by cosmic rays drifting

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16 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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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)
p
0

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




䵡杮整楣M瑵牢畬敮捥

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?





獨潵汤s扥b瑵牢畬敮t



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
d
䈯䈠㸾>1


䑥捡c 批 捡c捡c楮朠摯d湳瑲敡洡


(MP et al. 2005)

Magnetic


filaments


arise!


d
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 J1713
-
3946:


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
d
䈠縠~
0




No parallel mode seen




but
w
㰼<
W
g

not maintained!





CR back
-
reaction: drift disappears

d
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



G
CR

= 10


V
drift

= 0.3 c



g
max

/
W
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
w

㴠=⸲㔠
g
max


Peak MF ~ 12 B
0


Decays to ~ 6 B
0

Conclusions



New simulations with

w
㰼<
W
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