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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 R. Wielebinski: The `tomography' of the MISM %%�����ߣ���Ŀ���ƣ�
    \vspace{1.5mm}
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    \normalsize\rm
\zihao{5}\songti

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    \small\rm
    \zihao{6}\songti
    \noindent
    \\%%%[-0.5 true mm]
    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
    \hfill
      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf
The `Tomography' of the Magnetic Interstellar Medium   %%�����⣩
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Richard Wielebinski}  %%���������ߣ�

\vs ({\footnotesize {\it Max-Planck-Institut f\"ur
Radioastronomie, Auf dem H\"ugel 69,
       53121 Bonn, Germany}}) \\%%���������ߵ�λ��
\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
The Galaxy is a conglomerate of many components. We observe
starlight, radio continuum, HI radio line, various molecular
lines, UV and IR radiation, X-ray emissions and even $\gamma$-ray
events. Recent observations have shown that this interstellar
medium is permeated by magnetic fields.  Hence we should talk of
the magnetic interstellar medium (MISM) that will be the subject
of this paper.

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

\para{\large\bf{1}~~ Introduction}

\vs\vs We know little about the origin of magnetic fields. We have
just started to trace the distribution of magnetic fields in our
Galaxy, nearby galaxies, radio galaxies and clusters of galaxies.
At this stage we can say that every object seems to possess a
magnetic field. However we still cannot decide if the magnetic
fields are just a consequence of rotation only or if they play an
active role in the dynamics of galaxies. For a review of the
present situation the reader is referred to Han \& Wielebinski
(2002) or Beck (2000).

There are several methods of measuring magnetic fields. The
classical approach is to search for the signature of the Zeeman
effect. However considerable magnetic field intensity is needed to
``see'' the effect and hence observations of this type are limited
to molecular clouds, in particular to regions with masers.
Magnetic fields have been observed by polarization studies of both
optical and radio waves. In the optical wavelength range it is the
aligned dust that gives a measurable effect. At cm radio
wavelength the polarized synchrotron emission is caused by
electrons braked in magnetic fields and hence the measurement of
polarization gives us the information about the magnetic fields in
the emission regions. The emitted wave polarization is
perpendicular to the magnetic field orientation (i.e. $B_\perp$).
The polarized wave will however be rotated by the Faraday effect
in the intervening thermal regions. This has to be corrected for,
or else the Faraday effect can be used to give us information
about $B_\parallel$. In any direction of observation usually
several Faraday screens are seen as illustrated in Fig.\,1.

I will present some recent observations on our Milky Way and for nearby
galaxies. In particular I will show how different types of
observations can be combined, especially for our Galaxy, to
reconstitute the distribution of the magnetic fields in three
dimensions: hence perform a `tomography' of the MISM.

\begin{figure}[h]
\center {\includegraphics [bb=138 99 722
514,scale=0.3]{rw2-fig1.eps}}

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   %{\hspace{3.5cm}
   {\center
   \footnotesize

      Fig.\,1~~ Observation of two emission regions with two intervening
Faraday screens}

    \end{figure}

\para{\large\bf{2}~~ The Galactic Foreground}

\vs\vs The radio Galaxy is best seen as low-frequency radio
emission, like in the 408\,MHz survey of Haslam et al. (1982).
Polarization surveys of the northern sky have been made by Brouw
\& Spoelstra (1976) at several frequencies which led to the
suggestion by Speolstra (1984) that we have very low rotation
measures (�$\rm RM \sim \pm 8\, rad/m^2$) in our Galaxy. New
polarization surveys of the Galactic plane at 2.3/2.7~GHz by
Duncan et al. (1997; 1999) showed a considerable depolarization
implying  Faraday thick regions. More recently observations by
Uyaniker et al. (1999) at 1.4~GHz with an angular resolution of
$9^{\prime}$ showed deep nulls in polarized intensity, a
consequence of RMs of $\rm 70\, rad/m^2$ or more. Also Gaensler et
al. (2001) observed a section of the Galactic plane towards $l =
330^{\circ}$ and found RMs of $\rm \pm 150\, rad/m^2$. The Faraday
effects in the Galactic foreground can be considerable,
particularly at lower radio frequencies. The numerical values for
Faraday effects are given in Table~1. In the radio continuum of
the Milky Way we find RMs of $\sim\, 1$---$\rm 150\, rad/m^2$. RM
of pulsars or extragalactic radio sources (EGRS) can be as high as
$\rm 1000\, rad/m^2$. In the Galactic Center but also in AGNs we
have RMs of $\rm 5000\, rad/m^2$ or more. An example of a recent
high angular resolution map of a section of the Galaxy is shown in
Fig.\,2. We see highly polarized regions away from the Galactic
plane. Some polarized sources appear apparently not connected with
any total intensity emission. This can happen only when
considerable Faraday depth exists in the Galactic plane emission.

\begin{figure}[h]
\center {\includegraphics [bb=0 153 408
462,scale=0.7]{rw2-fig2.eps}}

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   %{\hspace{3.5cm}
   {\center
   \footnotesize

