Author name code: tian
ADS astronomy entries on 2022-09-14
author:"Tian, Hui"
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Title: Statistical investigation of the kinematic and thermal
properties of supra-arcade downflows observed during a solar flare
Authors: Tan, Guangyu; Hou, Yijun; Tian, Hui
Bibcode: 2022MNRAS.tmp.2292T
Altcode: 2022arXiv220814737T
Supra-arcade downflows (SADs) are dark structures descending towards
post-reconnection flare loops observed in extreme ultraviolet or X-ray
observations and are closely related to magnetic reconnection during
solar flares. Due to the lack of statistical study on SADs in a single
flare, evolutions of kinematic and thermal properties of SADs during
the flare process still remain obscure. In this work, we identified 81
SADs in a flare that occurred on 2013 May 22 using observations of the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO). The kinematic properties of each SAD, including the appearance
time, height, projective velocity, and acceleration were recorded. We
found that the appearance heights of SADs become larger during the
flare, which is likely due to the lift of the bottom of the plasma
sheet. In the flare decay phase, the region where SADs mainly appear
moves from the north part to the south side possibly related to
a secondary eruption in the south side. The trajectories of most
SADs can be fitted by one or two deceleration processes, while some
special ones have positive accelerations during the descent. For the
thermal properties, we selected 54 SADs, whose front and body could be
clearly distinguished from the surrounding during the entire descent,
to perform Differential Emission Measure analysis. It is revealed that
the temperatures of the SAD front and body tend to increase during
their downward courses, and the relationship between the density and
temperature indicates that the heating is mainly caused by adiabatic
compression.
Title: Magnetoseismology for the solar corona: from 10 Gauss to
coronal magnetograms
Authors: Yang, Zihao; Gibson, Sarah; He, Jiansen; Del Zanna, Giulio;
Tomczyk, Steven; Morton, Richard; McIntosh, Scott; Wang, Linghua;
Karak, Bidya Binay; Samanta, Tanmoy; Tian, Hui; Chen, Yajie; Bethge,
Christian; Bai, Xianyong
Bibcode: 2022cosp...44.2490Y
Altcode:
Magnetoseismology, a technique of magnetic field diagnostics based
on observations of magnetohydrodynamic (MHD) waves, has been widely
used to estimate the field strengths of oscillating structures in
the solar corona. However, previously magnetoseismology was mostly
applied to occasionally occurring oscillation events, providing
an estimate of only the average field strength or one-dimensional
distribution of field strength along an oscillating structure. This
restriction could be eliminated if we apply magnetoseismology to the
pervasive propagating transverse MHD waves discovered with the Coronal
Multi-channel Polarimeter (CoMP). Using several CoMP observations of
the Fe XIII 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of
the plasma density and wave phase speed in the corona, which allow us
to map both the strength and direction of the coronal magnetic field
in the plane of sky. We also examined distributions of the electron
density and magnetic field strength, and compared their variations
with height in the quiet Sun and active regions. Such measurements
could provide critical information to advance our understanding of the
Sun's magnetism and the magnetic coupling of the whole solar atmosphere.
Title: A new post-hoc flat field measurement method for the Solar
X-ray and Extreme Ultraviolet Imager onboard the Fengyun-3E satellite
Authors: Song, Qiao; Bai, Xianyong; Chen, Bo; Hu, Xiuqing; Chen,
Yajie; Hou, Zhenyong; Zhang, Xiaofan; He, Lingping; Song, Kefei;
Zhang, Peng; Wang, Jing-Song; Zhang, Xiaoxin; Zong, Weiguo; Dun,
Jinping; Tian, Hui; Deng, Yuanyong
Bibcode: 2022arXiv220701829S
Altcode:
The extreme ultraviolet (EUV) observations are widely used in
solar activity research and space weather forecasting since they
can observe both the solar eruptions and the source regions of
the solar wind. Flat field processing is indispensable to remove
the instrumental non-uniformity of a solar EUV imager in producing
high-quality scientific data from original observed data. Fengyun-3E
(FY-3E) is a meteorological satellite operated in Sun-synchronous orbit,
and the routine EUV imaging data from the Solar X-ray and Extreme
Ultraviolet Imager (X-EUVI) onboard FY-3E has the characteristics
of concentric rotation. Taking advantage of the concentric rotation,
we propose a post-hoc flat field measurement method for its EUV 195
channel in this paper. This method removes small-scale and time-varying
component of the coronal activities by taking the median value for
each pixel along the time axis of a concentric rotation data cube,
and then derives large-scale and invariable component of the quiet
coronal radiation, and finally generates a flat field image. Analysis
shows that our method is able to measure the instrumental spot-like
non-uniformity possibly caused by contamination on the detector, which
mostly disappears after the in-orbit self-cleaning process. It can
also measure the quasi-periodic grid-like non-uniformity, possibly
from the obscuration of the support mesh on the rear filter. After
flat field correction, these instrumental non-uniformities from the
original data are effectively removed. X-EUVI 195 data after dark and
flat field corrections are consistent with the 193 channel data from
SDO/AIA, verifying the suitability of the method. Our method is not
only suitable for FY-3E/X-EUVI but also a candidate method for the
flat field measurement of future solar EUV telescopes.
Title: Sun-as-a-star spectroscopic observations of the line-of-sight
velocity of a solar eruption on 2021 October 28
Authors: Xu, Yu; Yang, Zihao; Tian, Hui; Chamberlin, Phillip; Gao,
Yuhang; Bai, Xianyong
Bibcode: 2022cosp...44.1387X
Altcode:
The propagation direction and the actual speed of the coronal
mass ejection (CME), which are the most decisive factors for its
geo-effectiveness, are difficult to determine through single-perspective
imaging observations. Here we show that Sun-as-a-star spectroscopic
observations, together with imaging observations, could allow us
to achieve this goal. Using observations of the Extreme-ultraviolet
Variability Experiment (EVE) onboard the Solar Dynamics Observatory,
we found clear blue-shifted secondary emission components of
extreme ultraviolet spectral lines during a solar eruption on 2021
Oct. 28. From simultaneous imaging observations, we found that the
secondary components are caused by mass ejection from the flare
site. We estimated the line-of-sight (LOS) velocity of the ejecta
both from the double Gaussian fitting method and the red-blue (RB)
asymmetry analysis. The results of both methods agree well with
each other, and the average LOS velocity of the plasma is around 420
km/s. The plane-of-sky (POS) velocity, around 560 km/s, from the Solar
Terrestrial Relation Observatory-A (STEREO-A) view is measured using
the 30.4 nm images series taken by the Extreme Ultraviolet Imager
onboard STEREO-A. The actual velocity of the ejecta is then computed
by combining the imaging and spectroscopic observations, which turns
out to be around 569 km/s with an angel of around 42.4$^\circ$ to the
west of the Sun-Earth line. Our analysis suggests that the heating
of the plasma to a temperature of around 0.2 MK causes the filament
region to become as bright as the flare one, which leads to the obvious
blue-shifted components in the transition region line profiles.
Title: Cross-Loop Propagation of a Quasi-Periodic Extreme-Ultraviolet
Wave Triggered by a Solar Eruption
Authors: Sun, Zheng; Chen, P. F.; Tian, Hui; Yao, Shuo; Hou, Zhenyong;
Chen, Hechao; Gao, Yuhang
Bibcode: 2022cosp...44.2499S
Altcode:
Extreme-ultraviolet (EUV) waves refer to large-scale disturbances
propagating outward from sites of solar eruptions in EUV narrow-band
imaging observations of the Sun. These waves are normally single pulses
propagating radially from the eruption sites. Using observations
from the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory, we report a very rare case of EUV wave,
which is characterized by quasi-periodic propagation across coronal
loops. Excited by an M1.7-class solar flare occurring at NOAA active
region 12887 on 2 November 2021, the wave exhibits slow and fast
components. The slow component, whose average speed is about 103
km/s, appears to originate from the opening of coronal loops at one
footpoint. The fast component represents a relatively large disturbance,
propagating across the coronal loops at an average speed of about
308 km/s. There is no significant decrease in the speeds of both
components. A wavelet analysis reveals that the periods of the slow and
fast components are both ~132 s, which is close to the period of the
quasi-periodic pulsations (QPP) at the flare ribbon, ~158 s. We suggest
that the slow component is caused by successive opening and expansion
of coronal loops, and that the fast component represents fast-mode
magnetoacoustic wave trains triggered by the loop expansion. The
quasi-periodic loop opening is likely accompanied by an intermittent
energy release process that heats the flare ribbon quasi-periodically.
Title: Detection of flare-associated plasma flows in the stellar
corona of EV Lac with X-ray spectroscopy
Authors: Chen, Hechao; Tian, Hui; Lu, Hong-Peng; Hou, Zhenyong; Xu,
Yu; Wu, Yuchuan
Bibcode: 2022cosp...44.1385C
Altcode:
Stellar flares are characterized by sudden enhancement of
electromagnetic radiation from the atmospheres of stars. Compared
to their solar counterparts, our knowledge on the coronal plasma
dynamics during stellar flares and their connection to coronal mass
ejections remains very limited. With time-resolved high-resolution
X-ray spectroscopic observations of three strong stellar flares
on an active M dwarf EV Lac from the CHANDRA X-ray observatory, we
detected noticeable flare-associated plasma flows through Doppler shift
measurements of strong coronal emission lines from ions such as O VIII
(3 MK), Mg XII (10 MK), and Si XIV (16 MK). The plasma flow pattern in
the decay phase of one flare event, including a weak redshift (below
90 km s$ ^{-1}$) in the Si XIV line and persistent blueshifts (around
70-120 km s$ ^{-1}$) in the O VIII and Mg XII lines, indicates that
this event may occur with a possible filament eruption and shrinking
high-temperature post-flare loops. Moreover, evidences for explosive
and gentle chromospheric evaporation were found in another two flares,
respectively. For the explosive one, we simultaneously detected hot
evaporating flows with a velocity up to 150 km s$ ^{-1}$ in the Si XIV
line and cooler condensation downflows with a velocity of 40-75 km s$
^{-1}$ in the O VIII and Mg XII lines; for the gentle one, only weak
evaporating flows with a velocity of 50-80 km s$ ^{-1}$ appear in
the Si XIV and Mg XII lines. During this two flares, variable line
emission fluxes in He-like triplets of Si XIII further reveal an
increase of electron density from 10$ ^{12}$ up to 10$ ^{13}$ cm$
^{-3}$ and a temperature rise from 3 MK up to 6$-$7 MK.
Title: Transient small-scale brightenings in the quiet solar corona:
a model for campfires observed with Solar Orbiter
Authors: Chen, Yajie; Peter, Hardi; Berghmans, David; Tian, Hui;
Auchère, Frédéric; Przybylski, Damien
Bibcode: 2022cosp...44.2557C
Altcode:
Recent observations by the Extreme Ultraviolet Imager (EUI) on
board Solar Orbiter have revealed prevalent small-scale transient
brightenings in the quiet solar corona termed "campfires". To
understand the generation mechanism of these coronal brightenings,
we constructed a self-consistent and time-dependent quiet-Sun
model extending from the upper convection zone to the lower corona
using a realistic three-dimensional radiation magnetohydrodynamic
simulation. From the model we have synthesized the coronal emission
in the EUI 174 passband. We identified several transient coronal
brightenings similar to those in EUI observations. The size and
lifetime of these coronal brightenings are mostly 0.5-4 Mm and ~2 min,
respectively. These brightenings are generally located at a height of
2-4 Mm above the photosphere, and the local plasma is often heated
above 1 MK. By examining the magnetic field structures before and
after the occurrence of brightenings, we concluded that these coronal
brightenings are generated by component magnetic reconnection between
interacting bundles of magnetic field lines or neighboring field lines
within highly twisted flux ropes. Occurring in the coronal part of
the atmosphere, these events generally reveal no obvious signature
of flux emergence or cancellation in photospheric magnetograms. These
transient coronal brightenings may play an important role in heating
of the local coronal plasma.
Title: On the detectability of solar and stellar coronal mass
ejections through asymmetries of extreme-ultraviolet spectral line
profiles
Authors: Yang, Zihao; Guo, Yang; Tian, Hui; Chen, Yajie; Chen, Hechao;
Gao, Yuhang; Bai, Xianyong; Zhu, Yingjie; Zhang, Jiale
Bibcode: 2022cosp...44.1383Y
Altcode:
Coronal mass ejections (CMEs) are the largest-scale eruptive phenomena
in the solar system. Associated with enormous plasma ejections and
energy release, CMEs have an important impact on the solar-terrestrial
environment. CMEs can also occur on other stars and will greatly
impact the habitability of the orbiting exoplanets around the hosting
stars. Therefore, the detection of stellar CMEs and how stellar CMEs
affect the space environments are indispensable when evaluating the
habitability of exoplanets. Observationally, solar CMEs could result
in the asymmetries of spectral line profiles. However, few studies have
concentrated on whether we can detect solar and stellar CME signals and
accurately diagnose CME properties through line profile asymmetries. In
this work, we constructed a geometric CME model and derived the
analytical expressions for full-disk integrated extreme ultraviolet
(EUV) line profiles during CMEs. For different CME properties and
instrumental conditions, full disk-integrated line profiles were
synthesized. We further evaluated the detectability and diagnostic
potential of CMEs from the synthetic line profiles. Our investigations
provide important constraints on the future design of spectrographs
for solar and stellar CME detections through EUV line asymmetries.
Title: Decayless oscillations in solar coronal bright points
Authors: Gao, Yuhang; van Doorsselaere, Tom; Tian, Hui; Chen, Yajie
Bibcode: 2022cosp...44.2496G
Altcode:
Decayless kink oscillations of solar coronal loops (or decayless
oscillations for short) have attracted great attention since their
discovery. Coronal bright points (CBPs) are mini-active regions and
consist of loops with a small size. However, decayless oscillations
in CBPs have not been widely reported. In this study, we identified
this kind of oscillations in some CBPs using 171 \r{A}, images taken
by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO). After using the motion magnification algorithm to
increase oscillation amplitudes, we made time-distance maps to identify
the oscillatory signals. We also estimated the loop lengths and velocity
amplitudes. We analyzed 23 CBPs, and found 31 oscillation events in 16
of them. The oscillation periods range from 1 to 8 minutes (on average
about 5 minutes), and the displacement amplitudes have an average value
of 0.07 Mm. The average loop length and velocity amplitude are 23 Mm
and 1.57 km s$ ^{-1}$, respectively. Relationships between different
oscillation paraments are also examined. Additionally, we performed
a simple forward model to illustrate how these sub-pixel oscillation
amplitudes (less than 0.4 Mm) could be detected. Results of the model
confirm the reliability of our data processing methods. Our study
shows for the first time that decayless oscillations are common in
small-scale loops of CBPs. These oscillations may play an important
role in coronal heating in the quiet Sun and coronal holes. They also
allow for seismological diagnostics of the Alfv\'{e}n speed and magnetic
field strength in the corona.
Title: Coronal Microjets in Quiet-sun Regions Observed with the
Extreme Ultraviolet Imager Onboard Solar Orbiter
Authors: Hou, Zhenyong; He, Jiansen; Berghmans, David; Teriaca, Luca;
Wang, Linghua; Schuehle, Udo; Tian, Hui; Chen, Yajie; Chen, Hechao;
Gao, Yuhang; Bai, Xianyong
Bibcode: 2022cosp...44.2536H
Altcode:
We report the smallest coronal jets ever observed in the quiet Sun
with recent high resolution observations from the High Resolution
Telescopes (HRI-EUV and HRI-Lyα) of the Extreme Ultraviolet
Imager (EUI) onboard Solar Orbiter. In the HRI-EUV (174 Å) images,
these microjets usually appear as nearly collimated structures with
brightenings at their footpoints. Their average lifetime, projected
speed, width, and maximum length are 4.6 min, 62 km $s^{-1}$, 1.0 Mm,
and 7.7 Mm, respectively. Inverted-Y shaped structures and moving blobs
can be identified in some events. A subset of these events also reveal
signatures in the HRI-Lyα (H I Lyα at 1216 Å) images and the extreme
ultraviolet images taken by the Atmospheric Imaging Assembly onboard the
Solar Dynamics Observatory. Our differential emission measure analysis
suggests a multi-thermal nature and an average density of ~1.4x10^9
$cm^{-3}$ for these microjets. Their thermal and kinetic energies were
estimated to be ~3.9x10$^{24}$ erg and ~2.9x10$^{23}$ erg, respectively,
which are of the same order of the released energy predicted by the
nanoflare theory. Most events appear to be located at the edges of
network lanes and magnetic flux concentrations, suggesting that these
coronal microjets are likely generated by magnetic reconnection between
small-scale magnetic loops and the adjacent network field.
Title: Broadening and red asymmetry of Halpha line profiles observed
by LAMOST during a stellar flare on an M-type star
Authors: Wu, Yuchuan; He, Han; Zhang, Liyun; Tian, Hui; Lu, Hong-Peng;
Chen, Hechao; Xu, Yu; Shi, Jianrong; Wang, Haifeng
Bibcode: 2022cosp...44.1384W
Altcode:
Stellar flares are characterized by sudden enhancement of
electromagnetic radiation in stellar atmospheres. So far much of our
understanding of stellar flares comes from photometric observations,
from which plasma motions in flare regions could not be detected. From
the spectroscopic data of LAMOST DR7, we have found one stellar
flare that is characterized by an impulsive increase followed by a
gradual decrease in the H$\alpha$ line intensity on an M4-type star,
and the total energy radiated through H${\alpha}$ is estimated to be
on the order of $10^{33}$ erg. The H$\alpha$ line appears to have
a Voigt profile during the flare, which is likely caused by Stark
pressure broadening due to the dramatic increase of electron density
and/or opacity broadening due to the occurrence of strong non-thermal
heating. Obvious enhancement has been identified at the red wing of the
H$\alpha$ line profile after the impulsive increase of the H$\alpha$
line intensity. The red wing enhancement corresponds to plasma moving
away from the Earth at a velocity of 100$-$200 km s$^{-1}$. According
to the current knowledge of solar flares, this red wing enhancement
may originate from: (1) flare-driven coronal rain, (2) chromospheric
condensation, or (3) a filament/prominence eruption that either with a
non-radial backward propagation or with strong magnetic suppression. The
total mass of the moving plasma is estimated to be on the order of
$10^{15}$ kg.
Title: Possible detection of coronal mass ejections on late-type
main-sequence stars in LAMOST medium-resolution spectra
Authors: Lu, Hong-Peng; Tian, Hui
Bibcode: 2022cosp...44.1381L
Altcode:
Context. Stellar coronal mass ejections (CMEs) are the main source
of the stellar space weather and they could affect the habitability
of neighboring planets. Large-sample time-domain spectral data are
currently unavailable for the detection of stellar CMEs. Aims. This work
aims to detect stellar CMEs from spectra continuously observed through
the LAMOST Medium-Resolution Spectroscopic Survey (LAMOST-MRS). Our
sample includes 1,379,408 LAMOST-MRS spectra of 226,194 late-type
main-sequence stars (T$ _{eff}$ < 6000 K, log[g/(cm s$ ^{-2}$)] >
4). Methods. We first identified stellar CME candidates by examining
the asymmetry of H$\alpha$ lines, and then performed double Gaussian
fitting for H$\alpha$ contrast profiles (differences between CME
spectra and reference spectra) of the CME candidates to analyze the
temporal variation of the asymmetric component. Results. Four stellar
CME candidates were detected, one of which originates from a G-type
Sun-like star, and the other three are from M-type dwarfs. The Mg I
triplet (5168.94 Å, 5174.13 Å, 5185.10 Å) and the H$\alpha$ line of
candidate 1 exhibit a blue wing enhancement, and the corresponding
Doppler shift of this enhancement shows a gradually increasing
trend. The host star of candidate 2 is a G5-type Sun-like star. The
emission of the H$\alpha$ blue wing of this candidate gradually
increases, and the corresponding maximum Doppler velocity is close to
the stellar surface escape velocity. The H$\alpha$ line shows an obvious
blue wing enhancement in candidate 3. In candidate 4, the H$\alpha$
line shows an obvious red wing enhancement, and the corresponding
maximum Doppler velocity exceeds the surface escape velocity of the
host star. The lower limit of the CME mass was estimated to be 8.84
× $10^{17}$ g to 3.02 × $10^{20}$ g for these four candidates.
Title: Possible detection of coronal mass ejections on late-type
main-sequence stars in LAMOST medium-resolution spectra
Authors: Lu, Hong-peng; Tian, Hui; Zhang, Li-yun; Karoff, Christoffer;
Chen, He-chao; Shi, Jian-rong; Hou, Zhen-yong; Chen, Ya-jie; Xu, Yu;
Wu, Yu-chuan; Cao, Dong-tao; Wang, Jiang-tao
Bibcode: 2022A&A...663A.140L
Altcode: 2022arXiv220509972L
Context. Stellar coronal mass ejections (CMEs) are the primary driver
of exoplanetary space weather and may affect the habitability of
exoplanets. However, detections of possible stellar CME signatures are
extremely rare.
Aims: This work aims to detect stellar CMEs from
time-domain spectra observed through the LAMOST Medium-Resolution
Spec-troscopic Survey (LAMOST-MRS). Our sample includes 1 379
408 LAMOST-MRS spectra of 226 194 late-type main-sequence stars
(Teff < 6000 K, log[g/(cm s−2)] >
4.0).
Methods: We first identified stellar CME candidates by
examining the asymmetries of Hα line profiles and then performed double
Gaussian fitting for Hα contrast profiles (differences between the
CME spectra and reference spectra) of the CME candidates to analyse the
temporal variation in the asymmetric components.
Results: Three
stellar CME candidates were detected on three M dwarfs. The Hα and Mg I
triplet lines (at 5168.94 Å, 5174.13 Å, and 5185.10 Å) of candidate
1 all exhibit a blue-wing enhancement, and the corresponding Doppler
shift of this enhancement shows a gradually increasing trend. The Hα
line also shows an obvious blue-wing enhancement in candidate 2. In
candidate 3, the Hα line shows an obvious red-wing enhancement,
and the corresponding projected maximum velocity exceeds the surface
escape velocity of the host star. The lower limit of the CME mass
is estimated to be ~8 × 1017 g to 4 × 1018
g for these three candidates.
Title: Three-dimensional Propagation of the Global EUV Wave associated
with a solar eruption on 2021 October 28
Authors: Hou, Zhenyong; Wang, Jing-Song; Zhang, Xiaoxin; Zhang, Peng;
Chen, Bo; Tian, Hui; Chen, Yajie; Chen, Hechao; Bai, Xianyong; Xu, Yu
Bibcode: 2022cosp...44.2484H
Altcode:
We present a case study for the global extreme ultraviolet (EUV) wave
and its chromospheric counterpart `Moreton-Ramsey wave' associated
with the second X-class flare in Solar Cycle 25 and a halo coronal
mass ejection (CME). The EUV wave was observed in the H$\alpha$
and EUV passbands with different characteristic temperatures. In the
171\,\AA\ and 193/195\,\AA\ images, the wave propagates circularly
with an initial velocity of 600--720\,km\,s$^{-1}$ and a deceleration
of 110--320\,m\,s$^{-2}$. The local coronal plasma is heated from
log(T/K)$\approx$5.9 to log(T/K)$\approx$6.2 during the passage of the
wavefront. The H$\alpha$ and 304\,\AA\ images also reveal signatures
of wave propagation with a velocity of 310--540\,km\,s$^{-1}$. With
multi-wavelength and dual-perspective observations, we found that the
wavefront likely propagates forwardly inclined to the solar surface
with a tilt angle of $\sim$53.2$^{\circ}$. Our results suggest that
this EUV wave is a fast-mode magnetohydrodynamic wave or shock driven
by the expansion of the associated CME, whose wavefront is likely
a dome-shaped structure that could impact the upper chromosphere,
transition region and corona.
Title: Detection of Flare-induced Plasma Flows in the Corona of EV
Lac with X-Ray Spectroscopy
Authors: Chen, Hechao; Tian, Hui; Li, Hao; Wang, Jianguo; Lu, Hongpeng;
Xu, Yu; Hou, Zhenyong; Wu, Yuchuan
Bibcode: 2022ApJ...933...92C
Altcode: 2022arXiv220514293C
Stellar flares are characterized by sudden enhancement of
electromagnetic radiation from the atmospheres of stars. Compared to
their solar counterparts, our knowledge on the coronal plasma dynamics
of stellar flares and their connection to coronal mass ejections
remains very limited. With time-resolved high-resolution spectroscopic
observations from the Chandra X-ray Observatory, we detected noticeable
coronal plasma flows during several stellar flares on a nearby dMe
star EV Lac. In the observed spectra of O VIII (3 MK), Fe XVII (6
MK), Mg XII (10 MK), and Si XIV (16 MK) lines, these flare-induced
upflows/downflows appear as significant Doppler shifts of several tens
to 130 km s-1 , and the upflow velocity generally increases
with temperature. Variable line ratios of the Si XIII triplet reveal
that this plasma flows in most flares are accompanied by an increase
in the coronal plasma density and temperature. We interpret these
results as X-ray evidence of chromospheric evaporation on EV Lac. In
two successive flares, the plasma flow pattern and a sharp increase
of the measured coronal density are highly suggestive of explosive
evaporation. The transition from redshifts to blueshifts in such an
explosive evaporation occurs at a temperature of at least 10 MK,
much higher than that observed in solar flares (~1 MK). However,
in one flare the cool and warm upflows appear to be accompanied by
a decreasing plasma density, which might be explained by a stellar
filament/prominence eruption coupled to this flare. These results
provide important clues to understanding the coronal plasma dynamics
during flares on M dwarfs.
Title: Global Hg cycle over Ediacaran-Cambrian transition and its
implications for environmental and biological evolution
Authors: Wu, Yaowen; Yin, Runsheng; Li, Chao; Chen, Di; Grasby,
Stephen E.; Li, Tengfei; Ji, Sui; Tian, Hui; Peng, Ping'an
Bibcode: 2022E&PSL.58717551W
Altcode:
The Ediacaran-Cambrian (E-C) transition witnessed remarkable
environmental changes, the extinction of the Ediacaran biota, and
subsequent rapid diversification of Cambrian animals. However, the
linkages between environmental and biological evolution are still under
debate at this critical time. Here, we present new Hg abundance and Hg
isotopes in sediments from South China, which are then combined with
those published from the Indian craton to explore the co-evolution
of environment and complex life during the E-C transition. In
both areas, high Hg/TOC ratios and near-zero Δ199Hg of
the Late Ediacaran sediments suggest enhanced volcanism, whereas
relatively high Hg/TOC ratios and positive Δ199Hg shift
upsection indicate volcanic-sourced atmospheric Hg(II) deposition in
the earliest Cambrian. The dramatically decreasing Hg/TOC ratios and
positive Δ199Hg of early Cambrian Age 2 to 3 sediments
indicate scavenging of dissolved seawater Hg(II) by organic matter
particulates. Our Hg results suggest volcanism may have likely
played a significant role in the extinction of Ediacaran biota and
global negative carbon excursions near the E-C boundary. Furthermore,
our Hg data provides new evidence of extensive OM burial in the early
Cambrian Age 2 to 3 oceans, leading to a rapid increase of Earth-surface
O2 levels that coincided with appearance of more complex
large-body animals. Our study provides new insights of the global Hg
cycle into the co-evolution of the environment and complex life at
this critical time.
Title: Implications for Additional Plasma Heating Driving the
Extreme-ultraviolet Late Phase of a Solar Flare with Microwave
Imaging Spectroscopy
Authors: Zhang, Jiale; Chen, Bin; Yu, Sijie; Tian, Hui; Wei, Yuqian;
Chen, Hechao; Tan, Guangyu; Luo, Yingjie; Chen, Xingyao
Bibcode: 2022ApJ...932...53Z
Altcode: 2022arXiv220503518Z
Extreme-ultraviolet late phase (ELP) refers to the second
extreme-ultraviolet (EUV) radiation enhancement observed in certain
solar flares, which usually occurs tens of minutes to several hours
after the peak of soft X-ray emission. The coronal loop system that
hosts the ELP emission is often different from the main flaring arcade,
and the enhanced EUV emission therein may imply an additional heating
process. However, the origin of the ELP remains rather unclear. Here
we present the analysis of a C1.4 flare that features such an ELP,
which is also observed in microwave wavelengths by the Expanded Owens
Valley Solar Array. Similar to the case of the ELP, we find a gradual
microwave enhancement that occurs about 3 minutes after the main
impulsive phase microwave peaks. Radio sources coincide with both foot
points of the ELP loops and spectral fits on the time-varying microwave
spectra demonstrate a clear deviation of the electron distribution
from the Maxwellian case, which could result from injected nonthermal
electrons or nonuniform heating to the footpoint plasma. We further
point out that the delayed microwave enhancement suggests the presence
of an additional heating process, which could be responsible for the
evaporation of heated plasma that fills the ELP loops, producing the
prolonged ELP emission.
Title: Can We Detect Coronal Mass Ejections through Asymmetries of
Sun-as-a-star Extreme-ultraviolet Spectral Line Profiles?
Authors: Yang, Zihao; Tian, Hui; Bai, Xianyong; Chen, Yajie; Guo,
Yang; Zhu, Yingjie; Cheng, Xin; Gao, Yuhang; Xu, Yu; Chen, Hechao;
Zhang, Jiale
Bibcode: 2022ApJS..260...36Y
Altcode: 2022arXiv220403683Y
Coronal mass ejections (CMEs) are the largest-scale eruptive phenomena
in the solar system. Associated with enormous plasma ejections and
energy releases, CMEs have an important impact on the solar-terrestrial
environment. Accurate predictions of the arrival times of CMEs at
the Earth depend on precise measurements of their 3D velocities,
which can be achieved by using simultaneous line-of-sight (LOS)
and plane-of-sky (POS) observations. Besides the POS information from
routine coronagraph and extreme-ultraviolet (EUV) imaging observations,
spectroscopic observations could unveil the physical properties of
CMEs, including their LOS velocities. We propose that spectral line
asymmetries measured by Sun-as-a-star spectrographs can be used for
routine detections of CMEs and estimations of their LOS velocities
during their early propagation phases. Such observations can also
provide important clues for the detection of CMEs on other solar-like
stars. However, few studies have concentrated on whether we can detect
CME signals and accurately diagnose CME properties through Sun-as-a-star
spectral observations. In this work, we construct a geometric CME
model and derive the analytical expressions for full disk-integrated
EUV line profiles during CMEs. For different CME properties and
instrumental configurations, the full disk-integrated line profiles
are synthesized. We further evaluate the detectability and diagnostic
potential of CMEs from the synthetic line profiles. Our investigations
provide important constraints on the future design of Sun-as-a-star
spectrographs for CME detections through EUV line asymmetries.
Title: Sun-as-a-star Spectroscopic Observations of the Line-of-sight
Velocity of a Solar Eruption on 2021 October 28
Authors: Xu, Yu; Tian, Hui; Hou, Zhenyong; Yang, Zihao; Gao, Yuhang;
Bai, Xianyong
Bibcode: 2022ApJ...931...76X
Altcode: 2022arXiv220411722X
The propagation direction and true velocity of a solar coronal
mass ejection, which are among the most decisive factors for its
geo-effectiveness, are difficult to determine through single-perspective
imaging observations. Here we show that Sun-as-a-star spectroscopic
observations, together with imaging observations, could allow us
to solve this problem. Using observations of the Extreme Ultraviolet
Variability Experiment onboard the Solar Dynamics Observatory, we found
clear blueshifted secondary emission components in extreme-ultraviolet
spectral lines during a solar eruption on 2021 October 28. From
simultaneous imaging observations, we found that the secondary
components are caused by a mass ejection from the flare site. We
estimated the line-of-sight (LOS) velocity of the ejecta from both the
double Gaussian fitting method and the red-blue asymmetry analysis. The
results of both methods agree well with each other, giving an average
LOS velocity of the plasma of ~423 km s-1. From the 304 Å
image series taken by the Extreme ultraviolet Imager onboard the Solar
Terrestrial Relation Observatory-A (STEREO-A) spacecraft, we estimated
the plane-of-sky velocity from the STEREO-A viewpoint to be around
587 km s-1. The full velocity of the bulk motion of the
ejecta was then computed by combining the imaging and spectroscopic
observations, which turns out to be around 596 km s-1 with
an angle of 42.°4 to the west of the Sun-Earth line and 16.°0 south
to the ecliptic plane.
Title: Doppler shifts of spectral lines formed in the solar transition
region and corona
Authors: Chen, Yajie; Peter, Hardi; Przybylski, Damien; Tian, Hui;
Zhang, Jiale
Bibcode: 2022A&A...661A..94C
Altcode: 2022arXiv220304691C
Context. Emission lines formed in the transition region and corona
dominantly show redshifts and blueshifts, respectively.
Aims: We investigate the Doppler shifts in a 3D radiation
magnetohydrodynamic (MHD) model of the quiet Sun and compare these
to observed properties. We concentrate on Si IV 1394 Å originating
in the transition region and examine the Doppler shifts of several
other spectral lines at different formation temperatures.
Methods: We constructed a radiation MHD model extending from the upper
convection zone to the lower corona using the MURaM code. In this
quiet Sun model, the magnetic field is self-consistently maintained
by the action of a small-scale dynamo in the convection zone, and it
is extrapolated to the corona as an initial condition. We synthesized
the profiles of several optically thin emission lines, which formed at
temperatures from the transition region into the corona. We investigated
the spatial structure and coverage of redshifts and blueshifts and how
this changes with the line-formation temperature.
Results: The
model successfully reproduces the observed change of average net Doppler
shifts from redshifted to blueshifted from the transition region into
the corona. In particular, the model shows a clear imbalance of area
coverage of redshifts versus blueshifts in the transition region of
ca. 80% to 20%, even though it is even a bit larger on the real Sun. We
determine that (at least) four processes generate the systematic Doppler
shifts in our model, including pressure enhancement in the transition
region, transition region brightenings unrelated to coronal emission,
boundaries between cold and hot plasma, and siphon-type flows.
Conclusions: We show that there is not a single process that is
responsible for the observed net Doppler shifts in the transition
region and corona. Because current 3D MHD models do not yet fully
capture the evolution of spicules, which is one of the key ingredients
of the chromosphere, most probably these have yet to be added to the
list of processes responsible for the persistent Doppler shifts.
Title: VizieR Online Data Catalog: WD binary pathways survey. V. Gaia
WD+AFGK binaries (Ren+, 2020)
Authors: Ren, J. -J.; Raddi, R.; Rebassa-Mansergas, A.; Hernandez,
M. S.; Parsons, S. G.; Irawati, P.; Rittipruk, P.; Schreiber, M. R.;
Gansicke, B. T.; Torres, S.; Wang, H. -J.; Zhang, J. -B.; Zhao, Y.;
Zhou, Y. -T.; Han, Z. -W.; Wang, B.; Liu, C.; Liu, X. -W.; Wang, Y.;
Zheng, J.; Wang, J. -F.; Zhao, F.; Cui, K. -M.; Shi, J. -R.; Tian, H.
Bibcode: 2022yCat..19050038R
Altcode:
We observed 93 white dwarf and A-, F-, G-, or K-type main-sequence
star (WD+AFGK) candidates during 23 nights, spanning 2017-Dec-12 to
2018-Jun-27, from the Xinglong 2.16m telescope (XL216). The instrument
terminal we used was the High Resolution fiber-fed Spectrograph (HRS),
thus providing Echelle spectra of a 49800 resolving power for a fixed
slit width of 0.19mm, and covering the ~3650-10000Å wavelength
range. Eight nights of observations (between 2017-Jul-06 and
2018-Jan-16) were carried out using the Echelle spectrograph attached
to the 2.12m telescope at the San Pedro Martir Observatory in Baja
California, Mexico. The corresponding resolving power is R~20000 for
a slit width of 2.8", covering the 3650-7300Å wavelength range. Finally, we obtained 264 high-resolution spectra for 104 WD+AFGK
candidates, each of them having at least two spectra obtained on
different nights. (4 data files).
Title: Decayless Oscillations in Solar Coronal Bright Points
Authors: Gao, Yuhang; Tian, Hui; Van Doorsselaere, Tom; Chen, Yajie
Bibcode: 2022ApJ...930...55G
Altcode: 2022arXiv220317034G
Decayless kink oscillations of solar coronal loops (or decayless
oscillations for short) have attracted great attention since their
discovery. Coronal bright points (CBPs) are mini-active regions and
consist of loops with a small size. However, decayless oscillations
in CBPs have not been widely reported. In this study, we identified
this kind of oscillations in some CBPs using 171 Å images taken
by the Atmospheric Imaging Assembly on board the Solar Dynamics
Observatory. After using the motion magnification algorithm to increase
oscillation amplitudes, we made time-distance maps to identify the
oscillatory signals. We also estimated the loop lengths and velocity
amplitudes. We analyzed 23 CBPs and found 31 oscillation events in 16
of them. The oscillation periods range from 1 to 8 minutes (on average
about 5 minutes), and the displacement amplitudes have an average value
of 0.07 Mm. The average loop length and velocity amplitude are 23 Mm and
1.57 km s-1, respectively. Relationships between different
oscillation parameters are also examined. Additionally, we performed a
simple model to illustrate how these subpixel oscillation amplitudes
(less than 0.4 Mm) could be detected. Results of the model confirm
the reliability of our data processing methods. Our study shows for
the first time that decayless oscillations are common in small-scale
loops of CBPs. These oscillations allow for seismological diagnostics
of the Alfvén speed and magnetic field strength in the corona.
Title: Broadening and Redward Asymmetry of Hα Line Profiles Observed
by LAMOST during a Stellar Flare on an M-type Star
Authors: Wu, Yuchuan; Chen, Hechao; Tian, Hui; Zhang, Liyun; Shi,
Jianrong; He, Han; Lu, Hongpeng; Xu, Yu; Wang, Haifeng
Bibcode: 2022ApJ...928..180W
Altcode: 2022arXiv220302292W
Stellar flares are characterized by sudden enhancement of
electromagnetic radiation in stellar atmospheres. So far, much of our
understanding of stellar flares has come from photometric observations,
from which plasma motions in flare regions could not be detected. From
the spectroscopic data of LAMOST DR7, we have found one stellar
flare that is characterized by an impulsive increase followed by a
gradual decrease in the Hα line intensity on an M4-type star, and the
total energy radiated through Hα is estimated to be of the order of
1033 erg. The Hα line appears to have a Voigt profile during
the flare, which is likely caused by Stark pressure broadening due to
the dramatic increase in electron density and/or opacity broadening due
to the occurrence of strong nonthermal heating. Obvious enhancement
has been identified in the red wing of the Hα line profile after
the impulsive increase in the Hα line intensity. The red-wing
enhancement corresponds to plasma moving away from the Earth at
a velocity of 100-200 km s-1. According to our current
knowledge of solar flares, this red-wing enhancement may originate
from: (1) flare-driven coronal rain, (2) chromospheric condensation,
or (3) a filament/prominence eruption either with nonradial backward
propagation or with strong magnetic suppression. The total mass of
the moving plasma is estimated to be of the order of 1015 kg.
Title: Three-dimensional Propagation of the Global Extreme-ultraviolet
Wave Associated with a Solar Eruption on 2021 October 28
Authors: Hou, Zhenyong; Tian, Hui; Wang, Jing-Song; Zhang, Xiaoxin;
Song, Qiao; Zheng, Ruisheng; Chen, Hechao; Chen, Bo; Bai, Xianyong;
Chen, Yajie; He, Lingping; Song, Kefei; Zhang, Peng; Hu, Xiuqing;
Dun, Jinping; Zong, Weiguo; Song, Yongliang; Xu, Yu; Tan, Guangyu
Bibcode: 2022ApJ...928...98H
Altcode: 2022arXiv220213051H
We present a case study for the global extreme-ultraviolet (EUV) wave
and its chromospheric counterpart the Moreton-Ramsey Wave associated
with the second X-class flare in Solar Cycle 25 and a halo coronal
mass ejection (CME). The EUV wave was observed in the Hα and EUV
passbands with different characteristic temperatures. In the 171 Å
and 193/195 Å images, the wave propagates circularly with an initial
velocity of 600-720 km s-1 and a deceleration of 110-320
m s-2. The local coronal plasma is heated from log(T/K)
≍ 5.9 to log(T/K) ≍ 6.2 during the passage of the wave front. The
Hα and 304 Å images also reveal signatures of wave propagation with
a velocity of 310-540 km s-1. With multiwavelength and
dual-perspective observations, we found that the wave front likely
propagates forwardly inclined to the solar surface with a tilt angle
of ~53°.2. Our results suggest that this EUV wave is a fast-mode
magnetohydrodynamic wave or shock driven by the expansion of the
associated CME, whose wave front is likely a dome-shaped structure
that could impact the upper chromosphere, transition region, and corona.
Title: Coronal condensation as the source of transition-region
supersonic downflows above a sunspot
Authors: Chen, Hechao; Tian, Hui; Li, Leping; Peter, Hardi; Chitta,
Lakshmi Pradeep; Hou, Zhenyong
Bibcode: 2022A&A...659A.107C
Altcode: 2021arXiv211201354C
Context. Plasma loops or plumes rooted in sunspot umbrae often harbor
downflows with speeds of 100 km s−1. These downflows
are supersonic at transition region temperatures of ∼0.1 MK. The
source of these flows is not well understood.
Aims: We aim
to investigate the source of sunspot supersonic downflows (SSDs)
in active region 12740 using simultaneous spectroscopic and imaging
observations.
Methods: We identified SSD events from multiple
raster scans of a sunspot by the Interface Region Imaging Spectrograph,
and we calculated the electron densities, mass fluxes, and velocities
of these SSDs. The extreme-ultraviolet (EUV) images provided by the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory and
the EUV Imager onboard the Solar Terrestrial Relations Observatory were
employed to investigate the origin of these SSDs and their associated
coronal rain.
Results: Almost all the identified SSDs appear
at the footpoints of sunspot plumes and are temporally associated
with the appearance of chromospheric bright dots inside the sunspot
umbra. Dual-perspective EUV imaging observations reveal a large-scale
closed magnetic loop system spanning the sunspot region and a remote
region. We observed that the SSDs are caused by repeated coronal
rain that forms and flows along these closed magnetic loops toward the
sunspot. One episode of coronal rain clearly indicates that reconnection
near a coronal X-shaped structure first leads to the formation of
a magnetic dip. Subsequently, hot coronal plasma catastrophically
cools from ∼2 MK in the dip region via thermal instability. This
results in the formation of a transient prominence in the dip, from
which the cool gas mostly slides into the sunspot along inclined
magnetic fields under the gravity. This drainage process manifests
as a continuous rain flow, which lasts for ∼2 h and concurrently
results in a nearly steady SSD event. The total mass of condensation
(1.3 × 1014 g) and condensation rate (1.5 × 1010
g s−1) in the dip region were found to be sufficient to
sustain this long-lived SSD event, which has a mass transport rate of
0.7 − 1.2 × 1010 g s−1.
Conclusions:
Our results demonstrate that coronal condensation in magnetic dips
can result in the quasi-steady sunspot supersonic downflows. Movies associated to Figs. 1, 3, 6, 7, and 11 are available at https://www.aanda.org
Title: VizieR Online Data Catalog: Stellar parameters of a Young
Stellar Snake (Tian, 2020)
Authors: Tian, H. -J.
Bibcode: 2022yCat..19040196T
Altcode:
In this study, we mainly use the astrometric and photometric data from
Gaia DR2. (1 data file).
Title: VizieR Online Data Catalog: LAMOST Time-Domain survey, first
results (Wang+, 2021)
Authors: Wang, S.; Zhang, H. -T.; Bai, Z. -R.; Yuan, H. -L.; Xiang,
M. -S.; Zhang, B.; Hou, W.; Zuo, F.; Du, B.; Li, T. -D.; Yang, F.;
Cui, K. -M.; Wang, Y. -L.; Li, J.; Kovalev, M.; Li, C. -Q.; Tian,
H.; Zong, W. -K.; Han, H. -G.; Liu, C.; Luo, A. -L.; Shi, J. -R.;
Fu, J. -N.; Bi, S. -L.; Han, Z. -W.; Liu, J. -F.
Bibcode: 2022yCatp040002104W
Altcode:
We performed this survey with both the LRS and MRS observations. For
LRS observation, the wavelength coverage is 3650-9000Å (Luo et al.,
2015RAA....15.1095L). For MRS observation, the blue and red arms cover
wavelength ranges from 4950Å to 5350Å and from 6300Å to 6800Å,
respectively (Liu et al., 2020, in prep. arXiv:2005.07210). The LRS
survey of each plate was observed with 3-10 single 600s exposures in
one observation night; the MRS survey of each plate was observed with
3-8 single 1200s exposures. (6 data files).
Title: Research Progress of Decayless Oscillations in the Solar Corona
Authors: Gao, Y. H.; Tian, H.; Guo, M. Z.; Li, B.
Bibcode: 2022AcASn..63....1G
Altcode:
Decayless transverse kink oscillations of solar coronal loops (or
decayless oscillations for short) have attracted great attention since
their discovery. In contrast to the previously reported decaying kink
oscillations, they are not associated with any external impulsive
events and are found to show no obvious decay of amplitude. Besides,
their ubiquitous existence suggests that they might play an important
role in the process of coronal heating and diagnostics of the
properties of coronal loops. In this review, we summarize the research
progress of decayless oscillations. The observational results include
their amplitude, period, scaling rules, polarization and multiple
harmonics. We also introduce different types of theoretical models on
their generation mechanisms. Moreover, we also give some examples of
their application in coronal seismology. In the last section of this
review, we discuss future perspectives on the research of decayless
oscillations.
Title: Magnetoseismology for the solar corona: from 10 Gauss to
coronal magnetograms
Authors: Yang, Zihao; Bethge, Christian; Tian, Hui; Tomczyk, Steven;
Morton, Richard; Del Zanna, Giulio; McIntosh, Scott; Karak, Bidya
Binay; Gibson, Sarah; Samanta, Tanmoy; He, Jiansen; Chen, Yajie; Bai,
Xianyong; Wang, Linghua
Bibcode: 2021AGUFMSH12C..07Y
Altcode:
Magnetoseismology, a technique of magnetic field diagnostics based
on observations of magnetohydrodynamic (MHD) waves, has been widely
used to estimate the field strengths of oscillating structures in
the solar corona. However, previously magnetoseismology was mostly
applied to occasionally occurring oscillation events, providing
an estimate of only the average field strength or one-dimensional
distribution of field strength along an oscillating structure. This
restriction could be eliminated if we apply magnetoseismology to the
pervasive propagating transverse MHD waves discovered with the Coronal
Multi-channel Polarimeter (CoMP). Using several CoMP observations of
the Fe XIII 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of
the plasma density and wave phase speed in the corona, which allow us
to map both the strength and direction of the coronal magnetic field
in the plane of sky. We also examined distributions of the electron
density and magnetic field strength, and compared their variations
with height in the quiet Sun and active regions. Such measurements
could provide critical information to advance our understanding of the
Sun's magnetism and the magnetic coupling of the whole solar atmosphere.
Title: Magnitude and Uncertainty of Nitrous Oxide Emissions From
North America Based on Bottom-Up and Top-Down Approaches: Informing
Future Research and National Inventories
Authors: Xu, R.; Tian, H.; Pan, N.; Thompson, R. L.; Canadell,
J. G.; Davidson, E. A.; Nevison, C.; Winiwarter, W.; Shi, H.; Pan,
S.; Chang, J.; Ciais, P.; Dangal, S. R. S.; Ito, A.; Jackson, R. B.;
Joos, F.; Lauerwald, R.; Lienert, S.; Maavara, T.; Millet, D. B.;
Raymond, P. A.; Regnier, P.; Tubiello, F. N.; Vuichard, N.; Wells,
K. C.; Wilson, C.; Yang, J.; Yao, Y.; Zaehle, S.; Zhou, F.
Bibcode: 2021GeoRL..4895264X
Altcode:
We synthesized N2O emissions over North America using 17
bottom-up (BU) estimates from 1980-2016 and five top-down (TD) estimates
from 1998 to 2016. The BU-based total emission shows a slight increase
owing to U.S. agriculture, while no consistent trend is shown in TD
estimates. During 2007-2016, North American N2O emissions
are estimated at 1.7 (1.0-3.0) Tg N yr-1 (BU) and 1.3
(0.9-1.5) Tg N yr-1 (TD). Anthropogenic emissions were twice
as large as natural fluxes from soil and water. Direct agricultural
and industrial activities accounted for 68% of total anthropogenic
emissions, 71% of which was contributed by the U.S. Our estimates of
U.S. agricultural emissions are comparable to the EPA greenhouse gas
(GHG) inventory, which includes estimates from IPCC tier 1 (emission
factor) and tier 3 (process-based modeling) approaches. Conversely,
our estimated agricultural emissions for Canada and Mexico are twice
as large as the respective national GHG inventories.
