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Author name code: ding
ADS astronomy entries on 2022-09-14
author:"Ding, Mingde D."
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Title: The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed
by the Extreme Ultraviolet Imager aboard Solar Orbiter
Authors: Li, Ying; Li, Qiao; Song, De-Chao; Battaglia, Andrea
Francesco; Xiao, Hualin; Krucker, Säm; Schühle, Udo; Li, Hui; Gan,
Weiqun; Ding, M. D.
2022arXiv220806182L Altcode:
The hydrogen Lyman-$\alpha$ (H {\sc i} Ly$\alpha$) emission during solar
flares has rarely been studied in spatially resolved images and its
physical origin has not been fully understood. In this paper, we present
novel Ly$\alpha$ images for a C1.4 solar flare (SOL2021-08-20T22:00)
from the Extreme Ultraviolet Imager aboard Solar Orbiter, together
with multi-waveband and multi-perspective observations from the
Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics
Observatory spacecraft. It is found that the Ly$\alpha$ emission has
a good temporal correlation with the thermal emissions at 1--8 Å and
5--7 keV, indicating that the flaring Ly$\alpha$ is mainly produced by
a thermal process in this small event. However, nonthermal electrons
play a minor role in generating Ly$\alpha$ at flare ribbons during
the rise phase of the flare, as revealed by the hard X-ray imaging and
spectral fitting. Besides originating from flare ribbons, the Ly$\alpha$
emission can come from flare loops, likely caused by plasma heating
and also cooling that happen in different flare phases. It is also
found that the Ly$\alpha$ emission shows fairly similar features with
the He {\sc ii} 304 Å emission in light curve and spatio-temporal
variation along with small differences. These observational results
improve our understanding of the Ly$\alpha$ emission in solar flares
and also provide some insights for investigating the Ly$\alpha$
emission in stellar flares.
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Title: The Chinese Hα Solar Explorer (CHASE) mission: An overview
Authors: Li, Chuan; Fang, Cheng; Li, Zhen; Ding, MingDe; Chen,
PengFei; Qiu, Ye; You, Wei; Yuan, Yuan; An, MinJie; Tao, HongJiang;
Li, XianSheng; Chen, Zhe; Liu, Qiang; Mei, Gui; Yang, Liang; Zhang,
Wei; Cheng, WeiQiang; Chen, JianXin; Chen, ChangYa; Gu, Qiang;
Huang, QingLong; Liu, MingXing; Han, ChengShan; Xin, HongWei; Chen,
ChangZheng; Ni, YiWei; Wang, WenBo; Rao, ShiHao; Li, HaiTang; Lu,
Xi; Wang, Wei; Lin, Jun; Jiang, YiXian; Meng, LingJie; Zhao, Jian
2022SCPMA..6589602L Altcode: 2022arXiv220505962L
The Chinese Hα Solar Explorer (CHASE), dubbed "Xihe"—Goddess of
the Sun, was launched on October 14, 2021 as the first solar space
mission of China National Space Administration (CNSA). The CHASE
mission is designed to test a newly developed satellite platform and
to acquire the spectroscopic observations in the Hα waveband. The
Hα Imaging Spectrograph (HIS) is the scientific payload of the CHASE
satellite. It consists of two observational modes: raster scanning mode
and continuum imaging mode. The raster scanning mode obtains full-Sun or
region-of-interest spectral images from 6559.7 to 6565.9 å; and from
6567.8 to 6570.6 å with 0.024 å pixel spectral resolution and 1 min
temporal resolution. The continuum imaging mode obtains photospheric
images in continuum around 6689 å with the full width at half maximum
of 13.4 å. The CHASE mission will advance our understanding of the
dynamics of solar activity in the photosphere and chromosphere. In
this paper, we present an overview of the CHASE mission including the
scientific objectives, HIS instrument overview, data calibration flow,
and first results of on-orbit observations.
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Title: Coronal loop kink oscillation periods derived from the
information of density, magnetic field, and loop geometry
Authors: Chen, G. Y.; Chen, L. Y.; Guo, Y.; Ding, M. D.; Chen, P. F.;
Erdélyi, R.
2022A&A...664A..48C Altcode:
Context. Coronal loop oscillations can be triggered by solar eruptions,
for example, and are observed frequently by the Atmospheric Imaging
Assembly (AIA) on board Solar Dynamics Observatory (SDO). The
Helioseismic and Magnetic Imager (HMI) on board SDO offers us the
opportunity to measure the photospheric vector magnetic field and
carry out solar magneto-seismology (SMS). <BR /> Aims: By applying SMS,
we aim to verify the consistency between the observed period and the
one derived from the information of coronal density, magnetic field,
and loop geometry, that is, the shape of the loop axis. <BR /> Methods:
We analysed the data of three coronal loop oscillation events detected
by SDO/AIA and SDO/HMI. First, we obtained oscillation parameters
by fitting the observational data. Second, we used a differential
emission measure (DEM) analysis to diagnose the temperature and
density distribution along the coronal loop. Subsequently, we applied
magnetic field extrapolation to reconstruct the three-dimensional
magnetic field and then, finally, used the shooting method to compute
the oscillation periods from the governing equation. <BR /> Results:
The average magnetic field determined by magnetic field extrapolation
is consistent with that derived by SMS. A new analytical solution is
found under the assumption of exponential density profile and uniform
magnetic field. The periods estimated by combining the coronal density
and magnetic field distribution and the associated loop geometry are
closest to the observed ones, and are more realistic than when the
loop geometry is regarded as being semi-circular or having a linear
shape. <BR /> Conclusions: The period of a coronal loop is sensitive
to not only the density and magnetic field distribution but also the
loop geometry.
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Title: Editorial
Authors: Fang, Cheng; Ding, MingDe; Li, Chuan; Chen, PengFei;
Zhang, Wei
2022SCPMA..6589601F Altcode:
No abstract at ADS
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Title: Calibration procedures for the CHASE/HIS science data
Authors: Qiu, Ye; Rao, ShiHao; Li, Chuan; Fang, Cheng; Ding, MingDe;
Li, Zhen; Ni, YiWei; Wang, WenBo; Hong, Jie; Hao, Qi; Dai, Yu; Chen,
PengFei; Wan, XiaoSheng; Xu, Zhi; You, Wei; Yuan, Yuan; Tao, HongJiang;
Li, XianSheng; He, YuKun; Liu, Qiang
2022SCPMA..6589603Q Altcode: 2022arXiv220506075Q
The Hα line is an important optical line in solar observations
containing the information from the photosphere to the chromosphere. To
study the mechanisms of solar eruptions and the plasma dynamics in the
lower atmosphere, the Chinese Hα Solar Explorer (CHASE) was launched
into a Sun-synchronous orbit on October 14, 2021. The scientific payload
of the CHASE satellite is the Hα Imaging Spectrograph (HIS). The
CHASE/HIS acquires, for the first time, seeing-free Hα spectroscopic
observations with high spectral and temporal resolutions. It consists of
two observational modes. The raster scanning mode provides full-Sun
or region-of-interest spectra at Hα (6559.7-6565.9 Å) and Fe I
(6567.8-6570.6 Å) wavebands. The continuum imaging mode obtains
full-Sun photospheric images at around 6689 Å. In this paper, we
present detailed calibration procedures for the CHASE/HIS science
data, including the dark-field and flat-field correction, slit
image curvature correction, wavelength and intensity calibration,
and coordinate transformation. The higher-level data products can be
directly used for scientific research.
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Title: Imaging and Spectroscopic Observations of the Dynamic Processes
in Limb Solar Flares
Authors: Yu, Ke; Li, Y.; Hong, Jie; Song, De-Chao; Ding, M. D.
2022ApJ...935....8Y Altcode: 2022arXiv220705925Y
We investigate various dynamic processes including magnetic
reconnection, chromospheric evaporation, and coronal rain draining in
two limb solar flares through imaging and spectroscopic observations
from the Interface Region Imaging Spectrograph (IRIS) and the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory. In the early phase of the flares, a bright and dense
loop-top structure with a cusp-like shape can be seen in multiwavelength
images, which is cospatial with the hard X-ray 25-50 keV emission. In
particular, intermittent magnetic reconnection downflows are detected
in the time-space maps of AIA 304 Å. The reconnection downflows are
manifested as redshifts on one half of the loops and blueshifts on
the other half in the IRIS Si IV 1393.76 Å line due to a projection
effect. The Si IV profiles exhibit complex features (say, multipeak)
with a relatively larger width at the loop-top region. During the
impulsive phase, chromospheric evaporation is observed in both AIA
images and the IRIS Fe XXI 1354.08 Å line. Upward motions can be
seen from AIA 131 Å images. The Fe XXI line is significantly enhanced
and shows a good Gaussian shape. In the gradual phase, warm rains are
observed as downward moving plasmas in AIA 304 Å images. Both the Si
IV and Fe XXI lines show a relatively symmetric shape with a larger
width around the loop top. These results provide observational evidence
for various dynamic processes involved in the energy release process
of solar flares and are crucial to the understanding of this process.
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Title: Investigating Overexpansion of a Coronal Mass Ejection on
2021 May 07
Authors: Wang, Bitao; Song, Hong-Qiang; Cheng, Xin; Ding, Mingde
2022cosp...44.2429W Altcode:
Coronal Mass Ejection(CME) burst in 2021 May 07 shows a typical
three-part structure and is associated with a X-class flare, which
is continuously observed from 0 to 3 Rsun by SDO/AIA(0-1.3 Rsun) and
MLSO/KCor(1.05-3 Rsun). This event provides opportunity to study the
early evolution of this CME. We measure the aspect ratio of the leading
front and bright core to find there is a lateral overexpansion through
the early evolution of the CME. The minimum of the aspect ratio occurs
simultaneously with the peak of the SO/STIX 4-10 keV soft X-ray burst of
the flare. During the study we compare the observations of the bright
core between EUV(AIA) and white-light(KCor), the thermal structure
spread lower but wider than the dense structure, which may be caused
by the heat originating from the current sheet below conduct along
the magnetic field. What is more, the overexpansion of the CME make
it encounter a jacent pseudo-stream(PS) in the north. The interaction
between the CME and the PS causes many kinds of radio signals, i.e.,
a formal type II burst, a zero-drifted type II burst and a C-shaped
type III burst. To explain the abundant radio signals, A possible
scenario of the interaction is discussed.
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Title: Three-dimensional Structures and Heating Mechanism of Solar
Microflares
Authors: Li, Zhuofei; Cheng, Xin; Ding, Mingde; Chen, Feng; Chen, Jun
2022cosp...44.2558L Altcode:
Small-scale solar activities in the solar atmosphere, e.g., microflares
and campfires, are believed to be caused by magnetic reconnection,
with high similarity to major solar flares. Nevertheless, their
detailed three-dimensional (3D) structures and heating mechanisms are
still unsolved. In this Letter, based on high-resolution 3D radiative
magnetohydrodynamic simulation of the solar quiet region, conducted
with MURaM code including the upper convection zone, photosphere,
chromosphere, and low corona, we investigate the 3D magnetic structures
and heating mechanism of three homologous microflares. It is found that
the microflares originate from local hot plasma in the chromosphere. The
heated plasma is almost co-spatial with the maximal heating rate
normalized by the density. The 3D velocity field reveals a pair of
low-speed converging flows toward and high-speed outflows moving away
from the hot plasma. These features support that magnetic reconnection
plays a critical role in heating the local chromospheric plasma to
coronal temperature, giving rise to the observed microflares. The
magnetic topology analysis further discloses that the reconnection
is located near quasi-separators where both the current density and
squashing factors are maximal although the specific topology varies
from case to case.
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Title: Annihilation of Magnetic Islands at the Top of Solar Flare
Loops
Authors: Wang, Yulei; Lu, Quanming; Cheng, Xin; Ding, Mingde
2022cosp...44.1514W Altcode:
The dynamics of magnetic reconnection in the solar current sheet (CS)
is studied by high-resolution 2.5-dimensional MHD simulation. With the
commencing of magnetic reconnection, a number of magnetic islands are
formed intermittently and move quickly upward and downward along the
CS. Upon collision with the semi-closed flux of the flare loops, the
downflow islands cause a second reconnection with a rate comparable
with that in the main CS. Though the time-integrated magnetic energy
release is still dominated by the reconnection in the main CS,
the second reconnection can release substantial magnetic energy,
annihilating the main islands and generating secondary islands with
various scales at the flare loop top. The distribution function of the
flux of the secondary islands is found to follow a power law varying
from $f(\psi)\sim \psi^{-1}$ (small scale) to $\psi^{-2}$ (large scale),
which seems to be independent to background plasma $\beta$ and thermal
conduction (TC). However, the spatial scale and the strength of the
termination shocks driven by the main reconnection outflows or islands
decrease if $\beta$ increases or if TC is included. We suggest that
the annihilation of magnetic islands at the flare loop top, which is
not included in the standard flare model, plays a nonnegligible role in
releasing magnetic energy to heat flare plasma and accelerate particles.
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Title: Radiative Magnetohydrodynamic Simulation of the Confined
Eruption of a Magnetic Flux Rope: Magnetic Structure and Plasma
Thermodynamics
Authors: Wang, Can; Chen, Feng; Ding, Mingde; Lu, Zekun
2022ApJ...933L..29W Altcode: 2022arXiv220614188W
It is widely believed that magnetic flux ropes are the key structure
of solar eruptions; however, their observable counterparts are not
clear yet. We study a flare associated with flux rope eruption
in a comprehensive radiative magnetohydrodynamic simulation
of flare-productive active regions, especially focusing on the
thermodynamic properties of the plasma involved in the eruption and
their relation to the magnetic flux rope. The preexisting flux rope,
which carries cold and dense plasma, rises quasi-statically before the
onset of eruptions. During this stage, the flux rope does not show
obvious signatures in extreme ultraviolet (EUV) emission. After the
flare onset, a thin "current shell" is generated around the erupting
flux rope. Moreover, a current sheet is formed under the flux rope,
where two groups of magnetic arcades reconnect and create a group of
postflare loops. The plasma within the "current shell," current sheet,
and postflare loops are heated to more than 10 MK. The postflare loops
give rise to abundant soft X-ray emission. Meanwhile, a majority of
the plasma hosted in the flux rope is heated to around 1 MK, and the
main body of the flux rope is manifested as a bright arch in cooler
EUV passbands such as the AIA 171 Å channel.
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Title: Growth of a filament channel by intermittent small-scale
magnetic reconnection
Authors: Li, H. T.; Cheng, X.; Guo, J. H.; Yan, X. L.; Wang, L. F.;
Zhong, Z.; Li, C.; Ding, M. D.
2022A&A...663A.127L Altcode: 2022arXiv220309110L
Context. A filament channel (FC), a plasma volume where the magnetic
field is primarily aligned with the polarity inversion line,
is believed to be the pre-eruptive configuration of coronal mass
ejections. Nevertheless, evidence for how the FC is formed is still
elusive. <BR /> Aims: In this paper, we present a detailed study of the
build-up of a FC in order to understand its formation mechanism. <BR />
Methods: The New Vacuum Solar Telescope (NVST) of the Yunnan Observatory
and the Optical and Near-infrared Solar Eruption Tracer (ONSET)
of Nanjing University, as well as the Atmospheric Imaging Assembly
(AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar
Dynamics Observatory (SDO), are used to study the growth process of
the FC. Furthermore, we reconstruct the nonlinear force-free field
(NLFFF) of the active region using the regularized Biot-Savart laws
(RBSL) and the magnetofrictional method to reveal the three-dimensional
(3D) magnetic field properties of the FC. <BR /> Results: We find
that partial filament materials are quickly transferred to longer
magnetic field lines formed by small-scale magnetic reconnection, as
evidenced by dot-like Hα and extreme ultraviolet (EUV) brightenings
and subsequent bidirectional outflow jets, as well as untwisting
motions. The Hα and EUV bursts appear repeatedly at the same
location and are closely associated with flux cancelation, which
occurs between two small-scale opposite polarities and is driven by
shearing and converging motions. The 3D NLFFF model reveals that the
reconnection takes place in a hyperbolic flux tube that is located
above the flux-cancelation site and below the FC. <BR /> Conclusions:
The FC is gradually built up toward a twisted flux rope via a series
of small-scale reconnection events that occur intermittently prior
to the eruption. <P />Movie associated to Fig. 3 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/202243115/olm">https://www.aanda.org</A>
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Title: Current-sheet Oscillations Caused by the Kelvin-Helmholtz
Instability at the Loop Top of Solar Flares
Authors: Wang, Yulei; Cheng, Xin; Ren, Zining; Ding, Mingde
2022ApJ...931L..32W Altcode: 2022arXiv220510361W
Current sheets (CSs), long stretching structures of magnetic
reconnection above solar flare loops, are usually observed to oscillate;
their origins, however, are still puzzled at present. Based on a
high-resolution 2.5D MHD simulation of magnetic reconnection, we explore
the formation mechanism of CS oscillations. We find that large-amplitude
transverse waves are excited by the Kelvin-Helmholtz instability at
the highly turbulent cusp-shaped region. The perturbations propagate
upward along the CS with a phase speed close to local Alfvén speed thus
resulting in the CS oscillations we observe. Though the perturbations
damp after propagating for a long distance, the CS oscillations
are still detectable. In terms of detected CS oscillations, with a
combination of differential emission measure techniques, we propose
a new method for measuring the magnetic field strength of the CS and
its distribution in height.
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Title: An approximate recipe of chromospheric radiative losses for
solar flares
Authors: Hong, J.; Carlsson, M.; Ding, M. D.
2022A&A...661A..77H Altcode: 2022arXiv220307630H
Context. Radiative losses in the chromosphere are very important for the
energy balance of the Sun. There have been efforts to make simple lookup
tables for chromospheric radiative losses in the quiet Sun. During
solar flares, the atmospheric conditions are quite different, and
the currently available recipe is constructed from semi-empirical
models. How these recipes work in flare conditions remains to be
evaluated. <BR /> Aims: We aim to construct an approximate recipe
of chromospheric radiative losses for solar flares. <BR /> Methods:
We tabulate the optically thin radiative loss, escape probability,
and ionization fraction using a grid of flare models from radiative
hydrodynamic simulations as our dataset. <BR /> Results: We provide
new lookup tables to calculate chromospheric radiative losses for
flares. Compared with previous recipes, our recipe provides a better
approximation of the detailed radiative losses for flares.
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Title: Three-dimensional Magnetic and Thermodynamic Structures of
Solar Microflares
Authors: Li, Z. F.; Cheng, X.; Chen, F.; Chen, J.; Ding, M. D.
2022ApJ...930L...7L Altcode: 2022arXiv220407762L
Microflares, one of the small-scale solar activities, are believed to be
caused by magnetic reconnection. Nevertheless, their three-dimensional
(3D) magnetic structures, thermodynamic structures, and physical links
to reconnection are unclear. In this Letter, based on a high-resolution
3D radiative magnetohydrodynamic simulation of the quiet Sun spanning
from the upper convection zone to the corona, we investigate
the 3D magnetic and thermodynamic structures of three homologous
microflares. It is found that they originate from localized hot plasma
embedded in the chromospheric environment at the height of 2-10 Mm
above the photosphere and last for 3-10 minutes with released magnetic
energy in the range of 10<SUP>27</SUP>-10<SUP>28</SUP> erg. The heated
plasma is almost cospatial with the regions where the heating rate per
particle is maximal. The 3D velocity field reveals a pair of converging
flows with velocities of tens of km s<SUP>-1</SUP> moving toward and
outflows with velocities of about 100 km s<SUP>-1</SUP> moving away
from the hot plasma. These features support magnetic reconnection
playing a critical role in heating the localized chromospheric plasma
to coronal temperature, giving rise to the observed microflares. The
magnetic topology analysis further discloses that the reconnection
region is located near quasi-separators where both current density and
squashing factors are maximal although the specific topology may vary
from a tether-cutting to fan-spine-like structure.
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Title: Quantifying the Magnetic Structure of a Coronal Shock Producing
a Type II Radio Burst
Authors: Su, W.; Li, T. M.; Cheng, X.; Feng, L.; Zhang, P. J.; Chen,
P. F.; Ding, M. D.; Chen, L. J.; Guo, Y.; Wang, Y.; Li, D.; Zhang,
L. Y.
2022ApJ...929..175S Altcode: 2022arXiv220311042S
Type II radio bursts are thought to be produced by shock waves in the
solar atmosphere. However, what magnetic conditions are needed for the
generation of type II radio bursts is still a puzzling issue. Here,
we quantify the magnetic structure of a coronal shock associated with
a type II radio burst. Based on multiperspective extreme-ultraviolet
observations, we reconstruct the three-dimensional (3D) shock
surface. By using a magnetic field extrapolation model, we then derive
the orientation of the magnetic field relative to the normal of the
shock front (θ <SUB>Bn</SUB>) and the Alfvén Mach number (M <SUB>
A </SUB>) on the shock front. Combining the radio observations from
the Nancay Radio Heliograph, we obtain the source region of the type
II radio burst on the shock front. It is found that the radio burst
is generated by a shock with M <SUB> A </SUB> ≳ 1.5 and a bimodal
distribution of θ <SUB>Bn</SUB>. We also use the Rankine-Hugoniot
relations to quantify the properties of the shock downstream. Our
results provide a quantitative 3D magnetic structure condition of a
coronal shock that produces a type II radio burst.
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Title: Groove Formation on Phobos from Orbital Ejecta of Stickney
Crater
Authors: Xi, X.; Ding, M.; Zhu, M. -H.
2022LPICo2678.1062X Altcode:
Orbital ejecta from Stickney Crater can produce Phobos' grooves,
according to combined crater formation, ejecta trajectory, and groove
formation calculations.
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Title: Ejecta Pattern of Oblique Impacts on the Moon from Numerical
Simulation
Authors: Luo, X. -Z.; Zhu, M. -H.; Ding, M.
2022LPICo2678.1133L Altcode:
We use 3D numerical simulation to investigate the combined effect of
impact angles and impactor diameters on the ejecta pattern of oblique
impacts on the Moon.
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Title: Comparison of the energy spectra between pileup shock and
converging shock
Authors: Wang, X.; Feng, X.; Yan, Y.; Ding, M.; Lu, H.; Shan, H.
2022icrc.confE1361W Altcode: 2022PoS...395E1361W
No abstract at ADS
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Title: Automated Crater Detection and Absolute Model Age Estimation
for Craters in the South Pole-Aitken Basin of the Moon
Authors: Cui, X. -L.; Ding, M.; Wang, G.
2022LPICo2678.1900C Altcode:
We use a convolutional neural network to detect craters in the South
Pole-Aitken basin, and estimate model ages of mid-sized craters with
detection results.
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Title: Resolving Two Distinct Thermal X-Ray Components in a Compound
Solar Flare
Authors: Zhou, Zhenjun; Liu, Rui; Sun, Jianqing; Zhang, Jie; Ding,
Mingde; Cheng, Xin; Wang, Yuming; Yu, Xiaoyu; Liu, Lijuan; Cui, Jun
2022ApJ...925..132Z Altcode: 2021arXiv211204652Z
X-ray emission provides the most direct diagnostics of the energy
release process in solar flares. Occasionally, a superhot X-ray source
is found to be above hot flare loops of ~10 MK temperature. While the
origin of the superhot plasma is still elusive, it has conjured up an
intriguing image of in situ plasma heating near the reconnection site
high above the flare loops, in contrast to the conventional picture
of chromospheric evaporation. Here we investigate an extremely long
duration solar flare, in which EUV images show two distinct flare loop
systems that appear successively along a Γ-shaped polarity inversion
line (PIL). When both flare loop systems are present, the hard X-ray
spectrum is found to be well fitted by combining a hot component (T
<SUB> e </SUB> ~ 12 MK) and a superhot component (T <SUB> e </SUB>
~ 30 MK). Associated with a fast coronal mass ejection (CME), the
superhot X-ray source is located at the top of the flare arcade that
appears earlier, straddling and extending along the long "arm" of the
Γ-shaped PIL. Associated with a slow CME, the hot X-ray source is
located at the top of the flare arcade that appears later and sits
astride the short "arm" of the Γ-shaped PIL. Aided by observations
from a different viewing angle, we are able to verify that the superhot
X-ray source is above the hot one in projection, but the two sources
belong to different flare loop systems. Thus, this case study provides
a stereoscopic observation explaining the coexistence of superhot and
hot X-ray-emitting plasmas in solar flares.
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Title: Diagnosing the Optically Thick/Thin Features Using the
Intensity Ratio of Si IV Resonance Lines in Solar Flares
Authors: Zhou, Yi-An; Hong, Jie; Li, Y.; Ding, M. D.
2022ApJ...926..223Z Altcode: 2022arXiv220105305Z
In the optically thin regime, the intensity ratio of the two Si IV
resonance lines (1394 and 1403 Å) are theoretically the same as
the ratio of their oscillator strengths, which is exactly 2. Here,
we study the ratio of the integrated intensity of the Si IV lines
(R = ∫I <SUB>1394</SUB>(λ)dλ/∫I <SUB>1403</SUB>(λ)dλ)
and the ratio of intensity at each wavelength point (r(Δλ) = I
<SUB>1394</SUB>(Δλ)/I <SUB>1403</SUB>(Δλ)) in two solar flares
observed by the Interface Region Imaging Spectrograph. We find that at
flare ribbons, the ratio R ranges from 1.8 to 2.3 and would generally
decrease when the ribbons sweep across the slit position. In addition,
the distribution of r(Δλ) shows a descending trend from the blue
wing to the red wing. In loop cases, the Si IV line presents a wide
profile with a central reversal. The ratio R deviates little from 2,
but the ratio r(Δλ) can vary from 1.3 near the line center to greater
than 2 in the line wings. Hence we conclude that in flare conditions,
the ratio r(Δλ) varies across the line, due to the variation of the
opacity at the line center and line wings. We notice that, although the
ratio r(Δλ) could present a value that deviates from 2 as a result
of the opacity effect near the line center, the ratio R is still close
to 2. Therefore, caution should be taken when using the ratio of the
integrated intensity of the Si IV lines to diagnose the opacity effect.
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Title: Multiwavelength and Dual-perspective Observations of Eruption
and Untwisting of Two Homologous Magnetic Flux Ropes
Authors: Song, De-Chao; Li, Y.; Su, Y.; Ding, M. D.; Gan, W. Q.
2021ApJ...922..238S Altcode: 2021arXiv210911187S
In this paper, we present a detailed morphological, kinematic, and
thermal analysis of two homologous magnetic flux ropes (MFRs) from
NOAA 11515 on 2012 July 8-9. The study is based on multiwavelength
and dual-perspective imaging observations from the Solar Dynamics
Observatory and the Solar Terrestrial Relations Observatory Ahead
spacecraft, which can reveal the structure and evolution of the two
MFRs. We find that both of the MFRs show up in multiple passbands
and their emissions mainly consist of a cold component peaking at a
temperature of ~0.4-0.6 MK and a hot component peaking at ~7-8 MK. The
two MFRs exhibit erupting, expanding, and untwisting motions that
manifest distinctive features from two different viewpoints. Their
evolution can be divided into two stages-a fast-eruption stage with
speeds of about 105-125 km s<SUP>-1</SUP> for MFR-1 and 50-65 km
s<SUP>-1</SUP> for MFR-2-and a slow-expansion (or untwisting) stage
with speeds of about 10-35 km s<SUP>-1</SUP> for MFR-1 and 10-30 km
s<SUP>-1</SUP> for MFR-2 in the plane of the sky. We also find that
during the two-stage evolution, the high-temperature features mainly
appear in the interface region between MFRs and ambient magnetic
structures and also in the center of MFRs, which suggests that some
heating processes take place in such places as magnetic reconnection
and plasma compression. These observational results indicate that the
eruption and untwisting processes of MFRs are coupled with the heating
process, among which an energy conversion exists.
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Title: Annihilation of Magnetic Islands at the Top of Solar Flare
Loops
Authors: Wang, Yulei; Cheng, Xin; Ding, Mingde; Lu, Quanming
2021ApJ...923..227W Altcode: 2021arXiv211008526W
The dynamics of magnetic reconnection in the solar current sheet (CS)
is studied by high-resolution 2.5-dimensional MHD simulation. With the
commencing of magnetic reconnection, a number of magnetic islands are
formed intermittently and move quickly upward and downward along the
CS. Upon collision with the semi-closed flux of the flare loops, the
downflow islands cause a second reconnection with a rate comparable
with that in the main CS. Though the time-integrated magnetic energy
release is still dominated by the reconnection in the main CS,
the second reconnection can release substantial magnetic energy,
annihilating the main islands and generating secondary islands with
various scales at the flare loop top. The distribution function of
the flux of the secondary islands is found to follow a power law
varying from $f\left(\psi \right)\sim {\psi }^{-1}$ (small scale)
to ψ <SUP>-2</SUP> (large scale), which seems to be independent to
background plasma β and thermal conduction (TC). However, the spatial
scale and the strength of the termination shocks driven by the main
reconnection outflows or islands decrease if β increases or if TC
is included. We suggest that the annihilation of magnetic islands at
the flare loop top, which is not included in the standard flare model,
plays a nonnegligible role in releasing magnetic energy to heat flare
plasma and accelerate particles.
---------------------------------------------------------
Title: Revisiting the Spectral Features of Ellerman Bombs and UV
Bursts. I. Radiative Hydrodynamic Simulations
Authors: Hong, Jie; Li, Ying; Ding, M. D.; Hao, Qi
2021ApJ...921...50H Altcode: 2021arXiv210802699H
Ellerman bombs (EBs) and UV bursts are both small-scale solar activities
that occur in active regions. They are now believed to form at different
heights in the lower atmosphere. In this paper, we use one-dimensional
radiative hydrodynamic simulations to calculate various line profiles
in response to heating in different atmospheric layers. We confirm
that heating in the upper photosphere to the lower chromosphere can
generate spectral features of typical EBs, while heating in the mid
to upper chromosphere can generate spectral features of typical UV
bursts. The intensity evolution of the Hα line wing in EBs shows a
rise-plateau pattern, while that of the Si IV 1403 Å line center in UV
bursts shows a rise-fall pattern. However, the predicted enhancement
of the FUV continuum near 1400 Å for EBs is rarely reported and
requires further observations to check it. With two heating sources
or an extended heating source in the atmosphere, both EB and UV burst
features could be reproduced simultaneously.
---------------------------------------------------------
Title: Data-constrained Magnetohydrodynamic Simulation of a
Long-duration Eruptive Flare
Authors: Guo, Yang; Zhong, Ze; Ding, M. D.; Chen, P. F.; Xia, Chun;
Keppens, Rony
2021ApJ...919...39G Altcode: 2021arXiv210615080G
We perform a zero-β magnetohydrodynamic simulation for the C7.7
class flare initiated at 01:18 UT on 2011 June 21 using the Message
Passing Interface Adaptive Mesh Refinement Versatile Advection Code
(MPI-AMRVAC). The initial condition for the simulation involves a
flux rope, which we realize through the regularized Biot-Savart laws,
whose parameters are constrained by observations from the Atmospheric
Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and
the Extreme Ultraviolet Imager (EUVI) on the twin Solar Terrestrial
Relations Observatory (STEREO). This data-constrained initial state
is then relaxed to a force-free state by the magnetofrictional module
in MPI-AMRVAC. The further time-evolving simulation results reproduce
the eruption characteristics obtained by SDO/AIA 94 Å, 304 Å, and
STEREO/EUVI 304 Å observations fairly well. The simulated flux rope
possesses similar eruption direction, height range, and velocity to
the observations. In particular, the two phases of slow evolution
and fast eruption are reproduced by varying the density distribution
in the light of the draining process of the filament material. Our
data-constrained simulations also show other advantages, such as a
large field of view (about 0.76 R<SUB>⊙</SUB>). We study the twist
of the magnetic flux rope and the decay index of the overlying field,
and find that in this event, both the magnetic strapping force and the
magnetic tension force are sufficiently weaker than the magnetic hoop
force, thus allowing the successful eruption of the flux rope. We also
find that the anomalous resistivity is necessary to keep the correct
morphology of the erupting flux rope.
---------------------------------------------------------
Title: Extreme-ultraviolet Late Phase in Homologous Solar Flares
from a Complex Active Region
Authors: Zhong, Y.; Dai, Y.; Ding, M. D.
2021ApJ...916...37Z Altcode: 2021arXiv210510069Z
Recent observations in extreme-ultraviolet (EUV) wavelengths reveal a
new late phase in some solar flares, which is seen as a second peak in
warm coronal emissions (~3 MK) several tens of minutes to a few hours
after the soft X-ray peak. The origin of the EUV late phase (ELP)
is explained by either a long-lasting cooling process in the long
ELP loops or a delayed energy ejection into the ELP loops well after
the main flare heating. Using the observations with the Solar Dynamics
Observatory, we investigate the production of the ELP in six homologous
flares (F1-F6) originating from a complex active region (AR), NOAA
Active Region 11283, with an emphasis on the emission characteristics
of the flares. It is found that the main production mechanism of the
ELP changes from additional heating in flare F1 to long-lasting cooling
in flares F3-F6, with both mechanisms playing a role in flare F2. The
transition is evidenced by an abrupt decrease of the time lag of the ELP
peak, and the long-lasting cooling process in the majority of the flares
is validated by a positive correlation between the flare ribbon fluence
and the ELP peak intensity. We attribute the change in ELP production
mechanism to an enhancement of the envelope magnetic field above the
AR, which facilitates a more prompt and energetic heating of the ELP
loops. In addition, the last and the only confined flare F6 exhibits
an extremely large ELP. The different emission pattern revealed in this
flare may reflect a different energy partitioning inside the ELP loops,
which is due to a different magnetic reconnection process.
---------------------------------------------------------
Title: Thermodynamic Evolution of Solar Flare Supra-arcade Downflows
Authors: Li, Z. F.; Cheng, X.; Ding, M. D.; Reeves, Katharine K.;
Kittrell, DeOndre; Weber, Mark; McKenzie, David E.
2021ApJ...915..124L Altcode: 2021arXiv210709215L
Solar flares are rapid energy release phenomena that appear as bright
ribbons in the chromosphere and high temperature loops in the corona,
respectively. Supra-arcade Downflows (SADs) are plasma voids that first
come out above the flare loops and then move quickly toward the flare
loop top during the decay phase of the flare. In our work, we study
20 SADs appearing in three flares. By differential emission measure
(DEM) analysis, we calculate the DEM weighted average temperature and
emission measure of the front region and the main body of SADs. It is
found that the temperatures of the SAD front and body tend to increase
during the course of SADs flowing downwards. The relationship between
the pressure and temperature fits well with the adiabatic equation for
both the SAD front and body, suggesting that the heating of SADs is
mainly caused by adiabatic compression. Moreover, we also estimate
the velocities of SADs via the Fourier Local Correlation Tracking
method and find that increase of the temperature of the SAD front
presents a correlation with the decrease of the SAD kinetic energy,
while the SAD body does not, implying that the viscous process may
also heat the SAD front in spite of a limited role.
---------------------------------------------------------
Title: Revealing the structure of light pseudoscalar mesons at the
electron–ion collider
Authors: Arrington, J.; Gayoso, C. Ayerbe; Barry, P. C.; Berdnikov,
V.; Binosi, D.; Chang, L.; Diefenthaler, M.; Ding, M.; Ent, R.;
Frederico, T.; Furletova, Y.; Hobbs, T. J.; Horn, T.; Huber, G. M.;
Kay, S. J. D.; Keppel, C.; Lin, H. -W.; Mezrag, C.; Montgomery, R.;
Pegg, I. L.; Raya, K.; Reimer, P.; Richards, D. G.; Roberts, C. D.;
Rodríguez-Quintero, J.; Romanov, D.; Salmè, G.; Sato, N.; Segovia,
J.; Stepanov, P.; Tadepalli, A. S.; Trotta, R. L.
2021JPhG...48g5106A Altcode: 2021arXiv210211788A
The questions of how the bulk of the Universe's visible mass
emerges and how it is manifest in the existence and properties of
hadrons are profound, and probe the heart of strongly interacting
matter. Paradoxically, the lightest pseudoscalar mesons appear
to be key to a further understanding of the emergent mass and
structure mechanisms. These mesons, namely, the pion and kaon,
are the Nambu–Goldstone boson modes of quantum chromodynamics
(QCD). Unravelling their partonic structure and the interplay
between emergent and Higgs-boson mass mechanisms is a common goal
of three interdependent approaches—continuum QCD phenomenology,
lattice-regularised QCD, and the global analysis of parton
distributions—linked to experimental measurements of hadron
structure. Experimentally, the anticipated electron–ion collider will
enable a revolution in our ability to study pion and kaon structures,
accessed by scattering from the 'meson cloud' of the proton through the
Sullivan process. With the goal of enabling a suite of measurements that
can address these questions, we examine key reactions that identify the
critical detector-system requirements needed to map tagged pion and kaon
cross-sections over a wide range of kinematics. The excellent prospects
for extracting pion structural, functional, and form-factor data are
outlined, and similar prospects for kaon structures are discussed in
the context of a worldwide programme. The successful completion of the
programme outlined herein will deliver deep, far-reaching insights
into the emergence of pions and kaons, their properties, and their
role as QCD's Goldstone boson modes.
---------------------------------------------------------
Title: Evolution of the Toroidal Flux of CME Flux Ropes During
Eruption
Authors: Xing, C.; Cheng, X.; Ding, M.
2021AAS...23832206X Altcode:
Coronal mass ejections (CMEs) are large-scale explosions of the
coronal magnetic field. It is believed that magnetic reconnection
significantly builds up the core structure of CMEs, a magnetic flux
rope, during the eruption. However, the quantitative evolution of the
flux rope, particularly its toroidal flux, is still unclear. In this
work, we study the evolution of the toroidal flux of the CME flux rope
for four events. The toroidal flux is estimated as the magnetic flux
in the footpoint region of the flux rope, which is identified by a
method that simultaneously takes the coronal dimming and the hook of
the flare ribbon into account. We find that the toroidal flux of the
CME flux rope for all four events shows a two-phase evolution: a rapid
increasing phase followed by a decreasing phase. We further compare
the evolution of the toroidal flux with that of the Geostationary
Operational Environmental Satellites soft X-ray flux and find that
they are basically synchronous in time, except that the peak of the
former is somewhat delayed. The results suggest that the toroidal flux
of the CME flux rope may be first quickly built up by the reconnection
mainly taking place in the sheared overlying field and then reduced by
the reconnection among the twisted field lines within the flux rope,
as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.
---------------------------------------------------------
Title: Exploring the Nature of EUV Waves in a Radiative
Magnetohydrodynamic Simulation
Authors: Wang, Can; Chen, Feng; Ding, Mingde
2021ApJ...911L...8W Altcode: 2021arXiv210310326W
Coronal extreme-ultraviolet (EUV) waves are large-scale disturbances
propagating in the corona, whose physical nature and origin have been
discussed for decades. We report the first three-dimensional (3D)
radiative magnetohydrodynamic simulation of a coronal EUV wave and the
accompanying quasi-periodic wave trains. The numerical experiment is
conducted with the MURaM code and simulates the formation of solar
active regions through magnetic flux emergence from the convection
zone to the corona. The coronal EUV wave is driven by the eruption
of a magnetic flux rope that also gives rise to a C-class flare. It
propagates in a semicircular shape with an initial speed ranging
from about 550 to 700 km s<SUP>-1</SUP>, which corresponds to an
average Mach number (relative to fast magnetoacoustic waves) of about
1.2. Furthermore, the abrupt increase of the plasma density, pressure,
and tangential magnetic field at the wave front confirms fast-mode
shock nature of the coronal EUV wave. Quasi-periodic wave trains with
a period of about 30 s are found as multiple secondary wavefronts
propagating behind the leading wave front and ahead of the erupting
magnetic flux rope. We also note that the true wave front in the 3D
space can be very inhomogeneous; however, the line-of-sight integration
of EUV emission significantly smoothes the sharp structures in 3D and
leads to a more diffuse wave front.
---------------------------------------------------------
Title: Crustal Annulus of Impact Basins Controlled by Regional
Thermal State of the Moon
Authors: Ding, M.; Zhu, M. -H.
2021LPI....52.1097D Altcode:
We recognize that the PKT impact basins show no crustal annulus
that commonly exists for the FHT basins. We attribute the different
crustal annulus to regional thermal state and use impact dynamic and
viscoelastic models to test this hypothesis.