      Fig.\,2~~ A section of a 1.4~GHz survey from Uyaniker et al. (1999)}

    \end{figure}
%Fig. 2
\bc{\small\bf

 Table 1~~   Faraday rotation
}\\[2mm]

\fns \tabcolsep 4mm
\begin{tabular}{llllll}
\hline\\[-3.6mm]\hline
 \multicolumn{6}{l}{$\rm RM = (\Theta_1 - \Theta_2)/(\lambda^2_1 -
    \lambda^2_2)$, where $\Theta$ is in radians and $\lambda$ in
    meters.}\\
\multicolumn{6}{l}{Examples of rotation (in degrees) at several
    wavelengths are:}\\
    \\

$\lambda$ [cm] & RM [rad/m$^{-2}$] &1  &10   &100  &1000\\
 \hline
91           &$\Theta$ [degrees]   &70              &700 \\
74           &                     &31.2            &312
  &3120 \\
21           &                     &2.5     &25
  & 250  &2500 \\
11           &                     &0.69    & 6.9
  & 69    & 689 \\
 6    &                     & 0.205   & 2.05
  & 20.5  & 205 \\
 2.8  &                     & 0.045   & 0.45
  &  4.5 & 44.7 \\
 0.9  &                     & 0.0046  & 0.046
  &i 0.46 & 4.6 \\
  \hline
\multicolumn{6}{l}{electron density in $\rm cm^{-3}$ and dl is the
   path length in parsecs.}\\
 \hline\\[-3.6mm]\hline%\\[-4.5mm]\hline
 \end{tabular}
 \ec

\para{\large\bf{3}~~Pulsars and EGRSs}

\vs\vs Pulsars and extragalactic radio sources are observed
through the Galactic foreground. As seen in Table 1 the effects of
pulsar RM or on EGRS RM can be considerable if they pass through
dense thermal regions. These thermal regions can be identified
either from high frequency radio surveys or the more recent
H$\alpha$ surveys. We (Mitra et al. 2002) have studied the RM of
pulsars towards the anticentre of the Milky Way. Towards $l =
150^{\circ}$ several pulsars are seen through the HII region S205.
This HII region increases the Dispersion Measure of the observed
pulsar and changes the sign of the RM. It seems HII regions are
involved in a coupling process to the magnetic field. The possible
schematic model for this situation is shown in Figure~3. A winding
magnetic field has a $B_\parallel$ component in one region towards
the observer and in other region away from the observer.

\begin{figure}[h] \center {\includegraphics [bb=229 158 609 509
,scale=0.5]{rw2-fig3.eps}}

   %\parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {\center
   \footnotesize Fig.\,3~~ A schematic model showing the $B$ field orientation
towards the Perseus arm}

    \end{figure}




\para{\large\bf{4}~~The `Tomography' of the MISM}

\vs\vs In medical investigations a patient is scanned in thin
slices so that the doctor can in the end reconstruct a
three-dimensional picture of the patient. Using radio polarization
data at several frequencies and the Faraday rotation data from
pulsars and EGRS we may get enough information to reconstruct the
magnetic field structure in our Galaxy. In the Faraday data of
pulsars or EGRS we need good sampling of the Galaxy, a situation
that has not yet been reached. The surveys are just getting to a
situation that well sampled data becomes available. In Effelsberg
the observations of a Medium Latitude Survey ($b = \pm
20^{\circ}$) at 1.4\,GHz (with polarization) were completed. In
Australia a survey of the Galactic plane at 1.4\,GHz is in
progress. We still need a survey at some higher frequency, say
$\lambda = 6$\,cm, to have the good information about a wide band
of the Galactic plane and to understand the Faraday effects. This,
I hope, will be done with the Urumqi telescope. We also need more
RMs of pulsars. At present only some $\sim\,400$ RMs are known.
Also a large sample of  EGRS is needed, some of which is under way
in Canada (Brown \& Taylor 2001). Only through the combination of
all this data a model of the magnetic field of the Galaxy will be
possible.

\bc {\bf References} \ec

\vspace{-4mm} \no \fns

 \no Beck R., Phil. Trans. R. Soc. Lond. A,  2000, 358, 777\\
\no Brouw W.N., Spoelstra T.A.Th., A\&AS, 1976, 26, 129\\
\no Brown J.A., Taylor A.R.,ApJ,  2001, 563, L31\\
 \no Duncan A.R., Reich P., Reich W., et al., 1999,
           A\&A, 350, 447\\
\no Duncan A.R., Haynes R.F., Jones K.L., et al.,
            MNRAS, 1997, 277, 36\\
\no Geansler B.M., Dickey J.M., McClure-Griffith N.M., et
           al.  ApJ, 2001, 549, 959\\
\no Haslam C.G.T., Salter C.J., Stoffel H., et al.,  A\&AS, 1982,
47, 1\\
 \no Han J~L.,  Wielebinski R.  Chin. J.
Astron. Astrophys., 2002, 2, 293\\
 \no Mitra D.,
Wielebinski R., Kramer M., etal.,
           A\&A, 2002, (submitted)\\
\no Speolstra T.A.Th.,  A\&A, 1984, 135, 238\\
 \no
Uyaniker B., F\"urst E., Reich W., et al.  A\&AS, 1999, 138, 31\\

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