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Magnetohydrodynamic normal mode analysis of plasma with equilibrium pressure anisotropy
In this work, we generalise linear magnetohydrodynamic (MHD) stability theory
to include equilibrium pressure anisotropy in the fluid part of the analysis. A
novel 'single-adiabatic' (SA) fluid closure is presented which is complementary
to the usual 'double-adiabatic' (CGL) model and has the advantage of naturally
reproducing exactly the MHD spectrum in the isotropic limit. As with MHD and
CGL, the SA model neglects the anisotropic perturbed pressure and thus loses
non-local fast-particle stabilisation present in the kinetic approach. Another
interesting aspect of this new approach is that the stabilising terms appear
naturally as separate viscous corrections leaving the isotropic SA closure
unchanged. After verifying the self-consistency of the SA model, we re-derive
the projected linear MHD set of equations required for stability analysis of
tokamaks in the MISHKA code. The cylindrical wave equation is derived
analytically as done previously in the spectral theory of MHD and clear
predictions are made for the modification to fast-magnetosonic and slow ion
sound speeds due to equilibrium anisotropy.Comment: 19 pages. This is an author-created, un-copyedited version of an
article submitted for publication in Plasma Physics and Controlled Fusion.
IOP Publishing Ltd is not responsible for any errors or omissions in this
version of the manuscript or any version derived from i
Changes in the flagellar bundling time account for variations in swimming behavior of flagellated bacteria in viscous media
Although the motility of the flagellated bacteria, Escherichia coli, has been
widely studied, the effect of viscosity on swimming speed remains
controversial. The swimming mode of wild-type E.coli is often idealized as a
"run-and- tumble" sequence in which periods of swimming at a constant speed are
randomly interrupted by a sudden change of direction at a very low speed. Using
a tracking microscope, we follow cells for extended periods of time in
Newtonian liquids of varying viscosity, and find that the swimming behavior of
a single cell can exhibit a variety of behaviors including run-and-tumble and
"slow-random-walk" in which the cells move at relatively low speed. Although
the characteristic swimming speed varies between individuals and in different
polymer solutions, we find that the skewness of the speed distribution is
solely a function of viscosity and can be used, in concert with the measured
average swimming speed, to determine the effective running speed of each cell.
We hypothesize that differences in the swimming behavior observed in solutions
of different viscosity are due to changes in the flagellar bundling time, which
increases as the viscosity rises, due to the lower rotation rate of the
flagellar motor. A numerical simulation and the use of Resistive Force theory
provide support for this hypothesis
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