23,941 research outputs found
Metopolophium festucae cerealium (Hemiptera: Aphididae) : a new addition to the aphid fauna of North America
Metopolophium festucae cerealium (Stroyan) (Hemiptera: Aphididae) was found in wheat fields in the Pacific Northwest in 2011 and 2012. This is the first record of M. f. cerealium in North America. This subspecies can be a serious pest of cereal crops
Propagation of the 2012 March Coronal Mass Ejections from the Sun to Heliopause
In 2012 March the Sun exhibited extraordinary activities. In particular, the
active region NOAA AR 11429 emitted a series of large coronal mass ejections
(CMEs) which were imaged by STEREO as it rotated with the Sun from the east to
west. These sustained eruptions are expected to generate a global shell of
disturbed material sweeping through the heliosphere. A cluster of shocks and
interplanetary CMEs (ICMEs) were observed near the Earth, and are propagated
outward from 1 AU using an MHD model. The transient streams interact with each
other, which erases memory of the source and results in a large merged
interaction region (MIR) with a preceding shock. The MHD model predicts that
the shock and MIR would reach 120 AU around 2013 April 22, which agrees well
with the period of radio emissions and the time of a transient disturbance in
galactic cosmic rays detected by Voyager 1. These results are important for
understanding the "fate" of CMEs in the outer heliosphere and provide
confidence that the heliopause is located around 120 AU from the Sun.Comment: 13 pages, 5 figures, accepted for publication in ApJ Letter
Elastic metamaterials with simultaneously negative effective shear modulus and mass density
We propose a type of elastic metamaterial comprising fluid-solid composite
inclusions which can possess negative shear modulus and negative mass density
over a large frequency region. Such a solid metamaterial has a unique elastic
property that only transverse waves can propagate with a negative dispersion
while longitudinal waves are forbidden. This leads to many interesting
phenomena such as negative refraction, which is demonstrated by using a wedge
sample, and a significant amount of mode conversion from transverse waves to
longitudinal waves that cannot occur on the interface of two natural solids
Non-LTE Calculations of the Fe I 6173 {\AA} Line in a Flaring Atmosphere
The Fe I 6173 {\AA} line is widely used in the measurements of vector
magnetic fields by instruments including the Helioseismic and Magnetic Imager
(HMI). We perform non-local thermodynamic equilibrium calculations of this line
based on radiative hydrodynamic simulations in a flaring atmosphere. We employ
both a quiet-Sun atmosphere and a penumbral atmosphere as the initial one in
our simulations. We find that, in the quiet-Sun atmosphere, the line center is
obviously enhanced during an intermediate flare. The enhanced emission is
contributed from both radiative backwarming in the photosphere and particle
beam heating in the lower chromosphere. A blue asymmetry of the line profile
also appears due to an upward mass motion in the lower chromosphere. If we take
a penumbral atmosphere as the initial atmosphere, the line has a more
significant response to the flare heating, showing a central emission and an
obvious asymmetry. The low spectral resolution of HMI would indicate some loss
of information but the enhancement and line asymmetry are still kept. By
calculating polarized line profiles, we find that the Stokes I and V profiles
can be altered as a result of flare heating. Thus the distortion of this line
has a crucial influence on the magnetic field measured from this line, and one
should be cautious in interpreting the magnetic transients observed frequently
in solar flares.Comment: 12 pages, 5 figures, accepted by ApJ
Symmetry protected fractional Chern insulators and fractional topological insulators
In this paper we construct fully symmetric wavefunctions for the
spin-polarized fractional Chern insulators (FCI) and time-reversal-invariant
fractional topological insulators (FTI) in two dimensions using the parton
approach. We show that the lattice symmetry gives rise to many different FCI
and FTI phases even with the same filling fraction (and the same
quantized Hall conductance in FCI case). They have different
symmetry-protected topological orders, which are characterized by different
projective symmetry groups. We mainly focus on FCI phases which are realized in
a partially filled band with Chern number one. The low-energy gauge groups of a
generic FCI wavefunctions can be either or
the discrete group , and in the latter case the associated low-energy
physics are described by Chern-Simons-Higgs theories. We use our construction
to compute the ground state degeneracy. Examples of FCI/FTI wavefunctions on
honeycomb lattice and checkerboard lattice are explicitly given. Possible
non-Abelian FCI phases which may be realized in a partially filled band with
Chern number two are discussed. Generic FTI wavefunctions in the absence of
spin conservation are also presented whose low-energy gauge groups can be
either or . The constructed wavefunctions
also set up the framework for future variational Monte Carlo simulations.Comment: 24 pages, 13 figures, published versio
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