24,942 research outputs found
"EIT Waves" and Coronal Mass Ejections
Coronal "EIT waves" appear as EUV bright fronts propagating across a
significant part of the solar disk. The intriguing phenomenon provoked
continuing debates on their nature and their relation with coronal mass
ejections (CMEs). In this paper, we first summarize all the observational
features of "EIT waves", which should be accounted for by any successful model.
The theoretical models constructed during the past 10 years are then reviewed.
Finally, the implication of the "EIT wave" research to the understanding of
CMEs is discussed. The necessity is pointed out to revisit the nature of CME
frontal loop.Comment: 10 pages, 3 figures, a review paper to be published in the Proc. of
the 1st Asian-Pacific Solar Phys. Meetin
Imaging and Spectroscopic Observations of a Filament Channel and the Implications for the Nature of Counter-streamings
The dynamics of a filament channel are observed with imaging and
spectroscopic telescopes before and during the filament eruption on 2011
January 29. The extreme ultraviolet (EUV) spectral observations reveal that
there is no EUV counterparts of the H counter-streamings in the
filament channel, implying that the ubiquitous H counter-streamings
found by previous research are mainly due to longitudinal oscillations of
filament threads, which are not in phase between each other. However, there
exist larger-scale patchy counter-streamings in EUV along the filament channel
from one polarity to the other, implying that there is another component of
uni-directional flow (in the range of 10 km s) inside each filament
thread in addition to the implied longitudinal oscillation. Our results suggest
that the flow direction of the larger-scale patchy counter-streaming plasma in
the EUV is related to the intensity of the plage or active network, with the
upflows being located at brighter areas of the plage and downflows at the
weaker areas. Besides, we propose a new method to determine the chirality of an
erupting filament based on the skewness of the conjugate filament drainage
sites. It suggests that the right-skewed drainage corresponds to sinistral
chirality, whereas the left-skewed drainage corresponds to dextral chirality.Comment: 17 pages, 10 figures, published in ApJ, title changed slightly from
previous versio
Statistical Analysis of Filament Features Based on the H{\alpha} Solar Images from 1988 to 2013 by Computer Automated Detection Method
We improve our filament automated detection method which was proposed in our
previous works. It is then applied to process the full disk H data
mainly obtained by Big Bear Solar Observatory (BBSO) from 1988 to 2013,
spanning nearly 3 solar cycles. The butterfly diagrams of the filaments,
showing the information of the filament area, spine length, tilt angle, and the
barb number, are obtained. The variations of these features with the calendar
year and the latitude band are analyzed. The drift velocities of the filaments
in different latitude bands are calculated and studied. We also investigate the
north-south (N-S) asymmetries of the filament numbers in total and in each
subclass classified according to the filament area, spine length, and tilt
angle. The latitudinal distribution of the filament number is found to be
bimodal. About 80% of all the filaments have tilt angles within [0{\deg},
60{\deg}]. For the filaments within latitudes lower (higher) than 50{\deg} the
northeast (northwest) direction is dominant in the northern hemisphere and the
southeast (southwest) direction is dominant in the southern hemisphere. The
latitudinal migrations of the filaments experience three stages with declining
drift velocities in each of solar cycles 22 and 23, and it seems that the drift
velocity is faster in shorter solar cycles. Most filaments in latitudes lower
(higher) than 50{\deg} migrate toward the equator (polar region). The N-S
asymmetry indices indicate that the southern hemisphere is the dominant
hemisphere in solar cycle 22 and the northern hemisphere is the dominant one in
solar cycle 23.Comment: 51 pages, 12 tables, 25 figures, accepted for publication in ApJ
Improving Wireless Physical Layer Security via Exploiting Co-Channel Interference
This paper considers a scenario in which a source-destination pair needs to
establish a confidential connection against an external eavesdropper, aided by
the interference generated by another source-destination pair that exchanges
public messages. The goal is to compute the maximum achievable secrecy degrees
of freedom (S.D.o.F) region of a MIMO two-user wiretap network. First, a
cooperative secrecy transmission scheme is proposed, whose feasible set is
shown to achieve all S.D.o.F. pairs on the S.D.o.F. region boundary. In this
way, the determination of the S.D.o.F. region is reduced to a problem of
maximizing the S.D.o.F. pair over the proposed transmission scheme. The maximum
achievable S.D.o.F. region boundary points are obtained in closed form, and the
construction of the precoding matrices achieving the maximum S.D.o.F. region
boundary is provided. The obtained analytical expressions clearly show the
relation between the maximum achievable S.D.o.F. region and the number of
antennas at each terminal.Comment: 13 pages and 6 figure
Can a Fast-mode EUV Wave Generate a Stationary Front?
