
we first studied the source locations of seven distinct temporal peaks
observed in HXR and MW lightcurves of the 2011-02-15 X2.2 flare using
the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and
Nobeyama Radioheliograph. We found that the seven emission peaks did
not come from seven spatially distinct sites in HXR and MW, but rather
in HXR we observed a sudden change in location only between the second
and the third peak, with the same pattern occurring, but evolving more
slowly in MW, which is consistent with the tether-cutting model of solar
flares. Next, we closely examine the widely-used notion of a "common
population" of the accelerated electrons producing the HXR and the MW,
which has been challenged by some studies suggesting the differences in
the inferred energy spectral index and emitting energies of the HXR-
and MW- producing electrons. We use the Non-linear Force Free Field
model extrapolated from the observed photospheric magnetogram in the
three-dimensional, multi-wavelength modeling platform GX Simulator,
and attempt to create a unified electron population model that can
simultaneously reproduce the observed X-ray and MW observations of
the 2015-06-22 M6.5 flare. We constrain the model parameters by the
observations made by the highest-resolving instruments currently
available in two wavelengths, the RHESSI for X-ray and the Expanded
Owens Valley Solar Array for MW. The results suggest that the X-ray
emitting electron population model fits to the standard flare model with
the broken, hardening power-law spectrum at ~300 keV that simultaneously
produces the HXR footpoint emission and the MW high frequency emission,
and also reveals that there could be a “X-ray invisible” population
of nonthermal electrons that are trapped in a large magnetic field
volume above the X-ray emitting loops, that emits gyrosynchrotron
radiation mainly in MW low frequency range.

---------------------------------------------------------
Title: Multiwavelength observations of a flux rope formation by
series of magnetic reconnection in the chromosphere
Authors: Kumar, Pankaj; Yurchyshyn, Vasyl; Cho, Kyung-Suk; Wang, Haimin
2017A&A...603A..36K Altcode: 2017arXiv170309871K
Using high-resolution observations from the 1.6 m New Solar Telescope
(NST) operating at the Big Bear Solar Observatory (BBSO), we report
direct evidence of merging and reconnection of cool Hα loops in the
chromosphere during two homologous flares (B and C class) caused by a
shear motion at the footpoints of two loops. The reconnection between
these loops caused the formation of an unstable flux rope that showed
counterclockwise rotation. The flux rope could not reach the height of
torus instability and failed to form a coronal mass ejection. The HMI
magnetograms revealed rotation of the negative and positive (N1/P2)
polarity sunspots in the opposite directions, which increased the
right- and left-handed twist in the magnetic structures rooted at
N1/P2. Rapid photospheric flux cancellation (duration 20-30 min,
rate ≈3.44 × 1020 Mx h-1) was observed
during and even after the first B6.0 flare and continued until
the end of the second C2.3 flare. The RHESSI X-ray sources were
located at the site of the loop coalescence. To the best of our
knowledge, such a clear interaction of chromospheric loops along
with rapid flux cancellation has not been reported before. These
high-resolution observations suggest the formation of a small flux
rope by a series of magnetic reconnections within chromospheric
loops that are associated with very rapid flux cancellation. Movies attached to Figs. 2, 7, 8, and 10 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201629295/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Predicting Solar Flares Using SDO/HMI Vector Magnetic Data
Products and the Random Forest Algorithm
Authors: Liu, Chang; Deng, Na; Wang, Jason T. L.; Wang, Haimin
2017ApJ...843..104L Altcode: 2017arXiv170602422L
Adverse space-weather effects can often be traced to solar flares,
the prediction of which has drawn significant research interests. The
Helioseismic and Magnetic Imager (HMI) produces full-disk vector
magnetograms with continuous high cadence, while flare prediction
efforts utilizing this unprecedented data source are still limited. Here
we report results of flare prediction using physical parameters
provided by the Space-weather HMI Active Region Patches (SHARP)
and related data products. We survey X-ray flares that occurred from
2010 May to 2016 December and categorize their source regions into
four classes (B, C, M, and X) according to the maximum GOES magnitude
of flares they generated. We then retrieve SHARP-related parameters
for each selected region at the beginning of its flare date to build
a database. Finally, we train a machine-learning algorithm, called
random forest (RF), to predict the occurrence of a certain class of
flares in a given active region within 24 hr, evaluate the classifier
performance using the 10-fold cross-validation scheme, and characterize
the results using standard performance metrics. Compared to previous
works, our experiments indicate that using the HMI parameters and RF is
a valid method for flare forecasting with fairly reasonable prediction
performance. To our knowledge, this is the first time that RF has been
used to make multiclass predictions of solar flares. We also find that
the total unsigned quantities of vertical current, current helicity,
and flux near the polarity inversion line are among the most important
parameters for classifying flaring regions into different classes.

---------------------------------------------------------
Title: High-resolution Observations of Downflows at One End of a
Pre-eruption Filament
Authors: Li, Qin; Deng, Na; Jing, Ju; Wang, Haimin
2017ApJ...841..112L Altcode: 2017arXiv170508003L
Studying the dynamics of filaments at the pre-eruption phase can shed
light on the precursor of eruptive events. Such high-resolution studies
(of the order of 0.″1) are highly desirable yet very rare. In this
work, we present a detailed observation of a pre-eruption evolution of a
filament obtained by the 1.6 m New Solar Telescope (NST) at the Big Bear
Solar Observatory (BBSO). One end of the filament is anchored at the
sunspot in the NOAA active region (AR) 11515, which is well observed by
NST Hα off-bands from four hours before to one hour after the filament
eruption. A M1.6 flare is associated with the eruption. We observed
persistent downflowing materials along the Hα multi-threaded component
of the loop toward the AR end during the pre-eruption phase. We traced
the trajectories of plasma blobs along the Hα threads and obtained a
plane-of-sky velocity of 45 km s-1 on average. Furthermore,
we estimated the real velocities of the downflows and the altitude of
the filament by matching the observed Hα threads with magnetic field
lines extrapolated from a nonlinear force-free field model. Observations
of chromospheric brightenings at the footpoints of the falling plasma
blobs are also presented. The lower limit of the kinetic energy
per second of the downflows through the brightenings is found to be
∼1021 erg. Larger FOV observations from BBSO full-disk Hα
images show that the AR end of the filament started ascending four hours
before the flare. We attribute the observed downflows at the AR end of
the filament to the draining effect of the filament rising prior to
its eruption. During the slow-rise phase, the downflows continuously
drained away ∼1015g mass from the filament over a few
hours, which is believed to be essential for the instability, and
could be an important precursor of eruptive events.

---------------------------------------------------------
Title: Construction of a century solar chromosphere data set for
solar activity related research
Authors: Lin, Ganghua; Wang, Xiao Fan; Yang, Xiao; Liu, Suo; Zhang,
Mei; Wang, Haimin; Liu, Chang; Xu, Yan; Tlatov, Andrey; Demidov,
Mihail; Borovik, Aleksandr; Golovko, Aleksey
2017STP.....3b...5L Altcode: 2017SZF.....3b...5L
This article introduces our ongoing project "Construction of a Century
Solar Chromosphere Data Set for Solar Activity Related Research". Solar
activities are the major sources of space weather that affects
human lives. Some of the serious space weather consequences, for
instance, include interruption of space communication and navigation,
compromising the safety of astronauts and satellites, and damaging
power grids. Therefore, the solar activity research has both scientific
and social impacts. The major database is built up from digitized and
standardized film data obtained by several observatories around the
world and covers a time span of more than 100 years. After careful
calibration, we will develop feature extraction and data mining
tools and provide them together with the comprehensive database for
the astronomical community. Our final goal is to address several
physical issues: filament behavior in solar cycles, abnormal behavior
of solar cycle 24, large-scale solar eruptions, and sympathetic remote
brightenings. Significant signs of progress are expected in data mining
algorithms and software development, which will benefit the scientific
analysis and eventually advance our understanding of solar cycles.

---------------------------------------------------------
Title: Witnessing a Large-scale Slipping Magnetic Reconnection along
a Dimming Channel during a Solar Flare
Authors: Jing, Ju; Liu, Rui; Cheung, Mark C. M.; Lee, Jeongwoo; Xu,
Yan; Liu, Chang; Zhu, Chunming; Wang, Haimin
2017ApJ...842L..18J Altcode: 2017arXiv170601355J
We report the intriguing large-scale dynamic phenomena associated with
the M6.5 flare (SOL2015-06-22T18:23) in NOAA active region 12371,
observed by RHESSI, Fermi, and the Atmospheric Image Assembly (AIA)
and Magnetic Imager (HMI) on the Solar Dynamics Observatory (SDO). The
most interesting feature of this event is a third ribbon (R3) arising
in the decay phase, propagating along a dimming channel (seen in EUV
passbands) toward a neighboring sunspot. The propagation of R3 occurs
in the presence of hard X-ray footpoint emission and is broadly visible
at temperatures from 0.6 MK to over 10 MK through the differential
emission measure analysis. The coronal loops then undergo an apparent
slipping motion following the same path of R3, after a ∼80 minute
delay. To understand the underlying physics, we investigate the magnetic
configuration and the thermal structure of the flaring region. Our
results are in favor of a slipping-type reconnection followed by the
thermodynamic evolution of coronal loops. In comparison with those
previously reported slipping reconnection events, this one proceeds
across a particularly long distance (∼60 Mm) over a long period of
time (∼50 minutes) and shows two clearly distinguished phases: the
propagation of the footpoint brightening driven by nonthermal particle
injection and the apparent slippage of loops governed by plasma heating
and subsequent cooling.

---------------------------------------------------------
Title: High-resolution observations of flare precursors in the low
solar atmosphere
Authors: Wang, Haimin; Liu, Chang; Ahn, Kwangsu; Xu, Yan; Jing, Ju;
Deng, Na; Huang, Nengyi; Liu, Rui; Kusano, Kanya; Fleishman, Gregory
D.; Gary, Dale E.; Cao, Wenda
2017NatAs...1E..85W Altcode: 2017arXiv170309866W
Solar flares are generally believed to be powered by free magnetic
energy stored in the corona1, but the build up of
coronal energy alone may be insufficient to trigger the flare to
occur2. The flare onset mechanism is a critical but poorly
understood problem, insights into which could be gained from small-scale
energy releases known as precursors. These precursors are observed as
small pre-flare brightenings in various wavelengths3-13
and also from certain small-scale magnetic configurations such
as opposite-polarity fluxes14-16, where the magnetic
orientation of small bipoles is opposite to that of the ambient main
polarities. However, high-resolution observations of flare precursors
together with the associated photospheric magnetic field dynamics are
lacking. Here we study precursors of a flare using the unprecedented
spatiotemporal resolution of the 1.6-m New Solar Telescope, complemented
by new microwave data. Two episodes of precursor brightenings are
initiated at a small-scale magnetic channel17-20 (a form of
opposite-polarity flux) with multiple polarity inversions and enhanced
magnetic fluxes and currents, lying near the footpoints of sheared
magnetic loops. Microwave spectra corroborate that these precursor
emissions originate in the atmosphere. These results provide evidence
of low-atmospheric small-scale energy release, possibly linked to the
onset of the main flare.

---------------------------------------------------------
Title: The Energetics of White-light Flares Observed by SDO/HMI
and RHESSI
Authors: Huang, Neng-Yi; Xu, Yan; Wang, Haimin
2016RAA....16..177H Altcode: 2016arXiv160806015H
White-light (WL) flares have been observed and studied for more than a
century since their first discovery. However, some fundamental physics
behind the brilliant emission remains highly controversial. One
of the important facts in addressing the flare energetics is the
spatio-temporal correlation between the WL emission and the hard X-ray
(HXR) radiation, presumably suggesting that energetic electrons are the
energy sources. In this study, we present a statistical analysis of
25 strong flares (≥M5) observed simultaneously by the Helioseismic
and Magnetic Imager (HMI), on board the Solar Dynamics Observatory
(SDO), and the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). Among these events, WL emission was detected by SDO/HMI in
13 flares, associated with HXR emission. To quantitatively describe
the strength of WL emission, equivalent area (EA) is defined as the
integrated contrast enhancement over the entire flaring area. Our
results show that the EA is inversely proportional to the HXR power-law
index, indicating that stronger WL emission tends to be associated
with a larger population of high energy electrons. However, no obvious
correlation is found between WL emission and flux of non-thermal
electrons at 50 keV. For the other group of 13 flares without detectable
WL emission, the HXR spectra are softer (larger power-law index) than
those flares with WL emission, especially for the X-class flares in
this group.

---------------------------------------------------------
Title: Strong Blue Asymmetry in Hα Line as a Preflare Activity
Authors: Cho, Kyuhyoun; Lee, Jeongwoo; Chae, Jongchul; Wang, Haimin;
Ahn, Kwangsu; Yang, Heesu; Lim, Eun-kyung; Maurya, Ram Ajor
2016SoPh..291.2391C Altcode: 2016SoPh..tmp..131C; 2020arXiv200506404C
Chromospheric activities before solar flares provide important clues
to the mechanisms that initiate solar flares, but are as yet poorly
understood. We report a significant and rapid Hα line broadening
before the solar flare SOL2011-09-29T18:08 that was detected using
the unprecedented high-resolution Hα imaging spectroscopy with the
Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m New
Solar Telescope (NST) at Big Bear Solar Observatory. The strong Hα
broadening extends as a blue excursion up to −4.5 Å and as a red
excursion up to 2.0 Å, which implies a mixture of velocities in the
range of −130 kms−1 to 38 km s−1 derived
by applying the cloud model, comparable to the highest chromospheric
motions reported before. The Hα blueshifted broadening lasts for
about six minutes and is temporally and spatially correlated with the
start of a rising filament, which is later associated with the main
phase of the flare as detected by the Atmosphere Imaging Assembly (AIA)
onboard the Solar Dynamics Observatory (SDO). The potential importance
of this Hα blueshifted broadening as a preflare chromospheric activity
is briefly discussed within the context of the two-step eruption model.

---------------------------------------------------------
Title: Multi-wavelength Study of Transition Region Penumbral
Subarcsecond Bright Dots Using IRIS and NST
Authors: Deng, Na; Yurchyshyn, Vasyl; Tian, Hui; Kleint, Lucia; Liu,
Chang; Xu, Yan; Wang, Haimin
2016ApJ...829..103D Altcode: 2016arXiv160700306D
Using high-resolution transition region (TR) observations taken by
the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
al. revealed numerous short-lived subarcsecond bright dots (BDs)
above sunspots (mostly located in the penumbrae), which indicate yet
unexplained small-scale energy releases. Moreover, whether or not
these subarcsecond TR brightenings have any signature in the lower
atmosphere and how they are formed are still not fully resolved. This
paper presents a multi-wavelength study of the TR penumbral BDs using a
coordinated observation of a near disk center sunspot with IRIS and the
1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory. NST
provides high-resolution chromospheric and photospheric observations
with narrowband Hα imaging spectroscopy and broadband TiO images,
respectively, complementary to IRIS TR observations. A total of 2692
TR penumbral BDs are identified from a 37 minute time series of IRIS
1400 Å slit-jaw images. Their locations tend to be associated more
with downflowing and darker fibrils in the chromosphere, and weakly
associated with bright penumbral features in the photosphere. However,
temporal evolution analyses of the BDs show that there is no consistent
and convincing brightening response in the chromosphere. These results
are compatible with a formation mechanism of the TR penumbral BDs
by falling plasma from coronal heights along more vertical and dense
magnetic loops. The BDs may also be produced by small-scale impulsive
magnetic reconnection taking place sufficiently high in the atmosphere
that has no energy release in the chromosphere.

---------------------------------------------------------
Title: Flare differentially rotates sunspot on Sun's surface
Authors: Liu, Chang; Xu, Yan; Cao, Wenda; Deng, Na; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016NatCo...713104L Altcode: 2016arXiv161002969L
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere). It was considered implausible that solar flares,
as resulted from magnetic reconnection in the tenuous corona, would
cause a direct perturbation of the dense photosphere involving bulk
motion. Here we report the sudden flare-induced rotation of a sunspot
using the unprecedented spatiotemporal resolution of the 1.6 m New
Solar Telescope, supplemented by magnetic data from the Solar Dynamics
Observatory. It is clearly observed that the rotation is non-uniform
over the sunspot: as the flare ribbon sweeps across, its different
portions accelerate (up to ~50° h-1) at different times
corresponding to peaks of flare hard X-ray emission. The rotation may be
driven by the surface Lorentz-force change due to the back reaction of
coronal magnetic restructuring and is accompanied by a downward Poynting
flux. These results have direct consequences for our understanding of
energy and momentum transportation in the flare-related phenomena.

---------------------------------------------------------
Title: On the Rotation of Sunspots and Their Magnetic Polarity
Authors: Zheng, Jianchuan; Yang, Zhiliang; Guo, Kaiming; Wang, Haimin;
Wang, Shuo
2016ApJ...826....6Z Altcode: 2016arXiv160708343Z
The rotation of sunspots of 2 yr in two different solar cycles is
studied with the data from the Helioseismic and Magnetic Imager on
board the Solar Dynamics Observatory and the Michelson Doppler Imager
instrument on board the Solar and Heliospheric Observataory. We
choose the α sunspot groups and the relatively large and stable
sunspots of complex active regions in our sample. In the year of
2003, the α sunspot groups and the preceding sunspots tend to rotate
counterclockwise and have positive magnetic polarity in the northern
hemisphere. In the southern hemisphere, the magnetic polarity and
rotational tendency of the α sunspot groups and the preceding sunspots
are opposite to the northern hemisphere. The average rotational speed
of these sunspots in 2003 is about 0.°65 hr-1. From 2014
January to 2015 February, the α sunspot groups and the preceding
sunspots tend to rotate clockwise and have negative magnetic polarity
in the northern hemisphere. The patterns of rotation and magnetic
polarity of the southern hemisphere are also opposite to those of the
northern hemisphere. The average rotational speed of these sunspots
in 2014/2015 is about 1.°49 hr-1. The rotation of the
relatively large and stable preceding sunspots and that of the α
sunspot groups located in the same hemisphere have opposite rotational
direction in 2003 and 2014/2015.

---------------------------------------------------------
Title: Irreversible rapid changes of magnetic field associated with
the 2012 October 23 circular near-limb X1.8 Flare
Authors: Ye, Dan-Dan; Liu, Chang; Wang, Haimin
2016RAA....16...95Y Altcode: 2016RAA....16f..10Y; 2016arXiv160202080Y
It has been found that photospheric magnetic fields can change in
accordance with restructuring of the three-dimensional magnetic field
following solar eruptions. Previous studies mainly use vector magnetic
field data taken for events near the disk center. In this paper, we
analyze the magnetic field evolution associated with the 2012 October 23
X1.8 flare in NOAA AR 11598 that is close to the solar limb, using both
the 45 s cadence line-of-sight and 12 min cadence vector magnetograms
from the Helioseismic and Magnetic Imager on board Solar Dynamics
Observatory. This flare is classified as a circular-ribbon flare
with spine-fan type magnetic topology containing a null point. In
the line-of-sight magnetograms, there are two apparent polarity
inversion lines (PILs). The PIL closer to the limb is affected more
by the projection effect. Between these two PILs there lie positive
polarity magnetic fields, which are surrounded by negative polarity
fields outside the PILs. We find that after the flare, both the
apparent limb-ward and disk-ward negative fluxes decrease, while the
positive flux in-between increases. We also find that the horizontal
magnetic fields have a significant increase along the disk-ward
PIL, but in the surrounding area, they decrease. Synthesizing the
observed field changes, we conclude that the magnetic fields collapse
toward the surface above the disk-ward PIL as depicted in the coronal
implosion scenario, while the peripheral field turns to a more vertical
configuration after the flare. We also suggest that this event is
an asymmetric circular-ribbon flare: a flux rope is likely present
above the disk-ward PIL. Its eruption causes instability of the entire
fan-spine structure and the implosion near that PIL.

---------------------------------------------------------
Title: Ultra-Narrow Negative Flare Front Observed in Helium-10830
Å Using the1.6m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyung-Suk; Gary, Dale E.; Wang, Haimin
2016SPD....47.0633X Altcode:
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6-m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in solar flares.

---------------------------------------------------------
Title: BBSO/NST Observations of the Sudden Differential Rotation of
a Sunspot Caused by a Major Flare
Authors: Liu, Chang; Xu, Yan; Deng, Na; Cao, Wenda; Lee, Jeongwoo;
Hudson, Hugh S.; Gary, Dale E.; Wang, Jiasheng; Jing, Ju; Wang, Haimin
2016SPD....47.0615L Altcode:
Sunspots are concentrations of magnetic field visible on the solar
surface (photosphere), from which the field extends high into the
corona. Complex plasma motions that drag field in the photosphere can
build up free energy in the corona that powers solar eruptions. It
is known that solar flares and the often associated coronal ejections
(CMEs) can produce various radiations in the low atmosphere. However,
it was considered implausible that disturbances created in the tenuous
corona would cause a direct perturbation of the dense photosphere
involving bulk motion. Here we report the sudden rotational motion of
a sunspot clearly induced by a major solar flare (SOL2015-06-22T18:23
M6.6), using the unprecedented spatiotemporal resolution of the 1.6 m
New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO). It is
particularly striking that the rotation is not uniform over the sunspot:
as the flare ribbon sweeps across, its different portions accelerate
(up to ~50 degree per hour) at different times corresponding to peaks
of flare hard X-ray emission. The intensity and magnetic field of
the sunspot also change significantly associated with the flare. Our
results reveal an intrinsic relationship between the photospheric
plasma bulk motions and coronal energy release, with direct consequences
for our understanding of energy and momentum balance in the flare/CME
phenomenon. This work is mainly supported by NASA grants NNX13AF76G
and NNX13AG13G (LWS), and NNX16AF72G, and NSF grants AGS 1250818
and 1408703.

---------------------------------------------------------
Title: Hard X-ray and Microwave Simulation of 2015-06-22 M6.6 flare
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.; Fleishman,
Gregory D.; Nita, Gelu M.; Chen, Bin; Xu, Yan; Jing, Ju
2016SPD....47.0614K Altcode:
It is well known that the time profiles of the hard X-ray (HXR) emission
and the microwave (MW) emission during the impulsive phase of the solar
flare are well correlated, and this has led to the expectation that
these emissions come from a common population of flare-accelerated
electrons. However, the energy ranges of the electrons producing two
emissions are believed to be different (below and above several hundred
keV for HXR-producing and MW-producing electrons, respectively), and
some studies have shown that the indices of their energy spectra may
differ as well. To better understand the energy distributions of the
electrons producing these emissions, we present realistic forward-fit
simulations of the HXR and the MW emissions of 2015 June 22, M6.6 flare
using the newly developed, IDL-based platform GX simulator. We use the
3D magnetic field model extrapolated from magnetogram data from the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO), the images and the electron energy distribution
parameters deduced from the photon spectrum from the Reuven Ramaty
High Energy Solar Spectroscopic Imager (RHESSI), and the spatially
integrated MW spectrum and the cross-correlated amplitude data from the
Expanded Owens Valley Solar Array (EOVSA) to guide the modeling. We have
observed a possible above the-loop-top HXR source in 20-25 keV image,
well separated from the source seen in 6-12 keV that is typically
interpreted as a thermal loop-top source. Therefore, we simulate
the HXR emissions by combining two flux tubes at different heights:
the lower loop dominated by thermal electrons and the higher loop
dominated by nonthermal electrons. The MW and HXR emissions produced
from the forward-fit model are compared with observations to investigate
possible differences in the energy spectra of the HXR-producing and
the MW-producing electrons and what they can tell us about particle
acceleration.

---------------------------------------------------------
Title: High-Resolution Observations of Flare Precursors and Their
Relationship with Magnetic Channels
Authors: Wang, Haimin; Xu, Yan; Ahn, Kwangsu; Jing, Ju; Liu, Chang;
Deng, Na; Huang, Nengyi; Gary, Dale E.; Cao, Wenda
2016SPD....4720501W Altcode:
The study of precursors of flares is important for understanding
the basic magnetic instability leading to solar flares, which can
aid the forecasting of eruptions potentially related to severe
space weather effects. Although literatures reported many important
clues, high-resolution observations of pre-flare activities before a
well-observed solar flare have been rare. Even rarely, the associated
magnetic structures in fine scale (below 1") were also observed. In
this study we take advantage of multiwavelength high-resolution
observations completely covering the 2015 June 22 M6.6 flare, which
were obtained under excellent seeing condition with the 1.6 m New Solar
Telescope (NST) at Big Bear Solar Observatory. The NST data includes
observations of the H-alpha line in five spectral positions at a spatial
resolution of 0.1" and magnetograms at a resolution of 0.25". These
are complemented by IRIS UV observations with a resolution of 0.25". We
find that there are two episodes of pre-flare brightenings (precursors),
which are spatially associated with magnetic channels, i.e., elongated
structures comprising alternating magnetic polarity inversion lines
(Zirin &amp; Wang, 1993, Nature, 363, 426). The pre-flare chromospheric
and coronal features reflect an extremely sheared magnetic topology,
while the initiation of main flare brightenings correspond to a much
less sheared configuration. RHESSI HXR observations reveal that the
precursors have both thermal and nonthermal components, and the latter
is further evidenced by the microwave observations of the newly expanded
Solar Radio Array at Owens Valley.We further investigate the electric
current system above the magnetic channels using NLFFF extrapolations,
which show strong current sheets above the channel structure. This is
consistent with the MHD modeling of Kusano et al (2012, Ap.J., 760, 31),
who noted the importance of localized small-scale magnetic structure
in triggering the eruption of the whole active region. We suggest that
small-scale magnetic reconnection along the channels destabilizes the
magnetic structure of the active region and subsequently triggers the
main M6.6 flare and the associated halo CME.

---------------------------------------------------------
Title: The Energetics of Wight-light Flares Observed in Visible
Continuum and Hard X-ray
Authors: Huang, Nengyi; Xu, Yan; Wang, Haimin
2016SPD....4740601H Altcode:
White-light (WL) flares have been observed and studied more than a
century since the first discovery. However, some fundamental physics
behind the brilliant emission remains highly controversial. One
of the important facts in addressing the flare energetics is the
spatial-temporal correlation between the white-light emission and the
hard X-ray radiation, presumably suggesting that the energetic electrons
are the energy sources. In this study, we present a statistical analysis
of 26 strong flares (&gt;M5) observed simultaneously by the Helioseismic
and Magnetic Imager (HMI) on board the Solar Dynamics Observatory
(SDO) and the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI). Among these events, WL emission was detected by SDO/HMI in 13
flares, associated with HXR emission. To quantitatively describe the
strength of WL emission, equivalent area is defined as the integrated
contrast enhancement over the entire flaring area. Our results show that
the equivalent area is inversely proportional to the HXR power index,
indicating that stronger WL emission tends to be associated with larger
population of high energy electrons. Furthermore, we studied an M6.6
flare on 2015 June 22 observed by BBSO’s New Solar Telescope (NST),
showing WL emission in TiO continuum (705.7 nm), but no detectable WL
signal from SDO/HMI. The power index- equivalent area relationship of
this flare matches the trend found in the statistical analysis. In
addition, for the other group of 13 flares without detectable WL
emission, the HXR spectra are softer (larger power index) than those
flares with WL emission, especially for the X-class flares in this
group.

---------------------------------------------------------
Title: Multi-wavelength Study of Transition Region Penumbral
Bright Dots Using Interface Region Imaging Spectrograph and New
Solar Telescope
Authors: Deng, Na; Yurchyshyn, Vasyl B.; Tian, Hui; Kleint, Lucia;
Liu, Chang; Xu, Yan; Wang, Haimin
2016SPD....47.0101D Altcode:
Using high-resolution transition region (TR) observations taken by
the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
al. (2014b) revealed numerous short-lived sub-arcsecond bright dots
above sunspots (mostly located in the penumbrae), which indicate yet
unexplained small-scale energy releases. Moreover, whether these TR
brightenings have any signature in the lower atmosphere and how they
are formed are still not fully resolved. This paper presents a study of
these bright dots using a coordinated observation of a near disk-center
sunspot with IRIS and the 1.6 m New Solar Telescope (NST) at the Big
Bear Solar Observatory. NST provides high-resolution chromospheric
and photospheric observations with narrow-band H-alpha imaging
spectroscopy and broad-band TiO images, respectively, complementary
to IRIS TR observations. A total of 2692 TR penumbral bright dots
are identified from a 37-minute time series of IRIS 1400 A slitjaw
images. Their locations tend to be associated more with downflowing and
darker fibrils in the chromosphere, and weakly associated with bright
penumbral features in the photosphere. However, temporal evolution
analyses of the dots show that there is no consistent and convincing
brightening response in the chromosphere. These results are compatible
with a formation mechanism of the TR penumbral bright dots by falling
plasma from coronal heights along more vertical and dense magnetic
loops. The dots may also be produced by small-scale impulsive magnetic
reconnection taking place sufficiently high in the atmosphere that
has no energy release in the chromosphere.Acknowledgement: This work
is mainly supported by NASA grants NNX14AC12G, NNX13AF76G and by NSF
grant AGS 1408703.

---------------------------------------------------------
Title: Unprecedented Fine Structure of a Solar Flare Revealed by
the 1.6~m New Solar Telescope
Authors: Jing, Ju; Xu, Yan; Cao, Wenda; Liu, Chang; Gary, Dale E.;
Wang, Haimin
2016SPD....47.0601J Altcode:
Solar flares signify the sudden release of magnetic energy and are
sources of so called space weather. The fine structures (below 500
km) of flares are rarely observed and are accessible to only a few
instruments world-wide. Here we present observation of a solar flare
using exceptionally high resolution images from the 1.6~m New Solar
Telescope (NST) equipped with high order adaptive optics at Big Bear
Solar Observatory (BBSO). The observation reveals the process of the
flare in unprecedented detail, including the flare ribbon propagating
across the sunspots, coronal rain (made of condensing plasma) streaming
down along the post-flare loops, and the chromosphere's response to
the impact of coronal rain, showing fine-scale brightenings at the
footpoints of the falling plasma. Taking advantage of the resolving
power of the NST, we measure the cross-sectional widths of flare
ribbons, post-flare loops and footpoint brighenings, which generally lie
in the range of 80-200 km, well below the resolution of most current
instruments used for flare studies. Confining the scale of such fine
structure provides an essential piece of information in modeling the
energy transport mechanism of flares, which is an important issue in
solar and plasma physics.

---------------------------------------------------------
Title: Unprecedented Fine Structure of a Solar Flare Revealed by
the 1.6 m New Solar Telescope
Authors: Jing, Ju; Xu, Yan; Cao, Wenda; Liu, Chang; Gary, Dale;
Wang, Haimin
2016NatSR...624319J Altcode: 2016arXiv160408562J
Solar flares signify the sudden release of magnetic energy and
are sources of so called space weather. The fine structures (below
500 km) of flares are rarely observed and are accessible to only a few
instruments world-wide. Here we present observation of a solar flare
using exceptionally high resolution images from the 1.6 m New Solar
Telescope (NST) equipped with high order adaptive optics at Big Bear
Solar Observatory (BBSO). The observation reveals the process of the
flare in unprecedented detail, including the flare ribbon propagating
across the sunspots, coronal rain (made of condensing plasma) streaming
down along the post-flare loops, and the chromosphere’s response to
the impact of coronal rain, showing fine-scale brightenings at the
footpoints of the falling plasma. Taking advantage of the resolving
power of the NST, we measure the cross-sectional widths of flare
ribbons, post-flare loops and footpoint brighenings, which generally lie
in the range of 80-200 km, well below the resolution of most current
instruments used for flare studies. Confining the scale of such fine
structure provides an essential piece of information in modeling the
energy transport mechanism of flares, which is an important issue in
solar and plasma physics.

---------------------------------------------------------
Title: Ultra-narrow Negative Flare Front Observed in Helium-10830
Å Using the 1.6 m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyungsuk; Gary, Dale; Wang, Haimin
2016ApJ...819...89X Altcode: 2016arXiv160104729X
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6 m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in astronomical objects.

---------------------------------------------------------
Title: Structure, Stability, and Evolution of Magnetic Flux Ropes
from the Perspective of Magnetic Twist
Authors: Liu, Rui; Kliem, Bernhard; Titov, Viacheslav S.; Chen, Jun;
Wang, Yuming; Wang, Haimin; Liu, Chang; Xu, Yan; Wiegelmann, Thomas
2016ApJ...818..148L Altcode: 2015arXiv151202338L
We investigate the evolution of NOAA Active Region (AR) 11817 during
2013 August 10-12, when it developed a complex field configuration
and produced four confined, followed by two eruptive, flares. These
C-and-above flares are all associated with a magnetic flux rope (MFR)
located along the major polarity inversion line, where shearing and
converging photospheric flows are present. Aided by the nonlinear
force-free field modeling, we identify the MFR through mapping magnetic
connectivities and computing the twist number {{ T }}w
for each individual field line. The MFR is moderately twisted (|
{{ T }}w| \lt 2) and has a well-defined boundary of high
squashing factor Q. We found that the field line with the extremum |
{{ T }}w| is a reliable proxy of the rope axis, and that
the MFR's peak | {{ T }}w| temporarily increases within
half an hour before each flare while it decreases after the flare peak
for both confined and eruptive flares. This pre-flare increase in | {{
T }}w| has little effect on the AR's free magnetic energy or
any other parameters derived for the whole region, due to its moderate
amount and the MFR's relatively small volume, while its decrease after
flares is clearly associated with the stepwise decrease in the whole
region's free magnetic energy due to the flare. We suggest that {{ T
}}w may serve as a useful parameter in forewarning the onset
of eruption, and therefore, the consequent space weather effects. The
helical kink instability is identified as the prime candidate onset
mechanism for the considered flares.

---------------------------------------------------------
Title: Erratum: “Slow Rise and Partial Eruption of a
Double-decker Filament. I Observations and Interpretation'<A
href="/abs/2012ApJ...756...59L">(2012, ApJ, 756, 59)</A>
Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov,
Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin
2015ApJ...814..164L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Role of Erupting Sigmoid in Triggering a Flare with
Parallel and Large-scale Quasi-circular Ribbons
Authors: Joshi, Navin Chandra; Liu, Chang; Sun, Xudong; Wang, Haimin;
Magara, Tetsuya; Moon, Y. -J.
2015ApJ...812...50J Altcode: 2015arXiv150901792J
In this paper, we present observations and analysis of an interesting
sigmoid formation, eruption, and the associated flare that occurred
on 2014 April 18 using multi-wavelength data sets. We discuss the
possible role of the sigmoid eruption in triggering the flare, which
consists of two different sets of ribbons: parallel ribbons and a
large-scale quasi-circular ribbon. Several observational evidence and
nonlinear force-free field extrapolation results show the existence
of a large-scale fan-spine type magnetic configuration with a sigmoid
lying under a section of the fan dome. The event can be explained with
the following two phases. During the preflare phase, we observed the
formation and appearance of the sigmoid via tether-cutting reconnection
between the two sets of sheared fields under the fan dome. The second,
main flare phase features the eruption of the sigmoid, the subsequent
flare with parallel ribbons, and a quasi-circular ribbon. We propose
the following multi-stage successive reconnection scenario for the
main flare. First, tether-cutting reconnection is responsible for
the formation and the eruption of the sigmoid structure. Second, the
reconnection occurring in the wake of the erupting sigmoid produces
the parallel flare ribbons on the both sides of the circular polarity
inversion line. Third, the null-type reconnection higher in the corona,
possibly triggered by the erupting sigmoid, leads to the formation
of a large quasi-circular ribbon. For the first time, we suggest a
mechanism for this type of flare consisting of a double set of ribbons
triggered by an erupting sigmoid in a large-scale fan-spine-type
magnetic configuration.

---------------------------------------------------------
Title: A Circular-ribbon Solar Flare Following an Asymmetric Filament
Eruption
Authors: Liu, Chang; Deng, Na; Liu, Rui; Lee, Jeongwoo; Pariat,
Étienne; Wiegelmann, Thomas; Liu, Yang; Kleint, Lucia; Wang, Haimin
2015ApJ...812L..19L Altcode: 2015arXiv150908414L
The dynamic properties of flare ribbons and the often associated
filament eruptions can provide crucial information on the flaring
coronal magnetic field. This Letter analyzes the GOES-class X1.0 flare
on 2014 March 29 (SOL2014-03-29T17:48), in which we found an asymmetric
eruption of a sigmoidal filament and an ensuing circular flare
ribbon. Initially both EUV images and a preflare nonlinear force-free
field model show that the filament is embedded in magnetic fields with
a fan-spine-like structure. In the first phase, which is defined by a
weak but still increasing X-ray emission, the western portion of the
sigmoidal filament arches upward and then remains quasi-static for
about five minutes. The western fan-like and the outer spine-like
fields display an ascending motion, and several associated ribbons
begin to brighten. Also found is a bright EUV flow that streams down
along the eastern fan-like field. In the second phase that includes the
main peak of hard X-ray (HXR) emission, the filament erupts, leaving
behind two major HXR sources formed around its central dip portion
and a circular ribbon brightened sequentially. The expanding western
fan-like field interacts intensively with the outer spine-like field,
as clearly seen in running difference EUV images. We discuss these
observations in favor of a scenario where the asymmetric eruption of
the sigmoidal filament is initiated due to an MHD instability and
further facilitated by reconnection at a quasi-null in corona; the
latter is in turn enhanced by the filament eruption and subsequently
produces the circular flare ribbon.

---------------------------------------------------------
Title: Gradual Magnetic Evolution of Sunspot Structure and
Filament-Corona Dynamics Associated with the X1.8 Flare in AR11283
Authors: Ruan, Guiping; Chen, Yao; Wang, Haimin
2015ApJ...812..120R Altcode: 2015arXiv150904407R
In this paper, we present a study of the persistent and gradual
penumbral decay and the correlated decline of the photospheric
transverse field component 10-20 hr before a major flare (X1.8)
eruption on 2011 September 7. This long-term pre-eruption behavior
is corroborated by the well-imaged pre-flare filament rising, the
consistent expansion of the coronal arcades overlying the filament, and
the nonlinear force-free field modeling results in the literature. We
suggest that both the long-term pre-flare penumbral decay and the
transverse field decline are photospheric manifestations of the gradual
rise of the coronal filament-flux rope system. We also suggest that
the C3 flare and the subsequent reconnection process preceding the X1.8
flare play an important role in triggering the later major eruption.

---------------------------------------------------------
Title: Comparison between the eruptive X2.2 flare on 2011 February
15 and confined X3.1 flare on 2014 October 24
Authors: Jing, Ju; Xu, Yan; Lee, Jeongwoo; Nitta, Nariaki V.; Liu,
Chang; Park, Sung-Hong; Wiegelmann, Thomas; Wang, Haimin
2015RAA....15.1537J Altcode:
We compare two contrasting X-class flares in terms of magnetic free
energy, relative magnetic helicity and decay index of the active regions
(ARs) in which they occurred. The events in question are the eruptive
X2.2 flare from AR 11158 accompanied by a halo coronal mass ejection
(CME) and the confined X3.1 flare from AR 12192 with no associated
CME. These two flares exhibit similar behavior of free magnetic energy
and helicity buildup for a few days preceding them. A major difference
between the two flares is found to lie in the time-dependent change
of magnetic helicity of the ARs that hosted them. AR 11158 shows a
significant decrease in magnetic helicity starting ∼4 hours prior
to the flare, but no apparent decrease in helicity is observed in AR
12192. By examining the magnetic helicity injection rates in terms
of sign, we confirmed that the drastic decrease in magnetic helicity
before the eruptive X2.2 flare was not caused by the injection of
reversed helicity through the photosphere but rather the CME-related
change in the coronal magnetic field. Another major difference we find
is that AR 11158 had a significantly larger decay index and therefore
weaker overlying field than AR 12192. These results suggest that the
coronal magnetic helicity and the decay index of the overlying field
can provide a clue about the occurrence of CMEs.

---------------------------------------------------------
Title: Formation and Eruption of a Small Flux Rope in the Chromosphere
Observed by NST, IRIS, and SDO
Authors: Kumar, Pankaj; Yurchyshyn, Vasyl; Wang, Haimin; Cho, Kyung-Suk
2015ApJ...809...83K Altcode: 2015arXiv150701761K
Using high-resolution images from the 1.6 m New Solar Telescope
at Big Bear Solar Observatory, we report the direct evidence of
chromospheric reconnection at the polarity inversion line between
two small opposite polarity sunspots. Small jetlike structures
(with velocities of ∼20-55 km s-1) were observed at the
reconnection site before the onset of the first M1.0 flare. The slow
rise of untwisting jets was followed by the onset of cool plasma inflow
(∼10 km s-1) at the reconnection site, causing the onset
of a two-ribbon flare. The reconnection between two sheared J-shaped
cool Hα loops causes the formation of a small twisted (S-shaped) flux
rope in the chromosphere. In addition, Helioseismic and Magnetic Imager
magnetograms show the flux cancellation (both positive and negative)
during the first M1.0 flare. The emergence of negative flux and the
cancellation of positive flux (with shear flows) continue until the
successful eruption of the flux rope. The newly formed chromospheric
flux rope becomes unstable and rises slowly with a speed of ∼108 km
s-1 during a second C8.5 flare that occurred after ∼3
hr of the first M1.0 flare. The flux rope was destroyed by repeated
magnetic reconnection induced by its interaction with the ambient field
(fan-spine topology) and looks like an untwisting surge (∼170 km
s-1) in the coronal images recorded by the Solar Dynamics
Observatory/Atmospheric Imaging Assembly. These observations suggest
the formation of a chromospheric flux rope (by magnetic reconnection
associated with flux cancellation) during the first M1.0 flare and
its subsequent eruption/disruption during the second C8.5 flare.

---------------------------------------------------------
Title: Development of technique to detect and classify small-scale
magnetic flux cancellation and rapid blue-shifted excursions
Authors: Chen, Xin; Deng, Na; Lamb, Derek A.; Jing, Ju; Liu, Chang;
Liu, Rui; Park, Sung-Hong; Wang, Haimin
2015RAA....15.1012C Altcode: 2015arXiv150101226C
We present a set of tools for detecting small-scale solar magnetic
cancellations and the disk counterpart of type II spicules (the
so-called Rapid Blueshifted Excursions (RBEs)), using line-of-sight
photospheric magnetograms and chromospheric spectroscopic observations,
respectively. For tracking magnetic cancellation, we improve the
Southwest Automatic Magnetic Identification Suite (SWAMIS) so that
it is able to detect certain obscure cancellations that can be easily
missed. For detecting RBEs, we use a normalized reference profile to
reduce false-positive detections caused by the non-uniform background
and seeing condition. Similar to the magnetic feature tracking in
SWAMIS, we apply a dual-threshold method to enhance the accuracy of
RBE detection. These tools are employed to analyze our coordinated
observations using the Interferometric BIdimensional Spectrometer at the
Dunn Solar Telescope of the National Solar Observatory and Hinode. We
present the statistical properties of magnetic cancellations and RBEs,
and explore their correlation using this data set.

---------------------------------------------------------
Title: Observation of the 2011-02-15 X2.2 Flare in the Hard X-Ray
and Microwave
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2015ApJ...807..124K Altcode: 2015arXiv150601424K
Previous studies have shown that the energy release mechanism of some
solar flares follow the Standard magnetic-reconnection model, but the
detailed properties of high-energy electrons produced in the flare are
still not well understood. We conducted a unique, multi-wavelength
study that discloses the spatial, temporal and energy distributions
of the accelerated electrons in the X2.2 solar flare on 2011 February
15. We studied the source locations of seven distinct temporal peaks
observed in hard X-ray (HXR) and microwave (MW) light curves using the
RHESSI in 50-75 keV channels and Nobeyama Radioheliograph in 34 GHz,
respectively. We found that the seven emission peaks did not come
from seven spatially distinct sites in HXR and MW, but rather in HXR
we observed a sudden change in location only between the second and
the third peak, with the same pattern occurring, but evolving more
slowly in MW. Comparison between the HXR light curve and the temporal
variations in intensity in the two MW source kernels also confirmed that
the seven peaks came predominantly from two sources, each with multiple
temporal peaks. In addition, we studied the polarization properties of
MW sources, and time delay between HXR and MW. We discuss our results
in the context of the tether-cutting model.

---------------------------------------------------------
Title: Dynamical Thermal Structure of Super-arcade Downflows in
Solar Flares
Authors: Chen, Xin; Liu, Rui; Deng, Na; Wang, Haimin
2015TESS....140704C Altcode:
Super-arcade downflows (SADs) have been frequently observed during
the gradual phase of flares near the limb. In coronal emission lines
sensitive to flaring plasmas, they appear as tadpole-like dark voids
against the bright fan-shape “haze” above the well-defined flare
arcade and flow toward the arcade. We carefully studied several
selected SADs from two flare events using data observed by Solar
Dynamic Observatory / Atmospheric Imaging Assembly and calculated their
differential emission measures (DEMs) as well as the DEM-weighted
temperature. Our analysis shows that SADs are associated with a
substantially decreased DEMs, by 1~3 order of magnitude, compared with
the surrounding plasma. None of the SADs indicate DEM solutions above
20 MK, which implies that SADs are indeed density depletion rather than
very hot plasma. This depression in DEMs rapidly recovers as SADs pass
through, generally in a few minutes. In addition, we found that SADs
in one event appear spatio-temporally associated with the formation
of postflare loops. These results are examined against models and
numerical simulations.

---------------------------------------------------------
Title: Multi-stage filament eruption and the decay index distribution,
A case study
Authors: Xu, Yan; Liu, Chang; Jing, Ju; Wang, Haimin
2015TESS....130403X Altcode:
The general consensus of solar eruptions is the release of magnetic
free energy. However, it is still a big challenge to our understanding
of the initiation of such events. From the observing perspective, the
erupting material, basically filaments or magnetic flux ropes, usually
keep either their original shape or become more twisted. The former
case can be explained by torus instability and the latter represents
the scenario of eruptions caused by kink instability. The model of
torus instability requires a fast decay of the overlaying magnetic
field, quantitatively defined as the vertical magnetic gradient in
the log space. A specific range (1.1 to 2.0) of the decay index has
been identified by previous studies and can be used to distinguish
eruptive and confined eruptions. On the other hand, two phases of
filament eruptions, i.e., a slow rising motion followed by a fast
escape, were frequently observed. We present such a filament eruption
occurred on 2007 May 19, in which the deprojected heights of the
filament are measured by the stereoscopic reconstruction with STEREO
observations. During the slow rising phase, the filament remained
below 40 Mm. The decay indices are calculated using the extrapolated
SOHO/MDI magnetograms. We found that the decay indices increase as a
function of height and reaches the special range (1.1 to 2.0) around
30 to 50 Mm. This result reveals a picture of the two-stage filament
eruption, in which a small perturbation breaks the equilibrium and
leads the filament to lift up slowly. Once the filament reaches the
critical height, the overlying field drops significantly leading to
an acceleration of the filament eruption.

---------------------------------------------------------
Title: Structure and evolution of magnetic fields associated with
solar eruptions
Authors: Wang, Haimin; Liu, Chang
2015RAA....15..145W Altcode: 2014arXiv1412.8676W
This paper reviews the studies of solar photospheric magnetic field
evolution in active regions and its relationship to solar flares. It is
divided into two topics, the magnetic structure and evolution leading
to solar eruptions and rapid changes in the photospheric magnetic
field associated with eruptions. For the first topic, we describe
the magnetic complexity, new flux emergence, flux cancelation, shear
motions, sunspot rotation and magnetic helicity injection, which may
all contribute to the storage and buildup of energy that trigger solar
eruptions. For the second topic, we concentrate on the observations
of rapid and irreversible changes of the photospheric magnetic field
associated with flares, and the implication on the restructuring of
the three-dimensional magnetic field. In particular, we emphasize the
recent advances in observations of the photospheric magnetic field,
as state-of-the-art observing facilities (such as Hinode and Solar
Dynamics Observatory) have become available. The linkages between
observations, theories and future prospectives in this research area
are also discussed.

---------------------------------------------------------
Title: Chromospheric Rapid Blueshifted Excursions Observed with IBIS
and their Association with Photospheric Magnetic Field Evolution
Authors: Deng, Na; Chen, Xin; Liu, Chang; Jing, Ju; Tritschler,
Alexandra; Reardon, Kevin P.; Lamb, Derek A.; Deforest, Craig E.;
Denker, Carsten; Wang, Shuo; Liu, Rui; Wang, Haimin
2015ApJ...799..219D Altcode: 2014arXiv1412.4038D
Chromospheric rapid blueshifted excursions (RBEs) are suggested to
be the disk counterparts of type II spicules at the limb and believed
to contribute to the coronal heating process. Previous identification
of RBEs was mainly based on feature detection using Dopplergrams. In
this paper, we study RBEs on 2011 October 21 in a very quiet region at
the disk center, which were observed with the high-cadence imaging
spectroscopy of the Ca II 8542 Å line from the Interferometric
Bidimensional Spectrometer (IBIS). By using an automatic spectral
analysis algorithm, a total of 98 RBEs are identified during an 11
minute period. Most of these RBEs have either a round or elongated
shape, with an average area of 1.2 arcsec2. The detailed
temporal evolution of spectra from IBIS makes possible a quantitative
determination of the velocity (~16 km s-1) and acceleration
(~400 m s-2) of Ca II 8542 RBEs, and reveals an additional
deceleration (~-160 m s-2) phase that usually follows the
initial acceleration. In addition, we also investigate the association
of RBEs with the concomitant photospheric magnetic field evolution,
using coordinated high-resolution and high-sensitivity magnetograms
made by Hinode. Clear examples are found where RBEs appear to be
associated with the preceding magnetic flux emergence and/or the
subsequent flux cancellation. However, further analysis with the aid
of the Southwest Automatic Magnetic Identification Suite does not
yield a significant statistical association between these RBEs and
magnetic field evolution. We discuss the implications of our results
in the context of understanding the driving mechanism of RBEs.

