Author name code: fang-c
ADS astronomy entries on 2022-09-14
=author:"Fang, C."
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Title: Decayless longitudinal oscillations of a solar filament
maintained by quasi-periodic jets
Authors: Ni, Y. W.; Guo, J. H.; Zhang, Q. M.; Chen, J. L.; Fang, C.;
Chen, P. F.
Bibcode: 2022A&A...663A..31N
Altcode: 2022arXiv220315660N
Context. As a ubiquitous phenomenon, large-amplitude longitudinal
filament oscillations usually decay in 1-4 periods. Recently, we
observed a decayless case of such oscillations in the corona.
Aims: We try to understand the physical process that maintains the
decayless oscillation of the filament.
Methods: Multiwavelength
imaging observations and magnetograms were collected to study the
dynamics of the filament oscillation and its associated phenomena. To
explain the decayless oscillations, we also performed one-dimensional
hydrodynamic numerical simulations using the code MPI-AMRVAC.
Results: In observations, the filament oscillates without decay with
a period of 36.4 ± 0.3 min for almost 4 h before eruption. During
oscillations, four quasi-periodic jets emanate from a magnetic
cancellation site near the filament. The time interval between
neighboring jets is ∼68.9 ± 1.0 min. Numerical simulations
constrained by the observations reproduced the decayless longitudinal
oscillations. However, it is surprising to find that the period
of the decayless oscillations is not consistent with the pendulum
model.
Conclusions: We propose that the decayless longitudinal
oscillations of the filament are maintained by quasi-periodic jets,
which is verified by the hydrodynamic simulations. More importantly,
it is found that, when it is driven by quasi-periodic jets, the
period of the filament longitudinal oscillations also depends
on the driving period of the jets, not on the pendulum period
alone. With a parameter survey in simulations, we derived a formula
by which the pendulum oscillation period can be derived using the
observed period of decayless filament oscillations and the driving
periods of jets. Movie associated to Fig. 2 is available at https://www.aanda.org
Title: Spectral Diagnostics of Solar Photospheric Bright Points
Authors: Hao, Q.; Fang, C.; Ding, M. D.; Li, Z.; Cao, Wenda
Bibcode: 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 × 1027-2 × 1027 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
× 1010 erg cm-2 s-1. 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: Simulations of solar filament fine structures and their
counterstreaming flows
Authors: Zhou, Y. H.; Chen, P. F.; Hong, J.; Fang, C.
Bibcode: 2020NatAs...4..994Z
Altcode: 2021arXiv210413564Z; 2020NatAs.tmp..110Z
Solar filaments, also called solar prominences when appearing above the
solar limb, are cold, dense materials suspended in the hot tenuous solar
corona, consisting of numerous long, fibril-like threads. These threads
are the key to disclosing the physics of solar filaments. Similar
structures also exist in galaxy clusters. Besides their mysterious
formation, filament threads are observed to move with alternating
directions, which are called counterstreaming flows. However, the origin
of these flows has not been clarified yet. Here we report that turbulent
heating at the solar surface is the key, which randomly evaporates
materials from the solar surface to the corona, naturally reproducing
the formation and counterstreamings of the sparse threads in the solar
corona. We further suggest that while the cold Hα counterstreamings
are mainly due to longitudinal oscillations of the filament threads,
there are million-kelvin counterstreamings in the corona between
threads, which are alternating unidirectional flows.
Title: Damping Mechanisms of the Solar Filament Longitudinal
Oscillations in the Weak Magnetic Field
Authors: Zhang, L. Y.; Fang, C.; Chen, P. F.
Bibcode: 2019ApJ...884...74Z
Altcode: 2019arXiv190807148Z
Longitudinal oscillations of the solar filament have been
investigated via numerical simulations continuously, but mainly
in one dimension, where the magnetic field line is treated as a
rigid flux tube. Whereas those one-dimensional (1D) simulations
can roughly reproduce the observed oscillation periods, implying
that gravity is the main restoring force for filament longitudinal
oscillations, the decay time in 1D simulations is generally longer
than in observations. In this paper, we perform a two-dimensional (2D)
nonadiabatic magnetohydrodynamic simulation of filament longitudinal
oscillations, and compare it with the 2D adiabatic case and 1D adiabatic
and nonadiabatic cases. It is found that, whereas both nonadiabatic
processes (radiation and heat conduction) can significantly reduce the
decay time, wave leakage is another important mechanism to dissipate
the kinetic energy of the oscillating filament when the magnetic field
is weak so that gravity is comparable to Lorentz force. In this case,
our simulations indicate that the pendulum model might lead to an error
of ∼100% in determining the curvature radius of the dipped magnetic
field using the longitudinal oscillation period when the gravity to
Lorentz force ratio is close to unity.
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.
Bibcode: 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: Waiting time distributions of solar and stellar flares:
Poisson process or with memory?
Authors: Li, C.; Zhong, S. J.; Xu, Z. G.; He, H.; Yan, Y.; Chen,
P. F.; Fang, C.
Bibcode: 2018MNRAS.479L.139L
Altcode: 2018MNRAS.tmpL.121L
We present the waiting time statistics of solar flares observed during
the solar cycle 23 and of stellar flares produced by a solar-type star
(KIC 11551430). The Kolmogorov-Smirnov test is applied to examine the
reliability of Poisson hypothesis for the flare production. Different
degrees of departure from a Poisson process have been found, depending
on the flare intensities and levels of star activities. We then
fit the waiting time distributions with both the non-stationary
Poisson function and the Weibull function. It is evident that a
certain amount of memory is present during the period of high solar
activities, consistent with the result of Telloni et al. Furthermore,
our analysis indicates that the occurrence of relatively `weak' flares
are persistently correlated, while the production of `super flares'
is closer to a stochastic process statistically.
Title: Sixty-Year Career in Solar Physics
Authors: Fang, C.
Bibcode: 2018SoPh..293...72F
Altcode:
This memoir reviews my academic career in solar physics for 60 years,
including my research on non-LTE modeling, white-light flares, and
small-scale solar activities. Through this narrative, the reader
can catch a glimpse of the development of solar physics research in
mainland China from scratch. In the end, some prospects for future
development are given.
Title: Three-dimensional MHD Simulations of Solar Prominence
Oscillations in a Magnetic Flux Rope
Authors: Zhou, Yu-Hao; Xia, C.; Keppens, R.; Fang, C.; Chen, P. F.
Bibcode: 2018ApJ...856..179Z
Altcode: 2018arXiv180303385Z
Solar prominences are subject to all kinds of perturbations during
their lifetime, and frequently demonstrate oscillations. The
study of prominence oscillations provides an alternative way to
investigate their internal magnetic and thermal structures because
the characteristics of the oscillations depend on their interplay
with the solar corona. Prominence oscillations can be classified into
longitudinal and transverse types. We perform three-dimensional ideal
magnetohydrodynamic simulations of prominence oscillations along a
magnetic flux rope, with the aim of comparing the oscillation periods
with those predicted by various simplified models and examining the
restoring force. We find that the longitudinal oscillation has a period
of about 49 minutes, which is in accordance with the pendulum model
where the field-aligned component of gravity serves as the restoring
force. In contrast, the horizontal transverse oscillation has a period
of about 10 minutes and the vertical transverse oscillation has a
period of about 14 minutes, and both of them can be nicely fitted
with a two-dimensional slab model. We also find that the magnetic
tension force dominates most of the time in transverse oscillations,
except for the first minute when magnetic pressure overwhelms it.
Title: Automated Detection Methods for Solar Activities and an
Application for Statistic Analysis of Solar Filament
Authors: Hao, Q.; Chen, P. F.; Fang, C.
Bibcode: 2018IAUS..340..101H
Altcode: 2018arXiv180403320H
With the rapid development of telescopes, both temporal cadence and
the spatial resolution of observations are increasing. This in turn
generates vast amount of data, which can be efficiently searched only
with automated detections in order to derive the features of interest
in the observations. A number of automated detection methods and
algorithms have been developed for solar activities, based on the image
processing and machine learning techniques. In this paper, after briefly
reviewing some automated detection methods, we describe our efficient
and versatile automated detection method for solar filaments. It is
able not only to recognize filaments, determine the features such as
the position, area, spine, and other relevant parameters, but also to
trace the daily evolution of the filaments. It is applied to process
the full disk Hα data observed in nearly three solar cycles, and some
statistic results are presented.
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.
Bibcode: 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: Solar Filament Longitudinal Oscillations along a Magnetic
Field Tube with Two Dips
Authors: Zhou, Yu-Hao; Zhang, Li-Yue; Ouyang, Y.; Chen, P. F.; Fang, C.
Bibcode: 2017ApJ...839....9Z
Altcode: 2017arXiv170306560Z
Large-amplitude longitudinal oscillations of solar filaments have
been observed and explored for more than ten years. Previous studies
are mainly based on the one-dimensional rigid flux tube model with
a single magnetic dip. However, it has been noted that there might
be two magnetic dips, and hence two threads, along one magnetic
field line. Following previous work, we intend to investigate the
kinematics of the filament longitudinal oscillations when two threads
are magnetically connected, which is done by solving one-dimensional
radiative hydrodynamic equations with the numerical code MPI-AMRVAC. Two
different types of perturbations are considered, and the difference
from previous works resulting from the interaction of the two filament
threads is investigated. We find that even with the inclusion of the
thread-thread interaction, the oscillation period is modified weakly,
by at most 20% compared to the traditional pendulum model with one
thread. However, the damping timescale is significantly affected by
the thread-thread interaction. Hence, we should take it into account
when applying the consistent seismology to the filaments where two
threads are magnetically connected.
Title: Magnetic Separatrix as the Source Region of the Plasma Supply
for an Active-region Filament
Authors: Zou, P.; Fang, C.; Chen, P. F.; Yang, K.; Cao, Wenda
Bibcode: 2017ApJ...836..122Z
Altcode: 2017arXiv170101526Z
Solar filaments can be formed via chromospheric evaporation followed
by condensation in the corona or by the direct injection of cool
plasma from the chromosphere to the corona. We here confirm with
high-resolution Hα data observed by the 1.6 m New Solar Telescope of
the Big Bear Solar Observatory on 2015 August 21 that an active-region
filament is maintained by the continuous injection of cold chromospheric
plasma. We find that the filament is rooted along a bright ridge in Hα,
which corresponds to the intersection of a magnetic quasi-separatrix
layer with the solar surface. This bright ridge consists of many
small patches whose sizes are comparable to the width of the filament
threads. It is found that upflows originate from the brighter patches
of the ridge, whereas the downflows move toward the weaker patches of
the ridge. The whole filament is composed of two opposite-direction
streams, implying that longitudinal oscillations are not the only
cause of the counterstreamings, and unidirectional siphon flows with
alternative directions are another possibility.
Title: Chirality and Magnetic Configurations of Solar Filaments
Authors: Ouyang, Y.; Zhou, Y. H.; Chen, P. F.; Fang, C.
Bibcode: 2017ApJ...835...94O
Altcode: 2016arXiv161201054O
It has been revealed that the magnetic topology in the solar
atmosphere displays hemispheric preference, i.e., helicity is
mainly negative/positive in the northern/southern hemispheres,
respectively. However, the strength of the hemispheric rule and
its cyclic variation are controversial. In this paper, we apply a
new method based on the filament drainage to 571 erupting filaments
from 2010 May to 2015 December in order to determine the filament
chirality and its hemispheric preference. It is found that 91.6%
of our sample of erupting filaments follows the hemispheric rule of
helicity sign. It is also found that the strength of the hemispheric
preference of the quiescent filaments decreases slightly from ∼97%
in the rising phase to ∼85% in the declining phase of solar cycle 24,
whereas the strength of the intermediate filaments keeps a high value
around 96 ± 4% at all times. Only the active-region filaments show
significant variations. Their strength of the hemispheric rule rises
from ∼63% to ∼95% in the rising phase, and keeps a high value of
82% ± 5% during the declining phase. Furthermore, during a half-year
period around the solar maximum, their hemispheric preference totally
vanishes. Additionally, we also diagnose the magnetic configurations
of the filaments based on our indirect method and find that in our
sample of erupting events, 89% are inverse-polarity filaments with
a flux rope magnetic configuration, whereas 11% are normal-polarity
filaments with a sheared arcade configuration.
Title: Breaking of Magnetic Connectivity and Flare Initial Eruption
Authors: He, H.; Wang, H. N.; Yan, Y.; Chen, P. F.; Fang, C.
Bibcode: 2016AGUFMSH13C2307H
Altcode:
By quantitatively analyzing the 3-D coronal magnetic fields
(reconstructed using the nonlinear force-free field model) associated
with the X3.4-class solar flare event of AR 10930, a prominent
magnetic connectivity with strong negative α (force-free factor)
values and strong current density before the flare is revealed. This
magnetic connectivity extends along the main polarity inversion line
and is found to be totally broken after the flare. A cramped positive
α region appearing in the photosphere is identified as the cause
of the breaking of magnetic connectivity. The comparison with the
flare onset imaging observation exhibits that the breaking site of the
magnetic connectivity coincides with the location of the flare initial
eruption. A scenario for flare initial eruption is therefore proposed,
in which the Lorentz force acting on the isolated electric current at
the magnetic connectivity breaking site lifts the associated plasmas
and causes the initial ejection. The content of this presentation is
based on a published paper (He et al., 2014, JGR-Space Physics, 119,
3286-3315; also see arXiv:1605.00367).
Title: Can a Fast-Mode EUV Wave Generate a Stationary Front?
Authors: Chen, P. F.; Fang, C.; Chandra, R.; Srivastava, A. K.
Bibcode: 2016SoPh..291.3195C
Altcode: 2016arXiv160407982C; 2016SoPh..tmp...89C
The discovery of stationary "EIT waves" about 16 years ago posed a
big challenge to the then favorite fast-mode wave model for coronal
"EIT waves". It encouraged various non-wave models and played an
important role in convergence of the opposing viewpoints toward the
recent consensus that there are two types of EUV waves. However, it was
recently discovered that a stationary wave front can also be generated
when a fast-mode wave passes through a magnetic quasi-separatrix layer
(QSL). In this article, we perform a magnetohydrodynamic (MHD) numerical
simulation of the interaction between a fast-mode wave and a magnetic
QSL, and a stationary wave front is reproduced. The analysis of the
numerical results indicates that near the plasma beta ∼1 layer in
front of the magnetic QSL, part of the fast-mode wave is converted
to a slow-mode MHD wave, which is then trapped inside the magnetic
loops, forming a stationary wave front. Our results imply that we
have to be cautious in identifying the nature of a wave, since there
may be mode conversion during the propagation of the waves driven by
solar eruptions.
Title: Material Supply and Magnetic Configuration of an Active
Region Filament
Authors: Zou, P.; Fang, C.; Chen, P. F.; Yang, K.; Hao, Q.; Cao, Wenda
Bibcode: 2016ApJ...831..123Z
Altcode: 2017arXiv170102407Z
It is important to study the fine structures of solar filaments
with high-resolution observations, since it can help us understand
the magnetic and thermal structures of the filaments and their
dynamics. In this paper, we study a newly formed filament located
inside the active region NOAA 11762, which was observed by the 1.6 m
New Solar Telescope at Big Bear Solar Observatory from 16:40:19 UT to
17:07:58 UT on 2013 June 5. As revealed by the Hα filtergrams, cool
material is seen to be injected into the filament spine with a speed of
5-10 km s-1. At the source of the injection, brightenings
are identified in the chromosphere, which are accompanied by magnetic
cancellation in the photosphere, implying the importance of magnetic
reconnection in replenishing the filament with plasmas from the lower
atmosphere. Counter-streamings are detected near one endpoint of the
filament, with the plane-of-the-sky speed being 7-9 km s-1 in
the Hα red-wing filtergrams and 9-25 km s-1 in the blue-wing
filtergrams. The observations are indicative that this active region
filament is supported by a sheared arcade without magnetic dips, and
the counter-streamings are due to unidirectional flows with alternative
directions, rather than due to the longitudinal oscillations of filament
threads as in many other filaments.