---------------------------------------------------------
Title: The role of non-axisymmetry of magnetic flux rope in
constraining solar eruptions
Authors: Zhong, Ze; Guo, Yang; Ding, M. D.
2021NatCo..12.2734Z Altcode: 2021arXiv210507339Z
Whether a solar eruption is successful or failed depends on the
competition between different components of the Lorentz force exerting
on the flux rope that drives the eruption. The present models only
consider the strapping force generated by the background magnetic
field perpendicular to the flux rope and the tension force generated
by the field along the flux rope. Using the observed magnetic field
on the photosphere as a time-matching bottom boundary, we perform a
data-driven magnetohydrodynamic simulation for the 30 January 2015
confined eruption and successfully reproduce the observed solar flare
without a coronal mass ejection. Here we show a Lorentz force component,
resulting from the radial magnetic field or the non-axisymmetry of the
flux rope, which can essentially constrain the eruption. Our finding
contributes to the solar eruption model and presents the necessity
of considering the topological structure of a flux rope when studying
its eruption behaviour.
---------------------------------------------------------
Title: From Nonlinear Force-free Field Models to Data-driven
Magnetohydrodynamic Simulations
Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding,
Mingde; Yang, Kai; Zhong, Ze
2021cosp...43E1777G Altcode:
To study the origin, structures, and dynamics of various solar
activities, such as flares, prominences/filaments, and coronal
mass ejections, we have to know the 3D magnetic field in the solar
corona. Since many static phenomena in the corona live in a low
beta environment, they can be modelled as a force-free state. We
have implemented a new nonlinear force-free field (NLFFF) algorithm
in the Message Passing Interface Adaptive Mesh Refinement Versatile
Advection Code (MPI-AMRVAC), which could construct an NLFFF model in
both Cartesian and spherical coordinate systems, and in all uniform,
adaptive mesh refinement, and stretched grids. The NLFFF models
have been applied to observations to study the magnetic structures
of flux ropes, the coronal emission in extreme ultraviolet lines
and the morphology of flare ribbons. To further study the dynamic
eruption of a magnetic flux rope, we have developed a data-driven
magnetohydrodynamic (MHD) model using the zero-beta MHD equations. The
NLFFF model is served as the initial condition, and the time series
of observed magnetic field and velocity field provide the boundary
conditions. This model can reproduce the evolution of a magnetic flux
rope in its dynamic eruptive phase. We also find that a data-constrained
boundary condition, where the bottom boundary is fixed to the initial
values, reproduces a similar simulation result as the data-driven
simulation. The data-driven MHD model has also been applied to study
a failed eruption, where the torus instability, kink instability,
and additional components of Lorentz forces are studied in detail.
---------------------------------------------------------
Title: The magnetic flux rope structure of a triangulated solar
filament
Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding,
Mingde; Xu, Yu
2021cosp...43E1734G Altcode:
We construct a magnetic flux rope model for a prominence observed at
01:11 UT on 2011 June 21 in AR 11236 using the following methods,
triangulation from multi perspective observations, the flux
rope embedding method, the regularized Biot-Savart laws, and the
magnetofrictional method. First, the prominence path is reconstructed
with the triangulation with 304 Å images observed by the Atmospheric
Imaging Assembly on board Solar Dynamics Observatory (SDO) and by the
Extreme Ultraviolet Imager on board the twin Solar Terrestrial Relations
Observatory. Then, a flux rope is constructed with the regularized
Biot-Savart laws using the information of its axis. Next, it is embedded
into a potential magnetic field computed from the photospheric radial
magnetic field observed by the Helioseismic and Magnetic Imager on
board SDO. The combined magnetic field is finally relaxed by the
magnetofrictional method to reach a nonlinear force-free state. It
is found that both models constructed by the regularized Biot-Savart
laws and after the magnetofrictional relaxation coincide with the
304 Å images. The distribution of magnetic dips coincides with part
of the prominence material, and the quasi-separatrix layers wrap the
magnetic flux ropes, displaying hyperbolic flux tube structures. These
models have the advantages of constructing magnetic flux ropes in the
higher atmosphere and weak magnetic field regions, which could be used
as initial conditions for magnetohydrodynamic simulations of coronal
mass ejections.
---------------------------------------------------------
Title: Higgs modulation of emergent mass as revealed in kaon and
pion parton distributions
Authors: Cui, Z. -F.; Ding, M.; Gao, F.; Raya, K.; Binosi, D.; Chang,
L.; Roberts, C. D.; Rodríguez-Quintero, J.; Schmidt, S. M.
2021EPJA...57....5C Altcode: 2020arXiv200614075C
Strangeness was discovered roughly seventy years ago, lodged in a
particle now known as the kaon, K. Kindred to the pion, π ; both
states are massless in the absence of Higgs-boson couplings. Kaons
and pions are Nature's most fundamental Nambu-Goldstone modes. Their
properties are largely determined by the mechanisms responsible for
emergent mass in the standard model, but modulations applied by the
Higgs are crucial to Universe evolution. Despite their importance,
little is known empirically about K and π structure. This study
delivers the first parameter-free predictions for all K distribution
functions (DFs) and comparisons with the analogous π distributions,
i.e. the one-dimensional maps that reveal how the light-front momentum
of these states is shared amongst the gluons and quarks from which they
are formed. The results should stimulate improved analyses of existing
data and motivate new experiments sensitive to all K and π DFs.
---------------------------------------------------------
Title: Diquark correlations in hadron physics: Origin, impact and
evidence
Authors: Barabanov, M. Yu.; Bedolla, M. A.; Brooks, W. K.; Cates,
G. D.; Chen, C.; Chen, Y.; Cisbani, E.; Ding, M.; Eichmann, G.; Ent,
R.; Ferretti, J.; Gothe, R. W.; Horn, T.; Liuti, S.; Mezrag, C.;
Pilloni, A.; Puckett, A. J. R.; Roberts, C. D.; Rossi, P.; Salmé,
G.; Santopinto, E.; Segovia, J.; Syritsyn, S. N.; Takizawa, M.;
Tomasi-Gustafsson, E.; Wein, P.; Wojtsekhowski, B. B.
2021PrPNP.11603835B Altcode: 2020arXiv200807630B
The last decade has seen a marked shift in how the internal structure of
hadrons is understood. Modern experimental facilities, new theoretical
techniques for the continuum bound-state problem and progress with
lattice-regularised QCD have provided strong indications that soft
quark+quark (diquark) correlations play a crucial role in hadron
physics. For example, theory indicates that the appearance of such
correlations is a necessary consequence of dynamical chiral symmetry
breaking, viz. a corollary of emergent hadronic mass that is responsible
for almost all visible mass in the universe; experiment has uncovered
signals for such correlations in the flavour-separation of the proton's
electromagnetic form factors; and phenomenology suggests that diquark
correlations might be critical to the formation of exotic tetra- and
penta-quark hadrons. A broad spectrum of such information is evaluated
herein, with a view to consolidating the facts and therefrom moving
toward a coherent, unified picture of hadron structure and the role
that diquark correlations might play.
---------------------------------------------------------
Title: Spectroscopic Observations of High-speed Downflows in a C1.7
Solar Flare
Authors: Zhou, Yi-An; Li, Y.; Ding, M. D.; Hong, Jie; Yu, Ke
2020ApJ...904...95Z Altcode: 2020arXiv200906158Z
In this paper, we analyze the high-resolution UV spectra for a
C1.7 solar flare (SOL2017-09-09T06:51) observed by the Interface
Region Imaging Spectrograph (IRIS). We focus on the spectroscopic
observations at the locations where the cool lines of Si IV 1402.8 Å
(∼10<SUP>4.8</SUP> K) and C II 1334.5/1335.7 Å (∼10<SUP>4.4</SUP>
K) reveal significant redshifts with Doppler velocities up to ∼150 km
s<SUP>-1</SUP>. These redshifts appear in the rise phase of the flare,
then increase rapidly, reach the maximum in a few minutes, and proceed
into the decay phase. Combining the images from IRIS and Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory, we propose
that the redshifts in the cool lines are caused by the downflows in
the transition region and upper chromospheric layers, which likely
result from a magnetic reconnection leading to the flare. In addition,
the cool Si IV and C II lines show gentle redshifts (a few tens of km
s<SUP>-1</SUP>) at some other locations, which manifest some distinct
features from the above locations. This is supposed to originate from
a different physical process.
---------------------------------------------------------
Title: The Kinematic Evolution of Erupting Structures in Confined
Solar Flares
Authors: Huang, Z. W.; Cheng, X.; Ding, M. D.
2020ApJ...904L...2H Altcode: 2020arXiv201111929H
In this Letter, we study the kinematic properties of ascending hot
blobs associated with confined flares. Taking advantage of high-cadence
extreme-ultraviolet images provided by the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory, we find that for the 26
events selected here, the hot blobs are first impulsively accelerated
outward, but then quickly slow down to motionlessness. Their velocity
evolution is basically synchronous with the temporal variation of
the Geostationary Operational Environmental Satellite soft X-ray flux
of the associated flares, except that the velocity peak precedes the
soft X-ray peak by minutes. Moreover, the duration of the acceleration
phase of the erupting blobs is moderately correlated with that of the
flare rise phase. For nine of the 26 cases, the erupting blobs even
appear minutes prior to the onset of the associated flares. Our results
show that a fraction of confined flares also involve the eruption of
a magnetic flux rope, which sometimes is formed and heated prior to
the flare onset. We suggest that the initiation and development of
these confined flares are similar to that of eruptive ones, and the
main difference may lie in the background field constraint, which is
stronger for the former than for the latter.
---------------------------------------------------------
Title: The Lyα Emission in Solar Flares. I. A Statistical Study on
Its Relationship with the 1-8 ŠSoft X-Ray Emission
Authors: Jing, Zhichen; Pan, Wuqi; Yang, Yukun; Song, Dechao; Tian,
Jun; Li, Y.; Cheng, X.; Hong, Jie; Ding, M. D.
2020ApJ...904...41J Altcode: 2020arXiv200910358J
We statistically study the relationship between the Lyα and 1-8 Å soft
X-ray (SXR) emissions from 658 M- and X-class solar flares observed
by the Geostationary Operational Environmental Satellite during
2006-2016. Based on the peak times of the two wave band emissions,
we divide the flares into three types. Type I (III) has an earlier (a
later) peak time in the Lyα emission than that in the SXR emission,
while type II has nearly the same peak time (within the time resolution
of 10 s) between the Lyα and SXR emissions. In these 658 flares,
we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II
flares, and 143 (21.7%) type III flares, and that the three types
appear to have no dependence on the flare duration, flare location,
or solar cycle. Besides the main peak, the Lyα emission of the three
type flares also shows sub-peaks which can appear in the impulsive or
gradual phase of the flare. It is found that the main-peak (for type
I) and subpeak (for type III) emissions of Lyα that appear in the
impulsive phase follow the Neupert effect in general. This indicates
that such Lyα emissions are related to the nonthermal electron beam
heating. While the main-peak (for type III) and subpeak (for type I)
emissions of Lyα that appear in the gradual phase are supposed to be
primarily contributed by the thermal plasma that cools down.
---------------------------------------------------------
Title: Evolution of the Toroidal Flux of CME Flux Ropes during
Eruption
Authors: Xing, C.; Cheng, X.; Ding, M. D.
2020Innov...100059X Altcode: 2020arXiv201110750X
Coronal mass ejections (CMEs) are large-scale explosions of the coronal
magnetic field. It is believed that magnetic reconnection significantly
builds up the core structure of CMEs, a magnetic flux rope, during
the eruption. However, the quantitative evolution of the flux rope,
particularly its toroidal flux, is still unclear. In this paper,
we study the evolution of the toroidal flux of the CME flux rope
for four events. The toroidal flux is estimated as the magnetic flux
in the footpoint region of the flux rope, which is identified by a
method that simultaneously takes the coronal dimming and the hook of
the flare ribbon into account. We find that the toroidal flux of the
CME flux rope for all four events shows a two-phase evolution: a rapid
increasing phase followed by a decreasing phase. We further compare
the evolution of the toroidal flux with that of the Geostationary
Operational Environmental Satellites soft X-ray flux and find that
they are basically synchronous in time, except that the peak of the
former is somewhat delayed. The results suggest that the toroidal flux
of the CME flux rope may be first quickly built up by the reconnection
mainly taking place in the sheared overlying field and then reduced by
the reconnection among the twisted field lines within the flux rope,
as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.
---------------------------------------------------------
Title: The Magnetic Topology and Eruption Mechanism of a
Multiple-ribbon Flare
Authors: Qiu, Ye; Guo, Yang; Ding, Mingde; Zhong, Ze
2020ApJ...901...13Q Altcode: 2020arXiv200808866Q
Multiple-ribbon flares are usually complex in their magnetic topologies
and eruption mechanisms. In this paper, we investigate an X2.1 flare
(SOL2015-03-11T16:22) that occurred in active region 12297 near the
center of the solar disk by both potential and nonlinear force-free
field models extrapolated with the data observed by the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory. We calculate
the three-dimensional squashing degree distribution. The results
reveal that there are two flux ropes in this active region covered by
a large-scale hyperbolic flux tube (HFT), which is the intersection
of quasi-separatrix layers with a null point embedded in it. When
the background magnetic field diminishes due to the separation of the
northwest dipole and the flux cancellation, the central flux rope rises
up, forming the two brightest central ribbons. It then squeezes the
overlying HFT structure to generate further brightenings. This very
energetic flare with a complex shape is accompanied by a coronal mass
ejection (CME). We adopt the simplified line-tied force-balance equation
of the current ring model and assign the observed value of the decay
index to the equation to simulate the acceleration profile of the CME
in the early stage. It is found that the path with an inclination of
45° from radial best fits the profile of the actual acceleration.
---------------------------------------------------------
Title: Spectral Diagnostics of Solar Photospheric Bright Points
Authors: Hao, Q.; Fang, C.; Ding, M. D.; Li, Z.; Cao, Wenda
2020ApJ...900..130H Altcode: 2020arXiv200709675H
Through the use of the high-resolution spectral data and the broadband
imaging obtained with the Goode Solar Telescope at the Big Bear Solar
Observatory on 2013 June 6, the spectra of three typical photospheric
bright points (PBPs) have been analyzed. Based on the Hα and Ca II
8542 Å line profiles, as well as the TiO continuum emission, for
the first time, the non-LTE semiempirical atmospheric models for the
PBPs are computed. The attractive characteristic is the temperature
enhancement in the lower photosphere. The temperature enhancement is
about 200-500 K at the same column mass density as in the atmospheric
model of the quiet-Sun. The total excess radiative energy of a typical
PBP is estimated to be 1 × 10<SUP>27</SUP>-2 × 10<SUP>27</SUP> erg,
which can be regarded as the lower limit energy of the PBPs. The
radiation flux in the visible continuum for the PBPs is about 5.5
× 10<SUP>10</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>. Our result
also indicates that the temperature in the atmosphere above PBPs
is close to that of a plage. It gives clear evidence that PBPs may
contribute significantly to the heating of the plage atmosphere. Using
our semiempirical atmospheric models, we estimate self-consistently
the average magnetic flux density B in the PBPs. It is shown that the
maximum value is about 1 kG, and it decreases toward both higher and
lower layers, reminding us of the structure of a flux tube between
photospheric granules.
---------------------------------------------------------
Title: Non-LTE Calculations of the Mg I 12.32 μm Line in a Flaring
Atmosphere
Authors: Hong, Jie; Bai, Xianyong; Li, Ying; Ding, M. D.; Deng,
Yuanyong
2020ApJ...898..134H Altcode: 2020arXiv200606108H
The infrared Mg I lines near 12 μm are a pair of emission lines that
are magnetically sensitive and have been used to measure solar magnetic
fields. Here we calculate the response of the Mg I 12.32 μm line during
a flare and find that in our modeling this line has a complicated
behavior. At the beginning of the flare heating, this line shows an
intensity dimming at the line center. The intensity then increases
when heating continues, with increasing contributions from the heated
layers in the chromosphere. The line formation height and the line
width also increase as a result. As for the polarized line profiles,
we find that flare heating tends to decrease the Zeeman splitting
width and attenuates the Stokes V lobe intensity. The wider features
in the Stokes V profiles are more pronounced during flare heating,
which should be considered when performing magnetic field inversions.
---------------------------------------------------------
Title: What Determines Solar Flares Producing Interplanetary Type
III Radio Bursts?
Authors: Kou, Y. K.; Jing, Z. C.; Cheng, X.; Pan, W. Q.; Liu, Y.;
Li, C.; Ding, M. D.
2020ApJ...898L..24K Altcode: 2020arXiv200703852K
Energetic electrons accelerated by solar flares often give rise to type
III radio bursts at a broad wave band and even interplanetary type III
bursts (IT3) if the wavelength extends to a decameter-kilometer. In this
Letter, we investigate the probability of the flares that produce IT3,
based on the sample of 2272 flares above M-class observed from 1996
to 2016. It is found that only 49.6% of the flares are detected to
be accompanied with IT3. The duration, peak flux, and fluence of the
flares with and without IT3 both present power-law distributions in
the frequency domain, but the corresponding spectral indices for the
former (2.06 ± 0.17, 2.04 ± 0.18, and 1.55 ± 0.09) are obviously
smaller than that for the latter (2.82 ± 0.22, 2.51 ± 0.19, and 2.40
± 0.09), showing that the flares with IT3 have longer durations and
higher peak fluxes. We further examine the relevance of coronal mass
ejections (CMEs) to the two groups of flares. It is found that 58%
(655 of 1127) of the flares with IT3 but only 19% (200 of 1078) of the
flares without IT3 are associated with CMEs, and that the associated
CMEs for the flares with IT3 are inclined to be wider and faster. This
indicates that CMEs may also play a role in producing IT3, speculatively
facilitating the escape of accelerated electrons from the low corona
to the interplanetary space.
---------------------------------------------------------
Title: New Accurate Atomic Data for Iron-group Elements for
Astrophysics Applications
Authors: Concepcion Mairey, F.; Belmonte, M.; Clear, C.; Liggins,
F.; Pickering, J.; Ding, M.
2020AAS...23624706C Altcode:
Modern observations of astrophysical spectra are in many cases of
higher quality than those observed within a laboratory setting. This can
result in the possibility of inaccurate conclusions being drawn. There
is a need in astrophysics for improved atomic data for light and heavy
elements across the spectrum, from IR to vacuum-UV (VUV). Accurate
measurements of spectral line wavelengths and oscillator strengths
are required particularly for use in stellar models and chemical
abundance calculations, and in surveys such as Gaia-ESO or APOGEE
and future surveys. Laboratory astrophysicists aim to measure the
atomic data most useful for astronomers. Atomic data of iron-group
elements are particularly important due to their high abundance and
line-rich spectra. Our high-resolution Fourier Transform Spectrometry
(FTS) group at Imperial College London, UK, has, supported by STFC,
been providing accurate atomic data for use in astrophysics. Recent
results include new Fe I oscillator strengths for use in Galactic
surveys (Belmonte et al. 2017). There has been an order of magnitude
improvement in atomic data for Co III (Smillie et al. 2016) and in
the accuracy of energy levels and transition wavelengths for Mn II
(Liggins, PhD Physics, Imperial College, 2018) and Ni II (Clear,
PhD Physics, Imperial College, 2018). The first high-resolution
measurements of UV transition wavelengths of Cr III are being used
as wavelength standards (Smillie et al. 2008). Using new spectra
recorded at the National Institute of Standards and Technology (NIST)
and in the VUV at Imperial College, analysis is underway for accurate
wavelengths and atomic energy levels in Mn I and Fe III. The study of
the hyperfine structure of Co II is also ongoing using FTS data.
---------------------------------------------------------
Title: IRIS Si IV Line Profiles at Flare Ribbons as Indications of
Chromospheric Condensation
Authors: Yu, Ke; Li, Y.; Ding, M. D.; Li, D.; Zhou, Yi-An; Hong, Jie
2020ApJ...896..154Y Altcode: 2020arXiv200502029Y
We present temporal variations of the Si IV line profiles at the
flare ribbons in three solar flares observed by the Interface Region
Imaging Spectrograph. In the M1.1 flare on 2014 September 6 and the X1.6
flare on 2014 September 10, the Si IV line profiles evolve from wholly
redshifted to red-wing enhanced with the flare development. However,
in the B1.8 flare on 2016 December 2, the Si IV line profiles are
wholly redshifted throughout the flare evolution. We fit the wholly
redshifted line profiles with a single-Gaussian function, but fit
the red-asymmetric ones with a double-Gaussian function to deduce the
corresponding Doppler velocities. In addition, we find that hard X-ray
emission above 25 keV shows up in the two large flares, implying a
nonthermal electron beam heating. In the microflare, there only appears
weak hard X-ray emission up to 12 keV, indicative of a mostly thermal
heating. We interpret the redshifts or red asymmetries of the Si IV
line at the ribbons in the three flares as spectral manifestations of
chromospheric condensation. We propose that whether the line appears
to be wholly redshifted or red-asymmetric depends on the heating
mechanisms, as well as on the propagation of the condensation.
---------------------------------------------------------
Title: Initiation and Early Kinematic Evolution of Solar Eruptions
Authors: Cheng, X.; Zhang, J.; Kliem, B.; Török, T.; Xing, C.;
Zhou, Z. J.; Inhester, B.; Ding, M. D.
2020ApJ...894...85C Altcode: 2020arXiv200403790C
We investigate the initiation and early evolution of 12 solar eruptions,
including six active-region hot channel and six quiescent filament
eruptions, which were well observed by the Solar Dynamics Observatory,
as well as by the Solar Terrestrial Relations Observatory for the
latter. The sample includes one failed eruption and 11 coronal mass
ejections, with velocities ranging from 493 to 2140 km s<SUP>-1</SUP>. A
detailed analysis of the eruption kinematics yields the following main
results. (1) The early evolution of all events consists of a slow-rise
phase followed by a main-acceleration phase, the height-time profiles
of which differ markedly and can be best fit, respectively, by a linear
and an exponential function. This indicates that different physical
processes dominate in these phases, which is at variance with models
that involve a single process. (2) The kinematic evolution of the
eruptions tends to be synchronized with the flare light curve in both
phases. The synchronization is often but not always close. A delayed
onset of the impulsive flare phase is found in the majority of the
filament eruptions (five out of six). This delay and its trend to be
larger for slower eruptions favor ideal MHD instability models. (3)
The average decay index at the onset heights of the main acceleration
is close to the threshold of the torus instability for both groups
of events (although, it is based on a tentative coronal field model
for the hot channels), suggesting that this instability initiates and
possibly drives the main acceleration.
---------------------------------------------------------
Title: Erratum: "Modeling the IRIS Lines During a Flare. I. The
Blue-wing Enhancement in the Mg II k Line" (2020, ApJ, 890, 115)
Authors: Hong, Jie; Li, Ying; Ding, M. D.; Zhou, Yu-Hao
2020ApJ...894..160H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Control Parameters on Gravitational Signature of Large Impact
Craters on the Moon
Authors: Ding, M.; Soderblom, J. M.; Nimmo, F.; Bierson, C. J.; Zuber,
M. T.
2020LPI....51.1329D Altcode:
The lack of statistical distinction in the Bouguer anomaly between
large FHT and SPA craters is due to competing effects of thermal state
and crustal thickness.
---------------------------------------------------------
Title: Quantifying the Toroidal Flux of Preexisting Flux Ropes of
Coronal Mass Ejections
Authors: Xing, C.; Cheng, X.; Qiu, Jiong; Hu, Qiang; Priest, E. R.;
Ding, M. D.
2020ApJ...889..125X Altcode: 2019arXiv191210623X
In past decades, much progress has been achieved in understanding
the origin and evolution of coronal mass ejections (CMEs). In situ
observations of the counterparts of CMEs, especially magnetic clouds
(MCs) near the Earth, have provided measurements of the structure
and total flux of CME flux ropes. However, it has been difficult to
measure these properties in an erupting CME flux rope, in particular in
a preexisting flux rope. In this work, we propose a model to estimate
the toroidal flux of a preexisting flux rope by subtracting the flux
contributed by magnetic reconnection during the eruption from the
flux measured in the MC. The flux contributed by the reconnection
is derived from geometric properties of two-ribbon flares based on a
quasi-2D reconnection model. We then apply the model to four CME/flare
events and find that the ratio of toroidal flux in the preexisting flux
rope to that in the associated MC lies in the range 0.40-0.88. This
indicates that the toroidal flux of the preexisting flux rope makes an
important contribution to that of the CME flux rope and is usually at
least as large as the flux arising from the eruption process for the
selected events.
---------------------------------------------------------
Title: Modeling the IRIS Lines During a Flare. I. The Blue-wing
Enhancement in the Mg II k Line
Authors: Hong, Jie; Li, Ying; Ding, M. D.; Zhou, Yu-Hao
2020ApJ...890..115H Altcode: 2020arXiv200105677H
The Interface Region Imaging Spectrograph (IRIS) Mg II k line
serves as a very good tool to diagnose the heating processes in
solar flares. Recent studies have shown that apart from the usual
red asymmetries that are interpreted as the result of condensation
downflows, this line could also show a blue-wing enhancement. To
investigate how such a blue asymmetry is formed, we perform a grid of
radiative hydrodynamic simulations and calculate the corresponding line
profiles. We find that such a spectral feature is likely to originate
from the upward plasma motion in the upper chromosphere. However,
the formation region that is responsible for the blue-wing enhancement
could be located in an evaporation region, in an upward-moving blob, and
even an upward-moving condensation region. We discuss how the electron
beam parameters affect these different dynamics of the atmosphere.
---------------------------------------------------------
Title: Formation and Eruption of a Mini-sigmoid Originating in
Coronal Hole
Authors: Huang, Z. W.; Cheng, X.; Su, Y. N.; Liu, T.; Ding, M. D.
2019ApJ...887..130H Altcode: 2019arXiv191210404H
In this paper, we study in detail the evolution of a mini-sigmiod
originating in a cross-equatorial coronal hole, where the magnetic
field is mostly open and seriously distinct from the closed background
field above active-region sigmoids. The source region first appeared
as a bipole, which subsequently experienced a rapid emergence followed
by a long-term decay. Correspondingly, the coronal structure initially
appeared as arc-like loops, then gradually sheared and transformed into
continuously sigmoidal loops, mainly owing to flux cancellation near the
polarity inversion line. The temperature of J-shaped and sigmoidal loops
is estimated to be about 2.0 × 10<SUP>6</SUP> K, greater than that of
the background coronal hole. Using the flux-rope insertion method, we
further reconstruct the nonlinear force-free fields that well reproduces
the transformation of the potential field into a sigmoidal field. The
fact that the sheared and sigmoidal loops are mainly concentrated at
around the high-Q region implies that the reconnection most likely takes
place there to form the sigmoidal field and heat the plasma. Moreover,
the twist of sigmoidal field lines is estimated to be around 0.8, less
than the values derived for the sigmoids from active regions. However,
the sigmoidal flux may quickly enter an unstable regime at the very
low corona (<10 Mm) due to the open background field. The results
suggest that the mini-sigmoid, at least the one in our study, has the
same formation and eruption process as the large-scale one, but is
significantly influenced by the overlying flux.
---------------------------------------------------------
Title: Particle Acceleration at the Pileup Collision of the Twin Shock
Authors: Wang, Xin; Giacalone, Joe; Yan, Yihua; Ding, Mingde; Li,
Chuan; Lu, Hong; Shan, Hao
2019ApJ...885...66W Altcode:
Ground-level enhancement (GLE) events are often associated with
large gradual solar events such as fast coronal mass ejections
(CMEs), but not all fast CMEs lead to GLE events. Is there a type
of coordinated CME that could produce GLEs with larger intensity and
higher energies than those in the normal fast isolated CMEs? Here we
propose a twin-shock scenario driven by the twin CME coordinately,
in which the posterior shock catches up with the preceding shock and
has a pileup collision. In the present study, we chose the first GLE
event of the solar cycle 24 occurring on 2012 May 17 as an example to
investigate the probable association with the twin-shock scenario. We
use a dynamic Monte Carlo method to examine the energy spectrum with
relevance to the GLE event. In the twin-shock scenario, the seed
energetic particles produced by the normal preceding shock can be
injected into the posterior shock for reacceleration efficiently. As a
result, we obtain the detailed energy spectrum of the solar energetic
particles (SEPs) with different behaviors at the related episodes
of the twin-shock evolution. Therefore, we predict that the pileup
collision of the twin shock would dominate a concave energy spectral
slope in the 2012 May 17 SEP event.
---------------------------------------------------------
Title: The Magnetic Flux Rope Structure of a Triangulated Solar
Filament
Authors: Guo, Yang; Xu, Yu; Ding, M. D.; Chen, P. F.; Xia, Chun;
Keppens, Rony
2019ApJ...884L...1G Altcode:
Solar magnetic flux ropes are core structures driving solar
activities. We construct a magnetic flux rope for a filament/prominence
observed at 01:11 UT on 2011 June 21 in AR 11236 with a combination of
state-of-the-art methods, including triangulation from multiperspective
observations, the flux rope embedding method, the regularized
Biot-Savart laws, and the magnetofrictional method. First, the path
of the filament is reconstructed via the triangulation with 304 Å
images observed by the Atmospheric Imaging Assembly on board Solar
Dynamics Observatory (SDO) and by the Extreme Ultraviolet Imager on
board the twin Solar Terrestrial Relations Observatory. Then, a flux
rope is constructed with the regularized Biot-Savart laws using the
information of its axis. Next, it is embedded into a potential magnetic
field computed from the photospheric radial magnetic field observed by
the Helioseismic and Magnetic Imager on board SDO. The combined magnetic
field is finally relaxed by the magnetofrictional method to reach a
nonlinear force-free state. It is found that both models constructed
by the regularized Biot-Savart laws and after the magnetofrictional
relaxation coincide with the 304 Å images. The distribution of magnetic
dips coincides with part of the filament/prominence material, and
the quasi-separatrix layers wrap the magnetic flux ropes, displaying
hyperbolic flux tube structures. These models have the advantages of
constructing magnetic flux ropes in the higher atmosphere and weak
magnetic field regions, which could be used as initial conditions for
magnetohydrodynamic simulations of coronal mass ejections.
---------------------------------------------------------
Title: Measurement of the cosmic ray proton spectrum from 40 GeV to
100 TeV with the DAMPE satellite
Authors: An, Q.; Asfandiyarov, R.; Azzarello, P.; Bernardini, P.;
Bi, X. J.; Cai, M. S.; Chang, J.; Chen, D. Y.; Chen, H. F.; Chen,
J. L.; Chen, W.; Cui, M. Y.; Cui, T. S.; Dai, H. T.; D'Amone, A.;
De Benedittis, A.; De Mitri, I.; Di Santo, M.; Ding, M.; Dong, T. K.;
Dong, Y. F.; Dong, Z. X.; Donvito, G.; Droz, D.; Duan, J. L.; Duan,
K. K.; D'Urso, D.; Fan, R. R.; Fan, Y. Z.; Fang, F.; Feng, C. Q.;
Feng, L.; Fusco, P.; Gallo, V.; Gan, F. J.; Gao, M.; Gargano, F.; Gong,
K.; Gong, Y. Z.; Guo, D. Y.; Guo, J. H.; Guo, X. L.; Han, S. X.; Hu,
Y. M.; Huang, G. S.; Huang, X. Y.; Huang, Y. Y.; Ionica, M.; Jiang,
W.; Jin, X.; Kong, J.; Lei, S. J.; Li, S.; Li, W. L.; Li, X.; Li,
X. Q.; Li, Y.; Liang, Y. F.; Liang, Y. M.; Liao, N. H.; Liu, C. M.;
Liu, H.; Liu, J.; Liu, S. B.; Liu, W. Q.; Liu, Y.; Loparco, F.; Luo,
C. N.; Ma, M.; Ma, P. X.; Ma, S. Y.; Ma, T.; Ma, X. Y.; Marsella,
G.; Mazziotta, M. N.; Mo, D.; Niu, X. Y.; Pan, X.; Peng, W. X.;
Peng, X. Y.; Qiao, R.; Rao, J. N.; Salinas, M. M.; Shang, G. Z.;
Shen, W. H.; Shen, Z. Q.; Shen, Z. T.; Song, J. X.; Su, H.; Su, M.;
Sun, Z. Y.; Surdo, A.; Teng, X. J.; Tykhonov, A.; Vitillo, S.; Wang,
C.; Wang, H.; Wang, H. Y.; Wang, J. Z.; Wang, L. G.; Wang, Q.; Wang,
S.; Wang, X. H.; Wang, X. L.; Wang, Y. F.; Wang, Y. P.; Wang, Y. Z.;
Wang, Z. M.; Wei, D. M.; Wei, J. J.; Wei, Y. F.; Wen, S. C.; Wu, D.;
Wu, J.; Wu, L. B.; Wu, S. S.; Wu, X.; Xi, K.; Xia, Z. Q.; Xu, H. T.;
Xu, Z. H.; Xu, Z. L.; Xu, Z. Z.; Xue, G. F.; Yang, H. B.; Yang, P.;
Yang, Y. Q.; Yang, Z. L.; Yao, H. J.; Yu, Y. H.; Yuan, Q.; Yue, C.;
Zang, J. J.; Zhang, F.; Zhang, J. Y.; Zhang, J. Z.; Zhang, P. F.;
Zhang, S. X.; Zhang, W. Z.; Zhang, Y.; Zhang, Y. J.; Zhang, Y. L.;
Zhang, Y. P.; Zhang, Y. Q.; Zhang, Z.; Zhang, Z. Y.; Zhao, H.; Zhao,
H. Y.; Zhao, X. F.; Zhou, C. Y.; Zhou, Y.; Zhu, X.; Zhu, Y.; Zimmer, S.
2019SciA....5.3793A Altcode: 2019arXiv190912860A
The precise measurement of the spectrum of protons, the most abundant
component of the cosmic radiation, is necessary to understand the
source and acceleration of cosmic rays in the Milky Way. This work
reports the measurement of the cosmic ray proton fluxes with kinetic
energies from 40 GeV to 100 TeV, with two and a half years of data
recorded by the DArk Matter Particle Explorer (DAMPE). This is the
first time an experiment directly measures the cosmic ray protons up
to ~100 TeV with a high statistics. The measured spectrum confirms
the spectral hardening found by previous experiments and reveals a
softening at ~13.6 TeV, with the spectral index changing from ~2.60 to
~2.85. Our result suggests the existence of a new spectral feature of
cosmic rays at energies lower than the so-called knee, and sheds new
light on the origin of Galactic cosmic rays.
---------------------------------------------------------
Title: The Response of the Lyα Line in Different Flare Heating Models
Authors: Hong, Jie; Li, Ying; Ding, M. D.; Carlsson, Mats
2019ApJ...879..128H Altcode: 2019arXiv190513356H
The solar Lyα line is the strongest line in the ultraviolet waveband,
and is greatly enhanced during solar flares. Here we present
radiative hydrodynamic simulations of solar flares under different
heating models, and calculate the response of this line taking into
account nonequilibrium ionization of hydrogen and partial frequency
redistribution. We find that in nonthermal heating models, the Lyα line
can show a red or blue asymmetry corresponding to the chromospheric
evaporation or condensation, respectively. The asymmetry may change
from red to blue if the electron beam flux is large enough to produce
a significant chromospheric condensation region. In the Lyα intensity
light curve, a dip appears when the change of asymmetry occurs. In
thermal models, the Lyα line intensity peaks quickly and then falls,
and the profile has an overall red asymmetry, which is similar to the
profiles from heating by a soft electron beam. The Lyα profile shows
a single red peak at the end of thermal heating, and the whole line
is formed in a very small height range.
---------------------------------------------------------
Title: Solar Energetic particles at pileup collisions of the
multiple-shock
Authors: Wang, X.; Yan, Y.; Ding, M.
2019ICRC...36.1168W Altcode: 2019PoS...358.1168W
No abstract at ADS
---------------------------------------------------------
Title: Different Signatures of Chromospheric Evaporation in Two
Solar Flares Observed with IRIS
Authors: Li, Y.; Ding, M. D.; Hong, J.; Li, H.; Gan, W. Q.
2019ApJ...879...30L Altcode: 2019arXiv190509709L
We present different signatures of chromospheric evaporation in two
solar flares observed by the Interface Region Imaging Spectrograph
(IRIS). In the B1.6 flare on 2016 December 6 (SOL2016-12-06T10:40),
the transition region Si IV line and the chromospheric C II and Mg II
lines show blueshifts with low velocities up to 20 km s<SUP>-1</SUP>
at the flare loop footpoints in the rise phase, indicative of a gentle
chromospheric evaporation. While in the C1.6 flare on 2015 December
19 (SOL2015-12-19T10:51), the Si IV, C II, and Mg II lines exhibit
redshifts with velocities from several to tens of km s<SUP>-1</SUP>
at the footpoints, which might suggest an explosive chromospheric
evaporation. Explosive evaporation has been observed in many flares
that were captured by IRIS; however, gentle evaporation, especially
manifested as blueshifts in the cool Si IV, C II, and Mg II lines,
has scarcely been reported. Our results bring some new insights into
chromospheric evaporation in the IRIS era.
---------------------------------------------------------
Title: Chinese Sunspot Drawings and Their Digitization - (I)
Parameter Archives
Authors: Lin, G. H.; Wang, X. F.; Liu, S.; Yang, X.; Zhu, G. F.;
Deng, Y. Y.; Ji, H. S.; Zhou, T. H.; Sun, L. N.; Feng, Y. L.; Liu,
Z. Z.; Tao, J. P.; Ben, M. X.; Lin, J.; Ding, M. D.; Li, Z.; Zheng,
S.; Zeng, S. G.; He, H. L.; Zeng, X. Y.; Shu, Y.; Sun, X. B.
2019SoPh..294...79L Altcode: 2019arXiv190412316L
Based on the Chinese historical sunspots drawings, a data set consisting
of the scanned images and all their digitized parameters from 1925
to 2015 have been constructed. In this paper, we briefly describe
the developmental history of sunspots drawings in China. This paper
describes the preliminary handling processes that start from the initial
scanning to the extraction of parameters, and finally summarizes the
general features of this data set. It is the first systematic project
in the Chinese solar-physics community to digitize the historical series
of sunspot drawings. Our data set fills in an almost 90-year historical
gap, which originates from an area spanning 60 degrees from east to
west and 50 degrees from north to south and has no continuous and
detailed digital sunspot observation information. Being complementary
to other sunspot observations in the world, our data set provides
abundant information to the long-term solar activity cycles research.
---------------------------------------------------------
Title: 2.5 m wide-field and high-resolution telescope
Authors: Fang, Cheng; Gu, Bozhong; Yuan, Xiangyan; Ding, Mingde;
Chen, Pengfei; Dai, Zigao; Li, Xiangdong; Shi, Yong; Xie, Jiwei; Bai,
Jinming; Qu, Zhongquan; Hao, Qi; Quo, Yang; Cheng, Xin; Li, Zhen
2019SSPMA..49e9603F Altcode: 2019SSPMA..49e9603C
No abstract at ADS
---------------------------------------------------------
Title: A Statistical Study of the Magnetic Imprints of X-class
Solar Flares
Authors: Lu, Zekun; Cao, Weiguang; Jin, Gaoxiang; Zhang, Yining;
Ding, Mingde; Guo, Yang
2019ApJ...876..133L Altcode: 2018arXiv180308310L
Magnetic imprints, the rapid and irreversible evolution of photospheric
magnetic fields as feedback from flares in the corona, have been
confirmed by many previous studies. These studies showed that the
horizontal field will permanently increase at the polarity inversion
line (PIL) after eruptions, indicating that a more horizontal geometry
of the photospheric magnetic field is produced. In this study, we
analyze 20 X-class flares since the launch of the Solar Dynamics
Observatory in 15 active regions with heliographic angles no greater
than 45°. We observe clear magnetic imprints in 16 flares, whereas 4
flares are exceptional. The imprint regions of the horizontal field
are located not only at the PIL but also at sunspot penumbra with
strong vertical fields. Making use of the observed mass and speed
of the corresponding coronal mass ejections (CMEs), we find that
the CMEs with larger momenta are associated with stronger magnetic
imprints. Furthermore, a linear relationship, with a Kendall’s Tau-b
coefficient 0.54, between the CME momentum and the change of Lorentz
force, is revealed. Based on that, we quantify the back reaction time
to be ∼70 s, with a 90% confidence interval from about 50 to 90 s.