Title: Stellar Coronal Magnetic Field Measurements Based on the
Magnetic-field-induced Transition Theory
Authors: Liu, Xianyu; Chen, Yajie; Tian, Hui; Bai, Xianyong; Jin,
Meng; Li, Wenxian; Yang, Yang; Yang, Zihao; Deng, Yuanyong
Bibcode: 2021AGUFMSH15G2091L
Altcode:
Measurements of the magnetic field in the stellar coronae are extremely
difficult. Recently, it was proposed that the magnetic-field-induced
transition (MIT) of the Fe X 257 Å line can be used to measure the
coronal magnetic field of the Sun. We performed forward modeling with
a series of global stellar magnetohydrodynamics models to investigate
the possibility of extending this method to other late-type stars. We
first synthesized the emissions of several Fe X lines for each stellar
model, then calculated the magnetic field strengths using the intensity
ratios of Fe X 257 Å to several other Fe X lines based on the MIT
theory. Finally, we compared the derived field strengths with those in
the models, and concluded that this method can be used to measure at
least the coronal magnetic field strengths of stars with a mean surface
magnetic flux density about one order of magnitude higher than that of
the Sun. Our investigation suggests the need of an extreme ultraviolet
spectrometer to perform routine measurements of the stellar coronal
magnetic field.
Title: Transient small-scale brightenings in the quiet solar corona:
A model for campfires observed with Solar Orbiter
Authors: Chen, Yajie; Przybylski, Damien; Peter, Hardi; Tian, Hui;
Auchère, F.; Berghmans, D.
Bibcode: 2021A&A...656L...7C
Altcode: 2021arXiv210410940C
Context. Recent observations by the Extreme Ultraviolet Imager (EUI) on
board Solar Orbiter have characterized prevalent small-scale transient
brightenings in the corona above the quiet Sun termed campfires.
Aims: In this study we search for comparable brightenings in a numerical
model and then investigate their relation to the magnetic field and the
processes that drive these events.
Methods: We used the MURaM
code to solve the 3D radiation magnetohydrodynamic equations in a box
that stretches from the upper convection zone to the corona. The model
self-consistently produces a supergranular network of the magnetic
field and a hot corona above this quiet Sun. For the comparison with
the model, we synthesized the coronal emission as seen by EUI in its
174 Å channel, isolated the seven strongest transient brightenings,
and investigated the changes of the magnetic field in and around these
in detail.
Results: The transients we isolated have a lifetime of
about 2 min and are elongated loop-like features with lengths around 1
Mm to 4 Mm. They tend to occur at heights of about 2 Mm to 5 Mm above
the photosphere, a bit offset from magnetic concentrations that mark
the bright chromospheric network, and they reach temperatures of above
1 MK. As a result, they very much resemble the larger campfires found
in observations. In our model most events are energized by component
reconnection between bundles of field lines that interact at coronal
heights. In one case, we find that untwisting a highly twisted flux
rope initiates the heating.
Conclusions: Based on our study, we
propose that the majority of campfire events found by EUI are driven
by component reconnection and our model suggests that this process
significantly contributes to the heating of the corona above the quiet
Sun. Movies associated to Figs. 2, 4, 5, and A1 are available at https://www.aanda.org
Title: Possible Signature of Sausage Waves in Photospheric Bright
Points
Authors: Gao, Yuhang; Li, Fuyu; Li, Bo; Cao, Wenda; Song, Yongliang;
Tian, Hui; Guo, Mingzhe
Bibcode: 2021SoPh..296..184G
Altcode: 2021arXiv211211756G
Sausage waves have been frequently reported in solar magnetic structures
such as sunspots, pores, and coronal loops. However, they have not been
unambiguously identified in photospheric bright points (BPs). Using
high-resolution TiO image sequences obtained with the Goode Solar
Telescope at the Big Bear Solar Observatory, we analyzed four isolated
BPs. It was found that their area and average intensity oscillate
for several cycles in an in-phase fashion. The oscillation periods
range from 100 to 200 seconds. We interpreted the phase relation as a
signature of sausage waves, particularly slow waves, after discussing
sausage-wave theory and the opacity effect.
Title: Campfires observed by EUI: What have we learned so far?
Authors: Berghmans, David; Auchere, F.; Zhukov, Andrei; Mierla,
Marilena; Chen, Yajie; Peter, Hardi; Panesar, Navdeep; Chitta, Lakshmi
Pradeep; Antolin, Patrick; Aznar Cuadrado, Regina; Tian, Hui; Hou,
Zhenyong; Podladchikova, Olena
Bibcode: 2021AGUFMSH21A..02B
Altcode:
Since its very first light images of the corona, the EUI/HRIEUV
telescope onboard Solar Orbiter has observed small localised
brightenings in the Quiet Sun. These small localised brightenings,
have become known as campfires, and are observed with length scales
between 400 km and 4000 km and durations between 10 sec and 200
sec. The smallest and weakest of these HRIEUV brightenings have
not been previously observed. Simultaneous observations from the
EUI High-resolution Lyman- telescope (HRILYA) do not show localised
brightening events, but the locations of the HRIEUV events clearly
correspond to the chromospheric network. Comparisons with simultaneous
AIA images shows that most events can also be identified in the
17.1 nm, 19.3 nm, 21.1 nm, and 30.4 nm pass-bands of AIA, although
they appear weaker and blurred. Some of the larger campfires have
the appearance of small interacting loops with the brightening
expanding from the contact point of the loops. Our differential
emission measure (DEM) analysis indicated coronal temperatures. We
determined the height for a few of these campfires to be between 1
and 5 Mm above the photosphere. We interpret these events as a new
extension to the flare-microflare-nanoflare family. Given their low
height, the EUI campfires could stand as a new element of the fine
structure of the transition region-low corona, that is, as apexes
of small-scale loops that undergo internal heating all the way up to
coronal temperatures. 3D MHD simulations with the MURaM code revealed
brightenings that are in many ways similar to the campfires by EUI. The
brightenings in the simulations suggest that campfires are triggered by
component reconnection inside flux bundles rather than flux emergence
or cancellation. Nevertheless, some of the observed campfires can
be clearly linked to flux cancellation events and, intriguingly,
are preceded by an erupting cool plasma structure. Analysis of the
dynamics of campfires revealed that some have the appearance of coronal
microjets, the smallest coronal jets observed in the quiet Sun. The
HRIEUV images also reveal transient jets on a somewhat bigger scale
with repeated outflows on the order of 100 km s1. In this paper we
will provide an overview of the campfire related phenomena that EUI
has observed and discuss the possible relevance for coronal heating.
Title: Forward Modeling of Solar Coronal Magnetic Field Measurements
Based on a Magnetic-field-induced Transition in Fe X
Authors: Chen, Yajie; Li, Wenxian; Tian, Hui; Chen, Feng; Bai,
Xianyong; Yang, Yang; Yang, Zihao; Liu, Xianyu; Deng, Yuanyong
Bibcode: 2021AGUFMSH12C..06C
Altcode:
It was recently proposed that the intensity ratios of several extreme
ultraviolet spectral lines from the Fe X ion can be used to measure
the solar coronal magnetic field based on the magnetic-field-induced
transition (MIT) theory. To verify the suitability of this method,
we performed forward modeling with a three-dimensional radiation
magnetohydrodynamic model of a solar active region. Intensities of
several spectral lines from Fe X were synthesized from the model. Based
on the MIT theory, intensity ratios of the MIT line Fe x 257 A to
several other Fe X lines were used to derive the magnetic field
strengths, which were then compared with the field strengths in the
model. We also developed a new method to simultaneously estimate the
coronal density and temperature from the Fe X 174/175 and 184/345
A line ratios. Using these estimates, we demonstrated that the MIT
technique can provide reasonably accurate measurements of the coronal
magnetic field in both on-disk and off-limb solar observations. Our
investigation suggests that a spectrometer that can simultaneously
observe the Fe X 174, 175, 184, 257, and 345 A lines and allow an
accurate radiometric calibration for these lines is highly desired to
achieve reliable measurements of the coronal magnetic field. We have
also evaluated the impact of the uncertainty in the Fe X 3p4 3d 4D5/2
and 4D7/2 energy difference on the magnetic field measurements.
Title: Magnetohydrodynamic Waves in Open Coronal Structures
Authors: Banerjee, D.; Krishna Prasad, S.; Pant, V.; McLaughlin, J. A.;
Antolin, P.; Magyar, N.; Ofman, L.; Tian, H.; Van Doorsselaere, T.;
De Moortel, I.; Wang, T. J.
Bibcode: 2021SSRv..217...76B
Altcode: 2020arXiv201208802B
Modern observatories have revealed the ubiquitous presence of
magnetohydrodynamic waves in the solar corona. The propagating waves
(in contrast to the standing waves) are usually originated in the lower
solar atmosphere which makes them particularly relevant to coronal
heating. Furthermore, open coronal structures are believed to be the
source regions of solar wind, therefore, the detection of MHD waves
in these structures is also pertinent to the acceleration of solar
wind. Besides, the advanced capabilities of the current generation
telescopes have allowed us to extract important coronal properties
through MHD seismology. The recent progress made in the detection,
origin, and damping of both propagating slow magnetoacoustic waves and
kink (Alfvénic) waves is presented in this review article especially
in the context of open coronal structures. Where appropriate, we give
an overview on associated theoretical modelling studies. A few of the
important seismological applications of these waves are discussed. The
possible role of Alfvénic waves in the acceleration of solar wind is
also touched upon.
Title: Forward Modeling of Solar Coronal Magnetic-field Measurements
Based on a Magnetic-field-induced Transition in Fe X
Authors: Chen, Yajie; Li, Wenxian; Tian, Hui; Chen, Feng; Bai,
Xianyong; Yang, Yang; Yang, Zihao; Liu, Xianyu; Deng, Yuanyong
Bibcode: 2021ApJ...920..116C
Altcode: 2021arXiv210711783C
It was recently proposed that the intensity ratios of several extreme
ultraviolet spectral lines from Fe X ions can be used to measure the
solar coronal magnetic field based on magnetic-field-induced transition
(MIT) theory. To verify the suitability of this method, we performed
forward modeling with a three-dimensional radiation magnetohydrodynamic
model of a solar active region. Intensities of several spectral lines
from Fe X were synthesized from the model. Based on MIT theory, the
intensity ratios of the MIT line Fe X 257 Å to several other Fe X
lines were used to derive magnetic-field strengths, which were then
compared with the field strengths in the model. We also developed
a new method to simultaneously estimate the coronal density and
temperature from the Fe X 174/175 and 184/345 Å line ratios. Using
these estimates, we demonstrated that the MIT technique can provide
reasonably accurate measurements of the coronal magnetic field in both
on-disk and off-limb solar observations. Our investigation suggests that
a spectrometer that can simultaneously observe the Fe X 174, 175, 184,
257, and 345 Å lines and allow an accurate radiometric calibration
for these lines is highly desired to achieve reliable measurements of
the coronal magnetic field. We have also evaluated the impact of the
uncertainty in the Fe x 3p4 3d 4D5/2
and 4D7/2 energy difference on the magnetic-field
measurements.
Title: Non-thermal Electron Energization During the Impulsive Phase
of an X9.3 Flare Revealed by Insight-HXMT
Authors: Zhang, P.; Wang, W.; Su, Y.; Song, L. M.; Li, C. K.; Zhou,
D. K.; Zhang, S. N.; Tian, H.; Liu, S. M.; Zhao, H. S.; Zhang, S.
Bibcode: 2021ApJ...918...42Z
Altcode: 2021arXiv210609506Z
The X9.3 flare SOL20170906T11:55 was observed by the CsI detector aboard
the first Chinese X-ray observatory Hard X-ray Modulation telescope
(Insight-HXMT). Using the wavelets method, we report 22 s quasiperiodic
pulsations during the impulsive phase. The spectra from 100 keV to 800
keV show the evolution with the gamma-ray flux of a power-law photon
index from ~1.8 before the peak, ~2.0 around the flare peak, to ~1.8
again. The gyrosynchrotron microwave spectral analysis reveals a 36"6
± 0"6 radius gyrosynchrotron source with mean transverse magnetic
field around 608.2 Gauss. The penetrated ≥10 keV non-thermal electron
density is about 106.7 cm-3 at peak time. The
magnetic field strength followed the evolution of high-frequency radio
flux. Further gyrosynchrotron source modeling analysis implies that
there exists a quite steady gyrosynchrotron source, and the non-thermal
electron density and transverse magnetic field evolution are similar to
higher-frequency light curves. The temporal spectral analysis reveals
that those non-thermal electrons are accelerated by repeated magnetic
reconnection, likely from a lower corona source.
Title: Measurements of the Magnetic Field Strengths at the Bases of
Stellar Coronae Using the Magnetic-field-induced Transition Theory
Authors: Chen, Yajie; Liu, Xianyu; Tian, Hui; Bai, Xianyong; Jin,
Meng; Li, Wenxian; Yang, Yang; Yang, Zihao; Deng, Yuanyong
Bibcode: 2021ApJ...918L..13C
Altcode: 2021arXiv210808155C
Measurements of the magnetic field in the stellar coronae are extremely
difficult. Recently, it was proposed that the magnetic-field-induced
transition (MIT) of the Fe X 257 Å line can be used to measure the
coronal magnetic field of the Sun. We performed forward modeling with
a series of global stellar magnetohydrodynamics models to investigate
the possibility of extending this method to other late-type stars. We
first synthesized the emissions of several Fe X lines for each stellar
model, then calculated the magnetic field strengths using the intensity
ratios of Fe X 257 Å to several other Fe X lines based on the MIT
theory. Finally, we compared the derived field strengths with those
in the models, and concluded that this method can be used to measure
at least the magnetic field strengths at the coronal bases of stars
with a mean surface magnetic flux density about one order of magnitude
higher than that of the Sun. Our investigation suggests the need for
an extreme ultraviolet spectrometer to perform routine measurements
of the stellar coronal magnetic field.
Title: Coronal Microjets in Quiet-Sun Regions Observed with the
Extreme Ultraviolet Imager on Board the Solar Orbiter
Authors: Hou, Zhenyong; Tian, Hui; Berghmans, David; Chen, Hechao;
Teriaca, Luca; Schühle, Udo; Gao, Yuhang; Chen, Yajie; He, Jiansen;
Wang, Linghua; Bai, Xianyong
Bibcode: 2021ApJ...918L..20H
Altcode: 2021arXiv210808718H
We report the smallest coronal jets ever observed in the quiet Sun with
recent high-resolution observations from the High Resolution Telescopes
(HRIEUV and HRILyα) of the Extreme Ultraviolet
Imager on board the Solar Orbiter (SO). In the HRIEUV
(174 Å) images, these microjets usually appear as nearly collimated
structures with brightenings at their footpoints. Their average
lifetime, projected speed, width, and maximum length are 4.6 minutes,
62 km s-1, 1.0 Mm, and 7.7 Mm, respectively. Inverted-Y
shaped structures and moving blobs can be identified in some events. A
subset of these events also reveal signatures in the HRILyα
(H I Lyα at 1216 Å) images and the extreme ultraviolet images
taken by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO). Our differential emission-measure (EM)
analysis suggests a multithermal nature and an average density of ~1.4
× 109 cm-3 for these microjets. Their thermal
and kinetic energies were estimated to be ~3.9 × 1024 erg
and ~2.9 × 1023 erg, respectively, which are of the same
order of the released energy predicted by the nanoflare theory. Most
events appear to be located at the edges of network lanes and magnetic
flux concentrations, suggesting that these coronal microjets are likely
generated by magnetic reconnection between small-scale magnetic loops
and the adjacent network field.
Title: VizieR Online Data Catalog: Stellar groups in Taurus field
from Gaia DR2 & LAMOST (Liu+, 2021)
Authors: Liu, J.; Fang, M.; Tian, H.; Liu, C.; Yang, C.; Xue, X.
Bibcode: 2021yCat..22540020L
Altcode:
In order to fully explore the Taurus region, we use a large searching
area: 55°<=RA<=90° and 10°<=DEC<=35°. The
spectroscopic data used in this work are taken from the Large Sky Area
Multi-Object Fibre Spectroscopic Telescope (LAMOST) DR5. LAMOST is
a 4m Schmidt telescope of the National Astronomical Observatories of
China, located at Xinglong Observing Station, China. In this work, we
use the LAMOST DR5 data set. In order to construct the spectral
energy distribution of each source and estimate its extinction,
we used optical photometry in the g, r, i, z, and y bands from
Pan-STARRS (Cat. II/349) and G, GBP, and GRP
bands from Gaia DR2 (Cat. I/345), near-infrared photometry in the J,
H, and KS bands from the Two-Micron All Sky Survey (2MASS;
see Cat. II/246), and near- and mid-infrared photometry in the W1
(3.4um), W2 (4.6um), W3 (12um), and W4 (22um) bands from the WISE
(see Cat. II/311) all-sky survey. (2 data files).
Title: Flare-induced decay-less transverse oscillations in solar
coronal loops
Authors: Mandal, Sudip; Tian, Hui; Peter, Hardi
Bibcode: 2021A&A...652L...3M
Altcode: 2021arXiv210702247M
Evidence of flare-induced, large-amplitude, decay-less transverse
oscillations is presented. A system of multithermal coronal loops,
as observed with the Atmospheric Imaging Assembly (AIA), exhibit
decay-less transverse oscillations after a flare erupts nearby one
of the loop footpoints. Measured oscillation periods lie between 4.2
min and 6.9 min wherein the displacement amplitudes range from 0.17
Mm to 1.16 Mm. A motion-magnification technique has been employed
to detect the preflare decay-less oscillations. These oscillations
have similar periods (between 3.7 min and 5.0 min) similar to the
previous ones, but their amplitudes (0.04 Mm to 0.12 Mm) are found
to be significantly smaller. No phase difference is found among
oscillating threads of a loop when observed through a particular AIA
channel or when their multichannel signatures are compared. These
features suggest that the occurrence of a flare in this case neither
changed the nature of these oscillations (decaying versus decay-less),
nor the oscillation periods. The only effect the flare has is to
increase the oscillation amplitudes. Movie is available at https://www.aanda.org
Title: Statistical investigation on the formation of sunspot light
bridges
Authors: Li, Fu-Yu; Chen, Yu-Hao; Song, Yong-Liang; Hou, Zhen-Yong;
Tian, Hui
Bibcode: 2021RAA....21..144L
Altcode:
Light bridges (LBs) are bright lanes that divide one sunspot umbra
into two or more parts. Though frequently observed in sunspots,
their formation mechanisms have rarely been studied and thus are not
well understood. Here we present results from the first statistical
investigation on the formation of LBs. Using observations with
the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory, we identified 144 LBs within 71 active regions (ARs) over
the whole year of 2014. The formation processes of these LBs can be
categorized into three groups: penumbral intrusion (type-A), sunspot
merging (type-B) and umbral-dot emergence (type-C). The numbers of
events in these three categories are 74, 57 and 13, respectively. The
duration of the LB formation process is mostly less than 40 hours,
with an average of ~20 hours. Most LBs have a maximum length of less
than 20″. For type-A LBs, we found a positive correlation between
the LB length and the duration of the LB formation process, suggesting
a similar speed of penumbral intrusion in different sunspots.
Title: Quasi-Periodic Pulsations in Solar and Stellar Flares:
A Review of Underpinning Physical Mechanisms and Their Predicted
Observational Signatures
Authors: Zimovets, I. V.; McLaughlin, J. A.; Srivastava, A. K.;
Kolotkov, D. Y.; Kuznetsov, A. A.; Kupriyanova, E. G.; Cho, I. -H.;
Inglis, A. R.; Reale, F.; Pascoe, D. J.; Tian, H.; Yuan, D.; Li, D.;
Zhang, Q. M.
Bibcode: 2021SSRv..217...66Z
Altcode:
The phenomenon of quasi-periodic pulsations (QPPs) in solar and stellar
flares has been known for over 50 years and significant progress has
been made in this research area. It has become clear that QPPs are
not rare—they are found in many flares and, therefore, robust flare
models should reproduce their properties in a natural way. At least
fifteen mechanisms/models have been developed to explain QPPs in solar
flares, which mainly assume the presence of magnetohydrodynamic (MHD)
oscillations in coronal structures (magnetic loops and current sheets)
or quasi-periodic regimes of magnetic reconnection. We review the most
important and interesting results on flare QPPs, with an emphasis on
the results of recent years, and we present the predicted and prominent
observational signatures of each of the fifteen mechanisms. However,
it is not yet possible to draw an unambiguous conclusion as to
the correct underlying QPP mechanism because of the qualitative,
rather than quantitative, nature of most of the models and also due
to insufficient observational information on the physical properties
of the flare region, in particular the spatial structure of the QPP
source. We also review QPPs in stellar flares, where progress is
largely based on solar-stellar analogies, suggesting similarities in
the physical processes in flare regions on the Sun and magnetoactive
stars. The presence of QPPs with similar properties in solar and
stellar flares is, in itself, a strong additional argument in favor
of the likelihood of solar-stellar analogies. Hence, advancing our
understanding of QPPs in solar flares provides an important additional
channel of information about stellar flares. However, further work in
both theory/simulations and in observations is needed.
Title: VizieR Online Data Catalog: Ages of field stars from white
dwarf comp. in Gaia (Qiu+, 2021)
Authors: Qiu, D.; Tian, H. -J.; Wang, X. -D.; Nie, J. -L.; von Hippel,
T.; Liu, G. -C.; Fouesneau, M.; Rix, H. -W.
Bibcode: 2021yCat..22530058Q
Altcode:
In this work, we select 9589 MS-WD and 307 WD-WD candidate binaries
from the catalog released by Tian+ (2020, J/ApJS/246/4), which is a
catalog of 807611 binaries, by searching from Gaia DR2 in a range of
distance <4.0kpc and the projected separation s<1.0pc. According
to the contamination rates, we selected 4050 MS-WD binaries with
contamination rates <20%. See Section 2. We use an open-source
software suite, BASE-9 (von Hippel+ 2014arXiv1411.3786V), to constrain
the ages, ZAMS mass, AV, and distance modulus for the 4050
MS-WD candidate binaries. See Section 3. We build a catalog that
includes the ages and other parameters (see Table 2) of the 3551 field
MS components. We also provide a catalog that includes the ages and
other parameters of the 3551 MS-WD and 236 WD-WD binaries (see Table
4). (2 data files).
Title: Formation of Solar Quiescent Coronal Loops through Magnetic
Reconnection in an Emerging Active Region
Authors: Hou, Zhenyong; Tian, Hui; Chen, Hechao; Zhu, Xiaoshuai; Huang,
Zhenghua; Bai, Xianyong; He, Jiansen; Song, Yongliang; Xia, Lidong
Bibcode: 2021ApJ...915...39H
Altcode: 2021arXiv210503199H
Coronal loops are the building blocks of solar active regions. However,
their formation mechanism remains poorly understood. Here we present
direct observational evidence for the formation of coronal loops
through magnetic reconnection as new magnetic fluxes emerge into the
solar atmosphere. Extreme-ultraviolet observations by the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO)
clearly show the newly formed loops following magnetic reconnection
within a plasma sheet. Formation of the loops is also seen in the Hα
line-core images taken by the New Vacuum Solar Telescope. Observations
from the Helioseismic and Magnetic Imager on board SDO show that a
positive-polarity flux concentration moves toward a negative-polarity
one with a speed of ~0.4 km s-1 before the formation of
coronal loops. During the loop formation process, we found signatures
of flux cancellation and subsequent enhancement of the transverse
field between the two polarities. The three-dimensional magnetic
field structure reconstructed through a magnetohydrostatic model
shows field lines consistent with the loops in AIA images. Numerous
bright blobs with an average width of 1.37 Mm appear intermittently
in the plasma sheet and move upward with a projected velocity of ~114
km s-1. The temperature, emission measure, and density of
these blobs are about 3 MK, 2.0 × 1028 cm-5, and
1.2 × 1010 cm-3, respectively. A power spectral
analysis of these blobs indicates that the observed reconnection is
likely not dominated by a turbulent process. We have also identified
flows with a velocity of 20-50 km s-1 toward the footpoints
of the newly formed coronal loops.
Title: The Instability Strip of RRab Stars From LAMOST-Gaia Data
Authors: Luo, C.; Liu, C.; Zhang, X.; Chen, X.; Sun, W.; Zhang, X.;
Li, Y.; Zhang, C.; Tian, H.; Deng, L.
Bibcode: 2021ASPC..529..147L
Altcode:
We compile 995 RRab samples from various literature and cross-match
them with LAMOST to identify the boundaries of instability strip (IS)
at red and blue edges and to associate them with metallicity. We find
a significant gap at [Fe/H]=-0.5 dex, which separates the RRab stars
into metal-poor and metal-rich groups. For the metal-poor group, the IS
moves toward cooler Teff with declining [Fe/H]. The slopes,
dTeff/d[Fe/H], are 192±19 K/dex and 354±51 K/dex for
red and blue edges, respectively. For the metal-rich group, the blue
edge of IS moves toward hotter Teff with declining [Fe/H],
while the red edge is nearly independent on [Fe/H]. A similar trend
is also confirmed in Teff-luminosity diagram. By comparing
with various theoretical models of RRab stars, we find the correlation
between IS and metallicity is not consistent with the theoretical model.
Title: VizieR Online Data Catalog: Sgr stream K- & M-giants and
BHB stars (Yang+, 2019)
Authors: Yang, C.; Xue, X. -X.; Li, J.; Liu, C.; Zhang, Bo; Rix,
H. -W.; Zhang, L.; Zhao, G.; Tian, H.; Zhong, J.; Xing, Q.; Wu, Y.;
Li, C.; Carlin, J. L.; Chang, J.
Bibcode: 2021yCat..18860154Y
Altcode:
The Sgr stream sample consists of K-giants, M-giants, and blue
horizontal branch stars (BHBs). The K-giants are from Sloan Extension
for Galactic Understanding and Exploration 2 (SEGUE-2; Yanny+,
2009, J/AJ/137/4377) and the fifth data release of Large Sky Area
Multi-Object Fibre Spectroscopic Telescope (LAMOST DR5; see V/164),
and their distances were estimated by Bayesian method Xue+ (2014,
J/ApJ/784/170). The M-giants are picked up from LAMOST DR5 through
a 2MASS+WISE photometric selection criteria. The distances were
calculated through the (J-K)0 color-distance relation Li+
(2016, J/ApJ/823/59), Li+ (2019, J/ApJ/874/138), and Zhong+ (2019,
J/ApJS/244/8). The BHBs are chosen from SDSS by color and Balmer line
cuts, and their distances were easy to estimate because of the nearly
constant absolute magnitude of BHB stars (Xue+ 2011, J/ApJ/738/79). (2 data files).
Title: Mapping the global magnetic field in the solar corona through
magnetoseismology
Authors: Yang, Zihao; Bethge, Christian; Tian, Hui; Tomczyk, Steven;
Morton, Richard; Del Zanna, Giulio; McIntosh, Scott; Karak, Bidya
Binay; Gibson, Sarah; Samanta, Tanmoy; He, Jiansen; Chen, Yajie;
Wang, Linghua; Bai, Xianyong
Bibcode: 2021EGUGA..23..642Y
Altcode:
Magnetoseismology, a technique of magnetic field diagnostics based
on observations of magnetohydrodynamic (MHD) waves, has been widely
used to estimate the field strengths of oscillating structures in
the solar corona. However, previously magnetoseismology was mostly
applied to occasionally occurring oscillation events, providing
an estimate of only the average field strength or one-dimensional
distribution of field strength along an oscillating structure. This
restriction could be eliminated if we apply magnetoseismology to the
pervasive propagating transverse MHD waves discovered with the Coronal
Multi-channel Polarimeter (CoMP). Using several CoMP observations of
the Fe XIII 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of
the plasma density and wave phase speed in the corona, which allow us
to map both the strength and direction of the coronal magnetic field
in the plane of sky. We also examined distributions of the electron
density and magnetic field strength, and compared their variations
with height in the quiet Sun and active regions. Such measurements
could provide critical information to advance our understanding of the
Sun's magnetism and the magnetic coupling of the whole solar atmosphere.
Title: Transient small-scale brightenings in the quiet Sun corona:
a model for "campfires" observed with Solar Orbiter
Authors: Chen, Yajie; Przybylski, Damien; Peter, Hardi; Tian, Hui
Bibcode: 2021EGUGA..23.5061C
Altcode:
Recent observations by the Extreme Ultraviolet Imager (EUI)
onboard Solar Orbiter have revealed prevalent small-scale transient
brightenings in the quiet solar corona termed campfires. To understand
the generation mechanism of these coronal brightenings, we constructed
a self- consistent and time-dependent quiet-Sun model extending from
the upper convection zone to the lower corona using a realistic 3D
radiation MHD simulation. From the model we have synthesized the coronal
emission in the EUI 174 Å passband. We identified several transient
coronal brightenings similar to those in EUI observations. The size
and lifetime of these coronal brightenings are 2-4 Mm and ∼2 min,
respectively. These brightenings are located at a height of 2-4 Mm
above the photosphere, and the surrounding plasma is often heated
above 1 MK. These findings are consistent with the observational
characterisation of the campfires. Through a comparison of the magnetic
field structures before and after the occurrence of brightenings, we
conclude that these coronal brightenings are generated by component
magnetic reconnection between interacting bundles of field lines or
the relaxation of highly twisted flux ropes. Occurring in the coronal
part of the atmosphere, these events show no measurable signature
in the photosphere. These transient coronal brightenings may play an
important role in heating of the local coronal plasma.
Title: Formation of solar coronal loops through magnetic reconnection
in an emerging active region
Authors: Hou, Zhenyong; Tian, Hui; Chen, Hechao; Zhu, Xiaoshuai; He,
Jiansen; Bai, Xianyong; Huang, Zhenghua; Xia, Lidong
Bibcode: 2021EGUGA..23.1013H
Altcode:
Coronal loops are building blocks of solar active regions
(ARs). However, their formation is not well understood. Here we
present direct observational evidence for the formation of coronal loops
through magnetic reconnection as new magnetic fluxes emerge to the solar
atmosphere. Observations in the EUV passbands of SDO/AIA clearly show
the newly formed loops following magnetic reconnection within a vertical
current sheet. Formation of the loops is also seen in the Hα images
taken by NVST. The SDO/HMI observations show that a positive-polarity
flux concentration moves toward a negative-polarity one with a speed of
~0.5 km s-1 before the apparent formation of coronal loops. During the
formation of coronal loops, we found signatures of flux cancellation
and subsequent enhancement of the transverse field between the two
polarities. We have reconstructed the three-dimensional magnetic field
structure through a magnetohydrostatic model, which shows field lines
consistent with the loops in AIA images. Numerous bright blobs with a
width of ~1.5 Mm appear intermittently in the current sheet and move
upward with apparent velocities of ~80 km s-1. We have also identified
plasma blobs moving to the footpoints of the newly formed large loops,
with apparent velocities ranging from 30 to 50 km s-1. A differential
emission measure analysis shows that the temperature, emission measure
and density of the bright blobs are 2.5-3.5 MK, 1.1-2.3×1028 cm-5 and
8.9-12.9×109 cm-3, respectively. Power spectral analysis of these
blobs indicates that the magnetic reconnection is inconsistent with
the turbulent reconnection scenario.
Title: Upflows in the Upper Solar Atmosphere
Authors: Tian, Hui; Harra, Louise; Baker, Deborah; Brooks, David H.;
Xia, Lidong
Bibcode: 2021SoPh..296...47T
Altcode: 2021arXiv210202429T
Spectroscopic observations at extreme- and far-ultraviolet wavelengths
have revealed systematic upflows in the solar transition region and
corona. These upflows are best seen in the network structures of
the quiet Sun and coronal holes, boundaries of active regions, and
dimming regions associated with coronal mass ejections. They have been
intensively studied in the past two decades because they are likely to
be closely related to the formation of the solar wind and heating of the
upper solar atmosphere. We present an overview of the characteristics
of these upflows, introduce their possible formation mechanisms, and
discuss their potential roles in the mass and energy transport in the
solar atmosphere. Although past investigations have greatly improved
our understanding of these upflows, they have left us with several
outstanding questions and unresolved issues that should be addressed
in the future. New observations from the Solar Orbiter mission, the
Daniel K. Inouye Solar Telescope, and the Parker Solar Probe will
likely provide critical information to advance our understanding of
the generation, propagation, and energization of these upflows.
Title: In Situ Detection of the Solar Eruption: Lay a Finger on
the Sun
Authors: Lin, Jun; Huang, Shanjie; Li, Yan; Chong, Xiaoyu; Zhang,
Shenyi; Li, Mingtao; Zhang, Yiteng; Zhou, Bin; Ouyang, Gaoxiang; Xiang,
Lei; Dong, Liang; Ji, Haisheng; Tian, Hui; Song, Hongqiang; Liu, Yu;
Jin, Zhenyu; Feng, Jing; Zhang, Hongbo; Zhang, Xianguo; Zhang, Weijie;
Huang, Min; Lü, Qunbo; Deng, Lei; Fu, Huishan; Cheng, Xin; Wang, Min
Bibcode: 2021ChJSS..41..183L
Altcode:
No abstract at ADS
Title: Small-scale Bright Blobs Ejected from a Sunspot Light Bridge
Authors: Li, Fuyu; Chen, Yajie; Hou, Yijun; Tian, Hui; Bai, Xianyong;
Song, Yongliang
Bibcode: 2021ApJ...908..201L
Altcode: 2021arXiv210213545L
Light bridges (LBs) are bright lanes that divide an umbra into multiple
parts in some sunspots. Persistent oscillatory bright fronts at a
temperature of ∼105 K are commonly observed above LBs in
the 1400/1330 Špassbands of the Interface Region Imaging Spectrograph
(IRIS). Based on IRIS observations, we report small-scale bright blobs
ejected from the oscillating bright front above a light bridge. Some of
these blobs reveal a clear acceleration, whereas the others do not. The
average speed of these blobs projected onto the plane of sky is 71.7
± 14.7 km s-1, with an initial acceleration of 1.9 ± 1.3
km s-2. These blobs normally reach a projected distance of
3-7 Mm from their origin sites. They have an average projected area of
0.57 ± 0.37 Mm2. The blobs were also detected in multiple
extreme-ultraviolet passbands of the Atmospheric Imaging Assembly on
board the Solar Dynamics Observatory, but not in the Hα images taken
by the New Vacuum Solar Telescope. The typical temperature and electron
density of these blobs are around ${10}^{5.47}$ K and ${10}^{9.7}$
cm-3, respectively. The estimated kinetic and thermal
energies are on the order of ${10}^{22.8}$ erg and ${10}^{23.3}$
erg, respectively. These small-scale blobs appear to show three
different types of formation processes. They are possibly triggered
by induced reconnection or release of enhanced magnetic tension due
to interaction of adjacent shocks, local magnetic reconnection between
emerging magnetic bipoles on the light bridge and surrounding unipolar
umbral fields, and plasma acceleration or instability caused by upward
shocks, respectively.
Title: VizieR Online Data Catalog: LAMOST-Kepler/K2 survey (LK-MRS)
first year obs. (Zong+, 2020)
Authors: Zong, W.; Fu, J. -N.; De Cat, P.; Wang, J.; Shi, J.; Luo, A.;
Zhang, H.; Frasca, A.; Molenda-Zakowicz, J.; Gray, R. O.; Corbally,
C. J.; Catanzaro, G.; Cang, T.; Wang, J.; Chen, J.; Hou, Y.; Liu,
J.; Niu, H.; Pan, Y.; Tian, H.; Yan, H.; Zhang, Y.; Zuo, H.
Bibcode: 2021yCat..22510015Z
Altcode:
Phase II of the Large Sky Area Multi-Object Fibre Spectroscopic
Telescope (LAMOST)-Kepler/K2 survey (LK-MRS), initiated in 2018,
aims at collecting medium-resolution spectra (R~7500; hereafter MRS)
for more than 50000 stars with multiple visits (~60 epochs) over a
period of 5yr (2018 September to 2023 June). We selected 20 footprints
distributed across the Kepler field and six K2 campaigns, with each
plate containing a number of stars ranging from ~2000 to ~3000. During
the first year of observations, the LK-MRS has already visited 13
plates 223 times over 40 individual nights, and collected ~280000 and
~369000 high-quality spectra in the blue and red wavelength ranges,
respectively. The atmospheric parameters and radial velocities for
~259000 spectra of 21053 targets were successfully calculated by the
LAMOST stellar parameter pipeline. The internal uncertainties for the
effective temperature, surface gravity, metallicity, and radial velocity
are found to be 100K, 0.15dex, 0.09dex, and 1.00km/s, respectively, when
derived from a medium-resolution LAMOST spectrum with a signal-to-noise
ratio (S/N) in the g band of 10. All of the uncertainties decrease as
S/N increases, but they stabilize for S/N>100. We found 14997, 20091,
and 1514 stars in common with the targets from the LAMOST low-resolution
survey (LRS), Gaia, and the Apache Point Observatory Galactic Evolution
Experiment (APOGEE), respectively, corresponding to fractions of ~70%,
~95%, and ~7.2%. In general, the parameters derived from LK-MRS spectra
are consistent with those obtained from the LRS and APOGEE spectra,
but the scatter increases as the surface gravity decreases when
comparing with the measurements from APOGEE. A large discrepancy is
found with the Gaia values of the effective temperature. Comparisons
of the radial velocities of LK-MRS to Gaia and LK-MRS to APOGEE nearly
follow a Gaussian distribution with means of μ~1.10 and 0.73km/s,
respectively. We expect that the results from the LK-MRS spectra will
shed new light on binary stars, asteroseismology, stellar activity,
and other research fields. (1 data file).
Title: A magnetic reconnection model for hot explosions in the cool
atmosphere of the Sun
Authors: Ni, Lei; Chen, Yajie; Peter, Hardi; Tian, Hui; Lin, Jun
Bibcode: 2021A&A...646A..88N
Altcode: 2020arXiv201107692N
Context. Ultraviolet (UV) bursts and Ellerman bombs (EBs) are
transient brightenings observed in the low solar atmospheres of
emerging flux regions. Magnetic reconnection is believed to be the
main mechanism leading to formation of the two activities, which are
usually formed far apart from each other. However, observations also
led to the discovery of co-spatial and co-temporal EBs and UV bursts,
and their formation mechanisms are still not clear. The multi-thermal
components in these events, which span a large temperature range,
challenge our understanding of magnetic reconnection and heating
mechanisms in the partially ionized lower solar atmosphere.
Aims: We studied magnetic reconnection between the emerging magnetic
flux and back ground magnetic fields in the partially ionized and
highly stratificated low solar atmosphere. We aim to explain the
multi-thermal characteristics of UV bursts, and to find out whether
EBs and UV bursts can be generated in the same reconnection process
and how they are related with each other. We also aim to unearth the
important small-scale physics in these events.
Methods: We used
the single-fluid magnetohydrodynamic (MHD) code NIRVANA to perform
simulations. The background magnetic fields and emerging fields at the
solar surface are reasonably strong. The initial plasma parameters are
based on the C7 atmosphere model. We simulated cases with different
resolutions, and included the effects of ambipolar diffusion, radiative
cooling, and heat conduction. We analyzed the current density, plasma
density, temperature, and velocity distributions in the main current
sheet region, and synthesized the Si IV emission spectrum.
Results: After the current sheet with dense photosphere plasma emerges
and reaches 0.5 Mm above the solar surface, plasmoid instability
appears. The plasmoids collide and coalesce with each other, which
causes the plasmas with different densities and temperatures to be mixed
up in the turbulent reconnection region. Therefore, the hot plasmas
corresponding to the UV emissions and colder plasmas corresponding to
the emissions from other wavelengths can move together and occur at
about the same height. In the meantime, the hot turbulent structures
concentrate above 0.4 Mm, whereas the cool plasmas extend to much
lower heights to the bottom of the current sheet. These phenomena
are consistent with published observations in which UV bursts have a
tendency to be located at greater heights close to corresponding EBs
and all the EBs have partial overlap with corresponding UV bursts
in space. The synthesized Si IV line profiles are similar to that
observed in UV bursts; the enhanced wing of the line profiles can
extend to about 100 km s-1. The differences are significant
among the numerical results with different resolutions, indicating
that the realistic magnetic diffusivity is crucial to revealing the
fine structures and realistic plasmas heating in these reconnection
events. Our results also show that the reconnection heating contributed
by ambipolar diffusion in the low chromosphere around the temperature
minimum region is not efficient.
Title: Plasma heating induced by tadpole-like downflows in the
flaring solar corona
Authors: Samanta, T.; Tian, H.; Chen, B.; Reeves, K. K.; Cheung,
M. C. M.; Vourlidas, A.; Banerjee, D.
Bibcode: 2021Innov...200083S
Altcode: 2021arXiv210314257S
As one of the most spectacular energy release events in the solar
system, solar flares are generally powered by magnetic reconnection in
the solar corona. As a result of the re-arrangement of magnetic field
topology after the reconnection process, a series of new loop-like
magnetic structures are often formed and are known as flare loops. A
hot diffuse region, consisting of around 5-10 MK plasma, is also
observed above the loops and is called a supra-arcade fan. Often,
dark, tadpole-like structures are seen to descend through the bright
supra-arcade fans. It remains unclear what role these so-called
supra-arcade downflows (SADs) play in heating the flaring coronal
plasma. Here we show a unique flare observation, where many SADs collide
with the flare loops and strongly heat the loops to a temperature
of 10-20 MK. Several of these interactions generate clear signatures
of quasi-periodic enhancement in the full-Sun-integrated soft X-ray
emission, providing an alternative interpretation for quasi-periodic
pulsations that are commonly observed during solar and stellar flares.
Title: VizieR Online Data Catalog: LAMOST K giants in Galactic halo
substructures (Yang+, 2019)
Authors: Yang, C.; Xue, X. -X.; Li, J.; Zhang, L.; Liu, C.; Zhao,
G.; Chang, J.; Tian, H.; Li, C.
Bibcode: 2021yCat..18800065Y
Altcode:
LAMOST DR5 combining with Gaia DR2 enables us to construct a large
sample of 13554 halo K giants with distances up to 100kpc, radial
velocities, metallicities, and proper motions (see Section 2). Finally, we find 43 friends-of-friends (FoF) groups (1867 group
members), in which 25 groups belong to five known substructures: Sgr
Stream (13 groups), Monoceros Ring (4 groups), Virgo Overdensity (4
groups), Hercules-Aquila Cloud (3 groups), and Orphan Stream (1 group);
and 18 remaining groups are likely related to unknown substructures. See
Section 5. (3 data files).
Title: VizieR Online Data Catalog: The extended Gaia-PS1-SDSS (GPS1+)
proper motion catalog (Tian+, 2020)
Authors: Tian, H. -J.; Xu, Y.; Liu, C.; Rix, H. -W.; Sesar, B.;
Goldman, B.
Bibcode: 2021yCat.1351....0T
Altcode:
We still use the four basic imaging surveys, i.e., Gaia, PS1 (II/349),
SDSS, and 2MASS, to build the GPS1+ catalog. Unlike GPS1 (I/343),
GPS1+ will be based on the Gaia DR2 (I/345), but the other three
astrometric data sets remain the same as those used in GPS1, i.e.,
the same data version and treatment. (1 data file).
Title: Microwave diagnostics of magnetic field strengths in solar
flaring loops
Authors: Zhu, Rui; Tan, BaoLin; Su, YingNa; Tian, Hui; Xu, Yu; Chen,
XingYao; Song, YongLiang; Tan, GuangYu
Bibcode: 2021ScChE..64..169Z
Altcode: 2020arXiv200615014Z
We have performed microwave diagnostics of the magnetic field
strengths in solar flare loops based on the theory of gyrosynchrotron
emission. From Nobeyama Radioheliograph observations of three flare
events at 17 and 34 GHz, we obtained the degree of circular polarization
and the spectral index of microwave flux density, which were then used
to map the magnetic field strengths in post-flare loops. Our results
show that the magnetic field strength typically decreases from ~800 G
near the loop footpoints to ~100 G at a height of 10--25 Mm. Comparison
of our results with magnetic field modeling using a flux rope insertion
method is also discussed. Our study demonstrates the potential of
microwave imaging observations, even at only two frequencies, in
diagnosing the coronal magnetic field of flaring regions.
Title: Quiet-time Solar Wind Suprathermal Electrons of Different
Solar Origins
Authors: Yang, L.; Wang, L.; Zhao, L.; Tao, J.; Li, G.;
Wimmer-Schweingruber, R. F.; He, J.; Tian, H.; Bale, S. D.
Bibcode: 2020AGUFMSH008..07Y
Altcode:
The energy spectrum of solar wind strahl, halo, and superhalo electrons
likely carries crucial information on their possible origin and
acceleration at the Sun. Here we statistically investigate the energy
spectrum of solar wind strahl/ halo electrons at ∼ 0.1- 1.5 keV and
superhalo electrons at ∼ 20- 200 keV measured by Wind/ 3D Plasma
and Energetic Particle during quiet times from 1998 to 2014, according
to the types of their Potential Field Source Surface- mapped coronal
source regions ( CSRs) . We adopt the classifi cation scheme developed
by Zhao et al. to categorize the CSRs into four types: active region
( AR) , quiet Sun ( QS) , coronal hole ( CH) , and helmet-streamer
associated region ( HS) . We fi nd that for the quiet-time strahl,
the AR and HS ( QS and CH) correspond to a smaller ( larger) kappa
index κ strahl with the most frequent value of 7- 8.5
(8.5- 10) and a larger ( smaller) nstrahl with the most
frequent value of 0.013- 0.026 cm− 3 ( 0.006- 0.0013
cm− 3 ) . For the quiet-time halo, κ halo
behaves similarly to κ strahl , but nhalo
appears similar among the four CSR types. For the superhalo, the AR
( QS) corresponds to a larger ( smaller) power-law index β with the
most frequent value of 2.2- 2.4 ( 1.8- 2.0) , while the HS and CH have
a β not different from either the AR or QS; nsup appears
similar, with the most frequent value of 3x10− 8 - 3x
10− 7 cm− 3 , among the four
CSR types. These results suggest that the strahl ( superhalo) from
the hotter CSRs tends to be more ( less) effi ciently accelerated.
Title: The White Dwarf Binary Pathways Survey. V. The Gaia White Dwarf
Plus AFGK Binary Sample and the Identification of 23 Close Binaries
Authors: Ren, J. -J.; Raddi, R.; Rebassa-Mansergas, A.; Hernandez,
M. S.; Parsons, S. G.; Irawati, P.; Rittipruk, P.; Schreiber, M. R.;
Gänsicke, B. T.; Torres, S.; Wang, H. -J.; Zhang, J. -B.; Zhao, Y.;
Zhou, Y. -T.; Han, Z. -W.; Wang, B.; Liu, C.; Liu, X. -W.; Wang, Y.;
Zheng, J.; Wang, J. -F.; Zhao, F.; Cui, K. -M.; Shi, J. -R.; Tian, H.
Bibcode: 2020ApJ...905...38R
Altcode: 2020arXiv201002885R
Close white dwarf binaries consisting of a white dwarf and an A-, F-,
G-, or K-type main-sequence star, henceforth close WD+AFGK binaries,
are ideal systems to understand the nature of type Ia supernovae
progenitors and to test binary evolution models. In this work we
identify 775 WD+AFGK candidates from TGAS (The Tycho-Gaia Astrometric
Solution) and Gaia Data Release 2 (DR2), a well-defined sample of
stars with available parallaxes, and we measure radial velocities
(RVs) for 275 of them with the aim of identifying close binaries. The
RVs have been measured from high-resolution spectra obtained at the
Xinglong 2.16 m Telescope and the San Pedro Mártir 2.12 m Telescope
and/or from available LAMOST DR6 (low-resolution) and RAVE DR5
(medium-resolution) spectra. We identify 23 WD+AFGK systems displaying
more than 3σ RV variation among 151 systems for which the measured
values are obtained from different nights. Our WD+AFGK binary sample
contains both AFGK dwarfs and giants, with a giant fraction ∼43%. The
close binary fractions we determine for the WD+AFGK dwarf and giant
samples are ≃24% and ≃15%, respectively. We also determine the
stellar parameters (i.e., effective temperature, surface gravity,
metallicity, mass, and radius) of the AFGK companions with available
high-resolution spectra. The stellar parameter distributions of the
AFGK companions that are members of close and wide binary candidates
do not show statistically significant differences.
