17,122 research outputs found
Absolute and convective instabilities of parallel propagating circularly polarized Alfven waves: Beat instability
Ruderman and Simpson [Phys. Plasmas 11, 4178 (2004)] studied the absolute and convective decay instabilities of parallel propagating circularly polarized Alfven waves in plasmas where the sound speed c(S) is smaller than the Alfven speed upsilon(A). We extend their analysis for the beat instability which occurs in plasmas with c(S)>upsilon(A). We assume that the dimensionless amplitude of the circularly polarized Alfven wave (pump wave), a, is small. Applying Briggs' method we study the problem analytically using expansions in power series with respect to a. It is shown that the pump wave is absolutely unstable in a reference frame moving with the velocity U with respect to the rest plasma if U-lU-r, the instability is convective. The signaling problem is studied in a reference frame where the pump wave is convectively unstable. It is shown that the spatially amplifying waves exist only when the signaling frequency is in two narrow symmetric frequency bands with the widths of the order of a(3). These results enable us to extend for the case when c(S)>upsilon(A) the conclusions, previously made for the case when c(S)<upsilon(A), that circularly polarized Alfven waves propagating in the solar wind are convectively unstable in a reference frame of any spacecraft moving with the velocity not exceeding a few tens of km/s in the solar reference frame. The characteristic scale of spatial amplification for these waves exceeds 1 a.u
Development of Replacement Heifers using Combinations of Three Forage Types and Feed Supplements (with or without Broiler Litter)
The proper management of replacement heifers is an essential component of successful cow/calf operations. The level of management and nutrition applied to replacement heifers as calves and yearlings can impact their subsequent reproductive performance and productivity
Shallow Convection on Day 261 of GATE: Mesoscale Arcs
On 18 September 1974, a cloud cluster growing in the GATE [Global Atmospheric Research Program] ship array was examined using aircraft flying close to one another at different heights, the geostationary satellite SMS-1, and radar, rawinsonde and ship data, with a view to elucidating mechanisms of convection. In this paper we concentrate analysis on cloudy convection in the moist layer.
In and above southerly surface monsoon flow approaching the cluster, clouds indigenous to the moist layer took the form of rows of tiny cumulus, and of arcs of cumulus mediocris, with patterns different from those of deeper clouds. From satellite visible images, arcs were traced for periods exceeding 2 h. Airborne photography showed that the arcs were composed of many small clouds. Radar data showed that they originated after precipitation. Apparently, throughout their life cycle, they perpetuated the pattern of an initiating dense downdraft. Eventually they yielded isolated cumulus congestus, again bearing precipitation. Aircraft recorded the distribution of thermodynamic quantities and winds at altitudes within the mixed layer, and at 537 and 1067 m. These data indicated that the arcs persisted as mesoscale circulations driven by release of latent heat in the clouds, rather than being driven by the original density current at the surface. The cloudy circulations were vigorous near and above cloud base, becoming weaker upward through altitude 1 km. The entire mesoscale circulation systems were of horizontal scale roughly 40 km.
The mesoscale cloud patterns of the moist layer appeared to play a primary role in heat transfer upward within this layer, and contributed to the forcing of showering midtropospheric cloud
Simultaneous Border-Collision and Period-Doubling Bifurcations
We unfold the codimension-two simultaneous occurrence of a border-collision
bifurcation and a period-doubling bifurcation for a general piecewise-smooth,
continuous map. We find that, with sufficient non-degeneracy conditions, a
locus of period-doubling bifurcations emanates non-tangentially from a locus of
border-collision bifurcations. The corresponding period-doubled solution
undergoes a border-collision bifurcation along a curve emanating from the
codimension-two point and tangent to the period-doubling locus here. In the
case that the map is one-dimensional local dynamics are completely classified;
in particular, we give conditions that ensure chaos.Comment: 22 pages; 5 figure
The nuclear spectrum of the radio galaxy NGC 5128 (Centaurus A)
We present near-infrared spectra of the nuclear disk in the nearby radio
galaxy NGC 5128 (Centaurus A). On the basis of the observed strengths of the [S
III] 0.9532um and [Fe II] 1.2567um lines, we classify NGC 5128 as a LINER.
Modeling of the strengths of these and additional lines suggests that the
nuclear region is powered by shocks rather than photoionization.Comment: 12 pages including 2 figures, accepted by ApJ Letter
Degenerate dispersive equations arising in the study of magma dynamics
An outstanding problem in Earth science is understanding the method of
transport of magma in the Earth's mantle. Models for this process, transport in
a viscously deformable porous media, give rise to scalar degenerate,
dispersive, nonlinear wave equations. We establish a general local
well-posedness for a physical class of data (roughly ) via fixed point
methods. The strategy requires positive lower bounds on the solution. This is
extended to global existence for a subset of possible nonlinearities by making
use of certain conservation laws associated with the equations. Furthermore, we
construct a Lyapunov energy functional, which is locally convex about the
uniform state, and prove (global in time) nonlinear dynamic stability of the
uniform state for any choice of nonlinearity. We compare the dynamics to that
of other problems and discuss open questions concerning a larger range of
nonlinearities, for which we conjecture global existence.Comment: 27 Pages, 7 figures are not present in this version. See
http://www.columbia.edu/~grs2103/ for a PDF with figures. Submitted to
Nonlinearit
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