arXiv subjects

A. Kopp

Publications and source records attributed to A. Kopp.

At least 19 recordsLinked to original sources

arXiv2021

Numerical and experimental evidence for a new interpretation of residence times in space

We investigate the energy dependence of Jovian electron residence times, which allows for a deeper understanding of adiabatic energy changes that occur during charged particle transport, as well as of their significance for simulation approaches. Thereby we seek to further validate an improved approach to estimate residence times numerically by investigating the implications on previous analytical approaches, and possible effects detectable by spacecraft data. Utilizing a propagation model based on a Stochastic Differential Equation (SDE) solver written in CUDA, residence times for Jovian electrons are calculated over the whole energy range dominated by the Jovian electron source spectrum. We analyse the interdependences both with the magnetic connection between observer and the source as well as between the the distribution of the exit (simulation) times and the resulting residence times. We point out a linear relation between the residence time for different kinetic energies and the longitudinal shift of the 13 month periodicity typically observed for Jovian electrons and discuss the applicability of these findings to data. Furthermore, we utilize our finding that the simulated residence times are approximately linearly related to the energy loss for Jovian and Galactic electrons, and develop an improved analytical estimation in agreement with the numerical residence time and the longitudinal shift observed by measurements.

physics.space-ph↗

arXiv2020

Microstructure, mechanical properties, corrosion resistance and cytocompatibility of WE43 Mg alloy scaffolds fabricated by laser powder bed fusion for biomedical applications

Open-porous scaffolds of WE43 Mg alloy with a body-center cubic cell pattern were manufactured by laser powder bed fusion with different strut diameters. The geometry of the unit cells was adequately reproduced during additive manufacturing and the porosity within the struts was minimized. The microstructure of the scaffolds was modified by means of thermal solution and ageing heat treatments and was analysed in detail by means of X-ray microtomography, optical, scanning and transmission electron microscopy. Moreover, the corrosion rates and the mechanical properties of the scaffolds were measured as a function of the strut diameter and metallurgical condition. The microstructure of the as-printed scaffolds contained a mixture of Y-rich oxide particles and Rare Earth-rich intermetallic precipitates. The latter could be modified by heat treatments. The lowest corrosion rates of 2-3 mm/year were found in the as-printed and solution treated scaffolds and they could be reduced to ~0.1 mm/year by surface treatments using plasma electrolytic oxidation. The mechanical properties of the scaffolds improved with the strut diameter: the yield strength increased from 8 to 40 MPa and the elastic modulus improved from 0.2 to 0.8 GPa when the strut diameter increased from 275 μm to 800 μm. Nevertheless, the strength of the scaffolds without plasma electrolytic oxidation treatment decreased rapidly when immersed in simulated body fluid. In vitro biocompatibility tests showed surface treatments by plasma electrolytic oxidation were necessary to ensure cell proliferation in scaffolds with high surface-to-volume ratio.

cond-mat.mtrl-sci↗

arXiv2020

On the residence-time of Jovian electrons in the inner heliosphere

Jovian electrons serve as an important test-particle distribution in the inner heliosphere and have been used extensively in the past to study the (diffusive) transport of cosmic rays in the inner heliosphere. With new limits on the Jovian source function (i.e. the particle intensity just outside the Jovian magnetosphere), and a new set of in-situ observations at 1 AU for both cases of good and poor magnetic connection between the source and observer, we revisit some of these earlier simulations. We aim to find the optimal numerical set-up that can be used to simulate the propagation of 6 MeV Jovian electrons in the inner heliosphere. Using such a set-up, we further aim to study the residence (propagation) times of these particles for different levels of magnetic connection between Jupiter and an observer at Earth (1 AU). Using an advanced Jovian electron propagation model based on the stochastic differential equation (SDE) approach, we calculate the Jovian electron intensity for different model parameters. A comparison with observations leads to an optimal numerical set-up, which is then used to calculate the so-called residence (propagation) times of these particles. Comparing to in-situ observations, we are able to derive transport parameters that are appropriate to study the propagation of 6 MeV Jovian electrons in the inner heliosphere. Moreover, using these values, we show that the method of calculating the residence time applied in former literature is not suited to being interpreted as the propagation time of physical particles. This is due to an incorrect weighting of the probability distribution. We propose and apply a new method, where the results from each pseudo-particle are weighted by its resulting phase-space density (i.e. the number of physical particles that it represents). Thereby we obtain more reliable estimates for the propagation time.

