2,703 research outputs found
Electron Heating in 2D PIC Simulations of Quasi-Perpendicular Low-Beta Shocks
We measure the thermal electron energization in 1D and 2D particle-in-cell
(PIC) simulations of quasi-perpendicular, low-beta ()
collisionless ion-electron shocks with mass ratio , fast Mach
number -, and upstream magnetic field angle - from shock normal . It is known that
shock electron heating is described by an ambipolar, -parallel
electric potential jump, , that scales roughly linearly
with the electron temperature jump. Our simulations have
- in units of
the pre-shock ions' bulk kinetic energy, in agreement with prior measurements
and simulations. Different ways to measure , including the use
of de Hoffmann-Teller frame fields, agree to tens-of-percent accuracy.
Neglecting off-diagonal electron pressure tensor terms can lead to a systematic
underestimate of in our low- shocks. We further focus
on two shocks: a ()
case with a long, precursor of whistler waves along
, and a () case with
a shorter, precursor of whistlers oblique to both
and ; is the ion skin depth. Within the precursors,
has a secular rise towards the shock along multiple whistler
wavelengths and also has localized spikes within magnetic troughs. In a 1D
simulation of the , case,
shows a weak dependence on the electron plasma-to-cyclotron
frequency ratio , and decreases by a
factor of 2 as is raised to the true proton-electron value of 1836.Comment: 32 pages, 25 figures; submitted to Ap
Observation of the nonlinear Wood's anomaly on periodic arrays of nickel nanodimers
Linear and nonlinear magneto-photonic properties of periodic arrays of nickel
nanodimers are governed by the interplay of the (local) optical response of
individual nanoparticles and (non-local) diffraction phenomena, with a striking
example of Wood's anomaly. Angular and magnetic-field dependencies of the
second harmonic intensity evidence Wood's anomaly when new diffraction orders
emerge. Near-infrared spectroscopic measurements performed at different optical
wavelengths and grating constants discriminate between the linear and nonlinear
excitation mechanisms of Wood's anomalies. In the nonlinear regime the Wood's
anomaly is characterized by an order-of-magnitude larger effect in intensity
redistribution between the diffracted beams, as compared to the linear case.
The nonlinear Wood's anomaly manifests itself also in the nonlinear magnetic
contrast highlighting the prospects of nonlinear magneto-photonics.Comment: 8 pages, 6 figure
Port of Portland Pump Station
This project focused on the preliminary design of a pump station.https://pilotscholars.up.edu/egr_project/1024/thumbnail.jp
How to find the right postdoctoral position for you
The increasingly competitive academic job market has forced PhD graduates in psychology, neuroscience, and related fields to maximize their research output and secure grant funding during the early postdoctoral period of their careers. In the present article, based on a Q&A session presented at a research retreat (Brain and Behaviour Lab, University of Sydney) in February 2018, we draw on our firsthand experiences of navigating the transition from graduate student to postdoc. We offer practical advice to students who may be nearing the end of their PhDs and planning their first steps toward an academic career. Although the postdoc experience is varied, it is important for early-career researchers to make optimal choices to increase their chances of securing a continuing academic position. Ultimately, the goal of a postdoctoral position should be to develop all the facets of an academic career, but with a strong focus on the quantity and quality of research outputs.Australian Research Counci
A Heating Mechanism via Magnetic Pumping in the Intracluster Medium
Turbulence driven by AGN activity, cluster mergers and galaxy motion
constitutes an attractive energy source for heating the intracluster medium
(ICM). How this energy dissipates into the ICM plasma remains unclear, given
its low collisionality and high magnetization (precluding viscous heating by
Coulomb processes). Kunz et al. 2011 proposed a viable heating mechanism based
on the anisotropy of the plasma pressure (gyroviscous heating) under ICM
conditions. The present paper builds upon that work and shows that particles
can be gyroviscously heated by large-scale turbulent fluctuations via magnetic
pumping. We study how the anisotropy evolves under a range of forcing
frequencies, what waves and instabilities are generated and demonstrate that
the particle distribution function acquires a high energy tail. For this, we
perform particle-in-cell simulations where we periodically vary the mean
magnetic field . When grows (dwindles), a
pressure anisotropy ()
builds up ( and are, respectively, the pressures
perpendicular and parallel to ). These pressure anisotropies
excite mirror () and oblique firehose
() instabilities, which trap and scatter the
particles, limiting the anisotropy and providing a channel to heat the plasma.
The efficiency of this mechanism depends on the frequency of the large-scale
turbulent fluctuations and the efficiency of the scattering the instabilities
provide in their nonlinear stage. We provide a simplified analytical heating
model that captures the phenomenology involved. Our results show that this
process can be relevant in dissipating and distributing turbulent energy at
kinetic scales in the ICM.Comment: 24 pages, 17 figures, submitted to Ap
Polarized Narrow-Line Emission from the Nucleus of NGC 4258
The detection of polarized continuum and line emission from the nucleus of
NGC 4258 by Wilkes et al. (1995) provides an intriguing application of the
unified model of Seyfert nuclei to a galaxy in which there is known to be an
edge-on, rotating disk of molecular gas surrounding the nucleus. Unlike most
Seyfert nuclei, however, NGC 4258 has strongly polarized narrow emission lines.
