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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 W. Reich: Observations of Galactic Magnetic Fields  %%（作者：题目名称）
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\zihao{5}\songti

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    \begin{document}


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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 Observations of Galactic Magnetic Fields   %%（文题）
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Wolfgang Reich}  %%（本文作者）

\vs
({\footnotesize Max-Planck-Institut f\"ur Radioastronomie,
Auf dem H\"ugel 69 }~ 53121 Bonn, Germany) \\%%（本文作者单位）

\vs\vs \ec

\normalsize
\begin{quotation}

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
Radioastronomical methods are used to reveal the properties of Galactic magnetic
fields. Their strength
and regularity are derived from multi-wavelength observations of synchrotron
emission and its percentage
polarization. Faraday effects, which may cause depolarization and enhance
the rotation measure of extragalactic
sources, pulsars and supernova remnants, need to be taken into account. Rotation
measures carry information on the magnetic field strength
along the line of sight coupled to the warm interstellar medium. Recently, substantial
progress has been
made in Galactic polarimetry tracing magnetic fields on scales ranging from sub-pc to
several hundred pc.
This information is essential to understand the field properties on kpc scales, which
at present is easier to be observed in nearby galaxies.

New results from polarization surveys and follow-up observations made with the Effelsberg 100-m
telescope are discussed. Multi-frequency data
show a highly structured magneto-ionic interstellar medium. Analysis of Faraday
rotation effects probe
the properties of the interstellar medium. Locally large magnetic field and thermal
electron enhancements
compared to average interstellar values are noted. High-frequency
observations reveal the unusual properties of the poloidal magnetic field in the
Galactic Center region.
Polarized Galactic high latitude emission acts as a 'foreground' for observations
of the cosmic microwave
background and needs to be known at high frequencies.

\end{quotation}

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

\vs\vs
The emission at radio frequencies up to several GHz is mostly
synchrotron radiation. Relativistic electrons moving in a magnetic field
are deflected and
convert a small fraction of their kinetic energy into
radio emission. The strength of synchrotron radiation is highly dependent on
the magnetic field strength, while its spectrum reflects the energy distribution
of the radiating electrons, mostly well fitted by a power law. The
orientation of the linear polarization shows the direction
of the magnetic field, while its percentage is a measure of magnetic field
regularity. Faraday rotation occurs when linearly polarized radiation passes
through a magneto-ionic medium. The rotation measure (RM), defined as
RM\,$\rm\,[rad/m^{2}]$\,=\,0.81\,$\rm n_e\,[cm^{-3}]\,<B_{||}>\,[\mu G]\,l\,[pc] $,
quantifies the Faraday rotation and carries physical information of the
interstellar medium (ISM).

\para{\large\bf{2}~~The Large-Scale Field}

\vs\vs The magnetic field structure  is at present known for a fairly
large number of normal spiral galaxies (see the review of Beck (2001) and
references therein).
Galaxies seen face-on show the disk magnetic field to run
along the spiral arms, whereas it is more regular in the interarm regions.
Some galaxies
show thick disk or halo emission when seen
edge-on, whereas the polarized emission indicates the existence of
magnetic fields up to kpc-distances from the disk. The most reliable observations
of galaxies are
from combinations
of synthesis telescope data (e.g. VLA) with single-dish data from the Effelsberg
100-m telescope.
However, despite of
arcsecond angular resolution the spatial resolution is just of the order of a
kpc and any small-scale
structure is smoothed out.

\begin{figure}[h]
\center {\includegraphics [bb=0 89 408 479,scale=0.7]{f1.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,1~~ The mean absolute RM dependence of extragalactic sources
      from Galactic latitude b.
Extragalactic radio sources with detected X-ray emission by ROSAT
(Reich et al., 2000)
have higher RMs than those from the unbiased SKB-sample (Simard-Normandin,
Kronberg \& Button, 1982).
The lower envelope of the SKB-sample is the Galactic contribution to the observed RMs.}