---------------------------------------------------------
Title: Erratum: “Observations of Turbulent Magnetic
Reconnection within a Solar Current Sheet” (<A
href="http://doi.org/10.3847/1538-4357/aadd16">2018, ApJ, 866, 64</A>)
Authors: Cheng, X.; Li, Y.; Wan, L. F.; Ding, M. D.; Chen, P. F.;
Zhang, J.; Liu, J. J.
2019ApJ...874..108C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The on-orbit calibration of DArk Matter Particle Explorer
Authors: Ambrosi, G.; An, Q.; Asfandiyarov, R.; Azzarello, P.;
Bernardini, P.; Cai, M. S.; Caragiulo, M.; Chang, J.; Chen, D. Y.;
Chen, H. F.; Chen, J. L.; Chen, W.; Cui, M. Y.; Cui, T. S.; Dai, H. T.;
D'Amone, A.; De Benedittis, A.; De Mitri, I.; Ding, M.; Di Santo, M.;
Dong, J. N.; Dong, T. K.; Dong, Y. F.; Dong, Z. X.; Droz, D.; Duan,
K. K.; Duan, J. L.; D'Urso, D.; Fan, R. R.; Fan, Y. Z.; Fang, F.;
Feng, C. Q.; Feng, L.; Fusco, P.; Gallo, V.; Gan, F. J.; Gao, M.;
Gao, S. S.; Gargano, F.; Garrappa, S.; Gong, K.; Gong, Y. Z.; Guo,
J. H.; Hu, Y. M.; Huang, G. S.; Huang, Y. Y.; Ionica, M.; Jiang, D.;
Jiang, W.; Jin, X.; Kong, J.; Lei, S. J.; Li, S.; Li, X.; Li, W. L.;
Li, Y.; Liang, Y. F.; Liang, Y. M.; Liao, N. H.; Liu, C. M.; Liu,
H.; Liu, J.; Liu, S. B.; Liu, W. Q.; Liu, Y.; Loparco, F.; Ma, M.;
Ma, P. X.; Ma, S. Y.; Ma, T.; Ma, X. Q.; Ma, X. Y.; Marsella, G.;
Mazziotta, M. N.; Mo, D.; Niu, X. Y.; Pan, X.; Peng, X. Y.; Peng,
W. X.; Qiao, R.; Rao, J. N.; Salinas, M. M.; Shang, G. Z.; Shen,
W. H.; Shen, Z. Q.; Shen, Z. T.; Song, J. X.; Su, H.; Su, M.; Sun,
Z. Y.; Surdo, A.; Teng, X. J.; Tian, X. B.; Tykhonov, A.; Vitillo,
S.; Wang, C.; Wang, H.; Wang, H. Y.; Wang, J. Z.; Wang, L. G.; Wang,
Q.; Wang, S.; Wang, X. H.; Wang, X. L.; Wang, Y. F.; Wang, Y. P.;
Wang, Y. Z.; Wang, Z. M.; Wen, S. C.; Wei, D. M.; Wei, J. J.; Wei,
Y. F.; Wu, D.; Wu, J.; Wu, L. B.; Wu, S. S.; Wu, X.; Xi, K.; Xia,
Z. Q.; Xin, Y. L.; Xu, H. T.; Xu, Z. H.; Xu, Z. L.; Xu, Z. Z.; Xue,
G. F.; Yang, H. B.; Yang, P.; Yang, Y. Q.; Yang, Z. L.; Yao, H. J.;
Yu, Y. H.; Yuan, Q.; Yue, C.; Zang, J. J.; Zhang, D. L.; Zhang, F.;
Zhang, J. B.; Zhang, J. Y.; Zhang, J. Z.; Zhang, L.; Zhang, P. F.;
Zhang, S. X.; Zhang, W. Z.; Zhang, Y.; Zhang, Y. J.; Zhang, Y. Q.;
Zhang, Y. L.; Zhang, Y. P.; Zhang, Z.; Zhang, Z. Y.; Zhao, H.; Zhao,
H. Y.; Zhao, X. F.; Zhou, C. Y.; Zhou, Y.; Zhu, X.; Zhu, Y.; Zimmer, S.
2019APh...106...18A Altcode: 2019arXiv190702173A
The DArk Matter Particle Explorer (DAMPE), a satellite-based cosmic
ray and gamma-ray detector, was launched on December 17, 2015, and
began its on-orbit operation on December 24, 2015. In this work we
document the on-orbit calibration procedures used by DAMPE and report
the calibration results of the Plastic Scintillator strip Detector
(PSD), the Silicon-Tungsten tracKer-converter (STK), the BGO imaging
calorimeter (BGO), and the Neutron Detector (NUD). The results are
obtained using Galactic cosmic rays, bright known GeV gamma-ray
sources, and charge injection into the front-end electronics of each
sub-detector. The determination of the boundary of the South Atlantic
Anomaly (SAA), the measurement of the live time, and the alignments of
the detectors are also introduced. The calibration results demonstrate
the stability of the detectors in almost two years of the on-orbit
operation.
---------------------------------------------------------
Title: Transition from Circular-ribbon to Parallel-ribbon Flares
Associated with a Bifurcated Magnetic Flux Rope
Authors: Zhong, Z.; Guo, Y.; Ding, M. D.; Fang, C.; Hao, Q.
2019ApJ...871..105Z Altcode: 2018arXiv181210223Z
Magnetic flux ropes play a key role in triggering solar flares in
the solar atmosphere. In this paper, we investigate the evolution
of NOAA Active Region 12268 within 36 hr from 2015 January 29 to
30, during which a flux rope was formed and three M-class and three
C-class flares were triggered without coronal mass ejections. During
the evolution of the active region, the flare emission seen in the
Hα and ultraviolet wavebands changed from a circular shape (plus an
adjacent conjugated ribbon and a remote ribbon) to three relatively
straight and parallel ribbons. Based on a series of reconstructed
nonlinear force-free fields, we find sheared or twisted magnetic
field lines and a large-scale quasi-separatrix layer (QSL) associated
with 3D null points in a quadrupolar magnetic field. These features
always existed and constantly evolved during the 2 days. The twist
of the flux rope was gradually accumulated, eventually leading to
its instability. Around the flux rope, there were some topological
structures, including a bald patch, a hyperbolic flux tube, and a torus
QSL. We discuss how the particular magnetic structure and its evolution
produce the flare emission. In particular, the bifurcation of the flux
rope can explain the transition of the flares from circular to parallel
ribbons. We propose a two-stage evolution of the magnetic structure
and its associated flares. In the first stage, sheared arcades under
the dome-like large-scale QSL were gradually transformed into a flux
rope through magnetic reconnection, which produced the circular-ribbon
flare. In the second stage, the flux rope bifurcated to form the three
relatively straight and parallel flare ribbons.
---------------------------------------------------------
Title: Solar Magnetic Flux Rope Eruption Simulated by a Data-driven
Magnetohydrodynamic Model
Authors: Guo, Yang; Xia, Chun; Keppens, Rony; Ding, M. D.; Chen, P. F.
2019ApJ...870L..21G Altcode: 2018arXiv181210030G
The combination of magnetohydrodynamic (MHD) simulation and
multi-wavelength observations is an effective way to study the
mechanisms of magnetic flux rope eruption. We develop a data-driven MHD
model using the zero-β approximation. The initial condition is provided
by a nonlinear force-free field derived from the magneto-frictional
method based on vector magnetic field observed by the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory. The bottom
boundary uses observed time series of the vector magnetic field and the
vector velocity derived by the Differential Affine Velocity Estimator
for Vector Magnetograms. We apply the data-driven model to active
region 11123 observed from 06:00 UT on 2010 November 11 to about 2 hr
later. The evolution of the magnetic field topology coincides with
the flare ribbons observed in the 304 and 1600 Å wavebands by the
Atmospheric Imaging Assembly. The morphology, propagation path, and
propagation range of the flux rope are comparable with the observations
in 304 Å. We also find that a data-constrained boundary condition,
where the bottom boundary is fixed to the initial values, reproduces
a similar simulation result. This model can reproduce the evolution
of a magnetic flux rope in its dynamic eruptive phase.
---------------------------------------------------------
Title: Observations of Turbulent Magnetic Reconnection within a
Solar Current Sheet
Authors: Cheng, X.; Li, Y.; Wan, L. F.; Ding, M. D.; Chen, P. F.;
Zhang, J.; Liu, J. J.
2018ApJ...866...64C Altcode: 2018arXiv180806071C
Magnetic reconnection is a fundamental physical process in various
astrophysical, space, and laboratory environments. Many pieces of
evidence for magnetic reconnection have been uncovered. However, its
specific processes that could be fragmented and turbulent have been
short of direct observational evidence. Here, we present observations
of a super-hot current sheet during the SOL2017-09-10T X8.2-class solar
flare that display the fragmented and turbulent nature of magnetic
reconnection. As bilateral plasmas converge toward the current sheet,
significant plasma heating and nonthermal motions are detected
therein. Two oppositely directed outflow jets are intermittently
expelled out of the fragmenting current sheet, whose intensity shows a
power-law distribution in the spatial frequency domain. The intensity
and velocity of the sunward outflow jets also display a power-law
distribution in the temporal frequency domain. The length-to-width
ratio of current sheet is estimated to be larger than the theoretical
threshold and thus ensures its occurrence. The observations therefore
suggest that fragmented and turbulent magnetic reconnection occurs in
the long stretching current sheet.
---------------------------------------------------------
Title: Extremely Large Extreme-ultraviolet Late Phase Powered by
Intense Early Heating in a Non-eruptive Solar Flare
Authors: Dai, Yu; Ding, Mingde; Zong, Weiguo; Yang, Kai E.
2018ApJ...863..124D Altcode: 2018arXiv180701315D
We analyzed and modeled an M1.2 non-eruptive solar flare on
2011 September 9. The flare exhibited a strong late-phase peak of
extreme-ultraviolet (EUV) warm coronal emissions (∼3 MK), with peak
emission over 1.3 times that of the main flare peak. Multiple flare
ribbons are observed, whose evolution indicates a two-stage energy
release process. A nonlinear force-free field extrapolation reveals
the existence of a magnetic null point, a fan-spine structure, and two
flux ropes embedded in the fan dome. Magnetic reconnections involved
in the flare are driven by the destabilization and rise of one of the
flux ropes. In the first stage, the fast ascending flux rope drives
reconnections at the null point and the surrounding quasi-separatrix
layer (QSL), while in the second stage, reconnection mainly occurs
between the two legs of the field lines stretched by the eventually
stopped flux rope. The late-phase loops are mainly produced by the
first-stage QSL reconnection, while the second-stage reconnection is
responsible for the heating of main flaring loops. The first-stage
reconnection is believed to be more powerful, leading to an extremely
strong EUV late phase. We find that the delayed occurrence of the
late-phase peak is mainly due to the long cooling process of the long
late-phase loops. Using the model enthalpy-based thermal evolution of
loops, we model the EUV emissions from a late-phase loop. The modeling
reveals a peak heating rate of 1.1 erg cm<SUP>-3</SUP> s<SUP>-1</SUP>
for the late-phase loop, which is obviously higher than previous values.
---------------------------------------------------------
Title: Project of a New 2.5m Solar Telescope
Authors: Fang, Cheng; Chen, P. F.; Li, Zhen; Cao, Wenda; Hao, Qi;
Ding, Mingde; Gu, Baizhong; Yuan, Xiangyan
2018cosp...42E1041F Altcode:
A project of a 2.5m Solar Telescope has been worked out in China. It
is the first facility in the world with a special innovation design
and can conduct both high-resolution solar observations and the
large-field of view (FOV) night survey. Its scientific objectives
cover solar physics and time-domain astronomy, all of which are recent
hot topics in astronomy. As a large on-axis solar telescope in the
world with a larger FOV (7') than all the large solar telescopes
operating at present, it can provide unprecedented high-resolution
solar imaging and magnetic field data, which can help us obtain
breakthrough achievements on the study of solar active regions and
solar eruptions. The data are also very useful for the study of space
weather. As a large telescope in China, the telescope is very unique
in continuously monitoring short-time transient events by filling
the gap in the specific time-zone. Moreover, the telescope can make
essential contribution to training the graduate and undergraduate
students majoring in observational astronomy.
---------------------------------------------------------
Title: Ellerman bombs vs. UV bursts
Authors: Fang, Cheng; Li, Zhen; Hong, Jie; Ding, Mingde
2018cosp...42E1040F Altcode:
Ellerman bombs (EBs) are small brightening events in the solar
lower atmosphere. Most authors found that the temperature increase
of EBs around the temperature minimum region is in the range of
600K-3000K. With recent IRIS observations, people found UV brightening
events called UV bursts or IRIS bombs. Some authors proposed that
the temperature increase of EBs could be more than 10000K, and
the UV bursts could, in some cases, related to EBs. Using non-LTE
semi-empirical modeling, we investigate the line profiles, continuum
emission and the radiative losses for the EBs, and compare them with
observations. Our result indicates that if the EB maximum temperature
attains more than 10000K around the temperature minimum region, then the
resulted Hα and Ca II 8542 Å line profiles and the continuum emission
would be much stronger than that of EB observations. Moreover, due to
the high radiative losses, the high temperature EB would have a very
short lifetime, which is not comparable with the observations. Thus,
our study does not support the proposal that the EB temperatures are
higher than 10000K. Our non-LTE calculation also indicates that the
UV bursts are not the same as the EBs. The former are probably at
the higher layer of the solar atmosphere, while the latter are at the
lower atmosphere with lower temperature.
---------------------------------------------------------
Title: Project of a New 2.5m Solar Telescope
Authors: Fang, Cheng; Chen, P. F.; Li, Zhen; Cao, Wenda; Hao, Qi;
Ding, Mingde; Gu, Baizhong; Yuan, Xiangyan
2018cosp...42E1042F Altcode:
A project of a 2.5m Solar Telescope has been worked out in China. It
is the first facility in the world with a special innovation design
and can conduct both high-resolution solar observations and the
large-field of view (FOV) night survey. Its scientific objectives
cover solar physics and time-domain astronomy, all of which are recent
hot topics in astronomy. As a large on-axis solar telescope in the
world with a larger FOV (7') than all the large solar telescopes
operating at present, it can provide unprecedented high-resolution
solar imaging and magnetic field data, which can help us obtain
breakthrough achievements on the study of solar active regions and
solar eruptions. The data are also very useful for the study of space
weather. As a large telescope in China, the telescope is very unique
in continuously monitoring short-time transient events by filling
the gap in the specific time-zone. Moreover, the telescope can make
essential contribution to training the graduate and undergraduate
students majoring in observational astronomy.
---------------------------------------------------------
Title: Observations of white-light flares in NOAA active region 11515:
high occurrence rate and relationship with magnetic transients
Authors: Song, Y. L.; Tian, H.; Zhang, M.; Ding, M. D.
2018A&A...613A..69S Altcode: 2018arXiv180104371S
<BR /> Aims: There are two goals in this study. One is to investigate
how frequently white-light flares (WLFs) occur in a flare-productive
active region (NOAA active region 11515). The other is to investigate
the relationship between WLFs and magnetic transients (MTs). <BR />
Methods: We used the high-cadence (45 s) full-disk continuum filtergrams
and line-of-sight magnetograms taken by the Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) to identify
WLFs and MTs, respectively. Images taken by the Atmospheric Imaging
Assembly (AIA) on board SDO were also used to show the flare morphology
in the upper atmosphere. <BR /> Results: We found at least 20 WLFs out
of a total of 70 flares above C class (28.6%) in NOAA active region
11515 during its passage across the solar disk (E45°-W45°). Each of
these WLFs occurred in a small region, with a short duration of about 5
min. The enhancement of the white-light continuum intensity is usually
small, with an average enhancement of 8.1%. The 20 WLFs we observed
were found along an unusual configuration of the magnetic field that
was characterized by a narrow ribbon of negative field. Furthermore,
the WLFs were found to be accompanied by MTs, with radical changes
in magnetic field strength (or even a sign reversal) observed during
the flare. In contrast, there is no obvious signature of MTs in the 50
flares without white-light enhancements. <BR /> Conclusions: Our results
suggest that WLFs occur much more frequently than previously thought,
with most WLFs being fairly weak enhancements. This may explain why WLFs
are reported rarely. Our observations also suggest that MTs and WLFs are
closely related and appear cospatial and cotemporal, when considering
HMI data. A greater enhancement of WL emission is often accompanied by a
greater change in the line-of-sight component of the unsigned magnetic
field. Considering the close relationship between MTs and WLFs, many
previously reported flares with MTs may be WLFs. <P />The movie is
available at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Non-LTE Calculations of the Fe I 6173 Å Line in a Flaring
Atmosphere
Authors: Hong, Jie; Ding, M. D.; Li, Ying; Carlsson, Mats
2018ApJ...857L...2H Altcode: 2018arXiv180309912H
The Fe I 6173 Å line is widely used in the measurements of vector
magnetic fields by instruments including the Helioseismic and
Magnetic Imager (HMI). We perform non-local thermodynamic equilibrium
calculations of this line based on radiative hydrodynamic simulations
in a flaring atmosphere. We employ both a quiet-Sun atmosphere and
a penumbral atmosphere as the initial one in our simulations. We find
that, in the quiet-Sun atmosphere, the line center is obviously enhanced
during an intermediate flare. The enhanced emission is contributed from
both radiative backwarming in the photosphere and particle beam heating
in the lower chromosphere. A blue asymmetry of the line profile also
appears due to an upward mass motion in the lower chromosphere. If we
take a penumbral atmosphere as the initial atmosphere, the line has
a more significant response to the flare heating, showing a central
emission and an obvious asymmetry. The low spectral resolution of
HMI would indicate some loss of information, but the enhancement
and line asymmetry are still kept. By calculating polarized line
profiles, we find that the Stokes I and V profiles can be altered
as a result of flare heating. Thus the distortion of this line has
a crucial influence on the magnetic field measured from this line,
and one should be cautious in interpreting the magnetic transients
observed frequently in solar flares.
---------------------------------------------------------
Title: Probing the Production of Extreme-ultraviolet Late-phase
Solar Flares Using the Model Enthalpy-based Thermal Evolution of Loops
Authors: Dai, Yu; Ding, Mingde
2018ApJ...857...99D Altcode: 2018arXiv180307210D
Recent observations in extreme-ultraviolet (EUV) wavelengths reveal
an EUV late phase in some solar flares that is characterized by
a second peak in warm coronal emissions (∼3 MK) several tens of
minutes to a few hours after the soft X-ray (SXR) peak. Using the
model enthalpy-based thermal evolution of loops (EBTEL), we numerically
probe the production of EUV late-phase solar flares. Starting from two
main mechanisms of producing the EUV late phase, i.e., long-lasting
cooling and secondary heating, we carry out two groups of numerical
experiments to study the effects of these two processes on the emission
characteristics in late-phase loops. In either of the two processes an
EUV late-phase solar flare that conforms to the observational criteria
can be numerically synthesized. However, the underlying hydrodynamic and
thermodynamic evolutions in late-phase loops are different between the
two synthetic flare cases. The late-phase peak due to a long-lasting
cooling process always occurs during the radiative cooling phase,
while that powered by a secondary heating is more likely to take
place in the conductive cooling phase. We then propose a new method
for diagnosing the two mechanisms based on the shape of EUV late-phase
light curves. Moreover, from the partition of energy input, we discuss
why most solar flares are not EUV late flares. Finally, by addressing
some other factors that may potentially affect the loop emissions,
we also discuss why the EUV late phase is mainly observed in warm
coronal emissions.
---------------------------------------------------------
Title: Two Types of Long-duration Quasi-static Evolution of Solar
Filaments
Authors: Xing, C.; Li, H. C.; Jiang, B.; Cheng, X.; Ding, M. D.
2018ApJ...857L..14X Altcode: 2018arXiv180401232X
In this Letter, we investigate the long-duration quasi-static evolution
of 12 pre-eruptive filaments (four active region (AR) and eight
quiescent filaments), mainly focusing on the evolution of the filament
height in 3D and the decay index of the background magnetic field. The
filament height in 3D is derived through two-perspective observations
of Solar Dynamics Observatory (SDO) and Solar TErrestrial RElations
Observatory (STEREO). The coronal magnetic field is reconstructed
using the potential field source surface model. A new finding is that
the filaments we studied show two types of long-duration evolution:
one type comprises a long-duration static phase and a short, slow rise
phase with a duration of less than 12 hr and a speed of 0.1-0.7 km
s<SUP>-1</SUP>, while the other one only presents a slow rise phase
but with an extremely long duration of more than 60 hr and a smaller
speed of 0.01-0.2 km s<SUP>-1</SUP>. At the moment approaching the
eruption, the decay index of the background magnetic field at the
filament height is similar for both AR and quiescent filaments. The
average value and upper limit are ∼0.9 and ∼1.4, close to the
critical index of torus instability. Moreover, the filament height
and background magnetic field strength are also found to be linearly
and exponentially related with the filament length, respectively.
---------------------------------------------------------
Title: Observations of the CME shocks and the production of SEPs
Authors: Xu, Zigong; li, Chuan; Ding, Mingde
2018EGUGA..20.7722X Altcode:
The acceleration mechanism of SEPs is still a hot debated problem
especially during the onset of large SEPs. Here, we present a case
study on 2011-08-09 SEP event to clarify its acceleration source and
an 3D statistical study including 22 SEPs in order to know where and
how the particles are released. Based on the remote-sensing and in-situ
observations as well as the DSA theory, we found that the theoretical
particle spectrum was comparable to a observational one and a weak
correlation was obtained between the electrons producing HXR and the
in-situ observed electrons. These results indicated that the coronal
shock wave was the potential accelerator of SEPs. In statistical study,
we obtained the electrons and protons solar release and compared to
the 3D CME structures and HXR observation from RHESSI. It is found the
electrons are released 15 minutes earlier than the protons and both
electrons and protons release are in decay phase of flare eruptions. The
simulations of SEP propagation tell us that the time difference of the
electrons and protons release probably arises from their scattering
propagation in interplanetary space.
---------------------------------------------------------
Title: Period Increase and Amplitude Distribution of Kink Oscillation
of Coronal Loop
Authors: Su, W.; Guo, Y.; Erdélyi, R.; Ning, Z. J.; Ding, M. D.;
Cheng, X.; Tan, B. L.
2018NatSR...8.4471S Altcode: 2018arXiv180306848S
Coronal loops exist ubiquitously in the solar atmosphere. These loops
puzzle astronomers over half a century. Solar magneto-seismology (SMS)
provides a unique way to constrain the physical parameters of coronal
loops. Here, we study the evolution of oscillations of a coronal loop
observed by the Atmospheric Imaging Assembly (AIA). We measure geometric
and physical parameters of the loop oscillations. In particular, we
find that the mean period of the oscillations increased from 1048 to
1264 s during three oscillatory cycles. We employ the differential
emission measure method and apply the tools of SMS. The evolution
of densities inside and outside the loop is analyzed. We found that
an increase of density inside the loop and decrease of the magnetic
field strength along the loop are the main reasons for the increase
in the period during the oscillations. Besides, we also found that
the amplitude profile of the loop is different from a profile would
it be a homogeneous loop. It is proposed that the distribution of
magnetic strength along the loop rather than density stratification is
responsible for this deviation. The variation in period and distribution
of amplitude provide, in terms of SMS, a new and unprecedented insight
into coronal loop diagnostics.
---------------------------------------------------------
Title: Unambiguous Evidence of Filament Splitting-induced Partial
Eruptions
Authors: Cheng, X.; Kliem, B.; Ding, M. D.
2018ApJ...856...48C Altcode: 2018arXiv180204932C
Coronal mass ejections are often considered to result from the full
eruption of a magnetic flux rope (MFR). However, it is recognized
that, in some events, the MFR may release only part of its flux,
with the details of the implied splitting not completely established
due to limitations in observations. Here, we investigate two partial
eruption events including a confined and a successful one. Both
partial eruptions are a consequence of the vertical splitting of
a filament-hosting MFR involving internal reconnection. A loss of
equilibrium in the rising part of the magnetic flux is suggested
by the impulsive onset of both events and by the delayed onset of
reconnection in the confined event. The remaining part of the flux
might be line-tied to the photosphere in a bald patch (BP) separatrix
surface, and we confirm the existence of extended BP sections for
the successful eruption. The internal reconnection is signified by
brightenings in the body of one filament and between the rising and
remaining parts of both filaments. It evolves quickly into the standard
current sheet reconnection in the wake of the eruption. As a result,
regardless of being confined or successful, both eruptions produce
hard X-ray sources and flare loops below the erupting but above the
surviving flux, as well as a pair of flare ribbons enclosing the latter.
---------------------------------------------------------
Title: Observationally quantified reconnection providing a viable
mechanism for active region coronal heating
Authors: Yang, Kai E.; Longcope, Dana W.; Ding, M. D.; Guo, Yang
2018NatCo...9..692Y Altcode: 2018arXiv180206206Y
The heating of the Sun's corona has been explained by several different
mechanisms including wave dissipation and magnetic reconnection. While
both have been shown capable of supplying the requisite power, neither
has been used in a quantitative model of observations fed by measured
inputs. Here we show that impulsive reconnection is capable of producing
an active region corona agreeing both qualitatively and quantitatively
with extreme-ultraviolet observations. We calculate the heating power
proportional to the velocity difference between magnetic footpoints
and the photospheric plasma, called the non-ideal velocity. The
length scale of flux elements reconnected in the corona is found to be
around 160 km. The differential emission measure of the model corona
agrees with that derived using multi-wavelength images. Synthesized
extreme-ultraviolet images resemble observations both in their
loop-dominated appearance and their intensity histograms. This work
provides compelling evidence that impulsive reconnection events are
a viable mechanism for heating the corona.
---------------------------------------------------------
Title: Spectroscopic Observations of a Current Sheet in a Solar Flare
Authors: Li, Y.; Xue, J. C.; Ding, M. D.; Cheng, X.; Su, Y.; Feng,
L.; Hong, J.; Li, H.; Gan, W. Q.
2018ApJ...853L..15L Altcode: 2018arXiv180103631L
The current sheet is believed to be the region of energy dissipation
via magnetic reconnection in solar flares. However, its properties,
for example, the dynamic process, are not fully understood. Here,
we report a current sheet in a solar flare (SOL2017-09-10T16:06) that
was clearly observed by the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory as well as the EUV Imaging Spectrometer on
Hinode. The high-resolution imaging and spectroscopic observations show
that the current sheet is mainly visible in high-temperature (>10 MK)
passbands, particularly in the Fe XXIV 192.03 Å line with a formation
temperature of ∼18 MK. The hot Fe XXIV 192.03 Å line exhibits very
large nonthermal velocities up to 200 km s<SUP>-1</SUP> in the current
sheet, suggesting that turbulent motions exist there. The largest
turbulent velocity occurs at the edge of the current sheet, with some
offset with the strongest line intensity. At the central part of the
current sheet, the turbulent velocity is negatively correlated with the
line intensity. From the line emission and turbulent features we obtain
a thickness in the range of 7-11 Mm for the current sheet. These results
suggest that the current sheet has internal fine and dynamic structures
that may help the magnetic reconnection within it proceed efficiently.
---------------------------------------------------------
Title: A circular white-light flare with impulsive and gradual
white-light kernels
Authors: Hao, Q.; Yang, K.; Cheng, X.; Guo, Y.; Fang, C.; Ding, M. D.;
Chen, P. F.; Li, Z.
2017NatCo...8.2202H Altcode: 2017arXiv171207279H
White-light flares are the flares with emissions visible in the
optical continuum. They are thought to be rare and pose the most
stringent requirements in energy transport and heating in the lower
atmosphere. Here we present a nearly circular white-light flare on
10 March 2015 that was well observed by the Optical and Near-infrared
Solar Eruption Tracer and Solar Dynamics Observatory. In this flare,
there appear simultaneously both impulsive and gradual white-light
kernels. The generally accepted thick-target model would be responsible
for the impulsive kernels but not sufficient to interpret the gradual
kernels. Some other mechanisms including soft X-ray backwarming or
downward-propagating Alfvén waves, acting jointly with electron beam
bombardment, provide a possible interpretation. However, the origin of
this kind of white-light kernel is still an open question that induces
more observations and researches in the future to decipher it.
---------------------------------------------------------
Title: Spectroscopic Observations of Magnetic Reconnection and
Chromospheric Evaporation in an X-shaped Solar Flare
Authors: Li, Y.; Kelly, M.; Ding, M. D.; Qiu, J.; Zhu, X. S.; Gan,
W. Q.
2017ApJ...848..118L Altcode: 2017arXiv170808586L
We present observations of distinct UV spectral properties at different
locations during an atypical X-shaped flare (SOL2014-11-09T15:32)
observed by the Interface Region Imaging Spectrograph (IRIS). In
this flare, four chromospheric ribbons appear and converge at an
X-point where a separator is anchored. Above the X-point, two sets of
non-coplanar coronal loops approach laterally and reconnect at the
separator. The IRIS slit was located close to the X-point, cutting
across some of the flare ribbons and loops. Near the location of the
separator, the Si IV 1402.77 Å line exhibits significantly broadened
line wings extending to 200 km s<SUP>-1</SUP> with an unshifted line
core. These spectral features suggest the presence of bidirectional
flows possibly related to the separator reconnection. While at the
flare ribbons, the hot Fe xxi 1354.08 Å line shows blueshifts and the
cool Si IV 1402.77 Å, C II 1335.71 Å, and Mg II 2803.52 Å lines show
evident redshifts up to a velocity of 80 km s<SUP>-1</SUP>, which are
consistent with the scenario of chromospheric evaporation/condensation.
---------------------------------------------------------
Title: RADYN Simulations of Non-thermal and Thermal Models of
Ellerman Bombs
Authors: Hong, Jie; Carlsson, Mats; Ding, M. D.
2017ApJ...845..144H Altcode: 2017arXiv170705514H
Ellerman bombs (EBs) are brightenings in the Hα line wings that
are believed to be caused by magnetic reconnection in the lower
atmosphere. To study the response and evolution of the chromospheric
line profiles, we perform radiative hydrodynamic simulations of EBs
using both non-thermal and thermal models. Overall, these models can
generate line profiles that are similar to observations. However, in
non-thermal models we find dimming in the Hα line wings and continuum
when the heating begins, while for the thermal models dimming occurs
only in the Hα line core, and with a longer lifetime. This difference
in line profiles can be used to determine whether an EB is dominated by
non-thermal heating or thermal heating. In our simulations, if a higher
heating rate is applied, then the Hα line will be unrealistically
strong and there are still no clear UV burst signatures.
---------------------------------------------------------
Title: The Formation and Early Evolution of a CME and the Associated
Shock on 2014 January 8
Authors: Wan, Linfeng; Cheng, Xin; Shi, Tong; Su, Wei; Ding, Mingde
2017SPD....4820606W Altcode:
We study the formation and early evolution of a limb coronal mass
ejection (CME) and its associated shock wave that occurred on 2014
January 8. The extreme ultraviolet (EUV) images provided by AIA on
board \textit{Solar Dynamics Observatory} disclose that the CME first
appears as a bubble-like structure. Subsequently, its expansion forms
the CME and causes a quasi-circular EUV wave. Both the CME and the wave
front are clearly visible at all of the AIA EUV passbands. Through a
detailed kinematical analysis, it is found that the expansion of the
CME undergoes two phases: a first phase with a strong but transient
lateral over-expansion followed by a second phase with a self-similar
expansion. The temporal evolution of the expansion velocity coincides
very well with the variation of the 25--50 keV hard X-ray (HXR) flux of
the associated flare, which indicates that magnetic reconnection most
likely plays an important role in driving the expansion. Moreover, we
find that, when the velocity of the CME reaches $\sim$600 km s$^{-1}$,
the EUV wave starts to evolve into a shock wave, which is evidenced by
the appearance of a type II radio burst. Interestingly, we also notice
an unusual solar radio signal at $\sim$4 GHz that is similar to the
pattern of a type II radio burst but drifts to higher frequencies at a
rate of $\sim$0.3 MHz per second during about 7 minutes. Its derived
density is $\sim$5$\times$10$^{10}$ cm$^{-3}$ and increases slowly
with time. Joint imaging observations of HXR and EUV help to locate
the loop-top region and calculate its thermal proprieties, including
slowly increasing densities ($\sim$5$\times$10$^{10}$ cm$^{-3}$)
and temperatures ($\sim$14 MK). The similar results obtained from two
different ways above imply the possibility of this scenario: plasma
blobs that are ejected along the current sheet via magnetic reconnection
collide with underlying flare loops that are undergoing chromospheric
evaporation. Finally, we also study the thermal properties of the CME
and the EUV wave. We find that the plasma in the CME leading front
and the wave front has a temperature of $\sim$2 MK, while that in the
CME core region and the flare region has a much higher temperature of
$\ge$8 MK.
---------------------------------------------------------
Title: Origin and Structures of Solar Eruptions I: Magnetic Flux Rope
Authors: Cheng, Xin; Guo, Yang; Ding, MingDe
2017ScChD..60.1383C Altcode: 2017ScChE..60.1383C; 2017arXiv170508198C
Coronal mass ejections (CMEs) and solar flares are the large-scale
and most energetic eruptive phenomena in our solar system and able
to release a large quantity of plasma and magnetic flux from the
solar atmosphere into the solar wind. When these high-speed magnetized
plasmas along with the energetic particles arrive at the Earth, they may
interact with the magnetosphere and ionosphere, and seriously affect
the safety of human high-tech activities in outer space. The travel
time of a CME to 1 AU is about 1-3 days, while energetic particles
from the eruptions arrive even earlier. An efficient forecast of these
phenomena therefore requires a clear detection of CMEs/flares at the
stage as early as possible. To estimate the possibility of an eruption
leading to a CME/flare, we need to elucidate some fundamental but
elusive processes including in particular the origin and structures
of CMEs/flares. Understanding these processes can not only improve
the prediction of the occurrence of CMEs/flares and their effects on
geospace and the heliosphere but also help understand the mass ejections
and flares on other solar-type stars. The main purpose of this review
is to address the origin and early structures of CMEs/flares, from
multi-wavelength observational perspective. First of all, we start with
the ongoing debate of whether the pre-eruptive configuration, i.e.,
a helical magnetic flux rope (MFR), of CMEs/flares exists before the
eruption and then emphatically introduce observational manifestations
of the MFR. Secondly, we elaborate on the possible formation mechanisms
of the MFR through distinct ways. Thirdly, we discuss the initiation
of the MFR and associated dynamics during its evolution toward the
CME/flare. Finally, we come to some conclusions and put forward some
prospects in the future.
---------------------------------------------------------
Title: Using observations of slipping velocities to test the
hypothesis that reconnection heats the active region corona
Authors: Yang, Kai; Longcope, Dana; Guo, Yang; Ding, Mingde
2017SPD....4830301Y Altcode:
Numerous proposed coronal heating mechanisms have invoked magnetic
reconnection in some role. Testing such a mechanism requires a
method of measuring magnetic reconnection coupled with a prediction
of the heat delivered by reconnection at the observed rate. In the
absence of coronal reconnection, field line footpoints move at the
same velocity as the plasma they find themselves in. The rate of
coronal reconnection is therefore related to any discrepancy observed
between footpoint motion and that of the local plasma — so-called
slipping motion. We propose a novel method to measure this velocity
discrepancy by combining a sequence of non-linear force-free field
extrapolations with maps of photospheric velocity. We obtain both
from a sequence of vector magnetograms of an active region (AR). We
then propose a method of computing the coronal heating produced under
the assumption the observed slipping velocity was due entirely to
coronal reconnection. This heating rate is used to predict density
and temperature at points along an equilibrium loop. This, in turn,
is used to synthesize emission in EUV and SXR bands. We perform this
analysis using a sequence of HMI vector magnetograms of a particular
AR and compare synthesized images to observations of the same AR made
by SDO. We also compare differential emission measure inferred from
those observations to that of the modeled corona.
---------------------------------------------------------
Title: Origin and structures of solar eruptions II: Magnetic modeling
Authors: Guo, Yang; Cheng, Xin; Ding, MingDe
2017ScChD..60.1408G Altcode: 2017arXiv170605769G
The topology and dynamics of the three-dimensional magnetic field in the
solar atmosphere govern various solar eruptive phenomena and activities,
such as flares, coronal mass ejections, and filaments/prominences. We
have to observe and model the vector magnetic field to understand the
structures and physical mechanisms of these solar activities. Vector
magnetic fields on the photosphere are routinely observed via
the polarized light, and inferred with the inversion of Stokes
profiles. To analyze these vector magnetic fields, we need first to
remove the 180° ambiguity of the transverse components and correct
the projection effect. Then, the vector magnetic field can be served as
the boundary conditions for a force-free field modeling after a proper
preprocessing. The photospheric velocity field can also be derived from
a time sequence of vector magnetic fields. Three-dimensional magnetic
field could be derived and studied with theoretical force-free field
models, numerical nonlinear force-free field models, magnetohydrostatic
models, and magnetohydrodynamic models. Magnetic energy can be computed
with three-dimensional magnetic field models or a time series of vector
magnetic field. The magnetic topology is analyzed by pinpointing the
positions of magnetic null points, bald patches, and quasi-separatrix
layers. As a well conserved physical quantity, magnetic helicity can
be computed with various methods, such as the finite volume method,
discrete flux tube method, and helicity flux integration method. This
quantity serves as a promising parameter characterizing the activity
level of solar active regions.
---------------------------------------------------------
Title: Particle Acceleration in Two Converging Shocks
Authors: Wang, Xin; Giacalone, Joe; Yan, Yihua; Ding, Mingde; Wang,
Na; Shan, Hao
2017ApJ...842...74W Altcode:
Observations by spacecraft such as ACE, STEREO, and others show that
there are proton spectral “breaks” with energy E <SUB>br</SUB> at
1-10 MeV in some large CME-driven shocks. Generally, a single shock with
the diffusive acceleration mechanism would not predict the “broken”
energy spectrum. The present paper focuses on two converging shocks to
identify this energy spectral feature. In this case, the converging
shocks comprise one forward CME-driven shock on 2006 December 13 and
another backward Earth bow shock. We simulate the detailed particle
acceleration processes in the region of the converging shocks using the
Monte Carlo method. As a result, we not only obtain an extended energy
spectrum with an energy “tail” up to a few 10 MeV higher than that
in previous single shock model, but also we find an energy spectral
“break” occurring on ∼5.5 MeV. The predicted energy spectral
shape is consistent with observations from multiple spacecraft. The
spectral “break,” then, in this case is caused by the interaction
between the CME shock and Earth’s bow shock, and otherwise would not
be present if Earth were not in the path of the CME. <P />This work
is supported by the Xinjiang Natural Science Foundation No. 2014211A069.
---------------------------------------------------------
Title: Observations of a Coronal Shock Wave and the Production of
Solar Energetic Particles
Authors: Xu, Z. G.; Li, C.; Ding, M. D.
2017ApJ...840...38X Altcode:
We present a study that clarifies the acceleration source/mechanism
of the solar energetic particle (SEP) event on 2011 August 9. Based
on the assumption of scatter-free propagation of charged particles
along the interplanetary magnetic field, the solar particle release
times of the electrons and protons are derived and both found to be
in the decay phase of the flare emission. Furthermore, we compare the
peak-flux spectra of the in situ particles and the remote-sensing hard
X-ray photons and find a weak correlation between them. In particular,
we note that an extreme ultraviolet shock wave, presumed to be a
signature of coronal mass ejection (CME) shock front on the solar
surface, and an associated type II radio burst were observed alongside
this event. Under the framework of diffusive shock acceleration,
the derived shock compression ratio can accelerate particles with a
theoretical spectral index γ ={2.14}<SUB>-0.02</SUB><SUP>+0.01</SUP>,
which is comparable to the observational index of ∼2.0. Our results
appear to support the notion that the coronal shock wave was most
likely responsible for the SEP event. Specifically, we find that the
electrons were released in a low coronal site at ∼0.58 solar radii,
and protons were released when the CME-driven shock propagated to
∼1.38 solar radii. The multi-spacecraft observations, in addition,
reveal the connection between the acceleration of shock waves and the
release of SEPs.