Title: Statistical Analysis of Filament Features Based on the Hα
Solar Images from 1988 to 2013 by Computer Automated Detection Method
Authors: Hao, Q.; Fang, C.; Cao, W.; Chen, P. F.
Bibcode: 2015ApJS..221...33H
Altcode: 2015arXiv151104692H
We improve our filament automated detection method which was proposed
in our previous works. It is then applied to process the full disk
Hα data mainly obtained by the Big Bear Solar Observatory from 1988
to 2013, spanning nearly three solar cycles. The butterfly diagrams
of the filaments, showing the information of the filament area,
spine length, tilt angle, and the barb number, are obtained. The
variations of these features with the calendar year and the latitude
band are analyzed. The drift velocities of the filaments in different
latitude bands are calculated and studied. We also investigate the
north-south (N-S) asymmetries of the filament numbers in total and in
each subclass classified according to the filament area, spine length,
and tilt angle. The latitudinal distribution of the filament number
is found to be bimodal. About 80% of all the filaments have tilt
angles within [0°, 60°]. For the filaments within latitudes lower
(higher) than 50°, the northeast (northwest) direction is dominant
in the northern hemisphere and the southeast (southwest) direction is
dominant in the southern hemisphere. The latitudinal migrations of the
filaments experience three stages with declining drift velocities in
each of solar cycles 22 and 23, and it seems that the drift velocity
is faster in shorter solar cycles. Most filaments in latitudes lower
(higher) than 50° migrate toward the equator (polar region). The
N-S asymmetry indices indicate that the southern hemisphere is the
dominant hemisphere in solar cycle 22 and the northern hemisphere is
the dominant one in solar cycle 23.
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.
Bibcode: 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: 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.
Bibcode: 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: 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.
Bibcode: 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: Waiting Time Distribution of Solar Energetic Particle Events
Modeled with a Non-stationary Poisson Process
Authors: Li, C.; Zhong, S. J.; Wang, L.; Su, W.; Fang, C.
Bibcode: 2014ApJ...792L..26L
Altcode: 2014arXiv1408.2306L
We present a study of the waiting time distributions (WTDs) of solar
energetic particle (SEP) events observed with the spacecraft WIND and
GOES. The WTDs of both solar electron events (SEEs) and solar proton
events (SPEs) display a power-law tail of ~Δt -γ. The SEEs
display a broken power-law WTD. The power-law index is γ1
= 0.99 for the short waiting times (<70 hr) and γ2 =
1.92 for large waiting times (>100 hr). The break of the WTD of
SEEs is probably due to the modulation of the corotating interaction
regions. The power-law index, γ ~ 1.82, is derived for the WTD of
the SPEs which is consistent with the WTD of type II radio bursts,
indicating a close relationship between the shock wave and the
production of energetic protons. The WTDs of SEP events can be modeled
with a non-stationary Poisson process, which was proposed to understand
the waiting time statistics of solar flares. We generalize the method
and find that, if the SEP event rate λ = 1/Δt varies as the time
distribution of event rate f(λ) = Aλ-αexp (- βλ),
the time-dependent Poisson distribution can produce a power-law tail
WTD of ~Δt α - 3, where 0 <= α < 2.
Title: Imaging and Spectroscopic Observations of a Filament Channel
and the Implications for the Nature of Counter-streamings
Authors: Chen, P. F.; Harra, L. K.; Fang, C.
Bibcode: 2014ApJ...784...50C
Altcode: 2014arXiv1401.4514C
The dynamics of a filament channel are observed with imaging and
spectroscopic telescopes before and during the filament eruption on 2011
January 29. The extreme ultraviolet (EUV) spectral observations reveal
that there are no EUV counterparts of the Hα counter-streamings in the
filament channel, implying that the ubiquitous Hα counter-streamings
found by previous research are mainly due to longitudinal oscillations
of filament threads, which are not in phase between each other. However,
there exist larger-scale patchy counter-streamings in EUV along the
filament channel from one polarity to the other, implying that there
is another component of unidirectional flow (in the range of ±10
km s-1) inside each filament thread in addition to the
implied longitudinal oscillation. Our results suggest that the flow
direction of the larger-scale patchy counter-streaming plasma in the
EUV is related to the intensity of the plage or active network, with
the upflows being located at brighter areas of the plage and downflows
at the weaker areas. We propose a new method to determine the chirality
of an erupting filament on the basis of the skewness of the conjugate
filament drainage sites. This method suggests that the right-skewed
drainage corresponds to sinistral chirality, whereas the left-skewed
drainage corresponds to dextral chirality.
Title: Developing an Advanced Automated Method for Solar Filament
Recognition and Its Scientific Application to a Solar Cycle of MLSO
Hα Data
Authors: Hao, Q.; Fang, C.; Chen, P. F.
Bibcode: 2013SoPh..286..385H
Altcode: 2013arXiv1303.6367H
We developed a method to automatically detect and trace solar filaments
in Hα full-disk images. The program is able not only to recognize
filaments and determine their properties, such as the position, the
area, the spine, and other relevant parameters, but also to trace the
daily evolution of the filaments. The program consists of three steps:
First, preprocessing is applied to correct the original images; second,
the Canny edge-detection method is used to detect filaments; third,
filament properties are recognized through morphological operators. To
test the algorithm, we successfully applied it to observations from the
Mauna Loa Solar Observatory (MLSO). We analyzed Hα images obtained
by the MLSO from 1998 to 2009 and obtained a butterfly diagram
of filaments. This shows that the latitudinal migration of solar
filaments has three trends in Solar Cycle 23: The drift velocity was
fast from 1998 to the solar maximum, after which it became relatively
slow. After 2006, the migration became divergent, signifying the
solar minimum. About 60 % of the filaments with latitudes higher than
50∘ migrate toward the polar regions with relatively high
velocities, and the latitudinal migrating speeds in the northern and
the southern hemispheres do not differ significantly in Solar Cycle 23.
Title: Radiative Hydrodynamic Simulations of He I 10830 Å
Authors: Cheng, J. X.; Ding, M. D.; Fang, C.
Bibcode: 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: A new MHD code with adaptive mesh refinement and
parallelization for astrophysics
Authors: Jiang, R. -L.; Fang, C.; Chen, P. -F.
Bibcode: 2012CoPhC.183.1617J
Altcode: 2012arXiv1204.5849J
A new code, named MAP, is written in FORTRAN language for
magnetohydrodynamics (MHD) simulations with the adaptive mesh refinement
(AMR) and Message Passing Interface (MPI) parallelization. There
are several optional numerical schemes for computing the MHD part,
namely, modified Mac Cormack Scheme (MMC), Lax-Friedrichs scheme
(LF), and weighted essentially non-oscillatory (WENO) scheme. All of
them are second-order, two-step, component-wise schemes for hyperbolic
conservative equations. The total variation diminishing (TVD) limiters
and approximate Riemann solvers are also equipped. A high resolution can
be achieved by the hierarchical block-structured AMR mesh. We use the
extended generalized Lagrange multiplier (EGLM) MHD equations to reduce
the non-divergence free error produced by the scheme in the magnetic
induction equation. The numerical algorithms for the non-ideal terms,
e.g., the resistivity and the thermal conduction, are also equipped
in the code. The details of the AMR and MPI algorithms are described
in the paper.
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.
Bibcode: 2012A&A...544L..17H
Altcode: 2012arXiv1211.1751H
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).
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).
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.
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.
Bibcode: 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: Magnetic Reconnection in a Canopy-type Magnetic Configuration
for Solar Microflares
Authors: Jiang, R. -L.; Fang, C.; Chen, P. -F.
Bibcode: 2012EAS....55..107J
Altcode:
We performed 2.5D compressible resistive MHD simulations of magnetic
reconnection with gravity considered. The background magnetic field
is a canopy-type configuration which is rooted at the boundary of the
solar supergranule. By changing the bottom boundary conditions in the
simulations, an emerging flux rises up at the center of the supergranule
and reconnects with the canopy magnetic field. We successfully simulate
the coronal and chromospheric microflares, whose current sheets locate
at the corona and chromosphere respectively. The coronal microflare is
triggered by the reconnection at the corona, whose size and temperature
enhancement is bigger and higher than the chromospheric one. We also
found a hot jet (~1.8 × 106 K) relating to the observational
EUV/SXR jet and a cold jet (~104 K) corresponding to the
observational Hα/Ca surge or brightening in the coronal case. Whereas
there is only Hα/Ca bright point in the chromospheric one.
Title: Foreword
Authors: Faurobert, M.; Fang, C.; Corbard, T.
Bibcode: 2012EAS....55....1F
Altcode:
No abstract at ADS
Title: Numerical Simulation of Solar Microflares in a Canopy-type
Magnetic Configuration
Authors: Jiang, R. -L.; Fang, C.; Chen, P. -F.
Bibcode: 2012ApJ...751..152J
Altcode: 2012arXiv1204.5847J
Microflares are small activities in the solar low atmosphere; some are
in the low corona while others are in the chromosphere. Observations
show that some of the microflares are triggered by magnetic
reconnection between the emerging flux and a pre-existing background
magnetic field. We perform 2.5-dimensional, compressible, resistive
magnetohydrodynamic simulations of the magnetic reconnection
with gravity considered. The background magnetic field is a
canopy-type configuration that is rooted at the boundary of the
solar supergranule. By changing the bottom boundary conditions in
the simulation, a new magnetic flux emerges at the center of the
supergranule and reconnects with the canopy-type magnetic field. We
successfully simulate the coronal and chromospheric microflares
whose current sheets are located at the corona and the chromosphere,
respectively. The microflare with a coronal origin has a larger
size and a higher temperature enhancement than the microflare with a
chromospheric origin. In the microflares with coronal origins, we also
found a hot jet (~1.8 × 106 K), which is probably related
to the observational extreme ultraviolet or soft X-ray jets, and a cold
jet (~104 K), which is similar to the observational Hα/Ca
surges. However, there is only a Hα/Ca bright point in the microflares
that have chromospheric origins. The study of parameter dependence
shows that the size and strength of the emerging magnetic flux are the
key parameters that determine the height of the reconnection location,
and they further determine the different observational features of
the microflares.
Title: Understanding Solar Activity: Advances and Challenges
Authors: Faurobert, M.; Fang, C.; Corbard, T.
Bibcode: 2012EAS....55.....F
Altcode:
No abstract at ADS
Title: A Converging View on EIT Waves
Authors: Chen, P. F.; Fang, C.
Bibcode: 2012EAS....55..313C
Altcode:
Coronal "EIT wave" is a large-scale wavelike phenomenon propagating
across the major part of the Sun. Debates are being continued regarding
a more suitable name for it, its source driver, and its nature. "EIT
waves" have been widely explained in terms of fast-mode waves, despite
that the model contradicts with many observational features. Two-types
of EUV waves were predicted 10 years ago. With the recent high-cadence
observations, mainly from SDO/AIA telescope, more and more evidence
has been revealed in favor of the two-wave model. In this paper, after
summarizing different names used for EIT waves in the literature,
we show how the community are getting close to reaching a consensus
on the nature of EIT waves.
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.
Bibcode: 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: "EIT waves" and coronal mass ejections
Authors: Chen, P. F.; Fang, C.
Bibcode: 2011ASInC...2..229C
Altcode: 2011arXiv1108.5274C
Coronal "EIT waves" appear as EUV bright fronts propagating across
a significant part of the solar disk. The intriguing phenomenon
provoked continuing debates on their nature and their relation with
coronal mass ejections (CMEs). In this paper, we first summarize all
the observational features of "EIT waves", which should be accounted
for by any successful model. The theoretical models constructed during
the past 10 years are then reviewed. Finally, the implication of the
"EIT wave" research to the understanding of CMEs is discussed. The
necessity is pointed out to revisit the nature of CME frontal loop.
Title: History and progress of solar research in China
Authors: Fang, C.
Bibcode: 2011ASInC...2..343F
Altcode:
After a brief introduction of ancient Chinese records on the Sun, we
describe the beginnings of modern solar research in China in the 20th
century. The main contents are focused on the progress of solar research
in China after the 1950s, including the development of solar research
in Purple Mountain Observatory, National Astronomical Observatories,
Yunnan Astronomical Observatory, and some astronomy departments in the
universities. In particular, the constructions of solar observational
facilities, the increase of the numbers of solar researchers and
students, as well as the main topics of solar research since 1980s
are described in details. Some issues and prospects are being discussed.
Title: Numerical Simulations of Chromospheric Microflares
Authors: Jiang, R. L.; Fang, C.; Chen, P. F.
Bibcode: 2010ApJ...710.1387J
Altcode: 2010arXiv1012.0661J
With gravity, ionization, and radiation being considered, we perform
2.5 dimensional (2.5D) compressible resistive magnetohydrodynamic (MHD)
simulations of chromospheric magnetic reconnection using the CIP-MOCCT
scheme. The temperature distribution of the quiet-Sun atmospheric
model VALC and the helium abundance (10%) are adopted. Our 2.5D
MHD simulation reproduces qualitatively the temperature enhancement
observed in chromospheric microflares. The temperature enhancement
ΔT is demonstrated to be sensitive to the background magnetic field,
whereas the total evolution time Δt is sensitive to the magnitude
of the anomalous resistivity. Moreover, we found a scaling law,
which is described as ΔT/Δt ~ n H -1.5 B
2.1η0 0.88. Our results also indicate
that the velocity of the upward jet is much greater than that of the
downward jet, and the X-point may move up or down.
Title: Solar source of energetic particles in interplanetary space
during the 2006 December 13 event
Authors: Li, C.; Dai, Y.; Vial, J. -C.; Owen, C. J.; Matthews, S. A.;
Tang, Y. H.; Fang, C.; Fazakerley, A. N.
Bibcode: 2009A&A...503.1013L
Altcode: 2013arXiv1305.5593L
An X3.4 solar flare and a fast halo coronal mass ejection (CME)
occurred on 2006 December 13, accompanied by a high flux of energetic
particles recorded both in near-Earth space and at ground level. Our
purpose is to provide evidence of flare acceleration in a major solar
energetic particle (SEP) event. We first present observations from
ACE/EPAM, GOES, and the Apatity neutron monitor. It is found that the
initial particle release time coincides with the flare emission and
that the spectrum becomes softer and the anisotropy becomes weaker
during particle injection, indicating that the acceleration source
changes from a confined coronal site to a widespread interplanetary
CME-driven shock. We then describe a comprehensive study of the
associated flare active region. By use of imaging data from HINODE/SOT
and SOHO/MDI magnetogram, we infer the flare magnetic reconnection
rate in the form of the magnetic flux change rate. This correlates in
time with the microwave emission, indicating a physical link between
the flare magnetic reconnection and the acceleration of nonthermal
particles. Combining radio spectrograph data from Huairou/NOAC,
Culgoora/IPS, Learmonth/RSTN, and WAVES/WIND leads to a continuous
and longlasting radio burst extending from a few GHz down to several
kHz. Based on the photospheric vector magnetogram from Huairou/NOAC
and the nonlinear force free field (NFFF) reconstruction method,
we derive the 3D magnetic field configuration shortly after the
eruption. Furthermore, we also compute coronal field lines extending
to a few solar radii using a potential-field source-surface (PFSS)
model. Both the so-called type III-l burst and the magnetic field
configuration suggest that open-field lines extend from the flare
active region into interplanetary space, allowing the accelerated and
charged particles escape.
Title: Coronal Mass Ejection Induced Outflows Observed with Hinode/EIS
Authors: Jin, M.; Ding, M. D.; Chen, P. F.; Fang, C.; Imada, S.
Bibcode: 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: Spectral analysis of a micro-two-ribbon flare
Authors: Fang, C.; Jiang, R. L.; Tang, Y. H.
