arXiv subjects

Luke O'C. Drury

Publications and source records attributed to Luke O'C. Drury.

13 recordsLinked to original sources

arXiv2018

Divergent Perspectives on Expert Disagreement: Preliminary Evidence from Climate Science, Climate Policy, Astrophysics, and Public Opinion

We report the results of an exploratory study that examines the judgments of climate scientists, climate policy experts, astrophysicists, and non-experts (N = 3,367) about the factors that contribute to the creation and persistence of disagreement within climate science and astrophysics. We found that, as compared to educated non-experts, climate experts believe that there is less disagreement within climate science about climate change and that methodological factors and personal or institutional biases play less significant roles in generating existing disagreements than is commonly reported or assumed. We also found that, commensurate with the greater inherent uncertainty and data lacunae in their field, astrophysicists working on cosmic rays were generally more willing to acknowledge expert disagreement, more open to the idea that a set of data can have multiple valid interpretations, and generally less quick to dismiss someone articulating a non-standard view as non-expert, than climate scientists.

physics.ao-ph↗

arXiv2017

Galactic Cosmic Rays - Theory and Interpretation

The arguments surrounding the Galactic component of the cosmic rays, the energy budget, questions of composition, spectral features, anisotropy, sources etc, will be critically examined. We are moving into a new phase in the study of the Galactic cosmic rays where it is becoming clear that precision measurements are revealing new, and in some cases unexpected, features which are forcing us to develop more sophisticated models for their production and propagation. The fundamental concepts however appear to be quite solid and have changed remarkably little in the more than fifty years since Ginzburg and Syrovatskii's classic "The Origin of Cosmic Rays".

astro-ph.HE↗

arXiv2016

Power requirements for cosmic ray propagation models involving diffusive reacceleration; estimates and implications for the damping of interstellar turbulence

We make quantitative estimates of the power supplied to the Galactic cosmic ray population by second-order Fermi acceleration in the interstellar medium, or as it is usually termed in cosmic ray propagation studies, diffusive reacceleration. Using recent results on the local interstellar spectrum from the Voyager missions we show that for parameter values, in particular the Alfvén speed, typically used in propagation codes such as Galprop to fit the B/C ratio, the power contributed by diffusive reacceleration is significant and can be of order 50\% of the total Galactic cosmic ray power. The implications for the damping of interstellar turbulence are briefly considered.

astro-ph.HE↗

arXiv2015

Cosmic-ray diffusive reacceleration: a critical look

Cosmic-ray scattering on magnetic turbulence leads to spatial diffusive propagation; if the scattering medium is moving, this will inevitably also cause changes in the momentum of the particles, so-called diffusive reacceleration. This can be described as diffusion in momentum space. Diffusive reacceleration has often been invoked to explain the peak observed in secondary-to-primary ratios at a few GeV, in particular Boron-to-Carbon. This avoids the necessity to postulate an ad-hoc break in the spatial diffusive coefficient, and has become almost a standard in modelling cosmic-ray spectra. However, at the levels invoked, the process implies a significant input of energy from the interstellar medium into cosmic rays, so that in such models interstellar space competes with the usual accelerators like supernova remnants. The questions arise: is reacceleration really occurring at the high level required to explain secondary-to-primary ratios? and are the energy requirements physically plausible? We address this issue using both analytical and numerical models of cosmic-ray propagation.

astro-ph.HE↗

arXiv2014

Origin(s) of Cosmic Rays

The problem of the origin of Cosmic Rays is now over a century old and while there has been substantial progress, especially in the last decade, there are still open questions. The question of "origin" is open to at least three possible interpretations depending on whether one follows the energy powering the accelerator, the matter being accelerated, or the physics of the acceleration process; these approaches are reviewed in turn. Supernova remnants remain by far the most plausible candidates as dominant sources for the bulk of the Galactic cosmic rays, but contributions from other source populations remain possible. The transition at higher energies from Galactic to extra-galactic populations remains obscure.

astro-ph.HE↗

arXiv2014

Power requirements for cosmic ray propagation models involving re-acceleration and a comment on second order Fermi acceleration theory

We derive an analytic expression for the power transferred from interstellar turbulence to the Galactic cosmic rays in propagation models which include re-acceleration. This is used to estimate the power required in such models and the relative importance of the primary acceleration as against re-acceleration. The analysis provides a formal mathematical justification for Fermi's heuristic account of second order acceleration in his classic 1949 paper.

astro-ph.HE↗

arXiv2013

The problem of small angular scale structure in the cosmic ray anisotropy data

Recent observations have revealed structure on small angular scales in the anisotropy data of cosmic rays at multi TeV energies. Even though the absolute amplitudes are very small, these effects are somewhat surprising and a wide range of possible causes have been discussed. A possible origin associated with heliospheric electric fields is proposed.