Title: Coronal Heating by MHD Waves
Authors: Van Doorsselaere, Tom; Srivastava, Abhishek K.; Antolin,
Patrick; Magyar, Norbert; Vasheghani Farahani, Soheil; Tian, Hui;
Kolotkov, Dmitrii; Ofman, Leon; Guo, Mingzhe; Arregui, Iñigo; De
Moortel, Ineke; Pascoe, David
Bibcode: 2020SSRv..216..140V
Altcode: 2020arXiv201201371V
The heating of the solar chromosphere and corona to the observed high
temperatures, imply the presence of ongoing heating that balances
the strong radiative and thermal conduction losses expected in the
solar atmosphere. It has been theorized for decades that the required
heating mechanisms of the chromospheric and coronal parts of the active
regions, quiet-Sun, and coronal holes are associated with the solar
magnetic fields. However, the exact physical process that transport
and dissipate the magnetic energy which ultimately leads to the solar
plasma heating are not yet fully understood. The current understanding
of coronal heating relies on two main mechanism: reconnection and MHD
waves that may have various degrees of importance in different coronal
regions. In this review we focus on recent advances in our understanding
of MHD wave heating mechanisms. First, we focus on giving an overview
of observational results, where we show that different wave modes have
been discovered in the corona in the last decade, many of which are
associated with a significant energy flux, either generated in situ
or pumped from the lower solar atmosphere. Afterwards, we summarise
the recent findings of numerical modelling of waves, motivated by the
observational results. Despite the advances, only 3D MHD models with
Alfvén wave heating in an unstructured corona can explain the observed
coronal temperatures compatible with the quiet Sun, while 3D MHD wave
heating models including cross-field density structuring are not yet
able to account for the heating of coronal loops in active regions to
their observed temperature.
Title: Solar Energetic Electron Events Associated with Hard X-ray
Flares
Authors: Wang, W.; Wang, L.; Krucker, S.; Mason, G. M.; Bucik, R.;
Tian, H.; He, J.; Su, Y.; Bale, S.
Bibcode: 2020AGUFMSH045..05W
Altcode:
We presented a comparison survey of 16 solar energetic electron (SEE)
events measured by WIND/3DP with associated hard X-ray (HXR) flares
measured by RHESSI with good count statistics, from 2002 February to
2016 December. Among the 16 good cases, all show a double-power-law
shape with a downward break at ~ 68keV in the electron peak flux versus
energy spectrum at ~ 5-200keV; All cases show a clear single-power-law
in the HXR peak flux versus energy spectrum, and a single-power-law in
the flux versus energy spectrum of HXR-producing electrons derived via
bremsstrahlung mechanisms. For SEEs, their acceleration site at the
Sun should occur high (at a heliocentric altitude of ≧1.3 Rs
(the average solar radius)), to remain a power-law spectrum
extending down to ~ 5 keV during their anti-sunward escape into the
interplanetary medium (IPM). For 14 out of 16 cases, the spectral
index above the energy break of SEEs observed at 1AU basically agreed
with the derived spectral index of HXR-producing electrons. However,
the SEE spectral index below the energy break is all smaller than the
HXR-producing electron spectral index. These results suggest that SEEs
and HXR-producing electrons share a common origin and an additional
acceleration could exist between the SEE source and the flare region. In
addition, for cases with 3He/4He>0.01, the
observed 3He/4He shows a correlation with the
spectral index above the energy break of SEEs, indicating a possible
relation of the 3He-rich ion acceleration at high corona
with SEEs.
Title: Exploring the Perturbed Milky Way Disk and the Substructures
of the Outer Disk
Authors: Xu, Y.; Liu, C.; Tian, H.; Newberg, H. J.; Laporte, C. F. P.;
Zhang, B.; Wang, H. F.; Fu, X.; Li, J.; Deng, L. C.
Bibcode: 2020ApJ...905....6X
Altcode: 2020arXiv201014096X
The recent discovery of a spiral feature in the Z - VZ
phase plane in the solar neighborhood implies that the galactic disk
has been remarkably affected by a dwarf galaxy passing through it
some hundreds of millions of years ago. Using 429,500 Large Sky Area
Multi-Object Fibre Spectroscopic Telescope K giants stars, we show that
the spiral feature exists not only in the solar vicinity but it also
extends to about 15 kpc from the Galactic center and then disappears
beyond this radius. Moreover, we find that when the spiral features
in a plot of Vφ as a function of position in the Z -
VZ plane at various galactocentric radii are remapped to
the R - Z plane, the spiral can explain well the observed asymmetric
velocity substructures. This is evidence that the phase spiral features
are the same as the bulk motions found in previous work as well as
this work. Test particle simulations and N-body simulations show that
an encounter with a dwarf galaxy a few hundred million years ago will
induce a perturbation in the galactic disk. In addition, we find that
the last impact of Sgr dSph can also contribute to the flare. As a
consequence of the encounter, the distribution function of disk stars at
a large range of radii is imprinted by the gravitational perturbation.
Title: Untangling the global coronal magnetic field with
multiwavelength observations
Authors: Gibson, S. E.; Malanushenko, A.; de Toma, G.; Tomczyk, S.;
Reeves, K.; Tian, H.; Yang, Z.; Chen, B.; Fleishman, G.; Gary, D.;
Nita, G.; Pillet, V. M.; White, S.; Bąk-Stęślicka, U.; Dalmasse,
K.; Kucera, T.; Rachmeler, L. A.; Raouafi, N. E.; Zhao, J.
Bibcode: 2020arXiv201209992G
Altcode:
Magnetism defines the complex and dynamic solar corona. Coronal
mass ejections (CMEs) are thought to be caused by stresses, twists,
and tangles in coronal magnetic fields that build up energy and
ultimately erupt, hurling plasma into interplanetary space. Even the
ever-present solar wind possesses a three-dimensional morphology shaped
by the global coronal magnetic field, forming geoeffective corotating
interaction regions. CME evolution and the structure of the solar
wind depend intimately on the coronal magnetic field, so comprehensive
observations of the global magnetothermal atmosphere are crucial both
for scientific progress and space weather predictions. Although some
advances have been made in measuring coronal magnetic fields locally,
synoptic measurements of the global coronal magnetic field are not yet
available. We conclude that a key goal for 2050 should be comprehensive,
ongoing 3D synoptic maps of the global coronal magnetic field. This will
require the construction of new telescopes, ground and space-based,
to obtain complementary, multiwavelength observations sensitive
to the coronal magnetic field. It will also require development of
inversion frameworks capable of incorporating multi-wavelength data,
and forward analysis tools and simulation testbeds to prioritize and
establish observational requirements on the proposed telescopes.
Title: Generation of solar spicules and subsequent atmospheric heating
Authors: Chen, YaJie; Samanta, Tanmoy; Tian, Hui
Bibcode: 2020ScChE..63.2467C
Altcode:
No abstract at ADS
Title: Mapping the magnetic field in the solar corona through
magnetoseismology
Authors: Yang, ZiHao; Tian, Hui; Tomczyk, Steven; Morton, Richard;
Bai, XianYong; Samanta, Tanmoy; Chen, YaJie
Bibcode: 2020ScChE..63.2357Y
Altcode: 2020arXiv200803146Y
Magnetoseismology, a technique of magnetic field diagnostics based
on observations of magnetohydrodynamic (MHD) waves, has been widely
used to estimate the field strengths of oscillating structures in
the solar corona. However, previously magnetoseismology was mostly
applied to occasionally occurring oscillation events, providing
an estimate of only the average field strength or one-dimensional
distribution of field strength along an oscillating structure. This
restriction could be eliminated if we apply magnetoseismology to the
pervasive propagating transverse MHD waves discovered with the Coronal
Multi-channel Polarimeter (CoMP). Using several CoMP observations of
the Fe xiii 1074.7 nm and 1079.8 nm spectral lines, we obtained maps of
the plasma density and wave phase speed in the corona, which allow us
to map both the strength and direction of the coronal magnetic field
in the plane of sky. We also examined distributions of the electron
density and magnetic field strength, and compared their variations
with height in the quiet Sun and active regions. Such measurements
could provide critical information to advance our understanding of the
Sun's magnetism and the magnetic coupling of the whole solar atmosphere.
Title: VizieR Online Data Catalog: The ZTF catalog of periodic
variable stars (Chen+, 2020)
Authors: Chen, X.; Wang, S.; Deng, L.; de Grijs, R.; Yang, M.; Tian, H.
Bibcode: 2020yCat..22490018C
Altcode:
The Zwicky Transient Facility (ZTF) is a 48-inch Schmidt telescope at
the Palomar Observatory with a 47deg2 field of view. ZTF
DR2 contains data acquired between 2018 March and 2019 June, covering
a time span of around 470 days. The photometry is provided in the g
and r bands, with a uniform exposure time of 30s per observation. ZTF DR2 includes more than 1 billion stars, about half of which have
>20 epochs of observations. For the majority of stars located in the
northern Galactic plane, ZTF contains ~150 epochs of observations. (4 data files).
Title: Global maps of the magnetic field in the solar corona
Authors: Yang, Zihao; Bethge, Christian; Tian, Hui; Tomczyk, Steven;
Morton, Richard; Del Zanna, Giulio; McIntosh, Scott W.; Karak, Bidya
Binay; Gibson, Sarah; Samanta, Tanmoy; He, Jiansen; Chen, Yajie;
Wang, Linghua
Bibcode: 2020Sci...369..694Y
Altcode: 2020arXiv200803136Y
Understanding many physical processes in the solar atmosphere requires
determination of the magnetic field in each atmospheric layer. However,
direct measurements of the magnetic field in the Sun’s corona are
difficult to obtain. Using observations with the Coronal Multi-channel
Polarimeter, we have determined the spatial distribution of the
plasma density in the corona and the phase speed of the prevailing
transverse magnetohydrodynamic waves within the plasma. We combined
these measurements to map the plane-of-sky component of the global
coronal magnetic field. The derived field strengths in the corona,
from 1.05 to 1.35 solar radii, are mostly 1 to 4 gauss. Our results
demonstrate the capability of imaging spectroscopy in coronal magnetic
field diagnostics.
Title: Spectroscopic observations of the upper solar atmosphere
Authors: Tian, H.
Bibcode: 2020SPD....5120001T
Altcode:
Our understanding of the solar transition region and corona largely
relies on spectroscopic observations. Spectroscopy allows us to measure
key parameters such as plasma density, temperature, velocity, elemental
abundances and magnetic field, thus enabling us to quantitatively study
the mass and energy transport processes in the solar atmosphere. Since
2006, Hinode/EIS has been obtaining high-quality EUV spectra in active
regions, providing critical information to advance our understanding
of different types of coronal dynamics such as active region outflows,
CME-induced dimmings and persistent/decayless oscillations. Since 2013,
the simultaneous imaging and spectroscopic observations with IRIS
have uncovered a wide variety of fine structures and fast dynamics in
the transition region, both in the quiet Sun and above sunspots. In
addition, recent observations of the ground-based CoMP instrument have
demonstrated the capability of imaging spectroscopy in coronal magnetic
field diagnostics. With upcoming observations from Solar Orbiter/SPICE,
EUVST and UCoMP, spectroscopy will continue to be an important tool
for investigation of the magnetism and plasma dynamics in the upper
solar atmosphere.
Title: VizieR Online Data Catalog: Gaia and LAMOST DR4 M giant
members of Sgr stream (Li+, 2019)
Authors: Li, J.; Liu, C.; Xue, X.; Zhong, J.; Weiss, J.; Carlin,
J. L.; Tian, H.
Bibcode: 2020yCat..18740138L
Altcode:
In this paper, we map the 6D phase space of M giants from the
Sagittarius (Sgr) stream in LAMOST using LAMOST DR4 (V/153) radial
velocities combined with Gaia DR2 (I/345) proper motions and ALLWISE
(II/328) photometry. (1 data file).
Title: Quiet-time Solar Wind Suprathermal Electrons of Different
Solar Origins
Authors: Yang, Liu; Wang, Linghua; Zhao, Liang; Tao, Jiawei; Li,
Gang; Wimmer-Schweingruber, Robert F.; He, Jiansen; Tian, Hui; Bale,
Stuart D.
Bibcode: 2020ApJ...896L...5Y
Altcode:
The energy spectrum of solar wind strahl, halo, and superhalo electrons
likely carries crucial information on their possible origin and
acceleration at the Sun. Here we statistically investigate the energy
spectrum of solar wind strahl/halo electrons at ∼0.1-1.5 keV and
superhalo electrons at ∼20-200 keV measured by Wind/3D Plasma and
Energetic Particle during quiet times from 1998 to 2014, according to
the types of their Potential Field Source Surface-mapped coronal source
regions (CSRs). We adopt the classification scheme developed by Zhao et
al. to categorize the CSRs into four types: active region (AR), quiet
Sun (QS), coronal hole (CH), and helmet-streamer associated region
(HS). We find that for the quiet-time strahl, the AR and HS (QS and
CH) correspond to a smaller (larger) kappa index κstrahl
with the most frequent value of 7-8.5 (8.5-10) and a larger (smaller)
nstrahl with the most frequent value of 0.013-0.026
cm-3 (0.006-0.0013 cm-3). For the quiet-time
halo, κhalo behaves similarly to κstrahl, but
nhalo appears similar among the four CSR types. For the
superhalo, the AR (QS) corresponds to a larger (smaller) power-law
index β with the most frequent value of 2.2-2.4 (1.8-2.0), while
the HS and CH have a β not different from either the AR or QS;
nsup appears similar, with the most frequent value of 3
× 10-8-3 × 10-7 cm-3, among the
four CSR types. These results suggest that the strahl (superhalo)
from the hotter CSRs tends to be more (less) efficiently accelerated.
Title: A White-light Flare Powered by Magnetic Reconnection in the
Lower Solar Atmosphere
Authors: Song, Yongliang; Tian, Hui; Zhu, Xiaoshuai; Chen, Yajie;
Zhang, Mei; Zhang, Jingwen
Bibcode: 2020ApJ...893L..13S
Altcode: 2020arXiv200311747S
White-light flares (WLFs), first observed in 1859, refer to a
type of solar flare showing an obvious enhancement of the visible
continuum emission. This type of enhancement often occurs in most
energetic flares, and is usually interpreted as a consequence of
efficient heating in the lower solar atmosphere through nonthermal
electrons propagating downward from the energy release site in the
corona. However, this coronal-reconnection model has difficulty
in explaining the recently discovered small WLFs. Here we report a
C2.3 WLF, which is associated with several observational phenomena:
a fast decrease in opposite-polarity photospheric magnetic fluxes,
the disappearance of two adjacent pores, significant heating of the
lower chromosphere, a negligible increase of the hard X-ray flux, and an
associated U-shaped magnetic field configuration. All these suggest that
this WLF is powered by magnetic reconnection in the lower part of the
solar atmosphere rather than by reconnection higher up in the corona.
Title: VizieR Online Data Catalog: Catalog of ultrawide binary stars
from Gaia DR2 (Tian+, 2020)
Authors: Tian, H. -J.; El-Badry, K.; Rix, H. -W.; Gould, A.
Bibcode: 2020yCat..22460004T
Altcode:
Also available on the FTP, the file "All_candidates.fits" is
the original catalog in FITS format. "WBsDisklike.csv",
"WBsIntermediate.csv" and "WBs_Halolike.csv" are
the threee subsets in original CSV format for disk-like
(v{perp},tot<=40km/s), intermediate
(v{perp},tot=40-85km/s), and halo-like
(v{perp},tot>=85km/s) binaries, respectively. (1
data file).
Title: VizieR Online Data Catalog: Ages & masses for GPS1 WD-MS
binary systems (Fouesneau+, 2019)
Authors: Fouesneau, M.; Rix, H. -W.; von Hippel, T.; Hogg, D. W.;
Tian, H.
Bibcode: 2020yCat..18700009F
Altcode:
Observational tests of stellar and Galactic chemical evolution call
for the joint knowledge of a star's physical parameters, detailed
element abundances, and precise age. For cool main-sequence (MS) stars
the abundances of many elements can be measured from spectroscopy,
but ages are very hard to determine. The situation is different if
the MS star has a white dwarf (WD) companion and a known distance, as
the age of such a binary system can then be determined precisely from
the photometric properties of the cooling WD. As a pilot study for
obtaining precise age determinations of field MS stars, we identify
nearly 100 candidates for such wide binary systems: a faint WD whose
Gaia-PS1-SDSS (GPS1) proper motion (Tian+ 2017, I/343) matches that of
a brighter MS star in Gaia/TGAS (Gaia Collaboration 2016, I/337) with
a good parallax (σ{rho}/{rho}=<0.05). We model the WD's
multi-band photometry with the BASE-9 code using this precise distance
(assumed to be common for the pair) and infer ages for each binary
system. The resulting age estimates are precise to =<10% (=<20%)
for 42 (67) MS-WD systems. Our analysis more than doubles the number of
MS-WD systems with precise distances known to date, and it boosts the
number of such systems with precise age determination by an order of
magnitude. With the advent of the Gaia DR2 (Gaia Collaboration, 2018,
I/345) data, this approach will be applicable to a far larger sample,
providing ages for many MS stars (that can yield detailed abundances
for over 20 elements), especially in the age range of 2-8Gyr, where
there are only few known star clusters. (1 data file).
Title: VizieR Online Data Catalog: Radial velocity measurements in
LAMOST-II (Wang+, 2019)
Authors: Wang, R.; Luo, A. -L.; Chen, J. -J.; Bai, Z. -R.; Chen,
L.; Chen, X. -F.; Dong, S. -B.; Du, B.; Fu, J. -N.; Han, Z. -W.; Hou,
J. -L.; Hou, Y. -H.; Hou, W.; Jiang, D. -K.; Kong, X.; Li, L. -F.; Liu,
C.; Liu, J. -M.; Qin, L.; Shi, J. -R.; Tian, H.; Wu, H.; Wu, C. -J.;
Xie, J. -W.; Zhang, H. -T.; Zhang, S.; Zhao, G.; Zhao, Y. -H.; Zhong,
J.; Zong, W. -K.; Zuo, F.
Bibcode: 2020yCat..22440027W
Altcode:
The test observation for the LAMOST medium-resolution mode (MRS;
R=7500) began on 2017 September 1, and a total of 1,594,956 spectra
(each has two bands) of stars with S/Ns higher than 10 was collected
until 2017 December 31 (in LAMOST phase two; LAMOST-II). LAMOST
is a telescope possessing an effective aperture of 4m and 5° fields
of view, which is located at the Xinglong Observatory, Hebei Province,
China. (1 data file).
Title: Case Study of Solar Wind Suprathermal Electron Acceleration
at the Earth's Bow Shock
Authors: Liu, Zixuan; Wang, Linghua; Shi, Quanqi; Oka, Mitsuo; Yang,
Liu; Wimmer-Schweingruber, Robert F.; He, Jiansen; Tian, Hui; Bale,
Stuart D.
Bibcode: 2020ApJ...889L...2L
Altcode:
We present a case study of the in situ acceleration of solar wind
suprathermal electrons at the two quasi-perpendicular-bow-shock
crossings on 2015 November 4, combining the Wind 3D Plasma and
Energetic Particle measurements of ambient solar wind suprathermal
electrons and Magnetospheric Multiscale mission measurements of shocked
suprathermal electrons. In both cases, the omnidirectional differential
fluxes of shocked suprathermal electrons in the downstream exhibit
a double-power-law energy spectrum with a spectral index of ∼3 at
energies below a downward break ɛbrk near 40 keV and index
of ∼6 at energies above, different from the unshocked suprathermal
electrons observed in the ambient solar wind. At energies below
(above) ɛbrk, the observed electron flux ratio between the
downstream and ambient solar wind, JD/JA, peaks
near 90° PA (becomes roughly isotropic). Electrons at ɛbrk
have an average electron gyrodiameter (across bow shock) comparable
to the shock thickness. These suggest that the bow-shock acceleration
of suprathermal electrons is likely dominated by the shock drift
acceleration mechanism. For electrons at energies below (above)
ɛbrk, their estimated drift time appears to be roughly
energy independent (decrease with energy), leading to the formation
of a double-power-law spectrum substantially steepening at a break
that's determined by the shock thickness.
Title: Super Metal-rich Stars in the LAMOST Survey: A Test on Radial
Migration
Authors: Chen, Y. Q.; Zhao, G.; Zhao, J. K.; Liang, X. L.; Wu, Y. Q.;
Jia, Y. P.; Tian, H.; Liu, J. M.
Bibcode: 2019AJ....158..249C
Altcode:
Super metal-rich stars with [Fe/H] > 0.4 are selected from
LAMOST DR6, and two groups, the blue and the red, are found in the
T eff versus logg diagram with a temperature gap between
them. In combination with Gaia DR2, stellar positions, velocities,
and orbits are calculated, and spatial distributions, kinematical
properties, and orbital parameters are compared between the two
groups. The blue group shows mainly thin-disk kinematics and spans a
wide R range of 6-12 kpc, while the red group has both the thin-disk
and the thick-disk kinematics with a narrower range of R = 6-10 kpc. The
kinematical and orbital parameters of stars in the blue group indicate
that they could belong to the young population with age less than 1
Gyr, rather than blue stragglers of the old population. The orbital
parameters, R p , R a , and R g ,
of the red group with the thick-disk kinematics are smaller than those
with the thin-disk kinematics. The distributions of birth radius and
migration distance indicate that radial migration is a favorable origin
for the red group, especially those with the thick-disk kinematics,
but not for the blue group. The relative magnitude of radial migration
is of 51% for the whole sample and of 64% for the red group only. The
corotation radial migration caused by the bars and spiral arms at
resonances is the most likely mechanism for explaining these properties
of SMR stars in this work.
Title: On the Observations of Rapid Forced Reconnection in the
Solar Corona
Authors: Srivastava, A. K.; Mishra, S. K.; Jelínek, P.; Samanta,
Tanmoy; Tian, Hui; Pant, Vaibhav; Kayshap, P.; Banerjee, Dipankar;
Doyle, J. G.; Dwivedi, B. N.
Bibcode: 2019ApJ...887..137S
Altcode: 2019arXiv190107971S
Using multiwavelength imaging observations from the Atmospheric Imaging
Assembly on board the Solar Dynamics Observatory on 2012 May 3, we
present a novel physical scenario for the formation of a temporary
X-point in the solar corona, where plasma dynamics are forced externally
by a moving prominence. Natural diffusion was not predominant; however,
a prominence driven inflow occurred first, forming a thin current sheet,
thereafter enabling a forced magnetic reconnection at a considerably
high rate. Observations in relation to the numerical model reveal that
forced reconnection may rapidly and efficiently occur at higher rates
in the solar corona. This physical process may also heat the corona
locally even without establishing a significant and self-consistent
diffusion region. Using a parametric numerical study, we demonstrate
that the implementation of the external driver increases the rate
of the reconnection even when the resistivity required for creating
normal diffusion region decreases at the X-point. We conjecture that
the appropriate external forcing can bring the oppositely directed
field lines into the temporarily created diffusion region first via the
plasma inflows as seen in the observations. The reconnection and related
plasma outflows may occur thereafter at considerably larger rates.
Title: Generation of solar spicules and subsequent atmospheric heating
Authors: Samanta, Tanmoy; Tian, Hui; Yurchyshyn, Vasyl; Peter, Hardi;
Cao, Wenda; Sterling, Alphonse; Erdélyi, Robertus; Ahn, Kwangsu;
Feng, Song; Utz, Dominik; Banerjee, Dipankar; Chen, Yajie
Bibcode: 2019Sci...366..890S
Altcode: 2020arXiv200602571S
Spicules are rapidly evolving fine-scale jets of magnetized plasma in
the solar chromosphere. It remains unclear how these prevalent jets
originate from the solar surface and what role they play in heating
the solar atmosphere. Using the Goode Solar Telescope at the Big Bear
Solar Observatory, we observed spicules emerging within minutes of the
appearance of opposite-polarity magnetic flux around dominant-polarity
magnetic field concentrations. Data from the Solar Dynamics Observatory
showed subsequent heating of the adjacent corona. The dynamic
interaction of magnetic fields (likely due to magnetic reconnection)
in the partially ionized lower solar atmosphere appears to generate
these spicules and heat the upper solar atmosphere.
Title: Properties of Radial Velocities Measurement Based on LAMOST-II
Medium-resolution Spectroscopic Observations
Authors: Wang, R.; Luo, A. -L.; Chen, J. -J.; Bai, Z. -R.; Chen,
L.; Chen, X. -F.; Dong, S. -B.; Du, B.; Fu, J. -N.; Han, Z. -W.; Hou,
J. -L.; Hou, Y. -H.; Hou, W.; Jiang, D. -K.; Kong, X.; Li, L. -F.; Liu,
C.; Liu, J. -M.; Qin, L.; Shi, J. -R.; Tian, H.; Wu, H.; Wu, C. -J.;
Xie, J. -W.; Zhang, H. -T.; Zhang, S.; Zhao, G.; Zhao, Y. -H.; Zhong,
J.; Zong, W. -K.; Zuo, F.
Bibcode: 2019ApJS..244...27W
Altcode: 2019arXiv190804773W
The radial velocity (RV) is a basic physical quantity that can be
determined through the Doppler shift of the spectrum of a star. The
precision of the RV measurement depends on the resolution of the
spectrum we used and the accuracy of wavelength calibration. In this
work, radial velocities of the Large Sky Area Multi-Object Fibre
Spectroscopic Telescope-II (LAMOST-II) medium-resolution (R ∼ 7500)
spectra are measured for 1,594,956 spectra (each spectrum has two
wavebands) through matching with templates. A set of RV standard
stars are used to recalibrate the zero point of the measurement,
and some reference sets with RVs derived from medium-/high-resolution
observations are used to evaluate the accuracy of the measurement. By
comparing with reference sets, the accuracy of our measurement
can get 0.0277 km s-1 with respect to radial velocities
of standard stars. The intrinsic precision is estimated with the
multiple observations of single stars, which can be achieved to 1.36 km
s-1, 1.08 km s-1, and 0.91 km s-1 for
the spectra at signal-to-noise levels of 10, 20, and 50, respectively.
Title: Solar ultraviolet bursts in a coordinated observation of IRIS,
Hinode and SDO
Authors: Chen, YaJie; Tian, Hui; Zhu, XiaoShuai; Samanta, Tanmoy;
Wang, LingHua; He, JianSen
Bibcode: 2019ScChE..62.1555C
Altcode: 2019arXiv190204226C
Solar ultraviolet (UV) bursts are small-scale compact brightenings in
transition region images. The spectral profiles of transition region
lines in these bursts are significantly enhanced and broadened,
often with chromospheric absorption lines such as Ni~{\sc{ii}}
1335.203 and 1393.330 Å superimposed. We investigate the properties
of several UV bursts using a coordinated observation of the Interface
Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory (SDO),
and \textit{Hinode} on 2015 February 7. We have identified 12 UV
bursts, and 11 of them reveal small blueshifts of the Ni~{\sc{ii}}
absorption lines. However, the Ni~{\sc{ii}} lines in one UV burst
exhibit obvious redshifts of $\sim$20 km s$^{-1}$, which appear to
be related to the cold plasma downflows observed in the IRIS slit-jaw
images. We also examine the three-dimensional magnetic field topology
using a magnetohydrostatic model, and find that some UV bursts are
associated with magnetic null points or bald patches. In addition, we
find that these UV bursts reveal no obvious coronal signatures from
the observations of the Atmospheric Imaging Assembly (AIA) on board
SDO and the EUV Imaging Spectrometer (EIS) on board \textit{Hinode}.
Title: Modeling Mg II h, k and Triplet Lines at Solar Flare Ribbons
Authors: Zhu, Yingjie; Kowalski, Adam F.; Tian, Hui; Uitenbroek, Han;
Carlsson, Mats; Allred, Joel C.
Bibcode: 2019ApJ...879...19Z
Altcode: 2019arXiv190412285Z
Observations from the Interface Region Imaging Spectrograph often
reveal significantly broadened and non-reversed profiles of the Mg II
h, k and triplet lines at flare ribbons. To understand the formation
of these optically thick Mg II lines, we perform plane-parallel
radiative hydrodynamics modeling with the RADYN code, and then
recalculate the Mg II line profiles from RADYN atmosphere snapshots
using the radiative transfer code RH. We find that the current RH
code significantly underestimates the Mg II h and k Stark widths. By
implementing semiclassical perturbation approximation results of
quadratic Stark broadening from the STARK-B database in the RH code,
the Stark broadenings are found to be one order of magnitude larger
than those calculated from the current RH code. However, the improved
Stark widths are still too small, and another factor of 30 has to be
multiplied to reproduce the significantly broadened lines and adjacent
continuum seen in observations. Nonthermal electrons, magnetic fields,
three-dimensional effects, or electron density effects may account
for this factor. Without modifying the RADYN atmosphere, we have also
reproduced non-reversed Mg II h and k profiles, which appear when the
electron beam energy flux is decreasing. These profiles are formed at
an electron density of ∼8 × 1014 cm-3 and a
temperature of ∼1.4 × 104 K, where the source function
slightly deviates from the Planck function. Our investigation also
demonstrates that at flare ribbons the triplet lines are formed in the
upper chromosphere, close to the formation heights of the h and k lines.
Title: The Galactic spiral structure as revealed by O- and early
B-type stars
Authors: Chen, B. -Q.; Huang, Y.; Hou, L. -G.; Tian, H.; Li, G. -X.;
Yuan, H. -B.; Wang, H. -F.; Wang, C.; Tian, Z. -J.; Liu, X. -W.
Bibcode: 2019MNRAS.487.1400C
Altcode: 2019arXiv190505542B; 2019MNRAS.tmp.1307C
We investigate the morphology and kinematics of the Galactic spiral
structure based on a new sample of O- and early B-type stars. We
select 6858 highly confident OB star candidates from the combined
data of the VST Photometric H α Survey Data Release 2 (VPHAS+ DR2)
and the Gaia Data Release 2 (Gaia DR2). Together with the O-B2 stars
from the literature, we build a sample consisting of 14 880 O- and
early B-type stars, all with Gaia parallax uncertainties smaller
than 20 per cent. The new sample, hitherto the largest one of O- and
early B-type stars with robust distance and proper motion estimates,
covers the Galactic plane of distances up to 6 kpc from the Sun. The
sample allows us to examine the morphology of the Scutum, Sagittarius,
Local, and Perseus Arms in great detail. The spiral structure of the
Milky Way as traced by O- and early B-type stars shows flocculent
patterns. Accurate structure parameters, as well as the means and
dispersions of the vertical velocity distributions of the individual
spiral arms are presented.
Title: Evidence for Vortex Shedding in the Sun's Hot Corona
Authors: Samanta, Tanmoy; Tian, Hui; Nakariakov, Valery M.
Bibcode: 2019PhRvL.123c5102S
Altcode: 2019arXiv190708930S
Vortex shedding is an oscillating flow that is commonly observed
in fluids due to the presence of a blunt body in a flowing
medium. Numerical simulations have shown that the phenomenon of
vortex shedding could also develop in the magnetohydrodynamic (MHD)
domain. The dimensionless Strouhal number, the ratio of the blunt body
diameter to the product of the period of vortex shedding and the speed
of a flowing medium, is a robust indicator for vortex shedding, and,
generally of the order of 0.2 for a wide range of Reynolds number. Using
an observation from the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory, we report a wavelike or oscillating plasma
flow propagating upward against the Sun's gravitational force. A newly
formed shrinking loop in the postflare region possibly generates the
oscillation of the upflow in the wake of the hot and dense loop through
vortex shedding. The computed Strouhal number is consistent with the
prediction from previous MHD simulations. Our observation suggests
the possibility of vortex shedding in the solar corona.
Title: In situmeasurements of the solar eruption
Authors: Lin, Jun; Wang, Min; Tian, Hui; Song, Hongqiang; Fu, Huishan;
Huang, Min; Lv, Qunbo; Zhang, Weijie; Zhang, Xianguo; Zhang, Kunyi;
Li, Mingtao Zhang, Yiteng; Jin, Zhenyu; Chen, Dong; Yin, Zengshan;
Deng, Lei; Mao, Yufeng; Li, Yan; Mei, Zhixing; Ye, Jing
Bibcode: 2019SSPMA..49e9607L
Altcode: 2019SSPMA..49e9607J
No abstract at ADS
Title: Flame-like Ellerman Bombs and Their Connection to Solar
Ultraviolet Bursts
Authors: Chen, Yajie; Tian, Hui; Peter, Hardi; Samanta, Tanmoy;
Yurchyshyn, Vasyl; Wang, Haimin; Cao, Wenda; Wang, Linghua; He, Jiansen
Bibcode: 2019ApJ...875L..30C
Altcode: 2019arXiv190301981C
Ellerman bombs (EBs) are small-scale intense brightenings in Hα
wing images, which are generally believed to be signatures of
magnetic reconnection around the temperature minimum region of the
solar atmosphere. They have a flame-like morphology when observed
near the solar limb. Recent observations from the Interface Region
Imaging Spectrograph (IRIS) reveal another type of small-scale
reconnection event called an ultraviolet (UV) burst, in the lower solar
atmosphere. Though previous observations have shown a clear coincidence
between the occurrence of some UV bursts and EBs, the exact relationship
between these two phenomena is still debated. We investigate the spatial
and temporal relationship between flame-like EBs and UV bursts using
joint near-limb observations between the 1.6 m Goode Solar Telescope
(GST) and IRIS. In total, 161 EBs have been identified from the GST
observations, and ∼20 of them reveal signatures of UV bursts in
the IRIS images. Interestingly, we find that these UV bursts have a
tendency to appear at the upper parts of their associated flame-like
EBs. The intensity variations of most EB-related UV bursts and their
corresponding EBs match well. Our results suggest that some of these
UV bursts and EBs likely form at different heights during a common
reconnection process.
Title: Electron Acceleration by ICME-driven Shocks at 1 au
Authors: Yang, Liu; Wang, Linghua; Li, Gang; Wimmer-Schweingruber,
Robert F.; He, Jiansen; Tu, Chuanyi; Tian, Hui; Bale, Stuart D.
Bibcode: 2019ApJ...875..104Y
Altcode:
We present a comprehensive study of in situ electron acceleration
during 74 shocks driven by interplanetary coronal mass ejections
(ICMEs) with good suprathermal electron observations by the Wind
3DP instrument at 1 au from 1995 through 2014. Among the selected 59
quasi-perpendicular (15 quasi-parallel) shock cases, ∼86% (∼60%),
∼62% (∼36%), and ∼17% (∼7%) show significant electron flux
enhancements of J D /J A > 1.5 across
the shock, respectively at 0.43, 1.95, and 40 keV, where J D
and J A are the electron flux in the shock’s
downstream and the preceding ambient solar wind. For significantly
shocked suprathermal electrons, the differential flux J D
positively correlates most with the magnetosonic Mach number M
s , while the flux enhancement J D /J A
positively correlates most with the magnetic compression ratio r
B , among the shock parameters. Both J D and J
A generally fit well to a double-power-law spectrum at ∼0.4-100
keV, J ∝ E -β , with an index of β 1 ∼ 2-6
below a break energy of E br (which is typically ∼2 keV)
and an index of β 2 ∼ 2.0-3.2 at energies above. {β
}1D is similar to {β }1A
in all the shock cases, while {β }2D is similar
to (larger than) {β }2A in ∼60% (∼40%) of the
shock cases with significant electron enhancements. Furthermore, J
D /J A mostly peaks in the directions perpendicular
to the interplanetary magnetic field at ∼0.4-50 keV. These
results suggest that both quasi-parallel and quasi-perpendicular
shocks accelerate electrons in situ at 1 au mainly via shock drift
acceleration, with an acceleration efficiency probably affected by
the induced electric field at the shock surface.
Title: Classification of large-scale stellar spectra based on deep
convolutional neural network
Authors: Liu, W.; Zhu, M.; Dai, C.; He, D. Y.; Yao, Jiawen; Tian,
H. F.; Wang, B. Y.; Wu, K.; Zhan, Y.; Chen, B. -Q.; Luo, A. -Li;
Wang, R.; Cao, Y.; Yu, X. C.
Bibcode: 2019MNRAS.483.4774L
Altcode: 2018MNRAS.tmp.2885L
Classification of stellar spectra from voluminous spectra is a
very important and challenging task. In order to better classify
stellar spectra, inspired by the principle of deep convolutional
neural network (CNN), we propose a supervised algorithm for stellar
spectra classification based on 1-D stellar spectra convolutional
neural network (1-D SSCNN). In 1-D SSCNN, we modify the traditional
2-D convolutional neural network into 1-D network to adapt to the
spectral classification. On the basis of using convolution algorithm,
the spectral features are extracted and used for classification. We
firstly use the stellar spectra data to train a 1-D SSCNN to obtain
a well-trained model, and then we apply the well-trained model to
classify the unknown spectra. To evaluate the performance of the
proposed algorithms, we apply 1-D SSCNN to classify three spectral
types: F-type spectra, G-type spectra and K-type spectra and ten
subclasses of K-type spectra: A0-type, A5-type, F0-type, F5-type,
G0-type, G5-type, K0-type, K5-type, M0-type and M5-type spectra from
Sloan Digital Sky Survey (SDSS). Our 1-D SSCNN algorithm obtain higher
classification accuracy compared with Support Vector Machine (SVM),
Random Forest (RF) and Artificial neural network (ANN).
Title: Investigating the Transition Region Explosive Events and
Their Relationship to Network Jets
Authors: Chen, Yajie; Tian, Hui; Huang, Zhenghua; Peter, Hardi;
Samanta, Tanmoy
Bibcode: 2019ApJ...873...79C
Altcode: 2019arXiv190111215C
Recent imaging observations with the Interface Region Imaging
Spectrograph (IRIS) have revealed prevalent intermittent jets with
apparent speeds of 80-250 km s-1 from the network lanes
in the solar transition region (TR). Additionally, spectroscopic
observations of the TR lines have revealed the frequent presence of
highly non-Gaussian line profiles with enhanced emission at the line
wings, often referred to as explosive events (EEs). Using simultaneous
imaging and spectroscopic observations from IRIS, we investigate
the relationship between EEs and network jets. We first identify
EEs from the Si IV 1393.755 Å line profiles in our observations,
then examine related features in the 1330 Å slit-jaw images. Our
analysis suggests that EEs with double peaks or enhancements in both
wings appear to be located at either the footpoints of network jets or
transient compact brightenings. These EEs are most likely produced by
magnetic reconnection. We also find that EEs with enhancements only
at the blue wing are mainly located on network jets, away from the
footpoints. These EEs clearly result from the superposition of the
high-speed network jets on the TR background. In addition, EEs showing
enhancement only at the red wing of the line are often located around
the jet footpoints, which is possibly caused by the superposition of
reconnection downflows on the background emission. Moreover, we find
some network jets that are not associated with any detectable EEs. Our
analysis suggests that some EEs are related to the birth or propagation
of network jets, and that others are not connected to network jets.
Title: COronal Magnetism and Plasma ASsembled Scopes(COMPASS)
Authors: Qu, Zhongquan; Li, Hui; Zhong, Yue; Liang, Yu; Song, Zhiming;
Zhang, Haiying; Zhang, Hongxin; Chen, Yao; Tian, Hui; Cheng, Xin;
Xia, Lidong; Li, Bo; Chen, Bo; Yan, Xiaoli; Liu, Rui; Shen, Chenglong;
Feng, Li; Hou, Junfeng; Li, Hao; Li, Zheng; Li, Shaoying
Bibcode: 2019SSPMA..49e9606Q
Altcode:
No abstract at ADS
Title: Solar Ultraviolet Bursts
Authors: Young, Peter R.; Tian, Hui; Peter, Hardi; Rutten, Robert J.;
Nelson, Chris J.; Huang, Zhenghua; Schmieder, Brigitte; Vissers, Gregal
J. M.; Toriumi, Shin; Rouppe van der Voort, Luc H. M.; Madjarska, Maria
S.; Danilovic, Sanja; Berlicki, Arkadiusz; Chitta, L. P.; Cheung, Mark
C. M.; Madsen, Chad; Reardon, Kevin P.; Katsukawa, Yukio; Heinzel, Petr
Bibcode: 2018SSRv..214..120Y
Altcode: 2018arXiv180505850Y
The term "ultraviolet (UV) burst" is introduced to describe small,
intense, transient brightenings in ultraviolet images of solar active
regions. We inventorize their properties and provide a definition
based on image sequences in transition-region lines. Coronal signatures
are rare, and most bursts are associated with small-scale, canceling
opposite-polarity fields in the photosphere that occur in emerging flux
regions, moving magnetic features in sunspot moats, and sunspot light
bridges. We also compare UV bursts with similar transition-region
phenomena found previously in solar ultraviolet spectrometry and
with similar phenomena at optical wavelengths, in particular Ellerman
bombs. Akin to the latter, UV bursts are probably small-scale magnetic
reconnection events occurring in the low atmosphere, at photospheric
and/or chromospheric heights. Their intense emission in lines with
optically thin formation gives unique diagnostic opportunities
for studying the physics of magnetic reconnection in the low solar
atmosphere. This paper is a review report from an International Space
Science Institute team that met in 2016-2017.
Title: Investigation of White-light Emission in Circular-ribbon Flares
Authors: Song, Yongliang; Tian, Hui
Bibcode: 2018ApJ...867..159S
Altcode: 2018arXiv181002958S
Using observations by the Solar Dynamics Observatory from 2010 June to
2017 December, we have performed the first statistical investigation of
circular-ribbon flares (CFs) and examined the white-light emission in
them. We find 90 CFs occurring in 36 active regions (ARs), including
eight X-class, 34 M-class, and 48 C- and B-class flares. The occurrence
rate of white-light flares (WLFs) is 100% (8/8) for X-class CFs,
∼62% (21/34) for M-class CFs, and ∼8% (4/48) for C- and B-class
CFs. Sometimes we observe several CFs in a single AR, and nearly
all of them are WLFs. Compared to normal CFs, those with white-light
enhancement tend to have a shorter duration, smaller size, stronger
electric current and more complicated magnetic field. We find that for
X-class WLFs, the white-light enhancement is positively correlated
with the flare class, implying that it is largely determined by the
amount of released energy. However, there is no such correlation for M-
and C-class WLFs, suggesting that other factors such as the timescale,
spatial scale, and magnetic field complexity may play important roles
in the generation of white-light emission if the released energy is
not high enough.
Title: Dark Structures in Sunspot Light Bridges
Authors: Zhang, Jingwen; Tian, Hui; Solanki, Sami K.; Wang, Haimin;
Peter, Hardi; Ahn, Kwangsu; Xu, Yan; Zhu, Yingjie; Cao, Wenda; He,
Jiansen; Wang, Linghua
Bibcode: 2018ApJ...865...29Z
Altcode: 2018arXiv180900146Z
We present unprecedented high-resolution TiO images and Fe I 1565
nm spectropolarimetric data of two light bridges taken by the 1.6
m Goode Solar Telescope at Big Bear Solar Observatory. In the first
light bridge (LB1), we find striking knot-like dark structures within
the central dark lane. Many dark knots show migration away from the
penumbra along the light bridge. The sizes, intensity depressions,
and apparent speeds of their proper motion along the light bridges of
33 dark knots identified from the TiO images are mainly in the ranges
of 80 ∼ 200 km, 30% ∼ 50%, and 0.3 ∼ 1.2 km s-1,
respectively. In the second light bridge (LB2), a faint central dark
lane and striking transverse intergranular lanes were observed. These
intergranular lanes have sizes and intensity depressions comparable
to those of the dark knots in LB1 and also migrate away from the
penumbra at similar speeds. Our observations reveal that LB2 is made
up of a chain of evolving convection cells, as indicated by patches of
blueshift surrounded by narrow lanes of redshift. The central dark lane
generally corresponds to blueshifts, supporting the previous suggestion
of central dark lanes being the top parts of convection upflows. In
contrast, the intergranular lanes are associated with redshifts and
located at two sides of each convection cell. The magnetic fields are
stronger in intergranular lanes than in the central dark lane. These
results suggest that these intergranular lanes are manifestations of
convergent convective downflows in the light bridge. We also provide
evidence that the dark knots observed in LB1 may have a similar origin.
Title: Two Solar Tornadoes Observed with the Interface Region
Imaging Spectrograph
Authors: Yang, Zihao; Peter, Hardi; Su, Yang; Samanta, Tanmoy; Zhang,
Jingwen; Tian, Hui; Chen, Yajie
Bibcode: 2018cosp...42E3746Y
Altcode:
The barbs or legs of some prominences show an apparent motion of
rotation, which are often termed solar tornadoes. It is under debate
whether the apparent motion is a real rotating motion, or caused by
oscillations or counter-streaming flows. We present analysis results
from spectroscopic observations of two tornadoes by the Interface
Region Imaging Spectrograph. Each tornado was observed for more than 2.5
hours. Doppler velocities are derived through a single Gaussian fit to
the Mg II k 2796Å and Si IV 1393Å line profiles. We find coherent and
stable red and blue shifts adjacent to each other across the tornado
axes, which appears to favor the interpretation of these tornadoes
as rotating cool plasmas with temperatures of 10^4 K-10^5 K. This
interpretation is further supported by simultaneous observations of the
Atmospheric Imaging Assembly on board the Solar Dynamics Observatory,
which reveal periodic motions of dark structures in the tornadoes. Our
results demonstrate that spectroscopic observations can provide key
information to disentangle different physical processes in solar
prominences.
Title: Turbulence and Heating in the Flank and Wake Regions of a
Coronal Mass Ejection
Authors: He, Jiansen; Song, Hong-Qiang; Tomczyk, Steven; Wang, Linghua;
Tian, Hui; Fan, Siteng; Zhang, Lei; Yan, Limei
Bibcode: 2018cosp...42E1404H
Altcode:
As a coronal mass ejection (CME) passes, the flank and wake regions are
typically strongly disturbed. Various instruments, including the Large
Angle and Spectroscopic Coronagraph (LASCO), the Atmospheric Imaging
Assembly (AIA), and the Coronal Multi-channel Polarimeter (CoMP),
observed a CME close to the east limb on 26 October 2013.A hot (∼10
MK) rising blob was detected on the east limb, with an initial ejection
flow speed of ∼330 km/s. The magnetic structures on both sides and
in the wake of the CME were strongly distorted, showing initiation
of turbulent motions with Doppler-shift oscillations enhanced from
∼ ±3 km/s to ∼ ±15 km/s and effective thermal velocities from
∼30 km/s to ∼60 km/s, according to the CoMP observations at the
Fe XIII line. The CoMP Doppler-shift maps suggest that the turbulence
behaved differently at various heights; it showed clear wave-like
torsional oscillations at lower altitudes, which are interpreted as the
anti-phase oscillation of an alternating red/blue Doppler shift across
the strands at the flank. The turbulence seems to appear differently in
the channels of different temperatures. Its turnover time was ∼1000
seconds for the Fe 171 Å channel, while it was ∼500 seconds for the
Fe 193 Å channel. Mainly horizontal swaying rotations were observed in
the Fe 171 Å channel, while more vertical vortices were seen in the
Fe 193 Å channel. The differential-emission-measure profiles in the
flank and wake regions have two components that evolve differently:
the cool component decreased over time, evidently indicating a
drop-out of cool materials due to ejection, while the hot component
increased dramatically,probably because of the heating process, which
is suspected to be a result of magnetic reconnection and turbulence
dissipation. These results suggest a new turbulence-heating scenario
of the solar corona and solar wind.
Title: Diagnosing the magnetic field structure of a coronal cavity
observed during the 2017 total solar eclipse
Authors: Chen, Yajie; Su, Yingna; Tian, Hui; Qu, Zhongquan; Deng,
Linhua; Jibben, Patricia
Bibcode: 2018cosp...42E.607C
Altcode:
We present an investigation of a coronal cavity observed above the
western limb in the coronal red line Fe X 637.4 nm using a telescope
of Peking University and in the green line Fe XIV 530.3 nm using a
telescope of Yunnan Observatories, Chinese Academy of Sciences during
the total solar eclipse on 2017 August 21. A series of magnetic
field models are constructed based on the magnetograms taken by
the Helioseismic and Magnetic Imager onboard the Solar Dynamics
Observatory (SDO) one week before the eclipse. The model field lines
are then compared with coronal structures seen in images taken by
the Atmospheric Imaging Assembly on board SDO and in our coronal red
line images. The best-fit model consists of a flux rope with a twist
angle of 3.1pi, which is consistent with the most probable value
of the total twist angle of interplanetary flux ropes observed at 1
AU. Linear polarization of the Fe XIII 1074.7 nm line calculated from
this model shows a ``lagomorphic" signature that is also observed by the
Coronal Multichannel Polarimeter of the High Altitude Observatory. We
also find a ring-shaped structure in the line-of-sight velocity of
Fe XIII 1074.7 nm, which implies hot plasma flows along a helical
magnetic field structure, in the cavity. These results suggest that
the magnetic structure of the cavity is a highly twisted flux rope,
which may erupt eventually. The temperature structure of the cavity
has also been investigated using the intensity ratio of Fe XIII 1074.7
nm and Fe X 637.4 nm.