---------------------------------------------------------
Title: Three-dimensional Magnetic Restructuring in Two Homologous
Solar Flares in the Seismically Active NOAA AR 11283
Authors: Liu, Chang; Deng, Na; Lee, Jeongwoo; Wiegelmann, Thomas;
Jiang, Chaowei; Dennis, Brian R.; Su, Yang; Donea, Alina; Wang, Haimin
2014ApJ...795..128L Altcode: 2014arXiv1409.6391L
We carry out a comprehensive investigation comparing the
three-dimensional magnetic field restructuring, flare energy release,
and the helioseismic response of two homologous flares, the 2011
September 6 X2.1 (FL1) and September 7 X1.8 (FL2) flares in NOAA AR
11283. In our analysis, (1) a twisted flux rope (FR) collapses onto the
surface at a speed of 1.5 km s-1 after a partial eruption
in FL1. The FR then gradually grows to reach a higher altitude and
collapses again at 3 km s-1 after a fuller eruption in
FL2. Also, FL2 shows a larger decrease of the flux-weighted centroid
separation of opposite magnetic polarities and a greater change
of the horizontal field on the surface. These imply a more violent
coronal implosion with corresponding more intense surface signatures
in FL2. (2) The FR is inclined northward and together with the ambient
fields, it undergoes a southward turning after both events. This agrees
with the asymmetric decay of the penumbra observed in the peripheral
regions. (3) The amounts of free magnetic energy and nonthermal electron
energy released during FL1 are comparable to those of FL2 within the
uncertainties of the measurements. (4) No sunquake was detected in FL1;
in contrast, FL2 produced two seismic emission sources S1 and S2 both
lying in the penumbral regions. Interestingly, S1 and S2 are connected
by magnetic loops, and the stronger source S2 has a weaker vertical
magnetic field. We discuss these results in relation to the implosion
process in the low corona and the sunquake generation.

---------------------------------------------------------
Title: Interaction and Merging of two Sinistral Filaments
Authors: Jiang, Yunchun; Yang, Jiayan; Wang, Haimin; Ji, Haisheng;
Liu, Yu; Li, Haidong; Li, Jianping
2014ApJ...793...14J Altcode:
In this paper, we report the interaction and subsequent merging of two
sinistral filaments (F1 and F2) occurring at the boundary of AR 9720 on
2001 December 6. The two filaments were close and nearly perpendicular
to each other. The interaction occurred after F1 was erupted and
the eruption was impeded by a more extended filament channel (FC)
standing in the way, in which F2 was embedded. The erupted material
ran into FC along its axis, causing F1 and F2 to merge into a single
structure that subsequently underwent a large-amplitude to-and-fro
motion. A significant plasma heating process was observed in the
merging process, making the mixed material largely disappear from
the Hα passband, but appear in Extreme Ultraviolet Telescope 195 Å
images for a while. These observations can serve as strong evidence of
merging reconnection between the two colliding magnetic structures. A
new sinistral filament was formed along FC after the cooling of the
merged and heated material. No coronal mass ejection was observed to
be associated with the event; though, the eruption was accompanied
by a two-ribbon flare with a separation motion, indicating that the
eruption had failed. This event shows that, in addition to overlying
magnetic fields, such an interaction is an effective restraint to make
a filament eruption fail in this way.

---------------------------------------------------------
Title: Slow Rise and Partial Eruption of a Double-decker
Filament. II. A Double Flux Rope Model
Authors: Kliem, Bernhard; Török, Tibor; Titov, Viacheslav S.;
Lionello, Roberto; Linker, Jon A.; Liu, Rui; Liu, Chang; Wang, Haimin
2014ApJ...792..107K Altcode: 2014arXiv1407.2272K
Force-free equilibria containing two vertically arranged magnetic flux
ropes of like chirality and current direction are considered as a model
for split filaments/prominences and filament-sigmoid systems. Such
equilibria are constructed analytically through an extension of the
methods developed in Titov &amp; Démoulin and numerically through an
evolutionary sequence including shear flows, flux emergence, and flux
cancellation in the photospheric boundary. It is demonstrated that
the analytical equilibria are stable if an external toroidal (shear)
field component exceeding a threshold value is included. If this
component decreases sufficiently, then both flux ropes turn unstable
for conditions typical of solar active regions, with the lower rope
typically becoming unstable first. Either both flux ropes erupt upward,
or only the upper rope erupts while the lower rope reconnects with
the ambient flux low in the corona and is destroyed. However, for
shear field strengths staying somewhat above the threshold value,
the configuration also admits evolutions which lead to partial
eruptions with only the upper flux rope becoming unstable and the
lower one remaining in place. This can be triggered by a transfer of
flux and current from the lower to the upper rope, as suggested by
the observations of a split filament in Paper I. It can also result
from tether-cutting reconnection with the ambient flux at the X-type
structure between the flux ropes, which similarly influences their
stability properties in opposite ways. This is demonstrated for the
numerically constructed equilibrium.

---------------------------------------------------------
Title: An Unorthodox X-Class Long-duration Confined Flare
Authors: Liu, Rui; Titov, Viacheslav S.; Gou, Tingyu; Wang, Yuming;
Liu, Kai; Wang, Haimin
2014ApJ...790....8L Altcode: 2014arXiv1405.6774L
We report the observation of an X-class long-duration flare which is
clearly confined. It appears as a compact-loop flare in the traditional
EUV passbands (171 and 195 Å), but in the passbands sensitive to
flare plasmas (94 and 131 Å), it exhibits a cusp-shaped structure
above an arcade of loops like other long-duration events. Inspecting
images in a running difference approach, we find that the seemingly
diffuse, quasi-static cusp-shaped structure consists of multiple nested
loops that repeatedly rise upward and disappear approaching the cusp
edge. Over the gradual phase, we detect numerous episodes of loop
rising, each lasting minutes. A differential emission measure analysis
reveals that the temperature is highest at the top of the arcade and
becomes cooler at higher altitudes within the cusp-shaped structure,
contrary to typical long-duration flares. With a nonlinear force-free
model, our analysis shows that the event mainly involves two adjacent
sheared arcades separated by a T-type hyperbolic flux tube (HFT). One
of the arcades harbors a magnetic flux rope, which is identified with a
filament that survives the flare owing to the strong confining field. We
conclude that a new emergence of magnetic flux in the other arcade
triggers the flare, while the preexisting HFT and flux rope dictate
the structure and dynamics of the flare loops and ribbons during the
long-lasting decay phase, and that a quasi-separatrix layer high above
the HFT could account for the cusp-shaped structure.

---------------------------------------------------------
Title: Study of Two Successive Three-ribbon Solar Flares Using
BBSO/NST Observations
Authors: Wang, Haimin; Liu, Chang; Deng, Na; Zeng, Zhicheng; Xu, Yan;
Jing, Ju; Cao, Wenda
2014AAS...22412304W Altcode:
We studied two rarely observed three-ribbon flares (M1.9 and C9.2) on
2012 July 6 in NOAA AR 11515, which we found using Hα observations
of 0.1 arcsec resolution from the New Solar Telescope and Ca II
H images from Hinode. The flaring site is characterized by an
intriguing "fish-bone-like" morphology evidenced by both Halpha
images and a nonlinear force-free field (NLFFF) extrapolation,
where two semi-parallel rows of low-lying, sheared loops connect an
elongated, parasitic negative field with the sandwiching positive
fields. The NLFFF model also shows that the two rows of loops are
asymmetric in height and have opposite twists, and are enveloped by
large-scale field lines including open fields. The two flares occurred
in succession within half an hour and are located at the two ends of
the flaring region. The three ribbons of each flare run parallel to
the magnetic polarity inversion line, with the outer two lying in the
positive field and the central one in the negative field. Both flares
show surge-like flows in Halpha apparently toward the remote region,
while the C9.2 flare is also accompanied by EUV jets possibly along
the open field lines. Interestingly, the 12-25 keV hard X-ray sources
of the C9.2 flare first line up with the central ribbon then shift to
concentrate on the top of the higher branch of loops. These results
are discussed in favor of reconnection along the coronal null line,
producing the three flare ribbons and the associated ejections.

---------------------------------------------------------
Title: Confined Eruption of a Multi-thread Filament
Authors: Xu, Yan; Xu, Zhi; Liu, Zhong; Liu, Rui; Liu, Chang; Jing,
Ju; Wang, Haimin
2014AAS...22441401X Altcode:
Eruptive filaments are believed to be a major component, namely the
core, of coronal mass ejections (CMEs). In `confined eruptions',
the erupting materials eventually come back to the solar surface
instead of escaping away. We study a multi-thread filament, within
Active Region NOAA 11861, that splits into two parts. The upper
component erupts and comes down while the lower component remains
almost in the same location. The data set analyzed includes H$\alpha$
observation from the 1-meter New Vacuum Solar Telescope (NVST)
at Yunnan Astronomical Observatory (YNAO) and the EUV observations
taken by the Atmospheric Imaging Assembly (AIA) onboard the Solar
Dynamics Observatory (SDO). Both H$\alpha$ and EUV observations show
the erupting phase clearly. Fine details of the eruption are revealed
by the high resolution and high cadence H$\alpha$ observations. The
gradual expansion of the filament is followed by twisting of threads
that leads to a sudden eruption. The intuition of this process suggests
a helical kink instability. By extrapolating the vector magnetograms
obtained by Helioseismic and Magnetic Imager (HMI) on SDO, we calculate
the magnetic twist and discuss if the kink instability plays a major
role of initializing this eruption. In addition, we investigate
the overlying fields, also known as strapping fields, that confine
against the eruptions. The decay index, a quantitative measurement of
the strapping fields, is estimated to be 0.86. This value is smaller
than the critical value of 1.5 indicating a strong strapping field
preventing the materials from escaping.

---------------------------------------------------------
Title: Observation of 2011-02-15 X2.2 solar flare in Hard X-ray
and microwave
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale E.
2014AAS...22412332K Altcode:
Previous studies have shown that the energy release mechanism of some
solar flares follow the Standard magnetic-reconnection model, but the
detailed properties of high-energy electrons produced in the flare
are still not well understood. We conducted a unique, multi-wavelength
study that discloses the spatial, temporal and energy distributions of
the accelerated electrons in the X2.2 solar flare on Feb. 15, 2011. We
studied the source locations of the observed seven temporal peaks
in hard X-ray (HXR) and microwave (MW) lightcurves using the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in 50-75 keV
channels and Nobeyama Radioheliograph (NoRH) in 34 GHz, respectively. We
confirmed that peak emissions were coming from two spatially distinct
sites in HXR and MW, and in HXR we observed a sudden change between
these sites over the second and the third peak. Comparison between
the HXR lightcurve and the temporal variations in MW source kernel
intensity also confirmed that seven peak emissions were actually coming
from two sources, each with their own multiple peaks.

---------------------------------------------------------
Title: Investigating An X-Class Long-Duration Confined Flare
Authors: Liu, Rui; Titov, Viacheslav; Gou, Tingyu; Wang, Yuming; Liu,
Kai; Wang, Haimin
2014AAS...22411104L Altcode:
We report the observation of an X-class long-duration flare which is
clearly confined. It appears as a compact-loop 50 Mm high in projection)
flare in the traditional EUV passbands (171 and 195A}; ~1 MK), but
in the passbands sensitive to flare plasmas (94 and 131A; 6-10 MK),
it exhibits a cusp-shaped structure above an arcade of loops 100 Mm
high in projection) like other long-duration events. Inspecting images
in a running difference approach, we find that the seemingly diffuse,
quasi-static cusp-shaped structure actually consists of multiple nested
loops that repeatedly rise upward and disappear approaching the cusp
point. Over the gradual phase of the flare, we detect numerous episodes
of loop rising, each lasting minutes. A differential emission measure
analysis reveals that the temperature is highest at the top of the
arcade and becomes cooler at higher altitudes within the cusp-shaped
structure. These features are contrary to typical long-duration flares
that conform to the standard flare model. With a nonlinear force-free
reconstruction of the active region, our analysis of the photospheric
squashing factor $Q$ shows that the configuration has locally a
tri-polar structure, in the middle of which the negative polarity
is divided in half by a high-$Q$ line. The respective halves belong
to two adjacent sheared arcades, one of which harbors a magnetic flux
rope. The indicated high-Q line is a footprint of a hyperbolic flux tube
(HFT) where two quasi-separatrix layers (QSLs) adjoin each other at a
T-type junction passing through the joint arcades' apex. Comparing UV
1600A flare ribbons with the photospheric Q-maps at close times, we
conclude that the emergence of a new magnetic flux within the arcade
void of flux rope triggers the flare, while the preexisting T-type
HFT and flux rope dictate the structure and dynamics of the observed
flare loops and ribbons. The flux rope fails to escape owing to a
strong confining field but contributes to the long-lasting gradual
phase with the dissipation of QSL currents.

---------------------------------------------------------
Title: Comparison of Emission Properties of Two Homologous Flares
in AR 11283
Authors: Xu, Yan; Jing, Ju; Wang, Shuo; Wang, Haimin
2014ApJ...787....7X Altcode: 2014arXiv1404.6577X
Large, complex, active regions may produce multiple flares within a
certain period of one or two days. These flares could occur in the same
location with similar morphologies, commonly referred to as "homologous
flares." In 2011 September, active region NOAA 11283 produced a pair
of homologous flares on the 6th and 7th, respectively. Both of them
were white-light (WL) flares, as captured by the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory in
visible continuum at 6173 Å which is believed to originate from the
deep solar atmosphere. We investigate the WL emission of these X-class
flares with HMI's seeing-free imaging spectroscopy. The durations of
impulsive peaks in the continuum are about 4 minutes. We compare the
WL with hard X-ray (HXR) observations for the September 6 flare and
find a good correlation between the continuum and HXR both spatially
and temporally. In absence of RHESSI data during the second flare on
September 7, the derivative of the GOES soft X-ray is used and also
found to be well correlated temporally with the continuum. We measure
the contrast enhancements, characteristic sizes, and HXR fluxes
of the twin flares, which are similar for both flares, indicating
analogous triggering and heating processes. However, the September
7 flare was associated with conspicuous sunquake signals whereas no
seismic wave was detected during the flare on September 6. Therefore,
this comparison suggests that the particle bombardment may not play
a dominant role in producing the sunquake events studied in this paper.

---------------------------------------------------------
Title: A Solar Eruption Driven by Rapid Sunspot Rotation
Authors: Ruan, Guiping; Chen, Yao; Wang, Shuo; Zhang, Hongqi; Li, Gang;
Jing, Ju; Su, Jiangtao; Li, Xing; Xu, Haiqing; Du, Guohui; Wang, Haimin
2014ApJ...784..165R Altcode: 2014arXiv1402.6043R
We present the observation of a major solar eruption that is associated
with fast sunspot rotation. The event includes a sigmoidal filament
eruption, a coronal mass ejection, and a GOES X2.1 flare from
NOAA active region 11283. The filament and some overlying arcades
were partially rooted in a sunspot. The sunspot rotated at ~10°
hr-1 during a period of 6 hr prior to the eruption. In this
period, the filament was found to rise gradually along with the sunspot
rotation. Based on the Helioseismic and Magnetic Imager observation,
for an area along the polarity inversion line underneath the filament,
we found gradual pre-eruption decreases of both the mean strength
of the photospheric horizontal field (Bh ) and the mean
inclination angle between the vector magnetic field and the local
radial (or vertical) direction. These observations are consistent with
the pre-eruption gradual rising of the filament-associated magnetic
structure. In addition, according to the nonlinear force-free field
reconstruction of the coronal magnetic field, a pre-eruption magnetic
flux rope structure is found to be in alignment with the filament, and
a considerable amount of magnetic energy was transported to the corona
during the period of sunspot rotation. Our study provides evidence that
in this event sunspot rotation plays an important role in twisting,
energizing, and destabilizing the coronal filament-flux rope system,
and led to the eruption. We also propose that the pre-event evolution of
Bh may be used to discern the driving mechanism of eruptions.

---------------------------------------------------------
Title: Evolution of a Magnetic Flux Rope and its Overlying Arcade
Based on Nonlinear Force-free Field Extrapolations
Authors: Jing, Ju; Liu, Chang; Lee, Jeongwoo; Wang, Shuo; Wiegelmann,
Thomas; Xu, Yan; Wang, Haimin
2014ApJ...784L..13J Altcode:
Dynamic phenomena indicative of slipping reconnection and magnetic
implosion were found in a time series of nonlinear force-free field
(NLFFF) extrapolations for the active region 11515, which underwent
significant changes in the photospheric fields and produced five C-class
flares and one M-class flare over five hours on 2012 July 2. NLFFF
extrapolation was performed for the uninterrupted 5 hour period from
the 12 minute cadence vector magnetograms of the Helioseismic and
Magnetic Imager on board the Solar Dynamic Observatory. According
to the time-dependent NLFFF model, there was an elongated, highly
sheared magnetic flux rope structure that aligns well with an Hα
filament. This long filament splits sideways into two shorter segments,
which further separate from each other over time at a speed of 1-4 km
s-1, much faster than that of the footpoint motion of the
magnetic field. During the separation, the magnetic arcade arching over
the initial flux rope significantly decreases in height from ~4.5 Mm
to less than 0.5 Mm. We discuss the reality of this modeled magnetic
restructuring by relating it to the observations of the magnetic
cancellation, flares, a filament eruption, a penumbra formation,
and magnetic flows around the magnetic polarity inversion line.

---------------------------------------------------------
Title: Sudden Photospheric Motion and Sunspot Rotation Associated
with the X2.2 Flare on 2011 February 15
Authors: Wang, Shuo; Liu, Chang; Deng, Na; Wang, Haimin
2014ApJ...782L..31W Altcode: 2014arXiv1401.7957W
The Helioseismic and Magnetic Imager provides 45 s cadence intensity
images and 720 s cadence vector magnetograms. These unprecedented
high-cadence and high-resolution data give us a unique opportunity
to study the change of photospheric flows and sunspot rotations
associated with flares. By using the differential affine velocity
estimator method and the Fourier local correlation tracking method
separately, we calculate velocity and vorticity of photospheric
flows in the flaring NOAA AR 11158, and investigate their temporal
evolution around the X2.2 flare on 2011 February 15. It is found
that the shear flow around the flaring magnetic polarity inversion
line exhibits a sudden decrease, and both of the two main sunspots
undergo a sudden change in rotational motion during the impulsive
phase of the flare. These results are discussed in the context of the
Lorentz-force change that was proposed by Hudson et al. and Fisher
et al. This mechanism can explain the connections between the rapid
and irreversible photospheric vector magnetic field change and the
observed short-term motions associated with the flare. In particular,
the torque provided by the horizontal Lorentz force change agrees with
what is required for the measured angular acceleration.

---------------------------------------------------------
Title: Emerging Magnetic Flux as a Trigger of a Confined Flare
Authors: Titov, Viacheslav; Wang, Yuming; Liu, Kai; Liu, Rui; Wang,
Haimin; Gou, Tingyu
2014cosp...40E3349T Altcode:
We present the observation of an X-class long-duration confined
flare and the analysis of its magnetic structure evolution. Although
it appears as a compact-loop flare in the traditional EUV passbands
(171 and 195 {Å}), in the passbands sensitive to flare plasmas (94
and 131 {Å}; ∼6-10 MK), it exhibits a cusp-shaped structure above
an arcade of loops like other long-duration events. Inspecting images
in a running difference approach, we find that the seemingly diffuse,
quasi-static cusp-shaped structure actually consists of multiple nested
loops that repeatedly rise upward and disappear approaching the cusp
point. Over the gradual phase of the flare, we detect numerous episodes
of loop rising, each lasting tens of seconds to minutes. A differential
emission measure analysis reveals that the temperature is highest at
the top of the arcade and becomes cooler at higher altitudes within the
cusp-shaped structure. These features are opposite to what the standard
flare model predicts. To understand the actual scenario of this event,
we make first a nonlinear force-free reconstruction of the active region
for a sequence of vector magnetograms that covers the time period of the
event’s duration and then investigate the structure of the resulting
sequence of configurations, using our field-line mapping technique
based on the so-called squashing factor Q. Our analysis shows that the
configurations have locally a tri-polar structure, in the middle of
which the negative photospheric polarity is divided in half by a high-Q
line. The respective halves belong to two adjacent sheared arcades,
one of which harbors a magnetic flux rope. The indicated high-Q line is
a footprint of a hyperbolic flux tube (HFT) where two quasi-separatrix
layers adjoin each other at a T-type junction passing through the joint
arcades’ apex. Comparing UV 1600 {Å} images of the Sun's disk with
photospheric Q-maps at close times, we conclude that the emergence
of a new magnetic flux within one of the arcades triggers the flare,
while the preexisting T-type HFT and flux rope dictate the structure
and dynamics of the observed flare loops and ribbons in this event.

---------------------------------------------------------
Title: Study of Two Successive Three-ribbon Solar Flares on 2012
July 6
Authors: Wang, Haimin; Liu, Chang; Deng, Na; Zeng, Zhicheng; Xu, Yan;
Jing, Ju; Cao, Wenda
2014ApJ...781L..23W Altcode: 2013arXiv1312.6649W
This Letter reports two rarely observed three-ribbon flares (M1.9
and C9.2) on 2012 July 6 in NOAA AR 11515, which we found using Hα
observations of 0.”1 resolution from the New Solar Telescope and
Ca II H images from Hinode. The flaring site is characterized by an
intriguing "fish-bone-like" morphology evidenced by both Hα images
and a nonlinear force-free field (NLFFF) extrapolation, where two
semi-parallel rows of low-lying, sheared loops connect an elongated,
parasitic negative field with the sandwiching positive fields. The
NLFFF model also shows that the two rows of loops are asymmetric in
height and have opposite twists, and are enveloped by large-scale field
lines including open fields. The two flares occurred in succession
within half an hour and are located at the two ends of the flaring
region. The three ribbons of each flare run parallel to the magnetic
polarity inversion line, with the outer two lying in the positive
field and the central one in the negative field. Both flares show
surge-like flows in Hα apparently toward the remote region, while
the C9.2 flare is also accompanied by EUV jets possibly along the
open field lines. Interestingly, the 12-25 keV hard X-ray sources of
the C9.2 flare first line up with the central ribbon then shift to
concentrate on the top of the higher branch of loops. These results
are discussed in favor of reconnection along the coronal null line,
producing the three flare ribbons and the associated ejections.

---------------------------------------------------------
Title: Evidence for Solar Tether-cutting Magnetic Reconnection from
Coronal Field Extrapolations
Authors: Liu, Chang; Deng, Na; Lee, Jeongwoo; Wiegelmann, Thomas;
Moore, Ronald L.; Wang, Haimin
2013ApJ...778L..36L Altcode: 2013arXiv1310.5098L
Magnetic reconnection is one of the primary mechanisms for triggering
solar eruptive events, but direct observation of this rapid process has
been a challenge. In this Letter, using a nonlinear force-free field
(NLFFF) extrapolation technique, we present a visualization of field
line connectivity changes resulting from tether-cutting reconnection
over about 30 minutes during the 2011 February 13 M6.6 flare in NOAA
AR 11158. Evidence for the tether-cutting reconnection was first
collected through multiwavelength observations and then by analysis of
the field lines traced from positions of four conspicuous flare 1700
Å footpoints observed at the event onset. Right before the flare,
the four footpoints are located very close to the regions of local
maxima of the magnetic twist index. In particular, the field lines
from the inner two footpoints form two strongly twisted flux bundles
(up to ~1.2 turns), which shear past each other and reach out close to
the outer two footpoints, respectively. Immediately after the flare,
the twist index of regions around the footpoints diminishes greatly and
the above field lines become low-lying and less twisted (lsim0.6 turns),
overarched by loops linking the two flare ribbons formed later. About
10% of the flux (~3 × 1019 Mx) from the inner footpoints
undergoes a footpoint exchange. This portion of flux originates
from the edge regions of the inner footpoints that are brightened
first. These rapid changes of magnetic field connectivity inferred
from the NLFFF extrapolation are consistent with the tether-cutting
magnetic reconnection model.

---------------------------------------------------------
Title: Study of Rapid Formation of a δ Sunspot Associated with the
2012 July 2 C7.4 Flare Using High-resolution Observations of the
New Solar Telescope
Authors: Wang, Haimin; Liu, Chang; Wang, Shuo; Deng, Na; Xu, Yan;
Jing, Ju; Cao, Wenda
2013ApJ...774L..24W Altcode: 2013arXiv1308.2639W
Rapid, irreversible changes of magnetic topology and sunspot
structure associated with flares have been systematically observed
in recent years. The most striking features include the increase of
the horizontal field at the polarity inversion line (PIL) and the
co-spatial penumbral darkening. A likely explanation of the above
phenomenon is the back reaction to the coronal restructuring after
eruptions: a coronal mass ejection carries the upward momentum while
the downward momentum compresses the field lines near the PIL. Previous
studies could only use low-resolution (above 1”) magnetograms and
white-light images. Therefore, the changes are mostly observed for
X-class flares. Taking advantage of the 0.”1 spatial resolution
and 15 s temporal cadence of the New Solar Telescope at the Big Bear
Solar Observatory, we report in detail the rapid formation of sunspot
penumbra at the PIL associated with the C7.4 flare on 2012 July 2. It
is unambiguously shown that the solar granulation pattern evolves to
an alternating dark and bright fibril structure, the typical pattern
of penumbra. Interestingly, the appearance of such a penumbra creates
a new δ sunspot. The penumbral formation is also accompanied by the
enhancement of the horizontal field observed using vector magnetograms
from the Helioseismic and Magnetic Imager. We explain our observations
as being due to the eruption of a flux rope following magnetic
cancellation at the PIL. Subsequently, the re-closed arcade fields
are pushed down toward the surface to form the new penumbra. NLFFF
extrapolation clearly shows both the flux rope close to the surface
and the overlying fields.

---------------------------------------------------------
Title: He I D3 Observations of the 1984 May 22 M6.3 Solar Flare
Authors: Liu, Chang; Xu, Yan; Deng, Na; Lee, Jeongwoo; Zhang, Jifeng;
Prasad Choudhary, Debi; Wang, Haimin
2013ApJ...774...60L Altcode: 2013arXiv1306.6068L
The He I D3 line has a unique response to a flare impact on the low
solar atmosphere and can be a powerful diagnostic tool for energy
transport processes. Using images obtained from the recently digitized
films of the Big Bear Solar Observatory, we report D3 observations
of the M6.3 flare on 1984 May 22, which occurred in an active region
with a circular magnetic polarity inversion line (PIL). The impulsive
phase of the flare starts with a main elongated source that darkens in
D3, inside of which bright emission kernels appear at the time of the
initial small peak in hard X-rays (HXRs). These flare cores subsequently
evolve into a sharp emission strand lying within the dark halo; this
evolution occurs at the same time as the main peak in HXRs, reversing
the overall source contrast from -5% to 5%. The radiated energy in D3
during the main peak is estimated to be about 1030 erg,
which is comparable to that carried by nonthermal electrons above
20 keV. Afterward, the flare proceeds along the circular PIL in the
counterclockwise direction to form a dark circular ribbon in D3, which
apparently mirrors the bright ribbons in Hα and He I 10830 Å. All
of these ribbons last for over one hour in the late gradual phase. We
suggest that the present event resembles the so-called black-light flare
that was proposed based on continuum images, and that D3 darkening and
brightening features herein may be due to thermal conduction heating
and the direct precipitation of high-energy electrons, respectively.

---------------------------------------------------------
Title: Observation of a Moreton Wave and Wave-Filament Interactions
Associated with the Renowned X9 Flare on 1990 May 24
Authors: Liu, Rui; Liu, Chang; Xu, Yan; Liu, Wei; Kliem, Bernhard;
Wang, Haimin
2013ApJ...773..166L Altcode:
Using Big Bear Solar Observatory film data recently digitized at
NJIT, we investigate a Moreton wave associated with an X9 flare
on 1990 May 24, as well as its interactions with four filaments
F1-F4 located close to the flaring region. The interaction yields
interesting insight into physical properties of both the wave and
the filaments. The first clear Moreton wavefront appears at the
flaring-region periphery at approximately the same time as the peak
of a microwave burst and the first of two γ-ray peaks. The wavefront
propagates at different speeds ranging from 1500-2600 km s-1
in different directions, reaching as far as 600 Mm away from the flaring
site. Sequential chromospheric brightenings are observed ahead of the
Moreton wavefront. A slower diffuse front at 300-600 km s-1
is observed to trail the fast Moreton wavefront about one minute after
the onset. The Moreton wave decelerates to ~550 km s-1 as it
sweeps through F1. The wave passage results in F1's oscillation which
is featured by ~1 mHz signals with coherent Fourier phases over the
filament, the activation of F3 and F4 followed by gradual recovery,
but no disturbance in F2. Different height and magnetic environment
together may account for the distinct responses of the filaments to
the wave passage. The wavefront bulges at F4, whose spine is oriented
perpendicular to the upcoming wavefront. The deformation of the
wavefront is suggested to be due to both the forward inclination of
the wavefront and the enhancement of the local Alfvén speed within
the filament channel.

---------------------------------------------------------
Title: Study of ribbon separation and magnetic reconnection rates
Authors: Xie, Wenbin; Zhang, Hongqi; Lin, Jun; Wang, Haimin
2013IAUS..294..585X Altcode:
We study the correlation between the speed of two-ribbon separation
and the magnetic flux density during the 2001 April 10 solar flare. A
weak negative correlation is found between the ribbon separation
speed (Vr ) and the longitudinal magnetic flux density
(Bz ). In addition, we estimate the magnetic reconnection
rate (Erec ). Along the flare ribbons, Erec
fluctuates in the small range except near the HXR source. The localized
enhancement of the reconnection rate corresponds to the position of
the HXR source.

---------------------------------------------------------
Title: Observation of a Moreton Wave and Wave-Filament Interactions
Using Digitized Film Data at BBSO
Authors: Wang, Haimin; Liu, R.; Liu, C.; Xu, Y.; Liu, W.
2013SPD....4410402W Altcode:
We have finished digitizing all full disk and a fraction of high
resolution films obtained by Big Bear Solar Observatory (BBSO) from
1969 to 1997. Using high-cadence (10s) digitized data we investigate
a Moreton wave associated with an X9 flare on 1990 May 24, as well
as its interactions with four filaments F1-F4 located close to the
flaring active region. The interaction yields interesting insight
into physical properties of both the wave and the filaments. The
first Moreton wavefront appears at the active-region periphery at
21UT, about the same time as the peak of the microwave burst and the
first of the double-peak gamma-ray burst. The wavefront propagates at
2 Mm/s within five minutes of its initiation, reaching as far as 600
Mm away from the flaring site. Sequential chromospheric brightenings
(SCBs) are observed ahead of the Moreton wavefront, with similar
appearance as the subsequent sequential brightenings due to the wave
passage. A slower diffuse moving front at 300 o 600 km/s is observed
to trail the fast Moreton wavefront about 1 min after the onset. The
Moreton wave decelerates to 550 km/s as it sweeps through F1. The
wave passage results in oscillations throughout the entire filament,
predominantly perpendicular to F1’s spine, a temporary disappearance
of F3 and F4 followed by a gradual recovery, but no disturbance in
F2. Different height and magnetic configuration together may account
for the distinct responses of the filaments to the wave passage. The
wavefront bulges at F4 whose spine is oriented perpendicular to the
upcoming wavefront. The deformation of the wavefront is suggested to
be due both to the forward inclination of the wavefront and to the
enhancement of the local Alfven speed within the filament channel.

---------------------------------------------------------
Title: High-cadence and High-resolution Hα Imaging Spectroscopy of
a Circular Flare's Remote Ribbon with IBIS
Authors: Deng, Na; Tritschler, Alexandra; Jing, Ju; Chen, Xin; Liu,
Chang; Reardon, Kevin; Denker, Carsten; Xu, Yan; Wang, Haimin
2013ApJ...769..112D Altcode: 2013arXiv1304.4171D
We present an unprecedented high-resolution Hα imaging spectroscopic
observation of a C4.1 flare taken with the Interferometric Bidimensional
Spectrometer (IBIS) in conjunction with the adaptive optics system
at the 76 cm Dunn Solar Telescope on 2011 October 22 in the active
region NOAA 11324. Such a two-dimensional spectroscopic observation
covering the entire evolution of a flare ribbon with high spatial (0.”1
pixel-1 image scale), cadence (4.8 s), and spectral (0.1 Å
step size) resolution is rarely reported. The flare consists of a main
circular ribbon that occurred in a parasitic magnetic configuration and
a remote ribbon that was observed by the IBIS. Such a circular-ribbon
flare with a remote brightening is predicted in three-dimensional
fan-spine reconnection but so far has been rarely studied. During
the flare impulsive phase, we define "core" and "halo" structures in
the observed ribbon based on IBIS narrowband images in the Hα line
wing and line center. Examining the Hα emission spectra averaged in
the flare core and halo areas, we find that only those from the flare
cores show typical nonthermal electron beam heating characteristics that
have been revealed by previous theoretical simulations and observations
of flaring Hα line profiles. These characteristics include broad and
centrally reversed emission spectra, excess emission in the red wing
with regard to the blue wing (i.e., red asymmetry), and redshifted
bisectors of the emission spectra. We also observe rather quick
timescales for the heating (~30 s) and cooling (~14-33 s) in the flare
core locations. Therefore, we suggest that the flare cores revealed by
IBIS track the sites of electron beam precipitation with exceptional
spatial and temporal resolution. The flare cores show two-stage
motion (a parallel motion along the ribbon followed by an expansion
motion perpendicular to the ribbon) during the two impulsive phases
of the flare. Some cores jump quickly (30 km s-1) between
discrete magnetic elements implying reconnection involving different
flux tubes. We observe a very high temporal correlation (gsim 0.9)
between the integrated Hα and hard X-rays (HXR) emission during the
flare impulsive phase. A short time delay (4.6 s) is also found in the
Hα emission spikes relative to HXR bursts. The ionization timescale
of the cool chromosphere and the extra time taken for the electrons
to travel to the remote ribbon site may contribute to this delay.

---------------------------------------------------------
Title: Coordinated Observations of On-Disk Type II Spicules with
IBIS and Hinode
Authors: Chen, Xin; Deng, Na; Jing, Ju; Tritschler, Alexandria;
Reardon, Kevin; Wang, Haimin
2013enss.confE.147C Altcode:
Ubiquitous small-scale spicules/jets in the chromosphere are believed
to be an important ingredient contributing to coronal heating and
solar wind by supplying energy and mass upwards. In particular, type II
spicules discovered at the solar limb (De Pontieu et al. 2007) and their
highly probable chromospheric on disk counterpart "Rapid Blueshifted
Excursions" (RBEs; Langangen et al. 2008) have drawn much attention in
recent years. Their rapid heating, high speed upflow and association
with magnetic field indicate that the most possible underlying
driving mechanism is magnetic reconnection on small scales. In order
to understand the physical properties of these features, we carried
out a coordinated high resolution and high cadence observation of
chromospheric RBEs using the Interferometric BIdimensional Spectrometer
(IBIS) at the Dunn Solar Telescope and photospheric magnetic fields
using Hinode SOT/SP and SOT/NFI in October 2011. We identify RBEs based
on the IBIS observations, study their properties (velocity, density,
temperature etc.) by statistical analysis and show their relationship
with signatures of small-scale magnetic reconnection in the Hinode
magnetograms. Furthermore, we search for coronal counterpart of RBEs
from observations of the Atmospheric Imaging Assembly (AIA) onboard
the Solar Dynamics Observatory (SDO). References: De Pontieu, B. et
al. 2007, PASJ, 59, 655-662 Langangen, O. et al. 2008, ApJ, 679, L167

---------------------------------------------------------
Title: Properties of Reversed Helicity Injection in Active Region
11158
Authors: Jing, Ju; Chen, Xin; Lee, Jeongwoo; Wang, Shuo; Schuck,
Peter W.; Liu, Chang; Xu, Yan; Wang, Haimin
2013enss.confE.135J Altcode:
The MHD model by Kusano et al. (2004) shows that flares are triggered by
the injection of reversed helicity into the magnetic polarity inversion
line (PIL). Here we present observations of reversed helicity injection
in the active region NOAA 11158 from 2011 February 12 to 16 during
which two major flares with GOES class M6.6 and X2.2, respectively,
occurred. We have calculated magnetic helicity by counting the emerging
motion as well as shearing motion of the magnetic fields by applying the
Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM,
Schuck 2008) to a sequence of photospheric vector magnetograms of the
Helioseismic and Magnetic Imager (HMI) on the Solar Dynamic Observatory
(SDO). Amount of the accumulated helicity injected in this active region
over the five-day period is estimated to be ∼2×10^{43} Mx^2, mainly
contributed by the shearing motion of magnetic fields. By examining the
series of helicity density maps, we find the intermittent injection of
negative helicity as well as the persistent accumulation of positive
helicity in the region located around the magnetic PIL. We further
use the Southwest Automatic Magnetic Identification Suite (SWAMIS) to
detect and track each feature with reversed helicity (i.e., negative
helicity in this case). A total of 19 reversed-helicity features are
detected, and their location, lifetime, maximum area and flux variation
are presented in this paper. The two most noticeable injections of
reverse helicity appear prior to each of two major flares, M6.6 and
X2.2, respectively. The presented observation may have implications
on the tearing mode instability growing on the shear inversion layer
and magnetic reconnections leading to flare/CME initiation.

---------------------------------------------------------
Title: Sharp Acceleration of Sunspot Rotation Associated with Flares
in NOAA AR 11158
Authors: Wang, Shuo; Liu, Chang; Deng, Na; Wang, Haimin
2013enss.confE.137W Altcode:
The NOAA AR 11158 produced several major flares and showed spiral
penumbra that indicates rotation of sunspots. The Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory provides
both 45 seconds and 720 seconds cadence white-light images from two
cameras. These unprecedented high-cadence and high-resolution data
give us a unique opportunity to study the change of sunspot rotation
associated with flares. We calculated vorticity of selected spots
derived using the Differential Affine Velocity Estimator method. The
results of vorticity evolution from two different cameras match each
other, and are consistent with previous studies on spot rotation. We
find that some spots show sharp acceleration of rotation at the time
of the rising phase of GOES soft X-ray flux, and the faster-than-normal
spot rotation could last for about 5-60 minutes.

---------------------------------------------------------
Title: Time delays between radio and HXR observed during the
2011-02-15 X2.2 solar flare
Authors: Kuroda, Natsuha; Wang, Haimin; Gary, Dale
2013enss.confE..86K Altcode:
We report the second-scale delays observed between radio and HXR during
the X2.2 solar flare on February 15, 2011. The delays between different
energy channels from various instruments were previously reported by
Dolla et al. in 2012; they reported that the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) channels in the 25-100 keV range
lead all the other channels including the Nobeyama RadioPolarimeters
at 9 and 17 GHz and the Extreme Ultra-Violet (EUV) channels of the Euv
SpectroPhotometer (ESP) onboard the Solar Dynamic Observatory (SDO). We
looked at RHESSI channels in 25-250 keV and Nobeyama channels in 1-35
GHz and found that, in four data pairs that showed more than 80 %
correlation, the same several RHESSI peaks lead Nobeyama peaks by few
seconds. We discuss the possible physical meaning of this delay.

---------------------------------------------------------
Title: Fe I Spectra of Core-Halo Structures in a White-Light Flare
Authors: Xu, Yan; Jing, Ju; Wang, Haimin
2013enss.confE..52X Altcode:
In this study, we present the imaging spectroscopy of an X-class
flare with white-light emission on September 06, 2011, observed with
Helioseismic Magnetic Imager (HMI) on board Solar Dynamics Observatory
(SDO). The HMI provides seeing-free images at 6173 Å continuum with
a 45s cadence and six-point spectrograms centered at 6173.34 Å with
0.172 Å steps. Taking advantage of the 0.5 arcsecond image scale,
the flare kernels are fully resolved and fine structures, including the
core and halo, are able to be identified. We analyzed the line-profile,
constructed from six spectral positions, of the flare core and halo
pixels, respectively. We studied the morphology of the continuum
flare kernel comparing with previous white-light observations. The
resemblance and the discrepancy of the two kinds of spectra, which
could be related to different heating mechanisms, are then discussed.

---------------------------------------------------------
Title: Study of Black-Light Flares
Authors: Wang, Haimin; Liu, C.; Xu, Y.; Deng, N.
2013AAS...22131601W Altcode:
Black-light flares (BLFs; sometimes called "negative flares") is an
important physical problem for both stellar and solar communities. There
has been some observational evidence of BLFs in stars and the Sun. Such
observations were usually made in some continuum (e.g., 8542 A)
or spectral lines especially the He I D3 at 5876 A. The D3 line is
the strongest He line in the visible spectrum, and shows a unique
response often better for discriminating high-energy phenomena. The
only high-resolution helium observations of the Sun were taken in D3 at
Big Bear Solar Observatory (BBSO) and recorded on films. Part of these
film data have recently been digitized, which allows a detailed study
of the historic solar D3 observations. The first imaging evidences
of BLFs were presented by Zirin (1980) using BBSO D3 observations,
including an M8 flare on 1978 July 10 in which a shell-like D3 source
first darkened before the subsequent brightening, and another X2
flare on 1974 July 6. Here we present several other D3 BLFs. The
best observed event occurred on 1984 May 22. This M6 flare begins
with darkening and then brightening in a region presumably above the
magnetic neutral line, and the emission subsequently grows to become a
ridge-like structure. Meanwhile, a bright kernel starts to be seen at
the edge of the main sunspot umbra. Both the bright ridge and kernel
features in D3 further extend in the same direction, and apparently
evolve into two long dark ribbons mirroring the two long bright
ribbons in Halpha. Using the integrated flare contrast variation,
we discuss the feasibility of BLF observations in solar-type stars,
and the physical understanding of the phenomenon.

---------------------------------------------------------
Title: On the Relationship between the Coronal Magnetic Decay Index
and Coronal Mass Ejection Speed
Authors: Xu, Yan; Liu, Chang; Jing, Ju; Wang, Haimin
2012ApJ...761...52X Altcode: 2012arXiv1210.4437X
Numerical simulations suggest that kink and torus instabilities are two
potential contributors to the initiation and prorogation of eruptive
events. A magnetic parameter called the decay index (i.e., the coronal
magnetic gradient of the overlying fields above the eruptive flux
ropes) could play an important role in controlling the kinematics
of eruptions. Previous studies have identified a threshold range
of the decay index that distinguishes between eruptive and confined
configurations. Here we advance the study by investigating if there is
a clear correlation between the decay index and coronal mass ejection
(CME) speed. Thirty-eight CMEs associated with filament eruptions and/or
two-ribbon flares are selected using the Hα data from the Global Hα
Network. The filaments and flare ribbons observed in Hα associated
with the CMEs help to locate the magnetic polarity inversion line,
along which the decay index is calculated based on the potential field
extrapolation using Michelson Doppler Imager magnetograms as boundary
conditions. The speeds of CMEs are obtained from the LASCO C2 CME
catalog available online. We find that the mean decay index increases
with CME speed for those CMEs with a speed below 1000 km s-1
and stays flat around 2.2 for the CMEs with higher speeds. In addition,
we present a case study of a partial filament eruption, in which the
decay indices show different values above the erupted/non-erupted part.

---------------------------------------------------------
Title: Circular Ribbon Flares and Homologous Jets
Authors: Wang, Haimin; Liu, Chang
2012ApJ...760..101W Altcode: 2012arXiv1207.7345W
Solar flare emissions in the chromosphere often appear as elongated
ribbons on both sides of the magnetic polarity inversion line (PIL),
which has been regarded as evidence of a typical configuration of
magnetic reconnection. However, flares having a circular ribbon
have rarely been reported, although it is expected in the fan-spine
magnetic topology involving reconnection at a three-dimensional
(3D) coronal null point. We present five circular ribbon flares with
associated surges, using high-resolution and high-cadence Hα blue wing
observations obtained from the recently digitized films of Big Bear
Solar Observatory. In all the events, a central parasitic magnetic field
is encompassed by the opposite polarity, forming a circular PIL traced
by filament material. Consequently, a flare kernel at the center is
surrounded by a circular flare ribbon. The four homologous jet-related
flares on 1991 March 17 and 18 are of particular interest, as (1) the
circular ribbons brighten sequentially, with cospatial surges, rather
than simultaneously, (2) the central flare kernels show an intriguing
"round-trip" motion and become elongated, and (3) remote brightenings
occur at a region with the same magnetic polarity as the central
parasitic field and are co-temporal with a separate phase of flare
emissions. In another flare on 1991 February 25, the circular flare
emission and surge activity occur successively, and the event could be
associated with magnetic flux cancellation across the circular PIL. We
discuss the implications of these observations combining circular flare
ribbons, homologous jets, and remote brightenings for understanding
the dynamics of 3D magnetic restructuring.

---------------------------------------------------------
Title: Contracting and Erupting Components of Sigmoidal Active Regions
Authors: Liu, Rui; Liu, Chang; Török, Tibor; Wang, Yuming; Wang,
Haimin
2012ApJ...757..150L Altcode: 2012arXiv1208.0640L
It has recently been noted that solar eruptions can be associated with
the contraction of coronal loops that are not involved in magnetic
reconnection processes. In this paper, we investigate five coronal
eruptions originating from four sigmoidal active regions, using
high-cadence, high-resolution narrowband EUV images obtained by the
Solar Dynamic Observatory (SDO). The magnitudes of the flares associated
with the eruptions range from GOES class B to class X. Owing to the
high-sensitivity and broad temperature coverage of the Atmospheric
Imaging Assembly (AIA) on board SDO, we are able to identify both the
contracting and erupting components of the eruptions: the former is
observed in cold AIA channels as the contracting coronal loops overlying
the elbows of the sigmoid, and the latter is preferentially observed
in warm/hot AIA channels as an expanding bubble originating from the
center of the sigmoid. The initiation of eruption always precedes the
contraction, and in the energetically mild events (B- and C-flares),
it also precedes the increase in GOES soft X-ray fluxes. In the more
energetic events, the eruption is simultaneous with the impulsive phase
of the nonthermal hard X-ray emission. These observations confirm that
loop contraction is an integrated process in eruptions with partially
opened arcades. The consequence of contraction is a new equilibrium with
reduced magnetic energy, as the contracting loops never regain their
original positions. The contracting process is a direct consequence of
flare energy release, as evidenced by the strong correlation of the
maximal contracting speed, and strong anti-correlation of the time
delay of contraction relative to expansion, with the peak soft X-ray
flux. This is also implied by the relationship between contraction
and expansion, i.e., their timing and speed.

---------------------------------------------------------
Title: Slow Rise and Partial Eruption of a Double-decker
Filament. I. Observations and Interpretation
Authors: Liu, Rui; Kliem, Bernhard; Török, Tibor; Liu, Chang; Titov,
Viacheslav S.; Lionello, Roberto; Linker, Jon A.; Wang, Haimin
2012ApJ...756...59L Altcode: 2012arXiv1207.1757L
We study an active-region dextral filament that was composed of
two branches separated in height by about 13 Mm, as inferred from
three-dimensional reconstruction by combining SDO and STEREO-B
observations. This "double-decker" configuration sustained for days
before the upper branch erupted with a GOES-class M1.0 flare on 2010
August 7. Analyzing this evolution, we obtain the following main
results. (1) During the hours before the eruption, filament threads
within the lower branch were observed to intermittently brighten up,
lift upward, and then merge with the upper branch. The merging process
contributed magnetic flux and current to the upper branch, resulting
in its quasi-static ascent. (2) This transfer might serve as the
key mechanism for the upper branch to lose equilibrium by reaching
the limiting flux that can be stably held down by the overlying
field or by reaching the threshold of the torus instability. (3)
The erupting branch first straightened from a reverse S shape that
followed the polarity inversion line and then writhed into a forward S
shape. This shows a transfer of left-handed helicity in a sequence of
writhe-twist-writhe. The fact that the initial writhe is converted into
the twist of the flux rope excludes the helical kink instability as the
trigger process of the eruption, but supports the occurrence of the
instability in the main phase, which is indeed indicated by the very
strong writhing motion. (4) A hard X-ray sigmoid, likely of coronal
origin, formed in the gap between the two original filament branches
in the impulsive phase of the associated flare. This supports a model
of transient sigmoids forming in the vertical flare current sheet. (5)
Left-handed magnetic helicity is inferred for both branches of the
dextral filament. (6) Two types of force-free magnetic configurations
are compatible with the data, a double flux rope equilibrium and a
single flux rope situated above a loop arcade.

