Author name code: chae
ADS astronomy entries on 2022-09-14
author:"Chae, Jong Chul" OR author:"Chae, Jongchul"
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Title: Propagating Alfvénic Waves Observed in the Chromosphere
around a Small Sunspot: Tales of 3-minute Waves and 10-minute Waves
Authors: Chae, Jongchul; Cho, Kyuhyoun; Lim, Eun-Kyung; Kang, Juhyung
Bibcode: 2022ApJ...933..108C
Altcode:
Recent observations provided evidence that the solar chromosphere
of sunspot regions is pervaded by Alfvénic waves-transverse
magnetohydrodynamic (MHD) waves (Alfvén waves or kink waves). In order
to systematically investigate the physical characteristics of Alfvénic
waves over a wide range of periods, we analyzed the time series of
line-of-sight velocity maps constructed from the Hα spectral data of
a small sunspot region taken by the Fast Imaging Solar Spectrograph of
the Goode Solar Telescope at Big Bear. We identified each Alfvénic
wave packet by examining the cross-correlation of band-filtered
velocity between two points that are located a little apart presumably
on the same magnetic field line. As result, we detected a total of
279 wave packets in the superpenumbral region around the sunspot and
obtained their statistics of period, velocity amplitude, and propagation
speed. An important finding of ours is that the detected Alfvénic waves
are clearly separated into two groups: 3-minute period (<7 minutes)
waves and 10-minute period (>7 minutes) waves. We propose two tales
on the origin of Alfvénic waves in the chromosphere; the 3-minute
Alfvénic waves are excited by the upward-propagating slow waves in
the chromosphere through the slow-to-Alfvénic mode conversion, and
the 10-minute Alfvénic waves represent the chromospheric manifestation
of the kink waves driven by convective motions in the photosphere.
Title: Fast Spectral Inversion of the H And Ca II 8542 Line Spectra
Based on a Deep Learning Model
Authors: Lee, Kyoung SUN; Chae, Jongchul; Park, Eunsu; Moon, Yong-Jae;
Kwak, Hannah; Cho, Kyuhyoun
Bibcode: 2021AGUFMSH44A..01L
Altcode:
Recently a multilayer spectral inversion (MLSI) model has been
proposed to infer the physical parameters of plasmas in the solar
chromosphere. The inversion solves a three-layer radiative transfer
model using the H alpha and Ca II 8542 Å line profiles taken by
the Fast Imaging Solar Spectrograph (FISS). The model successfully
provides the physical plasma parameters, such as source functions,
Doppler velocities, and Doppler widths in the layers of photosphere
to chromosphere. However, it is quite expensive to apply the MLSI to
a huge number of line profiles. For example, the calculating time is
several hours for a scan raster. We apply deep-learning methods to
the inversion code to reduce the cost of calculating the physical
parameters. We train the models using pairs of absorption line
profiles (H alpha and Ca II 8542 Å) from FISS and their 13 physical
parameters (source functions, Doppler velocities, Doppler widths in
the chromosphere, and the pre-determined parameters for photosphere)
calculated from the spectral inversion code for 50 scan rasters
(~2,000,000 dataset) including quiet and active regions. We use a fully
connected dense layers for training the model. In addition, we utilize a
skip connections to avoid a problem of vanishing gradients. We evaluate
the model by comparing the pairs of absorption line profiles and their
inverted physical parameters from other quiet and active regions. Our
result shows that the deep learning model successfully reproduces
physical parameter maps of a scan raster observation per second within
15% of mean absolute percentage error and the mean squared errors of
0.3 to 0.003 depending on the parameters. Taking this advantage of
high-performance of the deep learning model, we plan to provide the
physical parameter maps from the FISS observations to understand the
chromospheric plasma conditions in various solar features.
Title: Investigation of the subsurface structure of a sunspot based
on the spatial distribution of oscillation centers inferred from
umbral flashes
Authors: Cho, Kyuhyoun; Chae, Jongchul; Madjarska, Maria S.
Bibcode: 2021A&A...656A..86C
Altcode: 2021arXiv210911185C
The subsurface structure of a solar sunspot is important for the
stability of the sunspot and the energy transport therein. Two
subsurface structure models have been proposed, the monolithic
and cluster models, but no clear observational evidence supporting
a particular model has been found to date. To obtain clues about
the subsurface structure of sunspots, we analyzed umbral flashes
in merging sunspots registered by IRIS Mg II 2796 Å slit-jaw
images. Umbral flashes are regarded as an observational manifestation
of magnetohydrodynamic shock waves originating from convection cells
below the photosphere. By tracking the motion of individual umbral
flashes, we determined the position of the convection cells that
are the oscillation centers located below the umbra. We found that
the oscillation centers are preferentially located at dark nuclei
in the umbral cores rather than in bright regions such as light
bridges or umbral dots. Moreover, the oscillation centers tend to
deviate from the convergent interface of the merging sunspots where
vigorous convection is expected to occur. We also found that the
inferred depths of the convection cells have no noticeable regional
dependence. These results suggest that the subsurface of the umbra
is an environment where convection can occur more easily than the
convergent interface, and hence support the cluster model. For more
concrete results, further studies based on umbral velocity oscillations
in the lower atmosphere are required. Movie is available at https://www.aanda.org
Title: Electric resistivity of partially ionized plasma in the lower
solar atmosphere
Authors: Chae, Jongchul; Litvinenko, Yuri E.
Bibcode: 2021RAA....21..232C
Altcode:
The lower solar atmosphere is a gravitationally stratified layer of
partially ionized plasma. We calculate the electric resistivity in the
solar photosphere and chromosphere, which is the key parameter that
controls the rate of magnetic reconnection in a Sweet-Parker current
sheet. The calculation takes into account the collisions between ions
and hydrogen atoms as well as the electron-ion collisions and the
electron-hydrogen atom collisions. We find that under the typical
conditions of the quiet Sun, electric resistivity is determined
mostly by the electron-hydrogen atom collisions in the photosphere,
and mostly by the ion-hydrogen collisions, i.e. ambipolar diffusion, in
the chromosphere. In magnetic reconnection events with strong magnetic
fields, the ambipolar diffusion, however, may be insignificant because
the heating by the reconnection itself may lead to the full ionization
of hydrogen atoms. We conclude that ambipolar diffusion may be the most
important source of electric resistivity responsible for the magnetic
flux cancelation and energy release in chromospheric current sheets
that can keep a significant fraction of neutral hydrogen atoms.
Title: Multilayer Spectral Inversion of Solar Hα and Ca II 8542
Line Spectra with Height-varying Absorption Profiles
Authors: Chae, Jongchul; Cho, Kyuhyoun; Kang, Juhyung; Lee, Kyoung-Sun;
Kwak, Hannah; Lim, Eun-Kyung
Bibcode: 2021JKAS...54..139C
Altcode:
We present an updated version of the multilayer spectral inversion
(MLSI) recently proposed as a technique to infer the physical parameters
of plasmas in the solar chromosphere from a strong absorption line. In
the original MLSI, the absorption profile was constant over each layer
of the chromosphere, whereas the source function was allowed to vary
with optical depth. In our updated MLSI, the absorption profile is
allowed to vary with optical depth in each layer and kept continuous
at the interface of two adjacent layers. We also propose a new set of
physical requirements for the parameters useful in the constrained
model fitting. We apply this updated MLSI to two sets of Hα and Ca
II line spectral data taken by the Fast Imaging Solar Spectrograph
(FISS) from a quiet region and an active region, respectively. We
find that the new version of the MLSI satisfactorily fits most of
the observed line profiles of various features, including a network
feature, an internetwork feature, a mottle feature in a quiet region,
and a plage feature, a superpenumbral fibril, an umbral feature, and
a fast downflow feature in an active region. The MLSI can also yield
physically reasonable estimates of hydrogen temperature and nonthermal
speed as well as Doppler velocities at different atmospheric levels. We
conclude that the MLSI is a very useful tool to analyze the Hα line and
the Ca II 8542 line spectral data, and will promote the investigation of
physical processes occurring in the solar photosphere and chromosphere.
Title: Ionization of Hydrogen in the Solar Atmosphere
Authors: Chae, Jongchul
Bibcode: 2021JASS...38...83C
Altcode:
The ionization degree of hydrogen is crucial in the physics of the
plasma in the solar chromosphere. It specifically limits the range of
plasma temperatures that can be determined from the Hα line. Given
that the chromosphere greatly deviates from the local thermodynamic
equilibrium (LTE) condition, precise determinations of hydrogen
ionization require the solving of the full set of non-LTE radiative
transfer equations throughout the atmosphere, which is usually a
formidable task. In many cases, it is still necessary to obtain a
quick estimate of hydrogen ionization without having to solve for
the non-LTE radiative transfer. Here, we present a simple method
to meet this need. We adopt the assumption that the photoionizing
radiation field changes little over time, even if physical conditions
change locally. With this assumption, the photoionization rate can be
obtained from a published atmosphere model and can be used to determine
the degree of hydrogen ionization when the temperature and electron
density are specified. The application of our method indicates that in
the chromospheric environment, plasma features contain more than 10%
neutral hydrogen at temperatures lower than 17,000 K but less than 1%
neutral hydrogen at temperatures higher than 23,000 K, implying that
the hydrogen temperature determined from the Hα line is physically
plausible if it is lower than 20,000 K, but may not be real, if it
is higher than 25,000 K. We conclude that our method can be readily
exploited to obtain a quick estimate of hydrogen ionization in plasma
features in the solar chromosphere.
Title: Spectroscopic Detection of Alfvénic Waves in the Chromosphere
of Sunspot Regions
Authors: Chae, Jongchul; Cho, Kyuhyoun; Nakariakov, Valery M.; Cho,
Kyung-Suk; Kwon, Ryun-Young
Bibcode: 2021ApJ...914L..16C
Altcode:
Transverse magnetohydrodynamic waves often called Alfvénic (or
kink) waves have been often theoretically put forward to solve the
outstanding problems of the solar corona like coronal heating, solar
wind acceleration, and chemical abundance enhancement. Here we report
the first spectroscopic detection of Alfvénic waves around a sunspot at
chromospheric heights. By analyzing the spectra of the Hα line and Ca
II 854.2 nm line, we determined line-of-sight velocity and temperature
as functions of position and time. As a result, we identified transverse
magnetohydrodynamic waves pervading the superpenumbral fibrils. These
waves are characterized by the periods of 2.5 to 4.5 minutes, and
the propagation direction parallel to the fibrils, the supersonic
propagation speeds of 45 to 145 km s-1, and the close
association with umbral oscillations and running penumbral waves in
sunspots. Our results support the notion that the chromosphere around
sunspots abounds with Alfvénic waves excited by the mode conversion
of the upward-propagating slow magnetoacoustic waves.
Title: The chromospheric component of coronal bright points. Coronal
and chromospheric responses to magnetic-flux emergence
Authors: Madjarska, Maria S.; Chae, Jongchul; Moreno-Insertis,
Fernando; Hou, Zhenyong; Nóbrega-Siverio, Daniel; Kwak, Hannah;
Galsgaard, Klaus; Cho, Kyuhyoun
Bibcode: 2021A&A...646A.107M
Altcode: 2020arXiv201209426M
Context. We investigate the chromospheric counterpart of small-scale
coronal loops constituting a coronal bright point (CBP) and its
response to a photospheric magnetic-flux increase accompanied by
co-temporal CBP heating.
Aims: The aim of this study is
to simultaneously investigate the chromospheric and coronal layers
associated with a CBP, and in so doing, provide further understanding on
the heating of plasmas confined in small-scale loops.
Methods:
We used co-observations from the Atmospheric Imaging Assembly and
Helioseismic Magnetic Imager on board the Solar Dynamics Observatory,
together with data from the Fast Imaging Solar Spectrograph taken
in the Hα and Ca II 8542.1 Å lines. We also employed both linear
force-free and potential field extrapolation models to investigate
the magnetic topology of the CBP loops and the overlying corona,
respectively. We used a new multi-layer spectral inversion technique
to derive the temporal variations of the temperature of the Hα loops
(HLs).
Results: We find that the counterpart of the CBP, as
seen at chromospheric temperatures, is composed of a bundle of dark
elongated features named in this work Hα loops, which constitute an
integral part of the CBP loop magnetic structure. An increase in the
photospheric magnetic flux due to flux emergence is accompanied by
a rise of the coronal emission of the CBP loops, that is a heating
episode. We also observe enhanced chromospheric activity associated
with the occurrence of new HLs and mottles. While the coronal emission
and magnetic flux increases appear to be co-temporal, the response of
the Hα counterpart of the CBP occurs with a small delay of less than
3 min. A sharp temperature increase is found in one of the HLs and
in one of the CBP footpoints estimated at 46% and 55% with respect
to the pre-event values, also starting with a delay of less than 3
min following the coronal heating episode. The low-lying CBP loop
structure remains non-potential for the entire observing period. The
magnetic topological analysis of the overlying corona reveals the
presence of a coronal null point at the beginning and towards the end
of the heating episode.
Conclusions: The delay in the response
of the chromospheric counterpart of the CBP suggests that the heating
may have occurred at coronal heights. Movies are available at https://www.aanda.org
Title: Detection of Opposite Magnetic Polarity in a Light Bridge:
Its Emergence and Cancellation in Association with LB Fan-shaped Jets
Authors: Lim, Eun-Kyung; Yang, Heesu; Yurchyshyn, Vasyl; Chae,
Jongchul; Song, Donguk; Madjarska, Maria S.
Bibcode: 2020ApJ...904...84L
Altcode: 2020arXiv201010713L
Light bridges (LBs) are relatively bright structures that divide
sunspot umbrae into two or more parts. Chromospheric LBs are known to
be associated with various activities including fan-shaped jet-like
ejections and brightenings. Although magnetic reconnection is
frequently suggested to be responsible for such activities, not many
studies present firm evidence to support the scenario. We carry out
magnetic field measurements and imaging spectroscopy of an LB where
fan-shaped jet-like ejections occur with co-spatial brightenings
at their footpoints. We study LB fine structure and magnetic field
changes using TiO images, Near-InfraRed Imaging Spectropolarimeter,
and Hα data taken by the 1.6 m Goode Solar Telescope. We detect
magnetic flux emergence in the LB that is of opposite polarity to that
of the sunspot. The new magnetic flux cancels with the pre-existing
flux at a rate of 5.6 × 1018 Mx hr-1. Both
the recurrent jet-like ejections and their base brightenings are
initiated at the vicinity of the magnetic flux cancellation, and show
apparent horizontal extension along the LB at a projected speed of up
to $18.4$ km s-1 to form a fan-shaped appearance. Based on
these observations, we suggest that the fan-shaped ejections may have
resulted from slipping reconnection between the new flux emerging in
the LB and the ambient sunspot field.
Title: Impulsive wave excitation by rapidly changing granules
Authors: Kwak, Hannah; Chae, Jongchul; Madjarska, Maria S.; Cho,
Kyuhyoun; Song, Donguk
Bibcode: 2020A&A...642A.154K
Altcode: 2020arXiv200812779K
It is not yet fully understood how magnetohydrodynamic waves in
the interior and atmosphere of the Sun are excited. Traditionally,
turbulent convection in the interior is considered to be the source of
wave excitation in the quiet Sun. Over the last few decades, acoustic
events observed in the intergranular lanes in the photosphere have
emerged as a strong candidate for a wave excitation source. Here we
report our observations of wave excitation by a new type of event:
rapidly changing granules. Our observations were carried out with the
Fast Imaging Solar Spectrograph in the Hα and Ca II 8542 Å lines
and the TiO 7057 Å broadband filter imager of the 1.6 m Goode Solar
Telescope at the Big Bear Solar Observatory. We identify granules in
the internetwork region that undergo rapid dynamic changes such as
collapse (event 1), fragmentation (event 2), or submergence (event
3). In the photospheric images, these granules become significantly
darker than neighboring granules. Following the granules' rapid
changes, transient oscillations are detected in the photospheric and
chromospheric layers. In the case of event 1, the dominant period of
the oscillations is close to 4.2 min in the photosphere and 3.8 min
in the chromosphere. Moreover, in the Ca II-0.5 Å raster image, we
observe repetitive brightenings in the location of the rapidly changing
granules that are considered the manifestation of shock waves. Based on
our results, we suggest that dynamic changes of granules can generate
upward-propagating acoustic waves in the quiet Sun that ultimately
develop into shocks. Movie attached to Fig. A.1 is available at https://www.aanda.org
Title: Inference of chromospheric plasma parameters on the
Sun. Multilayer spectral inversion of strong absorption lines
Authors: Chae, Jongchul; Madjarska, Maria S.; Kwak, Hannah; Cho,
Kyuhyoun
Bibcode: 2020A&A...640A..45C
Altcode:
The solar chromosphere can be observed well through strong absorption
lines. We infer the physical parameters of chromospheric plasmas
from these lines using a multilayer spectral inversion. This is a
new technique of spectral inversion. We assume that the atmosphere
consists of a finite number of layers. In each layer the absorption
profile is constant and the source function varies with optical depth
with a constant gradient. Specifically, we consider a three-layer
model of radiative transfer where the lowest layer is identified
with the photosphere and the two upper layers are identified with
the chromosphere. The absorption profile in the photosphere is
described by a Voigt function, and the profile in the chromosphere
by a Gaussian function. This three-layer model is fully specified
by 13 parameters. Four parameters can be fixed to prescribed values,
and one parameter can be determined from the analysis of a satellite
photospheric line. The remaining 8 parameters are determined from a
constrained least-squares fitting. We applied the multilayer spectral
inversion to the spectral data of the Hα and the Ca II 854.21 nm
lines taken in a quiet region by the Fast Imaging Solar Spectrograph
(FISS) of the Goode Solar Telescope (GST). We find that our model
successfully fits most of the observed profiles and produces regular
maps of the model parameters. The combination of the inferred Doppler
widths of the two lines yields reasonable estimates of temperature and
nonthermal speed in the chromosphere. We conclude that our multilayer
inversion is useful to infer chromospheric plasma parameters on the Sun.
Title: Inference of chromospheric plasma parameters on the Sun
Authors: Chae, Jongchul; Madjarska, Maria S.; Kwak, Hannah; Cho,
Kyuhyoun
Bibcode: 2020arXiv200611981C
Altcode:
The solar chromosphere can be observed well through strong absorption
lines. We infer the physical parameters of chromospheric plasmas
from these lines using a multilayer spectral inversion. This is a
new technique of spectral inversion. We assume that the atmosphere
consists of a finite number of layers. In each layer the absorption
profile is constant and the source function varies with optical depth
with a constant gradient. Specifically, we consider a three-layer
model of radiative transfer where the lowest layer is identified
with the photosphere and the two upper layers are identified with
the chromosphere. The absorption profile in the photosphere is
described by a Voigt function, and the profile in the chromosphere
by a Gaussian function. This three-layer model is fully specified
by 13 parameters. Four parameters can be fixed to prescribed values,
and one parameter can be determined from the analysis of a satellite
photospheric line. The remaining 8 parameters are determined from a
constrained least-squares fitting. We applied the multilayer spectral
inversion to the spectral data of the H$\alpha$ and the Ca II 854.21 nm
lines taken in a quiet region by the Fast Imaging Solar Spectrograph
(FISS) of the Goode Solar Telescope (GST). We find that our model
successfully fits most of the observed profiles and produces regular
maps of the model parameters. The combination of the inferred Doppler
widths of the two lines yields reasonable estimates of temperature and
nonthermal speed in the chromosphere. We conclude that our multilayer
inversion is useful to infer chromospheric plasma parameters on the Sun.
Title: The Application of the Filtered Backprojection Algorithm to
Solar Rotational Tomography
Authors: Cho, Kyuhyoun; Chae, Jongchul; Kwon, Ryun-Young; Bong,
Su-Chan; Cho, Kyung-Suk
Bibcode: 2020ApJ...895...55C
Altcode: 2020arXiv200506388C
Solar rotational tomography (SRT) is an important method to reconstruct
the physical parameters of the three-dimensional solar corona. Here
we propose an approach to apply the filtered backprojection (FBP)
algorithm to the SRT. The FBP algorithm is generally not suitable
for SRT due to the several issues with solar extreme ultraviolet
(EUV) observations—in particular, a problem caused by missing data
because of the unobserved back side of corona hidden behind the Sun. We
developed a method to generate a modified sinogram that resolves the
blocking problem. The modified sinogram is generated by combining
the EUV data at two opposite sites observed by the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory (SDO). We
generated the modified sinogram for about one month in 2019 February
and reconstructed the three-dimensional corona under the static state
assumption. In order to obtain the physical parameters of the corona,
we employed a differential emission measure inversion method. We
tested the performance of the FBP algorithm with the modified sinogram
by comparing the reconstructed data with the observed EUV image,
electron density models, previous studies of electron temperature,
and an observed coronagraph image. The results illustrate that the FBP
algorithm reasonably reconstructs the bright regions and the coronal
holes and can reproduce their physical parameters. The main advantage of
the FBP algorithm is that it is easy to understand and computationally
efficient. Thus, it enables us to easily probe the inhomogeneous coronal
electron density and temperature distribution of the solar corona.
Title: Source Depth of Three-minute Umbral Oscillations
Authors: Cho, Kyuhyoun; Chae, Jongchul
Bibcode: 2020ApJ...892L..31C
Altcode: 2020arXiv200310542C
We infer the depth of the internal sources giving rise to three-minute
umbral oscillations. Recent observations of ripple-like velocity
patterns of umbral oscillations supported the notion that there
exist internal sources exciting the umbral oscillations. We adopt the
hypothesis that the fast magnetohydrodynamic (MHD) waves generated at
a source below the photospheric layer propagate along different paths,
reach the surface at different times, and excited slow MHD waves by
mode conversion. These slow MHD waves are observed as the ripples that
apparently propagate horizontally. The propagation distance of the
ripple given as a function of time is strongly related to the depth
of the source. Using the spectral data of the Fe I 5435 Å line taken
by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope at
Big Bear Solar Observatory, we identified five ripples and determined
the propagation distance as a function of time in each ripple. From
the model fitting to these data, we obtained the depth between 1000
and 2000 km. Our result will serve as an observational constraint to
understanding the detailed processes of magnetoconvection and wave
generation in sunspots.
Title: A Study of Small Flares Associated with Plasma Blobs Outflowing
along Post-CME Rays
Authors: Kim, Yoo Jung; Kwon, Ryun-Young; Chae, Jongchul
Bibcode: 2020JKAS...53....1K
Altcode:
The recent study of Chae et al. (2017) found a one-to-one correspondence
between plasma blobs outflowing along a ray formed after a coronal
mass ejection (CME) and small X-ray flares. In the present work, we
have examined the spatial configuration and the eruption process of
the flares that are associated with the blobs by analyzing EUV images
and magnetograms taken by the SDO/AIA and HMI. We found that the main
flare and the successive small flares took place in a quadrupolar
magnetic configuration characterized by predominant magnetic fields of
positive polarity, two minor magnetic fragments of negative polarity,
and a curved polarity inversion line between them, which suggests that
the formation process of the blobs may be similar to that of the parent
CME. We also found that the successive flares resulted in a gradual
change of the quadrupolar magnetic configuration, and the relevant
migration of flaring kernels. The three-dimensional geometry and the
property of the current sheet, that is often supposed to be embedded
in an observed post-CME ray, seem to keep changing because of mutual
feedback between the successive flares and the temporal change of the
magnetic field configuration. Our results suggest that the observed
post-CME rays may not reflect the characteristics of the current sheet
responsible for the impulsive phase of the flare.
Title: Launch and early operation result of charged particle detector
in the range of 20 keV to 400 keV with deflecting electrostatic
field for in-situ measurement of suprathermal electrons, protons,
and neutrals
Authors: Seo, H.; Kim, E.; Ryu, K.; Shin, G. H.; Chae, J. S.; Shin, Y.;
Woo, J.; Seol, W.; Lee, C. H.; Lee, H.; Sohn, J. D.; Min, K.; Seon, J.
Bibcode: 2019AGUFMSH41E3306S
Altcode:
Medium Energy Particle Detector (MEPD) is an instrument capable
of measuring energy distribution of electrons, protons, and
neutrals in the energy range of 20 ~ 400 keV with 6.25 keV linear
or 2~24.32 keV pseudo-log energy resolution, 1~128 Hz cadence, and
106~108/cm2•sr•sec•keV dynamic
range of incident fluxes. The apparatus consists of two identical
telescopes, each with a field of view of 15 º x 70 º and total
geometric factor of about 0.01cm2·sr, with orthogonal
placement for the detection of different pitch angles of 0 º and 90
º. Each telescope utilizes an electrostatic field of 4,000 V / (3 ×
10-3 m) to deflect trajectories of incident particles and
a silicon detector with 4 identical pixels relatively placed along
the direction of the electric fields. MEPD produces spectral data
by recording number of detected particles with respect to individual
energy bins divided into 64. MEPD is a subset instrument of Instrument
for the Study of Space Storm (ISSS) launched on a sun-synchronous orbit
at an altitude of 575 km on December 4th, 2018. During the launch and
early operation period, commissioning observations are performed with
a measurement time of about 250 seconds in the sub-auroral, auroral,
and low-latitude regions. Typical spectra observed in sub-auroral
regions represent dominance of electrons with the pitch angle of 90 º
implying trapped electrons of the outer radiation belt. The measurements
in the auroral regions demonstrate downward suprathermal protons with
the pitch angle of 0 º in response to the electrostatic field. The
observations regarded as ENAs are found in both low-latitude and
high-latitude regions by examining the distinct spectra not affected by
the electrostatic field. The spectra acquired by pointing the telescope
toward the magneto-tail bound in the low-latitude regions are correlated
with Dst index indicating ENAs from the ring current,
whereas the spectra regarded as ENAs in the high-latitude regions are
measured with the pitch angle of 90 º indicating polar wind neutrals.
Title: Linear Acoustic Waves in a Nonisothermal
Atmosphere. II. Photospheric Resonator Model of Three-minute Umbral
Oscillations
Authors: Chae, Jongchul; Kang, Juhyung; Litvinenko, Yuri E.
Bibcode: 2019ApJ...883...72C
Altcode:
The velocity oscillations observed in the chromosphere of sunspot
umbrae resemble a resonance in that their power spectra are sharply
peaked around a period of about three minutes. In order to describe
the resonance that leads to the observed 3-minute oscillations, we
propose the photospheric resonator model of acoustic waves in the solar
atmosphere. The acoustic waves are driven by the motion of a piston
at the lower boundary, and propagate in a nonisothermal atmosphere
that consists of the lower layer (photosphere), where temperature
rapidly decreases with height, and the upper layer (chromosphere),
where temperature slowly increases with height. We have obtained
the following results: (1) The lower layer (photosphere) acts as
a leaky resonator of acoustic waves. The bottom end is established
by the piston, and the top end by the reflection at the interface
between the two layers. (2) The temperature minimum region partially
reflects and partially transmits acoustic waves of frequencies around
the acoustic cutoff frequency at the temperature minimum. (3) The
resonance occurs in the photospheric layer at one frequency around this
cutoff frequency. (4) The waves escaping the photospheric layer appear
as upward-propagating waves in the chromosphere. The power spectrum of
the velocity oscillation observed in the chromosphere can be fairly well
reproduced by this model. The photospheric resonator model was compared
with the chromospheric resonator model and the propagating wave model.
Title: The Observational Evidence for the Internal Excitation of
Sunspot Oscillations Inferred from the Fe I 5435 Å Line
Authors: Cho, Kyuhyoun; Chae, Jongchul; Lim, Eun-kyung; Yang, Heesu
Bibcode: 2019ApJ...879...67C
Altcode: 2019arXiv190601971C
The umbral oscillations of velocity are commonly observed in the
chromosphere of a sunspot. Their sources are considered to be
either the external p-mode driving or the internal excitation by
magnetoconvection. Even though the possibility of the p-mode driving
has been often considered, the internal excitation has been rarely
investigated. We report the identification of the oscillation patterns
that may be closely related to the events of internal excitation
from the observations of velocity oscillations in the temperature
minimum region of two sunspots. The velocities were determined from the
spectral data of the Fe I 5435 Å line, a magnetically insensitive line,
taken with the Fast Imaging Solar Spectrograph of the 1.6 m Goode Solar
Telescope at the Big Bear Solar Observatory. As a result, we identified
four oscillation patterns of 2.0 × 103 km coherent size
that were clearly identified for about 7.9 minutes with an oscillation
amplitude of 9.3 × 10-2 km s-1. The power of the
oscillations in these centers was concentrated in the 3 minute band. All
the oscillation centers were located above the umbral dots undergoing
noticeable morphological and dynamical changes that may be regarded
as an observable signature of small-scale magnetoconvection inside
the umbrae. Our results support the notion that magnetoconvection
associated with umbral dots inside sunspots can drive the 3 minute
umbral oscillations.
Title: The Physical Nature of Spiral Wave Patterns in Sunspots
Authors: Kang, Juhyung; Chae, Jongchul; Nakariakov, Valery M.; Cho,
Kyuhyoun; Kwak, Hannah; Lee, Kyeore
Bibcode: 2019ApJ...877L...9K
Altcode: 2019arXiv190508908K
Recently, spiral wave patterns (SWPs) have been detected in 3 minute
oscillations of sunspot umbrae, but the nature of this phenomenon has
remained elusive. We present a theoretical model that interprets the
observed SWPs as the superposition of two different azimuthal modes
of slow magnetoacoustic waves driven below the surface in an untwisted
and non-rotating magnetic cylinder. We apply this model to SWPs of the
line-of-sight (LOS) velocity in a pore observed by the Fast Imaging
Solar Spectrograph installed at the 1.6 m Goode Solar Telescope. One-
and two-armed SWPs were identified in instantaneous amplitudes of
LOS Doppler velocity maps of 3 minute oscillations. The associated
oscillation periods are about 160 s, and the durations are about 5
minutes. In our theoretical model, the observed spiral structures are
explained by the superposition of non-zero azimuthal modes driven 1600
km below the photosphere in the pore. The one-armed SWP is produced
by the slow-body sausage (m = 0) and kink (m = 1) modes, and the
two-armed SWP is formed by the slow-body sausage (m = 0) and fluting
(m = 2) modes of the magnetic flux tube forming the pore.
Title: Depth of Ellerman Burst Derived from High-resolution Hα and
Ca II 8542 Å Spectra
Authors: Seo, Minju; Quintero Noda, Carlos; Lee, Jeongwoo; Chae,
Jongchul
Bibcode: 2019ApJ...871..125S
Altcode:
High-resolution spectra of an Ellerman burst (EB) sampling the Hα
and the Ca II 8542 Å lines obtained with the Fast Imaging Solar
Spectrograph (FISS) installed on the 1.6 m Goode Solar Telescope
at the Big Bear Solar Observatory are compared with synthetic line
profiles constructed using the RH code for nonlocal thermodynamical
equilibrium radiative transfer. The EB heating is modeled by a local
temperature hump above the quiet-Sun temperature. Our first finding is
that FISS Hα and Ca II 8542 Å intensity profiles cannot be reproduced
simultaneously by a single hump model as far as the hump is thicker
than ≥100 km. Simultaneous reproduction of both line profiles is
possible when the EB temperature enhancement is confined to a layer
as thin as ≤20 km in the photosphere where the Hα wing response is
high and that of the Ca II 8542 Å is not. Moreover, when we examine
the EB spectra at different times, we find that the EB at a time of
weaker appearance is located at lower heights, ∼50 km, and moves
upward to ∼120 km at the time of maximum intensity. Complementary
calculations of the Na I D1 and Mg I b2 lines
as well as that of UV continuum at 1600 and 1700 Å with the deduced
EB atmosphere are also performed to test the result, which allows
us to discuss the shortcomings of this plane-parallel static model
atmosphere for understanding the physical properties of EBs.
Title: Development of the SNU Coelostat: Conceptual Design
Authors: Kang, Juhyung; Chae, Jongchul; Kwak, Hannah; Yang, Heesu
Bibcode: 2018JKAS...51..207K
Altcode:
A coelostat is often used for solar observations, because it corrects
the image rotation automatically by guiding sunlight into a fixed
telescope with two plane mirrors. For the purposes of education and
spectroscopic observation, the solar group at Seoul National University
(SNU) plans to develop the SNU coelostat (SNUC) and install it in the
SNU Astronomical Observatory (SAO). Requirements of the SNUC are <1''
positioning accuracy with 30 cm beam size on the entrance pupil in the
compact dome. To allow for installation in the small dome, we design a
compact slope type coelostat with a 45 cm primary plane mirror and a
39 cm secondary plane mirror. The motion of the SNUC is minimized by
fixing the position of the slope frame. Numerical simulations of the
available observational time of the designed coelostat shows that the
sun can be observed ay all times from June to early August and at least
three hours in other months. Since the high accuracy driving motors
installed in the SNUC can be affected by external environment factors
such as humidity and temperature variations, we design a prototype to
test the significance of these effects. The prototype consists of a
20 cm primary plane mirror, a 1 m slope rail, a direct drive motor,
a ballscrew, a linear motion guide, an AC servo motor, a reduction
gear and a linear encoder. We plan to control and test the accuracy of
the prototype with varying atmospheric conditions in early 2019. After
testing the prototype, the SNUC will be manufactured and installed in
SAO by 2020.
Title: Evolution of Photospheric Vector Magnetic Field Associated
with Moving Flare Ribbons as Seen by GST
Authors: Liu, Chang; Cao, Wenda; Chae, Jongchul; Ahn, Kwangsu; Prasad
Choudhary, Debi; Lee, Jeongwoo; Liu, Rui; Deng, Na; Wang, Jiasheng;
Wang, Haimin
Bibcode: 2018ApJ...869...21L
Altcode: 2018arXiv181011733L
The photospheric response to solar flares, also known as coronal
back reaction, is often observed as sudden flare-induced changes in
the vector magnetic field and sunspot motions. However, it remains
obscure whether evolving flare ribbons, the flare signature closest to
the photosphere, are accompanied by changes in vector magnetic field
therein. Here we explore the relationship between the dynamics of
flare ribbons in the chromosphere and variations of magnetic fields
in the underlying photosphere, using high-resolution off-band Hα
images and near-infrared vector magnetograms of the M6.5 flare on 2015
June 22 observed with the 1.6 m Goode Solar Telescope. We find that
changes of photospheric fields occur at the arrival of the flare ribbon
front, thus propagating analogously to flare ribbons. In general, the
horizontal field increases and the field lines become more inclined
to the surface. When ribbons sweep through regions that undergo a
rotational motion, the fields transiently become more vertical with
decreased horizontal field and inclination angle, and then restore
and/or become more horizontal than before the ribbon arrival. The
ribbon propagation decelerates near the sunspot rotation center,
where the vertical field becomes permanently enhanced. Similar magnetic
field changes are discernible in magnetograms from the Helioseismic and
Magnetic Imager (HMI), and an inward collapse of coronal magnetic fields
is inferred from the time sequence of nonlinear force-free field models
extrapolated from HMI magnetograms. We conclude that photospheric fields
respond nearly instantaneously to magnetic reconnection in the corona.
Title: Linear Acoustic Waves in a Nonisothermal Atmosphere. I. Simple
Nonisothermal Layer Solution and Acoustic Cutoff Frequency
Authors: Chae, Jongchul; Litvinenko, Yuri E.
Bibcode: 2018ApJ...869...36C
Altcode:
We investigate the behavior of acoustic waves in a nonisothermal
atmosphere based on the analytical solution of the wave
equation. Specifically, we consider acoustic waves propagating upwardly
in a simple nonisothermal layer where temperature either increases or
decreases monotonically with height. We present the solutions for both
velocity fluctuation and pressure fluctuation. In these solutions,
either velocity or pressure is spatially oscillatory in one part of
the layer and nonoscillatory in the other part, with the two parts
being smoothly connected to one another. Since the two parts transmit
the same amount of wave energy in each frequency, it is unreasonable to
identify the oscillating solution with the propagating solution and the
nonoscillating solution with the nonpropagating solution. The acoustic
cutoff frequency is defined as the frequency that separates the solution
that is spatially oscillatory for both velocity and pressure and the
solution that is not oscillatory for either velocity or pressure. The
cutoff frequency is found to be the same as the Lamb frequency at the
bottom in the temperature-decreasing layer but higher than this in the
temperature-increasing layer. Based on the transmission efficiency
introduced to quantify the wave propagation, we suggest that the
acoustic cutoff frequency should be understood as the center of the
frequency band where the transition from low acoustic transmission to
high transmission takes place, rather than as the frequency sharply
separating the propagating solution and the nonpropagating solution.
Title: Observation of the Kelvin-Helmholtz Instability in a Solar
Prominence
Authors: Yang, Heesu; Xu, Zhi; Lim, Eun-Kyung; Kim, Sujin; Cho,
Kyung-Suk; Kim, Yeon-Han; Chae, Jongchul; Cho, Kyuhyoun; Ji, Kaifan
Bibcode: 2018ApJ...857..115Y
Altcode:
Many solar prominences end their lives in eruptions or abrupt
disappearances that are associated with dynamical or thermal
instabilities. Such instabilities are important because they may be
responsible for energy transport and conversion. We present a clear
observation of a streaming kink-mode Kelvin-Helmholtz Instability (KHI)
taking place in a solar prominence using the Hα Lyot filter installed
at the New Vacuum Solar Telescope, Fuxian-lake Solar Observatory in
Yunnan, China. On one side of the prominence, a series of plasma blobs
floated up from the chromosphere and streamed parallel to the limb. The
plasma stream was accelerated to about 20-60 km s-1 and
then undulated. We found that 2″- and 5″-size vortices formed,
floated along the stream, and then broke up. After the 5″-size
vortex, a plasma ejection out of the stream was detected in the
Solar Dynamics Observatory/Atmospheric Imaging Assembly images. Just
before the formation of the 5″-size vortex, the stream displayed
an oscillatory transverse motion with a period of 255 s with the
amplitude growing at the rate of 0.001 s-1. We attribute
this oscillation of the stream and the subsequent formation of the
vortex to the KHI triggered by velocity shear between the stream,
guided by the magnetic field and the surrounding media. The plasma
ejection suggests the transport of prominence material into the upper
layer by the KHI in its nonlinear stage.