Title: Diagnosing Flare Dynamics Through the fe XXI 1354.08Å Line
Authors: Tian, Hui
Bibcode: 2018cosp...42E3385T
Altcode:
The Fe XXI 1354.08 line is the only strong emission line formed above
10 MK in the spectral range of IRIS. It has been demonstrated to be very
useful in diagnosing flare dynamics. For instance, the completeevolution
of chromospheric evaporation has been well observed and it appears
to correlate with the energy deposition rate in many flares, which
provides critical constraint to models of chromosphericevaporation. With
the Fe XXI line we have even observed multi-episode chromospheric
evaporations in a flare. The Fe XXI line has also been successful in
observing reconnection outflows, revealing important insight into the
flare reconnection process. Moreover, global MHD oscillations have
been detected in flare loops, both in the intensity and Doppler shift
of the Fe XXI line, allowing more accurate mode identification that
is crucial for coronal seismology.
Title: Observations of white-light flares in NOAA active region 11515:
high occurrence rate and relationship with magnetic transients
Authors: Song, Y. L.; Tian, H.; Zhang, M.; Ding, M. D.
Bibcode: 2018A&A...613A..69S
Altcode: 2018arXiv180104371S
Aims: There are two goals in this study. One is to investigate
how frequently white-light flares (WLFs) occur in a flare-productive
active region (NOAA active region 11515). The other is to investigate
the relationship between WLFs and magnetic transients (MTs).
Methods: We used the high-cadence (45 s) full-disk continuum filtergrams
and line-of-sight magnetograms taken by the Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO) to identify
WLFs and MTs, respectively. Images taken by the Atmospheric Imaging
Assembly (AIA) on board SDO were also used to show the flare morphology
in the upper atmosphere.
Results: We found at least 20 WLFs out
of a total of 70 flares above C class (28.6%) in NOAA active region
11515 during its passage across the solar disk (E45°-W45°). Each of
these WLFs occurred in a small region, with a short duration of about 5
min. The enhancement of the white-light continuum intensity is usually
small, with an average enhancement of 8.1%. The 20 WLFs we observed
were found along an unusual configuration of the magnetic field that
was characterized by a narrow ribbon of negative field. Furthermore,
the WLFs were found to be accompanied by MTs, with radical changes
in magnetic field strength (or even a sign reversal) observed during
the flare. In contrast, there is no obvious signature of MTs in the 50
flares without white-light enhancements.
Conclusions: Our results
suggest that WLFs occur much more frequently than previously thought,
with most WLFs being fairly weak enhancements. This may explain why WLFs
are reported rarely. Our observations also suggest that MTs and WLFs are
closely related and appear cospatial and cotemporal, when considering
HMI data. A greater enhancement of WL emission is often accompanied by a
greater change in the line-of-sight component of the unsigned magnetic
field. Considering the close relationship between MTs and WLFs, many
previously reported flares with MTs may be WLFs. The movie is
available at http://www.aanda.org
Title: Statistical Investigation of Supersonic Downflows in the
Transition Region above Sunspots
Authors: Samanta, Tanmoy; Tian, Hui; Prasad Choudhary, Debi
Bibcode: 2018ApJ...859..158S
Altcode: 2018arXiv180405054S
Downflows at supersonic speeds have been observed in the transition
region (TR) above sunspots for more than three decades. These downflows
are often seen in different TR spectral lines above sunspots. We
have performed a statistical investigation of these downflows using
a large sample that was missing previously. The Interface Region
Imaging Spectrograph (IRIS) has provided a wealth of observational
data of sunspots at high spatial and spectral resolutions in the
past few years. We have identified 60 data sets obtained with IRIS
raster scans. Using an automated code, we identified the locations
of strong downflows within these sunspots. We found that around
80% of our sample shows supersonic downflows in the Si IV 1403 Å
line. These downflows mostly appear in the penumbral regions, though
some of them are found in the umbrae. We also found that almost half of
these downflows show signatures in chromospheric lines. Furthermore,
a detailed spectral analysis was performed by selecting a small
spectral window containing the O IV 1400/1401 Å and Si IV 1403 Å
lines. Six Gaussian functions were simultaneously fitted to these
three spectral lines and their satellite lines associated with the
supersonic downflows. We calculated the intensity, Doppler velocity,
and line width for these lines. Using the O IV 1400/1401 Å line ratio,
we find that the downflow components are around one order of magnitude
less dense than the regular components. Results from our statistical
analysis suggest that these downflows may originate from the corona
and that they are independent of the background TR plasma.
Title: Multi-episode Chromospheric Evaporation Observed in a Solar
Flare
Authors: Tian, H.; Chen, N. -H.
Bibcode: 2018ApJ...856...34T
Altcode: 2018arXiv180104370T
With observations of the Interface Region Imaging Spectrograph (IRIS),
we study chromospheric heating and evaporation during an M1.6 flare
SOL2015-03-12T11:50. At the flare ribbons, the Mg II 2791.59 Å line
shows quasi-periodic short-duration red-wing enhancement, which is
likely related to repetitive chromospheric condensation as a result of
episodic heating. On the contrary, the Si IV 1402.77 Å line reveals
a persistent red-wing asymmetry in both the impulsive and decay
phases, suggesting that this line responds to both cooling downflows
and chromospheric condensation. The first two episodes of red-wing
enhancement occurred around 11:42 UT and 11:45 UT, when two moving
brightenings indicative of heating fronts crossed the IRIS slit. The
greatly enhanced red wings of the Si IV and Mg II lines at these
occasions are accompanied by an obvious increase in the line intensities
and the HXR flux, suggesting two episodes of energy injection into
the lower atmosphere in the form of nonthermal electrons. The Mg II
k/h ratio has a small value of ∼1.2 at the ribbons and decreases
to ∼1.1 at these two occasions. Correspondingly, the Fe XXI 1354
Å line reveals two episodes of chromospheric evaporation, which is
characterized as a smooth decrease of the blueshift from ∼300 km
s-1 to nearly zero within ∼3 minutes. The Fe XXI 1354 Å
line is entirely blueshifted in the first episode, while it appears to
contain a nearly stationary component and a blueshifted component in
the second episode. Additional episodes of blueshifted Fe XXI emission
are found around the northern ribbon in the decay phase, though no
obvious response is detected in the Si IV and Mg II emission. We have
also examined the Fe XXI emission at the flare loop top and identified
a secondary component with a ∼200 km s-1 redshift, which
possibly results from the downward moving reconnection outflow. Our
analysis also yields a rest wavelength of 1354.0878 ± 0.0072 Å for
this Fe XXI line.
Title: Diagnosing the Magnetic Field Structure of a Coronal Cavity
Observed during the 2017 Total Solar Eclipse
Authors: Chen, Yajie; Tian, Hui; Su, Yingna; Qu, Zhongquan; Deng,
Linhua; Jibben, Patricia R.; Yang, Zihao; Zhang, Jingwen; Samanta,
Tanmoy; He, Jiansen; Wang, Linghua; Zhu, Yingjie; Zhong, Yue; Liang, Yu
Bibcode: 2018ApJ...856...21C
Altcode: 2018arXiv180204432C
We present an investigation of a coronal cavity observed above the
western limb in the coronal red line Fe X 6374 Å using a telescope of
Peking University and in the green line Fe XIV 5303 Å using a telescope
of Yunnan Observatories, Chinese Academy of Sciences, during the total
solar eclipse on 2017 August 21. A series of magnetic field models is
constructed based on the magnetograms taken by the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory (SDO) one week
before the eclipse. The model field lines are then compared with coronal
structures seen in images taken by the Atmospheric Imaging Assembly
on board SDO and in our coronal red line images. The best-fit model
consists of a flux rope with a twist angle of 3.1π, which is consistent
with the most probable value of the total twist angle of interplanetary
flux ropes observed at 1 au. Linear polarization of the Fe XIII 10747
Å line calculated from this model shows a “lagomorphic” signature
that is also observed by the Coronal Multichannel Polarimeter of the
High Altitude Observatory. We also find a ring-shaped structure in the
line-of-sight velocity of Fe XIII 10747 Å, which implies hot plasma
flows along a helical magnetic field structure, in the cavity. These
results suggest that the magnetic structure of the cavity is a highly
twisted flux rope, which may erupt eventually. The temperature structure
of the cavity has also been investigated using the intensity ratio of
Fe XIII 10747 Å and Fe X 6374 Å.
Title: VizieR Online Data Catalog: Gaia-PS1-SDSS (GPS1) proper motion
catalog (Tian+, 2017)
Authors: Tian, H. -J.; Gupta, P.; Sesar, B.; Rix, H. -W.; Martin,
N. F.; Liu, C.; Goldman, B.; Platais, I.; Kudritzki, R. -P.; Waters,
C. Z.
Bibcode: 2018yCat.1343....0T
Altcode:
In order to construct proper motions, we analyze and model catalog
positions from four different imaging surveys, as discussed below. Gaia
DR1 is based on observations collected between 2014 July 25 and 2015
September 16. PS1 observations were collected between 2010 and 2014. The
SDSS DR9 data used here were obtained in the years between 2000 and
2008. The images from 2MASS were taken between 1997 and 2001. (1
data file).
Title: Two-stage Energy Release Process of a Confined Flare with
Double HXR Peaks
Authors: Ning, Hao; Chen, Yao; Wu, Zhao; Su, Yang; Tian, Hui; Li,
Gang; Du, Guohui; Song, Hongqiang
Bibcode: 2018ApJ...854..178N
Altcode: 2018arXiv180106641N
A complete understanding of the onset and subsequent evolution of
confined flares has not been achieved. Earlier studies mainly analyzed
disk events so as to reveal their magnetic topology and the cause of
confinement. In this study, taking advantage of a tandem of instruments
working at different wavelengths of X-rays, EUVs, and microwaves,
we present dynamic details about a confined flare observed on the
northwestern limb of the solar disk on 2016 July 24. The entire
dynamic evolutionary process starting from its onset is consistent
with a loop-loop interaction scenario. The X-ray profiles manifest an
intriguing double-peak feature. From the spectral fitting, it has been
found that the first peak is nonthermally dominated, while the second
peak is mostly multithermal with a hot (∼10 MK) and a super-hot
(∼30 MK) component. This double-peak feature is unique in that the
two peaks are clearly separated by 4 minutes, and the second peak
reaches up to 25-50 keV in addition, at energy bands above 3 keV,
the X-ray fluxes decline significantly between the two peaks. This,
together with other available imaging and spectral data, manifest a
two-stage energy release process. A comprehensive analysis is carried
out to investigate the nature of this two-stage process. We conclude
that the second stage with the hot and super-hot sources mainly involves
direct heating through a loop-loop reconnection at a relatively high
altitude in the corona. The uniqueness of the event characteristics
and the complete dataset make the study a nice addition to present
literature on solar flares.
Title: Observations of a White-light Flare Associated with a Filament
Eruption
Authors: Song, Y. L.; Guo, Y.; Tian, H.; Zhu, X. S.; Zhang, M.; Zhu,
Y. J.
Bibcode: 2018ApJ...854...64S
Altcode: 2018arXiv180104408S
We present observations of an M5.7 white-light flare (WLF) associated
with a small filament eruption in NOAA active region 11476 on 2012
May 10. During this flare, a circular flare ribbon appeared in the
east and a remote brightening occurred in the northwest of the active
region. Multi-wavelength data are employed to analyze the WLF, including
white light (WL), ultraviolet, extreme ultraviolet, hard X-ray (HXR),
and microwave. A close spatial and temporal relationship between the WL,
HXR, and microwave emissions is found in this WLF. However, the peak
time of the WL emission lagged that of the HXR and microwave emissions
by about 1-2 minutes. Such a result tends to support the backwarming
mechanism for the WL emission. Interestingly, the enhanced WL emission
occurred at the two footpoints of the filament. Through forced and
potential field extrapolations, we find that the 3D magnetic field in
the flare region has a fan-spine feature and that a flux rope lies
under the dome-like field structure. We describe the entire process
of flare evolution into several steps, each one producing the sequent
brightening below the filament, the circular flare ribbons, and the
WL enhancement, respectively. We suggest that a reconnection between
the magnetic field of the filament and the overlying magnetic field
or reconnection within the flux rope leads to the WL enhancement.
Title: Magnetic Braids in Eruptions of a Spiral Structure in the
Solar Atmosphere
Authors: Huang, Zhenghua; Xia, Lidong; Nelson, Chris J.; Liu, Jiajia;
Wiegelmann, Thomas; Tian, Hui; Klimchuk, James A.; Chen, Yao; Li, Bo
Bibcode: 2018ApJ...854...80H
Altcode: 2018arXiv180105967H
We report on high-resolution imaging and spectral observations of
eruptions of a spiral structure in the transition region, which
were taken with the Interface Region Imaging Spectrograph, and the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The eruption
coincided with the appearance of two series of jets, with velocities
comparable to the Alfvén speeds in their footpoints. Several pieces of
evidence of magnetic braiding in the eruption are revealed, including
localized bright knots, multiple well-separated jet threads, transition
region explosive events, and the fact that all three of these are
falling into the same locations within the eruptive structures. Through
analysis of the extrapolated 3D magnetic field in the region, we found
that the eruptive spiral structure corresponded well to locations
of twisted magnetic flux tubes with varying curl values along their
lengths. The eruption occurred where strong parallel currents,
high squashing factors, and large twist numbers were obtained. The
electron number density of the eruptive structure is found to be ∼3 ×
1012 cm-3, indicating that a significant amount
of mass could be pumped into the corona by the jets. Following the
eruption, the extrapolations revealed a set of seemingly relaxed loops,
which were visible in the AIA 94 Å channel, indicating temperatures
of around 6.3 MK. With these observations, we suggest that magnetic
braiding could be part of the mechanisms explaining the formation of
solar eruption and the mass and energy supplement to the corona.
Title: Frequently Occurring Reconnection Jets from Sunspot Light
Bridges
Authors: Tian, Hui; Yurchyshyn, Vasyl; Peter, Hardi; Solanki, Sami
K.; Young, Peter R.; Ni, Lei; Cao, Wenda; Ji, Kaifan; Zhu, Yingjie;
Zhang, Jingwen; Samanta, Tanmoy; Song, Yongliang; He, Jiansen; Wang,
Linghua; Chen, Yajie
Bibcode: 2018ApJ...854...92T
Altcode: 2018arXiv180106802T
Solid evidence of magnetic reconnection is rarely reported within
sunspots, the darkest regions with the strongest magnetic fields
and lowest temperatures in the solar atmosphere. Using the world’s
largest solar telescope, the 1.6 m Goode Solar Telescope, we detect
prevalent reconnection through frequently occurring fine-scale jets
in the Hα line wings at light bridges, the bright lanes that may
divide the dark sunspot core into multiple parts. Many jets have an
inverted Y-shape, shown by models to be typical of reconnection in a
unipolar field environment. Simultaneous spectral imaging data from
the Interface Region Imaging Spectrograph show that the reconnection
drives bidirectional flows up to 200 km s-1, and that the
weakly ionized plasma is heated by at least an order of magnitude up
to ∼80,000 K. Such highly dynamic reconnection jets and efficient
heating should be properly accounted for in future modeling efforts
of sunspots. Our observations also reveal that the surge-like activity
previously reported above light bridges in some chromospheric passbands
such as the Hα core has two components: the ever-present short surges
likely to be related to the upward leakage of magnetoacoustic waves
from the photosphere, and the occasionally occurring long and fast
surges that are obviously caused by the intermittent reconnection jets.
Title: Magnetic Reconnection at the Earliest Stage of Solar Flux
Emergence
Authors: Tian, Hui; Zhu, Xiaoshuai; Peter, Hardi; Zhao, Jie; Samanta,
Tanmoy; Chen, Yajie
Bibcode: 2018ApJ...854..174T
Altcode: 2018arXiv180106785T
On 2016 September 20, the Interface Region Imaging Spectrograph observed
an active region during its earliest emerging phase for almost 7 hr. The
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory
observed continuous emergence of small-scale magnetic bipoles with
a rate of ∼1016 Mx s-1. The emergence of
magnetic fluxes and interactions between different polarities lead to
the frequent occurrence of ultraviolet (UV) bursts, which exhibit as
intense transient brightenings in the 1400 Å images. In the meantime,
discrete small patches with the same magnetic polarity tend to move
together and merge, leading to the enhancement of the magnetic fields
and thus the formation of pores (small sunspots) at some locations. The
spectra of these UV bursts are characterized by the superposition of
several chromospheric absorption lines on the greatly broadened profiles
of some emission lines formed at typical transition region temperatures,
suggesting heating of the local materials to a few tens of thousands of
kelvin in the lower atmosphere by magnetic reconnection. Some bursts
reveal blue- and redshifts of ∼100 km s-1 at neighboring
pixels, indicating the spatially resolved bidirectional reconnection
outflows. Many such bursts appear to be associated with the cancellation
of magnetic fluxes with a rate of the order of ∼1015 Mx
s-1. We also investigate the three-dimensional magnetic
field topology through a magnetohydrostatic model and find that a
small fraction of the bursts are associated with bald patches (magnetic
dips). Finally, we find that almost all bursts are located in regions
of large squashing factor at the height of ∼1 Mm, reinforcing our
conclusion that these bursts are produced through reconnection in the
lower atmosphere.
Title: Two Solar Tornadoes Observed with the Interface Region
Imaging Spectrograph
Authors: Yang, Zihao; Tian, Hui; Peter, Hardi; Su, Yang; Samanta,
Tanmoy; Zhang, Jingwen; Chen, Yajie
Bibcode: 2018ApJ...852...79Y
Altcode: 2017arXiv171108968Y
The barbs or legs of some prominences show an apparent motion of
rotation, which are often termed solar tornadoes. It is under debate
whether the apparent motion is a real rotating motion, or caused by
oscillations or counter-streaming flows. We present analysis results
from spectroscopic observations of two tornadoes by the Interface
Region Imaging Spectrograph. Each tornado was observed for more than
2.5 hr. Doppler velocities are derived through a single Gaussian fit to
the Mg II k 2796 Å and Si IV 1393 Å line profiles. We find coherent
and stable redshifts and blueshifts adjacent to each other across
the tornado axes, which appears to favor the interpretation of these
tornadoes as rotating cool plasmas with temperatures of 104
K-105 K. This interpretation is further supported by
simultaneous observations of the Atmospheric Imaging Assembly on
board the Solar Dynamics Observatory, which reveal periodic motions
of dark structures in the tornadoes. Our results demonstrate that
spectroscopic observations can provide key information to disentangle
different physical processes in solar prominences.
Title: Turbulence and Heating in the Flank and Wake Regions of a
Coronal Mass Ejection
Authors: Fan, Siteng; He, Jiansen; Yan, Limei; Tomczyk, Steven; Tian,
Hui; Song, Hongqiang; Wang, Linghua; Zhang, Lei
Bibcode: 2018SoPh..293....6F
Altcode:
As a coronal mass ejection (CME) passes, the flank and wake regions are
typically strongly disturbed. Various instruments, including the Large
Angle and Spectroscopic Coronagraph (LASCO), the Atmospheric Imaging
Assembly (AIA), and the Coronal Multi-channel Polarimeter (CoMP),
observed a CME close to the east limb on 26 October 2013. A hot (≈10
MK) rising blob was detected on the east limb, with an initial ejection
flow speed of ≈330 kms−1. The magnetic structures
on both sides and in the wake of the CME were strongly distorted,
showing initiation of turbulent motions with Doppler-shift oscillations
enhanced from ≈±3 kms−1 to ≈±15 kms−1
and effective thermal velocities from ≈30 kms−1 to ≈60
kms−1, according to the CoMP observations at the Fe XIII
line. The CoMP Doppler-shift maps suggest that the turbulence behaved
differently at various heights; it showed clear wave-like torsional
oscillations at lower altitudes, which are interpreted as the antiphase
oscillation of an alternating red/blue Doppler shift across the strands
at the flank. The turbulence seems to appear differently in the channels
of different temperatures. Its turnover time was ≈1000 seconds for
the Fe 171 Å channel, while it was ≈500 seconds for the Fe 193 Å
channel. Mainly horizontal swaying rotations were observed in the Fe
171 Å channel, while more vertical vortices were seen in the Fe 193
Å channel. The differential-emission-measure profiles in the flank
and wake regions have two components that evolve differently: the
cool component decreased over time, evidently indicating a drop-out
of cool materials due to ejection, while the hot component increased
dramatically, probably because of the heating process, which is
suspected to be a result of magnetic reconnection and turbulence
dissipation. These results suggest a new turbulence-heating scenario
of the solar corona and solar wind.
Title: The Strongest Acceleration of >40 keV Electrons by
ICME-driven Shocks at 1 au
Authors: Yang, Liu; Wang, Linghua; Li, Gang; Wimmer-Schweingruber,
Robert F.; He, Jiansen; Tu, Chuanyi; Tian, Hui; Bale, Stuart D.
Bibcode: 2018ApJ...853...89Y
Altcode:
We present two case studies of the in-situ electron acceleration
during the 2000 February 11 shock and the 2004 July 22 shock, with
the strongest electron flux enhancement at 40 keV across the shock,
among all the quasi-perpendicular and quasi-parallel ICME-driven
shocks observed by the WIND 3DP instrument from 1995 through 2014 at
1 au. We find that for this quasi-perpendicular (quasi-parallel) shock
on 2000 February 11 (2004 July 22), the shocked electron differential
fluxes at ∼0.4-50 keV in the downstream generally fit well to a
double-power-law spectrum, J ∼ E -β , with an index of β
∼ 3.15 (4.0) at energies below a break at ∼3 keV (∼1 keV) and β
∼ 2.65 (2.6) at energies above. For both shock events, the downstream
electron spectral indices appear to be similar for all pitch angles,
which are significantly larger than the index prediction by diffusive
shock acceleration. In addition, the downstream electron pitch-angle
distributions show the anisotropic beams in the anti-sunward-traveling
direction, while the ratio of the downstream over ambient fluxes appears
to peak near 90° pitch angles, at all energies of ∼0.4-50 keV. These
results suggest that in both shocks, shock drift acceleration likely
plays an important role in accelerating electrons in situ at 1 au. Such
ICME-driven shocks could contribute to the formation of solar wind
halo electrons at energies ≲2 keV, as well as the production of solar
wind superhalo electrons at energies ≳2 keV in interplanetary space.
Title: A Study on Sunward-propagating Alfvénic Fluctuations with
a Power-law Spectrum (SAFP) Observed by the WIND Spacecraft
Authors: Wang, X.; Wu, H.; Tu, C. Y.; Wang, L.; He, J.; Tian, H.
Bibcode: 2017AGUFMSH33B2773W
Altcode:
Sunward-propagating Alfvénic fluctuations with a power-law spectrum
(SAFP) have been recently reported to be a significant physical
phenomenon in the solar wind. However, some characteristics of these
SAFPs are still unknown. Here we develop a new method for identifying
SAFPs. In this method, we can identify all SAFPs with any value of θRB
(angle between the global magnetic field and the radial direction). We
find 508 SAFPs using the WIND spacecraft observation from 1995 to
2014. We also find that SAFP occurs more frequently when θRB equals
90°. The spectral index with an average -1.77 changes continuously
from -2.18 for the parallel to -1.71 for the perpendicular. SAFPs
occur more at the maximum and tend to be observed in the slow solar
wind especially at solar minimum. We also apply the new method to
identify anti-sunward-propagating Alfvénic fluctuations with a
power-law spectrum (AFP) for comparison. The number of SAFPs is much
less than AFPs, and the cases with local bending account for about
half of all observed cases. SAFPs have a preference for negative σc
and ASFs for positive. The statistical results demonstrate that SAFP
has a steeper and weaker power spectrum and present a weaker power
anisotropy than that of AFP. These new results may reveal new insight
into the physical mechanism of the SAFP generation.
Title: Solar Wind 0.1-1 keV Electrons in the Corotating Interaction
Regions
Authors: Wang, L.; Tao, J.; Li, G.; Wimmer-Schweingruber, R. F.;
Jian, L. K.; He, J.; Tu, C.; Tian, H.; Bale, S. D.
Bibcode: 2017AGUFMSH34A..03W
Altcode:
Here we present a statistical study of the 0.1-1 keV suprathermal
electrons in the undisturbed and compressed slow/fast solar wind, for
the 71 corotating interaction regions (CIRs) with good measurements
from the WIND 3DP and MFI instruments from 1995 to 1997. For each of
these CIRs, we separate the strahl and halo electrons based on their
different behaviors in pitch angle distributions in the undisturbed and
compressed solar wind. We fit both the strahl and halo energy spectra
to a kappa function with an index κ index and effective temperature
Teff, and calculate the pitch-angle width at half-maximum (PAHM) of
the strahl population. We also integrate the electron measurements
between 0.1 and 1.0 keV to obtain the number density n and average
energy Eavg for the strahl and halo populations. We find that for
both the strahl and halo populations within and around these CIRs,
the fitted κ index strongly correlates with Teff, similar to the
quiet-time solar wind (Tao et al., ApJ, 2016). The number density of
both the strahl and halo shows a strong positive correlation with the
electron core temperature. The strahl number density ns is correlated
with the magnitude of interplanetary magnetic field, and the strahl
PAHM width is anti-correlated with the solar wind speed. These results
suggest that the origin of strahl electrons from the solar corona is
likely related to the electron core temperature and magnetic field
strength, while the production of halo electrons in the interplanetary
medium could depend on the solar wind velocity.
Title: A Study on Sunward Propagating Alfvénic Fluctuations With
a Power Law Spectrum Observed by the Wind Spacecraft
Authors: Wu, Honghong; Wang, Xin; Tu, Chuanyi; Wang, Linghua; He,
Jiansen; Tian, Hui
Bibcode: 2017JGRA..122.9768W
Altcode:
Sunward propagating Alfvénic fluctuations with a power law spectrum
(SAFP) have been recently observed in the upstream region of the
Earth's bow shock. However, some physical properties of these
fluctuations such as anisotropy remain unclear. Here we develop a
new method for identifying SAFPs, and present for the first time
the anisotropy of SAFPs power and spectral index. In this method,
the propagation direction determination of SAFPs does not rely on a
radial magnetic geometry but the pitch angle distribution of strahl
electron outflow. Therefore, the SAFPs with any value of θRB
(angle between the global mean magnetic field and the Sun-to-Earth
radial direction) can be identified, so that enables the study of
the spectral anisotropy. We find 508 SAFPs using the Wind spacecraft
measurements from 1995 to 2014. We show that the SAFP has an averaged
spectral index of -1.77 ± 0.28 and the index changes continuously
from -2.18 ± 0.21 when θRB=0°-10° to -1.71 ± 0.03 when
θRB=80°-90°. These SAFPs are observed more frequently
in the slow solar wind especially at solar minimum. We also select
antisunward propagating Alfvénic fluctuations with a power law spectrum
using the same method for comparison. The results indicate that the
power spectrum of SAFP is steeper, and the spectral intensity as well as
the power anisotropy of SAFP is weaker. These new findings may provide
information on the generation of turbulence in the upstream region.
Title: Probing the solar transition region: current status and
future perspectives
Authors: Tian, Hui
Bibcode: 2017RAA....17..110T
Altcode:
The solar transition region (TR) is the temperature regime from
roughly 0.02 MK to 0.8 MK in the solar atmosphere. It is the transition
layer from the collisional and partially ionized chromosphere to the
collisionless and fully ionized corona. The TR plays an important role
in the mass and energy transport in both the quiet solar atmosphere and
solar eruptions. Most of the TR emission lines fall into the spectral
range of far ultraviolet and extreme ultraviolet (∼ 400 Å-1600
Å). Imaging and spectroscopic observations in this spectral range are
the most important ways to obtain information about the physics of the
TR. Static solar atmosphere models predict a very thin TR. However,
recent high-resolution observations indicate that the TR is highly
dynamic and inhomogeneous. I will summarize some major findings about
the TR made through imaging and spectroscopic observations in the
past 20 years. These existing observations have demonstrated that the
TR may be the key to understanding coronal heating and origin of the
solar wind. Future exploration of the solar TR may need to focus on
the upper TR, since the plasma in this temperature regime (0.1 MK-0.8
MK) has not been routinely imaged before. High-resolution imaging and
spectroscopic observations of the upper TR will not only allow us to
track the mass and energy from the lower atmosphere to the corona,
but also help us to understand the initiation and heating mechanisms
of coronal mass ejections and solar flares.
Title: Multi-label Learning for Detection of CME-Associated Phenomena
Authors: Yang, Y. H.; Tian, H. M.; Peng, B.; Li, T. R.; Xie, Z. X.
Bibcode: 2017SoPh..292..131Y
Altcode:
Coronal mass ejections (CMEs) are considered as one of the driving
sources of space weather. They are usually associated with many physical
phenomena, e.g. flares, coronal dimmings, and sigmoids. To detect these
phenomena, traditional supervised-learning methods assumed that at most
one event occurred in a CME; therefore each CME instance is associated
with a single label and the phenomenon is processed in isolation. This
simplifying assumption does not fit well, as CMEs might have multiple
events simultaneously. We propose to detect multiple CME-associated
events by multi-label learning methods. With the data available
from the Atmospheric Imaging Assembly (AIA) and the Large Angle and
Spectrometric Coronagraph (LASCO), texture features representing the
events are extracted from all of the associated and not-associated CMEs
and converted into feature vectors for multi-label learning use. Then
a function is learned to predict the proper label sets for CMEs,
such that eight events, i.e. coronal dimming, coronal hole, coronal
jet, coronal wave, filament, filament eruption, flare, and sigmoid,
are detected explicitly. To test the proposed detection algorithm, we
adopt the four-fold cross-validation strategy on a set of 551 labeled
CMEs from AIA. Experimental results demonstrate the good performance
of the multi-label classification methods in terms of test error.
Title: Searching for a termination shock in a well-observed limb flare
Authors: Reeves, Kathy; Chen, Bin; White, Stephen; Tian, Hui
Bibcode: 2017shin.confE..66R
Altcode:
We examine a well-observed flare that occurred on the limb of the Sun
on March 7, 2015 in order to find possible signatures of a termination
shock due to outflows from reconnecting magnetic fields. Images
of this flare from Hinode/XRT and the SDO/AIA 131 bandpass show a
cusp-shaped morphology. The IRIS slit was positioned in the region of
the current sheet, above the flare loops. Fe XXI is detected in the
IRIS spectra with an average Doppler velocity of about 20 km/s. The
non-thermal widths in IRIS decrease steadily from 23:00 UT on the 7th
until 00:20 UT the next day, but there is no strong evidence that the
slit crossed a termination shock. Data from Nobeyama Radioheliograph
(NoRH) is primarily due to thermal free-free emission based on the
examination of NoRH images at 17 GHz and 34 GHz, and RHESSI data shows
a thermal looptop source at 6-10 keV. The lack of non-thermal emission
in this data implies that there is no shock region that accelerates
particles. However, neither of these instruments were observing during
the impulsive phase of the flare, which peaked at about 22:25 UT. We
also investigate the temperature structure of the flare using XRT and
AIA data, and do not find any evidence for a termination shock.
Title: Surge-like Oscillations above Sunspot Light Bridges Driven
by Magnetoacoustic Shocks
Authors: Zhang, Jingwen; Tian, Hui; He, Jiansen; Wang, Linghua
Bibcode: 2017ApJ...838....2Z
Altcode: 2017arXiv170208585Z
High-resolution observations of the solar chromosphere and transition
region often reveal surge-like oscillatory activities above sunspot
light bridges (LBs). These oscillations are often interpreted
as intermittent plasma jets produced by quasi-periodic magnetic
reconnection. We have analyzed the oscillations above an LB in a sunspot
using data taken by the Interface Region Imaging Spectrograph. The
chromospheric 2796 Å images show surge-like activities above the
entire LB at any time, forming an oscillating wall. Within the wall
we often see that the core of the Mg II k 2796.35 Å line first
experiences a large blueshift, and then gradually decreases to zero
shift before increasing to a redshift of comparable magnitude. Such a
behavior suggests that the oscillations are highly nonlinear and likely
related to shocks. In the 1400 Å passband, which samples emission
mainly from the Si IV ion, the most prominent feature is a bright
oscillatory front ahead of the surges. We find a positive correlation
between the acceleration and maximum velocity of the moving front,
which is consistent with numerical simulations of upward propagating
slow-mode shock waves. The Si IV 1402.77 Å line profile is generally
enhanced and broadened in the bright front, which might be caused
by turbulence generated through compression or by the shocks. These
results, together with the fact that the oscillation period stays
almost unchanged over a long duration, lead us to propose that the
surge-like oscillations above LBs are caused by shocked p-mode waves
leaked from the underlying photosphere.
Title: Dynamics of Subarcsecond Bright Dots in the Transition Region
above Sunspots and Their Relation to Penumbral Micro-jets
Authors: Samanta, Tanmoy; Tian, Hui; Banerjee, Dipankar; Schanche,
Nicole
Bibcode: 2017ApJ...835L..19S
Altcode: 2017arXiv170102531S
Recent high-resolution observations have revealed that subarcsecond
bright dots (BDs) with sub-minute lifetimes appear ubiquitously
in the transition region (TR) above sunspot penumbra. The presence
of penumbral micro-jets (PMJs) in the chromosphere was previously
reported. It was proposed that both the PMJs and BDs are formed due
to a magnetic reconnection process and may play an important role
in heating of the penumbra. Using simultaneous observations of the
chromosphere from the Solar Optical Telescope (SOT) on board Hinode and
observations of the TR from the Interface Region Imaging Spectrograph,
we study the dynamics of BDs and their relation to PMJs. We find two
types of BDs, one that is related to PMJs, and another that does not
show any visible dynamics in the SOT Ca II H images. From a statistical
analysis we show that these two types have different properties. The
BDs that are related to PMJs always appear at the top of the PMJs,
the vast majority of which show inward motion and originate before
the generation of the PMJs. These results may indicate that the
reconnection occurs at the lower coronal/TR height and initiates PMJs
at the chromosphere. This formation mechanism is in contrast with the
formation of PMJs by reconnection in the (upper) photosphere between
differently inclined fields.
Title: Observations of the solar corona during the total solar
eclipse on 21 August 2017
Authors: Tian, Hui; Qu, ZhongQuan; Chen, YaJie; Deng, LinHua; Huang,
ZhengHua; Li, Hao; Zhong, Yue; Liang, Yu; Zhang, JingWen; Zhang,
YiGong; Lun, BaoLi; Cheng, XiangMing; Yan, XiaoLi; Xue, ZhiKe; Xin,
YuXin; Song, ZhiMing; Zhu, YingJie; Samanta, Tanmoy
Bibcode: 2017E&PP....1...68T
Altcode:
No abstract at ADS
Title: Signatures of Reconnection Observed in a Candle-Flame Solar
Flare at the Limb
Authors: Reeves, K.; Chen, B.; White, S. M.; Schanche, N.; Tian, H.
Bibcode: 2016AGUFMSH31B2565R
Altcode:
We examine a well-observed flare that occurred on the limb of the Sun
on March 7, 2015 in order to find possible signatures of a termination
shock due to outflows from reconnecting magnetic fields. Images of this
flare from Hinode/XRT and the SDO/AIA 131 bandpass show a cusp-shaped
morphology. The IRIS slit was positioned in the region of the current
sheet, above the flare loops. Fe XXI is detected in the IRIS spectra
with an average Doppler velocity of about 20 km/s. The non-thermal
widths in IRIS decrease steadily from 23:00 UT on the 7th until 00:20
UT the next day. This decrease correlates well with the microwave
radio flux observed by the Nobeyama Radioheliograph (NoRH), which is
primarily due to thermal free-free emission based on the examination
of NoRH images at 17 GHz and 34 GHz. Temperatures of the loop-top
source derived from RHESSI and XRT also show a steady decrease during
this time. We measure downflow velocities in the cusp region in the
AIA 131 A bandpass, and find that from 22-23 UT the flows are about
300-400 km/s, and they slow down to about 100 km/s after 23 UT. This
work supported by NASA Grant NNX15AJ93G.
Title: The anisotropy of sunward-propagating Alfvénic fluctuations
with power-law spectra observed by Wind spacecraft
Authors: Wu, H.; Wang, X.; Tu, C.; Wang, L.; He, J.; Tian, H.;
Marsch, E.
Bibcode: 2016AGUFMSH41A2515W
Altcode:
Sunward-propagating Alfvén fluctuations with power-law spectra
(UPAFs) were reported to be the clue on understanding nonlinear inverse
cascade. Here we present for the first time the spectral anisotropy of
UPAFs with respect to the global mean magnetic field direction using
Wind spacecraft measurements within 20 years from 1995 to 2005. It is
found that the spectral anisotropy of UPAFs is different from that
of the anti-sunward-propagating Alfvén fluctuations (DPAFs). By
determining the propagation direction of the fluctuations straightly
by strahl electron outflow rather than the measured magnetic field
data, we find 1190 UPAFs and 37663 DPAFs. UPAFs spectrum has an
average spectral index of -2.14 when θRB is close to 0° and -1.77
when θRB is close to 90°, while for DPAFs they are -2.00 and -1.64,
respectively. The power of UPAFs shows no obvious anisotropy, although
DPAFs do have larger power at larger θRB. Here we also present the
first probable picture for illustrating inverse cascade in the solar
wind. Those results reveal that UPAFs could help us interpret the
physical process as a new objective.
Title: Multi-wavelength Study of Transition Region Penumbral
Subarcsecond Bright Dots Using IRIS and NST
Authors: Deng, Na; Yurchyshyn, Vasyl; Tian, Hui; Kleint, Lucia; Liu,
Chang; Xu, Yan; Wang, Haimin
Bibcode: 2016ApJ...829..103D
Altcode: 2016arXiv160700306D
Using high-resolution transition region (TR) observations taken by
the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
al. revealed numerous short-lived subarcsecond bright dots (BDs)
above sunspots (mostly located in the penumbrae), which indicate yet
unexplained small-scale energy releases. Moreover, whether or not
these subarcsecond TR brightenings have any signature in the lower
atmosphere and how they are formed are still not fully resolved. This
paper presents a multi-wavelength study of the TR penumbral BDs using a
coordinated observation of a near disk center sunspot with IRIS and the
1.6 m New Solar Telescope (NST) at the Big Bear Solar Observatory. NST
provides high-resolution chromospheric and photospheric observations
with narrowband Hα imaging spectroscopy and broadband TiO images,
respectively, complementary to IRIS TR observations. A total of 2692
TR penumbral BDs are identified from a 37 minute time series of IRIS
1400 Å slit-jaw images. Their locations tend to be associated more
with downflowing and darker fibrils in the chromosphere, and weakly
associated with bright penumbral features in the photosphere. However,
temporal evolution analyses of the BDs show that there is no consistent
and convincing brightening response in the chromosphere. These results
are compatible with a formation mechanism of the TR penumbral BDs
by falling plasma from coronal heights along more vertical and dense
magnetic loops. The BDs may also be produced by small-scale impulsive
magnetic reconnection taking place sufficiently high in the atmosphere
that has no energy release in the chromosphere.
Title: VizieR Online Data Catalog: WISE and 2MASS photometry of M
giant stars (Li+, 2016)
Authors: Li, J.; Smith, M. C.; Zhong, J.; Hou, J.; Carlin, J. L.;
Newberg, H. J.; Liu, C.; Chen, L.; Li, L.; Shao, Z.; Small, E.;
Tian, H.
Bibcode: 2016yCat..18230059L
Altcode:
We cross-matched our LAMOST spectroscopically confirmed sample of M
giants, M dwarfs, and QSOs (see section 2) to the ALLWISE Source Catalog
in the NASA/IPAC Infrared Science Archive, using a search radius of
3". We also applied some cuts detailed in section 3. (1 data file).
Title: The Stellar Age-T eff-Kinematical Asymmetry in
the Solar Neighborhood from LAMOST
Authors: Tian, H. J.; Liu, C.; Carlin, J. L.; Zhao, Y. H.; Chen, X. L.
Bibcode: 2016IAUS..317..354T
Altcode:
With the velocity de-projection technique, we derived the averaged 3
dimensional local velocity distribution using only the line-of-sight
velocity for the 200,000 FGK type main-sequence stars from the LAMOST
DR1 data. Taking the effective temperature as a proxy for age, we
investigate the variation of the velocity distribution as a function of
T eff and disk height within 100 < |z| < 500 pc. Using
the mean velocities of the cool stars, we derive the solar motion of
(U ⊙, V ⊙, W ⊙)=(9.58+/-2.39,
10.52+/-1.96, 7.01+/-1.67) kms-1 with respect to the
local standard of rest (LSR). Moreover, we find that the stars with T
eff > 6000 K show a net asymmetric motion of <U>~2
kms-1 and <W>~3 kms-1 compared to the
stars with T eff < 6000 K. And their azimuthal velocity
increases when |z| increases. The asymmetric motion in the warmer
stars is likely because they are too young and not completely relaxed.
Title: Source of Quasi-Periodic Brightenings of Solar Coronal Bright
Points: Waves or Repeated Reconnections
Authors: Samanta, Tanmoy; Tian, Hui; Banerjee, Dipankar
Bibcode: 2016cosp...41E1696S
Altcode:
Coronal bright points (BPs) are small-scale luminous features seen
in the solar corona. Quasi-periodic brightenings are frequently
observed in the BPs and are generally linked with underlying magnetic
flux changes. We study the dynamics of a BP seen in the coronal hole
using the Atmospheric Imaging Assembly images, the Helioseismic and
Magnetic Imager magnetogram on board the Solar Dynamics Observatory,
and spectroscopic data from the newly launched Interface Region
Imaging Spectrograph (IRIS). The detailed analysis shows that the
BP evolves throughout our observing period along with changes in
underlying photospheric magnetic flux and shows periodic brightenings
in different EUV and far-UV images. With the highest possible spectral
and spatial resolution of IRIS, we attempted to identify the sources
of these oscillations. IRIS sit-and-stare observation provided a
unique opportunity to study the time evolution of one footpoint of
the BP as the slit position crossed it. We noticed enhanced line
profile asymmetry, enhanced line width, intensity enhancements, and
large deviation from the average Doppler shift in the line profiles at
specific instances, which indicate the presence of sudden flows along
the line-of-sight direction. We propose that transition region explosive
events originating from small-scale reconnections and the reconnection
outflows are affecting the line profiles. The correlation between
all these parameters is consistent with the repetitive reconnection
scenario and could explain the quasi-periodic nature of the brightening.
Title: Are IRIS Bombs Connected to Ellerman Bombs?
Authors: Tian, Hui; Xu, Zhi; He, Jiansen; Madsen, Chad
Bibcode: 2016ApJ...824...96T
Altcode: 2016arXiv160405423T
Recent observations by the Interface Region Imaging Spectrograph
(IRIS) have revealed pockets of hot gas (∼2-8 × 104 K)
potentially resulting from magnetic reconnection in the partially
ionized lower solar atmosphere (IRIS bombs; IBs). Using joint
observations between IRIS and the Chinese New Vacuum Solar Telescope,
we have identified 10 IBs. We find that 3 are unambiguously and 3
others are possibly connected to Ellerman bombs (EBs), which show
intense brightening of the extended {{{H}}}α wings without
leaving an obvious signature in the {{{H}}}α core. These
bombs generally reveal the following distinct properties: (1) the O
IV 1401.156 Å and 1399.774 Å lines are absent or very weak; (2) the
Mn I 2795.640 Å line manifests as an absorption feature superimposed
on the greatly enhanced Mg II k line wing; (3) the Mg II k and h lines
show intense brightening in the wings and no dramatic enhancement in the
cores; (4) chromospheric absorption lines such as Ni II 1393.330 Å and
1335.203 Å are very strong; and (5) the 1700 Å images obtained with
the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory
reveal intense and compact brightenings. These properties support the
formation of these bombs in the photosphere, demonstrating that EBs
can be heated much more efficiently than previously thought. We also
demonstrate that the Mg II k and h lines can be used to investigate EBs
similarly to {{{H}}}α , which opens a promising new window
for EB studies. The remaining four IBs obviously have no connection
to EBs and they do not have the properties mentioned above, suggesting
a higher formation layer, possibly in the chromosphere.
Title: Global Sausage Oscillation of Solar Flare Loops Detected by
the Interface Region Imaging Spectrograph
Authors: Tian, Hui; Young, Peter R.; Reeves, Katharine K.; Wang,
Tongjiang; Antolin, Patrick; Chen, Bin; He, Jiansen
Bibcode: 2016ApJ...823L..16T
Altcode: 2016arXiv160501963T
An observation from the Interface Region Imaging Spectrograph
reveals coherent oscillations in the loops of an M1.6 flare on 2015
March 12. Both the intensity and Doppler shift of Fe xxi 1354.08 Å
show clear oscillations with a period of ∼25 s. Remarkably similar
oscillations were also detected in the soft X-ray flux recorded by
the Geostationary Operational Environmental Satellites (GOES). With
an estimated phase speed of ∼2420 km s-1 and a derived
electron density of at least 5.4 × 1010 cm-3,
the observed short-period oscillation is most likely the global
fast sausage mode of a hot flare loop. We find a phase shift of
∼π/2 (1/4 period) between the Doppler shift oscillation and the
intensity/GOES oscillations, which is consistent with a recent forward
modeling study of the sausage mode. The observed oscillation requires
a density contrast between the flare loop and coronal background of a
factor ≥42. The estimated phase speed of the global mode provides a
lower limit of the Alfvén speed outside the flare loop. We also find
an increase of the oscillation period, which might be caused by the
separation of the loop footpoints with time.
Title: Multi-wavelength Study of Transition Region Penumbral
Bright Dots Using Interface Region Imaging Spectrograph and New
Solar Telescope
Authors: Deng, Na; Yurchyshyn, Vasyl B.; Tian, Hui; Kleint, Lucia;
Liu, Chang; Xu, Yan; Wang, Haimin
Bibcode: 2016SPD....47.0101D
Altcode:
Using high-resolution transition region (TR) observations taken by
the Interface Region Imaging Spectrograph (IRIS) mission, Tian et
al. (2014b) revealed numerous short-lived sub-arcsecond bright dots
above sunspots (mostly located in the penumbrae), which indicate yet
unexplained small-scale energy releases. Moreover, whether these TR
brightenings have any signature in the lower atmosphere and how they
are formed are still not fully resolved. This paper presents a study of
these bright dots using a coordinated observation of a near disk-center
sunspot with IRIS and the 1.6 m New Solar Telescope (NST) at the Big
Bear Solar Observatory. NST provides high-resolution chromospheric
and photospheric observations with narrow-band H-alpha imaging
spectroscopy and broad-band TiO images, respectively, complementary
to IRIS TR observations. A total of 2692 TR penumbral bright dots
are identified from a 37-minute time series of IRIS 1400 A slitjaw
images. Their locations tend to be associated more with downflowing and
darker fibrils in the chromosphere, and weakly associated with bright
penumbral features in the photosphere. However, temporal evolution
analyses of the dots show that there is no consistent and convincing
brightening response in the chromosphere. These results are compatible
with a formation mechanism of the TR penumbral bright dots by falling
plasma from coronal heights along more vertical and dense magnetic
loops. The dots may also be produced by small-scale impulsive magnetic
reconnection taking place sufficiently high in the atmosphere that
has no energy release in the chromosphere.Acknowledgement: This work
is mainly supported by NASA grants NNX14AC12G, NNX13AF76G and by NSF
grant AGS 1408703.
Title: Erratum: "Modeling the Chromosphere of a Sunspot and the
Quiet Sun" (2015, ApJ, 811, 87)
Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wuelser, J. -P.