    \end{figure}
    \begin{figure}[tbph]
\center {\includegraphics [bb=0 275 397 461,scale=0.8]{f2.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,2~~ The 'Effelsberg 1.4\,GHz Medium Galactic Latitude Survey' area
with some prominent regions indicated.}

    \end{figure}

Our position inside the Galaxy makes the determination of its global
radio structure difficult. Beuermann, Kanbach
\& Berkhuijsen (1985) have deconvolved the 408-MHz all-sky survey of
Haslam et al. (1982) with the additional
assumption of a spiral arm pattern. They find a thin and a thick disk for the
Galaxy with the magnetic
field in the plane running along the spiral arms as expected. Its strength can
be estimated by different
methods and a smooth decrease from the inner Galaxy to its outer regions is
indicated. In the solar vicinity
a value for the total field strength of about 6\,$\mu$G
(Strong, Moskalenko \& Reimers 2000) seems most likely. Although the global
properties of
the Galaxy are less well known than those for some nearby galaxies, parsec-scale
objects can
only be studied in the Galaxy because of much larger spatial resolution.
Ideas on the generation and structure of large-scale magnetic fields
can be proven that way.

\para{\large\bf{3}~~ Rotation Measures of Polarized Sources}

\vs\vs
Rotation measures (RMs) of distinct sources shining through the Galaxy
are well suited to find the
magnetic field properties along the line of sight. Extragalactic sources
give data for the global magneto-ionic
medium, while pulsars trace the medium within a few kpc from the sun.
While pulsars have no intrinsic
RM that of extragalactic sources can be quite dominant. Fig.~1 illustrates
differences of intrinsic RMs for
two source samples: One with strong X-ray emission has systematically higher
RMs than
the other without any source selection.
Thus a check of the source properties is needed, and in addition the source
samples need to be sufficiently
large. A detailed analysis and interpretation of available pulsar RM data
was made by Han, Manchester \& Qiao (1999) and Han (this volume). In brief,
results for the magnetic
field direction in spiral arms and
systematic high-latitude variations of the Galactic magnetic field were
derived and compared with dynamo model
predictions.

\para{\large\bf{4}~~ Polarization Surveys of the Galactic Plane}

Polarization observations of Galactic emission trace the magnetic field perpendicular
to the line of sight and the percentage polarization is related to the regularity of
the field. A series of polarization surveys up to high Galactic latitudes were
carried out with the Dwingeloo 25-m telescope in the frequency range up to 1415\,MHz
(Brouw \& Spoelstra 1976).
They reveal a relatively smooth polarized emission and low RMs. This was somewhat
expected
from total intensity surveys showing smooth emission too. In consequence these
results did not
motivate more detailed polarization
surveys. The Effelsberg 2.695\,GHz survey measurements of the Galactic plane
(Reich et al. 1984)
simultaneously recorded polarized emission. These data were published by Junkes,
F\"urst \& Reich (1987)
showing patchy polarization related to the diffuse Galactic emission. These
results, together
with measurements from other telescopes, make it clear that
the polarized Galactic emission is much more structured on small scales
than previously believed.

Following the 2.695\,GHz survey, which covers the Milky Way within $10^{\circ}$, the
'Effelsberg 1.4\,GHz Medium Galactic Latitude Survey'
was carried out for a $40^{\circ}$ wide band including polarization (see Fig.~2 for
details).
This survey
ran between 1994 and 2001 and needed about 1000~h of net-observing time. About 70\% of
the data have been reduced by now and the analysis of the first fields was made.
The survey calibration onto an absolute scale and example maps of the
survey showing the principal properties in various directions were already
published by Uyan{\i}ker et al.
(1998, 1999). These first maps are accessible via the
web: $\tt http://www.mpifr-bonn.mpg.de/survey.html$. Also a number of other
surveys can
be accessed from there.


