5,269 research outputs found
Current systems of coronal loops in 3D MHD simulations
We study the magnetic field and current structure associated with a coronal
loop. Through this we investigate to what extent the assumptions of a
force-free magnetic field break down and where they might be justified. We
analyse a 3D MHD model of the solar corona in an emerging active region with
the focus on the structure of the forming coronal loops. The lower boundary of
this simulation is taken from a model of an emerging active region. As a
consequence of the emerging magnetic flux and the horizontal motions at the
surface a coronal loop forms self-consistently. We investigate the current
density along magnetic field inside (and outside) this loop and study the
magnetic and plasma properties in and around it. We find that the total current
along the loop changes its sign from being antiparallel to parallel to the
magnetic field. This is caused by the inclination of the loop together with the
footpoint motion. Around the loop the currents form a complex non-force-free
helical structure. This is directly related to a bipolar current structure at
the loop footpoints at the base of the corona and a local reduction of the
background magnetic field (i.e. outside the loop) caused by the plasma flow
into and along the loop. The locally reduced magnetic pressure in the loop
allows the loop to sustain a higher density, which is crucial for the emission
in extreme UV. The acting of the flow on the magnetic field hosting the loop
turns out to be also responsible for the observed squashing of the loop. The
complex magnetic field and current system surrounding it can be modeled only in
3D MHD models where the magnetic field has to balance the plasma pressure. A 1D
coronal loop model or a force-free extrapolation can not capture the current
system and the complex interaction of the plasma and the magnetic field in the
coronal loop, despite the fact that the loop is under low- conditions.Comment: 10 pages, 11 figures, published in A&
Polar features in the flagellar propulsion of E. coli bacteria
E. coli bacteria swim following a run and tumble pattern. In the run state
all flagella join in a single helical bundle that propels the cell body along
approximately straight paths. When one or more flagellar motors reverse
direction the bundle unwinds and the cell randomizes its orientation. This
basic picture represents an idealization of a much more complex dynamical
problem. Although it has been shown that bundle formation can occur at either
pole of the cell, it is still unclear whether this two run states correspond to
asymmetric propulsion features. Using holographic microscopy we record the 3D
motions of individual bacteria swimming in optical traps. We find that most
cells possess two run states characterised by different propulsion forces,
total torque and bundle conformations. We analyse the statistical properties of
bundle reversal and compare the hydrodynamic features of forward and backward
running states. Our method is naturally multi-particle and opens up the way
towards controlled hydrodynamic studies of interacting swimming cells
Application of digital particle image velocimetry to insect aerodynamics: measurement of the leading-edge vortex and near wake of a Hawkmoth.
Some insects use leading-edge vortices to generate high lift forces, as has been inferred from qualitative smoke visualisations of the flow around their wings. Here we present the first Digital Particle Image Velocimetry (DPIV) data and quantitative analysis of an insect’s leading-edge vortex and near wake at two flight speeds. This allows us to describe objectively 2D slices through the flow field of a tethered Tobacco Hawkmoth (Manduca sexta). The near-field vortex wake appears to braodly resemble elliptical vortex loops. The presence of a leading-edge vortex towards the end of the downstroke is found to coincide with peak upward force production measured by a six-component force–moment balance. The topology of Manduca’s leading-edge vortex differs from that previously described because late in the downstroke, the structure extends continuously from wingtip across the thorax to the other wingtip
Periodic orbits in tall laterally heated rectangular cavities
This study elucidates the origin of the multiplicity of stable oscillatory flows detected by time integration in tall rectangular cavities heated from the side. By using continuation techniques for periodic orbits, it is shown that initially unstable branches, arising at Hopf bifurcations of the basic steady flow, become stable after crossing Neimark-Sacker points. There are no saddle-node or pitchfork bifurcations of periodic orbits, which could have been alternative mechanisms of stabilization. According to the symmetries of the system, the orbits are either fixed cycles, which retain at any time the center symmetry of the steady flow, or symmetric cycles involving a time shift in the global invariance of the orbit. The bifurcation points along the branches of periodic flows are determined. By using time integrations, with unstable periodic solutions as initial conditions, we determine which of the bifurcations at the limits of the intervals of stable periodic orbits are sub- or supercritical.Postprint (author's final draft
Analysis of a Fragmenting Sunspot using Hinode Observations
We employ high resolution filtergrams and polarimetric measurements from
Hinode to follow the evolution of a sunspot for eight days starting on June 28,
2007. The imaging data were corrected for intensity gradients, projection
effects, and instrumental stray light prior to the analysis. The observations
show the formation of a light bridge at one corner of the sunspot by a slow
intrusion of neighbouring penumbral filaments. This divided the umbra into two
individual umbral cores. During the light bridge formation, there was a steep
increase in its intensity from 0.28 to 0.7 I_QS in nearly 4 hr, followed by a
gradual increase to quiet Sun (QS) values in 13 hr. This increase in intensity
was accompanied by a large reduction in the field strength from 1800 G to 300
G. The smaller umbral core gradually broke away from the parent sunspot nearly
2 days after the formation of the light bridge rendering the parent spot
without a penumbra at the location of fragmentation. The penumbra in the
fragment disappeared first within 34 hr, followed by the fragment whose area
decayed exponentially with a time constant of 22 hr. The depleted penumbra in
the parent sunspot regenerated when the inclination of the magnetic field at
the penumbra-QS boundary became within 40 deg. from being completely horizontal
and this occurred near the end of the fragment's lifetime. After the
disappearance of the fragment, another light bridge formed in the parent which
had similar properties as the fragmenting one, but did not divide the sunspot.
The significant weakening in field strength in the light bridge along with the
presence of granulation is suggestive of strong convection in the sunspot which
might have triggered the expulsion and fragmentation of the smaller spot.
Although the presence of QS photospheric conditions in sunspot umbrae could be
a necessary condition for fragmentation, it is not a sufficient one.Comment: Accepted for publication in ApJ; 15 pages, 15 figures, 1 tabl
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