Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the efficiency of Fermi acceleration at relativistic shocks   It is shown that Fermi acceleration at an ultra-relativistic shock wave
cannot operate on a particle for more than 1 1/2 Fermi cycle (i.e., u -> d -> u
-> d) if the particle Larmor radius is much smaller than the coherence length
of the magnetic field on both sides of the shock, as is usually assumed. This
conclusion is shown to be in excellent agreement with recent numerical
simulations. We thus argue that efficient Fermi acceleration at
ultra-relativistic shock waves requires significant non-linear processing of
the far upstream magnetic field with strong amplification of the small scale
magnetic power. The streaming or transverse Weibel instabilities are likely to
play a key role in this respect.

15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
26
The categories of this paper are: bayes-an
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ
19
The categories of this paper are: eess
23
The categories of this paper are: solv-int
11
The categories of this paper are: hep-lat
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
32
The categories of this paper are: atom-ph
28
The categories of this paper are: alg-geom
24
The categories of this paper are: dg-ga
2
The categories of this paper are: math