---------------------------------------------------------
Title: The Relationship between the Sudden Change of the Lorentz
Force and the Magnitude of Associated Flares
Authors: Wang, Shuo; Liu, Chang; Wang, Haimin
2012ApJ...757L...5W Altcode: 2012arXiv1208.3158W
The rapid and irreversible change of photospheric magnetic fields
associated with flares has been confirmed by many recent studies. These
studies showed that the photospheric magnetic fields respond to
coronal field restructuring and turn to a more horizontal state near
the magnetic polarity inversion line (PIL) after eruptions. Recent
theoretical work has shown that the change in the Lorentz force
associated with a magnetic eruption will lead to such a field
configuration at the photosphere. The Helioseismic Magnetic Imager has
been providing unprecedented full-disk vector magnetograms covering
the rising phase of the solar cycle 24. In this study, we analyze
18 flares in four active regions, with GOES X-ray class ranging from
C4.7 to X5.4. We find that there are permanent and rapid changes of
magnetic field around the flaring PIL, the most notable of which is
the increase of the transverse magnetic field. The changes of fields
integrated over the area and the derived change of Lorentz force both
show a strong correlation with flare magnitude. It is the first time
that such magnetic field changes have been observed even for C-class
flares. Furthermore, for seven events with associated coronal mass
ejections (CMEs), we use an estimate of the impulse provided by the
Lorentz force, plus the observed CME velocity, to estimate the CME
mass. We find that if the timescale of the back reaction is short, i.e.,
in the order of 10 s, the derived values of CME mass (~1015
g) generally agree with those reported in literature.

---------------------------------------------------------
Title: Recent progress on the portable solar adaptive optics
Authors: Ren, Deqing; Zhang, Xi; Penn, Matt; Wang, Haimin; Dou,
Jiangpei; Zhu, Yongtian; Rong, Li; Wang, Xue
2012SPIE.8447E..3KR Altcode:
The portable solar adaptive optics is a compact adaptive optics system
that will be the first visitor solar instrument in the world. As so,
it will be able to work with any solar telescope with a aperture
size up to ~ 2.0 meters, which will cover the largest solar telescope
currently operational. The portable AO features small physical size,
high-flexibility and high-performance, and is a duplicable and
affordable system. It will provide wave-front correction down to
the 0.5-μm wavelength, and will be used for solar high-resolution
imaging in the near infrared and the visible. It will be the first AO
system that uses LabVIEW based high quality parallel and block-diagram
programming, which fully takes advantage of today's multi-core CPUs, and
makes a rapid development of an AO system possible. In this publication,
we report our recent progress on the portable adaptive optics, which
includes the laboratory test for performance characterization, and
initial on-site scientific observations.

---------------------------------------------------------
Title: Evolution of Relative Magnetic Helicity and Current Helicity
in NOAA Active Region 11158
Authors: Jing, Ju; Park, Sung-Hong; Liu, Chang; Lee, Jeongwoo;
Wiegelmann, Thomas; Xu, Yan; Deng, Na; Wang, Haimin
2012ApJ...752L...9J Altcode:
Both magnetic and current helicities are crucial ingredients for
describing the complexity of active-region magnetic structure. In this
Letter, we present the temporal evolution of these helicities contained
in NOAA active region 11158 during five days from 2011 February 12
to 16. The photospheric vector magnetograms of the Helioseismic and
Magnetic Imager on board the Solar Dynamic Observatory were used as
the boundary conditions for the coronal field extrapolation under the
assumption of nonlinear force-free field, from which we calculated
both relative magnetic helicity and current helicity. We construct a
time-altitude diagram in which altitude distribution of the magnitude
of current helicity density is displayed as a function of time. This
diagram clearly shows a pattern of upwardly propagating current
helicity density over two days prior to the X2.2 flare on February
15 with an average propagation speed of ~36 m s-1. The
propagation is synchronous with the emergence of magnetic flux into
the photosphere, and indicative of a gradual energy buildup for the
X2.2 flare. The time profile of the relative magnetic helicity shows
a monotonically increasing trend most of the time, but a pattern
of increasing and decreasing magnetic helicity above the monotonic
variation appears prior to each of two major flares, M6.6 and X2.2,
respectively. The physics underlying this bump pattern is not fully
understood. However, the fact that this pattern is apparent in the
magnetic helicity evolution but not in the magnetic flux evolution
makes it a useful indicator in forecasting major flares.

---------------------------------------------------------
Title: Characteristic Size of Flare Kernels in the Visible and
Near-infrared Continua
Authors: Xu, Yan; Cao, Wenda; Jing, Ju; Wang, Haimin
2012ApJ...750L...7X Altcode:
In this Letter, we present a new approach to estimate the formation
height of visible and near-infrared emission of an X10 flare. The sizes
of flare emission cores in three wavelengths are accurately measured
during the peak of the flare. The source size is the largest in the
G band at 4308 Å and shrinks toward longer wavelengths, namely the
green continuum at 5200 Å and NIR at 15600 Å, where the emission is
believed to originate from the deeper atmosphere. This size-wavelength
variation is likely explained by the direct heating model as electrons
need to move along converging field lines from the corona to the
photosphere. Therefore, one can observe the smallest source, which in
our case is 0farcs65 ± 0farcs02 in the bottom layer (represented by
NIR), and observe relatively larger kernels in upper layers of 1farcs03
± 0farcs14 and 1farcs96 ± 0farcs27, using the green continuum and
G band, respectively. We then compare the source sizes with a simple
magnetic geometry to derive the formation height of the white-light
sources and magnetic pressure in different layers inside the flare loop.

---------------------------------------------------------
Title: The Occurrence and Speed of CMEs Related to Two Characteristic
Evolution Patterns of Helicity Injection in Their Solar Source Regions
Authors: Park, Sung-Hong; Cho, Kyung-Suk; Bong, Su-Chan; Kumar,
Pankaj; Chae, Jongchul; Liu, Rui; Wang, Haimin
2012ApJ...750...48P Altcode: 2012arXiv1203.1690P
Long-term (a few days) variation of magnetic helicity injection
was calculated for 28 solar active regions that produced 47 coronal
mass ejections (CMEs) to find its relationship to the CME occurrence
and speed using SOHO/MDI line-of-sight magnetograms. As a result, we
found that the 47 CMEs can be categorized into two different groups by
two characteristic evolution patterns of helicity injection in their
source active regions, which appeared for ~0.5-4.5 days before their
occurrence: (1) a monotonically increasing pattern with one sign of
helicity (Group A 30 CMEs in 23 active regions) and (2) a pattern
of significant helicity injection followed by its sign reversal
(Group B 17 CMEs in 5 active regions). We also found that CME speed
has a correlation with average helicity injection rate with linear
correlation coefficients of 0.85 and 0.63 for Group A and Group B,
respectively. In addition, these two CME groups show different
characteristics as follows: (1) the average CME speed of Group B
(1330 km s-1) is much faster than that of Group A (870
km s-1), (2) the CMEs in Group A tend to be single events
whereas those in Group B mainly consist of successive events, and (3)
flares related to the CMEs in Group B are relatively more energetic
and impulsive than those in Group A. Our findings therefore suggest
that the two CME groups have different pre-CME conditions in their
source active regions and different CME characteristics.

---------------------------------------------------------
Title: Evidence of 3-D Reconnection at Null Point from the
Observations of Circular Flares and Homologous Jets
Authors: Wang, Haimin; Liu, C.
2012AAS...22050906W Altcode:
In recent studies by Pariat, Antiochos and DeVore (2009, 2010),
fan-separatrix topology and magnetic reconnection at the null-point
were simulated and found to produce homologous jets. This motivates
us to search for axisymmetric magnetic structure and associated
flaring/jetting activity. Using high-resolution ( 0.15" per pixel)
and high-cadence ( 15 s) H-alpha center/offband observations obtained
from the recently digitized films of Big Bear Solar Observatory,
we were able to identify five large circular flares with associated
surges. All the events exhibit a central parasite magnetic field
surrounded by opposite polarity, forming a circular polarity inversion
line (PIL). Consequently, a compact flare kernel at the center is
surrounded by a circular ribbon, and together with the upward ejecting
dark surge, these seem to depict a dome-like magnetic structure. Very
interestingly, (1) the circular ribbon brightens sequentially rather
than simultaneously, (2) the central compact flare kernel shows
obvious motion, and (3) a remote elongated, co-temporal flare ribbon
at a region with the same polarity as the central parasite site is
seen in the series of four homologous events on 1991 March 17 and
18. The remote ribbon is 120" away from the jet location. Moreover,
magnetic reconnection across the circular PIL is evident from the
magnetic flux cancellation. These rarely observed homologous surges
with circular as well as central and remote flare ribbons provide
valuable evidence concerning the dynamics of magnetic reconnection in
a null-point topology. This study is dedicated to Professor Hal
Zirin, the founder of Big Bear Solar Observatory, who passed away on
January 3, 2012.

---------------------------------------------------------
Title: Rapid Transition of Uncombed Penumbrae to Faculae during
Large Flares
Authors: Wang, Haimin; Deng, Na; Liu, Chang
2012ApJ...748...76W Altcode: 2012arXiv1203.2267W
In the past two decades, the complex nature of sunspots has been
disclosed with high-resolution observations. One of the most important
findings is the "uncombed" penumbral structure, where a more horizontal
magnetic component carrying most of Evershed flows is embedded in a
more vertical magnetic background. The penumbral bright grains are
locations of hot upflows and dark fibrils are locations of horizontal
flows that are guided by a nearly horizontal magnetic field. On
the other hand, it was found that flares may change the topology of
sunspots in δ configuration: the structure at the flaring polarity
inversion line becomes darkened while sections of peripheral penumbrae
may disappear quickly and permanently associated with flares. The
high spatial and temporal resolution observations obtained with the
Hinode/Solar Optical Telescope provide an excellent opportunity to
study the evolution of penumbral fine structures associated with
major flares. Taking advantage of two near-limb events, we found that
in sections of peripheral penumbrae swept by flare ribbons the dark
fibrils completely disappear, while the bright grains evolve into
faculae that are signatures of vertical magnetic flux tubes. The
corresponding magnetic fluxes measured in the decaying penumbrae
show stepwise changes temporally correlated with the flares. These
observations suggest that the horizontal magnetic field component of the
penumbra could be straightened upward (i.e., turning from horizontal to
vertical) due to magnetic field restructuring associated with flares,
which results in the transition of penumbrae to faculae.

---------------------------------------------------------
Title: Response of the Photospheric Magnetic Field to Flares
Authors: Wang, Haimin
2012decs.confE.106W Altcode:
In this study, we present a near disk-center, GOES-class X2.2 flare,
which occurred in NOAA AR 11158 on 2011 February 15. Using the
magnetic field measurements made by SDO/HMI, we obtained the first
solid evidence of a rapid (in about 30 minutes) and irreversible
enhancement in the horizontal magnetic field at the flaring magnetic
polarity inversion line (PIL) by a magnitude of 30%. It is also shown
that the photospheric field becomes more sheared and more inclined. This
field evolution is unequivocally associated with the flare occurrence
in this sigmoidal active region, with the enhancement area located in
between the two chromospheric flare ribbons and the initial conjugate
hard X-ray footpoints. These results strongly corroborate our previous
conjecture that the photospheric magnetic field near the PIL must
become more horizontal after eruptions, which could be related to
the newly formed low-lying fields resulting from the tether-cutting
reconnection. The M6.6 flare on February 13 in the same active region
shows similar pattern.

---------------------------------------------------------
Title: Response of the Photospheric Magnetic Field to the X2.2 Flare
on 2011 February 15
Authors: Wang, Shuo; Liu, Chang; Liu, Rui; Deng, Na; Liu, Yang;
Wang, Haimin
2012ApJ...745L..17W Altcode: 2011arXiv1112.3948W
It is well known that the long-term evolution of the photospheric
magnetic field plays an important role in building up free energy
to power solar eruptions. Observations, despite being controversial,
have also revealed a rapid and permanent variation of the photospheric
magnetic field in response to the coronal magnetic field restructuring
during the eruption. The Helioseismic and Magnetic Imager instrument
(HMI) on board the newly launched Solar Dynamics Observatory
produces seeing-free full-disk vector magnetograms at consistently
high resolution and high cadence, which finally makes possible an
unambiguous and comprehensive study of this important back-reaction
process. In this study, we present a near disk-center, GOES-class X2.2
flare, which occurred in NOAA AR 11158 on 2011 February 15. Using the
magnetic field measurements made by HMI, we obtained the first solid
evidence of a rapid (in about 30 minutes) and irreversible enhancement
in the horizontal magnetic field at the flaring magnetic polarity
inversion line (PIL) by a magnitude of ~30%. It is also shown that
the photospheric field becomes more sheared and more inclined. This
field evolution is unequivocally associated with the flare occurrence
in this sigmoidal active region, with the enhancement area located in
between the two chromospheric flare ribbons and the initial conjugate
hard X-ray footpoints. These results strongly corroborate our previous
conjecture that the photospheric magnetic field near the PIL must become
more horizontal after eruptions, which could be related to the newly
formed low-lying fields resulting from the tether-cutting reconnection.

---------------------------------------------------------
Title: Rapid Changes of Photospheric Magnetic Field after
Tether-cutting Reconnection and Magnetic Implosion
Authors: Liu, Chang; Deng, Na; Liu, Rui; Lee, Jeongwoo; Wiegelmann,
Thomas; Jing, Ju; Xu, Yan; Wang, Shuo; Wang, Haimin
2012ApJ...745L...4L Altcode: 2011arXiv1112.3598L
The rapid, irreversible change of the photospheric magnetic field
has been recognized as an important element of the solar flare
process. This Letter reports such a rapid change of magnetic fields
during the 2011 February 13 M6.6 flare in NOAA AR 11158 that we found
from the vector magnetograms of the Helioseismic and Magnetic Imager
(HMI) with 12 minute cadence. High-resolution magnetograms of Hinode
that are available at ~-5.5, -1.5, 1.5, and 4 hr relative to the flare
maximum are used to reconstruct a three-dimensional coronal magnetic
field under the nonlinear force-free field (NLFFF) assumption. UV
and hard X-ray images are also used to illuminate the magnetic field
evolution and energy release. The rapid change is mainly detected by
HMI in a compact region lying in the center of the magnetic sigmoid,
where the mean horizontal field strength exhibited a significant
increase of 28%. The region lies between the initial strong UV and
hard X-ray sources in the chromosphere, which are cospatial with the
central feet of the sigmoid according to the NLFFF model. The NLFFF
model further shows that strong coronal currents are concentrated
immediately above the region, and that, more intriguingly, the
coronal current system underwent an apparent downward collapse after
the sigmoid eruption. These results are discussed in favor of both
the tether-cutting reconnection producing the flare and the ensuing
implosion of the coronal field resulting from the energy release.

---------------------------------------------------------
Title: Evidence of two-stage magnetic reconnection in the 2005
January 15 X2.6 flare
Authors: Wang, Pu; Li, Yixuan; Ding, Mingde; Ji, Haisheng; Wang, Haimin
2011NewA...16..470W Altcode:
We analyze in detail the X2.6 flare that occurred on 2005 January 15
in the NOAA AR 10720 using multiwavelength observations. There are
several interesting properties of the flare that reveal possible
two-stage magnetic reconnection similar to that in the physical
picture of tether-cutting, where the magnetic fields of two separate
loop systems reconnect at the flare core region, and subsequently
a large flux rope forms, erupts, and breaks open the overlying
arcade fields. The observed manifestations include: (1) remote H α
brightenings appear minutes before the main phase of the flare; (2)
separation of the flare ribbons has a slow and a fast phase, and the
flare hard X-ray emission appears in the later fast phase; (3) rapid
transverse field enhancement near the magnetic polarity inversion line
(PIL) is found to be associated with the flare. We conclude that the
flare occurrence fits the tether-cutting reconnection picture in a
special way, in which there are three flare ribbons outlining the
sigmoid configuration. We also discuss this event in the context of
what was predicted by Hudson et al. (2008), where the Lorentz force
near the flaring PIL drops after the flare and consequently the magnetic
field lines there turn to be more horizontal as we observed.

---------------------------------------------------------
Title: Nonpotentiality of Chromospheric Fibrils in NOAA Active
Regions 11092 and 9661
Authors: Jing, Ju; Yuan, Yuan; Reardon, Kevin; Wiegelmann, Thomas;
Xu, Yan; Wang, Haimin
2011ApJ...739...67J Altcode:
In this paper, we present a method to automatically segment
chromospheric fibrils from Hα observations and further identify their
orientation. We assume that chromospheric fibrils are aligned with the
magnetic field. By comparing the orientation of the fibrils with the
azimuth of the embedding chromospheric magnetic field extrapolated from
a potential field model, the shear angle, a measure of nonpotentiality,
along the fibrils is readily deduced. Following this approach, we make
a quantitative assessment of the nonpotentiality of fibrils in two NOAA
active regions (ARs): (1) the relatively simple AR 11092, observed with
very high resolution by Interferometric Bidimensional Spectrometer,
and (2) a β-γ-δ AR 9661, observed with median resolution by Big
Bear Solar Observatory before and after an X1.6 flare.

---------------------------------------------------------
Title: Solar flare forecasting using sunspot-groups classification
and photospheric magnetic parameters
Authors: Yuan, Yuan; Shih, Frank Y.; Jing, Ju; Wang, Haimin
2011IAUS..273..446Y Altcode:
In this paper, we investigate whether incorporating sunspot-groups
classification information would further improve the performance of our
previous logistic regression based solar flare forecasting method, which
uses only line-of-sight photospheric magnetic parameters. A dataset
containing 4913 samples from the year 2000 to 2005 is constructed,
in which 2721 samples from the year 2000, 2002 and 2004 are used as
a training set, and the remaining 2192 samples from the year 2001,
2003 and 2005 are used as a testing set. Experimental results show
that sunspot-groups classification combined with total gradient on the
strong gradient polarity neutral line achieve the highest forecasting
accuracy and thus it testifies sunspot-groups classification does help
in solar flare forecasting.

---------------------------------------------------------
Title: Three-dimensional Structure of Microwave Sources from Solar
Rotation Stereoscopy Versus Magnetic Extrapolations
Authors: Nita, Gelu M.; Fleishman, Gregory D.; Jing, Ju; Lesovoi,
Sergey V.; Bogod, Vladimir M.; Yasnov, Leonid V.; Wang, Haimin; Gary,
Dale E.
2011ApJ...737...82N Altcode: 2011arXiv1106.0262N
We use rotation stereoscopy to estimate the height of a steady-state
solar feature relative to the photosphere, based on its apparent motion
in the image plane recorded over several days of observation. The
stereoscopy algorithm is adapted to work with either one-
or two-dimensional data (i.e., from images or from observations
that record the projected position of the source along an arbitrary
axis). The accuracy of the algorithm is tested on simulated data, and
then the algorithm is used to estimate the coronal radio source heights
associated with the active region NOAA 10956, based on multifrequency
imaging data over seven days from the Siberian Solar Radio Telescope
near 5.7 GHz, the Nobeyama Radio Heliograph at 17 GHz, as well as
one-dimensional scans at multiple frequencies spanning the 5.98-15.95
GHz frequency range from the RATAN-600 instrument. The gyroresonance
emission mechanism, which is sensitive to the coronal magnetic field
strength, is applied to convert the estimated radio source heights
at various frequencies, h(f), to information about magnetic field
versus height B(h), and the results are compared to a magnetic field
extrapolation derived from photospheric magnetic field observations
obtained by Hinode and Michelson Doppler Imager. We found that the
gyroresonant emission comes from heights exceeding the location of the
third gyrolayer irrespective of the magnetic extrapolation method;
implications of this finding for coronal magnetography and coronal
plasma physics are discussed.

---------------------------------------------------------
Title: What determines the penumbral size and Evershed flow speed?
Authors: Deng, Na; Shimizu, Toshifumi; Choudhary, Debi Prasad;
Wang, Haimin
2011IAUS..273..216D Altcode: 2011arXiv1102.3164D
Using Hinode SP and G-band observations, we examined the relationship
between magnetic field structure and penumbral length as well as
Evershed flow speed. The latter two are positively correlated with
magnetic inclination angle or horizontal field strength within 1.5
kilogauss, which is in agreement with recent magnetoconvective
simulations of Evershed effect. This work thus provides direct
observational evidence supporting the magnetoconvection nature of
penumbral structure and Evershed flow in the presence of strong and
inclined magnetic field.

---------------------------------------------------------
Title: Study of sunspot motion and flow fields associated with
solar flares
Authors: Wang, Shuo; Liu, Chang; Wang, Haimin
2011IAUS..273..412W Altcode:
Evolution of sunspot structure and photospheric magnetic fields
are important to understand how the flare energy is built up and
released. With high-resolution optical data, it is possible to examine
in details the optical flows of the photosphere and their relationship
to the flaring process. Using G-band and Stokes-V data obtained with
Hinode Solar Optical Telescope (SOT), we study the sunspot motion and
flow fields associated with the 2006 December 13 X3.4 flare in NOAA
AR 10930. We calculate the centroids of the delta spot umbrae lying in
opposite magnetic polarities, and use two different methods to derive
the photospheric flow fields of the AR. We find that the shearing
motion before the flare changes to unshearing motion associated
with the eruption. A decrease of average velocity of shear flow is
found to be associated with the flare, with a magnitude of 0.2 km
s-1. As a related study, we also test implementing the
recently developed differential affine velocity estimator for vector
magnetograms (DAVE4VM; Schuck, P. W 2008) technique for the magnetic
field observations obtained by the Big Bear Solar Observatory (BBSO)
and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamic
Observatory (SDO). Using this method to analyze changes of active
region magnetic fields associated with flares may shed new light on
the cause and effect of flaring process.

---------------------------------------------------------
Title: Study of the change of surface magnetic field associated
with flares
Authors: Li, Yixuan; Jing, Ju; Fan, Yuhong; Wang, Haimin
2011IAUS..273..417L Altcode:
How magnetic field structure changes with eruptive events (e.g., flares
and CMEs) has been a long-standing problem in solar physics. Here we
present the analysis of eruption-associated changes in the magnetic
inclination angle, the transverse component of magnetic field and the
Lorentz force. The analysis is based on an observation of the X3.4
flare on Dec.13 2006 and a numerical simulation of a solar eruption
made by Yuhong Fan. Both observation and simulation show that (1) the
magnetic inclination angle in the decayed peripheral penumbra increases,
while that in the central area close to flaring polarity inversion
line (PIL) deceases after the flare; (2) the transverse component of
magnetic field increases at the lower altitude near flaring PIL after
the flare. The result suggests that the field lines at flaring neutral
line turn to more horizontal near the surface, that is in agreement
with the prediction of Hudson, Fisher &amp; Welsch (2008).

---------------------------------------------------------
Title: Rapid changes of sunspot structure associated with solar
eruptions
Authors: Wang, Haimin; Liu, Chang
2011IAUS..273...15W Altcode:
In this paper we summarize the studies of flare-related changes of
photospheric magnetic fields. When vector magnetograms are available,
we always find an increase of transverse field at the polarity
inversion line (PIL). We also discuss 1 minute cadence line-of-sight
MDI magnetogram observations, which usually show prominent changes of
magnetic flux contained in the flaring δ spot region. The observed
limb-ward flux increases while disk-ward flux decreases rapidly and
irreversibly after flares. These observations provides evidences,
either direct or indirect, for the theory and prediction of Hudson,
Fisher &amp; Welsch (2008) that the photospheric magnetic fields would
respond to coronal field restructuring and turn to a more horizontal
state near the PIL after eruptions. From the white-light observations,
we find that at flaring PIL, the structure becomes darker after an
eruption, while the peripheral penumbrae decay. Using high-resolution
Hinode data, we find evidence that only dark fibrils in the “uncombed”
penumbral structure disappear while the bright grains evolve to G-band
bright points after flares.

---------------------------------------------------------
Title: A Standard-to-blowout Jet
Authors: Liu, Chang; Deng, Na; Liu, Rui; Ugarte-Urra, Ignacio; Wang,
Shuo; Wang, Haimin
2011ApJ...735L..18L Altcode: 2011arXiv1105.3244L
The commonly observed jets provide critical information on the
small-scale energy release in the solar atmosphere. We report a near
disk-center jet on 2010 July 20, observed by the Solar Dynamics
Observatory. In this event, the standard interchange magnetic
reconnection between an emerging flux spanning 9 × 103
km and ambient open fields is followed by a blowout-like eruption. In
the "standard" stage, as the emerging negative element approached the
nearby positive network fields, a jet with a dome-like base in EUV grew
for 30 minutes before the jet spire began to migrate laterally with
enhanced flux emergence. In the "blowout" stage, the above converging
fields collided and the subsequent cancellation produced a UV microflare
lasting seven minutes, in which the dome of the jet seemed to be blown
out as (1) the spire swung faster and exhibited an unwinding motion
before vanishing, (2) a rising loop and a blob erupted leaving behind
cusped structures, with the blob spiraling outward in acceleration
after the flare maximum, and (3) ejecting material with a curtain-like
structure at chromospheric to transition-region temperatures also
underwent a transverse motion. It is thus suggested that the flare
reconnection rapidly removes the outer fields of the emerging flux to
allow its twisted core field to erupt, a scenario favoring the jet-scale
magnetic breakout model as recently advocated by Moore et al. in 2010.

---------------------------------------------------------
Title: Rapid Enhancement of Sheared Evershed Flow Along the Neutral
Line Associated with an X6.5 Flare Observed by Hinode
Authors: Deng, Na; Liu, Chang; Prasad Choudhary, Debi; Wang, Haimin
2011ApJ...733L..14D Altcode: 2011arXiv1104.3902D
We present G-band and Ca II H observations of NOAA AR 10930
obtained by Hinode/SOT on 2006 December 6 covering an X6.5
flare. The Local Correlation Tracking technique was applied to the
foreshortening-corrected G-band image series to acquire horizontal
proper motions in this complex βγδ active region. With the continuous
high-quality, spatial and temporal resolution G-band data, we not
only confirm the rapid decay of outer penumbrae and darkening of the
central structure near the flaring neutral line, but also unambiguously
detect for the first time the enhancement of the sheared Evershed
flow (average horizontal flow speed increased from 330 ± 3.1 to
403 ± 4.6 m s-1) along the neutral line right after the
eruptive white-light flare. Post-flare Ca II H images indicate that
the originally fanning out field lines at the two sides of the neutral
line get connected. Since penumbral structure and Evershed flow are
closely related to photospheric magnetic inclination or horizontal
field strength, we interpret the rapid changes of sunspot structure
and surface flow as the result of flare-induced magnetic restructuring
down to the photosphere. The magnetic fields turn from fanning out
to inward connection causing outer penumbrae decay, meanwhile those
near the flaring neutral line become more horizontal leading to
stronger Evershed flow there. The inferred enhancement of horizontal
magnetic field near the neutral line is consistent with recent magnetic
observations and theoretical predictions of flare-invoked photospheric
magnetic field change.

---------------------------------------------------------
Title: Uncombed Sunspot Penumbrae Are Combed by Large Flares
Authors: Wang, Haimin; Liu, C.; Deng, N.
2011SPD....42.2206W Altcode: 2011BAAS..43S.2206W
In the past two decades, the complex nature of sunspots is disclosed
with high resolution observations. One of the most important findings
is the “uncombed” penumbral structure, where the bright grains
are more vertical while dark fibrils are more horizontal (Solanki and
Montavon 1993). The Evershed flows are more closely associated with the
horizontal component. On the other hand, it was found that flares may
change the topology of the sunspot structure in delta configuration:
the central structure at the flaring polarity inversion line becomes
darkened while sections of peripheral penumbrae may disappear
permanently associated with flares (Liu et al. 2005). The high
spatial and temporal resolution observations obtained with Hinode/SOT on
December 6, 2006 and June 4, 2007 provide an excellent opportunity to
study the evolution of penumbral fine structure associated with major
flares. We found that in sections of penumbrae swept by flare ribbons,
the dark fibrils completely disappear, while the bright grains evolve
into faculae that resemble the structure of vertical magnetic flux
tubes. Therefore, the original uncombed penumbral structure seems to be
combed toward the vertical direction---the dark and bright components
are no longer separated in orientation after the flares. These results
provide a new insight into the possible impact by the coronal transients
on the photospheric magnetic structure of sunspots, and shed new
light on the obscure formation and decay mechanism of penumbrae. Reference Liu, C., Deng, N., Liu, Y., Falconer, D., Goode,
P.R., Denker, C. &amp; Wang, H., 2005, Ap.J., 622, 722 Solanki,
S.K. &amp; Montavon, C.A.P., 1993, A &amp; A, 275, 283

---------------------------------------------------------
Title: A Revisit of the Masuda Flare
Authors: Liu, Rui; Xu, Yan; Wang, Haimin
2011SoPh..269...67L Altcode: 2009arXiv0905.4309L; 2011SoPh..tmp....3L
We revisit the flare that occurred on 13 January 1992, which is now
universally termed the "Masuda flare". The new analysis is motivated
not just by its uniqueness despite the increasing number of coronal
observations in hard X-rays, but also by the improvement of Yohkoh
hard X-ray image processing, which was achieved after the intensive
investigations on this celebrated event. Using an uncertainty
analysis, we show that the hard X-ray coronal source is located
closer to the soft X-ray loop by about 5000 km (or 7 arcsec) in the
re-calibrated Hard X-ray Telescope (HXT) images than in the original
ones. Specifically, the centroid of the M1-band (23 - 33 keV) coronal
source is above the maximum brightness of the Soft X-ray Telescope
(SXT) loop by 5000±1000 km (9600 km in the original data) and above
the apex of the SXT loop represented by the 30% brightness contour
by 2000±1000 km (∼ 7000 km in the original data). The change is
obviously significant, because most coronal sources are above the
thermal loop by less than 6 arcsec. We suggest that this change may
account for the discrepancy in the literature, i.e., the spectrum of
the coronal emission was reported to be extremely hard below ∼ 20
keV in the pre-calibration investigations, whereas it was reported
to be considerably softer in the literature after the re-calibration
done by Sato, Kosugi, and Makishima (Pub. Astron. Soc. Japan51, 127,
1999). Still, the coronal spectrum is flatter at lower energies than
at higher energies, due to the lack of a similar, co-spatial source in
the L-band (14 - 23 keV), for which a convincing explanation is absent.

---------------------------------------------------------
Title: First Flare-related Rapid Change of Photospheric Magnetic
Field Observed by Solar Dynamics Observatory
Authors: Wang, Shuo; Liu, Chang; Liu, Rui; Deng, Na; Liu, Yang;
Wang, Haimin
2011arXiv1103.0027W Altcode:
Photospheric magnetic field not only plays important roles in
building up free energy and triggering solar eruptions, but also has
been observed to change rapidly and permanently responding to the
coronal magnetic field restructuring due to coronal transients. The
Helioseismic and Magnetic Imager instrument (HMI) on board the newly
launched Solar Dynamics Observatory (SDO) produces seeing-free full-disk
vector magnetograms at consistently high resolution and high cadence,
which finally makes possible an unambiguous and comprehensive study
of this important back-reaction process. In this study, we present a
near disk-center, GOES-class X2.2 flare occurred at NOAA AR 11158 on
2011 February 15 using the magnetic field measurements made by HMI. We
obtained the first solid evidence of an enhancement in the transverse
magnetic field at the flaring magnetic polarity inversion line (PIL)
by a magnitude of 70%. This rapid and irreversible field evolution is
unequivocally associated with the flare occurrence, with the enhancement
area located in between the two chromospheric flare ribbons. Similar
findings have been made for another two major flare events observed
by SDO. These results strongly corroborate our previous suggestion
that the photospheric magnetic field near the PIL must become more
horizontal after eruptions. In-depth studies will follow to further
link the photospheric magnetic field changes with the dynamics of
coronal mass ejections, when full Stokes inversion is carried out to
generate accurate magnetic field vectors.

---------------------------------------------------------
Title: Comparison Between Observation and Simulation of Magnetic
Field Changes Associated with Flares
Authors: Li, Yixuan; Jing, Ju; Fan, Yuhong; Wang, Haimin
2011ApJ...727L..19L Altcode:
It has been a long-standing question in solar physics how magnetic
field structure changes with eruptive events (e.g., flares and coronal
mass ejections). In this Letter, we present the eruption-associated
changes in the magnetic inclination angle, the horizontal component
of magnetic field vectors, and the Lorentz force. The analysis is
based on the observation of the X3.4 flare on 2006 December 13 and in
comparison to the numerical simulation of Fan. Both observation and
simulation show that (1) the magnetic inclination angle in the decayed
peripheral penumbra increases, while that in the central area close
to the flaring polarity inversion line (PIL) deceases after the flare;
(2) the horizontal component of magnetic field increases at the lower
altitude near the flaring PIL after the flare. The result suggests that
the field lines at the flaring neutral line turn to more horizontal near
the surface, that is in agreement with the prediction of Hudson et al.

---------------------------------------------------------
Title: Sigmoid-to-flux-rope Transition Leading to a Loop-like Coronal
Mass Ejection
Authors: Liu, Rui; Liu, Chang; Wang, Shuo; Deng, Na; Wang, Haimin
2010ApJ...725L..84L Altcode: 2010arXiv1011.1181L
Sigmoids are one of the most important precursor structures for solar
eruptions. In this Letter, we study a sigmoid eruption on 2010 August
1 with EUV data obtained by the Atmospheric Imaging Assembly (AIA)
on board the Solar Dynamic Observatory (SDO). In AIA 94 Å (Fe XVIII;
6 MK), topological reconfiguration due to tether-cutting reconnection
is unambiguously observed for the first time, i.e., two opposite
J-shaped loops reconnect to form a continuous S-shaped loop, whose
central portion is dipped and aligned along the magnetic polarity
inversion line (PIL), and a compact loop crossing the PIL. A causal
relationship between photospheric flows and coronal tether-cutting
reconnections is evidenced by the detection of persistent converging
flows toward the PIL using line-of-sight magnetograms obtained by the
Helioseismic and Magnetic Imager on board SDO. The S-shaped loop remains
in quasi-equilibrium in the lower corona for about 50 minutes, with the
central dipped portion rising slowly at ~10 km s-1. The speed
then increases to ~60 km s-1 about 10 minutes prior to the
onset of a GOES-class C3.2 flare, as the S-shaped loop speeds up its
transformation into an arch-shaped loop, which eventually leads to a
loop-like coronal mass ejection. The AIA observations combined with
Hα filtergrams as well as hard X-ray imaging and spectroscopy are
consistent with most flare loops being formed by reconnection of the
stretched legs of less-sheared J-shaped loops that envelopes the rising
flux rope, in agreement with the standard tether-cutting scenario.

---------------------------------------------------------
Title: A Reconnecting Current Sheet Imaged in a Solar Flare
Authors: Liu, Rui; Lee, Jeongwoo; Wang, Tongjiang; Stenborg, Guillermo;
Liu, Chang; Wang, Haimin
2010ApJ...723L..28L Altcode: 2010arXiv1009.4912L
Magnetic reconnection changes the magnetic field topology and powers
explosive events in astrophysical, space, and laboratory plasmas. For
flares and coronal mass ejections (CMEs) in the solar atmosphere,
the standard model predicts the presence of a reconnecting current
sheet, which has been the subject of considerable theoretical and
numerical modeling over the last 50 years, yet direct, unambiguous
observational verification has been absent. In this Letter, we show a
bright sheet structure of global length (&gt;0.25 R sun)
and macroscopic width ((5-10)×103 km) distinctly above
the cusp-shaped flaring loop, imaged during the flare rising phase in
EUV. The sheet formed due to the stretch of a transequatorial loop
system and was accompanied by various reconnection signatures. This
unique event provides a comprehensive view of the reconnection geometry
and dynamics in the solar corona.

---------------------------------------------------------
Title: Gradual Inflation of Active-region Coronal Arcades Building
up to Coronal Mass Ejections
Authors: Liu, Rui; Liu, Chang; Park, Sung-Hong; Wang, Haimin
2010ApJ...723..229L Altcode: 2010arXiv1008.4863L
The pre-coronal mass ejection (pre-CME) structure is of great
importance to understanding the origin of CMEs, which, however, has
been largely unknown for CMEs originating from active regions. In
this paper, we investigate this issue using the wavelet-enhanced EUV
Imaging Telescope (EIT) observations combined with the Large Angle and
Spectrometric Coronagraph, Michelson Doppler Imager, and GOES soft X-ray
observations. Selected for studying are 16 active-region coronal arcades
whose gradual inflation lead up to CMEs. Twelve of them clearly build
upon post-eruptive arcades resulting from a preceding eruption; the
remaining four are located high in the corona in the first place and/or
have existed for days. The observed inflation lasts for 8.7 ± 4.1 hr,
with the arcade rising from 1.15 ± 0.06 R sun to 1.36 ±
0.07 R sun within the EIT field of view (FOV). The rising
speed is less than 5 km s-1 most of the time. Only at the
end of this quasi-static stage does it increase to tens of kilometers
per second over tens of minutes. The arcade then erupts out of the
EIT FOV as a CME with similar morphology. This pre-CME structure is
apparently unaffected by the flares occurring during its quasi-static
inflation phase, but is closely coupled with the flare occurring during
its acceleration phase. For four events that are observed on the disk,
it is found that the gradual inflation of the arcade is accompanied by
significant helicity injection from the photosphere. In particular,
a swirling structure, which is reminiscent of a magnetic flux rope,
was observed in one of the arcades over 4 hr prior to the subsequent
CME, and the growth of the arcade is associated with the injection of
helicity of opposite sign into the active region via flux emergence. We
propose a four-phase evolution paradigm for the observed CMEs, i.e.,
a quasi-static inflation phase which corresponds to the buildup of
magnetic free energy in the corona, followed by the frequently observed
three-phase paradigm, including an initial phase, an acceleration phase,
and a gradual phase.

---------------------------------------------------------
Title: Motions of Hard X-ray Sources During an Asymmetric Eruption
Authors: Liu, Chang; Lee, Jeongwoo; Jing, Ju; Liu, Rui; Deng, Na;
Wang, Haimin
2010ApJ...721L.193L Altcode: 2010arXiv1008.5381L
Filament eruptions and hard X-ray (HXR) source motions are commonly
observed in solar flares, which provide critical information on the
coronal magnetic reconnection. This Letter reports an event on 2005
January 15, in which we found an asymmetric filament eruption and a
subsequent coronal mass ejection together with complicated motions
of HXR sources during the GOES-class X2.6 flare. The HXR sources
initially converge to the magnetic polarity inversion line (PIL),
and then move in directions either parallel or perpendicular to the
PIL depending on the local field configuration. We distinguish the
evolution of the HXR source motion in four phases and associate each of
them with distinct regions of coronal magnetic fields as reconstructed
using a nonlinear force-free field extrapolation. It is found that
the magnetic reconnection proceeds along the PIL toward the regions
where the overlying field decreases with height more rapidly. It is
also found that not only the perpendicular but the parallel motion of
the HXR sources correlates well with the HXR light curve. These results
are discussed in favor of the torus instability as an important factor
in the eruptive process.

---------------------------------------------------------
Title: Time Evolution of Coronal Magnetic Helicity in the Flaring
Active Region NOAA 10930
Authors: Park, Sung-Hong; Chae, Jongchul; Jing, Ju; Tan, Changyi;
Wang, Haimin
2010ApJ...720.1102P Altcode: 2010arXiv1008.1558P
To study the three-dimensional (3D) magnetic field topology and its
long-term evolution associated with the X3.4 flare of 2006 December 13,
we investigate the coronal relative magnetic helicity in the flaring
active region (AR) NOAA 10930 during the time period of December
8-14. The coronal helicity is calculated based on the 3D nonlinear
force-free magnetic fields reconstructed by the weighted optimization
method of Wiegelmann, and is compared with the amount of helicity
injected through the photospheric surface of the AR. The helicity
injection is determined from the magnetic helicity flux density proposed
by Pariat et al. using Solar and Heliospheric Observatory/Michelson
Doppler Imager magnetograms. The major findings of this study are the
following. (1) The time profile of the coronal helicity shows a good
correlation with that of the helicity accumulation by injection through
the surface. (2) The coronal helicity of the AR is estimated to be -4.3
× 1043 Mx2 just before the X3.4 flare. (3) This
flare is preceded not only by a large increase of negative helicity,
-3.2 × 1043 Mx2, in the corona over ~1.5 days
but also by noticeable injections of positive helicity through the
photospheric surface around the flaring magnetic polarity inversion
line during the time period of the channel structure development. We
conjecture that the occurrence of the X3.4 flare is involved with
the positive helicity injection into an existing system of negative
helicity.

---------------------------------------------------------
Title: The Formation of a Magnetic Channel by the Emergence of
Current-carrying Magnetic Fields
Authors: Lim, Eun-Kyung; Chae, Jongchul; Jing, Ju; Wang, Haimin;
Wiegelmann, Thomas
2010ApJ...719..403L Altcode: 2010arXiv1009.0420L
A magnetic channel—a series of polarity reversals separating elongated
flux threads with opposite polarities—may be a manifestation of
a highly non-potential magnetic configuration in active regions. To
understand its formation, we have carried out a detailed analysis of
the magnetic channel in AR 10930 using data taken by the Solar Optical
Telescope/Hinode. As a result, we found upflows (-0.5 to -1.0 km
s-1) and downflows (+1.5 to +2.0 km s-1) inside
and at both tips of the thread, respectively, and a pair of strong
vertical currents of opposite polarity along the channel. Moreover,
our analysis of the nonlinear force-free fields constructed from the
photospheric magnetic field indicates that the current density in the
lower corona may have gradually increased as a result of the continuous
emergence of the highly sheared flux along the channel. With these
results, we suggest that the magnetic channel originates from the
emergence of the twisted flux tube that has formed below the surface
before the emergence.

---------------------------------------------------------
Title: Nonlinear Force-free Modeling of Magnetic Fields in a Solar
Filament
Authors: Jing, Ju; Yuan, Yuan; Wiegelmann, Thomas; Xu, Yan; Liu, Rui;
Wang, Haimin
2010ApJ...719L..56J Altcode:
We present a striking filament pattern in the nonlinear force-free
(NLFF) chromospheric magnetic field of the active region NOAA
10956. The NLFF chromospheric field is extrapolated from the Hinode
high-resolution photospheric vector magnetogram using the weighted
optimization method. The modeled structure is characterized by a
highly sheared field with strong horizontal magnetic components and
has a virtually identical shape and location as the filament seen in
Hα. The modeled field strength agrees with the recent He I 10830 Å
observations by Kuckein et al.. The unequivocal resemblance between the
NLFF extrapolation and the Hα observation not only demonstrates the
ability of the NLFF field to reproduce chromospheric features, but also
provides a valuable diagnostic tool for the filament magnetic fields.

---------------------------------------------------------
Title: Productivity of Solar Flares and Magnetic Helicity Injection
in Active Regions
Authors: Park, Sung-hong; Chae, Jongchul; Wang, Haimin
2010ApJ...718...43P Altcode: 2010arXiv1005.3416P
The main objective of this study is to better understand how magnetic
helicity injection in an active region (AR) is related to the occurrence
and intensity of solar flares. We therefore investigate the magnetic
helicity injection rate and unsigned magnetic flux, as a reference. In
total, 378 ARs are analyzed using SOHO/MDI magnetograms. The 24 hr
averaged helicity injection rate and unsigned magnetic flux are compared
with the flare index and the flare-productive probability in the next
24 hr following a measurement. In addition, we study the variation of
helicity over a span of several days around the times of the 19 flares
above M5.0 which occurred in selected strong flare-productive ARs. The
major findings of this study are as follows: (1) for a sub-sample of
91 large ARs with unsigned magnetic fluxes in the range from (3-5)
× 1022 Mx, there is a difference in the magnetic helicity
injection rate between flaring ARs and non-flaring ARs by a factor
of 2; (2) the GOES C-flare-productive probability as a function of
helicity injection displays a sharp boundary between flare-productive
ARs and flare-quiet ones; (3) the history of helicity injection
before all the 19 major flares displayed a common characteristic:
a significant helicity accumulation of (3-45) × 1042
Mx2 during a phase of monotonically increasing helicity over
0.5-2 days. Our results support the notion that helicity injection is
important in flares, but it is not effective to use it alone for the
purpose of flare forecast. It is necessary to find a way to better
characterize the time history of helicity injection as well as its
spatial distribution inside ARs.

---------------------------------------------------------
Title: Direct Observation of a Y-Type Current Sheet during a Solar
Flare
Authors: Liu, Rui; Lee, Jeowoo; Wang, Tongjiang; Stenborg, Guillermo;
Liu, Chang; Wang, Haimin
2010shin.confE.160L Altcode:
A vertical current sheet is a crucial element in many flare/CME models,
yet direct, unambiguous observation of such a current sheet has been
absent. Here we present the observation of a Y-type current sheet,
which formed due to the gradual expansion and the subsequent eruption
of a group of transequatorial loops. As the loop legs approached each
other, plasma flew at ∼6 km/s into a local area where a cusp-shaped
flare loop subsequently formed, and the Y-type current sheet was seen as
a bright, elongated structure in the EIT 195A channel aboard SOHO. It
was at least 0.25 solar radius long and about (5-10)—10^3 km wide,
extending from 50 Mm above the cusp-shaped flare loop top. The inverse
Y-point initially moved downward, and then moved upward. Drifting
pulsating structures at metric frequencies were recorded during
the impulsive phase, implying tearing of the current sheet. A radio
Type III burst was recorded at the same time when the current sheet
was clearly seen in EUV, indicative of accelerated electrons beaming
upward from the upper tip of the current sheet. TRACE observed downward
propagating feature at ∼220 km/s along the cusp-shaped flare loop
during the impulsive phase, as well as upward jet-like feature at ∼100
km/s along the post-flare loop during the decay phase. A cusp-shaped
dimming region was seen in both EIT and TRACE 195A channels during
the decay phase, filling between the post-flare arcade and the Y-type
separatrix; both the arcade and the cusp expanded with time. With SOHO
CDS, a clear signature of chromospheric evaporation is detected during
the decay phase, i.e., the cusp-shaped dimming region is associated
with plasma upflow detected with EUV hot emission lines, such as Fe
XVI and Si XII, while the leg of the post-flare loop is associated
with downflow detected with cold lines, such as Ca X, O V, O IV and
He I. The current sheet orientation indicated by the cusp is well
aligned with a post-CME ray in white-light, apparently coaxial with
the CME. This unique event provides a first clear-cut observation of
the vertical current sheet during the flare, as well as a comprehensive
view of the reconnection geometry and dynamics, in the solar corona.

---------------------------------------------------------
Title: A portable solar adaptive optics system: software and
laboratory developments
Authors: Ren, Deqing; Penn, Matt; Plymate, Claude; Wang, Haimin;
Zhang, Xi; Dong, Bing; Brown, Nathan; Denio, Andrew
2010SPIE.7736E..3PR Altcode: 2010SPIE.7736E.124R
We present our recent process on a portable solar adaptive Optics
system, which is aimed for diffraction-limited imaging in the 1.0 ~
5.0-μm infrared wavelength range with any solar telescope with an
aperture size up to 1.6 meters. The realtime wave-front sensing, image
processing and computation are based on a commercial multi-core personal
computer. The software is developed in LabVIEW. Combining the power of
multi-core imaging processing and LabVIEW parallel programming, we show
that our solar adaptive optics can achieve excellent performance that
is competitive with other systems. In addition, the LabVIEW's block
diagram based programming is especially suitable for rapid development
of a prototype system, which makes a low-cost and high-performance
system possible. Our adaptive optics system is flexible; it can
work with any telescope with or without central obstruction with
any aperture size in the range of 0.6~1.6 meters. In addition, the
whole system is compact and can be brought to a solar observatory to
perform associated scientific observations. According to our knowledge,
this is the first adaptive optics that adopts the LabVIEW high-level
programming language with a multi-core commercial personal computer,
and includes the unique features discussed above.

---------------------------------------------------------
Title: Measurements of Filament Height in Hα and EUV 304 Å
Authors: Xu, Yan; Jing, Ju; Wang, Haimin
2010SoPh..264...81X Altcode: 2010SoPh..tmp...96X
In this study, we present the three-dimensional (3D) configuration of
a filament observed by STEREO and the Global High Resolution H-alpha
Network (GHN) in EUV 304 Å and Hα line center, respectively. This
was the largest filament located close to the active region NOAA
10956 that produced a small B9.6 flare and two Coronal Mass Ejections
(CMEs) on 19 May 2007. The 3D coordinates of multiple points traced
along this filament were reconstructed by triangulation from two
different aspect angles. The two STEREO (A and B) spacecraft had a
separation angle α of 8.6 degree on 19 May 2007. The "true" heights
of the filament were estimated using STEREO images in EUV 304 and
Hα images, respectively. Our results show that EUV emission of the
filament originates from higher locations than the Hα emission. We
also compare the measured reconstructed heights of the filaments in
EUV with those reported in previous studies.