Title: Nonlinear Effects in Three-minute Oscillations of the Solar
Chromosphere. II. Measurement of Nonlinearity Parameters at Different
Atmospheric Levels
Authors: Chae, Jongchul; Cho, Kyuhyoun; Song, Donguk; Litvinenko,
Yuri E.
Bibcode: 2018ApJ...854..127C
Altcode:
Recent theoretical studies suggest that the nonlinearity of three-minute
velocity oscillations at each atmospheric level can be quantified
by the two independent parameters—the steepening parameter and the
velocity amplitude parameter. For the first time, we measured these
two parameters at different atmospheric levels by analyzing a set of
spectral lines formed at different heights of sunspots ranging from
the temperature minimum to the transition region. The spectral data
were taken by the Fast Imaging Solar Spectrograph of the Goode Solar
Telescope, and by the Interface Region Imaging Spectrograph. As a
result, from the wavelet power spectra of the velocity oscillations
at different heights, we clearly identified the growth of the
second harmonic oscillations associated with the steepening of the
velocity oscillation, indicating that higher-frequency oscillations
of periods of 1.2 to 1.5 minutes originate from the nonlinearity
of the three-minute oscillations in the upper chromosphere. We also
found that the variation of the measured nonlinearity parameters is
consistent with the theoretical expectation that the nonlinearity of
the three-minute oscillations increases with height, and shock waves
form in the upper chromosphere. There are, however, discrepancies as
well between theory and observations, suggesting the need to improve
both theory and the measurement technique.
Title: Transient rotation of photospheric vector magnetic fields
associated with a solar flare
Authors: Xu, Yan; Cao, Wenda; Ahn, Kwangsu; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
Bibcode: 2018NatCo...9...46X
Altcode: 2018arXiv180103171X
As one of the most violent eruptions on the Sun, flares are believed to
be powered by magnetic reconnection. The fundamental physics involving
the release, transfer, and deposition of energy have been studied
extensively. Taking advantage of the unprecedented resolution provided
by the 1.6 m Goode Solar Telescope, here, we show a sudden rotation of
vector magnetic fields, about 12-20° counterclockwise, associated
with a flare. Unlike the permanent changes reported previously,
the azimuth-angle change is transient and cospatial/temporal with
Hα emission. The measured azimuth angle becomes closer to that in
potential fields suggesting untwist of flare loops. The magnetograms
were obtained in the near infrared at 1.56 μm, which is minimally
affected by flare emission and no intensity profile change was
detected. We believe that these transient changes are real and discuss
the possible explanations in which the high-energy electron beams or
Alfve'n waves play a crucial role.
Title: Observation of a Large-scale Quasi-circular Secondary Ribbon
Associated with Successive Flares and a Halo CME
Authors: Lim, Eun-Kyung; Yurchyshyn, Vasyl; Kumar, Pankaj; Cho,
Kyuhyoun; Jiang, Chaowei; Kim, Sujin; Yang, Heesu; Chae, Jongchul;
Cho, Kyung-Suk; Lee, Jeongwoo
Bibcode: 2017ApJ...850..167L
Altcode: 2017arXiv171100622L
Solar flare ribbons provide an important clue to the magnetic
reconnection process and associated magnetic field topology in the
solar corona. We detected a large-scale secondary flare ribbon of
a circular shape that developed in association with two successive
M-class flares and one coronal mass ejection. The ribbon revealed
interesting properties such as (1) a quasi-circular shape and enclosing
the central active region (AR); (2) the size as large as 500″ by
650″ (3) successive brightenings in the clockwise direction at a
speed of 160 km s-1 starting from the nearest position to
the flaring sunspots; (4) radial contraction and expansion in the
northern and the southern part, respectively, at speeds of ≤10
km s-1. Using multi-wavelength data from Solar Dynamics
Observatory, RHESSI, XRT, and Nobeyama, along with magnetic field
extrapolations, we found that: (1) the secondary ribbon location is
consistent with those of the field line footpoints of a fan-shaped
magnetic structure that connects the flaring region and the ambient
decaying field; (2) the second M2.6 flare occurred when the expanding
coronal loops driven by the first M2.0 flare encountered the background
decayed field; (3) immediately after the second flare, the secondary
ribbon developed along with dimming regions. Based on our findings,
we suggest that interaction between the expanding sigmoid field and
the overlying fan-shaped field triggered the secondary reconnection
that resulted in the field opening and formation of the quasi-circular
secondary ribbon. We thus conclude that interaction between the AR and
the ambient large-scale fields should be taken into account to fully
understand the entire eruption process.
Title: Thermal and Nonthermal Emissions of a Composite Flare Derived
from NoRH and SDO Observations
Authors: Lee, Jeongwoo; White, Stephen M.; Jing, Ju; Liu, Chang;
Masuda, Satoshi; Chae, Jongchul
Bibcode: 2017ApJ...850..124L
Altcode:
Differential emission measure (DEM) derived from the extreme ultraviolet
(EUV) lines of the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamic Observatory is used in the analysis of a solar flare observed
by the Nobeyama Radioheliograph (NoRH). The target was a composite event
consisting of an impulsive flare, SOL2015-06-21T01:42 (GOES class M2.0),
and a gradual flare, SOL2015-06-21T02:36 (M2.6), for which separation
of thermal plasma heating from nonthermal particle acceleration was of
major interest. We have calculated the thermal free-free intensity maps
with the AIA-derived DEM and compared them against the observed NoRH
maps to attribute the difference to the nonthermal component. In this
way, we were able to locate three distinct sources: the major source
with thermal and nonthermal components mixed, a nonthermal source
devoid of thermal particles, and a thermal source lacking microwave
emission. Both the first and the second nonthermal sources produced
impulsively rising 17 GHz intensities and moved away from the local
magnetic polarization inversion lines in correlation with the flare
radiation. In contrast, the thermal sources stay in fixed locations
and show temporal variations of the temperature and emission measure
uncorrelated with the flare radiation. We interpret these distinct
properties as indicating that nonthermal sources are powered by
magnetic reconnection and thermal sources passively receive energy
from the nonthermal donor. The finding of these distinct properties
between thermal and nonthermal sources demonstrates the microwave and
EUV emission measure combined diagnostics.
Title: Three-minute Sunspot Oscillations Driven by Magnetic
Reconnection in a Light Bridge
Authors: Song, Donguk; Chae, Jongchul; Kwak, Hannah; Kano, Ryouhei;
Yurchyshyn, Vasyl; Moon, Yong-Jae; Lim, Eun-Kyung; Lee, Jeongwoo
Bibcode: 2017ApJ...850L..33S
Altcode: 2017arXiv171106489S
We report a different type of three-minute chromospheric oscillation
above a sunspot in association with a small-scale impulsive event
in a light bridge (LB). During our observations, we found a transient
brightening in the LB. The brightening was composed of elementary bursts
that may be a manifestation of fast repetitive magnetic reconnections
in the LB. Interestingly, the oscillations in the nearby sunspot umbra
were impulsively excited when the intensity of the brightening reached
its peak. The initial period of the oscillations was about 2.3 minutes
and then gradually increased to 3.0 minutes with time. In addition,
we found that the amplitude of the excited oscillations was twice the
amplitude of oscillations before the brightening. Based on our results,
we propose that magnetic reconnection occurring in an LB can excite
oscillations in the nearby sunspot umbra.
Title: Nonlinear Effects in Three-minute Oscillations of the Solar
Chromosphere. I. An Analytical Nonlinear Solution and Detection of
the Second Harmonic
Authors: Chae, Jongchul; Litvinenko, Yuri E.
Bibcode: 2017ApJ...844..129C
Altcode:
The vertical propagation of nonlinear acoustic waves in an isothermal
atmosphere is considered. A new analytical solution that describes a
finite-amplitude wave of an arbitrary wavelength is obtained. Although
the short- and long-wavelength limits were previously considered
separately, the new solution describes both limiting cases within a
common framework and provides a straightforward way of interpolating
between the two limits. Physical features of the nonlinear waves
in the chromosphere are described, including the dispersive nature
of low-frequency waves, the steepening of the wave profile, and
the influence of the gravitational field on wavefront breaking and
shock formation. The analytical results suggest that observations of
three-minute oscillations in the solar chromosphere may reveal the
basic nonlinear effect of oscillations with combination frequencies,
superposed on the normal oscillations of the system. Explicit
expressions for a second-harmonic signal and the ratio of its amplitude
to the fundamental harmonic amplitude are derived. Observational
evidence of the second harmonic, obtained with the Fast Imaging Solar
Spectrograph, installed at the 1.6 m New Solar Telescope of the Big
Bear Observatory, is presented. The presented data are based on the
time variations of velocity determined from the Na I D2
and Hα lines.
Title: Magnetic vector rotation in response to the energetic electron
beam during a flare
Authors: Xu, Yan; Cao, Wenda; Kwangsu, Ahn; Jing, Ju; Liu, Chang;
Chae, Jongchul; Huang, Nengyi; Deng, Na; Gary, Dale E.; Wang, Haimin
Bibcode: 2017SPD....4810001X
Altcode:
As one of the most violent forms of eruption on the Sun, flares are
believed to be powered by magnetic reconnection, by which stored
magnetic energy is released. The fundamental physical processes
involving the release, transfer and deposition of energy in multiple
layers of the solar atmosphere have been studied extensively with
significant progress. Taking advantage of recent developments in
observing facilities, new phenomena are continually revealed, bringing
new understanding of solar flares. Here we report the discovery of
a transient rotation of vector magnetic fields associated with a
flare observed by the 1.6-m New Solar Telescope at Big Bear Solar
Observatory. After ruling out the possibility that the rotation is
caused by line profile changes due to flare heating, our observation
shows that the transverse field rotateded by about 12-20 degrees
counterclockwise, and returned quickly to previous values after the
flare ribbons swept through. More importantly, as a consequence of
the rotation, the flare loops untwisted and became more potential. The
vector magnetograms were obtained in the near infrared at 1560 nm, which
is minimally affected by flare emission and no intensity profile change
was detected. Therefore, we believe that these transient changes are
real, and conclude the high energy electron beams play an crucial role
in the field changes. A straightforward and instructive explanation is
that the induced magnetic field of the electron beam superimposed on
the pre-flare field leads to a transient rotation of the overall field.
Title: Evidence for a Magnetic Reconnection Origin of Plasma Outflows
along Post-CME Rays
Authors: Chae, Jongchul; Cho, Kyuhyoun; Kwon, Ryun-Young; Lim,
Eun-Kyung
Bibcode: 2017ApJ...841...49C
Altcode:
Bright rays are often observed after coronal mass ejections (CMEs)
erupt. These rays are dynamical structures along which plasmas
move outward. We investigated the outflows along the post-CME rays
observed by the COR2 on board STEREO Behind on 2013 September 21
and 22. We tracked two CMEs, two ray tips, and seven blobs using the
NAVE optical flow technique. As a result, we found that the departure
times of blobs and ray tips from the optimally chosen starting height
of 0.5 {R}⊙ coincided with the occurrence times of the
corresponding recurrent small flares within 10 minutes. These small
flares took place many hours after the major flares. This result
supports a magnetic reconnection origin of the outward flows along the
post-CME ray and the importance of magnetic islands for understanding
the process of magnetic reconnection. The total energy of magnetic
reconnection maintaining the outflows for 40 hr is estimated at 1.4×
{10}30 erg. Further investigations of plasma outflows
along post-CME rays will shed much light on the physical properties
of magnetic reconnection occurring in the solar corona.
Title: Performance of the Autoregressive Method in Long-Term
Prediction of Sunspot Number
Authors: Chae, Jongchul; Kim, Yeon Han
Bibcode: 2017JKAS...50...21C
Altcode:
No abstract at ADS
Title: Analytical description of nonlinear acoustic waves in the
solar chromosphere
Authors: Litvinenko, Yuri E.; Chae, Jongchul
Bibcode: 2017A&A...599A..15L
Altcode:
Aims: Vertical propagation of acoustic waves of finite
amplitude in an isothermal, gravitationally stratified atmosphere is
considered.
Methods: Methods of nonlinear acoustics are used to
derive a dispersive solution, which is valid in a long-wavelength limit,
and a non-dispersive solution, which is valid in a short-wavelength
limit. The influence of the gravitational field on wave-front breaking
and shock formation is described. The generation of a second harmonic
at twice the driving wave frequency, previously detected in numerical
simulations, is demonstrated analytically.
Results: Application
of the results to three-minute chromospheric oscillations, driven
by velocity perturbations at the base of the solar atmosphere, is
discussed. Numerical estimates suggest that the second harmonic signal
should be detectable in an upper chromosphere by an instrument such as
the Fast Imaging Solar Spectrograph installed at the 1.6-m New Solar
Telescope of the Big Bear Observatory.
Title: Photospheric Origin of Three-minute Oscillations in a Sunspot
Authors: Chae, Jongchul; Lee, Jeongwoo; Cho, Kyuhyoun; Song, Donguk;
Cho, Kyungsuk; Yurchyshyn, Vasyl
Bibcode: 2017ApJ...836...18C
Altcode:
The origin of the three-minute oscillations of intensity and velocity
observed in the chromosphere of sunspot umbrae is still unclear. We
investigated the spatio-spectral properties of the 3 minute oscillations
of velocity in the photosphere of a sunspot umbra as well as those
in the low chromosphere using the spectral data of the Ni I λ5436,
Fe I λ5435, and Na I D2 λ5890 lines taken by the Fast
Imaging Solar Spectrograph of the 1.6 m New Solar Telescope at the Big
Bear Solar Observatory. As a result, we found a local enhancement of
the 3 minute oscillation power in the vicinities of a light bridge
(LB) and numerous umbral dots (UDs) in the photosphere. These
3 minute oscillations occurred independently of the 5 minute
oscillations. Through wavelet analysis, we determined the amplitudes
and phases of the 3 minute oscillations at the formation heights of the
spectral lines, and they were found to be consistent with the upwardly
propagating slow magnetoacoustic waves in the photosphere with energy
flux large enough to explain the chromospheric oscillations. Our results
suggest that the 3 minute chromospheric oscillations in this sunspot may
have been generated by magnetoconvection occurring in the LB and UDs.
Title: Chromospheric Plasma Ejections in a Light Bridge of a Sunspot
Authors: Song, Donguk; Chae, Jongchul; Yurchyshyn, Vasyl; Lim,
Eun-Kyung; Cho, Kyung-Suk; Yang, Heesu; Cho, Kyuhyoun; Kwak, Hannah
Bibcode: 2017ApJ...835..240S
Altcode: 2017arXiv170106808S
It is well-known that light bridges (LBs) inside a sunspot produce
small-scale plasma ejections and transient brightenings in the
chromosphere, but the nature and origin of such phenomena are still
unclear. Utilizing the high-spatial and high-temporal resolution
spectral data taken with the Fast Imaging Solar Spectrograph
and the TiO 7057 Å broadband filter images installed at the 1.6
m New Solar Telescope of Big Bear Solar Observatory, we report
arcsecond-scale chromospheric plasma ejections (1.″7) inside a
LB. Interestingly, the ejections are found to be a manifestation of
upwardly propagating shock waves as evidenced by the sawtooth patterns
seen in the temporal-spectral plots of the Ca II 8542 Å and Hα
intensities. We also found a fine-scale photospheric pattern (1″)
diverging with a speed of about 2 km s-1 two minutes before
the plasma ejections, which seems to be a manifestation of magnetic
flux emergence. As a response to the plasma ejections, the corona
displayed small-scale transient brightenings. Based on our findings,
we suggest that the shock waves can be excited by the local disturbance
caused by magnetic reconnection between the emerging flux inside the
LB and the adjacent umbral magnetic field. The disturbance generates
slow-mode waves, which soon develop into shock waves, and manifest
themselves as the arcsecond-scale plasma ejections. It also appears
that the dissipation of mechanical energy in the shock waves can heat
the local corona.
Title: Solar Eruption and Local Magnetic Parameters
Authors: Lee, Jeongwoo; Liu, Chang; Jing, Ju; Chae, Jongchul
Bibcode: 2016ApJ...831L..18L
Altcode: 2017arXiv170804055L
It is now a common practice to use local magnetic parameters such as
magnetic decay index for explaining solar eruptions from active regions,
but there can be an alternative view that the global properties of the
source region should be counted as a more important factor. We discuss
this issue based on Solar Dynamics Observatory observations of the three
successive eruptions within 1.5 hr from the NOAA active region 11444 and
the magnetic parameters calculated using the nonlinear force-free field
model. Two violent eruptions occurred in the regions with relatively
high magnetic twist number (0.5-1.5) and high decay index (0.9-1.1)
at the nominal height of the filament (12″) and otherwise a mild
eruption occurred, which supports the local-parameter paradigm. Our
main point is that the time sequence of the eruptions did not go with
these parameters. It is argued that an additional factor, in the form
of stabilizing force, should operate to determine the onset of the
first eruption and temporal behaviors of subsequent eruptions. As
supporting evidence, we report that the heating and fast plasma flow
continuing for a timescale of an hour was the direct cause for the first
eruption and that the unidirectional propagation of the disturbance
determined the timing of subsequent eruptions. Both of these factors
are associated with the overall magnetic structure rather than local
magnetic properties of the active region.
Title: Strong Blue Asymmetry in Hα Line as a Preflare Activity
Authors: Cho, Kyuhyoun; Lee, Jeongwoo; Chae, Jongchul; Wang, Haimin;
Ahn, Kwangsu; Yang, Heesu; Lim, Eun-kyung; Maurya, Ram Ajor
Bibcode: 2016SoPh..291.2391C
Altcode: 2016SoPh..tmp..131C; 2020arXiv200506404C
Chromospheric activities before solar flares provide important clues
to the mechanisms that initiate solar flares, but are as yet poorly
understood. We report a significant and rapid Hα line broadening
before the solar flare SOL2011-09-29T18:08 that was detected using
the unprecedented high-resolution Hα imaging spectroscopy with the
Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m New
Solar Telescope (NST) at Big Bear Solar Observatory. The strong Hα
broadening extends as a blue excursion up to −4.5 Å and as a red
excursion up to 2.0 Å, which implies a mixture of velocities in the
range of −130 kms−1 to 38 km s−1 derived
by applying the cloud model, comparable to the highest chromospheric
motions reported before. The Hα blueshifted broadening lasts for
about six minutes and is temporally and spatially correlated with the
start of a rising filament, which is later associated with the main
phase of the flare as detected by the Atmosphere Imaging Assembly (AIA)
onboard the Solar Dynamics Observatory (SDO). The potential importance
of this Hα blueshifted broadening as a preflare chromospheric activity
is briefly discussed within the context of the two-step eruption model.
Title: Fine-scale Photospheric Connections of Ellerman Bombs
Authors: Yang, Heesu; Chae, Jongchul; Lim, Eun-Kyung; Song, Donguk;
Cho, Kyuhyoun; Kwak, Hannah; Yurchyshyn, Vasyl B.; Kim, Yeon-Han
Bibcode: 2016ApJ...829..100Y
Altcode:
We investigate the photospheric and magnetic field structures
associated with Ellerman bombs (EBs) using the 1.6 m New Solar
Telescope at Big Bear Solar Observatory. The nine observed EBs were
accompanied by elongated granule-like features (EGFs) that showed
transverse motions prior to the EBs with an average speed of about
3.8 km s-1. Each EGF consisted of a sub-arcsecond bright
core encircled by a dark lane around its moving front. The bright
core appeared in the TiO broadband filter images and in the far wings
of the Hα and Ca II 8542 Å lines. In four EBs, the bi-directional
expanding motion of the EGFs was identified in the TiO images. In
those cases, the EGFs were found to be accompanied by an emerging flux
(EF). In four other EBs, the EGF developed at the edge of a penumbra and
traveled in the sunspot’s radial direction. The EGFs in these cases
were identified as a moving magnetic feature (MMF). Our results show a
clear connection among the magnetic elements, photospheric features, and
EBs. This result suggests that the EBs result from magnetic reconnection
forced by EFs or MMFs that are frequently manifested by EGFs.
Title: Solar Multiple Eruptions from a Confined Magnetic Structure
Authors: Lee, Jeongwoo; Liu, Chang; Jing, Ju; Chae, Jongchul
Bibcode: 2016ApJ...829L...1L
Altcode: 2017arXiv170804056L
How eruption can recur from a confined magnetic structure is discussed
based on the Solar Dynamics Observatory observations of the NOAA
active region 11444, which produced three eruptions within 1.5 hr
on 2012 March 27. The active region (AR) had the positive-polarity
magnetic fields in the center surrounded by the negative-polarity fields
around. Since such a distribution of magnetic polarity tends to form
a dome-like magnetic fan structure confined over the AR, the multiple
eruptions were puzzling. Our investigation reveals that this event
exhibits several properties distinct from other eruptions associated
with magnetic fan structures: (I) a long filament encircling the AR
was present before the eruptions; (II) expansion of the open-closed
boundary (OCB) of the field lines after each eruption was suggestive
of the growing fan-dome structure, and (III) the ribbons inside the
closed magnetic polarity inversion line evolved in response to the
expanding OCB. It thus appears that in spite of multiple eruptions
the fan-dome structure remained undamaged, and the closing back field
lines after each eruption rather reinforced the fan-dome structure. We
argue that the multiple eruptions could occur in this AR in spite of
its confined magnetic structure because the filament encircling the AR
was adequate for slipping through the magnetic separatrix to minimize
the damage to its overlying fan-dome structure. The result of this
study provides a new insight into the productivity of eruptions from
a confined magnetic structure.
Title: Data Processing of the magnetograms for the Near InfraRed
Imaging Spectropolarimeter at Big Bear Solar Observatory
Authors: Ahn, Kwangsu; Cao, Wenda; Shumko, Sergiy; Chae, Jongchul
Bibcode: 2016SPD....47.0207A
Altcode:
We want to present the processing result of the vector magnetograms from
the Near InfraRed Imaging Spectropolarimeter (NIRIS) at Big Bear Solar
Observatory. The NIRIS is a successor of an old magnetograph system
at BBSO, which equips with the new infrared detector and the improved
Fabry-Perot filter system. While there are several upgrades to the new
hardware, there are also some challenges as the data acquisition rate
increases and we deal with the a larger detector array. The overall
process includes dark and flat correction, image alignment, de-stretch,
Stokes parameter selection, calibration of instrumental crosstalk,
and Milne-Eddington inversion.
Title: Ultra-Narrow Negative Flare Front Observed in Helium-10830
Å Using the1.6m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyung-Suk; Gary, Dale E.; Wang, Haimin
Bibcode: 2016SPD....47.0633X
Altcode:
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6-m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in solar flares.
Title: 2016 Total Solar Eclipse Expedition of KASI
Authors: Bong, Su-Chan; Choi, SeongHwan; Jang, Bi-Ho; Park, Jongyeob;
Jeon, Young-Beom; Cho, Kyuhyoun; Chae, Jongchul
Bibcode: 2016SPD....47.0311B
Altcode:
A total solar eclipse occurred on March 9 along the path through
Indonesia and the Pacific. KASI organized an expedition team for total
solar eclipse observation. The main purpose of this observation is to
test the coronal temperature and outflow velocity diagnostics based
on filter observation, which is proposed for the next generation
coronagraph. In addition, various white light observations were
tried. Although we could not get satisfactory data for the quantitative
diagnostics due to system problem and weather, we could get some useful
experimental data and nice white light images. We plan next expedition
for 2017 total solar eclipse in USA.
Title: Oscillatory Response of the Solar Chromosphere to a Strong
Downflow Event above a Sunspot
Authors: Kwak, Hannah; Chae, Jongchul; Song, Donguk; Kim, Yeon-Han;
Lim, Eun-Kyung; Madjarska, Maria S.
Bibcode: 2016ApJ...821L..30K
Altcode: 2016arXiv160402252K
We report three-minute oscillations in the solar chromosphere
driven by a strong downflow event in a sunspot. We used the Fast
Imaging Solar Spectrograph of the 1.6 m New Solar Telescope and the
Interface Region Imaging Spectrograph (IRIS). The strong downflow
event is identified in the chromospheric and transition region lines
above the sunspot umbra. After the event, oscillations occur at the
same region. The amplitude of the Doppler velocity oscillations is
2 km s-1 and gradually decreases with time. In addition,
the period of the oscillations gradually increases from 2.7 to 3.3
minutes. In the IRIS 1330 Å slit-jaw images, we identify a transient
brightening near the footpoint of the downflow detected in the Hα+0.5
Å image. The characteristics of the downflowing material are consistent
with those of sunspot plumes. Based on our findings, we suggest that
the gravitationally stratified atmosphere came to oscillate with a
three-minute period in response to the impulsive downflow event as
was theoretically investigated by Chae & Goode.
Title: Ultra-narrow Negative Flare Front Observed in Helium-10830
Å Using the 1.6 m New Solar Telescope
Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su,
Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho,
Kyuhyoun; Cho, Kyungsuk; Gary, Dale; Wang, Haimin
Bibcode: 2016ApJ...819...89X
Altcode: 2016arXiv160104729X
Solar flares are sudden flashes of brightness on the Sun and are often
associated with coronal mass ejections and solar energetic particles
that have adverse effects on the near-Earth environment. By definition,
flares are usually referred to as bright features resulting from excess
emission. Using the newly commissioned 1.6 m New Solar Telescope at
Big Bear Solar Observatory, we show a striking “negative” flare
with a narrow but unambiguous “dark” moving front observed in He I
10830 Å, which is as narrow as 340 km and is associated with distinct
spectral characteristics in Hα and Mg II lines. Theoretically, such
negative contrast in He I 10830 Å can be produced under special
circumstances by nonthermal electron collisions or photoionization
followed by recombination. Our discovery, made possible due to
unprecedented spatial resolution, confirms the presence of the required
plasma conditions and provides unique information in understanding
the energy release and radiative transfer in astronomical objects.
Title: a New Method to Determine the Temperature of CMES Using a
Coronagraph Filter System
Authors: Cho, Kyuhyoun; Chae, Jongchul; Lim, Eun-Kyung; Cho, Kyung-Suk;
Bong, Su-Chan; Yang, Heesu
Bibcode: 2016JKAS...49...45C
Altcode: 2016arXiv160307047C
The coronagraph is an instrument enables the investigation of faint
features in the vicinity of the Sun, particularly coronal mass ejections
(CMEs). So far coronagraphic observations have been mainly used to
determine the geometric and kinematic parameters of CMEs. Here, we
introduce a new method for the determination of CME temperature using
a two filter (4025 A and 3934 A) coronagraph system. The thermal motion
of free electrons in CMEs broadens the absorption lines in the optical
spectra that are produced by the Thomson scattering of visible light
originating in the photosphere, which affects the intensity ratio at two
different wavelengths. Thus the CME temperature can be inferred from
the intensity ratio measured by the two filter coronagraph system. We
demonstrate the method by invoking the graduated cylindrical shell
(GCS) model for the 3 dimensional CME density distribution and discuss
its significance.
Title: Observations of a Series of Flares and Associated Jet-like
Eruptions Driven by the Emergence of Twisted Magnetic Fields
Authors: Lim, Eun-Kyung; Yurchyshyn, Vasyl; Park, Sung-Hong; Kim,
Sujin; Cho, Kyung-Suk; Kumar, Pankaj; Chae, Jongchul; Yang, Heesu;
Cho, Kyuhyoun; Song, Donguk; Kim, Yeon-Han
Bibcode: 2016ApJ...817...39L
Altcode: 2015arXiv151201330L
We studied temporal changes of morphological and magnetic properties
of a succession of four confined flares followed by an eruptive flare
using the high-resolution New Solar Telescope (NST) operating at the Big
Bear Solar Observatory (BBSO) and Helioseismic and Magnetic Imager (HMI)
magnetograms and Atmospheric Image Assembly (AIA) EUV images provided by
the Solar Dynamics Observatory (SDO). From the NST/Hα and the SDO/AIA
304 Å observations we found that each flare developed a jet structure
that evolved in a manner similar to evolution of the blowout jet: (1)
an inverted-Y-shaped jet appeared and drifted away from its initial
position; (2) jets formed a curtain-like structure that consisted
of many fine threads accompanied by subsequent brightenings near
the footpoints of the fine threads; and finally, (3) the jet showed
a twisted structure visible near the flare maximum. Analysis of the
HMI data showed that both the negative magnetic flux and the magnetic
helicity have been gradually increasing in the positive-polarity region,
indicating the continuous injection of magnetic twist before and during
the series of flares. Based on these results, we suggest that the
continuous emergence of twisted magnetic flux played an important role
in producing successive flares and developing a series of blowout jets.
Title: Coronal Magnetic Structures for Homologous Eruptions
Authors: Lee, J.; Liu, C.; Jing, J.; Chae, J.
Bibcode: 2015AGUFMSH21C..05L
Altcode:
Many studies have been made on homologous eruptions for their importance
in understanding the flare energy build-up and release processes. We
study the homologous eruptions that occurred in three active regions,
NOAA 11444, 11283, and 12192, with emphasis on the coronal quantities
derived from the nonlinear force-free field (NLFFF) extrapolation. The
quantities include magnetic energy, electric current, and magnetic twist
number, and decay index, computed from the high cadence photospheric
vector magnetograms of the Helioseismic and Magnetic Imager (HMI) on
board the Solar Dynamic Observatory (SDO). In addition, photospheric
magnetic flux, flare ribbons and overlying field distribution are also
examined to determine the changes associated with each eruption. As
main results, we will present the difference between the homology of
confined eruptions and that of eruptive ones, and variations of the
coronal quantities with flare strength.
Title: Development of Space Science Instruments for Next Generation
Small Satellite -1
Authors: KIM, H.; Sohn, J.; Lee, J.; Kang, S. B.; Choi, C.; Woo,
C. H.; Seo, Y.; Woo, J.; Na, G. E.; Jo, G.; Min, K.; Seon, J.; Ryu,
K.; Kang, K. I.; Chae, J. S.; Shin, G. H.
Bibcode: 2015AGUFMSA51C2413K
Altcode:
A package of space science payloads is to be flown as a secondary
payload on board the first satellite of the Next Generation Small
Satellite-1 (NEXTSat-1) series of Korea, a microsatellite which is
scheduled for launch in 2017 into the polar orbit. It consists of Space
Radiation Detectors (SRD), which will measure the ring current and
radiation belt particles in the subauroral regions, and Space Plasma
Detectors (SPD), which will measure the ionospheric ions and electrons
in the middle and low latitude regions. SRD consists of two instruments:
Medium Energy Particle Detector (MEPD) will measure electrons and ions
in the range of ~25 to ~400 keV, and High Energy Particle Detector
(HEPD) will measure electrons of ~0.1 to ~2 MeV and ions up to ~20
MeV. With two and three telescopes for MEPD and HEPD, respectively,
together with many spectral bins, SRD, will discriminate the trapped and
precipitating particles with high spectral resolution. SPD is composed
of three conventional instruments: a Langmuir Probe (LP), a Retarding
Potential Analyzer (RPA), and an Ion Drift Meter (IDM). LP of a planar
shape will be used to determine the densities and temperatures of the
ionospheric electrons, while RPA and IDM will determine the densities,
temperatures, and the composition of the ionospheric ions as well as
their drift velocities. With its high temporal resolution, SPD aims to
observe substructures of plasma irregularities. This paper discusses
the designs and scientific objectives of these instruments.
Title: Detection of a Fine-scale Discontinuity of Photospheric
Magnetic Fields Associated with Solar Coronal Loop Brightenings
Authors: Song, Donguk; Chae, Jongchul; Park, Soyoung; Cho, Kyung-Suk;
Lim, Eun-Kyung; Ahn, Kwangsu; Cao, Wenda
Bibcode: 2015ApJ...810L..16S
Altcode:
We present the transient brightening of a coronal loop and an associated
fine-scale magnetic discontinuity detected in the photosphere. Utilizing
the high-resolution data taken with the Fast Imaging Solar Spectrograph
and InfraRed Imaging Magnetograph of the New Solar Telescope at Big
Bear Solar Observatory, we detect a narrow lane of intense horizontal
magnetic field representing a magnetic discontinuity. It was visible
as a dark lane partially encircling a pore in the continuum image,
and was located near one of the footpoints of a small coronal loop
that experienced transient brightenings. The horizontal field strength
gradually increased before the loop brightening, and then rapidly
decreased in the impulsive phase of the brightening, suggesting the
increase of the magnetic non-potentiality at the loop footpoint and the
sudden release of magnetic energy via magnetic reconnection. Our results
support the nanoflare theory that coronal heating events are caused
by magnetic reconnection events at fine-scale magnetic discontinuities.
Title: A Prestudy for the Development of a Compact Coronagraph
Authors: Bong, Su-Chan; Yang, Heesu; Cho, Kyuhyoun; Cho, Kyung-Suk;
Lim, Eun-Kyung; Park, Young-Deuk; Chae, Jongchul
Bibcode: 2015IAUGA..2254511B
Altcode:
We are conducting a prestudy for the development of a compact
coronagraph. The coronagraph is comprised of the external occulter,
lens, filter, and the CCD. We focus on the performance of the external
occulter, and the measurement of the coronal temperature and velocity
using a set of filters. We have tested the diffraction of a singlet
occulter and a cone occulter using laser. Compared to the singlet
occulter, the cone occulter gave rather symmetric, smooth and weak
diffraction pattern. We also have calculated the Thomson scattering of
the K corona to form coronal spectra in various temperature and velocity
conditions. We found the optimized wavelength set (393.4, 399.0, 402.5,
and 482.7 nm) for temperature and velocity measurement. We plan to
build a prototype coronagraph and make a test observation during the
total eclipse in 2016 without the occulter.
Title: Acoustic Waves Generated by a Disturbance in a
Gravitationally-Stratified Medium
Authors: Chae, Jongchul; Goode, Phil
Bibcode: 2015IAUGA..2232493C
Altcode:
Even though it is well-known from observations of the Sun that
three-minute period chromospheric oscillations persist in the
internetwork quiet regions and sunspot penumbrae, until now their
origin and persistence has defied clear explanation. Here we provide
a clear and simple explanation for it with a demonstration of how
such oscillations at the chromosphere's critical frequency naturally
arise in a gravitationally-stratified medium when it is disturbed. The
largest-wavenumber components of a chromospheric disturbance produce
the highest-frequency wave packets, which propagate out of the disturbed
region at the group speed, which is close to the sound speed. Meanwhile,
its smallest-wavenumber components develop into wave packets of
frequencies close to the critical frequency that propagate at a group
velocity that is much lower than the sound speed. Because of their
low propagation speed, these wave packets with the natural frequency
linger around the disturbed region and its surrounding regions for
a long time, and it is these that one would observationally identify
as the persistent, chromospheric three-minute oscillations. As well,
according to our explanation, the power of the persistent chromospheric
oscillations comes from disturbances with length scales that are greater
than twice the pressure scale height in the source region and their
frequencies are determined by the sound speed and the gravitational
acceleration of the region.
Title: Acoustic Waves Generated by Impulsive Disturbances in a
Gravitationally Stratified Medium
Authors: Chae, Jongchul; Goode, Philip R.
Bibcode: 2015ApJ...808..118C
Altcode:
Even though it is well-known from observations of the Sun that
three-minute period chromospheric oscillations persist in the
internetwork quiet regions and sunspot umbrae, until now their
origin and persistence has defied clear explanation. Here we provide
a clear and simple explanation for it with a demonstration of how
such oscillations at the chromosphere's cutoff frequency naturally
arise in a gravitationally stratified medium when it is disturbed. The
largest-wavenumber vertical components of a chromospheric disturbance
produce the highest-frequency wave packets, which propagate out of
the disturbed region at group speeds that are close to the sound
speed. Meanwhile, the smallest-wavenumber components develop into
wave packets of frequencies close to the acoustic cutoff frequency
that propagate at group speeds that are much lower than the sound
speed. Because of their low propagation speed, these low-frequency wave
packets linger in the disturbed region and nearby, and thus, are the
ones that an observer would identify as the persistent, chromospheric
three-minute oscillations. We emphasize that we can account for the
power of the persistent chromospheric oscillations as coming from the
repeated occurrence of disturbances with length scales greater than
twice the pressure scale height in the upper photosphere.
Title: Detection of Shock Merging in the Chromosphere of a Solar Pore
Authors: Chae, Jongchul; Song, Donguk; Seo, Minju; Cho, Kyung-Suk;
Park, Young-Deuk; Yurchyshyn, Vasyl
Bibcode: 2015ApJ...805L..21C
Altcode:
It was theoretically demonstrated that a shock propagating in the solar
atmosphere can overtake another and merge with it. We provide clear
observational evidence that shock merging does occur quite often in
the chromosphere of sunspots. Using Hα imaging spectral data taken by
the Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope
at the Big Bear Soar Observatory, we construct time-distance maps
of line-of-sight velocities along two appropriately chosen cuts in
a pore. The maps show a number of alternating redshift and blueshift
ridges, and we identify each interface between a preceding redshift
ridge and the following blueshift ridge as a shock ridge. The important
finding of ours is that two successive shock ridges often merge with
each other. This finding can be theoretically explained by the merging
of magneto-acoustic shock waves propagating with lower speeds of about
10 km s-1 and those propagating at higher speeds of about
16-22 km s-1. The shock merging is an important nonlinear
dynamical process of the solar chromosphere that can bridge the gap
between higher-frequency chromospheric oscillations and lower-frequency
dynamic phenomena such as fibrils.