Bibcode: 2016ApJ...821...70A
Altcode:
No abstract at ADS
Title: Statistical Study of Network Jets Observed in the Solar
Transition Region: a Comparison Between Coronal Holes and Quiet-Sun
Regions
Authors: Narang, Nancy; Arbacher, Rebecca T.; Tian, Hui; Banerjee,
Dipankar; Cranmer, Steven R.; DeLuca, Ed E.; McKillop, Sean
Bibcode: 2016SoPh..291.1129N
Altcode: 2016arXiv160406295N; 2016SoPh..tmp...56N
Recent IRIS observations have revealed a prevalence of intermittent
small-scale jets with apparent speeds of 80 -250 kms−1,
emanating from small-scale bright regions inside network boundaries
of coronal holes. We find that these network jets appear not only in
coronal holes but also in quiet-sun regions. Using IRIS 1330 Å (C
II) slit-jaw images, we extracted several parameters of these network
jets, e.g. apparent speed, length, lifetime, and increase in foot-point
brightness. Using several observations, we find that some properties of
the jets are very similar, but others are obviously different between
the quiet Sun and coronal holes. For example, our study shows that
the coronal-hole jets appear to be faster and longer than those in
the quiet Sun. This can be directly attributed to a difference in the
magnetic configuration of the two regions, with open magnetic field
lines rooted in coronal holes and magnetic loops often present in
the quiet Sun. We also detected compact bright loops that are most
likely transition region loops and are mostly located in quiet-Sun
regions. These small loop-like regions are generally devoid of network
jets. In spite of different magnetic structures in the coronal hole and
quiet Sun in the transition region, there appears to be no substantial
difference for the increase in footpoint brightness of the jets, which
suggests that the generation mechanism of these network jets is very
likely the same in both regions.
Title: The Enhancement of BAO in the SDSS MGS
Authors: Tian, H. J.; Neyrinck, M. C.; Budavári, T.; Szalay, A. S.
Bibcode: 2016IAUS..319..145T
Altcode:
We show that redshift-space distortions of galaxy correlations have a
strong effect on correlation functions with the signature of the Baryon
Acoustic Oscillations (BAO). Near the line of sight, the features become
sharper as a result of redshift-space distortions. We analyze the SDSS
DR7 main-galaxy sample (MGS), splitting the sample into slices 2.5 deg
on the sky in various rotations. Measuring 2D correlation functions
in each slice, we do see a sharp bump along the line of sight. Using
Mexican-hat wavelets, we localize it to (110 +/- 10) h - 1 Mpc and
estimate its significance at about 4σ.
Title: Observations of Small-scale IRIS Bombs (Reconnection Events)
in an Evolving Active Region
Authors: Madsen, C. A.; Tian, H.; DeLuca, E. E.
Bibcode: 2015AGUFMSH31B2414M
Altcode:
We present the first Interface Region Imaging Spectrograph (IRIS)
observations of small-scale bombs evolving with their host active
region. Bombs appear most clearly in the IRIS 1330 Å and 1400 Å
slit-jaw images as small (~1 arcsec), compact, intense brightenings at
transition region temperatures. Their NUV/FUV emission spectra exhibit
dramatic line splitting and strong absorption features indicative of
bidirectional flows from magnetic reconnection embedded deep within the
cool lower solar atmosphere. The bombs may contribute significantly
to the heating of the solar atmosphere in active regions; however,
it's unclear how prevalent the bombs are throughout the lifetime of an
active region. Using a semi-automated detection method, we locate bombs
within AR 11850 over the course of four observations from 06:00 UT on
September 25, 2013 until 11:30 UT the next day. The active region is
first observed in an emerging phase and rapidly grows into a mature
active region with well-developed sunspots. The bomb occurrence rate
drops dramatically as the active region fully emerges. We also find
that the bombs fall into two distinct populations: one appears largely
during active region emergence and contains a majority of the bombs,
while the other population is present regardless of active region
age. The first population of bombs is typically found embedded in the
low-lying loops prominent in the young active region. Furthermore,
we use Solar Dynamics Observatory/Helioseismic and Magnetic Imager
(SDO/HMI) line-of-sight magnetograms to show that the bombs associated
with the first population occur at the boundaries between the upward
and downward flux of small, isolated bipolar regions. These regions
dissipate as the active region emerges and reconfigures its magnetic
field into two large network patches of upward and downward flux with a
clear inversion line. The second, smaller population of bombs usually
occurs far from the active region loop structures in the plage and
sunspot penumbrae. They are associated with the boundaries between
strong upward or downward flux and regions where the magnetic field
is perpendicular to the line of sight.
Title: Temporal Evolution of Chromospheric Evaporation: Case
Studies of the M1.1 Flare on 2014 September 6 and X1.6 Flare on 2014
September 10
Authors: Tian, Hui; Young, Peter R.; Reeves, Katharine K.; Chen, Bin;
Liu, Wei; McKillop, Sean
Bibcode: 2015ApJ...811..139T
Altcode: 2015arXiv150502736T
With observations from the Interface Region Imaging Spectrograph, we
track the complete evolution of ∼11 MK evaporation flows in an M1.1
flare on 2014 September 6 and an X1.6 flare on 2014 September 10. These
hot flows, as indicated by the blueshifted Fe xxi 1354.08 Å line,
evolve smoothly with a velocity decreasing exponentially from ∼200
km s-1 to almost stationary within a few minutes. We find a
good correlation between the flow velocity and energy deposition rate
as represented by the hard X-ray flux observed with the Reuven Ramaty
High Energy Solar Spectroscopic Imager, or time derivative of the soft
X-ray flux observed with the Geostationary Operational Environmental
Satellites and the HINODE X-ray Telescope, which is in general agreement
with models of nonthermal electron heating. The maximum blueshift of
Fe xxi appears approximately at the same time as or slightly after
the impulsive enhancement of the ultraviolet continuum and the Mg ii
2798.8 Å line emission, demonstrating that the evaporation flow is
closely related to heating of the lower chromosphere. Finally, while
the hot Fe xxi 1354.08 Å line is entirely blueshifted with no obvious
rest component, cool chromospheric and transition region lines like
Si iv 1402.77 Å are often not entirely redshifted but just reveal an
obvious red wing enhancement at the ribbons, suggesting that the speed
of chromospheric condensation might be larger than previously thought.
Title: Modeling the Chromosphere of a Sunspot and the Quiet Sun
Authors: Avrett, E.; Tian, H.; Landi, E.; Curdt, W.; Wülser, J. -P.
Bibcode: 2015ApJ...811...87A
Altcode:
Semiempirical atmospheric modeling attempts to match an observed
spectrum by finding the temperature distribution and other physical
parameters along the line of sight through the emitting region such
that the calculated spectrum agrees with the observed one. In this
paper we take the observed spectrum of a sunspot and the quiet Sun
in the EUV wavelength range 668-1475 Å from the 2001 SUMER atlas
of Curdt et al. to determine models of the two atmospheric regions,
extending from the photosphere through the overlying chromosphere into
the transition region. We solve the coupled statistical equilibrium
and optically thick radiative transfer equations for a set of 32 atoms
and ions. The atoms that are part of molecules are treated separately,
and are excluded from the atomic abundances and atomic opacities. We
compare the Mg ii k line profile observations from the Interface
Region Imaging Spectrograph with the profiles calculated from the two
models. The calculated profiles for the sunspot are substantially
lower than the observed ones, based on the SUMER models. The only
way we have found to raise the calculated Mg ii lines to agree with
the observations is to introduce illumination of the sunspot from the
surrounding active region.
Title: Self-Absorption in the Solar Transition Region
Authors: Yan, Limei; Peter, Hardi; He, Jiansen; Tian, Hui; Xia,
Lidong; Wang, Linghua; Tu, Chuanyi; Zhang, Lei; Chen, Feng; Barczynski,
Krzysztof
Bibcode: 2015ApJ...811...48Y
Altcode: 2015arXiv150105706Y
Transient brightenings in the transition region of the Sun have
been studied for decades and are usually related to magnetic
reconnection. Recently, absorption features due to chromospheric
lines have been identified in transition region emission lines raising
the question of the thermal stratification during such reconnection
events. We analyze data from the Interface Region Imaging Spectrograph
in an emerging active region. Here the spectral profiles show clear
self-absorption features in the transition region lines of Si iv. While
some indications existed that opacity effects might play some role
in strong transition region lines, self-absorption has not been
observed before. We show why previous instruments could not observe
such self-absorption features, and discuss some implications of this
observation for the corresponding structure of reconnection events in
the atmosphere. Based on this we speculate that a range of phenomena,
such as explosive events, blinkers or Ellerman bombs, are just different
aspects of the same reconnection event occurring at different heights
in the atmosphere.
Title: Direct Observations of Magnetic Reconnection Outflow and CME
Triggering in a Small Erupting Solar Prominence
Authors: Reeves, Katharine K.; McCauley, Patrick I.; Tian, Hui
Bibcode: 2015ApJ...807....7R
Altcode: 2015arXiv150507307R
We examine a small prominence eruption that occurred on 2014 May 1 at
01:35 UT and was observed by the Interface Region Imaging Spectrometer
(IRIS) and the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics
Observatory. Pre- and post-eruption images were taken by the X-ray
Telescope (XRT) on Hinode. Pre-eruption, a dome-like structure exists
above the prominence, as demarcated by coronal rain. As the eruption
progresses, we find evidence for reconnection between the prominence
magnetic field and the overlying field. Fast flows are seen in AIA
and IRIS, indicating reconnection outflows. Plane-of-sky flows of
300 km s-1 are observed in the AIA 171 A channel along a
potentially reconnected field line. IRIS detects intermittent fast
line of sight flows of 200 km s-1 coincident with the
AIA flows. Differential emission measure calculations show heating
at the origin of the fast flows. Post-eruption XRT images show hot
loops probably due to reconfiguration of magnetic fields during the
eruption and subsequent heating of plasma in these loops. Although
there is evidence for reconnection above the prominence during the
eruption, high spatial resolution images from IRIS reveal potential
reconnection sites below the prominence. A height-time analysis of the
erupting prominence shows a slow initial rise with a velocity of 0.4 km
s-1 followed by a rapid acceleration with a final velocity
of 250 km s-1. Brightenings in IRIS during the transition
between these two phases indicate the eruption trigger for the fast
part of the eruption is likely a tether-cutting mechanism rather than
a break-out mechanism.
Title: Quasi-periodic Oscillation of a Coronal Bright Point
Authors: Samanta, Tanmoy; Banerjee, Dipankar; Tian, Hui
Bibcode: 2015ApJ...806..172S
Altcode: 2015arXiv150500587S
Coronal bright points (BPs) are small-scale luminous features seen
in the solar corona. Quasi-periodic brightenings are frequently
observed in the BPs and are generally linked with underlying magnetic
flux changes. We study the dynamics of a BP seen in the coronal hole
using the Atmospheric Imaging Assembly images, the Helioseismic and
Magnetic Imager magnetogram on board the Solar Dynamics Observatory,
and spectroscopic data from the newly launched Interface Region
Imaging Spectrograph (IRIS). The detailed analysis shows that the
BP evolves throughout our observing period along with changes in
underlying photospheric magnetic flux and shows periodic brightenings
in different EUV and far-UV images. With the highest possible spectral
and spatial resolution of IRIS, we attempted to identify the sources
of these oscillations. IRIS sit-and-stare observation provided a
unique opportunity to study the time evolution of one footpoint of
the BP as the slit position crossed it. We noticed enhanced line
profile asymmetry, enhanced line width, intensity enhancements, and
large deviation from the average Doppler shift in the line profiles at
specific instances, which indicate the presence of sudden flows along
the line-of-sight direction. We propose that transition region explosive
events originating from small-scale reconnections and the reconnection
outflows are affecting the line profiles. The correlation between
all these parameters is consistent with the repetitive reconnection
scenario and could explain the quasi-periodic nature of the brightening.
Title: Internetwork Chromospheric Bright Grains Observed With IRIS
and SST
Authors: Martínez-Sykora, Juan; Rouppe van der Voort, Luc; Carlsson,
Mats; De Pontieu, Bart; Pereira, Tiago M. D.; Boerner, Paul; Hurlburt,
Neal; Kleint, Lucia; Lemen, James; Tarbell, Ted D.; Title, Alan;
Wuelser, Jean-Pierre; Hansteen, Viggo H.; Golub, Leon; McKillop, Sean;
Reeves, Kathy K.; Saar, Steven; Testa, Paola; Tian, Hui; Jaeggli,
Sarah; Kankelborg, Charles
Bibcode: 2015ApJ...803...44M
Altcode: 2015arXiv150203490M
The Interface Region Imaging Spectrograph (IRIS) reveals small-scale
rapid brightenings in the form of bright grains all over coronal holes
and the quiet Sun. These bright grains are seen with the IRIS 1330,
1400, and 2796 Å slit-jaw filters. We combine coordinated observations
with IRIS and from the ground with the Swedish 1 m Solar Telescope
(SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H
3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å,
Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps
in Fe i 6302 Å at high spatial (0\buildrel{\prime\prime}\over{.}
33), temporal, and spectral resolution. We conclude that the IRIS
slit-jaw grains are the counterpart of so-called acoustic grains,
i.e., resulting from chromospheric acoustic waves in a non-magnetic
environment. We compare slit-jaw images (SJIs) with spectra from the
IRIS spectrograph. We conclude that the grain intensity in the 2796
Å slit-jaw filter comes from both the Mg ii k core and wings. The
signal in the C ii and Si iv lines is too weak to explain the presence
of grains in the 1300 and 1400 Å SJIs and we conclude that the grain
signal in these passbands comes mostly from the continuum. Although
weak, the characteristic shock signatures of acoustic grains can often
be detected in IRIS C ii spectra. For some grains, a spectral signature
can be found in IRIS Si iv. This suggests that upward propagating
acoustic waves sometimes reach all the way up to the transition region.
Title: The role of turbulence in coronal heating and solar wind
expansion
Authors: Cranmer, S. R.; Asgari-Targhi, M.; Miralles, M. P.; Raymond,
J. C.; Strachan, L.; Tian, H.; Woolsey, L. N.
Bibcode: 2015RSPTA.37340148C
Altcode: 2014arXiv1412.2307C
Plasma in the Sun's hot corona expands into the heliosphere as a
supersonic and highly magnetized solar wind. This paper provides an
overview of our current understanding of how the corona is heated and
how the solar wind is accelerated. Recent models of magnetohydrodynamic
turbulence have progressed to the point of successfully predicting
many observed properties of this complex, multi-scale system. However,
it is not clear whether the heating in open-field regions comes mainly
from the dissipation of turbulent fluctuations that are launched from
the solar surface, or whether the chaotic "magnetic carpet" in the
low corona energizes the system via magnetic reconnection. To help
pin down the physics, we also review some key observational results
from ultraviolet spectroscopy of the collisionless outer corona.
Title: Homologous Helical Jets: Observations By IRIS, SDO, and Hinode
and Magnetic Modeling With Data-Driven Simulations
Authors: Cheung, Mark C. M.; De Pontieu, B.; Tarbell, T. D.; Fu, Y.;
Tian, H.; Testa, P.; Reeves, K. K.; Martínez-Sykora, J.; Boerner,
P.; Wülser, J. P.; Lemen, J.; Title, A. M.; Hurlburt, N.; Kleint,
L.; Kankelborg, C.; Jaeggli, S.; Golub, L.; McKillop, S.; Saar, S.;
Carlsson, M.; Hansteen, V.
Bibcode: 2015ApJ...801...83C
Altcode: 2015arXiv150101593C
We report on observations of recurrent jets by instruments on board
the Interface Region Imaging Spectrograph, Solar Dynamics Observatory
(SDO), and Hinode spacecraft. Over a 4 hr period on 2013 July 21,
recurrent coronal jets were observed to emanate from NOAA Active Region
11793. Far-ultraviolet spectra probing plasma at transition region
temperatures show evidence of oppositely directed flows with components
reaching Doppler velocities of ±100 km s-1. Raster Doppler
maps using a Si iv transition region line show all four jets to have
helical motion of the same sense. Simultaneous observations of the
region by SDO and Hinode show that the jets emanate from a source
region comprising a pore embedded in the interior of a supergranule. The
parasitic pore has opposite polarity flux compared to the surrounding
network field. This leads to a spine-fan magnetic topology in the
coronal field that is amenable to jet formation. Time-dependent
data-driven simulations are used to investigate the underlying drivers
for the jets. These numerical experiments show that the emergence of
current-carrying magnetic field in the vicinity of the pore supplies
the magnetic twist needed for recurrent helical jet formation.
Title: The 2014 March 29 X-flare: Subarcsecond Resolution Observations
of Fe XXI λ1354.1
Authors: Young, Peter R.; Tian, Hui; Jaeggli, Sarah
Bibcode: 2015ApJ...799..218Y
Altcode: 2014arXiv1409.8603Y
The Interface Region Imaging Spectrometer (IRIS) is the first solar
instrument to observe ~10 MK plasma at subarcsecond spatial resolution
through imaging spectroscopy of the Fe XXI λ1354.1 forbidden line. IRIS
observations of the X1 class flare that occurred on 2014 March 29 at
17:48 UT reveal Fe XXI emission from both the flare ribbons and the
post-flare loop arcade. Fe XXI appears at all of the chromospheric
ribbon sites, although typically with a delay of one raster (75 s) and
sometimes offset by up to 1''. 100-200 km s-1 blue-shifts
are found at the brightest ribbons, suggesting hot plasma upflow into
the corona. The Fe XXI ribbon emission is compact with a spatial
extent of <2'', and can extend beyond the chromospheric ribbon
locations. Examples are found of both decreasing and increasing
blue-shift in the direction away from the ribbon locations, and
blue-shifts were present for at least six minutes after the flare
peak. The post-flare loop arcade, seen in Atmospheric Imaging Assembly
131 Å filtergram images that are dominated by Fe XXI, exhibited
bright loop-tops with an asymmetric intensity distribution. The sizes
of the loop-tops are resolved by IRIS at >=1'', and line widths
in the loop-tops are not broader than in the loop-legs suggesting the
loop-tops are not sites of enhanced turbulence. Line-of-sight speeds
in the loop arcade are typically <10 km s-1, and mean
non-thermal motions fall from 43 km s-1 at the flare peak
to 26 km s-1 six minutes later. If the average velocity
in the loop arcade is assumed to be at rest, then it implies a new
reference wavelength for the Fe XXI line of 1354.106 ± 0.023 Å.
Title: Observations of Umbral Flashes and Running Sunspot Waves with
the Interface Region Imaging Spectrograph
Authors: Madsen, Chad A.; Tian, Hui; DeLuca, Edward E.
Bibcode: 2015ApJ...800..129M
Altcode:
We present simultaneous, high-resolution, multi-temperature observations
of running waves and umbral flashes in a sunspot with the Interface
Region Imaging Spectrograph. We analyze intensity variations in
slit-jaw images to investigate the relationship between running
waves in the 1400 Å (Si IV/transition region) passband and umbral
flashes in the 2796 Å (Mg II/chromosphere) passband. Using global
wavelet analysis, we find that the dominant wave periods increase
from approximately 150 s near the sunspot center to about 300 s in
the penumbra in both passbands. This coincides extremely well with
the radially increasing inclination of magnetic field lines observed
with Solar Dynamics Observatory/Helioseismic and Magnetic Imager,
suggesting that the waves are propagating vertically along the inclined
field lines. Furthermore, apparent horizontal velocities for running
waves decrease from about 12 km s-1 in the umbra to about
4 km s-1 in the penumbra which is predicted by the same
inclined field geometry. Finally, we find that umbral flashes lead
running waves in both the spatial and temporal domains. The former
result is attributable to the inclined field geometry; however, the
geometry does not predict the radially increasing time lag which is
likely due to the opacity difference between the emission lines that
dominate the two passbands. These results suggest that running waves
and umbral flashes both originate from photospheric p-mode oscillations
and are manifestations of upward-propagating slow magnetoacoustic modes
at different altitudes and temperatures. The apparent trans-sunspot
propagation of running waves is not real but rather an image of these
upward-propagating modes traveling along field lines of radially
increasing inclination.
Title: Comparing High-speed Transition Region Jets in Coronal Holes
and Quiet Sun Regions
Authors: Tate Arbacher, Rebecca; Tian, Hui; Cranmer, Steven R.
Bibcode: 2015AAS...22513705T
Altcode:
The complicated energy transfer and plasma motion in the transition
region, between the photosphere and the corona, may play a significant
role in the formation and acceleration of the solar wind. New
observations from the Interface Region Imaging Spectrograph (IRIS)
have revealed unprecedented levels of detail in this less-studied
region. Coronal holes in particular are a likely source of solar
wind material, though the formation and acceleration mechanisms of
the fast solar wind are still largely unknown. In our previous work,
we have reported the prevalence of small-scale high-speed (~80-250
km/s) jets with transition region temperatures from the network
structures of coronal holes. Here we undertake a comparative study
of these short-lived episodic network jets in a coronal hole region
and a quiet sun region using IRIS sit-and-stare slit-jaw imaging in
the 1330 Angstrom (C II) passband. The pointing coordinates, exposure
time, observing cadence, and field of view of both observations are all
identical. Our preliminary study suggests that the speeds and lengths
of the network jets may differ between quiet sun and coronal hole
regions. The quiet sun region exhibits many compact bright regions with
sizes of 5-10 arcseconds which produce very few jets. The jets that do
exist tend to propagate at much slower speeds over smaller distances
than their coronal hole counterparts. Comparatively, in the coronal
hole, such compact regions are almost absent and all network patches
are permeated by the intermittent high-reaching jets. Such a difference
suggests that magnetic loops are much smaller in the coronal hole and
the network jets are produced at low heights. The recurrence frequency
seems to be higher in the coronal hole region, with many of the isolated
quiet sun region jets demonstrating curved trajectories.This work is
supported under contract 8100002705 from Lockheed-Martin to SAO and
by the NSF-REU solar physics program at SAO, grant number AGS-1263241.
Title: Comparing High-speed Transition Region Jets in Coronal Holes
and Quiet Sun Regions
Authors: Arbacher, R. T.; Tian, H.; Cranmer, S. R.
Bibcode: 2014AGUFMSH51C4181A
Altcode:
The complicated energy transfer and plasma motion in the transition
region, between the photosphere and the corona, may play a significant
role in the formation and acceleration of the solar wind. New
observations from the Interface Region Imaging Spectrograph (IRIS)
have revealed unprecedented levels of detail in this less-studied
region. Coronal holes in particular are a likely source of solar
wind material, though the formation and acceleration mechanisms of
the fast solar wind are still largely unknown. In our previous work,
we have reported the prevalence of small-scale high-speed (~80-250
km/s) jets with transition region temperatures from the network
structures of coronal holes. Here we undertake a comparative study
of these short-lived episodic network jets in a coronal hole region
and a quiet sun region using IRIS sit-and-stare slit-jaw imaging in
the 1330 Angstrom (C II) passband. The pointing coordinates, exposure
time, observing cadence, and field of view of both observations are
all identical. Our preliminary study suggests that the speeds and
lengths of the network jets may differ between quiet sun and coronal
hole regions. The quiet sun region exhibits many compact bright regions
with sizes of 5-10 arcseconds which produce very few jets. The jets
that do exist tend to propagate at much slower speeds over smaller
distances than their coronal hole counterparts. Comparatively, in the
coronal hole, such compact regions are almost absent and all network
patches are permeated by the intermittent high-reaching jets. Such
a difference suggests that magnetic loops are much smaller in the
coronal hole and the network jets are produced at low heights. The
recurrence frequency seems to be higher in the coronal hole region,
with many of the isolated quiet sun region jets demonstrating curved
trajectories. This work is supported under contract 8100002705 from
Lockheed-Martin to SAO and by the NSF-REU solar physics program at SAO,
grant number AGS-1263241.
Title: IRIS Observations of Magnetic Reconnection and Chromospheric
Evaporation in a Solar Flare
Authors: Tian, H.; Li, G.; Reeves, K.; Raymond, J. C.; Guo, F.
Bibcode: 2014AGUFMSH23A4145T
Altcode:
NASA's IRIS mission has observed signatures of the Fe XXI 1354 line
in tens of solar flares. In many of them, large blue shifts were
identified, supporting the scenario of chromospheric evaporation in
postflare loops. In the standard CSHKP flare model, the postflare
loops are a natual consequence of magnetic reconnection occurring at
the flare site. The CSHKP model also predicts downflow (and upflow)
plasma having a speed close to the Alfven speed. Yet, to date there
were no observations of fast moving downflow plasma in flares. Here, we
report the first detection of large red shift (~200 km/s along line of
sight) of the Fe XXI line with IRIS. Combined imaging and spectroscopic
observations of IRIS, together with SDO/AIA and RHESSI observations,
reveal that the redshifted Fe XXI feature co-located with the loop-top
hard X-Ray source and above the retracting loops. We intepret this large
redshift as signature of downward moving reconnection outflow/retracting
loops. Possible flux rope eruption and reconnection inflows are
also observed. Furthermore, we found that the entire Fe XXI line is
blueshifted by ~250 km/s at the loop footpoints. Cool lines of Si IV,
O IV, C II and Mg II all show obvious redshift at the same locations,
consistent with the scenario of chromospheric evaporation. The map of
electron temperature reconstructed from SDO/AIA observations shows
that the locations of ~10MK temperature generally coincide with the
observed Fe XXI feature very well. Hard X-rays up to ~100keV were found
from RHESSI observations, indicating an efficient electron acceleration
process in this event.
Title: Imaging and Spectroscopic Observations of Magnetic Reconnection
and Chromospheric Evaporation in a Solar Flare
Authors: Tian, Hui; Li, Gang; Reeves, Katharine K.; Raymond, John C.;
Guo, Fan; Liu, Wei; Chen, Bin; Murphy, Nicholas A.
Bibcode: 2014ApJ...797L..14T
Altcode: 2014arXiv1411.2301T
Magnetic reconnection is believed to be the dominant energy release
mechanism in solar flares. The standard flare model predicts both
downward and upward outflow plasmas with speeds close to the coronal
Alfvén speed. Yet, spectroscopic observations of such outflows,
especially the downflows, are extremely rare. With observations of
the newly launched Interface Region Imaging Spectrograph (IRIS), we
report the detection of a greatly redshifted (~125 km s-1
along the line of sight) Fe XXI 1354.08 Å emission line with a ~100 km
s-1 nonthermal width at the reconnection site of a flare. The
redshifted Fe XXI feature coincides spatially with the loop-top
X-ray source observed by RHESSI. We interpret this large redshift as
the signature of downward-moving reconnection outflow/hot retracting
loops. Imaging observations from both IRIS and the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory also reveal
the eruption and reconnection processes. Fast downward-propagating
blobs along these loops are also found from cool emission lines (e.g.,
Si IV, O IV, C II, Mg II) and images of AIA and IRIS. Furthermore,
the entire Fe XXI line is blueshifted by ~260 km s-1 at
the loop footpoints, where the cool lines mentioned above all exhibit
obvious redshift, a result that is consistent with the scenario of
chromospheric evaporation induced by downward-propagating nonthermal
electrons from the reconnection site.
Title: Prevalence of Micro-Jets from the Network Structures of the
Solar Transition Region and Chromosphere
Authors: DeLuca, E. E.; Tian, H.; Cranmer, S. R.; Reeves, K.; Miralles,
M. P.; McCauley, P.; McKillop, S.
Bibcode: 2014AGUFMSH51C4180D
Altcode:
IRIS observations in the 1330Å, 1400Å and 2796Å passbands have
revealed numerous small-scale jet-like features with speeds of ~80-250
km/s from the chromospheric network. These network jets occur in
both the quiet Sun and coronal holes. Their widths are often ~300
km or less. Many of these jets show up as elongated features with
enhanced line width in maps obtained with transition region (TR)
lines, suggesting that these jets reach at least TR temperatures and
they constitute an important element of TR structures. The ubiquitous
presence of these high-reaching (often >10 Mm) jets also suggests
that they may play a crucial role in the mass and energy budgets
of the corona and solar wind. The generation of these jets in the
network and the accompanying Alfven waves is also consistent with
the "magnetic furnace model" of solar wind proposed by Axford &
McKenzie (1992). The large speeds (greater than sound speed) suggest
that the Lorentz force (perhaps related to reconnection) must play
an important role in the generation and propagation of the network
jets. We believe that many network jets are the on-disk counterparts
and TR manifestation of type-II spicules.
Title: Hot explosions in the cool atmosphere of the Sun
Authors: Peter, H.; Tian, H.; Curdt, W.; Schmit, D.; Innes, D.;
De Pontieu, B.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Martínez-Sykora, Juan; Kleint,
L.; Golub, L.; McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.;
Kankelborg, C.; Jaeggli, S.; Carlsson, M.; Hansteen, V.
Bibcode: 2014Sci...346C.315P
Altcode: 2014arXiv1410.5842P
The solar atmosphere was traditionally represented with a simple
one-dimensional model. Over the past few decades, this paradigm shifted
for the chromosphere and corona that constitute the outer atmosphere,
which is now considered a dynamic structured envelope. Recent
observations by the Interface Region Imaging Spectrograph (IRIS) reveal
that it is difficult to determine what is up and down, even in the cool
6000-kelvin photosphere just above the solar surface: This region hosts
pockets of hot plasma transiently heated to almost 100,000 kelvin. The
energy to heat and accelerate the plasma requires a considerable
fraction of the energy from flares, the largest solar disruptions. These
IRIS observations not only confirm that the photosphere is more complex
than conventionally thought, but also provide insight into the energy
conversion in the process of magnetic reconnection.
Title: The unresolved fine structure resolved: IRIS observations of
the solar transition region
Authors: Hansteen, V.; De Pontieu, B.; Carlsson, M.; Lemen, J.; Title,
A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.; Pereira,
T. M. D.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martínez-Sykora, J.
Bibcode: 2014Sci...346E.315H
Altcode: 2014arXiv1412.3611H
The heating of the outer solar atmospheric layers, i.e., the transition
region and corona, to high temperatures is a long-standing problem
in solar (and stellar) physics. Solutions have been hampered by an
incomplete understanding of the magnetically controlled structure of
these regions. The high spatial and temporal resolution observations
with the Interface Region Imaging Spectrograph (IRIS) at the solar
limb reveal a plethora of short, low-lying loops or loop segments
at transition-region temperatures that vary rapidly, on the time
scales of minutes. We argue that the existence of these loops solves
a long-standing observational mystery. At the same time, based on
comparison with numerical models, this detection sheds light on a
critical piece of the coronal heating puzzle.
Title: Evidence of nonthermal particles in coronal loops heated
impulsively by nanoflares
Authors: Testa, P.; De Pontieu, B.; Allred, J.; Carlsson, M.; Reale,
F.; Daw, A.; Hansteen, V.; Martinez-Sykora, J.; Liu, W.; DeLuca, E. E.;
Golub, L.; McKillop, S.; Reeves, K.; Saar, S.; Tian, H.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kleint, L.; Kankelborg, C.; Jaeggli, S.
Bibcode: 2014Sci...346B.315T
Altcode: 2014arXiv1410.6130T
The physical processes causing energy exchange between the Sun’s
hot corona and its cool lower atmosphere remain poorly understood. The
chromosphere and transition region (TR) form an interface region between
the surface and the corona that is highly sensitive to the coronal
heating mechanism. High-resolution observations with the Interface
Region Imaging Spectrograph (IRIS) reveal rapid variability (~20 to
60 seconds) of intensity and velocity on small spatial scales (≲500
kilometers) at the footpoints of hot and dynamic coronal loops. The
observations are consistent with numerical simulations of heating by
beams of nonthermal electrons, which are generated in small impulsive
(≲30 seconds) heating events called “coronal nanoflares.” The
accelerated electrons deposit a sizable fraction of their energy
(≲1025 erg) in the chromosphere and TR. Our analysis
provides tight constraints on the properties of such electron beams
and new diagnostics for their presence in the nonflaring corona.
Title: Prevalence of small-scale jets from the networks of the solar
transition region and chromosphere
Authors: Tian, H.; DeLuca, E. E.; Cranmer, S. R.; De Pontieu, B.;
Peter, H.; Martínez-Sykora, J.; Golub, L.; McKillop, S.; Reeves,
K. K.; Miralles, M. P.; McCauley, P.; Saar, S.; Testa, P.; Weber,
M.; Murphy, N.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt, N.;
Tarbell, T. D.; Wuelser, J. P.; Kleint, L.; Kankelborg, C.; Jaeggli,
S.; Carlsson, M.; Hansteen, V.; McIntosh, S. W.
Bibcode: 2014Sci...346A.315T
Altcode: 2014arXiv1410.6143T
As the interface between the Sun’s photosphere and corona, the
chromosphere and transition region play a key role in the formation and
acceleration of the solar wind. Observations from the Interface Region
Imaging Spectrograph reveal the prevalence of intermittent small-scale
jets with speeds of 80 to 250 kilometers per second from the narrow
bright network lanes of this interface region. These jets have lifetimes
of 20 to 80 seconds and widths of ≤300 kilometers. They originate from
small-scale bright regions, often preceded by footpoint brightenings
and accompanied by transverse waves with amplitudes of ~20 kilometers
per second. Many jets reach temperatures of at least ~105
kelvin and constitute an important element of the transition region
structures. They are likely an intermittent but persistent source of
mass and energy for the solar wind.
Title: On the prevalence of small-scale twist in the solar
chromosphere and transition region
Authors: De Pontieu, B.; Rouppe van der Voort, L.; McIntosh, S. W.;
Pereira, T. M. D.; Carlsson, M.; Hansteen, V.; Skogsrud, H.; Lemen,
J.; Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser,
J. P.; De Luca, E. E.; Golub, L.; McKillop, S.; Reeves, K.; Saar,
S.; Testa, P.; Tian, H.; Kankelborg, C.; Jaeggli, S.; Kleint, L.;
Martinez-Sykora, J.
Bibcode: 2014Sci...346D.315D
Altcode: 2014arXiv1410.6862D
The solar chromosphere and transition region (TR) form an interface
between the Sun’s surface and its hot outer atmosphere. There,
most of the nonthermal energy that powers the solar atmosphere
is transformed into heat, although the detailed mechanism remains
elusive. High-resolution (0.33-arc second) observations with NASA’s
Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere
and TR that are replete with twist or torsional motions on sub-arc
second scales, occurring in active regions, quiet Sun regions, and
coronal holes alike. We coordinated observations with the Swedish
1-meter Solar Telescope (SST) to quantify these twisting motions and
their association with rapid heating to at least TR temperatures. This
view of the interface region provides insight into what heats the low
solar atmosphere.
Title: Open questions on prominences from coordinated observations
by IRIS, Hinode, SDO/AIA, THEMIS, and the Meudon/MSDP
Authors: Schmieder, B.; Tian, H.; Kucera, T.; López Ariste, A.;
Mein, N.; Mein, P.; Dalmasse, K.; Golub, L.
Bibcode: 2014A&A...569A..85S
Altcode: 2014arXiv1407.3171S
Context. A large prominence was observed by multiple instruments on the
ground and in space during an international campaign on September 24,
2013, for three hours (12:12 UT -15:12 UT). Instruments used in the
campaign included the newly launched (June 2013) Interface Region
Imaging Spectrograph (IRIS), THEMIS (Tenerife), the Hinode Solar
Optical Telescope (SOT), the Solar Dynamic Observatory's Atmospheric
Imaging Assembly (SDO/AIA), and the Multichannel Subtractive Double
Pass spectrograph (MSDP) in the Meudon Solar Tower. The movies obtained
in 304 Å with the EUV imager SDO/AIA, and in Ca II line by SOT show
the dynamic nature of the prominence.
Aims: The aim of this
work is to study the dynamics of the prominence fine structures in
multiple wavelengths to understand their formation.
Methods:
The spectrographs IRIS and MSDP provided line profiles with a high
cadence in Mg II h (2803.5 Å) and k (2796.4 Å) lines along four
slit positions (IRIS), and in Hα in a 2D field of view (MSDP). The
spectropolarimetry of THEMIS (Tenerife) allowed us to derive the
magnetic field of the prominence using the He D3 line
depolarization (Hanle effect combined with the Zeeman effect).
Results: The magnetic field is found to be globally horizontal with
a relatively weak field strength (8-15 Gauss). On the other hand,
the Ca II movie reveals turbulent-like motion that is not organized in
specific parts of the prominence. We tested the addition of a turbulent
magnetic component. This model is compatible with the polarimetric
observations at those places where the plasma turbulence peaks. On the
other hand, the Mg II line profiles show multiple peaks well separated
in wavelength. This is interpreted by the existence of small threads
along the line of sight with a large dispersion of discrete values of
Doppler shifts, from 5 km s-1 (a quasi-steady component) to
60-80 km s-1. Each peak corresponds to a Gaussian profile,
and not to a reversed profile as was expected by the present non-LTE
radiative transfer modeling. This is a very surprising behavior for
the Mg II line observed in prominences.
Conclusions: Turbulent
fields on top of the macroscopic horizontal component of the magnetic
field supporting the prominence give rise to the complex dynamics of
the plasma. The plasma with the high velocities (70 km s-1 to
100 km s-1 if we take into account the transverse velocities)
may correspond to condensation of plasma along more or less horizontal
threads of the arch-shape structure visible in 304 Å. The steady
flows (5 km s-1) would correspond to a more quiescent plasma
(cool and prominence-corona transition region) of the prominence packed
into dips in horizontal magnetic field lines. The very weak secondary
peaks in the Mg II profiles may reflect the turbulent nature of parts
of the prominence. Movies are available in electronic form at http://www.aanda.org
Title: An Interface Region Imaging Spectrograph First View on Solar
Spicules
Authors: Pereira, T. M. D.; De Pontieu, B.; Carlsson, M.; Hansteen,
V.; Tarbell, T. D.; Lemen, J.; Title, A.; Boerner, P.; Hurlburt,
N.; Wülser, J. P.; Martínez-Sykora, J.; Kleint, L.; Golub, L.;
McKillop, S.; Reeves, K. K.; Saar, S.; Testa, P.; Tian, H.; Jaeggli,
S.; Kankelborg, C.
Bibcode: 2014ApJ...792L..15P
Altcode: 2014arXiv1407.6360P
Solar spicules have eluded modelers and observers for decades. Since
the discovery of the more energetic type II, spicules have become
a heated topic but their contribution to the energy balance of the
low solar atmosphere remains unknown. Here we give a first glimpse of
what quiet-Sun spicules look like when observed with NASA's recently
launched Interface Region Imaging Spectrograph (IRIS). Using IRIS
spectra and filtergrams that sample the chromosphere and transition
region, we compare the properties and evolution of spicules as
observed in a coordinated campaign with Hinode and the Atmospheric
Imaging Assembly. Our IRIS observations allow us to follow the thermal
evolution of type II spicules and finally confirm that the fading
of Ca II H spicules appears to be caused by rapid heating to higher
temperatures. The IRIS spicules do not fade but continue evolving,
reaching higher and falling back down after 500-800 s. Ca II H type
II spicules are thus the initial stages of violent and hotter events
that mostly remain invisible in Ca II H filtergrams. These events
have very different properties from type I spicules, which show lower
velocities and no fading from chromospheric passbands. The IRIS spectra
of spicules show the same signature as their proposed disk counterparts,
reinforcing earlier work. Spectroheliograms from spectral rasters also
confirm that quiet-Sun spicules originate in bushes from the magnetic
network. Our results suggest that type II spicules are indeed the
site of vigorous heating (to at least transition region temperatures)
along extensive parts of the upward moving spicular plasma.
Title: Observations of Subarcsecond Bright Dots in the Transition
Region above Sunspots with the Interface Region Imaging Spectrograph
Authors: Tian, H.; Kleint, L.; Peter, H.; Weber, M.; Testa, P.;
DeLuca, E.; Golub, L.; Schanche, N.
Bibcode: 2014ApJ...790L..29T
Altcode: 2014arXiv1407.1060T
Observations with the Interface Region Imaging Spectrograph (IRIS)
have revealed numerous sub-arcsecond bright dots in the transition
region above sunspots. These bright dots are seen in the 1400 Å and
1330 Å slit-jaw images. They are clearly present in all sunspots we
investigated, mostly in the penumbrae, but also occasionally in some
umbrae and light bridges. The bright dots in the penumbrae typically
appear slightly elongated, with the two dimensions being 300-600 km and
250-450 km, respectively. The long sides of these dots are often nearly
parallel to the bright filamentary structures in the penumbrae but
sometimes clearly deviate from the radial direction. Their lifetimes
are mostly less than one minute, although some dots last for a few
minutes or even longer. Their intensities are often a few times stronger
than the intensities of the surrounding environment in the slit-jaw
images. About half of the bright dots show apparent movement with
speeds of ~10-40 km s-1 in the radial direction. Spectra of
a few bright dots were obtained and the Si IV 1402.77 Å line profiles
in these dots are significantly broadened. The line intensity can be
enhanced by one to two orders of magnitude. Some relatively bright
and long-lasting dots are also observed in several passbands of the
Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory,
and they appear to be located at the bases of loop-like structures. Many
of these bright dots are likely associated with small-scale energy
release events at the transition region footpoints of magnetic loops.
Title: The Interface Region Imaging Spectrograph (IRIS)
Authors: De Pontieu, B.; Title, A. M.; Lemen, J. R.; Kushner, G. D.;
Akin, D. J.; Allard, B.; Berger, T.; Boerner, P.; Cheung, M.; Chou,
C.; Drake, J. F.; Duncan, D. W.; Freeland, S.; Heyman, G. F.; Hoffman,
C.; Hurlburt, N. E.; Lindgren, R. W.; Mathur, D.; Rehse, R.; Sabolish,
D.; Seguin, R.; Schrijver, C. J.; Tarbell, T. D.; Wülser, J. -P.;
Wolfson, C. J.; Yanari, C.; Mudge, J.; Nguyen-Phuc, N.; Timmons,
R.; van Bezooijen, R.; Weingrod, I.; Brookner, R.; Butcher, G.;
Dougherty, B.; Eder, J.; Knagenhjelm, V.; Larsen, S.; Mansir, D.;
Phan, L.; Boyle, P.; Cheimets, P. N.; DeLuca, E. E.; Golub, L.;
Gates, R.; Hertz, E.; McKillop, S.; Park, S.; Perry, T.; Podgorski,
W. A.; Reeves, K.; Saar, S.; Testa, P.; Tian, H.; Weber, M.; Dunn, C.;
Eccles, S.; Jaeggli, S. A.; Kankelborg, C. C.; Mashburn, K.; Pust, N.;
Springer, L.; Carvalho, R.; Kleint, L.; Marmie, J.; Mazmanian, E.;
Pereira, T. M. D.; Sawyer, S.; Strong, J.; Worden, S. P.; Carlsson,
M.; Hansteen, V. H.; Leenaarts, J.; Wiesmann, M.; Aloise, J.; Chu,
K. -C.; Bush, R. I.; Scherrer, P. H.; Brekke, P.; Martinez-Sykora,
J.; Lites, B. W.; McIntosh, S. W.; Uitenbroek, H.; Okamoto, T. J.;
Gummin, M. A.; Auker, G.; Jerram, P.; Pool, P.; Waltham, N.
Bibcode: 2014SoPh..289.2733D
Altcode: 2014arXiv1401.2491D; 2014SoPh..tmp...25D
The Interface Region Imaging Spectrograph (IRIS) small explorer
spacecraft provides simultaneous spectra and images of the photosphere,
chromosphere, transition region, and corona with 0.33 - 0.4 arcsec
spatial resolution, two-second temporal resolution, and 1 km
s−1 velocity resolution over a field-of-view of up to
175 arcsec × 175 arcsec. IRIS was launched into a Sun-synchronous
orbit on 27 June 2013 using a Pegasus-XL rocket and consists of a
19-cm UV telescope that feeds a slit-based dual-bandpass imaging
spectrograph. IRIS obtains spectra in passbands from 1332 - 1358 Å,
1389 - 1407 Å, and 2783 - 2834 Å, including bright spectral lines
formed in the chromosphere (Mg II h 2803 Å and Mg II k 2796 Å) and
transition region (C II 1334/1335 Å and Si IV 1394/1403 Å). Slit-jaw
images in four different passbands (C II 1330, Si IV 1400, Mg II k
2796, and Mg II wing 2830 Å) can be taken simultaneously with spectral
rasters that sample regions up to 130 arcsec × 175 arcsec at a variety
of spatial samplings (from 0.33 arcsec and up). IRIS is sensitive to
emission from plasma at temperatures between 5000 K and 10 MK and will
advance our understanding of the flow of mass and energy through an
interface region, formed by the chromosphere and transition region,
between the photosphere and corona. This highly structured and dynamic
region not only acts as the conduit of all mass and energy feeding
into the corona and solar wind, it also requires an order of magnitude
more energy to heat than the corona and solar wind combined. The
IRIS investigation includes a strong numerical modeling component
based on advanced radiative-MHD codes to facilitate interpretation of
observations of this complex region. Approximately eight Gbytes of data
(after compression) are acquired by IRIS each day and made available
for unrestricted use within a few days of the observation.
Title: Detection of Supersonic Downflows and Associated Heating
Events in the Transition Region above Sunspots
Authors: Kleint, L.; Antolin, P.; Tian, H.; Judge, P.; Testa, P.;
De Pontieu, B.; Martínez-Sykora, J.; Reeves, K. K.; Wuelser, J. P.;
McKillop, S.; Saar, S.; Carlsson, M.; Boerner, P.; Hurlburt, N.; Lemen,
J.; Tarbell, T. D.; Title, A.; Golub, L.; Hansteen, V.; Jaeggli, S.;
Kankelborg, C.
Bibcode: 2014ApJ...789L..42K
Altcode: 2014arXiv1406.6816K
Interface Region Imaging Spectrograph data allow us to study the solar
transition region (TR) with an unprecedented spatial resolution of
0.''33. On 2013 August 30, we observed bursts of high Doppler shifts
suggesting strong supersonic downflows of up to 200 km s-1
and weaker, slightly slower upflows in the spectral lines Mg II h
and k, C II 1336, Si IV 1394 Å, and 1403 Å, that are correlated
with brightenings in the slitjaw images (SJIs). The bursty behavior
lasts throughout the 2 hr observation, with average burst durations
of about 20 s. The locations of these short-lived events appear to
be the umbral and penumbral footpoints of EUV loops. Fast apparent
downflows are observed along these loops in the SJIs and in the
Atmospheric Imaging Assembly, suggesting that the loops are thermally
unstable. We interpret the observations as cool material falling
from coronal heights, and especially coronal rain produced along the
thermally unstable loops, which leads to an increase of intensity
at the loop footpoints, probably indicating an increase of density
and temperature in the TR. The rain speeds are on the higher end of
previously reported speeds for this phenomenon, and possibly higher
than the free-fall velocity along the loops. On other observing days,
similar bright dots are sometimes aligned into ribbons, resembling
small flare ribbons. These observations provide a first insight into
small-scale heating events in sunspots in the TR.
Title: Persistent Doppler Shift Oscillations Observed with Hinode/EIS
in the Solar CoronA: Spectroscopic Signatures of Alfvénic Waves
and Recurring Upflows
Authors: Tian, Hui
Bibcode: 2014shin.confE.152T
Altcode:
Using data obtained by the EUV Imaging Spectrometer on board Hinode,
we have performed a survey of obvious and persistent (without
significant damping) Doppler shift oscillations in the corona. We
have found mainly two types of oscillations from February to April
in 2007. One type is found at loop footpoint regions, with a dominant
period around 10 minutes. They are characterized by coherent behavior
of all line parameters (line intensity, Doppler shift, line width,
and profile asymmetry), and apparent blueshift and blueward asymmetry
throughout almost the entire duration. Such oscillations are likely to
be signatures of quasi-periodic upflows (small-scale jets, or coronal
counterpart of type-II spicules), which may play an important role
in the supply of mass and energy to the hot corona. The other type of
oscillation is usually associated with the upper part of loops. They are
most clearly seen in the Doppler shift of coronal lines with formation
temperatures between one and two million degrees. The global wavelets
of these oscillations usually peak sharply around a period in the range
of three to six minutes. No obvious profile asymmetry is found and
the variation of the line width is typically very small. The intensity
variation is often less than 2%. These oscillations are more likely to
be signatures of kink/Alfvéen waves rather than flows. In a few cases,
there seems to be a ?/2 phase shift between the intensity and Doppler
shift oscillations, which may suggest the presence of slow-mode standing
waves according to wave theories. However, we demonstrate that such a
phase shift could also be produced by loops moving into and out of a
spatial pixel as a result of Alfvénic oscillations. In this scenario,
the intensity oscillations associated with Alfvénic waves are caused by
loop displacement rather than density change. These coronal waves may be
used to investigate properties of the coronal plasma and magnetic field.
