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Nereid
2007-Oct-28, 02:40 PM
This thread, and the several questions in this OP, was triggered by upriver's posts in the Breakdown of relativity? (also MAGIC delayed photon) Q&A thread (http://www.bautforum.com/questions-answers/63917-breakdown-relativity-also-magic-delayed-photon-2.html#post1099322).

To summarise: upriver asked about the extent to which a recent paper by Uchiyama et al (http://adsabs.harvard.edu/abs/2007Natur.449..576U) ("Extremely fast acceleration of cosmic rays in a supernova remnant") constituted observational support for the idea that (so-called 'galactic') cosmic rays are generated in plasma pinches in supernova remnants.

This is a general question, and involves, among other things, confusion over definitions (particularly 'plasma pinch').

As background, for readers unfamiliar with astrophysical research into the origins of 'galactic' cosmic rays, this recent review paper by Hoerendel (http://arxiv.org/abs/0710.4909), plus the references therein, may be a good place to start ("The origin of galactic cosmic rays", Invited talk given at the Roma International Conference on Astro-Particle physics (RICAP07) June 20th - 22nd, 2007. To be published in Nuclear Instruments and Methods A; the link is to the arXiv preprint abstract). In particular:
The actual mechanism of acceleration remained mysterious until Fermi [48] proposed a process that involved interaction of particles with large-scale magnetic fields in the Galaxy. Eventually, this lead to the currently accepted model of cosmic-ray acceleration by a first-order Fermi mechanism that operates in strong shock fronts which are powered by the explosions and propagate from the supernova remnant (SNR) into the interstellar medium [49,50,51,52].Yes, that's Enrico Fermi, and [48] is a 1949 publication.

Now to the specific questions:

1) What are the details of this 'first-order Fermi mechanism that operates in strong shock fronts', in terms of the physical mechanism(s)?

2) What is a 'plasma pinch'?

3) Why are the H.E.S.S. TeV gamma ray observations of the SNR RX J1713.7-3946 regarded as strongly supporting '[t]he theory of acceleration of (hadronic) cosmic rays at shock fronts in supernova remnants'?

4) In what way(s) does the Uchiyama et al. paper provide additional, independent support for this theory?

5) To what extent can a 'first-order Fermi mechanism that operates in strong shock fronts' be regarded as a plasma pinch?

upriver
2007-Oct-30, 01:52 AM
I ran across this interesting paper. I guess I wasnt the first with this question of
could GRB's be plasma pinches?


Polarization in Gamma-Ray Bursts Produced by Pinch Discharge
Authors:Wu, Mei; Chen, Li; Li, Ti-Pei
Affiliation:AA(Key Lab. of Particle Astrophys., Inst. of High Energy Phys., Chinese Academy of Sciences, Beijing 100039 ), AB(Department of Astronomy, Beijing Normal University, Beijing 100875 ), AC(Key Lab. of Particle Astrophys., Inst. of High Energy Phys., Chinese Academy of Sciences, Beijing 100039 ; Center for Astrophysics, Tsinghua University, Beijing 100084)
Publication:Chinese Journal of Astronomy and Astrophysics, Volume 5, Issue 1, pp. 57-64 (2005).
Publication Date:02/2005
Origin:IOP
DOI:10.1088/1009-9271/5/1/007
Bibliographic Code:2005ChJAA...5...57W

Abstract
Large-voltage, high-temperature plasma columns produced by pinch discharge can generate γ-ray flashes with energy spectra and spectral evolution consistent with what are observed in γ-ray bursts (GRBs), and the inverse Compton scattering (ICS) during the discharge process can produce high linear polarization. Our calculation indicates that the observed polarization depends on the angle between the line-of-sight to the GRB and the direction of the pinch discharge, but only weakly depends on observed γ-ray energy.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ChJAA...5...57W&db_key=AST&data_type=HTML&format=&high=42ca922c9c03905

Nereid
2007-Oct-30, 11:58 AM
I ran across this interesting paper. I guess I wasnt the first with this question of
could GRB's be plasma pinches?


Polarization in Gamma-Ray Bursts Produced by Pinch Discharge
Authors:Wu, Mei; Chen, Li; Li, Ti-Pei
Affiliation:AA(Key Lab. of Particle Astrophys., Inst. of High Energy Phys., Chinese Academy of Sciences, Beijing 100039 ), AB(Department of Astronomy, Beijing Normal University, Beijing 100875 ), AC(Key Lab. of Particle Astrophys., Inst. of High Energy Phys., Chinese Academy of Sciences, Beijing 100039 ; Center for Astrophysics, Tsinghua University, Beijing 100084)
Publication:Chinese Journal of Astronomy and Astrophysics, Volume 5, Issue 1, pp. 57-64 (2005).
Publication Date:02/2005
Origin:IOP
DOI:10.1088/1009-9271/5/1/007
Bibliographic Code:2005ChJAA...5...57W

Abstract
Large-voltage, high-temperature plasma columns produced by pinch discharge can generate γ-ray flashes with energy spectra and spectral evolution consistent with what are observed in γ-ray bursts (GRBs), and the inverse Compton scattering (ICS) during the discharge process can produce high linear polarization. Our calculation indicates that the observed polarization depends on the angle between the line-of-sight to the GRB and the direction of the pinch discharge, but only weakly depends on observed γ-ray energy.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005ChJAA...5...57W&db_key=AST&data_type=HTML&format=&high=42ca922c9c03905
Would you mind sticking to the topic?