Title: Intensity and Doppler Velocity Oscillations in Pore Atmospheres
Authors: Cho, K. -S.; Bong, S. -C.; Nakariakov, V. M.; Lim, E. -K.;
Park, Y. -D.; Chae, J. C.; Yang, H. -S.; Park, H. -M.; Yurchyshyn, V.
Bibcode: 2015ApJ...802...45C
Altcode:
We have investigated chromospheric traveling features running across two
merged pores from their centers at speeds of about 55 km s-1,
in the active region AR 11828. The pores were observed on 2013 August
24 by using high-time, spatial, and spectral resolution data from the
Fast Imaging Solar Spectrograph of the 1.6 m New Solar Telescope. We
infer a line-of-sight (LOS) velocity by applying the lambdameter method
to the Ca ii 8542 Å band and Hα band, and investigate intensity and
LOS velocity changes at different wavelengths and different positions
at the pores. We find that they have three-minute oscillations, and
the intensity oscillation from the line center (0.0 \overset{\circ}A
) is preceded by that from the core (-0.3 \overset{\circ}A ) of the
bands. There is no phase difference between the intensity and the
LOS velocity oscillations at a given wavelength. The amplitude of LOS
velocity from the near core spectra ({Δ }λ =0.10-0.21 \overset{\circ}A
) is greater than that from the far core spectra ({Δ }λ =0.24-0.36
\overset{\circ}A ). These results support the interpretation of the
observed wave as a slow magnetoacoustic wave propagating along the
magnetic field lines in the pores. The apparent horizontal motion and
a sudden decrease of its speed beyond the pores can be explained by
the projection effect caused by inclination of the magnetic field with
a canopy structure. We conclude that the observed wave properties of
the pores are quite similar to those from the sunspot observations.
Title: Variations of the electron density in the low and middle
latitude ionosphere due to high-speed solar wind streams observed
by the DEMETER satellite
Authors: Kim, H. E.; Lee, E.; Kim, K. H.; Lee, D. H.; Ryu, K.; Chae,
J.; Parrot, M.
Bibcode: 2014AGUFMSA11B3945K
Altcode:
Earth's ionosphere varies very dynamically according to the geomagnetic
activities and solar irradiance. Recently it has been studied that
the plasma and neutral densities in the thermosphere and ionosphere
show periodic oscillations with respect to the solar wind speed and Kp
index during the declining phase of the solar cycle. In this study,
we investigate the relationship between the electron density and
the solar wind speed during the declining phase of the solar cycle
using the observation from the Detection of Electro-Magnetic Emissions
Transmitted from Earthquake Regions (DEMETER) satellite at the altitude
of 660 km. The electron density near the dip equator tends to increase
with the solar wind speed from January to April, but there is no clear
tendency in the other times. Also, the electron density shows good
relationship with the solar wind speed at the magnetic longitudes from
240 to 300 degrees. However, there is poor relationship in the other
longitudes. We will discuss the seasonal and longitudinal dependences
of the electron density on the solar wind speed in the low and middle
latitude ionosphere.
Title: Suspected seismo-ionospheric coupling observed by satellite
measurements and GPS TEC related to the M7.9 Wenchuan earthquake of
12 May 2008
Authors: Ryu, K.; Parrot, M.; Kim, S. G.; Jeong, K. S.; Chae, J. S.;
Pulinets, S.; Oyama, K. -I.
Bibcode: 2014JGRA..11910305R
Altcode:
Anomalous changes in the ionospheric conditions related to the
Wenchuan earthquake of 12 May 2008 are investigated using electron
density (Ne) from Detection of Electro-Magnetic Emissions
Transmitted from Earthquake Regions (DEMETER) and CHAMP satellites,
electric field from DEMETER, and GPS-total electron content (TEC)
maps. The normalized Ne from the DEMETER satellite reveal
that the previously reported TEC increments before the earthquake
can be considered as fragments of the gradual equatorial ionization
anomaly (EIA) enhancements near the epicenter longitude that began
approximately 1 month before the earthquake and reached its maximum
with an exceptionally large strength index 8 days prior to the
main shock. This feature is indirectly confirmed through the CHAMP
Ne and GPS TEC data. Following the EIA intensity peak,
disturbances in the Ne and O+ density were
observed in the nightside. Based on the concurrent electric field and
Ne changes, it is suggested that EIA intensification could
be triggered by the E field disturbances over the epicenter.
Title: Active Region Coronal Rain Event Observed by the Fast Imaging
Solar Spectrograph on the NST
Authors: Ahn, Kwangsu; Chae, Jongchul; Cho, Kyung-Suk; Song, Donguk;
Yang, Heesu; Goode, Philip R.; Cao, Wenda; Park, Hyungmin; Nah,
Jakyung; Jang, Bi-Ho; Park, Young-Deuk
Bibcode: 2014SoPh..289.4117A
Altcode: 2014SoPh..tmp...98A
The Fast Imaging Solar Spectrograph (FISS) is being operated on the New
Solar Telescope of the Big Bear Solar Observatory. It simultaneously
records spectra of Hα and Ca II 8542 Å lines, and this dual-spectra
measurement provides an estimate of the temperature and nonthermal
speed components. We observed a loop structure in AR 11305 using the
FISS, SDO/AIA, and STEREO/EUVI in 304 Å, and found plasma material
falling along the loop from a coronal height into the umbra of a
sunspot, which accelerated up to 80 km s−1. We also
observed C2 and C7 flare events near the loop. The temperature of the
downflows was in the range of 10 000 - 33 000 K, increasing toward
the umbra. The temperature of the flow varied with time, and the
temperature near the footpoint rose immediately after the C7 flare,
but the temperature toward the umbra remained the same. There seemed
to be a temporal correlation between the amount of downflow material
and the observed C-class flares. The downflows decreased gradually soon
after the flares and then increased after a few hours. These high-speed
red-shift events occurred continuously during the observations. The
flows observed on-disk in Hα and Ca II 8542 Å appeared as fragmented,
fuzzy condensed material falling from the coronal heights when seen
off-limb with STEREO/EUVI at 304 Å. Based on these observations,
we propose that these flows were an on-disk signature of coronal rain.
Title: Seismo-ionospheric coupling appearing as equatorial
electron density enhancements observed via DEMETER electron density
measurements
Authors: Ryu, K.; Lee, E.; Chae, J. S.; Parrot, M.; Pulinets, S.
Bibcode: 2014JGRA..119.8524R
Altcode:
We report the processes and results of statistical analysis on
the ionospheric electron density data measured by the Detection
of Electro-Magnetic Emissions Transmitted from Earthquake Regions
(DEMETER) satellite over a period of 6 years (2005-2010), in
order to investigate the correlation between seismic activity and
equatorial plasma density variations. To simplify the analysis,
three equatorial regions with frequent earthquakes were selected and
then one-dimensional time series analysis between the daily seismic
activity indices and the equatorial ionization anomaly (EIA) intensity
indices, which represent relative equatorial electron density increase,
were performed for each region. The statistically significant values
of the lagged cross-correlation function, particularly in the region
with minimal effects of longitudinal asymmetry, indicate that some of
the very large earthquakes with M > 5.0 in the low-latitude region
can accompany observable precursory and concurrent EIA enhancements,
even though the seismic activity is not the most significant driver
of the equatorial ionospheric evolution. The physical mechanisms of
the seismo-ionospheric coupling is consistent with our observation,
and the possibility of earthquake prediction using the EIA intensity
variation is discussed.
Title: Magnetic-reconnection Generated Shock Waves as a Driver of
Solar Surges
Authors: Yang, Heesu; Chae, Jongchul; Lim, Eun-Kyung; Lee, Kyoung-sun;
Park, Hyungmin; Song, Dong-uk; Cho, Kyuhyoun
Bibcode: 2014ApJ...790L...4Y
Altcode:
We found that a surge consists of multiple shock features. In our
high-spatiotemporal spectroscopic observation of the surge, each shock
is identified with the sudden appearance of an absorption feature at the
blue wings of the Ca II 8542 Å line and Hα line that gradually shifts
to the red wings. The shock features overlap with one another with
the time interval of 110 s, which is much shorter than the duration
of each shock feature, 300-400 s. This finding suggests that the
multiple shocks might not have originated from a train of sinusoidal
waves generated by oscillations and flows in the photosphere. As we
found the signature of the magnetic flux cancelations at the base of
the surge, we conclude that the multiple shock waves in charge of the
surge were generated by the magnetic reconnection that occurred in
the low atmosphere in association with the flux cancelation.
Title: Superpenumbral Fibrils Powered by Sunspot Oscillations
Authors: Chae, Jongchul; Yang, Heesu; Park, Hyungmin; Ajor Maurya,
Ram; Cho, Kyung-Suk; Yurchysyn, Vasyl
Bibcode: 2014ApJ...789..108C
Altcode:
It is still a mystery how the solar chromosphere can stand high above
the photosphere. The dominant portion of this layer must be dynamically
supported, as is evident by the common occurrence of jets such as
spicules and mottles in quiet regions, and fibrils and surges in active
regions. Hence, revealing the driving mechanism of these chromospheric
jets is crucial for our understanding of how the chromosphere itself
exists. Here, we report our observational finding that fibrils in the
superpenumbra of a sunspot are powered by sunspot oscillations. We
find patterns of outward propagation that apparently originate from
inside the sunspot, propagate like running penumbral waves, and develop
into the fibrils. Redshift ridges seen in the time-distance plots of
velocity often merge, forming a fork-like pattern. The predominant
period of these shock waves increases, often jumping with distance,
from 3 minutes to 10 minutes. This short-to-long period transition
seems to result from the selective suppression of shocks by the
falling material of their preceding shocks. Based on our results,
we propose that the fibrils are driven by slow shock waves with long
periods that are produced by the merging of shock waves with shorter
periods propagating along the magnetic canopy.
Title: Magnetic-Reconnection Generated Shock Waves as a Driver of
Solar Surges
Authors: Yang, Heesu; Chae, Jongchul
Bibcode: 2014shin.confE.147Y
Altcode:
We found that a surge consists of multiple shock features. In our high
spatiotemporal spectroscopic observation of the surge using Fast Imaging
Solar Spectrograph of the 1.6 meter New Solar Telescope at Big Bear
Solar Observatory, each shock is identified with the sudden appearance
of an absorption feature at the blue wings of the Ca ii 854.2nm line
and H alpha line that gradually shifts to the red wings. The shock
features are overlapped with one another with an time interval of
110s, which is much shorter than the duration of each shock feature
300 - 400s. This finding suggests that the multiple shocks may not
have been originated from a train of sinusoidal waves generated by
oscillations and flows in the photosphere. As we found the signature of
the magnetic flux cancellations at the base of the surge, we conclude
that the multiple shock waves in charge of the surge were generated
by the magnetic reconnection in the low atmosphere associated with
the flux cancellation.
Title: Multisatellite observations of an intensified equatorial
ionization anomaly in relation to the northern Sumatra earthquake
of March 2005
Authors: Ryu, K.; Lee, E.; Chae, J. S.; Parrot, M.; Oyama, K. -I.
Bibcode: 2014JGRA..119.4767R
Altcode:
Here we report multisatellite observations of ionospheric disturbances
in relation to the occurrence of the M8.7 northern Sumatra earthquake of
28 March 2005. The DEMETER (Detection of Electro-Magnetic Emissions
Transmitted from Earthquake Regions) and CHAMP (Challenging
Minisatellite Payload) satellite data were investigated to find
possible precursory and postevent phenomena. It was found that EIA
(equatorial ionization anomaly) strength expressed in the apex height,
derived from the CHAMP plasma density profile, was intensified along the
orbits whose longitudes were close to the epicenter within about a week
before and after occurrence of the earthquake. Increases in electron and
O+ density along the orbits close to the epicenter were also
observed in the DEMETER measurements. The normalized equatorial plasma
density derived from the DEMETER measurements showed intensification
about a week before and after the earthquake reaching maximum the
day after the shock and afterward disappearing. In addition, similar
behavior of the EIA enhancements related to the M8.0 Pisco earthquake
of 15 August 2007 was observed. Surveys of space weather and geomagnetic
activities excluded the possibility that these fluctuations were caused
by changes in space weather or by a geomagnetic storm. Statistical
analyses of the longitudinal variation revealed that the EIA was
enhanced in the west of the epicenter and reduced in the east of the
epicenter, and this fits the "increased conductivity" model. Based on
these observations, we proposed a revised view of seismo-ionospheric
coupling in the region of the geomagnetic equator, to explain the EIA
features observed in this study.
Title: Shock Waves As a Driver of Surges
Authors: Yang, Heesu; Chae, Jongchul; Park, Hyungmin; Lim, Eun-Kyung;
Song, Dong-uk; Cho, Kyuhyoun
Bibcode: 2014cosp...40E3698Y
Altcode:
We found that a surge consists of multiple shock features for a interval
of 110 s with our high spatio-temporal spectroscopic observation using
the Fast Imaging Solar Spectrograph (FISS). Each shock is identified
with the sudden appearance of an absorption feature at the blue wing of
the Ca ii line and Halpha line, indicating the upward motion of 20-40
km s (-1) . Then the absorption feature gradually shifts to the red wing
with a constant acceleration, reaching the downward motion of 20-40 km s
(-1) within about 6 minutes. We also found magnetic flux cancellation
and an Ellerman bomb near the foot point of the surge, suggesting
that magnetic reconnection occurred in the low atmosphere. Our results
suggest that the waves produced by energy release due to the magnetic
reconnection in the low atmosphere propagates upward in the form of
the shock waves, which drive the surge.
Title: Signals of solar cosmic ray flux variations inferred from
the noise in raw CCD solar images taken by SOHO/EIT
Authors: Oh, Suyeon; Park, Hyungmin; Yi, Yu; Chae, Jongchul
Bibcode: 2014JGRA..119...88O
Altcode:
The noise embedded in the raw data in one scientific discipline has
sometimes been proven to be a valuable signal for another discipline,
and there are examples throughout science history. The solar images
taken by the solid state detectors (CCDs) of the Sun monitoring
satellites are usually cleaned by removing the traces of cosmic
rays on the raw CCD data files. Thus, while applying the method of
removing the cosmic ray traces, we may be able to estimate the cosmic
ray flux by counting the number of traces. The net cosmic ray flux is
the sum of galactic cosmic rays and solar-originating particles. The
latter are seen as highly transient flux changes related to solar
eruptions. We can identify this kind of "cosmic ray" event from the
association with phenomena revealed in processed solar images, and we
show this using the data of SOHO/extreme ultraviolet imaging telescope
(EIT). On the other hand, the estimated cosmic ray flux in the steady
state is anticorrelated with solar cycle sunspot number. The profiles
of estimated solar cosmic ray flux showing significant increase are
found to be strongly correlated with the ground neutron monitor ground
level enhancements. Additionally, the profile of estimated cosmic ray
flux is consistent with that of the GOES P6 channel. It indicates that
the particles with energy higher than 80 MeV may mainly produce the
tracks on CCD of EIT. In conclusion, the raw solar images are valuable
data for estimating both long-term cosmic ray variations and transient
solar particles events.
Title: Spectral Inversion of the Hα Line for a Plasma Feature in
the Upper Chromosphere of the Quiet Sun
Authors: Chae, Jongchul
Bibcode: 2014ApJ...780..109C
Altcode:
We propose a generalization of Becker's cloud model (BCM): an embedded
cloud model (ECM)—for the inversion of the core of the Hα line
spectrum of a plasma feature either lying high above the forest
of chromospheric features or partly embedded in the outermost part
of this forest. The fundamental assumption of the ECM is that the
background light incident on the bottom of the feature from below is
equal to the ensemble-average light at the same height. This light
is related to the observed ensemble-average light via the radiative
transfer that is described by the four parameters newly introduced in
addition to the original four parameters of the BCM. Three of these
new parameters are independently determined from the observed rms
contrast profile of the ensemble. We use the constrained χ2
fitting technique to determine the five free parameters. We find that
the ECM leads to the fairly good fitting of the observed line profiles
and the reasonable inference of physical parameters in quiet regions
where the BCM cannot. Our first application of this model to a quiet
region of the Sun indicates that the model can produce the complete
velocity map and Doppler width map of the region.
Title: Determination of Temperature in Solar Prominences/Filaments
Using FISS Observations
Authors: Chae, Jongchul; Park, Hyungmin; Song, Donguk
Bibcode: 2014IAUS..300...85C
Altcode:
Using the Fast Imaging Solar Spectrograph of the 1.6 meter New Solar
Telescope at Big Bear, we simultaneously took the spectral profiles
of the Hα line and the Ca ii line at 854.2 nm from prominences beyond
the solar limb and filaments on the disk. The spectral data were fitted
by the slab model of radiative transfer with constant source function,
either with zero background intensity profile (in prominences) or with
carefully constructed background intensity profile (in filaments). These
observations with different perspectives and different analyses produced
consistent results: temperature inside prominences/filaments ranges
from 4000 to 20000 K with a mean of about 9500 K. We expect that this
kind of observation and analysis with higher spatial resolution and
higher temporal resolution will allow us to study in detail the thermal
structure and evolution of plasma in prominences.
Title: Activity-related variations of high-degree p-mode amplitude,
width, and energy in solar active regions
Authors: Maurya, R. A.; Ambastha, A.; Chae, J.
Bibcode: 2014A&A...561A.123M
Altcode: 2013arXiv1310.6458M
Context. Solar energetic transients such as flares and coronal mass
ejections occur mostly within active regions (ARs) and release large
amounts of energy, which is expected to excite acoustic waves by
transferring the mechanical impulse of the thermal expansion of the
flare on the photosphere. On the other hand, strong magnetic fields of
AR sunspots absorb the power of the photospheric oscillation modes.
Aims: We study the properties of high-degree p-mode oscillations in
flaring and dormant ARs and compare them with those in corresponding
quiet regions (QRs) to find the association of the mode parameters with
magnetic- and flare-related activities.
Methods: We computed the
mode parameters using the ring-diagram technique. The magnetic-activity
indices (MAIs) of ARs and QRs were determined from the line-of-sight
magnetograms. The flare indices (FIs) of ARs were obtained from the GOES
X-ray fluxes. Mode parameters were corrected for foreshortening, duty
cycle, and MAI using multiple non-linear regression.
Results:
Our analysis of several flaring and dormant ARs observed during the
Carrington rotations 1980-2109 showed a strong association of the mode
amplitude, width, and energy with magnetic and flare activities,
although their changes are combined effects of foreshortening,
duty cycle, magnetic-activity, flare-activity, and measurement
uncertainties. We find that the largest reduction in mode amplitude
and background power of an AR are caused by the angular distance of
the AR from the solar disc centre. After correcting the mode parameters
for foreshortening and duty cycle, we find that the mode amplitudes of
flaring and dormant ARs are lower than in corresponding QRs reducing
with increasing MAI, suggesting a stronger mode power suppression in
ARs with larger magnetic fields. The mode widths in ARs are larger
than in corresponding QRs and increase with MAI, indicating shorter
lifetimes of modes in ARs than in QRs. The variations in mode amplitude
and width with MAI are not same in different frequency bands. The
largest amplification (reduction) in mode amplitude (mode width) of
dormant ARs is found in the five-minute frequency band. The average
mode energy of both the flaring and dormant ARs is smaller than in
their corresponding QRs, reducing with increasing MAI. But the average
mode energy reduction rate in flaring ARs is smaller than in dormant
ARs. Moreover, the increase in mode width rate in dormant (flaring)
ARs is followed by a decrease (increase) in the amplitude variation
rate. Furthermore, including the mode corrections for MAI shows that
mode amplitude and mode energy of flaring ARs escalate with FI, while
the mode width shows an opposite trend, suggesting excitations of modes
and growth in their lifetimes by flares. The increase (decrease) in mode
amplitude (width) is larger in the five-minute and higher-frequency
bands. The enhancement in width variation rate is followed by a rapid
decline in the amplitude variation rate.
Title: Temperature of Solar Prominences Obtained with the Fast Imaging
Solar Spectrograph on the 1.6 m New Solar Telescope at the Big Bear
Solar Observatory
Authors: Park, Hyungmin; Chae, Jongchul; Song, Donguk; Maurya, Ram
Ajor; Yang, Heesu; Park, Young-Deuk; Jang, Bi-Ho; Nah, Jakyoung; Cho,
Kyung-Suk; Kim, Yeon-Han; Ahn, Kwangsu; Cao, Wenda; Goode, Philip R.
Bibcode: 2013SoPh..288..105P
Altcode:
We observed solar prominences with the Fast Imaging Solar Spectrograph
(FISS) at the Big Bear Solar Observatory on 30 June 2010 and 15 August
2011. To determine the temperature of the prominence material, we
applied a nonlinear least-squares fitting of the radiative transfer
model. From the Doppler broadening of the Hα and Ca II lines, we
determined the temperature and nonthermal velocity separately. The
ranges of temperature and nonthermal velocity were 4000 - 20 000 K and
4 - 11 km s−1. We also found that the temperature varied
much from point to point within one prominence.
Title: Velocities and Temperatures of an Ellerman Bomb and Its
Associated Features
Authors: Yang, Heesu; Chae, Jongchul; Lim, Eun-Kyung; Park, Hyungmin;
Cho, Kyuhyoun; Maurya, Ram Ajor; Song, Donguk; Kim, Yeon-Han; Goode,
Philip R.
Bibcode: 2013SoPh..288...39Y
Altcode:
We investigated the velocity and temperature characteristics of an
Ellerman bomb (EB) and its associated features based on observations
made with the Fast Imaging Solar Spectrograph (FISS) and a broadband
TiO filter of the 1.6 meter New Solar Telescope at Big Bear Solar
Observatory. In the TiO images of the photospheric level, we found a
granular cell expanding in two opposite directions near the site of the
EB. When one end of this granule reached the EB site, the transverse
speed of the tip of the expanding granule rapidly decreased and the EB
brightened. The wings of the Hα profile of the EB indicated that the
EB was blueshifted up to 7 km s−1. About 260 s after the EB
brightening, a surge was seen in absorption and varied from a blueshift
of 20 km s−1 to a redshift of 40 km s−1 seen
in the Hα and Ca II 8542 Å lines. From the Doppler absorption width
of the two lines determined by applying the cloud model, we estimated
the mean temperature of the surge material to be about 29000 K and the
mean speed of nonthermal motion to be about 11 km s−1. We
discuss the physical implications of our results in terms of magnetic
reconnection and processes related to it.
Title: Fast Imaging Solar Spectrograph of the 1.6 Meter New Solar
Telescope at Big Bear Solar Observatory
Authors: Chae, Jongchul; Park, Hyung-Min; Ahn, Kwangsu; Yang, Heesu;
Park, Young-Deuk; Nah, Jakyoung; Jang, Bi Ho; Cho, Kyung-Suk; Cao,
Wenda; Goode, Philip R.
Bibcode: 2013SoPh..288....1C
Altcode: 2012SoPh..tmp..248C
For high resolution spectral observations of the Sun - particularly
its chromosphere, we have developed a dual-band echelle spectrograph
named Fast Imaging Solar Spectrograph (FISS), and installed it in a
vertical optical table in the Coudé Lab of the 1.6 meter New Solar
Telescope at Big Bear Solar Observatory. This instrument can cover any
part of the visible and near-infrared spectrum, but it usually records
the Hα band and the Ca II 8542 Å band simultaneously using two CCD
cameras, producing data well suited for the study of the structure and
dynamics of the chromosphere and filaments/prominences. The instrument
does imaging of high quality using a fast scan of the slit across the
field of view with the aid of adaptive optics. We describe its design,
specifics, and performance as well as data processing
Title: Doppler Shifts of the Hα Line and the Ca II 854.2 nm Line
in a Quiet Region of the Sun Observed with the FISS/NST
Authors: Chae, Jongchul; Park, Hyung-Min; Ahn, Kwangsu; Yang, Heesu;
Park, Young-Deuk; Cho, Kyung-Suk; Cao, Wenda
Bibcode: 2013SoPh..288...89C
Altcode:
The characteristics of Doppler shifts in a quiet region of the Sun
are compared between the Hα line and the Ca II infrared line at 854.2
nm. A small area of 16″×40″ was observed for about half an hour
with the Fast Imaging Solar Spectrograph (FISS) of the 1.6 meter New
Solar Telescope (NST) at Big Bear Solar Observatory. The observed area
contains a network region and an internetwork region, and identified
in the network region are fibrils and bright points. We infer Doppler
velocity vm from each line profile at each individual point
with the lambdameter method as a function of half wavelength separation
Δλ. It is confirmed that the bisector of the spatially averaged Ca II
line profile has an inverse C-shape with a significant peak redshift of
+ 1.8 km s−1. In contrast, the bisector of the spatially
averaged Hα line profile has a C-shape with a small peak blueshift of
− 0.5 km s−1. In both lines, the bisectors of bright
network points are significantly redshifted not only at the line
centers, but also at the wings. The Ca II Doppler shifts are found
to be correlated with the Hα ones with the strongest correlation
occurring in the internetwork region. Moreover, we find that here the
Doppler shifts in the two lines are essentially in phase. We discuss
the physical implications of our results in view of the formation of
the Hα line and Ca II 854.2 nm line in the quiet region chromosphere.
Title: Chromospheric Sunspot Oscillations in Hα and Ca II 8542 Å
Authors: Maurya, Ram Ajor; Chae, Jongchul; Park, Hyungmin; Yang,
Heesu; Song, Donguk; Cho, Kyuhyoun
Bibcode: 2013SoPh..288...73M
Altcode:
We study chromospheric oscillations including umbral flashes and running
penumbral waves in a sunspot of active region NOAA 11242 using scanning
spectroscopy in Hα and Ca II 8542 Å with the Fast Imaging Solar
Spectrograph (FISS) at the 1.6 meter New Solar Telescope at the Big Bear
Solar Observatory. A bisector method is applied to spectral observations
to construct chromospheric Doppler-velocity maps. Temporal-sequence
analysis of these shows enhanced high-frequency oscillations inside
the sunspot umbra in both lines. Their peak frequency gradually
decreases outward from the umbra. The oscillation power is found to be
associated with magnetic-field strength and inclination, with different
relationships in different frequency bands.
Title: Study of Magnetic Helicity Injection in the Active Region NOAA
9236 Producing Multiple Flare-associated Coronal Mass Ejection Events
Authors: Park, Sung-Hong; Kusano, Kanya; Cho, Kyung-Suk; Chae,
Jongchul; Bong, Su-Chan; Kumar, Pankaj; Park, So-Young; Kim, Yeon-Han;
Park, Young-Deuk
Bibcode: 2013ApJ...778...13P
Altcode: 2013arXiv1308.5774P
To better understand a preferred magnetic field configuration and its
evolution during coronal mass ejection (CME) events, we investigated
the spatial and temporal evolution of photospheric magnetic fields in
the active region NOAA 9236 that produced eight flare-associated CMEs
during the time period of 2000 November 23-26. The time variations
of the total magnetic helicity injection rate and the total unsigned
magnetic flux are determined and examined not only in the entire
active region but also in some local regions such as the main sunspots
and the CME-associated flaring regions using SOHO/MDI magnetogram
data. As a result, we found that (1) in the sunspots, a large amount
of positive (right-handed) magnetic helicity was injected during most
of the examined time period, (2) in the flare region, there was a
continuous injection of negative (left-handed) magnetic helicity during
the entire period, accompanied by a large increase of the unsigned
magnetic flux, and (3) the flaring regions were mainly composed of
emerging bipoles of magnetic fragments in which magnetic field lines
have substantially favorable conditions for making reconnection with
large-scale, overlying, and oppositely directed magnetic field lines
connecting the main sunspots. These observational findings can also
be well explained by some MHD numerical simulations for CME initiation
(e.g., reconnection-favored emerging flux models). We therefore conclude
that reconnection-favored magnetic fields in the flaring emerging flux
regions play a crucial role in producing the multiple flare-associated
CMEs in NOAA 9236.
Title: FISS Observations of Vertical Motion of Plasma in Tiny Pores
Authors: Cho, K. -S.; Bong, S. -C.; Chae, J.; Kim, Y. -H.; Park,
Y. -D.; Katsukawa, Y.
Bibcode: 2013SoPh..288...23C
Altcode:
Pores can be exploited for the understanding of the interaction
between small-scale vertical magnetic field and the surrounding
convective motions as well as the transport of mechanical energy into
the chromosphere along the magnetic field. For better understanding of
the physics of pores, we investigate tiny pores in a new emerging
active region (AR11117) that were observed on 26 October 2010
by the Solar Optical Telescope (SOT) on board Hinode and the Fast
Imaging Solar Spectrograph (FISS) of the 1.6 meter New Solar Telescope
(NST). The pores are compared with nearby small magnetic concentrations
(SMCs), which have similar magnetic flux as the pores but do not
appear dark. Magnetic flux density and Doppler velocities in the
photosphere are estimated by applying the center-of-gravity method to
the Hinode/Spectro-Polarimeter data. The line-of-sight motions in the
lower chromosphere are determined by applying the bisector method to
the wings of the Hα and the Ca II 8542 Å line simultaneously taken
by the FISS. The coordinated observation reveals that the pores are
filled with plasma which moves down slowly and are surrounded by
stronger downflow in the photosphere. In the lower chromosphere, we
found that the plasma flows upwards inside the pores while the plasma
in the SMCs is always moving down. Our inspection of the Ca II 8542 Å
line from the wing to the core shows that the upflow in the pores slows
down with height and turns into downflow in the upper chromosphere
while the downflow in the SMCs gains its speed. Our results are in
agreement with the numerical studies which suggest that rapid cooling
of the interior of the pores drives a strong downflow, which collides
with the dense lower layer below and rebounds into an upflow.
Title: Connection Between Chromospheric Events and Photospheric
Dynamics
Authors: Anđić, A.; Chae, J.; Park, H.; Yang, H.; Ahn, K.; Cao,
W.; Park, Y. D.
Bibcode: 2013SoPh..288...55A
Altcode: 2012SoPh..tmp..232A
We analyzed chromospheric events and their connection to oscillation
phenomena and photospheric dynamics. The observations were done with the
New Solar Telescope of Big Bear Solar Observatory using a broad-band
imager at the wavelength of a TiO band and FISS spectrograph scanning
Ca II and Hα spectral lines. The event in Ca II showed strong plasma
flows and propagating waves in the chromosphere. The movement of
the footpoints of flux tubes in the photosphere indicated flux tube
entanglement and magnetic reconnection as a possible cause of the
observed brightening and waves propagating in the chromosphere. An
upward propagating train of waves was observed at the site of the
downflow event in Hα. There was no clear relationship between
photospheric waves and the Ca II and Hα events. Our observations
indicate that chromospheric waves that were previously thought to
originate from the photosphere may be generated by some events in the
chromosphere as well.
Title: Global Coronal Seismology in the Extended Solar Corona through
Fast Magnetosonic Waves Observed by STEREO SECCHI COR1
Authors: Kwon, Ryun-Young; Kramar, Maxim; Wang, Tongjiang; Ofman,
Leon; Davila, Joseph M.; Chae, Jongchul; Zhang, Jie
Bibcode: 2013ApJ...776...55K
Altcode:
We present global coronal seismology for the first time, which allows
us to determine inhomogeneous magnetic field strength in the extended
corona. From the measurements of the propagation speed of a fast
magnetosonic wave associated with a coronal mass ejection (CME)
and the coronal background density distribution derived from the
polarized radiances observed by the STEREO SECCHI COR1, we determined
the magnetic field strengths along the trajectories of the wave at
different heliocentric distances. We found that the results have an
uncertainty less than 40%, and are consistent with values determined
with a potential field model and reported in previous works. The
characteristics of the coronal medium we found are that (1) the density,
magnetic field strength, and plasma β are lower in the coronal hole
region than in streamers; (2) the magnetic field strength decreases
slowly with height but the electron density decreases rapidly so that
the local fast magnetosonic speed increases while plasma β falls off
with height; and (3) the variations of the local fast magnetosonic
speed and plasma β are dominated by variations in the electron
density rather than the magnetic field strength. These results imply
that Moreton and EIT waves are downward-reflected fast magnetosonic
waves from the upper solar corona, rather than freely propagating
fast magnetosonic waves in a certain atmospheric layer. In addition,
the azimuthal components of CMEs and the driven waves may play an
important role in various manifestations of shocks, such as type II
radio bursts and solar energetic particle events.
Title: Helicity injection by flux motions and its role in flare
and CMEs
Authors: Panditi, Vemareddy; Ambastha, A.; Maurya, R.; Chae, J.;
Ambstha, A.; Maurya, R. A.; Chae, J.
Bibcode: 2013SPD....4430003P
Altcode:
An investigation of helicity injection by photospheric shear motions
is carried out for two active regions (ARs), NOAA 11158 and 11166,
using line-of-sight magnetic field observations obtained from
the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory. We derived the horizontal velocities in the ARs from the
differential affine velocity estimator (DAVE) technique. Persistent
strong shear motions at maximum velocities in the range of 0.6-0.9
km s-1 along the magnetic polarity inversion line and outward flows
from the peripheral regions of the sunspots were observed in the two
ARs. The helicities injected in NOAA 11158 and 11166 during their
six-day evolution period were estimated as 14.16x10$^42$ Mx$^2$
and 9.5x10$^42$ Mx$^2$, respectively. The estimated injection rates
decreased up to 13% by increasing the time interval between the
magnetograms from 12 minutes to 36 minutes, and increased up to 9%
by decreasing the DAVE window size from 21x18 to 9x6 pixel$^2$,
resulting in 10% variation in the accumulated helicity. In both
ARs, the flare-prone regions (R2) had inhomogeneous helicity flux
distribution with mixed helicities of both signs and coronal mass
ejection (CME) prone regions had almost homogeneous distribution of
helicity flux dominated by a single sign. The temporal profiles of
helicity injection showed impulsive variations during some flares/CMEs
due to negative helicity injection into the dominant region of positive
helicity flux. A quantitative analysis reveals a marginally significant
association of helicity flux with CMEs but not flares in AR 11158,
while for the AR 11166, we find a marginally significant association
of helicity flux with flares but not CMEs, providing evidence of the
role of helicity injection at localized sites of the events. These
short-term variations of helicity flux are further discussed in
view of possible flare-related effects. This study suggests that flux
motions and spatial distribution of helicity injection are important to
understanding the complex nature of the magnetic flux system of the AR,
and how it can lead to conditions favorable for eruptive events.
Title: Helicity Injection by the Shearing Motion of Fluxes in Relation
to Flares and Coronal Mass Ejections
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J.
Bibcode: 2013enss.confE...8V
Altcode:
An investigation of helicity injection by photospheric shear motions is
carried out for two active regions (ARs), NOAA 11158 and 11166, using
line-of-sight magnetic field observations obtained from the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. We derived
the horizontal velocities in the ARs from the differential affine
velocity estimator (DAVE) technique. Persistent strong shear motions at
maximum velocities in the range of 0.6-0.9 km s^{-1} along the magnetic
polarity inversion line and outward flows from the peripheral regions
of the sunspots were observed in the two ARs. The helicities injected
in NOAA 11158 and 11166 during their six-day evolution period were
estimated as 14.16x10^{42} Mx^2 and 9.5×10^{42} Mx^2, respectively. The
estimated injection rates decreased up to 13% by increasing the time
interval between the magnetograms from 12 minutes to 36 minutes, and
increased up to 9% by decreasing the DAVE window size from 21×18 to
9×6 pixel2, resulting in 10% variation in the accumulated helicity. In
both ARs, the flare-prone regions (R2) had inhomogeneous helicity
flux distribution with mixed helicities of both signs and coronal
mass ejection (CME) prone regions had almost homogeneous distribution
of helicity flux dominated by a single sign. The temporal profiles of
helicity injection showed impulsive variations during some flares/CMEs
due to negative helicity injection into the dominant region of positive
helicity flux. A quantitative analysis reveals a marginally significant
association of helicity flux with CMEs but not flares in AR 11158,
while for the AR 11166, we find a marginally significant association
of helicity flux with flares but not CMEs, providing evidence of the
role of helicity injection at localized sites of the events. These
short-term variations of helicity flux are further discussed in
view of possible flare-related effects. This study suggests that flux
motions and spatial distribution of helicity injection are important to
understanding the complex nature of the magnetic flux system of the AR,
and how it can lead to conditions favorable for eruptive events.
Title: Chromospheric Waves and Oscillations in Sunspots
Authors: Maurya, R. A.; Chae, J.
Bibcode: 2013enss.confE..45M
Altcode:
We studied the chromospheric oscillations in and around a sunspot
of the active region NOAA 11242 using high spectral and spatial
resolution observations in the spectral lines Hα and Ca II 8542Å
obtained from the Fast Imaging Solar Spectrograph (FISS) of 1.6 meter
New Solar Telescope (NST) at Big Bear Solar Observatory. A suitable
bisector method is applied to the spectral observations, to construct
the chromospheric Doppler Velocity maps. Time series analysis of
Doppler maps, in both the spectral bands, revealed enhanced high
frequency oscillations inside the umbra of the sunspot. The frequency
of oscillations gradually decreases from the umbra to outward. We have
found clear evidence of two boundaries for the peak power frequency
transformation, one of which occurs close to the umbral and penumbral
boundary, and the other near the penumbral and super-penumbral boundary
of the sunspot. The oscillation power is found to be associated with
magnetic field strength and inclination, although they showed different
relationships in different frequency bands.
