scispace - formally typeset
Search or ask a question
Author

O. Skjaeraasen

Bio: O. Skjaeraasen is an academic researcher from Max Planck Society. The author has contributed to research in topics: Poynting vector & Flux. The author has an hindex of 3, co-authored 3 publications receiving 300 citations.

Papers
More filters
Journal ArticleDOI
TL;DR: In this article, the authors investigated how rapidly this transition can take place by implementing into a global MHD model that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light, an upper limit follows from demanding that the disipation zone expand only subsonically in the comoving frame, and a further prescription is obtained by the growth rate of the relativistic tearing mode.
Abstract: Flows in which energy is transported predominantly as Poynting flux are thought to occur in pulsars, gamma-ray bursts, and relativistic jets from compact objects. The fluctuating component of the magnetic field in such a flow can in principle be dissipated by magnetic reconnection and used to accelerate the flow. We investigate how rapidly this transition can take place by implementing into a global MHD model that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light, an upper limit follows from demanding that the dissipation zone expand only subsonically in the comoving frame, and a further prescription is obtained by assuming that the expansion speed is limited by the growth rate of the relativistic tearing mode. In each case, solutions are presented that give the Lorentz factor of a spherical wind containing a transverse, oscillating magnetic field component as a function of radius. In the case of the Crab Pulsar, we find that the Poynting flux can be dissipated before the wind reaches the inner edge of the Nebula if the pulsar emits electron-positron pairs at a rate ± > 1040 s-1, thus providing a possible solution to the "σ-problem."

289 citations

01 Jan 2004
TL;DR: In this article, the authors considered the conversion of the Crab pulsar wind from one dominated by Poynting flux close to the star to a particle-born energy at the termination shock.
Abstract: The conversion of the Crab pulsar wind from one dominated by Poynting flux close to the star to one dominated by particle-born energy at the termination shock is considered. The idea put forward by Coroniti (1990) and criticised by Lyubarsky & Kirk (2001) that reconnection in a striped wind is responsible, is generalised to include faster prescriptions for the a priori unknown dissipation rate. Strong acceleration of the wind is confirmed, and the higher dissipation rates imply complete conversion of Poynting flux into particle-born flux within the unshocked wind.

11 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated how rapidly Poynting flux can be dissipated by magnetic reconnection, by implementing into a global MHD model, that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light.
Abstract: Flows in which energy is transported predominantly as Poynting flux are thought to occur in pulsars, gamma-ray bursts and relativistic jets from compact objects. The fluctuating component of the magnetic field in such a flow can in principle be dissipated by magnetic reconnection, and used to accelerate the flow. We investigate how rapidly this transition can take place, by implementing into a global MHD model, that uses a thermodynamic description of the plasma, explicit, physically motivated prescriptions for the dissipation rate: a lower limit on this rate is given by limiting the maximum drift speed of the current carriers to that of light, an upper limit follows from demanding that the dissipation zone expand only subsonically in the comoving frame and a further prescription is obtained by assuming that the expansion speed is limited by the growth rate of the relativistic tearing mode. In each case, solutions are presented which give the Lorentz factor of a spherical wind containing a transverse, oscillating magnetic field component as a function of radius. In the case of the Crab pulsar, we find that the Poynting flux can be dissipated before the wind reaches the inner edge of the Nebula if the pulsar emits electron positron pairs at a rate >1.E40 per second, thus providing a possible solution to the sigma-problem.

