Magnetic-field production by cosmic rays drifting upstream of SNR shocks Martin Pohl, ISU with Tom Stroman, ISU, Jacek Niemiec, PAN

Supernova remnants SNR can be resolved in TeV-band gamma rays! TeV band (HESS)   or IC keV band (ASCA) synchrotron

Supernova remnants Young SNR are ideal laboratories Important questions: Particle acceleration and magnetic turbulence What produces strong magnetic turbulence?

Supernova remnants Relative drift  Magnetic turbulence

Magnetic field amplification Observation: Nonthermal X-rays in filaments Requires strong magnetic field Magnetic turbulence related to particle acceleration?

Magnetic field amplification X-ray filaments involve strong magnetic field Origin unknown Fate unknown Shock? Energetic particles?  should be turbulent If persisting, MF must be very strong Turbulent field should cascade away … Not seen in radio polarimetry… How strong and where is it?

Magnetic field amplification X-ray filaments suggest  B/B >> 1 Decay by cascading downstream! (MP et al. 2005) Magnetic filaments arise!  B not determined

Magnetic field amplification Estimate magnetic-field strength using spectra? Depends on what electron spectrum you assume….. Factor 3 variation Voelk et al. 2008, modified by MP

Magnetic field amplification Clues from X-ray variability? (Uchiyama et al. 2007) Energy losses require a few milliGauss! BUT: Damping gives same timescale

Magnetic field amplification Strong field in entire SNR? No! RX J : X-ray variability  a few milliGauss (Uchiyama et al. 2007) Produces too much radio emission from secondaries (Huang & Pohl 2008)

Magnetic field amplification Radio polarization at rim of Tycho (Dickel 1991) Radial fields at 6cm Polarization degree 20-30% Doesn’t fit to turbulently amplified field! Models require homogeneous radial field (Stroman & Pohl, in prep.) Support for rapid damping?

Magnetic turbulence Level and distribution of amplified MF unclear What produces strong magnetic turbulence? Upstream: Relative motion of cosmic rays and cool plasma

Magnetic turbulence Most important: Saturation process and level Electrons and ions don’t form single fluid Coupling via electromagnetic fields Changes in the distribution functions Small-scale physics dominates large-scale structure  Particle-in-Cell simulations

Magnetic turbulence MHD simulations: B rms >> B 0 CR current assumed constant Knots and voids in NL phase MHD can’t do vacuum Analytical theory (e.g. Tony Bell): Streaming cosmic rays produce purely growing MF Wave-vector parallel to streaming

Magnetic turbulence Earlier PIC simulations: no B rms >> B 0 3-D 2-D, larger system Niemiec et al. 2008

Magnetic turbulence Magnetic-field growth seen Saturation near  B ~ B 0 No parallel mode seen but  <<  g not maintained! CR back-reaction: drift disappears  B larger when CR back-reaction turned off!

Particle distributions Establish common bulk motion

New simulations 2.5-D only! Parameters: N i / N CR = 50  CR = 10 V drift = 0.3 c  max /  g,i = 0.3 See poster by Tom Stroman

New simulations Parallel mode seen! B y N i

New simulations Drifts speeds align to 0.06 c Overshoot in drift speed? Im  = 0.25  max Peak MF ~ 12 B 0 Decays to ~ 6 B 0

Conclusions New simulations with  <<  g Parallel mode seen! Saturation still through changes in bulk speed Saturation level still at a few B 0 … may be enough Substantial density fluctuations Conclusions of Niemiec et al. (2008) still hold

Back-up slides

Particle distributions Energy transferred to background plasma

Particle distributions Isotropy roughly preserved Heating possibly artificial