714 research outputs found
Transverse oscillations of two coronal loops
We study transverse fast magnetohydrodynamic waves in a system of two coronal
loops modeled as smoothed, dense plasma cylinders in a uniform magnetic field.
The collective oscillatory properties of the system due to the interaction
between the individual loops are investigated from two points of view. Firstly,
the frequency and spatial structure of the normal modes are studied. The system
supports four trapped normal modes in which the loops move rigidly in the
transverse direction. The direction of the motions is either parallel or
perpendicular to the plane containing the axes of the loops. Two of these modes
correspond to oscillations of the loops in phase, while in the other two they
move in antiphase. Thus, these solutions are the generalization of the kink
mode of a single cylinder to the double cylinder case. Secondly, we analyze the
time-dependent problem of the excitation of the pair of tubes. We find that
depending on the shape and location of the initial disturbance, different
normal modes can be excited. The frequencies of normal modes are accurately
recovered from the numerical simulations. In some cases, because of the
simultaneous excitation of several eigenmodes, the system shows beating and the
phase lag between the loops is .Comment: Accepted for publication in The Astrophysical Journa
Stability of an MHD shear flow with a piecewise linear velocity profile
In this paper we present the results of the stability analysis of a simple shear flow of an incompressible fluid with a piecewise linear velocity profile in the presence of a magnetic field. In the flow, a finite transitional magnetic-free layer with a linear velocity profile is sandwiched by two semi-infinite regions. One of these regions is magnetic-free and the flow velocity in the region is constant. The other region is magnetic and the fluid in it is quiescent. The magnetic field is constant and parallel to the flow in the transitional layer. The fluid density is constant both in the magnetic as well as the magnetic-free regions, while it has a jump-type discontinuity at the boundary between the transitional layer and the magnetic region. The effect of gravity is included in the model, and it is assumed that the lighter fluid is overlaying the heavier one, thus no Rayleigh-Taylor instability is present. The dispersion equation governing the normal-mode stability of the flow is derived and its properties are analysed. We study stability of two cases: (i) magnetic-free flow in the presence of gravity, and (ii) magnetic flow without gravity. In the first case, the flow stability is controlled by the Rayleigh number, R. In the second case, the control parameter is the inverse squared Alfvénic Mach number, H . Stability of a particular monochromatic perturbation also depends on its dimensionless wavenumber α. We combine the analytical and numerical approaches to obtain the neutral stability curves in the (α,R)-plane in the case of the magnetic-free flow, and in the (α,H)-plane in the case of the magnetic flow. The dependence of the instability increment on R in the first case, and on H in the second case is treated. We apply the results of the analysis to the stability of a strongly subsonic portion of the heliopause. Our main conclusion is as follows: The inclusion of a transitional layer near the heliopause into the model increases by an order of magnitude the strength of the interstellar magnetic field required to stabilize this portion of the heliopause in comparison with the corresponding stabilizing strength of the magnetic field required when modelling the heliopause as a tangential discontinuity
Game semantic analysis of equivalence in IMJ
Using game semantics, we investigate the problem of verifying contextual equivalences in Interface Middleweight Java (IMJ), an imperative object calculus in which program phrases are typed using interfaces. Working in the setting where data types are non-recursive and restricted to finite domains, we identify the frontier between decidability and undecidability by reference to the structure of interfaces present in typing judgments. In particular, we show how to determine the decidability status of problem instances (over a fixed type signature) by examining the position of methods inside the term type and the types of its free identifiers. Our results build upon the recent fully abstract game semantics of IMJ. Decidability is proved by translation into visibly pushdown register automata over infinite alphabets with fresh-input recognition
Transverse oscillations of systems of coronal loops
We study the collective kinklike normal modes of a system of several
cylindrical loops using the T-matrix theory. Loops that have similar kink
frequencies oscillate collectively with a frequency which is slightly different
from that of the individual kink mode. On the other hand, if the kink frequency
of a loop is different from that of the others, it oscillates individually with
its own frequency. Since the individual kink frequency depends on the loop
density but not on its radius for typical 1 MK coronal loops, a coupling
between kink oscillations of neighboring loops take place when they have
similar densities. The relevance of these results in the interpretation of the
oscillations studied by \citet{schrijver2000} and \citet{verwichte2004}, in
which transverse collective loop oscillations seem to be detected, is
discussed. In the first case, two loops oscillating in antiphase are observed;
interpreting this motion as a collective kink mode suggests that their
densities are roughly equal. In the second case, there are almost three groups
of tubes that oscillate with similar periods and therefore their dynamics can
be collective, which again seems to indicate that the loops of each group share
a similar density. All the other loops seem to oscillate individually and their
densities can be different from the rest
Transverse oscillations of a multi-stranded loop
We investigate the transverse oscillations of a line-tied multi-stranded
coronal loop composed of several parallel cylindrical strands. First, the
collective fast normal modes of the loop are found with the T-matrix theory.
There is a huge quantity of normal modes with very different frequencies and a
complex structure of the associated magnetic pressure perturbation and velocity
field. The modes can be classified as bottom, middle, and top according to
their frequencies and spatial structure. Second, the temporal evolution of the
velocity and magnetic pressure perturbation after an initial disturbance are
analyzed. We find complex motions of the strands. The frequency analysis
reveals that these motions are a combination of low and high frequency modes.