---------------------------------------------------------
Title: Multi-wavelength Spectral Analysis of Ellerman Bombs Observed
by FISS and IRIS
Authors: Hong, Jie; Ding, M. D.; Cao, Wenda
2017ApJ...838..101H Altcode: 2017arXiv170304268H
Ellerman bombs (EBs) are a kind of solar activity that is suggested
to occur in the lower solar atmosphere. Recent observations using the
Interface Region Imaging Spectrograph (IRIS) show connections between
EBs and IRIS bombs (IBs), which imply that EBs might be heated to a much
higher temperature (8 × 10<SUP>4</SUP> K) than previous results. Here
we perform a spectral analysis of EBs simultaneously observed by the
Fast Imaging Solar Spectrograph and IRIS. The observational results
show clear evidence of heating in the lower atmosphere, indicated by
the wing enhancement in Hα, Ca II 8542 Å, and Mg II triplet lines and
also by brightenings in images of the 1700 Å and 2832 Å ultraviolet
continuum channels. Additionally, the intensity of the Mg II triplet
line is correlated with that of Hα when an EB occurs, suggesting the
possibility of using the triplet as an alternative way to identify
EBs. However, we do not find any signal in IRIS hotter lines (C II
and Si IV). For further analysis, we employ a two-cloud model to fit
the two chromospheric lines (Hα and Ca II 8542 Å) simultaneously,
and obtain a temperature enhancement of 2300 K for a strong EB. This
temperature is among the highest of previous modeling results, albeit
still insufficient to produce IB signatures at ultraviolet wavelengths.
---------------------------------------------------------
Title: Plasma Brightenings in a Failed Solar Filament Eruption
Authors: Li, Y.; Ding, M. D.
2017ApJ...838...15L Altcode: 2017arXiv170205136L
Failed filament eruptions are solar eruptions that are not associated
with coronal mass ejections. In a failed filament eruption, the
filament materials usually show some ascending and falling motions
as well as generating bright EUV emissions. Here we report a failed
filament eruption (SOL2016-07-22) that occurred in a quiet-Sun region
observed by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. In this event, the filament spreads out but gets confined
by the surrounding magnetic field. When interacting with the ambient
magnetic field, the filament material brightens up and flows along the
magnetic field lines through the corona to the chromosphere. We find
that some materials slide down along the lifting magnetic structure
containing the filament and impact the chromosphere, and through
kinetic energy dissipation, cause two ribbon-like brightenings in a
wide temperature range. There is evidence suggesting that magnetic
reconnection occurs between the filament magnetic structure and the
surrounding magnetic fields where filament plasma is heated to coronal
temperatures. In addition, thread-like brightenings show up on top
of the erupting magnetic fields at low temperatures, which might be
produced by an energy imbalance from a fast drop of radiative cooling
due to plasma rarefaction. Thus, this single event of a failed filament
eruption shows the existence of a variety of plasma brightenings that
may be caused by completely different heating mechanisms.
---------------------------------------------------------
Title: Imaging Observations of Magnetic Reconnection in a Solar
Eruptive Flare
Authors: Li, Y.; Sun, X.; Ding, M. D.; Qiu, J.; Priest, E. R.
2017ApJ...835..190L Altcode: 2016arXiv161209417L
Solar flares are among the most energetic events in the solar
atmosphere. It is widely accepted that flares are powered by magnetic
reconnection in the corona. An eruptive flare is usually accompanied
by a coronal mass ejection, both of which are probably driven by the
eruption of a magnetic flux rope (MFR). Here we report an eruptive
flare on 2016 March 23 observed by the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory. The extreme-ultraviolet
imaging observations exhibit the clear rise and eruption of an
MFR. In particular, the observations reveal solid evidence of
magnetic reconnection from both the corona and chromosphere during the
flare. Moreover, weak reconnection is observed before the start of the
flare. We find that the preflare weak reconnection is of tether-cutting
type and helps the MFR to rise slowly. Induced by a further rise of
the MFR, strong reconnection occurs in the rise phases of the flare,
which is temporally related to the MFR eruption. We also find that
the magnetic reconnection is more of 3D-type in the early phase,
as manifested in a strong-to-weak shear transition in flare loops,
and becomes more 2D-like in the later phase, as shown by the apparent
rising motion of an arcade of flare loops.
---------------------------------------------------------
Title: Comparison of the energy spectra between single shock and
converging double-shock
Authors: Wang, X.; Yin, Y.; Ding, M.; Wang, N.; Shan, H.
2017ICRC...35..102W Altcode: 2017PoS...301..102W
No abstract at ADS
---------------------------------------------------------
Title: Energy spectral properties of the twin-CME driven shocks
Authors: Wang, X.; Yin, Y.; Ding, M.; Wang, N.; Shan, H.
2017ICRC...35..100W Altcode: 2017PoS...301..100W
No abstract at ADS
---------------------------------------------------------
Title: Observation of Magnetic Reconnection at a 3D Null Point
Associated with a Solar Eruption
Authors: Sun, J. Q.; Zhang, J.; Yang, K.; Cheng, X.; Ding, M. D.
2016ApJ...830L...4S Altcode: 2016arXiv160906787S
Magnetic null has long been recognized as a special structure serving as
a preferential site for magnetic reconnection (MR). However, the direct
observational study of MR at null-points is largely lacking. Here, we
show the observations of MR around a magnetic null associated with an
eruption that resulted in an M1.7 flare and a coronal mass ejection. The
Geostationary Operational Environmental Satellites X-ray profile of
the flare exhibited two peaks at ∼02:23 UT and ∼02:40 UT on 2012
November 8, respectively. Based on the imaging observations, we find
that the first and also primary X-ray peak was originated from MR in the
current sheet (CS) underneath the erupting magnetic flux rope (MFR). On
the other hand, the second and also weaker X-ray peak was caused by MR
around a null point located above the pre-eruption MFR. The interaction
of the null point and the erupting MFR can be described as a two-step
process. During the first step, the erupting and fast expanding MFR
passed through the null point, resulting in a significant displacement
of the magnetic field surrounding the null. During the second step,
the displaced magnetic field started to move back, resulting in
a converging inflow and subsequently the MR around the null. The
null-point reconnection is a different process from the current sheet
reconnection in this flare; the latter is the cause of the main peak
of the flare, while the former is the cause of the secondary peak of
the flare and the conspicuous high-lying cusp structure.
---------------------------------------------------------
Title: Direct imaging of a classical solar eruptive flare
Authors: Li, Y.; Sun, X. D.; Ding, M. D.; Qiu, J.; Priest, E. R.;
Longcope, D. W.
2016usc..confE..21L Altcode:
Solar flares are the most energetic events in the solar system that
have a potential hazard on Earth. Although a standard model for
the eruptive flare accompanied by a coronal mass ejection has been
outlined and elaborated for decades, some key aspects are still under
debate, such as what drives the eruption, what is the role of magnetic
reconnection, and how the flare loops evolve. Here we present an
excellent event exhibiting nearly all the key elements involved in the
standard flare model. Using extreme-ultraviolet imaging observations,
we detect the unambiguous rise and eruption of a magnetic flux rope,
solid evidence for magnetic reconnection, and evident slipping and
rising motions in flare loops. Modeled coronal magnetic field supports
the interpretation of a pre-existing flux rope that persists after
the eruption with reduced twist. This flare, from the observational
view, shows a clear and comprehensive picture of how a classical solar
eruptive flare occurs and evolves, and helps to clarify some of the
controversial topics in the standard flare model.
---------------------------------------------------------
Title: Investigating the Conditions of the Formation of a Type II
Radio Burst on 2014 January 8
Authors: Su, W.; Cheng, X.; Ding, M. D.; Chen, P. F.; Ning, Z. J.;
Ji, H. S.
2016ApJ...830...70S Altcode: 2016arXiv160905633S
It is believed that type II radio bursts are generated by shock
waves. In order to understand the generation conditions of type II
radio bursts, we analyze the physical parameters of a shock front. The
type II radio burst we selected was observed by the Siberian Solar
Radio Telescope (SSRT) and Learmonth radio station and was associated
with a limb coronal mass ejection (CME) occurring on 2014 January 8
observed by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. The evolution of the CME in the inner corona presents a
double-layered structure that propagates outward. We fit the outer layer
(OL) of the structure with a partial circle and divide it into seven
directions from -45° to 45° with an angular separation of 15°. We
measure the OL speed along the seven directions and find that the
speed in the direction of -15° with respect to the central direction
is the fastest. We use the differential emission measure method to
calculate the physical parameters at the OL at the moment when the type
II radio burst was initiated, including the temperature (T), emission
measure (EM), temperature ratio ({T}<SUB>d</SUB>/{T}<SUB>{{u</SUB>}}),
compression ratio (X), and Alfvén Mach number (M <SUB>A</SUB>). We
compare the quantities X and M <SUB>A</SUB> to those obtained from
band-splitting in the radio spectrum, and find that this type II radio
burst is generated at a small region of the OL that is located at the
sector in the 45° direction. The results suggest that the generation
of type II radio bursts (shocks) requires larger values of X and M
<SUB>A</SUB> rather than simply a higher speed of the disturbance.
---------------------------------------------------------
Title: The Formation and Early Evolution of a Coronal Mass Ejection
and its Associated Shock Wave on 2014 January 8
Authors: Wan, Linfeng; Cheng, Xin; Shi, Tong; Su, Wei; Ding, M. D.
2016ApJ...826..174W Altcode: 2016arXiv160501132W
In this paper, we study the formation and early evolution of a limb
coronal mass ejection (CME) and its associated shock wave that occurred
on 2014 January 8. The extreme ultraviolet (EUV) images provided by
the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics
Observatory disclose that the CME first appears as a bubble-like
structure. Subsequently, its expansion forms the CME and causes a
quasi-circular EUV wave. Interestingly, both the CME and the wave
front are clearly visible at all of the AIA EUV passbands. Through a
detailed kinematical analysis, it is found that the expansion of the
CME undergoes two phases: a first phase with a strong but transient
lateral over-expansion followed by a second phase with a self-similar
expansion. The temporal evolution of the expansion velocity coincides
very well with the variation of the 25-50 keV hard X-ray flux of the
associated flare, which indicates that magnetic reconnection most likely
plays an important role in driving the expansion. Moreover, we find
that, when the velocity of the CME reaches ∼600 km s<SUP>-1</SUP>,
the EUV wave starts to evolve into a shock wave, which is evidenced
by the appearance of a type II radio burst. The shock’s formation
height is estimated to be ∼0.2 R <SUB>sun</SUB>, which is much lower
than the height derived previously. Finally, we also study the thermal
properties of the CME and the EUV wave. We find that the plasma in the
CME leading front and the wave front has a temperature of ∼2 MK,
while that in the CME core region and the flare region has a much
higher temperature of ≥8 MK.
---------------------------------------------------------
Title: The Characteristics of the Footpoints of Solar Magnetic Flux
Ropes during Eruptions
Authors: Cheng, X.; Ding, M. D.
2016ApJS..225...16C Altcode: 2016arXiv160504047C
We investigate the footpoints of four erupted magnetic flux ropes
(MFRs) that appear as sigmoidal hot channels prior to the eruptions
in the Atmospheric Imaging Assembly high temperature passbands. The
simultaneous Helioseismic and Magnetic Imager observations disclose
that one footpoint of the MFRs originates in the penumbra or penumbra
edge with a stronger magnetic field, while the other originates in the
moss region with a weaker magnetic field. The significant deviation
of the axes of the MFRs from the main polarity inversion lines and
associated filaments suggests that the MFRs have ascended to a high
altitude, thus becoming distinguishable from the source sigmoidal active
regions. Further, with the eruption of the MFRs, the average inclination
angle and direct current at the footpoints with stronger magnetic fields
tend to decrease, which is suggestive of a straightening and untwisting
of the magnetic field in the MFR legs. Moreover, the associated flare
ribbons also display an interesting evolution. They initially appear
as sporadic brightenings at the two footpoints of the MFRs and in
the regions below, and then quickly extend to two slender sheared
J-shaped ribbons with the two hooks corresponding to the two ends of
the MFRs. Finally, the straight parts of the two ribbons separate from
each other, evolving into two widened parallel ones. These features
mostly conform to and support the recently proposed three-dimensional
standard coronal mass ejection/flare model, I.e., the twisted MFR
eruption stretches and leads to the reconnection of the overlying
field that transits from a strong to weak shear with increasing height.
---------------------------------------------------------
Title: A few perspectives of solar physics research in China -
current status and future
Authors: Wang, Jingxiu; Ding, Mingde; Ji, Haisheng; Deng, Yuanyong;
Liu, Yu; Liu, Zhong; Qu, Zhongquan; Wang, Huaning; Xia, Lidong;
Yan, Yihua
2016AsJPh..25..461W Altcode:
Solar physics research as an important discipline in astrophysics in
China aims at improving the understanding of origin and variation of
solar magnetic field and magnetic activity, and founding the basis for
forecast of disastrous space weather. The current review is focused on
the solar physics research in China in recent three years. Highlights
in scientific research in solar magnetism, magnetic activity, coronal
plasma, and space weather forecast are briefly summarized. Key advances
in instrument development are reported in some necessary details. Future
tendency and working direction are considered and discussed.
---------------------------------------------------------
Title: Quantifying the Topology and Evolution of a Magnetic Flux
Rope Associated with Multi-flare Activities
Authors: Yang, Kai; Guo, Yang; Ding, M. D.
2016ApJ...824..148Y Altcode: 2016arXiv160407502Y
Magnetic flux ropes (MFRs) play an important role in solar
activities. The quantitative assessment of the topology of an MFR and
its evolution is crucial for a better understanding of the relationship
between the MFR and associated activities. In this paper, we investigate
the magnetic field of active region (AR) 12017 from 2014 March 28-29,
during which time 12 flares were triggered by intermittent eruptions
of a filament (either successful or confined). Using vector magnetic
field data from the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, we calculate the magnetic energy and helicity
injection in the AR, and extrapolate the 3D magnetic field with a
nonlinear force-free field model. From the extrapolations, we find
an MFR that is cospatial with the filament. We further determine the
configuration of this MFR from the closed quasi-separatrix layer (QSL)
around it. Then, we calculate the twist number and the magnetic helicity
for the field lines composing the MFR. The results show that the closed
QSL structure surrounding the MFR becomes smaller as a consequence of
flare occurrence. We also find that the flares in our sample are mainly
triggered by kink instability. Moreover, the twist number varies more
sensitively than other parameters with the occurrence of flares.
---------------------------------------------------------
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: Spectroscopic Diagnostics of Solar Magnetic Flux Ropes Using
Iron Forbidden Line
Authors: Cheng, X.; Ding, M. D.
2016ApJ...823L...4C Altcode: 2016arXiv160500195C
In this Letter, we present Interface Region Imaging Spectrograph Fe xxi
1354.08 Å forbidden line emission of two magnetic flux ropes (MFRs)
that caused two fast coronal mass ejections with velocities of ≥1000
km s<SUP>-1</SUP> and strong flares (X1.6 and M6.5) on 2014 September 10
and 2015 June 22, respectively. The extreme-ultraviolet images at the
131 and 94 Å passbands provided by the Atmospheric Imaging Assembly
on board Solar Dynamics Observatory reveal that both MFRs initially
appear as suspended hot channel-like structures. Interestingly, part
of the MFRs is also visible in the Fe xxi 1354.08 forbidden line, even
prior to the eruption, e.g., for the SOL2014-09-10 event. However, the
line emission is very weak and that only appears at a few locations
but not the whole structure of the MFRs. This implies that the MFRs
could be comprised of different threads with different temperatures
and densities, based on the fact that the formation of the Fe xxi
forbidden line requires a critical temperature (∼11.5 MK) and
density. Moreover, the line shows a non-thermal broadening and a
blueshift in the early phase. It suggests that magnetic reconnection
at that time has initiated; it not only heats the MFR and, at the same
time, produces a non-thermal broadening of the Fe xxi line but also
produces the poloidal flux, leading to the ascension of the MFRs.
---------------------------------------------------------
Title: Observations of an X-shaped Ribbon Flare in the Sun and Its
Three-dimensional Magnetic Reconnection
Authors: Li, Y.; Qiu, J.; Longcope, D. W.; Ding, M. D.; Yang, K.
2016ApJ...823L..13L Altcode: 2016arXiv160501833L
We report evolution of an atypical X-shaped flare ribbon that
provides novel observational evidence of three-dimensional (3D)
magnetic reconnection at a separator. The flare occurred on 2014
November 9. High-resolution slit-jaw 1330 Å images from the Interface
Region Imaging Spectrograph reveal four chromospheric flare ribbons
that converge and form an X-shape. Flare brightening in the upper
chromosphere spreads along the ribbons toward the center of the
“X” (the X-point), and then spreads outward in a direction more
perpendicular to the ribbons. These four ribbons are located in a
quadrupolar magnetic field. Reconstruction of magnetic topology in the
active region suggests the presence of a separator connecting to the
X-point outlined by the ribbons. The inward motion of flare ribbons in
the early stage therefore indicates 3D magnetic reconnection between two
sets of non-coplanar loops that approach laterally, and reconnection
proceeds downward along a section of vertical current sheet. Coronal
loops are also observed by the Atmospheric Imaging Assembly on board
the Solar Dynamics Observatory confirming the reconnection morphology
illustrated by ribbon evolution.
---------------------------------------------------------
Title: Observations of an X-shaped Ribbon Flare and Its
Three-dimensional Magnetic Reconnection with IRIS and SDO
Authors: Li, Ying; Qiu, Jiong; Longcope, Dana; Ding, Mingde
2016SPD....4730206L Altcode:
We report evolution of an atypical X-shaped flare ribbon which provides
novel observational evidence of three-dimensional (3D) magnetic
reconnection at a separator. The flare occurred on 2014 November 9, and
high-resolution slit-jaw 1330 images from IRIS reveal four chromospheric
flare ribbons that converge and form an X-shape. These four ribbons are
located in a quadrupolar magnetic field. Reconstruction of magnetic
topology in the active region suggests the presence of a separator
connecting to the X-point outlined by the ribbons. The inward motion
of flare ribbons, as well as coronal loops observed by the SDO/AIA,
indicates 3D magnetic reconnection between two sets of non-coplanar
loops that approach laterally, and the reconnection proceeds downward
to a very low height. We also study spectra of Si IV, C II, and Mg II
observed with the IRIS slit, which cuts across the flare ribbons near
the X-point. We have found two distinct types of line profiles. At the
flare ribbon, all the lines show evident redshifts with a velocity up to
50 km/s, and the redshifts are well correlated with the line intensity
and width. These redshifts suggest chromospheric condensation caused
by impulsive energy deposition from the separator reconnection. While
right outside the flare ribbon, the lines exhibit unshifted, symmetric,
yet broadened profiles; in particular, the Si IV line is significantly
broadened at the far wing. The line broadening persists for 20 minutes
till after the end of the flare. The distinct spectral features near
the X-point indicate different dynamics associated with the separator
reconnection.
---------------------------------------------------------
Title: Target Porosity Controls Crater Residual Bouguer Anomaly in
the Lunar Highlands
Authors: Ding, M.; Soderblom, J. M.; Zuber, M. T.; Bierson, C. J.;
Nimmo, F.; Milbury, C.
2016LPI....47.1359D Altcode:
We spatially map crater RBA, and find that the large-scale spatial
variations in crater RBA can be explained by variations in regional
porosity.
---------------------------------------------------------
Title: Bidirectional Outflows as Evidence of Magnetic Reconnection
Leading to a Solar Microflare
Authors: Hong, Jie; Ding, M. D.; Li, Ying; Yang, Kai; Cheng, Xin;
Chen, Feng; Fang, Cheng; Cao, Wenda
2016ApJ...820L..17H Altcode: 2016arXiv160300941H
Magnetic reconnection is a rapid energy release process that is believed
to be responsible for flares on the Sun and stars. Nevertheless, such
flare-related reconnection is mostly detected to occur in the corona,
while there have been few studies concerning the reconnection in the
chromosphere or photosphere. Here, we present both spectroscopic and
imaging observations of magnetic reconnection in the chromosphere
leading to a microflare. During the flare peak time, chromospheric
line profiles show significant blueshifted/redshifted components
on the two sides of the flaring site, corresponding to upflows and
downflows with velocities of ±(70-80) km s<SUP>-1</SUP>, comparable
with the local Alfvén speed as expected by the reconnection in
the chromosphere. The three-dimensional nonlinear force-free field
configuration further discloses twisted field lines (a flux rope)
at a low altitude, cospatial with the dark threads in He I 10830 Å
images. The instability of the flux rope may initiate the flare-related
reconnection. These observations provide clear evidence of magnetic
reconnection in the chromosphere and show the similar mechanisms of
a microflare to those of major flares.
---------------------------------------------------------
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: Response of chromospheric lines to different periodic
non-thermal electron beams
Authors: Cheng, Jianxia; Ding, Mingde
2016IAUS..320..239C Altcode:
Solar flares produce radiations in very broad wavelengths. Spectra
can supply us abundant information about the local plasma, such as
temperature, density, mass motion and so on. Strong chromospheric
lines, like the most studied Hα and Ca II 8542 Å lines are formed
under conditions of departures from local thermodynamic equilibrium
in the lower atmosphere subject to flare heating. Understanding how
these lines are formed is very useful for us to correctly interpret
the observations. In this paper, we try to figure out the response of
chromospheric lines heated by different periodic non-thermal electron
beams. Our results are based on radiative hydrodynamic simulations. We
vary the periods of electron beam injection from 1.25 s to 20 s. We
compare the response times to different heating parameters. Possible
explanations are discussed.
---------------------------------------------------------
Title: Toward an Accurate Prediction of the Arrival Time of
Geomagnetic-Effective Coronal Mass Ejections
Authors: Shi, T.; Wang, Y.; Wan, L.; Cheng, X.; Ding, M.; Zhang, J.
2015AGUFMSH51B2447S Altcode:
Accurately predicting the arrival of coronal mass ejections (CMEs)
to the Earth based on remote images is of critical significance for
the study of space weather. Here we make a statistical study of 21
Earth-directed CMEs, specifically exploring the relationship between CME
initial speeds and transit times. The initial speed of a CME is obtained
by fitting the CME with the Graduated Cylindrical Shell model and is
thus free of projection effects. We then use the drag force model to
fit results of the transit time versus the initial speed. By adopting
different drag regimes, i.e., the viscous, aerodynamics, and hybrid
regimes, we get similar results, with a least mean estimation error of
the hybrid model of 12.9 hr. CMEs with a propagation angle (the angle
between the propagation direction and the Sun-Earth line) larger than
their half-angular widths arrive at the Earth with an angular deviation
caused by factors other than the radial solar wind drag. The drag force
model cannot be reliably applied to such events. If we exclude these
events in the sample, the prediction accuracy can be improved, i.e.,
the estimation error reduces to 6.8 hr. This work suggests that it is
viable to predict the arrival time of CMEs to the Earth based on the
initial parameters with fairly good accuracy. Thus, it provides a method
of forecasting space weather 1-5 days following the occurrence of CMEs.
---------------------------------------------------------
Title: Chromospheric Evaporation in an X1.0 Flare on 2014 March 29
Observed with IRIS and EIS
Authors: Li, Y.; Ding, M. D.; Qiu, J.; Cheng, J. X.
2015ApJ...811....7L Altcode: 2015arXiv150803927L
Chromospheric evaporation refers to dynamic mass motions
in flare loops as a result of rapid energy deposition in the
chromosphere. These motions have been observed as blueshifts in X-ray
and extreme-ultraviolet (EUV) spectral lines corresponding to upward
motions at a few tens to a few hundreds of km s<SUP>-1</SUP>. Past
spectroscopic observations have also revealed a dominant stationary
component, in addition to the blueshifted component, in emission
lines formed at high temperatures (∼10 MK). This is contradictory to
evaporation models predicting predominant blueshifts in hot lines. The
recently launched Interface Region Imaging Spectrograph (IRIS) provides
high-resolution imaging and spectroscopic observations that focus
on the chromosphere and transition region in the UV passband. Using
the new IRIS observations, combined with coordinated observations
from the EUV Imaging Spectrometer, we study the chromospheric
evaporation process from the upper chromosphere to the corona
during an X1.0 flare on 2014 March 29. We find evident evaporation
signatures, characterized by Doppler shifts and line broadening, at
two flare ribbons that are separating from each other, suggesting that
chromospheric evaporation takes place in successively formed flaring
loops throughout the flare. More importantly, we detect dominant
blueshifts in the high-temperature Fe xxi line (∼10 MK), in agreement
with theoretical predictions. We also find that, in this flare, gentle
evaporation occurs at some locations in the rise phase of the flare,
while explosive evaporation is detected at some other locations near
the peak of the flare. There is a conversion from gentle to explosive
evaporation as the flare evolves.
---------------------------------------------------------
Title: The Response of Chromosphere Lines Heated by Different Periodic
Impulsive Non-thermal Electrons
Authors: Cheng, Jianxia; Ding, Mingde
2015IAUGA..2254236C Altcode:
Solar flares involve in radiations in very broad wavelengths. Spectra
can supply us abundant information about local plasma, such as
temperature, density, mass motion and so on. Strong chromosphere
lines, like the most studied Hα and Ca II 8542 Å are formed under
conditions of non-local thermal equilibrium which represent the
response of the lower atmosphere to flare heating. Understanding
the line formation is very useful for us to correctly interpret the
observations and evolution of line intensities and profiles. Numerical
simulation is a very important way to interpret observational data
and understand the flare mechanisms. In this paper, we try to figure
out the response of chromospheric lines heated by different periodic
impulsive non-thermal electrons. Our results are based on radiative
hydrodynamic simulations. We vary the non-thermal heating periods
from 1.25s to 20s. Different non-thermal electron fluxes are also
considered in our simulations. We compare the response times for
different heating parameters. Line centers and line wings also show
different signatures. Possible explanations are discussed in the
paper.This work is supported by NSFC under grants 11303073 ,11373023
and 11103008. This research is also supported by the open project of
Key Laboratory of Solar Activity, National Astronomical Observatories,
Chinese Academy of Sciences.
---------------------------------------------------------
Title: A Two-ribbon White-light Flare Associated with a Failed Solar
Eruption Observed by ONSET, SDO, and IRIS
Authors: Cheng, X.; Hao, Q.; Ding, M. D.; Liu, K.; Chen, P. F.; Fang,
C.; Liu, Y. D.
2015ApJ...809...46C Altcode: 2015arXiv150702109C
Two-ribbon brightenings are one of the most remarkable characteristics
of an eruptive solar flare and are often used to predict the
occurrence of coronal mass ejections (CMEs). Nevertheless, it was
recently called into question whether all two-ribbon flares are
eruptive. In this paper, we investigate a two-ribbon-like white-light
(WL) flare that is associated with a failed magnetic flux rope
(MFR) eruption on 2015 January 13, which has no accompanying CME in
the WL coronagraph. Observations by the Optical and Near-infrared
Solar Eruption Tracer and the Solar Dynamics Observatory reveal
that with the increase of the flare emission and the acceleration
of the unsuccessfully erupting MFR, two isolated kernels appear
at the WL 3600 Å passband and quickly develop into two elongated
ribbon-like structures. The evolution of the WL continuum enhancement
is completely coincident in time with the variation of Fermi hard
X-ray 26-50 keV flux. An increase of continuum emission is also
clearly visible at the whole FUV and NUV passbands observed by the
Interface Region Imaging Spectrograph. Moreover, in one WL kernel,
the Si iv, C ii, and Mg ii h/k lines display significant enhancement
and non-thermal broadening. However, their Doppler velocity pattern
is location-dependent. At the strongly bright pixels, these lines
exhibit a blueshift, while at moderately bright ones, the lines are
generally redshifted. These results show that the failed MFR eruption
is also able to produce a two-ribbon flare and high-energy electrons
that heat the lower atmosphere, causing the enhancement of the WL and
FUV/NUV continuum emissions and chromospheric evaporation.
---------------------------------------------------------
Title: Slow Patchy Extreme-ultraviolet Propagating Fronts Associated
with Fast Coronal Magneto-acoustic Waves in Solar Eruptions
Authors: Guo, Y.; Ding, M. D.; Chen, P. F.
2015ApJS..219...36G Altcode:
Using the high spatiotemporal resolution extreme ultraviolet (EUV)
observations of the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory, we conduct a statistical study of the
observational properties of the coronal EUV propagating fronts. We
find that it might be a universal phenomenon for two types of fronts
to coexist in a large solar eruptive event. It is consistent with the
hybrid model of EUV propagating fronts, which predicts that coronal
EUV propagating fronts consist of both a fast magneto-acoustic wave
and a nonwave component. We find that the morphologies, propagation
behaviors, and kinematic features of the two EUV propagating fronts are
completely different from each other. The fast magneto-acoustic wave
fronts are almost isotropic. They travel continuously from the flaring
region across multiple magnetic polarities to global distances. On
the other hand, the slow nonwave fronts appear as anisotropic and
sequential patches of EUV brightening. Each patch propagates locally
in the magnetic domains where the magnetic field lines connect to the
bottom boundary and stops at the magnetic domain boundaries. Within
each magnetic domain, the velocities of the slow patchy nonwave
component are an order of magnitude lower than that of the fast-wave
component. However, the patches of the slow EUV propagating front can
jump from one magnetic domain to a remote one. The velocities of such
a transit between different magnetic domains are about one-third to
one-half of those of the fast-wave component. The results show that the
velocities of the nonwave component, both within one magnetic domain
and between different magnetic domains, are highly nonuniform due to
the inhomogeneity of the magnetic field in the lower atmosphere.
---------------------------------------------------------
Title: On the Nature of the EUV Late Phase of Solar Flares
Authors: Li, Ying; Ding, Mingde; Guo, Yang; Dai, Yu
2015IAUGA..2258380L Altcode:
The extreme-ultraviolet (EUV) late phase of solar flares is a second
peak of warm coronal emissions (e.g., Fe XVI) for many minutes to a
few hours after the GOES soft X-ray peak. It was first observed by
the EUV Variability Experiment on board the Solar Dynamics Observatory
(SDO). The late-phase emission originates from a second set of longer
loops (late-phase loops) that are higher than the main flaring loops. It
is suggested to be caused by either additional heating or long-lasting
cooling. In this paper, we study the role of long-lasting cooling and
additional heating in producing the EUV late phase using the enthalpy
based thermal evolution of loops model. We find that a long cooling
process in late-phase loops can well explain the presence of the EUV
late-phase emission, but we cannot exclude the possibility of additional
heating in the decay phase. Moreover, we provide two preliminary methods
based on the UV and EUV emissions from the Atmospheric Imaging Assembly
on board SDO to determine whether or not additional heating plays a role
in the late-phase emission. Using nonlinear force-free field modeling,
we study the magnetic configuration of the EUV late phase. It is found
that the late phase can be generated either in hot spine field lines
associated with a magnetic null point or in large-scale magnetic loops
of multipolar magnetic fields. In this paper, we also discuss why the
EUV late phase is usually observed in warm coronal emissions and why
the majority of flares do not exhibit an EUV late phase.
---------------------------------------------------------
Title: Predicting the Arrival Time of Coronal Mass Ejections with
the Graduated Cylindrical Shell and Drag Force Model
Authors: Shi, Tong; Wang, Yikang; Wan, Linfeng; Cheng, Xin; Ding,
Mingde; Zhang, Jie
2015ApJ...806..271S Altcode: 2015arXiv150500884S
Accurately predicting the arrival of coronal mass ejections (CMEs) to
the Earth based on remote images is of critical significance for the
study of space weather. In this paper, we make a statistical study of 21
Earth-directed CMEs, specifically exploring the relationship between CME
initial speeds and transit times. The initial speed of a CME is obtained
by fitting the CME with the Graduated Cylindrical Shell model and is
thus free of projection effects. We then use the drag force model to
fit results of the transit time versus the initial speed. By adopting
different drag regimes, i.e., the viscous, aerodynamics, and hybrid
regimes, we get similar results, with a least mean estimation error of
the hybrid model of 12.9 hr. CMEs with a propagation angle (the angle
between the propagation direction and the Sun-Earth line) larger than
their half-angular widths arrive at the Earth with an angular deviation
caused by factors other than the radial solar wind drag. The drag force
model cannot be reliably applied to such events. If we exclude these
events in the sample, the prediction accuracy can be improved, i.e.,
the estimation error reduces to 6.8 hr. This work suggests that it is
viable to predict the arrival time of CMEs to the Earth based on the
initial parameters with fairly good accuracy. Thus, it provides a method
of forecasting space weather 1-5 days following the occurrence of CMEs.
---------------------------------------------------------
Title: Extreme ultraviolet imaging of three-dimensional magnetic
reconnection in a solar eruption
Authors: Sun, J. Q.; Cheng, X.; Ding, M. D.; Guo, Y.; Priest, E. R.;
Parnell, C. E.; Edwards, S. J.; Zhang, J.; Chen, P. F.; Fang, C.
2015NatCo...6.7598S Altcode: 2015NatCo...6E7598S; 2015arXiv150608255S
Magnetic reconnection, a change of magnetic field connectivity, is
a fundamental physical process in which magnetic energy is released
explosively, and it is responsible for various eruptive phenomena in the
universe. However, this process is difficult to observe directly. Here,
the magnetic topology associated with a solar reconnection event is
studied in three dimensions using the combined perspectives of two
spacecraft. The sequence of extreme ultraviolet images clearly shows
that two groups of oppositely directed and non-coplanar magnetic loops
gradually approach each other, forming a separator or quasi-separator
and then reconnecting. The plasma near the reconnection site is
subsequently heated from ~1 to >=5 MK. Shortly afterwards, warm flare
loops (~3 MK) appear underneath the hot plasma. Other observational
signatures of reconnection, including plasma inflows and downflows, are
unambiguously revealed and quantitatively measured. These observations
provide direct evidence of magnetic reconnection in a three-dimensional
configuration and reveal its origin.
---------------------------------------------------------
Title: On the 2012 October 23 Circular Ribbon Flare: Emission Features
and Magnetic Topology
Authors: Yang, Kai; Guo, Yang; Ding, M. D.
2015ApJ...806..171Y Altcode: 2015arXiv150502914Y
Circular ribbon flares are usually related to spine-fan type magnetic
topology containing null points. In this paper, we investigate
an X-class circular ribbon flare on 2012 October 23, using the
multiwavelength data from the Solar Dynamics Observatory, Hinode,
and RHESSI. In Ca ii H emission, the flare showed three ribbons with
two highly elongated ones inside and outside a quasi-circular one,
respectively. A hot channel was displayed in the extreme-ultraviolet
emissions that infers the existence of a magnetic flux rope. Two hard
X-ray (HXR) sources in the 12-25 keV energy band were located at the
footpoints of this hot channel. Using a nonlinear force-free magnetic
field extrapolation, we identify three topological structures: (1)
a three-dimensional null point, (2) a flux rope below the fan of the
null point, and (3) a large-scale quasi-separatrix layer (QSL) induced
by the quadrupolar-like magnetic field of the active region. We find
that the null point is embedded within the large-scale QSL. In our
case, all three identified topological structures must be considered
to explain all the emission features associated with the observed
flare. Besides, the HXR sources are regarded as the consequence of
the reconnection within or near the border of the flux rope.
---------------------------------------------------------
Title: A Type II Radio Burst without a Coronal Mass Ejection
Authors: Su, W.; Cheng, X.; Ding, M. D.; Chen, P. F.; Sun, J. Q.
2015ApJ...804...88S Altcode: 2015arXiv150300861S
Type II radio bursts are thought to be a signature of coronal shocks. In
this paper, we analyze a short-lived type II burst that started at 07:40
UT on 2011 February 28. By carefully checking white-light images, we
find that the type II radio burst is not accompanied by a coronal mass
ejection, only by a C2.4 class flare and narrow jet. However, in the EUV
images provided by the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory, we find a wave-like structure that propagated at
a speed of ∼600 km s<SUP>-1</SUP> during the burst. The relationship
between the type II radio burst and the wave-like structure is, in
particular, explored. For this purpose, we first derive the density
distribution under the wave by the differential emission measure method,
which is used to restrict the empirical density model. We then use the
restricted density model to invert the speed of the shock that produces
the observed frequency drift rate in the dynamic spectrum. The inverted
shock speed is similar to the speed of the wave-like structure. This
implies that the wave-like structure is most likely a coronal shock
that produces the type II radio burst. We also examine the evolution
of the magnetic field in the flare-associated active region and find
continuous flux emergence and cancellation taking place near the flare
site. Based on these facts, we propose a new mechanism for the formation
of the type II radio burst, i.e., the expansion of the strongly inclined
magnetic loops after reconnecting with a nearby emerging flux acts as
a piston to generate the shock wave.
---------------------------------------------------------
Title: Imaging and Spectroscopic Diagnostics on the Formation of
Two Magnetic Flux Ropes Revealed by SDO/AIA and IRIS
Authors: Cheng, X.; Ding, M. D.; Fang, C.
2015ApJ...804...82C Altcode: 2015arXiv150207801C
Helical magnetic flux rope (MFR) is a fundamental structure of
coronal mass ejections (CMEs) and has been discovered recently to
exist as a sigmoidal channel structure prior to its eruption in the
EUV high-temperature passbands of the Atmospheric Imaging Assembly
(AIA). However, when and where the MFR is built up are still elusive. In
this paper, we investigate two MFRs (MFR1 and MFR2) in detail, whose
eruptions produced two energetic solar flares and CMEs on 2014 April
18 and 2014 September 10, respectively. The AIA EUV images reveal that
for a long time prior to their eruption, both MFR1 and MFR2 are under
formation, which is probably through magnetic reconnection between two
groups of sheared arcades driven by the shearing and converging flows in
the photosphere near the polarity inversion line. At the footpoints of
the MFR1, the Interface Region Imaging Spectrograph Si iv, C ii, and Mg
ii lines exhibit weak to moderate redshifts and a non-thermal broadening
in the pre-flare phase. However, a relatively large blueshift and an
extremely strong non-thermal broadening are found at the formation site
of the MFR2. These spectral features consolidate the proposition that
the reconnection plays an important role in the formation of MFRs. For
the MFR1, the reconnection outflow may propagate along its legs,
penetrating into the transition region and the chromosphere at the
footpoints. For the MFR2, the reconnection probably takes place in the
lower atmosphere and results in the strong blueshift and non-thermal
broadening for the Mg ii, C ii, and Si iv lines.
---------------------------------------------------------
Title: Magnetohydrodynamic Seismology of a Coronal Loop System by
the First Two Modes of Standing Kink Waves
Authors: Guo, Y.; Erdélyi, R.; Srivastava, A. K.; Hao, Q.; Cheng,
X.; Chen, P. F.; Ding, M. D.; Dwivedi, B. N.
2015ApJ...799..151G Altcode: 2014arXiv1411.7095G
We report the observation of the first two harmonics of the horizontally
polarized kink waves excited in a coronal loop system lying southeast of
AR 11719 on 2013 April 11. The detected periods of the fundamental mode
(P <SUB>1</SUB>), its first overtone (P <SUB>2</SUB>) in the northern
half, and that in the southern one are 530.2 ± 13.3, 300.4 ± 27.7, and
334.7 ± 22.1 s, respectively. The periods of the first overtone in the
two halves are the same considering uncertainties in the measurement. We
estimate the average electron density, temperature, and length of
the loop system as (5.1 ± 0.8) × 10<SUP>8</SUP> cm<SUP>-3</SUP>,
0.65 ± 0.06 MK, and 203.8 ± 13.8 Mm, respectively. As a zeroth-order
estimation, the magnetic field strength, B = 8.2 ± 1.0 G, derived by
the coronal seismology using the fundamental kink mode matches with
that derived by a potential field model. The extrapolation model also
shows the asymmetric and nonuniform distribution of the magnetic field
along the coronal loop. Using the amplitude profile distributions of
both the fundamental mode and its first overtone, we observe that the
antinode positions of both the fundamental mode and its first overtone
shift toward the weak field region along the coronal loop. The results
indicate that the density stratification and the temperature difference
effects are larger than the magnetic field variation effect on the
period ratio. On the other hand, the magnetic field variation has a
greater effect on the eigenfunction of the first overtone than the
density stratification does for this case.
---------------------------------------------------------
Title: Variations in Crustal Structure, Lithospheric Flexural
Strength, and Isostatic Compensation Mechanisms of Mars
Authors: Ding, M.; Lin, J.; Zuber, M. T.