To further investigate the origin of the polarized emission, we have obtained
spectropolarimetric observations of the NGC 4258 nucleus at the Keck-II
telescope. The narrow-line polarizations range from 1.0% for [S II] 6716 to
13.9% for the [O II] 7319,7331 blend, and the position angle of polarization is
oriented nearly parallel to the projected plane of the masing disk. A
correlation between critical density and degree of polarization is detected for
the forbidden lines, indicating that the polarized emission arises from
relatively dense (n_e > 10^4 cm^-3) gas. An archival Hubble Space Telescope
narrow-band [O III] image shows that the narrow-line region has a compact,
nearly unresolved core, implying a FWHM size of <2.5 pc. We discuss the
possibility that the polarized emission might arise from the accretion disk
itself and become polarized by scattering within the disk atmosphere. A more
likely scenario is an obscuring torus or strongly warped disk surrounding the
inner portion of a narrow-line region which is strongly stratified in density.
The compact size of the narrow-line region implies that the obscuring structure
must be smaller than ~2.5 pc in diameter.Comment: To appear in the Astronomical Journal. 13 pages, including 1 table
and 4 figures. Uses emulateapj.st
VRLE: Lifelog Interaction Prototype in Virtual Reality:Lifelog Search Challenge at ACM ICMR 2020
The Lifelog Search Challenge (LSC) invites researchers to share
their prototypes for interactive lifelog retrieval and encourages
competition to develop and evaluate effective methodologies to
achieve this. With this paper we present a novel approach to visual
lifelog exploration based on our research to date utilising virtual
reality as a medium for interactive information retrieval. The VRLE
prototype presented is an iteration on a previous system which
won the first LSC competition at ACM ICMR 2018
Radiation shielding of protoplanetary discs in young star-forming regions
Protoplanetary discs spend their lives in the dense environment of a star
forming region. While there, they can be affected by nearby stars through
external photoevaporation and dynamic truncations. We present simulations that
use the AMUSE framework to couple the Torch model for star cluster formation
from a molecular cloud with a model for the evolution of protoplanetary discs
under these two environmental processes. We compare simulations with and
without extinction of photoevaporation-driving radiation. We find that the
majority of discs in our simulations are considerably shielded from
photoevaporation-driving radiation for at least 0.5 Myr after the formation of
the first massive stars. Radiation shielding increases disc lifetimes by an
order of magnitude and can let a disc retain more solid material for planet
formation. The reduction in external photoevaporation leaves discs larger and
more easily dynamically truncated, although external photoevaporation remains
the dominant mass loss process. Finally, we find that the correlation between
disc mass and projected distance to the most massive nearby star (often
interpreted as a sign of external photoevaporation) can be erased by the
presence of less massive stars that dominate their local radiation field.
Overall, we find that the presence and dynamics of gas in embedded clusters
with massive stars is important for the evolution of protoplanetary discs.Comment: 23 pages, 22 figures, 1 table, accepted for publication in MNRA
Redundant Function of REV-ERBα and β and Non-Essential Role for Bmal1 Cycling in Transcriptional Regulation of Intracellular Circadian Rhythms
The mammalian circadian clockwork is composed of a core PER/CRY feedback loop and additional interlocking loops. In particular, the ROR/REV/Bmal1 loop, consisting of ROR activators and REV-ERB repressors that regulate Bmal1 expression, is thought to “stabilize” core clock function. However, due to functional redundancy and pleiotropic effects of gene deletions, the role of the ROR/REV/Bmal1 loop has not been accurately defined. In this study, we examined cell-autonomous circadian oscillations using combined gene knockout and RNA interference and demonstrated that REV-ERBα and β are functionally redundant and are required for rhythmic Bmal1 expression. In contrast, the RORs contribute to Bmal1 amplitude but are dispensable for Bmal1 rhythm. We provide direct in vivo genetic evidence that the REV-ERBs also participate in combinatorial regulation of Cry1 and Rorc expression, leading to their phase-delay relative to Rev-erbα. Thus, the REV-ERBs play a more prominent role than the RORs in the basic clock mechanism. The cellular genetic approach permitted testing of the robustness of the intracellular core clock function. We showed that cells deficient in both REV-ERBα and β function, or those expressing constitutive BMAL1, were still able to generate and maintain normal Per2 rhythmicity. Our findings thus underscore the resilience of the intracellular clock mechanism and provide important insights into the transcriptional topologies underlying the circadian clock. Since REV-ERB function and Bmal1 mRNA/protein cycling are not necessary for basic clock function, we propose that the major role of the ROR/REV/Bmal1 loop and its constituents is to control rhythmic transcription of clock output genes
Mesoscale Simulations Reveal How Salt Influences Clay Particles Agglomeration in Aqueous Dispersions
The aggregation of clay particles is an everyday phenomenon of scientific and industrial relevance. However, it is a complex multiscale process that depends delicately on the nature of the particle-particle and particle-solvent interactions. Toward understanding how to control such phenomena, a multiscale computational approach is developed, building from molecular simulations conducted at atomic resolution to calculate the potential of mean force (PMF) profiles in both pure and saline water environments. We document how it is possible to use such a model to develop a fundamental understanding concerning the mechanism of particle aggregation. For example, using molecular dynamics simulations conducted at the mesoscale in implicit solvents, it is possible to quantify the size and shape of clay aggregates as a function of system conditions. The approach is used to emphasize the role of salt concentration, which directly affects the potentials of the mean forces between kaolinite particles. While particle agglomeration in pure water yields large aggregates, the presence of sodium chloride in the aqueous brine leads instead to a large number of small aggregates. These results are consistent with macroscopic experimental observations, suggesting that the simulation protocol developed could be relevant for preventing pore blocking in heterogeneous porous matrixes
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