886 research outputs found
On Hydrodynamic Motions in Dead Zones
We investigate fluid motions near the midplane of vertically stratified
accretion disks with highly resistive midplanes. In such disks, the
magnetorotational instability drives turbulence in thin layers surrounding a
resistive, stable dead zone. The turbulent layers in turn drive motions in the
dead zone. We examine the properties of these motions using three-dimensional,
stratified, local, shearing-box, non-ideal, magnetohydrodynamical simulations.
Although the turbulence in the active zones provides a source of vorticity to
the midplane, no evidence for coherent vortices is found in our simulations. It
appears that this is because of strong vertical oscillations in the dead zone.
By analyzing time series of azimuthally-averaged flow quantities, we identify
an axisymmetric wave mode particular to models with dead zones. This mode is
reduced in amplitude, but not suppressed entirely, by changing the equation of
state from isothermal to ideal. These waves are too low-frequency to affect
sedimentation of dust to the midplane, but may have significance for the
gravitational stability of the resulting midplane dust layers.Comment: 36 pages, 19 figures. ApJ accepte
Multiwavelength Observations of a Flux Rope Formation by Series of Magnetic Reconnection in the Chromosphere
Using high-resolution observations from the 1.6 m New Solar Telescope (NST)
operating at the Big Bear Solar Observatory (BBSO), we report direct evidence
of merging/reconnection of cool H loops in the chromosphere during two
homologous flares (B- and C-class) caused by a shear motion at the footpoint of
two loops. The reconnection between these loops caused the formation of an
unstable flux rope which showed counterclockwise rotation. The flux rope could
not reach the height of torus instability and failed to form a coronal mass
ejection. The HMI magnetograms revealed rotation of the negative/positive
(N1/P2) polarity sunspots in the opposite directions, which increased the right
and left-handed twist in the magnetic structures rooted at N1/P2. Rapid
photospheric flux cancellation (duration20-30 min,
rate3.4410 Mx h) was observed during and even
after the first B6.0 flare and continued until the end of the second C2.3
flare. The RHESSI X-ray sources were located at the site of the loop's
coalescence. To the best of our knowledge, such a clear interaction of
chromospheric loops along with rapid flux cancellation has not been reported
before. These high-resolution observations suggest the formation of a small
flux rope by a series of magnetic reconnection within chromospheric loops
associated with very rapid flux cancellation.Comment: A&A, in press, 12 pages, 12 figure
Bending fatigue tests on SiC-Al tapes under alternating stress at room temperature
The development of a testing method for fatigue tests on SiC-Al tapes containing a small amount of SiC filaments under alternating stress is reported. The fatigue strength curves resulting for this composite are discussed. They permit an estimate of its behavior under continuous stress and in combination with various other matrices, especially metal matrices
Universal classification of twisted, strained and sheared graphene moir\'e superlattices
Moir\'e superlattices in graphene supported on various substrates have opened
a new avenue to engineer graphene's electronic properties. Yet, the exact
crystallographic structure on which their band structure depends remains highly
debated. In this scanning tunneling microscopy and density functional theory
study, we have analysed graphene samples grown on multilayer graphene prepared
onto SiC and on the close-packed surfaces of Re and Ir with ultra-high
precision. We resolve small-angle twists and shears in graphene, and identify
large unit cells comprising more than 1,000 carbon atoms and exhibiting
non-trivial nanopatterns for moir\'e superlattices, which are commensurate to
the graphene lattice. Finally, a general formalism applicable to any hexagonal
moir\'e is presented to classify all reported structures.Comment: 14 pages, 6 figure
The Initiation Mechanism of the First On-disk X-Class Flare of Solar Cycle 25
In this paper we study the initiation mechanism of the first on-disk X-class
eruptive flare in solar cycle 25. Coronal magnetic field reconstructions reveal
a magnetic flux rope (MFR) with configuration highly consistent with a filament
existing for a long period before the flare, and the eruption of the whole
filament indicates that the MFR erupted during the flare. However, quantitative
analysis shows that the pre-flare MFR resides in a height too low to trigger a
torus instability (TI). The filament experienced a slow rise before the flare
onset, for which we estimate evolution of the filament height using a
triangulation method by combining the SDO and STEREO observations, and find it
is also much lower than the critical height for triggering TI. On the other
hand, the pre-flare evolution of the current density shows progressive thinning
of a vertical current layer on top of the flare PIL, which suggests that a
vertical current sheet forms before the eruption. Meanwhile, there is
continuously shearing motion along the PIL under the main branch of the
filament, which can drive the coronal field to form such a current sheet. As
such, we suggest that the event follows a reconnection-based initiation
mechanism as recently established using a high-accuracy MHD simulation, in
which an eruption is initiated by reconnection in a current sheet that forms
gradually within continuously-sheared magnetic arcade. The eruption should be
further driven by TI as the filament quickly rises into the TI domain during
the eruption
An evaluation of sea ice deformation and its spatial characteristics from the regional arctic system model
The Regional Arctic System Model (RASM) is used to investigate the process and frequency of extreme sea ice shear deformation events resulting in pycnocline upwelling due to Ekman pumping as described in McPhee et al. (2005). RASM is a fully coupled land, atmosphere, sea ice, and ocean model with high spatial and temporal resolution. Time series analysis of the upper ocean temperature structure, basal melt rate, total deformation rate, and ice-ocean stress curl yield the identification of individual events. Shear deformation events generate an upper ocean response given a positive ice-ocean stress curl, i.e., induced by counterclockwise rotation in the ice velocity field relative to the underlying ocean. Spatial and temporal characterization of the total deformation rate indicates that fine spatial and temporal resolution, on a statistical scale, is important for the energy budget of the Arctic. Results demonstrate a power law relationship between the mean deformation rate and length scale. This is hypothesized as being due to RASMs fully coupled system allowing for naturally occurring high frequency noise and the cascade of energy among model components. Simulated events are infrequent their relative impact on large scale energy exchange remains undetermined, which warrants further research of these phenomena.http://archive.org/details/anevaluationofse1094527872Lieutenant, United States NavyApproved for public release; distribution is unlimited
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