2014AGUFM.P51E3991D Altcode:
We analyze gravity and topography of Mars to investigate the spatial
variations in crustal thickness, lithospheric strength, and mechanisms
of support of prominent topographic features on Mars. The latest gravity
model JGMRO110c (released in 2012) from the Mars Reconnaissance Orbiter
mission has a spatial block size resolution of ~97 km (corresponding
to degree-110), enabling us to resolve crustal structures at higher
spatial resolution than those determined from previous degree-80 and
85 gravity models [Zuber et al., 2000; McGovern et al., 2002, 2004;
Neumann et al., 2004; Belleguic et al., 2005]. Using the latest gravity
data, we first inverted for a new version of crustal thickness model
of Mars assuming homogeneous crust and mantle densities of 2.9 and
3.5 g/cm3. We calculated "isostatic" topography for the Airy local
isostatic compensation mechanism, and "non-isostatic" topography after
removing the isostatic part. We find that about 92% of the Martian
surface is in relatively isostatic state, indicating either relatively
small lithospheric strength and/or small vertical loading. Relatively
isostatic regions include the hemispheric dichotomy, Hellas and Argyre
Planitia, Noachis and Arabia Terra, and Terra Cimmeria. In contrast,
regions with significant amount of non-isostatic topography include
the Olympus, Ascraeus, Arsia, Pavonis, Alba, and Elysium Mons, Isidis
Planitia and Valles Marineris. Their relatively large "non-isostatc
topography" implies relatively strong lithospheric strength and large
vertical loading. Spectral analysis of the admittance and correlation
relationship between gravity and topography were conducted for the
non-isostatic regions using the localized spectra method [Wieczorek and
Simons, 2005, 2007] and thin-shell lithospheric flexural approximation
[Forsyth, 1985; McGovern et al., 2002, 2004]. The best-fitting models
reveal significant variations in the effective lithospheric thickness
with the greatest values for the Olympus Mon, Valles Marineris, and
Isidis Planitia; reduced values for the Ascraeus, Arsis, and Pavonis
Mons; and smallest values for the Alba and Elysium Mons. Our models also
suggest that there could be significant sub-surface loading underneath
the Olympus, Ascraeus, Arsia, and Pavonis Mons, and Isidis Planitia.
---------------------------------------------------------
Title: On the Nature of the Extreme-ultraviolet Late Phase of
Solar Flares
Authors: Li, Y.; Ding, M. D.; Guo, Y.; Dai, Y.
2014ApJ...793...85L Altcode: 2014arXiv1407.6041L
The extreme-ultraviolet (EUV) late phase of solar flares is a second
peak of warm coronal emissions (e.g., Fe XVI) for many minutes to a
few hours after the GOES soft X-ray peak. It was first observed by
the EUV Variability Experiment on board the Solar Dynamics Observatory
(SDO). The late-phase emission originates from a second set of longer
loops (late-phase loops) that are higher than the main flaring loops. It
is suggested to be caused by either additional heating or long-lasting
cooling. In this paper, we study the role of long-lasting cooling and
additional heating in producing the EUV late phase using the enthalpy
based thermal evolution of loops model. We find that a long cooling
process in late-phase loops can well explain the presence of the EUV
late-phase emission, but we cannot exclude the possibility of additional
heating in the decay phase. Moreover, we provide two preliminary methods
based on the UV and EUV emissions from the Atmospheric Imaging Assembly
on board SDO to determine whether or not additional heating plays a role
in the late-phase emission. Using nonlinear force-free field modeling,
we study the magnetic configuration of the EUV late phase. It is found
that the late phase can be generated either in hot spine field lines
associated with a magnetic null point or in large-scale magnetic loops
of multipolar magnetic fields. In this paper, we also discuss why the
EUV late phase is usually observed in warm coronal emissions and why
the majority of flares do not exhibit an EUV late phase.
---------------------------------------------------------
Title: Recurrent Coronal Jets Induced by Magnetic Emergence in the
Solar Atmosphere
Authors: Guo, Y.; Démoulin, P.; Schmieder, B.; Ding, M. D.; Vargas
Domínguez, S.; Liu, Y.
2014RMxAC..44...45G Altcode:
Jets are part of the observed phenomenology in the solar corona. They
are thought to be a consequence of magnetic reconnection but the physics
involved is not completely understood. We study some recurrent jetting
events with unprecedented temporal and spatial resolutions.
---------------------------------------------------------
Title: Spectral Observations of Ellerman Bombs and Fitting with a
Two-cloud Model
Authors: Hong, Jie; Ding, M. D.; Li, Ying; Fang, Cheng; Cao, Wenda
2014ApJ...792...13H Altcode: 2014arXiv1407.3048H
We study the Hα and Ca II 8542 Å line spectra of four typical
Ellerman bombs (EBs) in the active region NOAA 11765 on 2013 June 6,
observed with the Fast Imaging Solar Spectrograph installed at the 1.6
m New Solar Telescope at Big Bear Solar Observatory. Considering that
EBs may occur in a restricted region in the lower atmosphere, and that
their spectral lines show particular features, we propose a two-cloud
model to fit the observed line profiles. The lower cloud can account
for the wing emission, and the upper cloud is mainly responsible for the
absorption at line center. After choosing carefully the free parameters,
we get satisfactory fitting results. As expected, the lower cloud shows
an increase of the source function, corresponding to a temperature
increase of 400-1000 K in EBs relative to the quiet Sun. This is
consistent with previous results deduced from semi-empirical models and
confirms that local heating occurs in the lower atmosphere during the
appearance of EBs. We also find that the optical depths can increase
to some extent in both the lower and upper clouds, which may result
from either direct heating in the lower cloud, or illumination by an
enhanced radiation on the upper cloud. The velocities derived from
this method, however, are different from those obtained using the
traditional bisector method, implying that one should be cautious when
interpreting this parameter. The two-cloud model can thus be used as
an efficient method to deduce the basic physical parameters of EBs.
---------------------------------------------------------
Title: Reciprocatory magnetic reconnection in a coronal bright point
Authors: Zhang, Q. M.; Chen, P. F.; Ding, M. D.; Ji, H. S.
2014A&A...568A..30Z Altcode: 2014arXiv1406.5328Z
Context. Coronal bright points (CBPs) are small-scale and long-duration
brightenings in the lower solar corona. They are often explained
in terms of magnetic reconnection. <BR /> Aims: We aim to study
the substructures of a CBP and clarify the relationship among the
brightenings of different patches inside the CBP. <BR /> Methods:
The event was observed by the X-ray Telescope (XRT) aboard the Hinode
spacecraft on 2009 August 22-23. <BR /> Results: The CBP showed repeated
brightenings (or CBP flashes). During each of the two successive CBP
flashes, that is, weak and strong flashes that were separated by ~2
hr, the XRT images revealed that the CBP was composed of two chambers,
patches A and B. During the weak flash, patch A brightened first, and
patch B brightened ~2 min later. During the transition, the right leg of
a large-scale coronal loop drifted from the right side of the CBP to the
left side. During the strong flash, patch B brightened first, and patch
A brightened ~2 min later. During the transition, the right leg of the
large-scale coronal loop drifted from the left side of the CBP to the
right side. In each flash, the rapid change of the connectivity of the
large-scale coronal loop is strongly suggestive of the interchange
reconnection. <BR /> Conclusions: For the first time we found
reciprocatory reconnection in the CBP, which means that reconnected
loops in the outflow region of the first reconnection process
serve as the inflow of the second reconnection process. <P />Movies
associated with Figs. 2 and 5 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201322815/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Formation of a Double-decker Magnetic Flux Rope in the
Sigmoidal Solar Active Region 11520
Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Sun, X. D.; Guo, Y.;
Wang, Y. M.; Kliem, B.; Deng, Y. Y.
2014ApJ...789...93C Altcode: 2014arXiv1405.4923C
In this paper, we address the formation of a magnetic flux rope
(MFR) that erupted on 2012 July 12 and caused a strong geomagnetic
storm event on July 15. Through analyzing the long-term evolution
of the associated active region observed by the Atmospheric Imaging
Assembly and the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, it is found that the twisted field of an MFR,
indicated by a continuous S-shaped sigmoid, is built up from two groups
of sheared arcades near the main polarity inversion line a half day
before the eruption. The temperature within the twisted field and
sheared arcades is higher than that of the ambient volume, suggesting
that magnetic reconnection most likely works there. The driver behind
the reconnection is attributed to shearing and converging motions
at magnetic footpoints with velocities in the range of 0.1-0.6 km
s<SUP>-1</SUP>. The rotation of the preceding sunspot also contributes
to the MFR buildup. Extrapolated three-dimensional non-linear force-free
field structures further reveal the locations of the reconnection to
be in a bald-patch region and in a hyperbolic flux tube. About 2 hr
before the eruption, indications of a second MFR in the form of an
S-shaped hot channel are seen. It lies above the original MFR that
continuously exists and includes a filament. The whole structure
thus makes up a stable double-decker MFR system for hours prior to
the eruption. Eventually, after entering the domain of instability,
the high-lying MFR impulsively erupts to generate a fast coronal mass
ejection and X-class flare; while the low-lying MFR remains behind
and continuously maintains the sigmoidicity of the active region.
---------------------------------------------------------
Title: On the Relationship Between a Hot-channel-like Solar Magnetic
Flux Rope and its Embedded Prominence
Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Srivastava, A. K.; Guo,
Y.; Chen, P. F.; Sun, J. Q.
2014ApJ...789L..35C Altcode: 2014arXiv1406.4196C
A magnetic flux rope (MFR) is a coherent and helical magnetic
field structure that has recently been found likely to appear as an
elongated hot channel prior to a solar eruption. In this Letter, we
investigate the relationship between the hot channel and the associated
prominence through analysis of a limb event on 2011 September 12. In
the early rise phase, the hot channel was initially cospatial with
the prominence. It then quickly expanded, resulting in a separation
of the top of the hot channel from that of the prominence. Meanwhile,
they both experienced an instantaneous morphology transformation from
a Λ shape to a reversed-Y shape and the top of these two structures
showed an exponential increase in height. These features are a good
indication of the occurrence of kink instability. Moreover, the onset
of kink instability is found to coincide in time with the impulsive
enhancement of flare emission underneath the hot channel, suggesting
that ideal kink instability likely also plays an important role in
triggering fast flare reconnection besides initiating the impulsive
acceleration of the hot channel and distorting its morphology. We
conclude that the hot channel is most likely the MFR system and the
prominence only corresponds to the cool materials that are collected
in the bottom of the helical field lines of the MFR against gravity.
---------------------------------------------------------
Title: Dynamic Evolution of an X-shaped Structure above a
Trans-equatorial Quadrupole Solar Active Region Group
Authors: Sun, J. Q.; Cheng, X.; Guo, Y.; Ding, M. D.; Li, Y.
2014ApJ...787L..27S Altcode: 2014arXiv1405.1563S
In the solar corona, magnetic reconnection usually takes place at the
singular configuration of the magnetic field, in particular near a
magnetic null, owing to its high susceptibility to perturbations. In
this Letter, we report a rare X-shaped structure, encompassing a
magnetic null, above a trans-equatorial quadrupole active region group
that is well observed by the Atmospheric Imaging Assembly (AIA). The
observations show that this X-shaped structure is visible in all AIA EUV
passbands and stably exists for days. However, possibly induced by flare
activities at the northern part of the quadrupole active region group,
the X-shaped structure starts to destabilize while a jet erupts near
its center at ~15:05 UT on 2013 October 7. Through nonlinear force-free
field modeling, we identify a magnetic null that is above the quadrupole
polarities and well corresponds to the X-shaped structure. After the
jet eruption, the temperature and emission measure of the plasma near
the X-shaped structure rise from ~2.3 MK and ~1.2 × 10<SUP>27</SUP>
cm<SUP>-5</SUP> at 15:01 UT to ~5.4 MK and ~3.7 × 10<SUP>27</SUP>
cm<SUP>-5</SUP> at 15:36 UT, respectively, revealed by the differential
emission measure analysis, indicating that magnetic reconnection most
likely takes place there to heat the plasma. Moreover, the height of
the null increases ~10 Mm, which is most likely due to the partial
opening of the field lines near the fan surface that makes the null
underneath rise to seek a new equilibrium.
---------------------------------------------------------
Title: On the Nature of the EUV Late Phase of Solar Flares
Authors: Li, Ying; Ding, Mingde; Guo, Yang; Dai, Yu
2014AAS...22411103L Altcode:
The EUV late phase of solar flares is a second peak of the warm coronal
emissions (e.g., Fe XVI) many minutes to a few hours after the GOES soft
X-ray peak, which was first observed by the EUV Variability Experiment
(EVE) on board the Solar Dynamics Observatory (SDO). The late phase
emission originates from a second set of longer loops (late phase
loops) that are higher than the main flaring loops. It is explained as
being caused by either additional heating or long-lasting cooling. In
this paper, we study the role of long-lasting cooling and additional
heating in producing the EUV late phase using the "enthalpy-based
thermal evolution of loops'" (EBTEL) experiments. We find that the
long cooling process in late phase loops, which definitely exists,
can sufficiently explain the emission property of the EUV late phase;
meanwhile, we cannot exclude the role of an additional heating of these
loops that possibly occurs. Moreover, we provide two preliminary methods
based on the UV and EUV emissions from the Atmospheric Imaging Assembly
(AIA) on board SDO to determine whether an additional heating plays
some role or not in the late phase emission. Through the nonlinear
force-free field modeling, we study the magnetic configuration related
to the EUV late phase. It is found that the late phase can be generated
either in hot spine field lines associated with a magnetic null point
or large-scale magnetic loops in multipolar magnetic fields. In this
paper, we also explain why the EUV late phase appears most obviously
in the warm coronal emissions and why the majority of flares do not
exhibit an EUV late phase.
---------------------------------------------------------
Title: Differential Emission Measure Analysis of A Limb Solar Flare
on 2012 July 19
Authors: Sun, J. Q.; Cheng, X.; Ding, M. D.
2014ApJ...786...73S Altcode: 2014arXiv1403.6202S
We perform Differential Emission Measure (DEM) analysis of an M7.7 flare
that occurred on 2012 July 19 and was well observed by the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamic Observatory. Using
the observational data with unprecedented high temporal and spatial
resolution from six AIA coronal passbands, we calculate the DEM of the
flare and derive the time series of maps of DEM-weighted temperature
and emission measure (EM). It is found that, during the flare,
the highest EM region is located in the flare loop top with a value
varying between ~8.4 × 10<SUP>28</SUP> cm<SUP>-5</SUP> and ~2.5 ×
10<SUP>30</SUP> cm<SUP>-5</SUP>. The temperature there rises from ~8
MK at about 04:40 UT (the initial rise phase) to a maximum value of
~13 MK at about 05:20 UT (the hard X-ray peak). Moreover, we find a
hot region that is above the flare loop top with a temperature even
up to ~16 MK. We also analyze the DEM properties of the reconnection
site. The temperature and density there are not as high as that in the
loop top and the flux rope, indicating that the main heating may not
take place inside the reconnection site. In the end, we examine the
dynamic behavior of the flare loops. Along the flare loop, both the
temperature and the EM are the highest in the loop top and gradually
decrease toward the footpoints. In the northern footpoint, an upward
force appears with a biggest value in the impulsive phase, which we
conjecture originates from chromospheric evaporation.
---------------------------------------------------------
Title: Heating and Dynamics of Two Flare Loop Systems Observed by
AIA and EIS
Authors: Li, Y.; Qiu, J.; Ding, M. D.
2014ApJ...781..120L Altcode: 2013arXiv1312.5056L
We investigate heating and evolution of flare loops in a C4.7 two-ribbon
flare on 2011 February 13. From Solar Dynamics Observatory/Atmospheric
Imaging Assembly (AIA) imaging observations, we can identify two sets of
loops. Hinode/EUV Imaging Spectrometer (EIS) spectroscopic observations
reveal blueshifts at the feet of both sets of loops. The evolution
and dynamics of the two sets are quite different. The first set of
loops exhibits blueshifts for about 25 minutes followed by redshifts,
while the second set shows stronger blueshifts, which are maintained
for about one hour. The UV 1600 observation by AIA also shows that
the feet of the second set of loops brighten twice. These suggest that
continuous heating may be present in the second set of loops. We use
spatially resolved UV light curves to infer heating rates in the few
tens of individual loops comprising the two loop systems. With these
heating rates, we then compute plasma evolution in these loops with the
"enthalpy-based thermal evolution of loops" model. The results show
that, for the first set of loops, the synthetic EUV light curves from
the model compare favorably with the observed light curves in six AIA
channels and eight EIS spectral lines, and the computed mean enthalpy
flow velocities also agree with the Doppler shift measurements by
EIS. For the second set of loops modeled with twice-heating, there
are some discrepancies between modeled and observed EUV light curves
in low-temperature bands, and the model does not fully produce the
prolonged blueshift signatures as observed. We discuss possible causes
for the discrepancies.
---------------------------------------------------------
Title: Tracking the Evolution of a Coherent Magnetic Flux Rope
Continuously from the Inner to the Outer Corona
Authors: Cheng, X.; Ding, M. D.; Guo, Y.; Zhang, J.; Vourlidas, A.;
Liu, Y. D.; Olmedo, O.; Sun, J. Q.; Li, C.
2014ApJ...780...28C Altcode: 2013arXiv1310.6782C
The magnetic flux rope (MFR) is believed to be the underlying
magnetic structure of coronal mass ejections (CMEs). However, it
remains unclear how an MFR evolves into and forms the multi-component
structure of a CME. In this paper, we perform a comprehensive study of
an extreme-ultraviolet (EUV) MFR eruption on 2013 May 22 by tracking
its morphological evolution, studying its kinematics, and quantifying
its thermal property. As EUV brightenings begin, the MFR starts to rise
slowly and shows helical threads winding around an axis. Meanwhile, cool
filamentary materials descend spirally down to the chromosphere. These
features provide direct observational evidence of intrinsically helical
structure of the MFR. Through detailed kinematical analysis, we find
that the MFR evolution has two distinct phases: a slow rise phase and
an impulsive acceleration phase. We attribute the first phase to the
magnetic reconnection within the quasi-separatrix layers surrounding
the MFR, and the much more energetic second phase to the fast magnetic
reconnection underneath the MFR. We suggest that the transition
between these two phases is caused by the torus instability. Moreover,
we identify that the MFR evolves smoothly into the outer corona and
appears as a coherent structure within the white-light CME volume. The
MFR in the outer corona was enveloped by bright fronts that originated
from plasma pile-up in front of the expanding MFR. The fronts are also
associated with the preceding sheath region followed by the outmost
MFR-driven shock.
---------------------------------------------------------
Title: Twist Accumulation and Topology Structure of a Solar Magnetic
Flux Rope
Authors: Guo, Y.; Ding, M. D.; Cheng, X.; Zhao, J.; Pariat, E.
2013ApJ...779..157G Altcode: 2013arXiv1311.1883G
To study the buildup of a magnetic flux rope before a major flare and
coronal mass ejection (CME), we compute the magnetic helicity injection,
twist accumulation, and topology structure of the three-dimensional
(3D) magnetic field, which is derived by the nonlinear force-free field
model. The Extreme-ultraviolet Imaging Telescope on board the Solar and
Heliospheric Observatory observed a series of confined flares without
any CME before a major flare with a CME at 23:02 UT on 2005 January 15
in active region NOAA 10720. We derive the vector velocity at eight
time points from 18:27 UT to 22:20 UT with the differential affine
velocity estimator for vector magnetic fields, which were observed by
the Digital Vector Magnetograph at Big Bear Solar Observatory. The
injected magnetic helicity is computed with the vector magnetic and
velocity fields. The helicity injection rate was (- 16.47 ± 3.52) ×
10<SUP>40</SUP> Mx<SUP>2</SUP> hr<SUP>-1</SUP>. We find that only about
1.8% of the injected magnetic helicity became the internal helicity of
the magnetic flux rope, whose twist increasing rate was -0.18 ± 0.08
Turns hr<SUP>-1</SUP>. The quasi-separatrix layers (QSLs) of the 3D
magnetic field are computed by evaluating the squashing degree, Q. We
find that the flux rope was wrapped by QSLs with large Q values, where
the magnetic reconnection induced by the continuously injected magnetic
helicity further produced the confined flares. We suggest that the flux
rope was built up and heated by the magnetic reconnection in the QSLs.
---------------------------------------------------------
Title: Production of the Extreme-ultraviolet Late Phase of an X
Class Flare in a Three-stage Magnetic Reconnection Process
Authors: Dai, Y.; Ding, M. D.; Guo, Y.
2013ApJ...773L..21D Altcode: 2013arXiv1307.4866D
We report on observations of an X class flare on 2011 September 6 by
the instruments on board the Solar Dynamics Observatory. The flare
occurs in a complex active region with multiple polarities. The
Extreme-Ultraviolet (EUV) Variability Experiment observations in
the warm coronal emission reveal three enhancements, the third of
which corresponds to an EUV late phase. The three enhancements have
a one-to-one correspondence to the three stages in flare evolution
identified by the spatially resolved Atmospheric Imaging Assembly
observations, which are characterized by a flux rope eruption, a
moderate filament ejection, and the appearance of EUV late phase
loops, respectively. The EUV late phase loops are spatially and
morphologically distinct from the main flare loops. Multi-channel
analysis suggests the presence of a continuous but fragmented energy
injection during the EUV late phase resulting in the warm corona
nature of the late phase loops. Based on these observational facts,
we propose a three-stage magnetic reconnection scenario to explain the
flare evolution. Reconnections in different stages involve different
magnetic fields but show a casual relationship between them. The EUV
late phase loops are mainly produced by the least energetic magnetic
reconnection in the last stage.
---------------------------------------------------------
Title: Recurrent coronal jets induced by repetitively accumulated
electric currents
Authors: Guo, Y.; Démoulin, P.; Schmieder, B.; Ding, M. D.; Vargas
Domínguez, S.; Liu, Y.
2013A&A...555A..19G Altcode: 2013arXiv1305.0902G
Context. Jets of plasma are frequently observed in the solar corona. A
self-similar recurrent behavior is observed in a fraction of them. <BR
/> Aims: Jets are thought to be a consequence of magnetic reconnection;
however, the physics involved is not fully understood. Therefore,
we study some jet observations with unprecedented temporal and
spatial resolutions. <BR /> Methods: The extreme-ultraviolet (EUV)
jets were observed by the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager
(HMI) on board SDO measured the vector magnetic field, from which
we derive the magnetic flux evolution, the photospheric velocity
field, and the vertical electric current evolution. The magnetic
configuration before the jets is derived by the nonlinear force-free
field extrapolation. <BR /> Results: Three EUV jets recurred in about
one hour on 17 September 2010 in the following magnetic polarity
of active region 11106. We derive that the jets are above a pair of
parasitic magnetic bipoles that are continuously driven by photospheric
diverging flows. The interaction drove the buildup of electric currents,
which we observed as elongated patterns at the photospheric level. For
the first time, the high temporal cadence of the HMI allows the
evolution of such small currents to be followed. In the jet region,
we found that the integrated absolute current peaks repetitively in
phase with the 171 Å flux evolution. The current buildup and its
decay are both fast, about ten minutes each, and the current maximum
precedes the 171 Å also by about ten minutes. Then, the HMI temporal
cadence is marginally fast enough to detect such changes. <BR />
Conclusions: The photospheric current pattern of the jets is found to
be associated with the quasi-separatrix layers deduced from the magnetic
extrapolation. From previous theoretical results, the observed diverging
flows are expected to continuously build such currents. We conclude
that the magnetic reconnection occurs periodically, in the current
layer created between the emerging bipoles and the large-scale active
region field. The periodic magnetic reconnection induced the observed
recurrent coronal jets and the decrease of the vertical electric
current magnitude. <P />Two movies are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Tests and applications of nonlinear force-free field
extrapolations in spherical geometry
Authors: Guo, Y.; Ding, M. D.
2013IAUS..294..553G Altcode:
We test a nonlinear force-free field (NLFFF) optimization code in
spherical geometry with an analytical solution from Low and Lou. The
potential field source surface (PFSS) model is served as the initial
and boundary conditions where observed data are not available. The
analytical solution can be well recovered if the boundary and
initial conditions are properly handled. Next, we discuss the
preprocessing procedure for the noisy bottom boundary data, and find
that preprocessing is necessary for NLFFF extrapolations when we use
the observed photospheric magnetic field as bottom boundaries. Finally,
we apply the NLFFF model to a solar area where four active regions
interacting with each other. An M8.7 flare occurred in one active
region. NLFFF modeling in spherical geometry simultaneously constructs
the small and large scale magnetic field configurations better than
the PFSS model does.
---------------------------------------------------------
Title: Radiative Hydrodynamic Simulations of He I 10830 Å
Authors: Cheng, J. X.; Ding, M. D.; Fang, C.
2013IAUS..294..545C Altcode:
We study the properties of the He I 10830 Å line under the bombardment
of nonthermal electron beam. Using radiative hydrodynamics method, we
obtain the line profiles from different model atmospheres by varying
the electron beam fluxes. Below a certain electron flux, the spectra
become much more absorptions as nonthemal flux increases. Above
the threshold, the spectral intensities increase as the flux goes
up. We also investigate the temporal evolution of the spectra under
the nonthermal heating. For weak electron flux, the profiles evolve
from weak absorptions to strong absorptions. For strong nonthermal
heating, the profiles can be significantly changed from absorptions
to emissions. The spectra also show red or blue asymmetries.
---------------------------------------------------------
Title: Heating and Dynamics of Two Flare Loop Systems Observed by
AIA and EIS
Authors: Li, Ying; Qiu, J.; Ding, M.
2013SPD....4420005L Altcode:
We focus on investigating heating and evolution of flare loops. A C4.7
two-ribbon flare on 2011 February 13 is analyzed and modeled. From
AIA imaging observations, we can identify two sets of loops in this
event. EIS spectroscopic observations reveal blueshifts at the feet
of both sets of loops during the impulsive phase. However, the dynamic
evolutions of the two sets of loops are quite different. The first set
of loops exhibits blueshifts 10 km/s) for about 25 minutes followed
by redshifts, while the second set shows stronger blueshifts 20 km/s)
which are maintained for about 1 hour. The long-lasting blueshifts
in the second set of loops are indicative of continuous heating. The
UV 1600 Å observation by AIA also shows that the feet of the loops
brighten twice with 15 minutes apart. The first set of loops, on the
other hand, brighten only once in the UV band. We construct heating
functions of the two sets of loops using spatially resolved UV light
curves at their footpoints, and model plasma evolution in these loops
with the EBTEL model. The results show that, for the first set of loops,
the synthetic EUV light curves from the model compare favorably with
the observed light curves in six AIA channels and eight EIS spectral
lines, and the computed mean enthalpy flow velocities also agree
with the Doppler shifts measured in EIS lines. For the second set
of loops modeled with twice-heating, there are some discrepancies
between modeled and observed EUV light curves at low-temperature
lines, and the model does not fully reproduce the prolonged blueshift
signatures as observed. The prominent discrepancies between model and
observations for the second set of loops may be caused by non-uniform
heating localized especially at the loop footpoints which cannot be
modeled by the 0D EBTEL model, or by unresolved fine flaring strands
in the loops with quite different heating rates and profiles.
---------------------------------------------------------
Title: Solar Flare Hard X-ray Spikes Observed by RHESSI: a Statistical
Study
Authors: Cheng, Jianxia; Qiu, J.; Ding, M.; Wang, H.
2013SPD....4440205C Altcode:
Hard X-ray (HXR) spikes refer to fine time structures on timescales
of seconds to milliseconds in high-energy HXR emission profiles
during solar flare eruptions. We present a preliminary statistical
investigation of temporal and spectral properties of HXR spikes. Using
a three-sigma spike selection rule, we detected 184 spikes in 94 out
of 322 flares with significant counts at given photon energies, which
were detected from demodulated HXR light curves obtained by the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth
of these spikes are also detected at photon energies higher than 100
keV. The statistical properties of the spikes are as follows. (1)
HXR spikes are produced in both impulsive flares and long-duration
flares with nearly the same occurrence rates. Ninety percent of
the spikes occur during the rise phase of the flares, and about 70%
occur around the peak times of the flares. (2) The time durations of
the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is
not dependent on photon energies. The spikes exhibit symmetric time
profiles with no significant difference between rise and decay times.(3)
Among the most energetic spikes, nearly all of them have harder count
spectra than their underlying slow-varying components. There is also
a weak indication that spikes exhibiting time lags in high-energy
emissions tend to have harder spectra than spikes with time lags in
low-energy emissions.
---------------------------------------------------------
Title: Investigating Two Successive Flux Rope Eruptions in a Solar
Active Region
Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Olmedo, O.; Sun, X. D.;
Guo, Y.; Liu, Y.
2013ApJ...769L..25C Altcode: 2013arXiv1304.7532C
We investigate two successive flux rope (FR1 and FR2) eruptions
resulting in two coronal mass ejections (CMEs) on 2012 January 23. Both
flux ropes (FRs) appeared as an EUV channel structure in the images
of high temperature passbands of the Atmospheric Imaging Assembly
prior to the CME eruption. Through fitting their height evolution
with a function consisting of linear and exponential components,
we determine the onset time of the FR impulsive acceleration with
high temporal accuracy for the first time. Using this onset time, we
divide the evolution of the FRs in the low corona into two phases:
a slow rise phase and an impulsive acceleration phase. In the slow
rise phase of FR1, the appearance of sporadic EUV and UV brightening
and the strong shearing along the polarity inverse line indicates
that the quasi-separatrix-layer reconnection likely initiates the slow
rise. On the other hand, for FR2, we mainly contribute its slow rise
to the FR1 eruption, which partially opened the overlying field and
thus decreased the magnetic restriction. At the onset of the impulsive
acceleration phase, FR1 (FR2) reaches the critical height of 84.4 ±
11.2 Mm (86.2 ± 13.0 Mm) where the decline of the overlying field with
height is fast enough to trigger the torus instability. After a very
short interval (~2 minutes), the flare emission began to enhance. These
results reveal the compound activity involving multiple magnetic FRs
and further suggest that the ideal torus instability probably plays
the essential role of initiating the impulsive acceleration of CMEs.
---------------------------------------------------------
Title: The Driver of Coronal Mass Ejections in the Low Corona:
A Flux Rope
Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Liu, Y.; Poomvises, W.
2013ApJ...763...43C Altcode: 2012arXiv1211.6524C
Recent Solar Dynamic Observatory observations reveal that coronal
mass ejections (CMEs) consist of a multi-temperature structure: a hot
flux rope and a cool leading front (LF). The flux rope first appears
as a twisted hot channel in the Atmospheric Imaging Assembly (AIA)
94 Å and 131 Å passbands. The twisted hot channel initially lies
along the polarity inversion line and then rises and develops into a
semi-circular flux-rope-like structure during the impulsive acceleration
phase of CMEs. In the meantime, the rising hot channel compresses the
surrounding magnetic field and plasma, which successively stack into the
CME LF. In this paper, we study in detail two well-observed CMEs that
occurred on 2011 March 7 and 2011 March 8, respectively. Each of them
is associated with an M-class flare. Through a kinematic analysis we
find that (1) the hot channel rises earlier than the first appearance
of the CME LF and the onset of the associated flare and (2) the speed
of the hot channel is always faster than that of the LF, at least in
the field of view of AIA. Thus, the hot channel acts as a continuous
driver of the CME formation and eruption in the early acceleration
phase. Subsequently, the two CMEs in white-light images can be well
reproduced by the graduated cylindrical shell flux rope model. These
results suggest that the pre-existing flux rope plays a key role in
CME initiation and formation.
---------------------------------------------------------
Title: Differential Emission Measure Analysis of Multiple Structural
Components of Coronal Mass Ejections in the Inner Corona
Authors: Cheng, X.; Zhang, J.; Saar, S. H.; Ding, M. D.
2012ApJ...761...62C Altcode: 2012arXiv1210.7287C
In this paper, we study the temperature and density properties of
multiple structural components of coronal mass ejections (CMEs)
using differential emission measure (DEM) analysis. The DEM analysis
is based on the six-passband EUV observations of solar corona from the
Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The
structural components studied include the hot channel in the core region
(presumably the magnetic flux rope of the CME), the bright loop-like
leading front (LF), and coronal dimming in the wake of the CME. We find
that the presumed flux rope has the highest average temperature (>8
MK) and density (~1.0 × 10<SUP>9</SUP> cm<SUP>-3</SUP>), resulting in
an enhanced emission measure over a broad temperature range (3 <=
T(MK) <= 20). On the other hand, the CME LF has a relatively cool
temperature (~2 MK) and a narrow temperature distribution similar to the
pre-eruption coronal temperature (1 <= T(MK) <= 3). The density
in the LF, however, is increased by 2%-32% compared with that of the
pre-eruption corona, depending on the event and location. In coronal
dimmings, the temperature is more broadly distributed (1 <= T(MK)
<= 4), but the density decreases by ~35%-~40%. These observational
results show that: (1) CME core regions are significantly heated,
presumably through magnetic reconnection; (2) CME LFs are a consequence
of compression of ambient plasma caused by the expansion of the CME
core region; and (3) the dimmings are largely caused by the plasma
rarefaction associated with the eruption.
---------------------------------------------------------
Title: Spectral diagnostics of the heating and dynamics of the
solar chromosphere
Authors: Ding, M.
2012IAUSS...6E.306D Altcode:
The solar chromosphere is a highly dynamic and inhomogeneous layer
in which spectral lines are formed under non-LTE conditions making
the spectral diagnostics relatively difficult. In particular, during
solar flares, the chromosphere is heated by non-thermal electrons (or
heat conduction) from the corona, which produce hard X-ray emission
and result in enhancements in chromospheric line emission, UV emission
and possibly white-light emission. The chromosphere is also believed
to be the energy release site of some small-scale activities like
Ellerman bombs. Here, I make a brief review of the recent progress
in diagnosing the heating and dynamics of the chromosphere in various
solar activities using multi-wavelength line and continuum observations
by especially ground-based telescopes.
---------------------------------------------------------
Title: Evolution and DEM analysis of Early Structures of Coronal
Mass Ejections by SDO-AIA
Authors: Cheng, X.; Zhang, J.; Ding, M.
2012AGUFMSH41A2096C Altcode:
Recent Solar Dynamic Observatory observations reveal that some coronal
mass ejections (CMEs) consist of multi-temperature structures:
a hot channel and a cool leading edge (LE). The channel first
appears as a twisted structure in the Atmospheric Imaging Assembly
94 ? and 131 ? passbands. The twisted hot channel initially lies
along the polarity inversion line and then gradually develops into
the less twisted semicircular structure during the impulsive phase
of CME acceleration. In the meantime, the hot channel compresses
the surrounding magnetic field and plasma, which successively stack
into the CME leading front. Further kinematic analysis shows that,
(1) the hot channel rises earlier than the first appearance of the
CME leading edge and the onset of the associated flare; (2) the speed
of the hot channel is always larger than that of the leading edge,
at least in the field of view of AIA. It seems that the hot channel
acts as a continuous driver of the CME formation and acceleration in
the early acceleration phase, like the behaviors of flux rope in CME
models and simulations. Moreover, through the differential emission
measure analysis, it is confirmed that the channel is dominated by the
plasma with high temperature (>8 MK) and high density (109cm-3),
while the leading edge and dimming region are mainly dominated by the
low temperature plasma (~2-3MK).
---------------------------------------------------------
Title: Modeling Magnetic Field Structure of a Solar Active Region
Corona Using Nonlinear Force-free Fields in Spherical Geometry
Authors: Guo, Y.; Ding, M. D.; Liu, Y.; Sun, X. D.; DeRosa, M. L.;
Wiegelmann, T.
2012ApJ...760...47G Altcode: 2012arXiv1210.0998G
We test a nonlinear force-free field (NLFFF) optimization code
in spherical geometry using an analytical solution from Low and
Lou. Several tests are run, ranging from idealized cases where exact
vector field data are provided on all boundaries, to cases where noisy
vector data are provided on only the lower boundary (approximating
the solar problem). Analytical tests also show that the NLFFF code
in the spherical geometry performs better than that in the Cartesian
one when the field of view of the bottom boundary is large, say, 20°
× 20°. Additionally, we apply the NLFFF model to an active region
observed by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory (SDO) both before and after an M8.7 flare. For
each observation time, we initialize the models using potential field
source surface (PFSS) extrapolations based on either a synoptic chart
or a flux-dispersal model, and compare the resulting NLFFF models. The
results show that NLFFF extrapolations using the flux-dispersal model
as the boundary condition have slightly lower, therefore better,
force-free, and divergence-free metrics, and contain larger free
magnetic energy. By comparing the extrapolated magnetic field lines with
the extreme ultraviolet (EUV) observations by the Atmospheric Imaging
Assembly on board SDO, we find that the NLFFF performs better than
the PFSS not only for the core field of the flare productive region,
but also for large EUV loops higher than 50 Mm.
---------------------------------------------------------
Title: Quadrature Observations of Wave and Non-wave Components and
their Decoupling in an Extreme-ultraviolet Wave Event
Authors: Dai, Y.; Ding, M. D.; Chen, P. F.; Zhang, J.
2012ApJ...759...55D Altcode: 2012arXiv1208.4539D
We report quadrature observations of an extreme-ultraviolet (EUV) wave
event on 2011 January 27 obtained by the Extreme Ultraviolet Imager on
board the Solar Terrestrial Relations Observatory, and the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory. Two
components are revealed in the EUV wave event. A primary front is
launched with an initial speed of ~440 km s<SUP>-1</SUP>. It appears
that significant emission enhancement occurs in the hotter channel
while deep emission reduction occurs in the cooler channel. When the
primary front encounters a large coronal loop system and slows down,
a secondary, much fainter, front emanates from the primary front with a
relatively higher starting speed of ~550 km s<SUP>-1</SUP>. Afterward,
the two fronts propagate independently with increasing separation. The
primary front finally stops at a magnetic separatrix, while the
secondary front travels farther until it fades out. In addition,
upon the arrival of the secondary front, transverse oscillations of
a prominence are triggered. We suggest that the two components are
of different natures. The primary front belongs to a non-wave coronal
mass ejection (CME) component, which can be reasonably explained with
the field-line stretching model. The multi-temperature behavior may be
caused by considerable heating due to nonlinear adiabatic compression
on the CME frontal loop. As for the secondary front, it is most likely
a linear fast-mode magnetohydrodynamic wave that propagates through
a medium of the typical coronal temperature. X-ray and radio data
provide us with complementary evidence in support of the above scenario.
---------------------------------------------------------
Title: Solar flare hard X-ray spikes observed by RHESSI: a statistical
study
Authors: Cheng, J. X.; Qiu, J.; Ding, M. D.; Wang, H.
2012A&A...547A..73C Altcode: 2012arXiv1210.7027C
Context. Hard X-ray (HXR) spikes refer to fine time structures on
timescales of seconds to milliseconds in high-energy HXR emission
profiles during solar flare eruptions. <BR /> Aims: We present
a preliminary statistical investigation of temporal and spectral
properties of HXR spikes. <BR /> Methods: Using a three-sigma spike
selection rule, we detected 184 spikes in 94 out of 322 flares with
significant counts at given photon energies, which were detected from
demodulated HXR light curves obtained by the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are
also detected at photon energies higher than 100 keV. <BR /> Results:
The statistical properties of the spikes are as follows. (1) HXR spikes
are produced in both impulsive flares and long-duration flares with
nearly the same occurrence rates. Ninety percent of the spikes occur
during the rise phase of the flares, and about 70% occur around the
peak times of the flares. (2) The time durations of the spikes vary
from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on
photon energies. The spikes exhibit symmetric time profiles with no
significant difference between rise and decay times. (3) Among the
most energetic spikes, nearly all of them have harder count spectra
than their underlying slow-varying components. There is also a weak
indication that spikes exhibiting time lags in high-energy emissions
tend to have harder spectra than spikes with time lags in low-energy
emissions.
---------------------------------------------------------
Title: Analysis and Modeling of Two Flare Loops Observed by AIA
and EIS
Authors: Li, Y.; Qiu, J.; Ding, M. D.