10 citations

08 Nov 2022
TL;DR: In this paper , a revised phenomenological model of the k − (cid:15) type was developed with the correct asymptotic limit of no turbulence modulation for small particles, and augmentation for larger diameter solids.
Abstract: A large amount of published data show that particles with diameter above 10% of the turbulence integral length scale ( D/l > 0 . 1) tend to increase the turbulent kinetic energy of the carrier fluid above the single-phase value, and smaller particles tend to suppress it. A revised phenomenological model of the k − (cid:15) type was developed to reproduce these effects with the correct asymptotic limit of no turbulence modulation for small particles, and augmentation for larger diameter solids. Particle-kinetic theory was used to derive the work exchanged between the particles and the fluid due to both drag and added mass forces to accomodate any particle/fluid density ratios including bubbles, droplets and heavy solids. For the larger particles, we devised a new model for vortex shedding induced by the slip between the particles and the turbulent flow, due to particle inertia. Simple approximate formulae for the turbulence modulation were obtained through asymptotic analysis, for the purpose of application. The overall augmentation around D/l = . a robust feature for a wide range of particle Reynolds and Stokes we not this general a Indeed, not at the larger diameters turbulence breakup, between particles and some data for solids in vertical gas very large turbulence augmentation that can only be due to gravitational settling rather than slip that is generated by the turbulent

Cited by
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors investigated anti-parallel reconnection in magnetically dominated electron-positron plasmas and proved that the late-time particle spectrum integrated over the whole reconnection region is a power law whose slope is harder than −2 for magnetizations σ 10.
Abstract: In magnetized astrophysical outflows, the dissipation of field energy into particle energy via magnetic reconnection is often invoked to explain the observed non-thermal signatures. By means of two- and three-dimensional particle-in-cell simulations, we investigate anti-parallel reconnection in magnetically dominated electron-positron plasmas. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic state of the system beyond the initial transients, and without any artificial limitation by the boundary conditions. At late times, the reconnection layer is organized into a chain of large magnetic islands connected by thin X-lines. The plasmoid instability further fragments each X-line into a series of smaller islands, separated by X-points. At the X-points, the particles become unmagnetized and they get accelerated along the reconnection electric field. We provide definitive evidence that the late-time particle spectrum integrated over the whole reconnection region is a power law whose slope is harder than –2 for magnetizations σ 10. Efficient particle acceleration to non-thermal energies is a generic by-product of the long-term evolution of relativistic reconnection in both two and three dimensions. In three dimensions, the drift-kink mode corrugates the reconnection layer at early times, but the long-term evolution is controlled by the plasmoid instability which facilitates efficient particle acceleration, analogous to the two-dimensional physics. Our findings have important implications for the generation of hard photon spectra in pulsar winds and relativistic astrophysical jets.

607 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that the observed radio bursts could be generated at shocks formed via the interaction of the magnetic pulse with the plasma within the nebula, which could be observed even from distant galaxies.
Abstract: Bursts of millisecond duration were recently discovered in the 1 GHz band. There is a strong evidence that they come from $\sim 1 $ Gpc distances, which implies extraordinary high brightness temperature. I propose that these bursts could be attributed to synchrotron maser emission from relativistic, magnetized shocks. At the onset of the magnetar flare, a strongly magnetized pulse is formed, which propagates away through the relativistic magnetar wind and eventually reaches the nebula inflated by the wind within the surrounding medium. I show that the observed radio bursts could be generated at shocks formed via the interaction of the magnetic pulse with the plasma within the nebula. The model predicts strong millisecond bursts in the TeV band, which could be observed even from distant galaxies.

420 citations

Journal ArticleDOI
TL;DR: In this paper, the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0? 10?1, was investigated by means of multi-dimensional particle-in-cell simulations.
Abstract: The afterglow emission from gamma-ray bursts (GRBs) is usually interpreted as synchrotron radiation from electrons accelerated at the GRB external shock that propagates with relativistic velocities into the magnetized interstellar medium. By means of multi-dimensional particle-in-cell simulations, we investigate the acceleration performance of weakly magnetized relativistic shocks, in the magnetization range 0 ? 10?1. The pre-shock magnetic field is orthogonal to the flow, as generically expected for relativistic shocks. We find that relativistic perpendicular shocks propagating in electron-positron plasmas are efficient particle accelerators if the magnetization is ? 10?3. For electron-ion plasmas, the transition to efficient acceleration occurs for ? 3 ? 10?5. Here, the acceleration process proceeds similarly for the two species, since the electrons enter the shock nearly in equipartition with the ions, as a result of strong pre-heating in the self-generated upstream turbulence. In both electron-positron and electron-ion shocks, we find that the maximum energy of the accelerated particles scales in time as ?maxt 1/2. This scaling is shallower than the so-called (and commonly assumed) Bohm limit ?maxt, and it naturally results from the small-scale nature of the Weibel turbulence generated in the shock layer. In magnetized plasmas, the energy of the accelerated particles increases until it reaches a saturation value ?sat/?0 mic 2 ~ ??1/4, where ?0 mic 2 is the mean energy per particle in the upstream bulk flow. Further energization is prevented by the fact that the self-generated turbulence is confined within a finite region of thickness ??1/2 around the shock. Our results can provide physically grounded inputs for models of non-thermal emission from a variety of astrophysical sources, with particular relevance to GRB afterglows.