---------------------------------------------------------
Title: Observational Evidence of Back Reaction on the Solar Surface
Associated with Coronal Magnetic Restructuring in Solar Eruptions
Authors: Wang, Haimin; Liu, Chang
2010ApJ...716L.195W Altcode: 2010arXiv1005.4137W
Most models of solar eruptions assume that coronal field lines are
anchored in the dense photosphere and thus the photospheric magnetic
fields would not have rapid, irreversible changes associated with
eruptions resulted from the coronal magnetic reconnection. Motivated
by the recent work of Hudson et al. on quantitatively evaluating the
back reaction due to energy release from the coronal fields, in this
Letter we synthesize our previous studies and present analysis of new
events about flare-related changes of photospheric magnetic fields. For
the 11 X-class flares where vector magnetograms are available, we
always find an increase of transverse field at the polarity inversion
line (PIL) although only four events had measurements with 1 minute
temporal resolution. We also discuss 18 events with 1 minute cadence
line-of-sight magnetogram observation, which all show prominent changes
of magnetic flux contained in the flaring δ spot region. Except in
one case, the observed limbward flux increases while diskward flux
decreases rapidly and irreversibly after flares. This observational
evidence provides support, either directly or indirectly, for the
theory and prediction of Hudson et al. that the photospheric magnetic
fields must respond to coronal field restructuring and turn to a more
horizontal state near the PIL after eruptions.

---------------------------------------------------------
Title: Fast Contraction of Coronal Loops at the Flare Peak
Authors: Liu, Rui; Wang, Haimin
2010ApJ...714L..41L Altcode:
On 2005 September 8, a coronal loop overlying the active region
NOAA 10808 was observed in TRACE 171 Å to contract at ~100 km
s-1 at the peak of an X5.4-2B flare at 21:05 UT. Prior to
the fast contraction, the loop underwent a much slower contraction at
~6 km s-1 for about 8 minutes, initiating during the flare
preheating phase. The sudden switch to fast contraction is presumably
corresponding to the onset of the impulsive phase. The contraction
resulted in the oscillation of a group of loops located below, with the
period of about 10 minutes. Meanwhile, the contracting loop exhibited
a similar oscillatory pattern superimposed on the dominant downward
motion. We suggest that the fast contraction reflects a suddenly
reduced magnetic pressure underneath due either to (1) the eruption
of magnetic structures located at lower altitudes or to (2) the rapid
conversion of magnetic free energy in the flare core region. Electrons
accelerated in the shrinking trap formed by the contracting loop can
theoretically contribute to a late-phase hard X-ray burst, which is
associated with Type IV radio emission. To complement the X5.4 flare
which was probably confined, a similar event observed in SOHO/EIT 195
Å on 2004 July 20 in an eruptive, M8.6 flare is briefly described,
in which the contraction was followed by the expansion of the same loop
leading up to a halo coronal mass ejection. These observations further
substantiate the conjecture of coronal implosion and suggest coronal
implosion as a new exciter mechanism for coronal loop oscillations.

---------------------------------------------------------
Title: Back-reaction on the Solar Surface Associated with Coronal
Magnetic Restructuring in Solar Eruptions
Authors: Wang, Haimin; Liu, C.
2010AAS...21640414W Altcode: 2010BAAS...41R.901W
Solar eruptions have been understood as the result of magnetic
reconnection in solar corona, therefore most models of flares and
coronal mass ejections assume that photospheric magnetic fields are
anchored and do not have rapid, irreversible changes associated with
the eruptions. Recently, we note the work by Hudson, Fisher and Welsch
(2008, ASP, 383, 221), who quantitatively assessed the back reaction
on the photosphere and solar interior by the coronal field evolution
required to release flare energy, and made the prediction that after
flares, the photospheric magnetic fields turn to a more horizontal
state. Here we summarize our studies of several papers and a few
new events that describe changes of magnetic fields associated with
flares. For the events that vector magnetograms are available, we
indeed find a rapid increase of transverse magnetic fields near the
polarity inversion line associated with large flares. For the other
events that only line-of-sight magnetograms are present, we always
observe that limb-ward flux increases while disk-ward flux decreases
rapidly and irreversibly associated with flares, which also indirectly
supports the theory of Hudson, Fisher and Welsch. Finally, we discuss
the possible relationship between the rapid changes of photospheric
magnetic fields and the excitation of seismic waves, the so-called
sunquakes (Kosovichev and Zharkova, 1998, Nature, 393, 317).

---------------------------------------------------------
Title: Free Magnetic Energy and Flare Productivity of Active Regions
Authors: Jing, Ju; Tan, Changyi; Yuan, Yuan; Wang, Benjamin;
Wiegelmann, Thomas; Xu, Yan; Wang, Haimin
2010ApJ...713..440J Altcode:
In this study, the photospheric vector magnetograms, obtained with the
Spectro-Polarimeter of the Solar Optical Telescope on board Hinode, are
used as the boundary conditions to extrapolate the three-dimensional
nonlinear force-free (NLFF) coronal magnetic fields. The observed
non-force-free photospheric magnetic fields are preprocessed toward the
nearly force-free chromospheric magnetic fields. The performance of the
preprocessing procedure is evaluated by comparing with chromospheric
magnetic fields obtained by the Vector SpectroMagnetograph instrument
located on the Synoptic Optical Long-term Investigations of the
Sun Tower. Then, the weighted optimization method is applied to the
preprocessed boundary data to extrapolate the NLFF fields with which
we are able to estimate the free magnetic energy stored in the active
regions. The magnitude scaling correlation between the free magnetic
energy and the soft X-ray flare index (FI) of active regions is then
studied. The latter quantifies the impending flare production of active
regions over the subsequent 1, 2, and 3 day time windows. Based on
75 samples, we find a positive correlation between the free energy
and the FI. We also study the temporal variation of free magnetic
energy for three active regions, of which two are flare-active and
one is flare-quiet during the observation over a period of several
days. While the magnitude of free magnetic energy unambiguously
differentiates between the flare-active and the flare-quiet regions,
the temporal variation of free magnetic energy does not exhibit a
clear and consistent pre-flare pattern. This may indicate that the
trigger mechanism of flares is as important as the energy storage in
active regions.

---------------------------------------------------------
Title: Dual-Stage Reconnection During Solar Flares Observed in
Hard X-ray
Authors: Xu, Yan; Jing, Ju; Cao, Wenda; Wang, Haimin
2010ApJ...709L.142X Altcode:
In this Letter, we present hard X-ray (HXR) observation by the Reuven
Ramaty High Energy Solar Spectroscopic Imager of the 2003 October 29
X10 flare. Two pairs of HXR conjugate footpoints have been identified
during the early impulsive phase. This geometric configuration is very
much in the manner predicted by the "tether-cutting" scenario first
proposed by Moore &amp; Roumeliotis. The HXR light curves show that
the outer pair of footpoints disappeared much faster than the other
pair. This temporal behavior further confirms that this event is a good
example of the "tether-cutting" model. In addition, we reconstructed
a three-dimensional magnetic field based on the nonlinear force-free
extrapolation and found that each pair of HXR footpoints were indeed
linked by corresponding magnetic field lines.

---------------------------------------------------------
Title: The contraction of flare loops
Authors: Ji, Haisheng; Wang, Haimin
2010cosp...38.1802J Altcode: 2010cosp.meet.1802J
In recent years, several authors have reported a contracting motion
for solar flare loops in many flares. That is, during the early
impulsive phase of solar flares, hard X-ray (HXR) looptop sources or
radio/extreme-ultraviolet (EUV) flaring loops have a descending or
shrinking motion and, at the same time, Hα ribbons or HXR footpoints
(FPs) are converging. Only after the impulsive phase does there begin
to appear an upward motion for the looptop sources and flaring loops
and a corresponding outward motion (the usual separation motion) for
the flare ribbons or FPs. In this talk, we will give a brief review to
all events in literature. We will try demonstrate that this new solar
flare phenomenon implies magnetic reconnection in a highly sheared
magnetic field.

---------------------------------------------------------
Title: Monitoring free magnetic energy in erupting active regions
Authors: Wiegelmann, Thomas; Thalmann, Julia; Jing, Ju; Wang, Haimin
2010cosp...38.2960W Altcode: 2010cosp.meet.2960W
In solar eruptions, like flares and coronal mass ejections, free
magnetic energy stored in the solar corona is converted into kinetic
energy. Unfortunately the coronal magnetic field cannot be measured
directly. We can, however, reconstruct the coronal magnetic field
from measurements of the photospheric magnetic field vector under
the reasonable assumption of a force-free coronal plasma. With
a procedure dubbed preprocessing we derive force-free consistent
boundary conditions, which are extrapolated into the solar corona
with a nonlinear force-free extrapolation code. The resulting 3D
coronal magnetic field allows us to derive the magnetic topology and
to computed the magnetic energy as well as an upper limited of the
free energy available for driving eruptive phenomena. We apply our
code to measurements from several ground based vector magnetographs,
e.g. the Solar Flare Telescope, SOLIS and the Big Bear Solar
Observatory. Within our studies we find a clear relationship between
the stored magnetic energy and the strength of eruptions. In most cases
not the entire free energy is converted to kinetic energy, but only a
fraction. Consequently, the post-flare magnetic field configuration
is usually not entirely current free, but significantly closer to a
potential field as before the flare.

---------------------------------------------------------
Title: The correlation between expansion speed and magnetic field
in solar flare ribbons
Authors: Xie, Wenbin; Wang, Haimin; Jing, Ju; Bao, Xingming; Zhang,
Hongqi
2009ScChG..52.1754X Altcode:
In this paper, we study the correlation between the expansion speed
of two-ribbon flares and the magnetic field measured in the ribbon
location, and compare such correlation for two events with different
magnetic configurations. These two events are: an M1.0 flare in the
quiet sun on September 12, 2000 and an X2.3 flare in Active Region
NOAA 9415 on April 10, 2001. The magnetic configuration of the M1.0
flare is simple, while that of X2.3 event is complex. We have derived
a power-law correlation between the ribbon expansion speed ( V 
r ) and the longitudinal magnetic field ( B z )
with an empirical relationship V r = A×B-{/z 
- δ }, where A is a constant and δ is the index of the
power-law correlation. We have found that δ for the M1.0 flare in the
simple magnetic configuration is larger than that for the X2.3 flare
in the complex magnetic configuration.

---------------------------------------------------------
Title: The change of magnetic inclination angles associated with
the X3.4 flare on December 13, 2006
Authors: Li, Yixuan; Jing, Ju; Tan, Changyi; Wang, Haimin
2009ScChG..52.1702L Altcode:
Recent studies showed a consistent pattern of changes in the sunspot
structure associated with major flares: part of the peripheral penumbral
regions vanishes during flares, and meanwhile, the umbral cores and/or
inner penumbral regions are darkened. To understand the underlying
physics of these observations, we compare the magnetic inclination angle
in the decayed peripheral and the enhanced inner penumbral regions
before and after the 4B/X3.4 flare of 2006 December 13 by using the
high-resolution vector magnetograms from Hinode. We find that the
mean inclination angle in the decayed penumbra increases after the
flare while that in the enhanced penumbra near flaring neutral line
decreases. The result confirms the previous idea that two components
of a δ sunspot become connected after flares. As a result of new
connection, peripheral penumbral fields change from a more inclined to
a more vertical configuration and transverse fields in umbral core and
inner penumbral regions increase substantially (Liu et al. 2005). The
flare-associated changes of Doppler width as well as other parameters
(the transverse field strength, continuum intensity and filling factor)
are also presented.

---------------------------------------------------------
Title: Coronal Implosion and Particle Acceleration in the Wake of
a Filament Eruption
Authors: Liu, Rui; Wang, Haimin
2009ApJ...703L..23L Altcode: 2009arXiv0908.1137L
We study the evolution of a group of TRACE 195 Å coronal loops
overlying a reverse S-shaped filament on 2001 June 15. These loops were
initially pushed upward with the filament ascending and kinking slowly,
but as soon as the filament rose explosively, they began to contract
at a speed of ~100 km s-1, and sustained for at least 12
minutes, presumably due to the reduced magnetic pressure underneath
with the filament escaping. Despite the contraction following the
expansion, the loops of interest remained largely intact during
the filament eruption, rather than formed via reconnection. These
contracting loops naturally formed a shrinking trap, in which hot
electrons of several keV, in an order of magnitude estimation, can be
accelerated to nonthermal energies. A single hard X-ray (HXR) burst,
with no corresponding rise in GOES soft X-ray (SXR) flux, was recorded
by the Hard X-ray Telescope (HXT) on board Yohkoh, when the contracting
loops expectedly approached the post-flare arcade originating from the
filament eruption. HXT images reveal a coronal source distinctly above
the top of the SXR arcade by ~15”. The injecting electron population
for the coronal source (thin target) is hardening by ~1.5 powers
relative to the footpoint emission (thick target), which is consistent
with electron trapping in the weak diffusion limit. Although we cannot
rule out additional reconnection, observational evidence suggests that
the shrinking coronal trap may play a significant role in the observed
nonthermal HXR emission during the flare decay phase.

---------------------------------------------------------
Title: Successive Solar Flares and Coronal Mass Ejections on 2005
September 13 from NOAA AR 10808
Authors: Liu, Chang; Lee, Jeongwoo; Karlický, Marian; Prasad
Choudhary, Debi; Deng, Na; Wang, Haimin
2009ApJ...703..757L Altcode: 2009arXiv0908.0487L
We present a multiwavelength study of the 2005 September 13 eruption
from NOAA AR 10808 that produced total four flares and two fast
coronal mass ejections (CMEs) within ~1.5 hr. Our primary attention
is paid to the fact that these eruptions occurred in close succession
in time, and that all of them were located along an S-shaped magnetic
polarity inversion line (PIL) of the active region. In our analysis,
(1) the disturbance created by the first flare propagated southward
along the PIL to cause a major filament eruption that led to the
first CME and the associated second flare underneath. (2) The first
CME partially removed the overlying magnetic fields over the northern
δ spot to allow the third flare and the second CME. (3) The ribbon
separation during the fourth flare would indicate reclosing of the
overlying field lines opened by the second CME. It is thus concluded
that these series of flares and CMEs are interrelated to each other via
magnetic reconnections between the expanding magnetic structure and
the nearby magnetic fields. These results complement previous works
made on this event with the suggested causal relationship among the
successive eruptions.

---------------------------------------------------------
Title: A portable solar adaptive optics system
Authors: Ren, Deqing; Penn, Matt; Wang, Haimin; Chapman, Gary;
Plymate, Claude
2009SPIE.7438E..0PR Altcode:
We are developing a portable adaptive optics system for solar
telescopes. The adaptive optics has a small physical size and is
optimized for diffraction-limited imaging in the 1.0 ~ 5.0-μm infrared
wavelength range for 1.5-m class solar telescopes. By replacing a
few optical components, it can be used with a solar telescope of any
aperture size that is currently available. The software is developed by
LabVIEW. LabVIEW's block diagram based programming makes it suitable for
rapid development of a prototype system. The portable adaptive optics
will be used with a 1.5-meter solar telescope for high-resolution
magnetic field investigation in the infrared. We discuss the design
philosophy for such a portable, low-cost, and high-performance
system. Estimated performances are also presented.

---------------------------------------------------------
Title: Reconnection Electric Field and Hardness of X-Ray Emission
of Solar Flares
Authors: Liu, Chang; Wang, Haimin
2009ApJ...696L..27L Altcode: 2009arXiv0903.3968L
Magnetic reconnection is believed to be the prime mechanism that
triggers solar flares and accelerates electrons up to energies of
MeV. In the classical two-dimensional reconnection model, the separation
motion of chromospheric ribbons, manifests the successive reconnection
that takes place higher up in the corona. Meanwhile, downward traveling
energetic electrons bombard the dense chromosphere and create hard
X-ray (HXR) emissions, which provide a valuable diagnostic of electron
acceleration. Analyses of ribbon dynamics and the HXR spectrum have
been carried out separately. In this Letter, we report a study of the
comparison of reconnection electric field measured from ribbon motion
and hardness (spectral index) of X-ray emission derived from X-ray
spectrum. Our survey of the maximum average reconnection electric field
and the minimum overall spectral index for 13 two-ribbon flares shows
that they are strongly anticorrelated. The former is also strongly
correlated with flare magnitude measured using the peak flux of soft
X-ray emissions. These provide strong support for electron acceleration
models based on the electric field generated at reconnecting current
sheet during flares.

---------------------------------------------------------
Title: Temporal Evolution of Free Magnetic Energy Associated with
Four X-Class Flares
Authors: Jing, Ju; Chen, P. F.; Wiegelmann, Thomas; Xu, Yan; Park,
Sung-Hong; Wang, Haimin
2009ApJ...696...84J Altcode:
We study the temporal variation of free magnetic energy
E free around the time of four X-class flares. The
high-cadence photospheric vector magnetograms obtained by the digital
vector magnegograph system at the Big Bear Solar Observatory are
used as the boundary conditions to reconstruct the three-dimensional
nonlinear force-free (NLFF) coronal field. In order to remove the
effect of the net Lorentz force and torque acting in the photosphere,
the vector magnetograms are preprocessed using the method devised by
Wiegelmann et al.. Then a well-tested multigrid-like optimization
code by Wiegelmann is applied to the preprocessed boundary data to
extrapolate the NLFF coronal field with which we are able to estimate
the free energy E free. In all the four events, we find
a significant drop of E free starting ~15 minutes before
the peak time of the associated nonthermal flare emission, although
long-term trend varies from event to event. We discuss the physical
implication of the result, i.e., the magnetic relaxation is already
going on in the corona well before the flare reconnection.

---------------------------------------------------------
Title: Implosion in a Coronal Eruption
Authors: Liu, Rui; Wang, Haimin; Alexander, David
2009ApJ...696..121L Altcode:
We present the observations of the contraction of the
extreme-ultraviolet coronal loops overlying the flaring region during
the preheating as well as the early impulsive phase of a GOES class
C8.9 flare. During the relatively long, 6 minutes, preheating phase,
hard X-ray (HXR) count rates at lower energies (below 25 keV) as well as
soft X-ray fluxes increase gradually and the flare emission is dominated
by a thermal looptop source with the temperature of 20-30 MK. After
the onset of impulsive HXR bursts, the flare spectrum is composed of a
thermal component of 17-20 MK, corresponding to the looptop emission,
and a nonthermal component with the spectral index γ = 3.5-4.5,
corresponding to a pair of conjugate footpoints. The contraction of
the overlying coronal loops is associated with the converging motion
of the conjugate footpoints and the downward motion of the looptop
source. The expansion of the coronal loops following the contraction
is associated with the enhancement in Hα emission in the flaring
region, and the heating of an eruptive filament whose northern end is
located close to the flaring region. The expansion eventually leads to
the eruption of the whole magnetic structure and a fast coronal mass
ejection. It is the first time that such a large scale contraction of
the coronal loops overlying the flaring region has been documented,
which is sustained for about 10 minutes at an average speed of ~5 km
s-1. Assuming that explosive chromospheric evaporation plays
a significant role in compensating for the reduction of the magnetic
pressure in the flaring region, we suggest that a prolonged preheating
phase dominated by coronal thermal emission is a necessary condition
for the observation of coronal implosion. The dense plasma accumulated
in the corona during the preheating phase may effectively suppress
explosive chromospheric evaporation, which explains the continuation
of the observed implosion up to ~7 minutes into the impulsive phase.

---------------------------------------------------------
Title: Study of the X2.6 Flare on 2005 January 15
Authors: Wang, Haimin; Liu, C.; Jing, J.; Li, Y.
2009SPD....40.1915W Altcode:
We analyzed in detail the X2.6 flare occurred on 2005 January 15 in
NOAA AR 10720. There are several interesting properties of the flare
that reveal the connection between two components of magnetic fields:
the core fields near the flaring neutral line and extended fields in the
large scale. The observed manifestations include: (1) magnetic channel
structure developed a few hours before the flare; (2) penumbral decay
at the AR periphery and neutral line field enhancement are found to
be associated with the flare; (3) Halpha brightening propagates to a
remote area outside the AR in the direction of coronal dimming; (4)
global activation of activities occurred in a number of ARs temporally
associated with the event. We further examine the results of non-linear
force free extrapolation, magnetic reconnection rate, and electron
acceleration in order to investigate the initiation, prime energy
release, and propagation of large-scale disturbance of this event.

---------------------------------------------------------
Title: Statistical Assessment of Photospheric Magnetic Features in
Imminent Solar Flare Predictions
Authors: Song, Hui; Tan, Changyi; Jing, Ju; Wang, Haimin; Yurchyshyn,
Vasyl; Abramenko, Valentyna
2009SoPh..254..101S Altcode:
In this study we use the ordinal logistic regression method to establish
a prediction model, which estimates the probability for each solar
active region to produce X-, M-, or C-class flares during the next
1-day time period. The three predictive parameters are (1) the total
unsigned magnetic flux Tflux, which is a measure of an
active region's size, (2) the length of the strong-gradient neutral
line Lgnl, which describes the global nonpotentiality of an
active region, and (3) the total magnetic dissipation Ediss,
which is another proxy of an active region's nonpotentiality. These
parameters are all derived from SOHO MDI magnetograms. The ordinal
response variable is the different level of solar flare magnitude. By
analyzing 174 active regions, Lgnl is proven to be the
most powerful predictor, if only one predictor is chosen. Compared
with the current prediction methods used by the Solar Monitor at the
Solar Data Analysis Center (SDAC) and NOAA's Space Weather Prediction
Center (SWPC), the ordinal logistic model using Lgnl,
Tflux, and Ediss as predictors demonstrated
its automatic functionality, simplicity, and fairly high prediction
accuracy. To our knowledge, this is the first time the ordinal logistic
regression model has been used in solar physics to predict solar flares.

---------------------------------------------------------
Title: Evolution of Optical Penumbral and Shear Flows Associated
with the X3.4 Flare of 2006 December 13
Authors: Tan, Changyi; Chen, P. F.; Abramenko, Valentyna; Wang, Haimin
2009ApJ...690.1820T Altcode:
The rapid and irreversible decay of penumbrae related to X-class flares
has been found in a number of studies. Since the optical penumbral
flows are closely associated with the morphology of sunspot penumbra,
we use state-of-the-art Hinode data to track penumbral flows in flaring
active regions as well as shear flows close to the flaring neutral
line. This paper concentrates on AR 10930 around the time of an X3.4
flare on 2006 December 13. We utilize the seeing-free solar optical
telescope G-band data as a tracer to obtain the horizontal component
of the penumbral and shear flows by local correlation tracking, and
Stokes-V data to register positive and negative magnetic elements along
the magnetic neutral line. We find that: (1) an obvious penumbral decay
appears in this active region intimately associated with the X3.4 flare;
(2) the mean magnitude of the horizontal speeds of the penumbral flows
within the penumbral decay areas temporally and spatially varies from
0.6 to 1.1 km s-1 (3) the penumbral flow decreases before
the flare eruption in two of the four penumbral decay areas; (4) the
mean shear flows along the magnetic neutral line of this δ-sunspot
started to decrease before the flare and continue to decrease for
another hour after the flare. The magnitude of this flow apparently
dropped from 0.6 to 0.3 km s-1. We propose that the decays
of the penumbra and the penumbral flow are related to the magnetic
rearrangement involved in the coronal mass ejection/flare events.

---------------------------------------------------------
Title: Study of Magnetic Channel Structure in Active Region 10930
Authors: Wang, Haimin; Jing, Ju; Tan, Changyi; Wiegelmann, Thomas;
Kubo, Masahito
2008ApJ...687..658W Altcode:
The concept of "magnetic channel" was first introduced by Zirin
&amp; Wang. They were defined as a series of oppositely directed
vertical-field inversions separated by extremely narrow elongated
transverse fields. In this paper, we utilized unprecedented filtergraph
and spectropolarimetry observations from Hinode, and studied the
evolution and physical properties of channel structure of AR 10930
in detail. We found the following: (1) Channels are associated with
new flux emergence in the middle of existing penumbra connecting
the δ sunspot. (2) The width of each channel is in the order of 1”
or less. (3) The line-of-sight magnetic gradient is highest in the
channel, 2.4-4.9 G km-1. (4) The fields are highly sheared
and inclined with a median shear angle around 64° and inclination angle
around 25°. (5) Using nonlinear force-free field (NLFF) extrapolation,
we derive a near surface current system carrying electric current
in the order of 5 × 1011 A. (6) The X3.4 flare on 2006
December 13 occurred during the period that the channels rapidly
formed, but a few hours before the maximum phase of channel structure
development. Based on the observational evidence, we propose that the
channels are formed during the emergence of a sequence of magnetic
bipoles that are squeezed in the compact penumbra of the δ sunspot
and they are highly nonpotential. Formation of channels might be a
precursor of major flares.

---------------------------------------------------------
Title: Automatic Detection of Magnetic Flux Emergings in the Solar
Atmosphere From Full-Disk Magnetogram Sequences
Authors: Fu, Gang; Shih, Frank Y.; Wang, Haimin
2008ITIP...17.2174F Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Variation of Relative Magnetic Helicity around Major Flares
Authors: Park, Sung-Hong; Lee, Jeongwoo; Choe, G. S.; Chae, Jongchul;
Jeong, Hyewon; Yang, Guo; Jing, Ju; Wang, Haimin
2008ApJ...686.1397P Altcode: 2010arXiv1004.2856P
We have investigated the variation of magnetic helicity over a span
of several days around the times of 11 X-class flares which occurred
in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696,
and 10720) using the magnetograms taken by the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory (SOHO). As a
major result we found that each of these major flares was preceded
by a significant helicity accumulation, (1.8-16) × 1042
Mx2 over a long period (0.5 to a few days). Another finding
is that the helicity accumulates at a nearly constant rate, (4.5-48)
× 1040 Mx2 hr-1, and then becomes
nearly constant before the flares. This led us to distinguish the
helicity variation into two phases: a phase of monotonically increasing
helicity and the following phase of relatively constant helicity. As
expected, the amount of helicity accumulated shows a modest correlation
with time-integrated soft X-ray flux during flares. However, the
average helicity change rate in the first phase shows even stronger
correlation with the time-integrated soft X-ray flux. We discuss the
physical implications of this result and the possibility that this
characteristic helicity variation pattern can be used as an early
warning sign for solar eruptions.

---------------------------------------------------------
Title: Early Abnormal Temperature Structure of X-Ray Loop-Top Source
of Solar Flares
Authors: Shen, Jinhua; Zhou, Tuanhui; Ji, Haisheng; Wang, Na; Cao,
Wenda; Wang, Haimin
2008ApJ...686L..37S Altcode: 2008arXiv0808.4047S
This Letter is to investigate the physics of a newly discovered
phenomenon—contracting flare loops in the early phase of solar
flares. In classical flare models, which were constructed based on
the phenomenon of the expansion of flare loops, an energy releasing
site is put above flare loops. These models can predict that there is
a vertical temperature gradient in the top of flare loops due to heat
conduction and cooling effects. Therefore, the centroid of an X-ray
loop-top source at higher energy bands will be higher in altitude,
which we can define as the normal temperature distribution. With
observations made by RHESSI, we analyzed 10 M- or X-class flares (9
limb flares). For all these flares, the movement of loop-top sources
shows an obvious U-shaped trajectory, which we take as the signature of
contraction-to-expansion of flare loops. We find that, for all these
flares, a normal temperature distribution does exist, but only along
the path of expansion. The temperature distribution along the path of
contraction is abnormal, showing no spatial order at all. The result
suggests that magnetic reconnection processes in the contraction and
expansion phases of these solar flares are different.

---------------------------------------------------------
Title: Intermittency in the Photosphere and Corona above an Active
Region
Authors: Abramenko, Valentyna; Yurchyshyn, Vasyl; Wang, Haimin
2008ApJ...681.1669A Altcode: 2009arXiv0903.2882A
Recent studies have demonstrated without doubt that the magnetic field
in the photosphere and corona is an intermittent structure, opening
new views of the underlying physics. In particular, such problems
as the existence in the corona of localized areas with extremely
strong resistivity (required to explain magnetic reconnection at all
scales) and the interchange between small and large scales (required
in the study of photospheric-coronal coupling), to name a few, can be
easily captured by the concept of intermittency. This study focuses
on simultaneous time variations of intermittency properties derived
in the photosphere, chromosphere, and corona. We analyze data for
NOAA Active Region 10930 acquired between 2006 December 8, 12:00 UT,
and December 13, 18:45 UT. Photospheric intermittency is inferred
from Hinode magnetic field measurements, while intermittency in the
transition region and corona is derived from Nobeyama 9 GHz radio
polarization measurements and high-cadence Hinode XRT (thin-Be) data,
as well as GOES 1-8 Å flux. The photospheric dynamics and its possible
relationship with the intermittency variations are also analyzed
by calculating the kinetic vorticity. In this case study, we find
the following chain of events: The intermittency of the photospheric
magnetic field peaked after the specific kinetic vorticity of plasma
flows in the active region reached its maximum (4 hr time delay). In
turn, a gradual increase of coronal intermittency occurred after the
peak of the photospheric intermittency. The time delay between the peak
of photospheric intermittency and the occurrence of the first strong
(X3.4) flare was approximately 1.3 days. Our analysis seems to suggest
that the enhancement of intermittency/complexity first occurs in the
photosphere and is later transported toward the corona.

---------------------------------------------------------
Title: Development of a correlation tracker system for the New
Solar Telescope
Authors: Choi, Seonghwan; Nah, Jakyoung; Moon, Yong-Jae; Wang, Haimin;
Coulter, Roy
2008SPIE.7015E..53C Altcode: 2008SPIE.7015E.138C
In this paper, we report on the development of a correlation tracker
system for the New Solar Telescope (NST). It consists of three
sub-systems: a tip-tilt mirror unit, a camera unit, and a control
unit. Its software has been developed via Microsoft Visual C++,
which enables us to take images from the high-speed CMOS camera in
order to measure the image motions induced by atmospheric turbulence
by using SAD algorithm and 2-D FFT cross-correlation, and to control
the high-dynamics Piezo tip-tilt mirror for tip-tilt correction. We
adopted the SIMD technology and parallel programming technology based
on the Intel Core 2 Quad processor without any additional processing
system (FPGA or DSP) for high-speed performance. As a result, we can
make a tip-tilt correction with about seven hundreds of Hz in a closed
loop mode. The prototype system has been successfully developed in a
laboratory and will be installed on the NST.

---------------------------------------------------------
Title: A Hard X-Ray Sigmoidal Structure during the Initial Phase of
the 2003 October 29 X10 Flare
Authors: Ji, Haisheng; Wang, Haimin; Liu, Chang; Dennis, Brian R.
2008ApJ...680..734J Altcode:
We find a hard X-ray (HXR) sigmoidal (\textsf{S}-shaped) structure
observed by RHESSI between 6 and 150 keV during the initial phase of the
X10 flare of 2003 October 29. Its counterparts are seen with the Solar
X-Ray Imager and TRACE. The flare evolves from a sigmoid to an arcade
phase as observed in TRACE 195 Å images. According to the spatial
structure of HXR emission, the flare process can be divided into two
different phases. During the first phase, HXR emission in different
energy ranges shares a similar sigmoidal evolving structure. The
structure appears to contract initially as shown by the time profile
of the separation between the two footpoints (FPs) at the ends of
the HXR sigmoid. During the second phase, HXR emission in the lower
energy range (lesssim30 keV) evolves into two sources located along the
neutral line. Meanwhile, the FPs in the higher energies (gtrsim30 keV)
move apart as usual. During the whole flaring process, the value of the
flare shear, defined as the angle between the line connecting two FPs
and the line perpendicular to the neutral line, decreases steadily. We
conclude that the flare was triggered by magnetic reconnection near
the center of the sigmoid. The converging and unshearing motion during
the sigmoid period is explained by continuing magnetic reconnection
driven by the erupting sigmoidal flux rope. The reconnection progresses
from highly sheared magnetic field lines to less sheared field lines
surrounding the flux rope.

---------------------------------------------------------
Title: Changes of Magnetic Structure in Three Dimensions Associated
with the X3.4 Flare of 2006 December 13
Authors: Jing, Ju; Wiegelmann, Thomas; Suematsu, Yoshinori; Kubo,
Masahito; Wang, Haimin
2008ApJ...676L..81J Altcode:
Recent observations demonstrated that sunspot structure can change
rapidly and irreversibly after flares. One of the most puzzling results
is the increase in magnetic shear around the flaring magnetic polarity
inversion line after flares. However, all these observations were
made at the photosphere level. In this Letter, we study the altitude
variation of the nonpotentiality of the magnetic fields associated
with the 4B/X3.4 flare of 2006 December 13. The vector magnetograms
with unprecedented quality from Hinode before and after the flare are
used as the boundary conditions to extrapolate the three-dimensional
nonlinear force-free magnetic fields and the potential fields. The
former are computed with the optimization algorithm and the latter with
the Green's function method. At the photosphere boundary, magnetic shear
increases after the flare in a local area close to the flaring magnetic
polarity inversion line. Two measures of the magnetic nonpotentiality,
the weighted mean shear θw and the total magnetic shear
θwB, are calculated in this area at progressively higher
altitude. By comparing their altitude variation profiles before and
after the flare, we find that the nonpotentiality of the local area
increases after the flare below ~8 Mm and decreases from that height
to ~70 Mm. Beyond 70 Mm, the magnetic fields approach potential for
both times.

---------------------------------------------------------
Title: The evolution of vector magnetic field associated with major
flares in NOAA AR10656
Authors: Wang, Shuo; Deng, Yuanyong; Jain, Rajmal; Yurchyshyn, Vasyl;
Wang, Haimin; Liu, Yuanyuan; Yang, Zhiliang
2008JApA...29...57W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spatial Distribution of Magnetic Reconnection in the 2006
December 13 Solar Flare as Observed by Hinode
Authors: Jing, Ju; Chae, Jongchul; Wang, Haimin
2008ApJ...672L..73J Altcode:
A massive two-ribbon flare and its source magnetic field region were
well captured by the Solar Optical Telescope (SOT) on board Hinode
in the Ca II H spectral line and by the Spectro-Polarimeter of SOT,
respectively. Using the high-resolution Hinode data sets, we compare
the spatial distribution of the local magnetic reconnection rate and
the energy release rate along the ribbons with that of G-band kernels
that serve as a proxy for the primary energy release. The G-band kernels
spatially coincide with the maximum of both modeled quantities, which
gives strong support for the reconnection model. We also investigate
the magnitude scaling correlation between the ribbon separation speed
Vr and magnetic field strength Bn at four 2
minute time bins around the maximum phase of the flare. It is found that
Vr is weakly and negatively correlated with Bn. An
empirical relation of Vr propto Bn-0.15
is obtained at the flare peak time with an correlation coefficient
~-0.33. The correlation is weaker at other time bins.

---------------------------------------------------------
Title: The Spatial Distribution of the Hard X-Ray Spectral Index
and the Local Magnetic Reconnection Rate
Authors: Liu, Chang; Lee, Jeongwoo; Jing, Ju; Gary, Dale E.; Wang,
Haimin
2008ApJ...672L..69L Altcode: 2007arXiv0711.1370L
The rare phenomenon of ribbon-like hard X-ray (HXR) sources up to 100
keV found in the 2005 May 13 M8.0 flare observed with the Reuven Ramaty
High Energy Solar Spectroscopic Imager provides detailed information
on the spatial distribution of flare HXR emission. In this Letter, we
further investigate the characteristics of HXR emission in this event
using imaging spectroscopy, from which we obtain spatially resolved HXR
spectral maps during the flare impulsive phase. As a result we found,
along a flare ribbon, an anticorrelation relationship between the local
HXR flux and the local HXR spectral index. We suggest that this can be
regarded as a spatial analog of the well-known temporal soft-hard-soft
spectral evolution pattern of the integrated HXR flux. We also found an
anticorrelation between the HXR spectral index and the local electric
field along the ribbon, which suggests electron acceleration by the
electric field during flares.

---------------------------------------------------------
Title: Successive Flaring during the 2005 September 13 Eruption
Authors: Wang, Haimin; Liu, Chang; Jing, Ju; Yurchyshyn, Vasyl
2007ApJ...671..973W Altcode:
We report a detailed analysis of successive flaring during the
X1.5 event in the NOAA AR 0808 on 2005 September 13. We identify a
filament lying at the southeast boundary of the active region as the
physical linkage between the two flares in close succession. It is
noticeable that the filament erupted ~13 minutes after the initial
flare onset at ~19:22 UT near the central magnetic polarity inversion
line (PIL). During this time period, the filament only showed a slow
rising; meanwhile, a spatially associated large magnetic loop with one
leg connecting to the initial flaring site began to brighten in the
TRACE 195 Å channel. After ~19:35 UT, the filament abruptly erupted
together with the bright TRACE loop. Besides the moving ribbons at
the first flaring site, the filament eruption caused a secondary flare
identified with another set of moving ribbons. This event thus provides
a clear evidence for the successive flaring where the initial flare
destabilizes the nearby flux loop system, leading to the filament
eruption with the second flare. We also identify the initial flare
core by finding rapid, irreversible enhancements of the photospheric
transverse magnetic fields at a section of the PIL.

---------------------------------------------------------
Title: Flow Field Evolution of a Decaying Sunspot
Authors: Deng, Na; Choudhary, Debi Prasad; Tritschler, Alexandra;
Denker, Carsten; Liu, Chang; Wang, Haimin
2007ApJ...671.1013D Altcode: 2007arXiv0709.3340D
We study the evolution of the flows and horizontal proper motions
in and around a decaying follower sunspot based on time sequences
of two-dimensional spectroscopic observations in the visible and
white-light imaging data obtained over 6 days from 2005 June 7 to
12. During this time period the sunspot decayed gradually to a pore. The
spectroscopic observations were obtained with the Fabry-Pérot-based
Visible-Light Imaging Magnetograph (VIM) in conjunction with the
high-order adaptive optics (AO) system operated at the 65 cm vacuum
reflector of the Big Bear Solar Observatory (BBSO). We apply local
correlation tracking (LCT) to the speckle-reconstructed time sequences
of white-light images around 600 nm to infer horizontal proper motions,
while the Doppler shifts of the scanned Fe I line at 630.15 nm are
used to calculate line-of-sight (LOS) velocities with subarcsecond
resolution. We find that the dividing line between radial inward and
outward proper motions in the inner and outer penumbra, respectively,
survives the decay phase. In particular the moat flow is still
detectable after the penumbra disappeared. Based on our observations,
three major processes removed flux from the sunspot: (1) fragmentation
of the umbra, (2) flux cancelation of moving magnetic features (MMFs; of
the same polarity as the sunspot) that encounter the leading opposite
polarity network and plages areas, and (3) flux transport by MMFs
(of the same polarity as the sunspot) to the surrounding network and
plage regions that have the same polarity as the sunspot.

---------------------------------------------------------
Title: The Eruption from a Sigmoidal Solar Active Region on 2005
May 13
Authors: Liu, Chang; Lee, Jeongwoo; Yurchyshyn, Vasyl; Deng, Na; Cho,
Kyung-suk; Karlický, Marian; Wang, Haimin
2007ApJ...669.1372L Altcode: 2007arXiv0707.2240L
This paper presents a multiwavelength study of the M8.0 flare and
its associated fast halo CME that originated from a bipolar NOAA
AR 10759 on 2005 May 13. The source active region has a conspicuous
sigmoid structure at the TRACE 171 Å channel as well as in the SXI
soft X-ray images, and we mainly concern ourselves with the detailed
process of the sigmoid eruption, as evidenced by the multiwavelength
data ranging from Hα, WL, EUV/UV, radio, and hard X-rays (HXRs). The
most important finding is that the flare brightening starts in the
core of the active region earlier than that of the rising motion of
the flux rope. This timing clearly addresses one of the main issues in
the magnetic eruption onset of sigmoid, namely, whether the eruption
is initiated by an internal tether cutting to allow the flux rope
to rise upward, or a flux rope rises due to a loss of equilibrium to
later induce tether cutting below it. Our high time cadence SXI and Hα
data show that the first scenario is relevant to this eruption. As in
other major findings, we have the RHESSI HXR images showing a change
of the HXR source from a confined footpoint structure to an elongated
ribbon-like structure after the flare maximum, which we relate to the
sigmoid-to-arcade evolution. The radio dynamic spectrum shows a type II
precursor that occurred at the time of expansion of the sigmoid and a
drifting pulsating structure in the flare rising phase in HXRs. Finally,
type II and III bursts are seen at the time of maximum HXR emission,
simultaneous with the maximum reconnection rate derived from the flare
ribbon motion in UV. We interpret these various observed properties
with the runaway tether-cutting model proposed by Moore et al. in 2001.

---------------------------------------------------------
Title: Hard X-Ray Intensity Distribution along Hα Ribbons
Authors: Jing, Ju; Lee, Jeongwoo; Liu, Chang; Gary, Dale E.; Wang,
Haimin
2007ApJ...664L.127J Altcode:
Unusual ribbon-like hard X-ray sources were found with the RHESSI
observation of a 2B/M8.0 flare on 2005 May 13. We use this unique
observation to investigate the spatial distribution of the hard
X-ray intensity along the ribbons and compare it with the local
magnetic reconnection rate and energy release rate predicted by
the standard magnetic reconnection model for two ribbon flares. In
the early phase of the flare, the hard X-ray sources appear to be
concentrated in strong field regions within the Hα ribbons, which
is explicable by the model. At and after the flare maximum phase,
the hard X-ray sources become spatially extended to resemble Hα
ribbons in morphology, during which the spatial distribution of hard
X-ray intensity lacks a correlation with that of the local magnetic
reconnection rate and energy release rate predicted by the model. We
argue that the magnetic reconnection during this event may involve
the rearrangement of magnetic field along the magnetic arcade axis
that is inevitably overlooked by the two-dimensional model and suggest
that this type of three-dimensional reconnection will be best seen in
so-called sigmoid-to-arcade transformations.

---------------------------------------------------------
Title: Statistical Correlations between Parameters of Photospheric
Magnetic Fields and Coronal Soft X-Ray Brightness
Authors: Tan, Changyi; Jing, Ju; Abramenko, V. I.; Pevtsov, A. A.;
Song, Hui; Park, Sung-Hong; Wang, Haimin
2007ApJ...665.1460T Altcode:
Using observations of more than 160 active regions, we investigate
the relationship between the coronal X-ray brightness, LB,
and five parameters derived from the photospheric magnetic fields. The
coronal X-ray brightness and the magnetic measures were obtained
from co-aligned SFD composite images from the Yohkoh SXT and
full-disk magnetograms from the SOHO MDI, respectively. The
magnetic parameters are (1) the length of strong-gradient
magnetic neutral lines, LGNL, (2) the magnetic energy
dissipation, ɛ, (3) the unsigned line-of-sight magnetic flux, Φ,
(4) the horizontal velocities, Vh, of random footpoint
motions in the photosphere, and (5) a proxy for the Poynting flux,
E=(1/4π)VhBz2, which characterizes
the energy flux from the photosphere into the corona due to random
footpoint motions. All measures except Vh were analyzed
in both the extensive (total) and intensive (average over an area)
forms. In addition, we used the area-averaged strong gradient
(&gt;50 G) of the magnetic field, ∇Bz, as an intensive
form of LGNL. We found that the Pearson correlation
coefficient between the total X-ray brightness and the total magnetic
measures decreases as 0.97, 0.88, 0.86, and 0.47 for Φ, E, ɛ, and
LGNL, respectively. The correlation coefficient between
the averaged X-ray brightness and the averaged magnetic measures
varied as 0.67, 0.71, 0.57, and 0.49 for &lt;Φ&gt;, , &lt;ɛ&gt;,
and &lt;∇Bz&gt;, respectively. We also found that the
velocities of the footpoint motions have no dependencies with Φ and
LB. We concluded that the observed high correlation between
LB and E is mainly due to the magnetic field. The energy
of the Poynting flux is in the range 106.7-107.6
ergs cm-2 s-1 for the majority of active regions,
which is sufficient to heat the corona due to footpoint random motions
of magnetic flux tubes.

---------------------------------------------------------
Title: Observation of Interactions and Eruptions of Two Filaments
Authors: Su, Jiangtao; Liu, Yu; Kurokawa, Hiroki; Mao, Xinjie; Yang,
Shangbin; Zhang, Hongqi; Wang, Haimin
2007SoPh..242...53S Altcode:
We present new observations of the interactions of two close, but
distinct, Hα filaments and their successive eruptions on 5 November
1998. The magnetic fields of the filaments are both of the sinistral
type. The interactions between the two filaments were initiated mainly
by an active filament of one of them. Before the filament eruptions, two
dark plasma ejections and chromospheric brightenings were observed. They
indicate that possible magnetic reconnection had occurred between the
two filaments. During the first filament eruption, salient dark mass
motions transferring from the left erupting filament into the right
one were observed. The right filament erupted 40 minutes later. This
second filament eruption may have been the result of a loss of stability
owing to the sudden mass injection from the left filament. Based on
the Hα observations, we have created a sketch for understanding the
interactions between two filaments and accompanying activities. The
traditional theory of filament merger requires that the filaments
share the same filament channel and that the reconnection occurs
between the two heads, as simulated by DeVore, Antiochos, and Aulanier
(Astrophys. J.629, 1122, 2005; 646, 1349, 2006). Our interpretation
is that the external bodily magnetic reconnection between flux ropes
of the same chirality is another possible way for two filament bodies
to coalesce.

---------------------------------------------------------
Title: Filament Eruption after the Onset of the X1.5 Flare on 2005
September 13
Authors: Wang, Haimin; Liu, C.; Jing, J.; Yurchyshyn, V.
2007AAS...210.9321W Altcode: 2007BAAS...39..214W
Erupting filaments usually play the role as the initial driver of
flaring process preceding the subsequent flare emissions. In this
Letter, we report a rare case that during the X1.5 flare on 2005
September 13, a filament at the boundary of the NOAA AR 0808 erupted
about 13 minutes after the flare onset at 19:22 UT near the central AR
neutral line. During this time period, the filament only showed a slow
rising; meanwhile, a spatially associated large magnetic loop with
one leg connecting to the initial flaring site began to brighten in
the TRACE 195A channel. After 19:35 UT, the filament abruptly erupted
together with the bright TRACE loop. Besides the moving ribbons at the
first flaring site, the filament eruption caused a secondary flare
identified with another set of moving ribbons. We suggest possible
triggering mechanisms for the initial flare and the eruption of the
peripheral flux loop system, which leads to the sympathetic flaring.

---------------------------------------------------------
Title: The Relaxation of Sheared Magnetic Fields: A Contracting
Process
Authors: Ji, Haisheng; Huang, Guangli; Wang, Haimin
2007ApJ...660..893J Altcode:
In recent years, several authors have reported a contracting motion for
solar flaring loops. That is, during the rising phase of solar flares,
hard X-ray (HXR) loop-top sources or radio/extreme-ultraviolet (EUV)
flaring loops have a descending motion and, at the same time, Hα
ribbons or HXR footpoints (FPs) are converging. The usual expansion
motion of flaring loops occurs only after the contraction. So far,
the contracting motion cannot be fully explained in a two-dimensional
flare model. The recent high-cadence Hα observation of an M-class
flare made by Ji et al. at GanYu Solar Station of Purple Mountain
Observatory may provide a clue leading to a proper understanding. The
observations show that the flare shear decreases steadily during
both the contraction and expansion phases. In this paper, we provide
supporting observational evidence by giving a detailed analysis of the
M6.8-class flare of 2003 June 17. For this flare, the EUV flaring loops,
Hα ribbons, and HXR loop-top source show well-correlated contraction
and subsequent expansion. The flare shear of this event, indicated
by Hα ribbons, HXR FPs, and EUV flaring loops, decreases steadily
throughout. The observations apparently imply that the contracting
motion of flaring loops may be the result of the relaxation of the
sheared magnetic field. In the framework of sheared linear force-free
arcades, we establish a quantitative model to show that the release
of magnetic energy will reduce magnetic shear of the arcades and less
sheared arcades will have smaller height and span.