This thread is about the proposed mechanisms for the production of galactic cosmic rays, in supernova remnants, not GRBs.

tusenfem
2007-Oct-30, 02:04 PM
1) What are the details of this 'first-order Fermi mechanism that operates in strong shock fronts', in terms of the physical mechanism(s)?


1st order Fermi acceleration can be seen as a pingpong ball traveling between two bats that are moving towards each other. Thus with every reflection the pingpong ball will gain energy. In the case of electrons and ions one needs something plasma physical, e.g. a shock front or magnetic mirror locations.

A particle can cross a shock multiple times (which first seems illogical, but I will not go into details, one can look it up in a book) and gain energy with each crossing.

A particle can be "trapped" between two moving magnetic mirrors which can accelerate it.



2) What is a 'plasma pinch'?


A plasma pinch is a location in a magnetized plasma where there is a contracting instability at work. One example is the Bennett pinch, in which the current flowing creates a compressional Lorentz force (J x B) which is able to overcome the thermal pressure of the filament and thus contracting at a certain location.

A generalization of this would be a sausage instability in a plasma.



3) Why are the H.E.S.S. TeV gamma ray observations of the SNR RX J1713.7-3946 regarded as strongly supporting '[t]he theory of acceleration of (hadronic) cosmic rays at shock fronts in supernova remnants'?

4) In what way(s) does the Uchiyama et al. paper provide additional, independent support for this theory?


No answers from me at the moment.



5) To what extent can a 'first-order Fermi mechanism that operates in strong shock fronts' be regarded as a plasma pinch?

This is not a plasma pinch, in my opinion, this is most likely multiple crossings of the shock front by the particle, which continuously accelerates them. One does not need a magnetic mirror, what in fact a plasma pinch would be.
Although the magnetic field comes into play, because the particle will cross back because of the larger Larmor radius in the downstream side of the shock. Here is an explanation (http://polywww.in2p3.fr/activites/gdr/pche06/doc/MLemoine_Shock.pdf)

upriver
2007-Nov-03, 03:08 AM
Magnetic cocoons power energetic cosmic rays.
http://space.newscientist.com/article/dn12818

upriver
2007-Nov-03, 05:59 AM
The mechanism for cosmic ray acceleration is Fermi acceleration.
What is that?
Here they give you and idea of what it is.
http://en.wikipedia.org/wiki/Fermi_acceleration

"Shock waves typically have moving magnetic inhomogeneities both preceding and following them. Consider the case of a charged particle traveling through the shock wave (from upstream to downstream)."

"Magnetic “inhomogeneities” or plasma waves "



The injection problem.
A mystery of first order Fermi processes is the injection problem. In the environment of a shock, only particles with energies that exceed the thermal energy by much (a factor of a few at least) can cross the shock and 'enter the game' of acceleration. It is not known at the time what mechanism causes the particles to initially have high enough energy to do so.


We will assume that there is enough energy for the electron to become caught by the magnetic mirror. The magnetic mirror in this case is the set of plasma waves in front of and behind the shock wave.

"Adiabatic invariants are useful to understand many interesting properties of collisionless plasma in cosmic magnetic fields: trapping and acceleration of charged particles in collapsing magnetic traps, the Fermi acceleration, “ cosmic rays” origin."

An adiabatic invariant is a property of a physical system which stays constant when changes are made slowly.

There is an adiabatic invariant associated with the periodic gyration of a charged particle around magnetic field-lines.



"Interactions with the background radiation field, in the presence of magnetic field inhomogeneities, will break the O’Neil’s adiabatic invariant. (F. Robicheaux) "

Consider a diamagnetic particle bouncing between magnetic mirrors and exchanging photons with the background radiation field.

The mirrors form an axial well whose “spring constant” is a function of the perpendicular energy of the particle.

Thus, the spring constant will change as the particle exchanges photons with the background field.
This will couple the parallel and perpendicular degrees of freedom."


As the shock wave moves outward, the magnetic mirror couples with particles and provides second order Fermi acceleration.

Slide 16 in the following presentation is where the section on Magnetic fields and acceleration starts. This gives a good background behind the idea with math.


Cosmic Rays and Galactic Field
http://research.amnh.org/~mordecai/g9001/class5.ppt


This would be my Google understanding of Fermi acceleration.