The discovery of stationary "EIT waves" about 16 years ago posed a big
challenge to the then favorite fast-mode wave model for coronal "EIT waves". It
encouraged the proposing of various non-wave models, and played an important
role in approaching the recent converging viewpoint, {\it i.e.} there are two
types of EUV waves. However, it was recently discovered that a stationary wave
front can also be generated when a fast-mode wave passes through a magnetic
quasi-separatrix layer (QSL). In this paper, we perform a magnetohydrodynamic
(MHD) numerical simulation of the interaction between a fast-mode wave and a
magnetic QSL, and a stationary wave front is reproduced. The analysis of the
numerical results indicates that near the plasma beta layer in front
of the magnetic QSL, part of the fast-mode wave is converted to a slow-mode MHD
wave, which is then trapped inside the magnetic loops, forming a stationary
wave front. Our research implies that we have to be cautious in identifying the
nature of a wave since there may be mode conversion during the propagation of
the waves driven by solar eruptions.Comment: 16 pages, 8 figures, accepted for publication in Solar Physic
On the Secrecy Capacity of a MIMO Gaussian Wiretap Channel with a Cooperative Jammer
We study the secrecy capacity of a helper-assisted Gaussian wiretap channel
with a source, a legitimate receiver, an eavesdropper and an external helper,
where each terminal is equipped with multiple antennas. Determining the secrecy
capacity in this scenario generally requires solving a nonconvex secrecy rate
maximization (SRM) problem. To deal with this issue, we first reformulate the
original SRM problem into a sequence of convex subproblems. For the special
case of single-antenna legitimate receiver, we obtain the secrecy capacity via
a combination of convex optimization and one-dimensional search, while for the
general case of multi-antenna legitimate receiver, we propose an iterative
solution. To gain more insight into how the secrecy capacity of a
helper-assisted Gaussian wiretap channel behaves, we examine the achievable
secure degrees of freedom (s.d.o.f.) and obtain the maximal achievable s.d.o.f.
in closed-form. We also derive a closed-form solution to the original SRM
problem which achieves the maximal s.d.o.f.. Numerical results are presented to
illustrate the efficacy of the proposed schemes.Comment: 13 pages, 7 figure
Can We Determine the Filament Chirality by the Filament Footpoint Location or the Barb-bearing?