---------------------------------------------------------
Title: The Ribbon-like Hard X-Ray Emission in a Sigmoidal Solar
Active Region
Authors: Liu, Chang; Lee, Jeongwoo; Gary, Dale E.; Wang, Haimin
2007ApJ...658L.127L Altcode: 2007astro.ph..2326L
Solar flare emissions at Hα and EUV/UV wavelengths often appear in
the form of two ribbons, and this has been regarded as evidence of a
typical configuration of solar magnetic reconnection. However, such
a ribbon structure has rarely been observed in hard X-rays (HXRs),
although it is expected. We report a ribbon-like HXR source observed
with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
at energies as high as 25-100 keV during the 2005 May 13 flare. For
a qualitative understanding of this unusual HXR morphology, we also
note that the source active region appeared in a conspicuous sigmoid
shape before the eruption and changed to an arcade structure afterward
as observed with the Transition Region and Coronal Explorer (TRACE) at
171 Å. We suggest that the ribbon-like HXR structure is associated with
the sigmoid-to-arcade transformation during this type of reconnection.

---------------------------------------------------------
Title: Magnetic Evolution and Temperature Variation in a Coronal Hole
Authors: Zhang, Jun; Zhou, Guiping; Wang, Jingxiu; Wang, Haimin
2007ApJ...655L.113Z Altcode: 2007arXiv0705.0598Z
We have explored the magnetic flux evolution and temperature variation
in a coronal hole region, using Big Bear Solar Observatory deep
magnetograms and SOHO EIT images observed from 2005 October 10 to
14. For comparison, we also investigated a neighboring quiet region
of the Sun. The coronal hole evolved from its mature stage to its
disappearance during the observing period. We have obtained the
following results: (1) When the coronal hole was well developed
on October 10, about 60% of the magnetic flux was positive. The
EUV brightness was 420 counts pixel-1, and the coronal
temperature, estimated from the line ratio of the EIT 195 and 171
Å images, was 1.07 MK. (2) On October 14, when the coronal hole
had almost disappeared, 51% of the magnetic flux was positive, the
EUV radiance was 530 counts pixel-1, and the temperature
was 1.10 MK. (3) In the neighboring quiet region, the fraction
of positive flux varied between 0.49 and 0.47. The EUV brightness
displayed an irregular variation, with a mean value of 870 counts
pixel-1. The temperature was almost constant at 1.11 MK
during the 5 day observation. Our results demonstrate that in a coronal
hole less imbalance of the magnetic flux in opposite polarities leads
to stronger EUV brightness and higher coronal temperatures.

---------------------------------------------------------
Title: Erratum: “The Statistical Relationship between the
Photospheric Magnetic Parameters and the Flare Productivity of
Active Regions” (<A href="/abs/2006ApJ...644.1273J">ApJ, 644, 1273
[2006]</A>)
Authors: Jing, Ju; Song, Hui; Abramenko, Valentyna; Tan, Changyi;
Wang, Haimin
2006ApJ...652.1796J Altcode:
Because of an error in generating math images, variables
for the derivative terms are missing from the published
versions of equations (1) and (2). The correct forms of these equations
are:∇Bz=[((dBz)/dx)2+((dBz)/dy)21/2
(1)ɛ(Bz)=4[((dBz)/dx)2+((dBz)/dy)2+2((dBz)/dx+(dBz)/dy)2.
(2)The Press sincerely regrets this error.

---------------------------------------------------------
Title: The Automatic Predictability of Super Geomagnetic Storms from
halo CMEs associated with Large Solar Flares
Authors: Song, Hui; Yurchyshyn, Vasyl; Yang, Guo; Tan, Changyi; Chen,
Weizhong; Wang, Haimin
2006SoPh..238..141S Altcode: 2006SoPh..tmp...30S
We investigate the relationship between magnetic structures of
coronal mass ejection (CME) source regions and geomagnetic storms, in
particular, the super storms when the Dst index decreases
below −200 nT. By examining all full halo CMEs that erupted
between 1996 and 2004, we selected 73 events associated with M-class
and X-class solar flares, which have a clearly identifiable source
region. By analyzing daily full-disk MDI magnetograms, we found that
the horizontal gradient of the line-of-sight magnetic field is a viable
parameter to identify a flaring magnetic neutral line and thus can be
used to predict the possible source region of CMEs. The accuracy of
this prediction is about 75%, especially for those associated with
X-class flares (up to 89%). The mean orientation of the magnetic
structures of source regions was derived and characterized by the
orientation angle θ, which is defined to be ≤ 90∘
in the case of the southward orientation and ≥ 90∘,
when the magnetic structure is northwardly oriented. The orientation
angle was calculated as the median orientation angle of extrapolated
field lines relative to the flaring neutral line. We report that for
about 92% of super storms (12 out of 13 events) the orientation angle
was found to be southward. In the case of intense and moderate storms
(Dst≥ −200 nT), the relationship is less pronounced
(70%, 21 out of 30 events). Our findings demonstrate that the approach
presented in this paper can be used to perform an automatic prediction
of the occurrence of large X-class flares and super geomagnetic storms.

---------------------------------------------------------
Title: First Light of the Near-Infrared Narrow-Band Tunable
Birefringent Filter at Big Bear Solar Observatory
Authors: Cao, Wenda; Hartkorn, Klaus; Ma, Jun; Xu, Yan; Spirock, Tom;
Wang, Haimin; Goode, Philip R.
2006SoPh..238..207C Altcode: 2006SoPh..tmp...53C
We discuss a near-infrared (NIR) narrow-band tunable birefringent
filter system newly developed by the Big Bear Solar Observatory
(BBSO). This is one of the first narrow-bandpass NIR filter systems
working at 1.56 μm which is used for the observation of the deepest
solar photosphere. Four stages of calcite were used to obtain a bandpass
of 2.5 Å along with a free spectral range (FSR) of 40 Å. Some unique
techniques were implemented in the design, including liquid crystal
variable retarders (LCVRs) to tune the bandpass in a range of ±100 Å,
a wide field configuration to provide up to 2° incident angle, and
oil-free structure to make it more compact and handy. After performing
calibration and characteristic evaluation at the Evans Facility of
the National Solar Observatory at Sacramento Peak (NSO/SP), a series
of high-resolution filtergrams and imaging polarimetry observations
were carried out with the Dunn Solar Telescope of NSO/SP and the
65-cm telescope of BBSO, in conjunction with the high-order adaptive
optics system and the Fabry-Pérot Interferometer (FPI). In this
paper, we describe the optical design and discuss the calibration
method. Preliminary observations show that it is capable of serving as
either a stand-alone narrow-band filter for NIR filtergram observations
or an order-sorting filter of a FPI applied to NIR two-dimensional
imaging spectro-polarimetry.

---------------------------------------------------------
Title: Rapid Changes of Photospheric Magnetic Fields around Flaring
Magnetic Neutral Lines
Authors: Wang, Haimin
2006ApJ...649..490W Altcode:
In this paper we study the short-term evolution of magnetic fields
associated with five flares in δ-sunspots. We concentrate on the
analysis of the magnetic gradient along the flaring neutral lines
(NLs). Obvious changes of the magnetic gradient occurred immediately
and rapidly following the onset of each flare. A rapid gradient
increase was found to be associated with three events, while a
decrease was associated with the other two. The changes were permanent,
and therefore not likely due to the flare emissions. In addition, we
evaluated the mean relative motions between the two magnetic polarities
in these δ-regions, in the directions parallel and perpendicular to
the flaring NLs. We derived the mean positions of the two magnetic
polarities using a center-of-mass (CoM) calculation and found that
(1) converging motions correspond to a gradient increase and diverging
motions, to a decrease; (2) for all the events, there appeared a sudden
release of magnetic shear associated with each flare, signified by
a decrease of CoM separation between 500 and 1200 km in the direction
parallel to the NLs. Combining the findings presented here with those in
previous papers, we propose that these results are evidence of magnetic
reconnection at or close to the photosphere. When an active region is
away from the solar disk center, the reconnected transverse fields
cause an apparent increase of the flux in the polarity toward the
limb and a decrease for the polarity closer to the disk center. This
observational pattern was indeed found for all 10 available events
that have been studied in the literature and in this paper. Finally,
we offer some predictions for future observations when high-quality
vector magnetogram sequences become available.

---------------------------------------------------------
Title: Automatic Detection and Classification of Coronal Mass
Ejections
Authors: Qu, Ming; Shih, Frank Y.; Jing, Ju; Wang, Haimin
2006SoPh..237..419Q Altcode: 2006SoPh..tmp...26Q
We present an automatic algorithm to detect, characterize, and
classify coronal mass ejections (CMEs) in Large Angle Spectrometric
Coronagraph (LASCO) C2 and C3 images. The algorithm includes three
steps: (1) production running difference images of LASCO C2 and C3;
(2) characterization of properties of CMEs such as intensity, height,
angular width of span, and speed, and (3) classification of strong,
median, and weak CMEs on the basis of CME characterization. In this
work, image enhancement, segmentation, and morphological methods are
used to detect and characterize CME regions. In addition, Support Vector
Machine (SVM) classifiers are incorporated with the CME properties
to distinguish strong CMEs from other weak CMEs. The real-time CME
detection and classification results are recorded in a database to
be available to the public. Comparing the two available CME catalogs,
SOHO/LASCO and CACTus CME catalogs, we have achieved accurate and fast
detection of strong CMEs and most of weak CMEs.

---------------------------------------------------------
Title: Comparison of Magnetic Flux Distribution between a Coronal
Hole and a Quiet Region
Authors: Zhang, Jun; Ma, Jun; Wang, Haimin
2006ApJ...649..464Z Altcode: 2007arXiv0705.0590Z
Employing Big Bear Solar Observatory (BBSO) deep magnetograms and Hα
images in a quiet region and a coronal hole, observed on 2004 September
14 and 16, respectively, we have explored the magnetic flux emergence,
disappearance, and distribution in the two regions. We obtain the
following results. (1) The evolution of magnetic flux in the quiet
region is much faster than that in the coronal hole, as the flux
appearing in the form of ephemeral regions in the quiet region is 4.3
times as large as that in the coronal hole, and the flux disappearing
in the form of flux cancellation is 2.9 times as fast as in the coronal
hole. (2) More magnetic elements with opposite polarities in the quiet
region are connected by arch filaments, estimating from magnetograms
and Hα images. (3) We measured the magnetic flux of about 1000 magnetic
elements in each observing region. The flux distribution of network and
intranetwork (IN) elements is similar in both polarities in the quiet
region. For network fields in the coronal hole, there are many more
negative elements than positive elements. However, for the IN fields,
there are many more positive elements than negative elements. (4)
In the coronal hole, the fraction of negative flux obviously changes
with a different threshold flux density; 73% of the magnetic fluxes
with a flux density larger than 2 G have negative polarity, and 95%
of the magnetic fluxes are negative, if we only consider the fields
with a flux density larger than 20 G. Our results demonstrate that in a
coronal hole, stronger fields are occupied by one predominant polarity,
but the majority of weaker fields are occupied by the opposite polarity.

---------------------------------------------------------
Title: On the Temporal and Spatial Properties of Elementary Bursts
Authors: Qiu, Jiong; Wang, Haimin
2006SoPh..236..293Q Altcode:
"Elementary bursts" refer to fine time structures on scales of tens
of milli-second to a few seconds in flare radiations. In this paper,
we investigate temporal and spatial properties of elementary bursts
by exploiting high-cadence Hα (100 ms) and hard X-ray (125 - 500 ms)
observations of an impulsive flare on March 16, 2000. We find that
the time scale of 2 - 3 s is likely an upper limit of the elementary
bursts in this event, at which hard X-ray emissions observed by
different instruments correlate, low energy (≤30 keV) hard X-rays
and Hα flux correlate, and Hα emissions at conjugate flare kernels
correlate. From our methods, and also largely limited by instrument
resolutions, there is a weak indication of existence of sub-second
structures. With the high-resolution Hα data, we also attempt to
explore the spatial structure of "elementary bursts" by determining the
average spatial displacement of Hα peak emission between successive
"elementary bursts" defined from hard X-ray light curves. We find that,
at the time scale of 3 s, the smallest spatial scale, as limited by
the imaging resolution, is about 0.4″. We discuss these results with
respect to mechanisms of fragmented magnetic energy release.

---------------------------------------------------------
Title: The Statistical Relationship between the Photospheric Magnetic
Parameters and the Flare Productivity of Active Regions
Authors: Jing, Ju; Song, Hui; Abramenko, Valentyna; Tan, Changyi;
Wang, Haimin
2006ApJ...644.1273J Altcode:
Using line-of-sight Michelson Doppler Imager (MDI) magnetograms of
89 active regions and Solar Geophysical Data (SGD) flare reports,
we explored, for the first time, the magnitude scaling correlations
between three parameters of magnetic fields and the flare productivity
of solar active regions. These parameters are (1) the mean value of
spatial magnetic gradients at strong-gradient magnetic neutral lines,
(∇Bz)NL (2) the length of strong-gradient
magnetic neutral lines, LGNL; and (3) the total magnetic
energy, ɛ(Bz)dA, dissipated in a layer of 1 m during 1 s
over the active region's area. The MDI magnetograms of active regions
used for our analysis are close to the solar central meridian (within
+/-10°). The flare productivity of active regions was quantified
by the soft X-ray flare index for different time windows from the
time interval of the entire disk passage down to +1 day from the
time of the analyzed magnetogram. Our results explicitly indicate
positive correlations between the parameters and the overall flare
productivity of active regions, and imminent flare production as
well. The correlations confirm the dependence of flare productivity
on the degree of nonpotentiality of active regions.

---------------------------------------------------------
Title: Diffraction-limited Polarimetry from the Infrared Imaging
Magnetograph at Big Bear Solar Observatory
Authors: Cao, Wenda; Jing, Ju; Ma, Jun; Xu, Yan; Wang, Haimin; Goode,
Philip R.
2006PASP..118..838C Altcode:
The Infrared Imaging Magnetograph (IRIM) system developed by Big Bear
Solar Observatory (BBSO) has been put into preliminary operation. It is
one of the first imaging spectropolarimeters working at 1565 nm and is
used for the observations of the Sun at its opacity minimum, exposing
the deepest photospheric layers. The tandem system, which includes a
4.2 nm interference filter, a unique 0.25 nm birefringent Lyot filter,
and a Fabry-Pérot etalon, is capable of providing a bandpass as low as
0.01 nm in a telecentric configuration. A fixed quarter-wave plate and
a nematic liquid crystal variable retarder are employed for analyzing
the circular polarization of the Zeeman components. The longitudinal
magnetic field is measured for the highly Zeeman-sensitive Fe I line
at 1564.85 nm (Landé factor g=3). The polarimetric data were taken
through a field of view of ~145”×145” and
were recorded by a 1024×1024 pixel, 14 bit HgCdTe CMOS focal plane
array camera. Benefiting from the correlation tracking system and a
newly developed adaptive optics system, the first imaging polarimetric
observations at 1565 nm were made at the diffraction limit on 2005 July
1 using BBSO's 65 cm telescope. After comparing the magnetograms from
IRIM with those taken by the Michelson Doppler Imager on board SOHO,
it was found that all the magnetic features matched very well in both
sets of magnetograms. In addition, Stokes V profiles obtained from the
Fabry-Pérot etalon scan data provide access to both the true magnetic
field strength and the filling factor of the small-scale magnetic
flux elements. In this paper, we present the design, fabrication, and
calibration of IRIM, as well as the results of the first scientific
observations.

---------------------------------------------------------
Title: Periodic Motion Along Solar Filaments
Authors: Jing, Ju; Lee, Jeongwoo; Spirock, Thomas J.; Wang, Haimin
2006SoPh..236...97J Altcode:
We present observations of four filaments that exhibit large-amplitude
periodic mass motion. Observations are obtained using the high
resolution (2″) and high cadence (1 min) Hα telescope system at the
Big Bear Solar Observatory (BBSO). The motions found in these events are
along the axis of the filaments, and are associated with the activity
of a nearby flare or filament. The most characteristic properties of
these motions are long period (≥ q80 min), large distance (≥ q 4
× 104 km) of mass transport at much higher velocity (≥ q
30 km s−1) than ever detected from filament motions. The
velocity, period, dimension and damping timescale measured for these
motions are presented, and discussed to identify the most plausible
restoring force and damping mechanism.

---------------------------------------------------------
Title: Multiwavelength Study of Flow Fields in Flaring Super Active
Region NOAA 10486
Authors: Deng, Na; Xu, Yan; Yang, Guo; Cao, Wenda; Liu, Chang; Rimmele,
Thomas R.; Wang, Haimin; Denker, Carsten
2006ApJ...644.1278D Altcode:
We present high-resolution observations of horizontal flow fields
measured by local correlation tracking from intensity images in three
wavelengths, i.e., G band (GB), white light (WL), and near-infrared
(NIR). The observations were obtained on 2003 October 29 within
the flaring super active region NOAA 10486, which was the source of
several X-class flares, including an X10 flare that occurred near
the end of the observing run. The data were obtained at National
Solar Observatory/Sacramento Peak (NSO/SP) using the newly developed
high-order adaptive optics (AO) system. We also use Dopplergrams
and magnetograms from MDI on board SOHO to study the line-of-sight
flow and magnetic field. We observe persistent and long-lived (at
least 5 hr) strong horizontal and vertical shear flows (both in the
order of 1 km s-1) along the magnetic neutral line (NL)
until the X10 flare occurred. From lower photospheric level (NIR),
the direction of the flows does not change up to the upper photosphere
(GB), while the flow speeds in the shear motion regions decrease and,
on the contrary, those in regions without shear motions increase with
increasing altitude. Right after the X10 flare, the magnetic gradient
decreased, while both horizontal and vertical shear flows dramatically
enhanced near the flaring NL. Our results suggest that photospheric
shear flows and local magnetic shear near the NL can increase after
the flare, which may be the result of shear release in the overlying
large-scale magnetic system or the reflection of a twisted or sheared
flux emergence carrying enough energy from the subphotosphere.

---------------------------------------------------------
Title: Evolution of Magnetic Gradients in Flaring Magnetic Neutral
Lines
Authors: Wang, Haimin
2006SPD....37.2002W Altcode: 2006BAAS...38Q.248W
We study the short-term evolution of magnetic fieldsassociated with five
flares in delta sunspots. We concentrate on theanalysis of magnetic
gradient along the flaring neutral lines. Rapidchanges of gradient
obviously occurred immediately following the onset ofeach flare. Rapid
gradient increase was found to be associated with threeevents, while
decrease was found for the other two. The changes werepermanent,
therefor, not likely due to the effect of flare emissions. Inaddition,
we evaluated the overall relative motions between two magneticpolarities
in the these delta regions, in the direction parallel andperpendicular
to the flaring neutral lines, respectively. Using the methodof
Center-of-Mass (CoM) separation calculation, we found that:
(1)converging motion corresponded to gradient increase, diverging
motion, thedecrease (2) for all the events, there appeared a sudden
release ofmagnetic shear associated with each flare, signified by a
decrease of CoMseparation between 500 and 1000 km in the direction
parallel to theneutral line. Combining the findings presented here
with those in previouspapers, such as penumbral decay in the outer
boundaries of delta spots,increase of transverse fields at the flaring
neutral lines and unbalancedfluxes changes associated with flares, we
propose that these are evidencesof quadrupolar magnetic reconnection
at or close to the photosphere. Wefurther propose that this is the
first stage of the two-stage energyrelease for flares and CMEs.

---------------------------------------------------------
Title: Large-Scale Activities Associated with the 2003 October 29
X10 Flare
Authors: Liu, Chang; Lee, Jeongwoo; Deng, Na; Gary, Dale E.; Wang,
Haimin
2006ApJ...642.1205L Altcode:
We present a multiwavelength study focusing on the large-scale
activities associated with the 2003 October 29 X10 flare and a halo
CME. This event was strong enough to clearly show several large-scale
activities, such as remote brightenings, Moreton waves at Hα off-bands,
and type II radio bursts, which offers an excellent opportunity to
clarify the relationship among them. The remote brightenings were found
near two coronal holes more than 2×105 km from the main
flare in eastern and southern directions, respectively. Coronal dimmings
were seen at the locus of the remote brightenings right after the flare
at both EUV and soft X-ray wavelengths. The Moreton waves propagated
both northeastward and southward, toward the aforementioned remote
regions, at speeds of approximately 1100 and 1900 km s-1,
respectively. Our analyses show that the Moreton waves, the type II
radio bursts, and the CME started almost simultaneously, but were not
cotemporal with the remote brightenings. The remote brightenings are
rather consistent with the flare hard X-ray emissions within the active
region, although they have much smaller scales. We therefore conclude
that the two remote brightening regions were magnetically connected to
the flaring active region, and that the remote brightenings, as well
as all other activities, were due to the interaction of an erupting
flux rope at the core of the flare with magnetic field overlying the
region. In this scenario, the large overlying loops should open to
allow the flare activity underneath them, which points to a picture
similar to the magnetic break-out process in such a large scale.

---------------------------------------------------------
Title: High-Resolution Observations of Multiwavelength Emissions
during Two X-Class White-Light Flares
Authors: Xu, Yan; Cao, Wenda; Liu, Chang; Yang, Guo; Jing, Ju; Denker,
Carsten; Emslie, A. Gordon; Wang, Haimin
2006ApJ...641.1210X Altcode:
We observed two X-class white-light flares (WLFs) on 2003 October 29
(~20:40 UT) and November 2 (~17:16 UT) using the Dunn Solar Telescope
(DST) and its High-Order Adaptive Optics (HOAO) system in several
wavelengths. The spatial resolution was close to the diffraction limit
of DST's 76 cm aperture, and the cadence was as high as 2 s. This
is the first time that WLFs have been observed in the near-infrared
(NIR) wavelength region. We present a detailed study in this paper
comparing photospheric continuum observations during the two events with
corresponding line-of-sight magnetograms from the Solar and Heliospheric
Observatory (SOHO) Michelson Doppler Imager (MDI) and hard X-ray (HXR)
data from the Ramaty High-Energy Solar Spectroscopic Imager (RHESSI). We
also discuss several models that provide possible mechanisms to explain
these continuum enhancements, especially in the NIR.

---------------------------------------------------------
Title: Spectral Subtraction: A New Approach to Remove Low- and
High-Order Speckle Noise
Authors: Ren, Deqing; Wang, Haimin
2006ApJ...640..530R Altcode:
We present a novel “spectral subtraction algorithm” (SSA) technique
to remove speckle noise. It consists of a low-order and a high-order
SSA and is based on a three-dimensional image spectroscopy in which
the three-dimensional data cube is available and thus the speckle noise
introduced by the wave-front error can be efficiently subtracted. For
the low-order SSA, speckles up to the second or third order can be
totally subtracted, leaving the residual speckles dominated only by the
third or fourth order, respectively, and imaging contrast is increased
consequently; for the high-order SSA, speckles up to the fourth or fifth
order can be subtracted, leaving the residual speckles dominated only by
the fifth or sixth order, respectively, and the performance is further
improved. This is the first demonstration that such high-order speckles
could be subtracted. Since the SSAs are conducted over a wide spectral
band, a white-light image can be re-assembled from the three-dimensional
data cube. The white-light image would increase the single-to-noise
ratio and reduce the exposure time, which are crucial for the search
of faint companion objects. Combined with a coronagraph, the SSA can
provide an extra contrast gain for the coronagraph imaging, relax the
requirement for the wave-front quality (no adaptive optics correction is
required for a space-borne imaging system), and significantly increase
the performance of exoplanet imaging and biomarker spectroscopy.

---------------------------------------------------------
Title: Converging Motion of Hα Conjugate Kernels: The Signature of
Fast Relaxation of a Sheared Magnetic Field
Authors: Ji, Haisheng; Huang, Guangli; Wang, Haimin; Zhou, Tuanhui;
Li, Youping; Zhang, Yanan; Song, Mutao
2006ApJ...636L.173J Altcode:
In this Letter, we present the results from a high-cadence (~40 ms)
Hα blue-wing observation of an M1.1-class solar flare, which occurred
in NOAA AR 10687 on 2004 November 1. In collaboration with RHESSI,
the observation was made with the Hα Fine Structure Telescope at the
GanYu Solar Station of the Purple Mountain Observatory. For this flare,
a pair of conjugate Hα kernels shows a kind of converging motion during
the impulsive phase. After the impulsive phase, there appears a normal
separation motion. The motion of one Hα kernel is perpendicular to
the magnetic neutral line, while another kernel's converging shows
both perpendicular and parallel components. Nevertheless, the shear
angle decreases during the converging motion, clearly showing the
relaxation of a sheared magnetic field. All of the above features are
confirmed with hard X-ray (HXR) footpoints observed by RHESSI. We also
obtained the time profiles of the rate of change of the shear angle and
the relative velocity of the two kernels with Hα observations. Both
of these time profiles show a good correlation with RHESSI HXR light
curves in the higher energy range (&gt;~50 keV). This indicates that,
during the peak times of the flare, the relaxation process may have
occurred rapidly. This event was also observed by the Nobeyama Radio
Heliograph (NoRH), showing a single microwave source. Using NoRH
maps at 17 GHz with 1 s cadence, we obtained the time profile of the
radio source's velocity using the same method that we used with Hα
images. The velocity-time curve of the microwave source shows a good
correlation with that obtained from the two Hα kernels.

---------------------------------------------------------
Title: Properties of Small Dark Features Observed in the Pure
Near-Infrared and Visible Continua
Authors: Xu, Yan; Cao, Wenda; Ma, Jun; Hartkorn, Klaus; Jing, Ju;
Denker, Carsten; Wang, Haimin
2005ApJ...628L.167X Altcode:
High-resolution images in the visible and near-infrared (NIR) continua
at around 1560 nm were obtained of solar active regions NOAA AR 10707
and AR 10486 with the Dunn Solar Telescope (DST) at the National Solar
Observatory/Sacramento Peak (NSO/SP) on 2004 December 1 and 2 and
2003 October 29. The images were taken with the high-order adaptive
optics (HOAO) system, and the spatial resolution was close to the
diffraction limit of the 76 cm aperture DST in both wavelengths. For
the 2004 December run, the NIR observations were made with a newly
developed Lyot filter system, which was designed at the Center for
Solar-Terrestrial Research (CSTR)/New Jersey Institute of Technology
(NJIT). The filter has a bandpass of 2.5 Å that allows us to observe
the pure NIR continuum at the opacity minimum. Our data show that all
dark features in the NIR are also dark in the visible light. There
is no evidence showing the existence of so-called dark faculae, i.e.,
faculae that have negative contrasts in the NIR but positive contrasts
in the visible. The negative peak contrasts of these small pores are
about 50% in the visible and 25% in the NIR, and their dimensions are
in the range of 1"-4".

---------------------------------------------------------
Title: Reevaluation of the Magnetic Structure and Evolution Associated
with the Bastille Day Flare on 2000 July 14
Authors: Wang, Haimin; Liu, Chang; Deng, Yuanyong; Zhang, Hongqi
2005ApJ...627.1031W Altcode:
The Bastille Day flare on 2000 July 14 was well observed by several
space- and ground-based observatories and studied extensively by many
researchers. Recently, we discovered that a large fraction of X-class
flares are associated with a very interesting evolutionary pattern
in δ sunspots: part of the outer δ spot structure decays rapidly
after major flares; in the meantime, central umbral and/or penumbral
structure becomes darker. These changes take place in about 1 hour and
are permanent. We find that the active region NOAA AR 9077 has sunspot
structure change similar to that associated with the 2000 July 14 X5.7
flare. We provide additional evidence supporting that we detected the
real change in the sunspot structure after the flare. The new evidence
presented in this paper include the following: (1) the Evershed velocity
of decayed penumbral segments was weakened significantly following
the flare, indicating actual weakening of penumbral structure; (2)
based on available vector magnetograms before and after the flare,
the transverse field strength decreased at the areas of penumbral
decay and increased significantly near the flaring neutral line;
(3) a new electric current system is found near the flare neutral
line after the flare; and (4) the center-of-mass positions of opposite
magnetic polarities converged toward magnetic neutral line immediately
following the onset of the flare, and magnetic flux of the active region
decreased steadily following the flare. There is no flare model capable
of interpreting all the aspects of observations. A simple quadrupolar
magnetic reconnection model may explain most of our observations: two
magnetic dipoles join at the δ configuration before the flare; magnetic
reconnection creates two new sets of loops: a compact flare loop and
a large-scale expanding loop that might be the source of the CME. The
outer penumbral fields become more vertical due to this reconnection,
corresponding to the penumbral decay. Following initiation of magnetic
reconnection associated with the flare, reconnected fields near the
magnetic neutral line are first enhanced, then gradually weakened as
it submerges. However, this model is questionable from one aspect of
the observations: we failed to identify two far-end footpoints of this
quadrupolar magnetic reconnection. We discuss other existing flare
models in the context of our observations as well.

---------------------------------------------------------
Title: Automatic Solar Filament Detection Using Image Processing
Techniques
Authors: Qu, Ming; Shih, Frank Y.; Jing, Ju; Wang, Haimin
2005SoPh..228..119Q Altcode:
We present an automatic solar filament detection algorithm based on
image enhancement, segmentation, pattern recognition, and mathematical
morphology methods. This algorithm cannot only detect filaments, but
can also identify spines, footpoints, and filament disappearances. It
consists of five steps: (1) The stabilized inverse diffusion equation
(SIDE) is used to enhance and sharpen filament contours. (2) A
new method for automatic threshold selection is proposed to extract
filaments from local background. (3) The support vector machine (SVM)
is used to differentiate between sunspots and filaments. (4) Once a
filament is identified, morphological thinning, pruning, and adaptive
edge linking methods are used to determine the filament properties. (5)
Finally, we propose a filament matching method to detect filament
disappearances. We have successfully applied the algorithm to Hα
full-disk images obtained at Big Bear Solar Observatory (BBSO). It has
the potential to become the foundation of an automatic solar filament
detection system, which will enhance our capabilities of forecasting
and predicting geo-effective events and space weather.

---------------------------------------------------------
Title: Rapid Penumbral Decay Associated with an X2.3 Flare in NOAA
Active Region 9026
Authors: Deng, Na; Liu, Chang; Yang, Guo; Wang, Haimin; Denker, Carsten
2005ApJ...623.1195D Altcode:
We present observations of rapid penumbral decay associated with a
major flare in solar NOAA Active Region 9026 on 2000 June 6. Within 1.5
hr, an X2.3 flare accompanied by an 11° long filament eruption and a
full-halo coronal mass ejection (CME) originated near the neutral line
of a large δ-spot region, which was associated with significant changes
in white-light structure and magnetic field topology: an increase
of moving magnetic features (MMFs), flux emergence and cancellation,
and, in particular, the rapid disappearance of two penumbral segments
located in opposite-polarity regions on the north and south sides of
the δ-spot. The rapid penumbral decay is believed to be the result
of magnetic field topology change that was caused by rapid magnetic
reconnection during the flare, rather than part of overall long-term
evolution. We present a possible explanation of this event, using a
“magnetic breakout” model for solar flares, considering its complex
multipolar δ-configuration and associated filament eruption and CME,
i.e., previously closed magnetic field lines opened up and reconnected
at a null point above the neutral line of this δ-spot. The magnetic
breakout caused an energy release from a highly sheared magnetic field
in the umbrae and a transition of the magnetic arcades from low lying
to high lying, which led to an increase of the inclination angle of the
magnetic field lines in the peripheral penumbrae; i.e., the magnetic
field turned from more inclined to more vertical and toward the inner
umbrae. Once the magnetic field in the penumbrae was vertical enough,
the Evershed flow ceased, the manifestation of which in white-light
structure is the disappearance of peripheral penumbrae. We also discuss
other possible flare models for this event and compare them in several
observational features. The present observations provide further
evidence that highly energetic events have a distinct associated
photospheric magnetic field signature and support the findings of
recent analyses of photospheric line-of-sight magnetograms from the
Big Bear Solar Observatory (BBSO) and the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory (SOHO) that show
rapid and permanent changes of photospheric magnetic fields associated
with flares.

---------------------------------------------------------
Title: High-Spatial-Resolution Imaging Combining High-Order Adaptive
Optics, Frame Selection, and Speckle Masking Reconstruction
Authors: Denker, Carsten; Mascarinas, Dulce; Xu, Yan; Cao, Wenda;
Yang, Guo; Wang, Haimin; Goode, Philip R.; Rimmele, Thomas
2005SoPh..227..217D Altcode:
We present, for the first time, high-spatial-resolution observations
combining high-order adaptive optics (AO), frame selection, and
post-facto image correction via speckle masking. The data analysis is
based on observations of solar active region NOAA 10486 taken with the
Dunn Solar Telescope (DST) at the Sacramento Peak Observatory (SPO) of
the National Solar Observatory (NSO) on 29 October 2003. The high Strehl
ratio encountered in AO corrected short-exposure images provides highly
improved signal-to-noise ratios leading to a superior recovery of the
object's Fourier phases. This allows reliable detection of small-scale
solar features near the diffraction limit of the telescope. Speckle
masking imaging provides access to high-order wavefront aberrations,
which predominantly originate at high atmospheric layers and are only
partially corrected by the AO system. In addition, the observations
provided qualitative measures of the image correction away from the
lock point of the AO system. We further present a brief inspection
of the underlying imaging theory discussing the limitations and
prospects of this multi-faceted image reconstruction approach in
terms of the recovery of spatial information, photometric accuracy,
and spectroscopic applications.

---------------------------------------------------------
Title: Rapid Change of δ Spot Structure Associated with Seven
Major Flares
Authors: Liu, Chang; Deng, Na; Liu, Yu; Falconer, David; Goode,
Philip R.; Denker, Carsten; Wang, Haimin
2005ApJ...622..722L Altcode:
A large fraction of major flares occur in active regions that exhibit a
δ configuration. The formation and disintegration of δ configurations
is very important in understanding the evolution of photospheric
magnetic fields. In this paper we study the relationship between the
change in δ spot structures and associated major flares. We present
a new observational result that part of penumbral segments in the
outer δ spot structure decay rapidly after major flares; meanwhile,
the neighboring umbral cores and/or inner penumbral regions become
darker. Using white-light (WL) observations from the Transition Region
and Coronal Explorer (TRACE), we study the short-term evolution of δ
spots associated with seven major flares, including six X-class flares
and one M-class flare. The rapid changes, which can be identified in
the time profiles of WL mean intensity are permanent, not transient,
and thus are not due to flare emission. The co-aligned magnetic field
observations obtained with the Michelson Doppler Imager (MDI) show
substantial changes in the longitudinal magnetic field associated with
the decaying penumbrae and darkened central areas. For two events for
which vector magnetograms were available, we find that the transverse
field associated with the penumbral decay areas decreased while it
increased in the central darkened regions. Both events also show an
increase in the magnetic shear after the flares. For all the events,
we find that the locations of penumbral decay are related to flare
emission and are connected by prominent TRACE postflare loops. To
explain these observations, we propose a reconnection picture in which
the two components of a δ spot become strongly connected after the
flare. The penumbral fields change from a highly inclined to a more
vertical configuration, which leads to penumbral decay. The umbral
core and inner penumbral region become darker as a result of increasing
longitudinal and transverse magnetic field components.

---------------------------------------------------------
Title: Magnetic Reconnection Rate and Flux-Rope Acceleration of
Two-Ribbon Flares
Authors: Jing, Ju; Qiu, Jiong; Lin, Jun; Qu, Ming; Xu, Yan; Wang,
Haimin
2005ApJ...620.1085J Altcode:
Forbes &amp; Lin derived simple equations to link the properties of
magnetic reconnection in the corona to observed signatures of solar
flares. We measured the photospheric magnetic fields and the flare
ribbon separation speeds then applied these equations to derive
two physical terms for the magnetic reconnection rates: the rate of
magnetic flux change ϕrec involved in magnetic reconnection
in the low corona and the electric field Erec inside the
reconnecting current sheet (RCS) that is generated during magnetic
reconnection. The central interest in this work is to investigate and
quantify the statistical correlation between the magnetic reconnection
rate and the corresponding flux-rope acceleration. From a sample of
13 well-observed two-ribbon flares, which are associated with filament
eruptions or coronal mass ejections (CMEs), the acceleration of erupting
filaments is found mainly in the range of 0.05-0.4 km s-2, up
to 3 km s-2. Correspondingly, the maximum Erec and
ϕrec mostly occur in the range of 0.2-5 V cm-1
and 0.5-6×1018 Mx s-1, respectively. A positive
and strong correlation is found with a cross-correlation coefficient of
0.94-0.97 between the magnetic reconnection rate and the acceleration
of erupting filaments that represents the early stages of flux-rope
eruptions in the low corona. However, the inferred reconnection rate is
not correlated to the acceleration of CME fronts measured by the Large
Angle and Spectrometric Coronagraph (LASCO) observations in the range
of 2-30 solar radii (the correlation coefficient is less than 0.2). A
reasonable correlation is found between the reconnection rate and
the velocity of CMEs, which indicates the cumulative acceleration of
CMEs from the low corona to the LASCO C2 field of view. The temporal
correlation between the magnetic reconnection rate and the flare
nonthermal emissions has also been verified in this paper.

---------------------------------------------------------
Title: Properties of Remote Flare Ribbons Associated with Coronal
Mass Ejections
Authors: Wang, Haimin
2005ApJ...618.1012W Altcode:
To understand the large-scale structure of flares and coronal mass
ejections, we studied the motion and properties of remote flare
ribbons of three major events: 2003 May 27 X1.3, 2000 November 24
X1.8, and 2000 November 25 X1.9 flares. The remote ribbons are in
addition to the typical two-ribbon pairs in the flare cores. We found
two common properties of these three events. First, the flare core
occurs at the edge of a major sunspot, associated with flux emergence
parallel to the edge. Such a flux emergence formed complicated magnetic
channel structure. Second, three flare ribbons are visible for each
event. The first two make up a typical two-ribbon structure near the
flare core. The third ribbon (remote ribbon) moves away from the flare
site at a speed between 30 and 100 km s-1, several times
larger than the nominal two-ribbon separation speed of flares. It is
interesting to note that for the 2000 November 25 event, the remote
ribbon first moved away in the same manner as the other two events;
it then retracted after the flare emission peaked. We compared such
an expansion/retraction motion with a limb event observed on 2001
April 15. We interpret the observed motion of remote ribbons by the
interaction between the erupting flux rope in the flare core region
and the overlying large-scale magnetic fields. For the 2000 November
25 event, the overlying field and flare core fields interacted twice,
during the outward and inward motions.

---------------------------------------------------------
Title: Magnetic Helicity Change Rate Associated with X-Class and
M-Class Flares
Authors: Hartkorn, Klaus; Wang, Haimin
2004SoPh..225..311H Altcode:
We investigate the total helicity change rate of active regions during
the time period of three X-class and five M-class flares using MDI
full-disk magnetograms which are sufficient to calculate the advection
and the shuffling terms. Two out of three regions with X-class flares
show a significant change in the helicity change rate, while none of
the five active regions with an associated M-class flare shows this
behavior. A closer investigation of the active regions associated
with a helicity change reveals certain peculiarities that point
to an artificial signal due to the magnetic reversal effect. This
is supported by the fact that a simulation of the reversal effect
reproduces the same shape of the helicity signal, although with an
amplitude one magnitude lower. We investigate active regions with
no flaring activity and determine the fluctuations of the helicity
change rate due to instrumental effects to be 12 × 1040
Mx2 h-1.

---------------------------------------------------------
Title: Photospheric Shear Flows along the Magnetic Neutral Line of
Active Region 10486 prior to an X10 Flare
Authors: Yang, Guo; Xu, Yan; Cao, Wenda; Wang, Haimin; Denker, Carsten;
Rimmele, Thomas R.
2004ApJ...617L.151Y Altcode:
We present high spatial resolution observations of proper motions in
the solar NOAA Active Region 10486 using a high-order adaptive optics
system, frame selection, and speckle-masking image reconstruction. The
data were obtained with the Dunn Solar Telescope of the National Solar
Observatory/Sacramento Peak on 2003 October 29. The resolution of the
images approaches the diffraction-limited resolution of the Dunn Solar
Telescope of about 0.14" at 527 nm. We analyzed a 2 hr time series with
a 1 minute cadence prior to an X10 white-light flare. Local correlation
tracking was used to measure the photospheric proper motions. We find
specific evidence of strong shear flows along the magnetic neutral line;
these shear flows are well defined and correlated with white-light
flare kernels in the visible and infrared. The speed along the flow
channels can reach up to 1.6 km s-1, and the separation of
channels with head-on flows can be less than 1". Counterstreaming and
complex flow patterns have been distinguishing characteristics of this
extraordinarily flare-productive active region.

---------------------------------------------------------
Title: On the Relation between Filament Eruptions, Flares, and
Coronal Mass Ejections
Authors: Jing, Ju; Yurchyshyn, Vasyl B.; Yang, Guo; Xu, Yan; Wang,
Haimin
2004ApJ...614.1054J Altcode:
We present a statistical study of 106 filament eruptions, which were
automatically detected by a pattern recognition program implemented
at Big Bear Solar Observatory using Hα full-disk data from 1999
to 2003. We compare these events with Geostationary Operational
Environmental Satellite soft X-ray time profiles, solar-geophysical
data (SGD) solar event reports, Michelson Doppler Imager magnetograms,
and Large Angle and Spectrometric Coronagraph (LASCO) data to determine
the relationship between filament eruptions and other phenomena of solar
activity. (1) Excluding eight events with no corresponding LASCO data,
55% or 56% of 98 events were associated with coronal mass ejections
(CMEs). (2) Active region filament eruptions have a considerably
higher flare association rate of 95% compared to quiescent filament
eruptions with 27%, but a comparable CME association rate, namely, 43%
for active region filament eruptions and 54% for quiescent filament
eruptions. (3) 54% or 68% of 80 disk events were associated with new
flux emergence. In addition, we derived the sign of magnetic helicity
and the orientation of the magnetic field associated with seven halo
CMEs and demonstrated that the geoeffectiveness of a halo CME can be
predicted by these two parameters.

---------------------------------------------------------
Title: Near infrared (NIR) achromatic phase retarder
Authors: Ma, Jun; Wang, Jingshan; Cao, W.; Denker, Carsten; Wang,
Haimin
2004SPIE.5523..139M Altcode:
The tunable near InfraRed Lyot filter (TNIRLF) is one of the focal plane
instruments for Advanced Technology Solar Telescope (ATST) project of
the National Solar Observatory (NSO). Achromatic half waveplate and
quarter waveplates working from 1000 nm to 1700 nm will be used in
this filter. In this paper, we give a description of the design and
development for the synthesized achromatic waveplates using quartz
plates. The retardance variation is within 1% over the full spectral
range and we discuss the variance of optical axis.

---------------------------------------------------------
Title: Instrumentation for the Advanced Technology Solar Telescope
Authors: Rimmele, Thomas R.; Hubbard, Robert P.; Balasubramaniam,
K. S.; Berger, Tom; Elmore, David; Gary, G. Allen; Jennings, Don;
Keller, Christoph; Kuhn, Jeff; Lin, Haosheng; Mickey, Don; Moretto,
Gilberto; Socas-Navarro, Hector; Stenflo, Jan O.; Wang, Haimin
2004SPIE.5492..944R Altcode:
The 4-m aperture Advanced Technology Solar Telescope (ATST) is the
next generation ground based solar telescope. In this paper we provide
an overview of the ATST post-focus instrumentation. The majority of
ATST instrumentation is located in an instrument Coude lab facility,
where a rotating platform provides image de-rotation. A high order
adaptive optics system delivers a corrected beam to the Coude lab
facility. Alternatively, instruments can be mounted at Nasmyth or
a small Gregorian area. For example, instruments for observing the
faint corona preferably will be mounted at Nasmyth focus where maximum
throughput is achieved. In addition, the Nasmyth focus has minimum
telescope polarization and minimum stray light. We describe the set of
first generation instruments, which include a Visible-Light Broadband
Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters,
Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter &amp;
Spectrometer and a UV-Polarimeter. We also discuss unique and efficient
approaches to the ATST instrumentation, which builds on the use of
common components such as detector systems, polarimetry packages and
various opto-mechanical components.

---------------------------------------------------------
Title: Hard X-Ray and Microwave Observations of Microflares
Authors: Qiu, Jiong; Liu, Chang; Gary, Dale E.; Nita, Gelu M.;
Wang, Haimin
2004ApJ...612..530Q Altcode:
In this paper, we study solar microflares using the coordinated hard
X-ray and microwave observations obtained by the Reuven Ramaty High
Energy Solar Spectroscopic Imager (RHESSI) during its open-shutter
operation mode and the Owens Valley Solar Array (OVSA). The events
in our study are selected in the energy range of 12-25 keV and are
relatively large microflares with an average GOES soft X-ray incremental
flux at the B2.0 level. A total of 760 microflares are identified from
the RHESSI burst catalog. Of the 200 microflares that fall into the
OVSA observing window, about 40% are detected in microwaves. Using
these hundreds of events as samples, we study the event distribution
with respect to the flux, the solar activity, and active regions,
in comparison with flares of larger scales. Nonthermal properties
of microflares are investigated through spectral analysis of X-rays
and microwaves. (1) We find that the event frequency distribution
with respect to the RHESSI peak count rates at 12-25 keV can be
accurately described with a power-law function down to 8 counts
s-1, the power-law index being 1.75+/-0.03, consistent with
previous studies. (2) Similar to large flares, the occurrence rate of
microflares is correlated with solar activity. The studied samples
of microflares are mostly produced by active regions, as suggested
by the large percentage of events detected by OVSA, which observes
target active regions. However, all active regions do not have equal
productivity, and certain active regions are a lot more productive
than other regions. (3) While some large and complex active regions are
predominantly productive in both very weak and strong events, we also
find an active region that produces many microflares and C-class events
but does not produce powerful events. (4) Analysis of energy-dependent
time profiles suggests that there is a pronounced temporal correlation
between the time derivative of soft X-rays and 14-20 keV hard X-rays,
i.e., the Neupert effect, in about one-half the studied events. (5)
Albeit small, many microflares exhibit hard X-ray emission at over
10 keV and microwave emission at around 10 GHz. Spectral analysis in
these two wavelengths corroborates the nonthermal nature of these
emissions. (6) In a limited number of samples, the RHESSI spectral
fitting yields a photon spectral index of 4.5-7, and microwave
spectral analysis on the same events shows that the power-law index
of the electron spectrum is in the range of 2-5. The discrepancy in
the electron spectrum index derived from hard X-rays and microwaves is
substantially greater than previously reported in big flares, hinting
at the existence of high-energy, microwave-emitting electrons that have
a much hardened spectrum compared with electrons emitting hard X-rays.

---------------------------------------------------------
Title: Automatic Solar Flare Tracking Using Image-Processing
Techniques
Authors: Qu, Ming; Shih, Frank; Jing, Ju; Wang, Haimin
2004SoPh..222..137Q Altcode:
Measurement of the evolution properties of solar flares through
their complete cyclic development is crucial in the studies of Solar
Physics. From the analysis of solar Hα images, we used Support Vector
Machines (SVMs) to automatically detect flares and applied image
segmentation techniques to compute their properties. We also present
a solution for automatically tracking the apparent separation motion
of two-ribbon flares and measuring their moving direction and speed in
the magnetic fields. From these measurements, with certain assumptions,
we inferred the reconnection of the electric field as a measure of the
rate of the magnetic reconnection in the corona. The automatic procedure
is a valuable tool for real-time monitoring of flare evolution.

---------------------------------------------------------
Title: Near-Infrared Observations at 1.56 Microns of the 2003 October
29 X10 White-Light Flare
Authors: Xu, Yan; Cao, Wenda; Liu, Chang; Yang, Guo; Qiu, Jiong;
Jing, Ju; Denker, Carsten; Wang, Haimin
2004ApJ...607L.131X Altcode:
We present high-resolution observations of an X10 white-light flare in
solar NOAA Active Region 10486 obtained with the Dunn Solar Telescope
(DST) at the National Solar Observatory/Sacramento Peak on 2003
October 29. Our investigation focuses on flare dynamics observed
in the near-infrared (NIR) continuum at 1.56 μm. This is the first
report of a white-light flare observed at the opacity minimum. The
spatial resolution was close to the diffraction limit of the 76 cm
aperture DST. The data benefited from a newly developed high-order
adaptive optics system and a state-of-the-art NIR complex metal oxide
semiconductor focal plane array. In addition, we compared hard X-ray
(HXR) data of RHESSI and magnetograms of the Michelson Doppler Imager on
board SOHO with the NIR continuum images. Although it is still possible
that some high-energy electrons penetrate deep to this layer, a more
likely explanation of the observed emission is back-warming. During
the impulsive phase of the flare, two major flare ribbons moved apart,
which were both temporally and spatially correlated with RHESSI HXR
ribbons. The maximum intensity enhancement of the two flare ribbons is
18% and 25%, respectively, compared to the quiet-Sun NIR continuum. The
separation speed of the ribbons is about 38 km s-1 in regions
with weak magnetic fields and decreases to about 19 km s-1,
where stronger magnetic fields are encountered. The derived electric
field in reconnection current sheet Ec is of the order of
45 V cm-1 during the flare maximum.

