Final Report Summary - CMR (Cosmic ray acceleration, magnetic field and radiation hydrodynamics)
Cosmic rays are energetic particles, mostly protons, arriving at the Earth with energies up to 200EeV. From general arguments the lower energy cosmic rays must be produced within our own Galaxy, probably by supernova remnants, but the highest energy cosmic rays must be produced outside our Galaxy. The transition from a Galactic to an extragalactic origin occurs somewhere between 1PeV and 1EeV. When first developed, the theory of diffusive shock acceleration gave a convincing account of how cosmic rays gain their energy apart from the major deficiency that the theory predicted a maximum cosmic ray energy very much smaller than that measured. The crucial breakthrough came ten years ago when it was understood theoretically how streaming cosmic rays drive a plasma instability that generates a large magnetic field and facilitates acceleration to the observed energies. Theory was confirmed by novel observations. The principal investigator of the CMR project was instrumental in the discovery of both diffusive shock acceleration and magnetic field amplification by cosmic rays. The aim of the present project was to combine the theories of shock acceleration and magnetic field amplification to construct a self-consistent model that can be compared in detail with observations and thereby further elucidate the origins of cosmic rays.
By advanced simulation and mathematical modelling we have shown that the basic model of magnetic field amplification and shock acceleration works for supernova remnants. We find that the maximum cosmic ray energy is determined by the time taken for the magnetic field to be amplified. Our simulations show that cosmic rays can be accelerated to the break in the energy spectrum at a few PeV only in supernova remnants that are younger than the ones known in our Galaxy (all greater than 100 years old). We find that cosmic ray acceleration begins at a very early stage and can even occur before a supernova becomes visible to the external world, probably producing a very early burst of energetic neutrinos in suitable circumstances.
We find that the escape of high energy cosmic rays into the interstellar medium is an integral part of the acceleration process and that variations in the escape process match features in the Galactic cosmic ray spectrum. Observations of very energetic supernova explosions show that the cosmic ray energy spectrum is steeper than the standard spectrum that holds at velocities less than 10,000 km/s. Theoretically we find that as the shock velocity increases, cosmic rays have an increasing tendency to get carried away downstream of the shock instead of being accelerated, thus steepening the spectrum. This becomes an increasing problem as the shock velocity approaches the speed of light. We show that except under exceptional conditions magnetic field growth is too slow to catch cosmic rays before they are carried away downstream of relativistic shocks. We have examined observations of the mildly relativistic shock at the termination of a quasar jet (4C74.26) and find that it accelerates cosmic rays only to relatively low energies in agreement with our theory. This combination of theory and observation suggests that relativistic shocks are probably unable to accelerate cosmic rays to EeV energies. This must be the subject of further work, but if our conclusions prove well founded it will place strong limitations on viable theories for the origins of the highest energy cosmic rays.
The basic plasma physics of magnetic field generation overlaps with the plasma physics of laser-produced plasmas. Lasers can be used to launch blast waves in a laboratory that are similar to supernova remnant blast waves. We collaborated in laser experiments that produce magnetic fields by the Biermann battery which are then amplified by turbulent motions in a process very similar to that which probably produces the ring of very strong magnetic field in the supernova remnant Cassiopeia A. Both the Biermann battery and resistive magnetic field generation (also active in laser-plasmas) produce magnetic field where there is no previous magnetic field and we have shown how these laboratory processes scale to the early universe where magnetic field must be produced in a previously unmagnetized plasma.
By advanced simulation and mathematical modelling we have shown that the basic model of magnetic field amplification and shock acceleration works for supernova remnants. We find that the maximum cosmic ray energy is determined by the time taken for the magnetic field to be amplified. Our simulations show that cosmic rays can be accelerated to the break in the energy spectrum at a few PeV only in supernova remnants that are younger than the ones known in our Galaxy (all greater than 100 years old). We find that cosmic ray acceleration begins at a very early stage and can even occur before a supernova becomes visible to the external world, probably producing a very early burst of energetic neutrinos in suitable circumstances.
We find that the escape of high energy cosmic rays into the interstellar medium is an integral part of the acceleration process and that variations in the escape process match features in the Galactic cosmic ray spectrum. Observations of very energetic supernova explosions show that the cosmic ray energy spectrum is steeper than the standard spectrum that holds at velocities less than 10,000 km/s. Theoretically we find that as the shock velocity increases, cosmic rays have an increasing tendency to get carried away downstream of the shock instead of being accelerated, thus steepening the spectrum. This becomes an increasing problem as the shock velocity approaches the speed of light. We show that except under exceptional conditions magnetic field growth is too slow to catch cosmic rays before they are carried away downstream of relativistic shocks. We have examined observations of the mildly relativistic shock at the termination of a quasar jet (4C74.26) and find that it accelerates cosmic rays only to relatively low energies in agreement with our theory. This combination of theory and observation suggests that relativistic shocks are probably unable to accelerate cosmic rays to EeV energies. This must be the subject of further work, but if our conclusions prove well founded it will place strong limitations on viable theories for the origins of the highest energy cosmic rays.
The basic plasma physics of magnetic field generation overlaps with the plasma physics of laser-produced plasmas. Lasers can be used to launch blast waves in a laboratory that are similar to supernova remnant blast waves. We collaborated in laser experiments that produce magnetic fields by the Biermann battery which are then amplified by turbulent motions in a process very similar to that which probably produces the ring of very strong magnetic field in the supernova remnant Cassiopeia A. Both the Biermann battery and resistive magnetic field generation (also active in laser-plasmas) produce magnetic field where there is no previous magnetic field and we have shown how these laboratory processes scale to the early universe where magnetic field must be produced in a previously unmagnetized plasma.