470 research outputs found
Evolutionary signatures in complex ejecta and their driven shocks
We examine interplanetary signatures of ejecta-ejecta interactions. To this end, two time intervals of inner-heliospheric (&le;1AU) observations separated by 2 solar cycles are chosen where ejecta/magnetic clouds are in the process of interacting to form complex ejecta. At the Sun, both intervals are characterized by many coronal mass ejections (CMEs) and flares. In each case, a complement of observations from various instruments on two spacecraft are examined in order to bring out the in-situ signatures of ejecta-ejecta interactions and their relation to solar observations. In the first interval (April 1979), data are shown from Helios-2 and ISEE-3, separated by ~0.33AU in radial distance and 28&deg; in heliographic longitude. In the second interval (March-April 2001), data from the SOHO and Wind probes are combined, relating effects at the Sun and their manifestations at 1AU on one of Wind's distant prograde orbits. At ~0.67AU, Helios-2 observes two individual ejecta which have merged by the time they are observed at 1AU by ISEE-3. In March 2001, two distinct Halo CMEs (H-CMEs) are observed on SOHO on 28-29 March approaching each other with a relative speed of 500kms<sup>-1</sup> within 30 solar radii. In order to isolate signatures of ejecta-ejecta interactions, the two event intervals are compared with expectations for pristine (isolated) ejecta near the last solar minimum, extensive observations on which were given by Berdichevsky et al. (2002). The observations from these two event sequences are then intercompared. In both event sequences, coalescence/merging was accompanied by the following signatures: heating of the plasma, acceleration of the leading ejecta and deceleration of the trailing ejecta, compressed field and plasma in the leading ejecta, disappearance of shocks and the strengthening of shocks driven by the accelerated ejecta. A search for reconnection signatures at the interface between the two ejecta in the March 2001 event was inconclusive because the measured changes in the plasma velocity tangential to the interface (&Delta;&nu;<sub>t</sub>) were not correlated with &Delta;(<i>B<sub>t</sub></i> /&rho;). This was possibly due to lack of sufficient magnetic shear across the interface. The ejecta mergers altered interplanetary parameters considerably, leading to contrasting geoeffects despite broadly similar solar activity. The complex ejecta on 31&nbsp;March 2001 caused a double-dip ring current enhancement, resulting in two great storms (<i>D<sub>st</sub></i>, corrected for the effect of magnetopause currents, &lt;-450nT), while the merger on 5 April 1979 produced only a corrected <i>D<sub>st</sub></i> of ~-100nT, mainly due to effects of magnetopause currents
The microcanonical thermodynamics of finite systems: The microscopic origin of condensation and phase separations; and the conditions for heat flow from lower to higher temperatures
Microcanonical thermodynamics allows the application of statistical mechanics
both to finite and even small systems and also to the largest, self-gravitating
ones. However, one must reconsider the fundamental principles of statistical
mechanics especially its key quantity, entropy. Whereas in conventional
thermostatistics, the homogeneity and extensivity of the system and the
concavity of its entropy are central conditions, these fail for the systems
considered here. For example, at phase separation, the entropy, S(E), is
necessarily convex to make exp[S(E)-E/T] bimodal in E. Particularly, as
inhomogeneities and surface effects cannot be scaled away, one must be careful
with the standard arguments of splitting a system into two subsystems, or
bringing two systems into thermal contact with energy or particle exchange. Not
only the volume part of the entropy must be considered. As will be shown here,
when removing constraints in regions of a negative heat capacity, the system
may even relax under a flow of heat (energy) against a temperature slope. Thus
the Clausius formulation of the second law: ``Heat always flows from hot to
cold'', can be violated. Temperature is not a necessary or fundamental control
parameter of thermostatistics. However, the second law is still satisfied and
the total Boltzmann entropy increases. In the final sections of this paper, the
general microscopic mechanism leading to condensation and to the convexity of
the microcanonical entropy at phase separation is sketched. Also the
microscopic conditions for the existence (or non-existence) of a critical
end-point of the phase-separation are discussed. This is explained for the
liquid-gas and the solid-liquid transition.Comment: 23 pages, 2 figures, Accepted for publication in the Journal of
Chemical Physic
Integrability and strong normal forms for non-autonomous systems in a neighbourhood of an equilibrium
The paper deals with the problem of existence of a convergent "strong" normal
form in the neighbourhood of an equilibrium, for a finite dimensional system of