---------------------------------------------------------
Title: Traces of the Dynamic Current Sheet during a Solar Flare
Authors: Ji, Haisheng; Wang, Haimin; Goode, Philip R.; Jiang, Yunchun;
Yurchyshyn, V.
2004ApJ...607L..55J Altcode:
High-cadence and high-resolution time sequences of far Hα
off-band images provide a unique tool to study the evolution of
the fine structure of flare kernels. The fine structure contains
important information on flare topology and the triggering
mechanism. In this Letter, we concentrate on the rapid changes
of the relative positions of two conjugate flare footpoints. In
order to carry out this study with the highest physical precision,
we use rc=ΣrjIj/ΣIj
(Ij is the Hα brightness at rj) to compute the
centroid of an Hα bright kernel region caused by solar flares. Using
this, we probe the fine temporal structures connected to the distance
between the centroids of two conjugate kernels of an M2.3 flare. The
flare, which occurred on 2002 September 9 in NOAA Active Region 0105,
was observed at Big Bear Solar Observatory at the far off-band center
wavelength of H α-1.3 Å, with a cadence of ~40 ms. The flare was also
observed by RHESSI. The time profile of the separation distance shows
an excellent anticorrelation to that of the hard X-ray (HXR) emissions
in 25-50 keV, which exhibit a number of separate spikes (the linear
Pearson correlation coefficient is found to be ~-0.83). The separation
between the two centroids decreases at the rising periods of four HXR
spikes, then it increases after the peak time of the flare to show
the expected separation motion. The most obvious decreasing, which
occurred during the first HXR peak, was confirmed by corresponding
images. This implies that during the impulsive phases, the energy
transported from the corona is deposited increasingly inwardly between
the two kernels. This new and perhaps surprising tendency for the
energy deposition can be explained as being caused by current sheet
pinch motions, which, at the same time, enhance the magnetic energy
reconnection rate to produce the observed HXR spikes.

---------------------------------------------------------
Title: Observations of Nonthermal and Thermal Hard X-Ray Spikes in
an M-Class Flare
Authors: Ji, Haisheng; Wang, Haimin; Schmahl, Edward J.; Qiu, Jiong;
Zhang, Yanan
2004ApJ...605..938J Altcode:
We report a study of an M2.3 flare, which occurred on 2002 September 9
in NOAA Active Region 0105. The observation was made at Big Bear Solar
Observatory (BBSO) at the wavelength of Hα-1.3 Å, with a cadence of
40 ms. The flare was also observed by the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI). Three kernels appear in Hα-1.3 Å
images, two of which are conjugate kernels. The light curves of the two
conjugate kernels show a correlation with that of hard X-rays (HXRs) in
the 25-50 keV band; however, the third one does not. We use HXR images
and spectra and the time delay between the Hα blue-wing emissions
and HXRs to distinguish between the thermal and nonthermal character
of spikes in the light curve. Among the four spikes we studied, the
second spike is thermal, while the other three are nonthermal. What
characterizes the thermal spike is the long time delay (~10 s), the
lack of HXR “footpoint” emission, and the absence of a power-law
component in HXR imaging spectra. In addition, the spectrum of this
spike is only well fitted by a double-temperature model with a hot
(~25.5 MK) component and a superhot (~47.1 MK) component with different
emission measures. The nonthermal spikes are characterized by short time
delays (&lt;~4 s, the RHESSI time resolution used in this paper), HXR
footpoint emission, and power-law spectra. The relative Hα blue-wing
intensities during the periods of the three nonthermal spikes are
evaluated in terms of beam parameters: power-law index and X-ray flux at
10 keV. Comparison with the observations shows that the first spike of
the early impulsive phase did not produce the predicted Hα intensity.

---------------------------------------------------------
Title: Magnetic Reconnection and Mass Acceleration in Flare-Coronal
Mass Ejection Events
Authors: Qiu, Jiong; Wang, Haimin; Cheng, C. Z.; Gary, Dale E.
2004ApJ...604..900Q Altcode:
An observational relationship has been well established among
magnetic reconnection, high-energy flare emissions and the rising
motion of erupting flux ropes. In this paper, we verify that the rate
of magnetic reconnection in the low corona is temporally correlated
with the evolution of flare nonthermal emissions in hard X-rays and
microwaves, all reaching their peak values during the rising phase of
the soft X-ray emission. In addition, however, our new observations
reveal a temporal correlation between the magnetic reconnection rate
and the directly observed acceleration of the accompanying coronal
mass ejection (CME) and filament in the low corona, thus establishing
a correlation with the rising flux rope. These results are obtained
by examining two well-observed two-ribbon flare events, for which we
have good measurements of the rise motion of filament eruption and
CMEs associated with the flares. By measuring the magnetic flux swept
through by flare ribbons as they separate in the lower atmosphere,
we infer the magnetic reconnection rate in terms of the reconnection
electric field Erec inside the reconnecting current sheet
(RCS) and the rate of magnetic flux convected into the diffusion
region. For the X1.6 flare event, the inferred Erec is ~5.8 V
cm-1 and the peak mass acceleration is ~3 km s-2,
while for the M1.0 flare event Erec is ~0.5 V cm-1
and the peak mass acceleration is 0.2-0.4 km s-2.

---------------------------------------------------------
Title: Magnetic Field, Hα, and RHESSI Observations of the 2002 July
23 Gamma-Ray Flare
Authors: Yurchyshyn, Vasyl; Wang, Haimin; Abramenko, Valentyna;
Spirock, Thomas J.; Krucker, Säm
2004ApJ...605..546Y Altcode:
In this paper we examine two aspects of the 2002 July 23 gamma-ray flare
by using multiwavelength observations. First, the data suggest that the
interaction of the erupted field with an overlying large-scale coronal
field can explain the offset between the gamma-ray and the hard X-ray
sources observed in this event. Second, we pay attention to rapid and
permanent changes in the photospheric magnetic field associated with
the flare. MDI and BBSO magnetograms show that the following magnetic
flux had rapidly decreased by 1×1020 Mx immediately after
the flare, while the leading polarity was gradually increasing for
several hours after the flare. Our study also suggests that the changes
were most probably associated with the emergence of new flux and the
reorientation of the magnetic field lines. We interpret the magnetograph
and spectral data for this event in terms of the tether-cutting model.

---------------------------------------------------------
Title: Evidence of Rapid Flux Emergence Associated with the M8.7
Flare on 2002 July 26
Authors: Wang, Haimin; Qiu, Jiong; Jing, Ju; Spirock, Thomas J.;
Yurchyshyn, Vasyl; Abramenko, Valentina; Ji, Haisheng; Goode,
Phillip R.
2004ApJ...605..931W Altcode:
In this paper, we present a detailed study of the M8.7 flare
that occurred on 2002 July 26 using data from the Big Bear Solar
Observatory (BBSO), Ramaty High Energy Solar Spectroscopic Imager
(RHESSI), the Transition Region and Coronal Explorer (TRACE), and the
Solar and Heliospheric Observatory (SOHO). This flare has interesting
properties similar to a number of flares that we studied previously,
such as a rapid increase of magnetic flux in one polarity and an
increase in transverse fields and magnetic shear associated with the
flare. However, this event had the most comprehensive observations; in
particular, the high-resolution high-cadence BBSO vector magnetograph
observations. At the time of the flare, across the flare neutral
line, there was a sudden emergence of magnetic flux at the rate
of 1020 Mx hr-1 in both the longitudinal and
transverse components. The emerging flux mostly occurred at the sites
of the flare. It was very inclined and led to impulsively enhanced
shear in the magnetic fields. We discuss these observations in the
context of magnetic reconnection triggered by rapid flux emergence. It
is also possible that the new flux signifies flare-related change in
the field line inclination.

---------------------------------------------------------
Title: Studies of Microflares in RHESSI Hard X-Ray, Big Bear Solar
Observatory Hα, and Michelson Doppler Imager Magnetograms
Authors: Liu, Chang; Qiu, Jiong; Gary, Dale E.; Krucker, Säm;
Wang, Haimin
2004ApJ...604..442L Altcode:
In this paper, we present a study of the morphology of 12 microflares
jointly observed by RHESSI in the energy range from 3 to 15 keV
and by Big Bear Solar Observatory (BBSO) at the Hα line. They
are A2-B3 events in GOES classification. From their time profiles,
we find that all of these microflares are seen in soft X-ray, hard
X-ray, and Hα wavelengths, and their temporal evolution resembles
that of large flares. Co-aligned hard X-ray, Hα, and magnetic field
observations show that the events all occurred in active regions and
were located near magnetic neutral lines. In almost all of the events,
the hard X-ray sources are elongated structures connecting two Hα
bright kernels in opposite magnetic fields. These results suggest
that, similar to large flares, the X-ray sources of the microflares
represent emission from small magnetic loops and that the Hα bright
kernels indicate emission at footpoints of these flare loops in the
lower atmosphere. Among the 12 microflares, we include five events that
are clearly associated with type III radio bursts as observed by the
radio spectrometer on board Wind. Spectral fitting results indicate
the nonthermal origin of the X-ray emission at over ~10 keV during
the impulsive phase of all the events, and the photon spectra of the
microflares associated with type III bursts are generally harder than
those without type III bursts. TRACE observations at EUV wavelengths are
available for five events in our list, and in two of these, coincident
EUV jets are clearly identified to be spatially associated with the
microflares. Such findings suggest that some microflares are produced
by magnetic reconnection, which results in closed compact loops and
open field lines. Electrons accelerated during the flare escape along
the open field lines to interplanetary space.

---------------------------------------------------------
Title: Low Atmosphere Reconnections Associated with AN Eruptive
Solar Flare
Authors: Moon, Y. -J.; Chae, Jongchul; Choe, G. S.; Wang, Haimin;
Park, Y. D.; Cheng, C. Z.
2004JKAS...37...41M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Characteristic evaluation of a near-infrared Fabry-Perot
filter for the InfraRed Imaging Magnetograph (IRIM)
Authors: Cao, Wenda; Denker, Carsten J.; Wang, Haimin; Ma, J.; Qu,
M.; Wang, Jinshan; Goode, Philip R.
2004SPIE.5171..307C Altcode:
The InfraRed Imaging Magnetograph (IRIM) is a high temporal
resolution, high spatial resolution, high spectral resolving power,
and high magnetic sensitivity solar two-dimensional narrow-band
spectro-polarimeter working in the near infrared from 1.0 μm to
1.7 μm at Big Bear Solar Observatory (BBSO). It consists of an
interference filter, a polarization analyzer, a birefringent filter,
and a Fabry-Perot etalon. As the narrowest filter of IRIM, the infrared
Fabry-Perot plays a very important role in achieving the narrow band
transmission of ~ 10 pm and high throughput between 85% and 95% for
the full wavelength range, maintaining wavelength tuning ability
from 1.0 to 1.7 μm, and assuring stability and reliability. As
the third of a series of publications describing IRIM, this paper
outlines a set of methods to evaluate the near infrared Fabry-Perot
etalon. Two-dimensional characteristic maps of the near infrared
Fabry-Perot etalon, including the bandpass ▵λ, effective finesse
Feff, peak transmission τmax, along with a free
spectral range, flatness, roughness, and stability and repeatability
were obtained with laboratory equipment. These measured results will
benefit the optimization of IRIM design and observational mode of
the future.

---------------------------------------------------------
Title: Correlation between speeds of coronal mass ejections and the
intensity of geomagnetic storms
Authors: Yurchyshyn, Vasyl; Wang, Haimin; Abramenko, Valentyna
2004SpWea...2.2001Y Altcode:
We studied the relationship between the projected speed of coronal
mass ejections (CMEs), determined from a sequence of Solar and
Heliospheric Observatory/Large Angle and Spectrometric Coronagraph
Experiment (SOHO/LASCO) images, and the hourly averaged magnitude of
the Bz component of the magnetic field in an interplanetary
ejecta, as measured by the Advanced Composition Explorer (ACE)
magnetometer in the Geocentric Solar Magnetospheric Coordinate System
(GSM). For CMEs that originate at the central part of the solar
disk we found that the intensity of Bz is correlated with
the projected speed of the CME, Vp. The relationship is
more pronounced for very fast ejecta (Vp &gt; 1200 km/s),
while slower events display larger scatter. We also present data which
support earlier conclusions about the correlation of Bz
and the Dst index of geomagnetic activity. A possible application of
the results to space weather forecasting is discussed.

---------------------------------------------------------
Title: Hα Dimmings Associated with the X1.6 Flare and Halo Coronal
Mass Ejection on 2001 October 19
Authors: Jiang, Yunchun; Ji, Haisheng; Wang, Haimin; Chen, Huadong
2003ApJ...597L.161J Altcode:
As the optical counterpart of coronal dimmings, we present the first
direct evidence of Hα dimmings associated with the X1.6 flare, filament
eruption, and coronal mass ejection (CME) that occurred on 2001 October
19. As two main Hα flare ribbons parallel to the erupting filament
slowly separated, two other ribbons first quickly expanded near the
feet of the filament, then their interiors substantially dimmed and
became darker than the preflare brightness. This formed two dimming
regions with sharp bright edges. The two Hα dimmings were different
from the flare nimbus and the Hα darkenings found by Ellison, McKenna,
&amp; Reid and Neidig et al., respectively, but similar to the coronal
“double dimmings” studied by Sterling &amp; Hudson. The coronal
dimmings of the event were also observed in 171 Å images from the
Transition Region and Coronal Explorer (TRACE). The 171 Å and Hα
dimmings showed very similar evolutions and occurred at the same two
sites with opposite magnetic polarities. They formed in the course of
the filament eruption, and after the eruption, some 171 Å loops of
the postflare arcade developed to connect their inner edges while some
loops with footpoints near their outer edges disappeared. Observations
from the Solar and Heliospheric Observatory Extreme-ultraviolet Imaging
Telescope (SOHO/EIT) showed more extended dimmings associated with the
eruption. Only a small portion of EIT dimmings was cospatial with the
Hα dimmings, while the major other portion was associated with the
disappeared 171 Å loops. These joint observations are interpreted by
using the flux rope model of CMEs, in which the Hα dimmings represent
the evacuated feet of the erupted flux rope. The mass supply of the halo
CME could be coming from both the erupted flux rope itself and the lost
material close to the flare footpoint regions as shown by Hα dimmings.

---------------------------------------------------------
Title: A New Method for Resolving the 180° Ambiguity in Solar
Vector Magnetograms
Authors: Moon, Y. -J.; Wang, Haimin; Spirock, Thomas J.; Goode, P. R.;
Park, Y. D.
2003SoPh..217...79M Altcode:
We present a new method to resolve the 180° ambiguity for solar
vector magnetogram measurements. The basic assumption is that the
magnetic shear angle (Δθ), which is defined as the difference
between the azimuth components of observed and potential fields,
approximately follows a normal distribution. The new method is
composed of three steps. First, we apply the potential field method
to determine the azimuthal components of the observed magnetic
fields. Second, we resolve the ambiguity with a new criterion:
−90°+ΔθmpleΔθle90°+Δθmp, where
Δθmp is the most probable value of magnetic shear angle
from its number distribution. Finally, to remove some localized field
discontinuities, we use the criterion Bt⋅Bmtge0,
where Bt and Bmt are an observed transverse field
and its mean value for a small surrounding region, respectively. For
an illustration, we have applied the new ambiguity removal method
(Uniform Shear Method) to a vector magnetogram which covers a highly
sheared region near the polarity inversion line of NOAA AR 0039. As a
result, we have found that the new ambiguity solution was successful
and removed spatial discontinuities in the transverse vector fields
produced in the magnetogram by the potential field method. It is
also found that our solution to the ambiguity gives nearly the same
results, for highly sheared vector magnetograms and vertical current
density distributions, of NOAA AR 5747 and AR 6233 as those of other
methods. The validity of the basic assumption for an approximate normal
distribution is demonstrated by the number distributions of magnetic
shear angle for the three active regions under consideration.

---------------------------------------------------------
Title: Observations of the Failed Eruption of a Filament
Authors: Ji, Haisheng; Wang, Haimin; Schmahl, Edward J.; Moon, Y. -J.;
Jiang, Yunchun
2003ApJ...595L.135J Altcode:
We have observed the fine temporal and spatial structure of a filament
eruption on 2002 May 27 following an M2-class flare. Our observations at
Big Bear Solar Observatory were made at the wavelength of Hα 1.3 Å,
with a cadence of 40 ms. The event was also observed by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at X-ray
energies from 3 to 50 keV and by the Transition Region and Coronal
Explorer (TRACE) in poFe XII 195 Å. The event appears to be a “failed
eruption,” as the filament material, seen in absorption by TRACE,
first accelerated then decelerated as it approached its peak height of
~8×104 km while the filament threads drained back to the
Sun. The fact that the eruption did not lead to a coronal mass ejection
indicates that the coronal magnetic field near ~8×104 km did
not open during the flare. The height-time curve obtained from the TRACE
195 Å images during the deceleration phase shows that the deceleration
of the filament exceeded the gravitational deceleration by more than
a factor of 10, which suggests that the filament material was pulled
back by magnetic tension. Also of importance are three sequential but
cospatial features-brightenings in EUV, a loop-top hard X-ray emission,
and “rupturing” of the Hα filament-that point to a release of energy
(and probably magnetic reconnection) above the initial filament's
location but well below its terminal height. Reconnection above a
filament does not appear in most models, with the notable exception of
quadrupolar and “breakout” models. These observations provide evidence
that at least two conditions are required for a successful eruption:
a reconnection very low in the corona (possibly above the filament)
and open or opening fields above that point.

---------------------------------------------------------
Title: Automatic Solar Flare Detection Using MLP, RBF, and SVM
Authors: Qu, Ming; Shih, Frank Y.; Jing, Ju; Wang, Haimin
2003SoPh..217..157Q Altcode:
The focus of automatic solar-flare detection is on the development of
efficient feature-based classifiers. The three principal techniques
used in this work are multi-layer perceptron (MLP), radial basis
function (RBF), and support vector machine (SVM) classifiers. We have
experimented and compared these three methods for solar-flare detection
on solar Hα images obtained from the Big Bear Solar Observatory in
California. The preprocessing step is to obtain nine principal features
of the solar flares for the classifiers. Experimental results show
that by using SVM we can obtain the best classification rate of the
solar flares. We believe our work will lead to real-time solar-flare
detection using advanced pattern recognition techniques.

---------------------------------------------------------
Title: Study of Ribbon Separation of a Flare Associated with a
Quiescent Filament Eruption
Authors: Wang, Haimin; Qiu, Jiong; Jing, Ju; Zhang, Hongqi
2003ApJ...593..564W Altcode:
In this paper, we present a detailed study of a two-ribbon flare
in the plage region observed by Kanzelhohe Solar Observatory (KSO),
which is one of the stations in our global Hα network. We select this
event due to its very clear filament eruption, two-ribbon separation,
and association with a fast coronal mass ejection (CME). We study
the separation between the two ribbons seen in Hα as a function of
time and find that the separation motion consisted of a fast stage of
rapid motion at a speed of about 15 km s-1 in the first
20 minutes and a slow stage with a separation speed of about 1 km
s-1 lasting for 2 hr. We then estimate the rate of the
magnetic reconnection in the corona, as represented by the electric
fields Ec in the reconnecting current sheet, by measuring
the ribbon motion speed and the magnetic fields obtained from MDI. We
find that there were two stages as well in evolution of the electric
fields: Ec=1Vcm-1 averaged over 20 minutes in
the early stage, followed by Ec=0.1Vcm-1 in
the subsequent 2 hr. The two stages of the ribbon motion and electric
fields coincide with the impulsive and decaying phases of the flare,
respectively, yielding clear evidence that the impulsive flare energy
release is governed by the fast magnetic reconnection in the corona. We
also measure the projected heights of the erupting filament from KSO
Hα and SOHO/EIT images. The filament started to rise 20 minutes before
the flare. After the flare onset, it was accelerated quickly at a rate
of 300 m s-2, and in 20 minutes, reached a speed of at least
540 km s-1, when it disappeared beyond the limb in the EIT
observations. The acceleration rate of the CME is estimated to be 58 m
s-2 during the decay phase of the flare. The comparison of
the height and velocity profiles between the filament and CME suggests
that fast acceleration of mass ejections occurred during the impulsive
phase of the flare, when the magnetic reconnection rate was also large,
with Ec=1Vcm-1.

---------------------------------------------------------
Title: Observational Evidence of a Magnetic Flux Rope Eruption
Associated with the X3 Flare on 2002 July 15
Authors: Liu, Yu; Jiang, Yunchun; Ji, Haishen; Zhang, Hongqi; Wang,
Haimin
2003ApJ...593L.137L Altcode:
We present the study of an eruption from the low solar atmosphere
(photosphere/chromosphere) as seen in Transition Region and Coronal
Explorer 1600 Å images and with the Solar and Heliospheric Observatory
Michelson Doppler Imager. The eruption reached its maximum at 20:08
UT on 2002 July 15 in the NOAA Active Region 10030 (N19°, W01°),
accompanied by an X3 flare and followed by a fast-halo coronal mass
ejection. The main observational results from the data are as follows:
(1) the erupting plasma was in a rapidly rising, twisted ropelike
structure; (2) the eruption occurred just preceding the onset of its
driven flare; and (3) the morphology and magnetic flux of one slender
footpoint (~9000 km in length) of the rope developed rapidly on the
photosphere. This structure disappeared in white light and in the
magnetograms within 60 minutes. This evidence supports the erupting
flux rope model. Our data favor the idea that a catastrophic loss of
MHD equilibrium can be the primary driving mechanism for the rapid
ejection of a flux rope. This conclusion is based on the judgment that
the ambient fields of the flux rope were partly opened as a result of
the magnetic reconnection.

---------------------------------------------------------
Title: 1.6 M Solar Telescope in Big Bear -- The NST
Authors: Goode, Philip R.; Denker, Carsten. J.; Didkovsky, Leonid I.;
Kuhn, J. R.; Wang, Haimin
2003JKAS...36S.125G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Relationship Between CME Kinematics and Flare Strength
Authors: Moon, Y. -J.; Choe, G. S.; Wang, Haimin; Park, Y. D.; Cheng,
C. Z.
2003JKAS...36...61M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Sun from Big Bear
Authors: Goode, Philip R.; Denker, Carsten; Wang, Haimin
2003ASSL..288..137G Altcode: 2003ASSL..287..437G
No abstract at ADS

---------------------------------------------------------
Title: Sympathetic Coronal Mass Ejections
Authors: Moon, Y. -J.; Choe, G. S.; Wang, Haimin; Park, Y. D.
2003ApJ...588.1176M Altcode:
We address the question whether there exist sympathetic coronal mass
ejections (CMEs), which take place almost simultaneously in different
locations with a certain physical connection. For this study, the
following three investigations are performed. First, we have examined
the waiting-time distribution of the CMEs that were observed by the SOHO
Large Angle and Spectrometric Coronagraph (LASCO) from 1999 February
to 2001 December. The observed waiting-time distribution is found to
be well approximated by a time-dependent Poisson distribution without
any noticeable overabundance at short waiting times. Second, we have
investigated the angular difference distribution of successive CME
pairs to examine their spatial correlations. A remarkable overabundance
relative to background levels is found within 10° of the position angle
difference, which supports the existence of quasi-homologous CMEs that
occur sequentially in the same active region. Both of the above results
indicate that sympathetic (interdependent) CMEs are far less frequent
than independent CMEs. Third, we have examined the EUV Imaging Telescope
running difference images and the LASCO images of quasi-simultaneous
CME pairs and found a candidate sympathetic CME pair, of which the
second CME may be initiated by the eruption of the first CME. Possible
mechanisms of the sympathetic CME triggering are discussed.

---------------------------------------------------------
Title: Magnetic Helicity Pumping by Twisted Flux Tube Expansion
Authors: Chae, Jongchul; Moon, Y. -J.; Rust, D. M.; Wang, Haimin;
Goode, Philip R.
2003JKAS...36...33C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: IRIM: An Imaging Magnetograph for High-Resoultion Solar
Observations in the Near-Infrared
Authors: Denker, Carsten J.; Ma, J.; Wang, Jingshan; Didkovsky,
Leonid V.; Varsik, John R.; Wang, Haimin; Goode, Philip R.
2003SPIE.4853..223D Altcode:
The InfraRed Imaging Magnetograph (IRIM) is an innovative magnetograph
system for near-infrared (NIR)observations of the Sun. IRIM will
provide high spatial resolution (0.2" per pixel image scale), high
temporal resolution (1-2 minutes), moderate spectral resolution
(14.0 pm), and high magnetic sensitivity covering a substantial
field-of-view (FOV: 170" circular). The bandpass of the instrument
is reduced in three steps while still providing high transmission:
(1) a 4 nm interference filter, (2) a 0.25 nm Lyot-filter, and (3)
a 14.0 pm tunable Fabry-Perot etalon. The innovative NIR Lyot-filter
was developed at the New Jersey Institute of Technology (NJIT) and
is currently being assembled at Cambridge Research Instruments. It is
the first of its kind and provides a large angle of acceptance, thus
solving many problems encountered with dual Fabry-Perot systems. The
two-dimensional line profiles will be recorded by a 1024 × 1024 pixel,
12-bit Complex Metal Oxide Semiconductor (CMOS) focal plane array (FPA)
manufactured by Rockwell Scientific Imaging, which can obtain images
at a rate of 50 fps. IRIM will utilize the remodelled Coude-feed of
the 65 cm vacuum telescope at the Big Bear Solar Observatory (BBSO)
and will benefit from an image stabilization and correction system of
independently operating Correlation Tracking (CT) and Adaptive Optics
(AO) systems.

---------------------------------------------------------
Title: Periodic Motion along a Solar Filament Initiated by a Subflare
Authors: Jing, Ju; Lee, Jeongwoo; Spirock, Thomas J.; Xu, Yan; Wang,
Haimin; Choe, G. S.
2003ApJ...584L.103J Altcode:
A type of mass motion in solar filaments, not previously reported,
is studied with high-cadence (1 minute) Hα observations made at
the Big Bear Solar Observatory on 2001 October 24. This oscillatory
motion is along the filament and extends over a long distance
(~1.4×105 km) with a period of ~80 minutes and a very
high velocity amplitude of ~92 km s-1. Another significant
property of this oscillation is that it is triggered by a subflare
that occurred near its footpoint. The oscillation completes three
cycles before it damps out over a timescale of ~210 minutes. We mainly
discuss whether this oscillation is an extreme form of the recently
discovered counterstreaming flows in filaments or is a form of the
large-amplitude filament oscillations (known as a “winking” filament)
discovered a half-century ago.

---------------------------------------------------------
Title: High-order adaptive optical system for Big Bear Solar
Observatory
Authors: Didkovsky, Leonid V.; Dolgushyn, Alexander; Marquette,
William; Nenow, Jeff; Varsik, John; Goode, Philip R.; Hegwer, Steven
L.; Ren, Deqing; Fletcher, Steve; Richards, Kit; Rimmele, Thomas;
Denker, Carsten J.; Wang, Haimin
2003SPIE.4853..630D Altcode:
We present a high-order adaptive optical system for the 26-inch vacuum
solar telescope of Big Bear Solar Observatory. A small elliptical
tip/tilt mirror is installed at the end of the existing coude
optical path on the fast two-axis tip/tilt platform with its resonant
frequency around 3.3 kHz. A 77 mm diameter deformable mirror with 76
subapertures as well as wave-front sensors (correlation tracker and
Shack-Hartman) and scientific channels for visible and IR polarimetry
are installed on an optical table. The correlation tracker sensor
can detect differences at 2 kHz between a 32×32 reference frame
and real time frames. The WFS channel detects 2.5 kHz (in binned
mode) high-order wave-front atmosphere aberrations to improve solar
images for two imaging magnetographs based on Fabry-Perot etalons in
telecentric configurations. The imaging magnetograph channels may work
simultaneously in a visible and IR spectral windows with FOVs of about
180×180 arc sec, spatial resolution of about 0.2 arc sec/pixel and
SNR of about 400 and 600 accordingly for 0.25 sec integration time.

---------------------------------------------------------
Title: Magnetic helicity change rate associated with three X-class
eruptive flares
Authors: Moon, Y. -J.; Chae, Jongchul; Wang, Haimin; Park, Y. D.
2003AdSpR..32.1953M Altcode:
In this paper we have examined the temporal evolution of magnetic
helicity change rate associated with three X-class eruptive flares that
occurred on November 24, 2000. By analyzing a set of 1 minute cadence
high-resolution magnetograms taken by Michelson Doppler Imager (MDI)
on board Solar and Heliospheric Observatory (SOHO), we have determined
the rates of magnetic helicity transport via horizontal photospheric
motions. Then we have compared the impulsive variations of helicity
injection rate with an initial eruption speed profile of a filament
associated with a X1.8 flare. As a result, we have found (1) that the
impulsive variations of magnetic helicity change rate were strongly
correlated with only three X-class flarings, (2) that the eruption of
the filament started about 10 minutes before the impulsive variation
of the helicity change rate, and (3) that horizontal velocity kernels
were located between Ha two flaring ribbons. Thus we suggest that
the observed impulsive helicity change is not a cause of the eruptive
solar flare but its result, i.e., photospheric response to the coronal
field restructuring.

---------------------------------------------------------
Title: How directions and helicity of erupted solar magnetic fields
define geoeffectiveness of coronal mass ejections
Authors: Yurchyshyn, Vasyl; Wang, Haimin; Abramenko, Valentyna
2003AdSpR..32.1965Y Altcode:
In this study we report on the relationship between the projected speed
of CMEs, measured at 20R from SOHO/LASCO images, and the hourly averaged
magnitude of the southwardly directed magnetic field, B z,
at the leading edge of interplanetary ejecta, as measured by the ACE
magnetometer. We found that those CMEs that originate at the central
part of the solar disk ( r &lt; 0.6 R⊙) are the most
geoeffective and the intensity of B z is an exponential
function of the CME's speeds. We propose an approach to estimate the
strength of the southward IMF at least one day in advance, immediately
after a CME started. The predicted value of the B z
component can be then used to estimate the intensity of a geomagnetic
storm caused by the eruption. The prediction method is based on the
correlation between the speeds of CMEs and magnitudes of the southward
IMF as well as the fact that the orientation and chirality of the
erupted solar filaments correspond to the orientation and chirality
of interplanetary ejecta.

---------------------------------------------------------
Title: A Statistical Study of Two Classes of Coronal Mass Ejections
Authors: Moon, Y. -J.; Choe, G. S.; Wang, Haimin; Park, Y. D.;
Gopalswamy, N.; Yang, Guo; Yashiro, S.
2002ApJ...581..694M Altcode:
A comprehensive statistical study is performed to address the question
of whether two classes of coronal mass ejections (CMEs) exist. A
total of 3217 CME events observed by SOHO/LASCO in 1996-2000 have
been analyzed. We have examined the distributions of CMEs according to
speed and acceleration, respectively, and investigated the correlation
between speed and acceleration of CMEs. This statistical analysis is
conducted for two subsets containing those CMEs that show a temporal and
spatial association either with GOES X-ray solar flares or with eruptive
filaments. We have found that CMEs associated with flares have a higher
median speed than those associated with eruptive filaments and that the
median speed of CMEs associated with strong flares is higher than that
of weak-flare-associated CMEs. The distribution of CME acceleration
shows a conspicuous peak near zero, not only for the whole data set,
but also for the two subsets associated either with solar flares or
with eruptive filaments. However, we have confirmed that the CMEs
associated with major flares tend to be more decelerated than the CMEs
related to eruptive filaments. The fraction of flare-associated CMEs
has a tendency to increase with the CME speed, whereas the fraction
of eruptive-filament-associated CMEs tends to decrease with the CME
speed. This result supports the concept of two CME classes. We have
found a possibility of two components in the CME speed distribution
for both the CME data associated with flares larger than M1 class and
the CME data related with limb flares. Our results suggest that the
apparent single-peak distribution of CME speed can be attributed to
the projection effect and possibly to abundance of small flares too. We
also note that there is a possible correlation between the speed of CMEs
and the time-integrated X-ray flux of the CME-associated limb flares.

---------------------------------------------------------
Title: Control and Acquisition Software for the Visible-Light
Fabry-Pérot Interferometer at the Big Bear Solar Observatory
Authors: Shumko, Sergiy; Denker, Carsten J.; Varsik, John; Didkovsky,
Leonid V.; Marquette, William; Goode, Philip R.; Wang, Haimin
2002SPIE.4848..483S Altcode:
We describe our progress in the development of a software package to
control a Fabry-Pérot interferometer (FPI) at the Big Bear Solar
Observatory (BBSO). The FPI is a key part of our new Visible-Light
Imaging Magnetograph (VIM). We describe the software libraries
and methods that we use to develop the software. We also present
specifications and characteristics of this new instrument.

---------------------------------------------------------
Title: Sudden Disappearance of a Small Sunspot Associated with the
2002 February 20 M2.4 Flare
Authors: Wang, Haimin; Ji, Haisheng; Schmahl, Edward J.; Qiu, Jiong;
Liu, Chang; Deng, Na
2002ApJ...580L.177W Altcode:
In this Letter, we present the evidence of rapid disappearance of a
small sunspot associated with a M2.4 flare, which occurred in NOAA
Active Region 9830, at 2106 UT, on 2002 February 20. The sunspot,
in the size of about 45 arcsec2, completely disappeared in
about 60 minutes, coinciding with the onset of the flare. The magnetic
flux of this sunspot disappeared in the similar timescale. Furthermore,
we found that the disappearing sunspot is located near the edge of the
main hard X-ray source observed by RHESSI. This hard X-ray source is
located in the magnetic neutral line dividing the disappearing spot
and the opposite polarity flux. Two possible explanations are offered:
(1) the disappearing sunspot is due to the rapid magnetic reconnection
and subsequent submergence; (2) the disappearing sunspot represents
impulsive emergence and detachment of a magnetic “bubble.”

---------------------------------------------------------
Title: Impulsive Variations of the Magnetic Helicity Change Rate
Associated with Eruptive Flares
Authors: Moon, Y. -J.; Chae, Jongchul; Wang, Haimin; Choe, G. S.;
Park, Y. D.
2002ApJ...580..528M Altcode:
In this paper, we investigate impulsive variations of the magnetic
helicity change rate associated with eruptive solar flares (three
X class flares and one M class flare) accompanying halo coronal
mass ejections. By analyzing four sets of 1 minute cadence full-disk
magnetograms taken by the Michelson Doppler Imager on board the Solar
and Heliospheric Observatory, we have determined the rates of magnetic
helicity transport due to horizontal photospheric motions. We have found
that magnetic helicity of the order of 1041 Mx2
was impulsively injected into the corona around the flaring peak time
of all the flares. We also found that there is a positive correlation
between the impulsively injected magnetic helicity and the X-ray peak
flux of the associated flare. The impulsive helicity variations are
attributed to horizontal velocity kernels localized near the polarity
inversion lines. Finally, we report that there is a close spatial
proximity between the horizontal velocity kernels and Hα bright points.

---------------------------------------------------------
Title: Temporal Change of Magnetic Shear Free from the 180arcdeg
Ambiguity
Authors: Moon, Y. -J.; Wang, Haimin; Spirock, Thomas J.; Park, Y. D.
2002JKAS...35..143M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Rapid Changes of Magnetic Fields Associated with Six X-Class
Flares
Authors: Wang, Haimin; Spirock, Thomas J.; Qiu, Jiong; Ji, Haisheng;
Yurchyshyn, Vasyl; Moon, Yong-Jae; Denker, Carsten; Goode, Philip R.
2002ApJ...576..497W Altcode:
In this paper, we present the results of the study of six X-class
flares. We found significant changes in the photospheric magnetic
fields associated with all of the events. For the five events in 2001,
when coronagraph data were available, all were associated with halo
coronal mass ejections. Based on the analyses of the line-of-sight
magnetograms, all six events had an increase in the magnetic flux
of the leading polarity of order of a few times 1020 Mx
while each event had some degree of decrease in the magnetic flux
of the following polarity. The flux changes are considered impulsive
because the “changeover” time, which we defined as the time to change
from preflare to postflare state, ranged from 10 to 100 minutes. The
observed changes are permanent. Therefore, the changes are not due
to changes in the line profile caused by flare emissions. For the
three most recent events, when vector magnetograms were available,
two showed an impulsive increase of the transverse field strength
and magnetic shear after the flares, as well as new sunspot area in
the form of penumbral structure. One of the events in this study was
from the previous solar cycle. This event showed a similar increase
in all components of the magnetic field, magnetic shear, and sunspot
area. We present three possible explanations to explain the observed
changes: (1) the emergence of very inclined flux loops, (2) a change
in the magnetic field direction, and (3) the expansion of the sunspot,
which moved some flux out of Zeeman saturation. However, we have no
explanation for the polarity preference; i.e., the flux of leading
polarity tends to increase while the flux of following polarity tends
to decrease slightly.

---------------------------------------------------------
Title: Active-Region Monitoring and Flare Forecasting I. Data
Processing and First Results
Authors: Gallagher, Peter T.; Moon, Y. -J.; Wang, Haimin
2002SoPh..209..171G Altcode:
This paper discusses a near real-time approach to solar active-region
monitoring and flare prediction using the Big Bear Solar Observatory
Active Region Monitor (ARM). Every hour, ARM reads, calibrates,
and analyses a variety of data including: full-disk Hα images from
the Global Hα Network; EUV, continuum, and magnetogram data from the
Solar and Heliospheric Observatory (SOHO); and full-disk magnetograms
from the Global Oscillation Network Group (GONG). For the first time,
magnetic gradient maps derived from GONG longitudinal magnetograms are
now available on-line and are found to be a useful diagnostic of flare
activity. ARM also includes a variety of active-region properties
from the National Oceanic and Atmospheric Administration's Space
Environment Center, such as up-to-date active-region positions, GOES
5-min X-ray data, and flare-to-region identifications. Furthermore,
we have developed a Flare Prediction System which estimates the
probability for each region to produce C-, M-, or X-class flares based
on nearly eight years of NOAA data from cycle 22. This, in addition
to BBSO's daily solar activity reports, has proven a useful resource
for activity forecasting.

---------------------------------------------------------
Title: Non-LTE Calculation of the Ni I 676.8 Nanometer Line in a
Flaring Atmosphere
Authors: Ding, M. D.; Qiu, Jiong; Wang, Haimin
2002ApJ...576L..83D Altcode:
The Ni I 676.8 nm line is used by the Solar and Heliospheric Observatory
Michelson Doppler Imager to measure the magnetic field and velocity
field in the solar atmosphere. We make non-LTE calculations of this
line in an atmosphere that is bombarded by an energetic electron
beam. This case is associated with the occurrence of solar flares. The
electron beam produces nonthermal ionization and excitation of the
hydrogen atoms and redistributes the level populations. This results
in an enhanced opacity near the Ni I line and an upward shift of its
formation height, as well as an increased line source function. We find
that the Ni I line may appear in emission in the presence of a fairly
strong electron beam and preferentially in a cool atmosphere (i.e.,
sunspot umbrae/penumbrae). On the other hand, if there is no bombarding
electron beam, the profile can hardly turn to emission even though the
atmosphere may be heated to higher temperatures through other ways. This
result implies that the sign reversal of the longitudinal magnetic
field observed in some flare events may not be a true reversal but
just an artifact associated with the production of an emission profile.

---------------------------------------------------------
Title: Flare Activity and Magnetic Helicity Injection by Photospheric
Horizontal Motions
Authors: Moon, Y. -J.; Chae, Jongchul; Choe, G. S.; Wang, Haimin;
Park, Y. D.; Yun, H. S.; Yurchyshyn, Vasyl; Goode, Philip R.
2002ApJ...574.1066M Altcode:
We present observational evidence that the occurrence of homologous
flares in an active region is physically related to the injection of
magnetic helicity by horizontal photospheric motions. We have analyzed
a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a
period of 6.5 hr by the Michelson Doppler Imager on board the Solar
and Heliospheric Observatory. During this observing time span, seven
homologous flares took place in the active region. We have computed
the magnetic helicity injection rate into the solar atmosphere by
photospheric shearing motions and found that a significant amount of
magnetic helicity was injected during the observing period. In a strong
M4.1 flare, the magnetic helicity injection rate impulsively increased
and peaked at the same time as the X-ray flux. The flare X-ray flux
integrated over the X-ray emission time strongly correlates with the
magnetic helicity injected during the flaring interval. The integrated
X-ray flux is found to be a logarithmically increasing function of
the injected magnetic helicity. Our results suggest that injection of
helicity and abrupt increase of helicity magnitude play a significant
role in flare triggering.

---------------------------------------------------------
Title: Statistical Evidence for Sympathetic Flares
Authors: Moon, Y. -J.; Choe, G. S.; Park, Y. D.; Wang, Haimin;
Gallagher, Peter T.; Chae, Jongchul; Yun, H. S.; Goode, Philip R.
2002ApJ...574..434M Altcode:
Sympathetic flares are a pair of flares that occur almost simultaneously
in different active regions, not by chance, but because of some
physical connection. In this paper statistical evidence for the
existence of sympathetic flares is presented. From GOES X-ray flare
data, we have collected 48 pairs of near simultaneous flares whose
positional information and Yohkoh soft X-ray telescope images
are available. To select the active regions that probably have
sympathetic flares, we have estimated the ratio R of actual flaring
overlap time to random-coincidence overlap time for 38 active region
pairs. We have then compared the waiting-time distributions for the
two different groups of active region pairs (R&gt;1 and R&lt;1) with
corresponding nonstationary Poisson distributions. As a result, we
find a remarkable overabundance of short waiting times for the group
with R&gt;1. This is the first time such strong statistical evidence
has been found for the existence of sympathetic flares. To examine
the role of interconnecting coronal loops, we have also conducted
the same analysis for two subgroups of the R&gt;1 group: one with
interconnecting X-ray loops and the other without. We do not find any
statistical evidence that the subgroup with interconnecting coronal
loops is more likely to produce sympathetic flares than the subgroup
without. For the subgroup with loops, we find that sympathetic flares
favor active region pairs with transequatorial loops.

---------------------------------------------------------
Title: Rapid Changes in the Longitudinal Magnetic Field Related to
the 2001 April 2 X20 Flare
Authors: Spirock, Thomas J.; Yurchyshyn, Vasyl B.; Wang, Haimin
2002ApJ...572.1072S Altcode:
Big Bear Solar Observatory observed the X20 flare that occurred at
approximately 21:50 UT on 2001 April 2 with its standard complement of
instruments. In this paper, we discuss the evolution of high-resolution
and high-cadence longitudinal magnetograph observations in the region
of the flare. The data reveal that there was a significant increase
in the magnetic field on the limbward side of the neutral line of the
active region at the location of the flare, while the magnetic field
on the side of the neutral line closer to the disk center remained
constant. We discuss possible rearrangements in the active region's
magnetic field that could lead to the observed changes.

---------------------------------------------------------
Title: Flux Cancellation Rates and Converging Speeds of Canceling
Magnetic Features
Authors: Chae, Jongchul; Moon, Yong-Jae; Wang, Haimin; Yun, H. S.
2002SoPh..207...73C Altcode:
Canceling magnetic features are commonly believed to result from
magnetic reconnection in the low atmosphere. According to the
Sweet-Parker type reconnection model, the rate of flux cancellation
in a canceling magnetic feature is related to the converging speed of
each pole. To test this prediction observationally, we have analyzed
the time variation of two canceling magnetic features in detail using
the high-resolution magnetograms taken by the Michelson Doppler Imager
(MDI) on the Solar and Heliospheric Observatory (SOHO). As a result,
we have obtained the rate and converging speed of flux cancellation
in each feature: 1.3×1018 Mx hr−1 (or
1.1×106 G cm s−1 per unit contact length) and
0.35 km s−1 in the smaller one, and 3.5×1018
Mx hr−1 (1.2×106 G cm s−1) and
0.27 km s−1 in the bigger one. The observed speeds are
found to be significantly bigger than the theoretically expected ones,
but this discrepancy can be resolved if uncertainty factors such as
low area filling factor of magnetic flux and low electric conductivity
are taken into account.

---------------------------------------------------------
Title: Core and Large-Scale Structure of the 2000 November 24 X-Class
Flare and Coronal Mass Ejection
Authors: Wang, Haimin; Gallagher, Peter; Yurchyshyn, Vasyl; Yang,
Guo; Goode, Philip R.
2002ApJ...569.1026W Altcode:
In this paper, we present three important aspects of the X1.8 flare
and the associated coronal mass ejection (CME) that occurred on 2000
November 24: (1) The source of the flare is clearly associated with
a magnetic channel structure, as was noted in a study by Zirin &amp;
Wang , which is due to a combination of flux emergence inside the
leading edge of the penumbra of the major leading sunspot and proper
motion of the sunspot group. The channel structure provides evidence for
twisted flux ropes that can erupt, forming the core of a CME, and may
be a common property of several superactive regions that have produced
multiple X-class flares in the past. (2) There are actually three flare
ribbons visible. The first can be seen moving away from the flare site,
while the second and third make up a stationary ribbon near the leader
spot. The moving ribbons could be due to a shock associated with the
erupting flux rope or due to the interaction of erupting rope and the
surrounding magnetic fields. In either case, the ribbon motion does
not fit the classical Kopp-Pneuman model, in which the separation
of ribbons is due to magnetic reconnection at successively higher
and higher coronal altitudes. (3) From the coronal dimming observed
with the EUV Imaging Telescope (EIT), the CME involved a much larger
region than the initial X-class flare. By comparing high-resolution
full-disk Hα and EIT observations, we found that a remote dimming
area is cospatial with the enhanced Hα emission. This result is
consistent with the recent model of Yokoyama &amp; Shibata that some
dimming areas near footpoints may be due to chromospheric evaporation.

---------------------------------------------------------
Title: Relationship between Flare Kernels in Hα Far-Blue Wing and
Magnetic Fields
Authors: Wang, Haimin; Qiu, Jiong
2002ApJ...568..408W Altcode:
Big Bear Solar Observatory (BBSO) recently developed fast camera
systems to observe solar flares with subsecond cadence. In most of our
observations, we observed at Hα-1.3 Å to study the chromospheric
emission from flare kernels, which are usually regarded as being
associated to nonthermal electron precipitation. For all four flares in
Active Region 8674 that we analyzed, we compared the initial brightening
of flare kernels at this wavelength with photospheric magnetograms and
found that initial brightenings avoided strong line-of-sight magnetic
regions; importantly, all nine flare kernels were within 10,000 km of
magnetic neutral lines. The observed flare morphology and evolution
suggest that emission near a magnetic neutral line may come from
unresolved footpoints of interacting flare loops, where nonthermal
electrons were precipitated, or from a low-lying compact loop due to
instantaneous heating in the early phase of the flare. Our current
observations cannot distinguish between these two mechanisms. It is
suggested that impulsive compact flares involve low-lying magnetic
loops or magnetic reconnection at small altitudes in contrast to
eruptive long-duration flares.

---------------------------------------------------------
Title: Temperatures of Extreme-Ultraviolet-emitting Plasma Structures
Observed by the Transition Region and Coronal Explorer
Authors: Chae, Jongchul; Park, Young-Deuk; Moon, Yong-Jae; Wang,
Haimin; Yun, H. S.
2002ApJ...567L.159C Altcode:
The Transition Region and Coronal Explorer has revealed, in
unprecedented detail, various kinds of EUV-emitting plasma structures
in the solar upper atmosphere. The filter ratio 195 Å/171 Å has been
conventionally used to determine the plasma temperatures, but this
method has a shortcoming in that it may not yield a unique temperature
value for a given ratio. Therefore, we introduce a new method employing
two filter ratios (195 Å/171 Å and 284 Å/195 Å). It is demonstrated
that this color-color method is effective in determining a wide range
of unambiguous plasma temperatures. We have obtained a temperature of
1×106 K for a loop that is bright in 171 Å but hardly
visible in 284 Å, a higher temperature of 2×106 K for
a loop that is clearly visible in 195 and 284 Å but not in 171 Å,
and a transition-region temperature of 2.5×105 K for a
low-lying loop that is clearly visible in all the EUV wavelengths. In
addition, we have found that “moss” structures have temperatures
of around 1×106 K and that EUV jets have temperatures of
about 2.5×105 K.