2012ApJ...758...52L Altcode: 2012arXiv1208.5440L
We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS
to investigate how flare loops are heated and evolve subsequently. The
flare is composed of two distinctive loop systems observed in extreme
ultraviolet (EUV) images. The UV 1600 Å emission at the feet of these
loops exhibits a rapid rise, followed by enhanced emission in different
EUV channels observed by the Atmospheric Imaging Assembly (AIA) and
the EUV Imaging Spectrometer (EIS). Such behavior is indicative of
impulsive energy deposit and the subsequent response in overlying
coronal loops that evolve through different temperatures. Using the
method we recently developed, we infer empirical heating functions
from the rapid rise of the UV light curves for the two loop systems,
respectively, treating them as two big loops with cross-sectional area
of 5” by 5”, and compute the plasma evolution in the loops using
the EBTEL model. We compute the synthetic EUV light curves, which,
with the limitation of the model, reasonably agree with observed light
curves obtained in multiple AIA channels and EIS lines: they show the
same evolution trend and their magnitudes are comparable by within a
factor of two. Furthermore, we also compare the computed mean enthalpy
flow velocity with the Doppler shift measurements by EIS during the
decay phase of the two loops. Our results suggest that the two different
loops with different heating functions as inferred from their footpoint
UV emission, combined with their different lengths as measured from
imaging observations, give rise to different coronal plasma evolution
patterns captured both in the model and in observations.
---------------------------------------------------------
Title: Understanding the white-light flare on 2012 March 9: evidence
of a two-step magnetic reconnection
Authors: Hao, Q.; Guo, Y.; Dai, Y.; Ding, M. D.; Li, Z.; Zhang, X. Y.;
Fang, C.
2012A&A...544L..17H Altcode: 2012arXiv1211.1751H
<BR /> Aims: We attempt to understand the white-light flare (WLF) that
was observed on 2012 March 9 with a newly constructed multi-wavelength
solar telescope called the Optical and Near-infrared Solar Eruption
Tracer (ONSET). <BR /> Methods: We analyzed WLF observations in radio,
Hα, white-light, ultraviolet, and X-ray bands. We also studied the
magnetic configuration of the flare via the nonlinear force-free field
(NLFFF) extrapolation and the vector magnetic field observed by the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO). <BR /> Results: Continuum emission enhancement
clearly appeared at the 3600 Å and 4250 Å bands, with peak contrasts
of 25% and 12%, respectively. The continuum emission enhancement closely
coincided with the impulsive increase in the hard X-ray emission and
a microwave type III burst at 03:40 UT. We find that the WLF appeared
at one end of either the sheared or twisted field lines or both. There
was also a long-lasting phase in the Hα and soft X-ray bands after the
white-light emission peak. In particular, a second, yet stronger, peak
appeared at 03:56 UT in the microwave band. <BR /> Conclusions: This
event shows clear evidence that the white-light emission was caused by
energetic particles bombarding the lower solar atmosphere. A two-step
magnetic reconnection scenario is proposed to explain the entire
process of flare evolution, i.e., the first-step magnetic reconnection
between the field lines that are highly sheared or twisted or both,
and the second-step one in the current sheet, which is stretched by
the erupting flux rope. The WLF is supposed to be triggered in the
first-step magnetic reconnection at a relatively low altitude.
---------------------------------------------------------
Title: ONSET-A New Multi-Wavelength Solar Telescope
Authors: Fang, C.; Chen, P. F.; Ding, M. D.; Dai, Y.; Li, Z.
2012EAS....55..349F Altcode:
A new multi-wavelength solar telescope, Optical and Near-infrared
Solar Eruption Tracer (ONSET), is constructed by Nanjing University,
being run in cooperation with Yunnan Astronomical Observatory. ONSET
is able to observe the Sun in three wavelength windows: He I 10830 Å,
Hα and white-light at 3600 Å or 4250 Å. Full-disk or partial solar
images with a field of 10 arcmin at three wavelengths can be obtained
nearly simultaneously. It is designed to trace solar eruptions with high
spatial and temporal resolutions. This telescope has been installed at
a new solar observing site near the Fuxian Lake, Yunnan Province. The
site is located at E102N24, with an altitude of 1722 m. The seeing is
stable and very nice. We give a brief description of the scientific
objectives and the basic structure of ONSET. Some preliminary results
are also shown.
---------------------------------------------------------
Title: Kinematics and Differential Emission Measure of the Flux Rope
during Coronal Mass Ejections
Authors: Ding, M. D.; Cheng, X.; Zhang, J.
2012EAS....55..287D Altcode:
Recent Solar Dynamic Observatory observations reveal that some coronal
mass ejections (CMEs) consist of multi-temperature structures: a hot
channel and a cool leading edge (LE). The channel first appears as
a twisted loop and lies along the polarity inversion line; it then
gradually develops into the less twisted semicircular structure
during the impulsive phase of CME acceleration. In the meantime,
the hot channel compresses the surrounding magnetic field and plasma,
which successively stack into the CME leading front. Here, we study
two well observed CMEs, which occurred on 2010 November 3 and 2011
March 8, respectively. We find that, the hot channel rises earlier
than the first appearance of the CME leading front and the onset of
the associated flare, and that the speed of the hot channel is always
larger than that of the leading front. Further differential emission
measure analysis shows that the channel often has significant emission
at high temperatures. We thus conclude that the observed hot channel
is likely to be magnetic flux rope that acts as a driver of the CME
acceleration in the early phase.
---------------------------------------------------------
Title: Silicon Detector Telescope for proton detection in electron
scattering reactions at MAMI
Authors: Makek, M.; Achenbach, P.; Ayerbe Gayoso, C.; Baumann, D.;
Bernauer, J. C.; Böhm, R.; Bosnar, D.; Denig, A.; Ding, M.; Distler,
M. O.; Doria, L.; Friedrich, J.; Friščić, I.; Gómez Rodríguez
de La Paz, M.; Merkel, H.; Müller, U.; Nungesser, L.; Pochodzalla,
J.; Potokar, M.; Seimetz, M.; Sánchez Majos, S.; Schlimme, B. S.;
Širca, S.; Walcher, Th.; Weinriefer, M.
2012NIMPA.673...82M Altcode:
A new Silicon Detector Telescope has been constructed and installed
within the experimental facility of the A1 collaboration at Mainz
Microtron, with the goal to detect low-energy protons. It consists of
seven silicon layers for energy and angle measurement and a plastic
scintillator for triggering purposes. The detector subtends a solid
angle up to 88 msr, depending on the distance from the target and
covers the proton kinetic energy range of 25-41 MeV with the mean
energy resolution σ<SUB>E</SUB>=0.47MeV, operating at 500 kHz. Digital
signal processing methods applied for energy reconstruction have been
important for keeping the acceptable energy resolution at high counting
rates. The Silicon Detector Telescope has been successfully used in
double and triple coincidence measurements along with the magnetic
spectrometers of the A1 collaboration.
---------------------------------------------------------
Title: Two Types of Magnetic Reconnection in Coronal Bright Points
and the Corresponding Magnetic Configuration
Authors: Zhang, Q. M.; Chen, P. F.; Guo, Y.; Fang, C.; Ding, M. D.
2012ApJ...746...19Z Altcode:
Coronal bright points (CBPs) are long-lived small-scale brightenings
in the solar corona. They are generally explained by magnetic
reconnection. However, the corresponding magnetic configurations are
not well understood. We carry out a detailed multi-wavelength analysis
of two neighboring CBPs on 2007 March 16, observed in soft X-ray
(SXR) and EUV channels. It is seen that the SXR light curves present
quasi-periodic flashes with an interval of ~1 hr superposed over the
long-lived mild brightenings, suggesting that the SXR brightenings of
this type of CBPs might consist of two components: one is the gentle
brightenings and the other is the CBP flashes. It is found that the
strong flashes of the bigger CBP are always accompanied by SXR jets. The
potential field extrapolation indicates that both CBPs are covered by
a dome-like separatrix surface, with a magnetic null point above. We
propose that the repetitive CBP flashes, as well as the recurrent SXR
jets, result from the impulsive null-point reconnection, while the
long-lived brightenings are due to the interchange reconnection along
the separatrix surface. Although the EUV images at high-temperature
lines resemble the SXR appearance, the 171 Å and 195 Å channels
reveal that the blurry CBP in SXR consists of a cusp-shaped loop and
several separate bright patches, which are explained to be due to the
null-point reconnection and the separatrix reconnection, respectively.
---------------------------------------------------------
Title: Evolution of Hard X-Ray Sources and Ultraviolet Solar Flare
Ribbons for a Confined Eruption of a Magnetic Flux Rope
Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Démoulin, P.; Li, H.
2012ApJ...746...17G Altcode: 2011arXiv1111.1790G
We study the magnetic field structures of hard X-ray (HXR) sources and
flare ribbons of the M1.1 flare in active region NOAA 10767 on 2005 May
27. We have found in a nonlinear force-free field extrapolation over the
same polarity inversion line, a small pre-eruptive magnetic flux rope
located next to sheared magnetic arcades. RHESSI and the Transition
Region and Coronal Explorer (TRACE) observed this confined flare in
the X-ray bands and ultraviolet (UV) 1600 Å bands, respectively. In
this event magnetic reconnection occurred at several locations. It
first started at the location of the pre-eruptive flux rope. Then,
the observations indicate that magnetic reconnection occurred between
the pre-eruptive magnetic flux rope and the sheared magnetic arcades
more than 10 minutes before the flare peak. This implies the formation
of the larger flux rope, as observed with TRACE. Next, HXR sources
appeared at the footpoints of this larger flux rope at the peak of the
flare. The associated high-energy particles may have been accelerated
below the flux rope in or around a reconnection region. Still, the close
spatial association between the HXR sources and the flux rope footpoints
favors an acceleration within the flux rope. Finally, a topological
analysis of a large solar region, including active regions NOAA 10766
and 10767, shows the existence of large-scale Quasi-Separatrix Layers
(QSLs) before the eruption of the flux rope. No enhanced emission was
found at these QSLs during the flare, but the UV flare ribbons stopped
at the border of the closest large-scale QSL.
---------------------------------------------------------
Title: Investigation of the Formation and Separation of an
Extreme-ultraviolet Wave from the Expansion of a Coronal Mass Ejection
Authors: Cheng, X.; Zhang, J.; Olmedo, O.; Vourlidas, A.; Ding, M. D.;
Liu, Y.
2012ApJ...745L...5C Altcode: 2011arXiv1112.4540C
We address the nature of EUV waves through direct observations of the
formation of a diffuse wave driven by the expansion of a coronal mass
ejection (CME) and its subsequent separation from the CME front. The
wave and the CME on 2011 June 7 were well observed by the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory. Following the
solar eruption onset, marked by the beginning of the rapid increasing
of the CME velocity and the X-ray flux of accompanying flare, the CME
exhibits a strong lateral expansion. During this impulsive expansion
phase, the expansion speed of the CME bubble increases from 100
km s<SUP>-1</SUP> to 450 km s<SUP>-1</SUP> in only six minutes. An
important finding is that a diffuse wave front starts to separate
from the front of the expanding bubble shortly after the lateral
expansion slows down. Also a type II burst is formed near the time of
the separation. After the separation, two distinct fronts propagate
with different kinematic properties. The diffuse front travels across
the entire solar disk, while the sharp front rises up, forming the CME
ejecta with the diffuse front ahead of it. These observations suggest
that the previously termed EUV wave is a composite phenomenon and
driven by the CME expansion. While the CME expansion is accelerating,
the wave front is cospatial with the CME front, thus the two fronts are
indiscernible. Following the end of the acceleration phase, the wave
moves away from the CME front with a gradually increasing distance
between them.
---------------------------------------------------------
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: Spectroscopic Analysis of Interaction between an
Extreme-ultraviolet Imaging Telescope Wave and a Coronal Upflow Region
Authors: Chen, F.; Ding, M. D.; Chen, P. F.; Harra, L. K.
2011ApJ...740..116C Altcode: 2011arXiv1107.5630C
We report a spectroscopic analysis of an EUV Imaging Telescope (EIT)
wave event that occurred in active region 11081 on 2010 June 12 and
was associated with an M2.0 class flare. The wave propagated nearly
circularly. The southeastern part of the wave front passed over an
upflow region near a magnetic bipole. Using EUV Imaging Spectrometer
raster observations for this region, we studied the properties of
plasma dynamics in the wave front, as well as the interaction between
the wave and the upflow region. We found a weak blueshift for the
Fe XII λ195.12 and Fe XIII λ202.04 lines in the wave front. The
local velocity along the solar surface, which is deduced from the
line-of-sight velocity in the wave front and the projection effect,
is much lower than the typical propagation speed of the wave. A more
interesting finding is that the upflow and non-thermal velocities
in the upflow region are suddenly diminished after the transit of
the wave front. This implies a significant change of magnetic field
orientation when the wave passed. As the lines in the upflow region
are redirected, the velocity along the line of sight is diminished
as a result. We suggest that this scenario is more in accordance with
what was proposed in the field-line stretching model of EIT waves.
---------------------------------------------------------
Title: A Comparative Study of Confined and Eruptive Flares in NOAA
AR 10720
Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Guo, Y.; Su, J. T.
2011ApJ...732...87C Altcode: 2011arXiv1103.2323C
We investigate the distinct properties of two types of flares:
eruptive flares associated with coronal mass ejections (CMEs) and
confined flares without CMEs. Our study sample includes nine M- and
X-class flares, all from the same active region (AR), six of which are
confined and three others which are eruptive. The confined flares tend
to be more impulsive in the soft X-ray time profiles and show slenderer
shapes in the Extreme-ultraviolet Imaging Telescope 195 Å images,
while the eruptive ones are long-duration events and show much more
extended brightening regions. The location of the confined flares is
closer to the center of the AR, while the eruptive flares are at the
outskirts. This difference is quantified by the displacement parameter,
which is the distance between the AR center and the flare location;
the average displacement of the six confined flares is 16 Mm, while
that of the eruptive ones is as large as 39 Mm. Further, through
nonlinear force-free field extrapolation, we find that the decay
index of the transverse magnetic field in the low corona (~10 Mm)
is larger for eruptive flares than for confined ones. In addition,
the strength of the transverse magnetic field over the eruptive flare
sites is weaker than it is over the confined ones. These results
demonstrate that the strength and the decay index of the background
magnetic field may determine whether or not a flare is eruptive or
confined. The implication of these results on CME models is discussed
in the context of torus instability of the flux rope.
---------------------------------------------------------
Title: Observing Flux Rope Formation During the Impulsive Phase of
a Solar Eruption
Authors: Cheng, X.; Zhang, J.; Liu, Y.; Ding, M. D.
2011ApJ...732L..25C Altcode: 2011arXiv1103.5084C
Magnetic flux ropes are believed to be an important structural
component of coronal mass ejections (CMEs). While there exists much
observational evidence of flux ropes after the eruption, e.g., as
seen in remote-sensing coronagraph images or in situ solar wind data,
the direct observation of flux ropes during CME impulsive phase has
been rare. In this Letter, we present an unambiguous observation
of a flux rope still in the formation phase in the low corona. The
CME of interest occurred above the east limb on 2010 November 3 with
footpoints partially blocked. The flux rope was seen as a bright blob
of hot plasma in the Atmospheric Imaging Assembly (AIA) 131 Å passband
(peak temperature ~11 MK) rising from the core of the source active
region, rapidly moving outward and stretching the surrounding background
magnetic field upward. The stretched magnetic field seemed to curve-in
behind the core, similar to the classical magnetic reconnection scenario
in eruptive flares. On the other hand, the flux rope appeared as a
dark cavity in the AIA 211 Å passband (2.0 MK) and 171 Å passband
(0.6 MK) in these relatively cool temperature bands, a bright rim
clearly enclosed the dark cavity. The bright rim likely represents the
pileup of the surrounding coronal plasma compressed by the expanding
flux rope. The composite structure seen in AIA multiple temperature
bands is very similar to that in the corresponding coronagraph images,
which consists of a bright leading edge and a dark cavity, commonly
believed to be a flux rope.
---------------------------------------------------------
Title: Observing Flux Rope Formation During the Impulsive Phase of
a Solar Eruption
Authors: Cheng, Xin; Zhang, J.; Yang, L.; Ding, M.
2011SPD....42.2315C Altcode: 2011BAAS..43S.2315C
Magnetic flux rope is believed to be an important structural
component of coronal mass ejections (CMEs). While there exist much
observational evidence of the flux rope post the eruption, e.g., as
seen in remote-sensing coronagraph images or in-situ solar wind data,
the direct observation of flux ropes during CME impulsive phase has
been rare or non-exist. In this Letter, we present an unambiguous
observation of a flux rope still in the formation phase in the low
corona. The CME of interest occurred above the east limb on 2010
November 03 with footpoints partially blocked. The flux rope was seen
as a blob of hot plasma in AIA 131 A passband (peak temperature 11
MK) rising from the core of the source active region, rapidly moving
outward and stretching upward the surrounding background magnetic
field. The stretched magnetic field seemed to curve-in, similar to the
classical magnetic reconnection scenario in eruptive flares. The flux
rope was also seen as a dark cavity in AIA 211 A passpand (2.0 MK)
and 171 A passband (0.6 MK); in these relatively cool temperature
bands, a bright rim clearly enclosed the dark cavity. The bright
rim likely represents the pile-up of the surrounding coronal plasma
compressed by the expanding flux rope. The composite structure seen
in AIA multiple temperature bands is very similar to that in the
corresponding coronagraph images, which consists of a bright leading
edge and a dark cavity, commonly believed to be a flux rope.
---------------------------------------------------------
Title: Different Patterns of Chromospheric Evaporation in a Flaring
Region Observed with Hinode/EIS
Authors: Li, Y.; Ding, M. D.
2011ApJ...727...98L Altcode: 2010arXiv1011.4562L
We investigate the chromospheric evaporation in the flare of 2007
January 16 using line profiles observed by the Exterme-UV Imaging
Spectrometer on board Hinode. Three points at flare ribbons of different
magnetic polarities are analyzed in detail. We find that the three
points show different patterns of upflows and downflows in the impulsive
phase of the flare. The spectral lines at the first point are mostly
blueshifted, with the hotter lines showing a dominant blueshifted
component over the stationary one. At the second point, however,
only weak upflows are detected; instead, notable downflows appear
at high temperatures (up to 2.5-5.0 MK). The third point is similar
to the second one only in that it shows evidence of multi-component
downflows. While the evaporated plasma falling back down as warm rain
is a possible cause of the redshifts at the second and third points,
the different patterns of chromospheric evaporation at the three
points imply the existence of different heating mechanisms in the
flaring active region.
---------------------------------------------------------
Title: Vector Magnetic Fields and Current Helicities in Coronal
Holes and Quiet Regions
Authors: Yang, Shuhong; Zhang, Jun; Li, Ting; Ding, Mingde
2011ApJ...726...49Y Altcode: 2010arXiv1011.0512Y
In the solar photosphere, many properties of coronal holes (CHs)
are not known, especially vector magnetic fields. Using observations
from Hinode, we investigate vector magnetic fields, current densities,
and current helicities in two CHs, and compare them with two normal
quiet regions (QRs) for the first time. We find that the areas where
large current helicities are located are mainly co-spatial with
strong vertical and horizontal field elements both in shape and in
location. In the CHs, horizontal magnetic fields, inclination angles,
current densities, and current helicities are larger than those in the
QRs. The mean vertical current density and current helicity in the CHs
and QRs, averaged over all the observed areas including the CHs and
QRs, are approximately 0.008 A m<SUP>-2</SUP> and 0.005 G<SUP>2</SUP>
m<SUP>-1</SUP>, respectively. The mean current density in magnetic flux
concentrations where the vertical fields are stronger than 100 G is as
large as 0.012 ± 0.001 A m<SUP>-2</SUP>, consistent with that in the
flare productive active regions. Our results imply that the magnetic
fields, especially the strong fields, both in the CHs and in the QRs
are nonpotential.
---------------------------------------------------------
Title: Driving mechanism of a failed eruption
Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Li, H.; Törö, T.;
Wiegelmann, T.
2011ASInC...2..307G Altcode:
We find a magnetic flux rope before the M1.1 flare in active
region 10767 on 2005 May 27 by a nonlinear force-free field
extrapolation. TRACE observations of the filament eruption show that the
erupting structure performed a writhing deformation and stopped rising
at a certain height, suggesting that the flux rope converted some of its
twist into writhe and was confined in the corona. After calculating the
twist of the flux rope, we find that it was comparable to thresholds
of the helical kink instability found in numerical simulations. We
conclude that the activation and rise of the flux rope were triggered
and initially driven by the kink instability. The decay index of
the external magnetic field stays below the threshold for the torus
instability within a long height range. The confinement of the eruption
could be explained by the failure of the torus instability. Hard X-ray
sources at the peak of the M1.1 flare coincided with the footpoints
of the erupting helical structure, which indicates a high possibility
that hard X-ray sources were produced more efficiently in the flux rope.
---------------------------------------------------------
Title: Driving Mechanism and Onset Condition of a Confined Eruption
Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Li, H.; Török, T.;
Wiegelmann, T.
2010ApJ...725L..38G Altcode:
We study a confined eruption accompanied by an M1.1 flare in solar
active region (AR) NOAA 10767 on 2005 May 27, where a pre-eruptive
magnetic flux rope was reported in a nonlinear force-free field (NLFFF)
extrapolation. The observations show a strong writhing motion of the
erupting structure, suggesting that a flux rope was indeed present
and converted some of its twist into writhe in the course of the
eruption. Using the NLFFF extrapolation, we calculate the twist of
the pre-eruptive flux rope and find that it is in very good agreement
with thresholds of the helical kink instability found in numerical
simulations. We conclude that the activation and rise of the flux
rope were triggered and driven by the instability. Using a potential
field extrapolation, we also estimate the height distribution of the
decay index of the external magnetic field in the AR 1 hr prior to the
eruption. We find that the decay index stays below the threshold for
the torus instability for a significant height range above the erupting
flux rope. This provides a possible explanation for the confinement
of the eruption to the low corona.
---------------------------------------------------------
Title: Evidence of Explosive Evaporation in a Microflare Observed
by Hinode/EIS
Authors: Chen, F.; Ding, M. D.
2010ApJ...724..640C Altcode: 2010arXiv1009.3193C
We present a detailed study of explosive chromospheric evaporation
during a microflare which occurred on 2007 December 7 as observed with
the Extreme-ultraviolet Imaging Spectrometer on board Hinode. We find
temperature-dependent upflows for lines formed from 1.0 to 2.5 MK and
downflows for lines formed from 0.05 to 0.63 MK in the impulsive phase
of the flare. Both the line intensity and the nonthermal line width
appear enhanced in most of the lines and are temporally correlated with
the evaporation velocity. Our results are consistent with the numerical
simulations of flare models, which take into account a strong nonthermal
electron beam in producing the explosive chromospheric evaporation. The
explosive evaporation observed in this microflare implies that the same
dynamic processes may exist in events with very different magnitudes.
---------------------------------------------------------
Title: Spectroscopic Analysis of an EIT Wave/dimming Observed by
Hinode/EIS
Authors: Chen, F.; Ding, M. D.; Chen, P. F.
2010ApJ...720.1254C Altcode: 2010arXiv1009.3190C
EUV Imaging Telescope (EIT) waves are a wavelike phenomenon propagating
outward from the coronal mass ejection source region, with expanding
dimmings following behind. We present a spectroscopic study of an EIT
wave/dimming event observed by the Hinode/Extreme-ultraviolet Imaging
Spectrometer. Although the identification of the wave front is somewhat
affected by the pre-existing loop structures, the expanding dimming
is well defined. We investigate the line intensity, width, and Doppler
velocity for four EUV lines. In addition to the significant blueshift
implying plasma outflows in the dimming region as revealed in previous
studies, we find that the widths of all four spectral lines increase
at the outer edge of the dimmings. We illustrate that this feature
can be well explained by the field line stretching model, which claims
that EIT waves are apparently moving brightenings that are generated
by the successive stretching of the closed field lines.
---------------------------------------------------------
Title: Re-flaring of a Post-flare Loop System Driven by Flux Rope
Emergence and Twisting
Authors: Cheng, X.; Ding, M. D.; Guo, Y.; Zhang, J.; Jing, J.;
Wiegelmann, T.
2010ApJ...716L..68C Altcode: 2010arXiv1005.1720C
In this Letter, we study in detail the evolution of the post-flare
loops on 2005 January 15 that occurred between two consecutive solar
eruption events, both of which generated a fast halo coronal mass
ejection (CME) and a major flare. The post-flare loop system, formed
after the first CME/flare eruption, evolved rapidly, as manifested by
the unusual accelerating rise motion of the loops. Through nonlinear
force-free field models, we obtain the magnetic structure over the
active region. It clearly shows that the flux rope below the loops also
kept rising, accompanied with increasing twist and length. Finally, the
post-flare magnetic configuration evolved to a state that resulted in
the second CME/flare eruption. This is an event in which the post-flare
loops can re-flare in a short period of ~16 hr following the first
CME/flare eruption. The observed re-flaring at the same location
is likely driven by the rapid evolution of the flux rope caused by
the magnetic flux emergence and the rotation of the sunspot. This
observation provides valuable information on CME/flare models and
their prediction.
---------------------------------------------------------
Title: A Study of the Build-up, Initiation, and Acceleration of 2008
April 26 Coronal Mass Ejection Observed by STEREO
Authors: Cheng, X.; Ding, M. D.; Zhang, J.
2010ApJ...712.1302C Altcode: 2010arXiv1002.4962C
In this paper, we analyze the full evolution, from a few days prior to
the eruption to the initiation, final acceleration, and propagation,
of the coronal mass ejection (CME) that occurred on 2008 April 26 using
the unprecedented high cadence and multi-wavelength observations by the
Solar Terrestrial Relations Observatory. There existed frequent filament
activities and EUV jets prior to the CME eruption for a few days. These
activities were probably caused by the magnetic reconnection in the
lower atmosphere driven by photospheric convergence motions, which
were evident in the sequence of magnetogram images from the Michelson
Doppler Imager on board the Solar and Heliospheric Observatory. The slow
low-layer magnetic reconnection may be responsible for the storage of
magnetic free energy in the corona and the formation of a sigmoidal
core field or a flux rope leading to the eventual eruption. The
occurrence of EUV brightenings in the sigmoidal core field prior to
the rise of the flux rope implies that the eruption was triggered by
the inner tether-cutting reconnection, but not the external breakout
reconnection. During the period of impulsive acceleration, the time
profile of the CME acceleration in the inner corona is found to be
consistent with the time profile of the reconnection electric field
inferred from the footpoint separation and the Reuven Ramaty High
Energy Solar Spectroscopic Imager 15-25 keV hard X-ray flux curve of
the associated flare. The full evolution of this CME can be described
in four distinct phases: the build-up phase, initiation phase, main
acceleration phase, and propagation phase. The physical properties
and the transition between these phases are discussed, in an attempt
to provide a global picture of CME dynamic evolution.
---------------------------------------------------------
Title: Radiative Hydrodynamic Simulation of the Continuum Emission
in Solar White-Light Flares
Authors: Cheng, J. X.; Ding, M. D.; Carlsson, Mats
2010ApJ...711..185C Altcode:
It is believed that solar white-light flares (WLFs) originate in the
lower chromosphere and upper photosphere. In particular, some recently
observed WLFs show a large continuum enhancement at 1.56 μm where the
opacity reaches its minimum. Therefore, it is important to clarify
how the energy is transferred to the lower layers responsible for
the production of WLFs. Based on radiative hydrodynamic simulations,
we study the role of non-thermal electron beams in increasing the
continuum emission. We vary the parameters of the electron beam and
disk positions and compare the results with observations. The electron
beam heated model can explain most of the observational white-light
enhancements. For the most energetic WLFs observed so far, however, a
very large electron beam flux and a high low-energy cutoff, which are
possibly beyond the parameter space in our simulations, are required
in order to reproduce the observed white-light emission.
---------------------------------------------------------
Title: A Statistical Study of the Post-impulsive-phase Acceleration
of Flare-associated Coronal Mass Ejections
Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Poomvises, W.
2010ApJ...712..752C Altcode: 2010arXiv1002.4960C
It is now generally accepted that the impulsive acceleration of a
coronal mass ejection (CME) in the inner corona is closely correlated in
time with the main energy release of the associated solar flare. In this
paper, we examine in detail the post-impulsive-phase acceleration of a
CME in the outer corona, which is the phase of evolution immediately
following the main impulsive acceleration of the CME; this phase is
believed to correspond to the decay phase of the associated flare. This
observational study is based on a statistical sample of 247 CMEs that
are associated with M- and X-class GOES soft X-ray flares from 1996 to
2006. We find that, from many examples of events, the CMEs associated
with flares with long-decay time (or so-called long-duration flares)
tend to have positive post-impulsive-phase acceleration, even though
some of them have already obtained a high speed at the end of the
impulsive acceleration but do not show a deceleration expected from
the aerodynamic dragging of the background solar wind. On the other
hand, the CMEs associated with flares of short-decay time tend to have
significant deceleration. In the scattering plot of all events, there
is a weak correlation between CME post-impulsive-phase acceleration
and flare decay time. The CMEs deviated from the general trend are
mostly slow or weak ones associated with flares of short-decay time;
the deviation is caused by the relatively stronger solar wind dragging
force for these events. The implications of our results on CME dynamics
and CME-flare relations are discussed.
---------------------------------------------------------
Title: Coronal Mass Ejection Induced Outflows Observed with Hinode/EIS
Authors: Jin, M.; Ding, M. D.; Chen, P. F.; Fang, C.; Imada, S.
2009ApJ...702...27J Altcode:
We investigate the outflows associated with two halo coronal mass
ejections (CMEs) that occurred on 2006 December 13 and 14 in NOAA
10930, using the Hinode/EIS observations. Each CME was accompanied by
an EIT wave and coronal dimmings. Dopplergrams in the dimming regions
are obtained from the spectra of seven EIS lines. The results show
that strong outflows are visible in the dimming regions during the
CME eruption at different heights from the lower transition region to
the corona. It is found that the velocity is positively correlated
with the photospheric magnetic field, as well as the magnitude of
the dimming. We estimate the mass loss based on height-dependent EUV
dimmings and find it to be smaller than the CME mass derived from
white-light observations. The mass difference is attributed partly to
the uncertain atmospheric model, and partly to the transition region
outflows, which refill the coronal dimmings.
---------------------------------------------------------
Title: Formation Heights of Extreme Ultraviolet Lines in an Active
Region Derived by Correlation of Doppler Velocity and Magnetic Field
Authors: Guo, Y.; Ding, M. D.; Jin, M.; Wiegelmann, T.
2009ApJ...696.1526G Altcode: 2009arXiv0903.4343G
We study the correlation heights, which indicate the formation height of
Extreme Ultraviolet (EUV) lines in an active region using observations
from the EUV Imaging Spectrometer and Solar Optical Telescope on
board Hinode. The nonlinear force-free field optimization method
is adopted to extrapolate the three-dimensional magnetic fields to
higher layers. Three subregions with different characteristics are
selected in the active region for this study. The results show that
the formation heights in different subregions vary with their different
magnetic fields or velocity patterns. After solving the line blending
problem between the He II 256.32 Å and Si X 256.37 Å lines by the
double Gaussian curve fitting, we find that the transition region
lies higher in a strong magnetic area. In a preflare heating area,
there possibly exist multithermal loops as implied by comparing the
Doppler velocity and the magnetic field on the solar disk.
---------------------------------------------------------
Title: Energy Spectrum of the Electrons Accelerated by a Reconnection
Electric Field: Exponential or Power Law?
Authors: Liu, W. J.; Chen, P. F.; Ding, M. D.; Fang, C.
2009ApJ...690.1633L Altcode: 2008arXiv0809.1212L
The direct current (DC) electric field near the reconnection region
has been proposed as an effective mechanism to accelerate protons and
electrons in solar flares. A power-law energy spectrum was generally
claimed in the simulations of electron acceleration by the reconnection
electric field. However in most of the literature, the electric and
magnetic fields were chosen independently. In this paper, we perform
test-particle simulations of electron acceleration in a reconnecting
magnetic field, where both the electric and magnetic fields are adopted
from numerical simulations of the MHD equations. It is found that the
accelerated electrons present a truncated power-law energy spectrum
with an exponential tail at high energies, which is analogous to the
case of diffusive shock acceleration. The influences of reconnection
parameters on the spectral feature are also investigated, such as the
longitudinal and transverse components of the magnetic field and the
size of the current sheet. It is suggested that the DC electric field
alone might not be able to reproduce the observed single or double
power-law distributions.
---------------------------------------------------------
Title: Loop-Like Hard X-Ray Emission in a 2005 January 20 Flare
Authors: Jin, Meng; Ding, Mingde
2008PASJ...60..835J Altcode:
We present analyses of hard X-ray (HXR) emission observed by the
Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
of an X7.1 flare on 2005 January 20. Generally, this flare shows
three HXR sources: two footpoint sources and a looptop one. During
some period of the flare evolution, HXR emission from the legs
of the loop was evident, and a loop-like HXR structure appeared,
especially at 25-50keV, which is unusual, and has been rarely reported
before. We consider this phenomenon to be observational evidence for
chromospheric evaporation. To confirm this point, we calculated the
height distribution of the energy deposition by an electron beam using
different atmospheric models. The results suggest that, in order to
engender the observed loop-like HXR emission in this flare, the coronal
mass density and the electron-beam flux should be extremely high, which
is a possible reason why loop-like HXR emission is rarely observed.
---------------------------------------------------------
Title: 3D Magnetic Field Configuration of the 2006 December 13 Flare
Extrapolated with the Optimization Method
Authors: Guo, Y.; Ding, M. D.; Wiegelmann, T.; Li, H.
2008ApJ...679.1629G Altcode:
The photospheric vector magnetic field of the active region NOAA
10930 was obtained with the Solar Optical Telescope (SOT) on board
the Hinode satellite with a very high spatial resolution (about
0.3”). Observations of the two-ribbon flare on 2006 December 13 in
this active region provide us a good sample to study the magnetic
field configuration related to the occurrence of the flare. Using the
optimization method for nonlinear force-free field (NLFFF) extrapolation
proposed by Wheatland et al. and recently developed by Wiegelmann, we
derive the three-dimensional (3D) vector magnetic field configuration
associated with this flare. The general topology can be described
as a highly sheared core field and a quasi-potential envelope arch
field. The core field clearly shows some dips supposed to sustain a
filament. Free energy release in the flare, calculated by subtracting
the energy contained in the NLFFF and the corresponding potential
field, is 2.4 × 10<SUP>31</SUP> ergs, which is ~2% of the preflare
potential field energy. We also calculate the shear angles, defined
as the angles between the NLFFF and potential field, and find that
they become larger at some particular sites in the lower atmosphere,
while they become significantly smaller in most places, implying that
the whole configuration gets closer to the potential field after the
flare. The Ca II H line images obtained with the Broadband Filter Imager
(BFI) of the SOT and the 1600 Å images with the Transition Region and
Coronal Explorer (TRACE) show that the preflare heating occurs mainly
in the core field. These results provide evidence in support of the
tether-cutting model of solar flares.
---------------------------------------------------------
Title: A complicated solar eruption event on 2003 October 26. Solar
flare
Authors: Ning, Zongjun; Ding, M. D.; Qiu, K. P.; Li, H.; Su, Y. N.;
Fang, C.
2008Ap&SS.315...45N Altcode: 2008Ap&SS.tmp...73N
We present multi-wavelength observations of a complicated solar
eruption event to associate with an X1.2 flare and a Coronal Mass
Ejection (CME) on 2003 October 26. The soft X-ray profile shows a
possibility for occurrence of two flares with peaks around 06:20 and
07:00 UT. According to our observations, there are many evidences to
show that they are corresponding to two energy releases. The first
one produces type II, type III, moving type IV continua, a decimetric
burst (DCIM) and strong emissions at H α, 195 and 1600 Å; While
the second energy release only produces a group of RS-III bursts,
DCIM and H α emissions. It appears that the first energy release is
associated with a CME, while the second CME is quiet. Such observational
difference between two energy releases is found indicating two magnetic
reconnection processes occurrence with different plasma situation.
---------------------------------------------------------
Title: Simulations of the periodic flaring rate on YY Gem
Authors: Gao, D. H.; Chen, P. F.; Ding, M. D.; Li, X. D.
2008MNRAS.384.1355G Altcode: 2008MNRAS.tmp..126G; 2007arXiv0712.2300G
The binary YY Gem shows many interesting properties, one of which
is the periodicity in its flaring rate. The period, which is about
48 +/- 3 min, was ever interpreted in terms of the oscillation of
a filament. In this paper, we propose a new model to explain this
phenomenon by means of 2.5D magnetohydrodynamic (MHD) numerical
simulations. It is found that magnetic reconnection is induced as the
coronal loops rooted on both stars inflate and approach each other,
which is driven by the differential stellar rotation. The magnetic
reconnection is modulated by fast-mode magnetoacoustic waves which are
trapped between the surfaces of the two stars, so that the reconnection
rate presents a periodic behaviour. With the typical parameters for the
binary system, the observed period can be reproduced. We also derive
an empirical formula to relate the period of the flaring rate to the
coronal temperature and density, as well as the magnetic field.
---------------------------------------------------------
Title: Correlation and asymmetry between solar flare hard X-ray
footpoints: a statistical study
Authors: Jin, M.; Ding, M. D.
2007A&A...471..705J Altcode:
Aims:We study the correlation and asymmetry between solar flare hard
X-ray footpoint time profiles that can be used to constrain the energy
release and particle acceleration processes in solar flares. <BR
/>Methods: We perform a statistical analysis of correlations and
asymmetries of hard X-ray footpoints for 29 solar flares that were
observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI) in 2002-2005. Using the CLEAN method, we derive the hard
X-ray images and then extract light curves for each footpoint. We
decompose the hard X-ray light curves into slowly-varying components
and fast-varying components using the Fourier method and check their
correlations separately. <BR />Results: Our analysis indicates that the
asymmetry between hard X-ray footpoints is a ubiquitous feature in solar
flares. However, in most asymmetric cases, the correlations between
the light curves of the two footpoints remain high. In particular,
the correlation coefficients between the slowly-varying components are
significantly higher than those between the fast-varying components. We
discuss the causes and implications of the asymmetries and correlations
between hard X-ray footpoints.
---------------------------------------------------------
Title: The Origin of Solar White-Light Flares
Authors: Ding, M. D.
2007ASPC..368..417D Altcode:
It is believed that solar white-light flares (WLFs) originate from the
lower atmosphere. In particular, some recently observed WLFs show a
very large continuum enhancement at 1.56 µ where the opacity reaches
minimum. Therefore, it is important to make clear how the energy is
transferred to the very low layers responsible for the production of
WLFs. We study the possible mechanisms that may directly or indirectly
help increase the continuum emission. We make calculations of model
atmospheres and compare the results with observations.
---------------------------------------------------------
Title: Microwave Spectral Evolution of Solar Flares
Authors: Ning, Zongjun; Ding, Mingde
2007PASJ...59..373N Altcode:
We analyze the time evolution of microwave spectral indices for 103
solar flares observed by Nobeyama Radio Polarimeters (NoRP) between
1998 and 2005. The microwave spectral index, γ(t), is derived from
the flux spectral index, δ(t). We assume a flux frequency dependence
of the form S(ν,t) = F<SUB>0</SUB>ν<SUP>δ(t)</SUP> in the optically
thin part, i.e., above the turnover frequency, f<SUB>max</SUB>. Here,
f<SUB>max</SUB> is defined as the frequency with the maximum flux among
six discrete frequencies (i.e., 1, 2, 3.75, 9.4, 17, and 35GHz). Based
on NoRP observations, we find two typical types of solar flares with
various microwave spectral behavior. Type I displays a soft-hard-soft
pattern, while the type II flare follows a soft-hard-harder behavior
in the rise-peak-decay phases at 35GHz. Our statistical results of
microwave flares confirm previous findings. Most microwave flares
appear to be of type II (91 out of the present samples), which is
different from the hard X-ray flares dominated by the soft-hard-soft
property. Such a difference is an observational constraint for electron
acceleration and transport theories in flare models.
---------------------------------------------------------
Title: Evolution of electron energy spectrum during solar flares
Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D.
2007AdSpR..39.1394L Altcode:
Particle acceleration by direct current electric field in the current
sheet has been extensively studied, in which an electric and a magnetic
field are generally prescribed, and a power law distribution of the
electron energy is obtained. Based on MHD numerical simulations of
flares, this paper aims at investigating the time evolution of the
electron energy spectrum during solar flares. It turns out that the
model reproduces the soft-hard-hard spectral feature which was observed
in some flares.