Title: Chromospheric Sunspot Oscillations in H-alpha and Ca II 8542A
Authors: Ajor Maurya, Ram; Chae, Jongchul; Park, Hyungmin; Yang,
Heesu; Song, Donguk; Cho, Kyuhyoun
Bibcode: 2013arXiv1302.1234A
Altcode:
We study chromospheric oscillations including umbral flashes and
running penumbral waves in a sunspot of active region (AR) using
scanning spectroscopy in H-alpha and Ca II 8542A, with the Fast Imaging
Solar Spectrograph (FISS) at the 1.6 meter New Solar Telescope at
Big Bear Solar Observatory. A bisector method is applied to spectral
observations to construct chromospheric Doppler velocity maps. Temporal
sequence analysis of these shows enhanced high-frequency oscillations
inside the sunspot umbra in both lines. Their peak frequency gradually
decreases outward from the umbra. The oscillation power is found to be
associated with magnetic-field strength and inclination, with different
relationships in different frequency bands.
Title: Solar Cosmic Ray Flux Variation Estimated from the Raw Solar
Images Taken by SOHO/EIT
Authors: Oh, Suyeon; Park, Hyungmin; Chae, Jongchul; Yi, Yu
Bibcode: 2013ICRC...33.3476O
Altcode:
The solar images are taken by the CCD detectors of the Sun monitoring
satellites such as SOHO, TRACE, STEREO, SDO, and so on. The solar images
are constructed after removing the traces of cosmic rays on the raw CCD
data files. Thus, while applying the method of removing the cosmic rays
traces, we can estimate the cosmic rays flux by counting the number
of traces. The cosmic ray flux in the steady state might be the sum
of the solar and galactic cosmic rays. However, the abrupt change in
the flux could be assumed to be originated from the Sun. Therefore,
we can identify the solar origins of the sudden solar cosmic ray flux
changes from the phenomena shown in the processed solar images. The
results using the data of SOHO/EIT are presented. The estimated cosmic
ray flux in the steady state is the anti-correlated with sunspot
numbers, which shows the minima in cosmic ray flux at the solar cycle
maxima defined by the sunspot numbers. The profiles of estimated solar
cosmic ray associated with ground level enhancements observed by the
ground neutron monitor have the significant increase in the cosmic
ray flux with good correlation. Thus, the solar images are valuable
data useful in estimating the solar cosmic ray long term and transient
flux variations.
Title: On the Injection of Helicity by the Shearing Motion of Fluxes
in Relation to Flares and Coronal Mass Ejections
Authors: Vemareddy, P.; Ambastha, A.; Maurya, R. A.; Chae, J.
Bibcode: 2012ApJ...761...86V
Altcode: 2012arXiv1202.5195V
An investigation of helicity injection by photospheric shear motions is
carried out for two active regions (ARs), NOAA 11158 and 11166, using
line-of-sight magnetic field observations obtained from the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory. We derived
the horizontal velocities in the ARs from the differential affine
velocity estimator (DAVE) technique. Persistent strong shear motions at
maximum velocities in the range of 0.6-0.9 km s-1 along the
magnetic polarity inversion line and outward flows from the peripheral
regions of the sunspots were observed in the two ARs. The helicities
injected in NOAA 11158 and 11166 during their six-day evolution
period were estimated as 14.16 × 1042 Mx2 and
9.5 × 1042 Mx2, respectively. The estimated
injection rates decreased up to 13% by increasing the time interval
between the magnetograms from 12 minutes to 36 minutes, and increased
up to 9% by decreasing the DAVE window size from 21 × 18 to 9 ×
6 pixel2, resulting in 10% variation in the accumulated
helicity. In both ARs, the flare-prone regions (R2) had inhomogeneous
helicity flux distribution with mixed helicities of both signs and
coronal mass ejection (CME) prone regions had almost homogeneous
distribution of helicity flux dominated by a single sign. The temporal
profiles of helicity injection showed impulsive variations during
some flares/CMEs due to negative helicity injection into the dominant
region of positive helicity flux. A quantitative analysis reveals
a marginally significant association of helicity flux with CMEs but
not flares in AR 11158, while for the AR 11166, we find a marginally
significant association of helicity flux with flares but not CMEs,
providing evidence of the role of helicity injection at localized
sites of the events. These short-term variations of helicity flux
are further discussed in view of possible flare-related effects. This
study suggests that flux motions and spatial distribution of helicity
injection are important to understanding the complex nature of the
magnetic flux system of the AR, and how it can lead to conditions
favorable for eruptive events.
Title: Three-dimensional Structure and Evolution of
Extreme-ultraviolet Bright Points Observed by STEREO/SECCHI/EUVI
Authors: Kwon, Ryun-Young; Chae, Jongchul; Davila, Joseph M.; Zhang,
Jie; Moon, Yong-Jae; Poomvises, Watanachak; Jones, Shaela I.
Bibcode: 2012ApJ...757..167K
Altcode:
We unveil the three-dimensional structure of quiet-Sun EUV bright
points and their temporal evolution by applying a triangulation method
to time series of images taken by SECCHI/EUVI on board the STEREO twin
spacecraft. For this study we examine the heights and lengths as the
components of the three-dimensional structure of EUV bright points
and their temporal evolutions. Among them we present three bright
points which show three distinct changes in the height and length:
decreasing, increasing, and steady. We show that the three distinct
changes are consistent with the motions (converging, diverging,
and shearing, respectively) of their photospheric magnetic flux
concentrations. Both growth and shrinkage of the magnetic fluxes
occur during their lifetimes and they are dominant in the initial
and later phases, respectively. They are all multi-temperature
loop systems which have hot loops (~106.2 K) overlying
cooler ones (~106.0 K) with cool legs (~104.9
K) during their whole evolutionary histories. Our results imply that
the multi-thermal loop system is a general character of EUV bright
points. We conclude that EUV bright points are flaring loops formed by
magnetic reconnection and their geometry may represent the reconnected
magnetic field lines rather than the separator field lines.
Title: Ca II Transient Brightenings Associated with Canceling
Magnetic Features
Authors: Park, Soyoung; Chae, Jongchul
Bibcode: 2012SoPh..280..103P
Altcode: 2012SoPh..tmp..158P
We analyzed transient Ca II H brightening associated with small-scale
canceling magnetic features (CMFs) in the quiet Sun near disk center
using Ca II H filter images and Na D1 magnetograms
of Hinode/SOT. We found that in most Ca II brightening events
related to CMFs the Ca II H intensity peaks after the magnetic flux
cancellation. Moreover, the brightening tends to appear as a pair
of bright points of similar size and brightness overlying a magnetic
bipole. Then a new opposite polarity fragment moves to them and cancels
out. These results imply that magnetic reconnection takes place there
and is responsible for CMFs.
Title: Rates of Magnetic Flux Cancellation Measured with Hinode/SOT
Authors: Park, S.; Chae, J.; Litvinenko, Y. E.
Bibcode: 2012ASPC..454...59P
Altcode:
The specific cancellation rate of canceling magnetic features,
defined as the rate of flux cancellation divided by the interface
length, is an observational quantity of magnetic reconnection model
in the low atmosphere. This value gives an accurate estimate of the
electric field in the reconnecting current sheet. We have measured
the specific rates using the magnetograms taken by the Solar Optical
Telescope (SOT)/Hinode. As a result, the specific rates determined
with SOT turned out to be systematically higher —about four times—
than those based on the data obtained by the Michelson Doppler Imager
(MDI) aboard the SOHO. This higher value is mainly due to the higher
resolution and better sensitivity of the SOT, resulting in magnetic
fluxes up to five times larger than those obtained from the MDI. The
higher rates of flux cancellation correspond to either faster inflows
or stronger magnetic fields of the inflow region in the current sheet,
which may have important consequences for the physics of photospheric
magnetic reconnection.
Title: The Formation of a Magnetic Channel by Emergence of
Current-Carrying Magnetic Fields
Authors: Lim, E. -K.; Chae, J.; Jing, J.; Wang, H.; Wiegelmann, T.
Bibcode: 2012ASPC..454..185L
Altcode:
The term magnetic channel indicates a series of polarity reversals
separating elongated flux threads with opposite polarities. We studied
the formation process of the magnetic channel in AR 10930 using time
sequence SP data taken by SOT/Hinode. As a result, we detected both
upflows (-0.5 to -1.0 km s-1) and downflows (+1.5 to +2.0 km
s-1) within the channel site, and a pair of strong vertical
current threads of opposite polarity along the channel. We also analyzed
coronal magnetic fields computed from the non-linear force-free field
extrapolation, and found that the current density in the lower corona
increased as the channel structure evolved. These findings support
the idea that the magnetic channel represents the emergence of the
twisted flux tube.
Title: First Stereoscopic Determinations of Heights and Their
Variations of EUV Bright Points Using SECCHI EUVI aboard STEREO
Authors: Kwon, R. -Y.; Chae, J.
Bibcode: 2012ASPC..454..163K
Altcode:
We measure the heights of an EUV BPs observed by EUVI onboard the
STEREO, and their time variation using a stereoscopic method developed
by Kwon, Chae, Zhang (2010). The lifetime of the BP was 25 h. There is
a tendency that the height of the BP is the highest at the formation
phase and it decreases through its temporal evolution while the
intensity is the strongest in the latter half of its lifetime. There
is strong correlation between height and length which implies that BP
is a group of semi-circular loops. Our results strongly suggest that
a BP originates from magnetic reconnection due to the interaction
between converging opposite magnetic poles.
Title: Waves in the Solar Chromosphere using FISS and SDO
Authors: Ajor Maurya, Ram; Chae, Jongchul; Park, Hyungmin
Bibcode: 2012cosp...39.1204A
Altcode: 2012cosp.meet.1204A
Waves in the solar atmosphere are believed to play a crucial role in
heating the chromosphere and corona. To explore the characteristic
properties of waves in different chromospheric features, e.g.,
sunspot, plages, etc., we analyzed the high resolution observational
data provided by the Fast Imaging Solar Spectrometer (FISS) and Solar
Dynamic Observatory (SDO). The high spatial and temporal resolution
multi-wavelength observations provided by these instruments enable us
to sample the chromosphere at different heights. The spectral data
provided by the FISS are used to derive physical parameters, viz.,
temperature, Doppler velocity, etc., which help us to understand the
wave excitation mechanism in different chromospheric features. We
have studied the running penumbral waves by constructing Doppler
images from the spectral observations in different wavelength bands
of H-alpha and Ca II obtained from the FISS. Details of our findings
will be presented in the paper.
Title: The Occurrence and Speed of CMEs Related to Two Characteristic
Evolution Patterns of Helicity Injection in Their Solar Source Regions
Authors: Park, Sung-Hong; Cho, Kyung-Suk; Bong, Su-Chan; Kumar,
Pankaj; Chae, Jongchul; Liu, Rui; Wang, Haimin
Bibcode: 2012ApJ...750...48P
Altcode: 2012arXiv1203.1690P
Long-term (a few days) variation of magnetic helicity injection
was calculated for 28 solar active regions that produced 47 coronal
mass ejections (CMEs) to find its relationship to the CME occurrence
and speed using SOHO/MDI line-of-sight magnetograms. As a result, we
found that the 47 CMEs can be categorized into two different groups by
two characteristic evolution patterns of helicity injection in their
source active regions, which appeared for ~0.5-4.5 days before their
occurrence: (1) a monotonically increasing pattern with one sign of
helicity (Group A 30 CMEs in 23 active regions) and (2) a pattern
of significant helicity injection followed by its sign reversal
(Group B 17 CMEs in 5 active regions). We also found that CME speed
has a correlation with average helicity injection rate with linear
correlation coefficients of 0.85 and 0.63 for Group A and Group B,
respectively. In addition, these two CME groups show different
characteristics as follows: (1) the average CME speed of Group B
(1330 km s-1) is much faster than that of Group A (870
km s-1), (2) the CMEs in Group A tend to be single events
whereas those in Group B mainly consist of successive events, and (3)
flares related to the CMEs in Group B are relatively more energetic
and impulsive than those in Group A. Our findings therefore suggest
that the two CME groups have different pre-CME conditions in their
source active regions and different CME characteristics.
Title: Magnetic Reconnection in the Photosphere and Chromosphere
Authors: Chae, J.
Bibcode: 2012ASPC..455..187C
Altcode:
Recent studies based on Hinode and high resolution ground-based
observations have produced significant results that shed more light
on the physical property of reconnection processes occurring in the
photosphere and chromosphere. For better insights on this issue,
I will review relevant published results together with previous
results, focusing on five questions: (1) Is flux cancellation a result
of magnetic reconnection? (2) At what atmospheric levels does such
reconnection occur? (3) How does magnetic flux retract? (4) How does
plasma respond to reconnection? (5) Is reconnection responsible for
the chromospheric and coronal heating?
Title: Automatic Solar Filament Segmentation and Characterization
Authors: Yuan, Y.; Shih, F. Y.; Jing, J.; Wang, H.; Chae, J.
Bibcode: 2011SoPh..272..101Y
Altcode: 2011SoPh..tmp..131Y; 2011SoPh..tmp..187Y; 2011SoPh..tmp..256Y
This paper presents a generic method to automatically segment and
characterize solar filaments from various Hα full-disk solar images,
obtained by different solar observatories, with different dynamic
ranges and statistical properties. First, a cascading Hough circle
detector is designed to find the center location and radius of the
solar disks. Second, polynomial surface fitting is adopted to correct
unbalanced luminance. Third, an adaptive thresholding method is put
forward to segment solar filaments. Finally, for each piece of a solar
filament, its centroid location, area, and length are characterized, in
which morphological thinning and graph theory are used for identifying
the main skeletons of filaments. To test the performance of the proposed
methods, a dataset composed of 125 Hα images is considered. These
images were obtained by four solar observatories from January 2000
to May 2010, one image per month. Experimental results show that the
accuracy rate is above 95% as measured by filament number and above 99%
as measured by filament area, indicating that only a few tiny filaments
are not detected.
Title: Substructure of quiet sun bright points
Authors: Andic, Aleksandra; Chae, Jongchul; Goode, Phillip R.
Bibcode: 2011IAUS..273..339A
Altcode: 2010arXiv1012.2080A
Since photospheric bright points (BPs) were first observed, there has
been a question as to how are they structured. Are they just single
flux tubes or a bundle of the flux-tubes? Surface photometry of the
quiet Sun (QS) has achieved resolution close to 0.1'' with the New Solar
Telescope at Big Bear Solar Observatory. This resolution allowed us to
detect a richer spectrum of BPs in the QS. The smallest BPs we observed
with TiO 705.68 nm were 0.13'', and we were able to resolve individual
components in some of the BPs clusters and ribbons observed in the QS,
showing that they are composed of the individual BPs. Average size of
observed BPs was 0.22''.
Title: Active Region High Velocity Events Observed by Fast Imaging
Solar Spectrograph on the NST
Authors: Ahn, Kwangsu; Chae, J.; Nah, J.; Park, H.; Jang, B.; Yang,
H.; Park, Y.; Cao, W.; Goode, P. R.
Bibcode: 2011SPD....42.1904A
Altcode: 2011BAAS..43S.1904A
The Fast Imaging Solar Spectrograph (FISS) is the only
spectrograph-based instrument on the NST that is currently
operational. With a high spectral resolution of 1.4 x 105
and simultaneous dual spectral band imaging, this instrument can
accurately determine the physical parameters of chromospheric features
--- filaments, jets and so on. Initial observations captured several
peculiar dynamic events that showed high line-of-sight velocities
of the order of 20 km s-1 in the vicinity of sunspots. We
expect that FISS will provide a better understanding of the physics
in the chromosphere with the aid of the unprecedentedly high spatial
resolution of NST.
Title: Propagating Transverse Wave In A Spicule Observed By The
Hinode Sot
Authors: Kim, Yeon-Han; Bong, S.; Cho, K.; Park, Y.; Cho, I.; Chae, J.
Bibcode: 2011SPD....42.0306K
Altcode: 2011BAAS..43S.0306K
We examined the wave signatures in a spicule observed by the Hinode SOT
to present a quantitative and clear evidence of propagating transverse
waves. Especially, we estimated the propagation wave speed in the
spicule observed on 2008 June 3 in the north polar limb of the Sun. For
this, we made time-slice stacks at different height of the spicule axis
using time series images of the spicule. All time-slice stacks for 9
different heights show oscillation patterns with a period of about 130
s. We also investigated the cross-correlation among time-slice stacks
to estimate the phase difference of the oscillation with height and
its propagating speed. We found that the mean phase delay over the
height difference of 3000 km is about 13 s. This result suggests that
the oscillation is a propagating transverse wave and the propagating
speed is about 220 km s-1. In addition, we found that the
estimated speeds increased from 100 to 230 km s-1 over 5000
km height range and this is well explained by the density variation
along the spicule structure.
Title: Chromospheric Response At The Photospheric Dynamic
Authors: Andic, Aleksandra; Chae, J.; Park, H.; Yang, H.
Bibcode: 2011SPD....42.1735A
Altcode: 2011BAAS..43S.1735A
We analysed the event in the chromosphere and its connection to
the photo- spheric dynamic. The observations were done at New Solar
Telescope at the Big Bear Solar Observatory using the photometry in TiO
spectral line and FISS spec- trograph scanning Ca II and Hα spectral
lines. The event showed strong plasma flows and propagating
oscillations that coin- cided with the photospheric dynamic. The
movement of the footprints of the flux tubes in photosphere indicated
a possible flux tube entanglement and mag- netic reconnection that
caused observed brightening. The waves, originating in the photosphere,
were propagating prior the event. The reconnection itself caused
additional burst of the oscillations originating at the place of the
reconnection. This event are complex and contain both jets and the
oscillatory propagation.
Title: Tiny Pores Observed by New Solar Telescope and Hinode
Authors: Cho, KyungSuk; Bong, S.; Chae, J.; Kim, Y.; Park, Y.; Ahn,
K.; Katsukawa, Y.
Bibcode: 2011SPD....42.1903C
Altcode: 2011BAAS..43S.1903C
Seoul National University and Korea Astronomy and Space Science
Institute installed Fast Imaging Solar Spectrograph (FISS) in the
Cude room of the 1.6 m New Solar Telescope (NST) at Big Bear Solar
Observatory on May 14, 2010. FISS is a unique system that can do
imaging of H-alpha and Ca II 8542 band simultaneously, which is quite
suitable for studying of dynamics of chromosphere. To investigate the
relationship between the photospheric and low-chromospheric motions at
the pore region, we took a coordinate observation with NST/FISS and
Hinode/SOT for new emerging active region (AR11117) on October 26,
2010. In the observed region, we could find two tiny pores and two
small magnetic concentrations (SMCs), which have similar magnetic
flux with the pores but do not look dark. Magnetic flux density and
Doppler velocities at the photosphere are estimated by applying the
center-of-gravity (COG) method to the HINODE/spectropolarimeter (SP)
data. The line-of-sight motions above the photosphere are determined
by adopting the bisector method to the wing spectra of Ha and CaII
8542 lines. As results, we found the followings. (1)There are upflow
motion on the pores and downflow motion on the SMCs. (2)Towards the
CaII 8542 line center, upflow motion decrease and turn to downward
motion in pores, while the speed of down flow motion increases in
the SMCs. (3)There is oscillating motion above pores and the SMCs,
and this motion keep its pattern along the height. (4) As height
increase, there is a general tendency of the speed shift to downward
on pores and the SMCs. In this poster, we will present preliminary
understanding of the coupling of pore dynamics between the photosphere
and the low-chromosphere.
Title: Response of Granulation to Small-scale Bright Features in
the Quiet Sun
Authors: Anđić, A.; Chae, J.; Goode, P. R.; Cao, W.; Ahn, K.;
Yurchyshyn, V.; Abramenko, V.
Bibcode: 2011ApJ...731...29A
Altcode: 2011arXiv1102.3404A
We detected 2.8 bright points (BPs) per Mm2 in the quiet
Sun with the New Solar Telescope at Big Bear Solar Observatory, using
the TiO 705.68 nm spectral line at an angular resolution ~0farcs1 to
obtain a 30 minute data sequence. Some BPs formed knots that were
stable in time and influenced the properties of the granulation
pattern around them. The observed granulation pattern within ~3''
of knots presents smaller granules than those observed in a normal
granulation pattern, i.e., around the knots a suppressed convection
is detected. Observed BPs covered ~5% of the solar surface and were
not homogeneously distributed. BPs had an average size of 0farcs22,
they were detectable for 4.28 minutes on average, and had an averaged
contrast of 0.1% in the deep red TiO spectral line.
Title: Development of the Fast Imaging Solar Spectrograph for 1.6
m New Solar Telescope
Authors: Nah, Ja-Kyoung; Chae, Jong-Chul; Park, Young-Deuk; Park,
Hyung-Min; Jang, Bi-Ho; Ahn, Kwang-Su; Yang, Hee-Su; Cho, Kyung-Suk;
Kim, Yeon-Han; Kim, Kwang-Dong; Cao, Wenda; Gorceix, Nicolas; Goode,
Philip. R.
Bibcode: 2011PKAS...26...45N
Altcode:
No abstract at ADS
Title: Recent activities of solar astronomers in Korea
Authors: Cho, K. -S.; Chae, J.; Moon, Y. J.; Choe, G. S.
Bibcode: 2011ASInC...2..383C
Altcode:
At present, about twenty PhDs are carrying out education, research, and
observation in the field of solar astronomy in Korea. The history and
recent activities of the Korean solar community are briefly reviewed
in this paper. We expect that the current efforts of Korean solar
astronomers contribute to the promotion of cooperative solar research
in the Asian-Pacific countries.
Title: Fast Imaging Solar Spectrograph System in New Solar Telescope
Authors: Park, Y. -D.; Kim, Y. H.; Chae, J.; Goode, P. R.; Cho, K. S.;
Park, H. M.; Nah, J. K.; Jang, B. H.
Bibcode: 2010nspm.conf..189P
Altcode:
In 2004, Big Bear Solar Observatory in California, USA launched
a project for construction of the world's largest aperture solar
telescope (D = 1.6m) called New Solar Telescope(NST). University
of Hawaii (UH) and Korea Astronomy and Space Science Institute(KASI)
partly collaborate on the project. NST is a designed off-axis parabolic
Gregorian reflector with very high spatial resolution(0.07 arcsec
at 5000A) and is equipped with several scientific instruments such as
Visible Imaging Magnetograph (VIM), InfraRed Imaging Magnetograph IRIM),
and so on. Since these scientific instruments are focused on studies of
the solar photosphere, we need a post-focus instrument for the NST to
study the fine structures and dynamic patterns of the solar chromosphere
and low Transition Region (TR) layer, including filaments/prominences,
spicules, jets, micro flares, etc. For this reason, we developed and
installed a fast imaging solar spectrograph(FISS) system on the NST
withadvantages of achieving compact design with high spectral resolution
and small aberration as well as recording many solar spectral lines in
a single and/or dual band mode. FISS was installed in May, 2010 and now
we carry out a test observation. In this talk, we introduce the FISS
system and the results of the test observation after FISS installation.
Title: Tiny Pores observed by HINODE/SOT
Authors: Cho, K.; Bong, S.; Chae, J.; Kim, Y.; Park, Y.
Bibcode: 2010AGUFMSH11B1647C
Altcode:
The study of pores, small penumbraless sunspots, can give us a chance to
understand how strong magnetic fields interact with convective motions
in the photosphere. For a better understanding of this interaction, we
investigate the temporal variation of several tiny pores smaller than
2“. These pores were observed by the Solar Optical Telescope (SOT)
onboard Hinode on 2006 December 29. We have analyzed the high resolution
spectropolarimetric (SP) data and the G-band filtergrams taken during
the observation. Magnetic flux density and Doppler velocities of the
pores are estimated by applying the center of gravity (COG) method to
the SP data. The horizontal motions in and around the pores are tracked
by adopting the Nonlinear Affine Velocity Estimator (NAVE) method to
the G-band filter images. As results, we found the followings. (1)
Darkness of pores is positively correlated with magnetic flux
density. (2) Downflows always exist inside and around the pores. (3)
The speed of downflows inside the pores is negatively correlated with
their darkness. (4) The pores are surrounded by strong downflows. (5)
Brightness changes of the pores are correlated with the divergence
of mass flow (correlation coefficient > 0.9). (6) The pores in the
growing phase are associated with the converging flow pattern and the
pores in the decay phase with the diverging flow pattern. Our results
support the idea that a pore grows as magnetic flux density increases
due to the convergence of ambient mass flow and it decays with the
decrease of the flux density due to the diverging mass flow.
Title: A small-scale H-alpha eruption in the north polar limb of
the Sun observed by New Solar Telescope
Authors: Kim, Y. -H.; Park, Y. -D.; Bong, S. -Ch.; Cho, K. -S.;
Chae, J.
Bibcode: 2010nspm.conf...73K
Altcode:
The 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory
(BBSO) is the recently constructed world's largest optical solar
telescope on the ground. Up to date it has been partly operated,
i.e., observations that have been made at Nasmyth focus only without
adaptive optic (AO) system. The AO system is planned to be installed
this summer. Using the NST, we have observed the north polar limb
in H-alpha line center wavelength on 2009 August 26. A remarkable
H-alpha eruption was observed from 18:20 UT to 18:45 UT with a
relatively slower speed of about 10 km/s in its early stage. The
eruption was then slightly accelerated up to 20-30 km/s and appeared
to be deflected along the pre-existing magnetic field. The eruption
also showed several interesting characteristics such as bifurcation,
rotation, horizontal oscillation, and direction and thickness changes
of its structure during its evolution. In this talk, we report on the
observational properties of the small-scale eruption observed by the
NST and discuss their implications for magnetic reconnection.
Title: Tiny Pores Observed by Hinode/Solar Optical Telescope
Authors: Cho, Kyung-Suk; Bong, Su-Chan; Chae, Jongchul; Kim, Yeon-Han;
Park, Young-Deuk
Bibcode: 2010ApJ...723..440C
Altcode:
The study of pores, small penumbraless sunspots, can give us a chance to
understand how strong magnetic fields interact with convective motions
in the photosphere. For a better understanding of this interaction,
we investigate the temporal variation of several tiny pores smaller
than 2''. These pores were observed by the Solar Optical Telescope on
board Hinode on 2006 December 29. We have analyzed the high-resolution
spectropolarimetric (SP) data and the G-band filtergrams taken during
the observation. Magnetic flux density and Doppler velocities of the
pores are estimated by applying the center-of-gravity method to the
SP data. The horizontal motions in and around the pores are tracked
by adopting the nonlinear affine velocity estimator method to the
G-band filter images. As a result, we found the following. (1) The
darkness of the pores is positively correlated with the magnetic flux
density. (2) Downflows always exist inside and around the pores. (3)
The speed of downflows inside the pores is negatively correlated with
their darkness. (4) The pores are surrounded by strong downflows. (5)
Brightness changes of the pores are correlated with the divergence
of mass flow (correlation coefficient >0.9). (6) The pores in
the growing phase are associated with the converging flow pattern
and the pores in the decay phase with the diverging flow pattern. Our
results support the idea that a pore grows as the magnetic flux density
increases due to the convergence of ambient mass flow and it decays
with the decrease of the flux density due to the diverging mass flow.
Title: Chromospheric Signatures of Small-scale Flux Emergence as
Observed with New Solar Telescope and Hinode Instruments
Authors: Yurchyshyn, V. B.; Goode, P. R.; Abramenko, V. I.; Chae,
J.; Cao, W.; Andic, A.; Ahn, K.
Bibcode: 2010ApJ...722.1970Y
Altcode: 2011arXiv1102.1034Y
With the ever-increasing influx of high-resolution images of the solar
surface obtained at a multitude of wavelengths, various processes
occurring at small spatial scales have become a greater focus of our
attention. Complex small-scale magnetic fields have been reported that
appear to have enough stored energy to heat the chromosphere. While
significant progress has been made in understanding small-scale
phenomena, many specifics remain elusive. We present here a detailed
study of a single event of disappearance of a magnetic dipole and
associated chromospheric activity. Based on New Solar Telescope Hα
data and Hinode photospheric line-of-sight magnetograms and Ca II
H images, we report the following. (1) Our analysis indicates that
even very small dipoles (elements separated by about 0farcs5 or less)
may reach the chromosphere and trigger non-negligible chromospheric
activity. (2) Careful consideration of the magnetic environment where
the new flux is deposited may shed light on the details of magnetic
flux removal from the solar surface. We argue that the apparent
collision and disappearance of two opposite polarity elements may
not necessarily indicate their cancellation (i.e., reconnection,
emergence of a "U" tube, or submergence of Ω loops). In our case, the
magnetic dipole disappeared by reconnecting with overlying large-scale
inclined plage fields. (3) Bright points (BPs) seen in off-band Hα
images are very well correlated with the Ca II H BPs, which in turn
are cospatial with G-band BPs. We further speculate that, in general,
Hα BPs are expected to be cospatial with photospheric BPs; however,
a direct comparison is needed to refine their relationship.
Title: Time Evolution of Coronal Magnetic Helicity in the Flaring
Active Region NOAA 10930
Authors: Park, Sung-Hong; Chae, Jongchul; Jing, Ju; Tan, Changyi;
Wang, Haimin
Bibcode: 2010ApJ...720.1102P
Altcode: 2010arXiv1008.1558P
To study the three-dimensional (3D) magnetic field topology and its
long-term evolution associated with the X3.4 flare of 2006 December 13,
we investigate the coronal relative magnetic helicity in the flaring
active region (AR) NOAA 10930 during the time period of December
8-14. The coronal helicity is calculated based on the 3D nonlinear
force-free magnetic fields reconstructed by the weighted optimization
method of Wiegelmann, and is compared with the amount of helicity
injected through the photospheric surface of the AR. The helicity
injection is determined from the magnetic helicity flux density proposed
by Pariat et al. using Solar and Heliospheric Observatory/Michelson
Doppler Imager magnetograms. The major findings of this study are the
following. (1) The time profile of the coronal helicity shows a good
correlation with that of the helicity accumulation by injection through
the surface. (2) The coronal helicity of the AR is estimated to be -4.3
× 1043 Mx2 just before the X3.4 flare. (3) This
flare is preceded not only by a large increase of negative helicity,
-3.2 × 1043 Mx2, in the corona over ~1.5 days
but also by noticeable injections of positive helicity through the
photospheric surface around the flaring magnetic polarity inversion
line during the time period of the channel structure development. We
conjecture that the occurrence of the X3.4 flare is involved with
the positive helicity injection into an existing system of negative
helicity.
Title: Patterns of Flows in an Intermediate Prominence Observed
by Hinode
Authors: Ahn, Kwangsu; Chae, Jongchul; Cao, Wenda; Goode, Philip R.
Bibcode: 2010ApJ...721...74A
Altcode:
The investigation of plasma flows in filaments/prominences gives
us clues to understanding their magnetic structures. We studied
the patterns of flows in an intermediate prominence observed by
Hinode/SOT. By examining a time series of Hα images and Ca II H images,
we have found horizontal flows in the spine and vertical flows in
the barb. Both of these flows have a characteristic speed of 10-20 km
s-1. The horizontal flows displayed counterstreaming. Our
detailed investigation revealed that most of the moving fragments in
fact reversed direction at the end point of the spine near a footpoint
close to the associated active region. These returning flows may be
one possible explanation of the well-known counterstreaming flows
in prominences. In contrast, we have found vertical flows—downward
and upward—in the barb. Most of the horizontal flows in the spine
seem to switch into vertical flows when they approach the barb,
and vice versa. We propose that the net force resulting from a small
deviation from magnetohydrostatic equilibrium, where magnetic fields
are predominantly horizontal, may drive these patterns of flow. In the
prominence studied here, the supposed magnetohydrostatic configuration
is characterized by magnetic field lines sagging with angles of 13°
and 39° in the spine and the barb, respectively.
Title: The Formation of a Magnetic Channel by the Emergence of
Current-carrying Magnetic Fields
Authors: Lim, Eun-Kyung; Chae, Jongchul; Jing, Ju; Wang, Haimin;
Wiegelmann, Thomas
Bibcode: 2010ApJ...719..403L
Altcode: 2010arXiv1009.0420L
A magnetic channel—a series of polarity reversals separating elongated
flux threads with opposite polarities—may be a manifestation of
a highly non-potential magnetic configuration in active regions. To
understand its formation, we have carried out a detailed analysis of
the magnetic channel in AR 10930 using data taken by the Solar Optical
Telescope/Hinode. As a result, we found upflows (-0.5 to -1.0 km
s-1) and downflows (+1.5 to +2.0 km s-1) inside
and at both tips of the thread, respectively, and a pair of strong
vertical currents of opposite polarity along the channel. Moreover,
our analysis of the nonlinear force-free fields constructed from the
photospheric magnetic field indicates that the current density in the
lower corona may have gradually increased as a result of the continuous
emergence of the highly sheared flux along the channel. With these
results, we suggest that the magnetic channel originates from the
emergence of the twisted flux tube that has formed below the surface
before the emergence.
Title: Productivity of Solar Flares and Magnetic Helicity Injection
in Active Regions
Authors: Park, Sung-hong; Chae, Jongchul; Wang, Haimin
Bibcode: 2010ApJ...718...43P
Altcode: 2010arXiv1005.3416P
The main objective of this study is to better understand how magnetic
helicity injection in an active region (AR) is related to the occurrence
and intensity of solar flares. We therefore investigate the magnetic
helicity injection rate and unsigned magnetic flux, as a reference. In
total, 378 ARs are analyzed using SOHO/MDI magnetograms. The 24 hr
averaged helicity injection rate and unsigned magnetic flux are compared
with the flare index and the flare-productive probability in the next
24 hr following a measurement. In addition, we study the variation of
helicity over a span of several days around the times of the 19 flares
above M5.0 which occurred in selected strong flare-productive ARs. The
major findings of this study are as follows: (1) for a sub-sample of
91 large ARs with unsigned magnetic fluxes in the range from (3-5)
× 1022 Mx, there is a difference in the magnetic helicity
injection rate between flaring ARs and non-flaring ARs by a factor
of 2; (2) the GOES C-flare-productive probability as a function of
helicity injection displays a sharp boundary between flare-productive
ARs and flare-quiet ones; (3) the history of helicity injection
before all the 19 major flares displayed a common characteristic:
a significant helicity accumulation of (3-45) × 1042
Mx2 during a phase of monotonically increasing helicity over
0.5-2 days. Our results support the notion that helicity injection is
important in flares, but it is not effective to use it alone for the
purpose of flare forecast. It is necessary to find a way to better
characterize the time history of helicity injection as well as its
spatial distribution inside ARs.
Title: Oscillatory Behavior in the Quiet Sun Observed with the New
Solar Telescope
Authors: Anđić, A.; Goode, P. R.; Chae, J.; Cao, W.; Ahn, K.;
Yurchyshyn, V.; Abramenko, V.
Bibcode: 2010ApJ...717L..79A
Altcode: 2010arXiv1007.0272A
Surface photometry of the quiet Sun has achieved an angular resolution
of 0farcs1 with the New Solar Telescope at Big Bear Solar Observatory,
revealing that a disproportionate fraction of the oscillatory events
appear above observed bright point-like structures. During the tracking
of these structures, we noted that the more powerful oscillatory events
are cospatial with them, indicating that observed flux tubes may be
the source of many observed oscillatory events.
Title: Non-Hydrostatic Support of Plasma in the Solar Chromosphere
and Corona
Authors: Chae, Jong-Chul
Bibcode: 2010JKAS...43...55C
Altcode:
No abstract at ADS
Title: Dynamics of Vertical Threads and Descending Knots in a
Hedgerow Prominence
Authors: Chae, Jongchul
Bibcode: 2010ApJ...714..618C
Altcode:
The existence and behavior of vertical fine structures of
plasma—threads and knots—are a significant observational clue
to understanding the magnetic structure and dynamics of quiescent
prominences on the quiet Sun. Based on the equation of motion in ideal
MHD, we reason that the non-hydrostatic support of plasma against
gravity in general requires either the motion of plasma with a high
value of downward acceleration (dynamical support) or the role of
horizontal magnetic fields (magnetic support). By carefully tracking
the motion of several bright threads seen in a hedgerow prominence
observed by the Solar Optical Telescope aboard Hinode, we confirm that
these threads are essentially static and stable, which negates the
dynamic support. The application of the Kippenhahn-Schlüter solution
suggests that they may be supported by sagged magnetic field lines with
a sag angle of about 43°. We also track several bright descending
knots and find that their descending speeds range from 10 to 30 km
s-1, with a mean value of 16 km s-1, and their
vertical accelerations from -0.10 to 0.10 km s-2, with a
mean of practically zero. This finding suggests that these knots are
basically supported by horizontal magnetic fields against gravity even
when they descend, and the complex variations of their descending speeds
should be attributed to small imbalances between gravity and the force
of magnetic tension. Furthermore, some knots are observed to impulsively
get accelerated downward from time to time. We conjecture that these
impulsive accelerations are a result of magnetic reconnection and the
subsequent interchange of magnetic configuration between a knot and its
surrounding structure. It is proposed that this process of reconnection
and interchange not only initiates the descending motion of the knots,
but also allows knots to keep falling long distance through the medium
permeated by horizontal magnetic fields.