---------------------------------------------------------
Title: Rapid Changes of Photospheric Magnetic Fields Associated with
CMEs and Flares
Authors: Wang, Haimin
2002smra.progE..24W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Motion of Flare Footpoint Emission and Inferred Electric
Field in Reconnecting Current Sheets
Authors: Qiu, Jiong; Lee, Jeongwoo; Gary, Dale E.; Wang, Haimin
2002ApJ...565.1335Q Altcode:
A systematic motion of Hα kernels during solar flares can be regarded
as the chromospheric signature of progressive magnetic reconnection in
the corona, in that the magnetic field lines swept through by the kernel
motion are those connected to the diffusion region at the reconnection
point. In this paper, we present high-cadence and high-resolution
Hα-1.3 Å observations of an impulsive flare that exhibits a systematic
kernel motion and relate them to the reconnecting current sheet (RCS)
in the corona. Through analyses of X-ray and microwave observations,
we further examine the role of the macroscopic electric field inside
the RCS in accelerating electrons. We measure the velocity of the
kernel motion to be 20-100 km s-1. This is used together
with the longitudinal magnetic field to infer an electric field as
high as 90 V cm-1 at the flare maximum. This event shows a
special magnetic field configuration and motion pattern of Hα kernels,
in that a light bridge divides a flare kernel into two parts that move
in different manners: one moving into the stronger magnetic field and
the other moving along the isogauss contour of the longitudinal magnetic
field. The temporal variation of the electric field inferred from the
former type of kernel motion is found to be correlated with 20-85
keV hard X-ray light curves during the rise of the major impulsive
phase. This would support the scenario of magnetic energy release
via current dissipation inside the RCS, along with the hypothesis of
the DC electric field acceleration of X-ray-emitting electrons below
100 keV. However, there is no good temporal correlation between the
hard X-ray emission and the inferred electric field from the other
motion pattern. Furthermore, the microwave emission, which supposedly
comes from higher energy electrons, shows a time profile and electron
spectrum that differs from those of the X-ray bursts. We conclude that
either the two-dimensional magnetic reconnection theory related to the
Hα kernel motion is applicable to only some part of the flare region
due to its special magnetic geometry, or the electron acceleration is
dominated by other mechanisms depending on the electron energy.

---------------------------------------------------------
Title: Development of an Automatic Filament Disappearance Detection
System
Authors: Gao, Jianlin; Wang, Haimin; Zhou, Mengchu
2002SoPh..205...93G Altcode:
This paper presents an efficient and automatic method for detecting
filament disappearances. This method was applied to the Big Bear
Solar Observatory's (BBSO) full-disk Hα images. The initial step is
to detect the filaments in the solar image, then determine if they
are growing, stable or disappearing. If a disappearing filament is
found, the solar community can be automatically alerted in near real
time. This system is proven to be accurate and fast. In addition,
three statistical studies of the appearance and disappearance of all
filaments in 1999 are presented.

---------------------------------------------------------
Title: Orientation of the Magnetic Fields in Interplanetary Flux
Ropes and Solar Filaments
Authors: Yurchyshyn, Vasyl B.; Wang, Haimin; Goode, Philip R.; Deng,
Yuanyong
2001ApJ...563..381Y Altcode:
Coronal mass ejections (CMEs) are often associated with erupting
magnetic structures or disappearing filaments. The majority of CMEs
headed directly toward the Earth are observed at 1 AU as magnetic
clouds-the region in the solar wind where the magnetic field strength
is higher than average and there is a smooth rotation of the magnetic
field vectors. The three-dimensional structure of magnetic clouds can
be represented by a force-free flux rope. When CMEs reach the Earth,
they may or may not cause magnetic storms, alter Earth's magnetic field,
or produce the phenomena known as auroras. The geoeffectiveness of a
solar CME depends on the orientation of the magnetic field in it. Two
M-class solar flares erupted on 2000 February 17. The second flare
occurred near a small active region, NOAA Active Region 8872. This
eruption was accompanied by a halo CME. However, the February 17 CME did
not trigger any magnetic activity when it arrived at the Earth. Another
powerful flare, on 2000 July 14, was also associated with a halo CME,
which caused the strongest geomagnetic activity of solar cycle 23. Using
ACE measurements of the interplanetary magnetic fields, we study the
orientation of the magnetic flux ropes in both sets of magnetic clouds
and compare them with the orientation of the solar magnetic fields
and disappearing filaments. We find that the direction of the axial
field and helicity of the flux ropes are consistent with those of
the erupted filaments. Thus, the geoeffectiveness of a CME is defined
by the orientation and structure of the erupted filament and by its
magnetic helicity as well. We also suggest that the geoeffectiveness
of a CME can be forecasted using daily full-disk Hα and Yohkoh images
and MDI magnetograms as well.

---------------------------------------------------------
Title: Hα Proxies for EIT Crinkles: Further Evidence for Preflare
“Breakout”-Type Activity in an Ejective Solar Eruption
Authors: Sterling, Alphonse C.; Moore, Ronald L.; Qiu, Jiong; Wang,
Haimin
2001ApJ...561.1116S Altcode:
We present Hα observations from Big Bear Solar Observatory of an
eruptive flare in NOAA Active Region 8210, occurring near 22:30 UT
on 1998 May 1. Previously, using the EUV Imaging Telescope (EIT)
on the SOHO spacecraft, we found that a pattern of transient,
localized brightenings, which we call “EIT crinkles,” appears in
the neighborhood of the eruption near the time of flare onset. These
EIT crinkles occur at a location in the active region well separated
from the sheared core magnetic fields, which is where the most intense
features of the eruption are concentrated. We also previously found
that high-cadence images from the Soft X-ray Telescope (SXT) on
Yohkoh indicate that soft X-ray intensity enhancements in the core
begin after the start of the EIT crinkles. With the Hα data, we find
remote flare brightening counterparts to the EIT crinkles. Light curves
as functions of time of various areas of the active region show that
several of the remote flare brightenings undergo intensity increases
prior to the onset of principal brightenings in the core region,
consistent with our earlier findings from EIT and SXT data. These timing
relationships are consistent with the eruption onset mechanism known
as the breakout model, introduced by Antiochos and colleagues, which
proposes that eruptions begin with reconnection at a magnetic null high
above the core region. Our observations are also consistent with other
proposed mechanisms that do not involve early reconnection in the core
region. As a corollary, our observations are not consistent with the
so-called tether-cutting models, which say that the eruption begins with
reconnection in the core. The Hα data further show that a filament in
the core region becomes activated near the time of EIT crinkle onset,
but little if any of the filament actually erupts, despite the presence
of a halo coronal mass ejection (CME) associated with this event.

---------------------------------------------------------
Title: Photospheric Plasma Flows Around a Solar Spot
Authors: Yurchyshyn, Vasyl B.; Wang, Haimin
2001SoPh..203..233Y Altcode:
We study photospheric plasma flows in an active region NOAA 8375, by
using uninterrupted high-resolution SOHO/MDI observations (137 intensity
images, 44 hours of observations). The active region consists of a
stable large spot and many small spots and pores. Analyzing horizontal
flow maps, obtained with local correlation tracking technique, we
found a system of stable persistent plasma flows existing in the
active region. The flows start on either side of the sunspot and
extend over 100” to the east. Our measurements show that the speed
of small sunspots and pores, averaged over 44 hours, was about 100 m
s−1, which corresponds to root-mean-square longitudinal
drifts of sunspots of 0.67°-0.76° day−1. We conclude
that these large-scale flows are due to faster proper motion of the
large sunspot relative to the ambient photospheric plasma. We suggest
that the flows may be a good carrier to transport magnetic flux from
eroding sunspots into the outer part of an active region.

---------------------------------------------------------
Title: Inter-Active Region Connection of Sympathetic Flaring on 2000
February 17
Authors: Wang, Haimin; Chae, Jongchul; Yurchyshyn, Vasyl; Yang, Guo;
Steinegger, Michael; Goode, Philip
2001ApJ...559.1171W Altcode:
We have analyzed high-resolution Hα full disk data from Big
Bear Solar Observatory (BBSO); magnetograph and EUV data from the
Michelson Doppler Imager, Large Angle and Spectrometric Coronagraph,
and Extreme Ultraviolet Imaging Telescope on board SOHO; and Yohkoh
soft X-ray data of 2000 February 17. Two sympathetic M-class solar
flares erupted in succession in NOAA Active Region 8869 and 8872,
respectively. The eruption from AR 8872 was followed by an extremely
symmetric halo coronal mass ejection (CME). We demonstrate the loop
activation, which appears to be the consequence of the first flare in
AR 8869 and the cause of the second flare in AR 8872. The activation
started in the form of a surge just after a filament eruption and its
associated flare in AR 8869. The surge quickly turned into a set of
disturbances that propagated at a speed of about 80 km s-1
toward the other active region AR 8872. The second flare followed in
less than an hour after the arrival of the disturbances at AR 8872. The
moving disturbances appeared in absorption in both Hα and EUV 195
Å images. The disturbances may represent mass transfer, which had
a significant velocity component perpendicular to the field lines
and, hence, caused the transport of field lines. In this case, the
disturbances may be considered to be a special kind of surge, which we
may call a “sweeping closed-loop surge.” We also demonstrated large
area dimmings associated with the CME in three active regions. The
dimming started from AR 8869 and AR 8872 and was extended to AR 8870,
which was on the opposite side of the solar equator. We believe that
both the activation of inter-active region loops and the large-scale
dimming are the signatures of large-scale restructuring associated
with the CME.

---------------------------------------------------------
Title: The Formation of a Prominence in Active Region NOAA
8668. I. SOHO/MDI Observations of Magnetic Field Evolution
Authors: Chae, Jongchul; Wang, Haimin; Qiu, Jiong; Goode, Philip R.;
Strous, Louis; Yun, H. S.
2001ApJ...560..476C Altcode:
We have studied the evolution of the photospheric magnetic field in
active region NOAA 8668 for 3 days while the formation of a reverse
S-shaped filament proceeded. From a set of full-disk line-of-sight
magnetograms taken by the Michelson Doppler Imager (MDI) on board
the Solar and Heliospheric Observatory (SOHO), we have found a
large canceling magnetic feature that was closely associated with
the formation of the filament. The positive flux of the magnetic
feature was initially 1.5×1021 Mx and exponentially
decreased with an e-folding time of 28 hr throughout the period of
observations. We also have determined the transverse velocities of
the magnetic flux concentrations in the active region by applying
local correlation tracking. As a result, a persistent pattern of shear
motion was identified in the neighborhood of the filament. The shear
motion had a speed of 0.2-0.5 km s-1 and fed negative
magnetic helicity of -3×1042 Mx2 into the
coronal volume during an observing run of 50 hr at an average rate
of -6×1040 Mx2 hr-1. This rate is
an order of magnitude higher than the rate of helicity change due to
the solar differential rotation. The magnetic flux of the field lines
created by magnetic reconnection and the magnetic helicity generated
by the photospheric shear motion are much more than enough for the
formation of the filament. Based on this result, we conjecture that the
filament formation may be the visible manifestation of the creation
of a much bigger magnetic structure that may consist of a flux rope
and an overlying sheared arcade.

---------------------------------------------------------
Title: Magnetic Flux Cancellation Observed in the Sunspot Moat
Authors: Yurchyshyn, Vasyl B.; Wang, Haimin
2001SoPh..202..309Y Altcode:
In this paper we study the evolution of magnetic fields of a 1F/2.4C
solar flare and following magnetic flux cancellation. The data are Big
Bear Solar Observatory and SOHO/MDI observations of active region NOAA
8375. The active region produced a multitude of subflares, many of them
being clustered along the moat boundary in the area with mixed polarity
magnetic fields. The study indicates a possible connection between the
flare and the flux cancellation. The cancellation rate, defined from
the data, was found to be 3×1019 Mx h−1. We
observed strong upward directed plasma flows at the cancellation
site. Suggesting that the cancellation is a result of reconnection
process, we also found a reconnection rate of 0.5 km s−1,
which is a significant fraction of Alfvén speed. The reconnection
rate indicates a regime of fast photospheric reconnection happening
during the cancellation.

---------------------------------------------------------
Title: Asymmetric Behavior of Hα Footpoint Emission during the
Early Phase of an Impulsive Flare
Authors: Qiu, Jiong; Ding, Ming D.; Wang, Haimin; Gallagher, Peter T.;
Sato, Jun; Denker, Carsten; Goode, Philip R.
2001ApJ...554..445Q Altcode:
We study the impulsive phase of a C9.0 solar flare using high temporal
and spatial resolution Hα images from Big Bear Solar Observatory (BBSO)
in conjunction with high-cadence hard X-ray (HXR) observations from
Yohkoh. During the early impulsive phase, HXR emission emerged from
two kernels K1 and K2 which were connected by coronal loops observed
in soft X-ray (SXR) images from Yohkoh. In Hα, the initial rise was
observed in one flare kernel K2, which was followed within 10 s by
enhanced emission in the associated kernel K1. Such a significant
asymmetry was not observed at HXR wavelengths. Our analysis shows
that the asymmetric Hα footpoint emission cannot be explained by the
magnetic mirroring effect in which strong field footpoints show lower
precipitation rates. Instead, we study this phenomenon by investigating
the atmospheric response of the lower chromosphere to nonthermal
beam heating. From numerical simulations, it is suggested that a cool
atmosphere does not respond rapidly to beam impact, which may explain
the missing Hα emission at K1 during the early impulsive phase. At K2,
the early-phase atmosphere may be preferentially heated resulting in
the Hα emission rapidly following the HXR emission. This is due to
the fact that K2 is a compact source which received persistent energy
deposition and consequent heating in a confined area during the early
phase. K1, on the other hand, is a diffused source which therefore
experienced a lower heating rate per unity area. We propose a scenario
in which the flare loop consists of multiple magnetic “threads”
connecting the compact footpoint K2 with the diffuse footpoint K1.

---------------------------------------------------------
Title: On the Fast Fluctuations in Solar Flare Hα Blue Wing Emission
Authors: Ding, M. D.; Qiu, Jiong; Wang, Haimin; Goode, Philip R.
2001ApJ...552..340D Altcode:
Fine temporal structures in hard X-ray and microwave emissions of
solar flares have been known for many years. Recent observations with
high time and spatial resolution revealed that emissions in the wings
of Hα could also exhibit fast (subsecond) fluctuations. We argue
that such fluctuations are physically related to the small-scale
injection of high-energy electrons. We explore this through numerical
calculations. The energy equation and the equations for energy-level
populations in hydrogen, in particular including the nonthermal
collisional excitation and ionization rates, are solved simultaneously
for an atmosphere impacted by a short-lived electron beam. We
determine the temporal evolution of the atmospheric temperature,
the atomic level populations, and the Hα line intensity. We find
that although the background Hα wing emission is mainly formed in
the photosphere, the fast fluctuations are probably produced in the
chromosphere, which is penetrated by ~20 keV electrons. To yield
Hα wing fluctuations of amplitude comparable to the observations,
a mean energy flux of ~(1-2)×1011 ergs cm-2
s-1 is required for the electron beam, if one adopts a
Gaussian macrovelocity of 25 km s-1. Such a burst contains a
total energy of 1025-1026 ergs. These parameters
are compatible with elementary flare bursts.

---------------------------------------------------------
Title: On the Correlation between the Orientation of Moving Magnetic
Features and the Large-Scale Twist of Sunspots
Authors: Yurchyshyn, Vasyl B.; Wang, Haimin; Goode, Philip R.
2001ApJ...550..470Y Altcode:
We present new results on the nature of moving magnetic features
(MMFs) deduced from Big Bear Solar Observatory observations of the
longitudinal magnetic fields of two large solar spots. MMFs are small
magnetic bipoles that move outward across the moat of an eroding
sunspot. We find that MMFs are not randomly oriented. To wit, in 21
out of 28 (75%) MMF pairs, the magnetic element having the polarity
of the sunspot was located farther from the sunspot. Furthermore,
there is a correlation between the orientation of the bipole and that
of the twist in a sunspot. For the two nearly round sunpots we studied,
we found that the bipoles were rotated counterclockwise in the case of a
clockwise twisted sunspot and clockwise for a spot with counterclockwise
twist. We also found a correlation between the orientation of MMF
bipole and the amount of twist in the spot. The MMF bipoles around
the highly twisted sunspot are oriented nearly tangential to the edge
of the sunspot, while in the slightly twisted sunspot the bipoles are
oriented nearly radially so that they point back to the spot.

---------------------------------------------------------
Title: Small Magnetic Bipoles Emerging in a Filament Channel
Authors: Chae, Jongchul; Martin, Sara F.; Yun, H. S.; Kim, Junghoon;
Lee, Sangwoo; Goode, Philip R.; Spirock, Tom; Wang, Haimin
2001ApJ...548..497C Altcode:
Observations have shown that quiescent prominences or filaments have a
hemispheric magnetic pattern of chirality. Motivated by the question
of whether the filament chirality is of subsurface origin or not,
we have studied small magnetic bipoles that emerged in a quiescent
filament channel at latitude N45°. During our 5 day observing run,
performed in 1999 October, a huge filament erupted and reformed shortly
in the same filament channel. Using high-cadence, long-integration
line-of-sight magnetograms taken at Big Bear Solar Observatory, we
identified a total of 102 bipoles that showed an average total flux
of 2.8×1019 Mx, an average separation of 7400 km at the
time of full development, and an emergence rate of 430 hr-1
per the entire solar surface area. These properties indicate that most
of the bipoles are ephemeral regions. The most important finding in
the present study is that the magnetic axes of the bipoles emerging in
the filament channel are systematically oriented; a negative (trailing)
pole is observed to be located preferentially to the south-east of its
companion positive (leading) pole. This preferred orientation does not
match either the Hale law of active region orientation or a theory that
attributes the axial field of a filament to emerging bipoles. We propose
two possible subsurface field configurations of bipoles consistent with
the observed preferential orientation and discuss physical implications
of our results for understanding filament magnetic fields.

---------------------------------------------------------
Title: A Rapid Change in Magnetic Connectivity Observed Before
Filament Eruption and Its Associated Flare
Authors: Kim, Jung-Hoon; Yun, H. S.; Lee, Sangwoo; Chae, Jongchul;
Goode, Philip R.; Wang, Haimin
2001ApJ...547L..85K Altcode:
To gain insight to the cause of filament eruptions and flares on
the Sun, we observed a filament that erupted in active region NOAA
8597. The observations consisted of Hα filtergrams at three wavelengths
(line center and +/-0.5 Å) and line-of-sight magnetograms. All
were taken on 1999 June 24 at Big Bear Solar Observatory. We found
from the time sequence of Hα images that the filament eruption was
preceded by a rapid change in connectivity in a bundle of filament
threads. The thread bundle was initially sharply curved near its one
end of the filament and suddenly flipped and then became straight in
the new orientation. The flipped segment of the thread bundle swept
over a 100”×50” area on the solar surface in
about half an hour. At the latter stage of the connectivity change,
we observed a downward draining of material along the thread bundle
that had a transverse component of 50 km s-1. After that,
the filament body split into two parallel parts, one part erupted
while the other part remained, and the two-ribbon flare occurred. We
also found canceling magnetic features in the vicinity of the initial
location of the thread end, which displayed a flux decrease during the
Hα connectivity change. Our results show clear and direct evidence
that magnetic reconnection takes place in the low atmosphere prior to
eruption. This preeruption reconnection seems to be very different
from a posteruption coronal reconnection, which is believed to lead
to a two-ribbon flare.

---------------------------------------------------------
Title: Correlation of Microwave and Hard X-Ray Spectral Parameters
Authors: Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.
2000ApJ...545.1116S Altcode:
We present the analysis of 27 solar flares with multiple peaks that
were observed at hard X-ray and microwave wavelengths. A total of
57 simultaneous peaks were observed by BATSE (hard X-rays) and Owens
Valley Radio Observatory (microwaves). Throughout the duration of a
flare, its spectra at both wavelengths are fitted independently at all
times. The hard X-ray spectra were fitted by a single power law in most
cases, whereas the microwave spectra were fitted as gyrosynchrotron
emission. For each individual peak, the parameters at both wavelengths
(peak flux, turnover frequency, spectral indices, and delays between
hard X-ray and microwave peak emission) were then compared and
correlated. We have also studied impulsive and nonimpulsive bursts
individually. The main results obtained were as follows. (1) In 75% of
the bursts, the inferred index of the electron energy distribution of
the microwave-emitting electrons, δr, is harder than that
of the lower energy hard X-ray-emitting electrons, δX,
on average by 0.5-2.0. This implies that there is a breakup in the
energy spectra of the electrons, as is sometimes observed in the hard
X-ray spectra of giant flares. (2) A soft-hard-harder spectral index
temporal evolution is more commonly seen in the microwave spectra
(47%) than in the hard X-ray observations (32%) and in nonimpulsive
flares than in impulsive ones. (3) Delays larger than 2 s were observed
between the radio and hard X-ray peaks in 65% of the bursts, with the
delays decreasing as the hard X-ray energy increased. (4) Nonimpulsive
flares are more microwave rich, have higher delays between their radio
emission and the hard X-ray peaks, and display harder spectral indices
than impulsive bursts.

---------------------------------------------------------
Title: Ultraviolet and Hα Emission in Ellerman Bombs
Authors: Qiu, Jiong; Ding, Ming D.; Wang, Haimin; Denker, Carsten;
Goode, Philip R.
2000ApJ...544L.157Q Altcode:
We present the first high-cadence time profiles of Ellerman bombs
(EBs) at two wavelengths, 1.3 Å in the blue wing of the Hα line and
the UV continuum at 1600 Å, and study their temporal correlation. Our
results demonstrate that 46 out of 75 EBs exhibit a good correlation at
the two wavelengths with a correlation coefficient greater than 50%,
suggesting that a common energy release produces emission at the two
wavelengths. We also find that the EBs with strong Hα emission tend
to show a good Hα-UV correlation but that the weakly correlated or
noncorrelated EBs are usually weak in Hα emission. More than half of
the Hα-UV well-correlated EBs are located at the boundaries of unipolar
magnetic areas; the others are located at, or close to, the magnetic
inversion lines. However, the majority of the weakly or noncorrelated
EBs are located at the magnetic inversion lines. Our results suggest
that the physical mechanisms and the energy distributions are quite
different in different types of EBs and that heating in the photosphere
and temperature minimum region is very important for producing EBs. The
high-cadence observations of EBs also confirm unambiguously that the
light curves of EBs generally demonstrate a fast rise and a fast decay,
with an average e-fold rising/decaying time of about 1 minute, which
distinguishes EBs from the flare phenomenon.

---------------------------------------------------------
Title: Dynamical Characteristics of Small-Scale Hα Upflow Events
on the Quiet Sun
Authors: Lee, Chik-Yin; Chae, Jongchul; Wang, Haimin
2000ApJ...545.1124L Altcode:
Studies of small-scale dynamical features on the quiet Sun are crucial
for understanding the energetics and mass supply in the chromosphere
and corona. Recent works by Wang and coworkers revealed the existence
of numerous short-lived upflow events in Hα on the quiet Sun. In
this paper we examine in detail the dynamical characteristics of these
Hα upflow events based on Hα spectrograph observations and compare
them with those of elongated dark mottles, which are often considered
to be the disk counterpart of limb spicules. Our observations were
performed at the Big Bear Solar Observatory on 1997 August 26 using
the Littrow Spectrograph on the 65 cm reflector. We repeated drift
scans of a quiet Sun region, which produced a four-dimensional (x, y,
t, λ) data array. We examine the spectral, temporal, and morphological
characteristics of upflow events and determine their physical parameters
like size, lifetime, birthrate, line-of-sight velocity, Doppler width,
and optical thickness. Our results show that upflow events appear
different from dark mottles in that (1) most of the line profiles of
upflow events show absorption in the blue wing only, while those of
dark mottles show absorptions in both wings, (2) the typical optical
thickness of upflow events is smaller than that of dark mottles, and
(3) unlike dark mottles, upflow events do not have returning flows at
the same site. We discuss possible physical relationships among upflow
events, elongated dark mottles, and spicules as observed at the limb.

---------------------------------------------------------
Title: High-Cadence Observations of an Impulsive Flare
Authors: Wang, Haimin; Qiu, Jiong; Denker, Carsten; Spirock, Tom;
Chen, Hangjun; Goode, Philip R.
2000ApJ...542.1080W Altcode:
No abstract at ADS

---------------------------------------------------------
Title: High-Resolution Hα Observations of Proper Motion in NOAA 8668:
Evidence for Filament Mass Injection by Chromospheric Reconnection
Authors: Chae, Jongchul; Denker, Carsten; Spirock, Tom J.; Wang,
Haimin; Goode, Philip R.
2000SoPh..195..333C Altcode:
There have been two different kinds of explanations for the source
of cool material in prominences or filaments: coronal condensations
from above and cool plasma injections from below. In this paper, we
present observational results which support filament mass injection
by chromospheric reconnection. The observations of an active filament
in the active region NOAA 8668 were performed on 17 August 1999 at a
wavelength of Hα−0.6 Å using the 65 cm vacuum reflector, a Zeiss Hα
birefringent filter, and a 12-bit SMD digital camera of Big Bear Solar
Observatory. The best image was selected every 12 s for an hour based
on a frame selection algorithm. All the images were then co-aligned and
corrected for local distortion due to the seeing. The time-lapse movie
of the data shows that the filament was undergoing ceaseless motion. The
Hα flow field has been determined as a function of time using local
correlation tracking. Time-averaged flow patterns usually trace local
magnetic field lines, as inferred from Hα fibrils and line-of-sight
magnetograms. An interesting finding is a transient flow field in a
system of small Hα loops, some of which merge into the filament. The
flow is associated with a cancelling magnetic feature which is located
at one end of the loop system. Initially a diverging flow with speeds
below 10 km s−1 is visible at the flux cancellation
site. The flow is soon directed along the loops and accelerated up
to 40 km s−1 in a few minutes. Some part of the plasma
flow then merges into and moves along the filament. This kind of
transient flow takes place several times during the observations. Our
results clearly demonstrate that reconnection in the photosphere and
chromosphere is a likely way to supply cool material to a filament,
as well as re-organizing the magnetic field configuration, and, hence,
is important in the formation of filaments.

---------------------------------------------------------
Title: Hard x-ray and Microwave Flux Spectra of the 2 November 1991
Solar Flare
Authors: Lee, Chik-Yin; Wang, Haimin
2000SoPh..195..149L Altcode:
We analysed the hard X-ray and microwave flux spectra of the solar flare
(BATSE No. 1791) on 2 November 1991, which started at 16:11:03 UT and
ended at 16:56:10 UT. This flare is particularly interesting because of
its deep cyclic intensity modulation. Data are available simultaneously
from the 16-channel BATSE/LAD hard X-ray and 45-frequency OVRO microwave
database. We quantitatively compare the time variations in profiles
of the hard X-ray spectral photon index, the 50 keV X-ray flux, and
microwave spectral indices (at both high and low frequency ends). As
expected, the X-ray photon spectral index decreases as the hard X-ray
flux increases. This pattern appears in all the sub-peaks. This is
consistent with previous observations that hard X-ray emission hardens
at the emission peak. However, the behaviour of the high-frequency
microwave index is unexpected. We observe an anti-correlation between
the high-frequency microwave index and the hard X-ray photon index
during the course of the flare. Finally, we study the arrival time
of microwave flux peaks as a function of frequency and find that
the microwave peak at a higher frequency comes earlier than that at
a lower frequency. A maximum delay of 72 s is found among the main
peaks at different frequencies. Shorter delays are found for the other
five sub-peaks.

---------------------------------------------------------
Title: Comparison of the 1998 April 29 M6.8 and 1998 November 5
M8.4 Flares
Authors: Wang, Haimin; Goode, Philip R.; Denker, Carsten; Yang, Guo;
Yurchishin, Vasyl; Nitta, Nariaki; Gurman, Joseph B.; St. Cyr, Chris;
Kosovichev, Alexander G.
2000ApJ...536..971W Altcode:
We combined, and analyzed in detail, the Hα and magnetograph data
from Big Bear Solar Observatory (BBSO), full-disk magnetograms from
the Michelson Doppler Imager (MDI) on board Solar and Heliospheric
Observatory (SOHO), coronagraph data from the Large Angle Spectrometric
Coronagraph (LASCO) of SOHO, Fe XII 195 Å data from the Extreme
ultraviolet Imaging Telescope (EIT) of SOHO, and Yohkoh soft X-ray
telescope (SXT) data of the M6.8 flare of 1998 April 29 in National
Oceanic and Atmospheric Administration (NOAA) region 8375 and the
M8.4 flare of 1998 November 5 in NOAA region 8384. These two flares
have remarkable similarities:1. Partial halo coronal mass ejections
(CMEs) were observed for both events. For the 1998 April 29 event,
even though the flare occurred in the southeast of the disk center,
the ejected material moved predominantly across the equator, and the
central part of the CME occurred in the northeast limb. The direction
in which the cusp points in the postflare SXT images determines the
dominant direction of the CMEs.2. Coronal dimming was clearly observed
in EIT Fe XII 195 Å for both but was not observed in Yohkoh SXT for
either event. Dimming started 2 hr before the onset of the flares,
indicating large-scale coronal restructuring before both flares.3. No
global or local photospheric magnetic field change was detected from
either event; in particular, no magnetic field change was found in the
dimming areas.4. Both events lasted several hours and, thus, could be
classified as long duration events (LDEs). However, they are different
in the following important aspects. For the 1998 April 29 event,
the flare and the CME are associated with an erupting filament in
which the two initial ribbons were well connected and then gradually
separated. SXT preflare images show the classical S-shape sheared
configuration (sigmoid structure). For the 1998 November 5 event, two
initial ribbons were well separated, and the SXT preflare image shows
the interaction of at least two loops. In addition, no filament eruption
was observed. We conclude that even though these two events resulted
in similar coronal consequences, they are due to two distinct physical
processes: eruption of sheared loops and interaction of two loops.

---------------------------------------------------------
Title: Extreme-Ultraviolet Flare Loop Emissions in an Eruptive Event
Authors: Qiu, Jiong; Wang, Haimin; Chae, Jongchul; Goode, Philip R.
2000SoPh..194..269Q Altcode:
The TRACE/BBSO joint campaign on 27 September 1998 observed an eruptive
flare event which lasted for half an hour. The observation covered
several ultraviolet (UV) and extreme-ultraviolet (EUV) lines and Hα
center and off-band emissions with very high spatial resolution. We find
the EUV emissions in different stages of the flare display different
characteristics. (1) During the `pre-flare' phase, when the SXR output
was weak, we observed simultaneous impulsive HXR peak at 25-100
keV and strong EUV emission. (2) In the impulsive phase, when Hα,
UV and SXR emissions were rising to the maxima, the EUV emission was
very weak. (3) During the main phase, when SXR emission was decaying,
a peak in the EUV emission was observed which was substantially delayed
by 7 min compared to emissions from other wavelengths. Based on our
observations, we propose that the `pre-flare' phase in this event was
a separate energy release process rather than a mere pre-cursor of the
flare, and it is likely that the `pre-flare' EUV emission was due to
weak in situ heating of low-lying coronal loops. The mechanism of the
EUV emission in the main phase is investigated. It is suggested that
the delayed EUV emission may come from cooling of SXR loops.

---------------------------------------------------------
Title: Macrospicules Observed with Hα Against the Quiet Solar Disk
Authors: Zhang, Jun; Wang, Jingxiu; Lee, Chik-Yin; Wang, Haimin
2000SoPh..194...59Z Altcode:
High-resolution Hα filtergrams and deep magnetograms were obtained from
the Big Bear Solar Observatory (BBSO) and Huairou Solar Observation
Station (HSOS) during 17-24 October 1997. The three days (17, 18,
and 19) with the best image quality were selected for this initial
research. We have found that macrospicules are triggered by interaction
either between intranetwork and network elements or among several
network magnetic elements. We present a model to explain the spatial
relationship between macrospicules and magnetic fields.

---------------------------------------------------------
Title: Active Region Loops Observed with SUMER on Board the SOHO
Authors: Chae, Jongchul; Wang, Haimin; Qiu, Jiong; Goode, Philip R.;
Wilhelm, Klaus
2000ApJ...533..535C Altcode:
We study the emission and dynamical characteristics of transition region
temperature plasmas in magnetic loops by analyzing a high-resolution,
limb observation of the active region NOAA 7962. The observations were
performed by the Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) instrument on board the Solar and Heliospheric Observatory
(SOHO). The SUMER observation produced a set of raster scans of the
region, in the four lines, H I Lyβ λ1025, O VI λλ1032, 1038,
and C II λ1037. The data are used to construct intensity, velocity,
and line width maps of the active region, from which more than 10
well-resolved loops are identified and classified into four different
groups. We determine several physical parameters of the loops in each
group such as diameter, length, temperature, line-of-sight plasma
velocity, and nonthermal line broadening. Our results indicate that
both kinds of temperature variations exist in active region loops:
variations from loop to loop and variations along each loop. It is
also found that there is a distinction between stationary loops and
dynamic loops. The dynamic loops have large bulk motions and large
nonthermal line broadenings. Some of the dynamic loops display large
velocity shears with the sign of line-of-sight velocities changing
across the loop axes. These velocity shears appear to represent
rotational motions around the loop axes with velocities of up to 50 km
s-1. There are indications that nonthermal line broadening
is the result of magnetohydrodynamic turbulence inside the loops. Based
on our observations, we postulate that when loops erupt, some of the
kinetic and magnetic energy cascades down to turbulent energy which
would be dissipated as heat.

---------------------------------------------------------
Title: Interaction between network and intranetwork magnetic fields
Authors: Zhang, Jun; Wang, Jingxiu; Lee, Chik-Yin; Wang, Haimin
2000SoPh..192..415Z Altcode:
Using high-resolution observations of deep magnetograms and Hα
filtergrams obtained at Big Bear Solar Observatory during 17-24
October 1997, we have studied the interaction of intranetwork and
network elements. The relationship between small-scale magnetic fields
and active phenomena is investigated. Most of the small-scale active
phenomena are triggered by the interaction either between intranetwork
and network magnetic elements or among several network elements. The
energy released due to the interaction of intranetwork-network elements
and network-network elements is large enough to heat the corona.

---------------------------------------------------------
Title: Minifilament Eruption on the Quiet Sun. I. Observations at
Hα Central Line
Authors: Wang, Jingxiu; Li, Wei; Denker, Carsten; Lee, Chikyin; Wang,
Haimin; Goode, Philip R.; McAllister, Alan; Martin, Sara F.
2000ApJ...530.1071W Altcode:
The eruption of miniature filaments on the quiet Sun has been analyzed
from time sequences of digital Hα filtergrams obtained at Big Bear
Solar Observatory during 1997 September 18-24. The 2 days with the
best image quality were selected for this initial study. During
13 hr of time-lapse observations on these 2 days, in an effective
640”x480” area of quiet Sun close to the disk
center, 88 erupting miniature filaments were identified. On average,
these small-scale erupting filaments have a projected length of 19,000
km, an observed ejection speed of 13 km s-1, and a mean
lifetime of 50 minutes from first appearance through eruption. The
total mass and kinetic energy involved in a miniature filament
eruption is estimated to be 1013 g and 1025
ergs, respectively. They are distinguished from macrospicules by the
same criteria that large-scale filaments, before and during eruption,
are distinguished from surges. Prior to eruption, one end, both ends,
or the midsection of a miniature filament is superposed over a polarity
reversal boundary on line-of-sight magnetograms. We conclude that
miniature filaments are the small-scale analog to large-scale filaments.

---------------------------------------------------------
Title: Comparison of Transient Network Brightenings and Explosive
Events in the Solar Transition Region
Authors: Chae, Jongchul; Wang, Haimin; Goode, Philip R.; Fludra,
Andrzej; Schühle, Udo
2000ApJ...528L.119C Altcode:
The relation between transient network brightenings, known as blinkers,
and explosive events is examined based on coordinated quiet Sun
observations in the transition region line O V λ630 recorded by
the Coronal Diagnostic Spectrometer (CDS), in the transition region
line Si IV λ1402 recorded by the Solar Ultraviolet Measurements of
Emitted Radiation (SUMER) instrument, and in photospheric magnetograms
taken by the Big Bear Solar Observatory videomagnetograph. From these
observations, we find that (1) explosive events, which are traditionally
defined as features with very broad UV line profiles, tend to keep
away from the centers of network brightenings and are mostly located
at the edges of such brightenings, (2) CDS blinkers consist of many
small-scale, short-lived SUMER “unit brightening events” with a
size of a few arcseconds and a lifetime of a few minutes, and most
importantly (3) each SUMER unit brightening event is characterized by
a UV line profile that is not as broad as those of explosive events,
but still has significantly enhanced wings. Our results imply that,
like explosive events, individual unit brightening events involve high
velocities, and, hence, blinkers may have the same physical origin
as explosive events. It is likely that transient network brightenings
and explosive events are both due to magnetic reconnection--but with
different magnetic geometries.

---------------------------------------------------------
Title: Counter-streaming Mass Flow and Transient Brightening in
Active Region Loops
Authors: Qiu, Jiong; Wang, Haimin; Chae, Jongchul; Goode, Philip R.
1999SoPh..190..153Q Altcode:
An active region loop system was observed in a decaying active region
for three hours by TRACE and BBSO in a joint campaign on September
27, 1998. Continuous mass motion was seen in Hα offband filtergrams
throughout the three hours, and some UV loops were exhibited transient
brightenings. We find that: (1) cool material was flowing along the
loops at a speed of at least 20 km s−1. Further, in Hα red
and blue wings, we see mass motion along different loops in opposite
directions. This is the first report of a counter-streaming pattern
of mass motion in an Hα loop system. (2) Transient brightenings
of different UV loops at different times were observed at C iv 1550
Å. These brightened UV loops were located in the same region and at
the same altitudes as the Hα loops. The observations show a clear
correlation between the transient brightenings of UV loops and mass
motion in Hα loops. (3) Both footpoints of the loop system were
located in regions of mixed magnetic polarities. Frequent micro-flares
at one footpoint of the loops with small-scale brightenings spreading
along the loop leg were observed before the brightening and rising
of one C iv loop. Similar to the case of a filament, the continuous
mass motion along the loops seems important for maintaining the cool
Hα loop system at coronal height. There may be an indication that the
mass motion in cool Hα loops and the correlated transient brightening
of the active region loops were due to the small-scale chromospheric
magnetic reconnection at the footpoint regions of the loop system.

---------------------------------------------------------
Title: Studies of Microflares and C5.2 flare of 27 September 1998
Authors: Wang, Haimin; Chae, Jongchul; Qiu, Jiong; Lee, Chik-Yin;
Goode, Philip R.
1999SoPh..188..365W Altcode:
On 27 September 1998, Big Bear Solar Observatory (BBSO) and Transition
Region and Coronal Explorer (TRACE) coordinated observations from
16:00 to 19:00 UT to study properties of microflares in AR NOAA
No. 8340. Fortuitously, a C5.2 flare occurred at 16:30 UT in this active
region. Hα and magnetograph movies were obtained at BBSO; C iv 1550
Å, Fe ix 171 Å, and Fe xii 195 Å movies were obtained by TRACE;
both with a cadence about 1 min. In this paper, we concentrate on the
study of magnetic properties of 70 C iv microflares, as well as their
relationship to the C5.2 flare. We obtained the following results: (1)
We found two kinds of microflares: microflares of transient brightenings
with a time scale of 1 to 5 min (impulsive events) and microflares
lasting half an hour or longer (persistent events). Ninety percent of
the microflares are impulsive events. Most of the event in this category
are associated with well defined magnetic neutral lines, but some are
found in non-neutral line areas. All of seven persistent events are
found at parasitic magnetic configurations with inclusions of small
magnetic flux within dominant magnetic flux of opposite polarity. (2)
More than a third of the impulsive microflares occurred near the C5.2
flare site indicating that a local instability is responsible for both
the C5.2 flare and microflares. This indirectly supports the avalanche
theory of flare energy release, which implies that a big flare may be
spatially associated with many small flares.

---------------------------------------------------------
Title: The Polarity Distribution of Intranetwork and Network Fields
Authors: Zhang, Jun; Wang, Jingxiu; Deng, Yuanyong; Wang, Haimin
1999SoPh..188...47Z Altcode:
We have studied the relative polarity distribution of intranetwork
(IN) and network (NW) fields for the first time, using very deep
magnetograms obtained at Big Bear Solar Observatory (BBSO) and
Huairou Solar Observation Station (HSOS). We found 80 network cells
and measured the polarities of intranetwork and network magnetic flux
within each cell. The analysis reveals that, in enhanced networks, the
signed sum of the IN flux in a cell and the signed sum of the network
flux on the boundary of the cell is opposite with 90% probability; in
mixed-polarity network, the corresponding signed fluxes are opposite
with a probability of 75%. We suggest that:

---------------------------------------------------------
Title: Extreme-Ultraviolet Jets and Hα Surges in Solar Microflares
Authors: Chae, Jongchul; Qiu, Jiong; Wang, Haimin; Goode, Philip R.
1999ApJ...513L..75C Altcode:
We analyzed simultaneous EUV data from the Transition Region and Coronal
Explorer and Hα data from Big Bear Solar Observatory. In the active
region studied, we found several EUV jets that repeatedly occurred where
pre-existing magnetic flux was “canceled” by newly emerging flux of
opposite polarity. The jets look like Yohkoh soft X-ray jets, but are
smaller and shorter lived than X-ray jets. They have a typical size
of 4000-10,000 km, a transverse velocity of 50-100 km s-1,
and a lifetime of 2-4 minutes. Each of the jets was ejected from a
looplike bright EUV emission patch at the moment that the patch reached
its peak emission. We also found dark Hα surges that are correlated
with these jets. A careful comparison, however, revealed that the Hα
surges are not cospatial with the EUV jets. Instead, the EUV jets are
identified with bright jetlike features in the Hα line center. Our
results support a picture in which Hα surges and EUV jets represent
different kinds of plasma ejection--cool and hot plasma ejections along
different field lines--which must be dynamically connected to each
other. We emphasize the importance of observed flux cancellation and
a small erupting filament in understanding the acceleration mechanisms
of EUV jets and Hα surges.

---------------------------------------------------------
Title: Interaction Between Intranetwork and Network Magnetic Fields
Authors: Zhang, Jun; Wang, Jingxiu; Wang, Haimin; Lee, Chik-Yin
1999soho....9E..90Z Altcode:
Using high resolutional observations of Ha filtergrams and deep
magnetograms of Huairou Solar Observation Station (HSOS) and Big Bear
Solar Observatory (BBSO), we study the interaction of intranetwork
and network elements, and analyse the relationship between magnetic
fields and small-scale active phenomena (network bright points,
mini-filament eruption, macrospicules etc.). We find all the active
phenomena are triggered by the interaction between intranetwork and
network elements. All small-scale active phenomena are located at the
position that the flux change fast, not the position that the flux is
large or the flux density is high.

---------------------------------------------------------
Title: Comparison of Hα and He II λ304 Macrospicules
Authors: Wang, Haimin
1998ApJ...509..461W Altcode:
Simultaneous observations of limb macrospicules were carried out in
Hα by the Big Bear Solar Observatory (BBSO) and in He II λ304 by
the Extreme-Ultraviolet Imaging Telescope (EIT) on board SOHO. For
the first time, Hα and He II macrospicules are compared, with high
spatial and temporal resolution and image enhancement. Data were
obtained on 1996 October 6, 7, and 8. The target of the first and
last day was the north pole; on October 7, it was the quiet west
limb. BBSO uses a 12 bit digital camera to obtain high-resolution
Hα filtergrams at -0.65 Å, line center, and 0.65 Å. The pixel
resolution ranges between 0.17" and 0.33", and temporal resolution
ranges between 30 and 90 s. EIT images have a fixed pixel resolution
of 2.5" and temporal resolution between 1 and 7 minutes. We found
the following: within the common field of view of BBSO and EIT,
all 53 identified He II λ304 macrospicules have counterparts in
Hα. However, morphologies of He II λ304 and Hα macrospicules are
completely different. He II λ304 macrospicules are typically in the
form of an elongated ejection, whereas Hα macrospicules are either
looplike bright features or much shorter jets. In the polar region,
55 (over 50%) Hα macrospicules do not show any corresponding He II
λ304 macrospicules. As expected, He II λ304 macrospicules occur
much more frequently in the polar coronal hole area than in the
equatorial region. However, Hα macrospicules occur at an equal rate
in the pole and in the equatorial limb. Equatorial Hα macrospicules
tend to be confined because of their looplike structure and the lack
of jetlike structure. Based on common properties of different events,
we propose that the disk counterparts of macrospicules might be Hα -
1.0 Å jets or a subset of “explosive events.” If this assumption
is true, macrospicules are the results of magnetic reconnections,
likely due to the network-ephemeral region or network-intranetwork
interactions. We propose that magnetic reconnection occurs at about
the same rate in the polar coronal hole and in the quiet regions. Hα
macrospicules are direct manifestation of magnetic reconnection. He
II λ304 images detect substantially taller structures that are
substantially hotter. Because of dominant vertical line configuration
near the pole, reconnection tends to produce He II λ304 macrospicules;
in the noncoronal hole regions, He II macrospicules become undetectable
because of the inclined magnetic field configuration.

---------------------------------------------------------
Title: Comparison of Prominences in Hα and He II 304 Å
Authors: Wang, Haimin; Chae, Jongchul; Gurman, Joseph B.; Kucera,
Therese A.
1998SoPh..183...91W Altcode:
In this letter, we bring attention to prominences which show different
morphology in Hα and He ii 304 Å, as observed simultaneously by
BBSO and EIT on board SOHO. Those two lines have been thought to
represent similar chromospheric structures although they are formed at
significantly different temperatures. We give two examples representing
two kinds of anomaly: (1) prominences showing strong Hα emissions in
the lower part and strong He ii emissions in the upper part, and (2)
erupting prominences showing extensive He ii emission, but nothing in
Hα. Our results indicate that a part or the whole of a prominence may
be too hot to emit Hα radiation, possibly due to heating or thermal
instability. Please note that these are not just two isolated cases,
many other prominences show the similar differences in Hα and He ii
304 Å.

---------------------------------------------------------
Title: The evolution of intranetwork magnetic elements
Authors: Zhang, Jun; Lin, Ganghua; Wang, Jingxiu; Wang, Haimin;
Zirin, Harold
1998A&A...338..322Z Altcode:
We have studied the evolution of Intranetwork (IN) magnetic elements,
using a particularly good series of very deep magnetograms obtained at
Big Bear Solar Observatory. The magnetograms span an interval 10 hours
long and cover an area of 310x 240 arcsec(2) . We are able to follow
528 intranetwork elements from birth to death. The analysis reveals the
following results: (1). The appearance of IN elements can be classified
into the following categories: half of the total IN elements emerge
as clusters of mixed polarities somewhere within the network cells,
one fifth appear as ephemeral regions (tiny bipoles), one fifth result
from the merging of several elements of a given polarity, and one tenth
appear by fragmentation of larger elements. (2). IN elements disappear
in four ways: one third of total IN elements cancel with elements of
opposite polarity, one third decay into weak fields without apparent
interaction with other elements, one fourth merge with IN or network
elements of the same polarity, and one tenth split into smaller IN
elements below detecting limit. (3). About one ninth (one sixth) of
the IN elements merge (cancel) with network features, consequently,
part of the flux in network features is built up from former IN magnetic
flux, and part is eliminated by IN elements. The net effect of merging
and cancellation is a gradual reduction of the total flux of network
elements in the 10 hours observational interval. It seems that not all
the network magnetic flux is the remnant of active region magnetic flux.

---------------------------------------------------------
Title: Chromospheric Upflow Events Associated with Transition Region
Explosive Events
Authors: Chae, Jongchul; Wang, Haimin; Lee, Chik-Yin; Goode, Philip
R.; Schühle, Udo
1998ApJ...504L.123C Altcode:
Transition region explosive events are considered to be a manifestation
of small-scale magnetic reconnection ubiquitously occurring--even in
the quiet Sun. In this paper, we report a close association between
transition region explosive events and chromospheric upflow events seen
in Hα. From a comparison of the Big Bear Solar Observatory (BBSO)
Hα spectrograph data and the Solar and Heliospheric Observatory
(SOHO) / Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
data, we found a succession of chromospheric upflow events at sites
where repeated explosive events occurred. Individual chromospheric
events appear as compact dark features that are best visible in
Hα-0.5 Å and that have a size of 2"-3" and a lifetime of 1-2
minutes. They are characterized by an upward motion of 15-30 km
s-1, a temperature of 104 K, a mass density
of 1×10-13 g, and a nonthermal velocity less than 10
km s-1. Unlike spicules, which display descending motion
following their ascending phase, these upflow events are not followed
by noticeable redshifts. “Hα jets” at -1.0 Å studied by Wang et
al. appear to be a special case of this kind of chromospheric upflow
event. The physical characteristics of chromospheric upflow events and
their close association with transition region explosive events suggest
that chromospheric upflow events may be the manifestation of cool plasma
material flowing into magnetically diffusive regions, while explosive
events represent hot plasma material flowing out of the same regions.

---------------------------------------------------------
Title: High Spatial Resolution Observations of a Small δ Spot
Authors: Denker, Carsten; Wang, Haimin
1998ApJ...502..493D Altcode:
The Big Bear Solar Observatory (BBSO) has a long tradition of flare
observations. In this paper, we would like to direct the reader's
attention to observations of a small δ spot that produced a moderate
flare activity characterized by 18 C-class and 2 M-class flares. Active
region NOAA 8076 (BBSO 3877) was one of the first active regions in
the new solar cycle 23. We present for the first time high spatial
resolution white-light observations obtained on 1997 August 31
with the speckle masking technique to study mechanisms that trigger
flares. Almost diffraction-limited speckle reconstructions revealed
the complex and highly dynamical behavior of a small emerging δ
configuration in the central part of NOAA 8076. We found strong shear
flows and indications of strong transverse fields in the small δ
spot. The flare-producing mechanism for this small activity complex was
very similar to that of the outstanding flare-producing region NOAA 5395
of 1989 March however, on a completely opposite spatial scale. As an
important by-product, the speckle-interferometric techniques provided
information about the seeing quality at a site. We used the spectral
ratio technique to estimate the Fried parameter r0. We
measured a maximum Fried parameter of rmax0=10.3
cm and an average Fried parameter of r0 = 9.0 +/- 0.7 cm
in which the standard deviation reflects the temporal variations of
the seeing, indicating good seeing conditions during our observations.

