Magneticfield production

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 gtgt 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 D
amping 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
• Doesnt 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 dont 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 gtgt B0 CR current assumed
constant Knots and voids in NL phase MHD cant
do vacuum
15
Magnetic turbulence
Earlier PIC simulations no Brms gtgt 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 ltlt 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 with w ltlt 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