Title: Stereoscopic Determination of Heights of Extreme Ultraviolet
Bright Points Using Data Taken by SECCHI/EUVI Aboard STEREO
Authors: Kwon, Ryun-Young; Chae, Jongchul; Zhang, Jie
Bibcode: 2010ApJ...714..130K
Altcode:
We measure the heights of EUV bright points (BPs) above the solar
surface by applying a stereoscopic method to the data taken by the Solar
TErrestrial RElations Observatory/SECCHI/Extreme UltraViolet Imager
(EUVI). We have developed a three-dimensional reconstruction method
for point-like features such as BPs using the simple principle that
the position of a point in the three-dimensional space is specified as
the intersection of two lines of sight. From a set of data consisting
of EUVI 171 Å, 195 Å, 284 Å, and 304 Å images taken on 11 days
arbitrarily selected during a period of 14 months, we have identified
and analyzed 210 individual BPs that were visible on all four passband
images and smaller than 30 Mm. The BPs seen in the 304 Å images have
an average height of 4.4 Mm, and are often associated with the legs
of coronal loops. In the 171 Å, 195 Å, and 284 Å images the BPs
appear loop-shaped, and have average heights of 5.1, 6.7, and 6.1 Mm,
respectively. Moreover, there is a tendency that overlying loops are
filled with hotter plasmas. The average heights of BPs in 171 Å, 195
Å, and 284 Å passbands are roughly twice the corresponding average
lengths. Our results support the notion that an EUV BP represents a
system of small loops with temperature stratification like flaring
loops, being consistent with the magnetic reconnection origin.
Title: Highest Resolution Observations of the Quietest Sun
Authors: Goode, Philip R.; Yurchyshyn, Vasyl; Cao, Wenda; Abramenko,
Valentyna; Andic, Aleksandra; Ahn, Kwangsu; Chae, Jongchul
Bibcode: 2010ApJ...714L..31G
Altcode:
Highest resolution observations made with the new 1.6 m aperture solar
telescope in Big Bear Solar Observatory during this time of historic
inactivity on the Sun reveal new insights into the small-scale dynamics
of the Sun's photosphere. The telescope's unprecedented resolution
enabled us to observe that the smallest scale photospheric magnetic
field seems to come in isolated points in the dark intergranular lanes,
rather than the predicted continuous sheets confined to the lanes, and
the unexpected longevity of the bright points implies a deeper anchoring
than predicted. Further, we demonstrated for the first time that the
photospheric plasma motion and magnetic fields are in equipartition
over a wide dynamic range, and both cascade energy to ever-smaller
scales according to classical Kolmogorov turbulence theory. Finally,
we discovered tiny jet-like features originating in the dark lanes that
surround the ubiquitous granules that characterize the solar surface.
Title: New Solar Telescope Observations of Magnetic Reconnection
Occurring in the Chromosphere of the Quiet Sun
Authors: Chae, Jongchul; Goode, P. R.; Ahn, K.; Yurchysyn, V.;
Abramenko, V.; Andic, A.; Cao, W.; Park, Y. D.
Bibcode: 2010ApJ...713L...6C
Altcode:
Magnetic reconnection is a process in which field-line connectivity
changes in a magnetized plasma. On the solar surface, it often
occurs with the cancellation of two magnetic fragments of opposite
polarity. Using the 1.6 m New Solar Telescope, we observed the
morphology and dynamics of plasma visible in the Hα line, which is
associated with a canceling magnetic feature (CMF) in the quiet Sun. The
region can be divided into four magnetic domains: two pre-reconnection
and two post-reconnection. In one post-reconnection domain, a small
cloud erupted, with a plane-of-sky speed of 10 km s-1,
while in the other one, brightening began at points and then tiny
bright loops appeared and subsequently shrank. These features support
the notion that magnetic reconnection taking place in the chromosphere
is responsible for CMFs.
Title: Magnetic Activity on the Sun Revealed by Hinode/SOT (Keynote)
Authors: Chae, J.; Park, S.
Bibcode: 2009ASPC..415..101C
Altcode:
Hinode observations have given us an unprecedented opportunity for
understanding fine-scale magnetic activities on the Sun. The data
produced by the Solar Optical Telescope (SOT) are particularly useful
in the investigation of the interaction between magnetic field and
plasma in the photosphere and chromosphere, opening the new era
of observational magnetohydrodynamics (MHD). As an illustration,
we would like to introduce new results and questions in three topics
we have worked on. One is the nature of magnetic reconnection in the
chromosphere as manifest in the flux cancellation observed in the
photosphere. Our new result is that the rates of flux cancellation
determined from Hinode/SOT are much higher than those determined from
SOHO/MDI. Another topic is the determination of scale-dependent magnetic
diffusivity in the photosphere, which seems to be closely related to
the turbulent cascade of magnetoconvection. Finally, we will present our
finding of a pattern of horizontal flows in a quiescent prominence that
consists of a number of vertical threads. We think this flow pattern
is an important clue to solving the mystery of vertical threads and
downflows along the threads in quiescent prominences.
Title: NST observations of H alpha features and the non-hydrostatic
support of the solar chromosphere
Authors: Chae, J.
Bibcode: 2009AGUFMSH51A1258C
Altcode:
The existence of many structures of cool plasma in the solar
chromosphere that extend much above the pressure scale height suggests
that these structures are supported against gravity by mechanisms other
than hydrostatic support. One possible mechanism is magnetic support,
and the other is dynamic support. To reveal which is the dominant
mechanism of non-hydrostatic support of chromospheric plasma, we have
taken high resolution images of the chromosphere at the limb and on
the disk by using the New Solar Telescope (NST) at Big Bear. We will
report different kinds of chromospheric features/events which may hold
a key to this problem.
Title: Rates of Photospheric Magnetic Flux Cancellation Measured
with Hinode
Authors: Park, Soyoung; Chae, Jongchul; Litvinenko, Yuri E.
Bibcode: 2009ApJ...704L..71P
Altcode:
Photospheric magnetic flux cancellation on the Sun is generally
believed to be caused by magnetic reconnection occurring in the
low solar atmosphere. Individual canceling magnetic features are
observationally characterized by the rate of flux cancellation. The
specific cancellation rate, defined as the rate of flux cancellation
divided by the interface length, gives an accurate estimate of the
electric field in the reconnecting current sheet. We have determined
the specific cancellation rate using the magnetograms taken by the
Solar Optical Telescope (SOT) aboard the Hinode satellite. The specific
rates determined with SOT turned out to be systematically higher than
those based on the data taken by the Michelson Doppler Imager (MDI)
aboard the Solar and Heliospheric Observatory. The median value of
the specific cancellation rate was found to be 8 × 106
G cm s-1—a value four times that obtained from the MDI
data. This big difference is mainly due to a higher angular resolution
and better sensitivity of the SOT, resulting in magnetic fluxes up
to five times larger than those obtained from the MDI. The higher
rates of flux cancellation correspond to either faster inflows or
stronger magnetic fields of the reconnection inflow region, which may
have important consequences for the physics of photospheric magnetic
reconnection.
Title: The Rotating Sunspot in AR 10930
Authors: Min, Soonyoung; Chae, Jongchul
Bibcode: 2009SoPh..258..203M
Altcode:
We study the pattern and behavior of a rotating sunspot in Active Region
10930. The rotational angular speed has been extracted from the apparent
motions of the sunspot determined by applying a new optical technique -
called non-linear affine velocity estimator (NAVE) - to high-resolution
G-band images taken by the Solar Optical Telescope (SOT) onboard the
Hinode satellite. The structure and dynamics of coronal loops in this
active region have been examined using the images obtained by the X-Ray
Telescope (XRT) and the spectral data taken by the Extreme-ultraviolet
Imaging Spectrometer (EIS), both also onboard Hinode. Our results are
summarized as follows: i) The small sunspot of positive polarity rotated
counterclockwise about its center by 540° during the period of five
days. ii) Its angular velocity varied with the azimuth angle as well
as the radial distance, being affected by the asymmetric shape of the
umbra. iii) The angular velocity increased up to 8° h−1
until 13 December as the sunspot grew, and then decreased rapidly down
to 3° h−1 on the next day as the sunspot decayed. iv)
The coronal loops that connected the two sunspots became sigmoidal in
shape. v) The coronal emissions from the regions around the rotating
sunspot were blueshifted, which may indicate the expansion of the
coronal loops. Our results suggest that the rotation of the sunspot
may be closely related to the dynamic development of emerging twisted
magnetic fields.
Title: A Comparison of the Initial Speed of Coronal Mass Ejections
with the Magnetic Flux and Magnetic Helicity of Magnetic Clouds
Authors: Sung, S. -K.; Marubashi, K.; Cho, K. -S.; Kim, Y. -H.; Kim,
K. -H.; Chae, J.; Moon, Y. -J.; Kim, I. -H.
Bibcode: 2009ApJ...699..298S
Altcode:
To investigate the relationship between the speed of a coronal mass
ejection (CME) and the magnetic energy released during its eruption,
we have compared the initial speed of CMEs (V CME) and the
two parameters of their associated magnetic clouds (MC), magnetic
flux (F MC), and magnetic helicity per unit length
(|H MC|/L), for 34 pairs of CMEs and MCs. The values
of these parameters in each MC have been determined by fitting
the magnetic data of the MC to the linear force-free cylindrical
model. As a result, we found that there are positive correlations
between V 2 CME and F MC, and
between V 2 CME and |H MC|/L. It
is also found that the kinetic energy of CMEs (E CME)
is correlated with F MC and |H MC|/L of the
associated MC. In contrast, we found no significant correlation between
langV MCrang2 and F MC, nor between
langV MCrang2 and |H MC|/L. Our
results support the notion that the eruption of a CME is related to
the magnetic helicity of the source active region.
Title: Estimation of Errors in the Transverse Velocity Vectors
Determined from HINODE/SOT Magnetograms Using the NAVE Technique
Authors: Chae, Jong-Chul; Moon, Yong-Jae
Bibcode: 2009JKAS...42...61C
Altcode:
No abstract at ADS
Title: Return Flows in a Counter-streaming Prominence
Authors: Ahn, Kwangsu; Chae, J.; Goode, P.
Bibcode: 2009SPD....40.1008A
Altcode:
It is known that the fine structures of filaments/prominences show
counter-streaming motion. We found that the counter-streaming could be
an outcome of return flows at the edge of filaments/prominences. We
picked an intermediate prominence that showed a dominant horizontal
motion. The trajectories of the plasma fragments in this prominence
showed that most of the plasma in this prominence changed their
direction near the edge of prominence structure. This implies that
the counter-streaming may be represented by plasma which move back and
forth along the horizontal magnetic field lines. We assumed that there
could be magnetic pillar-like structures near the edge, so that they
could increase magnetic pressure near the edge, pulling the plasma
back to the original position.
Title: Chirality of Intermediate Filaments and Magnetic Helicity of
Active Regions
Authors: Lim, Eun-Kyung; Chae, J.
Bibcode: 2009SPD....40.0912L
Altcode:
Filaments that form either between or around active regions (ARs)
are called intermediate filaments. Even though there have been many
theoretical studies, the origin of the chirality of filaments is still
unknown. We investigated how intermediate filaments are related to their
associated ARs, especially from the point of view of magnetic helicity
and the orientation of polarity inversion lines (PILs). The chirality
of filaments has been determined based on the orientations of barbs
observed in the full-disk Hα images taken at Big Bear Solar Observatory
during the rising phase of solar cycle 23. The sign of magnetic helicity
of ARs has been determined using S/inverse-S shaped sigmoids from Yohkoh
SXT images. As a result, we have found a good correlation between the
chirality of filaments and the magnetic helicity sign of ARs. Among
45 filaments, 42 filaments have shown the same sign as helicity sign
of nearby ARs. It has been also confirmed that the role of both the
orientation and the relative direction of PILs to ARs in determining
the chirality of filaments is not significant, against a theoretical
prediction. These results suggest that the chirality of intermediate
filaments may originate from magnetic helicity of their associated ARs.
Title: Signatures of Sweet-Parker magnetic reconnection in the
solar chromosphere
Authors: Litvinenko, Y. E.; Chae, J.
Bibcode: 2009A&A...495..953L
Altcode:
Aims: Predicted and observed signatures of magnetic reconnection in
the solar chromosphere are discussed.
Methods: Compressible
reconnection scalings for the Sweet-Parker current sheet in the
chromosphere are derived, which generalize earlier treatments by
including both the Lorentz force and the pressure gradient in the
reconnection outflow as possible drivers of reconnection jets. The
theoretical scalings were tested against the measured speed,
density, and temperature of an extreme-ultraviolet jet, observed
with TRACE. Model predictions for the reconnection jet parameters as
functions of height above the solar photosphere were computed.
Results: The predicted variation of reconnection jet parameters with
height appears to correspond to the observed wide range of parameters
of chromospheric jet-like features.
Title: Chirality of Intermediate Filaments and Magnetic Helicity of
Active Regions
Authors: Lim, Eun-Kyung; Chae, Jongchul
Bibcode: 2009ApJ...692..104L
Altcode: 2009arXiv0901.0360L
Filaments that form either between or around active regions (ARs)
are called intermediate filaments. Even though there have been many
theoretical studies, the origin of the chirality of filaments is still
unknown. We investigated how intermediate filaments are related to their
associated ARs, especially from the point of view of magnetic helicity
and the orientation of polarity inversion lines (PILs). The chirality
of filaments has been determined based on the orientations of barbs
observed in full-disk Hα images taken at Big Bear Solar Observatory
during the rising phase of solar cycle 23. The sign of magnetic helicity
of ARs has been determined using S/inverse-S shaped sigmoids from Yohkoh
SXT images. As a result, we have found good correlation between the
chirality of filaments and the magnetic helicity sign of ARs. Among
45 filaments, 42 filaments have shown the same sign as helicity sign
of nearby ARs. It has been also confirmed that the role of both the
orientation and the relative direction of PILs to ARs in determining
the chirality of filaments is not significant, against a theoretical
prediction. These results suggest that the chirality of intermediate
filaments may originate from magnetic helicity of their associated ARs.
Title: Magnetic Helicity Injection during the Formation of AN
Intermediate Filament
Authors: Jeong, Hye-Won; Chae, Jong-Chul; Moon, Y. J.
Bibcode: 2009JKAS...42....9J
Altcode:
No abstract at ADS
Title: A Test of Three Optical Flow Techniques—LCT, DAVE, and NAVE
Authors: Chae, Jongchul; Sakurai, Takashi
Bibcode: 2008ApJ...689..593C
Altcode:
A time sequence of high-quality images currently produced by
high-resolution observations either from the ground or in space may
be utilized to determine the transverse flow field on the plane of
the sky with the help of optical flow techniques. We have examined the
performance of three different methods—a well-known technique called
local correlation tracking (LCT), a recently developed technique called
the differential affine velocity estimator (DAVE), and a new technique
called the nonlinear affine velocity estimator (NAVE)—using three
kinds of image data: mapping-based synthetic images, a set of MHD
simulation data, and real images (magnetograms) taken by the Solar
Optical Telescope on board Hinode. We have generalized the model
equation of image evolution by adding to the continuity equation a
source term that is proportional to the image value. Synthetic images
were constructed based on the analytical solution of this equation
with different velocity profiles: uniform, affine, or nonaffine. The
tests with the synthetic data indicated that NAVE is very good at
detecting subpixel motions, superpixel motions, and nonuniform
motions, while LCT is not good at detecting nonuniform motions,
especially around critical points, and the performance of DAVE is
degraded in the presence of superpixel motions. In all the methods,
the performance became worse as the velocity field deviated more from
an affine one. We also found that the MHD simulation data we used
are not quite suited for discriminating between the three methods,
maybe because the data do not contain enough structural information
to be used for tracing. In contrast, the determination of velocity
fields from the real image data was somewhat sensitive to the technique
adopted. The technique of NAVE with the source term produced velocity
fields that are the most consistent with the data.
Title: Persistent Horizontal Flows and Magnetic Support of Vertical
Threads in a Quiescent Prominence
Authors: Chae, Jongchul; Ahn, Kwangsoo; Lim, Eun-Kyung; Choe, G. S.;
Sakurai, Takashi
Bibcode: 2008ApJ...689L..73C
Altcode:
There has been some controversy as to whether the magnetic fields
of vertical threads seen in quiescent prominences are predominantly
vertical or horizontal. We report finding special patterns of flow in
a quiescent prominence observed by the Solar Optical Telescope aboard
Hinode. This prominence is a small hedgerow prominence composed of many
vertical threads. To one side of it, we found a pattern of persistent
horizontal flows of Hα-emitting plasma. These flows originated from a
region in the chromosphere, rose to coronal heights, and then extended
horizontally for a long distance until they reached the main body of
the prominence. In the higher altitudes the flows either moved across
vertical threads or lifted them up, while in the lower altitudes
they often formed bright blobs of plasma and shed them, resulting in
a sudden change of flow direction from horizontal to vertical. The
observed persistent horizontal flows support a configuration of
initially horizontal magnetic fields, and our results appear to
be consistent with the traditional theory that vertical threads in
quiescent prominences are stacks of plasma supported against gravity
by the sagging of initially horizontal magnetic field lines.
Title: The Variation of Relative Magnetic Helicity around Major Flares
Authors: Park, Sung-Hong; Lee, Jeongwoo; Choe, G. S.; Chae, Jongchul;
Jeong, Hyewon; Yang, Guo; Jing, Ju; Wang, Haimin
Bibcode: 2008ApJ...686.1397P
Altcode: 2010arXiv1004.2856P
We have investigated the variation of magnetic helicity over a span
of several days around the times of 11 X-class flares which occurred
in seven active regions (NOAA 9672, 10030, 10314, 10486, 10564, 10696,
and 10720) using the magnetograms taken by the Michelson Doppler Imager
(MDI) on board the Solar and Heliospheric Observatory (SOHO). As a
major result we found that each of these major flares was preceded
by a significant helicity accumulation, (1.8-16) × 1042
Mx2 over a long period (0.5 to a few days). Another finding
is that the helicity accumulates at a nearly constant rate, (4.5-48)
× 1040 Mx2 hr-1, and then becomes
nearly constant before the flares. This led us to distinguish the
helicity variation into two phases: a phase of monotonically increasing
helicity and the following phase of relatively constant helicity. As
expected, the amount of helicity accumulated shows a modest correlation
with time-integrated soft X-ray flux during flares. However, the
average helicity change rate in the first phase shows even stronger
correlation with the time-integrated soft X-ray flux. We discuss the
physical implications of this result and the possibility that this
characteristic helicity variation pattern can be used as an early
warning sign for solar eruptions.
Title: Determination of Magnetic Diffusivity from High-Resolution
Solar Magnetograms
Authors: Chae, Jongchul; Litvinenko, Yuri E.; Sakurai, Takashi
Bibcode: 2008ApJ...683.1153C
Altcode:
The magnetic diffusivity in the solar photosphere is determined by
applying a new method to the magnetic induction equation. The magnetic
field evolution is specified by a time sequence of high-resolution
magnetograms of plage regions, taken by Hinode/SOT and SOHO/MDI. The
mean value of magnetic diffusivity determined from SOT magnetograms with
the smallest pixel size of 116 km is about 0.87 +/- 0.08 km2
s-1. This is the smallest value that has been empirically
determined so far. High-resolution and full-disk MDI magnetograms with
the pixel sizes of 440 and 1400 km yielded larger values of 4.4 +/-
0.4 and 18 +/- 7.4 km2 s-1, respectively. The
measured diffusivity values at different length scales are consistent
with a turbulent cascade that ends at a resistive dissipation scale of
about 30 km. The results suggest that turbulent magnetic diffusivity
should be taken into account in the analysis of the observed rate of
magnetic flux cancellation in the photosphere.
Title: Capability of the Fast Imaging Solar Spectrograph on NST/BBSO
for Observing Filaments/Prominences at the Spectral Lines Hα,
Ca II 8542, and Ca II K
Authors: Ahn, Kwang-Su; Chae, Jong-Chul; Park, Hyung-Min; Nah,
Jak-Young; Park, Young-Deuk; Jang, Bi-Ho; Moon, Yong-Jae
Bibcode: 2008JKAS...41...39A
Altcode:
No abstract at ADS
Title: Magnetic Twist of EUV Coronal Loops Well-traced in TRACE
Images: Evidence for Magnetic Reconnection Origin of Coronal Loops?
Authors: Kwon, Ryun Young; Chae, Jongchul
Bibcode: 2008ApJ...677L.141K
Altcode:
The measurements of magnetic twist of EUV coronal loops for 14 loops are
presented here. EUV coronal loops are thin, EUV-emitting structures of
hot plasma tracing magnetic field lines in the corona. The constriction
of plasma into a loop without dispersion may be explained if the
magnetic field of the loop is twisted. On the basis of this idea,
Chae and Moon developed a method of determining magnetic twist of
coronal loops by analyzing coronal images and photospheric magnetograms
together. By applying this method to as many coronal loops observed
by TRACE 171 Å as possible, we attempt to determine a statistically
meaningful value of magnetic twist of coronal loops for the first
time. We have selected a number of conspicuous loops which are bright
enough and well separated from other adjacent loops on TRACE EUV
images. We have constructed and examined coronal magnetic fields of
selected active regions containing the loops with consideration of the
projection effect, and we can identify 14 coronal loops whose magnetic
field lines are well represented by a linear force-free field. We
have found that these loops have absolute twist values from 0.22π to
1.73π, which suggests that the absolute winding number of EUV coronal
loop may be mostly less than one turn. Our results support the idea
that EUV coronal loops may be a product of magnetic reconnection of
magnetic flux tubes.
Title: Spatial Distribution of Magnetic Reconnection in the 2006
December 13 Solar Flare as Observed by Hinode
Authors: Jing, Ju; Chae, Jongchul; Wang, Haimin
Bibcode: 2008ApJ...672L..73J
Altcode:
A massive two-ribbon flare and its source magnetic field region were
well captured by the Solar Optical Telescope (SOT) on board Hinode
in the Ca II H spectral line and by the Spectro-Polarimeter of SOT,
respectively. Using the high-resolution Hinode data sets, we compare
the spatial distribution of the local magnetic reconnection rate and
the energy release rate along the ribbons with that of G-band kernels
that serve as a proxy for the primary energy release. The G-band kernels
spatially coincide with the maximum of both modeled quantities, which
gives strong support for the reconnection model. We also investigate
the magnitude scaling correlation between the ribbon separation speed
Vr and magnetic field strength Bn at four 2
minute time bins around the maximum phase of the flare. It is found that
Vr is weakly and negatively correlated with Bn. An
empirical relation of Vr propto Bn-0.15
is obtained at the flare peak time with an correlation coefficient
~-0.33. The correlation is weaker at other time bins.
Title: Determination of Magnetic Diffusivity from High Resolution
Solar Magnetograms
Authors: Chae, Jongchul; Litvinenko, Yuri E.; Sakurai, Takashi
Bibcode: 2008cosp...37..482C
Altcode: 2008cosp.meet..482C
The magnetic diffusivity in the solar photosphere is determined by
applying a model of magnetic induction to high resolution magnetograms
of plage regions, taken by HINODE/SOT and SOHO/MDI. The mean value
of magnetic diffusivity determined from SOT magnetograms with the
smallest pixel size of 116 km is about 0.84±0.34 km2 s-1 . This is
the smallest value that has been empirically determined so far. High
resolution and full-disk MDI magnetograms with the pixel sizes of 440
and 1400 km yielded larger values of 4.5±1.4 km2 s-1 and 13±10 km2
s-1 , respectively. The measured diffusivity values at different length
scales are consistent with a turbulent cascade that ends at a resistive
dissipation scale of about 25 km. The results suggest that turbulent
magnetic diffusivity should be taken into account in the analysis of
the observed rate of magnetic flux cancellation in the photosphere.
Title: Magnetic Twist of EUV Coronal Loops Observed by TRACE:
Evidence for Magnetic Reconnection Origin of Coronal Loops?
Authors: Chae, Jongchul
Bibcode: 2008cosp...37..483C
Altcode: 2008cosp.meet..483C
The measurements of magnetic twist of EUV coronal loops for 14
loops are presented here. EUV coronal loops are thin and emitting
structures of hot plasma tracing magnetic field line in the corona. The
constriction of plasma into a loop without dispersion may be explained
if the magnetic field of the loop is twisted. On the basis of this
idea, Chae and Moon (2005) developed a method of determining magnetic
twist of coronal loops by analyzing coronal images and photospheric
magnetograms together. By applying this method to as many coronal
loops observed by TRACE 171˚ as possible, we attempt to determine a
statistically meaningful A value of magnetic twist of coronal loops for
the first time. We have selected a number of conspicuous loops which
are bright enough and well separated from other adjacent loops on TRACE
EUV images. We have constructed and examined coronal magnetic fields
of selected active regions containing the loops with consideration of
the projection effect, and we could identify 14 coronal loops whose
magnetic field line is well represented by a linear force free field. We
have found that these loops have absolute twist values from 0.22 π to
1.73 π, which suggest that the absolute winding number of EUV coronal
loop may be mostly less than one turn. Our results support the idea
that EUV coronal loops may be a product of magnetic reconnection of
magnetic flux tubes.
Title: Relationship between CME initial speed and magnetic helicity
of magnetic clouds
Authors: Sung, S.; Marubashi, K.; Kim, K.; Cho, K.; Moon, Y.; Chae, J.
Bibcode: 2007AGUFMSH31A0228S
Altcode:
In order to understand the relationship between solar and interplanetary
phenomena, we have examined the initial properties of coronal mass
ejections (CMEs) and magnetic helicity of magnetic clouds (MCs) for 24
CME- MC pairs. MCs are fitted with the linear force-free cylindrical
model to obtain MC parameters (orientation, size, magnetic field
magnitude, impact parameter, chirality, etc). The relative helicity
per unit length of MC is calculated by \frac{HMC}{L}=\frac{4π
B02}{α}\int0RJ12(α r)rdr. Comparing the square of CME initial speeds
(VCME2) with the magnetic helicities (HMC, we find that there is a
positive correlation between VCME2 and HMC, and the linear correlation
coefficient (CC) between the two parameters is 0.52. We obtain a better
correlation (CC=0.61) for 17 events whose impact parameter (the shortest
distance of the satellite to the MC axis normalized by MC radius)
is less than 0.5. Considering that the magnetic force in a flux rope
is intimately related to magnetic helicity, our result supports that
the magnetic force is responsible for the CME eruption. From this
result we suggest that the high speed CME is associated with large
magnetic helicity.
Title: Magnetic Helicity Injection in Active Regions
Authors: Jeong, Hyewon; Chae, Jongchul
Bibcode: 2007ApJ...671.1022J
Altcode:
Magnetic helicity is now regarded as an important physical quantity in
understanding solar magnetic activities. We have studied the injection
of magnetic helicity through the photosphere of four active regions
during the long periods of their lifetimes. The rate of helicity
injection was determined as a function of time in each active region
by applying the local correlation tracking (LCT) method to full-disk,
96 minute cadence magnetograms taken by SOHO MDI. As a result, we have
found that most helicity was intensively injected while active regions
emerged and grew, suggesting that magnetic flux emergence may be the
major process of helicity injection. In all the active regions studied,
the rate of helicity injection during flux emergence was initially
low, increased and stayed high for a while, and then became low again,
while magnetic flux steadily increased at a more or less constant rate
all the time. Flux cancellation, as a minor process, resulted in some
loss of coronal magnetic helicity, or inverse helicity injection. The
contribution of differential rotation to helicity injection was found
to be insignificant in these active regions. It was also found that
the magnetic helicity budget of an active region strongly depends on
its average magnetic flux.
Title: Small-scale X-ray/EUV Jets seen in Hinode XRT and TRACE
Authors: Kim, Y.; Moon, Y.; Cho, K.; Bong, S.; Park, Y.; Sakurai,
T.; Chae, J.
Bibcode: 2007AGUFMSH53A1061K
Altcode:
In this study, we present the morphological and kinematic
characteristics of three small-scale X-ray/EUV jets that are
simultaneously observed by the X-ray telescope (XRT) onboard Hinode
(Solar-B) and the TRACE. For this study, we examined all XRT movies
with a thin Al/Poly filter from 2006 October 20 to April 8 to look
for small-scale X-ray eruptions and then found 34 eruptions. Next we
took a look at the corresponding TRACE 171/195 {Å} images associated
with these X-ray eruptions. As a result, we found three simultaneous
X-ray and EUV jets and their major characteristics can be summarized
as follows : (1) They all have no association with major flares. (2)
From the comparison between XRT and TRACE observations, we found that
they have similar characteristics in terms of projected speed, lifetime,
and size. (3) Their sizes range from 4 × 105 to 5 × 105 km. (4) Their
projected speeds are estimated to be 180-310 km s-1 with an average
speed of about 250 km s- 1 (5) Their lifetimes lie in the range of 100
to 600 s. (6) From the comparison between the XRT images with the SOHO
MDI maps for two events, all they are associated with the parasitic
polarity region. These characteristics are similar to the previous
observations for X-ray jets by the Yohkoh SXT but quite different
from EUV jets associated with Hα surges. In addition, from the EIS
four spectral lines for the last event, we found blueshift (up to -64
km s-1) and redshift (up to 20 km s-1) motions as well as nonthermal
velocities ranging from 57 to 106 km s-1 at the jet footpoint. We
note a tendency : the hotter the maximum ionization temperature is,
the larger the area of blueshift region is. These characteristics
are consistent with a typical jet model that includes the magnetic
reconnection between open coronal fields and emerging magnetic fields.
Title: Analysis of Korean Historical Comet Records
Authors: Park, So-Yeon; Chae, Jong-Chul
Bibcode: 2007PKAS...22..151P
Altcode:
No abstract at ADS
Title: Initial Results on Line-of-Sight Field Calibrations of SP/NFI
Data Taken by SOT/Hinode
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Suematsu, Yoshinori; Tsuneta, Saku;
Katsukawa, Yukio; Shimizu, Toshifumi; Shine, Richard A.; Tarbell,
Theodore D.; Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata,
Shin'ichi; Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.619C
Altcode:
We present initial results on the line-of-sight field calibration
of the two kinds of Stokes I and V data taken by the Solar Optical
Telescope on the satellite Hinode: spectral profiles of Stokes I and V
parameters recorded on the Spectro-polarimeter (SP), and monochromatic
images of the same parameters recorded on the Narrow-band Filter Imager
(NFI). By applying the center-of-gravity method to the SP data of
AR10930 taken on 2006 December 11, we determined the line-of-sight field
at every location in the active region. As a result, we found that the
line-of-sight field strength ranges up to 2kG in plages, even without
taking into account the filling factor, and up to 3.5kG or higher values
inside the umbra of the major sunspot. We calibrated the NFI data in
reference to the field determined from the SP data. In regions outside
the sunspots and the penumbral regions, we adopted a linear relation,
B|| = βV / I, between the circular polarization, V / I,
and the line-of-sight field strength, B||, and obtained β =
23.5kG in regions outside the sunspots, and β = 12.0kG in penumbral
regions. In umbral regions of sunspots, a first-order polynomial was
adopted to model the reversal of the polarization signal over the
field strength.
Title: Two-Step Reconnections in a C3.3 Flare and Its Preflare
Activity Observed by Hinode XRT
Authors: Kim, Sujin; Moon, Young-Jae; Kim, Khan-Hyuk; Kim, Yeon-Han;
Sakurai, Takashi; Chae, Jongchul; Kim, Kap-Sung
Bibcode: 2007PASJ...59S.831K
Altcode:
We investigated the evolution of a C3.3 impulsive flare and its
preflare activity, which occurred in NOAA Active Region 10923 on 2006
November 12, using Hinode X-Ray Telescope (XRT) data. For an extensive
investigation, we also used GOES X-ray flux, TRACE 171Å, and SOHO MDI
data. Examining the time-series of the XRT and TRACE images, we can
identify the following evolutionary sequences: (1) There were three
bundles of loops along the sheared polarity inversion line forming a
sigmoidal structure during the preflare phase. (2) Preflare brightening
occurred between two upper-loop bundles, and they consequently formed
one larger bundle. (3) The main flare occurred near the location
where this new loop bundle and the third bundle met together. (4) As a
result, a single stacked loop structure was formed. This morphological
evolution of the X-ray loops is quite consistent with a tether-cutting
model involving a single-bipole explosion. Our result shows that the
preflare and the main flare in this event are a two-step reconnection
process, which strongly suggests that the preflare activity plays an
important role in triggering the main flare.
Title: Hinode SP Vector Magnetogram of AR10930 and Its
Cross-Comparison with MDI
Authors: Moon, Yong-Jae; Kim, Yeon-Han; Park, Young-Deuk; Ichimoto,
Kiyoshi; Sakurai, Takashi; Chae, Jongchul; Cho, Kyung Suk; Bong,
Suchan; Suematsu, Yoshinori; Tsuneta, Saku; Katsukawa, Yukio; Shimojo,
Masumi; Shimizu, Toshifumi; Shine, Richard A.; Tarbell, Theodore D.;
Title, Alan M.; Lites, Bruce; Kubo, Masahito; Nagata, Shin'ichi;
Yokoyama, Takaaki
Bibcode: 2007PASJ...59S.625M
Altcode:
We present one Hinode Spectropolarimeter (SP) magnetogram of AR 10930
that produced several major flares. The inversion from Stokes profiles
to magnetic field vectors was made using the standard Milne-Eddington
code. We successfully applied the Uniform Shear Method for resolving
the 180° ambiguity to the magnetogram. The inversion gave very strong
magnetic field strengths (near 4500 gauss) for a small portion of area
in the umbra. Considering that the observed V-profile of 6301.5Å was
well-fitted as well as a direct estimation of the Zeeman splitting
results in 4300-4600 gauss, we think that the field strengths
should not be far from the actual value. A cross-comparison of the
Hinode SP and SOHO MDI high resolution flux densities shows that the
MDI flux density could be significantly underestimated by about a
factor of two. In addition, it has a serious negative correlation
(the so-called Zeeman saturation effect) with the Hinode SP flux
density for umbral regions. Finally, we could successfully obtain
a recalibrated MDI magnetogram that has been corrected for the
Zeeman saturation effect using not only a pair of MDI intensity and
magnetogram data simultaneously observed, but also the relationship
from the cross-comparison between the Hinode SP and MDI flux densities.
Title: Small-Scale X-Ray/EUV Jets Seen in Hinode XRT and TRACE
Authors: Kim, Yeon-Han; Moon, Young-Jae; Park, Young-Deuk; Sakurai,
Takashi; Chae, Jongchul; Cho, Kyung Suk; Bong, Su-Chan
Bibcode: 2007PASJ...59S.763K
Altcode:
We present the morphological and kinematic characteristics of three
small-scale X-ray/EUV jets simultaneously observed by the Hinode
XRT and the TRACE. For this, we examined all XRT movies with a thin
Al/Poly filter from 2006 October 20 to 2007 April 8, and then found
34 small-scale eruptions. We next took a look at the corresponding
TRACE 171/195Å images associated with the eruptions. As a result,
we found three simultaneous X-ray and EUV jets. For two different
bands, they have similar characteristics in terms of the projected
speed (90-310kms-1), lifetime (100-2000s), and size
(1.1-5×105km). These characteristics are similar to the
previous results by the Yohkoh SXT, but are quite different from EUV
jets associated with Hα surges. From the EUV Imaging Spectrometer,
four spectral lines for the last event, we found blueshift (up to
-64kms-1) and redshift (up to 20kms-1) motions as
well as nonthermal velocities ranging from 57 to 106kms-1
at the jet footpoint. We note a tendency: the hotter the maximum
ionization temperature is, the smaller the area of blueshift region
is. These characteristics are consistent with a typical jet model
that includes the magnetic reconnection between open coronal fields
and emerging magnetic fields.
Title: Chromospheric Magnetic Reconnection
Authors: Chae, J.
Bibcode: 2007ASPC..369..243C
Altcode:
There has been increasing observational evidence that magnetic
reconnection often occurs in the low atmosphere of the sun. I
will present a brief review of our recent observational results on
canceling magnetic features in the photosphere and jet-like events
in the chromosphere and discuss the expected physical properties of
magnetic reconnection occurring in the photosphere and chromosphere
based on an adiabatic current sheet model of Sweet-Parker type. A
special attention is paid to the temperature-speed relation in jet-like
events in the chromosphere, and the need of anomalous resistivity in
the model of chromospheric magnetic reconnection.
Title: Evolutionary Characteristics of Magnetic Helicity Injection
in Active Regions
Authors: Jeong, H.; Chae, J.
Bibcode: 2007ASPC..369..167J
Altcode:
Magnetic helicity is now regarded as an important physical quantity
in understanding the magnetic activities of solar active regions
such as flares and coronal mass ejections. A common wisdom is that
magnetic helicity is transported from the interior to the corona,
and then to the interplanetary space. In the present study, we are
interested in examining the temporal behavior of helicity injection
through the photospheric boundary that divides the interior and
the corona. Specifically we aim to see whether magnetic helicity
is supplied to the corona in a more or less steady way or not. We
determined the rate of helicity injection in each active region applying
Chae's method to the full-disk, 96 minute-cadence magnetograms taken by
SOHO/MDI. Using these data, each active region could be followed without
interruption for about 5 days while it was away from the limb. Some
active regions were followed at next rotations, too. We found that
magnetic helicity was supplied intensively during the period of flux
emergence, especially during the growth of active regions. The amount
of helicity injected during the growth period ranged from 1.7×10^{42}
Mx^{2} to 43×10^{42} Mx^{2}, depending on the active region flux. These
values are much larger than the estimated contributions of differential
rotation at the photospheric level. Our result suggests that most of
the magnetic helicity in active regions may be supplied for several
days during the early phase of active region evolution.
