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 ($\beta_p=0.25$) collisionless ion-electron shocks with mass ratio $m_i/m_e=200$, fast Mach number $\mathcal{M}_{ms}=1$-$4$, and upstream magnetic field angle $\theta_{Bn} = 55$-$85^\circ$ from shock normal $\hat{\boldsymbol{n}}$. It is known that shock electron heating is described by an ambipolar, $\boldsymbol{B}$-parallel electric potential jump, $\Delta\phi_\parallel$, that scales roughly linearly with the electron temperature jump. Our simulations have $\Delta\phi_\parallel/(0.5 m_i {u_\mathrm{sh}}^2) \sim 0.1$-$0.2$ in units of the pre-shock ions' bulk kinetic energy, in agreement with prior measurements and simulations. Different ways to measure $\phi_\parallel$, 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 $\phi_\parallel$ in our low-$\beta_p$ shocks. We further focus on two $\theta_{Bn}=65^\circ$ shocks: a $\mathcal{M}_s=4$ ($\mathcal{M}_A=1.8$) case with a long, $30 d_i$ precursor of whistler waves along $\hat{\boldsymbol{n}}$, and a $\mathcal{M}_s=7$ ($\mathcal{M}_A=3.2$) case with a shorter, $5d_i$ precursor of whistlers oblique to both $\hat{\boldsymbol{n}}$ and $\boldsymbol{B}$; $d_i$ is the ion skin depth. Within the precursors, $\phi_\parallel$ 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 $\mathcal{M}_s=4$, $\theta_{Bn}=65^\circ$ case, $\phi_\parallel$ shows a weak dependence on the electron plasma-to-cyclotron frequency ratio $\omega_{pe}/\Omega_{ce}$, and $\phi_\parallel$ decreases by a factor of 2 as $m_i/m_e$ 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 $\textbf{B}(t)$. When $\textbf{B}(t)$ grows (dwindles), a pressure anisotropy $P_{\perp}>P_{\parallel}$ ($P_{\perp}< P_{\parallel}$) builds up ($P_{\perp}$ and $P_{\parallel}$ are, respectively, the pressures perpendicular and parallel to $\textbf{B}(t)$). These pressure anisotropies excite mirror ($P_{\perp}>P_{\parallel}$) and oblique firehose ($P_{\parallel}>P_{\perp}$) 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