---------------------------------------------------------
Title: Diagnostics of the Heating Processes in Solar Flares Using
Chromospheric Spectral Lines
Authors: Cheng, J. X.; Ding, M. D.; Li, J. P.
2006ApJ...653..733C Altcode: 2006astro.ph..9674C
We have calculated the Hα and Ca II 8542 Å line profiles based
on four different atmospheric models, including the effects of
nonthermal electron beams with various energy fluxes. These two lines
have different responses to thermal and nonthermal effects, and can
be used to diagnose the thermal and nonthermal heating processes. We
apply our method to an X-class flare that occurred on 2001 October
19. We are able to identify quantitatively the heating effects during
the flare eruption. We find that the nonthermal effects at the outer
edge of the flare ribbon are more notable than that at the inner edge,
while the temperature at the inner edge seems higher. On the other hand,
the results show that nonthermal effects increase rapidly in the rise
phase and decrease quickly in the decay phase, but the atmospheric
temperature can still keep relatively high for some time after getting
to its maximum. For the two kernels that we analyze, the maximum
energy fluxes of the electron beams are ~10<SUP>10</SUP> and 10<SUP>
11</SUP> ergs cm<SUP>-2</SUP> s<SUP>-1</SUP>, respectively. However, the
atmospheric temperatures are not so high, i.e., lower than or slightly
higher than that of the weak flare model F1 at the two kernels. We
discuss the implications of the results for two-ribbon flare models.
---------------------------------------------------------
Title: Detection of Explosive Chromospheric Evaporation During a
Two-Ribbon Flare
Authors: Li, J. P.; Ding, M. D.
2006IAUJD...3E..15L Altcode:
We present two-dimensional spectral observations of an X1.2 two-ribbon
flare in which blue asymmetry of Hα profiles are found at one kernel
for more than 1 min. It is interesting that this kernel seems to move
along a flare loop seen on the Transition Region and Coronal Explorer
(TRACE) images since its appearance. While on the loop footpoints, the
Hα profiles are found to be red shifted. The statistical distribution
and the spatial distribution of the blue asymmetry at the kernel
are also studied. There seems to be a counterpart of the kernel on
TRACE 1600 Å images. We calculate the moving speed of the kernel by
measuring its moving front on TRACE 1600 images and find that it has
good temporal correlation with the line-of-sight velocity calculated
from the H_α profiles using a wing bisect method. The maximum
speed of the kernel measured from TRACE images is ~ 200 , which is
similar to the chromospheric evaporation speed deduced from soft X-ray
lines. We suppose that this kernel may be the plasma evaporated from
the chromosphere. If this is true, it will be the first time to get
direct evidence that supports the scenario of chromospheric evaporation
and condensation of solar flares in Hα line as we know.
---------------------------------------------------------
Title: Spectral Analysis of Ellerman Bombs
Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F.
2006ApJ...643.1325F Altcode:
By use of the high-resolution spectral data obtained with THEMIS on
2002 September 5, the characteristics of 14 well-observed Ellerman
bombs (EBs) have been analyzed. Our results indicate that 9 of the
14 EBs are located near the longitudinal magnetic polarity inversion
lines. Most of the EBs are accompanied by mass motions. The most obvious
characteristic of the EB spectra is the two emission bumps at the two
wings of both Hα and Ca II λ8542. For the first time both thermal
and nonthermal semiempirical atmospheric models for the conspicuous
and faint EBs are computed. In computing the nonthermal models, we
assume that the electron beam resulting from magnetic reconnection is
produced in the lower chromosphere. The reasons are that it requires
much lower energies for the injected particles and that it gives rise
to a more profound absorption at the Hα line center, in agreement with
our observations. The common characteristic is the heating in the lower
chromosphere and the upper photosphere. The temperature enhancement
is about 600-1300 K in the thermal models. If the nonthermal effects
are included, then the required temperature increase can be reduced
by 100-300 K. These imply that the EBs could probably be produced by
the magnetic reconnection in the solar lower atmosphere. The radiative
and kinetic energies of the EBs are estimated, and the total energy
is found to be 10<SUP>26</SUP> to 5×10<SUP>27</SUP> ergs. According
to the characteristics of EBs, we tentatively suggest that EBs could
be called “submicroflares.”
---------------------------------------------------------
Title: Footpoint Motion of the Continuum Emission in the 2002
September 30 White-Light Flare
Authors: Chen, Q. R.; Ding, M. D.
2006ApJ...641.1217C Altcode: 2005astro.ph.12496C
We present observations of the 2002 September 30 white-light
flare, in which the optical continuum emission near the Hα line is
enhanced by ~10%. The continuum emission exhibits a close temporal
and spatial coincidence with the hard X-ray (HXR) footpoint observed
by RHESSI. We find a systematic motion of the flare footpoint seen
in the continuum emission; the motion history follows roughly that
of the HXR source. This gives strong evidence that this white-light
flare is powered by heating of nonthermal electrons. We note that the
HXR spectrum in 10-50 keV is quite soft with γ~7 and there is no HXR
emission above 50 keV. The magnetic configuration of the flaring region
implies magnetic reconnection taking place at a relatively low altitude
during the flare. Despite a very soft spectrum of the electron beam,
its energy content is still sufficient to produce the heating in
the lower atmosphere, where the continuum emission originates. This
white-light flare highlights the importance of radiative back-warming
to transport the energy below when direct heating by beam electrons
is obviously impossible.
---------------------------------------------------------
Title: Assemblage of Diopside, Pyroxene, Akimotoite, and Ringwoodite
in the Heavily Shocked Sixiangkou L6 Chondrite: Further Constraints
of Shock Metamorphism
Authors: Zhang, A.; Hsu, W.; Wang, R.; Ding, M.
2006LPI....37.1069Z Altcode:
This study reports the occurrence of diopside, pyroxene, akimotoite,
and ringwoodite in the shock-induced melt veins of Sixiangkou and
discuss its implication to the shock metamorphism.
---------------------------------------------------------
Title: Periodicities in microwave radio bursts of solar flares
Authors: Ning, Zongjun; Ding, M. D.
2006IAUS..233..421N Altcode:
We analyze the periodicities for 294 solar flares observed by Nobeyama
Radioheliograph over 6 discrete frequencies of 1, 2, 3.75, 9.4, 17 and
35 GHz during the solar cycle 23 between 1998 and 2004. Using Fourier
analysis, we pick up the shortest period (T_{s}) for each event at
each frequency channel. We find that T_{s} varies in the range of
0.25-0.62 s over 6 frequency channels.
---------------------------------------------------------
Title: Diagnostics of thermal and non-thermal processes in solar
flares using chromospheric lines
Authors: Ding, M. D.; Cheng, J. X.; Li, J. P.
2006IAUS..233..397D Altcode:
The non-thermal electrons accelerated during solar flares can produce
enhanced and broadened chromospheric lines when they precipitate
into the chromosphere. In this paper, we propose a method to diagnose
the non-thermal processes using two chromospheric lines, Hα and Ca
II 8542 Å lines. First, we perform non-LTE calculations of these
two lines for various (thermal) model atmospheres and (non-thermal)
electron beams. Since the two lines have different sensitivities to the
non-thermal electrons, a set of line spectra can uniquely determine
a model atmosphere and an electron beam. We then apply this method
to a solar flare for which we have observed two-dimensional spectra
of the two lines. In particular, we examine the temporal variation of
thermal vs. non-thermal effects in flare bright kernels, as well as the
spatial variation across flare ribbons. The results show clearly that
the non-thermal effects appear most obviously at the flare maximum,
and preferentially at the outer edges of flare ribbons. The results
are consistent with flare theoretical models.
---------------------------------------------------------
Title: Altitude Dependence of Hard X-Ray Spectra in Solar Flares
Authors: Chen, Q. R.; Ding, M. D.; Chen, P. F.
2006IAUS..233..177C Altcode:
We investigate the altitude dependence of hard X-ray (HXR) spectra in
solar flares, i.e., whether the HXR spectra are related to the altitudes
of reconnection sites. We assume that the reconnection altitude can
be scaled by the distance between the two conjugate HXR footpoints in
the flare. By searching the RHESSI flare list from 2002 to 2004, we
find 42 solar flares below X-class that have enhanced 50-100 keV HXR
emission and two well-resolved HXR footpoints at the nonthermal peak
time. The preliminary results show that there is a weak correlation
(∼ -0.31) between the HXR spectral index and the HXR footpoint
distance. We further discuss the possible implications.
---------------------------------------------------------
Title: Occurrence of blue asymmetry in Halpha profiles of a two-ribbon
flare
Authors: Li, J. P.; Ding, M. D.
2006cosp...36.3085L Altcode: 2006cosp.meet.3085L
We present two-dimensional spectral observations of an X1 2 two-ribbon
flare in which blue asymmetry of H alpha profiles are seen at one
kernel for more than 1 min It is interesting that this kernel seems
to move along a flare loop seen on the emph Transition Region and
Coronal Explorer emph TRACE images since its appearance While on the
loop footpoints the H alpha profiles are found to be red shifted There
seems to have counterpart of the kernel on emph TRACE 195 and 1600 AA
images We calculate the moving speed of the kernel by measuring the
moving front on emph TRACE AA 1600 images and find that it has good
temporal correlation with the line-of-sight velocity calculated from
the H alpha profiles using a wing bisect method The maximum speed of
the kernel measured from emph TRACE 1600 AA images is sim 200 mbox km
s -1 that is similar to the chromospheric evaporation speed calculated
from soft X-ray lines We suppose that this kernel may be resulted from
the material evaporated from the chromosphere If this is true it will
be the first time to see direct evidence that supports the scenario
of chromospheric evaporation and condensation in solar flares as we know
---------------------------------------------------------
Title: Bright points in UV continuum in a M9.1 flare
Authors: Wang, L.; Fang, C.; Ding, M. D.
2006cosp...36.1547W Altcode: 2006cosp.meet.1547W
Using the data of RHESSI SOHO MDI SOHO EIT and TRACE white light
and 1700 AA images we analyze a gradual two-ribbon M9 1 solar flare
occurred on 2004 July 22 We find some bright points appeared in white
light and 1700 AA images alone the flare ribbons with the lifetime
of several minutes Some of them have rapid movement in weak magnetic
field regions with sharp contrast and small area We identify these
short-lived brightenings as mainly at the least UV continuum enhancement
Our primary result shows that the brightenings do not appear in the
HXR main sources These new phenomena may provide a new challenge for
the classical flare scenario
---------------------------------------------------------
Title: Statistical study of filament eruptions with emerging flux
Authors: Xu, X. Y.; Fang, C.; Chen, P. F.; Ding, M. D.
2006cosp...36.1148X Altcode: 2006cosp.meet.1148X
Observations indicate that solar coronal mass ejections CMEs are
closely associated with reconnection-favored flux emergence which
was explained in the emerging flux trigger mechanism for CMEs by Chen
Shibata 2000 Using numerical simulations we made a parametric survey
of the triggering factors the polarity orientation and the position
of the emerging flux and the amount of the unsigned flux Xu Chen Fang
2005 A diagram is presented to show the eruption and non-eruption
regimes in the parameter space In this paper a statistical study on
the filament eruptions related to emerging flux is performed and it
basically supports the theoretical results of our numerical simulation
Our results suggest that whether a CME can be triggered depends on
both the amount and the location of an emerging flux in addition to
its polarity orientation The work provides useful information for the
space weather forecast
---------------------------------------------------------
Title: Ellerman Bombs: Sub-Microflares in the Solar Atmosphere
Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F.
2006apri.meet...32F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A statistical study of the footpoint asymmetries in solar
flares
Authors: Ding, M. D.; Chen, Q. R.
2006cosp...36.2985D Altcode: 2006cosp.meet.2985D
Solar flares often show two footpoints that appear to be asymmetric
in particular in hard X-ray emissions The cause of such an asymmetry
is not fully known In this work we study the footpoint asymmetries
using a sample of flares observed by RHESSI that show clearly double
footpoint hard X-ray sources We compare in detail the hard X-ray
spectra the photon flux and the spectral index between each pair of
footpoints A statistical result is drawn and compared with some recent
theoretical computations
---------------------------------------------------------
Title: Investigation of Electron Energy Spectrum During Solar Flares
Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D.
2006cosp...36.2650L Altcode: 2006cosp.meet.2650L
Observations indicated that the energy spectrum of non-thermal
particles during the evolution of solar flares changes rapidly with
the power index alternating from soft to hard and soft again Based on
MHD numerical simulation of a flare with the typical prephase impulsive
and decay phases we perform the test-particle simulations in order to
investigate the time evolution of the electron spectrum The results
are compared with observations in detail
---------------------------------------------------------
Title: Synthesis of CME-Associated Moreton and EIT Wave Features
from MHD Simulations
Authors: Chen, P. F.; Ding, M. D.; Fang, C.
2005SSRv..121..201C Altcode:
Soft X-ray (SXR) waves, EIT waves, and Hα Moreton waves are all
associated with coronal mass ejections (CMEs). The knowledge of the
characteristics about these waves is crucial for the understanding
of CMEs, and hence for the space weather researches. MHD numerical
simulation is performed, with the consideration of the quiet Sun
atmosphere, to investigate the CME/flare processes. On the basis of
the numerical results, SXR, EUV, and Hα images of the eruption are
synthesized, where SXR waves, EIT waves, and Hα Moreton waves are
identified. It confirms that the EIT waves, which border the expanding
dimmming region, are produced by the successive opening (or stretching)
of the closed magnetic field lines. Hα Moreton waves are found to
propagate outward synchronously with the SXR waves, lagging behind
the latter spatially by ∼27 Mm in the simulated scenario. However,
the EIT wave velocity is only a third of the Moreton wave velocity. The
synthesized results also suggest that Hα± 0.45Å would be the best
off-band for the detection of Hα Moreton waves.
---------------------------------------------------------
Title: Microwave type III bursts and pulsation groups
Authors: Ning, Z.; Ding, M. D.; Wu, H. A.; Xu, F. Y.; Meng, X.
2005A&A...437..691N Altcode:
We analyse 22 continuous microwave reverse slope (RS) type III and 21
microwave pulsation groups observed by the Purple Mountain Observatory
(PMO) spectrometer over the frequency range of 4.5-7.5 GHz from
1999-2003. The RS type III groups have an average of 12 bursts per
group, a mean duration of 0.1-2 s around 6 GHz, a mean quasi-period
of 2.0 ± 0.6 s and a mean drift rate of 7.0 GHz s<SUP>-1</SUP>. The
shortest quasi-period is measurable down to 0.05 s thanks to the high
time resolution of this instrument. The microwave pulsation groups
have an average of 22 events per group, a mean duration of 3-4 s and
a mean quasi-period of 3.7 ± 1.1 s. The RS type III bursts have the
same quasi-period as the upward type III bursts documented before. We
discuss two possible origins of the quasi-periodicity of type III
bursts. One is the quasi-periodic process in magnetic reconnection
at the same site, and the other is spatially unresolved stochastic
processes in solar flares.
---------------------------------------------------------
Title: Kinetic Properties of CMEs Corrected for the Projection Effect
Authors: Yeh, Chin-Teh; Ding, M. D.; Chen, P. F.
2005SoPh..229..313Y Altcode:
Observations of coronal mass ejections (CMEs) with coronagraphs are
subject to a projection effect, which results in statistical errors in
many properties of CMEs, such as the eruption speed and the angular
width. In this paper, we develop a method to obtain the velocity and
angular width distributions of CMEs corrected for the projection
effect, and then re-examine the relationship between CMEs and the
associated flares. We find that (1) the mean eruption speed is 792
km s<SUP>−1</SUP> and the mean angular width is 59<SUP>∘</SUP>,
compared to the values of 549 km s<SUP>−1</SUP> and 77<SUP>∘</SUP>,
respectively before the correction; (2) after the correction, the
weak correlation between CME speeds and the GOES X-ray peak flux of
the flares gets unexpectedly poorer; and (3) before correction, there
is a weak correlation between the angular width and the speed of CMEs,
whereas the correlation is absent after the correction.
---------------------------------------------------------
Title: Occurrence of a White-Light Flare Over a Sunspot and the
Associated Change of Magnetic Flux
Authors: Li, J. P.; Ding, M. D.; Liu, Y.
2005SoPh..229..115L Altcode:
A white-light flare (WLF) on 10 March 2001 was well observed in the
Hα line and the Ca II λ8542 line using the imaging spectrograph
installed on the Solar Tower Telescope of Nanjing University. Three
small sunspots appeared in the infrared continuum image. In one sunspot,
the infrared continuum is enhanced by 4-6% compared to the preflare
value, making the sunspot almost disappear in the continuum image for
about 3 min. A hard X-ray (HXR) source appeared near the sunspot,
the flux of which showed a good time correlation with the profile
of the continuum emission. In the sunspot region, both positive and
negative magnetic flux suffered a substantial change. We propose that
electron precipitation followed by radiative back-warming may play the
chief role in heating the sunspot. The temperature rise in the lower
atmosphere and the corresponding energy requirement are estimated. The
results show that the energy released in a typical WLF is sufficient
to power the sunspot heating.
---------------------------------------------------------
Title: On the formation of the He I 10 830 Å line in a flaring
atmosphere
Authors: Ding, M. D.; Li, H.; Fang, C.
2005A&A...432..699D Altcode:
We explore the formation of the He i 10 830 Å line in a flaring
atmosphere, with special attention to the nonthermal effects of an
electron beam. Using non-LTE calculations we obtain the line profiles
from different model atmospheres. Without the nonthermal effects, the
line changes from weak absorption in a cool atmosphere to emission in
a hot and condensed atmosphere, as expected. However, the presence of
an electron beam can significantly change the line strength, producing
much stronger absorption and emission in these two cases. We find that
in the nonthermal case, the collisional ionization of He i followed by
recombinations becomes an important process in populating the triplet
levels corresponding to the He i 10 830 Å line. These results suggest
that the He i 10 830 Å line is also a potential diagnostic tool for
nonthermal effects in solar or stellar flares.
---------------------------------------------------------
Title: Parametric Survey of Emerging Flux for Triggering CMEs
Authors: Xu, X. Y.; Chen, P. F.; Fang, C.; Ding, M. D.
2005IAUS..226..217X Altcode:
Observations suggest that solar coronal mass ejections (CMEs) are
closely associated with reconnection-favored flux emergence, which
was explained as the emerging flux trigger mechanism for CMEs by Chen
and Shibata (2000) based on numerical simulations. This paper presents
a parametric survey of the CME-triggering environment. Our numerical
results show that whether the CMEs can be triggered depends on both
the amount and the location of the emerging flux. The results are
useful for space weather forecast.
---------------------------------------------------------
Title: On the impulsiveness of the Hα emission from a flaring
atmosphere
Authors: Ding, M. D.
2005AdSpR..35.1846D Altcode:
Emissions from solar flares may reveal fast fluctuations, which can be
attributed to small-scale injections of energetic electrons. In this
paper, we perform numerical calculations of the Hα emission from a
flaring atmosphere bombarded by a pulsating electron beam. We assume
that the variation of the electron beam flux consists of two components:
a fluctuation component and a background component. The results show
that the amplitude of Hα fluctuations varies depending on the magnitude
of the background flux of the electron beam. In the case of a higher
background flux, the Hα fluctuations are more significant than in
the case of a lower background flux. This result is compatible with
the observations in which the Hα fluctuations appear preferentially
near the hard X-ray maximum.
---------------------------------------------------------
Title: On the Relationship between the Continuum Enhancement and
Hard X-Ray Emission in a White-Light Flare
Authors: Chen, Q. R.; Ding, M. D.
2005ApJ...618..537C Altcode: 2004astro.ph.12171C
We investigate the relationship between the continuum enhancement and
the hard X-ray (HXR) emission of a white-light flare on 2002 September
29. By reconstructing the RHESSI HXR images in the impulsive phase,
we find two bright conjugate footpoints (FPs) on the two sides of the
magnetic neutral line. Using the thick-target model and assuming a
low-energy cutoff of 20 keV, the energy fluxes of nonthermal electron
beams bombarding FPs A and B are estimated to be 1.0×10<SUP>10</SUP>
and 0.8×10<SUP>10</SUP> ergs cm<SUP>-2</SUP> s<SUP>-1</SUP>,
respectively. However, the continuum enhancement at the two FPs is
not simply proportional to the electron beam flux. The continuum
emission at FP B is relatively strong with a maximum enhancement of
~8% and correlates temporally well with the HXR profile; however, the
continuum emission at FP A is less significant with an enhancement
of only ~4%-5%, regardless of the relatively strong beam flux. By
carefully inspecting the Hα line profiles, we ascribe such a contrast
to different atmospheric conditions at the two FPs. The Hα line profile
at FP B exhibits a relatively weak amplitude with a pronounced central
reversal, while the profile at FP A is fairly strong without a visible
central reversal. This indicates that in the early impulsive phase of
the flare, the local atmosphere at FP A has been appreciably heated
and the coronal pressure is high enough to prevent most high-energy
electrons from penetrating into the deeper atmosphere; while at FP
B, the atmosphere has not been fully heated, the electron beam can
effectively heat the chromosphere and produce the observed continuum
enhancement via the radiative back-warming effect.
---------------------------------------------------------
Title: Waiting Time Distribution of CMES
Authors: Yeh, C. -T.; Ding, M. D.; Chen, P. F.
2005ASSL..320..171Y Altcode: 2005smp..conf..171Y
No abstract at ADS
---------------------------------------------------------
Title: A CME and Related Phenomena on 2003 October 26
Authors: Ning, Zongjun; Fang, C.; Ding, M. D.; Yeh, C. -T.; Li, H.;
Xu, Y. N.; Zhang, Y.; Tan, C. M.
2005IAUS..226..123N Altcode:
We present the observational results of the solar bursts on the
band of 1-80 GHz (NORH) associated with both a CME and a flare on
Oct. 26 2003. This event shows two parts of radio bursts in the time
profile. The first part is associated with an X1.2 flare. However,
the following part seams related to both the flare and the CME, as
the radio emission is enhanced while the H α is decreasing. Thus,
these two parts of radio bursts may originate from different physical
processes, i.e., flare and CME shock. A primary study is performed on
the difference between this two parts.
---------------------------------------------------------
Title: Multiwavelength Analysis of an X1.6 Flare of 2001 October 19
Authors: Li, J. P.; Ding, M. D.
2004ApJ...606..583L Altcode:
An X1.6 two-ribbon flare was observed on 2001 October 19 using the
imaging spectrograph at the Solar Tower of Nanjing University. We
obtained a time series of two-dimensional Hα line spectra. Combining
the observations by the Nobeyama Radioheliograph (NoRH), the Transition
Region and Coronal Explorer (TRACE), the Yohkoh soft X-ray telescope
(SXT) and hard X-ray telescope (HXT), and the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory (SOHO), we
performed a multiwavelength analysis for this flare in detail. The hard
X-ray (HXR) and radio time profiles for this event have a double-peaked
structure. Four Hα bright kernels exist, two of which are identified
as the conjugate footpoints of a loop, while the other two are probably
the footpoints of two different loops that are closely related. We
compared the spatial distribution of the HXR sources with that of the
Hα kernels at the two peak times. The results show quite asymmetric
HXR emission in the Hα kernels. We ascribe this asymmetric behavior
to the magnetic mirroring effect. In addition, we found that different
heating mechanisms (nonthermal electron beam vs. heat conduction)
may apply to different Hα kernels. We also derived the velocity field
and found that the maximum velocity tends to occur at the outer edge
of the flare ribbons. This is consistent with the general scenario of
two-ribbon flare models.
---------------------------------------------------------
Title: Disappearance of a sunspot accompanying an M-Class flare
Authors: Li, J. P.; Ding, M. D.; Liu, Y.
2004IAUS..223..119L Altcode: 2005IAUS..223..119L
A white-light flare on 2001 March 10 was well observed in the Halpha
line and Ca II λ8542 line using the imaging spectrograph installed in
the Solar Tower Telescope of Nanjing University. Three small sunspots
appeared in the infrared continuum image, one of which showed that
the infrared continuum is enhanced by 4%-6% compared to the preflare
value and it almost disappeared in the continuum image for about 3
min. Near the sunspot, there appeared a hard X-ray (HXR) source, the
flux of which showed a good time correlation with the profile of the
continuum emission. We propose that electron precipitation followed
by radiative backwarming may play the chief role in heating the sunspot.
---------------------------------------------------------
Title: Two-dimensional spectroscopy of a white-light flare and its
relationship with high-energy electrons
Authors: Ding, M. D.; Chen, Q. R.; Fang, C.
2004cosp...35.2414D Altcode: 2004cosp.meet.2414D
We perform a multi-wavelength study of a white-light flare of September
29, 2002 that was simultaneously observed by a ground-based imaging
spectrograph and by the RHESSI. It is found that the enhanced continuum
emission corresponds both spatially and temporally with the hard X-ray
emission. There is a slight time delay between the peak of the continuum
emission and that of the hard X-ray emission, which can be explained
by the backwarming scenario, in which the chromosphere is directly
heated by a beam of high-energy electrons and the photosphere is then
heated through an enhanced radiation. We further deduce the electron
flux from the hard X-ray spectra. The relationship between the electron
flux and the continuum contrast is quantitatively compatible with the
theoretical prediction.
---------------------------------------------------------
Title: Diagnostics of Non-thermal Particles in Solar Chromospheric
Flares
Authors: Fang, C.; Xu, Z.; Ding, M. D.
2004IAUS..219..171F Altcode: 2003IAUS..219E..24F
Particle beam bombardments on the solar chromosphere will produce
non-thermal ionization and excitation. The effect on hydrogen lines has
been extensively investigated by using non-LTE theory and semi-empirical
flare models. It has been found that Halpha line is widely broadened and
shows a strong central reversal. Significant enhancements at the wings
of Lyalpha and Lybeta lines are also predicted. In the case of proton
bombardment less strong broadening and no large central reversal are
expected. We found that the coronal mass and the atmospheric condition
give much influence on the line profiles. The line profiles are good
tools for diagnosing the total flux of the particle beam but less
sensitive to the power index. One can use these factors to diagnose
the particle beam in the chromosphere.
---------------------------------------------------------
Title: Hα and Hard X-Ray Observations of a Two-Ribbon Flare
Associated with a Filament Eruption
Authors: Ding, M. D.; Chen, Q. R.; Li, J. P.; Chen, P. F.
2003ApJ...598..683D Altcode: 2003astro.ph..8085D
We perform a multiwavelength study of a two-ribbon flare on
2002 September 29 and its associated filament eruption, observed
simultaneously in the Hα line by a ground-based imaging spectrograph
and in hard X-rays by RHESSI. The flare ribbons contain several
Hα-bright kernels that show different evolutionary behaviors. In
particular, we find two kernels that may be the footpoints of a
loop. A single hard X-ray source appears to cover these two kernels
and to move across the magnetic neutral line. We explain this as a
result of the merging of two footpoint sources that show gradually
asymmetric emission owing to an asymmetric magnetic topology of
the newly reconnected loops. In one of the Hα kernels, we detect
a continuum enhancement at the visible wavelength. By checking its
spatial and temporal relationship with the hard X-ray emission, we
ascribe it to electron-beam precipitation. In addition, we derive the
line-of-sight velocity of the filament plasma based on the Doppler
shift of the filament-caused absorption in the Hα blue wing. The
filament shows rapid acceleration during the impulsive phase. These
observational features are in principal consistent with the general
scenario of the canonical two-ribbon flare model.
---------------------------------------------------------
Title: Interpretation of the infrared continuum in a solar white-light
flare
Authors: Ding, M. D.; Liu, Y.; Yeh, C. -T.; Li, J. P.
2003A&A...403.1151D Altcode:
We explain the origin of the infrared continuum in the solar flare of
10 March 2001 which shows a positive contrast in the maximum phase but
possibly a negative contrast in the early phase. The general feature
is consistent with a flare model heated by an electron beam. By making
non-LTE model calculations, we find that in the early phase, when
the electron beam bombards an unheated atmosphere, the non-thermal
ionization by the beam results in an increased H<SUP>-</SUP> opacity
which then reduces the emergent intensity. With the flare development,
the atmosphere is gradually heated. In particular, radiative backwarming
plays a chief role in the heating of the temperature minimum region and
upper photosphere. We estimate the temperature rise in these regions for
a fully heated atmosphere in which energy balance is attained. In this
case, the continuum emission rises above the quiescent value. Therefore,
this corresponds to the flare maximum phase. We further find that the
energy flux of the electron beam deduced from the hard X-ray emission
is large enough to account for the continuum contrast.
---------------------------------------------------------
Title: Observations and Spectral Analyses of Solar Flares
Authors: DING, M. D.
2003JKAS...36S..49D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral Diagnostics of Non-Thermal Particles in the Solar
Chromosphere
Authors: FANG, C.; XU, Z.; DING, M. D.
2003JKAS...36S..55F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Reconnection in the Solar Lower Atmosphere
Authors: Fang, C.; Chen, P. F.; Ding, M. D.
2003ASPC..289..425F Altcode: 2003aprm.conf..425F
Accumulating observational evidence indicates that magnetic reconnection
is a fundamental process in the solar lower atmosphere, which is
responsible for many localized activities and the global maintenance
of the hot dynamical corona. Meanwhile, qualitative theoretical
considerations and quantitative numerical simulations demonstrate
the applicability of the reconnection to a thin layer in the lower
atmosphere. This paper reviews the research progress in the related
observations, theories and numerical simulations.
---------------------------------------------------------
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: A statistical study of soft X-ray flares during solar cycle 22
Authors: Yin, Suying; Chen, Pengfei; Ding, Mingde; Fang, Cheng
2002JNUNS..38..457Y Altcode:
Solar flares are among the most energetic events on the Sun. The
eruption of solar flares has a great influence on the space environment
and the Earth. A statistical study of flares occurring in a whole solar
cycle is important in understanding the main characteristics of flares
and useful for further study on the flare mechanism. In this paper,
we make a complete statistics of the soft X-ray (SXR) flares during
solar cycle 22 according to the GOES satellite data. Among the 20930
flares, less than 1% belong to X-class, 10% to M-class, and about
60% to C-class. The flare activity in this cycle undergoes a similar
behavior to the sunspot number and experiences two peaks in 1989 and
1991, in which the mean flare index is 426 and 468, respectively. The
numbers of flares of C-, M-, and X-classes increase rapidly during the
ascending phase of the solar cycle, and decrease slowly during the
decay phase. The ascending phase takes only about 2 years while the
descending phase more than 4 years. The occurrence ratio of flares
has a power-law distribution in dependence on the peak soft X-ray
flux; the average power-law index for dN/dF, where N is the number
of flares and F is the peak flux, is about -2.135 and the correlation
coefficient is -0.987. Smaller (B-class and some C-class) flares are
affected by the background noise so that they are not included in the
derivation of flare index. Moreover, the percentage of SXR flares
that have optical counterparts increases with the soft X-ray flux
which amounts to 94% for X-class, 83% for M-class, 63% for C-class,
and only 30% for B-class, respectively.
---------------------------------------------------------
Title: Non-LTE Calculation of the Ni I 676.8 nm Line in a Flaring
Atmosphere
Authors: Ding, M. D.; Qiu, J.; Wang, H.
2002AAS...200.9103D Altcode: 2002BAAS...34..953D
The Ni I 676.8 nm line is used by SOHO/MDI to measure the magnetic
field and velocity field in the solar atmosphere. We make non-LTE
calculations of this line in a flaring atmosphere which is heated
by an energetic electron beam. 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. We find that the
line may appear emission in the presence of a fairly strong electron
beam. 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 through other ways. This result implies that at least
in some cases, 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: The Temperature and Density Structure of an Hα Flaring Loop
Authors: Ding, M. D.; Liu, Y.; Chen, P. F.
2002SoPh..207..125D Altcode:
We develop a simple method to deduce the temperature and density in
the loop of a limb flare from the spectral observations of two lines,
Hα and Ca ii 8542 Å. We first build a grid of homogeneous slab models
with various temperature and density values and compute the emergent
line intensities, and then find the relevant model that can match the
observed intensities. This is an approximate method because there are
several other factors which can influence the line intensities. We
apply this method to the limb flare of 11 November 1998 and deduce the
values of temperature and hydrogen number density at different spatial
points in the flaring loop, as well as their temporal variations. The
loop contains relatively high density and possibly the loop top is
slightly hotter and more condensed than the legs of the loop at the
flare maximum time. A favorable scenario to produce this result is
that magnetic reconnection occurs above the loop, and the reconnection
outflow may heat and condense the plasma near the loop top.
---------------------------------------------------------
Title: Non-LTE Inversion of an Hα Flaring Loop
Authors: Ding, M. D.; Liu, Y.; Chen, P. F.; Fang, C.
2002stma.conf...89D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Cyclic variations in the solar lower atmosphere
Authors: Fang, C.; Zhang, Y. X.; Ding, M. D.; Livingston, W. C.
2002AdSpR..29.1947F Altcode:
The Ca II K line has been measured regularly nearly every month since
1974 at Kitt Peak. It is well known that the K <SUB>1</SUB> component
of the Ca II K line is formed in the temperature minimum region (TMR)
of the solar atmosphere. Our study of the data of CaII K profiles over
two solar cycles indicates that both in full disc integrated spectra and
in center disc spectra, the distance between the red K <SUB>1</SUB> and
the blue K <SUB>1</SUB> of the profiles and its average intensity show
periodic variations. But the variation for the full disc integrated
spectra fluctuates in the same way as the sunspot number does,
while that for the center disc spectra has a time delay with respect
to sunspot number. Non-LTE computations yield a cyclic temperature
variation of about 17 K of the TMR in the quiet-Sun atmosphere and a
cyclic variation of about 15-20 km in the height position of the TMR.
---------------------------------------------------------
Title: Muti-wavelength Analysis of an X1.6 Flare on 2001 October 19
Authors: Li, J. P.; Ding, M. D.
2002aprm.conf..441L Altcode:
An X1.6 two-ribbon flare was observed on 2001 October 19, using the
imaging spectrograph in the solar tower of Nanjing University. We
obtained a time series of 2D spectra of the Hα line. Combined with
observations made with the Nobeyama radioheliograph (NoRH), Yohkoh/SXT
and HXT, SOHO/MDI, and the Huairou Solar Observing Station (HSOS),
we have made a detailed analysis of this flare event. The images show
a very complex flaring region. The X-ray and radio time profiles for
this event have a double-peaked structure. We find two bright kernels,
out of four in total, showing temporal variations of the line-of-sight
velocity, the line width and the line center intensity of Hα that
correspond respectively with the two peaks in the hard X-ray time
profile. These two kernels are found to be the foot points of two
loops of the flare. A discussion is made about this event based on
multi-wavelength analysis.
---------------------------------------------------------
Title: Waiting Time Distribution of Coronal Mass Ejections
Authors: Yeh, C. -T.; Ding, M. D.; Chen, P. F.
2002aprm.conf..471Y Altcode:
Coronal mass ejections (CME) and flares are believed to be the result of
rapid release of magnetic energy. The energy release process in solar
flares can be interpreted as avalanches of many small reconnection
events, that is, the statistical properties of the flaring events can
be described in a simple sandpile model. In this paper, we use the
avalanche model to investigate the frequency distribution of coronal
mass ejections, and we find the frequency distribution of coronal mass
ejections is similar to that of flares.
---------------------------------------------------------
Title: Magnetic Reconnection in the Solar Lower Atmosphere
Authors: Fang, C.; Chen, P. F.; Ding, M. D.
2002stma.conf....3F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Enhanced Emission at the Infrared Continuum in the Flare of
2001 March 10
Authors: Liu, Y.; Ding, M. D.; Fang, C.
2001ApJ...563L.169L Altcode:
We have obtained a time series of two-dimensional spectra of Hα
and Ca II λ8542 for a flare on 2001 March 10. This flare showed an
enhanced emission at the continuum near the Ca II 8542 Å line. The
continuum contrast is estimated to be 3%-5%. This emission lasted
about half a minute, showing a good time correlation with the peak of
the microwave radio flux at 7.58 GHz. The flare can be classified as a
type I white-light flare. A preliminary analysis shows that a nonthermal
electron beam cannot directly produce the continuum emission. Heating
in the lower atmosphere resulting from the radiative backwarming may
account for the continuum enhancement.
---------------------------------------------------------
Title: The role of non-thermal electrons in the hydrogen and calcium
lines of stellar flares
Authors: Ding, M. D.; Fang, C.
2001MNRAS.326..943D Altcode: 2001astro.ph..5097D
There is observational evidence showing that stellar and solar flares
occur with a similar circumstance, although the former are usually much
more energetic. It is expected that the bombardment by high-energy
electrons is one of the chief heating processes of the flaring
atmosphere. In this paper we study how a precipitating electron beam
can influence the line profiles of Lyα, Hα, Caii K and λ8542. We use
a model atmosphere of a dMe star and make non-LTE computations taking
into account the non-thermal collisional rates owing to the electron
beam. The results show that the four lines can be enhanced to different
extents. The relative enhancement increases with increasing formation
height of the lines. Varying the energy flux of the electron beam has
different effects on the four lines. The wings of Lyα and Hα become
increasingly broad with the beam flux; change of the Caii K and λ8542
lines, however, is most significant in the line centre. Varying the
electron energy (i.e. the low-energy cut-off for a power-law beam)
has a great influence on the Lyα line, but little on the Hα and
Caii lines. An electron beam of higher energy precipitates deeper,
thus producing less enhancement of the Lyα line. The Lyα/Hα flux
ratio is thus sensitive to the electron energy.
---------------------------------------------------------
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)×10<SUP>11</SUP> ergs cm<SUP>-2</SUP>
s<SUP>-1</SUP> is required for the electron beam, if one adopts a
Gaussian macrovelocity of 25 km s<SUP>-1</SUP>. Such a burst contains a
total energy of 10<SUP>25</SUP>-10<SUP>26</SUP> ergs. These parameters
are compatible with elementary flare bursts.
---------------------------------------------------------
Title: Physical Parameters of a Flare on 22 December 1999 Derived
from the Hα Line with a Modified Cloud Method
Authors: Liu, Y.; Ding, M. D.
2001SoPh..200..127L Altcode:
The flare of 22 December 1999 was observed in the Hα line using the
imaging spectrograph in the solar tower of Nanjing University. We
present a new technique different from the classical cloud model to
fit the Hα line profile which avoids using the background profile. We
obtain the four parameters of the flare chromosphere: the source
function, the optical thickness at line center, the line-of-sight
velocity and the Doppler width. The observed asymmetry profiles have
been reproduced well by the theoretical ones based on our model. A
discussion is made about the reliability of the results we have obtained
using the present method.
---------------------------------------------------------
Title: Quiet-Sun Variability In a Temperature Minimum Region
Authors: Zhang, Y. X.; Fang, C.; Ding, M. D.; Livingston, W. C.
2001ApJ...547L.179Z Altcode:
The results of monthly monitoring Ca II K line profiles at a quiet-Sun
region near solar disk center at Kitt Peak during about two solar cycles
since 1974 are presented to investigate the variations of K<SUB>1</SUB>
emission, which is strongly dependent on the temperature minimum region
(TMR). Cyclic variations are found in K<SUB>1</SUB> emission that are
similar to those in the sunspot numbers. However, there is an evident
time lag of about 1.2 yr between the K<SUB>1</SUB> emission and the
sunspot numbers. Non-LTE computations show that the corresponding
temperature variation and position fluctuation in the quiet-Sun
TMR region are estimated to be about 17 K and 1.7×10<SUP>-2</SUP>
g cm<SUP>-2</SUP>, respectively.
---------------------------------------------------------
Title: Cyclic Variation in the Solar Lower Atmosphere
Authors: Fang, C.; Zahng, Y. X.; Ding, M. D.; Livingston, W. C.
2001IAUS..203..387F Altcode:
CaII K line has been measured regularly nearly every month since 1974
at Kitt Peak. It is known that the K<SUB>1</SUB> component of the CaII
K line is formed in the temperature minimum region (TMR) of the solar
atmosphere. Our study on the data of CaII K line profiles over nearly
two solar cycles indicates that both in full disc integrated spectra and
in disc-center spectra, the distance between red K<SUB>1</SUB> and blue
K<SUB>1</SUB> and its mean intensity show periodical variations, but
the former fluctuates in the same way as the sunspot number does, while
the later has a time delay with respect to the sunspot number. Non-LTE
computation indicates a cyclic temperature variation of about 15 K of
the TMR in the quiet-Sun atmosphere and a cyclic variation of about
15-20 km of the position of the TMR.
