Publications by Tony Bell


Transport of high-energy charged particles through spatially-intermittent turbulent magnetic fields

Astrophysical Journal American Astronomical Society 892 (2020) 114

LE Chen, AFA Bott, P Tzeferacos, A Rigby, A Bell, R Bingham, C Graziani, J Katz, R Petrasso, G Gregori, F Miniati

Identifying the sources of the highest energy cosmic rays requires understanding how they are deflected by the stochastic, spatially intermittent intergalactic magnetic field. Here we report measurements of energetic charged-particle propagation through a laser-produced magnetized plasma with these properties. We characterize the diffusive transport of the particles experimentally. The results show that the transport is diffusive and that, for the regime of interest for the highest-energy cosmic rays, the diffusion coefficient is unaffected by the spatial intermittency of the magnetic field.


Cosmic ray acceleration in hydromagnetic flux tubes

Monthly Notices of the Royal Astronomical Society Oxford University Press 487 (2019) 4571-4579

AR Bell, J Matthews, KM Blundell, AT Araudo

We find that hydromagnetic flux tubes in back-flows in the lobes of radio galaxies offer a suitable environment for the acceleration of cosmic rays (CR) to ultra-high energies. We show that CR can reach the Hillas (1984) energy even if the magnetized turbulence in the flux tube is not sufficiently strong for Bohm diffusion to apply. First-order Fermi acceleration by successive weak shocks in a hydromagnetic flux tube is shown to be equivalent to second-order Fermi acceleration by strong turbulence.


Ultra-high energy cosmic rays from shocks in the lobes of powerful radio galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 482 (2018) 4303–4321-

J Matthews, B Bell, K Blundell, AT Araudo

The origin of ultra-high energy cosmic rays (UHECRs) has been an open question for decades. Here, we use a combination of hydrodynamic simulations and general physical arguments to demonstrate that UHECRs can in principle be produced by diffusive shock acceleration (DSA) in shocks in the backflowing material of radio galaxy lobes. These shocks occur after the jet material has passed through the relativistic termination shock. Recently, several authors have demonstrated that highly relativistic shocks are not effective in accelerating UHECRs. The shocks in our proposed model have a range of non-relativistic or mildly relativistic shock velocities more conducive to UHECR acceleration, with shock sizes in the range 1 − 10 kpc. Approximately 10% of the jet’s energy flux is focused through a shock in the backflow of M > 3. Although the shock velocities can be low enough that acceleration to high energy via DSA is still efficient, they are also high enough for the Hillas energy to approach 1019−20 eV, particularly for heavier CR composition and in cases where fluid elements pass through multiple shocks. We discuss some of the more general considerations for acceleration of particles to ultra-high energy with reference to giant-lobed radio galaxies such as Centaurus A and Fornax A, a class of sources which may be responsible for the observed anisotropies from UHECR observatories.


Cosmic ray acceleration by shocks: spectral steepening due to turbulent magnetic field amplification

Monthly Notices of the Royal Astronomical Society Oxford University Press 488* (2019) 2466-2472

A Bell, J Matthews, K Blundell

We show that the energy required to turbulently amplify magnetic field during cosmic ray (CR) acceleration by shocks extracts energy from the CR and steepens the CR energy spectrum.


Supersonic plasma turbulence in the laboratory

Nature Communications Nature Research 10 (2019) 1758

TG White, MT Oliver, P Mabey, AFA Bott, AA Schekochihin, G Gregori


Cosmic ray acceleration to ultrahigh energy in radio galaxies

EPJ Web of Conferences EDP Sciences 210 (2019) 04002

J Matthews, AR Bell, AT Araudo, KM Blundell

The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In this proceeding, we first review the general physical requirements that a source must meet for acceleration to 10-100 EeV, including the consideration that the shock is not highly relativistic. We show that shocks in the backflows of radio galaxies can meet these requirements. We discuss a model in which giant-lobed radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a more powerful past episode. We also show that Centaurus A, Fornax A and other radio galaxies may explain the observed anisotropies in data from the Pierre Auger Observatory, before examining some of the difficulties in associating UHECR anisotropies with astrophysical sources.