physics.space-ph↗

arXiv2018

The effect of turbulence strength on meandering field lines and Solar Energetic Particle event extents

Insights into the processes of Solar Energetic Particle (SEP) propagation are essential for understanding how solar eruptions affect the radiation environment of near-Earth space. SEP propagation is influenced by turbulent magnetic fields in the solar wind, resulting in stochastic transport of SEPs to Earth. Multi-spacecraft observations suggest that the cross-field propagation shapes the SEP fluxes at Earth strongly. However, modelling SEP cross-field transport as spatial diffusion has been shown to be insufficient without use of unrealistically large cross-field diffusion coefficients. Recent work has shown that the early-time propagation of energetic particles across the mean field direction in turbulent fields is not diffusive, as the particles propagating along meandering field lines. This early-time transport mode results in fast access of the particles across the mean field direction, in agreement with the SEP observations. In this work, we demonstrate the significance of turbulence strength on evolution of the SEP radiation environment near Earth. We calculate the transport parameters with a turbulence transport model, parametrised by the SEP parallel scattering mean free path at 1~AU, $λ_{\parallel}^{*}$, and show that the parallel and cross-field transport are connected, with conditions resulting in slow parallel transport corresponding to wider events. We find a scaling $σ_{ϕ,\, \mathrm{max}}\propto (1/λ_{\parallel}^{*})^{1/4}$ for the Gaussian fitting of the longitudinal distribution of maximum intensities. The longitudes with highest intensities are shifted towards the west for strong scattering conditions. Our results emphasise the importance of understanding both the SEP transport and the interplanetary turbulence conditions for modelling and predicting the SEP radiation environment at Earth.

astro-ph.SR↗

arXiv2016

Solar energetic particle access to distant longitudes through turbulent field-line meandering

Context. Current solar energetic particle (SEP) propagation models describe the effects of interplanetary plasma turbulence on SEPs as diffusion, using a Fokker-Planck (FP) equation. However, FP models cannot explain the observed fast access of SEPs across the average magnetic field to regions that are widely separated in longitude within the heliosphere without using unrealistically strong cross-field diffusion. Aims. We study whether the recently suggested early non-diffusive phase of SEP propagation can explain the wide SEP events with realistic particle transport parameters. Methods. We used a novel model that accounts for the SEP propagation along field lines that meander as a result of plasma turbulence. Such a non-diffusive propagation mode has been shown to dominate the SEP cross-field propagation early in the SEP event history. We compare the new model to the traditional approach, and to SEP observations. Results. Using the new model, we reproduce the observed longitudinal extent of SEP peak fluxes that are characterised by a Gaussian profile with $σ=30-50^\circ$, while current diffusion theory can only explain extents of 11$^\circ$ with realistic diffusion coefficients. Our model also reproduces the timing of SEP arrival at distant longitudes, which cannot be explained using the diffusion model. Conclusions. The early onset of SEPs over a wide range of longitudes can be understood as a result of the effects of magnetic field-line random walk in the interplanetary medium and requires an SEP transport model that properly describes the non-diffusive early phase of SEP cross-field propagation.

astro-ph.SR↗

arXiv2015

The effect of magnetic field modifications on the modulation of cosmic rays in the heliosphere

A numerical model for the solar modulation of cosmic rays, based on the solution of a set of stochastic differential equations, is used to illustrate the effects of modifying the heliospheric magnetic field, particularly in the polar regions of the heliosphere. To this end, the dfferences in the modulation brought about by each of three choices for the heliospheric magnetic field, i.e. the unmodified Parker field, the Smith-Bieber modified field, and the Jokipii-Kota modified field, are studied. It is illustrated that both the Jokipii-Kota and Smith-Bieber modifications are effective in modifying the Parker field in the polar regions. In addition, it is argued that the modification of Smith and Bieber is based on observational evidence and has a firm physical basis, while these motivations are lacking in the case of the Jokipii-Kota modification. From a cosmic ray modulation point of view, we found the Smith-Bieber modification to be the most suitable choice for modifying the heliospheric magnetic field. The features and effects of these three modifications are illustrated both qualitatively and quantitatively. It is also shown how the Smith-Bieber modified field can be applied in cosmic ray modulation models to reproduce observational cosmic ray proton spectra from the PAMELA mission during the solar minimum of 2006 - 2009. These results are compared with those obtained in previous studies of this unusual solar minimum activity period and found to be in good qualitative agreement.

