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Magnetic-field production by cosmic rays drifting upstream of SNR shocks

This research investigates the production of magnetic turbulence and the amplification of magnetic fields by cosmic rays upstream of supernova remnant shocks. The study explores the relationship between magnetic turbulence and particle acceleration, as well as the origins and fate of strong magnetic fields observed in X-ray filaments.

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Magnetic-field production by cosmic rays drifting upstream of SNR shocks

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

  2. Supernova remnants SNR can be resolved in TeV-band gamma rays! TeV band (HESS) p0 or IC keV band (ASCA) synchrotron

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

  4. Supernova remnants Relative drift  Magnetic turbulence

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

  6. 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?

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

  8. 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

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

  10. Magnetic field amplification • Strong field in entire SNR? • No! • RX J1713-3946: • X-ray variability • a few milliGauss • (Uchiyama et al. 2007) • Produces too much • radio emission from • secondaries • (Huang & Pohl 2008)

  11. 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?

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

  13. 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

  14. Magnetic turbulence • Analytical theory (e.g. Tony Bell): • Streaming cosmic rays produce purely growing MF • Wave-vector parallel to streaming MHD simulations: Brms >> B0 CR current assumed constant Knots and voids in NL phase MHD can’t do vacuum

  15. Magnetic turbulence Earlier PIC simulations: no Brms >> B0 3-D 2-D, larger system Niemiec et al. 2008

  16. Magnetic turbulence • Magnetic-field growth seen • Saturation near dB ~ B0 • No parallel mode seen • but w << Wg not maintained! • CR back-reaction: drift disappears dB larger when CR back-reaction turned off!

  17. Particle distributions Establish common bulk motion

  18. New simulations 2.5-D only! Parameters: Ni / NCR = 50 GCR= 10 Vdrift= 0.3 c gmax / Wg,i = 0.3 See poster by Tom Stroman

  19. New simulations Parallel mode seen! By Ni

  20. New simulations Drifts speeds align to 0.06 c Overshoot in drift speed? Im w = 0.25 gmax Peak MF ~ 12 B0 Decays to ~ 6 B0

  21. Conclusions • New simulations withw << Wg • Parallel mode seen! • Saturation still through changes in bulk speed • Saturation level still at a few B0 … may be enough • Substantial density fluctuations • Conclusions of Niemiec et al. (2008) still hold

  22. Back-up slides

  23. Particle distributions Energy transferred to background plasma

  24. Particle distributions Isotropy roughly preserved Heating possibly artificial

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