We attempt to propose a method for automatically detecting the solar filament
chirality and barb bearing. We first introduce the unweighted undirected graph
concept and adopt the Dijkstra shortest-path algorithm to recognize the
filament spine. Then, we use the polarity inversion line (PIL) shift method for
measuring the polarities on both sides of the filament, and employ the
connected components labeling method to identify the barbs and calculate the
angle between each barb and the spine to determine the bearing of the barbs,
i.e., left or right. We test the automatic detection method with H-alpha
filtergrams from the Big Bear Solar Observatory (BBSO) H-alpha archive and
magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board
the Solar Dynamics Observatory (SDO). Four filaments are automatically detected
and illustrated to show the results. The barbs in different parts of a filament
may have opposite bearings. The filaments in the southern hemisphere (northern
hemisphere) mainly have left-bearing (right-bearing) barbs and positive
(negative) magnetic helicity, respectively. The tested results demonstrate that
our method is efficient and effective in detecting the bearing of filament
barbs. It is demonstrated that the conventionally believed one-to-one
correspondence between filament chirality and barb bearing is not valid. The
correct detection of the filament axis chirality should be done by combining
both imaging morphology and magnetic field observations.Comment: 20 pages, 7 figures, accepted for publication in RA
Dependence of the Length of Solar Filament Threads on the Magnetic Configuration
High-resolution H observations indicate that filaments consist of an
assembly of thin threads. In quiescent filaments, the threads are generally
short, whereas in active region filaments, the threads are generally long. In
order to explain these observational features, we performed one-dimensional
radiative hydrodynamic simulations of filament formation along a dipped
magnetic flux tube in the framework of the chromospheric evaporation-coronal
condensation model. The geometry of a dipped magnetic flux tube is
characterized by three parameters, i.e., the depth (), the half-width (),
and the altitude () of the magnetic dip. The parameter survey in the
numerical simulations shows that allowing the filament thread to grow in 5
days, the maximum length () of the filament thread increases linearly
with , and decreases linearly with and . The dependence is fitted
into a linear function Mm. Such a relation can
qualitatively explain why quiescent filaments have shorter threads and active
region filaments have longer threads.Comment: 8 pages, 6 figures, accepted for publication in RA
Variations of the 3-D coronal magnetic field associated with the X3.4-class solar flare event of AR 10930
The variations of the 3-D coronal magnetic fields associated with the
X3.4-class flare of active region 10930 are studied in this paper. The coronal
magnetic field data are reconstructed from the photospheric vector magnetograms
obtained by the Hinode satellite and using the nonlinear force-free field
extrapolation method developed in our previous work (He et al., 2011). The 3-D
force-free factor , 3-D current density, and 3-D magnetic energy
density are employed to analyze the coronal data. The distributions of
and current density reveal a prominent magnetic connectivity with strong
negative values and strong current density before the flare. This
magnetic connectivity extends along the main polarity inversion line and is
found to be totally broken after the flare. The distribution variation of
magnetic energy density reveals the redistribution of magnetic energy before
and after the flare. In the lower space of the modeling volume the increase of
magnetic energy dominates, and in the higher space the decrease of energy
dominates. The comparison with the flare onset imaging observation exhibits
that the breaking site of the magnetic connectivity and site with the highest
values of energy density increase coincide with the location of flare initial
eruption. We conclude that a cramped positive region appearing in the
photosphere causes the breaking of the magnetic connectivity. A scenario for
flare initial eruption is proposed in which the Lorentz force acting on the
isolated electric current at the magnetic connectivity breaking site lifts the
associated plasmas and causes the initial ejection.Comment: 24 pages, 18 figures, 2 table
Solar Filament Longitudinal Oscillations along a Magnetic Field Tube with Two Dips
The large-amplitude longitudinal oscillations of solar filaments have been
observed and explored for more than ten years. Previous studies are mainly
based on the one-dimensional rigid flux tube model with a single magnetic dip.
However, it is noticed that there might be two magnetic dips, and hence two
threads, along one magnetic field line. Following the previous work, we intend
to investigate the kinematics of the filament longitudinal oscillations when
two threads are magnetically connected, which is done by solving
one-dimensional radiative hydrodynamic equations with the numerical code
MPI-AMRVAC. Two different types of perturbations are considered, and the
difference from previous works resulting from the filament thread-thread
interaction is investigated. We find that even with the inclusion of the
thread-thread interaction, the oscillation period is modified weakly, by at
most 20% compared to the traditional pendulum model with one thread. However,
the damping timescale is significantly affected by the thread-thread
interaction. Hence, we should take it into account when applying the consistent
seismology to the filaments where two threads are magnetically connected.Comment: 14 pages, 10 figures, accepted for publication in Ap
- …