Title: Magnetic Twist of EUV Coronal Loops Observed by TRACE
Authors: Kwon, R. -Y.; Chae, J.
Bibcode: 2007ASPC..369..299K
Altcode:
The constriction of plasma into a loop without dispersion may be
explained if the magnetic field of the loop is twisted. Based on this
idea, Chae and Moon (2005) developed a method of determining magnetic
twist of coronal loops by analyzing coronal images and photospheric
magnetograms together. We attempt to determine the number distribution
of magnetic twist of coronal loops, by expanding their method to more
number of EUV loops observed by TRACE 171 Å. Preliminary results we
have obtained from three loops indicate that these loops have twist
values from 1.5π to 2.5π, which suggests that the winding number of
EUV coronal loop may be around one.
Title: Low Atmosphere Reconnections Associated with a Filament
Eruption
Authors: Moon, Y. -J.; Chae, J.; Park, Y. D.
Bibcode: 2007ASPC..369..425M
Altcode:
We present preflare activity associated with an eruptive flare. For
this work we have analyzed high temporal (about 1 minute) and spatial
(about 1 arcsec) resolution images taken by SOHO/MDI, BBSO H-alpha
centerline and blue wing, and TRACE 1600 UV images. We found that
there were several transient brightenings seen around the preflare
phase. They took place near one footpoint of the filament and were
associated with cancelling magnetic features (CMFs) as well as
recurrent EUV ejections. The flux variations suggest that the flux
cancellation may have been driven by the flux emergence and/or shearing
motions. The above results imply that the preflare activity is caused
by low-atmospheric reconnections. For this event, we estimated the
ejection speeds of the associated filament ranging from 10 to 160 km/s
for the first twenty minutes. It is noted that the initiation of the
filament eruption (as defined by the rise speed of about 10 km/s) was
coincident with the preflare activity characterized by UV brightenings
and CMFs. Our results support that the initiation of the filament
eruption be physically related to low-atmosphere magnetic reconnection.
Title: Determination of Magnetic Helicity of a Solar Active Region
Using the Linear Force-Free Field Model
Authors: Lim, E. -K.; Jeong, H.; Chae, J.
Bibcode: 2007ASPC..369..175L
Altcode:
We aim to measure the helicity of the coronal magnetic field
of an active region based on the linear force-free field(LFFF)
assumption. With a value of the force-free α, the coronal field is
constructed from the extrapolation of SOHO/MDI magnetograms, and the
constructed field lines are compared with the coronal loops in the EUV
images taken by SOHO/EIT. The force-free field that best fits the loops
is used to calculate the helicity of the active region. By applying this
method to the solar active region AR 10696 during its first rotation,
we have examined the temporal variation of the magnetic helicity,
and have compared it with the accumulated amount of the helicity
transferred to the corona through the photosphere which was determined
independently. Our results are summarized as follows. 1) The magnetic
field of the active region is not fully LFF. The force-free α varies
from loop to loop, with a deviation of 30 to 45%, which gives us a
very conservative estimate of the uncertainty in the coronal helicity
determined using the LFF model. 2) The measured value of the coronal
magnetic helicity was close to the accumulated amount of injected
helicity with a discrepancy of 10∼30%. 3) The coronal magnetic
helicity decreased after CMEs occurred, with the amount of decrease
being close to what may have been carried away by the CMEs. Our results
support that the LFF field extrapolation method can be used to infer
the magnetic helicity of an active region within an uncertainty of a
few tens of percent.
Title: Hα Spectral Properties of Velocity Threads Constituting a
Quiescent Solar Filament
Authors: Chae, Jong-Chul; Park, Hyung-Min; Park, Young-Deuk
Bibcode: 2007JKAS...40...67C
Altcode:
No abstract at ADS
Title: Flux Pile-up Magnetic Reconnection in the Solar Photosphere
Authors: Litvinenko, Yuri E.; Chae, Jongchul; Park, So-Young
Bibcode: 2007ApJ...662.1302L
Altcode:
Magnetic reconnection in the temperature minimum region of the
solar photosphere, observationally manifested as canceling magnetic
features, is considered. Flux pile-up reconnection in the Sweet-Parker
current sheet is analyzed. It is shown that the standard Sweet-Parker
reconnection rate in the photosphere is too slow to explain the observed
cancellation. Flux pile-up reconnection scalings, however, are shown
to be in agreement with the speeds of canceling magnetic fragments,
magnetic fields in the fragments, and the rates of magnetic flux
cancellation, derived from SOHO MDI data. Pile-up factors in the range
between 1 and 5 and local reconnecting magnetic fields of a few hundred
G are calculated for the analyzed canceling features. The analysis
shows that flux pile-up is a likely mechanism for adjusting the local
parameters of reconnecting current sheets in the photosphere and for
sustaining the reconnection rates that are determined by large-scale
supergranular flows. The upward mass flux in the reconnection jet,
associated with a large canceling feature, is predicted to exceed
1015 g hr-1. Hence, cancellation in a few
photospheric patches over several hours can lead to the formation of
a solar filament in the corona.
Title: A Check for Consistency between Different Magnetic Helicity
Measurements Based on the Helicity Conservation Principle
Authors: Lim, Eun-Kyung; Jeong, Hyewon; Chae, Jongchul; Moon, Yong-Jae
Bibcode: 2007ApJ...656.1167L
Altcode:
Magnetic helicity is a useful quantity in characterizing the magnetic
systems of solar active regions. The purpose of the present work is
to check for consistency between the local correlation tracking (LCT)
method used to measure helicity injection through the photosphere, and
the linear force-free field (LFFF) method used to determine helicity
in the corona, based on the principle of helicity conservation in
the solar corona. We have calculated the amount of magnetic helicity
injected through the photosphere during the first disk passage of AR
10696 using the LCT method initially described by Chae. We have also
measured the coronal magnetic helicity as a function of time using the
LFFF method. With a value for the force-free α, the coronal field
is constructed from the extrapolation of the Solar and Heliospheric
Observatory (SOHO) MDI magnetograms, then compared with the coronal
loops in the EUV images taken by the SOHO EIT. The force-free α that
best fits the loops is used to calculate the coronal helicity. From a
careful comparison of different helicity measurements during each time
interval, we have reached the core conclusion that our measurements
follow the helicity conservation principle with an uncertainty of ~15%
and hence support the consistency between the two different methods
with the same amount of uncertainty.
Title: Measurements of magnetic helicity injected through the solar
photosphere
Authors: Chae, Jongchul
Bibcode: 2007AdSpR..39.1700C
Altcode:
The amount of magnetic helicity injected into the corona through
the photosphere can be measured by inferring the apparent motion
of photospheric footpoints of fieldlines from a time series of
line-of-sight magnetograms, as originally proposed by [Chae, J.,
2001. Observational determination of the rate of magnetic helicity
transport through the solar surface via the horizontal motion of
field line footpoints. Astrophysical Journal 560, L95-L98]. In this
paper, we present a revised version of Chae's original method by
adopting a better proxy for the flux density of magnetic helicity
that was proposed by [Pariat, E., Démoulin, P., Berger, M.A.,
2005. Photospheric flux density of magnetic helicity. Astronomy
and Astrophysics 439, 1191-1203]. Moreover it is allowed to choose
either the old technique of local correlation tracking (LCT) or the
new method called the different affine velocity estimator (DAVE)
method that was developed by [Schuck, P. W., 2006. Tracking magnetic
footpoints with the magnetic induction equation. Astrophysical Journal
646, 1358-1391.] for inferring transverse velocity. We have applied
the revised method using the DAVE method to a set of 96-min cadence,
full-disk MDI magnetograms for AR 10696 and compared the results with
those obtained using the LCT method. As a result, we have found that
the helicity measurements using the DAVE method yield systematically
higher values of helicity injection than those using the LCT method,
but the discrepancy is moderately small, being less than 10%.
Title: Magnetic helicity comparison between coronal magnetic field
of an active region and interplanetary magnetic cloud
Authors: Sung, S.; Chae, J.; Lim, E.; Kim, K.
Bibcode: 2006AGUFMSH33B0410S
Altcode:
We investigate several interplanetary magnetic cloud events associated
with halo coronal mass ejections to compare the content of helicity
in magnetic clouds with the coronal magnetic helicity variation after
coronal mass ejections occurred. We use a cylindrically symmetric
constant-alpha force-free model to derive magnetic helicity of magnetic
cloud events by determining the parameters as magnetic cloud orientation
and radius, magnetic magnitude, and helicity sign. For the helicity
of the coronal magnetic field of an active region, we also assume
the linear force-free field. The result of this study provide the
information whether the magnetic helicity is conserved or not between
sun and near Earth (1 AU).
Title: Progress on the 1.6-meter New Solar Telescope at Big Bear
Solar Observatory
Authors: Denker, C.; Goode, P. R.; Ren, D.; Saadeghvaziri, M. A.;
Verdoni, A. P.; Wang, H.; Yang, G.; Abramenko, V.; Cao, W.; Coulter,
R.; Fear, R.; Nenow, J.; Shoumko, S.; Spirock, T. J.; Varsik, J. R.;
Chae, J.; Kuhn, J. R.; Moon, Y.; Park, Y. D.; Tritschler, A.
Bibcode: 2006SPIE.6267E..0AD
Altcode: 2006SPIE.6267E..10D
The New Solar Telescope (NST) project at Big Bear Solar Observatory
(BBSO) now has all major contracts for design and fabrication in place
and construction of components is well underway. NST is a collaboration
between BBSO, the Korean Astronomical Observatory (KAO) and Institute
for Astronomy (IfA) at the University of Hawaii. The project will
install a 1.6-meter, off-axis telescope at BBSO, replacing a number
of older solar telescopes. The NST will be located in a recently
refurbished dome on the BBSO causeway, which projects 300 meters into
the Big Bear Lake. Recent site surveys have confirmed that BBSO is
one of the premier solar observing sites in the world. NST will be
uniquely equipped to take advantage of the long periods of excellent
seeing common at the lake site. An up-to-date progress report will
be presented including an overview of the project and details on the
current state of the design. The report provides a detailed description
of the optical design, the thermal control of the new dome, the optical
support structure, the telescope control systems, active and adaptive
optics systems, and the post-focus instrumentation for high-resolution
spectro-polarimetry.
Title: Two Phases of Helicity Variation Around Major Flares
Authors: Park, Sung-Hong; Chae, J.; Jeong, H.; Choe, G.; Lee, J.;
Yang, G.; Jing, J.; Wang, H.
Bibcode: 2006SPD....37.2201P
Altcode: 2006BAAS...38..249P
We have investigated the magnetic helicity injection rates in three
active regions (NOAA 10484, 10486 and 10696) around the times of four
X-class flares using the MDI/SOHO magnetograms. In all cases, the
total magnetic flux of the individual active region changed little
with time, and the helicity variation is directly related to the
variation of field line winding. This also justifies our measurement of
magnetic helicity injection rates using a local correlation tracking
method. Our analysis reveals that there were two distinct phases of
helicity variation around those flares. In the first phase that starts
a few days before the flare onset, the helicity accumulates at a nearly
constant rate. The second phase usually starts 3-12 hours before the
flare onset and lasts until 3-20 hours after the flare. During this
phase, the helicity injection rate is negligible and the magnitude
of helicity remains almost constant or increases only slightly. This
characteristic pattern in the helicity evolution may imply a physical
link between magnetic winding and flare occurrence, and could be used
as an early warning sign of impending flares.
Title: Imaging Spectroscopy of a Solar Filament Using a Tunable
Hα Filter
Authors: Chae, Jongchul; Park, Young-Deuk; Park, Hyung-Min
Bibcode: 2006SoPh..234..115C
Altcode:
Observations using a narrow band Hα filter still remain one of the
best ways to investigate the fine structures and internal dynamics of
solar filaments. Hα observations, however, have been usually carried
out with the peak response of the filter fixed at a single wavelength,
usually at the centerline, in which the investigation is limited to the
Hα morphology and its time evolution. In this paper, we demonstrate
that the Hα spectroscopy that takes Hα images successively at several
wavelengths is a useful tool in the study of solar filaments on the
solar disk. Our observation of a filament was carried out on August 3,
2004 at Big Bear Solar Observatory using the 10-inch refractor. The
Lyot Hα filter was successively tuned to five wavelengths: −0.6,
−0.3, 0.0, +0.3, and +0.6 Å from the Hα line center. Each set
of wavelength scan took 15 s. After several steps of data reduction,
we have constructed a five-wavelength spectral profile of intensity
contrast at every spatial point. The contrast profile at each spatial
point inside the filament was reasonably well fit by the cloud model
as far as the contrast is high enough, and allowed us to construct the
maps of τ0, v, Δ λD and S in the filament. We
also found that the line center method that is often used, always
yields line-of-sight velocities that are systematically lower than the
cloud model fit. Our result suggests that taking Hα images at several
wavelengths using a tunable filter provides an effective way of deriving
physically meaningful parameters of solar filaments. Particularly
constructing the time sequence of v maps appears to be a useful tool
for the study of internal dynamics, like counterstreaming, in filaments.
Title: Measurements of Magnetic Helicity Injected through the
Photosphere and Magnetic Helicity Content in the Corona
Authors: Chae, J.; Jeong, H.; Lim, E.
Bibcode: 2006cosp...36..880C
Altcode: 2006cosp.meet..880C
Magnetic helicity is a useful quantity in characterizing the magnetic
connection of the solar interior surface corona and the interplanetary
space It is a common wisdom that magnetic helicity is transferred from
the interior to the corona and then is expelled out of the Sun in
the form of coronal mass ejections From observations it is possible
to independently measure the accumulated amount of magnetic helicity
transferred through the photopshere the content of magnetic helicity in
the corona and the amount magnetic helicity carried away by coronal mass
ejections Chae 2001 fisrt proposed that the magnetic helicity injected
into the corona through the photopshere is determined from the local
correlation tracking of of magnetic fieldline footpoints The magnetic
helicity content of the corona is inferred from the three dimensional
structure of coronal magnetic field which is usually constructed
using the extrapolation of magnetic field The simplest extrapolation
model that can be used to determine magnetic helicity is a linear
force-free field We have applied the two indepdent approaches to the
active region 10696 to indepently determine the accumulated magnetic
helicity injected through the photosphere and the coronal magnetic
helicity content as functions of time Our results are as follows 1
most magnetic helicity was supplied intensively during the period
of flux emergence especially during the growth of active regions The
helicity injected in the early five days was about 2 times 10 43 Mx
2 2 The linear force-free assumption is not
Title: Spatio-Spectral Maximum Entropy Method: II. Solar Microwave
Imaging Spectroscopy
Authors: Bong, Su-Chan; Lee, Jeong-Woo; Gary, Dale E.; Yun, Hong-Sik;
Chae, Jong-Chul
Bibcode: 2005JKAS...38..445B
Altcode:
No abstract at ADS
Title: Magnetic Twist of an Extreme-Ultraviolet Coronal Loop Inferred
from Plasma Constriction
Authors: Chae, Jongchul; Moon, Yong-Jae
Bibcode: 2005ApJ...629.1110C
Altcode:
Magnetic twist is a very likely way of constricting coronal plasma
into loops. An attempt is made to determine the magnetic twist
that can explain the constriction of coronal loops indirectly
from observations. We decompose the coronal magnetic field into a
large-scale magnetic field and a small-scale magnetic field. The
large-scale field is assumed to be force-free, but the small-scale
field is not. We derive a magnetohydrostatic equation describing the
relationship between the small-scale field variation and the pressure
variation across the loop. We adopt a simple flux rope model with the
following properties. (1) There exists a close physical relationship
between the twist-creating process and the plasma-injecting process that
keeps the axial field constant as the loop forms. (2) The large-scale
force-free field is so close to the current-free configuration that
its associated current is negligible in the plasma constriction. (3)
Pressure peaks on the loop axis and monotonically decreases with the
distance from the axis. This flux tube model is characterized by three
independent parameters: axial field strength, peak pressure excess, and
loop width. The peak pressure excess and the loop width are determined
from an analysis of the EUV data taken by TRACE, and an estimate of the
axial field strength is found from the linear force-free extrapolation
of the photospheric magnetic field observed by SOHO MDI. By applying
this method to a specific coronal loop, we have found that the magnetic
twist on the loop axis is about 1.5π, and the twist per unit length
is more concentrated near its top than near its footpoints. This twist
value has been obtained using the simplifying assumption of uniform
axial field strength. In the case where the axial field is stronger
inside the loop than in the surroundings, the twist will turn out to
be greater than 1.5π, since the twist is required to constrict not
only the plasma but also the magnetic flux. On the other hand, if the
axial field is weaker than that in the surroundings, a smaller twist
may be enough to constrict the plasma.
Title: New extrapolation method for coronal mass ejection onset
time estimation
Authors: Moon, Y. -J.; Cho, K. -S.; Chae, Jongchul; Choe, G. S.; Kim,
Y. -H.; Bong, Su-Chan; Park, Y. -D.
Bibcode: 2005JGRA..110.7103M
Altcode: 2005JGRA..11007103M
The onset time of a coronal mass ejection (CME) is usually extrapolated
from its speed in a coronagraph using the constant speed method. In
this study, we present a new empirical method for extrapolating
the onset times of flare-associated CMEs. For this we reexamined
seven well-observed CME-flare events whose initial eruption speeds
are estimated to be about a few tens of kilometers per second from
LASCO C1 coronagraph or loop displacements seen in SOHO/EIT and TRACE
images. In this study, the CME onset time is assumed to be the earliest
observable time of eruption from these images. We estimated the onset
time differences between the CMEs and the associated flares observed
in soft X rays. We then compared them with the onset time differences
estimated by the constant speed extrapolation method from LASCO C2
positions. It is noted that there is a certain quadratic relation
between the CME speed first observed in the LASCO C2 field of view
and the observed CME travel time from near the solar surface to the
C2 field of view in units of the travel time estimated by the constant
speed method. This empirical relation thus enables us to determine the
onset times of CMEs without any assumption in CME kinematics. We have
applied the new empirical method to 91 flare-associated CMEs that were
accompanied by type II bursts. The onset time difference for these
events as well as the well-observed events shows that in most cases
(about 80%) the CME initiation precedes the onset of the associated
flare. This result provides an important clue to the mechanism and
relation of CMEs and flares.
Title: a Method for Determining Magnetic Helicity of Solar Active
Regions from Soho/mdi Magneto Grams
Authors: Chae, Jongchul; Jeong, Hyewon
Bibcode: 2005JKAS...38..295C
Altcode:
No abstract at ADS
Title: The Magnetic Structure of Filament Barbs
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, Young-Deuk
Bibcode: 2005ApJ...626..574C
Altcode:
There is a controversy about how features protruding laterally from
filaments, called barbs, are magnetically structured. On 2004 August 3,
we observed a filament that had well-developed barbs. The observations
were performed using the 10 inch refractor of the Big Bear Solar
Observatory. A fast camera was employed to capture images at five
different wavelengths of the Hα line and successively record them
on the basis of frame selection. The terminating points of the barbs
were clearly discernable in the Hα images without any ambiguity. The
comparison of the Hα images with the magnetograms taken by SOHO
MDI revealed that the termination occurred above the minor polarity
inversion line dividing the magnetic elements of the major polarity
and those of the minor polarity. There is also evidence that the flux
cancellation proceeded on the polarity inversion line. Our results
together with similar other recent observations support the idea that
filament barbs are cool matter suspended in local dips of magnetic
field lines, formed by magnetic reconnection in the chromosphere.
Title: The New Solar Telescope at Big Bear Solar Observatory -
A Progress Report
Authors: Denker, C.; Cao, W.; Chae, J.; Coulter, R.; Kuhn, J. R.;
Marquette, W. H.; Moon, Y.; Park, Y.; Ren, D.; Tritschler, A.; Varsik,
J. R.; Wang, H.; Yang, G.; Shoumko, S.; Goode, P. R.
Bibcode: 2005AGUSMSP43A..07D
Altcode:
The New Solar Telescope (NST) is a new 1.6-meter, off-axis telescope
for the Big Bear Solar Observatory (BBSO) in California. The NST is
collaboration between BBSO, the Korean Astronomical Observatory (KAO)
and Institute for Astronomy (IfA) at the University of Hawaii. BBSO
is an ideal site for high-spatial resolution observations, since this
mountain-lake site provides consistent seeing conditions with extended
periods of excellent seeing from sunrise to sunset. These unique seeing
characteristics make BBSO ideally suited for combined high-resolution
campaigns and synoptic observations, which are essential for studies
of solar activity and space weather. In this progress report, we
present the latest information on the optical design, the optical
support structure, the telescope control system and the requisite
instrumentation for the telescope. Acknowledgements: This work has been
supported by NSF under grants ATM-0236945, ATM-0342560, MRI-0320540,
and Air Force DURIP F-49620-03-1-0271.
Title: New Geoeffective Parameters of Very Fast Halo Coronal Mass
Authors: Moon, Y.; Cho, K.; Dryer, M.; Kim, Y.; Bong, S.; Chae, J.;
Park, Y.
Bibcode: 2005AGUSMSH23A..01M
Altcode:
We have examined the physical characteristics of very fast coronal
mass ejections (CMEs) and their geoeffective parameters. For this we
consider SOHO/LASCO CMEs whose speeds are larger than 1300 km s-1. By
examining all SOHO/EIT and SOHO/LASCO images of the CMEs, we selected
38 front-side very fast CMEs and then examined their associations with
solar activity such as X-ray flares and Type II bursts. As a result,
we found that among these frontside fast CMEs, 25 are halo (or full
halo) CMEs with span of 360 degrees; 12 are partial halo CMEs with
span greater than 130 degrees; only 1 is broadside CME with span of 53
degrees. There are 13 events that are shock deflected CMEs: 6 are full
halo CMEs and 7 are partial halo CMEs. It is found that about 60 %
(23/38) CMEs were ejected from the western hemisphere. We also note
that these very fast CMEs have very high associations with other solar
activities: all the CMEs are associated with X-ray flares (X-12, M-23,
C-3), and about 80 % of the CMEs (33/38) were accompanied by Type II
bursts. For the examination of CME geoeffectiveness, we select 12 halo
CMEs whose longitudes are less than 40 degrees, which are thought to
be the most plausible candidates of geoeffective CMEs. Then we examine
the relationship between their CME physical parameters (mass, column
density, location of an associated flare, and direction) and the Dst
index. Especially, a CME direction parameter, which is defined as the
maximum ratio of its shorter front from solar disk center and its longer
one, is proposed as a new geoeffective parameter. Its major advantage
is that it can be directly estimated from coronagraph observation. It
is found that while the location of the associated flare has a poor
relationship with the Dst index, the new direction parameter has a
relatively good relationship. In addition, the column density of a CME
also has a comparable good correlation with the Dst index. Noting that
the CME column density is strongly affected by the direction of a CME,
our results imply that the CME direction seems to be the most important
parameter that controls the geoeffectiveness of very fast halo CMEs.
Title: New Geoeffective Parameters of Very Fast Halo Coronal Mass
Ejections
Authors: Moon, Y. -J.; Cho, K. -S.; Dryer, M.; Kim, Y. -H.; Bong,
Su-chan; Chae, Jongchul; Park, Y. D.
Bibcode: 2005ApJ...624..414M
Altcode:
We have examined the physical characteristics of very fast coronal mass
ejections (CMEs) and their geoeffective parameters. For this we consider
SOHO LASCO CMEs whose speeds are larger than 1300 km s-1. By
examining all SOHO EIT and SOHO LASCO images of the CMEs, we selected
38 front-side very fast CMEs and then examined their associations with
solar activity such as X-ray flares and type II bursts. As a result,
we found that among these front-side fast CMEs, 25 are halo (or full
halo) CMEs with span of 360°, 12 are partial halo CMEs with span
greater than 130°, and only one is a broadside CME, with a span of
53°. There are 13 events that are shock-deflected CMEs: six are full
halo CMEs, and seven are partial halo CMEs. It is found that about 60%
(23/38) CMEs were ejected from the western hemisphere. We also note
that these very fast CMEs have very high associations with other solar
activities: all the CMEs are associated with X-ray flares (X-12, M-23,
C-3), and about 80% of the CMEs (33/38) were accompanied by type II
bursts. For the examination of CME geoeffectiveness, we select 12 halo
CMEs whose longitudes are less than 40°, which are thought to be the
most plausible candidates of geoeffective CMEs. Then we examine the
relation between their CME physical parameters (mass, column density,
location of an associated flare, and direction) and the Dst index. In
particular, a CME direction parameter, which is defined as the maximum
ratio of its shorter front from solar disk center and its longer one,
is proposed as a new geoeffective parameter. Its major advantage is
that it can be directly estimated from coronagraph observation. It
is found that while the location of the associated flare has a poor
correlation with the Dst index, the new direction parameter has a
relatively good correlation. In addition, the column density of a CME
also has a comparable good correlation with the Dst index. Noting that
the CME column density is strongly affected by the direction of a CME,
our results imply that the CME direction seems to be the most important
parameter that controls the geoeffectiveness of very fast halo CMEs.
Title: Technique for Inferring Magnetic Helicity of Active Regions
Authors: Chae, Jongchul; Moon, Yong-Jae
Bibcode: 2005HiA....13..109C
Altcode:
The magnetic helicity content of a coronal volume may change over time
at the presence of surface flows. When phtospheric magnetic fields are
predominantly vertical it is possible to determine the rate of change of
magnetic helicity using a time series of longitudinal magnetograms only
since one can determine the horizontal component of velocity by tracking
the displacements of magnetic flux fragments using the technique of
local correlation (LCT). This technique has turned out to be very
useful in revealing significant helicity changes that are associated
with prominence formation and major flares. When magnetic fields are
not vertical it is crucial to determine the vertical component of
velocity too. A clever way of inferring it from the induction equation
and the LCT-determined horizontal velocity has been proposed. However
the results should be interpreted with a caution since the LCT may
not yield even the correct value of the horizontal velocity component
when magnetic fields are inclined to the vertical. Alternatively we are
trying to make use of the full Stokes profile data taken by the NSO/ASP
to determine the velocity vectors as well as magnetic field vectors.
Title: Preliminary Feasibility Study of the Solar Observation Payloads
for STSAT-CLASS Satellites
Authors: Moon, Yong-Jae; Cho, Kyung-Seok; Jin, Ho; Chae, Jong-Chul;
Lee, Sung-Ho; Seon, Kwang-Il; Kim, Yeon-Han; Park, Young-Deuk
Bibcode: 2004JASS...21..329M
Altcode:
In this paper, we present preliminary feasibility studies on three
types of solar observation payloads for future Korean Science and
Technology Satellite (STSAT) programs. The three candidates are (1)
an UV imaging telescope, (2) an UV spectrograph, and (3) an X-ray
spectrometer. In the case of UV imaging telescope, the most important
constraint seems to be the control stability of a satellite in order
to obtain a reasonably good spatial resolution. Considering that
the current pointing stability estimated from the data of the Far
ultraviolet Imaging Spectrograph (FIMS) onboard the Korean STSAT-1, is
around 1 arc minutes/sec, we think that it is hard to obtain a spatial
resolution sufficient for scientific research by such an UV Imaging
Telescope. For solar imaging missions, we realize that an image
stabilization system, which is composed of a small guide telescope
with limb sensor and a servo controller of secondary mirror, is quite
essential for a very good pointing stability of about 0.1 arcsec. An UV
spectrograph covering the solar full disk seems to be a good choice in
that there is no risk due to poor pointing stability as well as that it
can provide us with valuable UV spectral irradiance data valuable for
studying their effects on the Earth's atmosphere and satellites. The
heritage of the FIMS can be a great advantage of developing the UV
spectrograph. Its main disadvantage is that two major missions are in
operation or scheduled. Our preliminary investigations show that an
X-ray spectrometer for the full disk Sun seems to be the best choice
among the three candidates. The reasons are : (1) high temporal and
spectral X-ray data are very essential for studying the acceleration
process of energetic particles associated with solar flares, (2)
we have a good heritage of X-ray detectors including a rocket-borne
X-ray detector, (3) in the case of developing countries such as India
and Czech, solar X-ray spectrometers were selected as their early
stage satellite missions due to their poor pointing stabilities, and
(4) there is no planned major mission after currently operating Reuven
Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) mission. Finally,
we present a preliminary design of a solar X-ray spectrometer covering
soft X-ray (2 keV) to gamma ray (10 MeV).
Title: Determination of magnetic helicity content of solar active
regions from SOHO/MDI magnetograms
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, Young-Deuk
Bibcode: 2004SoPh..223...39C
Altcode:
Chae (2001) first proposed a method of self-consistently determining the
rate of change of magnetic helicity using a time series of longitudinal
magnetograms only, such as taken by SOHO/MDI. Assuming that magnetic
fields in the photosphere are predominantly vertical, he determined
the horizontal component of velocity by tracking the displacements
of magnetic flux fragments using the technique of local correlation
tracking (LCT). In the present paper, after briefly reviewing the
recent advance in helicity rate measurement, we argue that the LCT
method can be more generally applied even to regions of inclined
magnetic fields. We also present some results obtained by applying
the LCT method to the active region NOAA 10365 under emergence during
the observable period, which are summarized as follows. (1) Strong
shearing flows were found near the polarity inversion line that were
very effective in helicity injection. (2) Both the magnetic flux and
helicity of the active region steadily increased during the observing
period, and reached 1.2 × 1022 Mx and 8 ×1042
Mx2, respectively, 4.5 days after the birth of the active
region. (3) The corresponding ratio of the helicity to the square of the
magnetic flux, 0.05, is roughly compatible with the values determined
by other studies using linear-force-free modeling. (4) A series of
flares took place while the rate of helicity injection was high. (5)
The choice of a smaller window size or a shorter time interval in
the LCT method resulted in a bigger value of the LCT velocity and a
bigger value of the temporal fluctuation of the helicity rate. (6)
Nevertheless when averaged over a time period of about one hour or
longer, the average rate of helicity became about the same within about
10%, almost irrespective of the chosen window size and time interval,
indicating that short-lived, fluctuating flows may be insignificant in
transferring magnetic helicity. Our results suggest that the LCT method
may be applied to 96-minute cadence full-disk MDI magnetograms or other
data of similar kind, to provide a practically useful, if not perfect,
way of monitoring the magnetic helicity content of active regions as
a function of time.
Title: Flat-Fielding of Solar Hα Observations Using Relatively
Shifted Images
Authors: Chae, Jongchul
Bibcode: 2004SoPh..221....1C
Altcode:
A new algorithm is proposed to determine the flat pattern from a
set of relatively shifted images. It simultaneously searches for the
flat pattern, the object image, the light levels, and optionally the
relative displacements that optimize the sum of the error squares. We
have applied the method to real Hα observations, and examined in detail
the dependence of the accuracy of the solution on the iteration number,
the light level change, the dither pattern, and the noise. It has been
found that the method can produce a flat pattern with an error down
to 0.25% of the mean level in Hα observations with low noise.
Title: Flat-Fielding of Solar Magnetograph Observations Using
Relatively Shifted Images
Authors: Chae, Jongchul
Bibcode: 2004SoPh..221...15C
Altcode:
I present a method to correct solar magnetograms for a non-uniform
detector pattern that depends on the sense of light polarization. The
application to Big Bear Solar Observatory magnetograph observations
demonstrates that the determination of a flat pattern can be done at
an accuracy imposed by the photon noise in the raw magnetograms.
Title: Low Atmosphere Reconnections Associated with AN Eruptive
Solar Flare
Authors: Moon, Y. -J.; Chae, Jongchul; Choe, G. S.; Wang, Haimin;
Park, Y. D.; Cheng, C. Z.
Bibcode: 2004JKAS...37...41M
Altcode:
No abstract at ADS
Title: Observational Evidence of Magnetic Flux Submergence in Flux
Cancellation Sites
Authors: Chae, Jongchul; Moon, Yong-Jae; Pevtsov, Alexei A.
Bibcode: 2004ApJ...602L..65C
Altcode:
Using high-resolution vector magnetograms of NOAA Active Region
10043, observed on 2002 July 26 with the Advanced Stokes Polarimeter
and low-order adaptive optics system, we studied the magnetic field
topology and line-of-sight velocities in two flux cancellation sites. We
found that the magnetic field is near horizontal at the place where
two opposite polarities cancel each other. In addition, we observed
significant downflows of about 1 km s-1 near the polarity
reversal line, where the field is horizontal. We interpret these
observations as the direct evidence of the magnetic flux submergence of
concave-down (Ω-shaped) magnetic loop at the flux cancellation sites.
Title: Small-Scale Hα Dynamic Features Supported by Chromo Spheric
Magnetic Reconnection
Authors: Lee, Sangwoo; Yun, Hong Sik; Chae, Jongchul; Goode, Philip R.
Bibcode: 2003JKAS...36S..21L
Altcode:
No abstract at ADS
Title: Magnetic Helicity Changes of Solar Active Regions by
Photospheric Horizontal Motions
Authors: Moon, Y. -J.; Chae, Jongchul; Park, Y. D.
Bibcode: 2003JKAS...36S..37M
Altcode:
No abstract at ADS
Title: Observational Tests of Chromospheric Magnetic Reconnection
Authors: Chae, Jongchul; Moon, Yong-Jae; Park, So-Young
Bibcode: 2003JKAS...36S..13C
Altcode:
No abstract at ADS
Title: Pre-flare Activity, Canceling Magnetic Features, and Filament
Initiation associated with an X-class Flare
Authors: Moon, Y. -J.; Chae, J.; Choe, G. S.; Wang, H.; Park, Y. D.
Bibcode: 2003SPD....34.1611M
Altcode: 2003BAAS...35R.834M
We have examined the relationship among pre-flare brightenings,
canceling magnetic features (CMFs), and filament initiation associated
with an X1.8 flare that occurred in NOAA AR 9236 on November 24,
2000. For this, we have inspected high temporal (about 1 minute)
and spatial (about 1 arcsec) resolutions of SOHO/MDI, BBSO Hα , and
TRACE 1600 Å UV images. Major results are as follows. First, these
pre-flare brightenings are the most clearly seen in the TRACE 1600 Å
images rather than in BBSO Hα with very weak corresponding features
in SOHO/EIT images. Second, two major pre-flare brightenings near one
of the filament footpoints are exactly located on CMFs seen in the
MDI images. Third, recurrent small-scale TRACE UV eruptive events,
which may be interpreted as upward outflows associated with the low
atmosphere magnetic reconnection, took place just at the site of the CMFs. Fourth, these CMFs show a common tendency of flux change:
first flux emergence and then flux cancellation. Fifth, the abrupt
UV enhancement of one major pre-flare brightening is responsible for
the flare precursor in the GOES X-ray curve. It is very interesting
to note that such an enhancement is coincident with the starting of
the eruption, which is about 20 minutes earlier than the starting
time of hard X-ray flaring. Our results support an idea that the low
atmosphere reconnection, supported by CMFs and pre-flare brightenings,
play important role in triggering the filament eruption. This
work has been supported by NASA grants NAG5-10894 and NAG5-7837,
by a MURI grant of AFOSR, and by NRL M10104000059-01J000002500 of
Korean government.
Title: Magnetic Helicity Pumping by Twisted Flux Tube Expansion
Authors: Chae, Jongchul; Moon, Y. -J.; Rust, D. M.; Wang, Haimin;
Goode, Philip R.
Bibcode: 2003JKAS...36...33C
Altcode:
No abstract at ADS
Title: The Formation of a Prominence in NOAA Active Region
8668. II. Trace Observations of Jets and Eruptions Associated with
Canceling Magnetic Features
Authors: Chae, Jongchul
Bibcode: 2003ApJ...584.1084C
Altcode:
Our previous study has shown that the formation of a reverse S-shaped
filament in NOAA active region 8668 was closely associated with a
large canceling magnetic feature. In the present paper we investigate
the response of the upper atmosphere at the region of this canceling
magnetic feature. The UV and EUV data taken by the Transition
Region and Coronal Explorer (TRACE) reveal that a series of jets
and small eruptions took place there during the formation of the
prominence. Plasma in each jet originated from a single site of flux
cancellation and moved in opposite directions at a transverse speed of
80-250 km s-1 across the plane of the sky. Plasma eruptions
showing complex morphology and dynamics started from two or more sites
of flux cancellation and appear to have the same physical origin as the
jets. The two filter ratio technique indicates that the EUV-emitting
plasma in the jets and eruptions have transition-region temperatures of
(2-3)×105 K. It is also found from emission-measure analysis
that the electron density is (0.7-1.9)×1010 cm-3
and that each jet carries plasma mass of (1.7-4.6)×1013
g and each eruption carries additional mass of (9-25)×1013
g. Our results are consistent with the current pictures that (1) flux
cancellation observed in the photosphere is a consequence of magnetic
reconnection occurring in the chromosphere and (2) that a series of
such magnetic reconnection events is able to supply the mass necessary
for the formation of a solar prominence.
Title: Magnetic helicity change rate associated with three X-class
eruptive flares
Authors: Moon, Y. -J.; Chae, Jongchul; Wang, Haimin; Park, Y. D.
