890 research outputs found

### Nonlinear Development of Streaming Instabilities In Strongly Magnetized Plasmas

The nonlinear development of streaming instabilities in the current layers
formed during magnetic reconnection with a guide field is explored. Theory and
3-D particle-in-cell simulations reveal two distinct phases. First, the
parallel Buneman instability grows and traps low velocity electrons. The
remaining electrons then drive two forms of turbulence: the parallel
electron-electron two-stream instability and the nearly-perpendicular lower
hybrid instability. The high velocity electrons resonate with the turbulence
and transfer momentum to the ions and low velocity electrons.Comment: Accepted by PR

### Second harmonic electromagnetic emission of a turbulent magnetized plasma driven by a powerful electron beam

The power of second harmonic electromagnetic emission is calculated for the
case when strong plasma turbulence is excited by a powerful electron beam in a
magnetized plasma. It is shown that the simple analytical model of strong
plasma turbulence with the assumption of a constant pump power is able to
explain experimentally observed bursts of electromagnetic radiation as a
consequence of separate collapse events. It is also found that the
electromagnetic emission power calculated for three-wave interaction processes
occurring in the long-wavelength part of turbulent spectrum is in
order-of-magnitude agreement with experimental results

### Cosmic-ray electron injection from the ionization of nuclei

We show that the secondary electrons ejected from the ionization of heavy
ions can be injected into the acceleration process that occurs at supernova
remnant shocks. This electron injection mechanism works since ions are ionized
during the acceleration when they move already with relativistic speed, just
like ejected electrons do. Using the abundances of heavy nuclei measured in
cosmic rays at Earth, we estimate the electron/proton ratio at the source to be
~10^-4, big enough to account for the nonthermal synchrotron emission observed
in young SNRs. We also show that the ionization process can limit the maximum
energy that heavy ions can reach.Comment: 4 pages, 1 figure, accepted for publication in Physical Review
Letter

### Turbulent Contributions to Ohm's Law in Axisymmetric Magnetized Plasmas

The effect of magnetic turbulence in shaping the current density in
axisymmetric magnetized plasma is analyzed using a turbulent extension of Ohm's
law derived from the self-consistent action-angle transport theory. Besides the
well-known hyper-resistive (helicity-conserving) contribution, the generalized
Ohm's law contains an anomalous resistivity term, and a turbulent
bootstrap-like term proportional to the current density derivative. The
numerical solution of the equation for equilibrium and turbulence profiles
characteristic of conventional and advanced scenarios shows that, trough
"turbulent bootstrap" effect and anomalous resistivity turbulence can generate
power and parallel current which are a sizable portion (about 20-25%) of the
corresponding effects associated with the neoclassical bootstrap effect. The
degree of alignment of the turbulence peak and the pressure gradient plays an
important role in defining the steady-state regime. In fully bootstrapped
tokamak, the hyper-resistivity is essential in overcoming the intrinsic
limitation of the hollow current profile.Comment: 19 pages, 6 figures, journal pape

### Artificial ionospheric layers driven by high-frequency radiowaves : an assessment

High-power ordinary mode radio waves produce artificial ionization in the F-region ionosphere at the European Incoherent Scatter (EISCAT at Tromsø, Norway) and High-frequency Active Auroral Research Program (HAARP at Gakona, Alaska, USA) facilities. We have summarized the features of the excited plasma turbulence and descending layers of freshly-ionized (“artificial”) plasma. The concept of an ionizing wavefront created by accelerated suprathermal electrons appears to be in accordance with the data. The strong Langmuir turbulence (SLT) regime is revealed by the specific spectral features of incoherent radar backscatter and stimulated electromagnetic emissions. Theory predicts that the SLT acceleration is facilitated in the presence of photoelectrons. This agrees with the intensified artificial plasma production and the greater speeds of descent but weaker incoherent radar backscatter in the sunlit ionosphere. Numerical investigation of propagation of O-mode waves and the development of SLT and descending layers have been performed. The greater extent of the SLT region at the magnetic zenith than at vertical appears to make magnetic zenith injections more efficient for electron acceleration and descending layers. At high powers, anomalous absorption is suppressed, leading to the Langmuir and upper hybrid processes during the whole heater-on period. The data suggest that parametric UH interactions mitigate anomalous absorption at heating frequencies far from electron gyroharmonics and also generate SLT in the upper hybrid layer. The persistence of artificial plasma at the terminal altitude depends on how close the heating frequency is to the local gyroharmonic

### Supersonic regime of the Hall-magnetohydrodynamics resistive tearing instability

An earlier analysis of the Hall-magnetohydrodynamics (MHD) tearing instability [E. Ahedo and J. J. Ramos, Plasma Phys. Controlled Fusion 51, 055018 (2009)] is extended to cover the regime where the growth rate becomes comparable or exceeds the sound frequency. Like in the previous subsonic work, a resistive, two-fluid Hall-MHD model with massless electrons and zero-Larmor-radius ions is adopted and a linear stability analysis about a force-free equilibrium in slab geometry is carried out. A salient feature of this supersonic regime is that the mode eigenfunctions become intrinsically complex, but the growth rate remains purely real. Even more interestingly, the dispersion relation remains of the same form as in the subsonic regime for any value of the instability Mach number, provided only that the ion skin depth is sufficiently small for the mode ion inertial layer width to be smaller than the macroscopic lengths, a generous bound that scales like a positive power of the Lundquist numbe

### The role of the blood group-related glycosyltransferases FUT2 and B4GALNT2 in susceptibility to infectious disease

The glycosylation profile of the gastrointestinal tract is an important factor mediating host-microbe interactions. Variation in these glycan structures is often mediated by blood group-related glycosyltransferases, and can lead to wide-ranging differences in susceptibility to both infectious- as well as chronic disease. In this review, we focus on the interplay between host glycosylation, the intestinal microbiota and susceptibility to gastrointestinal pathogens based on studies of two exemplary blood group-related glycosyltransferases that are conserved between mice and humans, namely FUT2 and B4GALNT2. We highlight that differences in susceptibility can arise due to both changes in direct interactions, such as bacterial adhesion, as well as indirect effects mediated by the intestinal microbiota. Although a large body of experimental work exists for direct interactions between host and pathogen, determining the more complex and variable mechanisms underlying three-way interactions involving the intestinal microbiota will be the subject of much-needed future research

### 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

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