The complexity of the strand motions produces a strong modulation of the whole
tube movement. We conclude that the presumed internal fine structure of a loop
influences its transverse oscillations and so its transverse dynamics cannot be
properly described by those of an equivalent monolithic loop.Comment: Accepted in Ap
Spatial contrasts in hepatic and biliary PAHs in Tilefish (Lopholatilus chamaeleonticeps) throughout the Gulf of Mexico, with comparison to the Northwest Atlantic
A multinational demersal longline survey was conducted on the Gulf of Mexico continental shelf over the years 2015 and 2016 to generate a Gulf-wide baseline of polycyclic aromatic hydrocarbon (PAH) concentrations in demersal fishes. Tilefish (Lopholatilus chamaeleonticeps) were sampled in all regions of the Gulf of Mexico for biometrics, bile, and liver. Tilefish liver was also obtained from surveys in the northwest Atlantic Ocean for comparison. Liver tissues (n ¼ 305) were analyzed for PAHs and select alkylated homologs using QuEChERS extractions and gas chromatography tandem mass spectrometry. Bile samples (n ¼ 225) were analyzed for biliary PAH metabolites using high-performance liquid chromatography with fluorescence detection. Spatial comparisons indicate the highest levels of PAH exposure and hepatic accumulation in the north central Gulf of Mexico, with decreasing concentrations moving from the north central Gulf counterclockwise, and an increase on the Yucatan Shelf. Hepatic PAH concentrations were similar between the Gulf of Mexico and the northwest Atlantic, however, Tilefish from the northwest Atlantic had higher concentrations and more frequent detection of carcinogenic high molecular weight PAHs. Overall, results demonstrate that PAH pollution was ubiquitous within the study regions, with recent exposure and hepatic accumulation observed in Tilefish from both the Gulf of Mexico and northwest Atlantic
Excitation of standing kink oscillations in coronal loops
In this work we review the efforts that have been done to study the
excitation of the standing fast kink body mode in coronal loops. We mainly
focus on the time-dependent problem, which is appropriate to describe flare or
CME induced kink oscillations. The analytical and numerical studies in slab and
cylindrical loop geometries are reviewed. We discuss the results from very
simple one-dimensional models to more realistic (but still simple) loop
configurations. We emphasise how the results of the initial value problem
complement the eigenmode calculations. The possible damping mechanisms of the
kink oscillations are also discussed
Density of States and Conductivity of Granular Metal or Array of Quantum Dots
The conductivity of a granular metal or an array of quantum dots usually has
the temperature dependence associated with variable range hopping within the
soft Coulomb gap of density of states. This is difficult to explain because
neutral dots have a hard charging gap at the Fermi level. We show that
uncontrolled or intentional doping of the insulator around dots by donors leads
to random charging of dots and finite bare density of states at the Fermi
level. Then Coulomb interactions between electrons of distant dots results in
the a soft Coulomb gap. We show that in a sparse array of dots the bare density
of states oscillates as a function of concentration of donors and causes
periodic changes in the temperature dependence of conductivity. In a dense
array of dots the bare density of states is totally smeared if there are
several donors per dot in the insulator.Comment: 13 pages, 15 figures. Some misprints are fixed. Some figures are
dropped. Some small changes are given to improve the organizatio
Local extinction and colonisation in native and exotic fish in relation to changes in land use
Distribution patterns of many native and exotic fish species are well documented, yet little is known about the temporal dynamics of native and exotic diversity in relation to changes in land use. We hypothesised that colonisation rates would be higher for exotic fish species and that extinction rates would be higher for native species in large stream systems. We also predicted that cold-water species would be more impacted than thermally tolerant species. To test these hypotheses, we used generalised linear mixed models to compare changes in native and exotic fish species richness over 10 years in a French drainage basin subjected to landscape alterations. Exotic fish were more susceptible to local extinction than the native ones. Extinction was greater among cold-tolerant species and at higher elevations. Colonisation by exotic species was higher at lower elevations. Although a decade of expanding urbanisation affected fish colonisation, agricultural lands experienced higher extinction rates. In the context of global changes in land use and population pressure, our study suggests that the temporal dynamics of fish diversity are driven by landscape alterations as well as by the thermal tolerance of species
Multiwavelength Observations of Supersonic Plasma Blob Triggered by Reconnection Generated Velocity Pulse in AR10808
Using multi-wavelength observations of Solar and Heliospheric Observatory
(SoHO)/Michelson Doppler Imager (MDI), Transition Region and Coronal Explorer
(TRACE) 171 \AA, and H from Culgoora Solar Observatory at Narrabri,
Australia, we present a unique observational signature of a propagating
supersonic plasma blob before an M6.2 class solar flare in AR10808 on 9th
September 2005. The blob was observed between 05:27 UT to 05:32 UT with almost
a constant shape for the first 2-3 minutes, and thereafter it quickly vanished
in the corona. The observed lower bound speed of the blob is estimated as
215 km s in its dynamical phase. The evidence of the blob with
almost similar shape and velocity concurrent in H and TRACE 171 \AA\
supports its formation by multi-temperature plasma. The energy release by a
recurrent 3-D reconnection process via the separator dome below the magnetic
null point, between the emerging flux and pre-existing field lines in the lower
solar atmosphere, is found to be the driver of a radial velocity pulse outwards
that accelerates this plasma blob in the solar atmosphere. In support of
identification of the possible driver of the observed eruption, we solve the
two-dimensional ideal magnetohydrodynamic equations numerically to simulate the
observed supersonic plasma blob. The numerical modelling closely match the
observed velocity, evolution of multi-temperature plasma, and quick vanishing
of the blob found in the observations. Under typical coronal conditions, such
blobs may also carry an energy flux of 7.0 ergs cm
s to re-balance the coronal losses above active regions.Comment: Solar Physics; 22 Pages; 8 Figure
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