---------------------------------------------------------
Title: The motion patterns of intranetwork magnetic elements
Authors: Zhang, Jun; Wang, Jingxiu; Wang, Haimin; Zirin, Harold
1998A&A...335..341Z Altcode:
By tracing individual elements, we have measured horizontal velocity
and studied motion patterns of Intranetwork (IN) magnetic elements for
the first time. The magnetograms obtained at Big Bear Solar Observatory
span an interval of 10-hour and cover an area of 310x 240 arc sec(2)
. In general, IN elements move radially and isotropically outwards from
emergence centers to boundaries of supergranule cells at first. However,
when they reach halfway between cell centers and boundaries, the motion
of IN elements is non-isotropic, there are prior directions. Most
of IN elements move towards the edges of network elements. There are
two components of the velocity fields: radial velocity and circular
velocity. &gt;From the centers to the boundaries of supergranule cells,
the magnitude of the radial velocity decreases gradually; but that of
the circular velocity increases obviously, at halfway between cell
center and boundary, the circular acceleration reachs the maximum,
about 10(-1) m s(-2) . The mean circular velocity near the boundary
is about 0.4 km s(-1) . The horizontal speeds deduced by tracing 768
intranetwork elements range from 0.05 km s(-1) to 0.8 km s(-1) with a
peak distribution at 0.4 km s(-1) . Both within the supergranule cells
and on the boundaries, there are convergence centers, but divergence
centers always exist within supergranule cells.

---------------------------------------------------------
Title: Photospheric Magnetic Field Changes Associated with Transition
Region Explosive Events
Authors: Chae, Jongchul; Wang, Haimin; Lee, Chik-Yin; Goode, Philip
R.; Schühle, Udo
1998ApJ...497L.109C Altcode:
From a comparison of the Solar and Heliospheric Observatory SUMER
spectral data and a time series of Big Bear Solar Observatory
magnetograms, we present observational clues to the physical origin of
transition region explosive events. First, explosive events rarely occur
in the interior of strong magnetic flux concentrations but rather are
preferentially found in regions with weak and mixed polarity fluxes that
display magnetic neutral lines. Second, the majority of explosive events
happen during the “cancellation” of photospheric magnetic flux. Third,
there is a strong tendency for explosive events to occur repeatedly, as
bursts, while local photospheric magnetic flux continuously decreases
because of cancellation. These results strongly support the idea that
transition region explosive events are a manifestation of magnetic
reconnection occurring in the quiet Sun. Furthermore, one may infer from
the third result that the explosive events represent repetitive fast
magnetic reconnections in the transition region, which are initiated
by slow magnetic reconnections occurring beneath.

---------------------------------------------------------
Title: Contrast of Faculae at 1.6 Microns
Authors: Wang, Haimin; Spirock, Thomas; Goode, Philip R.; Lee, Chikyin;
Zirin, Harold; Kosonocky, Walter
1998ApJ...495..957W Altcode:
We followed Active Region NOAA 7981 from 1996 July 27 to 1996 August
7 at Big Bear Solar Observatory. During the region's limb-to-limb
passage, images at 1.6 μm, 6103 Å, and CaK, as well as line-of-sight
magnetograms were obtained every day to study the variation of
facular/plage contrast and its relationship to magnetic fields. Our
1.6 μm images were observed by a high-quality 320 × 240 PtSi/Si
detector, which produces extremely uniform images. Our data agree with
the early results of Foukal et al. in several aspects: (1) at 1.6 μm,
some faculae are dark at solar disk center and all become bright when
they are close to the limb; (2) the changeover occurs approximately
at cos θ = 0.5-0.7 (3) the threshold of magnetic flux required to
produce a dark structure at 1.6 μm is about 5 × 1018 
Mx. Equally important, our result is different from that of Foukal et
al. on an important issue: among about 150 elements studied near the
disk center, only four of these IR dark faculae show no contrast in
the visible continuum. Other elements show dark contrasts in both 1.6
μm and the visible continuum, if there is sufficient resolution in
the data. However, darkening of weaker (lower flux) elements are more
obvious at IR and most bright points seen at red continuum disappear at
IR. These findings do confirm that 1.6 μm images reveal new aspects
of photospheric magnetic structure. In this paper, we also present a
quantitative relationship between the dark contrast of 1.6 μm faculae
and magnetic field strengths at the disk center, as well as the contrast
variation of IR faculae as a function of the disk position.

---------------------------------------------------------
Title: Lifetime of Intranetwork Magnetic Elements
Authors: Zhang, Jun; Lin, Ganghua; Wang, Jingxiu; Wang, Haimin;
Zirin, Harold
1998SoPh..178..245Z Altcode:
Using a 10-hour time sequence of very deep magnetograms of Big Bear
Solar Observatory, we have studied the lifetime of Intranetwork Magnetic
Elements for the first time. The analysis reveals the following results:

---------------------------------------------------------
Title: Study of Hα Jets on the Quiet Sun
Authors: Wang, Haimin; Johannesson, Anders; Stage, Mike; Lee, Chikyin;
Zirin, Harold
1998SoPh..178...55W Altcode: 1998SoPh..178..493W
High-speed jets of solar quiet regions have been observed at Big Bear
Solar Observatory in Hα−1.0 Ú, and compared with high-resolution
magnetograms. Over the whole Sun, the birthrate of the Hα−1.0 Ú
jets is about 19±3 events s−1, which is much lower than
the birthrate of spicules. The average lifetime of these jets is 2±1
min. Hα−1.0 Ú jets are very different from spicules, in the sense
of birthrate, lifetime, and shape. Jets tend to recur in the same sites,
always located in boundaries of supergranules. Under the best observing
conditions, we found that 80% of the major jet sites are associated
with converging magnetic dipoles - mainly the sites where intranetwork
elements are canceling with opposite polarity network elements. In order
to establish a possible relationship between the disk Hα jets and limb
macrospicules, we have also obtained time sequences of Hα center-line
images at the limb. These images are enhanced by median filtering so
that jet structures over the limb are easily studied. We found that
these limb Hα jets (above the spicule forest) repeatedly occur in
the same sites, which is the property shared by the disk Hα−1.0 Ú
jets. However, their mean lifetime is 10 min, substantially longer than
that of disk jets. Comparison with simultaneous SOHO/EIT Heii 304 Ú
images shows that every Heii 304 Ú jet over the limb coincides with an
Hα jet, although Heii 304 Ú jets extend much farther out. Some Hα
jets do not have associated He jets, probably due to the difference
in image resolutions. Hα spectra of selected jets are analyzed,
and we found that jets are not simply blue-shifted; instead, the
line profiles are broadened with significantly larger broadening on
the blue side. Two-component fitting finds that the velocity of the
blue-shifted component (an optically-thin component) is around 20 to
40 km s−1.

---------------------------------------------------------
Title: Filament Disappearances During the Period of September 1991
through September 1994
Authors: Wang, Haimin; Komenda, Arkadiusz E.; Tang, Frances; Zirin, Hal
1998SoPh..178..109W Altcode: 1998SoPh..178..547W
Continuous full-disk Hα images recorded by the Big Bear Solar
Observatory (BBSO) from 1 September 1991 to 19 September 1994 (the
first three years of Yohkoh mission) were digitized and analyzed. The
data set consists of nearly 10000 Hα images, one every half hour for
the period when the BBSO was observing. Two statistical studies of the
disappearing solar filaments based on this set of data are made: (1)
The disk latitude distribution of all larger disappearing filaments
with a minimum length of 70 arc sec, including the time of their
disappearance. Of the 1095 such filaments, 439 disappeared during our
continuous observations, 314 disappeared during the BBSO night gap,
162 disappeared during data gap (more than 94 hours) and 180 rotated
beyond the west limb. If we plot latitudes as a function of time for
all these disappeared filaments, it shows a uniform distribution in
latitude. However, if we plot the distribution of larger disappeared
filaments (200 arc sec or above), then the butterfly trend appears -
position of filaments tends to drift to lower latitude as solar activity
decreases. (2) The disk distribution of all detectable disappearing
filaments, large and small, for the 9-months period, January 1994 to
September 1994. We find that the size distribution of 351 collected
disappeared filaments follows a power law with a power index of −1.40.

---------------------------------------------------------
Title: Synoptic Observing Programs at Big Bear Solar Observatory
Authors: Wang, Haimin; Goode, Philip R.
1998ASPC..140..497W Altcode: 1998ssp..conf..497W
No abstract at ADS

---------------------------------------------------------
Title: Analyses of Vector Magnetograms in Flare-Productive Active
Regions
Authors: Wang, Haimin
1997SoPh..174..163W Altcode:
This paper reviews studies of the relationship between the evolution of
vector magnetic fields and the occurrence of major solar flares. Most of
the data were obtained by the video magnetograph systems at Big Bear
Solar Observatory (BBSO) and Huairou Solar Observatory (HSO). Due
to the favorable weather and seeing conditions at both stations,
high-resolution vector magnetograph sequences of many active regions
that produced major flares during last solar maximum (1989-1993) have
been recorded. We have analyzed several sequences of magnetograms
to study the evolution of vector magnetic fields of flare productive
active regions. The studies have focused on the following three aspects:
(1) processes which build up magnetic shear in active regions; (2)
the pre-flare magnetic structure of active regions; and (3) changes of
magnetic shear immediately preceding and following major flares. We
obtained the following results based on above studies: (1) Emerging
flux regions (EFRs) play very important roles in the production of
complicated photospheric flow patterns, magnetic shear and flares. (2)
Although the majority of flares prefer to occur in magnetically sheared
regions, many flares occurred in regions without strong photospheric
magnetic shear. (3) We found that photospheric magnetic shear increased
after all the 6 X-class flares studied by us. We want to emphasize
that this discovery is not contradictory to the energy conservation
principle, because a flare is a three-dimensional process, and the
photosphere only provides a two-dimensional boundary condition. This
argument is supported by the fact that if two initial ribbons of a
flare are widely separated (which may correspond to a higher-altitude
flare), the correlation of the flare with strong magnetic shear is
weak; if the two ribbons of a flare are close (which may correspond
to a lower-altitude flare), its correlation with the strong shear is
strong. (4) We have analyzed 18 additional M-class flares observed
by HSO in 1989 and 1990. No detectable shear change was found for all
the cases. It is likely that only the most energetic flares can affect
the photospheric magnetic topology.

---------------------------------------------------------
Title: Imaging the Chromospheric Evaporation of the 1994 June 30
Solar Flare
Authors: Silva, Adriana V. R.; Wang, Haimin; Gary, Dale E.; Nitta,
Nariaki; Zirin, Harold
1997ApJ...481..978S Altcode:
We analyze simultaneous Hα images (from the Big Bear Solar
Observatory), soft and hard X-ray images and spectra (from the soft
X-ray telescope [SXT], the Bragg Crystal Spectrometer [BCS], and the
hard X-ray telescope [HXT] on Yohkoh), and radio time profiles (from
the Owens Valley Radio Observatory) during the first 3 minutes of the
1994 June 30 flare. The strong blueshifts observed in the Ca XIX soft
X-ray line are interpreted as evidence of chromospheric evaporation,
with maximum up-flow velocities occurring 2 minutes prior to the hard
X-ray emission peak. In this study, we search for moving sources in
Hα, soft and hard X-ray images that correspond to the blueshifted
component. The chromospheric evaporation in this flare is divided
into two phases: an early phase with up-flow velocities of 350-450
km s-1, and a later phase (during the hard X-ray peak)
characterized by velocities of 100-200 km s-1. During the
first chromospheric evaporation phase, the footpoints of a loop seen
in HXT low-energy maps are seen to move toward the loop-top source. No
source displacement is observed in SXT images at this time. Images
of the later phase of chromospheric evaporation show a change in
the source morphology. The early HXT loop is no longer visible, and
HXT maps during this time display the two footpoints of a new loop
visible in SXT images. Now the HXT sources are stationary, and a SXT
footpoint source is seen to move toward the loop top. We interpret
the observed displacement of footpoint sources in HXT (early phase)
and SXT (later phase) maps to be the images of the evaporating front
projected onto the solar disk, while the up-flow velocities (inferred
from the blueshifts) are due to the movement of the same evaporating
material along the line of sight. By combining the up-flow velocities
with the proper motion of the footpoint sources seen in the maps,
we constructed a three-dimensional view of the magnetic loop for each
chromospheric evaporation phase. The early loop is almost semicircular,
with a height of 1.7 × 109 cm, whereas the later magnetic
loop is more elongated (a height of 3.2 × 109 cm), with
its apex closer to the footpoint where most of the evaporation took
place. The implications of these magnetic configurations and the
distinct evaporation phases are discussed.

---------------------------------------------------------
Title: New Digital Magnetograph at Big Bear Solar Observatory
Authors: Wang, Haimin; Denker, Carsten; Spirock, Thomas; Yang, Shu;
Goode, Philip
1997SPD....28.1503W Altcode: 1997BAAS...29..919W
A new magnetograph system has been installed and tested at Big Bear
Solar Observatory. The system uses part of BBSO's existing VMG
system: a quarter wave plate, a Ferro-Electric Liquid Crystal to
switch polarizations, and a 0.25A bandpass Zeiss filter tuned at CaI
6103A. A 256 by 256 12-bit Dalsa camera is used as the detector and
as the driver to switch the liquid crystal. The data rate of the
camera is 90 frames/s. The camera is interfaced by a Pentium-166
with a Mutech imaging board for data acquisition and analyses. The
computer has 128mb of ram, up to 700 live images can be stored in the
memory for a quick post-exposure image processing (image selection and
alignment). We have improved the sensitivity and spatial resolution
significantly over the old BBSO VMG system for the following reasons:
(1) new digital image data is in 12 bits while the video signal is below
8 bits. Polarizations weaker than 1% can not be detected by a single
pair subtraction in the video system. The digital system can detect a
polarization signal below 0.1% by a single pair subtraction. (2) Data
rate of the digital system is 90 frames/s, that of the video system
is 30 frames/s. So the time difference between two polarizations is
reduced in the new system. Under good seeing conditions, the data rate
of 90 frames/s ensures that the wavefront distortions are "frozen"
and approximately the same for the left and right circular polarized
image pairs. (3) Magnetograms are constructed after image selection and
alignment. The same system has potential for further imaging processing,
e.g. image de-stretch, and speckle interferometry. Preliminary results
will be presented at the meeting.

---------------------------------------------------------
Title: Filament Eruptions During the Period of Sept. 1991 through
Sept. 1994
Authors: Komenda, Arkadiusz E.; Wang, Haimin; Tang, Frances; Zirin, Hal
1997SPD....28.0103K Altcode: 1997BAAS...29..879K
From Sept. 1, 1991 to Sept 12, 1994, full disk Hα images were recorded
continuously at Big Bear Solar Observatory (BBSO) by super-VHS video
recorder. We digitized one image every half hour for that entire
period when the BBSO data are available. About 10,000 halpha images
are archived online, so anybody in the community can have an access
to such an archive, which are useful in studying the evolution of
solar active regions, flares and filament eruptions. In particular,
we are interested in the statistical properties of solar filaments
based on this set of data. In this paper, we present a complete list of
filament disappearance. In the first table, we list the disk location,
size and the time of disappearance of all significant filaments with
a minimum length of 70.1 arcsec. In the second table, we list above
properties of all the detectable filaments for the period of Jan. 1994
to Sept. 1994. The filaments disappear in four different ways: (1)
erupt during continuous observations, so its disappearance time is
accurate to within half hour. (2) disappear during the BBSO night gap
(12 to 14 hours); (3) disappear during date gap (most likely due to
bad weather) and (4) rotate beyond the West limb.

---------------------------------------------------------
Title: Study of Chromospheric Jets
Authors: Lee, Chik-Yin; Johannesson, Anders; Wang, Haimin; Stage,
Mike; Zirin, Hal
1997SPD....28.1305L Altcode: 1997BAAS...29Q.917L
In this paper we present a new fast spectral scan method to study Hα
jets, which are more energetic than regular spicules. The observations
achieved high two-dimensional spatial resolution, temporal resolution
and spectral resolution simultaneously. We used the Coude spectrograph
in the Big Bear Solar Observatory for the Hα data acquisition. The
procedure starts with fast spectral scans across the solar surface
(600" by 300") at the vicinity of the solar disk center. The guiding
system is turned off allowing the Sun to drift smoothly relative to
the spectrograph system. A total number of 600 spectral images at
6563A are recorded per scan on an optical disk recorder at a rate of
30 frames/sec. Tracking re-position the system to point to the same
initial coordinates on the Sun for new scans. The process repeats every
30 seconds until 60 scans are completed. The stored images are digitized
for off-line data analysis. A number of different image processing
techniques are used to improve the image quality. These include off-line
correlation tracking and running average among various constructed
images. From the four-dimensional (x,y,t,wavelength) Hα data arrays,
we can reconstruct spectroheliograms (600 by 240 pixels) at different
wavelengths. We calculate velocities of jets by measuring Doppler
shifts of the Hα absorption line, where we used an optically thin jet
model. Velocities of 30 to 50 km/sec for jets are derived. Velocity
maps are constructed thereafter for the scanned region, and are
compiled as movies. We measure the lifetime and size of the jets
from these movies. Lifetimes of a few minutes are observed. Further
approximations are made to find out temperature and density of jets
by fitting the Hα absorption line profile. The results will enable
us to estimate the total mechanical energy released by the jets.

---------------------------------------------------------
Title: Study of Quiet Sun Magnetic Fields and Chromospheric Jets
Authors: Wang, Haimin
1996AAS...188.3311W Altcode: 1996BAAS...28R.869W
Using best data sets of BBSO magnetograph and Hα observations on the
quiet Sun, we carefully studied the properties and flow patterns of
network and intranetwork magnetic fields and associated chromospheric
jets. We have obtained the following results: (a) We studied the data
for a quiet region with area of 300"x235", more than 2500 intranetwork
elements and 500 network elements are identified. Magnetic flux is
measured for each magnetic element from the magnetogram taken at the
best seeing. The flux of intranetwork elements falls in the range
from the detection limit, 10(16) Mx, to approximate 2x 10(18) Mx. The
peak distribution locates at the flux of (6 - 8)*E(16) Mx. (2) Using
the local correlation tracking techniques, we derived the horizontal
velocity fields of intranet and network magnetic fields. They consist of
two components: (a) the radial divergence flows which move intranetwork
fields from the network interior to the boundaries, and (b) the lateral
flows which move along the network boundaries and converge toward
stronger magnetic elements. Based on the number densities and flow
velocities of IN fields derived, we estimate that the lower limit of
total energy released from the recycling of intranetwork fields is 1.2x
10(28) ergs/s, which is comparable to the energy required for coronal
heatings. (3) Although we can not establish the correlation between
spicules and magnetic dipoles, strong correlation between Hα -1.0
Angstroms jets and magnetic dipoles has been found. This correlation
reveals reconnection nature of the jets. Based on the count rates of
these jets, they may play an important role in the generation of solar
wind and coronal heating.

---------------------------------------------------------
Title: Flares and separatrices between magnetic loops.
Authors: Zhi, Zhongxian; Wang, Jingxiu; Wang, Haimin
1995ChA&A..19..469Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Flux distribution of solar intranetwork magnetic fields
Authors: Wang, Jingxiu; Wang, Haimin; Tang, Frances; Lee, Jeongwoo W.;
Zirin, Harold
1995SoPh..160..277W Altcode:
Big Bear deep magnetograms of June 4, 1992 provide unprecedented
observations for direct measurements of solar intranetwork (IN) magnetic
fields. More than 2500 individual IN elements and 500 network elements
are identified and their magnetic flux measured in a quiet region
of 300 × 235 arc sec. The analysis reveals the following results:
IN element flux ranges from 1016 Mx (detection limit)
to 2 × 1018 Mx, with a peak flux distribution of 6 ×
1016 Mx.

---------------------------------------------------------
Title: High-Resolution Observation of Disk Spicules. I. Evolution
and Kinematics of Spicules in the Enhanced Network
Authors: Suematsu, Yoshinori; Wang, Haimin; Zirin, Harold
1995ApJ...450..411S Altcode:
We present measurements of a 75 minute sequence of CCD spicule
observations at Hα -0.65 Å, line center, and +0.65 Å. The
observations were made in a region of enhanced network near disk
center, where most spicules are longer and tilted further from the
vertical than those in truly quiet Sun. Images were reregistered with
a correlation coefficient greater than 0.9. We identify the spicules
as those elongated jets that radiate from elements of the magnetic
network and are the main chromospheric features seen in the wing of
Hα. Doppler images produced by red-blue subtraction show an upward
radial velocity during the extension phase and a downward velocity
during contraction for most spicules. Therefore, the spicules are
truly moving up and down. There were a few plagelike cells filled
with weak Hα emission, weak magnetic fields, and no spicules. We
also found many multiple spicules. The data are presented with
commentary on the accompanying videotape. We analyzed the
data to understand the spicule lifetimes and trajectories. We found
that the entire set of wavelengths and Dopplergrams was required to
separate overlapping spicules. Seventy-six of the 96 spicules studied
appear in complete upward and downward trajectories The evidence on
motions is not conclusive. Some proper motions are well represented
by ballistic trajectories with initial injection velocity about 40 km
s-1 for highly inclined spicules. The small decelerations
would require the spicules to be tilted typically 60°-70° from
the line of sight. Since limb observations favor tilts around 30°,
our observations must favor spicules tilted greatly from the radial,
as one finds in these enhanced field regions. The positive correlation
of lifetimes with projected lengths supports this model. However, the
Dopplergrams show that the entire spicule rises and falls as a whole,
which favors a fountain jet or some acceleration in the flux tube. The
downward trajectory may be slightly offset toward the network center
from the upward path. Bright points often appear at the bases of
spicules at Hα -0.65 Å, but during the peak extension or receding
phase of the spicule rather than the beginning; therefore, the spicule
is not a surgelike phenomenon (surges are usually initiated by Hα
brightening or a subflare, invariably in a bipolar feature). Further,
the magnetic elements, which match the Hα bright points exactly,
show no change associated with the brightening. The geometry creates
a problem for coronal heating models, as the spicule tops are at least
10" displaced from the bright-point bases, but no such offset appears
in the K line, for example. There is some evidence that the spicule
is generated several hundred kilometers above the photosphere.

---------------------------------------------------------
Title: Flares and separatrices between magnetic loops.
Authors: Shi, Zhongxian; Wang, Jingxiu; Wang, Haimin
1995AcASn..36..181S Altcode: 1995AcASn..36..181Z; 1995AcASn..36..187S
Based on the continuous observations of vector magnetic fields of NOAA
7469 from 4 to 12 April 1993, for the first time, the authors have
identified the magnetic loop systems and relevant separatrices for such
an active region. The observational signature of the cross-section
of separatrices on the photosphere is given. During the observed
period, flare activity took place repeatedly in the vicinity of the
separatrices.

---------------------------------------------------------
Title: Observations of Vector Magnetic Fields in Flaring Active
Regions
Authors: Chen, Jimin; Wang, Haimin; Zirin, Harold; Ai, Guoxiang
1994SoPh..154..261C Altcode:
We present vector magnetograph data of 6 active regions, all of
which produced major flares. Of the 20 M-class (or above) flares,
7 satisfy the flare conditions prescribed by Hagyard (high shear and
strong transverse fields). Strong photospheric shear, however, is not
necessarily a condition for a flare. We find an increase in the shear
for two flares, a 6-deg shear increase along the neutral line after a
X-2 flare and a 13-deg increase after a M-1.9 flare. For other flares,
we did not detect substantial shear changes.

---------------------------------------------------------
Title: The Roots of Coronal Structure in the Sun's Surface
Authors: Golub, Leon; Zirin, Harold; Wang, Haimin
1994SoPh..153..179G Altcode:
We have compared the structures seen on X-ray images obtained by a
flight of the NIXT sounding rocket payload on July 11, 1991 with
near-simultaneous photospheric and chromospheric structures and
magnetic fields observed at Big Bear. The X-ray images reflect
emission of both MgX and FeXVI, formed at 1 × 106 K
and 3 × 106 K, respectively. The brightest Hα sources
correspond to a dying sub-flare and other active region components,
all of which reveal coronal enhancements situated spatially well
above the Hα emission. The largest set of X-ray arches connected
plages of opposite polarity in a large bipolar active region. The
arches appear to lie in a small range of angle in the meridian plane
connecting their footpoints. Sunspots are dark on the surface and in
the corona. For the first time we see an emerging flux region in X-rays
and find the emission extends twice as high as the Hα arches. Many
features which we believe to correspond to `X-ray bright points'
(XBPs) were observed. Whether by resolution or spectral band, the
number detected greatly exceeds that from previous work. All of the
brighter XBPs correspond to bipolar Hα features, while unipolar Hα
bright points are the base of more diffuse comet-like coronal arches,
generally vertical. These diverge from individual features by less than
30°, and give a good measure of what the `canopies' must do. The Hα
data shows that all the Hα features were present the entire day, so
they are not clearly disappearing or reappearing. We find a new class
of XBPs which we call `satellite points', elements of opposite polarity
linked to nearby umbrae by invisible field lines. The satellite points
change rapidly in X-ray brightness during the flight. An M1.9 flare
occurred four hours after the flight; examination of the pre-flare
structures reveals nothing unusual.

---------------------------------------------------------
Title: Vector Magnetic Field Changes Associated with X-Class Flares
Authors: Wang, Haimin; Ewell, M. W., Jr.; Zirin, H.; Ai, Guoxiang
1994ApJ...424..436W Altcode:
We present high-resolution transverse and longitudinal magnetic field
measurements bracketing five X-class solar flares. We show that the
magnetic shear, defined as the angular difference between the measured
field and calculated potential field, actually increases after all
of these flares. In each case, the shear is shown to increase along a
substantial portion of the magnetic neutral line. For two of the cases,
we have excellent time resolution, on the order of several minutes,
and we demonstrate that the shear increase is impulsive. We briefly
discuss the theoretical implications of our results.

---------------------------------------------------------
Title: Flux Emergence and Umbra Formation after the X-9 Flare of
1991 March 22
Authors: Wang, Haimin; Tang, Frances
1993ApJ...407L..89W Altcode:
Vector magnetograms, H-alpha, D3, and white-light filtergrams were
obtained at the Big Bear Solar Observatory immediately after the X-9
flare on March 22, 1991. These observations show that the umbral area
increased by 2 x 10 exp 7 sq km, together with a magnetic flux increase
of 2 x 10 exp 20 Mx. The magnetic shear increased by 40 deg along the
neutral line. It is indicated, by the study of the evolution of spot
and magnetic structure of the March 22, 1991 region, that a pair of new
umbrae emerged suddenly on either side of the neutral line coinciding
with the shear increase immediately after the flare.

---------------------------------------------------------
Title: Strong transverse fields in δ-spots
Authors: Zirin, Harold; Wang, Haimin
1993SoPh..144...37Z Altcode:
Spectroscopic measurements of the strength and direction of transverse
magnetic fields in six δ-spots are presented. The field direction is
determined by the relative strength of the π- and σ-components at
different polarizer orientations, and is, with one exception, parallel
to the neutral line and as strong as the umbral field. Field strengths
determined by line splitting are as high as 3980 G.

---------------------------------------------------------
Title: The magnetic and velocity fields of solar active regions
Authors: Zirin, Harold; Ai, Guoxiang; Wang, Haimin
1993ASPC...46.....Z Altcode: 1993mvfs.conf.....Z; 1993IAUCo.141.....Z
Various papers on the magnetic and velocity fields of solar active
regions are reported. The general topics addressed include: structure
of active regions, theory of active region structure, techniques of
magnetic field measurements, the relation of the quiet sun to active
regions, fields in the chromosphere and corona, flares and transients,
magnetic shear and electric currents, structure and role of emerging
flux regions, convections and oscillation in active regions.

---------------------------------------------------------
Title: Lifetimes of the Magnetic Network Elements
Authors: Liu, Yang; Zhang, Hongqi; Wang, Haimin
1993ASPC...46..228L Altcode: 1993mvfs.conf..228L; 1993IAUCo.141..228L
No abstract at ADS

---------------------------------------------------------
Title: Flows; Evolution of Magnetic Fields; and Flares (Invited)
Authors: Wang, Haimin
1993ASPC...46..323W Altcode: 1993mvfs.conf..323W; 1993IAUCo.141..323W
No abstract at ADS

---------------------------------------------------------
Title: Joint vector magnetograph observations at BBSO, Huairou
Station and Mees Solar Observatory
Authors: Wang, Haimin; Varsik, John; Zirin, Harold; Canfield, Richard
C.; Leka, K. D.; Wang, Jingxiu
1992SoPh..142...11W Altcode:
Joint vector magnetograph observations were carried out at Big Bear
Solar Observatory (BBSO), Huairou Solar Observing Station (Huairou),
and Mees Solar Observatory (MSO) in late September 1989. Comparisons of
vector magnetograms obtained at the three stations show a high degree
of consistency in the morphology of both longitudinal and transverse
fields. Quantitative comparisons show the presence of noise, cross-talk
between longitudinal field and transverse field, Faraday rotation
and signal saturation effects in the magnetograms. We have tried to
establish how the scatter in measurements from different instruments
is apportioned between these sources of error.

---------------------------------------------------------
Title: Evolution of Magnetic Fields and Mass Flow in a Decaying
Active Region
Authors: Zhang, Hongqi; Ai, Guoxiang; Wang, Haimin; Zirin, Harold;
Patterson, Alan
1992SoPh..140..307Z Altcode:
Five days of coordinated observation were carried out
from 24-29 September, 1987 at Big Bear and Huairou Solar
Observatories. Longitudinal magnetic fields of an αp sunspot active
region were observed almost continuously by the two observatories. In
addition, vector magnetic fields, photospheric and chromospheric Doppler
velocity fields of the active region were also observed at Huairou
Solar Observatory. We studied the evolution of magnetic fields and
mass motions of the active region and obtained the following results:
(1) There are two kinds of Moving Magnetic Features (MMFs). (a) MMFs
with the same magnetic polarity as the center sunspot. These MMFs
carry net flux from the spot, move through the moat, and accumulate at
the moat's outer boundary. (b) MMFs in pairs of mixed polarity. These
MMFs are not responsible for the decay of the spot since they do not
carry away the net flux. MMFs in category (b) move faster than those
of (a). (2) The speed of the mixed polarity MMFs is larger than the
outflow measured by photospheric Dopplergrams. The uni-polar MMFs
are moving at about the same speed as the Doppler outflow. (3) The
chromospheric velocity is in approximately the opposite direction from
the photospheric velocity. The photospheric Doppler flow is outward;
chromospheric flow is inward. We also found evidence that downward
flow appears in the photospheric umbra; in the chromosphere there is
an upflow.

---------------------------------------------------------
Title: Evolution of vector magnetic fields and the August 27 1990
X-3 flare
Authors: Wang, Haimin
1992SoPh..140...85W Altcode:
This paper studies the evolution of vector magnetic fields in the
active region Boulder No. 6233 during an 11-hour observing period and
its relationship to an X-3 flare on August 27, 1990.

---------------------------------------------------------
Title: Flows around Sunspots and Pores
Authors: Wang, Haimin; Zirin, Harold
1992SoPh..140...41W Altcode:
We report on three sequences of high-resolution white-light and
magnetogram observations obtained in the summer of 1989. The duration
of sub-arcsecond seeing was three to four hours on each day. Study of
the white-light and magnetogram data yields the following results:
For all but one of the sunspots we have observed, both dark fibrils
and bright grains in the inner part of the penumbra of sunspots move
toward the umbra with a speed of about 0.5 km s-1. In the
outer part of the penumbra, movement is away from the umbra. The one
exception is a newly formed spot, which has inflow only in its penumbra.

---------------------------------------------------------
Title: Detection of “Invisible Sunspots”
Authors: Zirin, Harold; Wang, Haimin
1992ApJ...385L..27Z Altcode:
A new CCD system is used to detect tiny sunspots, which are termed
'micropores', associated with elements of the magnetic network far
from active regions. The smallest micropores detected are less than 1
arcsec in diameter and about half the size of the associated magnetic
feature. It is found that the pore size is systematically smaller
than the size of magnetic element seen in the magnetograms, but the
ratio of pore size versus magnetic element size is independent of pore
size, suggesting that the difference is real. For the smaller elements
there is a good linear relation between the brightness deficit of the
pore and the total flux of the magnetic element. The micropores are
distinguishable from dark lanes in the granulation by their long life,
large brightness deficit, and association with magnetic fields.

---------------------------------------------------------
Title: In Situ Disappearance of 6.Sunspots
Authors: Wang, Haimin
1992ASPC...27...97W Altcode: 1992socy.work...97W
No abstract at ADS

---------------------------------------------------------
Title: Motions, Fields, and Flares in the 1989 March Active Region
Authors: Wang, Haimin; Tang, Frances; Zirin, Harold; Ai, Guoxiang
1991ApJ...380..282W Altcode:
The results of observations of NOAA AR 5395 are presented. The region
was observed every day from limb to limb for significant periods, and
nine of the ten class-X flares were recorded. The region was found to
be a great Delta group, dominated by spots of following (f) polarity,
which moved rapidly westward, producing large changes in magnetic
structure which increased the shear and led to great flares. Aside
from its great size, the region was unusual in that normally p spots
dominate and move westward. In this case there was a 4:1 flux imbalance;
80 percent of the flux measured was of following polarity. The major
following spot in the region was found to move with a near-constant
acceleration, eventually reaching 0.25 km/s. Rapid spot motion was
discovered in all other superactive regions. Small p and f spots move
out from either side of the large f spot, and curl around it in curved
trajectories. The moving penumbral material coalesces into new umbrae.

---------------------------------------------------------
Title: Polar Fields during the Rising Phase of SOLAR-CYCLE-22
Authors: Tang, Frances; Wang, Haimin
1991SoPh..132..247T Altcode:
High-resolution magnetograph observations of the polar magnetic
fields have been obtained at intervals of time since the end of 1986
at Big Bear Solar Observatory. The Big Bear data differ from the
low-resolution, full-disk magnetograph observations in that the 2 arc
sec resolution makes it possible to resolve concentrated field upward
of 100 G. The purpose of this ongoing observation is to examine the
evolution of polar fields during the expected polarity reversal as
cycle 22 passes its maximum phase, and secondly, to study the polar
magnetic field: its true field strength, distribution, and how it
compares to other parts of the quiet Sun.

---------------------------------------------------------
Title: Magnetic Flux Transport of Decaying Active Regions and Enhanced
Magnetic Network
Authors: Wang, Haimin; Zirin, Harold; Ai, Guoxiang
1991SoPh..131...53W Altcode:
Several series of coordinated observations on decaying active regions
and enhanced magnetic network regions have been carried out jointly at
Big Bear Solar Observatory (BBSO) and Huairou Solar Observing Station of
the Bejing Astronomical Observatory in China. The evolution of magnetic
fields in several regions was followed closely for 3 to 7 days. The
transport of magnetic flux from the remnants of decayed active regions
was studied. Three related topics are included in this paper. (1)
We studied the evolution and lifetime of the magnetic network which
defines the boundaries of supergranules. The results are consistent with
our earlier studies: network cells have an average lifetime of about
70 hours; 68% of new cells appeared by growing from a single network
magnetic element; 50% of decaying cells disappeared by contracting
to a network element. (2) We studied the magnetic flux transport in
an enhanced network region in detail, and found the diffusion rate
to be negative, i.e., there was more flux moving towards the decayed
active region than away from it. We found several other cases where the
magnetic diffusion rate does not agree with Leighton's model. The slow
diffusion rate is likely due to the fact that the average velocity of
larger magnetic elements, which carry most of the magnetic flux, is
less than 0.1 km s−1; their average lifetime is longer
than 100 hours. (3) We briefly described some properties of Moving
Magnetic Features (MMFs) around a sunspot (detailed discussion on
MMFs will be presented in a separate paper). In this particular case,
the MMFs did not carry net flux away from the central spot. Instead,
the polarities of MMFs were essentially mixed so that outflowing
positive and negative fluxes were roughly balanced. During the 3-day
period, there was almost no net flux accumulation to form a moat. The
cancellation of MMFs of opposite polarities at the boundary of the
super-penumbra caused quite a few surges and Hα brightenings.

---------------------------------------------------------
Title: The Longer Term Evolution of Magnetic Field and Mass Flow in
a Decaying Active Region
Authors: Zhang, Hongqi; Ai, Guoxiang; Wang, Haimin; Zirin, Harold;
Patterson, Alan
1991LNP...380..271Z Altcode: 1991IAUCo.130..271Z; 1991sacs.coll..271Z
No abstract at ADS

---------------------------------------------------------
Title: Microwave Structure of the Quiet Sun at 8.5 GHz
Authors: Gary, Dale E.; Zirin, Harold; Wang, Haimin
1990ApJ...355..321G Altcode:
Multifrequency VLA observations of the quiet sun near 8.5 GHz are
presented. Two regions of the sun were observed, one dominated by an
enhanced network corresponding to a decayed active region, and the
other corresponding to an enhanced network with no active features. The
full-day synthesis maps for both show nearly perfect correspondence
to H-alpha images, and to longitudinal magnetograms. The coronal loops
were observed to appear as regions of radio emission with no underlying
longitudinal magnetic fields, being aligned with H-alpha fibrils in the
photosphere, and connecting regions of opposite magnetic polarity. The
emission can be modeled as optically thin free-free emission from a
coronal loop with a peak axial density of approximately 2.4-2.8 x 10
to the 9th/cu cm, for an assumed coronal temperature of 1-2 x 10 to the
6th K. The quiet chromosphere sources are measured, and the significance
of these measurements for existing chromospheric models is discussed.

---------------------------------------------------------
Title: Flows, flares, and formation of umbrae and light bridges in
BBSO region No. 1167
Authors: Zirin, Harold; Wang, Haimin
1990SoPh..125...45Z Altcode:
We present high-resolution observations of the large active region BBSO
No. 1167 (Boulder No. 5060) which cast new light on the structure
of sunspot regions. We obtained excellent data, highlighted by
videomagnetograms (VMG) obtained with our 65-cm telescope, which give
unprecedented spatial resolution, about 0.5” for much of two 11-hr
periods. This permitted us to see details of the field evolution and
flows in the AR. The Hα filtergrams and D3 filtergrams permit study
of these magnetic changes compared to spots and chromospheric structure.

---------------------------------------------------------
Title: Video image selection studies of granules, pores, and penumbral
flows near a large sunspot
Authors: Zirin, Harold; Wang, Haimin
1989SoPh..119..245Z Altcode:
An excellent high-resolution movie in the green continuum was produced
by shift-and-add treatment of two 60-min videotapes obtained at the
Big Bear Solar Observatory. We have studied the digitized images by
direct measurement, cross-correlation techniques, and correlation
tracking. The seeing-limited resolution was about 0.3 arc sec.

---------------------------------------------------------
Title: Seventy-Five Hours of Coordinated Videomagnetograph
Observations
Authors: Wang, Haimin; Zirin, Harold; Patterson, Alan; Al, Guoxiang;
Zhang, Hongqi
1989ApJ...343..489W Altcode:
Videomagnetograph observations obtained between September 24 and 29,
1987 are presented which illustrate the evolution of magnetic flux
surrounding a stable sunspot. It is found that the dominant sunspot
mainly ejects magnetic fields of opposite sign, and that the surrounding
plage fields steadily contract and retreat inward toward the umbra,
resulting in shrinking and weakening of the spot and plage. The extent
of the moat is shown to be reduced by 50 percent in a 75-hour period,
with the principal loss of flux probably due to concellation at the
main neutral line. Five subflares were noted, three occurring prior to
cancellation of the magnetic elements at the inversion line and two
occurring during the development and disappearance of an ephemeral
bipolar region.

---------------------------------------------------------
Title: Do mesogranules exist?
Authors: Wang, Haimin
1989SoPh..123...21W Altcode:
Applying spatial and temporal averaging techniques to several long
sequences of Dopplergrams obtained at the solar disk center and
near the limb, we confirmed the persistent supergranule velocity
pattern. After excluding the 5-min oscillation and supergranule velocity
fields from the disk center Doppler data, we find that the velocity
structure shows a typical scale of 7 Mm, consistent with the scale of
mesogranules found by November, Toomre, and Gebbie (1981) and November
et al. (1982). However, this velocity pattern does not show properties
of the cellular convection. It is not coherent for more than one hour,
the period during which the raw Doppler images are averaged to remove
the 5-min oscillation. Furthermore, we did not find convection patterns
in the scale of mesogranules from the Doppler data obtained near the
solar limb. We propose that the mesogranule velocity structure found by
November et al. might be the uncorrected part of the 5-min oscillation
and granule velocity.

---------------------------------------------------------
Title: The Association of Flares to Cancelling Magnetic Features on
the Sun
Authors: Livi, Silvia H. B.; Martin, Sara; Wang, Haimin; Ai, Guoxiang
1989SoPh..121..197L Altcode: 1989IAUCo.104..197L
Previous work relating flares to evolutionary changes of photospheric
solar magnetic fields are reviewed and reinterpreted in the light of
recent observations of cancelling magnetic fields. In line-of-sight
magnetograms and H-alpha filtergrams from Big Bear Solar Observatory,
we confirm the following 3 associations: (a) the occurrence of many
flares in the vicinity of emerging magnetic flux regions (Rust, 1974),
but only at locations where cancellation has been observed or inferred;
(b) the occurrence of flares at sites where the magnetic flux is
increasing on one side of a polarity inversion line and concurrently
decreasing on the other (Martres et al., 1968; Ribes, 1969); and
(c) the occurrence of flares at sites where cancellation is the only
observed change in the magnetograms for at least several hours before
a flare (Martin, Livi, and Wang, 1985). Because cancellation (or the
localized decrease in the line-of-sight component of magnetic flux)
is the only common factor in all of these circumstances, suggest
that cancellation is the more general association that includes the
other associations as special cases. We propose the hypothesis that
cancellation is a necessary, evolutionary precondition for flares. We
also confirm the observation of Martin, Livi, and Wang (1985) that the
initial parts of flares occur in close proximity to cancellation sites
but that during later phases, the flare emission can spread to other
parts of the magnetic field that are weak, strong, or not cancelling.

---------------------------------------------------------
Title: Coordinated videomagnetograph observations by the Big Bear
and Huairou Observatories
Authors: Wang, Haimin; Patterson, Alan; Zirin, Harold; Ai, Guoxiang;
Zhang, Hongqi
1988fnsm.work..279W Altcode:
A videomagnetograph patterned after the BBSO system was installed at
Huairou in 1987, and five days of coordinated observing were carried
out from September 24 to 29, 1987. The data were combined to make
a continuous movie of the fields abound a stable spot. A 57 hour
magnetograph run with two seven hour gaps were achieved from 2330 UT,
September 24 to 0830 UT, September 27. The frames were reregistered and
justified to eliminate the change of scale with meridian distance. The
intensities were corrected for cosine effect. Preliminary examination
of the data shows continuous decrease of the total magnetic field during
this period by more than 50 percent. The principal loss of flux appears
to be due to cancellation at the main neutral line. Some flux disappears
due to fragmentation, which makes the elements fall below the threshold,
while only a tiny loss due to diffusion can be detected. It is planned
to continue this program during Max 1991, including transverse field
measurements as well. Several long runs were already obtained in 1988.

---------------------------------------------------------
Title: On the Relationship Between Magnetic Fields and Supergranule
Velocity Fields
Authors: Wang, Haimin
1988SoPh..117..343W Altcode:
We studied the size, correlation lifetime and horizontal velocity
amplitude of supergranules in regions with different magnetic
activity. We found that the supergranule velocity cells have similar
scale, correlation lifetime and horizontal velocity amplitude in the
unipolar enhanced magnetic network regions and in the mixed-polarity
quiet Sun. However, the correlation lifetime of magnetic structure
is much longer in the enhanced network. We investigated the velocity
pattern of moving magnetic features (MMF) surrounding a decaying
sunspot. The velocity of MMFs is consistent with the outflow
surrounding the sunspot as measured by Dopplergrams. The velocity
cell surrounding the sunspot has a much larger velocity amplitude
and a longer lifetime than regular supergranule cells. We found that
ephemeral regions (ER) have a slight tendency to emerge at or near
boundaries of supergranules. Almost all the magnetic flux disappears
at the supergranule boundaries. In most cases, two poles of cancelling
features with opposite magnetic polarities approach along the boundaries
of supergranules.

---------------------------------------------------------
Title: Structure of Magnetic Fields on the Quiet Sun
Authors: Wang, Haimin
1988SoPh..116....1W Altcode:
To obtain quantitative temporal and spatial information on the network
magnetic fields, we applied auto- and cross-correlation techniques
to the Big Bear videomagnetogram (VMG) data. The average size of the
network magnetic elements derived from the auto-correlation curve is
about 5700 km. The distance between the primary and secondary peak in
the auto-correlation curve is about 17000 km, which is half of the size
of the supergranule as determined from the velocity map. The correlation
time is about 10 to 20 hours. The diffusion constant derived from the
cross-correlation curve is 150 km2 s-1. We also
found that in the quiet regions the total magnetic flux in a window 3'
× 4' changes very little in nearly 10 hours. That means the emergence
and the disappearance of magnetic flux are in balance. The cancelling
features and the emergence of ephemeral regions are the major sources
for the loss and replenishment of magnetic flux on the quiet Sun.

---------------------------------------------------------
Title: Magnetic Fields and Supergranule Velocity Fields on the
Quiet Sun.
Authors: Wang, Haimin
1988PhDT.........4W Altcode: 1988PhDT.......146W
I have carried out detailed study on the quiet sun magnetic fields and
supergranule velocity fields. This thesis consists of 6 themes. (1)
I studied the statistical properties of quiet sun magnetic fields,
including size distribution, evolution, flux budget of magnetic flux
elements, and the magnetic diffusion constant. From the observations,
I derived that the magnetic diffusion constant is &lt;=q150 km^2/sec in
the quiet region. I found that cancelling features and Ephemeral Regions
are major sources of magnetic flux disappearance and replenishment. (2)
I studied the supergranule velocity fields. Supergranule vertical
velocities have a r.m.s. speed of 0.03 km/s. By observing the evolution
of individual supergranule cells, I found that the average lifetime
of supergranules is &gt;=q50 hours. (3) I measured the contrast of
faculae near the solar limb. The measurements show no obvious contrast
increase or decrease near the solar limb. The observation fits neither
the "hot wall" nor "hot cloud" fluxtube model. (4) I measured the
separation velocities of new bipoles. The observed values are several
times smaller than the values estimated by the theory of magnetic
buoyancy. (5) I applied the local correlation tracking technique to BBSO
Videomagnetogram data and detected an approximate radial intranetwork
flow pattern. (6) I studied the relationship between magnetic fields
and convection velocity fields. I found that ephemeral regions have a
light tendency to emerge at or near the boundaries of supergranules;
supergranules have the same scale, correlation lifetime and mean
horizontal speed in the enhanced network region as in the mixed
polarity quiet sun; the velocity of moving magnetic features that
surround sunspots is consistent with the direct Doppler measurement.

---------------------------------------------------------
Title: High Resolution Studies of Penumbral and Pore Phenomena
Authors: Zirin, Harold; Wang, Haimin
1987BAAS...19..940Z Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Separation Velocity of Emerging Magnetic Flux
Authors: Chou, Dean-Yi; Wang, Haimin
1987SoPh..110...81C Altcode:
We measure the separation velocity of opposite poles from 24 new
bipoles on the Sun. We find that the measured velocities range from
about 0.2 to 1 km s−1. The fluxes of the bipoles range
over more than two orders of magnitude, and the mean field strength and
the sizes range over one order of magnitude. The measured separation
velocity is not correlated with the flux and the mean field strength of
the bipole. The separation velocity predicted by the present theory of
magnetic buoyancy is between 7.4Ba−1/4 cot θ and 13 cot
θ km s−1, where θ is the elevation angle of the flux
tube at the photosphere (see Figure 9), B is the mean field strength,
and a is the radius of the observed bipole. The rising velocity of
the top of flux tubes predicted by the theory of magnetic buoyancy
is between 3.7Ba−1/4 and 6.5 km s−1. The
predicted separation velocity is about one order of magnitude higher
than those measured, or else the flux tubes are almost vertical at the
photosphere. There is no correlation between the measured separation
velocity and the theoretical value, 7.4Ba−1/4. The
predicted rising velocity is also higher than the vertical velocity
near the line of inversion in emerging flux regions observed by other
authors.