Title: IRIS observations of sunspot oscillations
Authors: Tian, Hui
Bibcode: 2014shin.confE.151T
Altcode:
We present the first results of sunspot oscillations from observations
by the Interface Region Imaging Spectrograph. The strongly nonlinear
oscillation is identified in both the slit-jaw images and the
spectra of several emission lines formed in the transition region and
chromosphere. We first apply a single Gaussian fit to the profiles
of the MgII 2796.35 Å, CII 1335.71 Å, and Si IV 1393.76 Å lines
in the sunspot. The intensity change is ∼30%. The Doppler shift
oscillation reveals a sawtooth pattern with an amplitude of ∼10
km -1 in Si iv. The Si iv oscillation lags those of CII and MgII by
∼6 and ∼25 s, respectively. The line width suddenly increases
as the Doppler shift changes from redshift to blueshift. However,
we demonstrate that this increase is caused by the superposition of
two emission components. We then perform detailed analysis of the
line profiles at a few selected locations on the slit. The temporal
evolution of the line core is dominated by the following behavior:
a rapid excursion to the blue side, accompanied by an intensity
increase, followed by a linear decrease of the velocity to the red
side. The maximum intensity slightly lags the maximum blueshift in Si
IV, whereas the intensity enhancement slightly precedes the maximum
blueshift in MgII. We find a positive correlation between the maximum
velocity and deceleration, a result that is consistent with numerical
simulations of upward propagating magnetoacoustic shock waves.
Title: Analyzing an IRIS Blowout jet via Magnetofrictional Simulation
Authors: Savcheva, Antonia; Tian, Hui; Meyer, Karen
Bibcode: 2014AAS...22432310S
Altcode:
The imaging spectrograph, IRIS, offers unprecedented spatial and
temporal resolution of small-scale phenomena, which allows the
study of their spectral properties in the chromosphere and transition
region. This study present IRIS observations of a blowout coronal jet,
demonstrating the ability of IRIS to detect reconnection effects in
the low atmosphere in the available suite of spectral lines. We present
Doppler velocity and non-thermal width (NTW) maps of the jet and their
evolution in time. We interpret the results using MHD simulations of
jets. In addition, we present a data-driven magnetofrictional simulation
of the same jet and match the magnetic and current structure of the
jet to the observed NTW maps. We infer the height of the null point
and the extent of the region showing reconnection effects. We discuss
the implications of understanding reconnection effects in conjunction
with NTW maps.
Title: IRIS observations of the transition region above sunspots:
oscillations and moving penumbral dots
Authors: Tian, Hui; DeLuca, Ed; Weber, Mark A.; McKillop, Sean;
Reeves, Kathy; Kleint, Lucia; Martinez-Sykora, Juan; De Pontieu,
Bart; Carlsson, Mats
Bibcode: 2014AAS...22431306T
Altcode:
NASA's IRIS mission is providing high-cadence and high-resolution
observations of the solar transition region and chromosphere. We
present results from IRIS observation of the transition region above
sunspots. The major findings can be summarized as following: (1) The C
II and Mg II line profiles are almost Gaussian in the sunspot umbra and
clearly exhibit a deep reversal at the line center in the plage region,
suggesting a greatly reduced opacity in the sunspot atmosphere. (2)
Strongly nonlinear sunspot oscillations can be clearly identified
in not only the slit jaw images of 2796Å, 1400Å and 1330Å, but
also in spectra of the bright Mg II, C II and Si IV lines. The Si
iv oscillation lags those of C ii and Mg ii by 3 and 12 seconds,
respectively. The temporal evolution of the line core is dominated by
the following behavior: a rapid excursion to the blue side, accompanied
by an intensity increase, followed by a linear decrease of the velocity
to the red side. The maximum intensity slightly lags the maximum blue
shift in Si iv , whereas the intensity enhancement slightly precedes the
maximum blue shift in Mg ii . We find a positive correlation between
the maximum velocity and deceleration. These results are consistent
with numerical simulations of upward propagating magneto-acoustic
shock waves. We also demonstrate that the strongly nonlinear line
width oscillation, reported both previously and here, is spurious. (3)
Many small-scale bright dots are present in the penumbral filaments and
light bridges in SJI 1330Å and 1400Å images obtained in high-cadence
observations. They are usually smaller than 1" and often just a couple
of pixels wide. Some bright dots show apparent movement with a speed of
20-60 km/s(either outward or inward). The lifetime of these penumbral
dots is mostly less than 1 min. The most obvious feature of the Si IV
profiles in the bright dots is the enhanced line width. Besides that,
the profile looks normal and no obvious fast flows are detected. The
bright dots in the light bridges even show oscillation patterns. It's
not clear whether these oscillations are triggered by the umbral
oscillations or not.
Title: High-resolution Observations of the Shock Wave Behavior for
Sunspot Oscillations with the Interface Region Imaging Spectrograph
Authors: Tian, H.; DeLuca, E.; Reeves, K. K.; McKillop, S.; De Pontieu,
B.; Martínez-Sykora, J.; Carlsson, M.; Hansteen, V.; Kleint, L.;
Cheung, M.; Golub, L.; Saar, S.; Testa, P.; Weber, M.; Lemen, J.;
Title, A.; Boerner, P.; Hurlburt, N.; Tarbell, T. D.; Wuelser, J. P.;
Kankelborg, C.; Jaeggli, S.; McIntosh, S. W.
Bibcode: 2014ApJ...786..137T
Altcode: 2014arXiv1404.6291T
We present the first results of sunspot oscillations from observations
by the Interface Region Imaging Spectrograph. The strongly nonlinear
oscillation is identified in both the slit-jaw images and the
spectra of several emission lines formed in the transition region and
chromosphere. We first apply a single Gaussian fit to the profiles of
the Mg II 2796.35 Å, C II 1335.71 Å, and Si IV 1393.76 Å lines in the
sunspot. The intensity change is ~30%. The Doppler shift oscillation
reveals a sawtooth pattern with an amplitude of ~10 km s-1
in Si IV. The Si IV oscillation lags those of C II and Mg II by ~3 and
~12 s, respectively. The line width suddenly increases as the Doppler
shift changes from redshift to blueshift. However, we demonstrate
that this increase is caused by the superposition of two emission
components. We then perform detailed analysis of the line profiles at
a few selected locations on the slit. The temporal evolution of the
line core is dominated by the following behavior: a rapid excursion
to the blue side, accompanied by an intensity increase, followed by a
linear decrease of the velocity to the red side. The maximum intensity
slightly lags the maximum blueshift in Si IV, whereas the intensity
enhancement slightly precedes the maximum blueshift in Mg II. We find
a positive correlation between the maximum velocity and deceleration,
a result that is consistent with numerical simulations of upward
propagating magnetoacoustic shock waves.
Title: Dynamics of a prominence observed in Mg II lines by IRIS
Authors: Schmieder, Brigitte; Mein, Pierre; Dalmasse, Kévin; Tian,
Hui; Kucera, Therese; Lopez-Ariste, Arturo
Bibcode: 2014cosp...40E2927S
Altcode:
In September 2013 several prominences were observed with the IRIS
spectrograph during a 60 day-long international program. We will present
one set of observations obtained using multiple instruments on September
24. SDO/AIA and IRIS slit jaws provided images of the prominence
corresponding to different physical conditions of the transition
region between the cool plasma and the corona. The vector magnetic
field was derived from THEMIS (Tenerife) observations using the He D3
depolarisation due to the magnetic field. The inversion code (CPA) takes
into account the Hanle and the Zeeman effects. Movies from SDO/AIA in
304 A and Hinode/SOT in Ca II show the dynamics of the fine structures
in the plane of the sky. From Mg II and Si IV line spectra observed by
IRIS and H-alpha observed by the Multi-channel subtractive spectrograph
(MSDP) in the Meudon solar tower we derived the Dopplershifts of the
fine structures. The profiles of the Mg II lines are narrow (FHWM =0.15
A) and not reversed, contrary to the predictions of the theoretical
models (Paletou et al 1993). We could resolve the velocity of several
structures along the LOS with Dopplershifts as high as 60 km/s.
Title: Observations of Coronal Mass Ejections with the Coronal
Multichannel Polarimeter
Authors: Tian, H.; Tomczyk, S.; McIntosh, S. W.; Bethge, C.; de Toma,
G.; Gibson, S.
Bibcode: 2013SoPh..288..637T
Altcode: 2013arXiv1303.4647T
The Coronal Multichannel Polarimeter (CoMP) measures not only the
polarization of coronal emission, but also the full radiance profiles of
coronal emission lines. For the first time, CoMP observations provide
high-cadence image sequences of the coronal line intensity, Doppler
shift, and line width simultaneously over a large field of view. By
studying the Doppler shift and line width we may explore more of the
physical processes of the initiation and propagation of coronal mass
ejections (CMEs). Here we identify a list of CMEs observed by CoMP
and present the first results of these observations. Our preliminary
analysis shows that CMEs are usually associated with greatly increased
Doppler shift and enhanced line width. These new observations provide
not only valuable information to constrain CME models and probe
various processes during the initial propagation of CMEs in the low
corona, but also offer a possible cost-effective and low-risk means
of space-weather monitoring.
Title: The Evolving Magnetic Scales of the Outer Solar Atmosphere
and Their Potential Impact on Heliospheric Turbulence
Authors: McIntosh, Scott W.; Bethge, Christian; Threlfall, James;
De Moortel, Ineke; Leamon, Robert J.; Tian, Hui
Bibcode: 2013arXiv1311.2538M
Altcode:
The presence of turbulent phenomena in the outer solar atmosphere
is a given. However, because we are reduced to remotely sensing the
atmosphere of a star with instruments of limited spatial and/or spectral
resolution, we can only infer the physical progression from macroscopic
to microscopic phenomena. Even so, we know that many, if not all,
of the turbulent phenomena that pervade interplanetary space have
physical origins at the Sun and so in this brief article we consider
some recent measurements which point to sustained potential source(s)
of heliospheric turbulence in the magnetic and thermal domains. In
particular, we look at the scales of magnetism that are imprinted on
the outer solar atmosphere by the relentless magneto-convection of the
solar interior and combine state-of-the-art observations from the Solar
Dynamics Observatory (SDO) and the Coronal Multi-channel Polarimeter
(CoMP) which are beginning to hint at the origins of the wave/plasma
interplay prevalent closer to the Earth. While linking these disparate
scales of observation and understanding of their connection is near
to impossible, it is clear that the constant evolution of subsurface
magnetism on a host of scales guides and governs the flow of mass
and energy at the smallest scales. In the near future significant
progress in this area will be made by linking observations from high
resolution platforms like the Interface Region Imaging Spectrograph
(IRIS) and Advanced Technology Solar Telescope (ATST) with full-disk
synoptic observations such as those presented herein.
Title: VizieR Online Data Catalog: Galactic extinction from SDSS
BHB stars (Tian+, 2014)
Authors: Tian, H. J.; Liu, C.; Hu, J. Y.; Xu, Y.; Chen, X. L.
Bibcode: 2013yCat..35610142T
Altcode: 2013yCat..35619142T
File table3 contains the SDSS colors of the 94 template BHB stars
and their coordinates and the errors of colors. The 94 BHB stars are
selected from 7 globular clusters. (2 data files).
Title: Temperature dependence of ultraviolet line parameters in
network and internetwork regions of the quiet Sun and coronal holes
Authors: Wang, X.; McIntosh, S. W.; Curdt, W.; Tian, H.; Peter, H.;
Xia, L. -D.
Bibcode: 2013A&A...557A.126W
Altcode:
Aims: We study the temperature dependence of the average Doppler
shift and the non-thermal line width in network and internetwork
regions for both the quiet Sun (QS) and the coronal hole (CH), by
using observations of the Solar Ultraviolet Measurements of Emitted
Radiation instrument onboard the Solar and Heliospheric Observatory
spacecraft.
Methods: We obtain the average Doppler shift and
non-thermal line width in the network regions of QS, internetwork
regions of QS, network regions of CH, and internetwork regions of CH by
applying a single-Gaussian fit to the line profiles averaged in each
of the four regions. The formation temperatures of the lines we use
cover the range from 104 to 1.2 × 106 K. Two
simple scenarios are proposed to explain the temperature dependence of
the line parameters in the network regions. In one of the scenarios,
the spectral line consists of three components: a rapid, weak upflow
generated in the lower atmosphere, a nearly static background, and
a slow cooling downflow. In the other scenario, there are just two
components, which include a bright core component and a faint wide tail
one.
Results: An enhancement of the Doppler shift magnitude
and the non-thermal line width in network regions compared to the
internetwork regions is reported. We also report that most transition
region lines are less redshifted (by 0-8 km s-1) and broader
(by 0-5 km s-1) in CH compared to the counterparts of QS. In
internetwork regions, the difference in the Doppler shifts between the
coronal hole and the QS is slightly smaller, especially for the lines
with formation temperatures lower than 2 × 105 K. And the
two simple scenarios can reproduce the variation in the line parameters
with the temperature very well.
Conclusions: Our results suggest
that the physical processes in network and internetwork regions are
different and that one needs to separate network and internetwork when
discussing dynamics and physical properties of the solar atmosphere. The
agreement between the results of the observation and our scenarios
suggests that the temperature dependence of Doppler shifts and line
widths might be caused by the different relative contributions of the
three components at different temperatures. The results may shed new
light on our understanding of the complex chromosphere-corona mass
cycle. However, the existing observational results do not allow us to
distinguish between the two scenarios. At this stage, a high-resolution
instrument Interface Region Imaging Spectrograph is highly desirable.
Title: A Detailed Comparison between the Observed and Synthesized
Properties of a Simulated Type II Spicule
Authors: Martínez-Sykora, Juan; De Pontieu, Bart; Leenaarts, Jorrit;
Pereira, Tiago M. D.; Carlsson, Mats; Hansteen, Viggo; Stern, Julie
V.; Tian, Hui; McIntosh, Scott W.; Rouppe van der Voort, Luc
Bibcode: 2013ApJ...771...66M
Altcode: 2013arXiv1305.2397M
We have performed a three-dimensional radiative MHD simulation of the
solar atmosphere. This simulation shows a jet-like feature that shows
similarities to the type II spicules observed for the first time with
Hinode's Solar Optical Telescope. Rapid blueshifted events (RBEs) on the
solar disk are associated with these spicules. Observational results
suggest they may contribute significantly in supplying the corona
with hot plasma. We perform a detailed comparison of the properties
of the simulated jet with those of type II spicules (observed with
Hinode) and RBEs (with ground-based instruments). We analyze a wide
variety of synthetic emission and absorption lines from the simulations
including chromospheric (Ca II 8542 Å, Ca II H, and Hα) to transition
region and coronal temperatures (10,000 K to several million K). We
compare their synthetic intensities, line profiles, Doppler shifts,
line widths, and asymmetries with observations from Hinode/SOT and
EIS, SOHO/SUMER, the Swedish 1 m Solar Telescope, and SDO/AIA. Many
properties of the synthetic observables resemble the observations,
and we describe in detail the physical processes that lead to these
observables. Detailed analysis of the synthetic observables provides
insight into how observations should be analyzed to derive information
about physical variables in such a dynamic event. For example, we
find that line-of-sight superposition in the optically thin atmosphere
requires the combination of Doppler shifts and spectral line asymmetry
to determine the velocity in the jet. In our simulated type II spicule,
the lifetime of the asymmetry of the transition region lines is shorter
than that of the coronal lines. Other properties differ from the
observations, especially in the chromospheric lines. The mass density
of the part of the spicule with a chromospheric temperature is too low
to produce significant opacity in chromospheric lines. The synthetic
Ca II 8542 Å and Hα profiles therefore do not show signal resembling
RBEs. These and other discrepancies are described in detail, and we
discuss which mechanisms and physical processes may need to be included
in the MHD simulations to mimic the thermodynamic processes of the
chromosphere and corona, in particular to reproduce type II spicules.
Title: A Statistical Study on the Reconnection in Boundary Layers
of Small-scale Magnetic Flux Tubes in Solar Winds
Authors: Qi, Yu; Yao, Shuo; He, Jian-sen; Tian, Hui; Tu, Chuan-yi
Bibcode: 2013ChA&A..37..163Q
Altcode:
With the data from WIND satellite in 1995—2005, the small-scale
magnetic flux tubes determined with two methods, i.e., the artificial
recognition and program selection respectively, are compared. It is
found that there are magnetic reconnections in the boundary layers
of 41% small-scale magnetic flux tubes determined with the program
selection method, which is similar to the re- sult of small-scale
magnetic flux tubes determined with the method of artificial
recognition. The features of magnetic reconnections, such as the
magnetic shear angle, magnetic field strength, and duration of the
dissipation region of recon- nection at the small-scale flux tubes'
boundaries determined from both methods have the same statistical
tendency. This shows that there is no essential differ- ence in the
properties of reconnections in the boundary layers of the small-scale
magnetic flux tubes determined with the two methods. Hence the data
yielded by both methods can be used as the samples for statistically
studying the events of reconnection in the front and back boundary
layers of small-scale flux ropes. There are totally 71 magnetic
reconnection events selected in this paper. Our statistical result shows
that in 50 events (70%) the decrease of magnetic field strength in the
dissipation region of reconnection is larger than 20%, and in 47 events
(66%) the magnetic shear angle is larger than 90 degrees. These indicate
that the magnetic reconnections in boundaries of the small-scale
magnetic flux tubes are more likely to be anti-parallel. The statistics
has been performed sep- arately in the reconnections of the front
and back boundary layers of small-scale magnetic flux tubes. The
results show that the features of reconnections in the front and back
boundaries are similar to each other, which is different from the
various properties in the front and back boundaries of the magnetic
clouds, and this means that the expansion in the large-scale magnetic
flux tubes, such as magnetic clouds, does not happen in the small-scale
magnetic flux tubes.
Title: Global oscillations and small scale dynamics of a filament
Authors: Tian, Hui; Ofman, Leon; Wang, Tongjiang
Bibcode: 2013enss.confE.104T
Altcode:
Using AIA data, we observed global oscillatory motions along the axis
of a filament with detailed small scale dynamics. The global oscillation
lasted for more than two days before the eruption of the filament. Both
magnetic tension and gravity are likely to be involved in the global
oscillation, as predicted by theoretical models. A closer look of the
detailed dynamics within the filament suggests that torsional motions
around the prominence axis are also present. Torsional motions around
the two legs of the filament resemble the so-called "solar tornadoes"
observed at limb by AIA. Variable counter-streaming flows in the
prominence body were detected as well before the filament eruption. The
HMI magnetic field data and multi-line diagnostics are used to
investigate possible mechanisms responsible for the different types
of oscillations and flows before the eventual eruption of the filament.
Title: Formation of a hot plasma blob: observations of AIA, CoMP
and MK4
Authors: Tian, Hui; Gibson, Sarah
Bibcode: 2013enss.confE.107T
Altcode:
Using AIA observations, we found that a hot blob of plasma formed as
a trans-equatorial loop system rose and opened up. The hot blob was
most clearly seen in the AIA 94 passband and not obvious in any other
passbands. A cusp-like structure quickly developed below the hot blob
when the blob left the FOV of AIA 1.5 hours after its formation. This
event was also observed by the CoMP and MK4 instruments in Mauna Loa
Solar Observatory. The CoMP observation reveals a clear "bunny ear"
pattern suggestive of a magnetic flux rope around the AIA blob. The
hot blob observed by AIA seems to coincide with the region of enhanced
linear polarization between the two dark ears. A comparison between
these observations with MHD simulations of flux rope eruption suggest
that the hot blob might be the lower part of, or just below, the
erupted flux rope. In the MK4 data we see a clear three-part CME
propagating outward.
Title: Persistent Doppler Shift Oscillations Observed with Hinode/EIS
in the Solar Corona: Spectroscopic Signatures of Alfvénic Waves
and Recurring Upflows
Authors: Tian, Hui; McIntosh, Scott W.; Wang, Tongjiang; Ofman, Leon;
De Pontieu, Bart; Innes, Davina E.; Peter, Hardi
Bibcode: 2012ApJ...759..144T
Altcode: 2012arXiv1209.5286T
Using data obtained by the EUV Imaging Spectrometer on board Hinode,
we have performed a survey of obvious and persistent (without
significant damping) Doppler shift oscillations in the corona. We
have found mainly two types of oscillations from February to April
in 2007. One type is found at loop footpoint regions, with a dominant
period around 10 minutes. They are characterized by coherent behavior
of all line parameters (line intensity, Doppler shift, line width,
and profile asymmetry), and apparent blueshift and blueward asymmetry
throughout almost the entire duration. Such oscillations are likely to
be signatures of quasi-periodic upflows (small-scale jets, or coronal
counterpart of type-II spicules), which may play an important role
in the supply of mass and energy to the hot corona. The other type of
oscillation is usually associated with the upper part of loops. They are
most clearly seen in the Doppler shift of coronal lines with formation
temperatures between one and two million degrees. The global wavelets
of these oscillations usually peak sharply around a period in the range
of three to six minutes. No obvious profile asymmetry is found and
the variation of the line width is typically very small. The intensity
variation is often less than 2%. These oscillations are more likely to
be signatures of kink/Alfvén waves rather than flows. In a few cases,
there seems to be a π/2 phase shift between the intensity and Doppler
shift oscillations, which may suggest the presence of slow-mode standing
waves according to wave theories. However, we demonstrate that such a
phase shift could also be produced by loops moving into and out of a
spatial pixel as a result of Alfvénic oscillations. In this scenario,
the intensity oscillations associated with Alfvénic waves are caused by
loop displacement rather than density change. These coronal waves may be
used to investigate properties of the coronal plasma and magnetic field.
Title: Explosive Events: Swirling Transition Region Jets
Authors: Curdt, W.; Tian, H.; Kamio, S.
Bibcode: 2012SoPh..280..417C
Altcode: 2012SoPh..tmp...24C; 2012arXiv1201.3199C
In this paper, we extend our earlier work to provide additional evidence
for an alternative scenario to explain the nature of events called
`explosive events'. The bidirected, fast Doppler motion of explosive
events observed spectroscopically in the transition region emission is
classically interpreted as a pair of bidirectional jets moving upward
and downward from a reconnection site. We discuss the problems of this
model. In our previous work, we focused basically on the discrepancy of
fast Doppler motion without detectable motion in the image plane. We
now suggest an alternative scenario for the explosive events, based
on our observations of spectral line tilts and bifurcated structure
in some events. Both features are indicative of rotational motion
in narrow structures. We explain the bifurcation as the result of
rotation of hollow cylindrical structures and demonstrate that this
kind of sheath model can also be applied to explain the nature of the
puzzling `explosive events'. We find that the spectral tilt, the lack
of apparent motion, the bifurcation, and a rapidly growing number
of direct observations support an alternative scenario of linear,
spicular-sized jets with a strong spinning motion.
Title: Collimation of laser-driven energetic protons in a capillary
Authors: Chen, D. -P.; Yin, Y.; Ge, Z. -Y.; Xu, H.; Zhuo, H. -B.;
Ma, Y. -Y.; Shao, F. -Q.; Tian, C. -L.; Tian
Bibcode: 2012JPlPh..78..333C
Altcode:
Energetic divergent proton beams can be generated in the interaction
of ultra-intense laser pulses with solid-density foil targets via
target normal sheath acceleration (TNSA). In this paper, a scheme
using a capillary to reduce the proton beam divergence is proposed. By
two-dimensional particle-in-cell (PIC) simulations, it is shown that
strong transverse electric and magnetic fields rapidly grow at the
inner surface of the capillary when the laser-driven hot electrons
propagate through the target and into the capillary. The spontaneous
magnetic field collimates the electron flow, and the ions dragged from
the capillary wall by hot electrons neutralize the negative charge and
thus restrain the transverse extension of the sheath field set up by
electrons. The proton beam divergence, which is mainly determined by
the accelerating sheath field, is therefore reduced by the transverse
limitation of the sheath field in the capillary.
Title: Explosive events - swirling transition region jets
Authors: Curdt, Werner; Kamio, Suguru; Tian, Hui; Teriaca, Luca
Bibcode: 2012cosp...39..381C
Altcode: 2012cosp.meet..381C
The bi-directed, fast Doppler motion of explosive events observed
spectroscopically in the transition region emission is classically
interpreted as a pair of jets moving upward and downward from a
reconnection site. We discuss the problems of such a model and provide
evidence for an alternative scenario to explain the nature of so-called
'explosive events'. The discrepancy of fast Doppler motion without
detectable motion in the image plane is an old, unsolved problem. We
now provide the spectral tilt and the jet bifurcation as additional
arguments. Both features are indicative of rotational motion in narrow
structures. We interpret the bifurcation as the result of rotation of
hollow cylindrical structures and demonstrate that such a sheath model
can also be applied to explain the nature of the puzzling 'explosive
events'. We find that the spectral tilt, the lack of apparent motion,
the bifurcation, and a rapidly growing number of direct observations
support an alternative scenario of linear, spicular-sized jets with
a strong spinning motion.
Title: Temperature Dependence of UV Line Parameters in Network and
Internetwork Regions of the Quiet Sun and Coronal holes
Authors: Wang, Xin; McIntosh, Scott W.; Tian, Hui
Bibcode: 2012shin.confE...7W
Altcode:
By using observations of SUMER on board the SOHO spacecraft, we study
the temperature dependence of the Doppler shift, non-thermal width in
network and internetwork regions for both the quiet Sun (QS) and the
coronal hole (CH). In network regions, most of the transition region
(TR) line profiles are more red shifted (by 0-5km/s) and narrower (by
1-6km/s) in QS than in CH. Our results suggest that the mass cycle
between the chromosphere and corona mainly occurs in the network
and one needs to separate network and internetwork when discussing
thermal and dynamic properties of the solar atmosphere. In addition, a
three-component toy model is built to explain the temperature dependence
of the line parameters, which includes a rapid, weak upflow generated
in the lower atmosphere, a nearly static background, and a slow cooling
downflow. The agreement between the results of the observation and our
model suggests that the temperature dependence of Doppler shifts and
line widths might be caused by the different relative contributions of
the three components at different temperatures and will shed a new light
on our understanding of the complex chromosphere-coronal mass cycle.
Title: Two components of the coronal emission revealed by both
spectroscopic and imaging observations
Authors: Tian, Hui; McIntosh, Scott W.; De Pontieu, Bart
Bibcode: 2012shin.confE...1T
Altcode:
Boundaries of active regions have been suggested to be possible sources
of the slow solar wind. X-ray and EUV imaging observations often reveal
high-speed ( 100 km/s) quasi-periodic propagating disturbances (PDs)
along the fan-like structures at edges of active regions. Meanwhile EUV
spectroscopic observations of active region boundaries usually reveal
a blue shift of the order of 20 km/s and no periodicity. We think that
the key to solve these discrepancies is the asymmetry of the emission
line profile. The ubiquitous presence of blueward asymmetries of EUV
emission line profiles suggests at least two emission components:
a primary component accounting for the background coronal emission
and a weak secondary component associated with high-speed ( 100 km/s)
upflows. Through jointed imaging and spectroscopic observations, we have
demonstrated that the PDs are responsible for the secondary component
of line profiles and suggested that they may be an efficient means to
provide heated mass into the corona and solar wind. The intermittent
nature of these high-speed outflows (fine-scale jets) suggests that
the mass supply to the corona and solar wind is episodic rather than
continuous. Similar spectroscopic signatures have also been found in
CME-induced dimming regions, suggesting possible solar wind streams from
dimming regions. Unresolved problems include the production mechanism
of these high-speed outflows and the connection between these outflows
to the interplanetary space.
Title: New insight into CME processes revealed by CoMP observations
Authors: Tian, Hui; McIntosh, Scott W.; Bethge, Christian; Tomczyk,
Steve; Sitongia, Leonard E.
Bibcode: 2012shin.confE..11T
Altcode:
CoMP measures not only the polarization of coronal emission, but also
measures the full radiance profiles of coronal emission lines. For the
first time, CoMP observations provide high-cadence image sequences
of the coronal intensity, Doppler shift, line width and linear
polarization simultaneously. These measurements may help us explore
more of the physical processes at the onset of solar eruptions such
as CMEs and flares. They should also provide important constraints to
models of solar eruptions. Our preliminary analysis shows that CMEs are
usually associated with greatly increased Doppler shift and enhanced
line width. The linear polarization in CMEs measured by CoMP has also
been investigated.
Title: What can we Learn about Solar Coronal Mass Ejections,
Coronal Dimmings, and Extreme-Ultraviolet Jets Through Spectroscopic
Observations?
Authors: Tian, Hui; McIntosh, Scott W.; Xia, L. -D.; He, J. -S.;
Wang, X.
Bibcode: 2012shin.confE..10T
Altcode:
Solar eruptions, particularly coronal mass ejections (CMEs) and
extreme-ultraviolet (EUV) jets, have rarely been investigated with
spectroscopic observations. We analyze several data sets obtained by
the EUV Imaging Spectrometer onboard Hinode and find various types of
flows during CMEs and jet eruptions. We found weak high-speed ( 100
km/s) outflows in CME induced dimming regions, temperature-dependent
outflows (speed increases with temperature) immediately outside the
dimming region, and strong high-speed ( 200 km/s) outflows associated
with the CME ejecta and EUV jets. We have made plasma diagnostics
(density, temperature, mass) for the dimming regions and CME/jet
ejecta. Our results suggest that spectroscopic observations can provide
useful information on the kinematics and plasma properties of solar
mass eruptions.
Title: Erratum: "Pulsed Alfvén Waves in the Solar Wind" (2011, ApJ, 737, l35)
Authors: Gosling, J. T.; Tian, H.; Phan, T. D.
Bibcode: 2012ApJ...751L..22G
Altcode:
No abstract at ADS
Title: Recent Results from the Coronal Multi-Channel Polarimeter
Authors: Tomczyk, Steven; Bethge, C.; Gibson, S. E.; McIntosh, S. W.;
Rachmeler, L. A.; Tian, H.
Bibcode: 2012AAS...22031001T
Altcode:
The Coronal Multi-Channel Polarimeter (CoMP) instrument is a
ground-based filter/polarimeter which can image the solar corona at
wavelengths around the emission lines of FeXIII at 1074.7 and 1079.8
nm and the chromospheric emission line of HeI at 1083.0 nm. The
instrument consists of a 20-cm aperture coronagraph followed by a
Stokes polarimeter and a Lyot birefringent filter with a passband
of 0.14 nm width. Both the polarimeter and filter employ liquid
crystals for rapid electro-optical tuning. This instrument measures
the line-of-sight strength of the coronal magnetic field through the
Zeeman effect and the plane-of-sky direction of the magnetic field via
resonance scattering. The line-of-sight velocity can also be determined
from the Doppler shift. The CoMP has obtained daily observations from
the Mauna Loa Solar Observatory for almost one year. We will present
recent measurements of the polarization signatures seen with the
CoMP and a comparison with models that allow us to constrain coronal
structure. We also will present observations of coronal waves taken
with the CoMP and discuss their implications for the heating of the
solar corona and the acceleration of the solar wind.
Title: Hinode/EIS Line Profile Asymmetries and Their Relationship with
the Distribution of SDO/AIA Propagating Coronal Disturbance Velocities
Authors: Sechler, M.; McIntosh, S. W.; Tian, H.; De Pontieu, B.
Bibcode: 2012ASPC..455..361S
Altcode: 2012arXiv1201.5028S
Using joint observations from Hinode/EIS and the Atmospheric Imaging
Array (AIA) on the Solar Dynamics Observatory (SDO) we explore the
asymmetry of coronal EUV line profiles. We find that asymmetries exist
in all of the spectral lines studied, and not just the hottest lines
as has been recently reported in the literature. Those asymmetries
indicate that the velocities of the second emission component are
relatively consistent across temperature and consistent with the
apparent speed at which material is being inserted from the lower
atmosphere that is visible in the SDO/AIA images as propagating coronal
disturbances. Further, the observed asymmetries are of similar magnitude
(a few percent) and width (determined from the RB analysis) across the
temperature space sampled and in the small region studied. Clearly,
there are two components of emission in the locations where the
asymmetries are identified in the RB analysis, their characteristics
are consistent with those determined from the SDO/AIA data. There is
no evidence from our analysis that this second component is broader
than the main component of the line.
Title: Two Components of the Coronal Emission Revealed by Both
Spectroscopic and Imaging Observations
Authors: Tian, H.; McIntosh, S. W.; De Pontieu, B.
Bibcode: 2012ASPC..456...97T
Altcode:
X-ray and EUV imaging observations often reveal quasi-periodic
propagating disturbances along the fan-like structures at edges of
active regions. These disturbances have historically been interpreted
as being signatures of slow-mode magnetoacoustic waves propagating
into the corona. Recent spectroscopic observations have revealed
the ubiquitous presence of blueward asymmetries of EUV emission line
profiles. Such asymmetries suggest that there are at least two emission
components: a primary component accounting for the background emission
and a secondary component associated with high-speed upflows. Thus, a
single Gaussian fit can not reflect the real physics here. Through joint
imaging and spectroscopic observations, we find a clear association
of the secondary component with the upward propagating disturbances
and conclude that they are more likely to be real plasma outflows
(small-scale recurring jets) rather than slow waves. These outflows
may result from impulsive heating processes in the lower transition
region or chromosphere and could be an efficient means to provide hot
plasma into the corona and possibly also solar wind.
Title: On the Doppler Velocity of Emission Line Profiles Formed in
the "Coronal Contraflow" that Is the Chromosphere-Corona Mass Cycle
Authors: McIntosh, Scott W.; Tian, Hui; Sechler, Marybeth; De Pontieu,
Bart
Bibcode: 2012ApJ...749...60M
Altcode: 2012arXiv1202.1248M
This analysis begins to explore the complex chromosphere-corona mass
cycle using a blend of imaging and spectroscopic diagnostics. Single
Gaussian fits (SGFs) to hot emission line profiles (formed above 1 MK)
at the base of coronal loop structures indicate material blueshifts
of 5-10 km s-1, while cool emission line profiles (formed
below 1 MK) yield redshifts of a similar magnitude—indicating, to
zeroth order, that a temperature-dependent bifurcating flow exists
on coronal structures. Image sequences of the same region reveal
weakly emitting upward propagating disturbances in both hot and cool
emission with apparent speeds of 50-150 km s-1. Spectroscopic
observations indicate that these propagating disturbances produce a weak
emission component in the blue wing at commensurate speed, but that they
contribute only a few percent to the (ensemble) emission line profile
in a single spatio-temporal resolution element. Subsequent analysis of
imaging data shows material "draining" slowly (~10 km s-1)
out of the corona, but only in the cooler passbands. We interpret
the draining as the return flow of coronal material at the end of
the complex chromosphere-corona mass cycle. Further, we suggest that
the efficient radiative cooling of the draining material produces a
significant contribution to the red wing of cool emission lines that is
ultimately responsible for their systematic redshift as derived from an
SGF when compared to those formed in hotter (conductively dominated)
domains. The presence of counterstreaming flows complicates the line
profiles, their interpretation, and asymmetry diagnoses, but allows
a different physical picture of the lower corona to develop.
Title: What can We Learn about Solar Coronal Mass Ejections,
Coronal Dimmings, and Extreme-ultraviolet Jets through Spectroscopic
Observations?
Authors: Tian, Hui; McIntosh, Scott W.; Xia, Lidong; He, Jiansen;
Wang, Xin
Bibcode: 2012ApJ...748..106T
Altcode: 2012arXiv1201.2204T
Solar eruptions, particularly coronal mass ejections (CMEs) and
extreme-ultraviolet (EUV) jets, have rarely been investigated with
spectroscopic observations. We analyze several data sets obtained
by the EUV Imaging Spectrometer on board Hinode and find various
types of flows during CMEs and jet eruptions. CME-induced dimming
regions are found to be characterized by significant blueshift and
enhanced line width by using a single Gaussian fit, while a red-blue
(RB) asymmetry analysis and an RB-guided double Gaussian fit of
the coronal line profiles indicate that these are likely caused by
the superposition of a strong background emission component and a
relatively weak (~10%), high-speed (~100 km s-1) upflow
component. This finding suggests that the outflow velocity in the
dimming region is probably of the order of 100 km s-1, not
~20 km s-1 as reported previously. These weak, high-speed
outflows may provide a significant amount of mass to refill the corona
after the eruption of CMEs, and part of them may experience further
acceleration and eventually become solar wind streams that can serve
as an additional momentum source of the associated CMEs. Density and
temperature diagnostics of the dimming region suggest that dimming
is primarily an effect of density decrease rather than temperature
change. The mass losses in dimming regions as estimated from different
methods are roughly consistent with each other, and they are 20%-60%
of the masses of the associated CMEs. With the guide of RB asymmetry
analysis, we also find several temperature-dependent outflows (speed
increases with temperature) immediately outside the (deepest) dimming
region. These outflows may be evaporation flows that are caused by
the enhanced thermal conduction or nonthermal electron beams along
reconnecting field lines, or induced by the interaction between the
opened field lines in the dimming region and the closed loops in
the surrounding plage region. In an erupted CME loop and an EUV jet,
profiles of emission lines formed at coronal and transition region
temperatures are found to exhibit two well-separated components, an
almost stationary component accounting for the background emission and
a highly blueshifted (~200 km s-1) component representing
emission from the erupting material. The two components can easily
be decomposed through a double Gaussian fit, and we can diagnose the
electron density, temperature, and mass of the ejecta. Combining the
speed of the blueshifted component and the projected speed of the
erupting material derived from simultaneous imaging observations,
we can calculate the real speed of the ejecta.
Title: Spectroscopic observations of coronal mass ejections, coronal
dimming and EUV jets
Authors: Tian, Hui; McIntosh, Scott W.
Bibcode: 2012decs.confE..10T
Altcode:
Solar eruptions, particularly coronal mass ejections (CMEs) and
extreme-ultraviolet (EUV) jets, have rarely been investigated with
spectroscopic observations. We analyze several data sets obtained by
the EUV Imaging Spectrometer onboard Hinode and find various types of
flows during CMEs and jet eruptions. CME-induced dimming regions are
found to be characterized by significant blueshift and enhanced line
width by using a single Gaussian fit. While a red-blue (RB) asymmetry
analysis and a RB-guided double Gaussian fit of the coronal line
profiles indicate that these are likely caused by the superposition
of a strong background emission component and a relatively weak ( 10%)
high-speed ( 100 km s-1) upflow component. This finding suggests that
the outflow velocity in the dimming region is probably of the order
of 100 km s-1, not 20 km s-1 as reported previously. Density and
temperature diagnostics of the dimming region suggest that dimming
is primarily an effect of density decrease rather than temperature
change. The mass losses in dimming regions as estimated from different
methods are roughly consistent with each other and they are 20%-60%
of the masses of the associated CMEs. With the guide of RB asymmetry
analysis, we also find several temperature-dependent outflows (speed
increases with temperature) immediately outside the (deepest) dimming
region. These outflows may be evaporation flows which are caused by
the enhanced thermal conduction or nonthermal electron beams along
reconnecting field lines, or induced by the interaction between the
opened field lines in the dimming region and the closed loops in
the surrounding plage region. In an erupted CME loop and an EUV jet,
profiles of emission lines formed at coronal and transition region
temperatures are found to exhibit two well-separated components, an
almost stationary component accounting for the background emission
and a highly blueshifted ( 200 km s-1) component representing
emission from the erupting material. The two components can easily
be decomposed through a double Gaussian fit and we can diagnose the
electron density, temperature and mass of the ejecta. Combining the
speed of the blueshifted component and the projected speed of the
erupting material derived from simultaneous imaging observations,
we can calculate the real speed of the ejecta.
Title: Temperature dependence of EUV line parameters in network and
internetwork regions for quiet Sun and coronal hole
Authors: Wang, Xin; McIntosh, Scott W.; Tian, Hui
Bibcode: 2012decs.confE.107W
Altcode:
By using SUMER observations, we study the temperature dependence
of the intensity contrast, Doppler shift, non-thermal width and
profile asymmetry in network and internetwork regions for both the
quiet Sun (QS) and coronal holes (CHs). In network regions, most of
the transition region (TR) line profiles are more red shifted (by
0-5km/s) and narrower (by 1-6km/s) in QS than in CH. In the network,
the RB asymmetries of all the selected TR and coronal line profiles
are smaller (more blueward) in CH than in QS. While in the interwork
region the difference disappears. In addition, we also systematically
investigate differential emission measures (DEM) and electron densities
and found different behavior in network and internetwork regions by
using joint observations of SUMER and EIS. Our results suggest that
the mass cycle between the chromosphere and corona mainly occurs in
the network and one needs to separate network and internetwork when
discussing thermal and dynamic properties of the solar atmosphere.
Title: The mass cycle between the chromosphere and the corona/solar
wind
Authors: Tian, Hui
Bibcode: 2012decs.confE...9T
Altcode:
The plasma is not static but flows almost everywhere above the
chromosphere. EUV and FUV spectroscopy reveals a lot of information
of these mass flows, and thus enhances our understanding of coronal
heating and solar wind origin. Through a single Gaussian fit to
line profiles, emission lines formed in the transition region (TR)
are usually found to exhibit ubiquitous redshifts. While coronal
lines show predominant blueshifts in coronal holes (CHs), quiet-Sun
(QS) network boundaries and active region (AR) edges. However, careful
scrutiny of the line profiles indicates that they are obviously enhanced
in the blue wings, suggesting the presence of a secondary high-speed
upflow component besides the primary component. Meanwhile, imaging
observations of HINODE/SOT and SDO/AIA clearly reveal ubiquitous
episodic high-speed outflows in the form of type-II chromospheric
spicules and propagating coronal disturbances (PDs). It has been
suggested that the secondary component is associated with the type-II
spicules and PDs, although further detailed investigations are needed
to reach a solid conclusion. Moreover, recent AIA observations reveal
slow downflows in cool passbands, which may represent the cooling
of the previously heated plasma and should be embedded in TR line
profiles. These heating upflows and cooling downflows are natural
results of the mass cycling between the chromosphere and corona/solar
wind, and their different relative intensities at different temperatures
are likely to be responsible for the well-known temperature dependence
of TR Doppler shift. So far various double Gaussian fit algorithms and
red-blue asymmetry analysis techniques have been applied to coronal
line profiles to derive parameters of the upflow component. However, an
unambiguous decomposition of different components are still not possible
by using data acquired by current spectrographs. The IRIS instrument,
with a very small instrumental width and high spectral, temporal and
spatial resolutions, might be able to unambiguously decompose different
emission components in this continuous mass cycling process.
Title: Searching for the stellar streams.
Authors: Tian, H.; Jiang, B. -W.
Bibcode: 2012PABei..30...64T
Altcode:
No abstract at ADS
Title: Does the Kinetic Alfven Wave or Oblique Whistler Wave dominate
the Dissipation Range of Solar Wind Turbulence?
Authors: He, J.; Marsch, E.; Tu, C.; Wang, X.; Yao, S.; Tian, H.
Bibcode: 2011AGUFMSH42B..04H
Altcode:
Presently, there is an intense debate on which oblique wave mode is,
besides the parallel Alfven-cyclotron wave, responsible for further
energy cascading or damping in the dissipation range of solar wind
turbulence. From numerical simulation and dimensional analysis, the
power spectra in the dissipation range with a power law like k^(-7/3)
can be reproduced based on either the kinetic Alfven wave or whistler
wave. Here, we aim to show which one may be dominant in reality on
the basis of solar wind measurements by STEREO satellite. We extract
small-scale magnetic fluctuations in the dissipation range from
the time series by employing the wavelet decomposition method. We
then study two parameters (magnetic compressibility and magnetic
polarization) of the small-scale magnetic fluctuations in such time
intervals where the direction of the local mean magnetic field is nearly
orthogonal to the direction of the solar wind velocity. We find that
the magnetic compressibility in the dissipation range is less than
0.25, which is much smaller than the magnetic compressibility (~1.0)
of quasi-perpendicular whistler waves, but closer to the value of
kinetic Alfven waves. Moreover, we find that the small-scale magnetic
fluctuations seems to be elliptically polarized in the plane normal
to the solar wind velocity direction, and the major axis of the
polarization ellipse is perpendicular to local mean magnetic field
direction. This kind of observed magnetic polarization is consistent
with the characteristics of kinetic Alfven waves. Therefore, we may
conclude that, in the fast solar wind streams we have looked at,
oblique kinetic Alfven waves do exist and may play a role in the
dissipation range and for turbulence cascading and damping.
Title: Pulsed Alfvén Waves in the Solar Wind
Authors: Gosling, J. T.; Tian, H.; Phan, T.
Bibcode: 2011AGUFMSH11C..08G
Altcode:
Using 3-s plasma and magnetic field data from the Wind spacecraft
located in the solar wind well upstream from Earth, we report
observations of isolated, pulse-like Alfvénic disturbances in the solar
wind. These isolated events are characterized by roughly plane-polarized
rotations in the solar wind magnetic field and velocity vectors away
from the directions of the underlying field and velocity and then
back again. They pass over Wind on time scales ranging from seconds to
several minutes. These isolated, pulsed Alfvén waves are pervasive; we
have identified 175 such events over the full range of solar wind speeds
(320 - 550 km/s) observed in a randomly chosen 10-day interval. The
large majority of these events are propagating away from the Sun in the
solar wind rest frame. Maximum field rotations in the interval studied
ranged from 6 to 109 degrees. Similar to most Alfvénic-fluctuations in
the solar wind at 1 AU, the observed changes in velocity are typically
less than that predicted for pure Alfvén waves (Alfvénicity ranged
from 0.28 to 0.93). Most of the events are associated with small
enhancements or depressions in magnetic field strength and small
changes in proton number density and/or temperature. The pulse-like and
roughly symmetric nature of the magnetic field and velocity rotations
in these events suggests that these Alfvénic disturbances are not
evolving when observed. They thus appear to be, and probably are,
solitary waves. It is presently uncertain how these waves originate,
although they may evolve out of Alfvénic turbulence.
Title: Two Components of the Coronal Emission Revealed by
Extreme-Ultraviolet Spectroscopic Observations
Authors: Tian, H.; Mcintosh, S. W.; De Pontieu, B.; Martinez-Sykora,
J.; Wang, X.; Sechler, M.
Bibcode: 2011AGUFMSH33A2027T
Altcode:
Recent spectroscopic observations have revealed the ubiquitous presence
of blueward asymmetries of emission lines formed in the solar corona
and transition region. These asymmetries are most prominent in loop
footpoint regions, where a clear correlation of the asymmetry with the
Doppler shift and line width determined from the single-Gaussian fit
is found. Such asymmetries suggest at least two emission components: a
primary component accounting for the background emission and a secondary
component associated with high-speed upflows. The latter has been
proposed to play a vital role in the coronal heating process and there
is no agreement on its properties. Here we slightly modify the initially
developed technique of red-blue (RB) asymmetry analysis and apply it to
both artificial spectra and spectra observed by the Extreme-ultraviolet
Imaging Spectrometer on board Hinode, and demonstrate that the secondary
component usually contributes a few percent of the total emission, has
a velocity ranging from 50 to 150 km/s, and a Gaussian width comparable
to that of the primary one in loop footpoint regions. The results of
the RB asymmetry analysis are then used to guide a double-Gaussian fit
and we find that the obtained properties of the secondary component
are generally consistent with those obtained from the RB asymmetry
analysis. Through a comparison of the location, relative intensity,
and velocity distribution of the blueward secondary component with
the properties of the upward propagating disturbances revealed in
simultaneous images from the Atmospheric Imaging Assembly on board
the Solar Dynamics Observatory, we find a clear association of the
secondary component with the propagating disturbances.
Title: Two Components of the Solar Coronal Emission Revealed by
Extreme-ultraviolet Spectroscopic Observations
Authors: Tian, Hui; McIntosh, Scott W.; De Pontieu, Bart;
Martínez-Sykora, Juan; Sechler, Marybeth; Wang, Xin
Bibcode: 2011ApJ...738...18T
Altcode: 2011arXiv1106.1141T
Recent spectroscopic observations have revealed the ubiquitous presence
of blueward asymmetries of emission lines formed in the solar corona
and transition region. These asymmetries are most prominent in loop
footpoint regions, where a clear correlation of the asymmetry with the
Doppler shift and line width determined from the single-Gaussian fit
is found. Such asymmetries suggest at least two emission components: a
primary component accounting for the background emission and a secondary
component associated with high-speed upflows. The latter has been
proposed to play a vital role in the coronal heating process and there
is no agreement on its properties. Here we slightly modify the initially
developed technique of red-blue (RB) asymmetry analysis and apply it to
both artificial spectra and spectra observed by the Extreme-ultraviolet
Imaging Spectrometer on board Hinode, and demonstrate that the secondary
component usually contributes a few percent of the total emission,
and has a velocity ranging from 50 to 150 km s-1 and a
Gaussian width comparable to that of the primary one in loop footpoint
regions. The results of the RB asymmetry analysis are then used to
guide a double-Gaussian fit and we find that the obtained properties of
the secondary component are generally consistent with those obtained
from the RB asymmetry analysis. Through a comparison of the location,
relative intensity, and velocity distribution of the blueward secondary
component with the properties of the upward propagating disturbances
revealed in simultaneous images from the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory, we find a clear association
of the secondary component with the propagating disturbances.
