35 research outputs found
Low Mach-number collisionless electrostatic shocks and associated ion acceleration
The existence and properties of low Mach-number () electrostatic
collisionless shocks are investigated with a semi-analytical solution for the
shock structure. We show that the properties of the shock obtained in the
semi-analytical model can be well reproduced in fully kinetic Eulerian
Vlasov-Poisson simulations, where the shock is generated by the decay of an
initial density discontinuity. Using this semi-analytical model, we study the
effect of electron-to-ion temperature ratio and presence of impurities on both
the maximum shock potential and Mach number. We find that even a small amount
of impurities can influence the shock properties significantly, including the
reflected light ion fraction, which can change several orders of magnitude.
Electrostatic shocks in heavy ion plasmas reflect most of the hydrogen impurity
ions.Comment: In Plasma Physics and Controlled Fusio
Electron Energization in Reconnection: Eulerian versus Lagrangian Perspectives
Particle energization due to magnetic reconnection is an important unsolved
problem for myriad space and astrophysical plasmas. Electron energization in
magnetic reconnection has traditionally been examined from a particle, or
Lagrangian, perspective using particle-in-cell (PIC) simulations.
Guiding-center analyses of ensembles of PIC particles have suggested that Fermi
(curvature drift) acceleration and direct acceleration via the reconnection
electric field are the primary electron energization mechanisms. However, both
PIC guiding-center ensemble analyses and spacecraft observations are performed
in an Eulerian frame. For this work, we employ the continuum Vlasov-Maxwell
solver within the Gkeyll simulation framework to re-examine electron
energization from a kinetic continuum, Eulerian, perspective. We separately
examine the contribution of each drift energization component to determine the
dominant electron energization mechanisms in a moderate guide-field Gkeyll
reconnection simulation. In the Eulerian perspective, we find that the
diamagnetic and agyrotropic drifts are the primary electron energization
mechanisms away from the reconnection x-point, where direct acceleration
dominates. We compare the Eulerian (Vlasov Gkeyll) results with the wisdom
gained from Lagrangian (PIC) analyses.Comment: 10 pages, 7 figure
Isolation and Phase-Space Energization Analysis of the Instabilities in Collisionless Shocks
We analyze the generation of kinetic instabilities and their effect on the
energization of ions in non-relativistic, oblique collisionless shocks using a
3D-3V simulation by , a hybrid particle-in-cell code. At
sufficiently high Mach number, quasi-perpendicular and oblique shocks can
experience rippling of the shock surface caused by kinetic instabilities
arising from free energy in the ion velocity distribution due to the
combination of the incoming ion beam and the population of ions reflected at
the shock front. To understand the role of the ripple on particle energization,
we devise the new instability isolation method to identify the unstable modes
underlying the ripple and interpret the results in terms of the governing
kinetic instability. We generate velocity-space signatures using the
field-particle correlation technique to look at energy transfer in phase space
from the isolated instability driving the shock ripple, providing a viewpoint
on the different dynamics of distinct populations of ions in phase space. We
generate velocity-space signatures of the energy transfer in phase space of the
isolated instability driving the shock ripple using the field-particle
correlation technique. Together, the field-particle correlation technique and
our new instability isolation method provide a unique viewpoint on the
different dynamics of distinct populations of ions in phase space and allow us
to completely characterize the energetics of the collisionless shock under
investigation.Comment: 32 pages, 14 figures, accepted by the Journal of Plasma Physic
Revolutionizing our Understanding of Particle Energization in Space Plasmas Using On-Board Wave-Particle Correlator Instrumentation
The development of on-board wave-particle correlator instrumentation using existing instrumental capabilities makes it possible to probe the collisionless wave-particle interactions governing plasma heating and particle acceleration in the heliosphere at full measurement cadence while circumventing telemetry constraints, enabling us to maximize the scientific return from future missions
Phase Space Energization of Ions in Oblique Shocks
Examining energization of kinetic plasmas in phase space is a growing topic
of interest, owing to the wealth of data in phase space compared to traditional
bulk energization diagnostics. Via the field-particle correlation (FPC)
technique and using multiple means of numerically integrating the plasma
kinetic equation, we have studied the energization of ions in phase space
within oblique collisionless shocks. The perspective afforded to us with this
analysis in phase space allows us to characterize distinct populations of
energized ions. In particular, we focus on ions which reflect multiple times
off the shock front through shock-drift acceleration, and how to distinguish
these different reflected populations in phase space using the FPC technique.