Bibcode: 2009EGUGA..11.2658F
Altcode:
Using high resolution Halpha, CaII 8542 A and FeI 6302.5 A Stokes
spectral data obtained simultaneously with THEMIS, we analyze
the spectra of the micro-two-ribbon flare (MTRF) on 5th September
2002. The intensity, velocity and longitudinal magnetic field maps are
obtained. The hard X-ray emission observed by RHESSI provides evidence
of non-thermal particle acceleration in the MTRF. Using the Halpha and
CaII 8542 A line profiles and the non-LTE calculation, we obtain the
semi-empirical atmospheric models for two brightest kernels of the
MTRF. The result indicates that the temperature enhancement in the
chromosphere is more than 2500 K. The kinetic and radiative energy of
the MTRF are estimated.
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.
Bibcode: 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: 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.
Bibcode: 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: SMESE (SMall Explorer for Solar Eruptions): A microsatellite
mission with combined solar payload
Authors: Vial, J. -C.; Auchère, F.; Chang, J.; Fang, C.; Gan, W. Q.;
Klein, K. -L.; Prado, J. -Y.; Rouesnel, F.; Sémery, A.; Trottet,
G.; Wang, C.
Bibcode: 2008AdSpR..41..183V
Altcode:
The SMESE (SMall Explorer for Solar Eruptions) mission is a
microsatellite proposed by France and China. The payload of SMESE
consists of three packages: LYOT (a Lyman α imager and a Lyman α
coronagraph), DESIR (an Infra-red Telescope working at 35 80 and
100 250 μm), and HEBS (a High Energy Burst Spectrometer working
in X- and gamma-rays). The scientific objectives of the mission are
shortly presented. We describe the three instrumental packages and the
profile of the mission which accommodates them. With a launch around
2012 2013, the SMESE microsatellite mission will provide a unique
tool for detecting and understanding eruptions (flares and coronal
mass ejections). Observations should start around solar maximum, and
continue in the declining phase of activity, at a time when the Solar
Dynamics Observatory (SDO) should still be operating.
Title: Flare magnetic reconnection and relativistic particles in
the 2003 October 28 event
Authors: Li, C.; Tang, Y. H.; Dai, Y.; Fang, C.; Vial, J. -C.
Bibcode: 2007A&A...472..283L
Altcode: 2007arXiv0706.3648L
An X17.2 solar flare occurred on 2003 October 28, accompanied by
multi-wavelength emissions and a high flux of relativistic particles
observed at 1 AU. We present the analytic results of the TRACE, SOHO,
RHESSI, ACE, GOES, hard X-ray (INTEGRAL satellite), radio (Ondeřejov
radio telescope), and neutron monitor data. It is found that the
inferred magnetic reconnection electric field correlates well with the
hard X-ray, gamma-ray, and neutron emission at the Sun. Thus the flare's
magnetic reconnection probably makes a crucial contribution to the
prompt relativistic particles, which could be detected at 1 AU. Since
the neutrons were emitted a few minutes before the injection of protons
and electrons, we propose a magnetic-field evolution configuration to
explain this delay. We do not exclude the effect of CME-driven shock,
which probably plays an important role in the delayed gradual phase
of solar energetic particles.
Title: Reconstructing Spherical Nonlinear Force-free Field in the
Solar Corona
Authors: Song, M. T.; Fang, C.; Zhang, H. Q.; Tang, Y. H.; Wu, S. T.;
Zhang, Y. A.
Bibcode: 2007ApJ...666..491S
Altcode:
We present a spherical nonlinear force-free field (NFFF) reconstructing
method based on the photospheric vector magnetograms. The importance of
this method is its ability to reveal the NFFF configurations, which is
necessary for understanding the physical mechanisms of the initiation
of the large-scale solar eruptions, such as coronal mass ejections
and sympathetic flares. Using smooth continuous functions, the basic
NFFF-governing partial differential equations in spherical coordinates
reduce to a set of tractable ordinary differential equations. The
numerical scheme used in this paper is similar to the recent local
nonlinear force-free one developed by Song and coworkers. To illustrate
this method, we give two test examples. One is to compute a well-known
NFFF analytical solution given by Low & Lou. The other is for two
active regions NOAA 10486 and NOAA 10488 observed on 2003 October
29. The results show that the transequatorial magnetic loops are
revealed and coincided with some EUV Imaging Telescope loops.
Title: Statistical study on filament eruption caused by emerging flux
Authors: Xu, X. Y.; Fang, C.; Chen, P. F.
Bibcode: 2007AcASn..48..181X
Altcode:
Observations indicated that solar coronal mass ejections
(CMEs) are strongly associated with reconnection-favored flux
emergence. As suggested by observations, two types of emerging flux
with reconnection-favored direction can trigger filament eruptions
(and then CMEs): one is within the filament channel, and the other
is on the outer edge of the channel. Based on numerical simulations,
a physical model of the emerging flux trigger mechanism for CMEs is
proposed, which explained well the observational phenomena. Using
2.5 dimensional numerical simulations with the gravity and heat
conduction being omitted, the eruption and non-eruption regimes are
presented in parameter space, which indicate that whether a CME can be
triggered depends on both the amount and the location of an emerging
flux, besides its polarity orientation. Furthermore, by analyzing15
eruption events in 2002 and 2003 and 44 non-eruption events in 2002,
the relation between filament eruptions (and CMEs) and the properties
of emerging flux including its polarity orientation, its location,
and the amount of the unsigned flux is studied statistically. The
results show that not all the emerging flux can make a filament lose
equilibrium and then trigger the onset of a CME. The statistic results
basically support the theoretical numerical simulations, in which the
2.5 dimensional time dependent compressible resistive MHD equations
are numerically solved with a multi-step implicit scheme. This research
provides useful information for the space weather forecast.
Title: Sunspot Chromospheric Heating by Kinetic Alfvén Waves
Authors: Wu, D. J.; Fang, C.
Bibcode: 2007ApJ...659L.181W
Altcode:
Sunspot atmospheric models show that sunspots have a higher temperature
than the surrounding quiet Sun in the upper chromosphere, although
they are dark in the photosphere. This Letter presents a comparison
between acoustic wave heating and kinetic Alfvén wave (KAW) heating
in sunspots from the photosphere through the chromosphere based on
a semiempirical model calculated by non-LTE procedure. The result
suggests that acoustic wave heating is still a possible dominating
mechanism in the photosphere and in the lower chromosphere below 850
km, similar to previous works. But in the upper chromosphere above 850
km, KAW heating is a more promising candidate that dominates sunspot
chromospheric heating. We speculate that this probably relates to the
ionization exceeding one in a thousand in the upper chromosphere, so
that the plasma processes such as KAW dissipation play an important
role in the atmospheric dynamics.
Title: SMESE: A SMall Explorer for Solar Eruptions
Authors: Vial, J. -C.; Auchère, F.; Chang, J.; Fang, C.; Gan, W. Q.;
Klein, K. -L.; Prado, J. -Y.; Trottet, G.; Wang, C.; Yan, Y. H.
Bibcode: 2007AdSpR..40.1787V
Altcode:
The SMall Explorer for Solar Eruptions (SMESE) mission is a
microsatellite proposed by France and China. The payload of SMESE
consists of three packages: LYOT (a Lyman α imager and a Lyman α
coronagraph), DESIR (an Infra-Red Telescope working at 35-80 and
100-250 μm), and HEBS (a High-Energy Burst Spectrometer working in
X- and γ-rays). The status of research on flares and coronal mass
ejections is briefly reviewed in the context of on-going missions such
as SOHO, TRACE and RHESSI. The scientific objectives and the profile of
the mission are described. With a launch around 2012-2013, SMESE will
provide a unique tool for detecting and understanding eruptions (flares
and coronal mass ejections) close to the maximum phase of activity.
Title: Spectral analysis of a two-ribbon microflare
Authors: Xia, C.; Fang, C.; Chen, Q. R.; Tang, Y. H.
Bibcode: 2007AdSpR..39.1402X
Altcode:
Using high-resolution Hα, CaII 8542 Å and FeI 6302.5 Å Stokes
spectral data obtained simultaneously with THEMIS in 2002 September,
we have analyzed the spectra and the characteristics of a two-ribbon
microflare (MF). The hard X-ray emission provides evidence of
non-thermal particle acceleration in the microflare. The two-ribbons are
located on either sides of the magnetic polarity inversion line. The
non-thermal characteristics mainly appeared at the outer edges of the
flare ribbons. It indicates that the instantaneous magnetic reconnection
and the particle acceleration mainly took place at the outer edges
of the flare ribbons. Using the Hα and CaII 8542 Å line profiles
and the non-LTE calculation, we obtain the semi-empirical atmospheric
model for the bright kernel of the MF. The result indicates that the
temperature enhancement in the chromosphere is about 2000-2500 K.
Title: The acceleration characteristics of solar energetic particles
in the 2000 July 14 event
Authors: Li, C.; Tang, Y. H.; Dai, Y.; Zong, W. G.; Fang, C.
Bibcode: 2007A&A...461.1115L
Altcode: 2006astro.ph..9682L
Aims:In large gradual solar energetic particle (SEP) events, especially
the ground-level enhancement (GLE) events, where and how energetic
particles are accelerated is still a problem.
Methods: By
using imaging data from TRACE, Yohkoh/HXT, SOHO/MDI and SOHO/EIT,
along with the data from the GOES, Apatity NM, and SOHO/LASCO CME
catalog, the evolution of the X5.7 two-ribbon flare and the associated
SEP event on 14 July 2000 are studied.
Results: It is found
that the magnetic reconnection in this event consists of two parts,
and the induced electric field E_rec is temporally correlated with
the evolution of hard X-ray and γ-ray emission. In particular,
the first hard X-ray and γ-ray emission peak occurred at 10:22 UT,
corresponding to the magnetic reconnection in the western part of
the flare ribbons and the maximum E_rec of ~9.5 V/cm; the second
emission peak at 10:27 UT, corresponding to the eastern part and
the maximum E_rec of ~13.0 V/cm. We also analyze the SEP injection
profiles as functions of time and CME-height, and find two-component
injection which may result from different acceleration mechanisms.
Conclusions: .A reasonable conclusion is that reconnection electric
field makes a crucial contribution to the acceleration of relativistic
particles and the impulsive component of the large gradual SEP event,
while CME-driven shocks play a dominant role in the gradual component.
Title: Evolution of electron energy spectrum during solar flares
Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D.
Bibcode: 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: Magnetic reconnection configurations and particle acceleration
in solar flares
Authors: Chen, P. F.; Liu, W. J.; Fang, C.
Bibcode: 2007AdSpR..39.1421C
Altcode:
Numerical simulations of two types of flares indicate that magnetic
reconnection can provide environments favorable for various particle
acceleration mechanisms to work. This paper reviews recent test particle
simulations of DC electric field mechanism, and discusses how the flare
particles can escape into the interplanetary space under different
magnetic configurations.
Title: A New and Fast Way to Reconstruct a Nonlinear Force-free
Field in the Solar Corona
Authors: Song, M. T.; Fang, C.; Tang, Y. H.; Wu, S. T.; Zhang, Y. A.
Bibcode: 2006ApJ...649.1084S
Altcode:
We reexamine the method of upward integration of a nonlinear force-free
field (NFFF), which is, as is well known, an ill-posed problem. It
can be modified to a well-posed one by the following means: instead of
using finite difference to express partial derivatives, we use smooth
continuous functions to approach magnetic field values, write down
three field components consisting of amplitude functions multiplying
morphology functions, and reduce four basic NFFF equations to ordinary
differential ones. They are then solved in an asymptotic manner
(zeroth-order, first-order, etc.). Considering the physical meaning of
α, we found a self-consistent compatibility condition for the boundary
values. Furthermore, a computation algorithm is proposed, similar to
the usual time-dependent two-dimensional MHD simulation scheme. This
algorithm is steady and robust against the noise in the magnetic field
(in particular, the transverse field) measurement and is able to
deal with concentrated photospheric currents. The algorithm runs very
fast on an ordinary PC and lasts only 6 minutes for the 80×60 (x×y)
mesh up to a height of 80 (= 216,000 km~0.3 Rsolar). So it
provides a powerful tool for solar scientists to analyze the magnetic
field properties of solar active regions and to make predictions of
solar activity.
Title: First Results of Mg I (3p 1P1-4d
1D2) Line Linear Impact Polarization during
the Solar Flare on 2001 June 15
Authors: Xu, Z.; Henoux, J. C.; Chambe, G.; Petrashen, A. G.; Fang, C.
Bibcode: 2006ApJ...650.1193X
Altcode:
The results of the analysis of the first spectropolarimetric
observations of the 3p 1P1-4d
1D2 Mg I line at 5528.4 Å made during a solar
flare are presented in this paper. The line is found to be polarized
with a polarization degree at the line center that reaches up to 3%
and a direction of polarization nearly parallel to the local transverse
magnetic field. After eliminating scattering, the Zeeman effect, and the
intensity gradient as possible origins of the observed polarization,
this polarization is interpreted as due either to a low-energy proton
beam or to the return current associated with electron beams.
Title: On the CME velocity distribution
Authors: Chen, A. Q.; Chen, P. F.; Fang, C.
Bibcode: 2006A&A...456.1153C
Altcode:
Context: .Coronal mass ejections (CMEs) are often categorized into
flare-associated and filament-associated types, which logically is
incomplete since there are many CMEs of the intermediate type.
Aims: .With this new classification, this paper aims to reexamine
whether flare-associated CMEs and filament eruption-associated
CMEs have distinct velocity distributions and to investigate which
factors may affect the CME velocities.
Methods: .We divide
the CME events observed from 2001-2003 into three types, i.e., the
flare-associated type, the filament eruption-associated type, and the
intermediate type. The magnetic environments of the source regions,
e.g., the polarity orientation, the chirality of the filaments, etc.,
are examined.
Results: .Our results indicate that the P-value
of the likelihood between the flare-associated and the filament
eruption-associated CMEs is as high as 0.79, which strongly suggests
that they are a continuum of events rather than two distinct types. For
the filament eruption-associated CMEs, the speeds are found to be
strongly correlated with the average magnetic field in the filament
channel. It is also found that there is a slight tendency for the
filaments with the minority chirality to have weaker magnetic fields,
and hence the corresponding CMEs have smaller eruption speeds. A slight
tendency is also found for the CMEs associated with non-active region
filaments to have higher eruption speeds than those with active region
filaments. However, the polarity orientation of the filament channel
has little effect on the eruption speed.
Title: A Fast Extrapolation Method for Reconstructing the Coronal
Nonlinear Force-free Field
Authors: Song, M. T.; Fang, C.; Tang, Y. H.; Wu, S. T.; Zhang, Y. A.
Bibcode: 2006IAUJD...3E..51S
Altcode:
Introduction: We propose an improved method of upward integration
to reconstruct the nonlinear force-free field (NFFF) in the solar
corona. Method: The method of upward integration can be modified to a
well-posed one by the following ways: instead of using finite difference
to express partial derivatives, we use smooth continuous functions
to approach magnetic field. Three field components are expressed by
amplitude functions multiplying morphology functions, and the four basic
NFFF equations can be reduced to ordinary differential ones. They are
then solved in an asymptotic manner. Considering the physical meaning
of alpha, we found a self-consistent compatibility condition for the
boundary values. Furthermore, we propose a computation algorithm,
similar to the usual time-dependent 2D-MHD simulation scheme. Result:
This algorithm is steady and not sensitive to the noise in the magnetic
field (in particular the transverse field) measurement, and is able to
deal with concentrated photospheric currents. Our code runs very fast
in an usual PC-computer and lasts only 6 minutes for the mesh 80 × 60
(x : y) up to a height of 80 ( 216000 km). Discussion: Our new method
provides a powerful tool to analyze the magnetic field property of solar
active regions, and is useful for the prediction of solar activities.
Title: Spectral Analysis of Ellerman Bombs
Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F.
Bibcode: 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 1026 to 5×1027 ergs. According
to the characteristics of EBs, we tentatively suggest that EBs could
be called ``submicroflares.''
Title: Two candidate homologous CMEs on 2002 May 22
Authors: Cheng, J. X.; Fang, C.; Chen, P. F.