astro-ph.HE↗

arXiv2012

Turbulent magnetic field amplification driven by cosmic-ray pressure gradients

Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blastwave are much stronger than would be naively expected from simple shock compression of the field permeating the interstellar medium (ISM). We present a simple model which is capable of achieving sufficient magnetic field amplification to explain the observations. We propose that the cosmic-ray pressure gradient acting on the inhomogeneous ISM upstream of the supernova blastwave induces strong turbulence upstream of the supernova blastwave. The turbulence is generated through the differential acceleration of the upstream ISM which occurs as a result of density inhomogeneities in the ISM. This turbulence then amplifies the pre-existing magnetic field. Numerical simulations are presented which demonstrate that amplification factors of 20 or more are easily achievable by this mechanism when reasonable parameters for the ISM and supernova blastwave are assumed. The length scale over which this amplification occurs is that of the diffusion length of the highest energy non-thermal particles.

astro-ph.HE↗

arXiv2012

Origin of Cosmic Rays

It is argued that there are three `origins' of cosmic rays; the origin of the particles, the origin of the energy, and the site of the acceleration. The evidence for each origin is discussed and a plausible synthesis outlined for the particles of Galactic origin where the energy comes mainly (but not exclusively) from supernova explosions, the site of the acceleration is at strong collisionless shock waves, and the accelerated particles come from the interstellar and circumstellar material swept over by these shocks. If these shocks are capable (as indicated by recent observations and theoretical work) of significantly amplifying magnetic fields this picture appears capable of explaining the cosmic ray particles at all energies below the `ankle' at $3\times10^{18}\,\rm eV$. The particles above this energy are generally taken to be of extra-galactic origin and possible acceleration sites for these UHE particles are briefly discussed.

astro-ph.HE↗

arXiv2012

First order Fermi acceleration driven by magnetic reconnection

A box model is used to study first order Fermi acceleration driven by magnetic reconnection. It is shown, at least in this simple model, that the spectral index of the accelerated particles is related to the total compression in the same way as in diffusive shock acceleration and is not, as has been suggested, a universal $E^{-5/2}$ spectrum. The acceleration time-scale is estimated and some comments made about the applicability of the process.

astro-ph.HE↗

arXiv2011

Field amplification, vortex formation, and electron acceleration in a plasma protoshock: effect of asymmetric density profile

Gamma ray bursts (GRBs) are thought to originate from highly relativistic jets. The fireball model predicts internal shocks in the jets, causing magnetic field to be amplified and particles to be accelerated. We model the effects of an asymmetric density configuration for an internal plasma collision in a quasi-parallel magnetic field. We measured electron acceleration and found that a tenuous population of electrons is accelerated to Lorentz factors of \sim 300 - close to energy equipartition with ions. We found that the filaments did not remain static, but were deflected by the Lorentz force and rolled up into small vortices, which themselves merge to form a larger current vortex.

astro-ph.HE↗

arXiv2011

Escaping the accelerator; how, when and in what numbers do cosmic rays get out of supernova remnants?

The escape of charged particles accelerated by diffusive shock acceleration from supernova remnants is shown to be a more complex process than normally appreciated. Using a box model it is shown that the high-energy end of the spectrum can exhibit spectral breaks even with no formal escape as a result of geometrical dilution and changing time-scales. It is pointed out that the bulk of the cosmic ray particles at lower energies must be produced and released in the late stages of the remnant's evolution whereas the high energy particles are produced early on; this may explain recent observations of slight compositional variations with energy. Escape resulting from ion-neutral friction in dense and partially ionized media is discussed briefly and some comments made on the use of so-called "free escape boundary conditions". Finally estimates are made of the total production spectrum integrated over the life of the remnant.

astro-ph.GA↗

arXiv2010

Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock

The prompt emissions of gamma-ray bursts are seeded by radiating ultrarelativistic electrons. Internal shocks propagating through a jet launched by a stellar implosion, are expected to amplify the magnetic field & accelerate electrons. We explore the effects of density asymmetry & a quasi-parallel magnetic field on the collision of plasma clouds. A 2D relativistic PIC simulation models the collision of two plasma clouds, in the presence of a quasi-parallel magnetic field. The cloud density ratio is 10. The densities of ions & electrons & the temperature of 131 keV are equal in each cloud. The mass ratio is 250. The peak Lorentz factor of the electrons is determined, along with the orientation & strength of the magnetic field at the cloud collision boundary. The magnetic field component orthogonal to the initial plasma flow direction is amplified to values that exceed those expected from shock compression by over an order of magnitude. The forming shock is quasi-perpendicular due to this amplification, caused by a current sheet which develops in response to the differing deflection of the incoming upstream electrons & ions. The electron deflection implies a charge separation of the upstream electrons & ions; the resulting electric field drags the electrons through the magnetic field, whereupon they acquire a relativistic mass comparable to the ions. We demonstrate how a magnetic field structure resembling the cross section of a flux tube grows in the current sheet of the shock transition layer. Plasma filamentation develops, as well as signatures of orthogonal magnetic field striping. Localized magnetic bubbles form. Energy equipartition between the ion, electron & magnetic energy is obtained at the shock transition layer. The electronic radiation can provide a seed photon population that can be energized by secondary processes (e.g. inverse Compton).

astro-ph.HE↗