We further extend our analysis to simulations of three-dimensional shocks
undergoing more complicated dynamics, such as shock ripple, to demonstrate the
ability to recover the phase space signatures of this energization process in a
more general system. This work thus extends previous applications of the FPC
technique to more realistic collisionless shock environments, providing
stronger evidence of the technique's utility for simulation, laboratory, and
spacecraft analysis.Comment: 9 pages, 5 figure
CD8+ TÂ cells specific for an immunodominant SARS-CoV-2 nucleocapsid epitope display high naive precursor frequency and TCR promiscuity
To better understand primary and recall T cell responses during coronavirus disease 2019 (COVID-19), it is important to examine unmanipulated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells. By using peptide-human leukocyte antigen (HLA) tetramers for direct ex vivo analysis, we characterized CD8+ T cells specific for SARS-CoV-2 epitopes in COVID-19 patients and unexposed individuals. Unlike CD8+ T cells directed toward subdominant epitopes (B7/N257, A2/S269, and A24/S1,208) CD8+ T cells specific for the immunodominant B7/N105 epitope were detected at high frequencies in pre-pandemic samples and at increased frequencies during acute COVID-19 and convalescence. SARS-CoV-2-specific CD8+ T cells in pre-pandemic samples from children, adults, and elderly individuals predominantly displayed a naive phenotype, indicating a lack of previous cross-reactive exposures. T cell receptor (TCR) analyses revealed diverse TCRαβ repertoires and promiscuous αβ-TCR pairing within B7/N105+CD8+ T cells. Our study demonstrates high naive precursor frequency and TCRαβ diversity within immunodominant B7/N105-specific CD8+ T cells and provides insight into SARS-CoV-2-specific T cell origins and subsequent responses
Diagnosing the stratosphere-to-troposphere flux of ozone in a chemistry transport model
[1] Events involving stratosphere-troposphere exchange (STE) of ozone, such as tropopause folds and westerly ducts, are readily identified in observations and models, but a quantitative flux specifying where and when stratospheric ozone is mixed into the troposphere is not readily discerned from either. This work presents a new diagnostic based on determining when stratospheric air is mixed and diluted down to tropospheric abundances (< 100 ppb) and hence effectively participates in tropospheric chemistry. The method is applied to two years of high-resolution, global meteorological fields (1.9 degrees, 40 levels) from the ECMWF forecast model derived by U. Oslo for chemistry transport modeling and used in TRACE-P studies. The UCI CTM is run here with linearized stratospheric ozone chemistry (Linoz) and a parameterized tropospheric sink. In terms of events, the CTM accurately follows a March 2001 westerly duct stratospheric intrusion into the tropical eastern Pacific as observed by TOMS and calculates a 48-hour burst of STE O3 flux for that region. The influx associated with the event (0.3 Tg) is much less than the anomalous amount seen as an isolated island in column ozone (1.7 Tg). A climatology of monthly mean STE fluxes is similar for both years ( January to December 1997 and May 2000 to April 2001), but the warm phase of ENSO December 1997 is distinctly different from the cold phase of ENSO month December 2000. Global ozone fluxes are about 515 Tg ( year 1997) and 550 Tg ( year 2000/ 2001) with an equal amount into each hemisphere, and larger springtime fluxes for both hemispheres. In terms of geographical distribution, Northern Hemisphere regions of high ozone flux follow the jet streams over the oceans in the winter and over the continents in the summer, in agreement with many previous studies. In contrast, we find the largest STE flux is located in the subtropics during late spring, particularly over the Tibetan Plateau in May. This hot spot of STE is not a numerical artifact, it occurs in both meteorological years, and it appears to be caused by the rapid erosion of the tropopause. Ozone fluxes in the Southern Hemisphere have less variability ( either temporal or spatial), and they occur mainly in the subtropical region (25 degrees S - 35 degrees S) regardless of season. The poles, throughout the year, show minimal STE O3 flux