Bibcode: 2003AdSpR..32.1953M
Altcode:
In this paper we have examined the temporal evolution of magnetic
helicity change rate associated with three X-class eruptive flares that
occurred on November 24, 2000. By analyzing a set of 1 minute cadence
high-resolution magnetograms taken by Michelson Doppler Imager (MDI)
on board Solar and Heliospheric Observatory (SOHO), we have determined
the rates of magnetic helicity transport via horizontal photospheric
motions. Then we have compared the impulsive variations of helicity
injection rate with an initial eruption speed profile of a filament
associated with a X1.8 flare. As a result, we have found (1) that the
impulsive variations of magnetic helicity change rate were strongly
correlated with only three X-class flarings, (2) that the eruption of
the filament started about 10 minutes before the impulsive variation
of the helicity change rate, and (3) that horizontal velocity kernels
were located between Ha two flaring ribbons. Thus we suggest that
the observed impulsive helicity change is not a cause of the eruptive
solar flare but its result, i.e., photospheric response to the coronal
field restructuring.
Title: Techniques for Inferring Active Region Magnetic Helicity
Authors: Chae, Jongchul; Moon, Yong-Jae
Bibcode: 2003IAUJD...3E..25C
Altcode:
The magnetic helicity content of a coronal volume may change over time
at the presence of surface flows. When phtospheric magnetic fields are
predominantly vertical it is possible to determine the rate of change of
magnetic helicity using a time series of longitudinal magnetograms only
since one can determine the horizontal component of velocity by tracking
the displacements of magnetic flux fragments using the technique of
local correlation (LCT). This technique has turned out to be very
useful in revealing significant helicity changes that are associated
with prominence formation and major flares. When magnetic fields are
not vertical it is crucial to determine the vertical component of
velocity too. A clever way of inferring it from the induction equation
and the LCT-determined horizontal velocity has been proposed. However
the results should be interpreted with a caution since the LCT may
not yield even the correct value of the horizontal velocity component
when magnetic fields are inclined to the vertical. Alternatively we are
trying to make use of the full Stokes profile data taken by the NSO/ASP
to determine the velocity vectors as well as magnetic field vectors.
Title: Coronal Loops Heated by Magnetohydrodynamic Turbulence. I. A
Model of Isobaric Quiet-Sun Loops with Constant Cross Sections
Authors: Chae, Jongchul; Poland, Arthur I.; Aschwanden, Markus J.
Bibcode: 2002ApJ...581..726C
Altcode:
Several recent papers have presented new observational results
indicating that many coronal loops in active regions are nearly
isothermal. It is expected that quiet-Sun loops may have similar thermal
structures, since quiet-Sun differential emission measures look similar
to those in active regions. In the quiet Sun, it is well known from
observations that the nonthermal velocity inferred from the excess
broadening of a line over thermal broadening reaches a peak of about
30 km s-1 around 3×105 K and then decreases
with temperature, having a value of about 20 km s-1 at
1×106 K. In the present work, we make the assumption
that the observed nonthermal velocities are a manifestation of
magnetohydrodynamic (MHD) turbulence and present a model of static,
isobaric coronal loops heated by turbulence. Instead of solving the
MHD equations, we adopt simple energy spectra in MHD turbulence and
infer the heating rate as a function of temperature from the observed
nonthermal velocities. By solving the steady state energy equation
of a loop in which temperature monotonically increases with height,
we obtain the following results: (1) The heating rate is predominantly
near the footpoints and decreases with the loop arc length. (2) There
is a critical temperature above which the loop cannot be maintained
in a steady state. (3) The loop is denser and is more isothermal than
uniformly heated loops, being compatible with recent observations. (4)
The theoretical differential emission measures are in good agreement
with the empirical values at temperatures above 105 K. Below
this temperature, we still have a large discrepancy. (5) It is possible
to explain the observed strong correlation between intensity and
nonthermal velocity of a spectral line in the quiet Sun. Our results
support the idea that quasi-statically driven MHD turbulence of the
direct current (DC) type in the stratified medium (transition region
and corona) is a viable mechanism for coronal heating.
Title: Impulsive Variations of the Magnetic Helicity Change Rate
Associated with Eruptive Flares
Authors: Moon, Y. -J.; Chae, Jongchul; Wang, Haimin; Choe, G. S.;
Park, Y. D.
Bibcode: 2002ApJ...580..528M
Altcode:
In this paper, we investigate impulsive variations of the magnetic
helicity change rate associated with eruptive solar flares (three
X class flares and one M class flare) accompanying halo coronal
mass ejections. By analyzing four sets of 1 minute cadence full-disk
magnetograms taken by the Michelson Doppler Imager on board the Solar
and Heliospheric Observatory, we have determined the rates of magnetic
helicity transport due to horizontal photospheric motions. We have found
that magnetic helicity of the order of 1041 Mx2
was impulsively injected into the corona around the flaring peak time
of all the flares. We also found that there is a positive correlation
between the impulsively injected magnetic helicity and the X-ray peak
flux of the associated flare. The impulsive helicity variations are
attributed to horizontal velocity kernels localized near the polarity
inversion lines. Finally, we report that there is a close spatial
proximity between the horizontal velocity kernels and Hα bright points.
Title: Flare Activity and Magnetic Helicity Injection by Photospheric
Horizontal Motions
Authors: Moon, Y. -J.; Chae, Jongchul; Choe, G. S.; Wang, Haimin;
Park, Y. D.; Yun, H. S.; Yurchyshyn, Vasyl; Goode, Philip R.
Bibcode: 2002ApJ...574.1066M
Altcode:
We present observational evidence that the occurrence of homologous
flares in an active region is physically related to the injection of
magnetic helicity by horizontal photospheric motions. We have analyzed
a set of 1 minute cadence magnetograms of NOAA AR 8100 taken over a
period of 6.5 hr by the Michelson Doppler Imager on board the Solar
and Heliospheric Observatory. During this observing time span, seven
homologous flares took place in the active region. We have computed
the magnetic helicity injection rate into the solar atmosphere by
photospheric shearing motions and found that a significant amount of
magnetic helicity was injected during the observing period. In a strong
M4.1 flare, the magnetic helicity injection rate impulsively increased
and peaked at the same time as the X-ray flux. The flare X-ray flux
integrated over the X-ray emission time strongly correlates with the
magnetic helicity injected during the flaring interval. The integrated
X-ray flux is found to be a logarithmically increasing function of
the injected magnetic helicity. Our results suggest that injection of
helicity and abrupt increase of helicity magnitude play a significant
role in flare triggering.
Title: Statistical Evidence for Sympathetic Flares
Authors: Moon, Y. -J.; Choe, G. S.; Park, Y. D.; Wang, Haimin;
Gallagher, Peter T.; Chae, Jongchul; Yun, H. S.; Goode, Philip R.
Bibcode: 2002ApJ...574..434M
Altcode:
Sympathetic flares are a pair of flares that occur almost simultaneously
in different active regions, not by chance, but because of some
physical connection. In this paper statistical evidence for the
existence of sympathetic flares is presented. From GOES X-ray flare
data, we have collected 48 pairs of near simultaneous flares whose
positional information and Yohkoh soft X-ray telescope images
are available. To select the active regions that probably have
sympathetic flares, we have estimated the ratio R of actual flaring
overlap time to random-coincidence overlap time for 38 active region
pairs. We have then compared the waiting-time distributions for the
two different groups of active region pairs (R>1 and R<1) with
corresponding nonstationary Poisson distributions. As a result, we
find a remarkable overabundance of short waiting times for the group
with R>1. This is the first time such strong statistical evidence
has been found for the existence of sympathetic flares. To examine
the role of interconnecting coronal loops, we have also conducted
the same analysis for two subgroups of the R>1 group: one with
interconnecting X-ray loops and the other without. We do not find any
statistical evidence that the subgroup with interconnecting coronal
loops is more likely to produce sympathetic flares than the subgroup
without. For the subgroup with loops, we find that sympathetic flares
favor active region pairs with transequatorial loops.
Title: The Origin of Mass, Magnetic Flux, and Magnetic Helicity in
a Solar Prominence
Authors: Chae, J.; Yun, H. S.; Moon, Y. -J.; Wang, H.; Goode, P.
Bibcode: 2002AAS...200.3715C
Altcode: 2002BAAS...34..697C
Solar prominences are cloud-like cool and dense plasma supported by
highly non-potential, and very likely twisted magnetic fields in the
corona against the gravity. Therefore, the supply of mass, magnetic
flux, and magnetic helicity is the necessary condition for the formation
of solar prominences. We have been doing a series of case studies on
the formation of a prominence in active region NOAA 8668. As a result,
we have found a series of jets in Hα and EUV at the region where
significant amounts of magnetic fluxes of opposite polarity canceled
each other. We also found the existence of non-differential-rotation
photospheric shearing motion that prevailed during the period of
prominence formation. Both the magnetic flux associated with flux
cancellation, and magnetic helicity injected by the shearing motion
were more than enough for the formation of the prominence. Our results
support that chromospheric magnetic reconnection is in charge of
supplying mass and magnetic flux, and photospheric shearing motion
is in charge of supplying magnetic helicity. This work was supported
by the US-Korea Cooperative Science Program (KOSEF 995-0200-002-2,
NSF INT-98-16267), the NASA grant NAG 5-10894 and the the National
Research Laboratory project M10104000059-01J000002500 and the BK21
project of the Korean Government.
Title: Magnetic Helicity Injection By Transient Photospheric Shear
Flows in Flares and Prominences
Authors: Yun, H. S.; Chae, J.; Moon, Y. -J.; Wang, H.; Goode, P. R.
Bibcode: 2002AAS...200.3716Y
Altcode: 2002BAAS...34..697Y
There has been increasing interest in magnetic helicity since it is
a well-conserved measure of the non-potentiality of solar magnetic
fields. Magnetic helicity in the solar corona may change when magnetic
helicity is transported either across the photosphere or out of the
Sun. Of our specific interest is the magnetic helicity change due to
transient shear flows in the photosphere. We have developed a method
of determining its rate from a time series of magnetograms, and have
applied it to several active regions. We present one example showing
that transient shear flows may inject significant amount of magnetic
helicity for several days during the formation of a prominence,
and other examples showing that magnetic helicity injection by
shear flows may occur impulsively during strong flares. We do not
know the physical nature of transient shear flows, but there is a
possibility that they may represent the photospheric velocity field
of low frequency, large-scale torsional Alfvén waves passing through
the solar surface. This work was supported by the US-Korea Cooperative
Science Program (KOSEF 995-0200-002-2, NSF INT-98-16267), and the the
National Research Laboratory project M10104000059-01J000002500 and
the BK21 project of the Korean Government.
Title: Flux Cancellation Rates and Converging Speeds of Canceling
Magnetic Features
Authors: Chae, Jongchul; Moon, Yong-Jae; Wang, Haimin; Yun, H. S.
Bibcode: 2002SoPh..207...73C
Altcode:
Canceling magnetic features are commonly believed to result from
magnetic reconnection in the low atmosphere. According to the
Sweet-Parker type reconnection model, the rate of flux cancellation
in a canceling magnetic feature is related to the converging speed of
each pole. To test this prediction observationally, we have analyzed
the time variation of two canceling magnetic features in detail using
the high-resolution magnetograms taken by the Michelson Doppler Imager
(MDI) on the Solar and Heliospheric Observatory (SOHO). As a result,
we have obtained the rate and converging speed of flux cancellation
in each feature: 1.3×1018 Mx hr−1 (or
1.1×106 G cm s−1 per unit contact length) and
0.35 km s−1 in the smaller one, and 3.5×1018
Mx hr−1 (1.2×106 G cm s−1) and
0.27 km s−1 in the bigger one. The observed speeds are
found to be significantly bigger than the theoretically expected ones,
but this discrepancy can be resolved if uncertainty factors such as
low area filling factor of magnetic flux and low electric conductivity
are taken into account.
Title: Flare Activity and Magnetic Helicity Injection By Photospheric
Horizontal Motions
Authors: Moon, Y. -J.; Chae, J.; Choe, G.; Wang, H.; Park, Y. D.;
Yun, H. S.; Yurchyshyn, V.; Goode, P. R.
Bibcode: 2002AAS...200.2002M
Altcode: 2002BAAS...34..673M
We present observational evidence that the occurrence of homologous
flares in an active region is physically related to the injection
of magnetic helicity by horizontal photospheric motions. We have
analyzed a set of 1 minute cadence magnetograms of NOAA AR 8100 taken
over a period of 6.5 hours by Michelson Doppler Imager (MDI) on board
Solar and Heliospheric Observatory (SOHO). During this observing time
span, seven homologous flares took place in the active region. We have
computed the magnetic helicity injection rate into the solar atmosphere
by photospheric shearing motions, and found that a significant amount
of magnetic helicity was injected during the observing period. In a
strong M4.1 flare, the magnetic helicity injection rate impulsively
increased and peaked at the same time as the X-ray flux did. The flare
X-ray flux integrated over the X-ray emission time strongly correlates
with the magnetic helicity injected during the flaring interval. The
integrated X-ray flux is found to be a logarithmically increasing
function of the injected magnetic helicity. Our results suggest that
injection of helicity and abrupt increase of helicity magnitude play
a significant role in flare triggering. This work has been supported
by NASA grants NAG5-10894 and NAG5-7837, by MURI grant of AFOSR, by
the US-Korea Cooperative Science Program (NSF INT-98-16267), by NRL
M10104000059-01J000002500 of the Korean government, and by the BK 21
project of the Korean government.
Title: Temperatures of Extreme-Ultraviolet-emitting Plasma Structures
Observed by the Transition Region and Coronal Explorer
Authors: Chae, Jongchul; Park, Young-Deuk; Moon, Yong-Jae; Wang,
Haimin; Yun, H. S.
Bibcode: 2002ApJ...567L.159C
Altcode:
The Transition Region and Coronal Explorer has revealed, in
unprecedented detail, various kinds of EUV-emitting plasma structures
in the solar upper atmosphere. The filter ratio 195 Å/171 Å has been
conventionally used to determine the plasma temperatures, but this
method has a shortcoming in that it may not yield a unique temperature
value for a given ratio. Therefore, we introduce a new method employing
two filter ratios (195 Å/171 Å and 284 Å/195 Å). It is demonstrated
that this color-color method is effective in determining a wide range
of unambiguous plasma temperatures. We have obtained a temperature of
1×106 K for a loop that is bright in 171 Å but hardly
visible in 284 Å, a higher temperature of 2×106 K for
a loop that is clearly visible in 195 and 284 Å but not in 171 Å,
and a transition-region temperature of 2.5×105 K for a
low-lying loop that is clearly visible in all the EUV wavelengths. In
addition, we have found that ``moss'' structures have temperatures
of around 1×106 K and that EUV jets have temperatures of
about 2.5×105 K.
Title: Chromospheric Magnetic Reconnection on the Sun
Authors: Chae, Jongchul; Choi, Byung-Kyu; Park, Min-Ju
Bibcode: 2002JKAS...35...59C
Altcode:
No abstract at ADS
Title: Dip-Like Magnetic Field Structure Seen in Solar Prominences
Authors: Yun, H. S.; Lee, S.; Chae, J.; Kim, J. H.; Park, Y. D.; Kim,
S. E.; Goode, P. R.; Wang, H.
Bibcode: 2002stma.conf..103Y
Altcode:
No abstract at ADS
Title: Magnetic helicity injection associated with solar flares
Authors: Moon, Y.; Chae, J.; Wang, H.; Choe, G.; Park, Y.
Bibcode: 2002cosp...34E.471M
Altcode: 2002cosp.meetE.471M
We have examined the characteristics of magnetic helicity injections
associated with solar flares. We have analyzed a set of 1 minute
cadence magnetograms of NOAA AR 8100 taken over a period of 6.5 hours
by Michelson Doppler Imager (MDI) on board Solar and Heliospheric
Observatory (SOHO). During this observing time span, seven homologous
flares took place in the active region. We have computed the magnetic
helicity injection rate into the solar atmosphere by photospheric
shearing motions, and found that a significant amount of magnetic
helicity was injected during the observing period. In a strong M4.1
flare, the magnetic helicity injection rate impulsively increased and
peaked at the same time as the X-ray flux did. It is also noted that
the GOES X-ray flux integrated over the X-ray emission time strongly
correlates with the magnetic helicity injected during the flaring
interval.
Title: Inter-Active Region Connection of Sympathetic Flaring on 2000
February 17
Authors: Wang, Haimin; Chae, Jongchul; Yurchyshyn, Vasyl; Yang, Guo;
Steinegger, Michael; Goode, Philip
Bibcode: 2001ApJ...559.1171W
Altcode:
We have analyzed high-resolution Hα full disk data from Big
Bear Solar Observatory (BBSO); magnetograph and EUV data from the
Michelson Doppler Imager, Large Angle and Spectrometric Coronagraph,
and Extreme Ultraviolet Imaging Telescope on board SOHO; and Yohkoh
soft X-ray data of 2000 February 17. Two sympathetic M-class solar
flares erupted in succession in NOAA Active Region 8869 and 8872,
respectively. The eruption from AR 8872 was followed by an extremely
symmetric halo coronal mass ejection (CME). We demonstrate the loop
activation, which appears to be the consequence of the first flare in
AR 8869 and the cause of the second flare in AR 8872. The activation
started in the form of a surge just after a filament eruption and its
associated flare in AR 8869. The surge quickly turned into a set of
disturbances that propagated at a speed of about 80 km s-1
toward the other active region AR 8872. The second flare followed in
less than an hour after the arrival of the disturbances at AR 8872. The
moving disturbances appeared in absorption in both Hα and EUV 195
Å images. The disturbances may represent mass transfer, which had
a significant velocity component perpendicular to the field lines
and, hence, caused the transport of field lines. In this case, the
disturbances may be considered to be a special kind of surge, which we
may call a ``sweeping closed-loop surge.'' We also demonstrated large
area dimmings associated with the CME in three active regions. The
dimming started from AR 8869 and AR 8872 and was extended to AR 8870,
which was on the opposite side of the solar equator. We believe that
both the activation of inter-active region loops and the large-scale
dimming are the signatures of large-scale restructuring associated
with the CME.
Title: The Formation of a Prominence in Active Region NOAA
8668. I. SOHO/MDI Observations of Magnetic Field Evolution
Authors: Chae, Jongchul; Wang, Haimin; Qiu, Jiong; Goode, Philip R.;
Strous, Louis; Yun, H. S.
Bibcode: 2001ApJ...560..476C
Altcode:
We have studied the evolution of the photospheric magnetic field in
active region NOAA 8668 for 3 days while the formation of a reverse
S-shaped filament proceeded. From a set of full-disk line-of-sight
magnetograms taken by the Michelson Doppler Imager (MDI) on board
the Solar and Heliospheric Observatory (SOHO), we have found a
large canceling magnetic feature that was closely associated with
the formation of the filament. The positive flux of the magnetic
feature was initially 1.5×1021 Mx and exponentially
decreased with an e-folding time of 28 hr throughout the period of
observations. We also have determined the transverse velocities of
the magnetic flux concentrations in the active region by applying
local correlation tracking. As a result, a persistent pattern of shear
motion was identified in the neighborhood of the filament. The shear
motion had a speed of 0.2-0.5 km s-1 and fed negative
magnetic helicity of -3×1042 Mx2 into the
coronal volume during an observing run of 50 hr at an average rate
of -6×1040 Mx2 hr-1. This rate is
an order of magnitude higher than the rate of helicity change due to
the solar differential rotation. The magnetic flux of the field lines
created by magnetic reconnection and the magnetic helicity generated
by the photospheric shear motion are much more than enough for the
formation of the filament. Based on this result, we conjecture that the
filament formation may be the visible manifestation of the creation
of a much bigger magnetic structure that may consist of a flux rope
and an overlying sheared arcade.
Title: Are Non-thermal Motions in the Solar Corona Evidence for
Coronal Heating by MHD Turbulence?
Authors: Chae, Jongchul
Bibcode: 2001APS..DPPFM1005C
Altcode:
Recent solar observations suggest that the solar outer atmosphere is
turbulent. In the present talk, I focus on the SOHO/SUMER observations
of non-thermal motions --- unresolved plasma motions that are required
to explain the line width excess over thermal broadening --- of
UV/EUV emitting plasma whose temperature ranges from 10^4 to 10^6
K. It has been found from a thorough investigation of the quiet Sun
that the most probable speed of non-thermal motion depends on the
formation temperature of the spectral lines: it is smaller than 10 km
s-1 in the chromosphere (T<2 × 10^4 K), reaches a peak
value of 30 km s-1 in the transition region ( 3 × 10^5 K),
and decreases to about 20 km s-1 in the low corona (T>10^6
K). Non-thermal motions are essentially isotropic and remain unresolved
at a spatial resolution of 1000 km and at a temporal resolution of a
few tens of seconds. Moreover, a careful examination of profiles of the
S VI line formed at 2 × 10^5 K has revealed that a fraction of the
line-emitting plasma has non-thermal motions of 50 km s-1
or higher values. Non-thermal motions were also measured in each
of several spatially resolved loops in an active region. Some loops
have the most probable speed of 20 km s-1, others have a
much bigger value of 50 km s-1, suggesting that non-thermal
motion is an intrinsic property of a loop as temperature is. Non-thermal
motions may represent either one of or a combination of three different
kinds of physical processes: 1) a superposition of laminar flows along
very small magnetic loops, 2) MHD waves, especially Alfvénic waves,
and 3) MHD turbulence. Our results seem to be more compatible with the
last explanation than the others, and to support the coronal heating
by MHD turbulence.
Title: Observational Determination of the Rate of Magnetic Helicity
Transport through the Solar Surface via the Horizontal Motion of
Field Line Footpoints
Authors: Chae, Jongchul
Bibcode: 2001ApJ...560L..95C
Altcode:
Magnetic helicity may be transported to the solar corona through the
solar surface either via the passage of helical magnetic field lines
from below or via the shuffling of footpoints of preexisting coronal
field lines. In this Letter, we show how to observationally determine
the rate of magnetic helicity transport via photospheric footpoint
shuffling from a time series of line-of-sight magnetograms. Our
approach is not confined to the previously known shear motions,
such as differential rotation, but can be exploited to search for the
possible existence of physically significant shear motions other than
differential rotation. We have applied the method to a 40 hr run of
high-resolution magnetograms of a small active region (NOAA Active
Region 8011) taken by the Michelson Doppler Imager on board the Solar
and Heliospheric Observatory. In this region, we find that the rate
of magnetic helicity transport oscillates with periods of 1 to several
hours. Our result suggests that the time-series analysis of the helicity
transport rate might be a useful observational diagnostic for the role
of photospheric flows in the evolution of coronal magnetic fields in
solar active regions.
Title: A Steady Flow Model for the Differential Emission Measure in
the Solar Quiet Region
Authors: Bong, S.; Chae, J.; Yun, H.; Lee, J.
Bibcode: 2001AGUSM..SH41B21B
Altcode:
With high quality UV spectroscopy from the SoHO spacecraft, the
physical structure of the solar Transition Region (TR) is of renewed
interest. We have investigated the thermodynamic structure of the TR
using a one dimensional magnetic tube model constrained to Raymond
& Doyle's Differential Emission Measure (DEM) in the average quiet
sun. We have included the effect of the expansion of magnetic flux
tube and a heating which is required in addition to conductive heat,
convective energy and radiative cooling. From the resulting heating
and flux tube geometry, we also investigated upflows probable in the
transition region. To reproduce the Doppler shift of UV lines measured
using SoHO/SUMER (Chae, Yun, & Poland 1998), flux tube needs to
expand rapidly above T=105 K at a rate of radius increase
up to (7.4x 10-2 km-1)~ r4.1 where
r4.1 is the radius at log T = 4.1. To balance the energy,
an energy supply by more than (9.3x 104 erg cm-2
s-1)~π r4.12 is required at the region
between 1.3x 104 K and 2.5x 104 K regardless
of filling factor, suggesting a local heating in the chromosphere. As
for upflows, in subsonic flow cases, a model with the same additional
energy loss as in a downflow is probable. Also, supersonic flows could
be easily made and, in this case, supersonic upflows could carry extra
energy to corona without increasing DEM, showing the possibility that
upflows play a role in corona heating. This work was supported by
the Basic Science Research Institute Program, Ministry of Education
(BSRI-98-5408) and by the BK21 Project of the Korean Government.
Title: Inter-Active Region Connection of Sympathetic Flaring on 2000
February 17
Authors: Wang, H.; Chae, J.; Yurchyshyn, V.; Yang, G.; Steinegger,
M.; Goode, P. R.
Bibcode: 2001AGUSM..SP42A03W
Altcode:
We have analyzed high resolution Hα full disk data from Big Bear
Solar Observatory (BBSO), magnetograph and EUV data from MDI, LASCO
and EIT on board SOHO, and Yohkoh soft X-ray data of February 17,
2000. Two sympathetic M-class solar flares erupted in succession
in NOAA 8869 and 8872, respectively. The eruption from AR 8872 was
followed by an extremely symmetric halo CME. We demonstrate a new
kind of loop activation, which appears to be the consequence of
the first flare in AR 8869 and the cause of the second flare in
AR 8872. The activation started in the form of a surge just after
a filament eruption and its associated flare in AR 8869. The surge
quickly turned into a set of disturbances that propagated at a speed of
about 80 km/s toward the other active region AR 8872. The second flare
followed in less than an hour after the arrival of the disturbances
at AR 8872. The moving disturbances appeared in absorption in both
Hα and EUV 195A images. The disturbances may represent mass transfer
which had a significant velocity component perpendicular to the field
lines and, hence, caused the transport of field lines. In this case,
the disturbances may be considered to be a special kind of surge,
which we may call a "sweeping closed-loop surge". Alternatively, the
disturbances may represent fronts of compressive magnetohydrodynamic
waves that were driven by the surge. We also demonstrated large area
dimmings associated with the CME in three active regions. The dimming
started from ARs 8869 and 8872, and was extended to AR 8870 which was
on the opposite side of the solar equator. We believe that both the
activation of inter-active region loops and the large scale dimming are
the signatures of large scale re-structuring associated with the CME.
Title: Results from the Big Bear Solar Observatory's New Digital
Vector Magnetograph
Authors: Spirock, T. J.; Denker, C.; Varsik, J.; Shumko, S.; Qiu,
J.; Gallagher, P.; Chae, J.; Goode, P.; Wang, H.
Bibcode: 2001AGUSM..SP51B06S
Altcode:
During the past several years the Big Bear Solar Observatory has
been involved in an aggressive program to modernize the observatory's
instrumentation. At the forefront of this effort has been the upgrade
of the observatory's digital vector magnetograph (DVMG), which has been
recently integrated into the observatory's daily observing program. The
DVMG, which is mounted on the observatory's 25 cm vacuum refractor,
is a highly sensitive, high cadence magnetograph which studies the
FeI line at 630.1 nm. An easy to use GUI observing tool has been
written to aid instrument development and data acquisition. This
tool automatically calibrates the data and generates near real-time
vector magnetograms which will aid space weather forecasting and the
support of space weather missions. Also, our plan is to integrate the
DVMG data into the HESSI Synoptic Archive. The very sensitive quiet
Sun magnetograms, produced by the DVMG, will aid the study of small
scale magnetic reconnection at the intranetwork level and its possible
contribution to the coronal heating problem. Quiet sun longitudinal and
active region vector magnetograms will be presented. Image quality,
such as bias, cross-talk, noise levels and sensitivity, will be
discussed in addition to the improvements gained in post processing
such as image selection and image alignment.
Title: Small Magnetic Bipoles Emerging in a Filament Channel
Authors: Chae, Jongchul; Martin, Sara F.; Yun, H. S.; Kim, Junghoon;
Lee, Sangwoo; Goode, Philip R.; Spirock, Tom; Wang, Haimin
Bibcode: 2001ApJ...548..497C
Altcode:
Observations have shown that quiescent prominences or filaments have a
hemispheric magnetic pattern of chirality. Motivated by the question
of whether the filament chirality is of subsurface origin or not,
we have studied small magnetic bipoles that emerged in a quiescent
filament channel at latitude N45°. During our 5 day observing run,
performed in 1999 October, a huge filament erupted and reformed shortly
in the same filament channel. Using high-cadence, long-integration
line-of-sight magnetograms taken at Big Bear Solar Observatory, we
identified a total of 102 bipoles that showed an average total flux
of 2.8×1019 Mx, an average separation of 7400 km at the
time of full development, and an emergence rate of 430 hr-1
per the entire solar surface area. These properties indicate that most
of the bipoles are ephemeral regions. The most important finding in
the present study is that the magnetic axes of the bipoles emerging in
the filament channel are systematically oriented; a negative (trailing)
pole is observed to be located preferentially to the south-east of its
companion positive (leading) pole. This preferred orientation does not
match either the Hale law of active region orientation or a theory that
attributes the axial field of a filament to emerging bipoles. We propose
two possible subsurface field configurations of bipoles consistent with
the observed preferential orientation and discuss physical implications
of our results for understanding filament magnetic fields.
Title: The Big Bear Solar Observatory's Digital Vector Magnetograph
Authors: Spirock, T.; Denker, C.; Chen, H.; Chae, J.; Qiu, J.; Varsik,
J.; Wang, H.; Goode, P. R.; Marquette, W.
Bibcode: 2001ASPC..236...65S
Altcode: 2001aspt.conf...65S
No abstract at ADS
Title: A Rapid Change in Magnetic Connectivity Observed Before
Filament Eruption and Its Associated Flare
Authors: Kim, Jung-Hoon; Yun, H. S.; Lee, Sangwoo; Chae, Jongchul;
Goode, Philip R.; Wang, Haimin
Bibcode: 2001ApJ...547L..85K
Altcode:
To gain insight to the cause of filament eruptions and flares on
the Sun, we observed a filament that erupted in active region NOAA
8597. The observations consisted of Hα filtergrams at three wavelengths
(line center and +/-0.5 Å) and line-of-sight magnetograms. All
were taken on 1999 June 24 at Big Bear Solar Observatory. We found
from the time sequence of Hα images that the filament eruption was
preceded by a rapid change in connectivity in a bundle of filament
threads. The thread bundle was initially sharply curved near its one
end of the filament and suddenly flipped and then became straight in
the new orientation. The flipped segment of the thread bundle swept
over a 100''×50'' area on the solar surface in
about half an hour. At the latter stage of the connectivity change,
we observed a downward draining of material along the thread bundle
that had a transverse component of 50 km s-1. After that,
the filament body split into two parallel parts, one part erupted
while the other part remained, and the two-ribbon flare occurred. We
also found canceling magnetic features in the vicinity of the initial
location of the thread end, which displayed a flux decrease during the
Hα connectivity change. Our results show clear and direct evidence
that magnetic reconnection takes place in the low atmosphere prior to
eruption. This preeruption reconnection seems to be very different
from a posteruption coronal reconnection, which is believed to lead
to a two-ribbon flare.
Title: Dynamics of Quiet Sun Magnetic Fields
Authors: Denker, C.; Spirock, T.; Varsik, J. R.; Chae, J.; Marquette,
W. H.; Wang, H.; Goode, P. R.
Bibcode: 2001ASPC..236..463D
Altcode: 2001aspt.conf..463D
No abstract at ADS
Title: Dynamical Characteristics of Small-Scale Hα Upflow Events
on the Quiet Sun
Authors: Lee, Chik-Yin; Chae, Jongchul; Wang, Haimin
Bibcode: 2000ApJ...545.1124L
Altcode:
Studies of small-scale dynamical features on the quiet Sun are crucial
for understanding the energetics and mass supply in the chromosphere
and corona. Recent works by Wang and coworkers revealed the existence
of numerous short-lived upflow events in Hα on the quiet Sun. In
this paper we examine in detail the dynamical characteristics of these
Hα upflow events based on Hα spectrograph observations and compare
them with those of elongated dark mottles, which are often considered
to be the disk counterpart of limb spicules. Our observations were
performed at the Big Bear Solar Observatory on 1997 August 26 using
the Littrow Spectrograph on the 65 cm reflector. We repeated drift
scans of a quiet Sun region, which produced a four-dimensional (x, y,
t, λ) data array. We examine the spectral, temporal, and morphological
characteristics of upflow events and determine their physical parameters
like size, lifetime, birthrate, line-of-sight velocity, Doppler width,
and optical thickness. Our results show that upflow events appear
different from dark mottles in that (1) most of the line profiles of
upflow events show absorption in the blue wing only, while those of
dark mottles show absorptions in both wings, (2) the typical optical
thickness of upflow events is smaller than that of dark mottles, and
(3) unlike dark mottles, upflow events do not have returning flows at
the same site. We discuss possible physical relationships among upflow
events, elongated dark mottles, and spicules as observed at the limb.
Title: Steady Flow Model of a Magnetic Flux Tube Constrained to
Observed Differential Emission Measure
Authors: Bong, Su-Chan; Chae, Jongchul; Yun, Hong Sik
Bibcode: 2000JKAS...33..159B
Altcode:
We have investigated one dimensional steady flow model of a typical
magnetic flux tube in the solar transition region constrained
to observed Differential Emission Measure (DEM) for the average
quiet-Sun deduced by Raymond & Doyle (1981) with a flux tube
geometry conforming to Doppler shifts of UV lines measured by Chae,
Yun & Poland (1998). Because local heating and filling factor in the
transition region are not well known, we considered two extreme cases,
one characterized by the filling factor = 1 ("filled-up model") and
the other set by local heating = 0 ("not-heated model"). We examined
how much the heating is required for the flux tube by recomputing
a model through adjustment of the filling factor in such a way that
"not-heated model" accounts for the observed DEM.
Title: The Magnetic Helicity Sign of Filament Chirality
Authors: Chae, Jongchul
Bibcode: 2000ApJ...540L.115C
Altcode:
A solar prominence has either dextral or sinistral chirality depending
on its axial field direction. We determine the magnetic helicity
sign of filaments using high-resolution observations performed
by Transition Region And Coronal Explorer. At EUV wavelengths,
filaments sometimes appear as mixtures of bright threads and dark
threads. This characteristic has enabled us to discern overlying threads
and underlying ones and to determine the sign of magnetic helicity
based on the assumption that the helicity sign of two crossing thread
segments is the same as that of the filament. Our results support the
notion that dextral filaments have negative magnetic and that sinistral
filaments have positive helicity.
Title: High-Resolution Hα Observations of Proper Motion in NOAA 8668:
Evidence for Filament Mass Injection by Chromospheric Reconnection
Authors: Chae, Jongchul; Denker, Carsten; Spirock, Tom J.; Wang,
Haimin; Goode, Philip R.
Bibcode: 2000SoPh..195..333C
Altcode:
There have been two different kinds of explanations for the source
of cool material in prominences or filaments: coronal condensations
from above and cool plasma injections from below. In this paper, we
present observational results which support filament mass injection
by chromospheric reconnection. The observations of an active filament
in the active region NOAA 8668 were performed on 17 August 1999 at a
wavelength of Hα−0.6 Å using the 65 cm vacuum reflector, a Zeiss Hα
birefringent filter, and a 12-bit SMD digital camera of Big Bear Solar
Observatory. The best image was selected every 12 s for an hour based
on a frame selection algorithm. All the images were then co-aligned and
corrected for local distortion due to the seeing. The time-lapse movie
of the data shows that the filament was undergoing ceaseless motion. The
Hα flow field has been determined as a function of time using local
correlation tracking. Time-averaged flow patterns usually trace local
magnetic field lines, as inferred from Hα fibrils and line-of-sight
magnetograms. An interesting finding is a transient flow field in a
system of small Hα loops, some of which merge into the filament. The
flow is associated with a cancelling magnetic feature which is located
at one end of the loop system. Initially a diverging flow with speeds
below 10 km s−1 is visible at the flux cancellation
site. The flow is soon directed along the loops and accelerated up
to 40 km s−1 in a few minutes. Some part of the plasma
flow then merges into and moves along the filament. This kind of
transient flow takes place several times during the observations. Our
results clearly demonstrate that reconnection in the photosphere and
chromosphere is a likely way to supply cool material to a filament,
as well as re-organizing the magnetic field configuration, and, hence,
is important in the formation of filaments.
Title: Extreme-Ultraviolet Flare Loop Emissions in an Eruptive Event
Authors: Qiu, Jiong; Wang, Haimin; Chae, Jongchul; Goode, Philip R.
Bibcode: 2000SoPh..194..269Q
Altcode:
The TRACE/BBSO joint campaign on 27 September 1998 observed an eruptive
flare event which lasted for half an hour. The observation covered
several ultraviolet (UV) and extreme-ultraviolet (EUV) lines and Hα
center and off-band emissions with very high spatial resolution. We find
the EUV emissions in different stages of the flare display different
characteristics. (1) During the `pre-flare' phase, when the SXR output
was weak, we observed simultaneous impulsive HXR peak at 25-100
keV and strong EUV emission. (2) In the impulsive phase, when Hα,
UV and SXR emissions were rising to the maxima, the EUV emission was
very weak. (3) During the main phase, when SXR emission was decaying,
a peak in the EUV emission was observed which was substantially delayed
by 7 min compared to emissions from other wavelengths. Based on our
observations, we propose that the `pre-flare' phase in this event was
a separate energy release process rather than a mere pre-cursor of the
flare, and it is likely that the `pre-flare' EUV emission was due to
weak in situ heating of low-lying coronal loops. The mechanism of the
EUV emission in the main phase is investigated. It is suggested that
the delayed EUV emission may come from cooling of SXR loops.
