Tony Bell

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Tony Bell

Professor, FRS

RESEARCH INTERESTS

Astrophysical plasmas


Cosmic rays are high energy particles, mostly protons, arriving at the Earth and detected in many places in the Universe. Blast waves launched by supernova explosions (Cassiopeia A is a good example) are probably the main source of cosmic rays in our Milky Way Galaxy, reaching energies up to a few PeV. The highest energy cosmic rays with energies exceeding 100EeV are almost certainly accelerated in even more energetic events outside our own galaxy. Our aim is to understand the plasma processes responsible for cosmic ray acceleration and to explain cosmic ray origins.

Our recent paper on cosmic ray acceleration by relativistic shocks is attached.

See the tabs above for details of the CMR project (2010-15) funded by the European Research Council on 'Cosmic ray acceleration, magnetic field and radiation hydrodynamics'.

Laser-produced plasmas

Solids and gases can be heated and converted to a plasma with a very high energy density with high power lasers. An important potential application is Inertial Fusion Energy (IFE) for electricity generation. At very high laser intensities the plasma is relativistic. Lasers are now entering a regime in which QED is important. The picture below shows a simulation (Ridgers et al 2012, 2013) of the production of electron-positron pairs which should be possible when laser intensities exceed 1023 W cm -2.

Simulation (Ridgers et al 2012, 2013) of electron-positron (red) pairs and gamma rays (blue) when a thin aluminium foil (grey) is irradiated with a high power laser

Cosmic ray acceleration, magnetic field & radiation hydrodynamics (CMR)


The CMR project (2010-15) was funded by the European Research Council (project: 247039) in the area of activity PE9, Universe Sciences.

Cosmic rays are high energy particles, mostly protons, arriving at the Earth and detected in many places in the Universe. Blast waves launched by supernova explosions (see figure below) are probably the main source of cosmic rays in our Milky Way Galaxy, reaching energies up to a few PeV. The highest energy cosmic rays with energies exceeding 100EeV are almost certainly accelerated in even more energetic events outside our own galaxy. Our aim is to understand the plasma processes responsible for cosmic ray acceleration and to explain cosmic ray origins.

CHANDRA x-ray images of four historical supernova remnants
Credits: NASA/CXC/Rutgers/J.Hughes et al.; NASA/CXC/Rutgers/J.Warren & J.Hughes et al.; NASA/CXC/NCSU/S.Reynolds et al.; NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.

Details of the CMR project
The CMR project explores the various plasma processes contributing to cosmic ray acceleration by shocks. We aim to answer such questions as whether supernova remnants are able to account for acceleration to the ‘knee’ in the cosmic ray energy spectrum (where the spectrum steepens) at a few PeV, how cosmic rays escape the supernova remnant into the interstellar medium and why the Galactic cosmic ray spectrum is steeper than expected theoretically.
Magnetic field amplification is an essential part of the process since the maximum cosmic ray energy falls far short of the knee if the magnetic field is the few microGauss typical of the interstellar medium. Our results indicate that acceleration to a few PeV is only possible in the first few hundred years following the supernova explosion when the supernova blast wave can expand above 10,000 km/s. Consequently we are interested in the very early supernova expansion when the shock emerges from a dense plasma, possibly a circumstellar wind, when radiation may be important. We find that the cosmic ray spectrum can be especially steep at early times when the shock velocity is high, and this matches observations. Surprisingly we find that cosmic ray acceleration may even begin while the supernova shock passes through the outer layers of the star and its surrounding wind before the supernova becomes visible to the outside universe.
Our simulations of cosmic ray acceleration by supernova remnants show that a small proportion of cosmic rays escape ahead of the shock into the interstellar medium. Using novel numerical techniques for 3D simulation of cosmic rays interacting self-consistently with a magnetised fluid we have constructed the first computational model of the full shock acceleration process beyond the diffusion model. We have shown that the magnetic field increases to confine the cosmic rays close to the shock for rapid acceleration. At any one time, the highest energy cosmic rays being accelerated escape ahead of the shock while the majority of cosmic rays are swept away downstream. Consequently, Galactic cosmic rays must consist of two populations, the high energy cosmic rays that escape during acceleration and the lower energy cosmic rays that sit inside a supernova remnant as a bubble of cosmic rays that are released into the interstellar medium when the supernova remnant slows down and dissipates.
We find that supernova remnants appear to be able to accelerate cosmic rays to around 1PeV where the spectrum is observed to steepen. Very special Galactic supernova remnants may be able to accelerate protons a little beyond a PeV and heavy ions to higher energies because of their large electric charge, but it seems inevitable that the highest energy cosmic rays with energies in the EeV range must be accelerated outside our Galaxy. Relativistic shocks in active galaxies and their accompanying jets, or possibly in gamma-ray bursts, appear the most likely source of EeV cosmic rays. Hence we have been investigating the amplification of magnetic field in relativistic shocks. We find that the instabilities active at lower shock velocities in supernova remnants are unable to produce magnetic field on the desired scale. Our tentative conclusion is that relativistic shocks do not accelerate EeV particles, but this conclusion needs further investigation since there seem to be few other options for acceleration to EeV.
Very young supernova remnants are the most likely accelerators of Galactic PeV protons (so-called ‘Pevatrons’) so we are investigating the initial expansion of supernova remnants. We find that shock acceleration may begin even before the supernova becomes visible to the outside world.
We are also investigating the possibility of reproducing particle acceleration in the laboratory. We provide theoretical support for experiments led by Prof Gianluca Gregori that have demonstrated the generation of magnetic field by structured shocks in a scaled reproduction of the growth of magnetic field in the early universe. Further experiments have shown that magnetic field might be produced by turbulence generated by a shock passing through an inhomogeneous medium as might be happening in the supernova remnant Cassiopeia A.
Past and present members of the CMR team:
Prof Tony Bell, Dr Brian Reville, Dr Gwenael Giacinti, Dr Anabella Araudo
For a list of CMR publications, see the tab above.