Title: Pulsed Alfvén Waves in the Solar Wind
Authors: Gosling, J. T.; Tian, H.; Phan, T. D.
Bibcode: 2011ApJ...737L..35G
Altcode:
Using 3 s plasma and magnetic field data from the Wind spacecraft
located in the solar wind well upstream from Earth, we report
observations of isolated, pulse-like Alfvénic disturbances in the solar
wind. These isolated events are characterized by roughly plane-polarized
rotations in the solar wind magnetic field and velocity vectors away
from the directions of the underlying field and velocity and then
back again. They pass over Wind on timescales ranging from seconds to
several minutes. These isolated, pulsed Alfvén waves are pervasive;
we have identified 175 such events over the full range of solar wind
speeds (320-550 km s-1) observed in a randomly chosen 10
day interval. The large majority of these events are propagating away
from the Sun in the solar wind rest frame. Maximum field rotations
in the interval studied ranged from 6° to 109°. Similar to most
Alfvénic fluctuations in the solar wind at 1 AU, the observed changes
in velocity are typically less than that predicted for pure Alfvén
waves (Alfvénicity ranged from 0.28 to 0.93). Most of the events
are associated with small enhancements or depressions in magnetic
field strength and small changes in proton number density and/or
temperature. The pulse-like and roughly symmetric nature of the
magnetic field and velocity rotations in these events suggests that
these Alfvénic disturbances are not evolving when observed. They
thus appear to be, and probably are, solitary waves. It is presently
uncertain how these waves originate, although they may evolve out of
Alfvénic turbulence.
Title: Observation of High-speed Outflow on Plume-like
Structures of the Quiet Sun and Coronal Holes with Solar Dynamics
Observatory/Atmospheric Imaging Assembly
Authors: Tian, Hui; McIntosh, Scott W.; Habbal, Shadia Rifai; He,
Jiansen
Bibcode: 2011ApJ...736..130T
Altcode: 2011arXiv1105.3119T
Observations from the Atmospheric Imaging Assembly onboard the Solar
Dynamics Observatory reveal ubiquitous episodic outflows (jets)
with an average speed around 120 km s-1 at temperatures
often exceeding a million degree in plume-like structures, rooted in
magnetized regions of the quiet solar atmosphere. These outflows are not
restricted to the well-known plumes visible in polar coronal holes, but
are also present in plume-like structures originating from equatorial
coronal holes and quiet-Sun (QS) regions. Outflows are also visible
in the "inter-plume" regions throughout the atmosphere. Furthermore,
the structures traced out by these flows in both plume and inter-plume
regions continually exhibit transverse (Alfvénic) motion. Our finding
suggests that high-speed outflows originate mainly from the magnetic
network of the QS and coronal holes (CHs), and that the plume flows
observed are highlighted by the denser plasma contained therein. These
outflows might be an efficient means to provide heated mass into the
corona and serve as an important source of mass supply to the solar
wind. We demonstrate that the QS plume flows can sometimes significantly
contaminate the spectroscopic observations of the adjacent CHs—greatly
affecting the Doppler shifts observed, thus potentially impacting
significant investigations of such regions.
Title: Spectroscopic evidence for helicity in explosive events
Authors: Curdt, W.; Tian, H.
Bibcode: 2011A&A...532L...9C
Altcode: 2011arXiv1107.1969C
Aims: We report spectroscopic observations in support of a novel
view of transition region explosive events, observations that lend
empirical evidence that at least in some cases explosive events may
be nothing else but spinning narrow spicule-like structures.
Methods: Our spectra of textbook explosive events with simultaneous
Doppler flow of a red and a blue component are extreme cases of high
spectroscopic velocities that lack apparent motion, to be expected
if interpreted as a pair of collimated, linearly moving jets. The
awareness of this conflict led us to the alternative interpretation of
redshift and blueshift as a spinning motion of a small plasma volume. In
contrast to the bidirectional jet scenario, a small volume of spinning
plasma would be fully compatible with the observation of flows without
detectable apparent motion. We suspect that these small volumes could
be spicule-like structures and try to find evidence for this. We show
observations of helical motion in macrospicules and argue that these
features - if scaled down to a radius comparable to the slit size of
a spectrometer - should have a spectroscopic signature similar to that
observed in explosive events, which is admittedly not easily detectable
by imagers. Despite of this difficulty, evidence of helicity in spicules
has been reported in the literature. This led us to the new insight that
the same narrow spinning structures may be the drivers in both cases,
structures that imagers observe as spicules and that in spectrometers
cross the slit and are seen as explosive events.
Results:
We arrive at a concept that supports the idea that explosive events
and spicules are different manifestations of the same helicity-driven
scenario. In contrast to the conventional view of explosive events as
linear bidirectional jets that are triggered by a reconnection event
in the transition region, this new interpretation is compatible with
the observational results. Consequently, in this case a photospheric
or subphotospheric trigger has to be assumed.
Conclusions:
We suggest that explosive events/spicules are to be compared to the
unwinding of a loaded torsional spring.
Title: Observation of High-speed Outflow on Plume-like Structures
of the Quiet Sun and Coronal Holes with SDO/AIA
Authors: Tian, Hui; McIntosh, Scott W.; Habbal, Shadia Rifal; He,
Jiansen
Bibcode: 2011shin.confE.161T
Altcode:
Observations from the Atmospheric Imaging Assembly (AIA) onboard the
Solar Dynamics Observatory (SDO) reveal ubiquitous episodic outflows
(jets) with an average speed around 120 km s-1 at temperatures
often exceeding a million degree in plume-like structures, rooted in
magnetized regions of the quiet solar atmosphere. These outflows are not
restricted to the well-known plumes visible in polar coronal holes, but
are also present in plume-like structures originating from equatorial
coronal holes and quiet-Sun regions. Outflows are also visible in
the "interplume" regions throughout the atmosphere. Furthermore, the
structures traced out by these flows in both plume and inter-plume
regions continually exhibit transverse (Alfvéenic) motion. Our finding
suggests that high-speed outflows originate mainly from the magnetic
network of the quiet Sun and coronal holes, and that the plume flows
observed are highlighted by the denser plasma contained therein. These
outflows might be an efficient means to provide heated mass into the
corona and serve as an important source of mass supply to the solar
wind. We demonstrate that the quiet-Sun plume flows can sometimes
significantly contaminate the spectroscopic observations of the adjacent
coronal holes - greatly affecting the Doppler shifts observed, thus
potentially impacting significant investigations of such regions.
Title: Hydrogen Lyα and Lyβ Radiances and Profiles in Polar
Colornal Holes
Authors: Tian, Hui; Teriaca, Luca; Curdt, Werner; Vial, Jean-Claude
Bibcode: 2011shin.confE.166T
Altcode:
The hydrogen Lyα plays a dominant role in the radiative energy
transport in the lower transition region, and is important for
the studies of transition-region structure as well as solar wind
origin. We investigate the Lyα profiles obtained by the Solar
Ultraviolet Measurement of Emitted Radiation spectrograph on the
Solar and Heliospheric Observatory spacecraft in coronal holes and
the quiet Sun. In a subset of these observations, the Hi Lyβ, Si III,
and Ovi lines were also (quasi-)simultaneously recorded. We find that
the distances between the two peaks of Lyα profiles are larger in
coronal holes than in the quiet Sun, indicating a larger opacity in
coronal holes. This difference might result from the different magnetic
structures or the different radiation fields in the two regions. Most
of the Lyβ profiles in the coronal hole have a stronger blue peak,
in contrast to those in quiet-Sun regions while in both regions the
Lyα profiles are stronger in the blue peak. Although the asymmetries
are likely to be produced by differential flows in the solar atmosphere,
their detailed formation processes are still unclear. The radiance ratio
between Lyα and Lyβ decreases toward the limb in the coronal hole,
which might be due to the different opacity of the two lines. We also
find that the radiance distributions of the four lines are set by a
combined effect of limb brightening and the different emission level
between coronal holes and the quiet Sun.
Title: Two-dimensional correlation functions for density and magnetic
field fluctuations in magnetosheath turbulence measured by the
Cluster spacecraft
Authors: He, J. -S.; Marsch, E.; Tu, C. -Y.; Zong, Q. -G.; Yao, S.;
Tian, H.
Bibcode: 2011JGRA..116.6207H
Altcode:
Knowledge of multidimensional correlation functions is crucial for
understanding the anisotropy of turbulence. The two-dimensional (2-D)
spatial correlation functions (SCFs) obtained in previous studies
of space plasma turbulence were restricted to large-length scales
and covered a limited angular domain of the two-point separation
vector with respect to the mean magnetic field. Here we aim to
derive 2-D SCFs with smaller-length scale and nearly full angular
distribution for the fluctuations of the number density and magnetic
field in magnetosheath turbulence. We use the Cluster four-spacecraft
measurements of the fluctuations with respect to a temporally and
spatially varying background magnetic field to construct the 2-D
SCFs. We find that the correlation function of the density fluctuations
shows a pattern similar to that of the magnetic field fluctuations,
both of which appear to be composed of two populations, whereby the
major population extends along the coordinate parallel to mean magnetic
field (S$\parallel$) and the minor one deviates toward the
perpendicular coordinate (S$\perp$). This pattern of 2-D SCFs
implies that the energy of magnetosheath turbulence seems to cascade,
in the inertial range close to the ion scale, mostly transverse to the
background magnetic field and meanwhile partly along the field (i.e.,
k$\perp$ $\gg$ k$\parallel$).
Title: Possible Evidence of Alfvén-cyclotron Waves in the Angle
Distribution of Magnetic Helicity of Solar Wind Turbulence
Authors: He, Jiansen; Marsch, Eckart; Tu, Chuanyi; Yao, Shuo; Tian, Hui
Bibcode: 2011ApJ...731...85H
Altcode:
The fluctuating magnetic helicity is considered an important parameter
in diagnosing the characteristic modes of solar wind turbulence. Among
them is the Alfvén-cyclotron wave, which is probably responsible for
the solar wind plasma heating, but has not yet been identified from
the magnetic helicity of solar wind turbulence. Here, we present the
possible signatures of Alfvén-cyclotron waves in the distribution
of magnetic helicity as a function of θVB, which is
the angle between the solar wind velocity and local mean magnetic
field. We use magnetic field data from the STEREO spacecraft to
calculate the θVB distribution of the normalized reduced
fluctuating magnetic helicity σm. We find a dominant
negative σm for 1 s < p < 4 s (p is time period)
and for θVB < 30° in the solar wind outward magnetic
sector, and a dominant positive σm for 0.4 s < p
< 4 s and for θVB>150° in the solar wind inward
magnetic sector. These features of σm appearing around the
Doppler-shifted ion-cyclotron frequencies may be consistent with the
existence of Alfvén-cyclotron waves among the outward propagating
fluctuations. Moreover, right-handed polarized waves at larger
propagation angles, which might be kinetic Alfvén waves or whistler
waves, have also been identified on the basis of the σm
features in the angular range 40° < θVB < 140°. Our
findings suggest that Alfvén-cyclotron waves (together with other
wave modes) play a prominent role in turbulence cascading and plasma
heating of the solar wind.
Title: The Spectroscopic Signature of Quasi-periodic Upflows in
Active Region Timeseries
Authors: Tian, Hui; McIntosh, Scott W.; De Pontieu, Bart
Bibcode: 2011ApJ...727L..37T
Altcode: 2010arXiv1012.5112T
Quasi-periodic propagating disturbances are frequently observed in
coronal intensity image sequences. These disturbances have historically
been interpreted as being the signature of slow-mode magnetoacoustic
waves propagating into the corona. The detailed analysis of Hinode EUV
Imaging Spectrometer (EIS) timeseries observations of an active region
(known to contain propagating disturbances) shows strongly correlated,
quasi-periodic, oscillations in intensity, Doppler shift, and line
width. No frequency doubling is visible in the latter. The enhancements
in the moments of the line profile are generally accompanied by a faint,
quasi-periodically occurring, excess emission at ~100 km s-1
in the blue wing of coronal emission lines. The correspondence of
quasi-periodic excess wing emission and the moments of the line profile
indicates that repetitive high-velocity upflows are responsible for
the oscillatory behavior observed. Furthermore, we show that the same
quasi-periodic upflows can be directly identified in a simultaneous
image sequence obtained by the Hinode X-Ray Telescope. These results
are consistent with the recent assertion of De Pontieu & McIntosh
that the wave interpretation of the data is not unique. Indeed, given
that several instances are seen to propagate along the direction of
the EIS slit that clearly shows in-phase, quasi-periodic variations of
intensity, velocity, width (without frequency doubling), and blue wing
enhanced emission, this data set would appear to provide a compelling
example that upflows are more likely to be the main cause of the
quasi-periodicities observed here, as such correspondences are hard
to reconcile in the wave paradigm.
Title: Redshift-space Enhancement of Line-of-sight Baryon Acoustic
Oscillations in the Sloan Digital Sky Survey Main-galaxy Sample
Authors: Tian, H. J.; Neyrinck, Mark C.; Budavári, Tamás; Szalay,
Alexander S.
Bibcode: 2011ApJ...728...34T
Altcode: 2010arXiv1011.2481T
We show that redshift-space distortions of galaxy correlations have
a strong effect on correlation functions with distinct, localized
features, like the signature of the baryon acoustic oscillations
(BAO). Near the line of sight, the features become sharper as a result
of redshift-space distortions. We demonstrate this effect by measuring
the correlation function in Gaussian simulations and the Millennium
simulation. We also analyze the SDSS DR7 main-galaxy sample, splitting
the sample into slices 2fdg5 on the sky in various rotations. Measuring
two-dimensional correlation functions in each slice, we do see a sharp
bump along the line of sight. Using Mexican-hat wavelets, we localize
it to (110 ± 10) h -1 Mpc. Averaging only along the line of
sight, we estimate its significance at a particular wavelet scale and
location at 2.2σ. In a flat angular weighting in the (π, rp
) coordinate system, the noise level is suppressed, pushing the bump's
significance to 4σ. We estimate that there is about a 0.2% chance of
getting such a signal anywhere in the vicinity of the BAO scale from
a power spectrum lacking a BAO feature. However, these estimates of
the significances make some use of idealized Gaussian simulations,
and thus are likely a bit optimistic.
Title: The coronal convection
Authors: Curdt, W.; Tian, H.; Marsch, E.
Bibcode: 2011CEAB...35..187C
Altcode: 2011arXiv1101.2365C
We study the hydrogen Lyman emission in various solar features -- now
including Ly-α observations free from geocoronal absorption -- and
investigate statistically the imprint of flows and of the magnetic field
on the line profile and radiance distribution. As a new result, we found
that in Ly-α rasters locations with higher opacity cluster in the cell
interior, while the network has a trend to flatter profiles. Even deeper
self reversals and larger peak distances were found in coronal hole
spectra. We also compare simultaneous Ly-α and Ly-β profiles. There
is an obvious correspondence between asymmetry and redshift for both
lines, but, most surprisingly, the asymmetries of Ly-α and Ly-β
are opposite. We conclude that in both cases downflows determine
the line profile, in case of Ly-α by absorption and in the case of
Ly-β by emission. Our results show that the magnetically structured
atmosphere plays a dominating role in the line formation and indicate
the presence of a persisting downflow at both footpoints of closed
loops. We claim that this is the manifestation of a fundamental mass
transportation process, which Foukal back in 1978 introduced as the
'coronal convection'.
Title: Quasi-periodic outflows observed by the X-Ray Telescope
onboard Hinode in the boundary of an active region
Authors: Guo, Li-Jia; Tian, Hui; He, Jian-Sen
Bibcode: 2010RAA....10.1307G
Altcode: 2010arXiv1006.5258G
Persistent outflows have recently been detected at the boundaries
of some active regions. Although these outflows are suggested to be
possible sources of the slow solar wind, the nature of these outflows
is poorly understood. Through an analysis of an image sequence
obtained by the X-Ray Telescope onboard the Hinode spacecraft, we
found that quasi-periodic outflows are present in the boundary of an
active region. The flows are observed to occur intermittently, often
with a period of 5-10 min. The projected flow speed can reach more
than 200 km s-1, while its distribution peaks around 50 km
s-1. This sporadic high-speed outflow may play an important
role in the mass loading process of the slow solar wind. Our results
may imply that the outflow of the slow solar wind in the boundary of
the active region is intermittent and quasi-periodic in nature.
Title: Existence of Alfvén-cyclotron waves in solar wind turbulence
as identified from the angular distribution of magnetic helicity
Authors: He, J.; Marsch, E.; Tu, C.; Yao, S.; Tian, H.
Bibcode: 2010AGUFMSH43D..07H
Altcode:
The fluctuating magnetic helicity is a helpful mean for diagnosing
the wave characteristics in solar wind turbulence. However, the
signature of the Alfvén-cyclotron wave has not yet been revealed
from magnetic helicity, due to the limitations of the methods used
previously. We propose a new method to study the magnetic helicity,
σ_m, which is now considered as a function not only of the frequency
(f) but also the angle (Θ_VB) between the solar wind velocity
and local mean magnetic field B_0. We apply this method to STEREO
measurements of solar wind turbulence in both the magnetic-field
outward sectors and inward sectors. As a result, we find the following
remarkable features of the σ_m distributions. In outward sectors,
σ_m<0 for f∈[0.1,1.0]Hz and Θ_VB<30, and σ_m>0 for
f∈[0.1,1.0]Hz and 30<Θ_VB<150. In inward sectors, σ_m>0
for f∈[0.1,1.0]Hz and Θ_VB>150, and σ_m<0 for f∈[0.1,1.0]Hz
and 30<Θ_VB<150. These new findings indicate the existence of
Alfvén-cyclotron waves propagating quasi-parallel or anti-parallel
to B_0, besides the possible existence of kinetic-Alfvén or whistler
waves propagating obliquely to B_0. Therefore, we suggest that the
Alfvén-cyclotron wave, together with other wave modes, plays a crucial
role in turbulence cascading and heating of the solar wind.
Title: Signatures of transition region explosive events in hydrogen
Ly-beta profiles
Authors: Xia, L.; Zhang, M.; Tian, H.; Chen, Y.
Bibcode: 2010AGUFMSH11B1646X
Altcode:
Transition-region (TR) explosive events (EEs) are small-scale
dynamic phenomena often observed in the far and extreme ultraviolet
(FUV/EUV) spectral lines formed in the solar transition region. In this
contribution, we investigate the relationship between the peak emission
of hydrogen Ly-beta profiles and the emission of C II (103.7nm) and
O VI (103.2nm) lines to search for signatures of explosive events in
Ly-beta profiles. Two rasters made by the SUMER (Solar Ultraviolet
Measurements of Emitted Radiation) instrument on board SOHO in a
quiet-Sun region and an equatorial coronal hole were selected for our
study. Explosive events were identified from profiles of C II and O
VI, respectively. We compared Ly-beta profiles during EEs with those
averaged in the entire quiet-Sun and coronal-hole regions. We find that
the central part of Ly-beta profiles reverses more and the distance of
the two peaks becomes larger during EEs, both in the coronal hole and
in the quiet Sun. The average Ly-beta profile of the EEs detected by
C II has an obviously stronger blue peak. During EEs, there is a clear
correlation between the increased peak emission of Ly-beta profiles and
the enhanced wing emission of the C II and O VI lines. The correlation
is more pronounced for the Ly-beta peaks and C II wings, and less
significant for the Ly-beta blue peak and O VI blue wing. We also
find that the Ly-beta profiles are more reversed in the coronal hole
than in the quiet Sun. We suggest that the jets produced by EEs emit
Doppler-shifted Ly-beta photons, causing enhanced emission at positions
of the peaks of Ly-beta profiles. The more-reversed Ly-beta profiles
confirm the presence of a higher opacity in the coronal hole than in
the quiet Sun. The finding that EEs modify the Ly-beta line profile
in QS and CHs implies that one should be careful in the modeling and
interpretation of relevant observational data.
Title: Signatures of transition region explosive events in hydrogen
Lyβ profiles
Authors: Zhang, M.; Xia, L. -D.; Tian, H.; Chen, Y.
Bibcode: 2010A&A...520A..37Z
Altcode: 2010arXiv1006.2509Z
Aims: We search for signatures of transition region explosive
events (EEs) in hydrogen Lyβ profiles. The relationship between the
peak emission of Lyβ profiles and the wing emission of C ii and
O vi during EEs is investigated.
Methods: Two rasters made
by the SUMER (Solar Ultraviolet Measurements of Emitted Radiation)
instrument onboard SOHO in a quiet-Sun region and an equatorial
coronal hole were selected for our study. Transition-region explosive
events were identified from profiles of C ii 1037 Å and O vi 1032
Å, respectively. We compared Lyβ profiles during EEs with those
averaged in the entire quiet-Sun and coronal-hole regions.
Results: We find that the central part of Lyβ profiles reverses more
and the distance of the two peaks becomes larger during EEs, both in
the coronal hole and in the quiet Sun. The average Lyβ profile of the
EEs detected by C ii has an obviously stronger blue peak. During EEs,
there is a clear correlation between the increased peak emission of Lyβ
profiles and the enhanced wing emission of the C ii and O vi lines. The
correlation is more pronounced for the Lyβ peaks and C ii wings,
and less significant for the Lyβ blue peak and O vi blue wing. We
also find that the Lyβ profiles are more reversed in the coronal
hole than in the quiet Sun.
Conclusions: We suggest that the
jets produced by EEs emit the Doppler-shifted Lyβ photons, causing
enhanced emission at positions of the peaks of Lyβ profiles. The
more-reversed Lyβ profiles confirm the presence of higher opacity in
the coronal hole than in the quiet Sun. The finding that EEs modify
the Lyβ line profile in QS and CHs implies that one should be careful
in the modeling and interpretation of relevant observational data.
Title: Horizontal supergranule-scale motions inferred from TRACE
ultraviolet observations of the chromosphere
Authors: Tian, H.; Potts, H. E.; Marsch, E.; Attie, R.; He, J. -S.
Bibcode: 2010A&A...519A..58T
Altcode: 2010arXiv1006.0321T
Aims: We study horizontal supergranule-scale motions
revealed by TRACE observation of the chromospheric emission, and
investigate the coupling between the chromosphere and the underlying
photosphere.
Methods: A highly efficient feature-tracking
technique called balltracking has been applied for the first time to
the image sequences obtained by TRACE (transition region and coronal
explorer) in the passband of white light and the three ultraviolet
passbands centered at 1700 Å, 1600 Å, and 1550 Å. The resulting
velocity fields have been spatially smoothed and temporally averaged
in order to reveal horizontal supergranule-scale motions that may
exist at the emission heights of these passbands.
Results:
We find indeed a high correlation between the horizontal velocities
derived in the white-light and ultraviolet passbands. The horizontal
velocities derived from the chromospheric and photospheric emission
are comparable in magnitude.
Conclusions: The horizontal
motions derived in the UV passbands might indicate the existence of
a supergranule-scale magneto-convection in the chromosphere, which
may shed new light on the study of mass and energy supply to the
corona and solar wind at the height of the chromosphere. However, it
is also possible that the apparent motions reflect the chromospheric
brightness evolution as produced by acoustic shocks which might be
modulated by the photospheric granular motions in their excitation
process, or advected partly by the supergranule-scale flow towards
the network while propagating upward from the photosphere. To reach a
firm conclusion, it is necessary to investigate the role of granular
motions in the excitation of shocks through numerical modeling, and
future high-cadence chromospheric magnetograms must be scrutinized.
Title: Magnetic and spectroscopic properties of supergranular-scale
coronal jets and erupting loops in a polar coronal hole
Authors: He, J. -S.; Marsch, E.; Curdt, W.; Tian, H.; Tu, C. -Y.;
Xia, L. -D.; Kamio, S.
Bibcode: 2010A&A...519A..49H
Altcode:
Context. Coronal jets and mass ejections associated with erupting loops
are two distinct and frequently observed types of transient upflows
of plasma in coronal holes (CHs). But the magnetic and spectroscopic
properties of these events at the supergranular scale are not well
known.
Aims: Here we aim at studying in a polar hole the plasma
and field characteristics of coronal jets and erupting loops of a
supergranular size, for which we use observations from XRT, EIS and SOT
on Hinode as well as EUVI on STEREO.
Methods: The open magnetic
field structures related to the coronal jets are obtained by magnetic
field extrapolation into the corona from SOT magnetograms. Furthermore,
we use the EIS observations to analyze ultraviolet line intensities
and Doppler shifts in association with the erupting loops.
Results: We find that the coronal jet plasma is indeed ejected along
open field lines, thus confirming the conjecture of jet formation in
an open magnetic environment. The magnetic evolution at the jet base
is investigated, and the results indicate that the interaction between
two flux tubes of opposite magnetic polarities as well as the squeezing
of several tubes with identical polarities might be responsible for
the jet initiation. We reveal for the first time the spectroscopic
signatures of a supergranular-size erupting loop at its early stage,
which consists of three steps. The first step is the onset, which is
featured by a sudden brightening of one footpoint, as well as by the
occurrence of blueshifts along almost its entire path. The second step
is the initial expansion of the closed loop, which is estimated to move
upward at a speed of about 20 km s-1, as derived from the
line-of-sight (LOS) blueshift and the loop enlargement projected onto
the plane of the sky. In the third step, the loop's bright footpoint
is apparently diminishing its intensity and enhancing its blueshift,
which indicates that plasma upflow from the leg is filling the expanding
loop volume.
Conclusions: From our results we conclude that
in polar CHs, where the steady fast solar wind is known to emanate,
there are also at least two possible ways of causing transient plasma
outflows at supergranular scale. One is related to coronal jets
guided by open field lines, the other to the eruption of closed loops,
which is triggered by magnetic reconnection at their footpoints. 3 movies (for Figs. 2-4) are only available in electronic form at
http://www.aanda.org
Title: Signatures of Magnetic Reconnection at Boundaries of
Interplanetary Small-scale Magnetic Flux Ropes
Authors: Tian, Hui; Yao, Shuo; Zong, Qiugang; He, Jiansen; Qi, Yu
Bibcode: 2010ApJ...720..454T
Altcode: 2010arXiv1007.2247T
The interaction between interplanetary small-scale magnetic flux ropes
and the magnetic field in the ambient solar wind is an important topic
in the understanding of the evolution of magnetic structures in the
heliosphere. Through a survey of 125 previously reported small flux
ropes from 1995 to 2005, we find that 44 of them reveal clear signatures
of Alfvénic fluctuations and thus classify them as Alfvén wave trains
rather than flux ropes. Signatures of magnetic reconnection, generally
including a plasma jet of ~30 km s-1 within a magnetic field
rotational region, are clearly present at boundaries of about 42% of
the flux ropes and 14% of the wave trains. The reconnection exhausts
are often observed to show a local increase in the proton temperature,
density, and plasma beta. About 66% of the reconnection events at
flux rope boundaries are associated with a magnetic field shear angle
larger than 90° and 73% of them reveal a decrease of 20% or more in
the magnetic field magnitude, suggesting a dominance of anti-parallel
reconnection at flux rope boundaries. The occurrence rate of magnetic
reconnection at flux rope boundaries through the years 1995-2005 is also
investigated and we find that it is relatively low around the solar
maximum and much higher when approaching solar minima. The average
magnetic field depression and shear angle for reconnection events at
flux rope boundaries also reveal a similar trend from 1995 to 2005. Our
results demonstrate for the first time that boundaries of a substantial
fraction of small-scale flux ropes have properties similar to those
of magnetic clouds, in the sense that both of them exhibit signatures
of magnetic reconnection. The observed reconnection signatures could
be related either to the formation of small flux ropes or to the
interaction between flux ropes and the interplanetary magnetic fields.
Title: The evolution of the \vec{z} distribution of normal neutron
stars in the Galaxy
Authors: Wei, Y. -C.; Zhang, C. -M.; Zhao, Y. -H.; Peng, Q. -H.; Wu,
X. -J.; Wang, J.; Pan, Y. -Y.; Yin, H. -X.; Yan, Y.; Yan, T. -S.;
Tian, H. -J.; Esamdin, A.; Luo, A. L.; Cai, Y.; Taani, A.
Bibcode: 2010AN....331..817W
Altcode:
Under the two initial 1-D one parameter velocity distribution forms
(one is normal, the other is exponential), the z direction scale
height evolution of normal neutron stars in the Galaxy is studied by
numerical simulation. We do statistics for the cases at different time
segments, also do statistics for the cumulative cases made of each
time segment. The results show in the cumulative cases the evolution
curves of the scale heights are smoother than in the each time segment,
i.e., the cumulation improve the signal-to-noise ratio. Certainly the
evolution cases are different at different Galactic disk locations,
which also have very large difference from the average cases in the
whole disk. In the initial stages of z evolution of normal neutron
stars, after the beginning transient states, the cumulative scale
heights increase linearly with time, and the cumulative scale height
increasing rates have linear relationship with the initial velocity
distribution parameters, which have larger fluctuation in the vicinity
of the Sun than in the whole disk. We utilize the linear relationship of
the cumulative scale height increasing rates vs. the initial velocity
distribution parameters in the vicinity of the Sun to make comparison
with the observation near the Sun. The results show if there is no
magnetic decay, then the deserved initial velocity parameters are
obvious lower than the present well known results from some authors;
whereas if introducing magnetic decay, for the 1-D normal case we
can make consistence among concerning results using magnetic decay
time values which are supported by some authors, while for the 1-D
exponential case the results show the lackness of young pulsar samples
in the larger z in the vicinity of the Sun.
Title: Hydrogen Lyman Emission through the Solar Cycle
Authors: Curdt, W.; Tian, H.
Bibcode: 2010ASPC..428...81C
Altcode: 2010arXiv1002.3551C
We present observations and results of radiance and irradiance
studies completed by SOHO/SUMER during the past solar cycle. We
find that the cycle variation in Ly-α irradiance as observed by,
e.g., UARS/SOLSTICE cannot be explained by quiet Sun radiance data,
and conclude that the explanation must be related to differences in
the Ly-α radiance of various solar features and changes in their
fractional distribution over the solar cycle. Consequently, we studied
the emission of the hydrogen Ly-α line in various solar features—for
the first time observed by SUMER on disk in full resolution—to
investigate the imprint of the magnetic field on line profile and
radiance distribution. We also compare quasi-simultaneous Ly-α and
Ly-β line profiles. Such high-resolution observations—not hampered
by geocoronal absorption—have never been completed before.
Title: Intermittent outflows at the edge of an active region -
a possible source of the solar wind?
Authors: He, J. -S.; Marsch, E.; Tu, C. -Y.; Guo, L. -J.; Tian, H.
Bibcode: 2010A&A...516A..14H
Altcode:
Context. It has already been established that the solar wind may
originate at the edges of active regions (ARs), but the key questions
of how frequently these outflows occur, and at which height the
nascent solar wind originates have not yet been addressed.
Aims: We study the occurrence rate of these intermittent outflows,
the related plasma activities beneath in the low solar atmosphere,
and the interplanetary counterparts of the nascent solar wind
outflow.
Methods: We use the observations from XRT/Hinode
and TRACE to study the outflow patterns. The occurrence frequency of
the intermittent outflow is estimated by counting the occurrences of
propagating intensity enhancements in height-time diagrams. We adopt
observations of SOT/Hinode and EIS/Hinode to investigate the phenomena
in the chromosphere associated with the coronal outflows. The ACE
plasma and field in-situ measurements near Earth are used to study
the interplanetary manifestations.
Results: We find that in
one elongated coronal emission structure, referred to as strand,
the plasma flows outward intermittently, about every 20 min. The flow
speed sometimes exceeds 200 km s-1, which is indicative of
rapid acceleration, and thus exceeds the coronal sound speed at low
altitudes. The inferred flow speed of the soft-X-ray-emitting plasma
component seems a little higher than that of the Fe ix/x-emitting plasma
component. Chromospheric jets are found to occur at the root of the
strand. Upflows in the chromosphere are also confirmed by blue-shifts of
the He ii line. The heliospheric plasma counterpart close to the Earth
is found to be an intermediate-speed solar wind stream. The AR edge may
also deliver some plasmas to a fraction of the fast solar wind stream,
most of which emanate from the neighboring CH.
Conclusions:
The possible origin of the nascent solar wind in the chromosphere,
the observed excessive outflow speed of over 200 km s-1 in
the lower corona, and the corresponding intermediate-speed solar wind
stream in interplanetary space are all linked in our case study. These
phenomena from the low solar atmosphere to the heliosphere near Earth in
combination shed new light on the solar wind formation process. These
observational results will constrain future modeling of the solar
winds originating close to an AR.
Title: Solar Transition Region in the Quiet Sun and Active Regions
Authors: Tian, H.; Curdt, W.; He, J. -S.
Bibcode: 2010aogs...21..277T
Altcode: 2009arXiv0912.0345T
The solar transition region (TR), in which above the photosphere the
temperature increases rapidly and the density drops dramatically,
is believed to play an important role in coronal heating and solar
wind acceleration. Longlasting upflows are present in the upper TR
and interpreted as signatures of mass supply to large coronal loops
in the quiet Sun. Coronal bright points (BPs) are local heating
phenomena and we found a different Doppler-shift pattern at TR and
coronal temperatures in one BP, which might be related to the twisted
loop system. The dominant energy loss in the lower TR is the Ly-α
emission. It has been found that most Ly-α radiance profiles are
stronger in the blue peak, an asymmetry opposite to higher order Lyman
lines. This asymmetry is stronger when the downflow in the middle TR
is stronger, indicating that the TR flows play an important role in
the line formation process. The peak separation of Ly-α is found to be
larger in coronal holes than in the quiet Sun, reflecting the different
magnetic structures and radiation fields between the two regions. The
Lyman line profiles are found to be not reversed in sunspot plume and
umbra regions, while they are obviously reversed in the surrounding
plage region. At TR temperatures, the densities of the sunspot plume
and umbra are a factor of 10 lower than of the plage, indicating that
the sunspot plasma emitting at TR temperatures is higher and possibly
more extended in height above sunspots than above the plage region.
Title: New views on the emission and structure of the solar transition
region
Authors: Tian, Hui; Marsch, Eckart; Tu, Chuanyi; Curdt, Werner;
He, Jiansen
Bibcode: 2010PhDT.......178T
Altcode: 2010arXiv1004.3017T
The Sun is the only star that we can spatially resolve and it can
be regarded as a fundamental plasma laboratory of astrophysics. The
solar transition region (TR), the layer between the solar chromosphere
and corona, plays an important role in solar wind origin and coronal
heating. Recent high-resolution observations made by SOHO, TRACE,
and Hinode indicate that the TR is highly nonuniform and magnetically
structured. Through a combination of spectroscopic observations and
magnetic field extrapolations, the TR magnetic structures and plasma
properties have been found to be different in coronal holes and in the
quiet Sun. In active regions, the TR density and temperature structures
also differ in sunspots and the surrounding plage regions. Although
the TR is believed to be a dynamic layer, quasi-steady flows lasting
from several hours to several days are often present in the quiet
Sun, coronal holes, and active regions, indicating some kind of
plasma circulation/convection in the TR and corona. The emission of
hydrogen Lyman lines, which originates from the lower TR, has also
been intensively investigated in the recent past. Observations show
clearly that the flows and dynamics in the middle and upper TR can
greatly modify the Lyman line profiles.
Title: Upward and downward propagation of transverse waves due to
small-scale magnetic reconnection in the chromosphere
Authors: He, J. -S.; Marsch, E.; Tu, C. -Y.; Tian, H.
Bibcode: 2010AIPC.1216...32H
Altcode:
We aim at studying the connections between the propagation
of transverse waves on spicules and the small-scale magnetic
reconnections at the feet of the spicules. The observed transverse
wave may be interpreted as an Alfvén wave or a kink wave. We use the
observations from SOT/Hinode in the Ca II H line to study the spicule
dynamics above the reconnection sites. We found in three cases that
the transverse oscillations were excited by magnetic reconnection,
and steepened while propagating upwardly. In case-1, the steepening
oscillations were attenuated at greater heights in association with
the extension of the spicule plasma. In case-2, the transverse wave
seems to be initiated by a footpoint jump of the spicule trace in the
photosphere. The transverse wave in case-3 was found to be reflected,
and then to propagate downward. These results reveal the wave dynamics
in the low solar atmosphere, and provide observational evidence that is
useful for the modeling of coronal heating and solar wind acceleration,
involving wave generation by magnetic reconnection in the chromosphere.
Title: Upflows in the upper transition region of the quiet Sun
Authors: Tian, H.; Tu, C. -Y.; Marsch, E.; He, J. -S.; Zhou, C.;
Zhao, L.
Bibcode: 2010AIPC.1216...36T
Altcode: 2009arXiv0911.1833T
We investigate the physical meaning of the prominent blue shifts of
Ne VIII, which is observed to be associated with quiet-Sun network
junctions (boundary intersections), through data analyses combining
force-free-field extrapolations with EUV spectroscopic observations. For
a middle-latitude region, we reconstruct the magnetic funnel structure
in a sub-region showing faint emission in EIT-Fe 195. This funnel
appears to consist of several smaller funnels that originate from
network lanes, expand with height and finally merge into a single
wide open-field region. However, the large blue shifts of Ne VIII are
generally not associated with open fields, but seem to be associated
with the legs of closed magnetic loops. Moreover, in most cases
significant upflows are found in both of the funnel-shaped loop
legs. These quasi-steady upflows are regarded as signatures of
mass supply to the coronal loops rather than the solar wind. Our
observational result also reveals that in many cases the upflows in
the upper transition region (TR) and the downflows in the middle TR
are not fully cospatial. Based on these new observational results, we
suggest different TR structures in coronal holes and in the quiet Sun.
Title: Reconfiguration of the coronal magnetic field by means of
reconnection driven by photospheric magnetic flux convergence
Authors: He, J. -S.; Marsch, E.; Tu, C. -Y.; Tian, H.; Guo, L. -J.
Bibcode: 2010A&A...510A..40H
Altcode:
Context. Magnetic reconnection is commonly believed to be responsible
for flare-like events and plasma ejections in the solar atmosphere,
but the field-line reconfiguration observed in association with magnetic
reconnection has rarely been observed before.
Aims: We attempt to
reconstruct the configuration of the magnetic field during a magnetic
reconnection event, estimate the reconnection rate, and analyze the
resulting X-ray burst and plasma ejection.
Methods: We use the
local-correlation-tracking (LCT) method to track the convergence of
magnetic fields with opposite polarities using photospheric observations
from SOT/Hinode. The magnetic field lines are then extrapolated from
the tracked footpoint positions into the corona, and the changes in
field-line connections are marked. We estimate the reconnection rate
by calculating the convective electric field in the photosphere, which
is normalized to the product of the plasma jet speed and the coronal
magnetic field strength inside the inflow region. The observed X-ray
burst and plasma ejection are analysed with data from XRT/Hinode and
TRACE, respectively.
Results: We find that in this reconnection
event the two sets of approaching closed loops were reconfigured to
a set of superimposed large-scale closed loops and another set of
small-scale closed loops. Enhanced soft X-ray emission was seen to
rapidly fill the reconnected loop after the micro-flare occurred at
the reconnection site. Plasma was ejected from that site with a speed
between 27 and 40 km s-1. The reconnection rate is estimated
to range between 0.03 and 0.09.
Conclusions: Our work presents
a study of the magnetic field reconfiguration owing to magnetic
reconnection driven by flux convergence in the photosphere. This
observation of the magnetic structure change is helpful for future
diagnosis of magnetic reconnection. The results obtained for the
reconnection rate, the X-ray emission burst, and the plasma ejection
provides new observational evidence, and places constraints on future
theoretical study of magnetic reconnection in the Sun.
Title: The SUMER Ly-α line profile in quiescent prominences
Authors: Curdt, W.; Tian, H.; Teriaca, L.; Schühle, U.
Bibcode: 2010A&A...511L...4C
Altcode: 2010arXiv1002.1197C
Aims: As the result of a novel observing technique, we publish
for the first time SoHO-SUMER observations of the true spectral line
profile of hydrogen Lyman-α in quiescent prominences. Because SoHO is
not in Earth orbit, our high-quality data set is free of geocoronal
absorption. We studied the line profile to complement earlier
observations of the higher Lyman lines and to substantiate recent
model predictions.
Methods: We applied the reduced-aperture
observing mode to two prominence targets and did a statistical
analysis of the line profiles in both data sets. In particular, we
investigated the shape of the profile, the radiance distribution, and
the line shape-to-radiance interrelation. We also compared Ly-α data
to co-temporal λ 1206 Si iii data.
Results: We find that the
average profile of Ly-α has a blue-peak dominance and is reversed
more if the line-of-sight is perpendicular to the field lines. The
contrast of Ly-α prominence emission rasters is very low, and the
radiance distribution differs from the log-normal distribution of the
disk. Features in the Si iii line are not always co-spatial with Ly-α
emission.
Conclusions: Our empirical results support recent
multi-thread models, which predict that asymmetries and depths of the
self-reversal depend on the orientation of the prominence axis relative
to the line-of-sight.
Title: The coronal convection
Authors: Curdt, Werner; Tian, Hui; Marsch, Eckart
Bibcode: 2010cosp...38.2927C
Altcode: 2010cosp.meet.2927C
We studied with SUMER the hydrogen Lyman emission in various solar
features -now including Ly-α observations -and investigated the
imprints that coronal flows and the solar magnetic field have on the
line profile and radiance distribution. As a new result, we found
that in Ly-α raster maps the locations with higher line opacity
(evident by strong self-absorption dips) cluster in the cell interior
of the magnetic network, while the lines from the network lanes tend
to reveal flatter profiles. Even deeper self reversals and larger
peak separations were found in coronal hole spectra. We also compared
the quasi-simultaneously obtained Ly-α and Ly-β profiles and found
an obvious correspondence between asymmetry and redshift for both
lines, but, most surprisingly, the asymmetries of Ly-α and Ly-β are
opposite. We conclude that in both cases downflows determine the line
profile, in the case of Ly-α by absorption and in the case of Ly-β
by emission. Our results, which are further supported by recent Hinode
data, show that the magnetic structure of the solar atmosphere plays
a dominant role in the line formation, and indicate the prevalence of
persistent downflows at both footpoints of closed loops. We suggest
all this to be the manifestation of a continuous mass-transportation
process, which Foukal back in 1978 introduced as 'coronal convection'.
Title: Upward propagation and subsequent steepening of transverse
waves launched by small-scale magnetic reconnection in the
chromosphere
Authors: He, Jiansen; Marsch, Eckart; Tu, Chuanyi; Tian, Hui
Bibcode: 2010cosp...38.1805H
Altcode: 2010cosp.meet.1805H
SOT observations of chromospheric spicules have revealed that
Alfven-like transverse oscilla-tions on the spicules are prevalent
in the chromosphere. But the propagation features, e.g., the phase
speed and the propagation direction, have not been derived directly
from observations. It is still not clear how these transverse waves
are generated. Here we aim at identifying the propagation feature of
the transverse wave, studying its ampli-tude evolution as well as its
generation by magnetic reconnection underneath. The phase speed is
estimated to range between 20 and 150 km/s, based on time lags between
different transverse oscillation profiles at various heights along the
dynamic spicule. The transverse fluctuation is found to originate at
the cusp of an inverted Y-shaped emission structure, where apparently
magnetic reconnection occurred. The transverse wave steepened with
height probably due to density rarefaction in the stratified atmosphere,
and evolved into a nonlinear state with a large relative disturbance,
yielding a relative velocity-amplitude (dV/Vph) of 0.21 at 5.5 Mm. The
nonlinear transverse wave seems to be damped in velocity amplitude
beyond 5.5 Mm, with the transverse-fluctuation energy possibly being
converted to the longitudinal-motion energy. We also estimate the
energy flux density carried by the transverse wave, and find it is
still sufficient, in spite of possible attenuation in the transition
region, for heating the quiet corona or driving the solar wind. In
another case, we find that upward propagation (launched by magnetic
recon-nection) is subsequently followed by clear downward propagation,
which may indicate possible wave reflection at some height in the
transition region. Our findings shed new light on future modelling
of coronal heating and solar wind acceleration, involving magnetic
reconnection in the chromosphere. References: [1]. He, J.-S., Marsch,
E., Tu, C.-Y., Tian, H., Excitation of kink waves due to small-scale
magnetic reconnection in the chromosphere? ApJL, 705, L217-L222,
2009. [2]. He, J.-S., Marsch, E., Tu, C.-Y., Tian, H., Upward and
downward propagation of transverse waves due to small-scale magnetic
reconnection in the chromospehre, SolarWind-12, in press.
Title: Characteristics of the Expansion of Magnetic Funnels in Solar
Quiet Regions
Authors: Tan, Bo; Tian, Hui; He, Jian-sen
Bibcode: 2010ChA&A..34...40T
Altcode: 2010ChA&A..34...40B
Via the potential field extrapolation of the observed photospheric
magnetic field, the structure of the photospheric magnetic fields
above solar quiet regions is renewed. As revealed by the result,
below 20 Mm the open magnetic lines exhibit many obvious small funnel
structures. These funnels expand with height and at the height of about
20 Mm they combine into large funnel structures. By a systematic study
of the tendency of change of the cross section areas of funnels, it
is discovered that the cross section areas of funnels in solar quiet
regions expand approximately linearly. The velocity of expansion of
magnetic funnels at rather low altitudes (< 20 Mm) is larger than
that at high altitudes (> 20 Mm). This phenomenon has important
significance for the two-dimensional numerical simulations of the
origin of solar wind and the mass flow in magnetic loops. At the same
time it is found that the number of closed magnetic lines decreases
in the form of exponential function.
Title: Reconstruction of reconnecting magnetic field in both solar
corona and geo-magnetosphere and its application to reconnection
diagnosis
Authors: He, Jiansen; Marsch, Eckart; Tu, Chuanyi; Zong, Qiugang;
Tian, Hui; Guo, Lijia
Bibcode: 2010cosp...38.1942H
Altcode: 2010cosp.meet.1942H
Magnetic reconnection is a critical process of energy transfer in the
solar-terrestrial plasma. The related 3-D magnetic field geometry
and its temporal reconfiguration are important for reconnection
diagnosis. For example, we can estimate how fast the coronal
magnetic field is reconnected, based on temporal variation of the
reconnecting field lines, and we can learn about the micro-physics
in the reconnection region with the help of 3-D reconstruction of the
magnetic field in geo-space. For a solar micro-flare event, we study
the evolution of magnetic field lines, which are ex-trapolated from
dynamic footpoints showing convergence motions. We find that the two
sets of approaching closed loops were reconfigured to form a joint set
of superimposed large-scale closed loops and another set of small-scale
closed loops. We estimate the reconnection rate in two ways. One way
is to calculate the convective electric field in the photosphere,
which is normalized to the product of the plasma jet speed and the
coronal magnetic field strength inside the inflow region. Another
way is to directly calculate the reconnection rate based on Vin/VA,
where Vin is the motion of the convex segments on the reconnecting
field lines. The reconnection rate is estimated to range between 0.03
and 0.09. This is the first work illustrat-ing the reconfiguration
of the magnetic field geometry, owing to reconnection driven by flux
convergence in the photosphere. For a magnetic reconnection event in
the geo-magnetotail, we reconstruct the 3D magnetic field structure
containing a pair of nulls with a novel method based on Cluster
4-spacecraft measurements. We study the electron dynamics near one
null of the pair, and propose a new scenario of micro-physics in the
null region. In this scenario, electrons are temporarily trapped in the
central reconnection region, accelerated possibly by parallel electric
field and electron pressure gradient, and reflected from the magnetic
cusp mirrors leading to the bi-directional energetic electron beams,
which may excite the high-frequency electrostatic waves. Here, we
have demonstrated that reconstruction of the real-time magnetic field
structure is quite helpful for magnetic reconnection diagnosis, e.g.,
estimation of the reconnection rate and analysis of the micro-physics in
the inner reconnection region. References: [1]. He, J.-S., Marsch, E.,
Tu, C.-Y., Tian, H., Guo, L.-J., Reconfiguration of the coronal magnetic
field by means of reconnection driven by photospheric magnetic flux
convergence, AA, 510, A40, 2010. [2]. He, J.-S., Zong, Q.-G., Deng,
X.-H., Tu, C.-Y., et al., Electron trapping around a magnetic null,
Geophys. Res. Lett., 35, L14104, 2008.
Title: Solar wind origins in coronal holes and in the quiet Sun
Authors: He, J. -S.; Tu, C. -Y.; Tian, H.; Marsch, E.