Electron acceleration by wave turbulence in a magnetized plasma

Nature Physics Springer Nature 14 (2018) 475-479

A Rigby, F Cruz, B Albertazzi, R Bamford, A Bell, JE Cross, F Fraschetti, P Graham, Y Hara, PM Kozlowski, Y Kuramitsu, DQ Lamb, S Lebedev, F Miniati, T Morita, M Oliver, B Reville, Y Sakawa, S Sarkar, C Spindloe, R Trines, R Bingham, M Koenig, G Gregori

Astrophysical shocks are commonly revealed by the non-thermal emission of energetic electrons accelerated in situ1,2,3. Strong shocks are expected to accelerate particles to very high energies4,5,6; however, they require a source of particles with velocities fast enough to permit multiple shock crossings. While the resulting diffusive shock acceleration4 process can account for observations, the kinetic physics regulating the continuous injection of non-thermal particles is not well understood. Indeed, this injection problem is particularly acute for electrons, which rely on high-frequency plasma fluctuations to raise them above the thermal pool7,8. Here we show, using laboratory laser-produced shock experiments, that, in the presence of a strong magnetic field, significant electron pre-heating is achieved. We demonstrate that the key mechanism in producing these energetic electrons is through the generation of lower-hybrid turbulence via shock-reflected ions. Our experimental results are analogous to many astrophysical systems, including the interaction of a comet with the solar wind9, a setting where electron acceleration via lower-hybrid waves is possible.


Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma

Nature Communications Springer Nature 9 (2018) 591

P Tzeferacos, A Rigby, A Bott, A Bell, R Bingham, A Casner, F Cattaneo, EM Churazov, J Emig, F Fiuza, CB Forest, J Foster, C Graziani, J Katz, M Koenig, CK Li, J Meinecke, R Petrasso, HS Park, BA Remington, JS Ross, D Ryu, D Ryutov, TG White, B Reville

Magnetic fields are ubiquitous in the Universe. Diffuse radiosynchrotron emission observations and Faraday rotation measurements have revealed magnetic field strengths ranging from a few nG and tens of µG in extragalactic disks, halos and clusters [1], up to hundreds of TG in magnetars, as inferred from their spin-down [2]. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations [3–7]. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.


On the maximum energy of non-thermal particles in the primary hotspot of Cygnus A

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 473 (2018) 3500-3506

AT Araudo, AR Bell, KM Blundell, JH Matthews


Fornax A, Centaurus A and other radio galaxies as sources of ultra-high energy cosmic rays

Monthly Notices of the Royal Astronomical Society: Letters Oxford University Press 479 (2018) L76-L80

JH Matthews, AR Bell, KM Blundell, AT Araudo

The origin of ultra-high energy cosmic rays (UHECRs) is still unknown. It has recently been proposed that UHECR anisotropies can be attributed to starbust galaxies or active galactic nuclei. We suggest that the latter is more likely and that giant-lobed radio galaxies such as Centaurus A and Fornax A can explain the data.


Cosmic-ray acceleration by relativistic shocks: limits and estimates

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 473 (2018) 2364-2371

AR Bell, AT Araudo, JH Matthews, KM Blundell


Amplification of perpendicular and parallel magnetic fields by cosmic ray currents

Monthly Notices of the Royal Astronomical Society Oxford University Press 469 (2017) 1849-1860

JH Matthews, A Bell, KM Blundell, AT Araudo

Cosmic ray (CR) currents through magnetized plasma drive strong instabilities producing amplification of the magnetic field. This amplification helps explain the CR energy spectrum as well as observations of supernova remnants and radio galaxy hotspots. Using magnetohydrodynamic simulations, we study the behaviour of the non-resonant hybrid (NRH) instability (also known as the Bell instability) in the case of CR currents perpendicular and parallel to the initial magnetic field. We demonstrate that extending simulations of the perpendicular case to 3D reveals a different character to the turbulence from that observed in 2D. Despite these differences, in 3D the perpendicular NRH instability still grows exponentially far into the non-linear regime with a similar growth rate to both the 2D perpendicular and 3D parallel situations. We introduce some simple analytical models to elucidate the physical behaviour, using them to demonstrate that the transition to the non-linear regime is governed by the growth of thermal pressure inside dense filaments at the edges of the expanding loops. We discuss our results in the context of supernova remnants and jets in radio galaxies. Our work shows that the NRH instability can amplify magnetic fields to many times their initial value in parallel and perpendicular shocks.


Numerical modeling of laser-driven experiments aiming to demonstrate magnetic field amplification via turbulent dynamo

Physics of Plasmas AIP Publishing 24 (2017) 041404

P Tzeferacos, A Rigby, A Bott, A Bell, R Bingham, A Casner, F Cattaneo, EM Churazov, J Emig, N Flocke, F Fiuza, CB Forest, J Foster, C Graziani, J Katz, M Koenig, C-K Li, J Meinecke, R Petrasso, H-S Park, BA Remington, JS Ross, D Ryu, D Ryutov, TG White

The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model behind cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo to the values observed. We have conceived experiments that aim to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here, we describe the design of these experiments through simulation campaigns using FLASH, a highly capable radiation magnetohydrodynamics code that we have developed, and large-scale three-dimensional simulations on the Mira supercomputer at the Argonne National Laboratory. The simulation results indicate that the experimental platform may be capable of reaching a turbulent plasma state and determining the dynamo amplification. We validate and compare our numerical results with a small subset of experimental data using synthetic diagnostics.