physics.space-ph↗

arXiv2015

Cosmic-ray positrons from millisecond pulsars

Observations by the Fermi Large Area Telescope of gamma-ray millisecond pulsar light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ~10 GeV. Fermi has also uncovered many new millisecond pulsars, impacting Galactic stellar population models. We investigate the contribution of Galactic millisecond pulsars to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day millisecond pulsars. We simulate their pair spectra invoking an offset-dipole magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow and redback systems. Since millisecond pulsars are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of millisecond pulsars are only modest contributors around a few tens of GeV to the lepton fluxes measured by AMS-02, PAMELA, and Fermi, after which this component cuts off. The contribution by black widows and redbacks may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.

astro-ph.HE↗

arXiv2015

Modelling the average spectrum expected from a population of gamma-ray globular clusters

Millisecond pulsars occur abundantly in globular clusters. They are expected to be responsible for several spectral components in the radio through gamma-ray waveband (e.g., involving synchrotron and inverse Compton emission), as have been seen by Radio Telescope Effelsberg, Chandra X-ray Observatory, Fermi Large Area Telescope, and the High Energy Stereoscopic System (H.E.S.S.) in the case of Terzan 5 (with fewer spectral components seen for other globular clusters). H.E.S.S. has recently performed a stacking analysis involving 15 non-detected globular clusters and obtained quite constraining average flux upper limits above 230 GeV. We present a model that assumes millisecond pulsars as sources of relativistic particles and predicts multi-wavelength emission from globular clusters. We apply this model to the population of clusters mentioned above to predict the average spectrum and compare this to the H.E.S.S. upper limits. Such comparison allows us to test whether the model is viable, leading to possible constraints on various average cluster parameters within this framework.

astro-ph.HE↗

arXiv2015

The Contribution of Millisecond Pulsars to the Galactic Cosmic-Ray Lepton Spectrum

Pulsars are believed to be sources of relativistic electrons and positrons. The abundance of detections of gamma-ray millisecond pulsars by Fermi Large Area Telescope coupled with their light curve characteristics that imply copious pair production in their magnetospheres, motivated us to investigate this old pulsar population as a source of Galactic electrons and positrons and their contribution to the enhancement in cosmic-ray positron flux at GeV energies. We use a population synthesis code to predict the source properties (number, position, and power) of the present-day Galactic millisecond pulsars, taking into account the latest Fermi and radio observations to calibrate the model output. Next, we simulate pair cascade spectra from these pulsars using a model that invokes an offset-dipole magnetic field. We assume free escape of the pairs from the pulsar environment. We then compute the cumulative spectrum of transported electrons and positrons at Earth, following their diffusion and energy losses as they propagate through the Galaxy. Our results indicate that the predicted particle flux increases for non-zero offsets of the magnetic polar caps. Comparing our predicted spectrum and positron fraction to measurements by AMS-02, PAMELA, and Fermi, we find that millisecond pulsars are only modest contributors at a few tens of GeV, after which this leptonic spectral component cuts off. The positron fraction is therefore only slightly enhanced above 10 GeV relative to a background flux model. This implies that alternative sources such as young, nearby pulsars and supernova remnants should contribute additional primary positrons within the astrophysical scenario.