Bibcode: 2006AdSpR..38..470C
Altcode:
Two coronal mass ejections (CMEs) occurred on 2002 May 22, originating
from the same active region, AR 9948. Multi-wavelength data are
collected in order to clarify the relationship between the CMEs,
the associated flares and filament eruptions, and some other magnetic
activities, which is of great importance to understand the mechanism
of each phenomenon. It is tentatively suggested that the two CMEs are
probably homologous.
Title: A brief introduction to SMESE mission
Authors: Wang, C.; Fang, C.; Gan, W.; Prado, J. -Y.; Trottet, G.;
Vial, J. -C.; Yan, Y.; Auchere, F.; Chang, J.; Molodi, G.
Bibcode: 2006ilws.conf..211W
Altcode:
Small Exploration for Solar Eruptions (SMESE) is a joint mission
between France and China to investigate the two main types of eruption
events on the Sun: Coronal Mass Ejections (CME) and solar flares,
and their relationship. SMESE will provide a set of unprecedented
and complementary measurements including Ly-alpha imager, Ly-alpha
coronagraph, EUV imager, Detection of Solar Infra red radiation,
Hard X-ray/gamma ray spectrometry. SMESE aims to study, among others,
the CME triggering mechanism and its acceleration in the corona, the
particle acceleration by CME and solar flare, the physical association
of the CME and solar flare etc. SMESE will be launched in the next
solar maximum between 2010-2012.
Title: Why are there stationary EIT wave fronts
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2006AdSpR..38..456C
Altcode:
EIT waves are often observed to be propagating EUV enhancements followed
by an expanding dimming region after the launch of CMEs. It was widely
assumed that they are the coronal counterparts of the chromospheric
Moreton waves, though the former are three or more times slower. The
existence of a stationary “EIT wave” front in some events,
however, posed a big challenge to the wave explanation. Simulations
are performed to reproduce the stationary “EIT wave” front, which
is exactly located near the footpoint of the magnetic separatrix,
consistent with observations. The formation of the stationary front
is explained in the framework of our model where “EIT waves” are
supposed to be generated by successive opening of the field lines
covering the erupting flux rope in CMEs.
Title: Relation between the CME Velocity and the Magnetic Field
Authors: Chen, A. Q.; Chen, P. F.; Fang, C.
Bibcode: 2006cosp...36..426C
Altcode: 2006cosp.meet..426C
The velocities of Coronal mass ejections CMEs range from tens to more
than 2000 km s Earlier researches tended to fit them into a bimodal
distribution with the fast ones corresponding to flares and the slow
ones to filament eruptions However recent observations do not favor
such a classification and tend to imply that magnetic reconnection
plays an important role in the eruption of CMEs According to the
reconnection model the velocity of the ejecta is proportional to the
magnetic field strength in the inflow region We present a statistical
analysis of CME speeds with relation to the magnetic parameters It
is found that the CME velocity correlation with the average magnetic
field is significantly higher than with the total magnetic flux in
the source region It is also seen that filaments with the minority
chirality tend to erupt as slow CMEs
Title: SMESE, a SMall Explorer for the Study of solar Eruptions
Authors: Vial, J. -C.; Auchère, F.; Chang, J.; Fang, C.; Gan, W. Q.;
Molodij, G.; Prado, J. -Y.; Trottet, G.; Wang, C.; Yan, Y. H.
Bibcode: 2006cosp...36.3287V
Altcode: 2006cosp.meet.3287V
The SMESE Small Explorer for the Study of solar Eruptions mission is
a microsatellite proposed by France and China The payload of SMESE
consists of three packages LYOT a suite of two UV and EUV imagers and
a Lyman alpha coronagraph DESIR an Infra-Red Telescope working at 35
and 150 mu and HEBS a High Energy Burst Spectrometer working in X-
and gamma -rays The status of research on Coronal Mass Ejections and
flares will be briefly recalled in the context of on-going missions
such as SOHO TRACE and RHESSI The scientific objectives and the profile
of the mission will be described With a launch around 2011 SMESE will
provide a unique tool for detecting and understanding eruptions flares
and coronal mass ejections in the maximum phase of activity when the
Solar Dynamics Observatory SDO should still be operating
Title: Bright points in UV continuum in a M9.1 flare
Authors: Wang, L.; Fang, C.; Ding, M. D.
Bibcode: 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: SMESE: a Small Explorer for the Study of Solar Eruptions
Authors: Vial, J. -C.; Auchère, F.; Chang, J.; Fang, C.; Gan, W. Q.;
Molodij, G.; Prado, J. -Y.; Trottet, G.; Wang, C.; Yan, Y. H.
Bibcode: 2006cosp...36.3294V
Altcode: 2006cosp.meet.3294V
The SMESE Small Explorer for the Study of solar Eruptions mission is
a microsatellite proposed by France and China The payload of SMESE
consists of three packages LYOT a suite of two UV and EUV imagers and
a Lyman alpha coronagraph DESIR an Infra-Red Telescope working at 35
and 150 mu and HEBS a High Energy Burst Spectrometer working in X- and
gamma -rays The status of research on Coronal Mass Ejections will be
briefly recalled in the context of on-going missions such as SOHO TRACE
and RHESSI The scientific objectives and the profile of the mission
will be described With a launch around 2011 SMESE will provide a unique
tool for detecting and understanding eruptions flares and coronal mass
ejections in the maximum phase of activity and for the determination
of the morphology of the low corona from where the solar wind originates
Title: Multiwavelength analysis of the impact polarization of 2001
June 15 solar flare
Authors: Xu, Z.; Henoux, J. C.; Chambe, G.; Fang, C.
Bibcode: 2006cosp...36...49X
Altcode: 2006cosp.meet...49X
The linear polarization of H alpha H beta and MgI lines have been
found during the rise phase of soft X-ray emission of an M6 3 flare on
June 15th 2001 observed by THEMIS telescopes in the multi-wavelength
spectropolarimetric mode Here the linear polarization signals of MgI
line 5528 A are reported for the first time The polarization of these
three lines has a good spatial correspondence and is located at the
edges of flare kernels The maximum polarization degree of H alpha and
H beta lines can reach 4 - 6 at the line center and near line wings
For MgI line the polarization degree can exceed 3 5 concentrating
in the line center Polarization directions are not random but either
parallel or perpendicular to the local transverse magnetic field in
a large degree The origin of the observed short-time polarization is
discussed here with respect to the bombardment on the solar atmosphere
by low-energy protons or high-energy electrons associated with return
current
Title: Statistical study of filament eruptions with emerging flux
Authors: Xu, X. Y.; Fang, C.; Chen, P. F.; Ding, M. D.
Bibcode: 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: Magnetic reconnection and electron acceleration in the solar
lower atmosphere
Authors: Fang, C.; Xu, X. Y.; Chen, P. F.
Bibcode: 2006cosp...36..203F
Altcode: 2006cosp.meet..203F
Using high-resolution spectral data obtained with THEMIS on 2002
September 5 we have computed the semi-empirical atmospheric models of
Ellerman bombs EBs and microflares MFs Our results indicate that for
both of them there is a temperature bump up in the low-chromosphere
Compared to the quiet-Sun atmosphere the temperature bump up of the
bright MFs is about 2200K while that of the conspicuous EBs is about
1300K and locates deeper than that for MFs The temperature bump up can
be produced by the magnetic reconnection in the solar lower atmosphere
Assuming electrons being accelerated during the magnetic reconnection
we computed the non-thermal semi-empirical atmospheric models of EBs
and MFs which can well reproduce the observed H alpha and CaII 8542
AA lines The parameters and the energy deposit of the electron beams
have been discussed
Title: Ellerman Bombs: Sub-Microflares in the Solar Atmosphere
Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F.
Bibcode: 2006apri.meet...32F
Altcode:
No abstract at ADS
Title: Magnetic reconnection configurations and particle accelerations
on the Sun
Authors: Chen, P. F.; Liu, W. J.; Fang, C.
Bibcode: 2006cosp...36.3345C
Altcode: 2006cosp.meet.3345C
The electric field near the small-scale reconnection site has been
suggested to be a plausible mechanism for the nonthermal particles
that produce type III radio bursts and hard X-ray emissions in the
corona and chromosphere It has been demonstrated that the magnetic
configuration near the reconnection site plays an important role in
determining the final energy spectrum of the particles In this paper we
first review different reconnection configurations in the Sun which are
associated with various eruptive phenomena such as two-ribbon flares
emerging flux coronal loop-loop interactions and so on We then perform
test particle simulations with these magnetic configurations in order
to investigate their effect on the energy spectrum of the accelerated
particles Their application to observations is discussed
Title: Investigation of Electron Energy Spectrum During Solar Flares
Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D.
Bibcode: 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.
Bibcode: 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: Multiwavelength Analysis of the Impact Polarization of 2001
June 15 Solar Flare
Authors: Xu, Z.; Hénoux, J. -C.; Chambe, G.; Karlický, M.; Fang, C.
Bibcode: 2005ApJ...631..618X
Altcode:
We report here on the temporal and spatial evolution of the
impact polarization of the Hα and Hβ lines during an M6.3 solar
flare observed on 2001 June 15 with the THEMIS telescope in the
multiwavelength spectropolarimetric mode. Typical spectral intensity
and polarization profiles are presented. Both lines are linearly
polarized. The Hαline degree of polarization exceeds 4% at line
center and in the near line wings. The Hβ line is also linearly
polarized, with a degree of polarization reaching 6%. The directions
of polarization are either parallel or perpendicular to the local
transverse magnetic field (i.e., either radial or tangential because the
transverse magnetic field is directed almost in the flare-to-disk center
direction). However, contrary to Hα, the Hβ polarization direction
is radial only. The Hα and Hβ polarization islands are located at the
edges of flare kernels. Only for radial polarization are these islands
cospatial. No Hβ polarization is found at the places where tangential
Hα polarization is present. The origin of the observed polarization is
discussed. Bombardment by low-energy protons or high-energy electrons
associated with return currents can explain the radial polarization
observed in the lowest flare kernel. The tangential Hα polarization
observed in the surge near the upper flare location is interpreted
as due to the electric current at the origin of the electromagnetic
force that lifts the surge.
Title: A Full View of EIT Waves
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2005ApJ...622.1202C
Altcode:
Early observations by the EUV Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory indicated that propagating diffuse wave fronts,
now conventionally referred to as ``EIT waves,'' can often be seen on
the solar disk with a propagation velocity several times smaller than
that of Hα Moreton waves. They are almost always associated with
coronal mass ejections. We have previously confirmed the existence
of such a wave phenomenon with numerical simulations, which indicate
that there does exist a slower moving ``wave'' much behind the coronal
counterpart of the Hα Moreton wave. Further observations have disclosed
many new features of the EIT waves: the waves stop near the separatrix
between active regions, sometimes they experience acceleration from
the active region to the quiet region, and so on. Here we report on
MHD simulations performed to demonstrate how the typical features of
EIT waves can all be accounted for within our theoretical model, in
which the EIT waves are thought to be formed by successive stretching
or opening of closed field lines driven by an erupting flux rope. The
relationship between EIT waves, Hα Moreton waves, and type II radio
bursts is discussed, with an emphasis on reconciling the discrepancies
among different views of the ``EIT wave'' phenomenon.
Title: On the formation of the He I 10 830 Å line in a flaring
atmosphere
Authors: Ding, M. D.; Li, H.; Fang, C.
Bibcode: 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.
Bibcode: 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: EIT waves -- A signature of global magnetic restructuring
in CMEs
Authors: Chen, P. F.; Fang, C.
Bibcode: 2005IAUS..226...55C
Altcode:
The discovery of "EIT waves" after the launch of SOHO spacecraft sparked
wide interest among the coronal mass ejection (CME) community since
they may be crucial to the understanding of CMEs. However, the nature
of this phenomenon is still being hotly debated between fast-mode wave
explanation and non-wave explanation. Accumulating observations have
shown various features of the "EIT waves". For example, they tend to
be devoid of magnetic neutral lines and coronal holes; they may stop
near the magnetic separatrix between the source region and a nearby
active region; they may experience an acceleration from the vicinity of
the source active region to the quiet region, and so on. This paper is
aimed to review all these features, discuss how these observations may
provide constraints for the theoretical models, and point out their
implication to the understanding of CMEs.
Title: Information on particle acceleration and transport derived
from solar flare spectropolarimetry
Authors: Xu, Z.; Hénoux, J. -C.; Chambe, G.; Karlický, M.; Fang, C.
Bibcode: 2005AdSpR..35.1841X
Altcode:
The hydrogen H α line has been found to be linearly polarized at
some locations and times during a June 15th 2001 flare observed with
THEMIS. This flare was accompanied by radio pulses and hard X-ray
emission. Linear polarization is below the noise level in the flare
kernels. However, it is present at the edges of these kernels, in
the line center and near wings where the polarization degree exceeds
4%. The directions of polarization are not random but close within
±15° to the tangential and radial directions. This polarization can
be due either to electron beams and their associated return currents
or to electron and proton beams.
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.
Bibcode: 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: Observations of an active region filament
Authors: Zong, W. G.; Tang, Y. H.; Fang, C.; Xu, A. A.
Bibcode: 2005AdSpR..35.1752Z
Altcode:
An active region filament was well observed on September 4, 2002 with
THEMIS at the Teide observatory and SOHO/MDI. The full Stokes parameters
of the filament were obtained in Hα and FeI 6302 Å lines. Using
the data, we have studied the fine structure of the filament and
obtained the parameters at the barb endpoints, including intensity,
velocity and longitudinal magnetic field. Our results indicate: (a)
the Doppler velocities are quiet different at barb endpoints; (b)
the longitudinal magnetic fields at the barb endpoints are very weak;
(c) there is a strong magnetic field structure under the filament spine.
Title: Spectropolarimetric diagnostics of non-thermal particles in
the solar chromosphere
Authors: Xu, Z.; Fang, C.; Henoux, J. -C.
Bibcode: 2004cosp...35..278X
Altcode: 2004cosp.meet..278X
Linear impact polarization of chromospheric lines has already
been observed in the impulsive phase of solar flares, which
can be interpreted as the results of the bombardment of an
anisotropic non-thermal particle beam in the atmosphere. Using the
spectropolarimetric observations of THEMIS, we have measured the
degrees and the directions of the polarization from Hα and CaII
8542 Stokes line profiles of Ellerman Bombs, bright points and faint
flares, which occur mainly in the solar lower atmosphere. We attempt
to explore the main characters of the non-thermal energetic particles
from its effect on the impact linear polarization of the chromospheric
lines. The existence of this polarization due to non-thermal particles
can provide the evidence of reconnection in the solar lower atmosphere.
Title: Why are there stationary EIT wave fronts?
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2004cosp...35..276C
Altcode: 2004cosp.meet..276C
EIT waves are nearly circularly propagating emission enhancements
followed by expanding dimming regions, which have been found to be
closely related with coronal mass ejections. They are generally believed
to correspond to some kind of wave phenomenon. We have explained the
EIT waves as the propagating structures associated with the gradual
opening of coronal mass ejections. However, occasionally a stationary
front can be observed for several hours, which prompted the doubt
about whether the so-called "EIT waves" are pseudo-phenomena. Through
numerical simulations, this paper illustrates how a propagating EIT
wave stops to form the stationary front when it meets another active
region or a coronal hole.
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.
Bibcode: 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: Two homologous CMEs on 2002 May 22
Authors: Chen, J. X.; Fang, C.; Chen, P. F.
Bibcode: 2004cosp...35..233C
Altcode: 2004cosp.meet..233C
Two coronal mass ejections (CMEs) occurred on 2002 May 22, originating
from the same active region, AR 9948. Multi-wavelength data, including
LASCO, EIT, MDI/SOHO, GOES and Hαimages from National Observatory of
China, have been collected in order to clarify the relation between
the CMEs, the associated flares and filament eruptions, and some other
magnetic activities, which is of great importance to understand the
mechanism of each phenomenon. It is proposed that an emerging flux
may trigger the occurrence of the first CME, which in turn drives
the eruption of the second CME, i.e., the two homologous CMEs are
sympathetic. It is also confirmed that the speeds of CMEs are not
positively correlated with the soft X-ray peak flux of the associated
flares. The timing of these phenomena is also given.