CMR publications

Araudo, A.T., Bell, A.R., Blundell, K.M.
Astrophysical Journal, accepted for publication, arXiv:1505.02210
Particle acceleration and magnetic field amplification in the jets of 4C74.26
http://arxiv.org/pdf/1505.02210v2.pdf

Giacinti, G., Bell, A.R.
MNRAS 449, 3693 (2015) arXiv:1503.04170
Collisionless shocks and TeV neutrinos before Supernova shock breakout from an optically thick wind
http://arxiv.org/pdf/1503.04170v1.pdf

Bell, A.R.
MNRAS 447, 2224 (2015) arXiv:1412.7294
Cosmic Ray Origins in Supernova Blast Waves
http://arxiv.org/pdf/1412.7294v1.pdf

Bell, AR
Brazilian Journal of Physics 44, 415 (2014) arXiv:1311.5779
Particle Acceleration by Shocks in Supernova Remnants
http://arxiv.org/pdf/1311.5779v1.pdf

Giacinti, G., Kachelriess, M., Semikoz, D. V.
Physical Review D 90, 1302 (2014) arXiv:1403.3380
Explaining the spectra of cosmic ray groups above the knee by escape from the Galaxy
http://arxiv.org/pdf/1403.3380v2.pdf

Meinecke, J. plus 26 authors
Nature Physics 10, 520 (2014)
Turbulent amplification of magnetic fields in laboratory laser-produced shock waves
http://www2.physics.ox.ac.uk/sites/default/files/2014-11-24/meinecke_2014_pdf_30444.pdf

Reville, B., Bell, A. R.
MNRAS 439, 2050 (2014) arXiv:1401.2803
On the maximum energy of shock-accelerated cosmic rays at ultra-relativistic shocks
http://arxiv.org/pdf/1401.2803v2.pdf

Schure, K. M., Bell, A. R.
MNRAS 437, 2802 (2014) arXiv:1310.7027
From cosmic ray source to the Galactic pool
http://arxiv.org/pdf/1310.7027v1.pdf

Schure, K. M., Bell, A. R.
MNRAS 435, 1174 (2013) arXiv:1307.6575
Cosmic ray acceleration in young supernova remnants
http://arxiv.org/pdf/1307.6575v1.pdf

Giacinti, G., Kachelriess, M., Semikoz, D. V.
Physical Review D 88, 023010 (2013) arXiv:1306.3209
Anisotropic cosmic ray diffusion and its implications for gamma-ray astronomy
http://arxiv.org/pdf/1306.3209v2.pdf

Bell, A. R.; Schure, K. M.; Reville, B.; Giacinti, G.
MNRAS 431, 415 (2013) arXiv:1301.7264
Cosmic-ray acceleration and escape from supernova remnants
http://arxiv.org/pdf/1301.7264v1.pdf

Reville, B.; Bell, A. R.
MNRAS 430, 2873 (2013) arXiv:1301.3173
Universal behaviour of shock precursors in the presence of efficient cosmic ray acceleration
http://arxiv.org/pdf/1301.3173v1.pdf

Bell, A. R.
Astroparticle Physics 43, 56 (2013)
Cosmic ray acceleration
http://www.sciencedirect.com/science/article/pii/S0927650512001272

Reville, B.; Bell, A. R.; Gregori, G.
New Journal of Physics 15, 5015 (2013) arXiv:1211.3638
Diffusive shock acceleration at laser-driven shocks: studying cosmic-ray accelerators in the Laboratory
http://arxiv.org/pdf/1211.3638v1.pdf

Stawarz, L.; plus 16 co-authors
ApJ 766, 48 (2013) arXiv:1210.4237
Giant Lobes of Centaurus A Radio Galaxy Observed with the Suzaku X-Ray Satellite
http://arxiv.org/pdf/1210.4237v3.pdf

Kugland, N.L. plus 27 co-authors
Nature Physics 8, 809 (2012)
Self-organized electromagnetic field structure in laser-produced counter-streaming plasmas
http://www2.physics.ox.ac.uk/sites/default/files/2014-11-24/kugland_2012_pdf_27361.pdf

Schure, K. M.; Bell, A. R.; O'C Drury, L.; Bykov, A. M.
Space Science Reviews 173, 491 (2012) arXiv:1203.1637
Diffusive Shock Acceleration and Magnetic Field Amplification
http://arxiv.org/pdf/1203.1637v1.pdf

Gregori, G.; plus 24 co-authors
Nature 481, 480 (2012),
Generation of scaled protogalactic seed magnetic fields in laser-produced shock waves
http://www2.physics.ox.ac.uk/sites/default/files/2014-11-24/gregori_2012_pdf_89542.pdf

Reville, B.; Bell, A. R.
MNRAS 419, 2433 (2012) arXiv:1109.5690
A filamentation instability for streaming cosmic rays
http://arxiv.org/pdf/1109.5690v1.pdf

Bell, A. R.; Schure, K. M.; Reville, B.
MNRAS 418, 1208 (2011) arXiv:1108.0582
Cosmic ray acceleration at oblique shocks
http://arxiv.org/pdf/1108.0582v1.pdf

Miniati, F.; Bell, A. R.
ApJ 729, 73 (2011) arXiv:1001.2011
Resistive Magnetic Field Generation at Cosmic Dawn
http://arxiv.org/pdf/1001.2011v2.pdf