Bibcode: 2010AdSpR..45..303H
Altcode:
Coronal hole (CH) and the quiet Sun (QS) are considered to account
for sources of fast and slow solar wind streams, respectively. The
differences between the solar wind streams flowing out from the CH
and the QS are thought to be related with different plasma generation
and acceleration mechanisms in the respective source regions. Here we
review recent studies on the solar wind origin in the CH and the QS,
compare the possible flow geometries and magnetic structures in these
two kinds of solar regions, and summarize the physics associated with
two different origin scenarios.
Title: New views on the emission and structure of the solar transition
region
Authors: Tian, Hui; Marsch, Eckart; Tu, Chuanyi; Curdt, Werner;
He, Jiansen
Bibcode: 2010NewAR..54...13T
Altcode:
The Sun is the only star that we can spatially resolve and it can
be regarded as a fundamental plasma laboratory of astrophysics. The
solar transition region (TR), the layer between the solar chromosphere
and corona, plays an important role in solar wind origin and coronal
heating. Recent high-resolution observations made by SOHO, TRACE,
and Hinode indicate that the TR is highly nonuniform and magnetically
structured. Through a combination of spectroscopic observations and
magnetic field extrapolations, the TR magnetic structures and plasma
properties have been found to be different in coronal holes and in the
quiet Sun. In active regions, the TR density and temperature structures
also differ in sunspots and the surrounding plage regions. Although
the TR is believed to be a dynamic layer, quasi-steady flows lasting
from several hours to several days are often present in the quiet
Sun, coronal holes, and active regions, indicating some kind of
plasma circulation/convection in the TR and corona. The emission of
hydrogen Lyman lines, which originates from the lower TR, has also
been intensively investigated in the recent past. Observations show
clearly that the flows and dynamics in the middle and upper TR can
greatly modify the Lyman line profiles.
Title: The Nascent Fast Solar Wind Observed by the EUV Imaging
Spectrometer on Board Hinode
Authors: Tian, Hui; Tu, Chuanyi; Marsch, Eckart; He, Jiansen; Kamio,
Suguru
Bibcode: 2010ApJ...709L..88T
Altcode: 2009arXiv0912.4316T
The origin of the solar wind is one of the most important unresolved
problems in space and solar physics. We report here the first
spectroscopic signatures of the nascent fast solar wind on the basis
of observations made by the EUV Imaging Spectrometer on Hinode in a
polar coronal hole in which patches of blueshift are clearly present on
Dopplergrams of coronal emission lines with a formation temperature of
lg(T/K)>5.8. The corresponding upflow is associated with open field
lines in the coronal hole and seems to start in the solar transition
region and becomes more prominent with increasing temperature. This
temperature-dependent plasma outflow is interpreted as evidence of the
nascent fast solar wind in the polar coronal hole. The patches with
significant upflows are still isolated in the upper transition region
but merge in the corona, in agreement with the scenario of solar wind
outflow being guided by expanding magnetic funnels.
Title: A new view of solar wind origin near active regions and in
polar coronal holes on the basis of Hinode observations
Authors: He, Jiansen; Marsch, Eckart; Tu, Chuanyi; Tian, Hui; Guo,
Lijia; Curdt, Werner; Xia, Lidong; Kamio, Suguru
Bibcode: 2010cosp...38.2938H
Altcode: 2010cosp.meet.2938H
The possibility of full-range solar observations from Hinode with high
temporal and spatial resolution motivated us to revisit the solar wind
origin in the solar atmosphere. For the solar wind origin near active
regions, we reveal activity in the chromosphere to be responsible for
the coronal outflow at the AR edge; meanwhile we also succeeded in
finding the related solar wind stream in the heliosphere. Chromospheric
spicule-like jets are found to occur intermittently, and to flow
in similar direction than the intermittent coronal outflows. EIS
observations reveal that there are blue-shifts at the edge of AR
in both chromosphere and corona, and that the blue-shift increases
with temperature. Therefore, we suggest that the nascent solar wind
may probably originate in the chromosphere at the edge of ARs. The
connection between the source regions and their respective solar
wind streams is established through magnetic field lines, which are
extrapolated to reach the solar ecliptic plane at 2.5 Rs. We find that
the AR edge may be the source region of an intermediate-speed (400 km/s)
solar wind stream. For the solar wind origin in polar coronal holes
(CHs), we extend the blue-shift observations from the transition
region up to the corona, investigate the magnetic characteristics
in association with the polar coronal jets, and study in details the
initial phase of meso-scale loop eruption in polar CHs. We find that the
blue-shift becomes more and more dominant with increasing temperature
from the transition region to the corona. The polar coronal jets are
found to be elongated along the extrapolated open field lines, which
appear in fanning-out shape. The cancellation between bipolar magnetic
fields, or squeezing of monopolar magnetic fields, may be the driver for
a jet launch. The meso-scale loop started its eruption with a sudden
brightening at one footpoint, then expanded and moved upwardly with a
speed of 20 km/s, causing the mass at the footpoint to flow outwardly
and to fill in the expanded volume. We suggest that mass confined in
the coronal loops may be dynamically released into the outer corona
and even heliosphere via magnetic reconnection. References: [1]. He,
J.-S., Marsch, E., Tu, C.-Y., Guo, L.-J., Tian, H., Intermittent
outflows at the edge of an active region -a possible source of the
solar wind. Submitted to AA. [2]. Tian, H., Tu, C.-Y., Marsch, E.,
He, J.-S., Kamio, S., The nascent fast solar wind observed by the EUV
imaging spectrometer on board Hinode, ApJ, 709, L88-L93, 2010. [3]. He,
J.-S., Marsch, E., Curdt, W., Tu, C.-Y., Xia, L.-D., Tian, H., Kamio,
S., Meso-scale coronal jets and erupting loops guided by magnetic
fields in a polar coronal hole region, in preparation.
Title: Semiempirical Models of the Solar Atmosphere. III. Set of
Non-LTE Models for Far-Ultraviolet/Extreme-Ultraviolet Irradiance
Computation
Authors: Fontenla, J. M.; Curdt, W.; Haberreiter, M.; Harder, J.;
Tian, H.
Bibcode: 2009ApJ...707..482F
Altcode:
Semiempirical atmospheric models of solar surface features as observed
at moderate resolution are useful tools for understanding the observed
solar spectral irradiance variations. Paper I described a set of models
constructed to reproduce the observed radiance spectrum for solar
surface features at ~2 arcsec resolution that constitute an average
over small-scale features such as granulation. Paper II showed that a
revision of previous models of low-chromospheric inter-network regions
explains the observed infrared CO lines in addition to the UV and radio
continuum from submillimeter to centimetric wavelengths. The present
paper (1) shows that the Ca II H and K line wing observations are
also explained by the new quiet-Sun-composite model, (2) introduces
new low-chromospheric models of magnetic features that follow the
ideas in Paper II, (3) introduces new upper chromospheric structures
for all quiet-Sun and active-region models, and (4) shows how the new
set of models explains EUV/FUV observations of spectral radiance and
irradiance. This paper also discusses the chromospheric radiative-loss
estimates in each of the magnetic features. The new set of models
provides a basis for the spectral irradiance synthesis at EUV/FUV
wavelengths based on the features observed on the solar surface.
Title: Excitation of Kink Waves Due to Small-Scale Magnetic
Reconnection in the Chromosphere?
Authors: He, Jiansen; Marsch, Eckart; Tu, Chuanyi; Tian, Hui
Bibcode: 2009ApJ...705L.217H
Altcode:
The kink wave, which has often been observed in coronal loops, is
considered as a possibly important energy source contributing to coronal
heating. However, its generation has not yet been observed. Here, we
report the first observation of kink-wave excitation caused by magnetic
reconnection as inferred from Solar Optical Telescope measurements made
in the Ca II line. We observed transverse-displacement oscillations
on a spicule which propagated upwardly along the spicule trace and
originated from the cusp of an inverted Y-shaped structure, where
apparently magnetic reconnection occurred. Such transverse oscillation
of an individual spicule is interpreted by us to be the signature
of a kink wave that was excited by magnetic reconnection. We present
the height variations of the velocity amplitude, δv, and the phase
speed, C k, of the kink wave, starting from its source
region. The kink wave is found to steepen with height and to evolve
into a nonlinear state with a large relative disturbance, yielding a
(δv/C k) of 0.21 at 5.5 Mm. This nonlinear kink wave seems
to be damped in velocity amplitude beyond 5.5 Mm, which may result from
the conversion of transverse-fluctuation energy to longitudinal-motion
energy required to sustain the spicule. We also estimate the energy
flux density carried by the kink wave, and in spite of its attenuation
in the transition region conclude it to be sufficient for heating the
quiet corona. Our findings shed new light on future modeling of coronal
heating and solar wind acceleration involving magnetic reconnection
in the chromosphere.
Title: Upflows in Funnel-like Legs of Coronal Magnetic Loops
Authors: Tian, Hui; Marsch, Eckart; Curdt, Werner; He, Jiansen
Bibcode: 2009ApJ...704..883T
Altcode: 2009arXiv0909.0739T
The prominent blueshifts of Ne VIII associated with the junctions of the
magnetic network in the quiet Sun are still not well understood. By
comparing the coronal magnetic-field structures as obtained by a
potential-field reconstruction with the conspicuous blueshift patches
on the Dopplergram of Ne VIII as observed in an equatorial quiet-Sun
region, we find that most of the regions with significant upflow are
associated with the funnel-like legs of magnetic loops and cospatial
with increments of the line width. These quasi-steady upflows can be
regarded as the signatures of mass supply to coronal loops. By using the
square root of the line intensity as a proxy for the plasma density,
the mass flux of the upflow in each funnel can be estimated. We find
that the mass flux is anti-correlated with the funnel's expansion factor
as determined from the extrapolated magnetic field. One of the loop
systems is associated with a coronal bright point, which was observed
by several instruments and exhibited various morphologies in different
wavelengths and viewing directions. A remarkable agreement between its
magnetic structure and the associated EUV emission pattern was found,
suggesting an almost potential-field nature of the coronal magnetic
field. We also report the direct detection of a small-scale siphon flow
by both STEREO satellites. However, this transient siphon flow occurred
in a weak mixed-polarity-field region, which was outside the adjacent
magnetic funnel, and thus it is perhaps not related to plasma upflow
in the funnel. Based on these observations, we suggest that at upper
transition region (TR) temperatures the dominant flows in quiet-Sun
coronal loops are long-lasting upflows rather than siphon flows. We
also discuss the implications of our results for coronal heating and
unresolved magnetic structures.
Title: Hydrogen Lyα and Lyβ Radiances and Profiles in Polar
Coronal Holes
Authors: Tian, Hui; Teriaca, Luca; Curdt, Werner; Vial, Jean-Claude
Bibcode: 2009ApJ...703L.152T
Altcode: 2009arXiv0909.0735T
The hydrogen Lyα plays a dominant role in the radiative energy
transport in the lower transition region, and is important for
the studies of transition-region structure as well as solar wind
origin. We investigate the Lyα profiles obtained by the Solar
Ultraviolet Measurement of Emitted Radiation spectrograph on the
Solar and Heliospheric Observatory spacecraft in coronal holes and the
quiet Sun. In a subset of these observations, the H I Lyβ, Si III,
and O VI lines were also (quasi-)simultaneously recorded. We find
that the distances between the two peaks of Lyα profiles are larger
in coronal holes than in the quiet Sun, indicating a larger opacity in
coronal holes. This difference might result from the different magnetic
structures or the different radiation fields in the two regions. Most
of the Lyβ profiles in the coronal hole have a stronger blue peak,
in contrast to those in quiet-Sun regions while in both regions the
Lyα profiles are stronger in the blue peak. Although the asymmetries
are likely to be produced by differential flows in the solar atmosphere,
their detailed formation processes are still unclear. The radiance ratio
between Lyα and Lyβ decreases toward the limb in the coronal hole,
which might be due to the different opacity of the two lines. We also
find that the radiance distributions of the four lines are set by a
combined effect of limb brightening and the different emission level
between coronal holes and the quiet Sun.
Title: Solar transition region above sunspots
Authors: Tian, H.; Curdt, W.; Teriaca, L.; Landi, E.; Marsch, E.
Bibcode: 2009A&A...505..307T
Altcode: 2009arXiv0906.2211T
Aims: We study the transition region (TR) properties above sunspots and
the surrounding plage regions, by analyzing several sunspot reference
spectra obtained by the SUMER (Solar Ultraviolet Measurements of Emitted
Radiation) instrument in March 1999 and November 2006.
Methods:
We compare the SUMER spectra observed in the umbra, penumbra, plage,
and sunspot plume regions. The hydrogen Lyman line profiles averaged
in each of the four regions are presented. For the sunspot observed
in 2006, the electron densities, differential emission measure (DEM),
and filling factors of the TR plasma in the four regions are also
investigated.
Results: The self-reversals of the hydrogen Lyman
line profiles are almost absent in sunspots at different locations (at
heliocentric angles of up to 49°) on the solar disk. In the sunspot
plume, the Lyman lines are also not reversed, whilst the lower Lyman
line profiles observed in the plage region are obviously reversed, a
phenomenon found also in the normal quiet Sun. The TR densities of the
umbra and plume are similar and one order of magnitude lower than those
of the plage and penumbra. The DEM curve of the sunspot plume exhibits
a peak centered at log(T / K) ~ 5.45, which exceeds the DEM of other
regions by one to two orders of magnitude at these temperatures. We
also find that more than 100 lines, which are very weak or not observed
anywhere else on the Sun, are well observed by SUMER in the sunspot,
especially in the sunspot plume.
Conclusions: We suggest that the
TR above sunspots is higher and probably more extended, and that the
opacity of the hydrogen lines is much lower above sunspots, compared to
the TR above plage regions. Our result indicates that the enhanced TR
emission of the sunspot plume is probably caused by a large filling
factor. The strongly enhanced emission at TR temperatures and the
reduced continuum ensure that many normally weak TR lines are clearly
distinctive in the spectra of sunspot plumes. Tables 5 and 6 are
only available in electronic form at http://www.aanda.org
Title: Hydrogen Lyman-α and Lyman-β spectral radiance profiles in
the quiet Sun
Authors: Tian, H.; Curdt, W.; Marsch, E.; Schühle, U.
Bibcode: 2009A&A...504..239T
Altcode: 2009arXiv0907.1069T
Aims: We extend earlier work by studying the line profiles of the
hydrogen Lyman-α and Lyman-β lines in the quiet Sun. They were
obtained quasi-simultaneously in a raster scan with a size of about
150'' × 120'' near disk center.
Methods: The self-reversal depths
of the Ly-α and Ly-β profiles. we are quantified by measuring the
maximum spectral radiances of the two horns and the minimum spectral
radiance of the central reversal. The information on the asymmetries
of the Ly-α and Ly-β profiles is obtained through calculating
the 1st and 3rd-order moments of the line profiles. By comparing
maps of self-reversal depths and the moments with radiance images
of the Lyman lines, photospheric magnetograms, and Dopplergrams of
two other optically thin lines, we studied the spatial distribution
of the Ly-α and Ly-β profiles with different self-reversal depths,
and investigated the relationship between profile asymmetries and flows
in the solar atmosphere.
Results: We find that the emissions of
the Lyman lines tend to be more strongly absorbed in the internetwork,
as compared to those in the network region. Almost all of the Ly-α
profiles are self-reversed, while about 17% of the Ly-β profiles are
not reversed. The ratio of Ly-α and Ly-β intensities seems to be
independent of the magnetic field strength. Most Ly-α profiles are
stronger in the blue horn, whereas most Ly-β profiles are stronger
in the red horn. However, the opposite asymmetries of Ly-α and
Ly-β are not correlated pixel-to-pixel. We also confirm that when
larger transition-region downflows are present, the Ly-α and Ly-β
profiles are more enhanced in the blue and red horns, respectively. The
first-order moment of Ly-β, which reflects the combined effects of
the profile asymmetry and motion of the emitting material, strongly
correlates with the Doppler shifts of the Si iii and O vi lines, while
this correlation is much weaker for Ly-α. Our analysis shows that both
Ly-α and Ly-β might be more redshifted if stronger transition-region
downflows are present. We also find that the observed average Ly-β
profile is redshifted with respect to its rest position.
Title: Thermal stability and gas generation of petroleum in the
earth's subsurface based on kinetics modelling and field observations
Authors: Xiao, X. M.; Tian, H.; Song, Z. G.
Bibcode: 2009GeCAS..73R1460X
Altcode:
No abstract at ADS
Title: Kinetic modelling of gas generation and methane carbon isotope
fractionation during the cracking of aromatic hydrocarbons
Authors: Tian, H.; Xiao, X. M.; Guo, L. G.; Zhu, Y. F.
Bibcode: 2009GeCAS..73Q1330T
Altcode:
No abstract at ADS
Title: Upward Propagating Alfvén Wave and its Connection With
Magnetic Reconnection in the Chromosphere as Observed by SOT
Authors: Tu, C.; He, J.; Marsch, E.; Tian, H.; Guo, L.; Yao, S.
Bibcode: 2009AGUSMSH31B..02T
Altcode:
We identify high-frequency Alfvén waves propagating upward in the solar
chromosphere and transition region from observational data obtained
by the Solar Optical Telescope (SOT) onboard Hinode. We find that in
four cases the spicules are modulated by high-frequency (>0.02~Hz)
transverse fluctuations, which are inferred to be associated with
Alfvén waves. They apparently propagate upward along the spicules with
phase speeds ranging from 50 to 150~km/s. Three of these modulated
spicules show clear wave-like shapes, with short wavelengths being
less than 8~Mm. In our analysis we thus identified directly upward
propagation of Alfvén waves in the solar chromosphere and transition
region. In addition to the recently reported Alfvén waves with
very long wavelengths and wave periods, we find here four examples
of Alfvén waves with shorter wavelengths and periods. Moreover,
we report observational evidence for Alfvén-wave excitation due to
magnetic reconnection. These findings shed new light on the wave origin
and on the coronal and solar-wind heating by waves.
Title: Upward propagating high-frequency Alfvén waves as identified
from dynamic wave-like spicules observed by SOT on Hinode
Authors: He, J. -S.; Tu, C. -Y.; Marsch, E.; Guo, L. -J.; Yao, S.;
Tian, H.
Bibcode: 2009A&A...497..525H
Altcode:
Aims: We identify high-frequency Alfvén waves propagating upward in
the solar chromosphere and transition region from observation by Solar
Optical Telescope (SOT) onboard Hinode.
Methods: The spicule shape
is enhanced through application of a normal radial gradient filter and
an un-sharp mask on the images taken by SOT. The displaced position
of the spicule is at each height obtained by tracing the maximum
intensity after image processing. The dominant wave period is obtained
by the FFT method applied to the time variations of the displaced
position at a certain height. The phase speed is estimated with the
help of a cross-correlation analysis of two temporal sequences of the
displaced positions at two heights along the spicule.
Results:
We find in four cases that the spicules are modulated by high-frequency
(≥0.02 Hz) transverse fluctuations. Such fluctuations are suggested
to be Alfvén waves that propagate upward along the spicules with
phase speed ranges from 50 to 150 km s-1. Three of the
modulated spicules show clear wave-like shapes with short wavelengths
less than 8 Mm.
Conclusions: Our work identified directly upward
propagation of Alfvén waves in the solar chromosphere and transition
region. In addition to the recently reported Alfvén waves with very
long wavelength and wave period, we find here four examples of Alfvén
waves with shorter wavelengths and periods. These findings shed new
light on the wave origin and on coronal and solar-wind heating.
Title: The Emission Heights of Transition Region Lines in an
Equatorial Coronal Hole and the Surrounding Quiet Sun
Authors: Tian, Hui; Xia, Li-Dong; He, Jian-Sen; Tan, Bo; Yao, Shuo
Bibcode: 2008ChJAA...8..732T
Altcode:
Using the correlation between the radiance or Doppler velocity and
the extrapolated magnetic field, we determined the emission heights
of a set of solar transition region lines in an equatorial coronal
hole and in the surrounding quiet Sun region. We found that for
all of the six lower-transition-region lines, the emission height
is about 4 5 Mm in the equatorial coronal hole, and around 2 Mm in
the quiet Sun region. This result confirms the previous findings
that plasma with different temperature can coexist at the same layer
of transition region. In the quiet Sun region, the emission height
of the upper-transition-region line Ne VIII is almost the same that
of the lower-transition-region line, but in the coronal hole, it is
twice as high. This difference reveals that the outflow of Ne VIII
is a signature of solar wind in the coronal hole and is just a mass
supply to the large loops in the quiet Sun.
Title: The Ly-α profile and center-to-limb variation of the quiet Sun
Authors: Curdt, W.; Tian, H.; Teriaca, L.; Schühle, U.; Lemaire, P.
Bibcode: 2008A&A...492L...9C
Altcode: 2008arXiv0812.1441C
Aims: We study the emission of the hydrogen Lyman-α line in the quiet
Sun, its center-to-limb variation, and its radiance distribution. We
also compare quasi-simultaneous Ly-α and Ly-β line profiles.
Methods: We used the high spectral and spatial resolution of the
SUMER spectrometer and completed raster scans at various locations
along the disk. For the first time, we used a method to reduce the
incoming photon flux to a 20%-level by partly closing the aperture
door. We also performed a quasi-simultaneous observation of both Ly-α
and Ly-β at the Sun center in sit-and-stare mode. We infer the flow
characteristic in the Ly-α map from variations in the calibrated λ
1206 Si iii line centroids.
Results: We present the average
profile of Ly-α, its radiance distribution, its center-to-limb
behaviour, and the signature of flows on the line profiles. Little
center-to-limb variation and no limb brightening are observed in the
profiles of the Ly-α line. In contrast to all other lines of the
Lyman series, which have a red-horn asymmetry, Ly-α has a robust and
- except for dark locations - dominating blue-horn asymmetry. There
appears to be a brightness-to-asymmetry relationship. A similar
and even clearer trend is observed in the downflow-to-asymmetry
relationship. This important result is consistent with predictions
from models that include flows. However, the absence of a clear
center-to-limb variation in the profiles may be more indicative of
an isotropic field than a mainly radial flow.
Conclusions: It
appears that the ubiquitous hydrogen behaves in a similar way to a
filter that dampens all signatures of the line formation by processes
in both the chromosphere and transition region.
Title: The redshifted network contrast of transition region emission
Authors: Curdt, W.; Tian, H.; Dwivedi, B. N.; Marsch, E.
Bibcode: 2008A&A...491L..13C
Altcode: 2009arXiv0901.0808C
Aims: We study the VUV emission of the quiet Sun and the net redshift
of transition region lines in the SUMER spectral range. We aim at
establishing a link with atmospheric processes and interpreting the
observed downflow as the most evident part of the prevailing global
coronal mass transport.
Methods: We rank and arrange all pixels
of a monochromatic raster scan by radiance and define equally-sized
bins of bright, faint, and medium-bright pixels. Comparing the bright
pixels with the faint pixels, we determine the spectrally-resolved
network contrast for 19 emission lines. We then compare the contrast
centroids of these lines with the position of the line itself. We
establish a relationship between the observed redshift of the network
contrast with the line formation temperature.
Results: We find
that the network contrast is offset in wavelength compared to the
emission line itself. This offset, if interpreted as redshift, peaks
at middle transition region temperatures and is 10 times higher than
the previously reported net redshift of transition region emission
lines. We demonstrate that the brighter pixels are more redshifted,
causing both a significant shift of the network contrast profile and the
well-known net redshift. We show that this effect can be reconstructed
from the radiance distribution. This result is compatible with loop
models, which assume downflows near both footpoints.
Title: Radiance and Doppler shift distributions across the network
of the quiet Sun
Authors: Tian, H.; Tu, C. -Y.; Xia, L. -D.; He, J. -S.
Bibcode: 2008A&A...489.1297T
Altcode: 2009arXiv0907.1073T
Aims: The radiance and Doppler-shift distributions across the solar
network provide observational constraints of two-dimensional modeling
of transition-region emission and flows in coronal funnels. These
distributions have not, however, been studied in detail and we attempt
an investigation for a quiet Sun region.
Methods: Two different
methods, dispersion plots and average-profile studies, were applied to
investigate these distributions for three EUV lines. In the dispersion
plots, we divided the entire quiet Sun region scanned by SUMER into
a bright and a dark part according to an image of Fe xii taken by EIT
during the scanning; we plotted intensities and Doppler shifts in each
bin as determined according to a filtered intensity of Si ii. We also
studied the difference in height variations of the magnetic field as
extrapolated from the MDI magnetogram, in and outside network, in the
two parts. For the average-profile study, we selected 74 individual
cases and derived the average profiles of intensities and Doppler
shifts across the network. Cases with large values of blue shift of
Ne viii were further studied.
Results: The dispersion plots
reveal that the intensities of Si ii and C iv increase from network
boundary to network center in both the bright and dark parts. However,
the intensity of Ne viii shows different trends, namely increasing
in the bright part and decreasing in the dark part. In both parts,
the Doppler shift of C iv increases steadily from internetwork to
network center. The height variations in the magnetic field imply a
more homogeneous magnetic structure at greater heights and clearly
reflect the different magnetic structures in the two regions. The
average-profile study reveals that the intensities of the three lines
all decline from the network center to internetwork region. The binned
intensities of Si ii and Ne viii have a good correlation. We also find
that the large blue shift of Ne viii does not coincide with large red
shift of C iv.
Conclusions: Our results suggest that the network
structure is still prominent at the layer where Ne viii is formed in
the quiet Sun, and that the magnetic structures expand more strongly
in the dark part than in the bright part of this quiet Sun region. Our
results might also hint for a scenario of magnetic reconnection between
open funnels and side loops.
Title: Long-period oscillations in solar coronal bright points
Authors: Tian, H.; Xia, L. -D.; Li, S.
Bibcode: 2008A&A...489..741T
Altcode:
Aims: By analyzing TRACE (Transition Region and Coronal Explorer)
data, we investigate intensity oscillations in coronal bright points
(BPs) as well as the underlying chromosphere.
Methods: A Fourier
analysis is applied to a sequence of images observed in the 195 Å
and 1216 Å passbands of TRACE. We produce images of Fourier power in
three different period ranges (4-16 min, 20-40 min, 42-75 min). The
presence of periodic oscillations are further checked by applying
a wavelet analysis to a selected rectangular region in each of the
three BPs.
Results: We find a clear signature of oscillations
with different periods dominating in different strands of BP loops
in the 195 Å passband. The oscillatory power is stronger in BPs than
in the surrounding quiet Sun. In the chromospheric 1216 Å passband,
oscillations in these period ranges are found mainly above the magnetic
network elements, which are not necessarily associated with the coronal
BPs. The wavelet analyses reveal that oscillations with periods ranging
from 8 to 64 min can last for several cycles in both the BPs and the
underlying chromosphere.
Conclusions: It is still unclear whether
these oscillations are caused by propagating magneto-acoustic waves
in loop systems associated with the BPs, or result from recurrent
magnetic reconnection powering the BPs.
Title: Plasma Flows Guided by Strong Magnetic Fields in the Solar
Corona
Authors: Marsch, Eckart; Tian, Hui; Sun, Jian; Curdt, Werner;
Wiegelmann, Thomas
Bibcode: 2008ApJ...685.1262M
Altcode:
In this study new results are presented regarding the relationships
between the coronal magnetic field and the intensities and Doppler
shifts of ultraviolet emission lines. This combination of magnetic
field and spectroscopic data is used here to study material flows in
association with the coronal field. We introduce the term "coronal
circulation" to describe this flow, and to indicate that the plasma is
not static but flows everywhere in the extended solar atmosphere. The
blueshifts and redshifts often seen in transition region and coronal
ultraviolet emission lines are interpreted as corresponding to upflows
and downflows of the plasma on open (funnels) and closed (loops) coronal
magnetic field lines, which tightly confine and strongly lead the flows
in the low-beta plasma. Evidence for these processes exists in the
ubiquitous redshifts mostly seen at both legs of loops on all scales,
and the sporadic blueshifts occurring in strong funnels. Therefore,
there is no static magnetically stratified plasma in the corona, since
panta rhei, but rather a continuous global plasma circulation, being
the natural perpetuation of photospheric convection which ultimately
is the driver.
Title: Network oscillations at the boundary of an equatorial
coronal hole
Authors: Tian, H.; Xia, L. -D.
Bibcode: 2008A&A...488..331T
Altcode: 2009arXiv0907.0818T
Aims: We investigate intensity oscillations observed simultaneously in
the quiet chromosphere and in the corona, above an enhanced network
area at the boundary of an equatorial coronal hole.
Methods:
A Fourier analysis is applied to a sequence of images observed in
the 171 Å and 1600 Å passbands of TRACE. Four interesting features
above the magnetic network are further investigated by using a
wavelet analysis.
Results: Our results reveal that, in both
the 171 Å and 1600 Å passbands, oscillations above the magnetic
network show a lack of power at high frequencies (5.0-8.3 mHz), and
a significant power at low (1.3-2.0 mHz) and intermediate frequencies
(2.6-4.0 mHz). The global 5-min oscillation is clearly present in the
4 analyzed features when seen in the 1600 Å passband, and is also
found with enhanced power in feature 1 (leg of a large coronal loop)
and feature 2 (legs of a coronal bright point loop) when seen in
the 171 Å passband. Two features above an enhanced network element
(feature 3 and feature 4) show repeated propagating behaviors with a
dominant period of 10 min and 5 min, respectively.
Conclusions:
We suggest these oscillations are likely to be slow magneto-acoustic
waves propagating along inclined magnetic field lines, from the lower
solar atmosphere into the corona. The energy flux carried by these waves
is estimated of the order of 40~erg~ cm-2~s-1
for the 171 Å passband and is far lower than the energy required to
heat the quiet corona. For the 1600 Å passband, the energy flux is
about 1.4×10^6~erg~ cm-2~s-1, which is about
one third of the required energy budget for the chromosphere.
Title: Modeling the UV/EUV Spectrum with SRPM
Authors: Haberreiter, M.; Fontenla, J.; Curdt, W.; Tian, H.
Bibcode: 2008ESPM...12.3.12H
Altcode:
We present a new set of semi-empirical solar atmosphere structures
for different features on the solar disk that is consistent with the
latest inter- network model by Fontenla et al. (2007), which reproduces
the CO lines in the IR as well as the UV and radio continua. The
intensity distribution of SUMER observations is used to define a set
of solar disk features that accounts for the observed range of upper
chromospheric heating. Using these models the UV/EUV (as well as longer
wavelengths) radiance spectra are computed with SRPM in full-NLTE using
the latest available atomic data and considering 24 elements up to high
ionization stages with a total of approximately 14000 atomic levels
and approximately 170000 line transitions. The radiance spectra of
the new atmosphere structures allow us to study in detail the varying
contributions of these features to the solar spectral irradiance. The
synthetic spectra are compared with SUMER data and EUV irradiance
observations obtained with the EVE rocket-instrument during a recent
rocket flight.
Title: The line profile and center-to-limb variation of quiet-Sun
Lyman-alpha emission
Authors: Curdt, W.; Tian, H.; Teriaca, L.; Schühle, U.; Lemaire, P.
Bibcode: 2008ESPM...12.2.91C
Altcode:
We study the emission of the hydrogen Lyman-alpha line in the quiet
Sun, its center-to-limb variation, and its radiance distribution, which
we also compare to the Lyman-beta line. We use the high spectral and
spatial resolution of the SUMER spectrometer and take raster scans at
various locations on the disk. For the first time, we have used a new
method to reduce the incoming photon flux to a 20%-level by partly
closing the aperture door. We also performed a quasi-simultaneous
observation of both Ly-a and Ly-b at Sun centre in sit-and-stare
mode. We deduce the flow characteristic in Ly-a map from variations of
the calibrated Si III line centroids. We present the average profile
of Ly-a, its radiance distribution, its center-to-limb behaviour,
and the signature of flows on the line profiles. Different from all
other lines of the Lyman series, which have a red-horn asymmetry, Ly-a
has a robust and dominating blue-horn asymmetry. To our knowledge,
this result is only predicted by models which include flows.
Title: Cool and Hot Components of a Coronal Bright Point
Authors: Tian, H.; Curdt, W.; Marsch, E.; He, J. -S.
Bibcode: 2008ESPM...122.113T
Altcode:
We performed a systematic study of the Doppler shifts and electron
densities measured in an EUV bright point (hereafter BP) observed in
more than 10 EUV lines with formation temperatures from log(T=K) =
4.5 to 6.3. Those parts of a BP seen in transition region and coronal
lines are defined as its cool and hot components, respectively. We find
that the transition from cool to hot occurs at a temperature around
log(T=K) = 5.7. The two components of the BP reveal a totally different
orientation and Doppler-shift pattern, which might result from a twist
of the associated magnetic loop system. The analysis of magnetic-field
evolution and topology seems to favour a two-stage heating process,
in which magnetic cancelation and separator reconnection are powering,
respectively, the cool and hot components of the BP. We also found
that the electron densities of both components of the BP are higher
than those of the surrounding quiet Sun, and comparable to or smaller
than active-region densities.
Title: The Redshifted Network Contrast - Signature of Coronal
Convectio
Authors: Curdt, W.; Tian, H.; Dwivedi, B. N.; Marsch, E.
Bibcode: 2008ESPM...12.2.92C
Altcode:
Observations and interpretations of red- and/or blueshifted emission
lines from cosmic objects are crucial to understanding the physical
processes at work there. The ubiquituous net redshifts of solar
transition region (TR) emission lines are known since the Skylab
era. Brekke et al. and Chae et al. independently verified this result
by analyzing the high spectral resolution observations from the
SUMER/SoHO instrument. Both these groups found similar results for
the quantitative dependence of the net redshift on line formation
temperatures from 30 000 K to 1 MK. To our knowledge, however, a
satisfactory physical explanation of the redshift is still a matter
of debate. We present a new method to explain the TR red shift, which
is based on the radiance-redshift relationship, and which employs the
redshift of the network contrast as compared to the position of the
emission line itself. In contrast to the earlier work, our indirect
method is unique in several ways namely, (i) it does not require
a good wavelength calibration and thus avoids related problems, (ii) it is independent of the knowledge of the exact rest wavelength,
and (iii) it intimately relies on the physical processes in the
solar atmosphere.
Title: Electron trapping around a magnetic null
Authors: He, J. -S.; Zong, Q. -G.; Deng, X. -H.; Tu, C. -Y.; Xiao,
C. -J.; Wang, X. -G.; Ma, Z. -W.; Pu, Z. -Y.; Lucek, E.; Pedersen,
A.; Fazakerley, A.; Cornilleau-Wehrlin, N.; Dunlop, M. W.; Tian, H.;
Yao, S.; Tan, B.; Fu, S. -Y.; Glassmeier, K. -H.; Reme, H.; Dandouras,
I.; Escoubet, C. P.
Bibcode: 2008GeoRL..3514104H
Altcode:
Magnetic reconnection is an important process in astrophysical,
space and laboratory plasmas. The magnetic null pair structure is
theoretically suggested to be a crucial feature of the three-dimensional
magnetic reconnection. The physics around the null pair, however, has
not been explored in combination with the magnetic field configuration
deduced from in situ observations. Here, we report the identification of
the configuration around a null pair and simultaneous electron dynamics
near one null of the pair, observed by four Cluster spacecraft in the
geo-magnetotail. Further, we propose a new scenario of electron dynamics
in the null region, suggesting that electrons are temporarily trapped
in the central reconnection region including electron diffusion region
resulting in an electron density peak, accelerated possibly by parallel
electric field and electron pressure gradient, and reflected from the
magnetic cusp mirrors leading to the bi-directional energetic electron
beams, which excite the observed high frequency electrostatic waves.
Title: Cool and Hot Components of a Coronal Bright Point
Authors: Tian, Hui; Curdt, Werner; Marsch, Eckart; He, Jiansen
Bibcode: 2008ApJ...681L.121T
Altcode: 2009arXiv0906.3005T
We performed a systematic study of the Doppler shifts and electron
densities measured in an EUV bright point (hereafter BP) observed in
more than 10 EUV lines with formation temperatures from log (T/K) =
4.5 to 6.3. Those parts of a BP seen in transition region and coronal
lines are defined as its cool and hot components, respectively. We find
that the transition from cool to hot occurs at a temperature around log
(T/K) = 5.7. The two components of the BP reveal a totally different
orientation and Doppler-shift pattern, which might result from a twist
of the associated magnetic loop system. The analysis of magnetic field
evolution and topology seems to favor a two-stage heating process, in
which magnetic cancellation and separator reconnection are powering,
respectively, the cool and hot components of the BP. We also found
that the electron densities of both components of the BP are higher
than those of the surrounding quiet Sun, and comparable to or smaller
than active region densities.
Title: A magnetic null geometry reconstructed from Cluster spacecraft
observations
Authors: He, J. -S.; Tu, C. -Y.; Tian, H.; Xiao, C. -J.; Wang, X. -G.;
Pu, Z. -Y.; Ma, Z. -W.; Dunlop, M. W.; Zhao, H.; Zhou, G. -P.; Wang,
J. -X.; Fu, S. -Y.; Liu, Z. -X.; Zong, Q. -G.; Glassmeier, K. -H.;
Reme, H.; Dandouras, I.; Escoubet, C. P.
Bibcode: 2008JGRA..113.5205H
Altcode:
This paper reports for the first time the identification of a magnetic
structure around a magnetic null in a magnetic reconnection region
in the magnetotail. Magnetic reconnection is one of the fundamental
processes in astrophysical and solar-terrestrial plasmas. Though
the concept of reconnection has been studied for many years,
the process that really occurs has not been fully revealed by
direct measurements. In particular, the lack of a description of
three-dimensional (3-D) reconnecting magnetic field from observations
makes the task more difficult. The Cluster spacecraft array provide an
opportunity to reconstruct the 3-D magnetic reconnection structure based
on magnetic field vectors simultaneously measured at four positions. The
identification of this structure comes from a new method of analysis
of in situ measurements proposed here. Applying a fitting model of 10
spherical harmonic functions and a Harris current sheet function, plus
a constant field, we reconstruct a 3-D magnetic field configuration
around the magnetic null in an reconnection event observed by Cluster
in the geo-magnetotail.
Title: Sizes of transition-region structures in coronal holes and
in the quiet Sun
Authors: Tian, H.; Marsch, E.; Tu, C. -Y.; Xia, L. -D.; He, J. -S.
Bibcode: 2008A&A...482..267T
Altcode: 2009arXiv0906.3028T
Aims: We study the height variations of the sizes of chromospheric and
transition-region features in a small coronal hole and the adjacent
quiet Sun, considering images of the intensity, Doppler shift, and
non-thermal motion of ultraviolet emission lines as measured by SUMER
(Solar Ultraviolet Measurements by Emitted Radiation), together with
the magnetic field as obtained by extrapolation from photospheric
magnetograms.
Methods: In order to estimate the characteristic
sizes of the different features present in the chromosphere and
transition region, we have calculated the autocorrelation function
for the images as well as the corresponding extrapolated magnetic
field at different heights. The Half Width at Half Maximum (HWHM) of
the autocorrelation function is considered to be the characteristic
size of the feature shown in the corresponding image.
Results:
Our results indicate that, in both the coronal hole and quiet Sun,
the HWHM of the intensity image is larger than that of the images of
Doppler-shift and non-thermal width at any given altitude. The HWHM
of the intensity image is smaller in the chromosphere than in the
transition region, where the sizes of intensity features of lines at
different temperatures are almost the same. But in the upper part of
the transition region, the intensity size increases more strongly with
temperature in the coronal hole than in the quiet Sun. We also studied
the height variations of the HWHM of the magnetic field magnitude B
and its component \vert Bz \vert, and found they are equal
to each other at a certain height below 40 Mm in the coronal hole. The
height variations of the HWHM of \vert Bz/B \vert seem to be
consistent with the temperature variations of the intensity size.
Conclusions: Our results suggest that coronal loops are much lower,
and magnetic structures expand through the upper transition region
and lower corona much more strongly with height in the coronal hole
than in the quiet Sun.
Title: Signature of mass supply to quiet coronal loops
Authors: Tian, H.; Tu, C. -Y.; Marsch, E.; He, J. -S.; Zhou, G. -Q.
Bibcode: 2008A&A...478..915T
Altcode: 2009arXiv0906.3007T
Aims:The physical implication of large blue shift of Ne viii in the
quiet Sun region is investigated in this paper.
Methods: We
compare the significant Ne viii blue shifts, which are visible as large
blue patches on the Doppler-shift map of a middle-latitude quiet-Sun
region observed by SUMER, with the coronal magnetic-field structures
as reconstructed from a simultaneous photospheric magnetogram by means
of a force-free-field extrapolation.
Results: We show for the
first time that coronal funnels also exist in the quiet Sun. The
region studied contains several small funnels that originate from
network lanes, expand with height and finally merge into a single wide
open-field region. However, the large blue shifts of the Ne viii line
are not generally associated with funnels. A comparison between the
projections of coronal loops onto the solar x{-}y-plane and the Ne
viii dopplergram indicates that there are some loops that reveal large
Ne viii blue shifts in both legs, and some loops with upflow in one
and downflow in the other leg.
Conclusions: Our results suggest
that strong plasma outflow, which can be traced by large Ne viii blue
shift, is not necessarily associated with the solar wind originating
in coronal funnels but appears to be a signature of mass supply to
coronal loops. Under the assumption that the measured Doppler shift
of the Ne viii line represents the real outflow velocity of the neon
ions being markers of the proton flow, we estimate the mass supply
rate to coronal loops to be about 1034 s-1.
Title: Sizes and heights of magnetic structures in the solar
transition region as observed in ultraviolet emission lines at
different temperatures
Authors: Marsch, Eckart; Tian, Hui; Tu, Chuanyi; He, Jiansen; Zhou,
G. -Q.; Xia, Lidong
Bibcode: 2008cosp...37.1922M
Altcode: 2008cosp.meet.1922M
We investigate the height variations of the sizes of transition-region
(TR) features in a small coronal hole and the adjacent quiet Sun,
thereby considering images of the intensity, Doppler shift, and
non-thermal motion of ultraviolet emission lines as measured by SUMER
(Solar Ultraviolet Measurements by Emitted Radiation), together with
the magnetic field as obtained by extrapolation from photospheric
magnetograms. In order to estimate the sizes of different features
present in the upper chromosphere and TR, we have calculated the
autocorrelation function for the images as well as the corresponding
extrapolated magnetic field at different heights. The half width at
half maximum of the autocorrelation function is considered as the
characteristic size of a feature shown in the corresponding image, and
found in the intensity image to be smaller in the upper chromosphere
than the TR. Through a correlation analysis of the emission pattern
with the magnetic field (network and carpet of loops), the so-called
correlation height of the emission can be determined. A detailed
comparison is made of the coronal magnetic field obtained by
extrapolation with the radiances of many ultraviolet lines, which
are emitted by ions of various elements in different ionization
stages, corresponding to different local coronal temperatures. At
mesoscopic scales of several megameters the regions with strong emission
(originating from multiple small closed loops) are found to be located
at low heights, whereas weak emissions (coming from locally open,
i.e. far reaching fields) appear to originate at greater heights. Our
analysis confirms the notion that plasma at different temperature can
coexist at the same height, and that the TR is not thermally stratified
but strongly nonuniform and magnetically structured.
Title: A model of the solar wind driven by supergranular circulation
Authors: Tu, Chuanyi; Tian, Hui; He, Jiansen; Marsch, Eckart
Bibcode: 2008cosp...37.3238T
Altcode: 2008cosp.meet.3238T
The scenario for the origin of the solar wind driven by the
supergranular circulation as suggested by Tu et al. (2005) is tested
with model calculation and data analysis. This scenario assumes that the
fast solar wind originates at heights above 5 Mm in magnetically open
funnels in corona holes. Mass and energy are supplied to the solar wind
through reconnection of open field lines in the funnels with closed
loops advected by the supergranular circulation to the reconnection
sites. To test this scenario, we developed a one-dimensional one-fluid
model with mass flux and energy flux supplied at 5 Mm to mimic the
result expected from field-line reconnections. The upward flow and
the downward flow resulting from this model are shown to be consistent
with the blueshift of Ne VIII and redshift of Si II observed by SUMMER
on SOHO. The mass and energy supply rates required by the model are
shown to be consistent with the mass and energy delivery rates due to
reconnection between magnetic loops in the intra-network region and open
magnetic funnel at the network intersections. The model calculations
support the scenario of the solar wind being driven by supergranular
circulation. A discussion of a possible mechanism for the solar wind
origin in the quiet sun is also given, where the solar wind is suggested
to start flowing outward at a height of 20 Mm, which is higher than the
emission height of Ne VIII ( 5 Mm). We found that Ne VIII blueshifts
can occur at both legs of some closed loops, which suggests that mass
can be supplied upward to the corona from both footpoints. Tu, C.-Y.,
Zhou, C., Marsch, E., Xia, L.-D., Zhao, L., Wang, J.-X., and Wilhelm,
K., Solar wind origin in coronal funnels, Science, 308, 519, 2005.
Title: Boundary layer effects above a Himalayan valley near Mount
Everest
Authors: Sun, Fanglin; Ma, Yaoming; Li, Maoshan; Ma, Weiqiang; Tian,
Hui; Metzger, Stefan
Bibcode: 2007GeoRL..34.8808S
Altcode:
Periodical Wind Profiler and Radio Acoustic Sounding System observations
have been commenced at the Himalayas' northern slope nearby Mount
Everest in September 2005. Primarily data sets obtained 25 km remote
from the glacier edge are utilized for a preliminary discussion
of planetary boundary layer circulation resembling high alpine
mountainous regions. Substantial findings include the detection of
two wind shears and the phenomenon of glacier wind at a distance of
25 km from the glaciers. The latter lead to a reversed compensatory
flow in a vertical scale of up to 2000 m above ground level, pointing
at supra regional impact.
Title: Emission heights of coronal bright points on Fe XII radiance
map
Authors: Tian, H.; Tu, C. -Y.; He, J. -S.; Marsch, E.
Bibcode: 2007AdSpR..39.1853T
Altcode: 2009arXiv0907.4954T
The study of coronal bright points (BPs) is important for understanding
coronal heating and the origin of the solar wind. Previous studies
indicated that coronal BPs have a highly significant tendency to
coincide with magnetic neutral lines in the photosphere. Here we further
studied the emission heights of the BPs above the photosphere in the
bipolar magnetic loops that are apparently associated with them. As BPs
are seen in projection against the disk their true emission heights are
unknown. The correlation of the BP locations on the Fe XII radiance
map from EIT with the magnetic field features (in particular neutral
lines) was investigated in detail. The coronal magnetic field was
determined by an extrapolation of the photospheric field (derived from
2-D magnetograms obtained from the Kitt Peak observatory) to different
altitudes above the disk. It was found that most BPs sit on or near a
photospheric neutral line, but that the emission occurs at a height
of about 5 Mm. Some BPs, while being seen in projection, still seem
to coincide with neutral lines, although their emission takes place
at heights of more than 10 Mm. Such coincidences almost disappear
for emissions above 20 Mm. We also projected the upper segments of
the 3-D magnetic field lines above different heights, respectively,
on to the tangent x- y plane, where x is in the east-west and y in
the south-north direction. The shape of each BP was compared with the
respective field-line segment nearby. This comparison suggests that
most coronal BPs are actually located on the top of their associated
magnetic loops. Finally, we calculated for each selected BP region the
correlation coefficient between the Fe XII intensity enhancement and
the horizontal component of the extrapolated magnetic field vector at
the same x- y position in planes of different heights, respectively. We
found that for almost all the BP regions we studied the correlation
coefficient, with increasing height, increases to a maximal value and
then decreases again. The height corresponding to this maximum was
defined as the correlation height, which for most bright points was
found to range below 20 Mm.
Title: a Study on the Height of the Corona Bright Points on FeXII
Radiance Map
Authors: Tian, H.; Tu, C. -Y.; He, J. -S.; Marsch, E.
Bibcode: 2006cosp...36.1952T
Altcode: 2006cosp.meet.1952T
By registering SOHO EIT FeXII -19 5 nm coronal images with Kitt Peak
magnetograms D A Falconer et al 1998 found that the brightest features
in the network lanes which they called network coronal bright points
had a highly significant coincidence with magnetic network neutral lines
They found that most bright points sit on neutral lines in the network
magnetic flux We further studied the height information of the bright
points on the FeXII radiance map by comparing the individual pattern
of the coronal bright points with the distribution of the top segments
of the loops from the 3-D magnetic field force-free extrapolated from
the photospheric magnetogram If we assume the source regions of the
bright points are all on the top of the loops then the height of bright
points of FeXII radiance is between 5Mm-10Mm