Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Canals in Milky Way radio polarization maps   Narrow depolarized canals are common in maps of the polarized synchrotron
emission of the Milky Way. Two physical effects that can produce these canals
have been identified: the presence of Faraday rotation measure ($\RM$)
gradients in a foreground screen and the cumulative cancellation of
polarization known as differential Faraday rotation. We show that the behaviour
of the Stokes parameters $Q$ and $U$ in the vicinity of a canal can be used to
identify its origin. In the case of canals produced by a Faraday screen we
demonstrate that, if the polarization angle changes by $90\degr$ across the
canal, as is observed in all fields to-date, the gradients in $\RM$ must be
discontinuous. Shocks are an obvious source of such discontinuities and we
derive a relation of the expected mean separation of canals to the abundance
and Mach number of supernova driven shocks, and compare this with recent
observations by \citet{Haverkorn03}. We also predict the existence of less
common canals with polarization angle changes other than $90\degr$.
Differential Faraday rotation can produce canals in a uniform magneto-ionic
medium, but as the emitting layer becomes less uniform the canals will
disappear. We show that for moderate differences in emissivity in a two-layer
medium, of up to 1/2, and for Faraday depth fluctuations of standard deviation
$\lesssim 1 \mathrm{rad}$, canals produced by differential rotation will still
be visible.

17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
29
The categories of this paper are: funct-an
10
The categories of this paper are: q-bio
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
27
The categories of this paper are: comp-gas
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
18
The categories of this paper are: econ
26
The categories of this paper are: bayes-an
34
The categories of this paper are: chem-ph
11
The categories of this paper are: hep-lat