Particle acceleration and magnetic field amplification in hotspots of FR II galaxies: The case study 4C74.26

Astrophysical Journal University of Chicago Press (2015)

AT Araudo, AR Bell, KM Blundell

It has been suggested that relativistic shocks in extragalactic sources may accelerate the most energetic cosmic rays. However, recent theoretical advances indicating that relativistic shocks are probably unable to accelerate particles to energies much larger than a PeV cast doubt on this. In the present contribution we model the radio to X-ray emission in the southern hotspot of the quasar 4C74.26. The synchrotron radio emission is resolved near the shock with the MERLIN radio-interferometer, and the rapid decay of this emission behind the shock is interpreted as the decay of the downstream magnetic field as expected for small scale turbulence. If our result is confirmed by analyses of other radiogalaxies, it provides firm observational evidence that relativistic shocks at the termination region of powerful jets in FR II radiogalaxies do not accelerate ultra high energy cosmic rays.


Evidence that the maximum electron energy in hotspots of FR II galaxies is not determined by synchrotron cooling

Monthly Notices of the Royal Astronomical Society Oxford University Press 460 (2016) 3554-3562

AT Araudo, A Bell, A Crilly, K Blundell

It has been suggested that relativistic shocks in extragalactic sources may accelerate the highest energy cosmic rays. The maximum energy to which cosmic rays can be accelerated depends on the structure of magnetic turbulence near the shock but recent theoretical advances indicate that relativistic shocks are probably unable to accelerate particles to energies much larger than a PeV. We study the hotspots of powerful radiogalaxies, where electrons accelerated at the termination shock emit synchrotron radiation. The turnover of the synchrotron spectrum is typically observed between infrared and optical frequencies, indicating that the maximum energy of non-thermal electrons accelerated at the shock is ≲ TeV for a canonical magnetic field of ~100 μG. Based on theoretical considerations we show that this maximum energy cannot be constrained by synchrotron losses as usually assumed, unless the jet density is unreasonably large and most of the jet upstream energy goes to non-thermal particles. We test this result by considering a sample of hotspots observed with high spatial resolution at radio, infrared and optical wavelengths.


Contemporary particle-in-cell approach to laser-plasma modelling

PLASMA PHYSICS AND CONTROLLED FUSION 57 (2015) ARTN 113001

TD Arber, K Bennett, CS Brady, A Lawrence-Douglas, MG Ramsay, NJ Sircombe, P Gillies, RG Evans, H Schmitz, AR Bell, CP Ridgers


Cosmic ray origins in supernova blast waves

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 447 (2015) 2224-2234

AR Bell


Collisionless shocks and TeV neutrinos before Supernova shock breakout from an optically thick wind

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 449 (2015) 3693-3699

G Giacinti, AR Bell


Developed turbulence and nonlinear amplification of magnetic fields in laboratory and astrophysical plasmas.

Proceedings of the National Academy of Sciences of the United States of America National Academy of Sciences 112 (2015) 8211-8215

J Meinecke, P Tzeferacos, AR Bell, R Bingham, RJ Clarke, EM Churazov, R Crowston, H Doyle, RP Drake, R Heathcote, M Koenig, Y Kuramitsu, CC Kuranz, D Lee, MJ MacDonald, CD Murphy, MM Notley, H-S Park, A Pelka, A Ravasio, B Reville, Y Sakawa, WC Wan, NC Woolsey, R Yurchak

The visible matter in the universe is turbulent and magnetized. Turbulence in galaxy clusters is produced by mergers and by jets of the central galaxies and believed responsible for the amplification of magnetic fields. We report on experiments looking at the collision of two laser-produced plasma clouds, mimicking, in the laboratory, a cluster merger event. By measuring the spectrum of the density fluctuations, we infer developed, Kolmogorov-like turbulence. From spectral line broadening, we estimate a level of turbulence consistent with turbulent heating balancing radiative cooling, as it likely does in galaxy clusters. We show that the magnetic field is amplified by turbulent motions, reaching a nonlinear regime that is a precursor to turbulent dynamo. Thus, our experiment provides a promising platform for understanding the structure of turbulence and the amplification of magnetic fields in the universe.


Particle acceleration and magnetic field amplification in the jets of 4C74.26

Astrophysical Journal American Astronomical Society 806 (2015) ARTN 243-

AT Araudo, A Bell, K Blundell

We model the multi-wavelength emission in the southern hotspot of the radio quasar 4C74.26. The synchrotron radio emission is resolved near the shock with the MERLIN radio-interferometer, and the rapid decay of this emission behind the shock is interpreted as the decay of the amplified downstream magnetic field as expected for small scale turbulence. Electrons are accelerated to only 0.3 TeV, consistent with a diffusion coefficient many orders of magnitude greater than in the Bohm regime. If the same diffusion coefficient applies to the protons, their maximum energy is only ~100 TeV.

Pages