372 citations

Journal ArticleDOI
TL;DR: In this paper, two-dimensional (2D) and three-dimensional kinetic simulations were carried out to investigate relativistic magnetic reconnection and the associated particle acceleration, and it was shown that the acceleration is accelerated by the curvature drift of particles along the electric field induced by the relativist flows.
Abstract: Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two- and three-dimensional (3D) kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron–positron plasmas starting with a magnetically dominated, force-free current sheet (σ ≡ B2 / (4πnemec2) >> 1). For this limit, we demonstrate that relativistic reconnection is highly efficient at accelerating particles through a first-order Fermi process accomplished by the curvature drift of particles along the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra f α (γ - 1)-p and approaches p = 1 for sufficiently large σ and system size. Eventually most of the available magnetic free energy is converted into nonthermal particle kinetic energy. An analytic model is presented to explain the key results and predict a general condition for the formation of power-law distributions. The development of reconnection in these regimes leads to relativistic inflow and outflow speeds and enhanced reconnection rates relative to nonrelativistic regimes. In the 3Dmore » simulation, the interplay between secondary kink and tearing instabilities leads to strong magnetic turbulence, but does not significantly change the energy conversion, reconnection rate, or particle acceleration. This paper suggests that relativistic reconnection sites are strong sources of nonthermal particles, which may have important implications for a variety of high-energy astrophysical problems.« less

313 citations

Journal ArticleDOI
TL;DR: In this article, a power law-type energy spectrum of the combined data set can be approximated by a power-law type energy spectrum: d?/dE =?0?,?0 = 10-11?photons?cm-2 s-1 TeV-1, and? = -2.62? 0.02stat? 0.05sys.
Abstract: The Crab supernova remnant has been observed regularly with the stereoscopic system of five imaging air Cerenkov telescopes that was part of the High Energy Gamma Ray Astronomy (HEGRA) experiment. In total, close to 400 hr of useful data have been collected from 1997 to 2002. The differential energy spectrum of the combined data set can be approximated by a power law-type energy spectrum: d?/dE = ?0 ?, ?0 = 10-11?photons?cm-2 s-1 TeV-1, and ? = -2.62 ? 0.02stat ? 0.05sys. The spectrum extends up to energies of 80?TeV and is well matched by model calculations in the framework of inverse Compton scattering of various seed photons in the nebula, including for the first time a recently detected compact emission region at millimeter wavelengths. The observed indications for a gradual steepening of the energy spectrum in data is expected in the inverse Compton emission model. The average magnetic field in the emitting volume is determined to be 161.6 ? 0.8stat ? 18sys ?G. The presence of protons in the nebula is not required to explain the observed flux, and upper limits on the injected power of protons are calculated to be as low as 20% of the total spin-down luminosity for bulk Lorentz factors of the wind in the range of 104-106. The position and size of the emission region have been studied over a wide range of energies. The position is shifted by 13'' to the west of the pulsar, with a systematic uncertainty of 25''. No significant shift in the position with energy is observed. The size of the emission region is constrained to be less than 2' at energies between 1 and 10?TeV. Above 30?TeV the size is constrained to be less than 3'. No indication of pulsed emission has been found, and upper limits in differential bins of energy have been calculated reaching typically 1%-3% of the unpulsed component.

299 citations