Title: What have we learned on non-thermal particle acceleration
and transport by optical spectropolarimetry
Authors: Hénoux, J. -C.; Karlicky, M.; Xu, Z.; Fang, C.
Bibcode: 2004cosp...35..647H
Altcode: 2004cosp.meet..647H
Most of the information on non-thermal particles is derived from
hard X-ray, gamma-ray or radio observations. Optical spectroscopic
polarimetry provides complementary information on the particle nature
and velocity distributions. Information is also provided on their
propagation conditions at chromospheric level. We will report here
on the information derived from measurements of the impact linear
polarization made in the Halpha and Hbeta lines with the French-Italian
solar telescope THEMIS.
Title: Diagnostics of Non-thermal Particles in Solar Chromospheric
Flares
Authors: Fang, C.; Xu, Z.; Ding, M. D.
Bibcode: 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: The THMIS-MTR observation of a active region filament
Authors: Zong, W. G.; Tang, Y. H.; Fang, C.
Bibcode: 2004cosp...35.1633Z
Altcode: 2004cosp.meet.1633Z
We present some THMIS-MTR observations of a active region filament
on September 4, 2002. The full stokes parameters of the filament
were obtained in Hα, CaII 8542 and FeI 6302. By use of the data
with high spatial resolution(0.44" per pixel), we probed the fine
structure of the filament and gave out the parameters at the barbs'
endpoints, including intensity, velocity and longitudinal magnetic
field. Comparing the quiescent filament which we have discussed
before, we find that: 1)The velocities of the barbs' endpoints are
much bigger in the active region filament, the values are more than
one thousand meters per second. 2)The barbs' endpoints terminate at
the low logitudinal magnetic field in the active region filament, too.
Title: Spectro-polarimetric observation of the fine structure of a
quiescent filament
Authors: Zong, W. G.; Tang, Y. H.; Fang, C.; Mein, P.; Mein, N.; Xu,
A. A.
Bibcode: 2003A&A...412..267Z
Altcode:
This paper presents the spectro-polarimetric measurements of a big
quiescent filament observed by the MSDP mode of the THEMIS on August
24, 2000. The Hα , CaII 8542 and NaI D2 line profiles of a segment of
the filament were obtained. By use of the Hα images with high spatial
resolution, the two barb endpoints were identified. The parameters at
the barbs' endpoints, including intensity, velocity and longitudinal
magnetic field were measured. Using the data with high spatial
resolution (0.16'' per pixel), we have found the following results. 1)
There was mass motion at the barb endpoints in the chromosphere, the
values and the directions of the mass motion at the barb endpoints
change in several minutes. 2) The two barb endpoints are located
between the majority polarities and the minority polarities.
Title: Coronal Plume Heating and Kinetic Dissipation of Kinetic
Alfvén Waves
Authors: Wu, D. J.; Fang, C.
Bibcode: 2003ApJ...596..656W
Altcode:
Coronal plumes are steady-state (for periods of at least 24 hr) bright
rays that extend nearly radially from the photosphere to approximately
12 Rsolar in coronal holes and are believed to be denser
than the surrounding media. In a low-β plasma such as coronal holes,
kinetic dissipation of Alfvén waves due to the wave-particle resonant
interaction can directly lead to electron heating. On the basis of
an empirical model of coronal plumes, we investigate the kinetic
dissipation of Alfvén waves in a plume embedded in the coronal
hole. The results show that in the main body of the dense plume,
which is embedded in a nearly uniformly magnetized coronal hole,
the dissipation of the wave energy can provide an additional local
electron heating that is enough to balance the extra radiative loss
of the dense bright plume.
Title: Hα Line Polarization in the Major Flare of 2002 July
23. I. Observations and Data Analysis
Authors: Firstova, N. M.; Xu, Z.; Fang, C.
Bibcode: 2003ApJ...595L.131F
Altcode:
On 2002 July 23, a major 2B/X4.8-class flare was observed in Hα lines
with the Large Solar Vacuum Telescope of the Baikal Astrophysical
Observatory using the spectropolarimetric method. Linear polarization
of 3%-10% has been detected in Hα lines of the flare, particularly
in the Hα lines with a central reversal. They are mainly radial on
the solar disk and appear at the impulsive phase of hard X-ray and
γ-ray bursts. The polarization changes its direction in a spatially
limited small region (~4"-5") and within a short period of time
(~10 s). Moreover, the linear polarization is limited to only some
relatively small regions of the flare.
Title: Spectral Diagnostics of Non-Thermal Particles in the Solar
Chromosphere
Authors: FANG, C.; XU, Z.; DING, M. D.
Bibcode: 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.
Bibcode: 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: Evidence of EIT and Moreton Waves in Numerical Simulations
Authors: Chen, P. F.; Wu, S. T.; Shibata, K.; Fang, C.
Bibcode: 2002ApJ...572L..99C
Altcode:
Solar coronal mass ejections (CMEs) are associated with many dynamical
phenomena, among which EIT waves have always been a puzzle. In this
Letter MHD processes of CME-induced wave phenomena are numerically
simulated. It is shown that as the flux rope rises, a piston-driven
shock is formed along the envelope of the expanding CME, which sweeps
the solar surface as it propagates. We propose that the legs of the
shock produce Moreton waves. Simultaneously, a slower moving wavelike
structure, with an enhanced plasma region ahead, is discerned, which
we propose corresponds to the observed EIT waves. The mechanism for
EIT waves is therefore suggested, and their relation with Moreton
waves and radio bursts is discussed.
Title: Projects of the 23rd solar cycle study in China
Authors: Fang, C.
Bibcode: 2002AdSpR..29.1561F
Altcode:
The main projects of the 23rd solar cycle study in China, including
the main topics to be studied and the instrumentations, are briefly
described. Several key projects in China are also presented.
Title: Non-LTE Inversion of an Hα Flaring Loop
Authors: Ding, M. D.; Liu, Y.; Chen, P. F.; Fang, C.
Bibcode: 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.
Bibcode: 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 1 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 1 and
the blue K 1 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: The characteristics of microwave burst source and energy
budget in disappering filament
Authors: Tang, Y. H.; Schmieder, B.; Fang, C.
Bibcode: 2002AdSpR..29.1461T
Altcode:
On 7 May, 1992 a complex event with the disappearence of a filament
and a two ribbon flare was well observed in Hα filtergrams, Yorkoh
soft X-ray data, and radio data. This event is studied by using the
flare-filament current model. The momentum equation and the energy
equation of the filament current have been solved. By using the measured
time variation of the filament height, the corresponding evolution of
the current intensity and the total energy of the current filament
were computed. The magnetic field strength and the energy flux of
energetic electrons in the source region of microwave bursts have also
been estimated by using the microwave spectrum. During the main phase,
the mean magnetic strength and the energy flux of energetic electrons
are about 300-400G and 1 × 10 11 erg cm -2s
-1, respectively. By using these physical parameters, the
energy budget has been made. These results show that the flare of 7
May, 1992 may be the result of reconnection of current sheet beneath
the disappearing filament and the flare energy is provided by current
filament.
Title: Energetics of the 18 May 1994 brightening event
Authors: Tang, Y. H.; Li, Y. N.; Fang, C.; Schmieder, B.; Aulanier,
G.; Demoulin, P.
Bibcode: 2002AdSpR..30..557T
Altcode:
By using YOHKOH soft X-ray images, vector magnetograms and Hα
filtergrams, the energetics of the brightening event of May 18, 1994 has
been studied. It occurred in a nearly potential magnetic configuration
as shown by the comparison between the magnetic extrapolation(linear
force free field) and the large scale soft X-ray loops. This event
is related to the emergence of a new magnetic flux. The brightening
points of Hα and soft X-ray are located at computed separatrices
associated with field lines which are tangent to the photosphere. This
brightening may be a signature of reconnection taking place between the
pre-existing non-potential loops and the new emerging small loops. The
magnetic energy provided by reconnection is likely to be stored in
the non-potential loops and shearing emerging flux. A phenomenological
model is offered.
Title: A numerical study of flaring loop dynamics during magnetic
reconnection
Authors: Fang, C.; Chen, P. F.; Tang, Y. H.; Di, X. H.
Bibcode: 2002AdSpR..29.1445F
Altcode:
2.5-dimensional magnetic reconnection is numerically simulated for
two cases, one with a high altitude of the reconnection point, the
other with a low altitude. In the former case, bright loops appear to
rise for a long time, with footpoints separating and the field lines
below the bright loops shrinking. In the latter case, the bright loops
cease to rise after a short period of reconnection and become rather
stable. The results imply that the two types of solar flares, i.e.,
two-ribbon flares and compact flares, might be unified under a single
magnetic reconnection model, where the height of the reconnection
point leads to the bifurcation.
Title: Coronal mass ejections and emerging flux
Authors: Chen, P. F.; Fang, C.; Shibata, K.; Tang, Y. H.
Bibcode: 2002AdSpR..30..535C
Altcode:
This paper reviews our recent progress in the numerical study of
coronal mass ejections (CMEs) based on flux rope model, which shows
that when the reconnection-favored emerging flux appears either within
or on the outer edge of the filament channel, the flux rope would lose
its equilibrium, and be ejected, while a current sheet is formed below
the flux rope. For the case with emergence within the filament channel,
even small flux is enough to trigger the loss of equilibrium, however,
there is a threshold for the emerging flux on the outer edge of the
filament channel. Given that anomalous resistivity sets in (e.g. when
the current density exceeds a critical value), fast reconnection is
resulted in, leading to fast eruption of the flux rope and localized
flare (either impulsive-type or LDE-type depending on the height of
the reconnection point) near the solar surface. The numerical results
can well explain why CMEs are not centered on flares and provide hints
for CME-flare spatial and temporal relationships.
Title: Mechanism of coronal mass ejections triggered by emerging flux
Authors: Chen, P. F.; Fang, C.; Shibata, K.
Bibcode: 2002HiA....12..394C
Altcode:
No abstract at ADS
Title: Magnetic Reconnection in the Solar Lower Atmosphere
Authors: Fang, C.; Chen, P. F.; Ding, M. D.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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: Quiet-Sun Variability In a Temperature Minimum Region
Authors: Zhang, Y. X.; Fang, C.; Ding, M. D.; Livingston, W. C.
Bibcode: 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 K1
emission, which is strongly dependent on the temperature minimum region
(TMR). Cyclic variations are found in K1 emission that are
similar to those in the sunspot numbers. However, there is an evident
time lag of about 1.2 yr between the K1 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 1.7×10-2
g cm-2, respectively.
Title: Cyclic Variation in the Solar Lower Atmosphere
Authors: Fang, C.; Zahng, Y. X.; Ding, M. D.; Livingston, W. C.
Bibcode: 2001IAUS..203..387F
Altcode:
CaII K line has been measured regularly nearly every month since 1974
at Kitt Peak. It is known that the K1 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 K1 and blue
K1 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.
Bibcode: 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,
F1~1012ergcm-2s-1),
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
F1<~1011ergcm-2s-1,
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.
Bibcode: 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 F1 and F2, 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: Hα and Soft X-Ray Brightening Events Caused by Emerging Flux
Authors: Tang, Y. H.; Li, Y. N.; Fang, C.; Aulanier, G.; Schmieder,
B.; Demoulin, P.; Sakurai, T.
Bibcode: 2000ApJ...534..482T
Altcode:
By using Yohkoh soft X-ray images, vector magnetograms, and Hα
filtergrams, the brightening event that occurred on 1994 May 18 has
been studied in detail. It occurred in a nearly potential large-scale
magnetic configuration as shown by the comparison between the magnetic
extrapolation (linear force-free field) and the large-scale soft
X-ray loops. This event is related to the emergence of a new magnetic
flux of about 3×1020 Mx. The impulsive enhancement of
the emerging flux occurs about 20 minutes before the peaks of the
Hα and soft X-ray brightening and lasts for about 10 minutes. This
brightening may be a signature of reconnection taking place between the
preexisting nonpotential loops and the new emerging small loops. The
magnetic energy provided by reconnection is likely to be stored in
the nonpotential loops and the emerging flux as implied by the vector
magnetograms. By using the electron temperature and the electron density
of the brightening event derived from the analysis of the Yohkoh data,
an energy budget has been estimated. The result indicates that the
energy needed can be reasonably provided by magnetic reconnection.
Title: On the origin of solar white-light flares
Authors: Gan, W. Q.; Hénoux, J. C.; Fang, C.
Bibcode: 2000A&A...354..691G
Altcode:
Using the Hα line intensity as a constraint, we study the role of
a chromospheric condensation and the role of non-thermal effects in
producing the continuum enhancement of white-light flares. Within
an acceptable range of Hα line intensities and electron energy
flux, it is shown that neither a chromospheric condensation nor
non-thermal effects alone can directly explain the observed continuum
enhancement. The hybrid role of both chromospheric condensation and
non-thermal effects can only lead to a Balmer jump, but still not to a
significant continuum intensity increase between 4000 and 7000 Ä. A
possible picture is discussed, that is, the Balmer jump is directly
produced by the association of a chromospheric condensation and of
non-thermal effects, while the continuum enhancement between 4000 and
7000 Ä, is indirectly produced by the condensation and the non-thermal
effects via radiative heating to the deeper photospheric layers.
Title: Brightening Event in Hα and Soft X-Ray on May 18, 1994
Authors: Tang, Y. H.; Li, Y. N.; Schmieder, B.; Aulanier, G.; Demoulin,
P.; Fang, C.; Sakurai, T.
Bibcode: 2000AdSpR..25.1829T
Altcode:
By using Yohkoh soft X-ray images, vector magnetograms and Hα
filtergrams, a brightening event that occurred on May 18, 1994 has
been studied in detail. It occurred in a nearly potential large-scale
magnetic configuration as shown by the comparisons between the magnetic
extrapolations (linear force-free field) and the large-scale soft
X-ray loops. This brightening event observed in Hα line and soft
X-ray seems to be related to new emerging magnetic flux with an
amount of photospheric magnetic flux of about 3× 1020
Mx. The emerging flux increases obviously about 20 minutes before the
Hα and soft X-ray brightening, and lasts for about 103
s. This brightening may be a signature of reconnection taking place
between the pre-existing loop and the new emerging small loops
Title: Numerical Study on the Effect of Heat Conduction on Magnetic
Reconnection
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
Bibcode: 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: 3-D Magnetic Configurations for Filaments and Flares: The
Role of ``Magnetic Dips'' and ``Bald Patches''
Authors: Aulanier, G.; Schmieder, B.; van Driel-Gesztelyi, L.; Kucera,
T.; Démoulin, P.; Fang, C.; Mein, N.; Vial, J. -C.; Mein, P.; Tang,
Y. H.; Deforest, C.
Bibcode: 2000AdSpR..26..485A
Altcode:
The 3-D magnetic configuration of a filament and of a low energy
flare is reconstructed, using linear mag- netohydrostatic (lmhs)
extrapolations. In both cases, we find observational signatures
of energy release at the locations of computed ``bald patches''
separatrices, characterised by field lines which are tangent to
the photosphere.The filament was observed on Sept. 25, 1996, in Hα
with the MSDP on the German VTT, Tenerife, as well as in Si IV with
SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the
magnetic polarities observed with SOHO/MDI. The shape and location of
the computed dipped field lines are in good agreement with the shape of
the filament and its feet observed in Hα. Some ``bald patches'' (BPs)
are present where the distribution of dips reaches the photosphere. We
show that some of the large scale field lines rooted in BPs can be
related to bright fine structures in Si IV. We propose that the plasma
there is heated by ohmic dissipation from the currents expected to be
present along the BP separatrices.The flare was observed on May 18,
1994, in soft X-rays with Yohkoh/SXT, and in Hα at Mitaka (Japan). The
magnetic field is directly extrapolated from a photospheric magnetogram
from Kitt Peak Observatory. The intersections with the photosphere of
the computed separatrices match well the bright Hα ribbons. The later
are associated to three BPs, with overlaying dipped field lines. We
show that enhanced densities are present in these dips, which can be
correlated with dark Hα fibrils.Both cases show the importance of
dipped field lines and BPs in the solar atmosphere. Energy release
via ohmic dissipation as well as reconnection along BP separatrices
is proposed to provide heating observed as UV brightenings in filament
channels and even as small flares
Title: Commission 12: Solar Radiation and Structure (Radiation et
Structure Solaires)
Authors: Foukal, Peter; Solanki, Sami; Mariska, J.; Baliunas, S.;
Dravins, D.; Duvall, T.; Fang, C.; Gaizauskas, V.; Heinzel, P.;
Kononovich, E.; Koutchmy, S.; Melrose, D.; Stix, M.; Suematsu, Y.;
Deubner, F.
