1,065 research outputs found
Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma
The kinetic evolution of the Orszag-Tang vortex is studied using
collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex
leads rapidly to broadband turbulence. Significant differences from MHD arise
at small scales, where the fluid scale energy dissipates into heat almost
exclusively through the magnetic field because the protons are decoupled from
the magnetic field. Although cyclotron resonance is absent, the protons heat
preferentially in the plane perpendicular to the mean field, as in the corona
and solar wind. Effective transport coefficients are calculated.Comment: 4 pages, 4 figures. Submitted to PR
Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence
Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection
conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta
(ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic
shear (strong guide field). Changing the simulation domain size, we find that
the ion response varies greatly. For reconnecting regions with scales
comparable to the ion Larmor radius, the ions do not respond to the
reconnection dynamics leading to ''electron-only'' reconnection with very large
quasi-steady reconnection rates. The transition to more traditional
''ion-coupled'' reconnection is gradual as the reconnection domain size
increases, with the ions becoming frozen-in in the exhaust when the magnetic
island width in the normal direction reaches many ion inertial lengths. During
this transition, the quasi-steady reconnection rate decreases until the ions
are fully coupled, ultimately reaching an asymptotic value. The scaling of the
ion outflow velocity with exhaust width during this electron-only to
ion-coupled transition is found to be consistent with a theoretical model of a
newly reconnected field line. In order to have a fully frozen-in ion exhaust
with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width
of at least several ion inertial lengths is needed. In turbulent systems with
reconnection occurring between magnetic bubbles associated with fluctuations,
using geometric arguments we estimate that fully ion-coupled reconnection
requires magnetic bubble length scales of at least several tens of ion inertial
lengths
Characterization of the Prion Protein (PRP) Gene in Ten Breeds of Sheep
Transmissible Spongiform Encephalopathies (TSE\u27s) are neurodegenerative disorders characterized by a long generation time, spongy degeneration in the cerebral gray matter, neuronal loss and proliferation and hypertrophy of glial cells. An abnormal form of the prion protein (PrP) plays a major part in TSE pathogenesis and has been hypothesized to be the only component of the infectious agent. Some animals exposed to scrapie, the TSE affecting sheep and goats, seem to be resistant to development of the disease. Alleles encoding amino acid substitutions at codons 136 (A/V) and 171 (Q/R/H) have been associated with scrapie resistance. Other amino acid substitutions at codons 112 (M/T), 137 (M/T), 141 (L/F), 154 (R/H), and 211 (R/Q) have been reported but not associated with scrapie resistance. It may be possible to reduce the incidence of ovine scrapie by increasing the frequency of resistant genotypes (AA-136, RR-171, or QR-171). Thus, an important consideration is the frequency of these genotypes in different breeds of sheep. In this study, the genetic sequence for codons 104-175 was determined for at least ten animals of ten sheep breeds (n=207). Genotypes at codons 112, 136, 154, and 171 were determined. For codon 136, the frequency of the susceptible allele (V) was less than 0.20 in all breeds. In contrast, the frequency of the susceptible allele (Q) at codon 171 ranged from 0.27 (St. Croix) to 0.96 (Hampshire). In addition, a previously unreported substitution was found at codon 143 (H/R), with frequencies as high as 0.40
A Survey of Local Group Galaxies Currently Forming Stars: III. A Search for Luminous Blue Variables and Other H-alpha Emission-Lined Stars
We describe a search for H-alpha emission-lined stars in M31, M33, and seven
dwarfs in or near the Local Group (IC 10, NGC 6822, WLM, Sextans B, Sextans A,
Pegasus and the Phoenix dwarf) using interference filter imaging with the KPNO
and CTIO 4-m telescope and Mosaic cameras. The survey is aimed primarily at
identifying new Luminous Blue Variables (LBVs) from their spectroscopic
similarity to known LBVs, avoiding the bias towards photometric variability,
which may require centuries to manifest itself if LBVs go through long
quiescent periods. Followup spectroscopy with WIYN confirms that our survey
detected a wealth of stars whose spectra are similar to the known LBVs. We
"classify" the spectra of known LBVs, and compare these to the spectra of the
new LBV candidates. We demonstrate spectacular spectral variability for several
of the new LBV candidates, such as AM2, previously classified as a Wolf-Rayet
star, which now shows FeI, FeII and Balmer emission lines but neither the NIII
4634,42 nor HeII 4686 emission that it did in 1982. Profound spectral changes
are also noted for other suspected and known LBVs. Several of the LBV
candidates also show >0.5 mag changes in V over the past 10-20 years. The
number of known or suspected LBVs is now 24 in M31, 37 in M33, 1 in NGC 6822,
and 3 in IC 10. We estimate that the total number of LBVs in M31 and M33 may be
several hundred, in contrast to the 8 known historically through large-scale
photometric variability. This has significant implications for the time scale
of the LBV phase. We also identify a few new WRs and peculiar emission-lined
objects.Comment: Accepted by the Astronomical Journal. Version with higher quality
figures may be downloaded from http://www.lowell.edu/users/massey/has.pdf.g
Ongoing monitoring of Tortugas Ecological Reserve: Assessing the consequences of reserve designation
Over the past five years, a biogeographic characterization of Tortugas Ecological Reserve(TER) has been carried out to measure the post-implementation effects of TER as a refuge for exploited species. Our results demonstrate that there is substantial microalgal biomass at depths between 10 and 30 m in the soft sediments at the coral reef interface, and that this community may play an important role in the food web supporting reef organisms. In addition, preliminary
stable isotope data, in conjunction with prior results from the west Florida shelf, suggest that the shallow water benthic habitats surrounding the coral reefs of TER will prove to be an important source of the primary production ultimately fueling fish production throughout TER. The
majority of the fish analyzed so far have exhibited a C isotope signature consistent with a food web which relies heavily on benthic primary production. Fish counts indicate a marked increase in the abundance of large fish (>20 cm) within the Reserve relative to the Out and Park strata,
across years. Faunal collections from open and protected soft bottom habitat near the northern boundary of Tortugas North strongly suggest that relaxation of trawling pressure has increased benthic biomass and diversity in this area of TER. These data, employing an integrated Before -
After Control Impact (BACI) design at multiple spatial scales, will allow us to continue to document and quantify the post-implementation effects of TER. (PDF contains 58 pages
Localizability of Tachyonic Particles and Neutrinoless Double Beta Decay
The quantum field theory of superluminal (tachyonic) particles is plagued
with a number of problems, which include the Lorentz non-invariance of the
vacuum state, the ambiguous separation of the field operator into creation and
annihilation operators under Lorentz transformations, and the necessity of a
complex reinterpretation principle for quantum processes. Another unsolved
question concerns the treatment of subluminal components of a tachyonic wave
packets in the field-theoretical formalism, and the calculation of the
time-ordered propagator. After a brief discussion on related problems, we
conclude that rather painful choices have to be made in order to incorporate
tachyonic spin-1/2 particles into field theory. We argue that the field theory
needs to be formulated such as to allow for localizable tachyonic particles,
even if that means that a slight unitarity violation is introduced into the S
matrix, and we write down field operators with unrestricted momenta. We find
that once these choices have been made, the propagator for the neutrino field
can be given in a compact form, and the left-handedness of the neutrino as well
as the right-handedness of the antineutrino follow naturally. Consequences for
neutrinoless double beta decay and superluminal propagation of neutrinos are
briefly discussed.Comment: 12 pages, 5 figure
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