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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill J. L. Han: The Global Structure of
    Magnetic Fields in Our Galaxy  %%�����ߣ���Ŀ���ƣ�
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    \zihao{6}\songti
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    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf The Global Structure of
    Magnetic Fields in Our Galaxy   %%�����⣩
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\vs\vs{\normalsize J. L. Han}  %%���������ߣ�

\vs

(\nao)
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
The global structure of magnetic fields in our Galaxy is hard to
know completely. However, we have obtained some knowledge from
rotation measures of pulsars and extragalactic radio sources.
Pulsars provide unique probes for the {\it large-scale}
interstellar magnetic field in the Galactic disk. The magnetic
field in the nearby half of the Galactic disk are found to
be {\it coherent in directions} over a linear scale of $\sim
10$\,kpc between the Carina-Sagittarius and Crux-Scutum arms from
$l\sim45\dg$ to $l\sim305\dg$. The coherent spiral structures and
field direction reversals, including the newly determined
counterclockwise field near the Norma arm, are consistent with the
bi-symmetric spiral model for the disk field. In the Galactic halo
and Galactic center, the antisymmetric rotation measure sky
indicates the dipole field in the Galactic center and the
azimuthal field with reversed field directions below and above the
Galactic plane, which suggests that the A0 dynamo is operating in
our Galaxy.

 %\vn
 %\no{\bf Key words}~~  pulsar-mode  change-spectrum
\end{quotation}

%%%% ����
\vs\vs
The Faraday rotation of pulsars and extragalactic radio
sources are in fact the best probes of the Galactic magnetic field
on the large scale!

The rotation measure (RM) of a source is proportional to the
integration along its line of sight of the product of the radial
magnetic field and electron density, $RM=\int_{\rm obs}^{\rm obj}
B_{||} n_e dl$. Marvellously our Galaxy is the visually largest edge-on
Galaxy covering all the sky. There are so many sources, either the
extragalactic polarized radio sources or pulsars in our Galaxy, in all
directions that contain information of the field $B_{||}$ in all parts
of the Galaxy. This gives us a unique chance to study the detailed
structure of the magnetic field in the Galactic halo and disk, with a
principle similar to the CT technology used medically. I emphasize
that this is not possible for any other galaxies.

The antisymmetric RM
distribution of extragalactic radio sources as well as pulsars,
covering all the inner Galaxy, reflects the azimuthal field with
reversed directions above and below the Galactic plane (Fig.1),
entirely consistent with the field configuration of an A0 dynamo,
so that the field origin probably lies in a dynamo in the halo
(Han et al. 1997).

The polar field in the Galactic center strongly supports this
conclusion.

\begin{figure}[tbph]
\center {\includegraphics [bb=0 178 408
456,scale=0.65]{a0_dynamo.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
 Fig.\,1~~ The RMs of extragalactic radio sources show the antisymmetric
field structure of the Galactic halo, which is the same as that of
an A0 dynamo (see Han et al. 1997 for details).  Filled symbols
denote positive RMs and open symbols negative RMs. The
distribution of pulsar RMs is very similar but with generally
smaller RM values.}
\end{figure}
\begin{figure}[tbph]
\center {\includegraphics [bb=0 162 408
460,scale=0.8]{psr_rm.eps}}
\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize
 Fig.\,2~~ The distribution of pulsar RMs projected onto the Galactic plane
reveals the field structure in the Galactic disk, which has
direction reversals from arm to arm (after Han et al. 2002). The
square symbols are newly determined RMs from Parkes observations,
and filled symbols indicate positive RMs. The well-determined
field structure is illustrated by thick lines and arrows. The
thick dashed lines indicate the controversial field structure
which needs further data for confirmation. The newly determined
field directions near the Norma arm and the Crux arm are indicated
by thin arrows.}

    \end{figure}

Han et al. (1997) first noticed that the RM distribution of high
latitude pulsars is dominated by the halo azimuthal field (Fig.1).
Afterwards, any analysis of pulsar RMs for the disk field was
confined to the pulsars at lower Galactic latitudes. Han et al.
(1999) then observed a good number of pulsar RMs and divided all
known pulsar RMs into those lying within higher and lower latitude
ranges for studies of the halo and disk field,  respectively. Up
to now, among about 1385 known pulsars, 523 pulsars have measured
values of RM and 365 of them lie at lower latitudes ($|b|<8\dg$).

At lower latitudes, the pulsar RMs can be used to diagnosis the
large-scale field structure in Galactic disk. What we measured is the
integration of B-fields along the line of sight. One normally
assumes that the azimuthal field component $B_{\phi}$ is greater than
vertical and radial components $B_z$ or $B_r$.  This is reasonable and
has been justified (Han \& Qiao 1994, Han et al.  1999). In most lines
of sights, this integration reflects the major components $B_{\phi}$
of the large-scale fields. For distant region, one can use  the gradient
of the average or general tendency of RM variations versus pulsar DMs
to trace the large-scale field.  The scatter of
the data about this general tendency is probably mostly due to the
effect of smaller scale interstellar structure, such as the HII
regions.

Using all available pulsar RM data, together with our more than 200
new measurements of pulsars discovered in Parkes multibeam pulsar
survey (Manchester et al. 2001), we recently report the detection of
counterclockwise magnetic fields near the Norma arm, which are {\it
coherent in field direction} over more than 5 kpc along the arm (Han et al.
2002). Our
present understanding of the structure of the {\it large-scale} magnetic
field in the Galactic disk is summarised in Fig.2, which is almost all
revealed by pulsar RMs.

We conclude that based on all available information, the
global field structure is best described by two distinct components:
a bi-symmetric spiral field in the disk  with reversed direction from
arm to arm, and an azimuthal field in the halo with reversed directions
below and above the Galactic plane.  The latter, together with
a probably dipole field in the Galactic center, corresponds to an A0
dynamo field configuration. Pulsars are the most important probes for
detection of the {\it large-scale} interstellar fields.

\vs\vn{\bf ACKNOWLEDGEMENTS}~~ The project is supported by the
National Natural Science Foundation of China (19903003 and
10025313), the National Key Basic Research Science Foundation of
China (G19990754) and the partner group of MPIfR at NAOC.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Han J.L., Manchester R.N., et al., A\&A, 1997,  322: 98\\
\no Han J.L., Manchester R.N., Lyne A.G., et al.,
 ApJ, 2002, 570: L17\\
\no Han J.L., Manchester R.N., Qiao G.J., MNRAS, 1999, 306: 371\\
\no Han J.L., Qiao G.J., A\&A, 1994, 288: 759\\
 \no
Manchester R.N., Lyne A.G., Camilo F., et al., MNRAS, 2001, 328:
17\\

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