Title: Orientation of Emerging Bipoles in a Filament Channel
Authors: Chae, J.; Goode, P. R.; Spirock, T. J.; Wang, H.; Martin,
S. F.; Yun, H. S.; Kim, Jung-Hoon; Lee, Sangwoo
Bibcode: 2000SPD....31.0405C
Altcode: 2000BAAS...32R.835C
Observations have shown that quiescent prominences or filaments have
a hemispheric magnetic pattern of chirality or handedness. Motivated
by the question of whether the filament chirality is of sub-surface
origin or not, we have studied magnetic bipoles emerging in a quiescent
filament channel at latitude N45° . During our 5 day observing run
performed in 1999 October, a huge filament erupted and another began to
form in the same filament channel. Using high cadence deep line-of-sight
magnetograms, we identified a total of 102 small emerging bipoles,
which display the following statistical properties: 1) an average flux
of 1.2x 1019 Mx and an average separation of 7200 km; 2) an
inferred global emergence frequency of 600 hr-1 all over the
solar surface; and 3) a preferred orientation that a negative (trailing)
pole is located at the south-east of the companion positive (leading)
pole. The majority of the bipoles appear to be ephemeral regions which
are systematically smaller than those previously studied with Kitt
Peak full disk daily magnetograms. The preferred orientation of these
bipoles differs greatly from both the filament axial field direction
and the active region polarity law. We conclude that factors other than
the Hale polarity law are the cause of asymmetry in the orientation of
small bipoles having total magnetic fluxes below 2 x 1019 Mx.
Title: Observational Evidence for Magnetic Dips in Solar Prominences
Authors: Lee, Sangwoo; Yun, H. S.; Kim, Jung-Hoon; Chae, J.; Goode,
P. R.; Choe, G. S.
Bibcode: 2000SPD....31.0148L
Altcode: 2000BAAS...32R.809L
It has been a long-standing mystery in the study of the Sun how cool and
dense plasma material in prominences can be supported against gravity. A
common wisdom has been to assume magnetic field configurations with
magnetic dips which supply an upward magnetic tension. A number of
proposed theoretical models have regarded magnetic dips as a supporting
mechanism of the dense plasma material. Nevertheless, the existence
of magnetic dips in prominences has a scant observational underpinning
mainly because of the difficulty in determining 3-D magnetic fields in
prominences. For the first time, we report observational evidence for
magnetic dips based on the mass motion seen in prominences. We have
found an oscillatory overshooting out of a prominence body, which is
very naturally explained as mass motion along dipped magnetic field
lines sagging under gravity.
Title: A Rapid Magnetic Connectivity Change Observed Before a
Filament Eruption
Authors: Kim, Jung-Hoon; Yun, H. S.; Lee, Sangwoo; Chae, J.; Goode,
P. R.
Bibcode: 2000SPD....31.0267K
Altcode: 2000BAAS...32Q.823K
It has been a mystery what causes a solar prominence eruption. To get an
insight on this problem, we observed an active region filament eruption
in Hα lines (line center, +/- 0.5 { Angstroms}) and longitudinal
magnetograms at Big Bear Solar Observatory. The cadence is about 1
minute each for Hα and magnetogram. During 8 hours of observing run,
we covered complete phases of the filament eruption and an associated
two-ribbon flare. From Hα movies, we found a rapid connectivity
change in a filament thread before its eruption. The thread was
initially highly curved. It suddenly became straightened due to the
rotation of its end. The duration of change was about 30 minutes and
the rotated thread swept 100{\arcsec}x50{\arcsec} area on the solar
surface. After this connectivity change, the filament body split into
two parts, one part erupted, and the two-ribbon flare occurred. Being
associated with the connectivity change, transverse mass motion of about
50 km/s was observed in Hα along the newly connected field lines, and
a cancelling magnetic feature was observed in the vicinity where the
connectivity changed. Flux cancellation began before the connectivity
change, and continued while the connectivity change proceeded. Within 2
hours, the flux decreased by the amount of about 2.0x 1020
Mx. These observational results clearly give direct evidence that
magnetic reconnection takes place prior to eruption. This reconnection
is distinct from the post-eruption reconnection which is believed to
lead to the two-ribbon flare.
Title: Active Region Loops Observed with SUMER on Board the SOHO
Authors: Chae, Jongchul; Wang, Haimin; Qiu, Jiong; Goode, Philip R.;
Wilhelm, Klaus
Bibcode: 2000ApJ...533..535C
Altcode:
We study the emission and dynamical characteristics of transition region
temperature plasmas in magnetic loops by analyzing a high-resolution,
limb observation of the active region NOAA 7962. The observations were
performed by the Solar Ultraviolet Measurements of Emitted Radiation
(SUMER) instrument on board the Solar and Heliospheric Observatory
(SOHO). The SUMER observation produced a set of raster scans of the
region, in the four lines, H I Lyβ λ1025, O VI λλ1032, 1038,
and C II λ1037. The data are used to construct intensity, velocity,
and line width maps of the active region, from which more than 10
well-resolved loops are identified and classified into four different
groups. We determine several physical parameters of the loops in each
group such as diameter, length, temperature, line-of-sight plasma
velocity, and nonthermal line broadening. Our results indicate that
both kinds of temperature variations exist in active region loops:
variations from loop to loop and variations along each loop. It is
also found that there is a distinction between stationary loops and
dynamic loops. The dynamic loops have large bulk motions and large
nonthermal line broadenings. Some of the dynamic loops display large
velocity shears with the sign of line-of-sight velocities changing
across the loop axes. These velocity shears appear to represent
rotational motions around the loop axes with velocities of up to 50 km
s-1. There are indications that nonthermal line broadening
is the result of magnetohydrodynamic turbulence inside the loops. Based
on our observations, we postulate that when loops erupt, some of the
kinetic and magnetic energy cascades down to turbulent energy which
would be dissipated as heat.
Title: Nonpotential Broadening of UV Lines Observed at the Limb of
the Quiet Sun
Authors: Lee, Hyunsook; Yun, Hong Sik; Chae, Jongchul
Bibcode: 2000JKAS...33...57L
Altcode:
We have done a spectroscopic study of the solar transition region
using high resolution UV & EUV data obtained by SUMER) on board
SOHO. Optically thin and conspicuous emission lines observed at the
solar limb are carefully selected to acquire average values of physical
parameters for the quiet region as a function of radial distance. Our
main results found from the present study can be summarized as
follows. 1) Nonthermal velocities estimated from various UV lines
do not decrease with height at least within one total line intensity
scale height above the limb. 2) Nonthermal velocity distribution with
temperature is very similar to that of the disk center, in the sense
that its peak is located around 2E+5 K, but the value is systematically
larger than that of the disk. 3) It is found that nonthermal velocity
is inversely proportional to quadratic root of electron density up to
about 10 arc seconds above the limb, i.e. ~ Ne**(-1/4), implying that
the observed nonthermal broadening can be attributed to Alfven waves
passing through the medium. 4) Electron density estimated from the O
V 629/760 line ratio is found to range from about 1e+10 cm-3 to 2e+10
cm-3 in the transition region.
Title: Comparison of Transient Network Brightenings and Explosive
Events in the Solar Transition Region
Authors: Chae, Jongchul; Wang, Haimin; Goode, Philip R.; Fludra,
Andrzej; Schühle, Udo
Bibcode: 2000ApJ...528L.119C
Altcode:
The relation between transient network brightenings, known as blinkers,
and explosive events is examined based on coordinated quiet Sun
observations in the transition region line O V λ630 recorded by
the Coronal Diagnostic Spectrometer (CDS), in the transition region
line Si IV λ1402 recorded by the Solar Ultraviolet Measurements of
Emitted Radiation (SUMER) instrument, and in photospheric magnetograms
taken by the Big Bear Solar Observatory videomagnetograph. From these
observations, we find that (1) explosive events, which are traditionally
defined as features with very broad UV line profiles, tend to keep
away from the centers of network brightenings and are mostly located
at the edges of such brightenings, (2) CDS blinkers consist of many
small-scale, short-lived SUMER ``unit brightening events'' with a
size of a few arcseconds and a lifetime of a few minutes, and most
importantly (3) each SUMER unit brightening event is characterized by
a UV line profile that is not as broad as those of explosive events,
but still has significantly enhanced wings. Our results imply that,
like explosive events, individual unit brightening events involve high
velocities, and, hence, blinkers may have the same physical origin
as explosive events. It is likely that transient network brightenings
and explosive events are both due to magnetic reconnection--but with
different magnetic geometries.
Title: Counter-streaming Mass Flow and Transient Brightening in
Active Region Loops
Authors: Qiu, Jiong; Wang, Haimin; Chae, Jongchul; Goode, Philip R.
Bibcode: 1999SoPh..190..153Q
Altcode:
An active region loop system was observed in a decaying active region
for three hours by TRACE and BBSO in a joint campaign on September
27, 1998. Continuous mass motion was seen in Hα offband filtergrams
throughout the three hours, and some UV loops were exhibited transient
brightenings. We find that: (1) cool material was flowing along the
loops at a speed of at least 20 km s−1. Further, in Hα red
and blue wings, we see mass motion along different loops in opposite
directions. This is the first report of a counter-streaming pattern
of mass motion in an Hα loop system. (2) Transient brightenings
of different UV loops at different times were observed at C iv 1550
Å. These brightened UV loops were located in the same region and at
the same altitudes as the Hα loops. The observations show a clear
correlation between the transient brightenings of UV loops and mass
motion in Hα loops. (3) Both footpoints of the loop system were
located in regions of mixed magnetic polarities. Frequent micro-flares
at one footpoint of the loops with small-scale brightenings spreading
along the loop leg were observed before the brightening and rising
of one C iv loop. Similar to the case of a filament, the continuous
mass motion along the loops seems important for maintaining the cool
Hα loop system at coronal height. There may be an indication that the
mass motion in cool Hα loops and the correlated transient brightening
of the active region loops were due to the small-scale chromospheric
magnetic reconnection at the footpoint regions of the loop system.
Title: Non-Coplanar Magnetic Reconnection as a Magnetic Twist Origin
Authors: Chae, Jongchul
Bibcode: 1999JKAS...32..137C
Altcode:
Recent studies show the importance of understanding three-dimensional
magnetic reconnection on the solar surface. For this purpose, I consider
non-coplanar magnetic reconnection, a simple case of three-dimensional
reconnection driven by a collision of two straight flux tubes which
are not on the same plane initially. The relative angle theta between
the two tubes characterizes such reconnection, and can be regarded
as a measure of magnetic shear. The observable characteristics of
non-coplanar reconnection are compared between the two cases of small
and large angles. An important feature of the non-coplanar reconnection
is that magnetic twist can be produced via the re-ordering of field
lines. This is a consequence of the conversion of mutual helicity
into self helicities by reconnection. It is shown that the principle
of energy conservation when combined with the production of magnetic
twist puts a low limit on the relative angle between two flux tubes for
reconnection to occur. I provide several observations supporting the
magnetic twist generation by reconnection, and discuss its physical
implications for the origin of magnetic twist on the solar surface
and the problem of coronal heating.
Title: Small-Scale Dynamics on the Quiet Sun
Authors: Wang, H.; Chae, J.; Lee, C. -Y.
Bibcode: 1999ESASP.446..695W
Altcode: 1999soho....8..695W
No abstract at ADS
Title: Studies of Microflares and C5.2 flare of 27 September 1998
Authors: Wang, Haimin; Chae, Jongchul; Qiu, Jiong; Lee, Chik-Yin;
Goode, Philip R.
Bibcode: 1999SoPh..188..365W
Altcode:
On 27 September 1998, Big Bear Solar Observatory (BBSO) and Transition
Region and Coronal Explorer (TRACE) coordinated observations from
16:00 to 19:00 UT to study properties of microflares in AR NOAA
No. 8340. Fortuitously, a C5.2 flare occurred at 16:30 UT in this active
region. Hα and magnetograph movies were obtained at BBSO; C iv 1550
Å, Fe ix 171 Å, and Fe xii 195 Å movies were obtained by TRACE;
both with a cadence about 1 min. In this paper, we concentrate on the
study of magnetic properties of 70 C iv microflares, as well as their
relationship to the C5.2 flare. We obtained the following results: (1)
We found two kinds of microflares: microflares of transient brightenings
with a time scale of 1 to 5 min (impulsive events) and microflares
lasting half an hour or longer (persistent events). Ninety percent of
the microflares are impulsive events. Most of the event in this category
are associated with well defined magnetic neutral lines, but some are
found in non-neutral line areas. All of seven persistent events are
found at parasitic magnetic configurations with inclusions of small
magnetic flux within dominant magnetic flux of opposite polarity. (2)
More than a third of the impulsive microflares occurred near the C5.2
flare site indicating that a local instability is responsible for both
the C5.2 flare and microflares. This indirectly supports the avalanche
theory of flare energy release, which implies that a big flare may be
spatially associated with many small flares.
Title: Extreme-Ultraviolet Jets and Hα Surges in Solar Microflares
Authors: Chae, J.; Qiu, J.; Wang, H.; Goode, P. R.
Bibcode: 1999AAS...194.7906C
Altcode: 1999BAAS...31..963C
We analyzed simultaneous EUV data from the Transition Region And
Coronal Explorer (TRACE) and Hα data from Big Bear Solar Observatory
(BBSO). In the active region studied, we found several EUV jets that
repeatedly occurred where pre-existing magnetic flux was canceled by
newly emerging flux of opposite polarity. The jets look like, but are
usually smaller and shorter lived than Yohkoh soft X-ray jets. The EUV
jets have a typical size of 4000-10,000 km, a transverse velocity of
50-100 km s(-1) , and a lifetime of 2-4 minutes. Each of the jets was
ejected from a loop-like bright EUV emission patch at the moment when
the patch reached its peak emission. We also found dark Hα surges
that are correlated with these jets. A careful comparison, however,
revealed that the Hα jets are not cospatial with the EUV jets. Instead,
the EUV jets are are identified with bright jetlike features in the Hα
line center, which are distinct from dark surges. Our results support
a picture in which Hα surges and EUV jets represent different kinds
of plasma ejection --- cool and hot plasma ejections along different
field lines --- which must be dynamically connected to each other. We
emphasize the importance of observed flux cancellation and a small
erupting filament in understanding the acceleration mechanisms of EUV
jets and Hα surges. This work is partially supported by NSF under
grant ATM-97-14796 and NASA under grants NAG5-4919, NAG5-7349, and
NAG5-7350 to BBSO.
Title: Dynamical Characteristics of Hα Fine Features on the Quiet Sun
Authors: Lee, C. -Y.; Wang, H.; Chae, J.
Bibcode: 1999AAS...194.2306L
Altcode: 1999BAAS...31..861L
The Littrow spectrograph in the Big Bear Solar Observatory (BBSO) is an
unique grating system which can produce a 4-dimensional (x,y,lambda ,t)
data array. We are mainly interested in studying Hα fine features like
upflow events, spicules/fibrils, and dark grains by means of constructed
spectroheligrams. We have found from upflow events: (1) The typical
profile indicates a spectral character in which absorption is only
observed in the blue wing. (2) The line-center optical thickness
is lower than that of spicules. (3) The motion has no receding
phase as seen in Hα as shown by Doppler signals. The lifetime,
size, and velocity of upflow events are found to be 1.4 minutes,
2.5 arcsec (FWHM), and 15 km s(-1) respectively. Dark grains may be
considered to be the least energetic upflow events in the sense that
its lifetime and size are about half that of upflow events. The total
birthrate is estimated to be 200 s(-1) , which is about one third of
that of explosive events. The association of the magnetic origin of
explosive events has been reported by a previous study on the use of
BBSO/magnetograms and SOHO/SUMER UV images. The electron temperature
(10(4) K) and density (10(10) cm(-3) ) of upflow events can easily be
found. The global particle and kinetic energy flux are determined to be
10(15) cm(-2) s(-1) and 10(5) ergs cm(-2) s(-1) respectively. Judged by
the favorable amount of particle flux, we propose a scenario in that
the predominant downflow observed in UV is due to hot and diffusive
returning material of Hα upflow events. Comparisons with TRACE/EUV
images are in process.
Title: Modeling of Solar Transition Region Outflows
Authors: Poland, A. I.; Chae, J.
Bibcode: 1999AAS...194.9312P
Altcode: 1999BAAS...31..990P
The problem of observing outflow of material from the Sun has been an
interesting issue in solar physics for several decades. The problem is
that we know material flows from the Sun since there is a solar wind
measured at Earth and in interplanetary space. However, observations in
the chromosphere, transition region, and lower corona show mostly inflow
with almost no outflow. In a recent paper by Hassler and coworkers it
was shown that there is a significant outflow at network boundaries
as seen in NeVIII (Te 800,000K), but no significant outflows have been
observed at other temperatures. In our earlier work we demonstrated why
one would expect to see only inflows below 100,000K. The energy balance
between conduction, radiation, and enthalpy resulted in gradients
being too steep to observe outflowing material. We have extended that
work to 1,000,000K and included the observed effects of the spreading
magnetic field with temperature. These new calculation results seem
to be in agreement with the observations and indicate that outflows
should only be observable in the 800,000K range.
Title: Ultraviolet Flare Loop Systems in an Eruptive Event
Authors: Qiu, J.; Wang, H.; Chae, J.; Lee, C.; Goode, P. R.
Bibcode: 1999AAS...194.7904Q
Altcode: 1999BAAS...31..963Q
The BBSO/TRACE joint campaign on September 27, 1998 observed an
eruptive flare event which lasted for half an hour. The observation
covered several ultraviolet lines from transition region and Hα
line center and off-band emissions from chromosphere with high
spatial resolution which allows detailed study on the flare plasma
at wide temperature range. Various flare loop systems are found in
the active region depicting different scenarios of the event. The
flare was started by low atmosphere explosive reconnection which
triggered vehement loop eruption, heating of both large scale and
low-lying pre-existent loops, and mass ejection of both hot (up to
10(6) K) and cool (10(4) K) plasmas. Due to reconnection of open field
lines, new flaring loop archade was formed after the eruption and was
clearly seen in EUV emissions. From high resolution UV, EUV and Hα
filtergrams, fine structure of both low-lying and coronal loops can be
identified. The multi-wavelength observations on this event enable us
to distinguish different mechanisms of energy transfer in different
loop systems displaying different radiative and dynamic behaviours,
and to investigate the early stage of the flare when lower atmosphere
instability is essential in triggering the major flare and the relation
between lower and higher atmospheres is complex. This work is supported
by NSF under grants ATM-9628862 and ATM-9713359, and NASA under grants
NAG5-5036 and NAG5-7085, and ONR under grant N00014-97-1-1037.
Title: Studies of Microflares and C5.2 Flare of September 27, 1998
Authors: Wang, H.; Qiu, J.; Chae, J.; Lee, C.; Goode, P.
Bibcode: 1999AAS...194.7907W
Altcode: 1999BAAS...31..964W
On September 27, 1998, Big Bear Solar Observatory (BBSO) and Transition
Region and Coronal Explorer (TRACE) coordinated observations from
16:00 to 19:00 UT to study properties of microflares on AR NOAA
#8340. Fortuitously, a C5.2 flare occurred at 16:30UT in this active
region. Hα and magnetograph movies were obtained at BBSO; CIV 1550
Angstroms, FeIX 171 Angstroms, and FeXII 195 Angstroms movies were
obtained by TRACE; both with a cadence about 1 minute. In this
paper, we concentrate on the study of magnetic properties of 70
CIV microflares, as well as their relationship to the C5.2 flare. We
obtained the following results: (1) We found two kinds of microflares:
(a) microflares of transient brightenings with a time scale of
1 to 5 minutes (impulsive events). 90% of the microflares are of
this form. Most of the events in this category are associated with
well defined magnetic neutral lines, although a few of them are not
located near a neutral line; and (b) microflares lasting half an hour
or longer (persistent events). Seven events in three clusters belong to
this category. All three sites are associated with a dominant magnetic
polarity plus a very small element of opposite polarity. (2) More than
a third of the impulsive microflares occurred near the C5.2 flare site
indicating that a local instability is responsible for both the C5.2
flare and microflares. This indirectly support the avalanche theory of
flare energy release, which predicts that a big flare may be associated
with many small flares. Based on the morphologies of those events, we
postulate that the persistent events may be due to loop interaction;
while impulsive events may be associated with sheared loop structure.
Title: Extreme-Ultraviolet Jets and Hα Surges in Solar Microflares
Authors: Chae, Jongchul; Qiu, Jiong; Wang, Haimin; Goode, Philip R.
Bibcode: 1999ApJ...513L..75C
Altcode:
We analyzed simultaneous EUV data from the Transition Region and Coronal
Explorer and Hα data from Big Bear Solar Observatory. In the active
region studied, we found several EUV jets that repeatedly occurred where
pre-existing magnetic flux was ``canceled'' by newly emerging flux of
opposite polarity. The jets look like Yohkoh soft X-ray jets, but are
smaller and shorter lived than X-ray jets. They have a typical size
of 4000-10,000 km, a transverse velocity of 50-100 km s-1,
and a lifetime of 2-4 minutes. Each of the jets was ejected from a
looplike bright EUV emission patch at the moment that the patch reached
its peak emission. We also found dark Hα surges that are correlated
with these jets. A careful comparison, however, revealed that the Hα
surges are not cospatial with the EUV jets. Instead, the EUV jets are
identified with bright jetlike features in the Hα line center. Our
results support a picture in which Hα surges and EUV jets represent
different kinds of plasma ejection--cool and hot plasma ejections along
different field lines--which must be dynamically connected to each
other. We emphasize the importance of observed flux cancellation and
a small erupting filament in understanding the acceleration mechanisms
of EUV jets and Hα surges.
Title: Evidence for non-potential magnetic fields in the quiet Sun
Authors: Woodard, M. F.; Chae, Jongchul
Bibcode: 1999SoPh..184..239W
Altcode:
A comparison of BBSO Hα centerline filtergrams and videomagnetograms
was made to investigate the existence of non- potential magnetic fields
in the quiet Sun near magnetic network. We use the fibril structure
in the Hα images as a proxy for the horizontal chromospheric magnetic
field which we compare with the horizontal field obtained by potential
extrapolation of the observed, line-of-sight photospheric field. The
quiet-Sun field was found to be consistently and significantly
non-potential in each of the three fields of view studied. A
transient extreme ultraviolet (EUV) brightening, known as a blinker,
occurred during the observations of a region where the field is highly
non-potential, suggesting a connection between magnetic reconnection
and non-potentiality.
Title: Small-Scale Magnetic Reconnection in the Quiet Sun
Authors: Chae, J.
Bibcode: 1999ASPC..183..375C
Altcode: 1999hrsp.conf..375C
No abstract at ADS
Title: Stray-Light Effect on Magnetograph Observations
Authors: Chae, Jongchul; Yun, Hong Sik; Sakurai, Takashi; Ichimoto,
Kiyoshi
Bibcode: 1998SoPh..183..229C
Altcode:
To examine the stray-light effect in magnetograph observations, we
have determined the point spread functions of the vector magnetograph
mounted on the Japanese Solar Flare Telescope based on two indirect
methods, one analyzing the solar limb intensity profile, and the
other using the Fourier power spectra of photospheric intensity
distributions. Point spread functions consist of two parts: a blurring
part which describes seeing and small-spread-angle stray light, and
a scattering part which describes large-spread-angle stray light. The
FWHM spatial resolution is typically 3.0'', and the amount of scattered
light is about 15% on clear days. We find that the blurring part is
well described by a Moffat function whose Fourier transform is given
by an exponential function. Our results indicate that polarization
measurements of low-intensity magnetic elements like sunspots may be
significantly underestimated due to the large-spread-angle stray light,
and polarization measurements of magnetic elements which are smaller
than 5-7'' appear to be disturbed by small-spread-angle stray light.
Title: Stray-light correction in magnetograph observations using
the maximum entropy method
Authors: Chae, Jongchul; Yun, Hong Sik; Sakurai, Takashi; Ichimoto,
Kiyoshi
Bibcode: 1998SoPh..183..245C
Altcode:
We have developed a method of stray-light correction which is applicable
to filter-based magnetograph observations. Stray-light-corrected Stokes
images are obtained by performing the deconvolution of observed Stokes
images by the point spread function which is determined from the Stokes
I image. For image deconvolution, the maximum entropy principle is
used to guarantee that intensity should be positive and polarization
degrees should be less than unity. We present an iterative algorithm
for the maximum entropy method, which seeks the solution in Fourier
space and thus accomplishes fast convergence. We find that our method
is effective in correcting stray light which has a spread angle greater
than the full width at half maximum of the point spread function. We
also discuss the effect of stray light on magnetograph calibration.
Title: Comparison of Prominences in Hα and He II 304 Å
Authors: Wang, Haimin; Chae, Jongchul; Gurman, Joseph B.; Kucera,
Therese A.
Bibcode: 1998SoPh..183...91W
Altcode:
In this letter, we bring attention to prominences which show different
morphology in Hα and He ii 304 Å, as observed simultaneously by
BBSO and EIT on board SOHO. Those two lines have been thought to
represent similar chromospheric structures although they are formed at
significantly different temperatures. We give two examples representing
two kinds of anomaly: (1) prominences showing strong Hα emissions in
the lower part and strong He ii emissions in the upper part, and (2)
erupting prominences showing extensive He ii emission, but nothing in
Hα. Our results indicate that a part or the whole of a prominence may
be too hot to emit Hα radiation, possibly due to heating or thermal
instability. Please note that these are not just two isolated cases,
many other prominences show the similar differences in Hα and He ii
304 Å.
Title: SUMER Measurements of Nonthermal Motions: Constraints on
Coronal Heating Mechanisms
Authors: Chae, Jongchul; Schühle, Udo; Lemaire, Philippe
Bibcode: 1998ApJ...505..957C
Altcode:
We have determined nonthermal velocities in the quiet Sun at
temperatures between 104 K and 2 × 106 K by
measuring the widths of a number of EUV and far-ultraviolet (FUV)
lines taken with SUMER on board the SOHO spacecraft. The broadenings
owing to the SUMER instrument and the finite opacity in each line
have been carefully examined. The nonthermal velocity at temperatures
below 2 × 104 K is smaller than 10 km s-1. The
velocity increases with temperature, reaches a peak value of 30
km s-1 around 3 × 105 K, and then decreases
with the temperature. The coronal nonthermal velocity is about 20 km
s-1. There exists a strong correlation between intensity
and nonthermal velocity at temperatures 2 × 104-1 ×
105 K. The correlation at higher temperatures weakens as
temperature increases. Furthermore, there is a spatial correlation
between the nonthermal velocities inferred from a set of any two lines
with temperatures below 2 × 105 K. Neither significant
center-to-limb variation nor meaningful dependence on the integration
time was found from the measured nonthermal velocities. We have
discovered the existence of high-velocity components in the observed
S VI λ933.4 line profiles. The average nonthermal velocity and
intensity fraction of this S VI line high-velocity component are found
to be 55 km s-1 and 0.25, respectively. Observational
characteristics of nonthermal motions carry some problems that should be
solved when interpreting observed nonthermal motions in terms of either
unresolved loop flows or Alfvén waves. The isotropic and very small
scale nature of the observed nonthermal motions appears to be suited to
the MHD turbulence interpretation of nonthermal motions. The turbulent
heating rates inferred from the measured nonthermal motions can account
for the radiative loss throughout the transition region and corona if
the nonthermal motions are truly turbulent motions whose mechanical
energy is injected at a scale of 1000 km (Kolmogorov-type turbulence)
or 15 km (Kraichnan-type turbulence). The existence of high-velocity
components at temperatures 6 × 104-2 × 105
K appears as observational evidence supporting nanoflare heating at
these temperatures.
Title: Chromospheric Upflow Events Associated with Transition Region
Explosive Events
Authors: Chae, Jongchul; Wang, Haimin; Lee, Chik-Yin; Goode, Philip
R.; Schühle, Udo
Bibcode: 1998ApJ...504L.123C
Altcode:
Transition region explosive events are considered to be a manifestation
of small-scale magnetic reconnection ubiquitously occurring--even in
the quiet Sun. In this paper, we report a close association between
transition region explosive events and chromospheric upflow events seen
in Hα. From a comparison of the Big Bear Solar Observatory (BBSO)
Hα spectrograph data and the Solar and Heliospheric Observatory
(SOHO) / Solar Ultraviolet Measurements of Emitted Radiation (SUMER)
data, we found a succession of chromospheric upflow events at sites
where repeated explosive events occurred. Individual chromospheric
events appear as compact dark features that are best visible in
Hα-0.5 Å and that have a size of 2"-3" and a lifetime of 1-2
minutes. They are characterized by an upward motion of 15-30 km
s-1, a temperature of 104 K, a mass density
of 1×10-13 g, and a nonthermal velocity less than 10
km s-1. Unlike spicules, which display descending motion
following their ascending phase, these upflow events are not followed
by noticeable redshifts. ``Hα jets'' at -1.0 Å studied by Wang et
al. appear to be a special case of this kind of chromospheric upflow
event. The physical characteristics of chromospheric upflow events and
their close association with transition region explosive events suggest
that chromospheric upflow events may be the manifestation of cool plasma
material flowing into magnetically diffusive regions, while explosive
events represent hot plasma material flowing out of the same regions.
Title: Photospheric Magnetic Field Changes Associated with Transition
Region Explosive Events
Authors: Chae, Jongchul; Wang, Haimin; Lee, Chik-Yin; Goode, Philip
R.; Schühle, Udo
Bibcode: 1998ApJ...497L.109C
Altcode:
From a comparison of the Solar and Heliospheric Observatory SUMER
spectral data and a time series of Big Bear Solar Observatory
magnetograms, we present observational clues to the physical origin of
transition region explosive events. First, explosive events rarely occur
in the interior of strong magnetic flux concentrations but rather are
preferentially found in regions with weak and mixed polarity fluxes that
display magnetic neutral lines. Second, the majority of explosive events
happen during the ``cancellation'' of photospheric magnetic flux. Third,
there is a strong tendency for explosive events to occur repeatedly, as
bursts, while local photospheric magnetic flux continuously decreases
because of cancellation. These results strongly support the idea that
transition region explosive events are a manifestation of magnetic
reconnection occurring in the quiet Sun. Furthermore, one may infer from
the third result that the explosive events represent repetitive fast
magnetic reconnections in the transition region, which are initiated
by slow magnetic reconnections occurring beneath.
Title: Dynamical Characteristics of the Quiet Transition Region:
Spatial Correlation Studies of H I 931 and S VI 933 UV Lines
Authors: Yun, Hong Sik; Chae, Jong Chul; Poland, A. J.
Bibcode: 1998JKAS...31....1Y
Altcode:
To understand the basic physics underlying large spatial fluctuations
of intensity and Doppler shift, we have investigated the dynamical
charctersitics of the transition region of the quiet sun by analyzing
a raster scan of high resolution UV spectral band containing H
Lyman lines and a S VI line. The spectra were taken from a quiet
area of 100" X 100" located near the disk center by SUMER on board
SOHO. The spectral band ranges from 906 A to 950 A with spatial and
spectral resolution of 1" and 0.044 A, respectively. The parameters of
individual spectral lines were determined from a single Gaussian fit
to each spectral line. Then, spatial correlation analyses have been
made among the line parameters. Important findings emerged from the
present analysis are as follows. (1) The integrated intensity maps of
the observed area of H I 931 line (1 X 10^4 K) and S VI 933 line (2 X
10^5 K) look very similar to each other with the same characterstic
size of 5". An important difference, however, is that the intensity
ratio of brighter network regions to darker cell regions is much
larger in S VI 933 line than that in H I 931 line. (2) Dynamical
features represented by Doppler shifts and line widths are smaller
than those features seen in intensity maps. The features are found
to be changing rapidly with time within a time scale shorter than the
integration time, 110 seconds, while the intensity structure remains
nearly unchanged during the same time interval. (3) The line intensity
of S VI is quite strongly correlated with that of H I lines, but the
Doppler shift correlation between the two lines is not as strong as
the intensity correlation. The correlation length of the intensity
structure is found to be about 5.7"(4100 km), which is at least 3 times
larger than that of the velocity structure. These findings support the
notion that the basic unit of the transition region of the quiet sun
is a loop-like structure with a size of a few 10^3 km, within which
a number of unresolved smaller velocity structures are present.
Title: Temperature Dependence of Ultraviolet Line Average Doppler
Shifts in the Quiet Sun
Authors: Chae, J.; Yun, H. S.; Poland, A. I.
Bibcode: 1998ApJS..114..151C
Altcode:
The existence of prevailing redshifts in the UV lines formed in the
solar transition region raises an important question concerning its
physical origin and its role in the mass and energy balance of the
outer solar atmosphere. A series of UV spectral lines observed by
SUMER has been analyzed to obtain the spatial average of Doppler
shifts in the quiet Sun as a function of temperature. The UV lines
used for the analysis cover temperatures ranging from 104
to 106 K. The wavelength calibration has been done in
reference to the coolest chromospheric lines such as neutral lines
of silicon and sulfur. The positioning of the line center in blended
lines has been made by employing a constrained multi-Gaussian fitting
technique. The error in the measured average of the Doppler shifts is
estimated to be smaller than 1 km s-1. Our results show
that the average Doppler shift at the base of the transition region is
about 1-2 km s-1, increasing with temperature with a peak
value of 11 km s-1 near T = 2.3 × 105 K. Then
it decreases but remains still above zero (5 km s-1 in
Ne VIII lines and 4 km s-1 in Mg X lines). We find that
this behavior can be explained by the dominance of emission from
plasma flowing downward from the upper hot region to the lower cool
region along flux tubes with varying cross section. Assuming that
pressure and mass flux are constant along a flux tube, the cross
section of a typical flux tube has been estimated as a function of
temperature. It turns out that the cross section is nearly constant
below T = 105 K and then expands by a factor of about 30 at
T = 106 K. This behavior is fairly well represented by an
analytical functional form, A(T)/A(Th) = [1 + (Γ2
- 1)(T/Th)ν]1/2/Γ with parameters
of Th = 106 K, Γ = 30, and ν = 3.6.
Title: Effects of Non-LTE Radiative Loss and Partial Ionization on
the Structure of the Transition Region
Authors: Chae, J.; Yun, H. S.; Poland, A. I.
Bibcode: 1997ApJ...480..817C
Altcode:
In this paper we address the question of how non-LTE radiative losses
with partial ionization of hydrogen and helium affects the energetics
and structure of the solar transition region. To accomplish this we
have constructed theoretical models of a thin rigid magnetic flux
tube with a steady material flow, which is embedded vertically in
the solar atmosphere. These models include the effects of material
flow, conduction, non-LTE radiative transfer in H and He, and partial
ionization. We find from this study that the effect of non-LTE radiative
transfer with partial ionization is significant near the base of the
transition region at temperatures less than 2.5 × 104
K. This leads to a 1 order of magnitude increase in the differential
emission measure in comparison with the optically thin approximation
with complete ionization in the low (less than 2.5 × 104 K)
temperature regime. Above this region the non-LTE and opacity effects
are small. In the upflow case the conductive and convective energy
processes dominate to such a large extent that non-LTE radiative
process and partial ionization are not important. In this work
we also confirm the previous work of other authors who provided the
explanation for why downflowing transition region material is much
more visible than upflowing material. We present the results in a
manner that gives a good physical understanding as to why this occurs.
Title: Power Spectra of Solar Network and Non-Network Fields
Authors: Lee, Jeongwoo; Chae, J. -C.; Yun, H. S.; Zirin, H.
Bibcode: 1997SoPh..171..269L
Altcode: 1997SoPh..171..269J
We report new properties of solar magnetic fields in a quiet region
as found from their magnetic power spectra. The power spectra of
network and intranetwork fields (non-network fields) are separately
calculated from a Big Bear magnetogram obtained with moderately high
spatial resolution of 1.5 arc sec and a high sensitivity reaching 2
Mx cm-2. The effect of seeing on the power spectrum has
been corrected using Fried's (1966) Modulation Transfer Function
with the seeing parameter determined in our previous analysis of
the magnetogram. As a result, it is found that the two-dimensional
power spectra of network and non-network fields appear in a form: Γ(
≲ ≲ 1) ∼ -1 and Γ( ≳ 1)
∼ -3.5. Here 0 ≈ 0.47 Mm-1
for network fields and 0 ≈ 0.69 Mm-1 for
non-network fields, the latter of which corresponds to the size
of mesogranulation; 1 ≈ 3.0 Mm-1 for both,
which is about the size of a large granule. The network field spectrum
below 0 appears nearly flat, whereas that of non-network
fields instead decreases towards lower wave numbers as Γ( ) ∼
1.3. The turnover behavior of magnetic field spectra around
1 coincides with that found for the velocity power spectrum,
which may justify the kinetic approach taken in previous theoretical
studies of the solar magnetic power spectra.
Title: The Effect of Seeing on Solar Magnetic Flux Measurements
Authors: Lee, Jeongwoo; Chae, J. -C.; Yun, H. S.; Zirin, H.
Bibcode: 1997SoPh..171...35L
Altcode: 1997SoPh..171...35J
We investigate the influence of seeing upon measurement of magnetic flux
of photospheric fields. For this purpose we quantify seeing variation
in one day's observation at Big Bear Solar Observatory in terms of
the Fried function, a Modulation Transfer Function for the atmospheric
seeing. The temporal variation of seeing quality is compared with that
of magnetic flux measured in a quiet region with size 5' × 4' near
the solar disk center. A good correlation is found between the seeing
change and apparent evolution of magnetic flux values, implying, as a
possibility, that magnetic flux measurement might have been modulated
by seeing. Based on a simple model of ensembles of Gaussian magnetic
elements we argue that even the net flux as well as the total flux
can change due to seeing variation if the magnetograph has a finite
detection threshold and if the intrinsic fluxes in one and the other
polarities are unbalanced.