Bibcode: 2000IAUTA..24...73F
Altcode:
No abstract at ADS
Title: Evolution of Magnetic Field and CMEs
Authors: Fang, C.; Chen, P. F.; Tang, Y. H.; Shibata, K.
Bibcode: 2000IAUJD...7E..17F
Altcode:
Observations show that some evolving magnetic structures occur in
solar active regions before the onset of CMEs. In 2D framework, through
changing the bottom boundary of magnetic field, we simulate numerically
the dynamics of the coronal plasma with a magnetic configuration similar
to a filament. In some cases, we find the flux rope (or filament) loses
its equilibrium, and moves upward. A current sheet is formed below the
filament. If there is no reconnection or the reconnection is not so
fast, the filament will finally stop motion and fall down, while when
fast reconnection occurs, the filament erupts. The characteristics of
its motion are well consistent with filament eruptions and CMEs. After
the reconnection, a cusp shaped hot X-ray loop is formed as the
signature of solar flares. This model provides deep insight on the
relation between the photospheric magnetic field variation and the
onset of CMEs, and on the relation between CMEs and solar flares.
Title: Multi-line two-dimensional spectroscopy of a limb flare
Authors: Ding, M. D.; Fang, C.; Yin, S. Y.; Chen, P. F.
Bibcode: 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.
Bibcode: 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 B0, to a medium extent on the density
ρ0, and weakly on the temperature T0, the
resistivity η, and the length scale L0, by which the size
of current sheet and the height of the X-point are both scaled. The
strong B0 dependence means that the Lorentz force is the
dominant factor; the inertia of the plasma may account for the moderate
ρ0 dependence; and the weak η dependence may imply that
``fast reconnection'' occurs; the weak L0 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: Analysis Of The Disappearing Filament And Flare Of 7 May 1992
Authors: Tang, Y. H.; Mouradian, Z.; Schmieder, B.; Fang, C.;
Sakurai, T.
Bibcode: 1999SoPh..185..143T
Altcode:
By using Yohkoh soft X-ray data, Hα filtergrams, and radio data, the
activation of the disappearing filament and the flare eruption on 7
May 1992 have been studied. Main conclusions are as follows: (1) the
emergence of new magnetic flux tends to affect the pre-existing X-ray
loops, which usually appear in arcades spanning Hα filament, changing
the magnetic environment of the filament, and then enhance the current
in the filament. Therefore newly emerging flux plays a fundamental
role in the destabilization of this filament. (2) According to the Hα
data and the rising motion of the filament, the corresponding current
variation in the filament has been calculated. It seems that the current
interruption may be a possible trigger mechanism for this filament
disappearance. (3) The magnetic field strength and the energy flux
of energetic electrons in the source region of microwave bursts have
been estimated by using the microwave spectrum. During the main phase,
the mean magnetic strength and the energy flux of energetic electrons
are about 300-400 G and 1×1011 erg cm−2 s −1, respectively. (4)
The energy provided by reconnection of the current sheet and the total
energy of the current filament are estimated and we show that there
is enough energy stored in the filament to feed the 7 May, 1992 flare.
Title: Simulation of Magnetic Reconnection with Heat Conduction
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
Bibcode: 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-1, 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: Two-Fluid Motion of Plasma in Alfvén Waves and the Heating
of Solar Coronal Loops
Authors: Wu, D. J.; Fang, C.
Bibcode: 1999ApJ...511..958W
Altcode:
Taking the effect of two-fluid motion of plasma in Alfvén waves
into account, a nonzero parallel electric field can rise within
the wave itself under collisionless conditions. This leads to a
kinetic dissipation of Alfvén waves by the wave-particle resonant
interaction and electron heating along the ambient magnetic field
lines. Employing the drift kinetic equation, we investigated this
electron-heating mechanism under the cool ion approximation. The
result shows that the damping rate and the heating rate obviously
depend on the strength distribution of the ambient magnetic field, and
that they reach their maximum values at B0=Bm
and B0=(1+2α)1/2Bm, respectively,
for the perturbed field of δB~Bα0,
where Bm is the ambient magnetic field strength when
vA=vTe. Finally, we propose that this heating
mechanism can be applied to explain the brightness distribution of
solar soft X-ray coronal loops.
Title: Heating in the Lower Atmosphere and the Continuum Emission
of Solar White-Light Flares
Authors: Ding, M. D.; Fang, C.; Yun, H. S.
Bibcode: 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: A Measure of Magnetic Field Discontinuity
Authors: Moon, Y. -J.; Yun, H. S.; Lee, S. W.; Kim, J. -H.; Choe,
G. S.; Park, Y. D.; Ai, G.; Zhang, H. Q.; Fang, C.
Bibcode: 1999SoPh..184..323M
Altcode:
In this paper we introduce a measure of magnetic field discontinuity,
MAD, defined as Maximum Angular Difference between two adjacent
magnetic field vectors. To examine the characteristics of the MAD,
we have considered several active region models having a quadrupolar
field configuration and computed MADs over these active regions
by approximating the 3-D magnetic fields as an ensemble of charge
potential fields or linear force-free fields. The computed MAD fields
are studied in comparison with other flare activity indicators such
as separators. It is found that (1) the region of high level MAD
corresponds well with the separator, or practically the intersection
of the separator with the plane of measurement, (2) it singles out
local discontinuities of magnetic fields, and (3) the MAD can also be
a measure of the evolutionary status of an active region.
Title: Magnetic Reconnection and Flare Loop Motion
Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D.
Bibcode: 1999ASSL..240..337C
Altcode: 1999numa.conf..337C
No abstract at ADS
Title: Magnetohydrostatic Model of a Bald-Patch Flare
Authors: Aulanier, G.; Démoulin, P.; Schmieder, B.; Fang, C.; Tang,
Y. H.
Bibcode: 1998SoPh..183..369A
Altcode:
On 18 May, 1994, a subflare was observed in AR 7722 in X-rays by
Yohkoh/SXT and in Hα at National Astronomical Observatory of Japan. The
associated brightenings are due to small-scale magnetic energy release,
triggered by parasitic fluxes emerging and moving at the edge of leading
sunspots. Using the magnetohydrostatic equations derived by Low (1992),
we model the magnetic field configuration by extrapolation of the Kitt
Peak photospheric field, taking into account the effects of pressure
and gravity. Hα flare kernels are shown to be located at computed
separatrices associated with field lines which are tangent to the
photosphere, namely 'bald patches' (BPs). This is evidence that BPs
can be involved in flares, and that current sheets can be dissipated
in low levels of the solar atmosphere. The presence of dense plasma
which is supported against gravity in the magnetic dips above BPs is
correlated to dark elongated features observed in Hα. Mass flows in
these flat fibrils are discussed in the context of energy release in
the BP separatrices. The effect of the plasma on the computed magnetic
configuration is shown to be of secondary importance with respect to
the topology of the field.
Title: Spectral Analysis of a Thermal Flare on 27 October 1993
Authors: Schmieder, B.; Fang, C.; Harra-Murnion, L. K.
Bibcode: 1998SoPh..182..447S
Altcode:
A thermal subflare occurring on 27 October 1993 was observed during
a multi-wavelength campaign with the Yohkoh spacecraft and the
Multi-channel Subtractive Double-Pass spectrograph (MSDP) at Pic du
Midi. The various instruments provided us with 2-D Hα spectra, X-ray
spectra and X-ray images. A non-LTE computation (including chromospheric
condensation) was carried out to determine the dynamic parameters of
the flaring chromosphere. By combining these results with the Yohkoh
data, we illustrate that momentum is balanced between the upflowing
plasma and the downflowing cool plasma during the flare. This result
is consistent with the evaporation model.
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.
Bibcode: 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 E0, close to 20 keV,
for which the electron flux d F1/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=E0, 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.
Bibcode: 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: Line profiles in moustaches produced by an impacting energetic
particle beam
Authors: Ding, M. D.; Henoux, J. -C.; Fang, C.
Bibcode: 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: Non-thermal hydrogen line emission caused by an oblique
incident proton beam through charge exchange
Authors: Zhao, X.; Fang, C.; Henoux, J. -C.
Bibcode: 1998A&A...330..351Z
Altcode:
In this paper, formulae are given for computing the non-thermal emission
of superthermal hydrogen atoms generated through charge exchange by
the bombardment of the solar atmosphere by an oblique incident proton
beam with a given pitch angle. Specifically, we discuss the non-thermal
emission of hydrogen in Lyalpha , Lybeta , and Hα lines and find that
the profiles of these lines are quite different from the ones caused by
bombardment by a proton beam moving vertically. The intensity and the
asymmetry of the non-thermal emission profiles strongly depend on the
beam pitch angle alpha and on the angle theta between the direction
of magnetic field and the line of sight. By computing the thermal
emission under the semi-empirical flare atmospheric models F1, F2, and
the quiet-Sun atmospheric model C, we compare the relative importance
of thermal and non-thermal emission. For the Hα line, the non-thermal
emission, with the proton flux used, is too small to be detectable; for
the Lyalpha line, the contribution of non-thermal emission to the line
wings is smaller than the one of a vertical beam; while for the Lybeta
line, line wing enhancement and broadening are significant. Thus, Lybeta
line is a good diagnostic tool for non-thermal proton beam bombardment.
Title: Particle Beam Diagnostics Based on UV and Optical Spectra of
Solar Flares (invited)
Authors: Fang, C.; Ding, M. D.; Hénoux, J. C.
Bibcode: 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.
Bibcode: 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 × 106 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 109 cm-3,
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: Magnetic reconnection mechanism for Type II white-light flares.
Authors: Li, X. Q.; Song, M. T.; Hu, F. M.; Fang, C.
Bibcode: 1997A&A...320..300L
Altcode:
Based on the concept that the heating source for Type II white-light
flares (WLFs) may be located in the photosphere, we suggest that Type II
WLF is reduced by magnetic reconnection in a weakly ionized plasma. A
plasmoid may rise by magnetic buoyancy from the subphotosphere into a
sunspot region and then expand to form a current sheet in which a type
of resistive instability takes place. The effect of neutral atoms on
the resistive instability is estimated and found to produce a growth
time of typically ten minutes that agrees well with observations.
Title: Influence of non-thermal processes on line asymmetries in
solar flares.
Authors: Ding, M. D.; Fang, C.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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: Time-dependent Flare Signature in Hydrogen Balmer Lines
Authors: Lee, Sang-Woo; Lee, Jeongwoo; Yun, H. S.; Fang, C.; Hu, J.
Bibcode: 1996ApJ...470L..65L
Altcode:
In recent studies of H alpha flare imaging spectroscopy performed
by Leka et al. and de la Beaujardiere et al. in 1993, H alpha line
profiles have been investigated as a function of space to locate the
physical process underlying observed spectral profiles. In this Letter,
we discuss time evolution of hydrogen Balmer line profiles at a fixed
spatial point recorded by the multichannel spectrograph at Nanjing
University in China during the 1991 October 27 white-light flare which
occurred in NOAA 6891. It is found that the Balmer emission lines
with a central reversal show up at the beginning of the H alpha flare
but change to single-peaked profiles in about 30 s. This result may,
according to Canfield et al.'s model in 1993, indicate that the Balmer
line enhancement even at one flare site could be caused by two physical
processes, namely, initially by the high-energy particle precipitation
and subsequently by the high coronal pressure. We also present evidence
for downward motion and the Stark broadening effect on the observed
Balmer line wings to examine further the physical consequences of the
particle precipitation. Time correlation of the Balmer line emission
with hard X-ray and continuum brightening is briefly discussed based
on the spectral information.
Title: On a Possible Explanation of Chromospheric Line Asymmetries
of Solar Flares
Authors: Ding, M. D.; Fang, C.
Bibcode: 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: Spectral analysis of Hα, Hβ, and Hγ lines of a white-light
flare (3B/X6.1) on October 27, 1991
Authors: Lee, S. W.; Yun, H. S.; Hu, J.; Fang, C.; Wang, J. L.
Bibcode: 1996SoPh..163..361L
Altcode:
We have analyzed time series of Hα, Hβ, and Hγ line profiles taken
from a 3B/X6.1 flare which occurred on October 27, 1991 in active
region NOAA 6891. Each set of the spectra was taken simultaneously for
the first 10 min of the flare event with a low and non-uniform time
resolution of 10-40 s. A total of 22 sets of Hα, Hβ, and Hγ were
scanned by a PDS with absolute intensity calibration to derive the
dynamics and energetics of material in the flare region. Our results
are as follows: (1) The Balmer line emitting region is accelerated
downward to about 50 km s-1 for the first 50 s and then
is decelerated to about 10 km s-1 for the next 150 s. (2)
The radial velocity peak precedes the Balmer line intensity peak by
about 40 s. (3) The total energy radiated from these Balmer lines is
estimated to be 4.9 × 1029 erg.
Title: Chromospheric Evaporation in Four Solar Flares Observed
by YOHKOH
Authors: Ding, M. D.; Watanabe, T.; Shibata, K.; Sakurai, T.; Kosugi,
T.; Fang, C.
Bibcode: 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-1. The durations of upflows with observable velocities
(≥ 100 km s-1) 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.
Bibcode: 1996mpsa.conf..553F
Altcode: 1996IAUCo.153..553F
No abstract at ADS
Title: Effects on Non-Thermal Particles on the UV and Optical Spectra
of Solar Flares
Authors: Henoux, J. -C.; Fang, C.
Bibcode: 1996mpsa.conf..561H
Altcode: 1996IAUCo.153..561H
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.
Bibcode: 1996gbaa.conf..303H
Altcode:
No abstract at ADS
Title: On Thermal Hydrodynamical Models of Solar Flares with a High
Initial Coronal Density
Authors: Gan, W. Q.; Cheng, C. C.; Fang, C.
Bibcode: 1995ApJ...452..445G
Altcode:
We present some hydrodynamical results for a thermal model of solar
flares with a high initial coronal density (5-11 × 1010
cm-3) in the flaring loop. The thermal energy is assumed to
be released around the top of the loop. The results of the hydrodynamic
simulation show that, in the initial phase of the flare, there is
always a strong downward motion in the corona. This motion with a
velocity as large as 700 km s--1 can last a few tens of
seconds, leading to a pronounced redshift of the Ca XIX W line. This
is not consistent with the observations of majority of flares, which
show predominantly blueshifted or blue-asymmetric Ca XIX and Fe
XXV lines. However, the results could account for the redshifted Ca
XIX emission observed in a small number of flares, indicating a high
initial coronal density in these flares. We conclude that only when the
initial coronal density is smaller than 1010 cm-3
can the strong downward motion of the coronal material at the early
phase of the flare be suppressed. When we combine the present and
previous results of flare simulations, it appears that a model with
both thermal and nonthermal energy sources may be more realistic in
describing the flare hydrodynamics.
Title: Spectral Analysis of the White Light Flare on 1974OCT11
Authors: Yin, S. Y.; Fang, C.; Ding, M. D.; Hiei, E.; Fu, W. J.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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
K1 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: Hydrogen line spectra of a nonthermal proton beam. due to
charge exchange in solar flares
Authors: Fang, C.; Feautrier, N.; Henoux, J. -C.
