4,392 research outputs found
Thermodynamics and kinetics of boundary friction
A deterministic theory describing the behavior of an ultrathin lubricant film
between two atomically-smooth solid surfaces is proposed. For the description
of lubricant state the parameter of excess volume arising due to chaotization
of solid medium structure in the course of melting is introduced. Thermodynamic
and shear melting is described consistently. Dependences of friction force on
temperature of lubricant, shear velocity of rubbing surfaces, and pressure upon
surfaces are analyzed. Within the framework of a simple tribological model the
stick-slip mode of friction, when the lubricant periodically melts and
solidifies, is described. The obtained results are qualitatively compared with
the experimental data.Comment: 14 pages, 6 figures, 33 reference
Three-dimensional numerical simulations of fast-to-Alfven conversion in sunspots
The conversion of fast waves to the Alfven mode in a realistic sunspot
atmosphere is studied through three-dimensional numerical simulations. An
upward propagating fast acoustic wave is excited in the high-beta region of the
model. The new wave modes generated at the conversion layer are analyzed from
the projections of the velocity and magnetic field in their characteristic
directions, and the computation of their wave energy and fluxes. The analysis
reveals that the maximum efficiency of the conversion to the slow mode is
obtained for inclinations of 25 degrees and low azimuths, while the Alfven wave
conversions peaks at high inclinations and azimuths between 50 and 120 degrees.
Downward propagating Alfven waves appear at the regions of the sunspot where
the orientation of the magnetic field is in the direction opposite to the wave
propagation, since at these locations the Alfven wave couples better with the
downgoing fast magnetic wave which are reflected due to the gradients of the
Alfven speed. The simulations shows that the Alfven energy at the chromosphere
is comparable to the acoustic energy of the slow mode, being even higher at
high inclined magnetic fields.Comment: Accepted for publication in The Astrophysical Journa
Simulations of magneto-hydrodynamic waves in atmospheres of roAp stars
We report 2D time-dependent non-linear magneto-hydrodynamical simulations of
waves in the atmospheres of roAp stars. We explore a grid of simulations in a
wide parameter space. The aim of our study is to understand the influence of
the atmosphere and the magnetic field on the propagation and reflection
properties of magneto-acoustic waves, formation of shocks and node layers.Comment: 2 pages, Proceedings of the IAU Symposium 259, "Cosmic Magnetic
Fields: From Planets, to Stars and Galaxies", November 200
Numerical simulations of conversion to Alfven waves in sunspots
We study the conversion of fast magneto-acoustic waves to Alfven waves by
means of 2.5D numerical simulations in a sunspot-like magnetic configuration. A
fast, essentially acoustic, wave of a given frequency and wave number is
generated below the surface and propagates upward though the Alfven/acoustic
equipartition layer where it splits into upgoing slow (acoustic) and fast
(magnetic) waves. The fast wave quickly reflects off the steep Alfven speed
gradient, but around and above this reflection height it partially converts to
Alfven waves, depending on the local relative inclinations of the background
magnetic field and the wavevector. To measure the efficiency of this conversion
to Alfven waves we calculate acoustic and magnetic energy fluxes. The
particular amplitude and phase relations between the magnetic field and
velocity oscillations help us to demonstrate that the waves produced are indeed
Alfven waves. We find that the conversion to Alfven waves is particularly
important for strongly inclined fields like those existing in sunspot
penumbrae. Equally important is the magnetic field orientation with respect to
the vertical plane of wave propagation, which we refer to as "field azimuth".
For field azimuth less than 90 degrees the generated Alfven waves continue
upwards, but above 90 degrees downgoing Alfven waves are preferentially
produced. This yields negative Alfven energy flux for azimuths between 90 and
180 degrees. Alfven energy fluxes may be comparable to or exceed acoustic
fluxes, depending upon geometry, though computational exigencies limit their
magnitude in our simulations.Comment: Accepted for publication in Ap
THE STUDY OF THE INFLUENCE OF PERIODIC LATERAL VELOCITY VARIATIONS ON THE TIME-DISTANCE GRAPH IN THE REFLECTION METHOD
To study the effect of insufficient consideration of lateral and
sinusoidal velocity variations on the obtained features of the reflecting boundary
behaviour under the standard interpretation scheme in the CMP method
Magneto-acoustic waves in sunspots: first results from a new 3D nonlinear magnetohydrodynamic code
Waves observed in the photosphere and chromosphere of sunspots show complex
dynamics and spatial patterns. The interpretation of high-resolution sunspot
wave observations requires modeling of three-dimensional non-linear wave
propagation and mode transformation in the sunspot upper layers in realistic
spot model atmospheres. Here we present the first results of such modeling. We
have developed a 3D non-linear numerical code specially designed to calculate
the response of magnetic structures in equilibrium to an arbitrary
perturbation. The code solves the 3D nonlinear MHD equations for perturbations;
it is stabilized by hyper-diffusivity terms and is fully parallelized. The
robustness of the code is demonstrated by a number of standard tests. We
analyze several simulations of a sunspot perturbed by pulses of different
periods at subphotospheric level, from short periods, introduced for academic
purposes, to longer and realistic periods of three and five minutes. We present
a detailed description of the three-dimensional mode transformation in a
non-trivial sunspot-like magnetic field configuration, including the conversion
between fast and slow magneto-acoustic waves and the Alfv\'en wave, by
calculation of the wave energy fluxes. Our main findings are the following: (1)
the conversion from acoustic to the Alfv\'en mode is only observed if the the
driving pulse is located out of the sunspot axis, but this conversion is
energetically inefficient; (2) as a consequence of the cut-off effects and
refraction of the fast magneto-acoustic mode, the energy of the evanescent
waves with periods around 5 minutes remains almost completely below the level
beta=1; (3) waves with frequencies above the cut-off propagate field-aligned to
the chromosphere and their power becomes dominating over that of evanescent
5-minute oscillations, in agreement with observations
Self-organization of adatom adsorption structure at interaction with tip of dynamic force microscope
The formation of an adatom adsorption structure in dynamic force microscopy
experiment is shown as a result of the spontaneous appearance of shear strain
caused by external supercritical heating. This transition is described by the
Kelvin-Voigt equation for a viscoelastic medium, the relaxation
Landau-Khalatnikov equation for shear stress, and the relaxation equation for
temperature. It is shown that these equations formally coincide with the
synergetic Lorenz system, where the shear strain acts as the order parameter,
the conjugate field is reduced to the stress, and the temperature is the
control parameter. Within the adiabatic approximation, the steady-state values
of these quantities are found. Taking into account the sample shear modulus vs
strain dependence, the formation of the adatom adsorption configuration is
described as the first-order transition. The critical temperature of the tip
linearly increases with the growth of the effective value of the sample shear
modulus and decreases with the growth of its typical value.Comment: 10 pages, 3 figure
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