In Fig.~3 one of the example maps in the Galactic anticentre is
displayed. It shows rather smooth total intensity (with
extragalactic sources superimposed) and highly structured linear
polarization. Because the polarization structures have no distinct
counterparts in total intensity they do not result from polarized
emission regions, but reflect Faraday rotation and depolarization
of polarized background emission, which in a complex way adds
 with emission from the foreground.
Polarization vectors are rotated and/or scaled additionally and
finally added to an unaffected vector. This way enhancements or
depressions relative to the surroundings of the Faraday screen
occur. It is also quite important whether the measurements are on
an absolute or a relative scale. This makes a significant
difference as shown in Fig.~5. Nevertheless, Faraday screens are a
new powerful tool to investigate the magneto-ionic component of
the interstellar medium.
\begin{figure}[tbph]
\center {\includegraphics [bb=0 60 408 488,scale=0.8]{f3.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,3~~ Section of the Galactic anticentre taken from Uyan{\i}ker et al. (1999).
The total intensities are rather smooth and rise towards the Galactic plane.
Numerous extragalactic sources are superimposed (top). In contrast highly structured
polarized intensities are seen with no relation to total intensity structures (bottom).}

    \end{figure}
\begin{figure}[tbph]
\center {\includegraphics [bb=0 242 408 438,scale=0.85]{f4.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,4~~ Polarized emission at 1.4\,GHz and 2.695\,GHz (Duncan et al., 1999)
convolved to 9.35' angular resolution.
Contours show total intensities, with numerous intense sources concentrated
along the Galactic plane. Polarized intensities (greyscale) and
polarization E-vectors are superimposed.}

    \end{figure}

Towards the Galactic anticentre (Fig.~3) the observed polarization
structures are more clearly pronounced than those seen towards the
inner Galaxy (Fig.~4). This obviously reflects the difference in
the length of sight. Towards the inner Galaxy structures rapidly
change with frequency as is demonstrated in Fig.~4, where
polarized regions visible at 1.4\,GHz disappear at 2.7\,GHz and
new features show up at the higher frequency. This behaviour
clearly indicates that a number of Faraday screens along the line
of sight act at different distances. Multi-frequency observations
are needed for an analysis and decomposition. To reveal the
intrinsic structure of the magnetic field at large distances
high-frequency observations are essential, where RMs have
decreasing influence.

\vs\vn{\bf{4.1~~Analysis of Faraday Screens}}

The most severe limitation in analysing the physical properties of Faraday screens
is their unknown distance. A recent study of polarized emission from the
Effelsberg 1.4\,GHz survey towards the Taurus molecular cloud complexes showed clear
relations and thus constrained the distance of Faraday screens (Wolleben, 2001).
Adding 1.7\,GHz observations an analysis of the Faraday screens was possible, and
a number
of high-RM structures at the surface of molecular clouds were investigated.
The typical sizes of these Faraday screens are about 2~pc and RMs up to about
40\,rad $\rm m^{-2}$
were derived. Excessive $\rm H{\alpha}$ emission is missing and also no excess
of thermal radio emission is seen. The magnetic field strength along
the line of sight must exceed 20\,$\mu$G. It is not obvious which effect
causes such strong magnetic field enhancements.

The determination of the polarized component up to a certain distance sets a
lower limit for
the synchrotron emission up to that distance for the case of a perfectly regular
magnetic field.
Towards the Taurus clouds with about 140~pc distance polarized emission ranging
between
150 and 200\,mK at 1.4\,GHz at least implies about 300\,mK for the total intensity.
This is quite
a large value and should be compared with previous estimates which give about 0.3\,K/kpc.
If this excess is not a local anomaly of the Taurus complex, it will affect Galactic synchrotron
models (e.g. Beuermann, Kanbach \& Berkhuijsen 1985), in particular by reducing the high
latitude (halo or thick disk) emission.