Bibcode: 1995A&A...297..854F
Altcode:
Using more recent values of the atomic parameters and a more refined
atmospheric model, the nonthermal line emission resulting from
proton-hydrogen charge exchange has been recomputed. Confirming the
results of Canfield & Chang (1985), the hydrogen Hα nonthermal
line emission was found too small to be detectable. On the other hand,
the nonthermal emission in the red wings of the Lyα and Lyβ lines
is significantly high and can be used as a diagnostic of low energy
protons. However, this emission is at least one order of magnitude
lower than the value derived by Canfield and Chang.
Title: Diagnostics of non-thermal processes in chromospheric
flares. III. Lyα and Lyβ spectra for an atmosphere bombarded by
electron or proton beams.
Authors: Henoux, J. C.; Fang, C.; Gan, W. Q.
Bibcode: 1995A&A...297..574H
Altcode:
The Lyα and Lyβ line profiles, including the nonthermal excitation
and ionization of hydrogen by electron or proton bombardment, have
been computed for atmospheres with the temperature distributions of the
semi-empirical models F_1_ and F_2_. These computations indicate that
the intensities at the line wings are greatly enhanced by nonthermal
effects, especially for an electron beam bombardment. These effects are
the most obvious when the coronal column mass is relatively small. Thus,
nonthermal effects should be the most pronounced just at the beginning
of the impulsive phase. After the maximum of the impulsive phase,
due to a fast increase of the coronal column mass, these effects will
rapidly decrease.
Title: Analysis of 2-d flare spectra: Velocity fields derived from
Hα line asymmetries
Authors: Ding, M. D.; Fang, C.; Huang, Y. R.
Bibcode: 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−1 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: Linear Current Instability in Loop Prominences
Authors: Tang, Y. H.; Fang, C.; Cui, L. S.
Bibcode: 1995SoPh..158...71T
Altcode:
By use of the dispersion equation given by Song, Wu, and Dryer
(1987) for a cylinder plasma with mass motion and gravity included,
we investigate the linear current instabilities developed in loop
prominences. The results indicate that the mode of linear instability
depends mainly on whethervs2 > Φ or not,
wherevs is the sonic velocity at heightz, Φ =GM/(R +z)
is the gravity potential,G the gravitational constant,M andR the mass
and the radius of the Sun respectively. Ifvs2
> Φ, then the sausage instability will be dominant. Otherwise,
the kink instability will be more important. A possible explanation of
`knot' structure, which appears sometimes in solar loop prominences
has been given.
Title: On the spectral characteristics and atmospheric models of
two types of white-light flares.
Authors: Fang, C.; Ding, M. D.
Bibcode: 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: Semi-Empirical Models of the White-Light Flare on October
24, 1991
Authors: Fang, C.; Henoux, J. C.; Ju, Hu; Yin-Zhang, Xue; Xiu-Fa,
Gao; Qi-Jun, Fu
Bibcode: 1995SoPh..157..271F
Altcode:
On October 24, 1991, a white-light flare was observed both from space
and from the ground. A multi-waveband spectral analysis shows that
the peak time of the continuum emission coincides well with that of
a radio burst at 2840 MHz and with the hard X-ray emission. Three
semi-empirical models, corresponding to the pre-flare condition and to
the peak time of continuum emission both with and without non-thermal
excitation and ionization of hydrogen by an electron beam, have been
obtained. The results indicate that there is fast heating both in the
chromosphere and the photosphere. Some evidence is given that this WLF
is very likely a result of bombardment by an electron beam. By taking
into account non-thermal effects, the chromospheric temperature of
the semi-empirical model is significantly reduced.
Title: VizieR Online Data Catalog: White-light flares models
(Fang+, 1995)
Authors: Fang, C.; Ding, M. D.
Bibcode: 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.
Bibcode: 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- 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: Magnetohydrodynamic phenomena in the solar atmosphere.
Authors: Fang, C.; Tang, Y. H.
Bibcode: 1994PPMtO..13...87F
Altcode:
Solar atmosphere is a rotating plasma shell filled by magnetic
field. The coupling between the magnetic and the movement of
plasma makes a variety of magnetohydrodynamic phenomena in the solar
atmosphere. After a brief introduction on the basic theoretical regime
of solar MHD, the authors describe mainly the magnetohydrodynamic
aspects of solar flares, solar prominences (filaments) and flux tubes
with different scales.
Title: The CCD Imaging Spectral Observational System in the Solar
Tower of Nanjing University
Authors: Huang, Y. R.; Fang, C.; Zu, Z. G.; Xue, Y. Z.; Gao, X. F.;
Yin, S. Y.; Hu, J.; Ji, G. P.
Bibcode: 1994AcASn..35..176H
Altcode:
No abstract at ADS
Title: On the Propagation of Chromospheric Condensations in Solar
Flares (II). Evolution of the Mass Motion Velocity
Authors: Ding, M. D.; Fang, C.
Bibcode: 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.
Bibcode: 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: High-Resolution Spectral Observation during the Impulsive
Phase of a Flare
Authors: Ji, G. P.; Kurokawa, H.; Fang, C.; Huang, Y. R.
Bibcode: 1994SoPh..149..195J
Altcode:
High-resolution observations of the flare on October 21, 1989 were
made with the Domeless Solar Telescope of the Hida Observatory. The
following new results have been obtained: (a) during the impulsive phase
of the flare, the spectral line asymmetry has spatial fine structures of
1″-2″; (b) for several points in the flare region the line profile
alternatively changes between blue asymmetry and red asymmetry within
a few seconds. A possible explanation has been suggested.
Title: The Propagation of Chromospheric Condensations and the
Asymmetry of Spectral Lines in Solar Flares
Authors: Ding, M. D.; Fang, C.
Bibcode: 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.
Bibcode: 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.
Bibcode: 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: Semiempirical Flare Models with Chromospheric Condensation
Authors: Gan, W. Q.; Rieger, E.; Fang, C.
Bibcode: 1993ApJ...416..886G
Altcode:
Using Hα line profiles observed for two flares, we have constructed
series of semiempirical models with chromospheric condensation. From
the semiempirical point of view, it is shown for the first time
that chromospheric condensations are responsible for the Hα
line asymmetries, not only for the red-asymmetry, but also for
the blue-asymmetry with a central reversal. The properties of the
condensation and its calculated evolutionary tendency seem to be
consistent with the results of current hydrodynamical models of solar
flares. The results also show that for the first flare, the Hα can
be explained merely by introducing the condensation without any need
to heat the chromosphere, while for the second flare, the condensation
has to overlap a heated chromosphere.
Title: Hα Emission Line Profiles of Solar Flares with Turbulent
Velocity Fields
Authors: Ding, M. D.; Fang, C.
Bibcode: 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: Hα and Ca II K line profiles of solar flare in turbulent
velocity fields
Authors: Fang, C.; Ding, M. D.
Bibcode: 1993AdSpR..13i.321F
Altcode: 1993AdSpR..13..321F
No abstract at ADS
Title: Diagnostics of non-thermal processes in chromospheric
flares. I. H alpha and CaII K line profiles of an atmosphere bombarded
by hecta keV electrons.
Authors: Fang, C.; Henoux, J. C.; Gan, W. Q.
Bibcode: 1993A&A...274..917F
Altcode:
The significance of non-thermal excitation and ionization of hydrogen
and CaII, by an electron beam, on the line profiles of Hα and CaII K
line has been investigated. Non-LTE Hot and CaII K line profiles have
been calculated for the temperature distributions of semi-empirical
flares models F1 and F2 (Machado et al. t980). For
reasonable values of the beam energy flux and power index, the hydrogen
lines are greatly strengthened and broadened, and an obvious central
reversal appears. The effects are weaker for the CaII K line. These
effects can be used to diagnose electron beam bombardment during a
solar flare, especially at its early phase. Any semi-empirical flare
model that does not take into account non-thermal effects overestimates
the heating of the solar atmosphere.
Title: Diagnostics of non-thermal processes in chromospheric
flares. II. H alpha and CaII K line profiles of an atmosphere
bombarded by 100 KeV-1 MeV protons.
Authors: Henoux, J. C.; Fang, C.; Gan, W. Q.
Bibcode: 1993A&A...274..923H
Altcode:
The effects on the Hα line profile of non-thermal excitation
and ionization of hydrogen by a 100 KeV- 1 MeV proton beam have
been investigated. Non-LTE Hα line profiles have been calculated
for the temperature distributions of semi- empirical flares models
F1 and F2 (Machado et al. 1980). For reasonable
values of the beam energy flux and power index, the hydrogen lines are
greatly strengthened. However, contrary to the case of an electron
bombarded atmosphere, the line is not broadened, and no central
reversal appears. These effects, alone or in conjunction with linear
polarization measurements or Lyα red wing observations, can be used
to diagnose proton beam bombardment.
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.
Bibcode: 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.
Bibcode: 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: The Influence of Chromospheric Condensation on Hα Line
Profiles
Authors: Gan, W. Q.; Rieger, E.; Fang, C.; Zhang, H. Q.
Bibcode: 1993SoPh..143..141G
Altcode:
The influence of the chromospheric condensation on Hα line profiles
for the thermal model of a solar flare has been empirically studied
in this paper. The so-called thermal model here means that there
is no temperature increase relative to the quiet-Sun chromosphere
but with a chromospheric condensation in the lower part of its
transition region, which case is assumed to represent the early
stage of the impulsive phase. The main results include: when the
temperature within the condensation region is assumed to be equal
to that in front of it, the influence is to create an additional
absorption profile overlapping on the original one; by increasing
the condensation strength, the Hα line profile changes from a little
line-center increase to broadened red asymmetry, then to the reversed
red asymmetry, and finally to two independent absorption profiles;
the thickness of the condensation determines the absorption of the
additional profile; descending the transition region has no obvious
effect on the basic characteristic of the Hα line profile except
a little increase in the line center. Assuming that the temperature
within the condensation is higher than that ahead of the condensation,
the calculated Hα line profiles may be strong enough to be comparable
with the observations. This means that if the condensation in the
purely thermal model can reach a higher temperature, we may also use
only thermal origin to explain a chromospheric flare.
Title: Spectral diagnostics for a thermal hydrodynamical model of
a solar flare loop
Authors: Gan, W. Q.; Rieger, E.; Fang, C.; Zhang, H. Q.
Bibcode: 1992A&A...266..573G
Altcode:
Based on a thermal hydrodynamic model of solar flares, we have for
the first time simultaneously computed the theoretical Ca XIX spectra
(3.163-3.22 A) as well as H-alpha and Ca II K line profiles. The results
show that the soft X-ray lines are to a large degree qualitatively
consistent with the observations. The only deficiency is the
predominant blueshifted component in the early impulsive phase. The
explanations for this difference are discussed. In the chromosphere,
although mostly unaffected by heating, the H-alpha and Ca II K lines
are rather strong. The asymmetric characteristics calculated show to
be also compatible with the observations. We therefore conclude that
the thermal hydrodynamic model can describe the observed spectral
characteristics of solar flares rather well.
Title: The Role of Chromospheric Condensations in the Continuum
Emission of White-Light Flares
Authors: Gan, W. Q.; Rieger, E.; Zhang, H. Q.; Fang, C.
Bibcode: 1992ApJ...397..694G
Altcode:
A non-LTE atmospheric modeling technique has been used to study the
relations between the chromospheric condensation and the continuum
emission in order to explore the possibility that the chromospheric
condensations may be responsible for white-light flare emissions. It is
found that the chromospheric condensation could produce the continuum
enhancement only when it has a temperature of 9000-10,000 K and only
when the transition region moves downward below to generate than 0.0001
g/sq cm. If the temperature within the condensation is equal to or
lower than that of neighboring region, the continuum is either not
affected or even decreased. The role of the condensation in producing
the continuum seems to be independent of the chromospheric state. Taking
the quiet-sun model as a background, the enhanced continuum usually
exhibits a reddish distribution with a Balmer jump. Taking the penumbra
model as background, the calculated continuum enhancement may exhibit
a flat or a bluish distribution with a Balmer jump.
Title: Ca ii K line asymmetries in two well-observed solar flares
of October 18, 1990
Authors: Fang, C.; Hiei, E.; Okamoto, T.
Bibcode: 1991SoPh..135...89F
Altcode:
Two-dimensional evolutions of two flares of October 18, 1990 have been
well observed in the CaII K line with a CCD camera at Norikura station
of National Astronomical Observatory in Japan. There are two common
characteristics for the flares: 3 ∼- 5 min before the impulsive phase,
the heating already begins at the footpoints of the flares, but no
asymmetry in line emission has been detected. After the onset of the
impulsive phase, CaII K line emission at the footpoints shows strong
red asymmetry, with the maximum asymmetry occurring at the same time as
the peak of the radio bursts. The maximum downward velocity is about
30 ∼ 50 km s−1. For flare 1, blue and red asymmetries
were observed in two sides of the footpoint area. They developed and
attained a maximum nearly at the same time and the inferred Doppler
velocities are comparable (30 ∼ 40 km s−1). This implies
that two mass jets started from a small region and ejected along a
loop but in opposite directions with roughly equivalent momentum. A
possible mechanism has been discussed.
Title: A hydrodynamic model of the impulsive phase of a solar
flare loop
Authors: Gan, W. Q.; Zhang, H. Q.; Fang, C.
Bibcode: 1991A&A...241..618G
Altcode:
With improvements in modeling of the chromospheric radiative loss
and in the resolution of the transition region, together with the
introduction of the mechanism of chromospheric heating by coronal soft
X-rays, a numerically simulated hydrodynamic response of a solar flare
loop to the thermal energy deposition around the top of the loop is
described. Detailed evolutionary pictures have been obtained during the
early stage of the impulsive phase for the conductively heated model,
showing the fast heating of the flaring atmosphere, the formation and
the development of chromospheric evaporation and condensation, the
motion of the transition region, the consequences of the high-speed
collision of evaporated material at the top.
Title: A Numerical Simulation for the Cooling Process of the Solar
Flare Loop
Authors: Gan, W. Q.; Fang, C.; Zhang, H. Q.
Bibcode: 1991PASA....9..317G
Altcode: 1991PASAu...9..317G
No abstract at ADS
Title: A Hydrodynamic Thermal Model of the Impulsive Phase of
Solar Flares
Authors: Fang, C.; Gan, W. Q.; Zhang, H. Q.
Bibcode: 1991LNP...387..157F
Altcode: 1991fpsa.conf..157F
No abstract at ADS
Title: AR:5395 - Observations on the Asymmetries of the Hα and CAII
K Line Profiles of the Flares on 1989MAR9 and 1989MAR14
Authors: Fang, C.; Hu, J.; Ji, G. P.; Yin, S. Y.; Zhang, Q. Z.
Bibcode: 1990PYunO...4...38F
Altcode:
No abstract at ADS
Title: A Hydrodynamic Model of the Gradual Phase of the Solar
Flare Loop
Authors: Gan, W. Q.; Fang, C.
Bibcode: 1990ApJ...358..328G
Altcode:
The hydrodynamic cooling process after the maximum phase of a
solar flare has been simulated numerically, with improvements on
the chromospheric radiative loss and the resolution of the transition
region, together with the introduction of the mechanism of chromospheric
heating by coronal soft X-rays. At the early stage of the gradual phase,
chromospheric evaporation continues to take place and the transition
region moves further downward. At the middle stage of the gradual phase,
the corona atmosphere appears to have a quasi-periodic oscillation,
and the transition region remains at the same position for a long
time. At the later stage to of the gradual phase, the descent of
coronal material does not happen, and the atmosphere evolves into a
state with high density and low temperature. Finally, the atmosphere
does not reach the quiet-sun conditions directly, and it would perhaps
undergo a process of instability and develop into the stage of postflare
loop. The efficiency of flare soft X-ray heating of the chromosphere
is also discussed.