astro-ph.HE↗

arXiv2014

Glow-in-the-dark globular clusters: modelling their multiwavelength lanterns

Globular clusters (GCs) are astronomical tapestries embroidered with an abundance of exotic stellar-type objects. Their high age promises a rich harvest of evolved stellar products, while the deep potential wells and high mass densities at their centres probably facilitate the formation of multiple-member stellar systems via increased stellar encounter rates. The ubiquity of low-mass X-ray binaries, thought to be the progenitors of millisecond pulsars (MSPs), explain the large number of observed GC radio pulsars and X-ray counterparts. The Fermi Large Area Telescope (LAT) recently unveiled the first gamma-ray GC pulsar (PSR J1823-3021A). The first observations of GCs in the GeV and TeV bands furthermore created much excitement, and in view of the above, it seems natural to explain these high-energy lanterns by investigating an MSP origin. An MSP population is expected to radiate several pulsed spectral components in the radio through gamma-ray wavebands, in addition to being sources of relativistic particles. The latter may interact with background photons in the clusters producing TeV excesses, while they may also radiate synchrotron photons as they traverse the cluster magnetic field. We present multiwavelength modelling results for Terzan 5. We also briefly discuss some alternative interpretations for the observed GC gamma-ray signals.

astro-ph.HE↗

arXiv2013

Pitch-angle scattering in magnetostatic turbulence. I. Test-particle simulations and the validity of analytical results

Context. Spacecraft observations have motivated the need for a refined description of the phase-space distribution function. Of particular importance is the pitch-angle diffusion coefficient that occurs in the Fokker-Planck transport equation. Aims. Simulations and analytical test-particle theories are compared to verify the diffusion description of particle transport, which does not allow for non-Markovian behavior. Methods. A Monte-Carlo simulation code was used to trace the trajectories of test particles moving in turbulent magnetic fields. From the ensemble average, the pitch-angle Fokker-Planck coefficient is obtained via the mean square displacement. Results. It is shown that, while excellent agreement with analytical theories can be obtained for slab turbulence, considerable deviations are found for isotropic turbulence. In addition, all Fokker-Planck coefficients tend to zero for high time values.

astro-ph.HE↗

arXiv2013

Simultaneous Analysis of Recurrent Jovian Electron Increases and Galactic Cosmic Ray Decreases

The transport environment for particles in the heliosphere, e.g. galactic cosmic rays (GCRs) and MeV electrons (including those originating from Jupiters magnetosphere), is defined by the solar wind flow and the structure of the embedded heliospheric magnetic field. Solar wind structures, such as co-rotating interaction regions (CIR), can result in periodically modulation of both particles species. A detailed analysis of this recurrent Jovian electron events and galactic cosmic ray decreases measured by SOHO EPHIN is presented here, showing clearly a change of phase between both phenomena during the cause of the years 2007 and 2008. This effect can be explained by the change of difference in heliolongitude between the Earth and Jupiter, which is of central importance for the propagation of Jovian electrons. Furthermore, the data can be ordered such that the 27-day Jovian electron variation vanishes in the sector which does not connect the Earth with Jupiter magnetically using observed solar wind speeds.

astro-ph.HE↗

arXiv2012

Multiwavelength Modelling of the Globular Cluster Terzan 5

Diffuse X-ray emission has recently been detected from the globular cluster (GC) Terzan 5, extending out to ~2.5' from the cluster centre. This emission may arise from synchrotron radiation (SR) by energetic leptons being injected into the cluster by the resident millisecond pulsar (MSP) population that interact with the cluster field. These leptons may also be reaccelerated in shocks created by collisions of pulsar winds, and may interact with bright starlight and cosmic microwave background (CMB) photons, yielding gamma rays at very high energies (VHE) through the inverse Compton (IC) process. In the GeV range, Fermi Large Area Telescope (LAT) has detected a population of GCs, very plausibly including Terzan 5, their spectral properties and energetics being consistent with cumulative magnetospheric emission from a population of MSPs. H.E.S.S. has furthermore detected a VHE excess in the direction of Terzan 5. One may derive constraints on the number of MSPs, N_tot, and the radial profiles of the GC B-field, stellar energy density, as well as the diffusion coefficient using the spatially-resolved X-ray, high-energy (HE), and VHE fluxes. If the Fermi LAT flux is due to magnetospheric processes, it will scale with the number of visible gamma-ray MSPs, N_vis. The HE spectrum therefore provides an independent way of constraining the number of MSPs (since N_tot >= N_vis). Consequently, the synthesis of available multiwavelength data presents a unique opportunity to constrain several parameters of the GC Terzan 5.

