92 research outputs found
On the extrapolation of magneto-hydro-static equilibria on the sun
Modeling the interface region between solar photosphere and corona is
challenging, because the relative importance of magnetic and plasma forces
change by several orders of magnitude. While the solar corona can be modeled by
the force-free assumption, we need to take care about plasma forces (pressure
gradient and gravity) in photosphere and chromosphere, here within the
magneto-hydro-static (MHS) model. We solve the MHS equations with the help of
an optimization principle and use vector magnetogram as boundary condition.
Positive pressure and density are ensured by replacing them with two new basic
variables. The Lorentz force during optimization is used to update the plasma
pressure on the bottom boundary, which makes the new extrapolation works even
without pressure measurement on the photosphere. Our code is tested by using a
linear MHS model as reference. From the detailed analyses, we find that the
newly developed MHS extrapolation recovers the reference model at high
accuracy. The MHS extrapolation is, however, numerically more expensive than
the nonlinear force-free field (NLFFF) extrapolation and consequently one
should limit their application to regions where plasma forces become important,
e.g. in a layer of about 2 Mm above the photosphere.Comment: accepted for publication in Ap
Magnetic reconnection at the earliest stage of solar flux emergence
On 2016 September 20, the Interface Region Imaging Spectrograph observed an
active region during its earliest emerging phase for almost 7 hours. The
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory
observed continuous emergence of small-scale magnetic bipoles with a rate of
10 Mx~s. The emergence of magnetic fluxes and interactions
between different polarities lead to frequent occurrence of ultraviolet (UV)
bursts, which exhibit as intense transient brightenings in the 1400 \AA{}
images. In the meantime, discrete small patches with the same magnetic polarity
tend to move together and merge, leading to enhancement of the magnetic fields
and thus 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 red shifts of
100~km~s 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
10 Mx~s. We also investigate the three-dimensional magnetic
field topology through a magneto-hydrostatic 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.Comment: ApJ, 10 figure
Does the non-force-freeness matter for the extrapolation of solar magnetic field?
Magnetic field extrapolation is a fundamental tool to reconstruct the
three-dimensional solar coronal magnetic field. However, the prevalently used
force-free field model might not be applicable in the lower atmosphere, where
plasma \b{eta} is greater than 1. In this work, we perform extrapolation in
active region 12158, based on an updated magnetohydrostatic (MHS) method. By
comparing the results with those from the force-free field method of
Current-Field Iteration in Spherical Coordinates (CFITS), we find that the
overall properties, which are characterized by the magnetic free energy and
helicity, are roughly the same after volume integral. The major differences lie
in the magnetic configuration and the twist number of magnetic flux rope (MFR).
A coherent MFR with twist around 1 is reproduced from CFITS. In another manner,
two sets of MFR, which are highly twisted and slightly coupled, are derived by
the MHS method. The latter one is better constrained by the high-resolution
observations, such as the filament fibrils, pre-eruptive braiding
characteristics and the eruptive double-J shaped hot channel. Overall, our work
shows the MHS method is more promising to reproduce the magnetic fine
structures that can well match the observations not only in the chromosphere
but also in the corona. This initiates the necessity of reconsidering the
simplification of low atmosphere for currently widely used nonlinear force-free
extrapolation method, since such assumption will not only omit the magnetic
structures at low atmosphere but also affect those obtained in the corona, and
therefore bringing in ambiguity in interpreting the solar eruption.Comment: 19 pages, 6 figures, accepted by Ap
Solar Ultraviolet Bursts in a Coordinated Observation of IRIS, Hinode and SDO
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 {\AA} 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 20 km s, 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}.Comment: will appear in the journal of Science China Technological Science
The high helium abundance and charge states of the interplanetary CME and its material source on the Sun
Identifying the source of the material within coronal mass ejections (CMEs)
and understanding CME onset mechanisms are fundamental issues in solar and
space physics. Parameters relating to plasma composition, such as charge states
and He abundance (\ahe), may be different for plasmas originating from
differing processes or regions on the Sun. Thus, it is crucial to examine the
relationship between in-situ measurements of CME composition and activity on
the Sun. We study the CME that erupted on 2014 September 10, in association
with an X1.6 flare, by analyzing AIA imaging and IRIS spectroscopic
observations and its in-situ signatures detected by Wind and ACE. We find that
during the slow expansion and intensity increase of the sigmoid, plasma
temperatures of 9 MK, and higher, first appear at the footpoints of the
sigmoid, associated with chromospheric brightening. Then the high-temperature
region extends along the sigmoid. IRIS observations confirm that this extension
is caused by transportation of hot plasma upflow. Our results show that
chromospheric material can be heated to 9 MK, and above, by chromospheric
evaporation at the sigmoid footpoints before flare onset. The heated
chromospheric material can transport into the sigmoidal structure and supply
mass to the CME. The aforementioned CME mass supply scenario provides a
reasonable explanation for the detection of high charge states and elevated
\ahe\ in the associated ICME. The observations also demonstrate that the
quasi-steady evolution in the precursor phase is dominated by magnetic
reconnection between the rising flux rope and the overlying magnetic field
structure.Comment: 10 pages, 5 figures, accepted for publication in ApJ
An Integrative Framework for Bayesian Variable Selection with Informative Priors for Identifying Genes and Pathways
The discovery of genetic or genomic markers plays a central role in the development of personalized medicine. A notable challenge exists when dealing with the high dimensionality of the data sets, as thousands of genes or millions of genetic variants are collected on a relatively small number of subjects. Traditional gene-wise selection methods using univariate analyses face difficulty to incorporate correlational, structural, or functional structures amongst the molecular measures. For microarray gene expression data, we first summarize solutions in dealing with ‘large p, small n’ problems, and then propose an integrative Bayesian variable selection (iBVS) framework for simultaneously identifying causal or marker genes and regulatory pathways. A novel partial least squares (PLS) g-prior for iBVS is developed to allow the incorporation of prior knowledge on gene-gene interactions or functional relationships. From the point view of systems biology, iBVS enables user to directly target the joint effects of multiple genes and pathways in a hierarchical modeling diagram to predict disease status or phenotype. The estimated posterior selection probabilities offer probabilitic and biological interpretations. Both simulated data and a set of microarray data in predicting stroke status are used in validating the performance of iBVS in a Probit model with binary outcomes. iBVS offers a general framework for effective discovery of various molecular biomarkers by combining data-based statistics and knowledge-based priors. Guidelines on making posterior inferences, determining Bayesian significance levels, and improving computational efficiencies are also discussed
A PLSPM-Based Test Statistic for Detecting Gene-Gene Co-Association in Genome-Wide Association Study with Case-Control Design
For genome-wide association data analysis, two genes in any pathway, two SNPs in the two linked gene regions respectively or in the two linked exons respectively within one gene are often correlated with each other. We therefore proposed the concept of gene-gene co-association, which refers to the effects not only due to the traditional interaction under nearly independent condition but the correlation between two genes. Furthermore, we constructed a novel statistic for detecting gene-gene co-association based on Partial Least Squares Path Modeling (PLSPM). Through simulation, the relationship between traditional interaction and co-association was highlighted under three different types of co-association. Both simulation and real data analysis demonstrated that the proposed PLSPM-based statistic has better performance than single SNP-based logistic model, PCA-based logistic model, and other gene-based methods
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