Title: Semiempirical Models at Different Heights of a Quiescent
Prominence
Authors: Fang, C.; Zhang, Q. H.; Yin, S. Y.; Livingston, W.
Bibcode: 1990LNP...363..284F
Altcode: 1990doqp.coll..284F; 1990IAUCo.117..284F
Semi-empirical models of a quiescent prominence observed on Dec. 12,
1972 with the McHath Telescope at Kitt Peak have been deduced for
different heights. The transfer, statistical equilibrium equations as
coupled with the hydrostatic equilibrium, and the partical conservtion
equations have been solved. The models reproduce well the simultaneously
observed H , H , Hr, CaII K,H and infrared triplet line profiles. The
study indicates that the pressure near the edge of prominences and
the microturbulence velocity in the prominences basically do not vary
with height, but the temperature decreases monotonously from the edge
toward the center. It is found that the temperature near the edge
of prominences increases only slightly with height, but the central
temperature decreases significantly. The results also indicate that
near the edge of prominences there is no radiative equilibrium and
the total radiative loss has a maxium which is mainly due to L. The
radiative loss due to CaII is negligible in comparison with that due
to hydrogen. The ionization problem of Calcium has also been discussed.
Title: A semi-empirical model of sunspot penumbra
Authors: Ding, M. D.; Fang, C.
Bibcode: 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.
Title: Frontiers in Solar Physics
Authors: Fang, C.
Bibcode: 1989PrA.....7...19F
Altcode:
No abstract at ADS
Title: Dynamics of Solar Flare Atmosphere and its Spectral Diagnostics
Authors: Fang, C.; Gan, W. Q.
Bibcode: 1989PrA.....7...90F
Altcode:
No abstract at ADS
Title: Semiempirical models of the impulsive phase of the
chromospheric flare.
Authors: Gan, W. -Q.; Fang, C.
Bibcode: 1988ChJSS...8...12G
Altcode:
No abstract at ADS
Title: Studies of the solar prominence of October 18, 1984 (I).
Authors: Ye, S. -H.; Hu, J.; Zhang, Q. -Z.; Fang, C.
Bibcode: 1988PPMtO...7...33Y
Altcode:
This paper reports on the spectrophotometric studies of the solar
prominence of October 18, 1984. It mainly describes the observation,
the spectrograms and their reduction as well as the results of research
on the mechanisms of broadening of hydrogen lines and the regions of
formation of spectral lines belonging to various chemical elements.
Title: Spectral Analysis and the Two-Dimensional Distribution of
Physical Parameters in a Quiescent Prominence
Authors: Zhang, Q. Z.; Livingston, W. C.; Hu, J.; Fang, C.
Bibcode: 1987SoPh..114..245Z
Altcode:
The profiles of Hα and Ca II K lines of a arch quiescent prominence on
April 1, 1971 have been analyzed and the two-dimensional distributions
of electron temperature Te, micro-turbulence velocity
vt and the column number density of hydrogen along the
line-of-sight NH have been obtained. Te,
υt, and NH are found to be 7500 K, 6 km
s−1 and 2.2 × 1018 cm−2
on an average, respectively. The electron temperature at the central
part of the prominence and along the two arcades are greater than that
at the edges, while the distribution of the micro-turbulence velocity
in these regions is opposite. There is no systematic variation in
Te and vt, from the center to the periphery as
described by Hirayama (1971). The column number density in the central
region is lower than that at the two edges.
Title: Semi-empirical models of a quiescent prominence
Authors: Zhang, Q. Z.; Fang, C.
Bibcode: 1987A&A...175..277Z
Altcode:
In the paper semi-empirical quiescent prominence models which satisfy
the constraints of the radiative transfer equation, hydrostatic and
statistical equilibrium are proposed. The atom is assumed to have
five levels plus one continuum for hydrogen as well as for ionized
calcium. The models reproduce well the observed Hα, Hβ, Ca II H and
K line profiles. The results indicate that the temperature from the
outer layer to the centre of the prominence decreases and the source
functions of Hα, Hβ, Hγ, Ca II H and K lines increase. By computing
radiation losses, it is found that the radiative loss due to hydrogen
is much larger than that due to ionized calcium.
Title: A study of the energy deposits in flaring atmosphere with
semiempirical models of flares.
Authors: Fang, C.; Gan, W. -Q.
Bibcode: 1987ChJSS...7...29F
Altcode:
No abstract at ADS
Title: Unresolved small magnetic fluxtubes on the sun.
Authors: Fang, C.
Bibcode: 1987PPMtO...6....1F
Altcode:
Since the 1970s many observational facts have shown that most of solar
magnetic fluxes, except in sunspots, are concentrated in the form of
so-called magnetic fluxtubes, the diameters of which are believed to
be less than 0.1arcsec. The existence of small magnetic fluxtubes has
made a great influence upon the research of solar physics. This review
describes the history of these developments, the methods of diagnostics,
the results obtained so far and theoretical models.
Title: Numerical modeling of the arch eruptive prominence on Dec. 20,
1982.
Authors: Hu, Y. -Q.; Jin, S. -P.; Ni, X. -B.; Cui, L. -S.; Ji, G. -P.;
Fang, C.
Bibcode: 1986AcASn..27...47H
Altcode:
The present paper presents a numerical modeling of the eruptive
prominence on Dec. 20, 1982, based on one-dimensional hydrodynamical
equations. The effect of various factors, such as front deceleration,
thermal conduction, radiation loss and heating, on the formation of the
eruptive prominence is preliminarily discussed. The result shows that
arch eruptive prominences have completely different dynamic characters
from coronal arches.
Title: Chromospheric `active region loops'
Authors: Fang, C.; Martres, M. J.
Bibcode: 1986SoPh..105...51F
Altcode:
Archshaped structures above or around sunspot groups are considered as
tracers of the magnetic lines of force. A study of the chromospheric
contribution to the 3D general pattern is necessary to quantify
this relationship. The emissive features detected in nine different
active regions (AR) and observed on the disk at different levels
in the chromosphere have been analysed (6 maps/AR). A good spatial
correspondence is found between the maxima of Ca II K3 and
Hα emissions. Eleven archshaped structures may be easily interpreted as
loops. The footpoints are located on both sides of an inversion region
in the magnetic field. They always avoid the local maxima and minima
of the photospheric line-of-sight magnetic fields (H∥)
pattern independent of the heliographic longitude. This suggests that
the magnetic lines of force may have an oblique direction relative to
the solar surface.
Title: Semiempirical time-varying models of chromospheric flares.
Authors: Fang, C.; Gan, W. Q.; Huang, Y. R.; Hu, J.
Bibcode: 1986lasf.conf..117F
Altcode: 1986lasf.symp..117F
It is proposed to use Balmer lines and Ca II lines as good diagnostics
of the models of chromospheric flares. By use of an iteration method to
solve the equations describing hydrostatic, radiative and statistical
equilibrium for hydrogen and ionized calcium atoms, the semiempirical
time-varying models of chromospheric flares can be computed; the
observed line profiles are well reproduced. As an application, the
authors obtained the semiempirical time-varying models of the flare
that occurred on 2 Dec., 1982.
Title: White light flares and atmospheric modeling (Working Group
report).
Authors: Machado, M. E.; Avrett, E. H.; Falciani, R.; Fang, C.;
Gesztelyi, L.; Henoux, J. -C.; Hiei, E.; Neidig, D. F.; Rust, D. M.;
Sotirovski, P.; Svestka, Z.; Zirin, H.
Bibcode: 1986lasf.conf..483M
Altcode:
The authors give a short summary of their discussions, and a set
of recommendations which may help in the study of white light flare
emission processes.
Title: Semiempirical time-varying models of the chromospheric flare
on Dec. 2, 1982.
Authors: Fang, C.; Huang, Y. -R.; Hu, J.; Gan, W. -Q.
Bibcode: 1986AcApS...6..322F
Altcode:
Using the multichannel spectrograph installed in the solar tower
telescope of Nanjing University, the authors obtained the whole
process spectra of an SB flare (S15W11) in Hα, Hβ and Ca II K line
simultaneously. A series of Hα, Hβ and Ca II K line profiles and
relevant parameters corresponding to seven moments are given. The
non-LTE semiempirical time-varying models of the flare have been
computed.
Title: Continuum emission flares on Dec. 30, 1982 and on Feb. 3, 1983.
Authors: Huang, Y. R.; Fang, C.; Hu, J.
Bibcode: 1986lasf.conf..178H
Altcode:
The results indicated that the variations of the continuum emission of
the two flares are more than 8 - 10% of the undisturbed values. Besides,
the continuum emission coincided well in space and in time with the
Hα kernel emission.
Title: A possible mechanism of formation of faculae
Authors: Ni, X. -B.; Jiang, Y. -T.; Chen, Z. -Z.; Fang, C.
Bibcode: 1985AcASn..26..107N
Altcode:
It is shown that the formation of faculae is the result of Joule
dissipation of the Hall current generated by the interaction of a
convection field of granules and the inter-granular magnetic field. The
characteristic length of the region generating the Hall current must
correspond to a Reynolds number of less than 1. The energy balance
equation for a typical faculae region is derived and the results are
used to calculate the velocity fields for five observational models
of faculae. The proposed mechanism can also be used to explain the
dependence of the facula brightness on the strength of the magnetic
fields, as well as the apparent distribution of faculae on the solar
disk. A schematic diagram showing an inter-granular magnetic flux tube
in a convection field of granules is provided.
Title: Dynamic models of flux tubes in the interpretation of
polarization measurements.
Authors: Ribes, E.; Rees, D.; Fang, C.
Bibcode: 1985NASCP2374..300R
Altcode:
Dynamic models lead to solutions quite different from hydrostatic
ones. In view of the detection of mass motions with large velocity
gradients, polarization measurements have to be interpreted in terms
of dynamic models. The main properties of the intensity line profiles
as well as the asymmetries of the V Stokes profiles can be explained
best in terms of magnetic elements with moderate field strength.
Title: Structure and Physics of Solar Faculae - Part Four -
Chromospheric Granular Structure
Authors: Fang, C.; Mouradian, Z.; Banos, G.; Dumont, S.; Pecker, J. C.
Bibcode: 1984SoPh...91...61F
Altcode:
The methods used and the results obtained in the measurement of
the distances between the centers of chromospheric granules are
described. A coincidence of these structures at two different altitudes
was observed. Observations made in the K2v, or in the K3 and CN lines
permit the comparison of two different altitudes: the upper and the
lower chromosphere. These results include flocculi on the edge of
the supergranules as well as plages. Two main results are obtained:
(l)the most likely distance between two neighboring granules is, at
the minimum of the solar cycle, of about 2″. 60 for K3 and 2″.45
for CN, and (2) this distance is decreasing with growing solar activity.
Title: Photometric catalogue for stars in Selected Areas and other
fields inthe RGU-system (X). Photometry of fields in and near to the
Milky Way: Anticenter 5, Carina (IC 2581), Centaurus III, Aquila II,
Cassiopeia(NGC 7654).
Authors: Becker, W.; Fenkart, R. P.; Spaenhauer, A.; Alfaro, E. A.;
Kandemir, G.; Karaali, S.; Fang, C.; Garcia-Pelayo, J. M.; Steppe,
H.; Topaktas, L.
Bibcode: 1984pcss.book.....B
Altcode:
The tenth volume of the Basle Photometric Catalogues contains the
G-magnitudes, as well as the long- (G-R) and short-wave (U-G) colour
indices of the stars in five galactic fields. Additionally, data as
obtained from the interpretationof the two-colour diagrams are given,
i.e.: if the star in question has been identified as a main-sequence
star of either population, or asa late-type giant, or if it is blended
on the photographic plates of one or more colours.
Title: Photospheric and Chromospheric Magnetic Field Structure
Authors: Mouradian, Z.; Dumont, S.; Fang, C.; Pecker, J. C.; Banos, G.
Bibcode: 1984apoa.conf...63M
Altcode:
No abstract at ADS
Title: Simultaneous monochromatic and spectral observations of two
large loop prominence groups
Authors: Cui, L. -S.; Hu, J.; Ji, G. -P.; Ni, X. -B.; Huang, Y. -R.;
Fang, C.
Bibcode: 1984AcApS...4..279C
Altcode: 1984AcApS...4..285C
Using the solar tower telescope of Nanjing University, the two
large loop-prominence groups of Dec. 20, 1982 and Feb. 9, 1983, were
observed. H-alha photographs and spectra around the H-alpha and H and
K lines were obtained simultaneously. From these data, a line-of-sight
velocity distribution which agrees perfectly with the distribution for
matter falling freely without viscosity is derived. From the widths
of the H-alpha and the K lines, the loop material is found to have a
uniform kinetic temperature and a turbulent velocity that increases with
height. From the central intensities of the lines a density of n(H) =
(1.3-2.6) x 10 to the 10th/cu cm is derived. A possible mechanism of
the formation of loop-prominence groups and their relation with flares
are discussed.
Title: The Solar Tower Telescope of Nanjing University and its
Multiple Solar Spectrograph
Authors: Fang, C.; Huang, Y. R.
Bibcode: 1983AcASn..24..189F
Altcode:
No abstract at ADS
Title: Self-consistent models of flare heated solar chromospheres
Authors: Fang, C.; Henoux, J. C.
Bibcode: 1983A&A...118..139F
Altcode:
For the first time self consistent models of an X-ray irradiated
flaring chromosphere have been derived, together with a model of
an electron heated chromosphere. The atmospheric radiation losses
have been computed by including Lα, Lβ, LyC, Hα, Mg II and Ca
II and H- emission. The hydrogen radiation losses were
computed using complete linearization of the transfer, energy balance
and hydrostatic equilibrium equations coupled with the statistical
equilibrium equations. This method permits a substantial increase in
the accuracy of the radiation loss estimate without requiring excessive
computing time.
Title: RGU three colour photometry of a field in Centaurus
Authors: Spaenhauer, A.; Fang, C.
Bibcode: 1982A&AS...47..441S
Altcode:
RGU system measurements are presented for 2425 stars in a field in
Centaurus, to the limiting magnitudes of 16.6 m, 15.2 m, and 19.0 m for
G, R and U, respectively. Density functions of main sequence stars for
different intervals of absolute magnitudes, and of late type giants, are
derived on the basis of statistical photometric parallaxes. In addition,
the relation color excess E(G-R) versus distance is established up
to 2 kpc, along with the luminosity function for different intervals
of distance.
Title: Short description of new catalogues available.
Authors: Warren, W. H., Jr.; Becker, W.; Svolopoulos, S. N.; Fang, C.;
Fenkart, R.; Schaltenbrand, R.; Wagner, R.; Yilmaz, F.; Fünfschilling,
H.; Svolopoulos, N.; Topaktas, L.; Wooden, W.; Jaschek, M.; Buscombe,
W.; Evans, D. S.; Eichendorf, W.; Reinhardt, M.
Bibcode: 1978BICDS..15..116W
Altcode:
No abstract at ADS
Title: Photometric Catalogue for stars in selected areas and other
fields in the RGU-system (III). Milky Way fields: Perseus_I, NGC_1664,
M37_I, M37_II, NGC_6067, Sagittarius_I.
Authors: Becker, W.; Fenkart, R.; Fang, C.; Fünfschilling, H.;
Svolopoulos, N.; Topaktas, L.; Wooden, W.; Yilmaz, F.
Bibcode: 1977pcss.book.....B
Altcode:
No abstract at ADS
Title: Eclipse Comet Search of 7 March 1970
Authors: Bonner, George P.; Fang, C.; Kovar, N. S.; Kovar, R. P.
Bibcode: 1970BAAS....2T.297B
Altcode:
No abstract at ADS
Title: The estimates of the galactic clusters NGC 1664 and NGC 1605.
Authors: Fang, C.
Bibcode: 1970A&A.....4...75F
Altcode:
The distances of the two galactic clusters NGC 1664 and NGC 1604 are
determined with the method of three-color photometry. The distances
are found to be 1240 and 2650 po, respectively. Key words: star cluster
- galactic