\para{\large\bf{5}~~High Latitude Polarization}

\begin{figure}[tbph]
\center {\includegraphics [bb= 0 257 408 433,scale=0.85]{f5.eps}}

   \parbox[tb]{12cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,5~~ Polarized high latitude emission at 1.4~GHz. The left image shows an
Effelsberg map with relative zero levels in Stokes U and Q. The right map has
added large-scale structures
as taken from the Dwingeloo survey (Brouw \& Spoelstra, 1976).}

    \end{figure}

Although RM on average decreases with Galactic latitudes (Fig.~1),
Faraday screens on small scales are known to exist  also at high
Galactic latitudes (e.g. Wieringa et al. 1993; Haverkorn, Katgert
\& de Bruyn  2000; Reich et al. 2002). In Fig.~5 we display as an
example an Effelsberg 1.4\,GHz image of a $5^{\circ} \times
5^{\circ}$ large
 field at $l,b = 109^{\circ}, 73^{\circ}$
(Reich et al. 2002). Polarized emission features
on scales up to
about $1^{\circ}$ are visible on the Effelsberg map. They are not correlated
to total intensity
enhancements or depressions and
therefore are acting as Faraday screens affecting polarized background  emission.
These features
are not characterised by excessive H$\alpha$ emission (although deep observations
are missing)
that an upper limit for the thermal electron column density is set by the noise in
the radio continuum image.

It is rather important to add the large-scale smooth emission to
the Effelsberg image, which is on a relative level due to
zero-level definition at the edges of the map. The absolutely
calibrated large-scale emission is estimated from the Dwingeloo
survey of Brouw \& Spoelstra (1976). For the field displayed in
Fig.~5 the large-scale component dominates by more than 90\%
(although limitations in the Dwingeloo survey imply about 30\%
uncertainties). However, different to total intensities the
inclusion of large-scale structures is not a simple addition as
for total intensities but a vector addition by adding Stokes U and
Q which may have different signs. Thus features in the Effelsberg
map become $enhanced$ or $depressed$, depending on their vector
orientation relative to the main field direction. The actual
amount of RM imposed by the Faraday screen depends on its distance
and the relative strength and magnetic field direction of the
foreground and background emission components.

\vs\vn{\bf{5.1~~Influence on the Cosmic Microwave Background
Emission}}

Cosmic microwave background (CMB) experiments have nowadays
sensitivities to study details of CMB fluctuations. The angular
power spectrum derived from these experiments reflects the
evolution of the early universe. CMB studies have to take into
account the Galactic foreground. Contamination by the rather
smooth synchrotron emission observed in cold high Galactic
latitudes does not seem to be a serious limitation. Possibly
undetected flat-spectrum extragalactic sources may be problematic.
However, the next generation of CMB experiments will include
studies of the polarized fluctuations to decide on cosmological
models. These signals are much fainter than total intensity
fluctuations. Extrapolations of angular power spectra of the
polarized emission indicate that measurements up to about 60\,GHz
might be affected by the Galactic foreground (Tucci et al. 2000).
However, the large unknown in these estimates is the amount of
Faraday rotation, which obviously distributes power from large
scales into small scales. As discussed above, Faraday screens even
act at very high Galactic latitudes so that new high-frequency
measurements are needed. It is planned to use the Urumqi 25-m
telescope for 5\,GHz polarization measurements to be combined with
Effelsberg 1.4\,GHz polarization data at the same angular
resolution. This way it is expected to significantly improve
predictions for the polarized Galactic foreground at high
frequencies.
\begin{figure}[tbhp]
\center {\includegraphics [bb=114 0 369 504,scale=0.75]{f6.eps}}
\parbox[tb]{10cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize

      Fig.\,6~~ The vertical Arc structure in the Galactic Centre region
traces a strong poloidal magnetic field as seen by high-frequency
observations. Very high RMs ranging up to several thousand $\rm
rad\,m^{-2}$ were derived. The magnetic field strength of the Arc
is a few hundred $\rm \mu G$ in the diffuse and up to 1\,mG within
its narrow filaments. The 32\,GHz image is from the Effelsberg
100-m telescope convolved to an angular resolution of 36'' (Reich,
in prep.). The Sgr A emission complex is located west to the Arc
and shows weak but rather complex polarization. The Galactic
coordinates are relative.}