differential equations with analytic and time-dependent non-linear term. The
problem can be solved either under some non-resonance hypotheses on the
spectrum of the linear part or if the non-linear term is assumed to be (slowly)
decaying in time. This paper "completes" a pioneering work of Pustil'nikov in
which, despite under weaker non-resonance hypotheses, the nonlinearity is
required to be asymptotically autonomous. The result is obtained as a
consequence of the existence of a strong normal form for a suitable class of
real-analytic Hamiltonians with non-autonomous perturbations.Comment: 10 page
Interaction of a vortex ring with the free surface of ideal fluid
The interaction of a small vortex ring with the free surface of a perfect
fluid is considered. In the frame of the point ring approximation the
asymptotic expression for the Fourier-components of radiated surface waves is
obtained in the case when the vortex ring comes from infinity and has both
horizontal and vertical components of the velocity. The non-conservative
corrections to the equations of motion of the ring, due to Cherenkov radiation,
are derived.Comment: LaTeX, 15 pages, 1 eps figur
Interplay between bending and stretching in carbon nanoribbons
We investigate the bending properties of carbon nanoribbons by combining
continuum elasticity theory and tight-binding atomistic simulations. First, we
develop a complete analysis of a given bended configuration through continuum
mechanics. Then, we provide by tight-binding calculations the value of the
bending rigidity in good agreement with recent literature. We discuss the
emergence of a stretching field induced by the full atomic-scale relaxation of
the nanoribbon architecture. We further prove that such an in-plane strain
field can be decomposed into a first contribution due to the actual bending of
the sheet and a second one due to edge effects.Comment: 5 pages, 6 figure
Lagrangian Variational Framework for Boundary Value Problems
A boundary value problem is commonly associated with constraints imposed on a
system at its boundary. We advance here an alternative point of view treating
the system as interacting "boundary" and "interior" subsystems. This view is
implemented through a Lagrangian framework that allows to account for (i) a
variety of forces including dissipative acting at the boundary; (ii) a
multitude of features of interactions between the boundary and the interior
fields when the boundary fields may differ from the boundary limit of the
interior fields; (iii) detailed pictures of the energy distribution and its
flow; (iv) linear and nonlinear effects. We provide a number of elucidating
examples of the structured boundary and its interactions with the system
interior. We also show that the proposed approach covers the well known
boundary value problems.Comment: 41 pages, 3 figure
Derivation of Boltzmann Principle
We present a derivation of Boltzmann principle
based on classical mechanical models of thermodynamics. The argument is based
on the heat theorem and can be traced back to the second half of the nineteenth
century with the works of Helmholtz and Boltzmann. Despite its simplicity, this
argument has remained almost unknown. We present it in a modern, self-contained
and accessible form. The approach constitutes an important link between
classical mechanics and statistical mechanics
Correction of seafloor magnetotelluric data for topographic effects during inversion
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B12105, doi:10.1029/2004JB003463.The large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields. For marine magnetotelluric (MT) studies, correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle. Previously, correction for topography was based on the thin sheet approximation which breaks down at periods under ∼1000 s in the deep ocean. This paper introduces an analysis method for seafloor MT data which combines removal of three-dimensional (3-D) topographic effects with inversion of the data for 2-D structure. The observed MT impedance is first corrected to a flat-lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation. The corrected MT response is then inverted in a flat seafloor model space. Because of coupling between topographic effects and deeper structure, the correction and inversion steps are iterated until changes in each become small. The procedure is verified using synthetic and real data. Tests for synthetic 3-D topography over a half-space show that the method closely recovers the true half-space model after a few iterations. The procedure is also applied to real data collected in the Mantle Electromagnetic and Tomography (MELT) experiment on the East Pacific Rise at 17°S.This work was supported by NSF grants
OCE9402324 and OCE0118254 and Research Program on Mantle Core
Dynamics, Institute for Research on Earth Evolution (IFREE), Japan
Agency for Marine-Earth Science and Technology (JAMSTEC)
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