---------------------------------------------------------
Title: The role of non-thermal electrons in the optical continuum
of stellar flares
Authors: Ding, M. D.; Fang, C.
2000MNRAS.317..867D Altcode: 2000astro.ph..5388D
The continuum emission of stellar flares in UV and visible bands can
be enhanced by two or even three orders of magnitude relative to the
quiescent level and is usually characterized by a blue colour. It
is difficult for thermal atmospheric models to reproduce all these
spectral features. If the flaring process involves the acceleration
of energetic electrons which then precipitate downwards to heat the
lower atmosphere, collisional excitation and ionization of ambient
hydrogen atoms by these non-thermal electrons could be important
in powering the continuum emission. To explore such a possibility,
we compute the continuum spectra from an atmospheric model for a dMe
star, AD Leo, at its quiescent state, when considering the non-thermal
effects by precipitating electron beams. The results show that if
the electron beam has an energy flux large enough (for example,
F<SUB>1</SUB>~10<SUP>12</SUP>ergcm<SUP>-2</SUP>s<SUP>-1</SUP>),
the U-band brightening and, in particular, the U-B colour are
roughly comparable with observed values for a typical large
flare. Moreover, for electron beams with a moderate energy flux
F<SUB>1</SUB><~10<SUP>11</SUP>ergcm<SUP>-2</SUP>s<SUP>-1</SUP>,
a decrease of the emission at the Paschen continuum appears. This
can explain at least partly the continuum dimming observed in some
stellar flares. Adopting an atmospheric model for the flaring state
can further raise the continuum flux, but it yields a spectral colour
incomparable with observations. This implies that the non-thermal
effects may play the chief role in powering the continuum emission in
some stellar flares.
---------------------------------------------------------
Title: Diagnostics of non-thermal processes in chromospheric
flares --- IV. Limb flare spectra for an atmosphere bombarded by an
electron beam
Authors: Fang, C.; Hénoux, J. -C.; Ding, M. D.
2000A&A...360..702F Altcode:
Hα, Lyα, Lyβ, CaII K and λ8542 Å line profiles have been computed
for limb flares with height distributions of temperatures as given by
the semi- empirical models F<SUB>1</SUB> and F<SUB>2</SUB>, by including
the non-thermal collisional excitation and ionization of hydrogen and
of ionized calcium that results from electron bombardment. In agreement
with observations, the computed profiles of the hydrogen lines are very
broad, especially at the height where the source function reaches its
maximum. Non-thermal effects are less pronounced for CaII lines.
---------------------------------------------------------
Title: XPS studies on the electronic structure of bonding between
solid and solutes: adsorption of arsenate, chromate, phosphate,
Pb<SUP>2+</SUP>, and Zn<SUP>2+</SUP> ions on amorphous black ferric
oxyhydroxide
Authors: Ding, M.; de Jong, B. H. W. S.; Roosendaal, S. J.;
Vredenberg, A.
2000GeCoA..64.1209D Altcode:
The adsorption of three anions (AsO<SUB>4</SUB><SUP>3-</SUP>,
PO<SUB>4</SUB><SUP>3-</SUP>, and CrO<SUB>4</SUB><SUP>2-</SUP>)
and two cations (Zn<SUP>2+</SUP> and Pb<SUP>2+</SUP>) has been
determined on a new form of amorphous black ferric oxyhydroxide
that resembles in local structure β-FeOOH, akaganeite. The nature
of the interaction between FeOOH substrate and the above mentioned
adsorbates is characterised with X-ray photoelectron spectroscopy on
the core and valence band levels of trivalent iron and oxygen using
frontier molecular orbital theory as theoretical framework. Our findings
indicate that substantial and variable charge transfer occurs in which
the FeOOH substrate can function either as a Lewis acid or base in its
interaction with different adsorbates. Our findings suggest that the
valence band spectra can be used to estimate qualitatively the energy
level separation between the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) of the surface complexes
and to assess the chemical affinity between substrate and adsorbate.
---------------------------------------------------------
Title: Cyclic variation in the temperature minimum region of solar
atmosphere.
Authors: Zhang, Yunxiang; Fang, Cheng; Livingston, W. C.; Ding, Mingde
2000AcASn..41...44Z Altcode:
Calcium K line as well as some other solar spectral lines have been
measured regularly nearly every month since 1974 at Kitt Peak. It is now
widely accepted that K<SUB>1</SUB> is formed in the temperature minimum
region (TMR) and that the distance of the component K<SUB>1</SUB> from
the line center and the intensity of K<SUB>1</SUB> is well related to
the position and temperature of TMR, respectively. The authors' study
on the data of K line profiles and on the sunspot numbers over decades
shows: in full disc integrated spectra the distance of K<SUB>1</SUB>
from the line center fluctuates in the same way as the sunspot number
does, indicating a solar cyclic variation.
---------------------------------------------------------
Title: Numerical Study on the Effect of Heat Conduction on Magnetic
Reconnection
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
2000AdSpR..26..525C Altcode:
The effect of heat conduction on 2.5D magnetic reconnection, similar
to that in Kopp-Pneuman model, is numerically studied. It is shown
that the heat conduction accelerates the reconnection, increases
the amount of shrinkage of the closed field lines, and increases the
average rise speed of the SXR loop. MHD slow shocks contribute to the
SXR loop heating. When the timescale of heat conduction is shorter
than the Alfvén timescale, an adiabatic slow shock is dissociated
into an isothermal slow shock and a heat conduction front
---------------------------------------------------------
Title: An unusually intense CA II K line wing: possible role of
particle acceleration in the lower atmosphere
Authors: Ding, M. D.
1999A&A...351..368D Altcode:
The white-light flare on 1974 October 11 showed an unusual behavior:
its Ca ii K line reached at flare maximum an intensity of K<SUB>1</SUB>
as high as half of the continuum. This spectral feature cannot be
explained by ordinary flare models. We present an abnormal model
with an extremely hot temperature minimum region (TMR) that can
reproduce the unusually high K<SUB>1</SUB> intensity. However, this
poses a very severe constraint on the energy requirement in the lower
atmosphere. Since canonical heating models are insufficient to provide
enough heating energy in lower layers, an in situ energy source may
be required. We further investigate the possible role of a non-thermal
particle beam injected from a lower layer. A beam of hecta-keV electrons
(or MeV protons) can sufficiently heat the TMR and lead to the formation
of a very hot TMR provided that the energy flux of the beam is large
enough. Whether such kinds of particle beams exist needs to be checked
by more theoretical and observational studies.
---------------------------------------------------------
Title: Numerical simulation of the magnetic reconnection in solar
flares.
Authors: Fang, Cheng; Chen, Pengfei; di, Xiaohua; Ding, Mingde;
Tang, Yuhua
1999PPMtO..18..330F Altcode:
2D magnetic reconnection in solar flares is numerically simulated. The
results show that when the reconnection occurs at a high altitude,
the evolution can reproduce the main characteristics (such as flaring
loop motions and footpoints separation, etc.) of two-ribbon flares;
while that with a low reconnection X-point shows a large similarity
to compact flares. The results also indicate that there is no direct
relation between the rises of soft X-ray loops and the X-point.
---------------------------------------------------------
Title: Multi-line two-dimensional spectroscopy of a limb flare
Authors: Ding, M. D.; Fang, C.; Yin, S. Y.; Chen, P. F.
1999A&A...348L..29D Altcode:
We present the result of a preliminary analysis of the 2D spectra of Hα
and Ca ii lambda 8542 for a limb flare on 11 November, 1998. Near the
top of the flaring loop, the Hα line is extraordinarily broadened. The
effect of line opacity (or the saturation of line core) cannot fully
account for the observed line width since it requires an extremely high
loop density (n_H>~ 10(13) cm(-3) when T=10(4) K). The remaining
possibility is the broadening by micro-turbulence or inhomogeneous mass
motions. Since the two lines demonstrate different broadening effects,
it is quite possible that they are formed in different fine structures
which cannot be spatially resolved by observations.
---------------------------------------------------------
Title: Flaring Loop Motion and a Unified Model for Solar Flares
Authors: Chen, P. F.; Fang, C.; Ding, M. D.; Tang, Y. H.
1999ApJ...520..853C Altcode:
We performed 2.5-dimensional numerical simulations of magnetic
reconnection for several models, some with the reconnection point at a
high altitude (the X-type point in magnetic reconnection), and one with
the reconnection point at a low altitude. In the high-altitude cases,
the bright loop appears to rise for a long time, with its two footpoints
separating and the field lines below the bright loop shrinking, which
are all typical features of two-ribbon flares. The rise speed of the
loop and the separation speed of its footpoints depend strongly on
the magnetic field B<SUB>0</SUB>, to a medium extent on the density
ρ<SUB>0</SUB>, and weakly on the temperature T<SUB>0</SUB>, the
resistivity η, and the length scale L<SUB>0</SUB>, by which the size
of current sheet and the height of the X-point are both scaled. The
strong B<SUB>0</SUB> dependence means that the Lorentz force is the
dominant factor; the inertia of the plasma may account for the moderate
ρ<SUB>0</SUB> dependence; and the weak η dependence may imply that
“fast reconnection” occurs; the weak L<SUB>0</SUB> dependence implies
that the flaring loop motion has geometrical self-similarity. In the
low-altitude case, the bright loops cease rising only a short time
after the impulsive phase of the reconnection and then become rather
stable, which shows a distinct similarity to the compact flares. The
results imply that the two types of solar flares, i.e., the two-ribbon
flares and the compact ones, might be unified into the same magnetic
reconnection model, where the height of the reconnection point leads
to the bifurcation.
---------------------------------------------------------
Title: Numerical study on the flaring loop dynamics.
Authors: Fang, Cheng; Chen, Pengfei; Tang, Yuhua; Ding, Mingde
1999PPMtO..18...93F Altcode:
Numerical simulations are performed to study the flaring loop dynamics
during magnetic reconnection. Two models are considered with different
reconnection point heights. In the model with high reconnection point,
the results can reproduce the loop motions in the two-ribbon flares,
while in the model with low reconnection point, the evolution shows a
large similarity to the compact flares. It implies that the two types
of solar flares could be unified into the same model, where the height
of the reconnection point leads to the bifurcation.
---------------------------------------------------------
Title: Simulation of Magnetic Reconnection with Heat Conduction
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
1999ApJ...513..516C Altcode:
Magnetohydrodynamic (MHD) equations are numerically solved to study
2.5-dimensional magnetic reconnection with field-aligned heat
conduction, which is also compared with the adiabatic case. The
dynamical evolution starts after anomalous resistivity is introduced
into a hydrostatic solar atmosphere with a force-free current sheet,
which might be similar to the configuration before some solar
flares. The results show that two jets (i.e., the outflows of the
reconnection region) appear. The downward jet collides with the closed
line-tied field lines, and a bright loop is formed with a termination
shock at the loop top. As the reconnection goes on, the loop rises
almost uniformly with a speed of tens of km s<SUP>-1</SUP>, and the two
footpoints of the loop separate with a speed comparable to the loop rise
speed. Besides the apparent loop motion, the magnetic loops below the
loop top shrink weakly. Such a picture is consistent with that given by
observations of two-ribbon solar flares. Moreover, the results indicate
that the slow MHD shock contributes to the bright loop heating. Some
detailed structures of the reconnection process are also discussed.
---------------------------------------------------------
Title: Heating in the Lower Atmosphere and the Continuum Emission
of Solar White-Light Flares
Authors: Ding, M. D.; Fang, C.; Yun, H. S.
1999ApJ...512..454D Altcode:
Observationally, there is a small fraction of solar white-light flares
(WLFs), the so-called type II WLFs, showing an increased visible
continuum but no significant Balmer jump and less strong chromospheric
line emission in comparison with type I WLFs. The classical point of
view, that the flare energy is initially released in the corona and
then transported downward, can hardly explain WLFs of this kind. In
this paper we explore the possibility that type II WLFs originate from
a deeper layer. Assuming an in situ energy release, in particular in
the form of high-energy particles, in a region around the temperature
minimum, the continuum emission is computed in different time stages
during the flare evolution. At first, nonthermal excitation and
ionization of hydrogen atoms caused by bombarding particles result in
a decline of the visible continuum. Later on, the lower atmosphere
is gradually heated through radiative transfer, mitigating the
continuum decline. In the final stage, when the particle bombardment
stops while the atmosphere still keeps a heated state, we obtain a
positive continuum contrast without an obvious Balmer jump. This meets
the condition required for type II WLFs. The presence or absence of a
continuum decline in the early stage of the flare provides a diagnostic
tool for nonthermal processes in the lower atmosphere.
---------------------------------------------------------
Title: Magnetic Reconnection and Flare Loop Motion
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
1999ASSL..240..337C Altcode: 1999numa.conf..337C
No abstract at ADS
---------------------------------------------------------
Title: 2D spectra analysis of a SN/C5.6 flare on 19 March 1993.
Authors: Yin, Suying; Fang, Cheng; Ding, Mingde; Huang, Youran;
Gao, Xiufa
1999JNanU..35...21Y Altcode:
On 19 March 1993, a SN/C5.6 flare occurred near the center of the
solar disk at ≡02:19 UT. Twodimensional Hα spectra were obtained by
using an imaging spectrograph installed at the Solar Tower Telescope
of Nanjing University. The 2D line center intensity contours show
that the flare has three main kernels. Their morphology, structure
and evolution are different, while the red asymmetry at the Hα wings
is common for the three regions. Based on the asymmetries of the Hα
lines, the authors derive a time series of 2D velocity fields for the
flare. The results indicate that the inferred downward velocities are
various in different regions.
---------------------------------------------------------
Title: Multi-Wavelength Analysis of the Flare on 2 October 1993
Authors: Fang, C.; Tang, Y. H.; Hénoux, J. -C.; Huang, Y. R.; Ding,
M. D.; Sakurai, T.
1998SoPh..182..163F Altcode:
By use of Yohkoh hard X-ray flux and soft X-ray images, and of vector
magnetograms and 2D spectral observations, a 1N/C6.5 flare observed on
2 October 1993 is analysed in detail. Evidence is provided not only
morphologically but also quantitatively that the dynamics at kernels
A and C of the flare in the impulsive phase were controlled mainly by
electron beam bombardment, while the heating of kernel B is mainly due
to heat conduction. By plotting the energy gradient of the electron
energy flux as a function of energy for the various spectral indexes
observed during the flare, the acceleration mechanism is found to be
such that there is a constant energy E<SUB>0</SUB>, close to 20 keV,
for which the electron flux d F<SUB>1</SUB>/dE is constant. It is shown
that such a conclusion can be reached more directly by using the photon
flux, which in that case must be constant for E=E<SUB>0</SUB>, whatever
the value of the power index. This result implies also that the electron
spectrum is represented by a power law and that the X-ray photons are
produced in a thick target. Instantaneous momentum balance is shown to
exist between the upflowing soft X-ray-emitting and the downflowing
Hα- emitting plasma at the kernels of the flare. The observed Hα
red asymmetry is well reproduced by the non-LTE computation, with the
down-moving condensation included. The observation of the magnetic
field suggests that the flare was triggered probably by magnetic
flux emergence.
---------------------------------------------------------
Title: A possible mechanism for the Hα broad wings emission of
Ellerman bombs
Authors: Henoux, J. -C.; Fang, C.; Ding, M. D.
1998A&A...337..294H Altcode:
In this paper, the non-thermal emission of Lyalpha , Lybeta and Hα
lines generated through charge exchange by protons accelerated in
the low chromosphere and moving with a small pitch angle around an
horizontal magnetic field has been computed. Computations have been
done for observations made at the center of the solar disk, i.e. in a
situation where the solar magnetic field is perpendicular to the line
of sight, for non zero pitch angles. In such conditions, the photons
emitted through charge exchange are Doppler shifted symmetrically in
both wings of the lines. The Hα far line wing intensities increase
significantly, making possible to reproduce the characteristics of
the spectra of Ellerman bombs. Thus it is proposed that a proton beam
accelerated in the low chromosphere, and losing its energy there,
is a viable candidate for explaining the observed EB spectra.
---------------------------------------------------------
Title: Two-dimensional spectral observations of sunspots in
chromospheric lines. I. Asymmetries of CA II lines
Authors: Ding, M. D.; Schleicher, H.
1998A&A...332..767D Altcode:
We present two-dimensional spectral observations of two closely
located small sunspots in chromospheric lines. Line asymmetries and
shifts are derived for the Caii K line in particular. It is found that
the majority of Caii K profiles show a blue asymmetry. Red asymmetry
may also appear, but only in some tiny regions. Some ephemeral blue
asymmetry patches are found in the umbrae, which seem to be related to
umbral flashes. On the limb side of the penumbra, the Caii K profiles
show a distinguishably blue asymmetry of K<SUB>1</SUB>. In contrast,
the profiles on the center side are more likely to possess a blue
asymmetry of K<SUB>2</SUB>, together with a slight red shift of
K<SUB>3</SUB>. We discuss possible causes for such a peculiar spatial
distribution of line asymmetries. Further line profile computations
are needed to get a consistent and satisfactory explanation.
---------------------------------------------------------
Title: Line profiles in moustaches produced by an impacting energetic
particle beam
Authors: Ding, M. D.; Henoux, J. -C.; Fang, C.
1998A&A...332..761D Altcode:
The Hα line profile in moustaches is characterized by enhanced wings
and a deep central absorption. We explore the possibility that such a
profile may be due to the effect of energetic particles bombarding the
atmosphere. Computations show that the characteristics of moustache
line profiles can be qualitatively reproduced in two extreme cases,
either injection from the corona of high energy particles (ga 60 keV
electrons or ga 3 MeV protons) or injection in a low-lying site, in
middle chromosphere or deeper, of less energetic particles ( ~ 20 keV
electrons or ~ 400 keV protons). The requirements on the energy and on
the depth of the injection site of energetic particles are reduced in
the case of observations close to the solar limb. The role of protons
of energies below 1 MeV is slightly less significant than that of
deka-keV electrons in the case of a high particle injection site,
but such protons remain to be viable candidates in the case of lower
particle injection sites and of observations at larger heliocentric
angles. Observations at various wavelengths are needed to find which
of these hypotheses is convenient for explaining a given event.
---------------------------------------------------------
Title: Particle Beam Diagnostics Based on UV and Optical Spectra of
Solar Flares (invited)
Authors: Fang, C.; Ding, M. D.; Hénoux, J. C.
1998asct.conf..497F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Loops Above a Sunspot Region
Authors: Fang, C.; Tang, Y. H.; Ding, M. D.; Zhao, J.; Sakurai, T.;
Hiei, E.
1997SoPh..176..267F Altcode:
By analysing the data of Yohkoh soft X-ray images, vector magnetograms
and 2D spectral observations, coronal loops above a large sunspot on
16-19 May 1994 have been studied. It is shown that the loops follow
generally the alignment of concentrated magnetic flux. The results
indicate that the soft X-ray emission is low just above the sunspot,
while some loops connecting regions with opposite magnetic polarities
show strong soft X-ray emission. Especially, the part of the loops
near the weaker magnetic field region tends to be brighter than the
one near the stronger magnetic field. The temperature around the top
of the loops is typically ∼3 × 10<SUP>6</SUP> K, which is higher
than that at the legs of the loops by a factor of 1.5-2.0. The density
near the top of the loops is about 5 x 10<SUP>9</SUP> cm<SUP>-3</SUP>,
which is higher than that of the leg parts of the loops. These loops
represent probably the sites where strong magnetic flux and/or current
are concentrated.
---------------------------------------------------------
Title: Lyman continuum as a diagnostic for nonthermal processes in
solar flares.
Authors: Ding, M. D.; Schleicher, H.
1997A&A...322..674D Altcode:
We have computed the Lyman continuum intensities for different flare
circumstances, namely, the temperature rise in the chromosphere, the
downward shift of the transition region, and the nonthermal effect of a
precipitating beam of electrons. All the three factors can enhance the
intensity by various amounts. The third effect additionally lowers the
color temperature of the Lyman continuum. It thus provides a diagnostic
tool to discriminate whether there exists an obvious nonthermal effect
of electron beam bombardment during solar flares. It is also suggested
that the relative magnitudes of Lyman and Balmer continuum enhancements
can be used to infer the energy distribution type of the beam electrons.
---------------------------------------------------------
Title: Influence of non-thermal processes on line asymmetries in
solar flares.
Authors: Ding, M. D.; Fang, C.
1997A&A...318L..17D Altcode:
We make line profile calculations for a flare atmosphere considering
simultaneously the effect of non-thermal excitation and ionization of
the hydrogen atoms caused by precipitating high-energy electrons, and
the effect of possible chromospheric downflows. The results confirm the
earlier finding that a downflow, if confined to the upper chromosphere,
can sometimes produce a blue asymmetry of the Hα line. In addition,
we find that the existence of such non-thermal effects tends to enhance
the blue-asymmetry magnitude. In particular, the larger the energy
flux, or the harder the energy spectrum for the beam electrons, the
easier it will be to produce a blue-asymmetric Hα profile, which is
simultaneously intensified and broadened. Possible reasons which make
the blue asymmetry less popular than the red asymmetry in observations
are discussed.
---------------------------------------------------------
Title: Spectral Characteristics and Mechanisms of Two Types of Solar
White-Light Flares
Authors: Fang, C.; Ding, M. D.
1997tcca.conf..340F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral Diagnostics of the Energetic Particles in Solar Flares
Authors: Fang, C.; Ding, M. D.; Henoux, J. C.; Gan, W. Q.
1996JKASS..29..295F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Parametric study of the continuum emission of white-light
flares.
Authors: Ding, M. D.; Fang, C.
1996A&A...314..643D Altcode:
In this paper, we study the effect of different flare-associated
disturbances on the white-light continuum emission, using a non-LTE
computation code. A systematic study is made to show the variational
behavior of the continuum contrast in dependence on different input
parameters and different locations on the Sun. The results show that
(1) a purely chromospheric temperature rise is not sufficient, under
the assumed physical conditions, to produce a significant continuum
enhancement, especially at visible wavelengths, except for some extreme
cases when the chromospheric temperature is rather high and the event
occurs near the disk limb, (2) a downward shift of the transition
region would have some effect, if the top column mass density becomes
sufficiently large, and (3) the effect of nonthermal electron beam
bombardment is obvious only when the transition region has not suffered
a great shift. For these three cases, the continuum contrast in the
Balmer continuum is much larger than that beyond the Balmer continuum,
implying there is a Balmer jump in the modeled spectra. In some special
cases, the chromospheric temperature rise enhances the absorption
of the photospheric radiation, thus producing a spurious negative
continuum contrast. A case for possible temperature rises around the
temperature minimum region is also considered. The continuum contrast
in the case of enhanced temperature minimum is found to be much flatter
than for the above cases, implying a significant emission process in
the lower atmosphere.
---------------------------------------------------------
Title: The Lyman continuum as a diagnostic for nonthermal processes
in solar flares.
Authors: Ding, M. D.; Schleicher, H.
1996AGAb...12..163D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On a Possible Explanation of Chromospheric Line Asymmetries
of Solar Flares
Authors: Ding, M. D.; Fang, C.
1996SoPh..166..437D Altcode:
We discuss the relationship between the chromospheric downward motions
and the line asymmetries in solar flares by using a simple model. It is
found that both the blue asymmetry and red asymmetry of the Hα line
can be caused by downward motions, as long as the moving material is
confined to different heights in the chromosphere. The Ca II K line,
however, mainly shows a red asymmetry. The results can qualitatively
explain some observations.
---------------------------------------------------------
Title: Chromospheric Evaporation in Four Solar Flares Observed
by YOHKOH
Authors: Ding, M. D.; Watanabe, T.; Shibata, K.; Sakurai, T.; Kosugi,
T.; Fang, C.
1996ApJ...458..391D Altcode:
We present an analysis of the Ca XIX soft X-ray spectra of four
flares observed by the Yohkoh spacecraft. These spectra show evident
blueshifted signatures. Using a two-component fitting technique,
we obtain the time variations of the fitting parameters. The
peak line-of-sight upflow velocities range from 180 to 320 km
s<SUP>-1</SUP>. The durations of upflows with observable velocities
(≥ 100 km s<SUP>-1</SUP>) are found to be 4-10 minutes. Particularly,
for two of the four flares, the emission measure of the blueshifted
component is larger than that of the stationary component at early
times. This fact agrees qualitatively with the predictions of numerical
simulations of chromospheric evaporation. There is a rough correlation
between the nonthermal turbulent velocity and the upflow velocity,
consistent with the results of Fludra et al. We also find that most Ca
xix resonance profiles show no significant shift of the profile peak,
regardless of the blue asymmetry which sometimes appears. The possible
causes of this phenomenon are discussed.
---------------------------------------------------------
Title: Are White-Light Flares Related to High Energy Particles?
Authors: Fang, C.; Ding, M. D.; Hu, J.; Yin, S. Y.
1996mpsa.conf..553F Altcode: 1996IAUCo.153..553F
No abstract at ADS
---------------------------------------------------------
Title: The Improved Solar Tower of Nanjing University and a CCD
Imaging Spectrograph
Authors: Huang, Y. R.; Fang, C.; Ding, M. D.; Ji, G. P.; Gao, X. F.;
Ying, S. Y.; Hu, J.; Xue, Y. Z.
1996gbaa.conf..303H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Spectral Analysis of the White Light Flare on 1974OCT11
Authors: Yin, S. Y.; Fang, C.; Ding, M. D.; Hiei, E.; Fu, W. J.
1995AcASn..36..279Y Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A CCD Imaging Spectrograph in the Improved Solar Tower of
Nanjing-University
Authors: Huang, Y. R.; Fang, C.; Ding, M. D.; Gao, X. F.; Zhu, Z. G.;
Ying, S. Y.; Hu, J.; Xue, Y. Z.
1995SoPh..159..127H Altcode: 1995SoPh..159..127Y
Since 1992 the solar tower telescope of Nanjing University (118°51' E,
32°03' N) as well as its multichannel solar spectrograph, originally
established in 1982, have been reconstructed and a two-channel imaging
spectrograph has been operated successfully. The apertures of the
coelostat and the secondary mirror are both 60 cm. The spherical
objective mirror, having an aperture of 43 cm and a focal length
of 2170 cm, produces a solar image of 20 cm diameter. Two auxiliary
telescopes using a small fraction of the coelostat's aperture were
set up for guiding and Hα monochromatic monitoring. A multichannel
spectrograph can be operated in six wavebands simultaneously. A CCD
imaging spectrograph can be used for data acquisition at Hα and CaII K
line wavebands automatically and simultaneously. The instrument consists
of two CCD cameras, an image processor (SR-151), a personal computer,
and a mechanical scanning device. The principal characteristics of the
instruments are described. Some observational results are presented
as examples.
---------------------------------------------------------
Title: An Unusual Behaviour of the White-Light Flare on 11 October,
1974
Authors: Fang, C.; Yin, S. Y.; Hiei, E.; Ding, M. D.; Fu, Q. J.
1995SoPh..158..387F Altcode:
It was found that in the spectrum of the white-light flare on 11
October, 1974, at the time of continuum maximum, the intensity of
K<SUB>1</SUB> in the CaII K line increased very significantly and
reached nearly half of the continuum intensity. The duration of
this unusual increase is less than 4 min. It seems that existing
semi-empirical models can reproduce neither this characteristic nor
the chromospheric condensation.
---------------------------------------------------------
Title: Analysis of 2-d flare spectra: Velocity fields derived from
Hα line asymmetries
Authors: Ding, M. D.; Fang, C.; Huang, Y. R.
1995SoPh..158...81D Altcode:
We derive a time series of two-dimensional velocity fields for a
flare region on 1992 December 16, based on the asymmetries of the
Hα line. The Hα spectra were obtained by an imaging spectrograph
at the Solar Tower Telescope of Nanjing University. Four sites
with evident chromospheric downflows are found to appear and decay
consecutively in the studied region. The value of maximum velocities
is 30-40 km s<SUP>−1</SUP> and the lifetime of downflows is 2-3
min at these sites. It is also shown that the asymmetries only exist
at the line wing, while the line center has nearly no shifts for
this flare. Finally, we make a discussion on the characteristics of
the velocity distribution and its correlations with the intensity
distribution, as well as with the hard X-ray emission.
---------------------------------------------------------
Title: Asymmetry and Doppler shift of the metallic lines of the
solar flare on 11 October 1974.
Authors: Yin, Suying; Fang, Cheng; Ding, Mingde
1995JNanU..31..221Y Altcode:
On 11 October 1974 an 1N flare occurred near the solar disk center. The
asymmetries and the Doppler shifts of the 41 metallic lines of the
flare are analysed in detail. The results are helpful to understand
the dynamic process in the lower atmosphere of solar flares. It is
proposed that the movements around the temperature minimum region
caused by the flare could explain these observational results.
---------------------------------------------------------
Title: On the spectral characteristics and atmospheric models of
two types of white-light flares.
Authors: Fang, C.; Ding, M. D.
1995A&AS..110...99F Altcode:
Based on detailed analyses of the solar white-light flares (WLFs)
of 1974 September 10, 1979 September 19 and 1991 October 24, the
characteristics of two types of WLFs have been distinguished. For
the Type I WLF, there is a good time correlation between the maximum
of continuum emission and the peaks of hard X-rays and microwave
radiations; there is also a strong Balmer jump in the spectra;
the Balmer lines are strong and very broad. For the Type II WLF,
the situations are quite different. For these two types of WLFs,
the atmospheric models and the contributions to the continuum from
the hydrogen atoms and negative hydrogen ions, as well as the net
radiative cooling rates, have been compared in detail. The possible
mechanisms for the two types of WLFs have also been discussed.
---------------------------------------------------------
Title: VizieR Online Data Catalog: White-light flares models
(Fang+, 1995)
Authors: Fang, C.; Ding, M. D.
1994yCat..41100099F Altcode:
Based on detailed analyses of the solar white-light flares (WLFs)
of 1974 September 10, 1979 September 19 and 1991 October 24, the
characteristics of two types of WLFs have been distinguished. (3
data files).
---------------------------------------------------------
Title: Optical Spectra and Semi-Empirical Model of a White-Light Flare
Authors: Ding, M. D.; Fang, C.; Gan, W. Q.; Okamoto, T.
1994ApJ...429..890D Altcode:
We derive a semi-empirical atmospheric model for the 1979 September 19
white-light flare (WLF), based on the observed continuum at 3590 to
3990 A and the Ca II K line profile. The model has a strongly heated
photosphere, but has a chromospheric structure within the range of
ordinary flare models. According to this model, the white-light
continuum is mainly produced by the H<SUP>-</SUP> emission in
the photosphere. A negative Balmer jump appears in the continuum
spectrum (without considering the line blanketing effect), due to the
Balmer continuum absorption in the chromosphere. From the spectral
features and model results, the traditional heating mechanisms that
are proposed to transport the energy from the corona down to deep
layers seem insufficient to fully explain the photospheric heating
for this WLF. The heating energy may originate within the photosphere
itself. The possible energy release processes are also discussed,
but a satisfactory explanation needs further quantitative studies.
---------------------------------------------------------
Title: Recent progress in the study of white-light flares.
Authors: Fang, Cheng; Ding, Mingde
1994PABei..12..100F Altcode:
This article reviews the recent progress in the study of solar and
stellar white-light flares (WLFs). The following topics are mainly
discussed: (1) spectral features of two different types of WLFs; (2)
semi-empirical models of the WLF atmosphere; and (3) heating mechanisms
of the WLF atmosphere. A brief comparison of stellar WLFs and solar
WLFs is also presented.
---------------------------------------------------------
Title: On the Propagation of Chromospheric Condensations in Solar
Flares (II). Evolution of the Mass Motion Velocity
Authors: Ding, M. D.; Fang, C.
1994Ap&SS.213..247D Altcode:
We study the evolution of the mass motion velocity in the chromospheric
condensation, when it propagates into the deeper atmosphere. The
condensation is represented by a shock-like structure. Its momentum
equation can be solved after some approximations. The computations
are carried out for two cases, i.e., the case that the gas pressure
just behind the condensation front is constant and the case that the
pressure increase at the top of the condensation is constant. The
results show that the duration of the condensation in the second case
is considerably longer than that in the first case. The most evident
difference of the velocity evolution between the two cases appears in
their later phase. A comparison of the results in this paper with the
dynamic simulations indicates that the second case may be closer to
the real situation.
---------------------------------------------------------
Title: On the Propagation of Chromospheric Condensations in Solar
Flares (I). Dynamic Simulations
Authors: Ding, M. D.; Fang, C.
1994Ap&SS.213..233D Altcode:
We simulate dynamically the downward propagation of the chromospheric
condensation, which originates following the chromospheric evaporation
during solar flares. Our attention is concentrated on the lower
part of the atmosphere. The top of the chromosphere (base of the
transition region) is regarded as the top boundary. The condensation
is mimicked by assuming an impulsive pressure increase at the top
boundary. Using such a method, we compute in detail the evolution
process of a condensation. The results show that the condensation can
penetrate into the deeper atmosphere, though it becomes very weak at
the later phase. Moreover, we also discuss the possibility that the
mass motions in the condensation may cause the asymmetries of some
spectral lines as observations have indicated.
---------------------------------------------------------
Title: The Propagation of Chromospheric Condensations and the
Asymmetry of Spectral Lines in Solar Flares
Authors: Ding, M. D.; Fang, C.
1994AcApS..14..172D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Study of the White Light Flare of 1974SEP10
Authors: Hu, J.; Fang, C.; Ding, M. D.; Hiei, E.
1994AcASn..35..396H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Doppler Shifts of Metallic Lines for a White-Light Flare
Authors: Ding, M. D.; Fang, C.; Okamoto, T.
1994SoPh..149..143D Altcode:
In this paper we investigate the Doppler shifts of the metallic lines
for a 3B white-light flare on September 19, 1979. The results show
that there is no systematic shift at the line center, while there may
appear some asymmetry at the line wing. A possible heating process of
this flare is proposed to interpret the observed spectral features.
---------------------------------------------------------
Title: Hα Emission Line Profiles of Solar Flares with Turbulent
Velocity Fields
Authors: Ding, M. D.; Fang, C.
1993SoPh..147..305D Altcode:
The Hα line profile in a flare atmospheric model superposed by a
spatially correlated velocity field is studied in detail in this
paper. The computations are carried out with the assumption that
the velocity field is represented by a Kubo-Anderson process. It
is found that the shape and the intensity of the Hα line profile
depend greatly on the parameters of the velocity field. The variation
of the profile and its relative deviation with respect to different
correlation lengths is more complex than in the case of absorption
lines. It is also shown that such a profile cannot be matched by those
produced in the usually-used micro- and macro-turbulent approaches,
especially for the velocity field with an intermediate correlation
length. The results imply that the flare atmosphere should be highly
turbulent in order to explain the observed Hα line profile with only
weak central reversal in the flare spectra. Particularly, the effects
of meso-turbulent fields should be taken into account in order to
improve the spectral diagnostics and modelling of the flare atmosphere.
---------------------------------------------------------
Title: Ca II K emission line profile in a flare atmosphere with
turbulent velocity fields.
Authors: Ding, Mingde; Fang, Cheng
1993AcASn..34..279D Altcode: 1993AcASn..34..287D
The authors studied in detail the properties of the Ca II K emission
line profile in a flare atmosphere with the existence of turbulent
velocity fields. The results show that (1) the intensity and shape of
the emergent line profile depend strongly on the parameters of the
turbulent field, (2) when the correlation length and the turbulent
velocity are large enough, the theoretical profile has no central
reversal, which can match the observed result, and (3) the profile
in meso-turbulent fields can not be fitted by that in micro- and
macro-turbulent approximations. These results will be useful in
semi-empirical model calculations and spectral analyses of solar flares.
---------------------------------------------------------
Title: Hα and Ca II K line profiles of solar flare in turbulent
velocity fields
Authors: Fang, C.; Ding, M. D.
1993AdSpR..13i.321F Altcode: 1993AdSpR..13..321F
No abstract at ADS
---------------------------------------------------------
Title: Spectral analysis of the 3B flare on September 19, 1979:
asymmetry and Doppler shift of the metal lines.
Authors: Ding, Mingde; Fang, Cheng; Okamoto, T.
1993AcApS..13..245D Altcode:
The asymmetry and Doppler shift of metal lines of the 3B flare on
September 19, 1979 are investigated. The results show that: (1) red
asymmetries dominate in strong metal lines, but blue asymmetries also
exist in some weak lines; (2) the maximum of the line asymmetry always
precedes the maximum of the line intensity; (3) the blue asymmetry
occurs generally in the early phase and can turn to a red one in the
later phase; and (4) the line center has no obvious shift, regardless
of the line asymmetry. It is proposed that the mass motions around
the temperature minimum region caused by the impulsive heating or
the propagation of the chromospheric condensation can explain these
observational results. The study on metal lines can help understanding
the dynamic process in the lower atmosphere of solar flares.
---------------------------------------------------------
Title: Spectral Analysis of the 3b Flare on 1979SEP19 - Asymmetry
and Doppler Shift of the Metal Lines
Authors: Ding, M. D.; Fang, C.; Tomizo, O.
1993AcApS..13..252D Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mean Optical Depth and Contribution Function of Spectral
Lines in a Turbulent Atmosphere
Authors: Ding, M. D.; Fang, C.
1993Ap&SS.204..283D Altcode:
In this paper we present a new definition and its analytic expressions
for the mean optical depth and the mean contribution function
of spectral lines in a turbulent atmosphere. These mean values are
based on the radiative transfer equation and thus satisfy the general
properties of the radiation field. They can be used to study the line
formation process in turbulent atmospheres.
---------------------------------------------------------
Title: Semi-empirical model of the white light flare on September
19, 1979.
Authors: Fang, Cheng; Ding, Mingde; Okamoto, T.; Gan, Weiqun
1993ScChA..36..217F Altcode:
The intensities of the continuum emission and the Ca II Ka line of the
white light flare on Sep 19, 1979 are measured and analyzed. Their
variations with time are given. It is indicated that the continuum
emission of this flare appeared in the early impulsive phase and
lasted for about 5 - 6 min, with the time of maximum intensity 2 -
3 min earlier than that for the microwave radio burst. Based on the
non-LTE theory, a semi-empirical model at a time with the continuum
emission being relatively intensive is presented. The results show that
the temperature in the flare photosphere is increased by 150 - 250K,
and that the continuum emission is produced mainly by the negative
hydrogen ion.
---------------------------------------------------------
Title: Center-to-limb variation of spectral lines and quiet sun
atmospheric model.
Authors: Ding, Mingde; Fang, Cheng
1991ChJSS..11..241D Altcode:
Spectral lines of hydrogen and ionized calcium, as well as their
center-to-limb variations, observed at Kitt Peak National Soalr
Observatory, are used to diagnose the VAL-C', quiet Sun atmospheric
model. The PANDORA computer program is used to perform the detailed
non-LTE model calculations, especially for hydrogen and calcium. The
main discrepanicies between the computed spectral lines and the observed
data are: (1) The residual intensity in Hβ and λ8498 line centers
is evidently higher, while that in Ca II H and K lines are relatively
lower than the observed values. (2) The center-to-limb enhancement
coefficient of the residual intensities of Hβ, and all the Ca II lines
(resonance and infrared lines) near Δλ = 0.5 Å is a little larger
than indicated by the observation. However, all other spectral features
observed can be reproduced fairly well. Finally, the possible causes
which can make the computed values deviate from the oberved ones is
discussed, and a comment on revising the VAL-C' model is given.
---------------------------------------------------------
Title: A semi-empirical model of sunspot penumbra
Authors: Ding, M. D.; Fang, C.
1989A&A...225..204D Altcode:
A new semiempirical model of a sunspot penumbra is presented in this
paper. The model is based on the profiles of H-alpha, H-beta, Ca II H
and K and infrared lines observed at Kitt Peak Observatory on July 3,
1985. Non-LTE calculations are made to obtain the atmospheric model. In
the temperature minimum region there is a notable negative value of the
net radiative loss rate. By analyzing the magnetohydrostatic equilibrium
of the model atmosphere, it is shown that the magnetic force has a
great impact on the distribution of particle densities at and above
the upper chromosphere, whereas it does not change substantially the
emergent intensities of the seven lines except those of the line cores
of Ca II H and K.