astro-ph.HE↗

arXiv2011

Modeling High-energy and Very-high-energy gamma-rays from the Terzan 5 Cluster

The Fermi Large Area Telescope (LAT) has recently detected a population of globular clusters (GCs) in high-energy (HE) gamma-rays. Their spectral properties and energetics are consistent with cumulative emission from a population of millisecond pulsars (MSPs) hosted by these clusters. For example, the HE spectra exhibit fairly hard power-law indices and cutoffs around a few GeV, typical of pulsed spectra measured for the gamma-ray pulsar population. The energetics may be used to constrain the number of visible MSPs in the cluster (N_vis), assuming canonical values for the average gamma-ray efficiency and spin-down power. This interpretation is indeed strengthened by the fact that the first gamma-ray MSP has now been identified in the GC NGC 6624, and this MSP is responsible for almost all of the HE emission from this cluster. On the other hand, it has been argued that the MSPs are also sources of relativistic leptons which may be reaccelerated in shocks originating in collisions of stellar winds in the cluster core, and may upscatter bright starlight and cosmic microwave background photons to very high energies. Therefore, this unpulsed component may give an independent constraint on the total number of MSPs (N_tot) hosted in the GC, for a given cluster magnetic field B and diffusion coefficient k_0. Lastly, the transport properties of the energetic leptons may be further constrained using multiwavelength data, e.g., to infer the radial dependence of k_0 and B. We present results on our modeling of the pulsed and unpulsed gamma-ray fluxes from the GC Terzan 5.

astro-ph.HE↗

arXiv2011

Cross section in deuteron-proton elastic scattering at 1.25 GeV/u

First results of the differential cross section in dp elastic scattering at 1.25 GeV/u measured with the HADES over a large angular range are reported. The obtained data corresponds to large transverse momenta, where a high sensitivity to the two-nucleon and three-nucleon short-range correlations is expected.

nucl-ex↗

arXiv2011

Single and double pion production in np collisions at 1.25 GeV with HADES

The preliminary results on charged pion production in np collisions at an incident beam energy of 1.25 GeV measured with HADES are presented. The np reactions were isolated in dp collisions at 1.25 GeV/u using the Forward Wall hodoscope, which allowed to register spectator protons. The results for np -> pppi-, np -> nppi+pi- and np -> dpi+pi- channels are compared with OPE calculations. A reasonable agreement between experimental results and the predictions of the OPE+OBE model is observed.

nucl-ex↗

arXiv2007

Radiation-condensation instability in a four-fluid dusty plasma

In this work linear stability analysis of a four-fluid optically thin plasma consisting of electrons, ions, neutral atoms, and charged dust particles is performed with respect to the radiation-condensation (RC) instability. The energy budget of the plasma involves the input from heating through photo-electron emission by dust particles under external ultraviolet radiation as well as radiative losses in inelastic electron-neutral, electron-ion, neutral-neutral collisions. It is shown that negatively charged particles stimulate the RC instability in the sense that the conditions for the instability to hold are wider than similar conditions in a single-fluid description.

physics.plasm-ph↗

arXiv2004

Cosmic-ray propagation properties for an origin in SNRs

We have studied the impact of cosmic-ray acceleration in SNR on the spectra of cosmic-ray nuclei in the Galaxy using a series expansion of the propagation equation, which allows us to use analytical solutions for part of the problem and an efficient numerical treatment of the remaining equations and thus accurately describes the cosmic-ray propagation on small scales around their sources in three spatial dimensions and time. We found strong variations of the cosmic-ray nuclei flux by typically 20% with occasional spikes of much higher amplitude, but only minor changes in the spectral distribution. The locally measured spectra of primary cosmic rays fit well into the obtained range of possible spectra. We further showed that the spectra of the secondary element Boron show almost no variations, so that the above findings also imply significant fluctuations of the Boron-to-Carbon ratio. Therefore the commonly used method of determining CR propagation parameters by fitting secondary-to-primary ratios appears flawed on account of the variations that these ratios would show throughout the Galaxy.

astro-ph↗