    \end{figure}

\para{\large\bf{6}~~The Magnetic Field in the Galactic Center Region}

\vs\vs The magnetic field in the Galactic Center deviates largely
from that in the Galactic disk. Strong and very thin synchrotron
filaments were detected, which trace a vertical magnetic field.
Its strength has been estimated to be one or two orders of
magnitude larger than in the Galactic disk. Fig.\,6 displays an
Effelsberg 32\,GHz image. This frequency is high enough to observe
the polarized Galactic Center emission without being largely
affected by depolarization. Most of the numerous filaments are
visible only at low frequencies as they have a rather steep
spectrum, except for the most intense Arc structure which was
shown to have a flat or even inverted spectrum (Reich et al.
1988). Recently, enhanced 150~GHz emission was detected from that
part of the Arc (Reich, Sofue \& Matsuo 2000), which is in
interaction with the molecular cloud G0.108-0.108 (Tsuboi, Ukita
\& Handa 1997). Here the most energetic particles show up.
Magnetic reconnection at the surface of the molecular cloud moving
perpendicularly to the strong vertical field is proposed as the
most likely acceleration process for a quasi-monoenergetic
electron spectrum, which is required to illuminate the Arc (Reich,
Sofue \& Matsuo 2000).

\para{\large\bf{7}~~Summary and Outlook}

\vs\vs
The analysis of polarized synchrotron emission is a powerful tool to study magnetic field
structures and the magneto-ionic component of the interstellar medium.
Large-scale surveys are on the
way and these trigger follow-up measurements at high frequencies to investigate the
nature of Faraday screens. Additional data are needed to constrain their distance.
Sensitive measurements are required to estimate the amount of thermal electron densities.
The results obtained so far
indicate strong local enhancements of magnetic fields and/or electron densities in
the interstellar medium. Their origin largely seems unknown so far. Beyond the
properties of the Galactic emission these investigations are also relevant to
estimate the foreground influence on sensitive CMB studies. Future observational
needs are absolutely calibrated polarization measurements, which require substantial
efforts, and extensions of systematic surveys towards higher frequencies.

\bc
{\bf References}
\ec

\vspace{-4mm}
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Duncan A R, Reich P, Reich W, et al. 1999, A\&A, 350: 447 \nref
Han J L, Manchester R N, Qiao G J.  MNRAS, 1999, 306: 371 \nref
Haslam C G T, Salter C J, Stoffel H, et al. A\&AS, 1982, 47: 1
\nref  Haverkorn M, Katgert P, de Bruyn A G. A\&A, 2000, 356: L13
\nref  Junkes N, F\"urst E, Reich W A\&AS, 1987, 69: 451 \nref
Reich W, F\"urst E, Steffen P, et al. A\&AS, 1984, 58: 197 \nref
Reich W, F\"urst E, Reich P, et al. A\&A, 2000, 363:  141 \nref
Reich W, Sofue Y, Wielebinski R, et al. A\&A, 1988, 191: 303 \nref
Reich W, Sofue Y, Matsuo H. 2000, PASJ, 52: 355 \nref  Reich W,
F\"urst E, Reich P, et al, 2002, in "Astrophysical Polarized
Backgrounds", eds. S. Cecchini, S. Cortiglioni, R. Sault \& C.
Sbarra, p. 3 \nref  Simard-Normandin M, Kronberg K, Button S.
ApJS, 1982, 45: 97 \nref  Strong A W, Moskalenko I V, Reimers O.
ApJS, 2000, 537: 763 \nref  Tsuboi M, Ukita N, Handa T. ApJ, 1997,
481: 263 \nref  Tucci M, Caretti E, Cecchini S, et al. New
Astronomy, 2000, 5: 181 \nref  Uyan{\i}ker B, F\"urst E, Reich W,
et al.  A\&AS, 1998, 132: 401 \nref  Uyan{\i}ker B, F\"urst E,
Reich W, et al, A\&AS, 1999, 138: 31 \nref  Wieringa M H, de Bryn
A G, Jansen D, et al. A\&A, 1993, 268: 215 \nref  Wolleben M.
2001, Diploma Thesis, Bonn University


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